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Electrical System Analysis Of Hybrid Electric Vehicle

Introduction

The trend to save the environment for future generations while at the same time maintain our current lifestyle has proved to be a constant struggle. One of the most discussed and debated issue of modern time is the increased use of petroleum based products for automobiles. Cars are considered consumer goods. Automobiles are run using an internal combustion (IC) engine that burn hydrocarbons to generate energy that helps move the vehicle. Currently, the two most commonly used hydrocarbons are gasoline and diesel.

The growing dependence on imported oil, along with a heightened concern about the environment, has led to our increased interest in electric cars as an alternative to traditional gas-powered automobiles. Battery systems for electric vehicles are improving, but with their limited range of travel, they are still not feasible for most people. In addition, we believe that the average person making the decision to purchase environmentally friendly vehicles would demand that those vehicles be comfortable, attractive, convenient, and affordable to purchase and maintain. Newly available automotive technology, known as hybrid electric vehicles (HEV), appears to meet these requirements. Hybrid power systems were conceived as a way to compensate for the shortfall in battery technology (Office of Transportation Technologies, HEV program). Hybrid electric vehicles recharge as it is driven, get approximately double the miles per gallon of gas than current vehicles (Toyota, technology) and can be refueled at any gas station. Each hybrid vehicle will produce thousands fewer pounds of pollutants than the vehicles currently on the road. According to Department of Energy estimates, a hybrid car driven 12,000 miles per year will cut carbon dioxide emissions by 4,700 pounds over its predecessor, says the National Resources Defense Council article on earth smart cars.

Hybrids will allow drivers to get between 20 and 30 miles per gallon more than standard automobiles. With this kind of savings, it won't take long to make up the additional cost of the hybrid. Hybrids save on gas in a number of ways. All hybrids shut off the gas engine automatically when the car is stopped. The engine turns back on when the driver presses the gas pedal. The gas engine will also come on to start charging the batteries when they become low on power. Typically, when a consumer buys products to help the environment the consumer pays more. Hybrids are a refreshing exception where the consumer actually saves money by doing something good for the environment. Not only does fuel efficiency save the drive money, burning less gasoline means that there is less pollution causing emissions released into the atmosphere. There is also a lower level of carbon dioxide, a major contributor to global warming, released into the atmosphere.

HEVs are growing leaders in transportation technology development. Hybrids have great potential for growth in improving the automotive industry, while also reducing serious resource consumption, reliance on foreign oil, air pollution, and traffic congestion. The hybrid's complexity, and the fact that some of the best storage and conversion systems have yet to be fully developed, ignites varied opinions on hybrids' ultimate impact in the marketplace.

In conclusion, hybrid cars are better than traditional gasoline powered vehicles, however they still have problems. Currently hybrid cars seem to be the best solution in combating the devastating global effects of exhaust emissions. With lower emissions and improved fuel economy, hybrids are a great way to travel. However, these lightweight cars are more vulnerable to traffic fatalities and still give off some emissions. They also accelerate at a slower pace than conventional vehicles. Hybrids have a short battery life, and their parts are expensive and not easily accessible, but hopefully as hybrids become more popular, this will change.

Overview

Hybrid SystemsThe hybrid system is the wave of the future. In its simplest form, a hybrid system combines the best operating characteristics of an internal combustion engine and an electric motor. More sophisticated hybrid systems recover energy otherwise lost to heat in the breaks and use it to supplement the power of its fuel-burning engine. These sophisticated techniques allow the hybrid system to achieve superior fuel efficiency. On continuum that is hybrid technology, we typically break things down into full or strong, mild and micro hybrids which are also known as simply stop-start engine hybrid. A mild hybrid relies on the internal combustion engine to provide constant power for moving the vehicle but is incapable of propelling the vehicle alone. Full hybrids use a gasoline engine as the primary source of power like solar, fuel cells, battery etc; and electric motor provides additional power when needed. All these have been in chapter 1.

Battery TechnologyCentral to the discussion regarding the relative merits of the various hybrids is the big box that stores the energy to propel the electric motor—the battery. The battery is responsible for 25 - 75% of the increased weight, volume, and cost associated with the various hybrid configurations. Today most hybrid car batteries are Nickel metal hydride or Lithium-ion; both are regarded as more environment friendly then lead-based batteries which constitutes the bulk of car batteries today. The Lithium-ion battery has attracted attention due to its potential for use in hybrid electric vehicles. In addition its smaller size and lighter weight, Lithium-ion batteries deliver performance that helps to protect the environment with features such as improved charge efficiency without memory effect. The battery industry is currently working on the development of better performing and more sophisticated technology that costs less. In chapter 2, all the battery aspects are elaborated. Electrical and Thermal managementThe battery charger is a bidirectional ac-dc converter, recharge mode is ac to dc conversion and inverter mode is dc to ac conversion. To implement the plug-in function a single phase bidirectional ac-dc converter interfacing with the grid is essential. A dc-dc converter balances the voltage between the electric motor and the energy storage device, in a hybrid, boosting or reducing the voltage as necessary, which provides more of the energy under braking and under acceleration.

Regenerative braking means capturing the vehicles momentum (kinetic energy) and turning it into electricity that recharges the on board battery as the vehicle is slowing down or stopping. The super capacitor is an electro-chemical capacitor that has usually high energy density compared to common capacitors. These can quickly store large amount of electricity and discharge the electricity on demand to batteries.

The thermal management system delivers a battery pack an optimum average temperature with only small variations between the modules and within the pack. An ideal thermal management system should be able to maintain the desired uniform temperature in pack by rejecting heat in hot climates, adding heat in cold climates and providing ventilation if the battery generates potentially hazardous gases. The entire chapter 3 is about electrical and thermal management of the hybrid system.

Chapter 01

Hybrid System

1.1 Stop-Starts Hybrids

A stop-start hybrid is the simplest kind, but this minimal technology may become the most common within a few years.

Figure 01: Start-stop hybrid

It is composed simply of an energy storage device—like a battery—and a beefed-up starter-motor that can also act as a generator.

Stop-Start systems are also called idle-stop—because it puts an end to burning fuel and emitting pollutants when a conventional car would be idling.

In practice, the car’s engine control unit shuts off the engine when the car slows down or comes to a stop. As soon as the driver puts in the clutch, moves the shift lever, or accelerates, the battery powers the starter motor, which quickly switches on the engineStart-stop systems are the lowest-cost hybrid alternative, but if fitted to large numbers of cars, they could substantially reduce fuel consumption and air pollution from idling vehicles, especially in crowded city centers.

1.2 Mild Hybrid

A mild hybrid is a type of gasoline-electric hybrid that uses an internal combustion engine to power the vehicle at all times. An electric motor is incorporated only as a power booster of sorts, as a starter-generator, or both. While some mild hybrids use an electric drive motor to provide a gasoline engine with extra power, it cannot ever propel the vehicle on its own. Mild hybrids save fuel by shutting engine power off under most circumstances when the vehicle is stopped, braking, or coasting. The engine restarts seamlessly and efficiently. Electric accessories like the radio or GPS navigation continue to function with the engine off.

How They Are DifferentAll mild hybrids are less expensive than full hybrid systems because they require less sophisticated components and less battery power. Some, but not all, mild hybrids use regenerative braking to recharge the battery. Different mild hybrid configurations exist including Integrated Starter-Generator (ISG) and Belt Alternator Starter (BAS) systems.

The basic premise of the mild hybrid is same as the strong hybrid. An electric motor/generator operates in parallel with the internal combustion engine to provide additional drive torque as well as regenerative braking. The primary difference lies in the power and energy capacity of the electrical side of the system.

Figure 02: Next generation GM hybrid system

The GM system uses what is essentially a beefed up alternator and modified belt drive system to provide some additional drive torque to the engine as well as re-start it. During deceleration the mild hybrid system can also provide some regenerative braking capability.

Benefits of Mild HybridsMainly to get some of the benefits of a hybrid system at a significantly lower cost and weight Mild hybrids typically have a much smaller battery than a strong hybrid and a smaller, weaker motor/generator. Mild hybrids do provide a modest improvement in fuel efficiency of 10 to 15 percent because they're not burning gas when stopped. While a mild hybrid system can't drive the vehicle on electricity alone, it still provides benefits. Like direct injection and turbo charging, it allows the automaker to downsize the base engine while maintaining the same performance level. The combination of the reduced peak output of the engine and eliminating engine idle can contribute fuel consumption savings of up to 15 percent in urban driving and 8-10 percent overall. Because they are much less costly than full hybrid systems, a greater number of drivers are likely to choose mild hybrids and realize better fuel efficiency than would otherwise be the case. This easy entry into the world of hybrids will serve to familiarize drivers with hybrid technology and potentially encourage drivers to choose a full hybrid for their next vehicle.

1.3 Full Hybrid

A hybrid vehicle classification in which the arrangement of the electric drive motor, the internal combustion engine and battery system allow the hybrid vehicle to be powered solely by the electric motor under certain operating conditions—generally low speed maneuvering and light cruising. When additional power is needed, the engine kicks in and both power plants work together to propel the vehicle.

In addition, full hybrids can use the electric motor as the sole source of propulsion for low-speed, low-acceleration driving, such as in stop-and-go traffic or for backing up. This electric-only driving mode can further increase fuel efficiency under some driving conditions.

Figure 03: full hybrid vehicle

Starting: When a full hybrid vehicle is initially started, the battery typically powers all accessories.

The gasoline engine only starts if the battery needs to be charged or the accessories require more power than available from the battery.

Figure 04: initially started full hybrid vehicle

The battery stores energy generated from the gasoline engine or during regenerative braking from the electric motor. Since the battery powers the vehicle at low speed, it is larger and holds much more energy than batteries used to start conventional vehicles.

The gasoline engine in a hybrid is much like those in conventional vehicles, except that it is usually much smaller and more efficient.

The electric motor powers the vehicle at low speed and assists the gasoline engine when additional power is required. It also otherwise converts wasted energy from braking into electricity and stores it in the battery.

The generator converts mechanical energy from the engine into electricity which can be used by the electric motor or stored in the battery. It is also used to start the gasoline engine instantly when needed.

The power split device is a gear box connecting the gasoline engine, generator and electric motor. It allows the engine and motor to power the car independently or in tandem and allows the gasoline engine to charge the batteries or provide power to the wheels as needed.

Low speed: For initial acceleration and slow-speed driving, as well as reverse, the electric motor uses electricity from the battery to power the vehicle. If the battery needs to be recharged, the generator starts the engine and converts energy from the engine into electricity, which is stored in the battery.

Figure 05: converting energy from the engine into electricity

Cruising part 1: At speeds above mid-range, both the engine and electric motor are used to propel the vehicle. The gasoline engine provides power to the drive-train directly and to the electric motor via the generator.

Figure 06: the engine and electric motor for propel the vehicle

Cruising Part 2: The generator can also convert energy from the engine into electricity and send it to the battery for storage.

Figure 07: converting energy from the engine into electricity

Passing Part 1: During heavy accelerating or when additional power is needed, the gasoline engine and electric motor are both used to propel the vehicle.

Figure 08: During heavy acceleration the gasoline engine and electric motor for propel the vehicle

Passing Part 2: Additional electricity from the battery may be used to power the electric motor.

Figure 09: battery for power the electric motor

Braking Part 1: Regenerative braking converts otherwise wasted energy from braking into electricity and stores it in the battery. In regenerative braking, the electric motor is reversed so that, instead of using electricity to turn the wheels, the rotating wheels turn the motor and create electricity. Using energy from the wheels to turn the motor slows the vehicle down.

Figure 10: reversing electric motor during the regenerative braking

Braking Part 2: If additional stopping power is needed, conventional friction brakes (e.g., disc brakes) are also applied automatically.

Figure 11: Need of additional stopping power

Stopped: When the vehicle is stopped, such as at a red light, the gasoline engine and electric motor shut off automatically so that energy is not wasted in idling. All other systems, including the electric air conditioning, continue to run.

Figure 12: shut off gasoline engine and electric motor

1.3.1 Power Sources

(a) SolarRenewable energy sources are being used all over the world. For example, wind energy, solar power, Hybrid cars and using methane gas for stove fuel. These are all excellent examples of renewable energy as they are all affordable and convenient as well as being efficient. Solar power is an excellent energy resource. Cars can now be solely power by solar panels, creating huge possibilities. If solar energy was used worldwide, along with other renewable sources, the world would be cleaner and more pleasurable to live in, knowing that the greenhouse gases are under control.

Solar Powered Cars or Solar Cars are first step towards developing nature friendly vehicles with the help of solar power. Solar Cars also gain energy from Sun. Hundreds of photo voltaic cells are used by these cars to convert sunlight into electricity. Each cell is capable of producing about one half volt of Electricity

Electrical systems in Solar Cars are designed so as to allow for variations in sunlight. The motor and battery of car is charged by energy from sun. This charged battery is used when Sun is hidden by a dense cloud. It takes lot of efforts and meticulous plan to design such Solar powered vehicles because if all energy is diverted towards driving then nothing will be kept in reserve and car will come to halt once when it is taken out on cloudy day. Engine of solar car slows down if too much energy is diverted to the battery. So Solar Powered Cars are designed so as to make it more efficient to run at top speed.

Solar powered vehicles deliver high performance due to their extreme lightness of weight and excellent aerodynamics. But that is also one of the big drawbacks of these vehicles because any vehicle which is designed to ferry passenger in their routine life is heavier and less aerodynamic in nature so that they can achieve more speeds. So it would be better not to design cars which are purely powered by solar energy. The average speed achieved by Solar Powered Cars is 37 miles per hour.

(b) Fuel cellsA fuel cell is an electricity generation system that does not convert the chemical energy of fuel into heat by combustion, but electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. Such a fuel cell can be applied to the supply of electric power for small-sized electrical/electronic devices such as portable devices, as well as to the supply of electric power for industry, homes, and vehicles.

At present, the most preferred fuel cell for a vehicle is a polymer electrolyte membrane fuel cell (PEMFC), also called a proton exchange membrane fuel cell, that preferably comprises: a membrane electrode assembly (MEA) including a polymer electrolyte membrane (PEM) for transporting hydrogen ions and an electrode catalyst layer, in which an electrochemical reaction takes place, disposed on both sides of the PEM; a gas diffusion layer (GDL) for uniformly diffusing reactant gases and transmitting generated electricity; a gasket and a sealing member for maintaining air tightness of the reactant gases and coolant and providing an appropriate bonding pressure; and a bipolar plate for transferring the reactant gases and coolant.

In the fuel cell having the above-described configuration, hydrogen as a preferred fuel and oxygen (air) as a preferred oxidizing agent are supplied to an anode and a cathode through

flow fields of the bipolar plate, respectively. The hydrogen is suitably supplied to the anode (also called a “fuel electrode”, “hydrogen electrode”, and “oxidation electrode”) and the oxygen (air) is suitably supplied to the cathode (also called an “air electrode”, “oxygen electrode”, and “reduction electrode”). The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H + ) and electrons (e − ) by catalyst of the electrode catalyst layer preferably provided on both sides of the electrolyte membrane. At this time, only the hydrogen ions are selectively transmitted to the cathode through the electrolyte membrane, which is preferably a cation exchange membrane and, at the same time, the electrons are transmitted to the anode through the GDL and the bipolar plate, which are conductors. At the cathode, the hydrogen ions supplied through the electrolyte membrane and the electrons transmitted through the bipolar plate meet the oxygen in the air supplied to the cathode by an air supplier and cause a reaction that produces water. Due to the movement of hydrogen ions occurring at this time, the flow of electrons through an external conducting wire occurs, and thus a current is suitably generated.

If the fuel cell is used as the only power source of an electric vehicle, the fuel cell powers all loads of the vehicle, which results in performance deterioration during operation where the efficiency of the fuel cell is low. Moreover, during high speed operation where a high voltage is required, a sufficient voltage required by a drive motor is not supplied due to a rapid decrease in output voltage, thus decreasing acceleration performance. Furthermore, if a sudden load is applied to the vehicle, the output voltage of the fuel cell suddenly drops and sufficient power is not supplied to the drive motor, thus decreasing vehicle performance and accordingly, a sudden change in load imposes a heavy burden on the fuel cell since electricity is generated by an electrochemical reaction. In addition, since the fuel cell preferably has unidirectional output characteristics, it is impossible to recover energy from the drive motor during braking of the vehicle, thus decreasing the efficiency of the vehicle system.

Accordingly, a fuel cell hybrid vehicle has been developed. Exemplary fuel cell hybrid vehicles include large vehicles, such as a bus, as well as small vehicles that are preferably equipped with storage means such as a high voltage battery or a super capacitor as an auxiliary power source for suitably providing the power required for driving the motor in addition to the fuel cell as a main power source. At present, a fuel cell-storage means hybrid vehicle that does not employ a power converter has been studied, and the fuel cell-storage means hybrid vehicle has high fuel efficiency (e.g. high regenerative braking, high efficiency of super capacitor, and without the use of the power converter), an increase in durability of the fuel cell, suitably high reliability control, and the like.

In the hybrid vehicle in which the fuel cell and the storage means are preferably directly connected, the fuel cell continuously outputs power at a suitably constant level during driving. If electric power is suitably sufficient, the storage means is charged with surplus power, whereas, if the electric power is insufficient, the storage means supplies the insufficient power to drive the vehicle.

The driving mode of the hybrid vehicle including the fuel cell as the main power source and the super capacitor (or a high voltage battery which is a secondary battery) as the auxiliary power source preferably includes an electric vehicle (EV) mode in which the motor is driven only by the power of the fuel cell, a hybrid electric vehicle (HEV) mode in which the motor is driven by the fuel cell and the super capacitor at the same time, and a regenerative braking (RB) mode in which the super capacitor is charged.

However, in fuel cell-super capacitor hybrid vehicle the super capacitor is automatically charged by the fuel cell, which thus restricts the regenerative braking. Accordingly, stopping the operation of the fuel cell during low power operation and during regenerative braking will overcome this restriction. Moreover, it is possible to improve the fuel efficiency by restricting the use of the fuel cell during low power operation where the efficiency of the fuel cell is low.

According to the present invention, the air and hydrogen supply is suitably cut off in the low efficiency region of the fuel cell, and the fuel cell voltage drops by consuming residual oxygen and hydrogen, thus stopping the operation of the fuel cell (EV mode or regenerative braking mode). If the conditions for restarting the fuel cell are suitably satisfied, in which the voltage of the storage means (super capacitor or battery) is below a predetermined reference voltage or the load required by the vehicle is above a reference load, the air and hydrogen supply is restarted to restart the fuel cell (HEV mode). As a result, the present invention has the following effects.

1. Since the operation of the fuel cell BOP components (especially, the air blower) is preferably stopped during low power operation where the efficiency of the fuel cell is low, it is possible to suitably improve the fuel efficiency and the efficiency of the fuel cell system.

2. Since the automatic charge from the fuel cell to the super capacitor is prevented and thereby an increase in the voltage of the super capacitor is suitably prevented, the amount of regenerative braking is increased, thus improving the fuel efficiency.

3. It is possible to suitably improve the durability of the fuel cell by reducing the open circuit voltage (OCV).

4. It is possible to prevent the deterioration of the fuel cell stack, which occurs when oxygen is introduced into the anode, without a loss of improvement in fuel efficiency, and further improve the durability of the fuel cell stack by the control process of the non-power generation region, in which, if the air supply to the fuel cell stack is suitably cut off as the fuel cell stop mode is started, preferably, the hydrogen supply is not immediately cut off and, if the fuel cell voltage is eliminated after maintaining the anode pressure at an optimal level through the hydrogen supply, the load device for voltage elimination is driven and the hydrogen supply is cut off.

5. Since the voltage unexpectedly generated in the fuel cell stack can be immediately or substantially immediately eliminated by driving the load device for voltage elimination along with the hydrogen supply cut off, it is possible to prevent the deterioration of the fuel cell stack and improve the durability of the fuel cell stack.

(c) BatteryA hybrid car basically uses the hybrid car battery for its electrical power, which can make the car run. These hybrid batteries are used in the hybrid car as the second power source. The hybrid battery was previously used in electric cars where there was no other power source except the hybrid batteries. Unfortunately, those hybrids weren't very practical due to the short distance they could go on battery power only.

Hybrid Cars use a rechargeable energy storage system to supplement fossil fuel energy for vehicle propulsion. Hybrid engines are smaller and more efficient than traditional fuel

engines. Some hybrid vehicles use regenerative braking to generate electricity while travelling. The term "Hybrid Vehicle" can also refer to a vehicle engine that uses a combination of different fuels such as petroleum and ethanol.

The most popular hybrid batteries that are being developed are Li-ion, more formally known as Lithium ion batteries. This hybrid battery will soon be used in a lot of hybrid cars as more and more car manufacturers are using the Li-ion battery as a way to power up these hybrid vehicles. The Lithium ion batteries are seen as the power source for the next generation cars and other hybrid vehicles. These Lithium ion batteries cost a lot because they are not only rechargeable but they also last for a long period of time. With the help of these hybrid batteries, hybrid cars will be able to drive well over 100,000 miles before replacement.

These recycling batteries are very important for any hybrid or electric car. The main reason for this is the fact that the electric or the hybrid car should be able to run several miles on electricity after which it needs to be recharged again to ensure again that the car is running. If there is no option for recycling batteries, then it would be a terrible loss considering the fact that the batteries need to be changed again and again - every time the power in the battery is drained. Thus, rechargeable batteries are very important to ensure that the battery can be charged again and again and used for a long period of time. Once a customer can buy a plug-in hybrid electric car, pre-equipped with a hybrid car battery, it will likely be recommended that they charge the hybrid battery cell/pack to its fullest before driving it. The Li-ion battery should be fully charged to ensure that it gives power to its full capacity. Also, the batteries will be built to last for the whole life of the car. So, most of the time, it won’t be necessary to change the hybrid car battery at all, but sometimes the Lithium ion batteries will become damaged due to some external factors and might need to be changed. This situation is very rare and most of the time, the hybrid batteries will not require a change at all. When the batteries need to be changed, the replacement costs are a little high. The life time of a hybrid car battery as said earlier will be up to the life of the car and only in some cases, the life of the battery will reduce.

In such cases when the life of the batteries is over, then it is necessary to take care of the disposal of the hybrid batteries. The disposal should be done properly to ensure that the batteries do not cause any leakage and do not pollute the environment. The hybrid cars are being manufactured more and more each and every day because of the demands for these cars are increasing. Thus, with the help of these hybrid batteries, one is able to save on fuel and one also contributes in reducing pollution.

Chapter: 02

Battery of Hybrid Vehicles

2.1 Battery Technology

Of all recent futuristic technology, the hybrid battery takes the cake as far as most complex. Hybrid cars rely extensively on their batteries, more so than other types of vehicles because they provide power for one of the car’s two main engines. With that being said, there are many types of batteries out there and still more are being developed every day.

Hybrid cars are powered by two types of technologies working together at once: the electric motor and the internal combustion (gasoline) engine. These engines work together to make the car extremely fuel efficient, often turning off the combustion engine when not in use such as at a red light and using regenerative brake technology to recharge the battery for the electric motor. Because the usage of the battery itself is so extensive, it becomes absolutely necessary to charge the battery at all times when not in use. The type of batteries used in a hybrid car must also have a higher capacity than regular batteries.

Like all batteries, hybrid batteries have two electrodes (which collect or emit an electric charge) that sit in an ion-rich solution called the electrolyte. (An ion, by the way, is an atom or group of atoms with an electrical charge.)

The electrodes are typically very close, so a polymer film, called a separator, prevents them from touching, which would create a short circuit. An on-off switch in whatever device is powered by the battery- phone or laptop, bridges the cell’s electrodes to generate power. That’s when the electrochemical reaction begins.

Ionized elements in one electrode are in a chemical state where they are easily attracted to combine with other molecules, emitting electrons (energy) in the process. Those elements are tugged through the electrolyte and the separator toward the opposing electrode. The ions of the negative electrode (anode) give up electrons; the positive ions coming toward the anode accept them. The electrons released during this process travel through the external circuit (e.g. phone), producing a flow of charge in the opposite direction to the flow of ions. During recharge, current is forced into the cell, reversing the process.

2.1.1 Early Revolution

Nickel-Cadmium Batteries Fortunately for us and for the environment, hybrid cars do not use the typically problematic Nickel-Cadmium batteries, which you most commonly see as rechargeable batteries in small devices such as cell phones, digital cameras and remote-controlled toys. These batteries contain lead, which is highly toxic, harmful to the environment, and difficult to recycle. They also have a small energy capacity, which makes them inappropriate for the heavy-duty usage needed to run a hybrid car. These types of batteries can be found under the hood of almost every conventional gasoline-run vehicle, the image of which comes to mind when picturing what’s under the hood of a typical car.

NiMH Batteries (Nickel Metal-Hydride)Currently, this type of battery is being used in hybrid cars. Like its Nickel-Cadmium counterpart, this battery uses the same chemical nickel oxyhydroxide (NiOH) to help it hold a charge. But unlike the Nickel-Cadmium batteries, which uses cadmium for the negative electrode, this type of battery uses a Nickel alloy called Nickel Metal-Hydride for the negative electrode. Due to the absence of Cadmium, which is considered environmentally toxic, this type of battery is more “green.” It is also safer to use and has a higher capacity than traditional Nickel-Cadmium batteries. The downside of this battery is that it still does not have the high-capacity to run a hybrid’s sophisticated electric motor without being charged as much as possible. It is also more expensive than most batteries due to the cost of Nickel. This is currently what keeps the cost of a hybrid car at a premium.

Lithium Batteries

Next on the list of up-and-coming hybrid vehicle technology, the Lithium battery is currently powering small handheld devices such as laptops and cell phones. Lithium batteries have a higher capacity than other types of batteries, and they are also made more cheaply. Lithium batteries could enable hybrid cars to go much longer distances without using a single drop of gasoline, for distances anywhere from 50-100 miles. Although most hybrids are not plug-in models, this would be required with the current technology if these types of batteries were to be used. Another problem with these batteries is that they use cobalt in their formulas, which tends to explode. Automakers are scrambling to find an alternative to cobalt which would provide the same amount of power.

Lithium Ion Battery - For Next Generation Hybrids CarsLithium ion (or Li-ion) batteries are important because they have a higher energy density the amount of energy they hold by weight, or by volume than any other type. The rule of thumb is that Li-ion cells hold roughly twice as much energy per pound as do the previous generation of advanced batteries, nickel-metal-hydride (NiMH) which are used in all current hybrids including the Toyota Prius. NiMH, in turn, holds about twice the energy per pound of the conventional lead-acid (PbA) 12-Volt battery that powers your car’s starter motor. It’s Li-ion’s ability to carry so much energy that makes electric cars possible.

2.1.2 Cathode Contenders

Cobalt DioxideCobalt Dioxide is the most popular choice today for small cells (those in your mobile phone or laptop). It’s been on the market for 15 years, so it’s proven and its costs are known, though like nickel, cobalt is pricey. Cobalt is more reactive than nickel or manganese, meaning it offers high electrical potential when paired with graphite anodes, giving higher voltage. It has the highest energy density but when fully charged, it is the most prone to oxidation (fire) caused by internal shorts. This can lead to thermal runaway, where one cell causes its neighbors to combust, igniting the whole pack almost instantly. Also, the internal impedance of a cobalt cell, the extent to which it “pushes back” against an alternating current, increases not just with use but with time as well. That means an unused five year old cobalt cell holds less energy than a brand-new one.

Cobalt dioxide cells are manufactured by dozens of Japanese, South Korean, and Chinese companies, but only Tesla Motors uses them in an electric car. Their pack uses sensors, cell isolation, and liquid cooling to ensure that any energy released if a cell shorts out can’t ignite any of its neighbors.

Nickel-cobalt-manganese (NCM)Nickel-cobalt-manganese (NCM) is somewhat easier to make. Manganese is cheaper than cobalt, but it dissolves slightly in electrolytes which gives it a shorter life. Substituting nickel and manganese for some of the cobalt lets manufacturers tune the cell either for higher power (voltage) or for greater energy density, though not both at the same time. NCM remains susceptible to thermal runaway, though less so than cobalt dioxide. Its long-term durability is still unclear, and nickel and manganese are both still expensive now. Manufacturers include Hitachi, Panasonic, and Sanyo.

Nickel-cobalt-aluminum (NCA)

Nickel-cobalt-aluminum (NCA) is similar to NCM, with lower-cost aluminum replacing the manganese. Companies that make NCA cells include Toyota and Johnson Controls Saft, a joint venture between a Milwaukee automotive supplier and a French battery firm.

Oxide spinel (MnO Manganese)Manganese oxide spinel (MnO) offers higher power at a lower cost than cobalt, because its three-dimensional crystalline structure provides more surface area, permitting better ion flow between electrodes. But the drawback is a much lower energy density. GS Yuasa, LG Chem, NEC-Lamilion Energy, and Samsung offer cells with such cathodes; LG Chem is one of two companies competing to have its cells used in the Chevrolet Volt.

Iron phosphate (FePo)Iron phosphate (FePo) might be the most promising new cathode, thanks to its stability and safety. The compound is inexpensive, and because the bonds between the iron, phosphate, and oxygen atoms are far stronger than those between cobalt and oxygen atoms, the oxygen is much harder to detach when overcharged. So if it fails, it can do so without overheating. Unfortunately, iron phosphate cells work at a lower voltage than cobalt, so more of them must be chained together to provide enough power to turn a motor. A123 Systems which is competing for the Volt contract as well uses nanostructures in their FePo cathodes, which it says produces better power and longer life. Other manufacturers include Gaia and Valence Technology.

2.1.3 Recently Used BatteriesToyota Prius Hybrid BatteryThe battery pack of the second generation Toyota Prius consists of 28 Panasonic prismatic nickel metal hydride modules each containing six 1.2 volt cells connected in series to produce a nominal voltage of 201.6 volts. The total number of cells is 168, compared with 228 cells packaged in 38 modules in the first generation Prius. The pack is positioned behind the back seat.

The weight of the complete battery pack is 53.3 kg. The discharge power capability of the Prius pack is about 20 kW at 50 percent state-of-charge. The power capability increases with higher temperatures and decreases at lower temperatures. The Prius has a computer that’s solely dedicated to keeping the Prius battery at the optimum temperature and optimum charge level. The Prius supplies conditioned air from the cabin as thermal management for cooling the batteries. The air is drawn by a 12-volt blower installed above the driver’s side rear tire well.

Highlander Hybrid Battery ToyotaThe nickel metal hydride battery used in Highlander Hybrid and the Lexus RX 400h is packaged in a newly developed metal battery casing. The 240 cells can deliver high voltage of 288 volts but the motor-generators units can operate on variable voltage anywhere from 280 volts to 650 volts. The battery pack supplies 288 volts, but the boost converter, a part of the inverter above the transaxle, changes this to 500 volts. This battery pack provides 40 percent more power than the Prius battery, despite being 18 percent smaller.

Each of the modules has its own monitoring and cooling control system. The cooling performance reduces efficiency losses due to excessive heat, ensuring that the battery can supply required electric power to the motors at all times. The battery-monitoring unit

manages discharge and recharging by the generator and motors to keep the charge level constant while the car is running. The battery pack is stowed under the rear seats.

Ford Escape Hybrid BatteryThe Ford Escape Hybrid’s battery pack, made by Sanyo, consists of 250 individual nickel metal hydride cells. As with other hybrid battery packs, the cells are similar in shape to a size D flashlight battery. Each individual battery cell, contained in a stainless steel case, is 1.3 volts. The cells are welded and wrapped together in groups of five to form a module. There are 50 modules in the battery pack. The total voltage of the battery pack is 330 volts.

Honda Insight BatteryThe Honda Insight’s battery pack, made up of 120 Panasonic 1.2-volt nickel metal hydride D cells is capable of 100A discharge, and 50A charge rates. The system limits the usable capacity to 4ah to extend battery life. Total battery pack output is 144 volts. The batteries are located under the cargo compartment floor, along with the Honda Integrated Motor Assist’s power control unit. Honda used technology developed for its EV Plus electric car for the original development of the Insight’s battery system.

2.2 Battery Capacity

The valve regulated lead acid-battery (VRLA) is a maintenance-free lead acid battery operating on the principle known as “sealed, recombination,” wherein all the electrolyte is stored in absorptive glass mats (AGM) separators. The battery must remain sealed for its entire operating life and, to achieve maximum cycle life, must be properly recharged to prevent any excessive overcharge. Excessive overcharge results in excessive gas pressure build-up inside the battery, which is relieved by the opening of the pressure relief valve (typically set at 1.5 psi ± 0.5 psi). Every time the valve opens, water vapor is lost, which in turn reduces battery life.

The battery has been developed from extruded lead onto glass-fiber filaments that are woven into grids (mats) for use as electrode plates. This process provides the desirable crystal structure of lead oxide (PbO2) active material. The battery must be maintained, however, under optimal driving conditions.

The USABC has outlined the performance requirements for VRLA batteries for the near term and the next few years, especially for use in electric vehicle applications. VRLA battery provides up to 95 Whr/L of energy, while the requirements are to increase the energy density to 135 Whr/L over the next few years. This increase in the energy density means that there has to be a significant increase in the battery capacity.

The useful available capacity of the battery (in Ahr) is dependent on the discharge current. This relationship can be expressed in the form

* t = K

where I is the discharge current in A, t (0.1 < t < 3) is the duration of the discharge in hours and n and K are constants for a particular battery type.

Figure 13: The estimated Peukert plot at 80°F for an 80Ahr battery

For example, an 80 Ahr VRLA battery, Peukert constants n may vary between 1.123 to 1.33 and K may vary from 138 to 300 respectively. The graph in Figure 13 is a Peukert plot at room temperature, 80°F.

Temperature Dependence of Battery CapacityThe useful Ahr capacity available from the VRLA is dependent on battery temperature and may be represented by the following equationCt = C77 * (1 - 0.065(77 - t))

Where t is the temperature in °F, Ct is the battery capacity at t °F and C77 is the capacity of the battery at 77°F (room temperature). For example, C3 capacity at 32°F, for an 80 Ahr VRLA battery is expressed as C3 (32°F).C3 (32°F) = 80 * (1 - 0.0065(77 - 32)) = 56.6Ah

Similarly C0.1 at 80°F for an 80 Ahr VRLA battery is 36.3Ahr. Thus C0.1 at 120°F is 45.74Ahr.

Thus under a constant current discharge and variation of temperature the battery pack capacity changes the performance of the electric vehicle (EV). This is observed as a variation of the driving distance before an EV recharge.

As illustrated in Figure 14, the graph is the estimated VRLA battery capacity with respect to the battery pack temperature. A 80Ahr VRLA battery above room temperature, 77°F, exhibits a larger than rated battery capacity. This increase is larger at higher temperatures. A fully charged battery pack when discharged at 100°F can deliver approximately two times the rated battery pack current than a battery pack at room temperature under similar discharge conditions.

Figure 14: Variation of estimated VRLA battery capacity with temperature

Hybrid car battery lifeManufacturers claim the battery life to be of 8-10 years or 80,000 to 100,000 miles, but can even pass this limit. If used with proper maintenance, hybrid car battery life expectancy can go well beyond what the manufacturers claim. Some hybrid car owners say the average car battery life extended from around 150,000 to 200,000. As the battery is used over time, their capacity to hold the charge may deteriorate. Let us get to know more about the cost of hybrid car batteries.

Hybrid Car Battery CostA negative thing about hybrid car batteries is their cost which may even go up to a few thousand dollars. Companies providing extended battery warranties are a good alternative to lower the cost of hybrid car battery replacement. The cost of hybrid car batteries may range from $3000 to $6000. If you are thinking of lessening the hybrid car battery replacement cost, a good option is to go in for batteries from out of-service automobiles. In this way you can purchase quality batteries at a much discounted price.

2.3 Plug-in Hybrid Electric Vehicle

A plug-in hybrid electric vehicle (PHEV or PHV), also known as a plug-in hybrid, is a hybrid vehicle with rechargeable batteries that can be restored to full charge by connecting a plug to an external electric power source (usually simply a normal electric wall socket). A PHEV shares the characteristics of both a conventional hybrid electric vehicle, having an electric motor and an internal combustion engine; and of an all-electric vehicle, also having a plug to connect to the electrical grid. Most PHEVs on the road today are passenger cars, but there are also PHEV versions of commercial vehicles and vans, utility trucks, buses, trains, motorcycles, scooters, and military vehicles.

The cost for electricity to power plug-in hybrids for all-electric operation has been estimated at less than one quarter of the cost of gasoline. Compared to conventional vehicles, PHEVs reduce air pollution locally and dependence on petroleum. They may reduce greenhouse gas emissions that contribute to global warming, compared with conventional vehicles. PHEVs also eliminate the problem of "range anxiety" associated to all-electric vehicles, because the combustion engine works as a backup when the batteries are depleted. Plug-in hybrids use no fossil fuel during their all-electric range and produce lower greenhouse gas emissions if their batteries are charged from renewable electricity. Other benefits include improved national energy security, fewer fill-ups at the filling station, the convenience of home recharging,

opportunities to provide emergency backup power in the home, and vehicle-to-grid (V2G) applications.

PHEVs are based on the same three basic power train architectures as conventional electric hybrids:

Series hybridsSeries hybrids use an internal combustion engine (ICE) to turn a generator, which in turn supplies current to an electric motor, which then rotates the vehicle’s drive wheels. A battery or super capacitor pack, or a combination of the two, can be used to store excess charge. With an appropriate balance of components this type can operate over a substantial distance with its full range of power without engaging the ICE. As is the case for other architectures, series hybrids can operate without recharging as long as there is liquid fuel in the tank.

Parallel hybridsParallel hybrids can simultaneously transmit power to their drive wheels from two distinct sources for example, an internal combustion engine and a battery powered electric drive. Although most parallel hybrids incorporate an electric motor between the vehicle's engine and transmission, a parallel hybrid can also use its engine to drive one of the vehicle's axles, while its electric motor drives the other axle and/or a generator used for recharging the batteries. (This type is called a road-coupled hybrid). The Parallel hybrids can be programmed to use the electric motor to substitute for the ICE at lower power demands as well as to substantially increase the power available to a smaller ICE, both of which substantially increase fuel economy compared to a simple ICE vehicle.

Series-parallel hybridsSeries-parallel hybrids have the flexibility to operate in either series or parallel mode. Hybrid power trains currently used by Ford, Lexus, Nissan, and Toyota, which some refer to as “series-parallel with power-split,” can operate in both series and parallel mode at the same time.

General Motors announced plans to release a vehicle that will be able to go long distances in electric-only mode. It thus became the first U.S. company to commit to producing a so-called plug-in hybrid design one that has batteries so capacious that they can be recharged not only by the engine but also from wall current in the garage. It represents the next way station along the path to all-electric vehicles.

2.4 Safety Battery System

Although the lithium-ion cells in laptops and mobile phones pack twice as much energy per pound as the next-best kind, they haven't found their way into hybrid cars because they're worryingly prone to fires. A123, a Watertown, Mass. startup, believes it has solved the problem with a lithium-ion design using a special formulation for the battery's cathode, or positive plate.

The safety problem that has stood in the way of lithium-ion batteries became notorious last year when laptops using such batteries were shown spouting flames in video clips that circulated on the Internet. Millions of lithium-ion batteries had to be recalled, even though no one was hurt. If masses of such batteries had been crammed into automobiles, however, the fires would likely have resulted in the deaths of the passengers.

The fires seem to begin when a small manufacturing defect, perhaps compounded by overcharging, causes oxygen to separate from the compound making up the cathode, a heat-releasing process known as oxidation. As the cell overheats, it can prime oxidation in neighboring cells, a process known as thermal runaway.

A123 overcomes the problem by making its cathodes out of iron phosphate, which bonds to the oxygen far more powerfully than does the cobalt dioxide found in conventional lithium-ion batteries. Its cells are thus far less subject to oxidation, and thus less prone to thermal runaway.

DisposalRechargeable batteries contain heavy metals such as nickel that can cause neurological or kidney damage with enough exposure. However, they are a preferable alternative to lead-acid batteries present in combustion cars, which are one of the most harmful consumer products on the market. Rechargeable batteries should be taken to a recycling facility at the end of their lives. Many car manufacturers will take batteries back. Toyota puts a phone number on each battery.

Heat DischargeLithium ion batteries are lighter and store more electricity than nickel metal hydride, but in laptops and small power tools they discharge heat and are a potential fire danger. In landfills they have even been known to explode from hot temperatures. However, Jeff Boyd, CEO of Miles Automotive Group, guarantees that its lithium ion batteries do not generate heat because of a change in chemical composition.

HandlingWith an electric vehicle, avoid leaving connections open and exposed so that they do not come into contact with people or tools. According to the website EV Convert, use a short wrench when working with EV batteries, or a tool that can't accidentally bridge from one terminal to the next, and possibly insulate the handle.

Fail SafeThe circuits in electric vehicles should be designed to disconnect the high voltage line from the battery component in case a failure or some kind of problem occurs. A heavy-duty relay connected to the ignition key can disconnect the voltage line when the key is off, and another relay attached to the accelerator pot in the engine compartment can also disconnect the voltage when you let go of the gas pedal.

Chapter: 03

System Requirements3.1 Electrical Management

The major components like converters, regenerative braking system and super capacitor are discussed under electrical management of the hybrid system. The converters are combined into one unit to manage the power and recharging circuits in hybrids electric vehicles. Regenerative braking takes energy normally wasted during braking and turns it into usable energy and super capacitors are used to boost up the acceleration.

3.1.1 AC to DC Converters

A plug-in hybrid electrical vehicle (PHEV) is an electric-drive hybrid vehicle with an all electric operating range. It combines batteries and internal combustion engines in an efficient manner. The PHEV provides a fuel tank and combustion engine to be used when an extended driving range is needed. A battery charger is essential for PHEV.

The battery charger should have two main functions: one is charging the battery to a proper state of charge (SOC). This operation mode is called recharge mode. The other operation mode is called inverter mode, which means the battery energy can be inverted and flows back to the grid or for possibly supplying ac electricity locally. Therefore, the battery charger is a bidirectional ac-dc converter, recharge mode is ac to dc conversion and inverter mode is dc to ac conversion. There are two types of battery chargers: off-board and on-board. An off-board charger is separated from the PHEV and can allow for higher weight and volume at a lower cost to PHEV efficiency. In the PHEVs product development, the cost, volume and weight of the power electronics and electric machine (PEEM) system are important. The bidirectional ac-dc converter belongs to this PEEM system. On-board charger is designed to combine with the whole PHEV system, which can benefit from the system optimization consideration; this can lead to higher performance. There are two strategies for the bidirectional ac-dc converter design: one is that the bidirectional converter separates from the driving system. The other one is to combine the motor driving inverter with the converter and lower the cost.

General Requirements

A. PHEV Operation Duty Cycle: PHEVs are designed for the majority of typical daily driving distance requirement. PHEVs have fuel tanks and internal combustion engines for the needs of longer trips.

Figure 15: PHEV operation duty cycle There are three operation modes-1) Charge Depletion (CD) Mode: During this operation mode, the net energy stored in batteries will decrease over a driving profile. The depletion process will be ended at about 20% SOC.

2) Charge Sustaining (CS) Mode: During this operation mode, the net energy stored in batteries may increase and decrease over a driving profile. However, by the end of operation duty cycle the energy stored in batteries will be the same as that of at the beginning of the period.

3) Regular Recharge Mode: In this mode, the batteries will be recharged by plug-in outlet. The grid ac energy is then converted to dc energy stored in the batteries. Usually, the recharge mode will be ended at 100% SOC.

B. Battery Power/Energy Requirements: It can be seen that in PHEV, battery energy needed is less than that of EV and greater than HEV, which is in the range ~5-10kWh, that depends on the driving distance supporting by the battery energy. It is easy to know the power/energy (P/E) ratio of different kinds of vehicles for the same acceleration time periods. Typically, the PHEV P/E ratio is about ~5-18.

C. Typical PHEV Bidirectional AC-DC Converter Profile: Compared with battery electric vehicles (BEVs) and electric vehicles (EVs), PHEVs do not need fast charging because of the relatively smaller battery capacity in the PHEVs as well as the use of internal combustion engine and fuel tank for unexpected charging scenarios PHEVs can be recharged during the whole night or during the day when they are parked. The time periods for PHEVs recharging can last several hours. Table I gives the example of PHEV20 typical bidirectional ac-dc converter profile, which includes battery capacity, converter (charger) ratings and recharging time of four car models. it is interesting to note that parameters of power ratings and charging time may be conditioned by 1.2-1.4kW and 1or 2 hours respectively.

Table 1: PHEV20 typical battery charger profile

D. Grid Connection and Other Requirements:In recharge operation mode, since the bidirectional ac-dc converter operates as a nonlinear load on the grid, the related standards should be met, such as safety, reliability, EMC and harmonics requirements. The charger should operate correctly under the PHEV temperature environment: (1) Air temperature: 20°F to +120°F; (2) Paved surface temperatures: up to150°F; (3) Occupant compartment temperatures: up to170°F. The charger should operate normally under 120Vor 208/240V single phase 60Hz ac source, with ±10% tolerance at rated input voltage.

The power factor should be not less than 0.95 and the THD (total harmonic distortion) current should be no more than 20% at rated load. In inverter operation mode, for the battery energy depletion, if the battery energy is feedback to the grid, then the bidirectional ac-dc converters served as grid connected inverter. Then all the grid connection inverter standards should be satisfied by the bidirectional converter. Other aspects to be considered during the bidirectional ac-dc converter design include the efficiency, cost, volume and weight.

Typical Topologies Analysis

PHEV 20Vehicles Pack Size Charger

CircuitCharging Time 20% SOC

Compact Sedan 5.1 kWh 120VAC/15A 3.9-5.4 hrsMid-size Sedan 5.9 kWh 120VAC/15A 4.4-5.9 hrsMid-size SUV 7.7 kWh 120VAC/15A 5.4-7.1 hrsFull-size SUV 9.3 kWh 120VAC/15A 6.3-8.2 hrs

Generally, a PHEV battery charger not only includes a bidirectional ac-dc converter, it also includes an EMI filter, and/or isolation transformer, control circuits unit and software. Based on the connection with the motor power electronics unit of PHEV, the topologies can be classified to two types:

A. independent circuit topologyB. Combination circuit topology.

A. Independent Circuit TopologyFig. 16 is the block diagram of the independent circuit topology, which indicates the bidirectional ac-dc converter is an independent circuit unit. There is no relationship with the motor driving inverter. As shown in Fig. 3, the bidirectional converter which is parallel with the motor driving inverter is connected to the battery bus.

Figure 16: Independent battery charger topology

Figure 17: Bidirectional ac-dc converter as battery charger

Several kinds of bidirectional ac-dc converters can be used for this topology. Fig. 17 is a full bridge bidirectional ac-dc converter application example. By implementing proper control strategy, the full bridge bidirectional converter can be operated in battery charge mode (ac-dc rectifier mode) or in inverter mode (dc-ac mode) respectively. In the battery charge mode, the line current is in phase with the line voltage. Thus the input power factor is unity. In the inverter mode, the output can be connected to the utility grid or ac load. By using the instantaneous voltage control and the average voltage control techniques, the ac output voltage is sinusoidal. If it is connected with the utility grid, the amplitude, frequency and phase of output voltage will be the same as the grid voltage (by synchronous circuit). The bidirectional ac-dc converter topology is applied to implement a small battery energy storage system (BESS).

Since the bidirectional ac-dc converter is independent of motor drive inverter, the converter components such as power switches, capacitors and inductors can be easily designed. Further,

the implementations of battery management and converter control strategies are simpler than the combination topologies.

On the other hand, there are obvious disadvantages. Since the bidirectional ac-dc converter is design independent of the motor driving system, this increases the components which lead to higher cost and larger volume/ weight.

B. Combination Circuit Topology

When batteries are recharged or operate in inverter mode for battery depletion, the PHEV motor is in off the state, thus the motor driving inverter is off. One can use the motor driving system (includes motor driving inverter and motor windings) to complete the function of the bidirectional ac-dc converter. Fig. 18 is the combination topology which means that there is no independent bidirectional ac-dc converter. In this topology, the motor winding(s) is used for the boost energy storage inductor (in ac-dc battery recharge mode), whereas, in dc-ac inverter mode, the motor winding(s) is used as the filter inductor. At the same time, the motor drive inverter is served as the bidirectional ac-dc converter. For this topology, there are two sub-types:

two motor-driving inverter system One motor-driving inverter system.

Figure 18: Combined battery charger topology

1) Topology of Two Motor-Driving-Inverters with Two MotorsFig. 19 is the typical two-motor driving system topology. There are two motors M1 & M1’and two motor driving inverters A &A’. During the motor driving mode, the contactors K1, K2 and K3 are all turned off, while during battery recharge and inverter modes, the contactors K1, K2 and K3 are all closed. To accomplish the bidirectional ac-dc converter functions, the two switches in one leg of each motor inverter are controlled on and/or off, and the other switches in other two legs should be in the off states. The two controlled legs of the two inverters are composed of the full bridge bidirectional ac-dc converter (the related two windings are served as energy storage inductors/filter inductors). As an example, one can let S3&S4, S5&S6, S3’& S4’ and S5’&S6’ off, while S1&S2 and S1’&S2’ are controlled on/off. The motor windings L1 and L1’ are used for the energy storage inductors/filter inductors. Similarly, on the proper control basis, in battery recharge mode, the line current can be controlled in phase with the line voltage (Power factor is close to unity and harmonics is low); in grid connection inverter mode, the output voltage can be controlled in phase with the grid voltage, while the amplitude is the same as the grid voltage.

Figure 19: Two motor-driving-inverters with one motor

2) Topology of Two Motor-Driving-Inverters with One MotorIn Fig. 20, motor M has two sets connecting to the two inverters respectively. The operation principle is the same as the two-motor driving system topology. To summarize case 1) and case 2), since the bidirectional ac-dc converter is the combination of the motor inverter and motor windings, there is no need other additional components. The cost is saved and the volume and weight are less than the independent topology. The main disadvantage is the control complexity of the battery and the bidirectional ac-dc converter operation modes. Fortunately, most of the control work can be accomplished by software. Since the motor parameters and the motor driving inverts are designed based on the whole PHEV system, the components optimization is for the whole motor driving system, thus not based on the bidirectional ac-dc converter operation modes. However this will not affect the reliability and efficiency since the inverter power ratings are greater than the battery recharge mode/inverter mode requirements.

Figure 20: One motor-driving-inverter system

3) Topology of One Motor-Driving-Inverter SystemFig. 21 depicts the topology of a one-motor driving system. During the motor driving mode, contactor K1is closed and contactors K2 & K3 are opened, while during battery recharge/ inverter modes, contactor K1 is opened and contactors K2 &K3 are closed. Inverter switches S5&S6 and S1&S2 (or S3&S4) are controlled on/off, which composed of full bridge bidirectional ac-dc converter. Motor windings L3 and L1 (or L2) are served as the energy storage inductor/filter inductor. The battery management and bidirectional operation modes are the same as that of the above topologies

Figure 21: One motor-driving-inverter system

The bidirectional ac-dc converter is the key power electronic unit for the plug-in function in PHEV. The basic electrical requirements and product specifications are summarized in this paper. Further, in the PHEV product development, performance, the cost, volume and weight are important indexes. A high performance lower cost bidirectional ac-dc converter with less volume and weight can benefit to the whole PHEV system.

3.1.2. DC to DC Converters

Hybrid electric vehicles are attracting more and more interest. Growing fuel costs and environmental concerns are just two factors that push the research activities in this area, and there are many car manufacturers that develop hybrid vehicles. There exist various possibilities for the set up of the electric power system of a hybrid vehicle, but the basic structure often has voltage levels and interconnections as shown in Figure 22.

Figure 22: A typical possibility of a conventional electrical power distribution system architecture for hybrid electric vehicles

Conventionally, hybrid electric vehicles have two different voltage levels. A 12V battery supplies a 14V dc bus over a battery charge/discharge unit. Traditional loads such as a heater, audio and lighting systems are supplied by this low voltage bus.

Due to the high power levels the propulsion system is connected to a high-voltage 200V...600V dc bus for increasing the efficiency, whereas the supply voltages of the electric machine are generated by a dc-ac converter connected to this high-voltage bus. The low- and the high-voltage bus are interconnected bidirectionally via a dc-dc converter, which needs to be galvanically isolated for safety reasons.

Also in conventional cars, industry considered the introduction of an additional 42V dc bus powered by a 36V battery due to the growing number of electrical loads. There, the 200V to 600V high-voltage level could be omitted and the propulsion system could be supplied with 42V. In this system is compared to the conventional one. Basically, also other combinations are possible, but the advantages of the high-voltage solution outweigh those of the 42V

system. Therefore, it is assumed in the following that the propulsion system is fed via a 200V to 600V high-voltage bus.

To increase the power density and reduce the costs of the electric system, the dc-dc converter, interconnecting the low and high-voltage buses, could be functionally integrated into the inverter/machine. For example, a possible integration of the dc-dc converter in the inverter system is presented, but it is applicable only for two split-phase motors.

This concept, however, does not provide galvanic isolation, which is required for safety reasons as already mentioned above. In addition, the output voltage Vout of the dc-dc converter cannot be controlled in every operation region of the drive system. If the inverter operates for example in the six step mode, the switching states of the inverter are completely determined. Consequently, there is no degree of freedom for controlling the output voltage of the dc-dc converter.

To solve these problems, an integrated high-voltage to low voltage dc-dc converter with galvanic isolation is proposed. Depending on the actual distribution system, this converter can be used to convert 200-600V to 14V or 42V bidirectionally.

Multi Functional Converter System I (MFCS- I)The starting point for the following considerations is a conventional full-bridge converter feeding a high-frequency transformer [Figure 23(a)], whose high frequency output is rectified to the output voltage. The secondary side rectifier could be a conventional full-bridge configuration, or a diode rectifier, in case only unipolar power transfer is required.

In a first step, the fifth leg [Figure 23(a)] of the conventional system is replaced by the zero-sequence voltage of the inverter. There, the transformer is connected between the star point and the midpoint of the primary-side leg four [Figure 23(b)]. This concept is referred to as Multi Functional Converter System I (MFCS-I).

Figure 23: Multi Functional Converter Systems (MFCS)

Multi Functional Converter System II (MFCS- II)In the MFCS-II, the galvanic isolation of is integrated into the motor [Figure 23(c)] in order to reduce the costs and the weight compared to the conventional system and the MFCS-I. To do so, a small number of additional windings with relatively small cross section must be placed in the slots together with the conventional motor windings.

In Figure 24 a simplified sketch of the transformer winding integration used in the MFCS-II is shown. The conventional motor windings act as the primary windings of the three transformers. The additional windings are the secondary ones. The machine stator iron is also used as transformer core.

Figure 24: Simplified schematic of the integrated transformer, where the motor windings are used as primary windings and the stator iron as core

Figure 25 shows the general equivalent circuit of the motor with integrated transformer, which will be explained in the following. First, it is assumed for simplicity that the machine is not rotating. If a voltage is applied to phase P1, a flux P1 begins to build up, inducing a voltage vP12 in the secondary winding P12, which is proportional to vP11 (ignoring losses). If the machine is rotating, the rotor additionally induces the voltage eP1 in phase P1. The same applies to phases P2 and P3.

Figure 25: Equivalent circuit of the motor with integrated transformer

Comparison of the MFCS-I and the MFCS-II

The machine’s zero sequence inductance in the MFCS-I correspond to the sum of the transformer stray and magnetizing inductance. Therefore, both the stray and the magnetizing inductance typically will be smaller for the MFCS-II as for the MFCS-I, where the external transformer can be designed to meet the requirements. The small magnetizing inductance might increase the losses, depending on the operating point and the switching schemes. On the other hand, the MFCS-II machine sees only the flux generated by the magnetizing current, which is only a share of the zero-sequence current. Thus, the additional burden of the machine caused through the dc-dc converter is smaller in the MFCS-II, also leading to smaller magnetizing losses. With optimized switching strategies, the efficiencies of the two concepts are comparable. Of course, the Dual Active Bridge can be designed more easily for minimal losses, but if an optimized switching strategy and an appropriate motor is chosen for the MFCS’s, its efficiency is similar to the conventional system.

Table 2: Comparison of the conventional Dual Active Bridge,the MFCS-I and the MFCS-II

DAB (Dual Active Bridge)

MFCS-I MFCS-II

4legs 3 legs 3 legsexternaltransformer

externaltransformer

No externalTransformer

external inductance No external inductance No external inductance

conventionalmachine

machine with accessiblestar point

machine with accessible star point and additionalwindings

no additional flux additional flux from additional flux

zero-phase current from magnetizingcurrent

An advantage of the MFCS-II is of course its minimized number of components. The system saves not only one switch leg, as the MFCS-I, but also the external transformer. Besides the cost savings from the reduced number of components, the integration of the transformer can be accomplished by a supplier earlier in the production chain. Table 2 summarizes the comparison, where the number of legs is only applying to the DC/DC converter.

Prototype converter

A ZVS converter prototype was designed to supply a 40 kW, 500 V load. The input inductor has a value of 50 μH and was wound on Metglas C-cores. The differential inductance of the IPT is 35 μH, and has a maximum peak-to-peak current value of 238 A for a duty ratio of 0.5. To limit the core losses in the IPT, a ferrite core with a distributed gap structure was used to reduce the eddy current losses in the windings. The construction of the IPT is depicted in Fig. 26. Each winding limb was formed with a stack of 15 ring cores of 34 mm diameter with small spaces between the cores to create a distributed air gap. The top and bottom of the core were formed with ferrite 'I' pieces. The windings were made with 0.4 mm litz wire. Semikron IGBT modules SKM400GB125D were used for the switching devices and the current sharing between the phases of the converter was achieved by using a peak current mode controller.

Figure 26: IPT core with distributed gap

Experimental resultsFig. 27 shows the steady-state waveforms of the lower switch, Q1, for the converter operating at 40 kW and vo= 500 V. The switching frequency is 31 kHz, and the duty ratio is 0.4.

It can be seen that as soon as the voltage falls, the device has a negative current that is conducted by its anti-parallel diode. Clearly, the voltage is zero when the transistor starts conducting the positive current.

Figure 27: ZVS operation in IGBT Q1

Table 3 shows the estimated power losses in the components of the prototype. The efficiency was approximately 97%. Table 3: Estimated power losses for = 300 V, 40kW

Losses in: Estimated (W)Semiconductors, conduction 300Semiconductors, switching 500IPT 200Input inductor 50Total 1050Estimated efficiency 97%

PrognosticsAnother requirement of an automotive system is its ability to sense failure of part or all of the system and where possible predicts this failure. Current power converters, including DC-DC converters, are usually tested to failure and the mean time to failure deduced. This value then becomes the normal operating life including some safety margin. For real world use this value is not practical as it is based on uniform operating parameters or unrealistic operating cycles. Real world operation, especially that of automotive systems is very random, in particular with regard to duty cycle and environment. It is therefore necessary to develop a health monitoring system that can predict the remaining life of the converter in real time. Research was carried out looking at the failure modes and likelihood of failure of the main components of a DC-DC converter. The results can be seen in Table 4. Table 4 – Failure cause and likelihood for DC-DCconverter main components

These results show that the capacitors in a converter are the most likely cause of failure and there is much research done determining how they fail and how to predict their failure. The health monitoring system therefore uses capacitor ageing prediction as described in the referenced papers and the work described in this paper concentrates on predicting the failure of the semiconductor devices. The failure mode of the semiconductor devices is due to their construction by materials with dissimilar coefficients of expansion. During their normal operating life the devices heat up and cool down and this causes thermal stress in the device and eventually leads to failure. In order to develop a strategy for predicting failure an accelerated method to age and fail the device was required. In order to accelerate the test, the flow of heat out of the IGBT module needs to be controlled. This is done by the use of thermoelectric coolers based on the Peltier- Seebeck effect. Thermoelectric coolers are solid state devices which move heat from one side of the cooler to another depending on the current flowing through the cooler. If the current is reversed the heat will flow in the opposite direction.

This controllable heat flow is used in the test rig to accelerate the temperature rise and fall of the IGBT module and thus reduce the time to failure of the module. The thermoelectric coolers are mounted between the IGBT module and heat sink. The base plate of the IGBT module is drilled to allow for a thermocouple to be placed as close as possible to the semiconductor junction and heatsink compound is used between all contact surfaces to ensure best thermal conductivity. The test cycle is as follows: The IGBT gate is switched on and the thermoelectric coolers activated so as to take heat out of the heatsink and into the base plate of the IGBT module. When the junction temperature reaches the maximum for the device the IGBT is switched off and the polarity of the current to the thermoelectric coolers reversed. When the junction temperature reaches the minimum a cycle counter is incremented and the cycle repeats. During the heating or ON cycle the Vce of the IGBT is also monitored and if this value increases by 20% from the original value the IGBT is regarded as having failed and the test stops.

In order to validate the accelerated lifeing methodology a Fuji Electric IGBT was used. This IGBT was rated at 600V and 50A, therefore the load that the IGBT was driving during the on phase of the experiment was set to 50A.

Figure 28: IGBT Junction temperature, gate voltage and

Figure 28 shows the junction temperature for three cycles along with the gate pulse and measured . Initial testing of the IGBT modules uses a minimum temperature of 10ºC and a maximum temperature of 150ºC, which is the maximum specified in the manufacturer’s data sheet. The test rig is achieving a temperature rise of 140ºC in about 12 seconds giving a

temperature rise rate of 12ºC per second. The test rig was able to fail an IGBT module in 1427 cycles. The point of failure can be seen in Figures 29 and 30.

Figure 29: IGBT Junction temperature at point of failure

Figure 30: IGBT at point of failure

This failure time is around 20 times faster than test methods which don’t use thermoelectric coolers and the number of cycles to failure is consistent with results from these other test methods. This life data can then be used by prognostic algorithms to predict the failure of the IGBT modules in real time.

By integrating the dc-dc converter, which connects the low and high-voltage buses of a hybrid vehicle, costs and weight of the hybrid propulsion system can be reduced. In this paper, a new concept for integrating the converter is presented which allows to replace one leg of the dc-dc converter by the inverter stage and to fully integrate the transformer in the machine.

3.1.3 Regenerative Braking

Regenerative braking is an effective method to improve fuel economy in heavy traffic and urban areas for hybrid electric vehicles. In regenerative braking, the energy recuperation takes place via kinetic and potential energy conversion to electric energy, which is stored in the energy storage device, such as an ultra capacitor in this study. Later, the stored energy can be re-used to propel the vehicle during subsequent acceleration. However, a conventional vehicle converts the useful energy into heat that is dissipated into atmosphere.

Regenerative braking has to be carried out together with the conventional friction braking. The first reason is that the braking power in an emergency case is huge and the electric motor (EM) power is not big enough to supply such a large amount of power. The second reason is that the regenerative braking cannot be used in conditions when the ultra capacitor voltage is high in order to increase its lifetime, etc.

The current research has three aims:i. Formulating a regenerative torque distribution (RTD) strategy to make maximum use

of the braking energy without compromising the braking performance and drivability.ii. Developing an emulated engine compression braking (EECB) during coasting.

iii. Proposing a regenerative torque optimization strategy (RTO) to maximize the actual electric power recuperated by the ultra capacitor.

Traditional Braking SystemsIn a traditional braking system, pressing on the brake pedal will cause a pair of brake pads in each wheel to come into contact with the surface of a brake rotor. This contact produces friction which is what slows down and eventually stops the vehicle. The friction itself produces heat as an energy byproduct. Automotive engineers and designers generally perceive heat as a loss. This is the reason why, especially in high performance cars, brake cooling systems such as air dams are employed to dissipate heat from the brakes so that they can quickly regain their efficiency.

Regenerative Braking SystemsFigure 31 depicts a diagram of a brake system and an electronic hydraulic brake system developed for the hybrid electric vehicle. When a driver depresses the brake pedal, the brake pedal force is transmitted to the master cylinder through a vacuum booster. The master cylinder pressure is supplied to the rear wheel cylinder through a proportional valve and the rear braking must be forced at all periods to prevent vehicle instability. The electronic hydraulic brake system consists of a motor, a pump, an accumulator, and a pressure-regulating valve. An on–off switch controls the motor work to maintain the accumulator pressure within a proper scope in order to save electric energy. A brake control unit (BCU) distributes the regenerative braking force and the front wheel friction braking force depending on the vehicle information, which come directly from the sensors and other control units over a controller area network (CAN) bus. If the regenerative braking force provided by the EM is not sufficient enough to achieve the demand of the front wheel braking force, the pressure-regulating valve works simultaneously to supply the insufficient force. A pedal feel simulator is added to provide a similar brake pedal feeling to the driver.

Fig 31: Diagram of a brake system and an electronic hydraulic brake system

i. Regenerative Torque Distribution (RTD) The available regenerative braking force

During braking, the available regenerative braking force applied to the front wheel can be obtained as

(1)

where i is the CVT gear ratio, is the final reduction gear ratio, is the transmission

efficiency, which is assumed constant, r is the tyre radius, and is the EM available regenerative braking torque, expressed as

(2)

where and are the EM generation and ultracapacitor charging capacities respectively, and is the weight factor of the RTD. They are calculated as

(3)

(4)

(5)

where is the EM speed, is the EM maximal generation power, is the ultracapacitor maximal charging power as a function of the ultracapacitor voltage U and its temperature and is a negative value and transmitted from the ultracapacitor control module to the BCU over the CAN bus, is the weight factor of U, is the weight factor of the vehicle velocity V, and is the weight factor of the EM state, which is defined as S and sent to the BCU from the dynamic motor control module over the CAN bus. They are represented as

In equations (3) and (4), and are calculated by two different formulas depending on the EM speed, because the base speed of the EM is 1500 r/min. Moreover, the factor 9550 exists to balance the different units of the torque, speed, and power, which are N m, r/min, and kW respectively.

The working range of U is from 30 to 50 V. The variable protects the ultracapacitor from overcharging, which may deteriorate its lifetime. For U above 46 V, starts to decrease from 1 to 0 until U reach 48 V. For V above 10 km/h, w2 increases from 0 to 1 until V reaches 30 km/h in order to recover as much regenerative energy as possible. Below 10 km/h, no regenerative torque is generated. The reasons are that the vehicle kinetic energy is small, the regeneration energy is not considerable, and the energy recover efficiency is not a concern, but may influence negatively on drive comfort.

The demand of the front wheel braking forceFigure 32(a) shows the relationship between the brake pedal position and the master cylinder pressure and Fig. 32(b) shows the relationship among the master cylinder pressure , the total braking force , the front wheel braking force , and the rear wheel braking force of the conventional vehicle. The magnitude of the demand of the front wheel braking force , which serves as a benchmark for distributing the regenerative braking force and the front wheel friction braking force, can be expressed as

Fig 32: The braking characteristic of the conventional vehicle ( the function of and , the function of and )

Braking force distributionThe amount of the front wheel lock-up force can be obtained as,

The actual regenerative braking force applied to the front wheel , the actual EM

regenerative braking torque , and power can be calculated as

The actual front wheel friction braking force and cylinder pressure are calculated as,

Where is the inverse function of .

The position of the pressure regulating valve a is achieved depending on its characteristics, obtained as

where is the function of a and . Then, the BCU sends the pulse width module command to the pressure-regulating valve in order to achieve the target .

For the RTD, at the beginning of braking, if the ICE speed is higher than the speed of stopping fuel injection as a function of the ICE coolant temperature TECT, the ICE is

directed to stop fuel injection in order to enhance fuel economy, shown in Fig. 5. Clutch one is engaged and ICE compression braking is used. The variable decreases as V decreases. When decreases to the speed of recovering fuel injection , then the ICE is directed to recover fuel injection and the idle throttle is commanded, shown in Fig. 33. The reason for recovering fuel injection is that, during braking, the ICE control unit equipped in a conventional vehicle directs the ICE to recover fuel injection at . Thereby, this control method is chosen as the reference for the hybrid electric vehicle in order to provide the same braking feeling as the conventional vehicle to the driver.

Fig. 33 The characteristic of stopping and recovering fuel injection

During the stopping fuel injection period of braking, once the accelerator pedal is depressed, the ICE recovers fuel injection immediately.

ii. Emulated Engine Compression Braking (EECB)At the beginning of coasting, if is higher than mentioned above, the ICE is directed to stop fuel injection and the ICE produces drag torque; then the EECB is applied. The process of the EECB is that clutch one is disengaged and the EM provides the negative torque to emulate the ICE drag torque to charge the ultracapacitor. The amount of the EM torque of the EECB is equivalent to what the ICE can provide at current speed in order to provide the same coasting feeling to the driver as when being performed by the ICE.

The ICE drag torque at 295 K is about two times the torque at 365 K and increases with

increased speed. Figure 34 shows the ICE drag torque as a function of and , expressed as

The available EM generation torque can be calculated as

where is the weight factor of the EECB, represented as

Fig 34: The ICE drag torque and available EM generation torque

For example, when and equal 360 K and 40 V respectively, the values of and at different speeds are shown in Fig. 34. The intersection of the two curves indicates the speed of beginning of the EECB , 2000 r/min in this example. Above 2000

r/min, is smaller than , clutch one cannot be disengaged, and the EECB cannot be carried out either. Below 2000 r/min, the EECB begins. Clutch one is disengaged and the ICE is directed to stop fuel injection; then is ramped to zero. The variable decreases as V decreases due to vehicle inertia. The speed of engaging clutch one as a function of is bigger than . When decreases to , 1100 r/min in this example, clutch one is engaged to restart the ICE, shown in Fig. 35.

Fig 35: The speed of engaging clutch one

iii. Regenerative Torque Optimization (RTO)For a given or defined as obtained by the above-mentioned RTD or EECB, the actual electric power recuperated by the ultracapacitor can be calculated as

where is the EM generation efficiency and is the ultracapacitor charging efficiency, which can be obtained by their efficiency contours. The variable is gained depending on

and , and and are multiplied and converted into the electric power flowing into the ultracapacitor ; then is obtained depending on and .

Therefore

The RTO is designed to find the optimal EM torque and the desired CVT ratio with maximal . Therefore, the objective function of the RTO can be expressed as

Then, the goal is to find

The constraint equations of the RTO can be represented as

The RTO has been formulated and implemented in Matlab/Simulink. Then, and can be obtained for various and after offline calculations. The results (maps) are essentially three-dimensional look-up table based control, shown in Fig. 36.

Fig. 36: Inputs and outputs of the RTO

Regenerative Braking EfficiencyThe energy efficiency of a conventional car is only about 20 percent, with the remaining 80 percent of its energy being converted to heat through friction. The miraculous thing about regenerative braking is that it may be able to capture as much as half of that wasted energy and put it back to work. This could reduce fuel consumption by 10 to 25 percent. Hydraulic regenerative braking systems could provide even more impressive gains, potentially reducing fuel use by 25 to 45 percent.

One of the most promising methods to improve fuel economy for hybrid electric vehicles involves regeneration of braking energy. In this study, a regenerative torque distribution strategy (RTD) is written to retrieve aggressively and store as much available vehicle kinetic energy as possible. An emulated engine compression braking (EECB) is suggested during coasting. In addition, a regenerative torque optimization strategy (RTO) is implemented to maximize the actual electric power recuperated by the ultracapacitor.

3.1.4. Super capacitors

Super capacitors are a new technology that allows storing 20 times more energy than conventional electrolytic capacitors. Despite this important advance in energy storage, they are still far from being compared with electrochemical batteries. Even Lead-acid batteries can store at least ten times more energy than super capacitors. However, they present a lot better performance in specific power than any battery, and can be charged and discharged thousands of times without performance deterioration. These very good characteristics can be used in combination with normal electrochemical batteries, to improve the transient performance of an electric vehicle, and to increase the useful life of the batteries. Fast and sudden battery discharge during acceleration, or fast charge during regenerative braking can be avoided with the help of super capacitors. Besides, super capacitors allow regenerative braking even with the batteries fully charged.

Super capacitors merged with batteries (hybrid battery) will become the new super battery. Just about everything that is now powered by batteries will be improved by this much better energy supply.  They can be made in most any size, from postage stamp to hybrid car battery pack. Their light weight and low cost make them attractive for most portable electronics and phones, as well as aircraft and automobiles.

The new ones are flexible and biodegradable and can be powered by body fluids. (Since body fluids can act as an electrolyte, the battery can be used for medical devices and could be installed into a patient fully charged but dry and feed off bodily fluids to allow it to re-power and discharge energy. 

Direct Supercapacitor ConnectionSupercapacitor integration with the battery-load circuit can be challenging when trying to optimize the presence of this additional sub-system. The simplest way is to connect the supercapacitor directly in parallel with the battery bank, after first pre-charging it to the battery terminal voltage. Such a connection is shown in Fig. 37 where the load current denoted by is defined to flow downward (i.e., positive during acceleration and coasting, and negative during regenerative breaking).

Figure 37. Parallel connection of supercapacitor bank, battery bank, andelectrical load

Given a certain load current profile representing some short drive cycle, the battery current and supercapacitor current are found by basic circuit rules such Kirchoff’s voltage and current laws: (6)

(7)

(8)

where and represent the internal capacitor and battery voltages, respectively. Subsititution of (6) and (8) in (7) yields the first order equation of :

(9)

Where

(10)

The solution to (10) can be written as shown in Eqn. (11) below:

(11)

where K is determined by setting the initial value of to . Note that it is not possible to control power flow in and out of the supercapacitor bank in the circuit above since its terminal voltage is forced to be equal to the that at the battery terminals at all time. Current division between the battery and super capacitor bank is determined solely by the two branch internal resistances and internal voltages.

Mathematical Model of the DC-DC ConverterWe want to analyze the energy transfer from the super-capacitor (the voltage of which is rather low) into the capacitor (the voltage of which is rather high). The aim is to find what output can be transferred at what voltage ratio / and how it is influenced by inductance L and resistance R and by different duty factors. We assume in the first state ideal transistors with zero switching time.

Figure 38: Circuit diagram

Ideal Transistors:Two current shapes can occur. In the first case current is un-interrupted; in the second case it is interrupted. Both cases are depicted in the Fig. 39.

Current is uninterrupted. Current is interrupted (Continuous)

Figure 39: Current shapes in case of ideal transistor

Let us denote:

is time when reaches the value zero

is the minimal duty factor at which current is un-interrupted

is the minimal value of un-interrupted current pulsation.

is the maximal value of pulsation.

is duty factor for the wanted .

for interrupted

for un-interrupted .

is average current value from or into appropriate capacitor.

;for interrupted current from super-capacitor

; for continuous current from super-capacitor

; for interrupted current into capacitor .

; for continuous current into capacitor .

is output power from the capacitor ,

is input power into the capacitor ,

Real TransistorsReal transistor cannot switch on and off immediately. How is the real transistor simulated is shown on the Fig.40. The switching off time consists of delay time voltage rise time , rapid drop and tailing time and . The process takes together approximately 10μs.

The time at which the transistor T2 should be off is at switching frequency 5 kHz and duty factor 0.9T 20μs. We shall see that the features of transistor are important.

Figure 40: Switch off and switch on transistor characteristics

Advantages of a Super Capacitor Virtually unlimited life cycle - cycles millions of time -10 to 12 year life Low impedance Charges in seconds No danger of overcharge Very high rates of charge and discharge

High cycle efficiency (95% or more)

Super capacitors and ultra capacitors are relatively expensive in terms of cost per watt

Super Capacitor Disadvantages Linear discharge voltage prevents use of the full energy spectrum Low energy density – typically holds one-fifth to one-tenth the energy of an

electrochemical battery

Cells have low voltages – serial connections are needed to obtain higher voltages. Voltage balancing is required if more than three capacitors are connected in series

High self-discharge – the rate is considerably higher than that of an electrochemical battery

Requires sophisticated electronic control and switching equipment

Figure 41: electronic control of super capacitor

Super capacitors or Ultra Capacitors were initially used by the US military to start the engines of tanks and submarines. Most applications now are in small appliances, handheld electronics and hybrid electric vehicles.

NASA has a research project to use super capacitors in an electric bus called the Hybrid Electric Transit Bus. The energy used to start the engine and accelerate the bus is regenerated from braking. During test runs, a bus loaded with 30 super capacitors, each of them weighing 32 kg and releasing energy of 50 kJ at 200 V managed to run for four miles.

In most hybrid vehicles, 42 V super capacitors are used. General Motors has developed a pickup truck with a V8 engine that uses the super capacitor / ultra capacitor to replace the battery. The efficiency of the engine rose by 14%. The super capacitor supplies energy to the alternator. Toyota has developed a diesel engine using the same technology and it is claimed to use just 2.7 liters of fuel per 100 km.

Super capacitor Versus BatteryA super capacitor by itself cannot totally replace the battery. But, by merging a super capacitor and a battery together - like a "Hybrid Battery" it will be possible for super capacitors to replace the battery as we know it today.

Super capacitors need batteries to store the energy and are basically used as a buffer between the battery and the device.  Super capacitors can be charged and discharged hundreds of thousands of times where a battery cannot do that. Soon the price point will be where most every electronic device will use them. As a hybrid battery, it will be the new super battery.

3.2 Thermal Management

Battery performance, life, and cost directly affect the performance, life, and cost of the hybrid electric vehicles (HEVs). Battery temperature influences the availability of discharge power (for start up and acceleration), energy, and charge acceptance during energy recovery from regenerative braking. These affect vehicle drive-ability and fuel economy. Temperature also affects the life of the battery. Therefore, ideally, batteries should operate within a temperature range that is optimum for performance and life. The desired operating temperature range is different for different battery types (with different electro chemistry). Usually, the optimum temperature range for the battery operation (desired by the battery manufacturer) is much narrower than the specified operating range for the vehicle (identified by the vehicle manufacturer). For example, the desired operating temperature for a lead acid battery is 25°C to 45°C; however the specified vehicle operating range could be -30°C to 60°C. In addition to considering the (absolute) temperature of a battery pack, uneven temperature distribution in a pack should be also considered. Temperature variation from module to module in a pack could lead to different charge/discharge behavior for each module. This, in turn, could lead to electrically unbalanced modules/packs, and reduced pack performance. For high temperature batteries such as ZEBRA and lithium metal polymer batteries, thermal management is considered an integral part of the battery pack and has been included in the design by the battery manufacturers. The need for battery thermal management for ambient temperature batteries such as valve regulated lead acid (VRLA), nickel metal hydride (NiMH), and lithium ion (Li-Ion) was not obvious initially, however, EV and HEV battery and vehicle manufacturers have come to realize such a need. Current prototype or production EVs and HEVs with ambient temperature batteries have battery thermal management systems - some more elaborate than others. Over the last several years, with support from US Department of Energy, NREL has been working with U.S. automobile manufacturers and their battery pack suppliers (as part of the Partnership for a New Generation of Vehicles (PNGV) program) to identify and resolve thermal issues associated with battery packs for HEVs. To evaluate battery pack designs and provide solutions for battery thermal issues, we have used heat transfer and fluid flow principles, finite element thermal analysis, and heat transfer and fluid flow experiments.

Desired Attributes of a Thermal Management System The goal of a thermal management system is to deliver a battery pack at an optimum average temperature (dictated by life and performance trade-off) with even temperature distribution (or only small variations between the modules and within the pack) as identified by the battery manufacturer. However, the pack thermal management system has to meet the requirements of the vehicle as specified by the vehicle manufacturer-it must be compact, lightweight, low cost, easily packaged, and compatible with location in the vehicle. In addition, it must be reliable, and easily accessible for maintenance. It must also use low parasitic power, allow the pack to operate under a wide range of climate conditions (very cold to very hot), and provide ventilation if the battery generates potentially hazardous gases. A thermal management system may use air for heat/cooling/ventilation-

A thermal management system may also use liquid for cooling/heating (Figure 42), insulation, thermal storage such as phase change materials, or a combination of these methods. The thermal management system may be passive (i.e., only the ambient environment is used) or active (i.e., a built-in source provides heating and/or cooling at cold or hot temperatures). The thermal management control strategy is done through the battery electronic control unit.

Figure 42: General Schematic of Thermal Management using Air

Designing Battery Thermal Management SystemAs with any system, there are several approaches to designing a BTMS. The approach depends on the desired level of sophistication, availability of information, and timetable/budget for a particular project. Based on our learning experience, we have proposed a systematic approach to designing and evaluating a BTMS. A summary of the steps is provided here –

1. Define the BTMS design objective and constraints. These are dictated by the battery type, acceptable temperature range, acceptable temperature variation, and the packaging.

2. Obtain module/pack heat generation and heat capacity. These will affect the size of the cooling/heating system and how fast the pack responds to temperature fluctuations.

3. Perform a first-order module and BTMS evaluation. Preliminary analysis is performed to determine the transient and steady-state thermal response of the module

and pack in order to select an initial strategy. Various options, choices of heat transfer medium (air or liquid), and different flow paths (direct or indirect, series or parallel) are evaluated. We believe that to design having a good BTMS starts with a designing a module with thermal behavior in mind.

4. Predict the battery module and pack thermal behavior. Detailed analysis is done to evaluate the impact of various parameters under various conditions and driving duty cycles for both battery module and pack.

5. Design a preliminary BTMS. Based on the packaging and expected performance, the system parameters are specified.

6. Build and test the BTMS. A prototype BTMS is built and then tested on the bench and in the vehicle under various loads and conditions.

7. Improve the BTMS. Based on the test data and analysis, the design is fine tuned or modified for the next step.

Heat Generation and Heat CapacityThe magnitude of the overall heat generation rate from a battery pack under load dictates the size and design of the cooling system. The heat generation (due to electrochemical enthalpy change and electrical resistive heating) depends on the chemistry type, construction, temperature, state of charge, and charge/discharge profile. At NREL, we have been using a large custom-built calorimeter to measure the heat generation from cells/modules with various cycles, state of charge, and temperature. Table 5 shows some typical results for various batteries. These and other data show that that, for the same current draw, a NiMH battery generatesmore heat than a VRLA or Li-Ion batteries at elevated temperatures (> 40°C). Heat generation from VRLA and Li-Ion is roughly the same for similar currents. At room temperature, less heat is generated for NiMH for the same current, but NiMH is not as energy efficient. Generally, as temperature decreases more heat is generated because of an increase in resistance in the cells. As the discharge rate increases, more heat is generated. Under certain conditions, the battery electrochemical reaction could be endothermic, as shown in Table 5 for Li-Ion battery at C/1 discharge rate at 50°C.

Table 5: Heat generation from Typical HEV/EV Modules using NREL’s CalorimeterHeat Generation (W)/Cell

Battery Type Cycle 0°C 22-

25°C40-50°C

VRLA, 16.5 Ah

C/1 Discharge, 100% to 0% State of Charge 1.21 1.28 0.4

VRLA, 16.5 Ah

5C Discharge, 100% to 0% State of Charge 16.07 14.02 11.17

NiMH, 20 Ah

C/1 Discharge, 70% to 35% State of Charge - 1.19 1.11

NiMH, 20 Ah

5C Discharge, 70% to 35% State of Charge - 22.79 25.27

Li-Ion, 6 Ah C/1 Discharge, 80% to 50% State of Charge 0.6 0.04 -0.18

Li-Ion, 6 Ah 5C Discharge, 80% to 50% State of Charge 12.07 3.50 1.22

In order to do any reasonable transient thermal analysis, the designer needs to know the heat capacity of a module in order to determine the thermal mass of the pack. Overall or average heat capacity can either be measured in a calorimeter or calculated from knowledge of the heat capacity of individual components using a mass-weighted average of cell/module components. Typical heat capacity for a VRLA (16.5 Ah) is 660 J/kg/K, for a NiMH (20 Ah) heat capacity is 677.4 J/kg/K, and for Li-Ion (6 Ah) heat capacity is 795 J/kg/K.

3.3 Cost aspects

Purchase PriceMost Hybrid cars or SUVs can cost between $3000 and $8000 more than their gas-powered only counterparts. However, when factoring in maintenance costs and lower fuel costs, the savings are eventually passed on to the consumer. Additionally, the government has stepped up and offered tax breaks to those who decide to go with the environmentally friendly Hybrid.

Service and Maintenance CostsJust like conventional gas-powered vehicles, regular maintenance is necessary in hybrid electronic vehicles. There is, however, cause to believe that the overall cost of maintaining Hybrid may be equal to or less than maintaining a conventional car. Currently, the maintenance costs, comparatively, for a Hybrid versus a conventional gas-powered vehicle are nominal. In fact, engine wear may turn out to be less for a Hybrid due to its ability to shut itself down during idling. This, in turn, will create less wear on the engine. This is especially favorable for city drivers. However, it remains true for drivers in any environment, city, suburb or country. The other bonus feature of a Hybrid has that a conventional gas-powered vehicle does not have is its braking system. In a Hybrid, when slowing to a stop, the electric motor slows the vehicle taking much of the strain off the braking system. To sum up, engine use and brake use is less in a Hybrid as compared to a gas-only powered conventional car. These two factors weigh in on the maintenance list when comparison-shopping or simply when owning a Hybrid car or Hybrid SUV. The one component to be most concerned with when owning a Hybrid is the life of the battery. Keeping in mind that the battery provides life to the electric motor, the battery is an essential piece of the puzzle and one third of the three main components of the power sources in a Hybrid vehicle. The battery is constantly drained and recharged by the electric motor.

Although replacement of the battery is a necessary part of owning a Hybrid car or SUV, many manufacturers sell their vehicles with excellent warranty packages. It is expensive to replace hybrid batteries—it can cost in the neighborhood of $3,000 for a full hybrid battery replacement. But on the other hand, hybrid batteries have proven themselves to be extremely reliable. And as long as they are not abused and the vehicle charging control system operates effectively, they can be not unrealistically expected to last for nearly the life of the vehicle.

Manufacturers are providing generous battery warranties (generally about 8-10 years and 80,000 to 100,000 miles), but as with most warranted components, they are designed to last well beyond the coverage period. It would not be unreasonable to expect the life of a battery pack to exceed 150,000 miles

Battery Costs: Current Trend and Expectations

Prospective high first costs were one major argument against use of lithium ion batteries in the electric vehicles developed under the California ZEV mandate. Although the battery cost issue is much reduced for HEVs because of their smaller batteries, it will remain an important concern of prospective HEV manufacturers until the achievement of acceptable costs through mass production of batteries meeting PHEV performance, life and safety requirements is demonstrated. To help resolve the cost issue ahead of mass production and thus remove one important barrier to HEVs, a number studies projecting lithium ion battery costs for various rates of production have been undertaken over the past ten years. These studies differ in methodology, basic assumptions, and in the specific lithium ion chemistries and manufacturing techniques assumed. However, the newer results tend to converge, especially at true mass production rates for which materials costs dominate battery costs. Table 6 lists approximate costs for HEV batteries derived from the work of Santini and Nelson presented at EVS24 that represents the newest and most detailed Li Ion battery cost analysis. The Santini and Nelson battery cost and capacity data were used to determine battery specific costs as a function of battery capacity. From that relationship, specific and total costs were determined for the batteries in the capacities used in this paper of Table 6.

Table 6: Li Ion Battery Cost Projections

VehicleType

BatteryCapacity(kWh)

SpecificCost($/kWh)

BatteryCost($)

CostDifference(PHEV-HEV)

Full HEV 2 700 1400 0PHEV(10) 4-4.5 395 ~1680 ~280

PHEV(20) 7-9 255 ~2040 ~640

PHEV(40) 14-18 210 ~3460 ~2060

Although these costs are substantial, the battery cost increments for each step of increasing nominal electric range capability are only modest. In particular, the battery cost increments for shorter-range PHEVs over HEVs are quite small. All of the battery costs appear to be lower than the energy cost savings that can be expected over the life of the vehicles from the displacement of fuel energy by electricity. It is also noticeable that the cost of Li-ion batteries (i.e. at cell, module or pack level) includes material cost (e.g. anode/cathode materials), manufacturing cost and other costs (e.g. R&D, marketing, transportation) Material costs account for ~75% of the total battery pack cost while manufacturing and other costs represent around 5% and 20% respectively. A list of current Li-ion battery costs is given in Table 7. Current prices are in the range of 700-1000 $/kWh or even higher.

Table 7: Current price of Li-ion batteries

Manufacturer Chemistry Current Price($/kWh)

Target Price($/kWh)

Enert 1 (HEV) Li-Polymer 660 N/AValence Technologies(VLNC) Li-phosphate 1000 500

Altair Nanotechnologies (ALTI) Li-titanate 1000 N/AA123 Systems (power tool packs) Li-phosphate 1228 N/A2008 DOE SEGIS-ES Estimates (PV solar battery packs) Various 1333 780

2009 NEDO survey results (Avg of Japanese products) Various 2018 1000

The high production volumes already achieved today suggest that Li-ion battery costs could significantly decline in the short term. It is for instance expected that Li-ion battery cost would fall as low as 395 $/kWh and 260 $/kWh for a PHEV10 and a PHEV40 respectively with 100000 units produced. The battery cost goal set by the USABC range from 300 $/kWh to $200/kWh for the PHEV10 and PHEV40 respectively. The MIT estimates that the commercialization of a PHEV30 requires a cost as low as 320 $/kWh. The U.S. Department of Energy' goal is 250 $/kWh by the year 2015. PHEVs would become cost efficient to consumers if battery prices would decrease from 1300 $/kWh to about 500 $/kWh (so that the battery may pay for itself). It is however not yet proven that costs will reduce in such a scale. Despite already important production volumes, costs remained constant over last 9 years. It is thus not guaranteed that the above-mentioned targets will be met. Li-ion battery costs are expected to remain lower than NiMH batteries but the range of 600-700 $/kWh is seen more realistic in the short to medium term. Figure 43 shows a possible (rather optimistic) evolution of Li-ion battery pack cost for PHEV-40 by 2010 and 2020.

Figure 43: Battery Pack Supply Chain Cost Breakdown

3.4 Safety Hazards

Though hybrids get better fuel economy than conventional vehicles, they also present some unique hazards when they are involved in an accident or when they have to be serviced or repaired. The high voltage hybrid battery and hybrid powertrain components create a potential shock hazard. We haven't heard of anyone actually being electrocuted while working on a hybrid, or while being extracted from a wrecked hybrid, but the danger is real. Consequently, certain precautions must be used when working on a hybrid car or truck.

As a rule, hybrids that have a full electric driving mode use higher voltage batteries than those that do not have a full electric mode. The battery pack in the Honda Insight and Civic

Hybrid at rated at 144 volts. The 1st generation 2001-2003 Toyota Prius battery is rated at 273.6 volts while the 2nd generation 2004-2008 Prius is rated at 201.6 volts. The Ford Escape Hybrid has the most potent battery of all, rated at 330 volts! By comparison, the battery in the Saturn Vue hybrid is rated at 36 volts.

Hybrid Electrocaution Hazard The voltage in most hybrid batteries can deliver a lethal shock, much like that of an electric chair. What's more, the voltage from a hybrid battery is Direct Current (DC), which carries more of a wallop than Alternating Current (AC). The threshold voltage where DC becomes dangerous can be as low as 55 to 60 volts, compared to 110 volts for AC. Ordinary 12 volt DC car batteries and electrical systems pose no danger, but the high voltage secondary ignition system can give you a nasty shock (though the current is usually too low to cause serious harm).

Electromagnetic FieldsThere are concerns about the risk of developing cancer as a result of exposure to electromagnetic fields. HybridCars.com reports that magnetic fields are produced with the flow of current from the hybrid batteries, and some reports have linked cancer to exposure to electromagnetic fields. However, according to the National Institutes of Health, scientists have concluded that the overall risk of getting cancer from an electromagnetic field is weak.

Pacemakers and DefibrillatorsDue to the electromagnetic field that is produced by an electric hybrid vehicle, people with pacemakers and defibrillators should avoid getting too close to the engine. According to the relevant websites, operating a hybrid car shouldn't pose a risk unless anyone get within a few inches of the engine. There is no risk being inside of the vehicle.

Hybrid SafetyHigh voltage cables in hybrid vehicles are usually color-coded to warn you of their potential danger. On most, the high voltage cables are color-coded ORANGE. On the Saturn, with its 36-volt system, the cables are color-coded BLUE. Avoid contact with these cables unless the high voltage battery in the back of the vehicle has first been disconnected.

All hybrid batteries have a safety switch or disconnect mechanism to disconnect the battery from the vehicle's electrical system. The location of the battery disconnect safety switch and the disconnect procedure will vary from one application to another, so refer to your owner's manual or service literature for the specifics.

Another often overlooked precaution is to make sure the ignition is OFF and the key or key fob is away from the vehicle before it is serviced or repaired. It has to make sure the READY light is not on. If the power is on, the high voltage system is live and poses a shock hazard should you come into direct contact with any of its uninsulated electrical components (such as the inverter under the hood).

Another suggestion is to wait 15 minutes before working on the vehicle after the battery has been isolated or disconnected. The high voltage capacitors inside the inverter need time to bleed off their stored power.

Most hybrids are designed to isolate the high voltage battery if the vehicle is involved in an accident that is serious enough to deploy the air bags. On the Prius, the high voltage battery

and wiring circuits are separate from the other electrical circuits in the vehicle, and do not use the body or chassis as a ground. The Prius has a ground fault sensor that will disconnect the hybrid battery and turn on a warning light (an exclamation mark inside a triangle) if it detects any high voltage leakage to to the body. A DTC P3009 fault code would indicate such a problem on the Prius.

The good news is that nobody has yet been injured or electrocuted by a hybrid electric vehicle. An extensive search of news archives failed to turn up any reports of service technicians, emergency responders or motorists being zapped by a high voltage hybrid.

Chapter 04

Reliability and Further Research

4.1 Hybrid Car Pros

a) Environment FriendlyProbably most documented about hybrid cars are their green credentials - and if more of us drove hybrid vehicles, it would ultimately significantly reduce CO2 emissions. A hybrid car produces 25 to 35% less in CO2 emissions than regular cars, because it is has a second electric, battery powered engine, which recharges via the petrol engine. This is a much more energy efficient engine for town and city driving, or driving in traffic. Then, when driving at higher speeds, the power of the petrol engine kicks in.

b) Fuel EfficientThe dual engines help to maintain the most efficient energy consumption during all driving conditions - which means cars will need to be filled with petrol far less often than with a regular car. Because the car is able to utilize the battery powered engine when driving at lower speeds or in traffic (which is often when needless amounts of petrol is burned), little or no fuel is needed during these driving conditions.

Plus, hybrid cars are designed specifically to maximize fuel efficiency. The materials used for the body of the vehicle are lighter, and the design is aerodynamic to reduce air resistance.

c) Financial BenefitsBecause hybrid vehicles are better for the environment, the government - keen to be seen to be embracing green policy - offer incentives for driving them. Expect to have much lower annual car tax bills, and exemption from congestion charges or low emission zones.

d) Reduced Oil DemandWith greater use of hybrid cars the demand of on fossil fuels like oil will reduce which leads to overall reduction in demand of the oil throughout the world leading to reduction in its prices.

4.2 Limitations

a) PriceHybrid cars typically cost several thousand dollars more than the non-hybrid version of the same vehicle. If your vehicle has a problem, replacement parts needed to repair the vehicle may be more expensive as well.

b) BatteriesThe battery for a hybrid vehicle adds more weight to the car. Some batteries need to recharge every few weeks. In addition, the cost of replacing a battery is expensive. On the other hand, many batteries come with warranties for extended periods of time.

c) PollutionHybrid cars require more parts and components than traditional cars. The extra steps needed during the manufacturing process of hybrid cars may cause more pollution than regular vehicles during manufacturing.

d) NoiseHybrid electric vehicles running on electric batteries alone are significantly quieter than traditional vehicles. The lack of noise may cause problems for pedestrians, neighborhood children, or those who are impaired and rely on the noise of the car to warn them of a nearby vehicle.

e) PerformanceA hybrid electric vehicle often has a slower acceleration and may also have a lower top speed than its traditional counterpart.

4.3 Future Research

Plug-in HybridIn the not-too-distant future, we hope to have plug-in electric hybrid vehicles available on the market. These cars run on electric powered batteries. The batteries are charged over night, or during the day when they are parked in garages or at park-and-ride facilities, plugged into converted electrical outlets. These are not wimpy cars, either, with limited ranges of 20 miles. Technologies exist (but for some reason have not yet been implemented by the major vehicle manufacturers yet) to allow up to 40 miles on a single charge, at which point the vehicle switches over to fuel. Estimates are that these vehicles get from 100-125 miles per gallon. This could be much higher if you have a short commute and do not drive far enough to switch from battery to fuel power.

The ride is silent, and acceleration capabilities are satisfactory to quite good. The most pleasing prospect, however, is the fact that we can save at the pump, cut back on greenhouse gas emissions, and reduce our dependence on foreign oil production.

Breakthrough in battery systemIn the near future hybrid batteries are going to be one of the biggest areas of hybrid vehicle development. Currently, hybrid vehicles utilize NiMH battery technology, but it appears that the future will almost certainly be dominated by Lithium-ion batteries.

In a decade or less, it isn't inconceivable to imagine hybrid batteries that are two or three times as efficient as today's batteries, but half as heavy and half as expensive.

All future hybrids will benefit significantly from these developments in hybrid battery technology, but especially full hybrids,

This breakthrough include that these batteries that charge in minutes, albeit with special chargers, affordable and luxury pure EV cars with 250 miles range and a wide choice of plug in hybrids very soon. Many new battery chemistries look promising for the future. Some of the new electric cars generate at least some of their electric power from solar cells on the vehicle. In future, they may generate electricity in part from shock absorbers, transparent solar cells over windows and - thermoelectrically - from the engine and exhaust in hybrids.

Fuel cell hybridWhile many see hydrogen hybrids as a 'bridge' to fuel cell vehicles, the truth is, most fuel cell vehicles will inevitably be fuel cell hybrid vehicles.

Ultimately, hybrid vehicles aren't a bridge to fuel cell vehicles; they represent an integral piece of fuel cell vehicle technology.

Hydrogen hybridsAlready Ford and Toyota have been working on hybrids that utilize hydrogen rather than gasoline. In fact several hydrogen hybrids are already on the road in California.

These hydrogen hybrids are seen as a bridge to fuel cell hybrids because they could start laying the foundation for hydrogen fueling stations.

Diesel of hybridsIn the short term, there is a lot of potential for diesel hybrids, especially outside of the United States. Since diesel offers better fuel economy than gasoline, diesel hybrids would be more fuel efficient than gasoline hybrids.

Many “next generation” gas-electric hybrids were unveiled. While dual-mode hybrids are efficient at stop and go traffic, they don’t quite add as much on the highways. A new concept plug-in hybrid from Saturn can potentially go for 10 miles on pure electric power, before needing to use gas.

But whether its battery technology, plugging into the grid, powering vehicles from solar, nuclear, or wind, all renewable fuels come into play. Automakers say that having a diverse set of alternatives is the best way to reduce our dependence on gasoline.

4.4 Outlook

The outlook for hybrid vehicles is very positive for a variety of reasons. The HEV can easily be designed to equal or surpass the performance of conventional cars, and it can meet or exceed customer expectations. The extra cost of the car, currently about US$5000, can be substantially reduced as manufacturing experience is acquired for the batteries, which currently cost about US$3000, and for the other components. In countries with relatively high gasoline prices, the savings in gasoline costs currently pay back the extra purchase costs of the car in about 8 years. As manufacturing volumes and gasoline prices increase, this payback period could be reduced to 3-5 years, and consumers would have an economic incentive to buy HEVs. For all these reasons, some of the major motor manufacturers strongly support

hybrid vehicle technology; for example Toyota considers it to be the key enabling technology for the coming decades.

Hybrid vehicle technology can be used in most segments of the car market, including all mid- and high-priced passenger cars, city buses, and delivery trucks. It has fewer advantages for heavy trucks that drive long distances at constant speed and for low -cost small passenger cars, which are already fuel efficient and for which the extra purchase cost would be a major impediment.

Hybrid vehicle technology is a “step-out” technology in that it modifies an existing and proven technology, rather than replacing it completely with something that is totally different. This is a major advantage from the market introduction perspective. The changes can be introduced gradually, and the risk of major technical problems and high warranty costs after a few years is reduced. Another advantage is that no new infrastructure is required.

Hybrid vehicles have strong environmental advantages like they reduce noxious pollution, emissions of greenhouse gases, and energy consumption by half. They will be able to use all of the advanced fuels, such as ethanol, bio-diesel, and natural gas, which will make them even more environmentally friendly. In countries where most of the electricity is generated by coal-fired power plants, hybrid vehicles will be environmentally friendlier than battery electric vehicles.

Great potential for improvement in hybrid technology still remains, not only in cost reduction but also in technical performance. On the other hand, strategies that increase the performance of hybrid vehicles could degrade their potential fuel efficiency. The key will be further improvements in battery performance; so that the battery will easily last for 8-10 years and the car will be able to drive on battery power for 20-30 km. This would allow hybrid vehicles to drive on battery power with zero pollution in city centers and use their internal combustion engines only on the outskirts of a city or on long trips.

Conclusion

Nowadays, research of Hybrid Electric Vehicle is one of the most meaningful ways to solve the problems of pollution and energy. Motor and its control technology are one of main components of Hybrid Electric Vehicle, and largely affect vehicle’s power performance, fuel economy, and emission. This paper carries on the research of Hybrid system, Battery technology, Electrical and Thermal management with a brief idea of safety and cost aspect.

In case of the hybrid battery life expectancy, although at present the batteries used in this vehicle have a limited number of charging cycle, they are proved to be extremely reliable. In comparison with regular car batteries, a study shows that regular car batteries will generally need replacing on average about every three or four years. Hybrid car battery packs, on the other hand, are commonly warranted to last for about eight to ten years. Testing indicates that they may eventually actually outlast the car itself and in another estimation we can say that the batteries are designed to last between 150,000 and 200,000 miles. Moreover, hybrid car batteries are highly recyclable and have been designed to have a reduced toxic waste effect as compared to other, similar products. It is expected that hybrid battery packs of the future will probably last even longer and be even more environmentally friendly than their counterparts of today. In case of complex inverter design there are some easy solutions available. Inverter

motor control applications are increasing at a tremendous speed. Their main advantages are better control, energy saving, longer lifetime. Regenerative braking does more than simply stop the car. Electric motors and electric generators (such as a car's alternator) are essentially two sides of the same technology. Both use magnetic fields and coiled wires, but in different configurations. Regenerative braking systems take advantage of this duality. In this paper the role of ultra capacitor to improve the system performance in acceleration and regenerative braking times has been inspected and the results show remarkable advantages for the proposed hybrid system configuration and energy sharing strategy.

Hybrid cars will be the next alternative to fuel vehicle. These cars are one among the promising types of new generation cars. Hybrid cars are more reliable than electric cars and they have gasoline as an alternate fuel. Hybrid cars are currently more expensive to buy than conventional cars. But these cars are also known to emit far lower levels of pollutants in the air. So, people may pay more now but they should see the great gains and thus yields is solving current and long term energy needs.

As in Bangladesh, compressed natural gas (CNG) is being used along with petroleum oil to reduce the usage of conventional fuel. Similarly, in HEV, batteries are replacing the CNG to provide sufficient power to the system. Concerning this fact, HEVs can put a remarkable impact on the transportation system of Bangladesh and are very likely to claim an ever larger percentage of the road in forthcoming years.

References

[1] Pesaran, A.A., Vlahinos, A., Burch, S.D., "Thermal Performance of EV and HEV Battery Modules and Packs," Proceedings of the 14th International Electric Vehicle Symposium, Orlando, Florida, December 15–17, 1997.

[2] Oswald, L.J. and Skellenger, G.D. "The GM/DOE Hybrid Vehicle Propulsion Systems Program: A Status Report,” Proceedings of the 14th International Electric Vehicle Symposium, Orlando, Florida, and December 15–17, 1997.

[3] Pesaran, A.A., Burch, S.D., Keyser, M., “An Approach for Designing Thermal Management Systems for Electric and Hybrid Vehicle Battery Packs,” May 24-27, 1999, Proceedings of the 4th Vehicle Thermal Management Systems, London, UK.

[4] Pesaran, A.A., Swan, D., Olson, J., Guerin, J.T., Burch, S., Rehn, R., Skellenger, G.D., "Thermal Analysis and Performance of a Battery Pack for a Hybrid Electric Vehicle," Proceedings of the 15th International Electric Vehicle Symposium, Brussels, Belgium, October 1–October 3, 1998.

[5] Pesaran, A.A., Keyser, K., "Thermal Characteristics of selected EV and HEV Batteries,” Proceedings of the 16th Annual Battery Conference: Applications and Advances, Long Beach, California, January 9–12, 2001.

[6] Keyser, M., Pesaran, A.A., Mihalic, A., and Zolot, M, “Thermal Characterization of Advanced Battery Technologies for EV and HEV Applications,” NREL Report, August 2000.

[7] F Wang and B Zhuo; Regenerative braking strategy for hybrid electric.

[8] Vehicles based on regenerative torque optimization control; Institute of Auto Electronic Technology, School of Mechanical Engineering, Shanghai Jiaotong University, Shanghai, People’s Republic of China.

[9] Juan W. Dixon, Micah Ortúzar and Eduardo Wiechmann; Regenerative Braking for an Electric Vehicle Using Ultracapacitorsand a Buck-Boost Converter; Department of Electrical Engineering,Catholic University of Chile , Casilla 306, Correo 22, Santiago, Chile.

[10] F. Nemry, G. Leduc, A. Muñoz, Plug-in Hybrid and Battery-Electric Vehicles: State of the research and development and comparative analysis of energy and cost efficiency.

[11] Fritz R. Kalhammer, Haresh Kamath, Mark Duvall, Mark Alexander and Bryan Jungers.

[12] F. Kalhammer, Status of Batteries for PHEVs, presented at the Plug-In 2008 Pre-Conference Battery Workshop, Anaheim, California, July 2008.

[13] M. Anderman, F. Kalhammer and D. MacArthur., Advanced Batteries for Electric Vehicles: An Assessment of Performance, Cost and Availability, Report of the California Air Resources Board Battery Technical Advisory Panel, June 2000, available from the California Air Resources Board, Sacramento, California, U.S.A.

[14] D. Santini and P. Nelson, 24th Electric Vehicle Symposium, Stavanger, Norway, May 2009.

[15] P. Mock and S. Schmid,“Brennstoffzellen- und Batteriefahrzeuge – Kurzfristiger Hype oder langfristiger Trend?” Innovative Fahrzeugantriebe, Dresden, Germany 6-7 November 2008. [16] P.James, A.Forsyth, G.Calderon-Lopez, V.Pickert ; DC-DC converter for hybrid and all electric vehicles; Stavanger, Norway, May 13-16, 2009 .

[17] Ciappa, M and Fichtner, W, Lifetime Prediction of IGBT Modules for Traction Applications, IEEE 38th Annual International Reliability Physics symposium, 2000.

[18] H. Plesko, J. Biela, and J. W. Kolar;” Design and Analysis of a New Drive-Integrated Auxiliary Dc-Dc Converter for Hybrid Vehicles; Power Electronic Systems Laboratory”, ETH Zurich ETH-Zentrum, ETL I16, Physikstrasse 3CH-8092 Zurich, Switzerland.

[19] M. Kheraluwala, R. Gasgoigne, D. Divan, and E. Bauman, “Performance characterization of a high power dual active bridge dc/dc converter,” in Conference Record of the 1990 IEEE Industry Applications Society Annual Meeting, vol. 2, 7-12 Oct. 1990.

Appendix

A

A123 SystemA123 Systems develops and manufactures advanced lithium-ion (lithium iron phosphate) batteries and battery systems for the transportation, electric grid services and commercial markets.

Anti lock braking system An anti-lock braking system (ABS) is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up (that is, ceasing rotation) and therefore avoiding skidding.

B

BAS-Belt Alternator StarterThe BAS system operates with a “start-stop system”, in that it shuts down the engine as the vehicle comes to a stop and instantly restarts it when the brake pedal is released.

Brake Cooling System A brake cooling system for cooling vehicle brakes having fascia for mounting in front area of the vehicle. The brake cooling system further has a channel structure communicating with the aperture to allow the air to pass through the channel structure.

Battery Charger A battery charger is a device used to put energy into a secondary cell or rechargeable (battery) by forcing an electric current through it.

Bi directional ac/dc converter AC/DC converter designed for high-output direct Currents and tight output voltage regulations. This converter exhibits good efficiency and Very low output voltage level.

C

Compression braking A compression brake, frequently call Jake brake or Jacobs brake, is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves in the cylinders, releasing the compressed air trapped in the cylinders, and slowing the vehicle.

D

Duty cycle The time intervals devoted to starting, running, stopping, and idling when a device is used for intermittent duty. The ratio of working time to total time for an intermittently operating device, usually expressed as a percent. Also known as duty factor.

E

EV-Electric VehicleAn electric vehicle (EV), also referred to as an electric drive vehicle, uses one or more electric motors for propulsion. Electric vehicles include electric cars, electric trains, electric lorries, electric aeroplanes, electric boats, electric motorcycles and scooters and electric spacecraft.

Endothermic In thermodynamics, the word endothermic ("within-heating") describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat.

G

Galvanic Isolation Galvanic isolation is the principle of isolating functional sections of electrical systems preventing the moving of charge-carrying particles from one section to another, i.e. there is no electric current flowing directly from one section to the next. Galvanic isolation is used in situations where two or more electric circuits must communicate, but their grounds may be at different potentials. It is an effective method of breaking ground loops by preventing unwanted current from travelling between two units sharing a ground conductor.

GPS-Global Processing SystemThe Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites.

H

HEV-Hybrid Electric VehicleA hybrid electric vehicle (HEV) is a type of hybrid vehicle and electric vehicle which combines a conventional internal combustion engine (ICE) propulsion system with an electric propulsion system

Hybrid Battery The hybrid car battery---a rechargeable battery used for hybrid vehicles---is responsible for running every function of the car. The hybrid battery powers the electric motor---the component responsible for propelling the vehicle---while reducing the pressure on the gasoline engine, thus reducing the amount of fuel the car needs. The hybrid car battery is designed for the car to drive for longer periods of time in between trips to a gas station.

HPA It stands for Hydraulic power assist. It means that a hydraulic system is incorporated with mechanical steering.

I

ISG-Integrated Starter GeneratorThe electronically controlled integrated starter-generator (ISG), as its name implies, replaces both the conventional starter and alternator (generator) in a single electric device.

L

Lithium Battery Lithium batteries are disposable (primary) batteries that have lithium metal or lithium compounds as an anode.

M

MEA-Membrane Electrode Assembly

A membrane electrode assembly (MEA) is an assembled stack of proton exchange membranes (PEMs) or alkali anion exchange membrane (AAEMs), catalyst and flat plate electrode used in a fuel cell.

P

PEM-Polymer Electrolyte MembraneA proton exchange membrane or polymer electrolyte membrane (PEM) is a semi permeable membrane generally made from ionomers and designed to conduct protons while being impermeable to gases such as oxygen or hydrogen. This is their essential function when incorporated into a membrane electrode assembly (MEA) of a proton exchange membrane fuel cell or of a proton exchange membrane electrolyser separation of reactants and transport of protons.

PEMFC-Polymer Electrolyte Membrane Fuel CellProton exchange membrane fuel cells, also known as polymer electrolyte membrane (PEM) fuel cells (PEMFC), are a type of fuel cell being developed for transport applications as well as for stationary fuel cell applications and portable fuel cell applications. Their distinguishing features include lower temperature/pressure ranges (50 to 100 °C) and a special polymer electrolyte membrane.

Peltier see beck effect Two fundamental principles in the relationship between heat and electricity are the Peltier and See beck effects.  Both relate temperature to voltage intensity and current direction and have been essential in many cooling / heating applications. This is especially true in high cooling environments such as refrigeration units, advanced computer system cooling, and cooling for high-powered lasers, etc.

PHEV A plug-in hybrid electric vehicle (PHEV or PHV), also known as a plug-in hybrid, is a hybrid vehicle with rechargeable batteries that can be restored to full charge by connecting a plug to an external electric power source (usually simply a normal electric wall socket).

R

Regenerative Braking Technology Regenerative braking is used on hybrid gas/electric automobiles to recoup some of the energy lost during stopping. This energy is saved in a storage battery and used later to power the motor whenever the car is in electric mode.

Regenerative torque A complementary regenerative torque system exists for a vehicle including an engine having an accelerator pedal position sensor; a transmission unit; and a drive shaft for driving a pair of wheels for propelling the vehicle. The regenerative torque system selectively stores and supplies energy to the drive train to provide on demand complementary torque thereto. The regenerative torque system may be either hydraulic or electric in nature and may be disposed upstream or downstream of the transmission relative to the engine.

S

Super capacitor/Ultra capacitor

The super capacitor resembles a regular capacitor with the exception that it offers very high capacitance in a small package. Energy storage is by means of static charge rather than of an electro-chemical process that is inherent to the battery. Applying a voltage differential on the positive and negative plates charges the super capacitor.

Starter motor A starter motor (also starting motor or starter) is an electric motor for rotating an internal-combustion engine so as to initiate the engine's operation under its own power.

T

Thermoelectric cooler Thermoelectric cooling uses the Peltier effect to create a heat flux between the junctions of two different types of materials.

Z

ZVS converter It stands for zero voltage switching converters. The ZVS function is achieved at the commutation stage of two complementary switches.