Energy and the Environment

Energy and the Environment

Fall 2013Instructor: Xiaodong Chu

Email ： [email protected] Tel.: 81696127

mailto:[email protected]

Flashbacks of Last Lecture

• The development of the internal combustion engineinternal combustion engine (ICE) (内燃机 ) and its supply of liquid fossil fuel transformed both urbanurban (城市 ) and rural rural (农村 ) communities in the twentieth century

• The most common engine in road vehicles is the gasoline-gasoline-fueled spark ignition (SI) reciprocating enginefueled spark ignition (SI) reciprocating engine (火花点火往复式汽油发动机 ) and the diesel-fueled compression ignition (CI) reciprocating engine engineengine (压缩点火往复式柴油发动机 ) is less common

• Working principles of ICE: two-stroke cycle and four-stroke cycle


• The burning of fuel in the cylinder of a reciprocating internal combustion engine produces mechanical power by exerting a force on the moving piston

• The total work done by the gas in one cycle of the engine is

• The power input to the piston

dW pdV

cW pdV pV

cpV NNP W

n n


• Brake powerBrake power (制动功率 )• Brake mean effective pressureBrake mean effective pressure (制动平均有效压力 ) • Brake specific fuel consumption Brake specific fuel consumption (bsfc) (制动油耗率 , 燃油消耗率 , 制动单位油耗量 )

Transportation: Vehicle Power and Performance

• Moving a vehicle along a highway requires the expenditure of mechanical power to turn the wheels– Part of this power is needed to overcome the drag force drag force (气动阻力 )

exerted by the air on the moving vehicle– Another part is needed to counter the resistance of the tires moving

over the ground, called the rolling resistancerolling resistance (滚动阻力 )– If the vehicle is climbing a hill, additional power is needed to lift it

vertically in the earth’s gravitational fieldearth’s gravitational field (地球重力场 )– When the vehicle accelerates to a higher speed, power is needed to

increase the kinetic energykinetic energy (动能 ) of the vehicle


• The aerodynamic drag force aerodynamic drag force (气动阻力 ) acting on a vehicle is given as the ρCD AV2/2, where ρ is the mass density of air (kg/m3), A is the frontal areafrontal area (迎风面积 ) (m2) of the vehicle, V is the vehicle speed (m/s), and CD is the drag coefficient

• The drag coefficient is determined by testing vehicle models in a wind tunnelwind tunnel (风洞 ), and its value (in the range of 0.25–0.5) depends very much on the vehicle shape and external smoothness

• The corresponding power needed to overcome the air resistance is the product of the force times the vehicle velocity

2)(

3VACP Dair


• The source of rolling resistance lies in the deflectiondeflection (偏转 ) of the tire where it comes into contact with the ground– This deflection is necessary to support the weight of the vehicle and to provide a

contact area between the tire and the road to prevent the tire slippingslipping (打滑 ) along the road surface

– The deflection is such that, when the wheel rotates, the road surface exerts a retarding torqueretarding torque (减速力矩 ) on the wheel, which must be overcome by the engine drive system, and a corresponding retarding force on the wheel axlewheel axle (轮轴 )

• This force is usually specified as CRmg, and the corresponding power Proll becomes

where m is the vehicular mass, g is the acceleration of gravity, V is the vehicle speed and CR is a small dimensionless constant whose value depends upon the tire construction and pressure

mgVCP Rroll


• When climbing a hill of rise angle θ, the force of gravity acting on the vehicle, mg, has a component mg sin θ opposing the forward motionforward motion (向前运动 )

• The power required to maintain a steady climb rate issinmgVPhill


• The instantaneous power instantaneous power (瞬时功率 ) required to accelerate the vehicle, Pacc, is simply the time rate time rate (时间变化率 ) of increase of vehicular kinetic energy, m(1 +ε)V2/2, or

where ε is a small dimensionless constant that accounts for the rotational inertia rotational inertia (转动惯量 ) of the wheels and drive traindrive train (传动系统 ), and dV/dt is the acceleration of the vehicle

• When the vehicle decelerates (dV/dT < 0) by using the brakes or closing the throttle, the negative power of equation does not put power back into the engine, so that Pacc is effectively zero during deceleration

dt

dVVm

Vm

dt

dPacc )1(

2

)1( 2


• The total power thus becomes

• For a given driving cycle, heavy cars will require more powerful engines and will consume more fuel, more or less in proportion to vehicle mass, than will light vehicles – A very fuel efficient vehicle is necessarily a light one

• For driving at a steady speed, only rolling and aerodynamic resistance must be overcome, the power required being

2sin)1(

3VACggC

dt

dVmV

PPPPP

DR

airhillrollacc

2)(

3VACmgVCP DRsteady


• The parameters of vehicle design that lead to increased fuel economy include low values of vehicle mass m, drag area product CDA, and rolling resistance coefficient CR

• Recovery of vehicle kinetic energy during deceleration for reuse during other portions of the driving cycle will also improve vehicle fuel economy


• The transmission is a device attached to the engine that provides stepped speeds to the drive shaft that connects it to the wheels

• There are two forms of transmission: manual and automatic– In a manual transmission, the vehicle operator disengages a clutch disengages a clutch (脱开离合器 ) and manually shifts to a different gear before engaging the clutch again

– In an automatic transmission, a fluid coupling fluid coupling (液压耦合装置 ) replaces the clutch and gear shifting gear shifting (换档 ) is accomplished by computer-controlled hydraulic actuatorshydraulic actuators (液压执行器 )

• The more operator-friendly automatic transmission is less efficient than the manual one by about 10 percentage points


Transmission with a five-speed + reverse gearbox


• The drive shaft drive shaft (驱动轴 ) connecting the engine/transmission to the wheels, either front or rear (or both in the case of four-wheel drive), utilizes a differential gear (差速器 ) to apply equal torque to both drive wheels drive wheels (驱动轮 ) while allowing different wheel speeds during maneuvering

• The drive shaft provides for the vertical motion of wheels with respect to the chassischassis (底盘 ) and for the steering steering motion motion (转向运动 ) when using a front-wheel drive


• For constant-speed travel, the engine power demand rises rapidly with vehicle speed, as the aerodynamic drag outpaces rolling resistance

• At any vehicle speed, the vertical distance between the “demand” (dash–dot) curve and the “maximum supply” (solid) curve is the maximum power available for vehicle acceleration and hill climbing


• At low vehicular speeds, this difference is greatest for the lowest shift level shift level (变速级 ), while at the highest speed this difference is least, shrinking to zero at the vehicle’s top speed

• The lower gear levels are needed to provide high acceleration at low vehicular speed


• Only a small fraction of the maximum engine power is used at steady vehicle speeds, reaching about 40% at cruising speed cruising speed (巡航速度 ) and the remainder is available for acceleration and hill climbing

• For average vehicle duty cycles, only a fraction of the time is used in acceleration, and the portion of that at which the engine power is a maximum is small

• The vehicle capacity factor, the time-averaged fraction of installed power that is utilized by a moving vehicle, is less than 50% for the average duty cycle

• Nevertheless, for satisfactory performance reserve power is required over the normal speed range of the vehicle


• For a steady vehicle speed, the engine relative efficiency is at best 50% for speeds less than half the cruise speed, rising to about 80% at cruise speed

• The normal practice of downshiftingdownshifting (降档 ) at low speeds leads to even lower relative efficiencies

• Taking into consideration the short periods of acceleration within a typical driving cycle, the time-averaged relative engine efficiency is certain to be less than 80%

• A method is to employ a continuously variable transmission continuously variable transmission (连续变速传动 ) that can reach higher relative engine efficiencies over a range of steady speeds, but will downshift when acceleration is needed


Variable-diameter pulley for CVT

Transportation: Vehicle Fuel Efficiency

• Traditionally, the efficiency of use of fuel by a vehicle is measured by the distance it moves in a trip divided by the fuel consumed

• The vehicle’s fuel efficiency determines both the fuel cost of the trip and the accompanying carbon emissions to the atmosphere

• The vehicle operator is concerned with the fuel cost while national authorities are concerned with the effects of aggregate vehicular fuel consumption on the problems of maintaining a reliable fuel supply and, most recently, on the contribution of vehicles to the national budget of greenhouse gas emissions


• In the United States, fuel efficiency of new passenger vehicles and light trucks is regulated by the National Highway Traffic Safety Administration of the U.S. Department of Transportation, which had its origin in the oil shortages of the 1970s caused by an embargo on oil exports by OPEC nations

• To ameliorate the nation’s future dependence upon imported oil, Corporate Average Fuel Economy (CAFE) standards were promulgated, beginning with the 1978 model year

• These standards required automobile manufacturers to design vehicles so that their sales-averaged vehicle fuel economy did not exceed the level designated for each of two vehicle classes: passenger vehicles and light trucks (pickup trucks, vans, sport utility vehicles)


• Could you please make a survey on China vehicle fuel efficiency regulations?


• The measurement of vehicle fuel economy is based upon dynamometer (测功机 ) simulations of typical driving cycles for urban and highway travel, originally devised by the U.S. EPA for evaluating vehicle emissions (Urban Dynamometer Driving Schedule, UDDS)

• The test vehicle is operated in a stationary position, while the drive wheels turn a dynamometer that is adjusted to provide the air resistance and acceleration/deceleration loads

• While the dynamometer does not precisely simulate these forces at each point in the test, it suffices to provide a reasonable average for characterizing the emissions and fuel economy of the test cycle


• Two test cycles are used for measurements of vehicle fuel efficiency, one for urban driving and the other for highway travel


• There are two major methods to improve vehicle fuel efficiency: improvements to the vehicle design vehicle design and to the power sourcepower source


• There are three major parameters for improving vehicle performance, which are vehicle mass m, rolling resistance coefficient CR, and the vehicle drag area CDA


• Vehicle Mass: In current conventional vehicles, mass is the parameter that best correlates fuel economy– Large, heavy vehicles require big engines to perform well; they

consequently consume more fuel– For a given vehicle size, reducing vehicle mass will permit reductions

in engine and transmission mass, tire and wheel mass, braking system mass, fuel storage mass, steering system mass, engine radiator engine radiator (发动机散热器 ) mass, and so on, compounding the gains in direct mass reduction of the vehicle framevehicle frame (车架 )


• The principal means for reducing mass is the substitution of lighter materials of equal strength and stiffnessstrength and stiffness (强度和刚度 ), such as aluminum alloys or fiber-reinforced plastic for steel and plastic for window glass, as well as the redesign of the vehicle structure to minimize structural mass

• Reducing vehicle mass by material substitution material substitution (材料替代 ) may have implications for vehicle safety– Vehicle frames are designed to absorb the vehicle’s kinetic energy in a

crash while protecting the occupants from harm – In a two-vehicle collision of vehicles of unequal mass, the lighter

vehicle absorbs more than its own kinetic energy and thereby suffers a safety disadvantage


• Aerodynamic Resistance: Reduction of aerodynamic resistance is limited to lowering the drag coefficient CD by careful streamlining of the entire body, because the frontal area A is essentially fixed by the requirements of providing an enclosure for the passengers

• The bulky form of the automobile limits what can be done to reduce aerodynamic drag, but attention to details can bring the drag coefficient into the range of 0.20–0.25 for passenger vehicles


• Rolling Resistance: The rolling resistance coefficient CR can be lowered to 0.005 from current values of 0.010–0.014 by improvements in tire design, but it is difficult to maintain durability, performance, and safety (traction) while reducing rolling resistance

• Light alloy wheels reduce sprung mass, which is desirable, but more complex suspension systems suspension systems (悬挂系统 ) will be needed to recover normal performance with low-resistance tires


• There are several paths to improving the efficiency of engines while supplying the requisite power needed by the vehicle– One is to improve the fuel (or thermal) efficiency of the engine,

especially at off-optimum conditions where the engine is usually operated

– A second is to utilize transmissions that permit the engine to maximize its efficiency for a required power output

– A third is to reduce engine mass, for a given power, so as to improve vehicle efficiency

• However, there are serious constraints imposed on engine efficiency improvements by the requirements for limiting exhaust pollutant emissions


• Reducing Intake Stroke Losses in SI Engines: At partial load the cylinder pressure during the intake stroke is lowered to reduce the fuel amount in the cylinder, resulting in a loss of efficiency– This loss may be offset by varying the timing of the inlet and exhaust

valves with the engine load Variable valve timing (VVT) systems Variable valve timing (VVT) systems (可变气门正时系统 ) are currently used in four-

valve engines, adding to the peak power output and engine mass reduction benefit

– Another alternative is direct fuel injection (DI) into the cylinder during the intake stroke, forming a non-uniform fuel–air mixture at higher overall pressure and lower intake stroke power loss

– The recirculation of exhaust gas into the cylinder during the intake stroke so as to reduce exhaust pollutant emissions can be arranged to reduce intake losses and improve engine efficiency at part load


• Replacing SI Engines by CI Engines: The indirect injection CI engine enjoys about a 25% advantage in fuel economy over the SI engine and does not suffer as much efficiency loss at part load as does the SI engine

• Direct injection CI engines have even higher fuel economy advantage, about 30–40%

Direct injection CI Indirect injection CI


• On the other hand, the CI engine is heavier, for a given power, thereby incurring a vehicle mass efficiency penalty, and is more expensive

• It is more difficult to reduce NOx and particulate emissions in the CI engine


• SuperchargingSupercharging (增压 ): Both SI and CI engines can be supercharged (or turbocharged) to increase maximum engine power for a given displacement and engine mass, with some improvement in engine efficiency at higher loads


• Continuously Variable Transmission: The traditional multistep transmission does not permit the engine to operate at maximum thermal efficiency over the entire vehicle duty cycle

• Indeed, the engine seldom operates at best efficiency• A continuously variable transmission (CVT) can be controlled

to maximize the engine efficiency at any power level needed by the vehicle

• Current CVT transmissions have shown that overall vehicle efficiency can be improved, provided that the transmission efficiency is close to that of conventional transmissions


• Engine Idle-OffEngine Idle-Off (发动机怠速关闭 ): In congested urban driving, considerable time is spent with the engine idling while the vehicle is stationary

• If the engine is stopped during these intervals, fuel will be saved and exhaust emissions will be reduced

• However, stopping the engine and restarting also consumes fuel, so that the length of the idle period has to be sufficient for there to be a fuel reduction in idle-off control

Energy and the Environment

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