Continuously Variable Transmission (CVT)

1. General Notions about Transmissions

The purpose of any transmission is to translate engine performance into vehicle performance. A transmission accomplishes this task by providing a variety of gear ratios between the engine crankshaft and the output axle of the vehicle.

The transmission adapts the output of the internal combustion engine to the drive wheels and reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Often, a transmission has multiple gear ratios (or simply "gears"), with the ability to switch between them as speed varies. The gears can be engaged manually (by the operator) or automatically. Directional (forward and reverse) control may also be provided.

The goal of the transmission is to allow the engine to operate within an ideal state of power production, and to apply this power effectively to the track through the use of the appropriate gear ratio.

In a traditional automatic transmission, the gears are interlocking toothed wheels that help transmit and modify rotary motion and torque. A combination of planetary gears creates all of the different gear ratios that the transmission can produce, typically four forward gears and one reverse gear.

2. Continuously variable transmissions

A continuously variable transmission is a transmission which can change through an infinite number of effective gear ratios between maximum and minimum values.

A continuously variable transmission, or CVT, is a type of automatic transmission that provides more useable power, better fuel economy and a smoother driving experience than a traditional automatic transmission.

Unlike traditional automatic transmission, that only allow a fixed number of gear ratios to be selected, CVTs don’t have a gearbox with a set number of gears, which means they don’t have interlocking tooth wheels.

A gear refers to a ratio of engine shaft speed to driveshaft speed. Although CVTs change this ratio without using a set of planetary gears, they are still described as having low and high "gears" for the sake of convention.

Fig. 2.1 Example of continuously variable transmission (Geuns, 2003)

3. Classification of CVTs

There are different types of CVTs: pulley-based, toroidal and hydrostatic.

3.1. Pulley-based CVTs

The most common type of CVT operates on an ingenious pulley system that allows an infinite variability between highest and lowest gears with no discrete steps or shifts.

Most CVTs only have three basic components:

A high-power metal or rubber belt A variable-input "driving" pulley An output "driven" pulley

CVTs also have various microprocessors and sensors, but the three components described above are the key elements that enable the technology to work.

The variable-diameter pulleys are the heart of a CVT. Each pulley is made of two 20-degree cones facing each other. A belt rides in the groove between the two cones. V-belts are preferred if the belt is made of rubber. V-belts get their name from the fact that the belts bear a V-shaped cross section, which increases the frictional grip of the belt.

When the two cones of the pulley are far apart (when the diameter increases), the belt rides lower in the groove, and the radius of the belt loop going around the pulley gets smaller. When the cones are close together (when the diameter decreases), the belt rides higher in the groove, and the radius of the belt loop going around the pulley gets larger. CVTs may use hydraulic pressure, centrifugal force or spring tension to create the force necessary to adjust the pulley halves.

Variable-diameter pulleys must always come in pairs. One of the pulleys, known as the drive pulley (or driving pulley), is connected to the crankshaft of the engine. The driving pulley is also called the input pulley because it's where the energy from the engine enters the transmission. The second pulley is called the driven pulley because the first pulley is turning it. As an output pulley, the driven pulley transfers energy to the driveshaft.

The distance between the center of the pulleys to where the belt makes contact in the groove is known as the pitch radius. When the pulleys are far apart, the belt rides lower and the pitch radius decreases. When the pulleys are close together, the belt rides higher

and the pitch radius increases. The ratio of the pitch radius on the driving pulley to the pitch radius on the driven pulley determines the gear

When one pulley increases its radius, the other decreases its radius to keep the belt tight. As the two pulleys change their radii relative to one another, they create an infinite number of gear ratios -- from low to high and everything in between. For example, when the pitch radius is small on the driving pulley and large on the driven pulley, then the rotational speed of the driven pulley decreases, resulting in a lower “gear.” When the pitch radius is large on the driving pulley and small on the driven pulley, then the rotational speed of the driven pulley increases, resulting in a higher “gear.” Thus, in theory, a CVT has an infinite number of "gears" that it can run through at any time, at any engine or vehicle speed.

Metal belts don't slip and are highly durable, enabling CVTs to handle more engine torque. They are also quieter than rubber-belt-driven CVTs.

3.2. Toroidal CVTs

It replaces the belts and pulleys with discs and power rollers. Although such a system seems drastically different, all of the components are analogous to a belt-and-pulley system and lead to the same results -- a continuously variable transmission.

Working steps:

One disc connects to the engine. This is equivalent to the driving pulley. Another disc connects to the drive shaft. This is equivalent to the driven pulley. Rollers, or wheels, located between the discs act like the belt, transmitting power from

one disc to the other.

The wheels can rotate along two axes. They spin around the horizontal axis and tilt in or out around the vertical axis, which allows the wheels to touch the discs in different areas. When the wheels are in contact with the driving disc near the center, they must contact the driven disc near the rim, resulting in a reduction in speed and an increase in torque (i.e., low gear). When the wheels touch the driving disc near the rim, they must contact the driven disc near the center, resulting in an increase in speed and a decrease in torque (i.e., overdrive gear). A simple tilt of the wheels, then, incrementally changes the gear ratio, providing for smooth, nearly instantaneous ratio changes.

An outstanding example of friction CVT transmission is Extroid Nissan CVT transmission. This revolutionary type of transmission uses no chain or belt for transmitting motion, but a set of rollers as shown below.

3.3. Hydrostatic CVTs

Both the pulley-and-V-belt CVT and the toroidal CVT are examples of frictional CVTs, which work by varying the radius of the contact point between two rotating objects. There is another type of CVT, known as a hydrostatic CVT, that uses variable-displacement pumps to vary the fluid flow into hydrostatic motors. In this type of transmission, the rotational motion of the engine operates a hydrostatic pump on the driving side. The pump converts rotational motion into fluid flow. Then, with a hydrostatic motor located on the driven side, the fluid flow is converted back into rotational motion.

Often, a hydrostatic transmission is combined with a planetary gearset and clutches to create a hybrid system known as a hydromechanical transmission. Hydromechanical transmissions transfer power from the engine to the wheels in three different modes. At a low speed, power is transmitted hydraulically, and at a high speed, power is transmitted mechanically. Between these extremes, the transmission uses both hydraulic and mechanical means to transfer power. Hydromechanical transmissions are ideal for heavy-duty applications, which is why they are common in agricultural tractors and all-terrain vehicles.

4. Advantages and disadvantages of using CVTsIn comparison to the other types of automatic or manual transmissions, CVTs offer a series of advantages.Continuously variable transmissions are becoming more popular for good reason. They boast several advantages that make them appealing both to drivers and to environmentalists.

Wear and tear is minimal compared to the wear that takes place in a traditional transmission with set gears. That’s because the CVT doesn’t require nearly as many moving parts as a traditional transmission. Less moving parts means that there’s less friction, and therefore less heat working on the parts and breaking them down.

Low maintenance costs - The only part of the CVT that requires routine maintenance is the belt, as it’s constantly moving around. However, the price of a new belt is not expensive at all. Even when you add up the cost of all the belts that you may need during the life of the vehicle, when you consider the fact that the CVT is much more durable, and won’t likely require major repairs like a traditional transmission

Better fuel consumption than a regular automatic transmission as the CVT is able to keep the car in its optimum power range regardless of speed. They can save up to 30% in fuel consumption.

Improved comfort – they provide a smoother ride than automatic transmission as they allow linear and constant accelerations, without interrupting the flow of power between engine and wheels. Thus, it is eliminated the shock that occurs when changing the gear.

Better emission control and less green house gas emissions because of improved control of the engine’s speed range.

Improved acceleration due to the lower power loss experienced, as the power flux engine – wheels is not interrupted.

Adapts to varying road conditions and power demands to allows for a better ride.

Lighter and smaller than automatic transmissions, making them suitable especially for small vehicles with front-wheel-drive traction

The hydraulic motor of the Hydrostatic CVT can be mounted directly onto the wheel hub and this allows for a more flexible suspension system and this in turn eliminates efficiency losses due to the friction from the driveshaft.

On the other hand, the following disadvantages can be mentioned:

No shift shock and some people prefer that feeling. Therefore, automakers are beginning to make CVTs that feel like a traditional transmission, but that still operate as efficiently as a basic CVT.

Belt-driven CVTs (VDP system) have a limited amount of torque, however the technology is constantly being improved.

Not faster or more reliable than automatic transmissions. Cannot cope with big engines or high torque loads (engines above 3.5 liters). Require special oil and other material

5. Kinematic scheme and componentsThis is the first representation of the kinematic scheme of a Friction Continuously

Variable Transmission.

Components of a friction CVT

6. Gear ratioAs discussed, gear ratio is just as important as engine production during establishment of desired vehicle dynamics. Once operating engine speeds are determined, it is the active gear ratio that translates the engine speed into vehicle speed and the engine power into vehicle performance. All changes in diameter between the input cranckshaft and output axel or tire affect the gear ratio and thus the relationship of engine speed to vehicle speed including within the CVT. For any transmission, the gear ratio is defined as:

where wi and wo are the rotational speeds of the input and output shafts. A continuously variable transmission is a transmission able to change the speed ratio without any steps. The speed ratio can be continuously varied by the CVT in a range limited by the extreme values of the transmission ratio (minimum and maximum ratio). The transmission is well characterized by there extreme ratios but also can be defined by the ratio spread D:

The last important parameter of the CVT is the efficiency, which is computed as the ratio of the output power and input power:

From the measurements of the kinematic scheme the gear ratios are as follows:For the minimum gear ratio we measure the lowest value of the input gear and the highest of the output:

i=Din/Dout

i=45/100imin=0.45

For the maximum gear ratio we measure the highest value of the input gear and the lowest of the output:

i=Din/Dout

i=100/45imax= 2.22

Torque flux

Kinematic scheme created using a CAD program:

7. 3D modelling

In order to proper understand how the friction CVT works, we created a simplified 3D model. In the following images are the main components of the assembly:

The complete assembly:

References

https://autotehnic.wordpress.com/2013/05/15/transmisii-automate-cu-variatie-continua-cvt/http://www.nissan-global.com/PDF/tcvt_e.pdfhttp://www.autospeed.com/cms/article.html?&title=Performance-Variable-Transmissions&A=111487http://www.nsk.com/company/presslounge/news/2011/press111129.htmlhttp://en.wikipedia.org/wiki/Transmission_%28mechanics%29http://cars.about.com/od/thingsyouneedtoknow/a/CVT.htmhttp://auto.howstuffworks.com/cvt1.htmhttp://www.odec.ca/projects/2007/viva7s2/proscons2.htmhttp://www.cvt.co.nz/cvt_advantages_and_disadvantages.htm