HOW CVT TRANSMISSION WORKS

CVT Basics
Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears, which means they don't have interlocking toothed wheels. 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.

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.

CVT Benefits

Continuously variable transmissions are becoming more popular for good reason. They boast several advantages that make them appealing both to drivers and to environmentalists. The table below describes some of the key features and benefits of CVTs.

Advantages of CVTs
Feature	Benefit
Constant, stepless acceleration from a complete stop to cruising speed	Eliminates "shift shock" -- makes for a smoother ride
Works to keep the car in its optimum power range regardless of how fast the car is traveling	Improved fuel efficiency
Responds better to changing conditions, such as changes in throttle and speed	Eliminates gear hunting as a car decelerates, especially going up a hill
Less power loss in a CVT than a typical automatic transmission	Better acceleration
Better control of a gasoline engine's speed range	Better control over emissions
Can incorporate automated versions of mechanical clutches	Replace inefficient fluid torque converters

TOYOTA LATEST CVT TRANSMISSION TECHNOLOGY

TOYOTA CVT TRANSMISSION TECHNOLOGY

Figure 1. TOYOTA SUPER CVT-I

The current trend for Toyota transmission is CVT transmission.

The Super CVT-I 7 speed automatic transmission of the Corolla Altis 2011 varies in functionality from the conventional 4-speed torque converter based automatic transmission gearbox you find in the conventional corolla automatic.

The ideal CVT system works on infinite gear ratios which vary continuously over wide range and give a low loss transmission performance. The conventional automatic with fluid based torque converter transmission causes some losses, so its not very fuel efficient. However the CVT may also cause some practical problems and slip in due to the nature of the transmission mechanism. To overcome the disadvantages of these two system, Toyota has developed a 7-Speed CVT-I transmission which is not the ideal implementation of the CVT, but it makes use of 7 gears controlled on which the gear shift is intelligently controlled by a control system to keep the engine RPM of the car in the optimum range for superior fuel efficiency and power response of the car.

Figure 2 :The current Toyota Corolla or Toyota Altis in Malaysia is using Toyota Super CVT-i in all its variant.

Hybrid Transmission

A hybrid transmission that uses Toyota’s original power split device.

  

  The hybrid transmission consists of the power split device, the generator, the electric motor and the reduction gears, etc. The power from the engine is split into two by the power split device. One of the output shafts is connected to the motor and the wheels while the other is connected to the generator. In this way, the motive power from the engine is transmitted through two routes, i.e., a mechanical route and an electrical route.

  An electronically controlled continuously variable transmission is also provided, which can change speed while continuously varying the rpm of the engine and the rpm of the generator and the electric motor (in relation to vehicle speed).

  THS II also reduces friction loss by about 30% by using ball bearings in the transmission and low-friction.

 

 Power Split Device
   The power split device uses a planetary gear. The rotational shaft of the planetary carrier inside the gear mechanism is directly linked to the engine, and transmits the motive power to the outer ring gear and the inner sun gear via pinion gears. The rotational shaft of the ring gear is directly linked to the motor and transmits the drive force to the wheels, while the rotational shaft of the sun gear is directly linked to the generator.

Transmission

  A worn-out automatic transmission will not only dampen your driving experience, it can adversely affect fuel economy as well. If your transmission shudders, slips or the fluid has turned from pink to brown, it could be time for transmission replacement. Get the durability and reliability you expect from a Genuine Toyota automatic transmission.

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