Continuously Variable Transmission (CVT) (Automobile)


Continuously Variable Transmission (CVT)

The power output of a normal engine varies with the engine speed. At low speeds the output is very less. For better vehicle performance, the engine must run at higher speed at which it develops its maximum power. The same situation also repeats when the torque output and fuel economy, the other two performance factors, are considered. Maximum torque occurs at a different speed from that for maximum economy and also neither of these speeds coincides with the point of maximum power.
The attainment of the constant engine speed required to achieve any one of the three performance factors is not possible with a conventional gearbox, because the engine speed is required to be continually changed to match the vehicle speed. Therefore, the engine only performs its best at the vehicle speed appropriate to the point of maximum engine torque, power or economy.
A continuously variable transmis­sion (CVT) is a particular automatic transmission capable of providing a smoothly varying gear ratio. Unlike a conventional automatic gearbox the CVT has no fixed gears. It varies the drive ratio continuously by changing the operating diameters of two pulleys that are linked by a steel V-belt. The trans­mission can alter its ratio imperceptibly, without any interruption of drive. Figure 25.70 illustrates the basic principle of the CVT system.
The CVT offers the driving comforts associated with an automatic gearbox as well as the efficiency of a manual transmission, without any loss of vehicle performance. Unlike a conven­tional automatic, no torque converter is used and hence no hydraulic slippage losses occur in this system. The CVT is capable of responding instantly to throttle pressure, providing smooth and rapid acceleration. Unfortunately, due to the difficulties of manufacturing a reliable and durable system only a few car makers incorporate such a transmission.

The History of CVT

The idea of employing a belt drive system with variable diameter pulleys is as old as about 1908, when the Rudge company developed their ‘Multi’ motorcycle. The Rudge Multi incor­porated a CVT system using a variable-diameter drive pulley fitted to the engine, and a fixed-diameter pulley mounted on the rear wheel, both are coupled by a leather belt. By rotating a small hand wheel positioned next to the fuel tank, the running diameter of the drive pulley could be varied to alter the gear ratio. At the same time, the position of the rear wheel was also slightly altered to maintain the drive-belt tension. This system was so successful that once the Rudge Multi had to be banned from the Isle of Man TT race in order to provide other motorcycles some chance of winning.
The first practical CVT system for cars came in 1955 by the Van Doorne brothers of Eindhoven in Holland. It was built by DAF (Van Doorne’s Automobile fabrick NV) with the name “Variomatic” and launched in their tiny Daffodil saloon car. The system incorporated a pair of V-section rubber belts ran under tension between primary and secondary pulleys having variable groove widths. Control of the pulley running diameters, and therefore the drive ratio, was achieved through servos operated by an inlet manifold vacuum. A centrifugal clutch was used for setting off from rest (Fig. 25.71).
Although it remained in production until 1992 the Variomatic suffered from several drawbacks, in particular it had limited torque capacity and it occupied a large installation volume. DAF engineers, meanwhile, established a new company called VDT (Van Doorne’s Transmissie BV) to continue work on CVT with an objective to improve the technology. They launched a new CVT system in 1979 using a segmented steel thrust belt. This new system offered higher torque capacity, and was much lighter and more compact than the Variomatic system. Subsequently the system was used in small cars manufactured by Rover, Ford, Fiat, Subaru and Nissan.

Principle of operation of CVT system.
Fig. 25.70. Principle of operation of CVT system.
Variomatic CVT system.
Fig. 25.71. Variomatic CVT system.


Ford Continuously Variable Transaxle (CTX)

The Ford CTX works on the same belt-drive principle of the Van Doorne unit, but instead of using two rubber drive belts in tension, this system incorporates a single steel belt that works in compression. The transmission unit is suitable for front-wheel drive cars upto 1.6 litres capacity of engine and it has proved to be more economical than a conventional automatic gearbox.
Figure 25.72 illustrates the Ford CTX, which has been built around an enclosed V-belt running between two adjustable cone-shaped pulleys. The primary pulley is connected to the engine through a single-stage epicyclic gear train having compound planets, and the secondary pulley is connected through a pair of reduction gears to the final drive.
One half of the primary pulley, connected directly to a hydraulic servo, is moved first to have a change in gear ratio. Then the width of the pulley is altered, so that the effective diameter is gradually changed and at the same time drive is being transmitted. The diameter of the secondary pulley is varied to maintain belt tension. To achieve this, a strong spring is used to push the two havles of the secondary pulley together.
The epicyclic gear train is controlled by two multi-plate clutches immersed in oil to provide forward and reverse motions. One clutch connects the sun wheel and planet carrier together to give forward, and the other locks the annulus to the transmission casing to provide reverse. These clutches are designed to slip at low speed so that the car can creep when maneuvering. The thrust belt uses a large number of V-shaped steel elements retained on two steel bands. Each steel element of the belt during driving transmits thrust by pushing against its neighbour.
The hydraulic system and driver controls used with the Ford CTX system are similar to a conventional automatic transmission. When the driving mode is selected, hydraulic pressure from an engine-driven pump acts on the appropriate clutch actuator. With the vehicle in motion, to change a gear ratio the hydraulic valve unit varies the pressure applied to the pulley servo
Ford CTX transaxle.
Fig. 25.72. Ford CTX transaxle.
in accordance with the signals received from the engine, and vehicle-speed sensors. When the vehicle is accelerated or decelerated, the engine initially behaves in a strange manner to drivers, normally experienced with conventional systems. Since the CVT system selects the best ratio to suit the operating conditions, the engine is not required to be accelerated through each gear before the vehicle reaches its operating speed.

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