CVT is an automatic transmission that can select any desired drive ratio within its operating range. Unlike a conventional four or five speed transmission, CVT is an 'infinite speed ratio' transmission.

Although in 1490, Leonardo Da Vinci made a sketch of a stepless continuously variable transmission, it was only in 1958 that Daf in Netherlands produced a CVT for a car, which was developed and improved over the years, initially its use was limited and that too in small cars, yet the numerous advantages which a modern CVT offers, have made it quite popular in the modern cars. 'Honda Multimatic' used in the Toyota hybrid 'Insight' and Audi 'Multitronic' fitted in their 'A6' model are recent examples.

Principles of working :-

A simple CVT has three main components :-

1. A variable input driving pulley
2. An output (driven) pulley
3. A metal belt

Apart from the above basic components, there are also various microprocessors and sensors and even epicyclic gearing and clutch. Both the driving as well as the driven pulleys are of variable diameter type. Each pulley consists of a spline shaft with two 20° cones facing each other. A belt is there in the groove between the two cones. Earlier when rubber belt was used, they were of V-shaped section. However, the modern metal belts are specially constructed to provide for the required flexibility and long life.

The driving pulley is connected to the engine crankshaft, while the driven pulley transfers motion to the drive shaft. When the two cones of the pulley are close together, the belt rides higher in the groove and the pulley diameter apparently increases, whereas the belt rides lower in the groove, making the effective diameter decrease, when the two cones of the pulley are far apart. The two cones are moved closer or far apart using hydraulic pressure, centrifugal force or spring tension (all through electronic control). Moreover, when diameter of driving pulley increases, the diameter of the driven pulley decreases, and vice versa. This is done to keep the belt always tight.

As the two pulleys change their diameter relative to each other, infinite number of gear ratios are obtained. For example, when diameter is small on the driving pulley and large on the driven pulley, the speed of the driven pulley is decreased, resulting in 'low gear'. Similarly 'high gear' is obtained when diameter on the driving pulley is larger than the diameter of the driven pulley.

A conventional CVT uses the information from engine management ECU to determine the torque being delivered by the engine and accordingly the pressure exerted on the belt to avoid slipping, usually, exerting excessive pressure. However, in Audi 'Multitronic', the torque is measured only when it is entering the CVT so that the pressure exerted is just enough for the torque transmission, and no more, thus improving its efficiency. Further the drive is transmitted separately to forward gears and to reverse, through two multiplate clutches. There are also two small hydraulic chambers for moving the variator pulleys instead of one larger chamber in case of conventional CVTs, this reduces the power consumed.

Advantages :-

1. Constant stepless acceleration from start to high speed, eliminating 'shift shock' to provide a smoother ride. In fact the operation is much smoother than even conventional hydraulic automatic transmission.

2. It keeps the car in optimum power range under all conditions, thus reducing fuel consumption. Besides, it gives better fuel economy than hydraulic transmission by avoiding torque converter slippage.

3. Responds better to changing conditions, example throttle and speed changes, which eliminates gear hunting while moving up an incline.

4. Less emissions due to better engine control under all conditions.

Disadvantages :-

1. Torque handling capability of a CVT is limited by the strength of the belt or chain inside it, limiting its use in small cars. Audi's Multitronic is limited to 300 Nm of torque. Perhaps the biggest vehicle with a CVT is the Nissan Murano, which is a four wheel drive vehicle with a V6 engine. At the present technology level CVTs can handle maximum torque of 350 Nm only.

2. CVTs have not been used successfully for series production standard drivelines due to packaging. A CVT needs a relatively large centre distance which cannot be provided in an inline configuration, example with longitudinal engines in rear wheel drive vehicles. However, CVTs have been successfully employed in small and medium sized vehicles with front transverse drivelines.

3. The life of a CVT automatic in light duty use has been found to be about the same as that of a conventional automatic transmission. However, when the vehicle is subjected to heavy stop and go driving, as in case of delivery vehicles, CVT is found to have much shorter life, say, about 150000 km.

Miscellaneous :-

1. In actual CVTs special provisions, such as following, have to be made :-

(i) A simple CVT as described above, does not have a neutral position, which is provided by some form of clutch arrangement.

(ii) Some means of reversing the car is also required. The easiest method is to use a single epicyclic gear train, which would reverse the output from the variator pulley.

2. Next generation CVT design combines the conventional CVT principle with an auxiliary gearbox and a significantly increased gear ratio. This has been developed by Nissan Motor company of Japan and their transmission supplier Jatco Limited and is expected to be employed in Nissan vehicles soon. It is reported that this design has resulted in the decrease of overall length of the gearbox by 10% and of its weight by 13% compared to its previous version.