Continuously Variable Transmission (CVT)

A continuously variable transmission (CVT) is a transmission which can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that only allow a few different discrete gear ratios to be selected. The flexibility of a CVT allows the driving shaft to maintain a constant angular velocity over a range of output velocities. This can provide better fuel economy than other transmissions by enabling the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds.

Many small tractors for home and garden use have simple hydrostatic or rubber belt CVTs. For example, the John Deere Gator line of small utility vehicles use a belt with a conical pulley system. They can deliver a lot of power and can reach speeds of 10-15 MPH, all without need for a clutch or shift gears. Many new snowmobiles and motor scooters use CVTs. Virtually all snowmobile and motor scooter CVTs are rubber belt/variable pulley CVTs.

Some combine harvesters have CVTs. The CVT allows the forward speed of the combine to be adjusted independently of the engine speed. This allows the operator to slow down and speed up as needed to accommodate variations in thickness of the crop.

CVTs have been used in aircraft electrical power generating systems since the 1950s and in SCCA Formula 500 race cars since the early 1970s. More recently, CVT systems have been developed for go-karts and have proven to increase performance and engine life expectancy. The Tomcar range of off-road vehicles also utilizes the CVT system.

Some older drill presses contain a pulley-based CVT where the output shaft has a pair of manually-adjustable conical pulley halves which a wide drive belt from the motor loops through. The pulley on the motor, however, is usually fixed in diameter, or may have a series of given-diameter steps to allow a selection of speed ranges. A handwheel on the drill press, marked with a scale corresponding to the desired machine speed, is mounted to a reduction gearing system for the operator to precisely control the width of the gap between the pulley halves. This gap width thus adjusts the gearing ratio between the motor's fixed pulley and the output shaft's variable pulley, changing speed of the chuck; a tensioner pulley is implemented in the belt transmission to take up or release the slack in the belt as the speed is altered. In most cases, however, the drill press' speed cannot be changed without the motor running.

Advantages

• CVTs can compensate for changing vehicle speeds, allowing the engine speed to remain at its level of peak efficiency. This improves fuel economy and by effect, exhaust emissions.
• CVTs operate smoothly since there are no gear changes which cause sudden jerks.
• Very few problems have been reported with the CVT transmission, lowering the cost of ownership
• There are 25% fewer moving parts to a CVT transmission.
• They are also cheaper, but still expensive to repair
• The fluids do not have to be changed as often as an automatic transmission

Disadvantages

• Many early CVT driven cars had disappointing performance. The original Ford Fiesta 1.1 CTX, for example, used 8.2 percent more gasoline than the manual transmission version in city driving.
• CVTs operate smoothly. This can give a perception of low power, because many drivers expect a jerk when they begin to move the vehicle. The expected jerk of a non-CVT can be emulated by CVT control software though, eliminating this marketing problem.
• Since the CVT keeps the engine turning at constant RPM over a wide range of vehicle speeds, pressing on the accelerator pedal will make the car move faster but doesn't change the sound coming from the engine as much as a conventional automatic transmission gear-shift. This confuses some drivers and, again, leads to an impression of a lack of power.
• CVT torque handling capability is limited by the strength of their transmission medium (usually a belt or chain), and by their ability to withstand friction wear between torque source and transmission medium for friction-driven CVTs. CVTs in production prior to 2005 are predominantly belt or chain driven and therefore typically limited to low powered cars and other light duty applications. Units using advanced lubricants, however, have been proven to support any amount of torque in production vehicles, including that used for buses, heavy trucks, and earth moving equipment.
• Any kind of automatic transmission may be rejected in some areas, because of reasons of cost and tradition. The small car market in Europe seems to be one of the most suspicious. In the USA and in Japan automatic cars have been widely accepted in all size classes.

Source : www.wikipedia.com