Hydrostatic CVTsBoth 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.
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.
|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|
In the next section, we'll look at what it's like to drive a CVT-based car.