How a CVT Transmission Worksby Richard Rowe
The continuously variable transmission isn't exactly new. In fact, it predates the automobile by about 400 years, for none other than Leonardo DaVinci once described a functional CVT in one of his journals. There are at least half a dozen different types of CVT in use today, and odds are good that you have at least one of them in your garden shed right this moment.
Purpose and Function
Most transmissions use some kind of multiple-gear arrangement to vary engine rpm relative to vehicle speed. As the transmission shifts up, rpm rises to a certain point, then suddenly drops again as the next gear engages. This arrangement is far from ideal for most gasoline engines, which tend to either produce maximum power or deliver maximum fuel efficiency in a fairly narrow rpm range. A continuously variable transmission doesn't use fixed gear ratios, so there's no "step" up or down as vehicle road speed increases or decreases. This allows the engine to stay at its ideal power or efficiency rpm, regardless of speed.
Common Types - The VDP
Garden tractors, snowmobiles, mopeds, golf carts and other low-power engine applications often use a belt-operated, variable diameter pulley CVT. A VDP system uses a par of pulleys made of two opposing pulleys, facing each other in an hourglass shape. One of the pulleys is mobile, capable of moving closer to or further away from the other. A belt sits in-between these two pulleys; as the mobile pulley moves, the belt either drops toward the center of the pulley or rides up near the circumference. A VDP is simple and reliable, but belt slippage reduces its ability to efficiently transfer power. The "single tooth" variant of the VDP design adds a ridge to the cone to reduce slippage, which greatly enhances the transmission's efficiency.
Common Types - Hydrostatic CVTs
The hydrostatic or hydraulic CVT is probably the second most common type, often found on any large commercial, military or construction vehicle that uses an hydraulic system. The hydrostatic CVT uses a variable displacement pump connected to the engine to drive a number of matching hydraulic pumps in the wheels or accessory drives. The variable displacement pump integrates a swash plate that can change angle on demand, thus allowing the pump to push more or less fluid per revolution. Hydrostatic CVTs are uncommon primarily because their associated hydraulic systems are fairly heavy and because they're somewhat inefficient at high rpm, but you can often find them under zero-turn lawnmowers.
Ford began experimenting with mass-market CVTs in European-market Fiestas in the mid-1980s, opting for a fairly conventional belt-driven design. Ford later switched to a stronger, chain-driven CVT in 2005. Nissan's been using CVTs for decades, beginning with the N-CVT introduced in the 1992 Nissan March. Nissan later switched to a Toroidal CVT, which helped to bring this transmission into the automotive mainstream. Nissan has since upgraded the old belt system, installing it in their newest Murano and Altima. Ratcheting CVTs offer extremely high efficiency and torque capability, but they're complicated, expensive to build and tend to transmit more mechanical vibration and harshness through the drivetrain.
Pros and Cons
The CVT's ability to hold an engine at any given rpm gives it a hypothetical leg up over any traditional gear-type transmission in both fuel efficiency and acceleration. However, much of the CVT's efficiency comes from the fact that the computer is a better driver than you are, and because the CVT computer can interact with the engine control computer to continuously optimize drivetrain performance as a whole. Driving a CVT is a bit weird if you're used to a standard transmission -- more like operating a boat than a car, since there's no direct correlation between engine rpm and road speed. Driving enthusiasts tend to loathe them and their inherent lack of driver involvement, but soccer moms generally like the CVT's seamless acceleration and efficiency.
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