Continuously variable and other types of transmissions

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A continuously variable transmission

Manual or automatic transmissions have given way to paddle-shifted auto and continuously variable transmissions (pictured) in autos. Image from Wikimedia Commons.

Some people might have noticed the term “continuously variable transmission” and other buzzwords being bandied about concerning automotive transmissions. There’s nothing magical about them, but there are some fundamental differences people should be aware of.

Fringe transmissions gaining ground

Automobile transmissions used to come in two flavors: manual and automatic. However, some may have noticed a greater variety these days, as paddle-operated automatics and continuously variable transmissions are being offered in cars. There are some fundamental differences.

The basics

The function of a transmission is to transmit the mechanical energy of the engine’s crankshaft, which spins as the engine operates, to the driveshaft, which is connected to the differential. The differential sends power to the front, rear or all wheels.

An engine’s crankshaft can only rotate at so many revolutions per minute. A transmission selects one of a number of gears that increase or decrease the revolutions per minute being sent to the driveshaft and thus to the wheels, increasing or decreasing the speed of the car.

The driver selects gears by hand in a manual transmissions . Automatic transmissions change gears by themselves. In a paddle-operated automatic, the driver has to select an ascending or descending gear via the paddle shifters. Many paddle-operated autos employ a double-system as well. Continuously variable transmissions, though, are something else.

New trend, old system

The first CVT, according to How Stuff Works, was devised by none other than Leonardo Da Vinci. The modern iteration was devised and patented in 1886. It wasn’t widely used for many years, though a CVT was found, according to Edmunds, on the Subaru Justy, a small SUV that was sold worldwide from 1989 to 1993.

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The most common type is a pulley-operated CVT. The way gears are selected is by lengthening or compressing the distance between the inner and outer plates of the two pulleys used to send power to the driveshaft. One side of each pulley is fixed; the shaft connecting the inner and outer plates, both of which are tapered, is lengthened or shortened via automation. The length between the plates of the input or output shaft changes, depending on speed.

There are also hydrostatic and toroidal CVTs, which work a bit differently.

Benefits

The effects are, among other things, that the transition between gears is seamless; the shock of the next gear being selected is nonexistent. The other effect is that it keeps engine RPMs in the optimum range for power and, more importantly, for efficiency. In other words, it keeps the engine in a “sweet spot” for acceleration and for using fuel.

The Environmental Protection Agency estimates a CVT improves efficiency by 6 percent, adding up to savings of $2,000 over the lifetime of a vehicle.

A bit more on how CVTs work

Sources

How Stuff Works

Edmunds

EPA Fuel Economy


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