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What is a CVT gearbox?

November 24, 2014 by

Buying a new car doesn’t have to be difficult – after all, that’s why we started carwow. But sometimes just choosing the equipment for your new wheels can be tough, thanks to indecipherable acronyms and new technology.

To help, we’re here with a series of guides to help explain some of the more modern technologies that you might not have seen on a car. This week, we’re looking at CVT gearboxes.

CVT stands for continually variable transmission (also called infinitely variable). Like DSG gearboxes, CVTs are, in the simplest terms, automatic gearboxes. You start the car, hold the brake, slot it into ‘D’ and you can pull away and come to a standstill without using a clutch or fearing stalling. And like an auto, it alters the gear ratio as you gather speed to keep the engine operating at the correct rpms.

What are they?

Traditional automatic gearboxes have a fluid connection between the engine and the gears. Torque (the turning force of the engine) passes through the fluid connection, through the gears to the wheels. Typically these gearboxes have between five and eight fixed gears or ratios that spread the engine’s power across the ranges of speeds the car will be driving at. This means the car can pull away smoothly in first gear and won’t destroy itself at motorway speeds using a higher one.

A CVT differs because, instead of having fixed gears, it has the capacity to infinitely (or continuously) vary the gear ratio between the engine and the wheels. This means, in theory, when the driver doesn’t require that much power from the engine the CVT can let the engine rev lower than a ‘top gear’ would. It also means that when the driver requires power, the engine stays spinning at its most powerful speed and the acceleration happens by changing the CVT’s ratio.

How does it work?

In one of the most common varieties of CVT, one end is connected to the engine and the other is connected to the wheels. Joining the two ends is, usually, a reinforced band made of rubber or metal. Each end is formed of two shallow cones facing one another, with the band running in the gap between them. By varying the distance between the two cones the gear ratio can be changed – i.e. when the cones are close together the band spins a lot in one revolution, when they are further apart the band spins much less in one revolution. Other CVT types use more robust planetary gear sets to the same effect.

A computer controls the ratio at the engine and at the wheels to ensure that the most sensible ‘gear’ is selected for a given driving condition. When acceleration is demanded, the engine spins up to its most powerful point then the CVT alters the ratios to increase the speed. This is in contrast to cars with traditional gearboxes, which accelerate through their rev ranges, then have to change gear and repeat the process.

What are the advantages?

Theoretically, there are numerous advantages. Combustion engines have a particular point in their rev ranges where they make the most power, unlike electric motors which always make their maximum output. A CVT allows the engine to remain in this zone while accelerating, and then drop down to low rpm almost instantly when power is no longer required.

CVTs are most useful, and most commonly found, in hybrid cars where their design makes them well suited to combining the power from combustion engines and electric motors. Though no CVT has fixed gears, some are programmed to mimic having set gears to make the car’s acceleration feel more natural and progressive.

Any drawbacks?

If you peruse reviews of cars fitted with CVTs, then the gearbox typically becomes a point of contention. Many reviewers dislike the way that CVTs cause revs to rise and fall in an unnatural manner, and the way that their engines stay noisy during acceleration. Some CVTs are also criticised for being unresponsive to inputs from the pedals.

Most criticisms, though, seem to be a matter of taste and what the driver is used to. Certainly, CVTs won’t suit die-hard petrolheads, but for the majority of buyers just looking for efficient, automatic transportation, a CVT will work perfectly fine.

Anything else I need to consider?

Automatic gearboxes are becoming more and more common as manufacturers realise that computers can do the job of shifting gear faster and more efficiently than a human. Many reviewers, however, prefer traditional torque converter or twin-clutch DSG-style units because they fit more naturally into the rhythm of driving.

What about other types of gearbox?

We’ve also produced an easy-to-understand guide to DSG gearboxes – the sort more traditionally found in petrol and diesel cars.