Torque shop

My new car has something called CVT for a transmission. It does not seem to change gears when I move off or even on the expressways. When I want to accelerate quickly, I do not feel the gears changing down. Instead, I hear the engine speed rising as the car accelerates. What exactly is this “CVT” and is this typically how it works?

CVT stands for Continuously Variable Transmission and as its name suggests, the ratios vary continuously as opposed to “stepped” that you find in other transmissions.

The idea is for the engine to maintain its optimum rpm at all speeds. There are no actual gears in a CVT. Instead, the system uses two pulleys – one driven by the engine and the other driving the wheels.

A steel drive-belt is the transmission element between the two pulleys which effect the different ratios by varying their diameters over the speed range.

If you look at the chain and sprockets on a multi-speed bicycle, you will see how the drive-torque and wheel speed vary depending on the sprocket diameter.

Conceptually, the CVT is brilliant. There are fewer parts than in an automatic – it is more compact than even a manual gearbox and devoid of any shift shock. It is usually more efficient too.

Under normal urban conditions, a CVT offers a smooth and seamless drive. But while its technology has improved over the years, a CVT is still not as responsive as a geared transmission.

Also, it often gives the impression that a clutch or torque-converter is slipping.

When accelerating hard, a CVT tends to be characterised by a whiny engine noise because of the sustained high rpm.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on October 14, 2017, with the headline 'Torque shop'. Print Edition | Subscribe