Continuously Variable Transmissions

Continuously Variable Transmissions, or CVT's, as they are called in the trade, are used in snowmobiles, some all terrain vehicles (ATV's), and a few automobiles. A continuously variable transmission works just like it sounds. It continuously varies the gear ratios between the engine and the final drive. There is no need to select different gears: just place the shifter in forward and away you go.

Why use a CVT rather than a conventional automatic or manual transmission? The answer is efficiency; engine efficiency. With a CVT, the engine goes from an idle to a pre-programmed rpm immediately so the engine input is constant, and the transmission varies the output speed for smooth, seamless acceleration. Keeping the engine at a constant rpm allows the engineers to optimise ignition timing, camshaft design, and manifold tuning for excellent volumetric efficiency and low emissions.

Other reasons for using CVT's include simplicity of design, and smooth power application to the ground. The simplicity part comes by using fewer parts. Vehicles with reverse gear obviously have more parts than a simple forward drive, but there are still fewer parts and gears to manufacture.

The smooth power application of CVT's is useful in off-road vehicles. Power can be applied without any jerks or surges that could cause the vehicle to lose traction on steep climbs or loose terrain.

So how do they work? The two most common systems use a belt or a chain. Let's look at snowmobiles first. They use a belt. On a snowmobile there are two pulleys connected by a rubber drive belt. Both pulleys are "V" shaped and the width of the "V" can be varied. The drive pulley is connected to the engine and the width of the pulley is controlled by engine speed. At idle, the pulley is wide and the belt is not gripped. Push on the throttle, and the governor causes the pulley sides to move together. When engine speed reaches it's maximum, the pulley continues to become narrower as vehicle speed increases, causing the drive belt to climb up and keeping the engine at its peak power.

The driven pulley also varies its width, but its main job is to keep the slack out of the belt. The driven pulley halves are spring loaded to force them together, and as the drive pulley closes up, the belt climbs up the drive pulley causing the belt to tighten, forcing the driven pulley apart. The variable width of the pulleys and the variable distance of the belt from the centre of the pulleys provide the continuously variable drive.

Automobiles with CVT's use a drive chain instead of a rubber belt. The chain is stronger and more durable on a much heavier vehicle. Although much more complex, automotive CVT's use variable width pulleys just like snowmobiles. Hydraulics and computer controls are used to vary the width of the drive pulley.