A gearbox with no gears lets the engine hum at one steady speed
Most gearboxes offer a handful of fixed steps. A continuously variable transmission has no steps at all: it slides smoothly through every ratio in between. That lets a car's engine sit at whatever speed suits it best, most efficient or most powerful, while the wheels speed up or slow down underneath.
The commonest design is surprisingly simple. Two pulleys are each made of a pair of cones facing one another, with a V-shaped belt running between them. Push the cones together and the belt is forced outward, riding on a wider circle; pull them apart and it drops lower. Because the belt's length and the gap between pulleys stay fixed, one pulley must widen while the other narrows at the same moment, which keeps the belt taut and shifts the ratio.
Light machines such as snowmobiles and utility vehicles get by with steel-reinforced rubber belts, but cars need something tougher. Their belts are built from many tiny metal elements threaded onto stacked, flexible steel bands that slide over one another and need careful lubrication. In some designs the input pulley actually pushes the elements towards the output pulley rather than pulling on them. A belt drive runs at roughly 88 per cent efficiency, below a manual gearbox, though keeping the engine in its sweet spot can make up the difference.
Other designs trade in different strengths. Toroidal versions tilt rollers between a pair of shaped discs, handle more torque than belts, and in some cases can reverse direction without a separate reverse gear. Chain drives with thin sliding plates lock into grooved cones for heavy industrial work, and ratcheting designs stack one-way clutches that cannot slip, at the cost of vibration.
Hydrostatic transmissions skip mechanical links entirely. An engine-driven pump pushes oil to hydraulic motors at the wheels, and varying how much oil flows changes the effective ratio. Hoses let motors sit almost anywhere, crawl speeds can be as slow as you like, and one lever handles forward and reverse. The price is weight, cost and efficiency: few manage much more than 65 per cent, against as much as 90 per cent for gears.
Source: Continuously variable transmission
