Automatic Transmission

An automatic transmission is a gearbox that selects and changes gear ratios on its own, freeing the driver from operating a clutch or shifting manually. The driver chooses a direction with the selector, typically Park, Reverse, Neutral or Drive, and thereafter controls the car with only the accelerator and brake pedals; the transmission reads speed, load and throttle and decides when to change gear. This simplicity, particularly welcome in heavy traffic and on hills, is why the automatic has come to dominate sales in many of the world's largest car markets.

The term covers several quite distinct mechanisms rather than a single design. The traditional automatic uses a torque converter, a fluid coupling that multiplies torque at low speed and lets the engine idle while the car is stationary, feeding a planetary gearset whose ratios are selected by hydraulically operated clutches and bands. A dual-clutch transmission instead uses two clutches and what are essentially two manual gearboxes operated by computer, pre-selecting the next gear for very fast changes. The continuously variable transmission dispenses with fixed steps altogether, using a belt or chain running between variable-diameter pulleys to provide a seamless range of ratios, while a battery-electric car often needs only a single fixed reduction gear because its motor delivers torque across a wide speed range.

For the driver the everyday benefit is ease and smoothness. There is no clutch to balance when moving off on a slope, no need to take a hand from the wheel to change gear, and no risk of stalling, all of which lowers the workload and makes stop-start driving markedly less tiring. Modern units are also genuinely quick: a good dual-clutch or eight- or nine-speed torque-converter automatic now shifts faster and more smoothly than most drivers can manage with a manual, and the close spacing of their many ratios keeps the engine in its most efficient or most responsive band.

The history of the technology runs from the first mass-produced fluid-coupled automatics of the late 1930s and 1940s, through three- and four-speed units, to today's transmissions with eight, nine or ten ratios, dual clutches and sophisticated control software. Improvements in lock-up clutches, which mechanically connect the engine to the gearbox at cruising speed to eliminate the slip and inefficiency of the fluid coupling, together with adaptive control software, have erased much of the old fuel-economy penalty that once counted against the automatic.

There remain trade-offs to weigh. Automatics generally cost more to buy than the equivalent manual, can be more expensive to repair, and the more complex types may need particular fluids and periodic servicing. Some drivers also miss the direct mechanical engagement of a manual, and a poorly matched continuously variable unit can produce the so-called rubber-band sensation, with engine speed rising ahead of road speed under acceleration.

The automatic transmission stands in contrast to the manual transmission, which requires the driver to work the clutch and gear lever. As a category it embraces the torque-converter automatic, the dual-clutch transmission and the continuously variable transmission, each a different engineering answer to the same goal of changing gear without the driver's intervention.