How Automatic Transmissions Work

Transmissions have a tough job. They must help keep the engine in its powerband (usable rpm range) and transmit that power to the drive wheels over a huge speed range. Pulling power must be available when towing a trailer from a dead stop, yet the engine rpm must be at a comfortably low speed for efficient high-speed cruising.

The focus here is on "juice" transmissions.

In an accompanying story, we covered the ins and outs of the age-old standard/manual transmission. The focus here is on "juice" transmissions: automatics that use hydraulics to change gears. Let's shift into the discussion, shall we?

All vehicles need some type of clutch and transmission. The clutch disconnects the engine's output shaft from the transmission (and, in turn, the drive wheels) so the vehicle can idle at a stop. Then, the clutch gradually re-engages the engine to the transmission to smoothly allow the vehicle to accelerate away from a stop. The transmission must provide a wide enough range of gear ratios to accommodate the vehicle's typical operating speed range.

Gearing
A transmission has to convert the engine's power output into useful torque that can be fed to the wheels at varying speeds. A choice of gear ratios between the engine and the drive wheels allows this to happen. In First gear, the transmission allows the engine to rev freely even though the vehicle is traveling slowly. This provides the maximum torque multiplication for accelerating away from a stop or pulling a heavy load at low speed. Low gear gives you maximum pulling power but very little speed.


In higher gears, the transmission puts the engine rpm at a comfortable and fuel-efficient level, even though the road speed is very high. The compromise here is relatively little pulling power: The slow-turning engine may not have the power to maintain the speed you want without downshifting to a lower gear. Between these high and low ratios, a transmission needs to have enough gear choices to deal with all the likely driving situations the vehicle will encounter. In modern cars with automatic transmissions, that means at least three gears, but as many as five are becoming common (BMW even has a 6-speed automatic).