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How Transmissions Work

Inside auto-shifters

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.

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).

Auto Pilot

Automatic transmissions have been with us since 1939. Today, they are the predominant choice among American drivers. Far more sophisticated than their predecessors, today's automatic transmissions have electronic adaptive shift logic and can even mimic the feel and driver involvement of a manual transmission.

In place of a manual clutch, automatic transmissions have a torque converter positioned between the engine and transmission. A type of fluid coupling, inside the torque converter's oil-filled housing is an impeller that is connected to the end of the engine's crankshaft. The fluid pressure generated by the impeller in turn drives a turbine that is connected to the transmission's input shaft.

The design of the torque converter determines how fast the engine can rev before the vehicle begins to move. The faster the engine revs, the less slip the torque converter allows. For greater fuel efficiency, most torque converters automatically lock the impeller and turbine together during top-gear cruising to eliminate slip.

Instead of gearsets driving each other on parallel shafts, typical automatic transmissions use planetary gears to create a variety of ratios. A planetary gearset is comprised of a large outer ring gear with teeth cut into its inner face. Centered inside the ring gear is a much smaller "sun" gear, which is connected to the transmission's input shaft.

Between the sun and ring gears are a set of "planet" gears that are oriented by a carrier. Planetary gearsets are neat little devices because by clamping the ring gear, the planet gear carrier, the planet gears or the sun gear together in various combinations, you can get several ratios on the output side—even Reverse. By adding planetary gearsets to the transmission, still more ratios can be added.

A combination of electronics and hydraulics orchestrate shifting to suit the conditions. Depending on the throttle position, the speed with which the throttle pedal is moved, vehicle speed and other factors, the shift mapping is varied to suit the conditions. Based on an indication of a closed throttle and steady or increasing speed, some vehicles can sense they're on a downhill and will automatically downshift from top gear to develop engine braking.