Torque-converter automatic

A conventional automatic connects the engine to the gearbox through a torque converter, a sealed doughnut of oil. Inside, an impeller driven by the engine flings oil at a turbine attached to the gearbox input. At idle the oil just slips past and the car can sit in Drive with the brakes on; open the throttle and the oil flow drags the turbine round. A stator between the two redirects the returning oil so that, at low speed, the output torque is higher than the input, which gives these boxes their strong, smooth pull from rest. Once cruising, a lock-up clutch inside the converter bolts the two halves together to stop the slip and save fuel.

The ratios come from planetary gearsets: a sun gear, planet gears on a carrier and a ring gear. Holding any one member with a hydraulic clutch pack or brake band while driving another gives a different ratio, and stacking two or three gearsets yields six, eight or ten speeds in a compact package.

• The valve body, a maze of oil passages and solenoids, decides which clutches to pressurise.
• Each shift is a hand-over between clutch packs, cushioned by the oil, so it is felt as a soft surge rather than a jolt.
• The engine computer and gearbox computer talk constantly, trimming torque during shifts.

The unit sits where a manual would. Aisin's six-speed units in the Grand Vitara, Brezza, Scorpio-N, XUV700 and Fortuner, ZF's eight-speed in BMWs, and the Allison boxes in some city buses are all this type.

What you notice: smoothness, effortless creep in traffic, and reliability over very high mileages.