Automatic Transmission

The torque converter has three elements: the impeller (pump), bolted to the engine flexplate; the turbine, splined to the gearbox input shaft; and the stator, mounted on a one-way clutch. At high slip — pulling away — fluid leaving the turbine is redirected by the stationary stator back into the impeller in a helpful direction, multiplying torque; a stall torque ratio of roughly 2:1 is typical. As turbine speed approaches impeller speed the stator freewheels and the unit behaves as a fluid coupling. A lock-up clutch then joins impeller and turbine mechanically to eliminate slip losses at cruise, and modern units may lock up as early as second gear, often with controlled micro-slip to damp vibration.

A simple planetary set obeys the Willis relation (S + R) ω_c = S ω_s + R ω_r, where S and R are sun and ring tooth counts. With the ring held, sun as input and carrier as output, the reduction is 1 + R/S; holding the sun and driving the ring gives a smaller reduction; locking any two members together gives 1:1; holding the carrier reverses rotation. Compound arrangements (Simpson, Ravigneaux, Lepelletier) combine sets so that five or six shift elements provide six to ten forward ratios.

The valve body routes line pressure to the clutch packs and band brakes. A transmission control unit commands pressure-control solenoids, modulating apply and release pressures so that the off-going element releases precisely as the on-coming element takes up torque (a clutch-to-clutch shift), while the engine ECU briefly trims torque to smooth the change. Shift maps balance economy, performance and comfort; adaptive logic learns clutch wear. Alternatives include dual-clutch transmissions (two automated dry or wet clutches feeding odd and even gear shafts, pre-selecting the next gear) and CVTs (a steel push-belt or chain running between variable-width pulleys for continuously variable ratio).