Other types of automatic gearbox

In other types of automatic transmission a fluid converter or hydraulic torque converter is used instead of a fluid coupling.

Whereas the fluid coupling merely transmits the engine torque to the mechanical gearbox, the torque converter converts the low torque of the rapidly rotating engine into a high torque in conjunction with low speed of rotation at the output shaft of the converter.

 A torque converter of this kind therefore itself constitutes a transmission stage. In the fluid converter is an impeller which imparts its speed to the oil and forces it into the turbine. At low speeds of rotation the efficiency of such a device is very low, and for this reason the oil flowing out of the turbine is passed through a so-called stator (or reactor) whose fixed vanes are so curved that they redirect the oil flow into the impeller and thereby boost the action of the latter.

The stator vanes thus assist the conversion of the low torque of the rapidly rotating engine shaft into the high torque of the slowly rotating output shaft of the converter, so that the energy of the oil emerging from the turbine is not dissipated, but can be re-utilised. The speed of the turbine gradually becomes equal to that of the impeller, however, with the result that the efficiency diminishes again. Various arrangements are used to counteract this. In one of these the stator vanes are made adjustable, i.e., the deflection of the redirected flow is suited to the speed of rotation of the turbine at any particular time (in stationary systems).

Another arrangement comprises a freewheel which thrusts against the casing during starting up. The force which restrains the freewheel is produced by the difference between the engine torque and the output torque of the transmission. When these torques finally become equal at the end of starting-up, the stator detaches itself from the housing, through the agency of a freewheel, and rotates along with the turbine. The converter has thus become a fluid coupling.

The freewheel may be of the grip roller type. The transmission casing and the cage of the freewheel are rigidly interconnected. When the thrusting force of the stator acts in one direction, the grip rollers are jammed into narrowing gaps in the freewheel, so that the latter becomes locked and transmits force.

When the thrusting force decreases or reverses its direction, the rollers are released, thus allowing the collar to rotate freely.

The remaining gear ratios in an automatic gearbox equipped with torque converters are obtained by means of planetary gear sets additionally provided.