Semi-automatic transmissions are designed to reduce the skill and effort that a manual gearbox requires of a driver and to do so without either the cost or complexity of a fully automatic transmission.
A semi-automatic transmission has an automatic clutch and a simplified, assisted gear change mechanism. The automatic clutch allows for two-pedal control, the conventional clutch pedal being eliminated. The automatic clutch allows the car to be driven smoothly away from rest after first gear has been selected and it also disconnects the engine from the gearbox during gear changes as well as when the vehicle is brought to a stop.
Many different types of automatic clutch have been used in car transmission systems over the years. All that the driver generally has to do when driving a car with semi-automatic transmission is to select the required gear when it is needed, either with a conventional gear lever operating in the usual “gate” or with a selector lever mounted on the floor or the steering column. In other words, a semi-automatic system differs from a fully automatic unit in that it requires the driver to decide which gear ratio to use and when to change gear and then to operate a lever to initiate the change. In neither case is a conventional clutch pedal necessary.
The gearbox in a semi-automatic system can be either a conventional layshaft synchromesh box or an epicyclic unit similar to those used in fully automatic systems. The synchromesh type is described here since this is more common in semi-automatic systems. Epicyclic designs are discussed in other articles on fully automatic transmission systems.
A wide range of clutches have been used in semi-automatic transmissions. Centrifugal friction clutches have been used on DAF, Volvo and some Citroen models, while electro-magnetic clutches, made largely by Fcrodo and Cotal, have been used on some cars in the Renault range. Ferodo also produced a combined electro-magnetic and centrifugal clutch, though it saw little practical use. An otherwise orthodox friction clutch was employed with a semi-automatic, hydraulic-servo release mechanism on the Citroen DS19. A friction clutch has also been used in conjunction with a fluid coupling, notably in the Mercedes-Benz Hydrak transmission and the Chrysler Simplimatic. Centrifugal and electro-magnetic clutches and fluid couplings are described in other articles.
A combination of clutches is sometimes used because certain types of clutch are more suitable than others for starting from rest or disengaging rapidly for gear changing, the two main functions. The fluid coupling and the torque converter, for example, are excellent for the first of these requirements but are unsuitable for disconnecting the engine from a synchromesh gearbox during gear changing. Consequently they are normally combined with a friction clutch in a single assembly on such cars as the Citroen GS and CX range of passenger cars. ratio. Its value varies according to the design of the torque converter but is normally in the region of two to two and a half times the engine torque for the type described above, i.e.. with 100 lb/ft available from the engine, a stall torque ratio of 250 lb/ft will be achieved.