Another episode of our occasional series attempting to de- mystify the latest technology, although the subject this time - the automatic gearbox - is hardly new having been in common use in American cars for many years, but still all to little understood by European drivers.

The number of European cars fitted with automatic gearboxes has risen rapidly in the last ten years, a growth which seems destined to increase exponentially as stop-start driving conditions and traffic densities make the easier driving conferred by auto's more desirable and their inherent smoothness and flexibility improves.

There are two types of autobox in common usage, the CVT - Continuously Variable Transmission - and the increasingly sophisticated conventional auto.

CVT gearboxes are now used by Ford, Volvo and Fiat for their smaller cars and derive from the DAF belt transmission of the early Sixties. As the name suggests, drive is transmitted from the engine to the drive shafts by a belt or series of belts which adjust their relative positions according to engine speed and load as determined by complex electronics, effectively offering just one infinitely variable gear.

The conventional automatic is also electronically controlled and selects from a preset number of gear ratios the one most appropriate to the engine speed and load being placed on it, the " kickdown"  on the accelerator enabling selection of a lower gear for more acceleration, the electronics selecting when to change up a gear. Originally only 3 forward gears were available but more recent versions allow up to 5 forward speeds and, increasingly commonly, the ability to vary - in sport or economy modes - the rev limit at which change ups occur, the holding of a gear to a higher engine speed giving greater acceleration.

Some also allow the driver to select a manual mode and drive the autobox as though it were an ordinary gearbox yet without the need of a clutch. Probably the optimimum compromise between control and effort. 

Article © Graham Benge 2007

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