Firstly, what are often termed “semi-automatic transmissions” can actually be one of several things, hence a degree of understandable confusion amongst buyers.
In past times, the term was applied to transmissions that did not change gears automatically, but merely facilitated manual gear changes (using a selector) by dispensing with the need to press a clutch pedal to change gear.
Few of these now exist in the basic form and most now offer an added facility for leaving the transmission’s computer to initiate and execute the actual gear changes, if required.
The term is also misleadingly used to describe true, torque converter equipped, automatic transmissions (e.g. Tiptronic) that allow full manual override and gear selection using the selector lever, or steering wheel paddles.
The term “semi-automatic” transmission is now properly applied to the breed of variants of conventional manual gearboxes, normally using a conventional single dry plate clutch. These are also correctly termed “automated manual transmissions”, which to some degree explains their construction.
They use electronic sensors, pneumatics, processors and actuators to execute the gear shifts prompted by a computer, or at the command of the driver, (using paddles or a floor mounted lever) whilst the clutch is actuated by electronic and mechanical equipment.
In theory, a computer synchronises the timing and actions required to make quick, smooth gear shifts although, in practice, changes can sometimes be laboriously slow, or jerky.
This is usually down to the programming intended to protect the transmission from being subjected to abuse in the form of extreme torque; smooth changes are best accomplished manually (somewhat defeating the objective) or with suitably timed easing off of the accelerator input by the driver, to coax the car’s electronics into deciding to change gear, which takes a little practice.
But drivers wanting a truly automatic gear selection experience are probably best avoiding these types of transmission.
Totally different from all other geared transmissions, and fully automatic when not manually overridden, we have what are known as CVTs, or “continually variable transmissions”. Here the gearing is determined by a theoretically simple system of two variable-diameter pulleys, each shaped like a pair of opposing cones, with a metal belt running between them, not on teeth, but driving like a rubber drive belt.
The halves of each pulley are moveable on their shaft and, as the pulley halves come closer together or expand further apart, the belt rides higher or lower on the pulleys, making their effective diameter larger or smaller. As one pulley expands the other contracts, in order to maintain constant chain tension.
The gearing itself is stepless and wider ratios can generally be obtained than from conventional gearboxes, offering the prospect of high gearing and better economy at cruising speed. A computer monitors engine data, power demand, and other factors and completely masterminds the gearing selection.
When provided with a manual override facility, as with the most widely available diesel CVT, Audi’s Multitronic system, the manual option provides eight stepped gears, from eight specific pulley positions.
Multitronic, also used in the SEAT Exeo, employs an oil-cooled, computer-controlled multi-plate clutch, and the drive belt operates in compression, not in tension as with a bicycle chain, using a complex and robust multi-link metal belt lubricated by transmission fluid.
A Ford Durashift CVT transmission, supplied by the German transmission company ZF, was offered on several 1.6 TDCi models for some years, but is now no longer available.
CVT units are widely used in Toyota hybrid vehicles, but the Mercedes-Benz Autotronic CVT transmission used in the last generation of A and B-Class cars has now been abandoned in the new models, in favour of a new dual clutch automatic transmission, which is an increasingly popular form of transmission, of which more in next month’s issue.