Diagnostics

Most people think that there are two kinds of transmissions for a car – the manual gearbox and the automatic transmission. But as soon as you have a better understanding of what a transmission does then you will realise there is a little more to it. The definition of a transmission is the transfer of rotational force from the vehicle’s engine to the wheels. It is a crucial part of the driveline via the differential to the wheels either configured front or rear 2WD or 4WD, which splits the power to the driving wheels.

This torque uses the appropriate gear ratio when selected either manually, or automatically. Manual selection of the gear requires depressing the clutch pedal to disengage the connection between the engine and transmission to allow a gear change. There are several manual gearbox types that range generally between four – six forward speeds plus reverse.

The lower gear ratios provide greater rotational torque to climb steep grades meaning more power, but less vehicle velocity. The higher gear ratios result in less wheel torque but faster rotational wheel speed which in turn is reduces Engine RPM providing greater fuel economy at a higher speed. All transmissions connected to an internalcombustion engine (ICE), whether a manual or an automatic, work on this basic premise.

Manual transmissions – the standard manual transmission is the least popular of the transmission types these days as it requires more work for the driver. The driver must manually move a lever through a gate (normally H design) to select the gear they want, while depressing a pedal that operates the clutch to disconnect and reconnect the drive. This causes a momentary reduction in acceleration, while this may mean some manuals do not feel as smooth or swift in between each shift, many still want the option to oversee gear selection especially in a performance car.

Most modern manual transmissions are of the constant-mesh type, with the shift lever operated by the driver connected to shift forks that move around dog clutches to engage the different ratios. To prevent the crunching associated with gear changes on old-style unsynchronised gearboxes, synchronization rings or cones are used. The friction clutch is a lossless, direct drive to the wheels.

Automatic transmissions – as the word ‘automatic’ suggests, this transmission does not need the driver’s input to change gears, it makes use of hydraulic fluid to shift; the required gear is selected based on feedback from sensors that determine how much power is needed. Standard automatic gearboxes comprise a planetary gearset that make use of a torque converter, which takes over the job of a standard clutch found on a manual transmission. Not that long ago automatic transmissions had three or four gears, however today they can have many more gear ratios. Various trade names for this transmission types include General Motors’ Hydra-Matic, BMW’s Steptronic and Audi’s Tiptronic. Automatic transmission designs are also used on many trucks and SUVs, especially those with high tow ratings. Some Automatic transmissions also offer a manual mode to give the driver some control and provide a sportier on road experience. This manual mode can simulate a manual transmission, with the driver in control of the gearshifts – but without a clutch pedal.

A continuously variable transmission (CVT) – is a different type of automatic transmission without gears at all. It uses a special belt or chain that runs between two pulleys, shaped like cones to continuously vary gear ratios. As the belt or chain moves up one cone, it moves down the other, effectively creating an infinite number of gear ratios to always keep the engine in its most efficient RPM range. They can also have a pseudo manual mode to provide shifting between certain preset positions on the cones to create the feel of a traditional planetary automatic gearbox.

More new cars are equipped with a CVT, even larger sedans and has become common on some makes of hybrid cars. The benefits are many, but there are some disadvantages also.

Dual-clutch transmissions (DCTs) are like two normal manual transmissions with two clutches but integrated into the same unit to work as one transmission. One clutch is connected to the odd gears and the second to the even gears. As the vehicle moves with the first gear engaged on one clutch and a preselector already engages second gear on the other, open clutch. The gearshift is nothing more than the first clutch opening and the second closing. Then while in second gear, the third gear is preselected again on the first clutch, awaiting its turn, and so on. Just like an automatic, driver does not shift gears – the shifts are initiated by an electronic control module. DCT’s have been made popular with the

Direct Shift Gearbox, or DSG, from the Volkswagen Group, as used on the VW Golf series.

So, which is the best automatic transmission variation or manual transmission?

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