The modern automobile transmission system found on rear wheel drives consists of a clutch, a gearbox (either manual or automatic) and a driveshaft.
The clutch transmits the power of the rotating flywheel to the gearbox. This in turn gears the rotating speed and turns the wheels via the drive shaft.
The clutch is operated by a foot pedal within the vehicle. As the clutch pedal is released, the clutch plate within the clutch moves onto the face of the engine's rotating flywheel. The friction between clutch plate and flywheel brings them both up to equal speed without judder.
In a modern four-speed gearbox, the gears are fitted to three shafts - the input, layshaft and output shaft - are meshed together at all times. Gears on the input and layshaft are fixed whilst output shafts are collars with dog clutches fitted either side; these have projections or dogs at regular intervals around them and when meshed together allow the gear wheel and collar to turn together. On moving the gearstick and selecting a gear, the collar is moved along the output shaft and engages with a gear in mesh on the layshaft, so turning the flywheel. In low gears the gear on the layshaft is small and the output gear is large, giving the greatest speed reduction. As each gear is selected, the ratio between layshaft and output shaft decreases until in top gear the two gears are of equal proportion. In reverse another collar with a small idler gear is meshed between those on the layshaft, and output shaft reversing the output shaft allowing the vehicle to reverse.
Using automatic transmission alleviates the need to change gears manually. With this type of system there is no clutch pedal for the driver to use. The selection of gears is done automatically. The only operation the driver has to perform is to move a control lever, which then works a number of guiding positions. They are a follows: N for neutral, P for Park, R for reverse and D for forward drive. There is sometimes also an L position, which has the ability to hold a lower gear. As a safety precaution there are stopping devices within the system, to prevent the selection of either park or reverse when the vehicle is in motion. Another feature to ensure safety is that the engine is only able to start if the vehicle is in neutral or park. If fully automated gear changes are required, the D position must be the one selected.
