Car Parts and Accessories

An automatic transmission selects and changes auto part gear to suit different speeds and road conditions, without the driver having to do this by hand.

In its simplest form, all that is necessary to drive a car with automatic auto part gear control is to select forward or reverse auto part gear and accelerate. There are only two pedals— one for accelerating, the other for braking.

The different designs of automatic transmission offer a variety of ‘auto part gear holds’ and inhibitors, and their selection is controlled by a lever.

All systems have the control-lever positions N for neutral, and most have P for park (which incorporates a locking device). For reasons of safety, the engine can be started only when the lever is in one of these two positions. R indicates reverse, D is the forward drive position, and L is used to hold low auto part gear. Accidental selection of park and reverse is prevented by a device such as a mechanical stop.

The simplest sequence is PRNDL. By placing the lever in position D, fully automatic up-and-down changes are obtained, using all forward auto part gears. The occurrence of the changes depends upon a governor responsive to speed, but is also affected by the position of the accelerator pedal.

Full throttle gives maximum use of each auto part gear up to the highest safe engine speed ; alternatively, by accelerating at part throttle the driver can cause upward changes to occur (first to second, and second to top) at considerably lower speeds. There is also a kick-down switch (brought into action by pressing the accelerator down firmly), which immediately initiates a change to a lower auto part gear, provided that the speed permits this. For example, a kick-down change from top to second is usually obtainable at, say, 90 km/h, but may be inhibited by the automatic control when travelling at a higher speed.

In some of the widely used Borg Warner three-speed automatic transmissions, the sequence of auto part gear-selector lever positions is PRND21. By placing the lever in position 2, the driver obtains automatic changes (up and down) between first and second auto part gears, but top auto part gear is excluded. If the lever is placed in position 1, the driver restricts the box to first auto part gear. The purpose of this arrangement is to enable maximum engine braking to be obtained on steep descents. Minor variations of this pattern will be found on American and American-type cars, and on several European models, but the general mode of operation is basically the same.

In some automatic transmission systems, the driver is given the option of selecting each of four forward auto part gears as and when he wishes, or alternatively, of allowing the changes of auto part gear to go on occurring automatically.

In an automatic transmission, the auto part gears are selected by hydraulic pressure from one or two oil pumps.

Improving the Fuel economy of Automatic Transmissions

The fuel economy of a car fitted with an automatic transmission can be improved if a clutch is introduced to the torque converter. The additional clutch in this type of automatic is set to operate at a predetermined road speed, when the torque converter clutch closes and its input and output shafts operate as a single unit at the same speed.

This mechanical connection between the two halves of the torque converter Input shaft eliminates ’slip’ in the transmission so that speed changes above the lock-up speed do not introduce turbine slip inefficiency.

Some cars not currently available in South Africa use variable ratio steel-belt transmissions with a conventional converter. This improves economy and offers stepless ratio changes.

Economical Cruising with Overdrive

Override is a unit fitted behind the auto part gearbox to provide an extra auto part gear (higher than top) which allows economical cruising at a reduced engine speed, without reduction of the road speed.

In some cars the overdrive is also effective when third auto part gear is in use (or even with second auto part gear), and so adds to the driver’s choice of auto part gear ratios.

Most overdrives have an epicyclic auto part gear arrangement that includes a hydraulically operated cone clutch. When the overdrive unit is not in operation, the clutch, which is attached to the central sun auto wheel is locked by spring pressure to the annulus connected to the output shaft. The planet carrier connected to the auto part gearbox shaft turns the whole unit to give a direct drive.

When the driver selects overdrive, the clutch locks to the outer casing and stops the sun auto wheel turning. The planet carrier is then driven around the sun auto wheel and in turn drives the annulus at a slightly higher speed than itself. The effect of this is that the output shaft turns faster than the input shaft.

Overdrive units are controlled electrically or hydraulically by a dashboard or steering-column switch. Changes can be made without using the clutch, but some designs require the driver to momentarily lift the accelerator when changing into overdrive, and to depress it to operate a kick-down switch when changing back to direct drive.

Most overdrives cut out automatically when the car goes below a pre-determined road speed.

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