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Driving Automatic Vehicles

 

Automatic car gearboxes are less common than manual gearboxes in Great Britain and as a result drivers are often uncertain in which position the gear shift lever should be in any given set of circumstances.

This article is designed to assist the driver to decide the optimal position and explains the workings of the components to facilitate understanding.

Modern automatics now have 5 gears.
The majority of gear selection positions are marked:

 

P Park Must never be engaged whilst the vehicle is in motion as it locks the transmission and prevents the car from moving.
R Reverse
N Neutral
D Automatic use of 1st, 2nd, 3rd, 4th & 5th gears.
4 Automatic use of 1st, 2nd, 3rd & 4th gears.
3 Automatic use of 1st, 2nd & 3rd gears.
2 Automatic use of 1st & 2nd gears.
1 1st gear hold.
 

For normal driving the lever may be placed in D and the transmission will automatically change up or down according to road speed and accelerator position.

It is necessary to manually change down to a lower gear, this may be done by moving the gear lever to the required position, but only if the vehicle is travelling at a speed which is within the range of the gear chosen.
This facility must not be used excessively.

When maximum acceleration is required, the accelerator should be pushed to the full throttle position, overcoming the built-in resistance. This brings into operation the "kick down" that causes an immediate downshift into the correct gear for maximum acceleration, provided that the road speed is within the speed range of the lower gear. When accelerator pedal is released, the gearbox will automatically change up again. Some gearboxes have a "kick down" system that also works at part throttle. It is because of the different methods in which gear changes can occur that drivers are sometimes uncertain as to what action they should take to negotiate a hazard in the correct gear.

In most cases, the automatic transmission consists of a torque converter and a set of gears called a planetary or epicycle gear train. These are fitted to the car in place of a conventional clutch and gearbox.

A torque converter consists of an impeller that is driven by the engine, and a turbine that drives the gearbox input shaft. Each is bowl-shaped and contains a number of partitions called vanes. The two bowls are placed face-to-face in a casing filled with oil and they are separated by a small clearance so that there is no friction between them.

When the engine is idling, oil is flung by centrifugal force from the impeller and it enters the turbine, which remains stationary because the force of the oil is not yet sufficient to turn it. When the driver depresses the accelerator, the speed of the impeller increases and the turning effort derived from the fast-moving oil becomes great enough to overcome the resistance of the turbine and this begins to rotate the gearbox-input shaft. If the vehicle is in gear, it starts to move off.

After passing its energy to the turbine, the oil re-enters the impeller and is circulated back to the turbine again. As the engine speeds up some turning effort is transmitted, but there is still a degree of slip in the unit so that the gearbox-input shaft is rotating more slowly than the engine.

The torque converter, as the name implies, converts the torque or turning effort of the engine into the higher torque needed by the car at low road speeds. An increase in torque has the same effect as changing to a lower gear, so a torque converter is also gear reducer, acting like a set of extra gears before the engine’s drive reaches the gearbox. It is able to deliver this higher torque because there is a small vaned wheel, known as a reactor or stator, placed between the impeller and turbine that, according to engine speed, directs the oil along a more favourable path towards the impeller, enabling it to give extra thrust to the turbine blades. At pull-away speeds, the torque converter can be double the turning effort produced by the engine and applied to the gearbox.

As the engine speed increases, this 2:1 increase in turning effort is reduced so that when holding a steady throttle opening and cruising, there is no increase at all and the parts of the torque converter rotate at the same speed.

Leaving the gear lever in D when negotiating a hazard in a car fitted with an automatic gearbox in no way equates with a vehicle with a manual gearbox in top gear when negotiating a hazard.

When stationary in traffic, even for many minutes, it is not necessary to move the gear lever into neutral because the torque converter absorbs the engine propulsion force but does not transmit it all to the gearbox as the engine revs are too low. No wear is taking place.
In fact, more wear will take place if the driver engages neutral and then engages a gear when he is able to move off.
When stationary, but in gear for any period, the hand brake should be applied and the foot kept clear of the accelerator.

Here are some recommendations as to the correct action to be taken in some common sets of circumstances:

  • At traffic lights.
    Leave in D. If red, stop and apply hand brake.
  • At Roundabouts.
    Use D unless the roundabout is very large or in exceptional circumstances.
  • On bends.
    Normally leave the car in D, unless the car is likely to change gear on its own when the driver does not want it to only then should manual selection of a gear be made.
  • Overtaking.
    Normally use "kick down" if a quick overtake is needed. If progressing along a line of vehicles where quick acceleration is needed, followed by deceleration to fit into a gap, the manual selection of a lower gear may be beneficial.
  • General.
    In unusual circumstances, when the gearbox is continually changing up and down between two gears, the manual selection of the lower of these tow gears may be beneficial.

It is not necessary to either "kick down" or change down manually to engage a lower gear for a hazard simply because one would change down if driving a car with a manual gearbox when negotiating the same hazard. The torque converter and gear train are designed to select the correct ratio.

On certain gearboxes, the manual engagement of ‘2’ eliminates the "kick down" facility completely. Therefore, the gearbox may be prevented from getting the car out of a potentially dangerous low speed situation by rapid acceleration in the first gear because the driver engaged ‘2’ when he would have been better leaving it in ‘D’.

 
 
 

In This Section:

Driving Automatic Vehicles

Gear Selection Advice

Overtaking with an Auto

 

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