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
Modern automatics now have 5 gears.
The majority of gear selection positions are marked:
Must never be
engaged whilst the vehicle is in motion as it
locks the transmission and prevents the car from
of 1st, 2nd, 3rd, 4th & 5th gears.
of 1st, 2nd, 3rd & 4th gears.
of 1st, 2nd & 3rd gears.
of 1st & 2nd gears.
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
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
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
engines 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
- 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.
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
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.