What is an Automatic Transmission / Gearbox?
An automatic transmission is an automobile
gearbox that can change gear ratios automatically as the vehicle moves, thus freeing
the driver from having to shift gears manually (similar but larger devices are also
used for railroad locomotives).
Most cars sold in the United States since the 1950s have been equipped with an automatic
transmission. This has, however, not been the case in Europe and much of the rest
of the world. Automatic transmissions, particularly earlier ones, reduce fuel efficiency
and power. Where fuel is expensive and, thus, engines generally smaller, these penalties
are more burdensome. In recent years, automatic transmissions have significantly
improved in their ability to support high fuel efficiency but manual transmissions
are still generally more efficient.
A cut-away model of a torque converter - Most automatic transmissions have a set selection
of possible gear ranges, often with a parking pawl feature that will lock the output
shaft of the transmission.
However, some simple machines with limited speed ranges and/or fixed engine speeds
only use a torque converter to provide a variable gearing of the engine to the wheels.
Typical examples include forklift trucks and some modern lawn mowers.
Recently manufacturers have begun to make continuously variable transmissions commonly
available (earlier models such as the
Subaru Justy did not popularize
CVT). These designs can
change the ratios over a range rather than between set gear ratios. Even though
CVTs have been used for decades in a few vehicles (e.g. DAF saloons and the
Volvo 340 series that succeeded
them, and later the Subaru Justy), the technology has recently gained greater acceptance
among manufacturers and customers.
Automatic transmission modes
Conventionally, in order to select the mode, the driver would have to move a gear
shift lever located on the steering column or on the floor next to him. In order
to select gears/modes the driver must push a button in (called the shift lock button)
or pull the handle (only on column mounted shifters) out. Some vehicles (like the
DB9) position selector buttons for each mode on the cockpit instead, freeing up
space on the central console. Vehicles conforming to US Government standards must
have the modes ordered P-R-N-D-L (left to right, top to bottom, or clockwise).
Automatic Transmissions have
various modes depending on the model and make of the transmission. Some of the common
Park (P) – This selection mechanically locks the transmission,
restricting the car from moving in any direction. A pin prevents the transmission
from moving forward (although wheels, depending on the drive train, can still spin
freely), it is recommended to use the hand brake (or emergency brake) because this
actually locks the wheels and prevents them from moving, and increases the life
of the transmission and the park mechanism. A car should be allowed to come to a
complete stop before setting transmission into park to prevent damage. Park is one
of only two selections in which the car can be started. In some cars (notably those
sold in the US), the driver must have the footbrake depressed before the transmission
can be taken out of park.
Reverse (R) – This
puts the car into the reverse gear, giving the ability for the car to drive backwards.
In order for the driver to select reverse they must come to a complete stop, and
push the shift lock button in and select reverse. Not coming to a complete stop
can cause severe damage to the transmission. Many modern automatic gearboxes have
a safety mechanism in place, which does to some extent prevent (but doesn't completely
avoid) inadvertently putting the car in reverse when the vehicle is moving. This
mechanism usually consists of a moveable physical barrier on either side of the
Reverse position, and is electronically linked to the brake pedal, which needs to
be pressed in order to allow putting the car in reverse.
Neutral/No gear (N)
– This disconnects the transmission from the wheels so the car can move freely under
its own weight. This is the only other selection in which the car can be started.
Drive (D) – This allows
the car to move forward and accelerate through a range of gears. The number of gears
a transmission has depends on the model, but they can commonly range from 3, 4 (the
most common), 5, 6 (found in
VW / Audi
Direct Shift Gearbox), 7 (found in
Mercedes 7G gearbox) and 8 in the new model of
Lexus cars. Some cars when put
into D will automatically lock the doors or turn on the Daytime Running Lamps.
As well as the above modes,
there are also other modes dependant on the manufacturer and model. Some examples
D4 – In Honda and Acura automatics this mode is used commonly for highway
use (as stated in the manual) and uses all 4 forward gears.
D3 – This is also found in Honda and Acura automatics and only uses
the first 3 gears and according to the manual it is used for stop & go traffic
such as city driving.
+ − and M – This is the manual selection of gears for automatics,
such as Porsche's
Tiptronic. The driver can shift up and down at their will, like in a semi-automatic
transmission. This mode may be engaged either through a selector/position or by
actually changing gear (e.g. tipping the gear-down paddle).
OverDrive ([D], OD, or a boxed D) - This mode is used in some transmissions
(including late 1980s Chevrolet) to allow early Computer Controlled Transmissions
to engage the Automatic Overdrive; in these transmissions, Drive (D) locks the Automatic
Overdrive off, but is identical otherwise. OD in these cars engaged under steady
speeds or low acceleration at 45mph; it would automatically come on at 65 under
Second (2 or S) – This mode limits the transmission to the first
two gears, or more commonly locks the transmission in second gear. This can be used
to drive in adverse conditions such as snow and ice, as well as climbing or going
down hills in the winter time.
First (1 or L) – This mode locks the transmission in first gear
only. It will not accelerate through any gear range. This, like second, can be used
during the winter season, or towing.
Hydraulic automatic transmissions
The predominant form of automatic transmission is hydraulically operated, using
a fluid coupling or torque converter and a set of planetary gearsets to provide
a range of torque multiplication.
Parts and operation
A hydraulic automatic transmission consists of the following parts:
- Fluid coupling or torque
converter: A hydraulic device connecting the engine and the transmission.
It takes the place of a mechanical clutch, allowing the engine to remain running
at rest without stalling. A torque converter is a fluid coupling that also provides
a variable amount of torque multiplication at low engine speeds, increasing "breakaway"
- Planetary gearset:
A compound planetary set whose bands and clutches are actuated by hydraulic servos
controlled by the valve body, providing two or more gear ratios.
- Valve body: hydraulic
control center that receives pressurised fluid from a main pump operated by the
fluid coupling/torque converter. The pressure coming from this pump is regulated
and used to run a network of spring-loaded valves, check balls and servo pistons.
The valves use the pump pressure and the pressure from a centrifugal governor on
the output side (as well as hydraulic signals from the range selector valves and
the throttle valve or modulator) to control which ratio is selected on the gearset;
as the car and engine change speed, the difference between the pressures changes,
causing different sets of valves to open and close. The hydraulic pressure controlled
by these valves drives the various clutch and brake band actuators, thereby controlling
the operation of the planetary gearset to select the optimum gear ratio for the
current operating conditions. However, in many modern automatic transmissions, the
valves are controlled by electro-mechanical servos which are controlled by the Engine
Management System or a separate transmission controller.
- Hydraulic & Lubricating
Oil: called Automatic Transmission Fluid, or ATF, this component of the
transmission provides lubrication, corrosion prevention, and a hydraulic medium
to convey mechanical power. Primarily made from refined petroleum and processed
to provide properties that promote smooth power transmission and increase service
life, the ATF is one of the few parts of the automatic transmission that needs routine
service as the vehicle ages.
The multitude of parts, along with the complex design of the valve body, originally
made hydraulic automatic transmissions much more complicated (and expensive) to
build and repair than manual transmissions. In most cars (except US family, luxury,
sport-utility vehicle, and minivan models) they have usually been extra-cost options
for this reason. Mass manufacturing and decades of improvement have reduced this