AUTOMATIC TRANSMISSION

An automatic transmission, also called auto, self-shifting transmission, n-speed

automatic (where n is its number of forward gear ratios), or AT, is a type of motor
vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the
driver from having to shift gears manually. Like other transmission systems on vehicles, it allows
an internal combustion engine, best suited to run at a relatively high rotational speed, to provide
a range of speed and torque outputs necessary for vehicular travel. The number of forward gear
ratios is often expressed for manual transmissions as well (e.g., 6-speed manual).
The most popular form found in automobiles is the hydraulic automatic transmission. Similar but
larger devices are also used for heavy-duty commercial and industrial vehicles and equipment.
This system uses a fluid coupling in place of a friction clutch, and accomplishes gear changes by
hydraulically locking and unlocking a system of planetary gears. These systems have a defined
set of gear ranges, often with a parking pawl that locks the output shaft of the transmission to
keep the vehicle from rolling either forward or backward. Some machines with limited speed
ranges or fixed engine speeds, such as some forklifts and lawn mowers, only use a torque
converterto provide a variable gearing of the engine to the wheels.
Besides the traditional hydraulic automatic transmissions, there are also other types of automated
transmissions, such as a continuously variable transmission (CVT) and semi-automatic
transmissions, that free the driver from having to shift gears manually, by using the
transmission's computer to change gear, if for example the driver were redlining the engine.
Despite superficial similarity to other transmissions, traditional automatic transmissions differ
significantly in internal operation and driver's feel from semi-automatics and CVTs. In contrast
to conventional automatic transmissions, a CVT uses a belt or other torque transmission scheme
to allow an "infinite" number of gear ratios instead of a fixed number of gear ratios. A semi-
automatic retains a clutch like a manual transmission, but controls the clutch
through electrohydraulic means. The ability to shift gears manually, often via paddle shifters, can
also be found on certain automated transmissions (manumatics such as Tiptronic), semi-
automatics (BMW SMG, VW Group DSG), and CVTs (such as Lineartronic).
The obvious advantage of an automatic transmission to the driver is the lack of a clutch pedal
and manual shift pattern in normal driving. This allows the driver to operate the car with as few
as two limbs (possibly using assist devices to position controls within reach of usable limbs),
allowing amputees and other disabled individuals to drive. The lack of manual shifting also
reduces the attention and workload required inside the cabin, such as monitoring the tachometer
and taking a hand off the wheel to move the shifter, allowing the driver to ideally keep both
hands on the wheel at all times and to focus more on the road. Control of the car at low speeds is
often easier with an automatic than a manual, due to a side effect of the clutchless fluid-coupling
design called "creep" that causes the car to want to move while in a driving gear, even at idle.
The primary disadvantage of the most popular hydraulic designs is reduced mechanical
efficiency of the power transfer between engine and drivetrain, due to the fluid coupling
connecting the engine to the gearbox. This can result in lower power/torque ratings for
automatics compared to manuals with the same engine specs, as well as reduced fuel efficiency
in city driving as the engine must maintain idle against the resistance of the fluid coupling.
Advances in transmission and coupler design have narrowed this gap considerably, but clutch-
based transmissions (manual or semi-automatic) are still preferred in sport-tuned trim levels of
various production cars, as well as in many auto racing leagues.

Automatic transmission modes

Conventionally, in order to select the transmission operating mode, the driver moves a selection
lever located either on the steering column or on the floor (as with a manual on the floor, except
that automatic selectors on the floor do not move in the same type of pattern as manual levers
do). In order to select modes, or to manually select specific gear ratios, the driver must push a
button in (called the shift-lock button) or pull the handle (only on column mounted shifters) out.
Some vehicles 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). Previously, quadrant-selected automatic transmissions
often used a P-N-D-L-R layout, or similar.[13] Such a pattern led to a number of deaths and
injuries owing to driver error causing unintentional gear selection, as well as the danger of
having a selector (when worn) jump into reverse from low gear during engine
braking maneuvers.

A floor selection lever in a 1992 Ford Escort showing the P-R-N-[D]-D-L modes as well as the
shift lock button on the top of the lever
Depending on the model and make of the transmission, these controls can take several forms.
However most include the following:

Park (P)
This selection mechanically locks the output shaft of transmission, restricting the vehicle from
moving in any direction. A parking pawlprevents the transmission from rotating, and therefore
the vehicle from moving. However, the vehicle's non-driven wheels are still free to rotate, and
the driven wheels may still rotate individually (because of the differential). For this reason, it is
recommended to use the hand brake (parking brake) because this actually locks (in most cases)
the wheels and prevents them from moving. It is typical of front-wheel-drive vehicles for the
parking brake to lock the rear (non-driving) wheels, so use of both the parking brake and the
transmission park lock provides the greatest security against unintended movement on slopes.
This also increases the life of the transmission and the park pin mechanism, because parking on
an incline with the transmission in park without the parking brake engaged will cause undue
stress on the parking pin, and may even prevent the pin from releasing. A hand brake should also
prevent the car from moving if a worn selector accidentally drops into reverse gear while idling.
A car should be allowed to come to a complete stop before setting the transmission into park to
prevent damage. Usually, Park (P) is one of only two selections in which the car's engine can be
started, the other being Neutral (N). This is typically achieved via a normally open inhibitor
switch (sometimes called a "neutral safety switch") wired in series with the starter motor
engagement circuit, which is closed when P or N is selected, completing the circuit (when the
key is turned to the start position). In many modern cars and trucks, the driver must have the foot
brake applied before the transmission can be taken out of park. The Park position is omitted on
buses/coaches (and some road tractors) with automatic transmission (on which a parking pawl is
not practical), which must instead be placed in neutral with the air-operated parking brakes set.
Reverse (R)
This engages reverse gear within the transmission, permitting the vehicle to be driven backward,
and operates a switch to turn on the white backup lights for improved visibility (the switch may
also activate a beeper on delivery trucks or other large vehicles to audibly warn other drivers and
nearby pedestrians of the driver's reverse movement). To select reverse in most transmissions,
the driver must come to a complete stop, depress the shift-lock button (or move the shift lever
toward the driver in a column shifter, or move the shifter sideways along a notched channel in a
console shifter) and select reverse. The driver should avoid engaging reverse while the vehicle is
moving forwards, and likewise avoid engaging any forward gear while travelling backwards. On
transmissions with a torque converter, doing so at very low speed (walking pace) is not harmful,
but causes unnecessary wear on clutches and bands, and a sudden deceleration that not only is
uncomfortable, but also uncontrollable since the brakes and the throttle contribute in the same
direction. This sudden acceleration, or jerk, can still be felt when engaging the gear at standstill,
but the driver normally suppresses this by holding the brakes. Travelling slowly in the right
direction while engaging the gear minimizes the jerk further, which is actually beneficial to the
wearing parts of the transmission. Electronically controlled transmissions may behave
differently, as engaging a gear at speed is essentially undefined behaviour. Some modern
transmissions have a safety mechanism that will resist putting the car in reverse when the vehicle
is moving forward; such a mechanism may consist of a solenoid-controlled physical barrier on
either side of the reverse position, electronically engaged by a switch on the brake pedal, so that
the brake pedal needs to be depressed in order to allow the selection of reverse. Some electronic
transmissions prevent or delay engagement of reverse gear altogether while the car is moving.
Some shifters with a shift button allow the driver to freely move the shifter
from R to N or D without actually depressing the button. However, the driver cannot shift back
to R without depressing the shift button, to prevent accidental shifting which could damage the
transmission, especially at high speeds.
Neutral / No gear (N)
This disengages all gear trains within the transmission, effectively disconnecting the
transmission from the driven wheels, allowing the vehicle to coast freely under its own weight
and gain momentum without the motive force from the engine. Coasting in idle down long
grades (where law permits) should be avoided, though, with many transmission designs as the
transmission's lubrication pump is commonly driven by the input (engine) side, which may not
provide sufficient fluid flow at engine idle speed for high-speed travel. Similarly, emergency
towing (with the driven wheels of the disabled and non-running vehicle on the ground) with an
automatic transmission in neutral is not permitted by the manufacturer for many vehicles.
Manufacturers understand emergency situations and list limitations of towing a vehicle in neutral
(usually not to exceed 55 mph and 50 miles). This is the only other selection in which the
vehicle's engine may be started.
Drive (D)
This position allows the transmission to engage the full range of available forward gear ratios,
allowing the vehicle to move forward and accelerate through its range of gears. The number of
gear ratios within the transmission depends on the model, but three was initially predominant. In
the 1990s four and five speeds became common. Six-speed automatic transmissions were
probably the most common offering in cars and trucks from about 2010, and were still common
(especially in older or less expensive models of vehicles) in 2017. However, seven-speed
automatics had become available in some high-performance production luxury cars (found
in Mercedes 7G gearbox, Infiniti), as are eight-speed autos in models from 2006 introduced
by Aisin Seiki Co. in Lexus, ZF, Hyundai Motor Company and General Motors. From 2013 are
available nine speeds transmissions produced by ZF and Mercedes 9G. In the 2017 model
year Ford and General Motors introduced a 10-speed transmission.

Overdrive ('D', 'OD', or a boxed [D] or the absence of an illuminated 'O/D OFF')
This mode is used in some transmissions 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 (Overdrive) in these cars is engaged under steady speeds or
low acceleration at approximately 35–45 mph (56–72 km/h). Under hard acceleration or below
35–45 mph (56–72 km/h), the transmission will automatically downshift. Other vehicles with
this selector (for example light trucks) will not only disable up-shift to the overdrive gear, but
keep the remaining gears available for use of engine braking. Drivers should verify the behaviour
of this switch and consider the benefits of reduced friction brake use when city driving where
speeds typically do not necessitate the overdrive gear.