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General Information About Transmissions

Automatic Transmission

An automatic transmission (also known as an automatic gearbox) is one type of motor that can automatically change gears as the automobile moves, freeing the driver from having to manually shift gears. Most automotive transmissions have a defined set of gear ranges, often with a parking gear feature that locks the drive shaft of the transmission.

Similar but large devices are also used for heavy-duty, commercial and industrial vehicles and equipment. Some machines with limited speed range or fixed engine speeds, such as some forklifts, only use a torque converter to provide a changeable gearing of the engine to the wheels.

Besides automatics, there are also other types of automated transmissions such as Continuous Variable Transmissions (CVTs) and semi-automatic transmissions, that enable or free-up the driver from having to shift gears manually, by using the transmission's computer to change gear, if for example the driver were "redlining" (driving the car's engine at or above its rated maximum Revolutions Per Minute also known as RPM) the engine. Despite superficial similarities to other transmissions, automatic transmissions differ significantly in internal operation from semi-automatic and CVTs. An automatic uses a torque converter (On automatic transmissions, the coupling device between the engine and transmission. A fluid-filled turbine that allow the engine to continue to run while the vehicle is stopped. As engine speed increases, torque is transmitted to the transmission. Torque converters serve much the same function as the clutch on a manual transmission) instead of a clutch to manage the connection between the transmission gearing and the engine. In contrast, a CVT uses a belt or other torque transmission design 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 electro-hydraulic means.

A Conventional Manual Transmission is frequently the base or standard equipment in a car, with the option being an automated transmission or Manual Transmission. The ability to shift gears manually, often via paddle shifters, can also be found on certain automated transmissions, semi-automatics (BMW), and Continuous Variable Transmissions (CVTs) such as Subaru.

Comparison with Manual Transmission

Most cars sold in North America since the 1950s have been available with an automatic transmission. Where as, automatic transmissions are less popular in Europe, with 80% of drivers choosing manual transmissions. In some Asian markets and in Australia, automatic transmission owners will not be licensed to drive a manual transmission vehicle. And reversely, a manual license will allow the driver to drive both manual and automatic vehicles.

Automatic Transmission Modes

Ordinarily, in order to select the transmission operating mode, the driver moves a selection lever located either on the steering column (a shaft that connects the steering wheel of a vehicle to the rest of the steering mechanism) or on the floor. In order to select modes or to manually select specific gear speeds, the driver must push a button in (called the shift lock button) or pull the handle out. Some vehicles position the selector button for each mode on the cockpit dashboard, freeing up space on the central console. Automobiles conforming to US Government standards mush have the modes ordered "P", "R", "N", "D", "L" (left to right, top to bottom or clockwise). Prior to this, automatic transmissions often used a "P", "N", "D", "L", "R" layout or similar. Such a pattern led to a number of deaths and injuries due to driver error causing unintentional gear selection, as well as the danger of having a selector (when worn) jump into Reverse from Low gear.

Automatic transmissions have various modes depending on the model and make of the transmission. Some of the common modes include:

Park (P)

This selection mechanically locks the drive shaft, restricting the vehicle from moving in any direction. A parking lever prevents the transmission from rotating and therefore the vehicle from moving, although the vehicle's non-driven road wheels may still rotate freely. For this reason, it is recommended to use the parking brake because this actually locks (in most cases) the rear wheels and prevents them from moving. This also increases the life of the transmission, because parking on an incline with the transmission in park without the parking brake engaged will cause unnecessary stress on the parking lever. An efficiently adjusted parking brake should also prevent the car from moving if a worn selector accidentally drops into Reverse, during early morning fast-idle engine warm-ups. It should be noted that locking the transmission drive shaft does not positively lock the driving wheels. If one driving wheel slips while the transmission is in park, the other will roll freely as the slipping wheel rotates in the opposite direction. Only a parking brake or hand brake can be relied upon to positively lock both of the parking-braked wheels. This is not the case with certain 1950's Chrysler products that carried their parking brake on the transmission shaft. It is typical of front-wheel-drive vehicles for the parking brake to be on the rear (non-driving) wheels, so the use of both the parking brake and the transmission park lever or gear, provides the greatest security against unintended movement on slopes or inclines.

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 the only two selections in which the car's engine can be started, the other being Neutral (N). In several modern automobiles and trucks, the driver must apply the foot brake before the transmission can be taken out of Park.

Reverse (R)

This engages the Reverse gear within the transmission, giving the vehicle the ability to drive backwards. In order for the driver to select reverse in modern transmissions, they must come to a complete stop, push the shift lock button in (or pull the shift lever forward in the case of a column shifter or steering wheel) and select reverse. Not coming to a complete stop can cause severe damage to your transmission. Many modern automatic transmissions have a safety mechanism in place, which to some extent prevents (but does not completely avoid) inadvertently putting the car in reverse when the vehicle is moving forward. This mechanism usually consists of a controlled physical barrier on either side of the Reverse position, which is electronically engaged by a switch on the brake pedal. Therefore, the brake pedal needs to be pressed down in order to allow the selection of the Reverse gear. Some electronic transmissions prevent or delay engagement of Reverse gear altogether while the car is moving.

Neutral/No Gear (N)

This disengages all gear shafts within the transmission, effectively disconnecting the transmission from the driven road wheels, so the automobile is able to freely move under its own weight and grain momentum without the motive force from the engine (engine braking). This is the only other selection or gear in which the vehicle's engine can be started.

Drive (D)

This position allows the transmission to engage to full range of available forward gear shafts and therefore allows the vehicle to move forward and accelerate through its range of gears. The number of gear ratios a transmission has depends on the model, but they initially ranged from three (most prominent before the 1990's), to four and five speeds (loosing popularity to six-speed autos, though still favored by Chrysler and Honda/Acura) Six-speed automatic transmissions are now probably the most common transmissions offered by Toyota, Chevrolet, Corvette, GM, Pontiac, Ford/Lincoln/Mercury vehicles.

However, seven-speed autos are becoming available (found in Mercedes, Infiniti, VW, as well as eight-speed autos in Lexus, BMW, VW, etc.

Continuously Variable Transmissions

A fundamentally different type of automatic transmission is the Continuously Variable Transmission or CVT, which can smoothly and steplessly alter its gear shafts by varying the diameter of a pair of belt or chain-linked pulleys, wheels or cones. CVT designs are usually as fuel efficient as manual transmissions in city driving, but early designs loose efficiency as engine speeds increase.

Some current hybrid vehicles, notably those of Toyota, Lexus and Ford, have an electronically controlled CVT or E-CVT. In this engine system, the transmission has fixed gears but the ratio of wheel-speed to engine-speed can be continuously varied by controlling the speed of the third input by using an electric motor-generator.

Automatic Transmission Models

Some of the best known automatic families include:

  • General Motors
  • Ford
  • Chrysler
  • Honda
  • Nissan
  • Volkswagen
  • Hyundai
  • Mini Cooper - Automatic or manual transmission, all models

Many of the above Automated Manual Transmissions (AMT) exist in modified states, which were created by racing enthusiasts and their mechanics by systematically re-designing the transmission to achieve higher levels of performance. These are known as "performance transmissions". An example of a manufacturer of high performance transmissions of General Motors and Ford transmissions is PerformBuilt.