Cars stuff

Automatic Transmission

12.14.2008 | 12:39 am | Car Modifications

Automatic transmission (commonly known as AT) is an automobile gearbox that can change gear ratios automatically. The changes are made as the car is moving and frees the driver from making manual shifts. Almost all of the automatic transmissions have a preset gear range and often feature a parking pawl system which locks the output shaft of the transmission. The first car to use automatic transmission was Oldsmobile’s 1940 models. These cars featured Hydra-Matic drive and were the first fully automatic transmissions. Hydra-Matic used a fluid coupling (not a torque converter like modern cars) and three planetary gearsets. The system provided four speeds plus reverse. By the late 1960s most fluid-coupling four-speeds and two-speed transmissions had disappeared. Their place was taken by the three-speed units featuring torque converters. As engine computers became more powerful, more and more of the automatic transmission’s control was offloaded to them.

Automatic transmission provides a lower power and fuel efficiency than manual transmission, but is easier for drivers. Beginners benefit the most from automatic transmission. However, in the UK, if a beginner passes his driving test on an automatic transmission car, the driver license is only for automatic transmission cars. So the driver can’t drive a manual transmission car. A driver that possesses a manual license can drive both manual and automatic transmission cars.

Automatic transmission has various modes, depending on the model of the car and the car’s producer. Here are some of the most common modes:

* Park (P) – Locks the car from moving by locking the transmission. However, note that in Park mode, the non-drive wheels can still spin. Because of this you are highly advised to use the hand brake, which immobilizes these wheels. Using the hand brake also increases the lifetime of the transmission as it frees the park pin from stress when parking on an incline. The car has to come to a complete stop before you should engage the Park mode to prevent damages to the transmission. In most cases, Park mode is one of two modes in which a car can be started.

* Reverse (R) – This mode places the car in reverse gear and allows it to move backwards. Before engaging R mode, the car must come to a complete stop to prevent any damages to the transmission system. Some cars feature devices that prevent you from making this mistake, but some don’t.

* Neutral (N) – This is the No Gear mode. The car can move freely in any direction because the transmission is disconnected from the wheels. This is the second mode in which the car can be started.

* Drive (D) – In this mode, the car can accelerate forward through its complete set of gears. Some cars will automatically lock the doors when places on D, to prevent accidents.

* Second (S or 2) – This mode locks the transmission on the first two gears. The car might shift in other gears if the rpm is too high, in order to prevent engine damages. The S mode is very useful when driving on snow or other adverse conditions.

* First (L or 1) – This mode locks the transmission on the first gear. It is very useful on snow, ice, or when towing.

There are also some different types of automatic transmissions. One of them is the continuously variable transmission (known as CVT). This type of transmission can smoothly alter its gear ratio by varying the diameter of a pair of belt or chain-linked pulleys, wheels or cones. Some of these CVTs use a hydrostatic drive, which consists of a variable displacement pump and a hydraulic motor used to transmit power to the wheels without gears. CVT are very fuel efficient, compared to manual transmissions in city driving. However, early designs lose efficiency when the engine speed increased. Some hybrid vehicles (from Toyota, Lexus and Ford Motor Company) have a so called “electronically-controlled CVT” (or E-CVT). The transmission has some fixed gears, but the ratio of wheel-speed to engine-speed can be continuously modified just by controlling the speed of the third input to a differential. This is accomplished by using an electric motor-generator.

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