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Automatic Transmission allows the driver to be free from having to change gears manually.
The most common type of automatic transmission is hydraulic. This system uses fluid coupling in place of the manual’s friction clutch, and changes gears by locking and unlocking a system of planetary gears.
The normal lettering you see on an automatic are P R N D (Park, Reverse, Neutral, Drive)
This selection mechanically locks the output shaft of transmission, restricting the vehicle from moving in any direction.
A parking pawl prevents the transmission from rotating, and therefore the vehicle from moving. However, it should be noted that 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 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).
In many modern vehicles, the driver must have the foot brake applied before the transmission can be taken out of park.
This engages reverse gear within the transmission, permitting the vehicle to be driven backward, and operates a switch to turn on the white reversing lights for improved visibility. To select reverse in most transmissions, the driver must come to a complete stop, depress the shift lock button or move the shift stick sideways along a notched channel and select reverse. Not coming to a complete stop may cause severe damage to the transmission. Some modern automatic transmissions have a safety mechanism in place, which does, to some extent, prevent (but not completely avoid) inadvertently 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. Therefore, 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.
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, as the transmission’s lubrication pump is driven by non-idle engine RPMs. Similarly, emergency towing with an automatic transmission in neutral should be a last resort. 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.
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 they initially ranged from three, to four and five speeds although now losing popularity due to the six-speed autos. Six-speed automatic transmissions are probably the most common in vehicles from 2010 by car manufacturers like Toyota and Ford. However, seven-speed automatics are becoming available in some high-performance production luxury cars from Mercedes 7G, as are eight-speed autos in models from 2006 introduced by Lexus and some larger Hyundai vehicles. From 2013 are available nine speeds transmissions produced ZF and Mercedes 9G.