|The transmission is a device that is linked to the back of the engine and transfers the power from the engine to the drive wheels. An automobile engine operates at its optimum at a certain RPM (Revolutions Per Minute) range and it is the transmission’s responsibility to make ensure that the power is given to the wheels while keeping the engine within that range. It does this through different gear combinations. In first gear, the engine turns much faster in relation to the drive wheels, while in high gear the engine is running even though the car may be going in excess of 70 MPH. In addition to the different forward gears, a transmission also has a neutral position which disconnects the engine from the drive wheels, and reverse, which causes the drive wheels to turn in the opposite direction. Finally, is the Park position. In this position, a latch mechanism (not unlike a deadbolt lock on a door) is inserted into a slot in the output shaft to lock the drive wheels and stop them from turning, thus preventing the vehicle from rolling.
There are two basic classes of automatic transmissions dependent on whether the vehicle is rear wheel drive or front wheel drive.
First on a rear wheel drive car, the transmission is normally mounted to the back of the engine and is located under the hump in the center of the floorboard alongside the accelerator pedal position. A drive shaft links the rear of the transmission to the final drive which is positioned in the rear axle and is used to transfer power to the rear wheels. Power flow on this unit is simple and straight forward going from the engine, via the torque converter, then through the transmission and drive shaft until it gets to the final drive where it is split and sent to the two rear wheels.
| On a front wheel drive vehicle, the transmission is usually added with the final drive to form what is called a transaxle. The engine on a front wheel drive car is usually positioned sideways in the car with the transaxle tucked under it on the side of the engine facing the rear of the car. Front axles are attached directly to the transaxle and it provides power to the front wheels. In this example, power passes from the engine, through the torque converter to a large chain that sends the power through a 180 degree turn to the transmission that is alongside the engine. From there, the power is sent through the transmission to the final drive where it is divided and sent to the two front wheels through the drive axles.
There are a number of other arrangements including front drive vehicles where the engine is mounted front to back instead of the sideways and there are other systems that drive all four wheels but the two systems described here are the most popular. A less popular rear drive system has the transmission positioned directly to the final drive at the rear and is connected by a drive shaft to the torque converter which is still positioned on the engine. This system is seen on the new Corvette and is used in order to balance the weight evenly between the front and rear wheels for improved performance and driving. Another rear drive system mounts everything, the engine, transmission and final drive in the rear. This rear engine arrangement is popular and available on the Porsche cars.
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