How an automatic transmission works

The automobile has become so sophisticated and the automatic transmissions so reliable; that automatic transmissions are the most popular option, or are even standard on many models. Over 85% of all new vehicles are ordered with an automatic transmission. All the driver has to do is start the engine, select a gear and operate the accelerator and brakes. It may not be as much fun as shifting gears, but it is far more efficient if you haul heavy loads or pull a trailer.

The automatic transmission anticipates the engines needs and selects gears in response to various inputs (engine vacuum, road speed, throttle position, etc.) to maintain the best application of power. The operations usually performed by the clutch and manual transmission are accomplished automatically, through the use of the fluid coupling, which allows a very slight, controlled slippage between the engine and transmission. Tiny hydraulic valves control the application of different gear ratios on demand by the driver (position of the accelerator pedal), or in a preset response to engine conditions and road speed.

The automatic transmission allows engine torque and power to be transmitted to the drive wheels within a narrow range of engine operating speeds. The transmission will allow the engine to turn fast enough to produce plenty of power and torque at very low speeds, while keeping it at a sensible rpm at high vehicle speeds.

The transmission uses a light fluid as the medium for the transmission of power. This fluid also operates the hydraulic control circuits and acts as a lubricant. Because the transmission fluid performs all of these three functions, trouble within the unit can easily travel from one part to another.

The automatic transmission operates on a principle that fluids cannot be compressed, and that when put into motion, will cause a similar reaction upon any resisting force. To understand this law of fluids, think of two fans placed opposite each other. If one fan is turned on, it will begin to turn the opposite fan blades. This principle is applied to the operation of the fluid coupling and torque converter by using driving and driven members in place of fan blades.

Every type of automatic transmission has two sections. The front section contains the fluid coupling or torque converter and takes the place of the driver operated clutch. The rear section contains the valve body assembly and the hydraulically controlled gear units, which take the place of the manually shifted standard transmission.

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