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A torque converter is a mechanical device, used mainly in automobiles, that transfers the rotating power generated by an vehicle's engine to the transmission. It is part of the family of mechanisms known as fluid couplings, which use hydraulic fluid to transmit mechanical power. A torque converter is installed in automatic transmissions and does the job a clutch would do in a manual transmission, which is allowing the power created by the engine to be distributed to the wheels.
A torque converter consists of three mechanical parts — a pump, a turbine, and a stator. The pump is attached to directly to the engine, and spins at the same speed as the motor. Inside the pump are many fins, which, as the pump spins, direct hydraulic fluid outward to the turbine. The turbine then spins at close to the same speed as the engine, but in the opposite direction. The spinning of the turbine causes the transmission to rotate and drive the wheels. The hydraulic fluid exits the turbine at its center, moving in the direction opposite to how it was forced in by the pump.
At this point, the stator, which is similarly located in the center of the converter, reverses the direction of the fluid a second time. This greatly increases the efficiency of the overall design, but only occurs at relatively low speeds. Depending on the precise specifications of the torque converter, the stator begins to freewheel at a particular speed, because the pump and turbine begin moving at almost exactly the same speed, and the fluid no longer changes direction.
One particular advantage a torque converter has over a conventional fluid coupling — and what makes it ideal for use in automatic transmissions — is the fact that it can multiply the amount of torque it generates as the engine provides more power. A real-world example of this is the comparison of the relatively light pressure that must be applied to a brake pedal to keep a car stationary while idling, when compared to the increase in pressure needed to keep it still when gas is also applied. At very low speeds torque can be multiplied two or three times by a torque converter.
One of the major downsides to torque converters, as opposed to normal fluid couplings, is that, given how the pump and turbine never spin at exactly the same speed, some power is always wasted. This, along with its typically heavier weight, is the reason that manual transmission vehicles tend to get better fuel mileage than those with automatic transmission.
@Soulfox -- That isn't as true as it used to be. Automatic transmissions have gotten a lot better of the years and can rev higher during hard acceleration than they would normally, thus resulting in better performance. Also, most automatic transmissions are computer controlled to an extent and allow drivers to select different modes (sports, economy, comfort, etc.) for different applications.
There are still advantages to a manual transmission, but they are no longer as essential for high performance as they once were. You can get by just fine with an automatic transmission most of the time.
Another disadvantage is that you can control when your engine shifts with a standard transmission. Going for power? Wind it up to high RPMs to get more power in a hurry. You can't do that with an automatic transmission as those shift at virtually the same time every time.
Having more control over when the car shifts can result in significant performance boosts.
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