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    Hot Rod Tech and Tools Main Page

    Understanding Automatic Transmissions


    The Unders is a vehicle's gearbox that is able to change the gear ratios automatically as the vehicle moves. This then allows the driver to be free from having to shift gears manually.

    Most vehicles that have been sold in the United States since the 1950s have been equipped with an automatic transmission. Although, this isn't the case in Europe and much of the rest of the world. Automatic transmissions reduce the fuel efficiency and the power of the vehicle. The fuel is more expensive and thus, the engines are generally smaller. In some recent years, the automatic transmissions have significantly improved in their ability to support high fuel efficiency, were manual transmissions are still generally more efficient.

    There are two basic types of automatic transmission:

    • The front wheel drive version called a Transaxle because it incorporates a differential
    • A rear wheel drive version called a Transmission that does not.

    A transaxle is also called a transmission because it is a fully automatic electronically controlled front wheel drive transaxle with four forward gear ranges that include overdrive and one reverse gear range.

    The normal automatic transmission is fully automatic with four forward and one reverse speed. The predominant form of automatic transmission is hydraulically operated, using:

    • A fluid coupling or torque converter — a hydraulic device used to connect the engine and the transmission. It takes the place of the manual clutch, which allows the engine to remain running without stalling. A torque converter is a hydraulic fluid coupling used to transmit power from one or more engines or motors to a driveshaft. Therefore it also provides a variable amount of torque multiplication at low engine speeds, and increases the "breakaway" acceleration.
    • Planetary gearset — a compound planetary set is a gear system that consists of one or more outer gears, or planet gears, that rotate around a central gear to provide a range of torque multiplication.
    • Valve body — a hydraulic control center that receives pressurized fluid from a main pump that is operated by the fluid coupling or torque converter. The pressure that comes from this pump is regulated and is used to run a network of spring loaded valves. The valves use the pump pressure and the pressure from a centrifugal governor on the output side to control which ratio is selected on the gearset. So, as the vehicle's engine speeds up, the difference between the pressure changes, and this causes the different sets of valves to open and close. The hydraulic pressure that is controlled by these valves will drive the various clutch and brake band actuators, thereby controlling the operation of the planetary gearset to select the correct gear ratio for those current operating conditions.

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