Torque converter

From Academic Kids

A torque converter is a hydraulic device prevalent in automatic transmissions and marine propulsion systems, such as diesel powered ships, where two or more engines are driving a common shaft. It consists of a toroidal chamber containing two elements, a centrifugal pump (forming the front part of the housing and attached to the motor shaft with a flex plate), and a similar hydraulic motor, contained within (but not bound to) the after part of the housing and attached to the remaining portion of the transmission with an output shaft. The remaining portion of the transmission will usually contain gears in the form of planetary gearsets for changing the output ratio and for reverse operation.

Simplistically, a torque converter can be thought of as two fans, where one fan is spun by the engine, which then blows the hydraulic fluid to the other fan causing it to spin. A torque converter does not have angled blades like a fan, but rather radial chambers which cause the fluid to move from the center to the outside on the engine side and from the outside of the transmission side to the center, rather then front to back as in a fan. The two segments may be thought of as an engine driven pump connected directly to an hydraulic motor.

The hydraulic fluid (usually a light oil) is drawn from the inner portion of the pump and coerced to move with the pump by radial impeller segments within the pump. This action imparts both a radial flow (by centrifugal force) and an angular momentum to the fluid. The fluid exits the pump at the outer portion and impinges upon similar impeller segments in the hydraulic motor part of the converter. The fluid then returns toward the axis of the device and is recirculated into the inner portion of the pump, completing the fluid cycle.

Even with the hydraulic motor portion motionless, the redirection of the fluid mass from its spiral motion in the pump to the radial motion in the (converter) motor, will cause a torque (twisting force) to be applied to the output shaft. This torque can exceed the torque applied to the input, hence the name of the device. A lower torque at high rotational speed is converted to a higher torque at lower speed. This conversion is the essential function of any vehicle transmission, manual or automatic.

With the input shaft at low speed, the converter becomes quite inefficient (it is said to be "stalled"), and so only a small torque is applied to the output shaft. This property enables the elimination of the clutch, an essential part of drivelines using manual transmissions, but requires the addition of a parking pawl to lock the output shaft when the vehicle is unattended.

When a large displacement motor capable of high torque is used it is possible to build an effective transmission with only two forward gear ratios. In the interest of higher efficiencies and better performance, transmissions are now designed with four to six forward ratios, so the torque converter will operate only within a narrow range of input to output speed ratios.

Torque converters, by their nature, exhibit "slip", which tends to waste energy by heating the fluid. To obtain higher efficiencies modern torque converters contain an additional clutch-like mechanism to mechanically lock the pump and motor portions together. This is called a "locking" torque converter. The segments are locked when the transmission's computer elements determine that the vehicle is cruising at near constant speed.

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