A torque converter’s primary function is to transfer the rotating power generated by the engine to the car’s transmission.  In cars with an automatic transmission, it generally takes the place of a manually operated clutch.

There are four main components to an automatic transmission torque converter:

  • Housing.
  • Impeller. This is the driving section of the converter, which contains curved blades placed on the inside cover of the housing. As it rotates, driven by the engine’s crankshaft, it sends the transmission fluid through curved passages toward the turbine.
  • Turbine. This is the driven section of the converter, whose design is similar to that of the impeller, but with blades curved in the opposite direction. When the fluid from the impeller strikes the turbine blades, it turns and drives the transmission input shaft in the same direction as the engine’s crankshaft.
  • Stator. The stator consists of a third set of blades that send fluid leaving the turbine back to the impeller, giving the impeller an added boost.

The extent to which the converter multiplies the torque generated by the engine depends on the difference between the rotational speeds of the impeller and the turbine. The maximum torque increase is produced when the engine is sending power to the impeller, but the turbine is stalled (for instance, when the car is in gear, but its brakes are being applied). The multiplication of torque tapers off as the turbine speed approaches impeller speed and essentially ceases when the turbine attains 90% of impeller speed. This is called the point of coupling. Some modern torque converters employ a clutch pressure plate to connect the front of the converter to the turbine and lock them together mechanically at the point of coupling in order to increase efficiency.