A general description of torque converter operation when the shift lever is shifted into a “D”, “2”, “L” or “R” is given below.
1. Engine idling, vehicle stopped
When the engine is idling, the torque generated by the engine itself is at a minimum. If the brakes (parking brake and/or foot brake) are applied, the load on the turbine runner is great because it cannot rotate. Because the vehicle is stopped, however, the speed ratio of the turbine runner to the pump impeller is 0 while the torque ratio is at a maximum. Therefore, the turbine runner is always ready to be rotated with a higher torque than the torque generated by the engine.
2. Vehicle starting off
When the brakes are released, the turbine runner is able to rotate with the transaxle input shaft. Therefore, the turbine runner rotates with a torque greater than that generated by the engine by depressing the accelerator pedal. Thus, the vehicle starts to move.
3. Vehicle running at low speed
As the vehicle speed increases, the rotational speed of the turbine runner quickly approaches that of the pump impeller. Therefore, the torque ratio quickly approaches 1.0. When the ratio of the turbine runner speed to that of the pump impeller speed reaches the clutch point, the stator starts to rotate, and torque multiplication decreases. In other words, the torque converter starts to operate as a fluid coupling. Therefore, the vehicle speed increases almost in linear proportion to the engine speed.
4. Vehicle running constantly at medium or high
speed
The torque converter functions only as a fluid coupling. The turbine runner is rotating at a speed almost identical to that of the pump impeller.
HINT:
Under normal conditions when the vehicle starts off, the torque converter reaches the clutch point 2 to 3 seconds. However, if the load is heavy even while the vehicle is running at a medium or high speed, the torque converter may operate in the converter range.
When the brakes are released, even if the accelerator pedal is not depressed, the vehicle slowly starts to move. This is called creep phenomenon.