Stages of Operation
http://drive2.subaru.com/Fall02/Fall02_TorqueConvert/Diagram.gif
Before explaining why this all works it is important to understand the stages the system goes through when a car goes from a stop to cruising speed.
By design, torque converters switch between these three stages automatically, based on input speed and resistance on drivetrain (e.g. brakes, vehicle begins up a hill, etc.)
To see how these parts interact, view the following videos.
- Stage one is stall. While the vehicle is not moving, the engine runs at its idle speed, but the tires must remain stationary. Here, the force of the brakes overcome the force of the fluid from the impeller on the turbine, and the car doesn't move. This is a very important feature of the torque converter, it allows engine to continue to run and automatically be "disconnected" from the rest of the drivetrain.
- Step two is acceleration. Here the impeller rotates far faster than the turbine, this ratio is known as stall ratio, and this is where the torque converter acts as a variable reduction gear, thus earning the name torque converter. The stall ratio is relative to each torque converter and is based on many components of the design, particularly the angle and size of stator fins.
- Stage three is coupling. At this stage the vehicle is no longer accelerating and is at cruising speed. Here, torque conversion, or in other words, a reduction gear isn't desired for maximum efficiency. Ideally, the impeller and the turbine will spin at equal rates, but due to factors such as friction, this isn't possible. To solve this, many modern torque converters include a lock up clutch which is used to tie the output shaft to the body of the torque converter, which rotates with the engine, so that the engine and output shaft of the torque converter turn at a 1:1 ratio, and effectively zero energy loss occurs between engine crankshaft and transmission input. However, this lock up clutch isn't necessary for torque converter overall performance, it simply increased overall vehicle efficiency by reducing necessary engine rotations.
By design, torque converters switch between these three stages automatically, based on input speed and resistance on drivetrain (e.g. brakes, vehicle begins up a hill, etc.)
To see how these parts interact, view the following videos.