When a car is stationary, where does the engine’s power go?


So I know that the combustion moves the pistons which turns the crank shaft and sends power to the wheels via the drive train (is that right?). But when the wheels aren’t moving, such as when the car is in drive but stationary, where is all that power going? The engine is still running, and the crank shaft is still turning, so somewhere along the line some parts must be slipping, or not turning when they normally would, right?

Basically I’m a little confused as to how the mechanical rotation of the engine is translated into “power” or applying pressure to rotate the wheels.

In: Engineering

That is where a clutch comes into play.

When you are idle the flywheel disengages from the clutch plate and stops the engine from transferring power to the drive train. When you start moving again, the clutch and flywheel press together and power is transferred back to the drive train.

There is some slippage there. That is how a clutch gets worn out. It is rubbing against the flywheel.

If the vehicle is equipped with a manual transmission, this job is performed by the clutch which is actuated by a separate pedal by the driver. When the pedal is depressed the clutch releases and allows the motor to spin free of the transmission. In the case of an automatic transmission this job is performed by the torque converter. With one exception, the torque converter does not actually physically couple the motor to the transmission, it uses a hydraulic connection. The exception to this is the case of a lockup torque converter, which most vehicles are equipped with. At cruising speed, and under minimal load (i.e. flat ground or downhill) the torque converter will physically couple the motor to the transmission. This typically results in reduced engine RPM which also usually results in higher fuel efficiency. The downside is that if significant load is applied while the torque converter is locked it frequently produces significant heat which causes premature wear of the transmission fluid.