How can a clutch connect a running engine smoothly?

A running engine can keep turning while a car is stopped. A clutch makes that possible by controlling the connection between the engine and the transmission. In a common dry clutch, the engine turns a heavy disc called a flywheel. A separate friction disc connects to the transmission's input shaft. A pressure plate can clamp that disc against the flywheel.

When the clamping force is released, the driven disc is no longer firmly gripped. The engine side can turn without forcing the transmission side to match it. Bringing the surfaces into contact begins to transfer torque through friction. At first, the two sides may still rotate at different speeds. This is called slipping.

During slip, friction tends to reduce the difference in rotation speed. It also converts some mechanical energy into thermal energy. That is why a slipping clutch can become hot. In the illustration, 1200 rpm on the engine side and 600 rpm on the driven side are supplied snapshots showing a difference of 600 rpm; they are not a prediction of how fast a particular car accelerates.

Once the surfaces grip without slipping, the disc and flywheel rotate together. Springs in the pressure-plate assembly maintain the clamping force. Releasing the connection again allows the transmission to be separated from the running engine. The clutch is therefore a controllable friction connection, with a gradual slipping stage between separation and shared rotation.