Clutch control

With the clutch pedal completely pressed, there is no direct link between the engine and the driveshaft, so no power can pass from the engine to the drive shaft and wheels. With the pedal entirely released, there is full contact between the engine and the drive shaft, via the clutch plate, which means that the engine can apply power directly to the drive shaft. However, it is possible to have the clutch plate partially engaged, allowing the clutch to slip. As a result, only a fraction of the power from the engine reaches the drive shaft.

Low gear and low speed

When a car is in first gear, small variations in engine speed translate to large changes in acceleration and engine braking. However, with a combination of clutch control and careful use of engine speed, a much smoother ride can be achieved - by allowing the clutch to slip, variations in engine revs are not immediately translated into changes in drive shaft rotation speed, but rather the friction on the clutch plate allows the drive shaft to gradually equalize with the speed of the engine.

Once the car reaches a suitable speed, the clutch can be fully engaged and speed can then be controlled either by varying the engine speed or by partially disengaging the clutch again if necessary.

This particular use of clutch control is frequently taught to learner drivers as a way to control acceleration when pulling away from a complete stop or when driving at very slow speeds while minimizing the chance of stalling the engine.

Changing from first to second gear under maximum power

As the clutch slips, engine speed is lost but torque is not lost except through the effect of the new engine speed on the engine torque itself. In most car engines the torque output is higher as the engine speed increases up to 4500RPM or more [1]. So slipping the clutch actually gives more torque to the wheels even though the fraction of power wasted in the clutch increases much faster.

It is not necessary to maintain high engine speed to properly operate a motor vehicle. Any vehicle with a standard transmission can operate with an engine speed slightly above idle when following the proper gear sequence. The clutch can be operated normally with no sacrifice in performance; however, in certain off-road and racing conditions, to attain higher speed and better performance, the practice of clutch control plays an integral part in doing so.

Balancing the clutch

Normally when a vehicle is stationary on an uphill slope it is necessary to use the handbrake in conjunction with clutch control to prevent the vehicle from rolling backwards when pulling away. However, in situations where the vehicle must be stopped briefly, for example in slow moving traffic, the clutch can be used to balance the uphill force from the engine with the downhill force of gravity. The benefit of this is that there is no need for the hand- or foot-brake, and the driver can pull away more quickly.

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