The vacuum clutch is a type of clutch system worked by the vacuum existing in the engine manifold. The figure shows the mechanism of a vacuum clutch.
It consists of a vacuum cylinder with a piston, reservoir and a non-return valve, a solenoid-operated valve. The reservoir is linked to the engine manifold through a non-return valve. The vacuum cylinder is connected to the reservoir through the solenoid-operated valve.
The solenoid is operated by the battery and the circuit consists of a switch that is placed in the gear lever. The switch is on when the driver holds the gear lever to change gears.
When the throttle is opened wide, the pressure in the inlet manifold increases, because of which the non-return valve closes and isolating the reservoir from the manifold. Thus, a vacuum exists in the reservoir all the time.
In the normal operating condition, the switch in the gear lever remains always open and the solenoid-operated valve remains always in its bottom position. In this position, the atmospheric pressure acts on both the sides of the piston of the vacuum cylinder, because the vacuum cylinder is open to the atmosphere through a vent. When the driver holds the gear lever to change the gear, the switch is closed and thus energizing the solenoid which pulls the valve up.
This connects one side of the vacuum cylinder to the reservoir. Due to the pressure difference on the vacuum cylinder piston, it moves. This movement of the piston is transmitted by a linkage system to the clutch, causing the clutch to disengage. When the driver is not holding the gear lever, the switch remains open and the clutch also remains engaged due to the force of springs.