The partial vacuum existing in the engine manifold is put to use for operating the clutch. A Reservoir is connected to the engine manifold through a non return valve. The reservoir is further connected to a vacuum cylinder through a solenoid operated valve. The solenoid itself is operated from the battery and the circuit incorporates a switch which is placed in the gear lever and is operated when the driver holds the lever to change gears. Vacuum cylinder contains a piston which is exposed to atmospheric pressure on one side. The piston is further connected through linkage To The clutch. The movement of the Piston thus operates The clutch.
In the part throttle position there is sufficient vacuum in the engine inlet manifold. When the throttle is opened wide, the pressure in the manifold increases, but due to this increase of pressure the non return valve closes, isolating the Reservoir from the manifold. Thus a vacuum exists in the reservoir all the time.
In the normal operation the switch in the gear lever remains open and the solenoid operated valve renains in its bottom position. In this position, the atmospheric pressure acts on both sides of the Piston in the vacuum cylinder. However, when the driver is to change gears, he holds the gear lever. The action of the driver closes the switch, energizing the solenoid which pulls the valve up, connecting the vacuum cylinder to the vacuum in the Reservoir. Thus the piston is subjected to unequal pressures on two sides, which causes it to move. This movement is transmitted by linkage to disengage the clutch. The clutch used in this case is an ordinary friction clutch, which remains engaged due to the force of the springs provided in the clutch itself. The gear lever switch is opened as soon as driver releases the lever after changing the gear and clutch is again engaged.
Advantages :-
There are no linkages therefore clutch arrangement becomes simply and easy.
Compact arrangement is possible
No friction thus wear and tear reduces
Life of components is high
Working fluid that is Air is easily and readily available.
Disadvantages :-
High cost of compressor and other parts
Engine performance reduces as more energy is wasted in running the compressor
Any fault in compressor or other components may stall the vehicle
Applications :-
Heavy duty operations like trucks, buses and other transport vehicles
High performance cars
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