Theoretical Valve Timing Diagram
The exact moment at which the inlet and outlet valve opens and closes with reference to the position of piston and crank shown diagrammatically is known as Valve Timing Diagram. The timing is expressed in terms of degrees of crank rotation.
Suction Stroke: Inlet valve is open. Piston moves from the Top Dead Centre(TDC) to Bottom Dead Centre(BDC). Air-fuel mix is sucked in by negative pressure in cylinder.
Compression Stroke: Inlet and outlet valves closed. Piston moves upwards from BDC to TDC. Air-fuel mix is compressed.
Expansion/Power Stroke: Inlet and outlet remains closed here also. Piston moves from down from TDC to BDC. This happens as a result of ignition of the mixture inside the cylinder. Ignition is started by spark plug.
Exhaust Stroke: Exhaust valve opens. Piston moves up from BDC to TDC. Exhaust gases are pushed out of the cylinder.
To know the details about the four strokes of the IC engine click here.
Actual Valve Timing Diagram
The Actual Valve Timing Diagram has slight variations with respect to the Theoretical Valve Timing Diagram. The variations are made in order to maximize the engine performance.
Refer the figure given and compare it with the Theoretical VTD to the see the difference. The reason for making these deviations from theoretical VTD is explained below.
Opening and closing of Inlet Valve:
The inlet valve is made to open 10degree to 30degree before the piston reaches the Top Dead Center (TDC) during Suction Stroke and is allowed to close only after 30degree to 40degree after the piston reaches and leaves the BDC in the beginning of compression stroke.
Reason – The reason for doing this is to facilitate silent operation of the engine under high speeds. The inlet valves are made to operate slowly to avoid noise and hence sufficient time should be provided for the air-fuel mix to get into the cylinder. Thus valves are made to open before the actual BDC.
Since the inlet valve is a small opening sufficient mixture doesn’t enter the cylinder in such short time, as the piston reaches BDC. Thus the inlet valve is kept open for some time period of time after BDC, to facilitate sufficient flow of charge into the cylinder.
Opening and closing of Exhaust Valve:
The exhaust valve is made to open 30degree to 60degree before the TDC in the exhaust stroke and allowed to close only after 80 to 10 in0 the beginning of the suction stroke.
Reason – The gases inside the cylinder posses a higher pressure even after the expansion stroke. This higher pressure enables it to move out of the cylinder through the exhaust valve reducing the work that needs to be done by the engine piston in pushing out these gases. Thus the exhaust valve is made to open before the piston reaches the BDC thus enabling the gases to escape outside on its own and the remaining gases are pushed out by the upward motion of the piston.
When the piston reaches the TDC, if the exhaust valve is closed like in actual timing diagram, a certain amount of exhaust gases will get compressed and remain inside the cylinder and will be carried to the next cycle also. To prevent this, the exhaust valves are allowed to close only a certain time after the piston reaches the TDC.