### Valve Timing Diagram of Four Stroke Engine / Otto Cycle

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.

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.

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Felix sila said...

Thenks

Anonymous said...

Will burnt gases not escape through the inlet valve cos on the diagram the inlet opens when the exhaust valve is about to close.?????

cuddlyable3 said...

There is a fast moving stream of exhaust gas from the exhaust valve through the exhaust pipeware and muffler. The stream has inertia and draws out the last remaining burnt gas from the cylinder during the short time that both inlet and exhaust valves are open; this also helps draw fresh air/fuel in. The failure that Anonymous describes would happen only if the engine were turning so slowly that it stalls.

mayank said...

thanksss

mayank said...

thanksss

mayank said...

thanksss

Unknown said...

thanks

veronica said...

d reasons were nt satisfactory fr d actual vt diagram

Sayantan Datta said...

Inlet valve will open before 8 degrees and at that time exhaust stroke is running.So there will be no vacuum inside the cylinder.At that time how the carburettor will make air-fuel mixture without any engine vacuum?

Anonymous said...

Clean explanation. Thanks.

Anonymous said...

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Good explanation , although I felt that in the explanation of exhaust valve open there is a mistake in the opening position which have been mentioned as 10 -30 degree b4 TDC in exhaust stroke instead should be 10 -40 degree b4 BDC prior commencement of exhaust stroke. Same also reflected correctly in ur diagram also.
Thank u.

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Racing Components said...

There will be no vacuum inside the cylinder.At that time how the carburetor will make air-fuel mixture without any engine vacuum?

Roshan said...

Thanks a lot For explain about valve timing diagrame.