Table of Contents

Theoretical and Actual valve timing diagram For Four Stroke SI Engine

Theoretical valve timing diagram:

The exact moment at which each of the valves opens and closes with reference to the position of piston and crank can be shown graphically in a diagram. This diagram is known as “valve timing diagram”.
In theoretical valve timing diagram, inlet and exhaust valves open and close at both dead centers. Similarly, all processes are sharply completed at TDC or BOC. Figure 1.72 shows theoretical valve timing diagram for four stroke SI engines.

IS => Ignition Start
EVO => Exhaust Valve Open
EVC => Exhaust Valve Close
TDC => Top Dead Center
BOC => Bottom Dead Center.

Actual valve timing diagram:

Figure 1.73 shows actual valve timing diagram for four stroke SI engine. The inlet valve opens 10-30° before TOC. The air-fuel mixture is sucked into the cylinder till the inlet valve closes. The inlet valve closes 30-40°’ or ‘even 60° after BOC. The charge is compressed till the spark occurs. The spark is produced 20-40° before TDC. It gives sufficient time for the fuel to burn. Both pressure and temperature increase. The burnt gases are expanded till the exhaust valve opens.

The exhaust valve opens 30-60° before BOC. The exhaust gases are forced out from the cylinder till the exhaust valve closes. The exhaust valve closes 8-20° after TOC. Before closing, the inlet valve again opens 10-30° before TOC. If the piston is positioned between fourth stroke (exhaust) and the first stroke (intake) at TOC, both valves open. The period between NO and EVC is known as valve overlap period. The angle of valve overlap is the sum of the angle of opening the intake valve before TOC and closing of the exhaust valve after TOC. In Figure 1.73, the angle of overlap is specified as 10°.