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Working of Valves in Four stroke S.I Engine and Valve Timing Diagram




 The exact moment at which each of  the valves open and close with reference to the position of  the  piston and crank can be shown graphically in a diagram. This diagram is known as  " Valve timing   Diagram".

 Theoretical Valve Timing  Diagram :



 In Theoretical valve timing diagram, inlet and exhaust valves open and  close at both the dead centers. Similarly, all the processes are sharply completed at the TDC or BDC. shows a theoretical valve timing diagram for the four stroke S.I Engine. 

 Actual Valve Timing Diagram : 




 Actual valve timing Diagram for four stroke S.I Engine. The inlet valve open 10* - 30* before the TDC. The air - fuel mixture is sucked into the cylinder till the inlet valves closes. The inlet valve closes 30 - 40* or even 60* after the BDC. The charge is compressed till the spark occurs. The Spark is produced 20* - 40* before the TDC. It gives sufficient time for the fuel to burn. The pressure and Temperature increase. The burnt gases are expanded till the exhaust valve opens 

The Exhaust Valve opens 30* - 60*  before the BDC. The exhaust gases are forced out from the cylinder till the exhaust valve closes. The exhaust valve closes 8 - 20* after the TDC. Before closing, again the inlet valve opens 10 - 30 * before the TDC. The period between the IVO and EVC is known as valve overlap period. The angle between the inlet valve opening and exhaust valve closing is known as angle of valve overlap.   

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