Valve Timing Diagram of Two Stroke Engine

                   Valve Timing Diagram of Two Stroke Engine

THEORETICAL VALVE TIMING DIAGRAM

 

In theoretical valve timing diagram for a two-stroke engine, the fuel is fired at A and the expansion of gases takes place from A to B. The crankshaft revolves through approximately 120º and the piston moves from TDC towards BDC. At B both the valves open and suction and exhaust takes place from B to C. The crankshaft revolves through approximately 120º and the piston moves from first to BDC and then  little upwards. At C, both the valves close and compression takes place from C to A. The crankshaft revolves through approximately 120º and the piston moves to TDC.

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.

 

In actual valve timing diagram the expansion of charge starts as the piston moves from TDC towards BDC. The exhaust port opens before the piston reaches the BDC and the burnt gases start leaving the cylinder. After a small fraction of the crank revolution, the transfer port also opens and the fresh fuel-air mixture enters into the engine cylinder. This is done as the fresh incoming charge helps in pushing out the burnt gases. Now the piston reaches BDC and then starts moving upwards. As the crank moves a little beyond BDC, first the transfer port closes and then the exhaust port also closes. This is done to suck the fresh charge through the transfer port and to exhaust the burnt gases through the exhaust port simultaneously. Now the charge is compressed with both ports closed, and then ignited with the help of a spark plug before the end of compression stroke. This is done as the charge requires some time to ignite. By the time the piston reaches TDC, the burnt gases push the piston downwards with full  force and expansion of the burnt gases takes place.