Actual Valve Timing Diagram For 2 Stroke Cycle And 4 Stroke Cycle Cl Engine.

Valve Timing Diagram for Two-stroke Cycle

Firstly, the exhaust port opens as the piston reaches BDC(Bottom Dead Center ) during the expansion and the burnt gases start leaving the cylinder. After a small fraction of crank revolution, the transfer port opens and the fresh air enters into the cylinder. This is done to push the burnt gases by fresh air. 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 air and exhaust the burnt gases simultaneously. 

Now the charge is compressed and the fuel valve is opened before the piston reaches TDC (Top Dead Center). Fuel is injected in the form of spray and ignited due to the temperature of compressed air. The fuel valve closes after the piston has come down a little from the TDC. Now the burnt gases push the piston downward with full force and expansion of gases takes place. And again the above-said cycle continues. In this cycle, the exhaust and transfer ports open and close at equal angles on either side of the BDC position.

Valve Timing Diagram for Four-stroke Cycle

 engine. We see that the inlet valve opens before the beginning of the suction stroke. After reaching TDC the suction stroke starts. The piston reaches BDC and then starts moving up. When the crank moves a little beyond the BDC, the inlet valve closes. This is done as the incoming air continues to flow into the cylinder although the piston is moving upward from BDC.

Now the air is compressed with both valves closed. The fuel valve opens before the piston reaches TDC. Fuel is injected in the form of fine spray and gets ignited due to the high temperature of compressed air. The fuel valve closes after the piston has come down a little from TDC. Burnt gases push the piston downward and expansion takes place. Now exhaust valve opens before the piston again reaches BDC and the burnt gases start leaving the engine cylinder. The inlet valve opens before TDC to start a suction stroke. This is done as the fresh air helps in removing the burnt gases, and again the process continues.