Theoretically the valve should open and close at the end or start of the stroke. But in actual practice it is not desirable to do so in order to obtain better results. The valves are generally opened earlier and closed later .
Valve timing Diagram comprises of a 360 degrees figure which represent the movement of the piston from Top dead centre to bottom dead centre . In all the stroke of the engine cycle , which is measured in degrees and the opening and closing of the valves is controlled according to these degrees.
Starting from the suction stroke , the intake valve is opened about 10 ° to 30 ° begore the TDC . Exhaust valve is also open at this movement . By opening the intake valve earlier, more chances to fresh charge coming in are made . As fresh chagre move in , it helps in expelling the exhaust gases also from the chamber. 
Fig. (a) Inlet valve open, inlet valve close
The inlet valve does not close at B.D.C. , but is closed at after it . It generally closes 30° to 40 ° after B.D.C. by doing so , a large amount of fresh chagre is ensured, as the charge during suction stroke was get going in and compression by piston has not yet been able to push out the charge.
During compression , the spark is ignited before the T.D.C. in case of otto . This is about 10 ° before T.D.C. or tge fuel is injected about 5 to 10 ° before T.D.C. . This is done so , because the whole of the fuel can't burn instantaneously. Fuel also has some delay period , ( i.e. period between spark and actual ignition ). Also time is required for the spark to travel and spread throughout the cylinder. 
In expansion stroke , the exhaust valve opens about 30 to 40 ° before B.D.C. and closes about 10 ° after T.D.C. . Thus the period of exhaust is increased and complete and effective scavenging is ensured. If this is not done , then residual combustible mixture results in the dilution of fresh chagre.
Fig . ( c ) expansion stroke
No comments:
Post a Comment