Question

At what angle valve is situated in ic and si engine

Answer 1

We consider that the valves open and close at the dead centre positions of the piston. But, in actual practice they do not open and close instantaneously at dead centres. The valves operate some degrees before or after the dead centres. The ignition is also timed to occur a little before the top dead centre. The timings of these sequence of events can be shown graphically in terms of crank angles from dead centre position. This diagram is known as valve timing diagram.

Valve timing diagram for four-stroke petrol engine:

diagram shows the valve timing diagram for a four-stroke cycle petrol engine. The inlet valve opens 10-30° before the top dead centre position. The air-fuel mixture is suked into the engine cylinder till the inlet valve closes. The inlet valve closes 30-40° or even 60° after the bottom dead centre position. The air-fuel mixture is compressed till the spark occurs.

The spark is produced 20-40° before the t.d.c. position. This gives sufficient time for the fuel to burn. The pressure and temperature increases. The burning gases expand and force the piston to do useful work. The burning gases expand till the exhaust valve opens. The exhaust valve opens 30-60° before the b.d.c. position. The exhaust gases are forced out of the cylinder till the exhaust valve closes. The exhaust valve closes 8-20° after the t.d.c. position. Before it closes, again the inlet valve opens 10-30° before the t.d.c. position. The period between the inlet valve opening and exhaust valve closing is known as valve overlap period. The angle between the inlet valve opening and exhaust valve closing is known as angle of valve overlap.

Valve timing diagram for four-stroke diesel engine:

The actual valve timing diagram for four-stroke diesel engine is shown in figure The inlet valve opens 10-25° before the top dead centre position. Fresh air is sucked into the engine cylinder till the inlet valve closes. The inlet valve closes 25-50° after the bottom dead centre position. The air is compressed till the fuel is injected. The fuel injection starts 5-10° before the t.d.c. position in the compression stroke. The air fuel mixture burns. The temperature and pressure increases.

The burning gases expand till the exhaust valve opens. The exhaust valve opens 30-50° before the b.d.c. position. The exhaust gases are forced out of the engine cylinder till the exhaust valve closes. The exhaust valve closes 10-15° after the t.d.c. position. Before the exhaust valve closes, again the inlet valve opens 10-25° before the t.d.c. position. The period between the inlet valve opening the exhaust valve closing is known as valve overlap period. The angle between these two events is known as angle of valve overlap.