Valve Timing of Engine

Valve Timing

Let's show piston motion as a circle. Each stroke is shown as semi circle. Intake valve open at Top Dead Centre (TDC) and closes at Bottom Dead Centre (BDC). Exhaust  valve open at Bottom Dead Centre (BDC) and closes at Top Dead Centre (TDC). A few mixture induce the cylinder. In practice the intake valve usually open early before the piston reaches top dead centre and stage open a little first bottom dead centre. The exhaust valve opens a little first before the piston reaches bottom dead centre and stage open a little of the top dead centre. Valves actually open and close can't imagine by angles. To make this angles easy to read.

Fig : Valve Timing Diagram

 Let's use spiral instead of a circle. This intake valve open 12° before the piston reaches top dead centre and closes 40°  after bottom dead centre. The exhaust valve opens 47° before bottom dead centre and stage open until 21° first top dead centre. This gives exhaust gases more time to leave. By the time the piston is 47° before bottom dead centre on the power stroke. A little power is lost by leaking the exhaust gases have more time to exit.

 

When an intake valve opens before top dead centre and exhaust valve opens before bottom dead centre, it is called LEAD. When an intake valve closes after bottom dead  centre and exhaust valve closes after top dead centre, it is called LAG. Before the end of exhaust stroke the intake and exhaust  valves are open at the same time for few degrees around top dead centre. This is called Valve Overlap.

Various Engines use different valve timing. Manufacturers specification contain the exact information.