Valve
Timing

The opening and closing of the exhaust and inlet valve
overlap each other. The following figure shows the overlapping of the two
valves.
Figure 3. Intake and Exhaust Valve Timing
The
inlet valve begins to open before top dead center so that there will be a large
effective area, Ae, at the start of induction. The exhaust valve
begins to open before bottom dead center to give the combustion products time to
escape
The speed of the engine determines how early the exhaust
valve opens. At low RPM's the valve opens early and at high RPM's the valve
opens late.