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.