When a cylinder reaches the end of its exhaust stroke, with the exhaust valve(s) open and the piston at top dead centre, there will still be some exhaust gas left in the cylinder. If the valve remains open exhaust gas will continue to flow out of the exhaust port due to its own momentum even after the piston has started to descend on the induction stroke. Keeping the valve open results in better scavenging of exhaust gas.
The opposite effect occurs on the intake side. When the intake valve is opened the air in the intake system is essentially static, and it takes a measurable time to begin flowing into the cylinder. If the valve is opened early, before the piston has reached top dead centre, extra time is allowed for the air to begin flowing in and ultimately more air is admitted during the induction stroke.
Because the intake valves open early (before top dead centre on the induction stroke) and the exhaust valves stay open late (after top dead centre on the induction stroke) there is a period where both valves are open together. This period is known as the valve overlap, and can be expressed in degrees of crankshaft rotation or degrees of camshaft rotation.
The effects of inertia on the intake and exhaust gases vary little with engine speed, which means they take up a greater and greater angle of engine rotation as engine speed rises. Engines optimised for higher rotational speeds will generally have greater valve overlap. However, excessive valve overlap compromises low-speed running.
Variable valve timing systems like Porsche's Variocam and BMW's Vanos vary valve overlap as well as the timing, optimising the valve events for a wider range of engine speeds.
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