What does the term "valve overlap" refer to in engine timing?

The term "valve overlap" specifically refers to the period when both the intake and exhaust valves are open at the same time during the engine's cycle. This overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke. During this phase, some remaining exhaust gases are expelled while fresh air-fuel mixture is being drawn into the cylinder. This design helps improve engine efficiency and can enhance performance by allowing better scavenging of exhaust gases and optimizing the filling of the combustion chamber with the air-fuel mixture. Such timing is critical for high-performance engines, where maximizing air intake and improving exhaust flow can lead to better power output.