When does maximum valve overlap occur in an engine?

Maximum valve overlap occurs at top dead center during the exhaust stroke. This is the moment when both the intake and exhaust valves are open simultaneously, allowing for the maximum exchange of exhaust gases from the combustion chamber and the intake of fresh air-fuel mixture into the cylinder for the next cycle.

During this phase, the engine takes advantage of the negative pressure created by the exiting exhaust gases to help draw in the incoming mixture, enhancing the scavenging process. This overlap is crucial for improving the engine's performance and efficiency by ensuring that the combustion chamber is cleared of exhaust gases before the intake process begins anew.

The specific timing at top dead center is critical because it represents a moment when the piston is at its highest point in the cylinder, and the geometry of the engine allows for the maximum amount of both gases to flow due to pressure differences. This is essential for achieving optimal engine function and performance.