Excessive valve clearance in a piston engine results in what effect?

Excessive valve clearance in a piston engine leads to a condition where the valves do not close properly, which can result in decreased valve overlap. Valve overlap refers to the period in the combustion cycle when both the intake and exhaust valves are open simultaneously. When excessive clearance is present, the points at which the valves open and close are altered, often causing them to close later than designed. This timing change decreases the amount of time both valves are open at the same time, thus diminishing valve overlap.

This can negatively impact the engine's performance, as proper valve overlap is crucial for efficient scavenging of exhaust gases and maximizing the intake of fresh air-fuel mixture. A balance must be struck with valve clearance to ensure optimal engine operation, and excessive clearance disrupts this balance by reducing the effective overlap between the intake and exhaust strokes.