FAA Powerplant Written Practice Test

What is the effect of excessive valve clearance in a piston engine?

1. Increases valve overlap

2. Decreases valve overlap

3. Enhances fuel mixing

4. Increases engine noise

The correct answer is: Decreases valve overlap

Excessive valve clearance in a piston engine is primarily characterized by a gap between the valve and its seat that is wider than specified. This condition directly interferes with the timing and duration that valves are open during the engine cycle. When valve clearance is excessive, valves close later and open sooner than intended, which effectively reduces the amount of time they are in contact with their seats. This premature opening of the intake valves and delayed closing of both intake and exhaust valves results in decreased valve overlap. Valve overlap is the period in the engine cycle when both the intake and exhaust valves are open simultaneously, allowing for better scavenging of exhaust gases and improved intake of the air-fuel mixture. Therefore, excessive clearance disrupts this synchronization, ultimately diminishing the overlap period. In addition to changes in valve timing, excessive clearance typically results in mechanical noise due to the increased amount of play in the valve train components. Many engines are designed with specific tolerances, and when those tolerances are exceeded, it can lead to additional wear and tear and potentially harm engine operation. Therefore, while it does lead to increased engine noise, the more immediate and critical effect is the decrease in valve overlap, making the reduction in effective engine performance a more pressing concern.