FAA Powerplant Written Practice Test

What effect does excessive valve clearance have on the duration of valve opening?

1. Decrease for both intake and exhaust valves

2. Increase for both intake and exhaust valves

3. Remain the same for both valves

4. Only affect the exhaust valve

The correct answer is: Decrease for both intake and exhaust valves

Excessive valve clearance refers to the distance between the valve stem and the rocker arm when the valve is in its closed position. When the clearance is too great, it causes the valve to remain closed longer than it should, which leads to a decrease in the duration of valve opening. Specifically, during the engine's operation, when the valve clearance is excessive, the rocker arm has to move further to achieve the same lift at the valve due to the increased gap. This may result in the valve not opening fully or not opening for the expected duration during the engine cycle. As a result, both the intake and exhaust valves would have shortened periods of opening, affecting engine performance by limiting airflow into and out of the combustion chamber. Hence, the correct answer indicates that both the intake and exhaust valves experience a decrease in the duration of their opening when excessive clearance is present.