FAA Powerplant Written Practice Test

Which of the following factors will decrease volumetric efficiency in a reciprocating engine?

1. Excessive fuel pressure

2. Improper valve timing

3. High RPM operation

4. Low ambient temperatures

The correct answer is: Improper valve timing

Improper valve timing negatively impacts volumetric efficiency in a reciprocating engine because it affects the engine's ability to intake air-fuel mixture efficiently. Proper valve timing ensures that the intake valve opens at the right moment to allow the maximum amount of air-fuel mixture into the combustion chamber during the intake stroke. If the timing is off, the intake valve may open too late or close too soon, which hampers the engine's ability to fill the cylinder completely. This results in reduced air density in the cylinder and can lead to a decrease in power output. In contrast, excessive fuel pressure does not significantly hinder the volumetric efficiency as it primarily affects the distribution of fuel rather than the volume of air intake. High RPM operation can sometimes enhance volumetric efficiency due to increased airflow, while low ambient temperatures could actually improve air density, potentially benefiting volumetric efficiency as well. Therefore, improper valve timing is a critical factor when assessing volumetric efficiency in a reciprocating engine.