FAA Powerplant Written Practice Test

Which factor would lead to a decrease in volumetric efficiency in a reciprocating engine?

1. Improper valve timing

2. Correct air-fuel mixture

3. Optimum engine temperature

4. High compression ratio

The correct answer is: Improper valve timing

Improper valve timing is a key factor that can significantly lead to a decrease in volumetric efficiency in a reciprocating engine. Volumetric efficiency refers to the engine's ability to fill its cylinders with air (and fuel) during the intake cycle. When the timing of the opening and closing of the intake and exhaust valves is not aligned properly with the engine’s cycle, it can result in a loss of airflow into the cylinders. If the intake valve opens too late or closes too early, or if the exhaust valve opens too soon or closes too late, the engine will not have sufficient time to draw in the maximum volume of air-fuel mixture. This premature or delayed timing can create a scenario where some of the intake charge is lost during the scavenging process or where intake pressure may not be optimized, both of which reduce the amount of air and fuel that actually enters the cylinder. As a result, the combustion process becomes inefficient, leading to a drop in performance. In contrast, factors such as a correct air-fuel mixture, optimum engine temperature, and high compression ratio are all conditions that can either maintain or improve the engine's volumetric efficiency. A proper air-fuel mixture ensures complete combustion, optimal engine temperature aids in efficient combustion and vapor