FAA Powerplant Written Practice Test

What does low compression in an engine cylinder often suggest about the engine's condition?

1. A malfunction in the fuel system.

2. Possible valve seat leakage or worn piston rings.

3. A clean and optimal running condition.

4. Improper throttle position.

The correct answer is: Possible valve seat leakage or worn piston rings.

Low compression in an engine cylinder is typically indicative of mechanical wear or failure within the engine components, particularly relating to the sealing surfaces within the cylinder. One of the primary causes of low compression is valve seat leakage, which occurs when the valves do not seal properly against their seats, allowing the compressed air-fuel mixture to escape during the compression stroke. This can lead to insufficient pressure for combustion, resulting in poor engine performance. Additionally, worn piston rings can also contribute to low compression. The piston rings are critical for sealing the combustion chamber and maintaining pressure. If these rings are worn, they can fail to provide an adequate seal against the cylinder walls, allowing combustion gases to escape into the crankcase, which reduces the overall compression within the cylinder. This understanding of low compression aligns significantly with diagnosing issues in aircraft engines, as maintaining proper compression levels is essential for efficient operation and performance.