FAA Powerplant Written Practice Test

Why are cam-ground pistons used in some aircraft engines?

1. To improve oil circulation

2. To provide a better fit at operating temperatures

3. To enhance combustion efficiency

4. To simplify installation

The correct answer is: To provide a better fit at operating temperatures

Cam-ground pistons are designed with a specific shape that allows for a better fit at operating temperatures. As the piston heats up during operation, it expands. A cam-ground piston accounts for this expansion by having an elliptical cross-section instead of a perfectly circular one. This shape enables the piston to maintain an effective radial clearance in the cylinder, optimizing the fit as it expands. This engineering feature minimizes the risk of piston scuffing against the cylinder wall, as well as reduces the wear and tear that can occur in high-temperature operating conditions. The enhanced fit not only contributes to efficient engine performance but also helps in controlling oil consumption and reducing emissions by ensuring that the piston maintains its proper alignment and seal within the cylinder throughout its operational range. The other options, while suggesting various benefits, do not correctly highlight the primary purpose of cam-ground pistons. For instance, improving oil circulation is more related to the design of the lubrication system itself, while combustion efficiency is predominantly influenced by fuel-air mixture and ignition characteristics. Simplifying installation does not directly correspond to the cam-ground design but instead refers to broader compatibility and designs in engine architecture.