FAA Powerplant Written Practice Test

What is the main consequence of grinding the valves of a reciprocating engine to a feather head edge?

1. Reduced fuel efficiency

2. Increased horsepower

3. Pre-ignition and burned valves

4. Improved cooling

The correct answer is: Pre-ignition and burned valves

Grinding the valves of a reciprocating engine to a feather edge can lead to pre-ignition and burned valves. When valves are excessively ground, their sealing surfaces become thinner and more susceptible to overheating. This condition can result in improper valve seating, which affects the combustion chamber's ability to maintain the correct pressures and temperatures needed for efficient operation. As the valve heats up, especially under high-stress operating conditions, it may not be able to dissipate heat effectively due to its reduced mass. This can lead to a situation where the valve doesn't close properly, allowing fuel-air mixture to ignite before the spark plug fires, resulting in pre-ignition. Moreover, a valve that has been ground down too much can become thinner to the point that it fails or warps, leading to burnt edges and eventual valve failure. Understanding this consequence is crucial for engine maintenance and performance, as ensuring proper grinding techniques are employed is essential for the longevity and efficient operation of the engine.