FAA Powerplant Written Practice Test

What can cause detonation in a piston engine?

1. Too cool a temperature of the combustion chamber

2. Excessive carbon build-up in the cylinders

3. High pressure and high temperature within the combustion chamber

4. Improper fuel octane rating

The correct answer is: High pressure and high temperature within the combustion chamber

Detonation in a piston engine occurs primarily due to high pressure and high temperature conditions within the combustion chamber. When the air-fuel mixture is compressed to a point where the temperature and pressure rise significantly, it can ignite spontaneously before the spark plug fires. This leads to a rapid and uncontrolled combustion event, which is what we refer to as detonation. Factors contributing to these high pressure and temperature conditions might include overly aggressive ignition timing and high levels of compression, but the essence of detonation arises from the physical state of the gases being at a temperature and pressure that facilitate auto-ignition. Thus, understanding the relationship between pressure, temperature, and the combustion process is crucial for managing and preventing detonation in piston engines. The other options discuss various aspects of engine operation, but they do not directly relate to the core cause of detonation. While excessive carbon build-up can influence combustion characteristics and an improper octane rating can lead to pre-ignition, they are not the primary causes of detonation itself. Similarly, having too cool a combustion chamber temperature is generally unlikely to lead to detonation, as cooler temperatures actually reduce the likelihood of such spontaneous ignition.