FAA Ground School Practice Test

What causes detonation in a reciprocating aircraft engine?

1. The fuel burns progressively and evenly

2. The unburned charge in the cylinders explodes

3. The fuel ignites slowly

4. The engine runs too cool

The correct answer is: The unburned charge in the cylinders explodes

Detonation in a reciprocating aircraft engine occurs when the unburned mixture in the cylinders ignites suddenly and explosively rather than burning progressively and evenly. This phenomenon can lead to a rapid rise in pressure within the cylinder, potentially causing significant engine damage. In a normal combustion process, the fuel-air mixture ignites as the piston approaches the top of its stroke, leading to smooth combustion. However, if conditions such as excessive combustion chamber temperatures or improper fuel octane are present, the mixture can ignite too early or an unburned part of the mixture can detonate. Such scenarios result in the “explosive” characteristic of detonation, as opposed to the controlled burn that typically occurs. The other options describe situations that do not lead to this explosive combustion. For instance, progressive and even burning of fuel indicates normal combustion without detonation, while slow ignition does not contribute to the sudden pressure surge that characterizes detonation. Running the engine too cool is also not a direct cause of detonation; in fact, an engine that is too cool might have other performance issues, but it doesn't contribute to the explosive characteristics of detonation.