12 / 20

In relation to the combustion process within a piston engine, what is meant by 'detonation'?

  • A

    When fuel ignites too late, due to retarded timing

  • B

    The normal burning of fuel and oxygen in the cylinders

  • C

    Rough running if the mixture is leaned slightly too far with cruise power set

  • D

    An uncontrolled and explosive burning of fuel, causing damage

Refer to figure.
For the best results, the fuel air mixture should be ignited at the right moment to ensure maximum energy extraction from the fuel, and transmission into mechanical energy to rotate the engine and propeller. This relies on a slow, controlled burn of the fuel after ignition.

Detonation describes when the fuel is not combusted in a controlled manner, and instead burns rapidly leading to a violent, uneven and uncontrolled increase in pressure within the cylinder. The mixture essentially explodes, instead of being burned in a controlled manner. The high pressures and temperatures can cause damage to the engine, eventually leading to failure of the piston crowns.

Detonation is also referred to as "knocking" due to the sound the engine makes when detonation occurs inside the cylinders. Detonation can be caused by a few different things, including:

  • Using a fuel with poor anti-knock properties (too low an octane rating)
  • Weak mixture
  • Hot mixture or engine temperature

The inclusion of lead in aviation fuel is mainly to improve resistance to knocking and detonation.

Your Notes (not visible to others)



This question has appeared on the real examination, you can find the related countries below.

Keyboard Shortcut Use Keyboard Shortcuts