Detonation occurs when air/fuel mixture in the cylinder has been ignited by the spark plug and the smooth burning is interrupted by the unburned mixture in the combustion chamber exploding before the flame front can reach it.
Detonation is also called knocking, pinking or pinging.
The fuel to air mixture is normally ignited slightly before the point of maximum compression to allow a small time for the flame-front of the burning fuel to expand throughout the mixture so that maximum pressure occurs at the optimum point. The flame-front moves at roughly 33.5 m/s during normal combustion. It is only when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time that the detonation occurs. It is caused by an instantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinder pressure rises dramatically beyond its design limits and if allowed to persist detonation will damage or destroy engine parts.
Detonation can be prevented by:
- The use of a fuel with higher octane rating
- The addition of octane-increasing additives.
- Increasing the amount of fuel injected/inducted (resulting in lower Air to Fuel Ratio)
- Reduction of cylinder pressure by increasing the engine revolutions (lower gear), decreasing the manifold pressure (throttle opening) or reducing the load on the engine, or any combination.
- Reduction of charge (in-cylinder) temperatures (such as through cooling, water injection or compression ratio reduction).
- Retardation of spark plug ignition.
- Improved combustion chamber design that concentrates mixture near the spark plug and generates high turbulence to promote fast even burning.
- Use of a spark plug of colder heat range in cases where the spark plug insulator has become a source of pre-ignition leading to detonation.
Correct ignition timing is essential for optimum engine performance and fuel efficiency. Modern car engines have sensors that can detect knock and retard (delay) the ignition (spark plug firing) to prevent it, allowing engines to safely use petrol of below-design octane rating, with the consequence of reduced power and efficiency.
A knock sensor consists of a small piezoelectric microphone, on the engine block, connected to the engine's ECU. Spectral analysis is used to detect the trademark frequency produced by detonation at various RPM. When detonation is detected the ignition timing is retarded, reducing the knocking and protecting the engine.