Detonation and pre-ignition

Detonation (knock)

Uncontrolled, abrupt combustion of the unburnt charge ahead of the normal flame front. The pressure wave produces the characteristic knock sound (often inaudible in aircraft cockpits but visible on instruments).

Causes:

• Octane too low for the engine
• Excessive cylinder pressure (high manifold pressure on constant-speed engines)
• Excessive cylinder temperature (CHT)
• Too-lean mixture at high power
• High OAT

Symptoms:

• High CHT (often the only cockpit indication)
• Loss of power
• Rough running
• Potential engine damage (piston, rod, cylinder head)

Actions:

1. Enrich mixture (more fuel cools)
2. Reduce power (throttle/pitch back)
3. Monitor CHT and land if persisting

Pre-ignition

Self-ignition of the mixture before the regular spark, triggered by a hot spot in the cylinder:

• Glowing carbon deposits (coking on piston or spark plug)
• Glowing exhaust valve
• Defective spark plug whose electrode runs abnormally hot

Pre-ignition is often the consequence of repeated detonation but can occur in isolation. Symptoms similar (high CHT, loss of power); diagnosis differs.

Correct fuel grade

Avoid knock by using the correct grade of avgas:

Never use mogas unless the AFM or an STC explicitly authorises it. Mogas has a different vapour-pressure profile (vapour lock!) and different additives (ethanol can damage seals).