Detonation and pre-ignition

Learning objective

You can separate detonation / knock (uncontrolled end-gas burn after the spark, ahead of the normal flame front) from pre-ignition (ignition at a hot spot before the spark) and name cockpit actions without invented CHT limits.

60 seconds

Detonation = the remaining mixture explodes. Pre-ignition = a glow spot fires too early. Both: high CHT, power loss. First teaching: enrich, reduce power, watch CHT — limits AFM. Octane and grade belong with the fuel-grades lesson.

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).

Exam knowledge

• Detonation ≠ pre-ignition.
• Detonation: end-gas after the spark, abrupt; favoured by low octane, high pressure, high CHT, too lean at high load.
• Pre-ignition: hot spot before the spark.
• Cockpit: enrich, reduce power; numbers AFM.
• Mogas only with AFM/STC (knock and vapour pressure — other lesson).

Check yourself

1. What is detonation in a piston engine?

• A) Auto-ignition at a glowing carbon spot before the spark
• B) The normal two-flame-front burn from dual plugs
• C) A delayed impulse-coupling spark
• D) Uncontrolled, explosive burn of the remaining mixture ahead of the flame front

2. What triggers pre-ignition?

• A) Low octane alone, with no hot spot
• B) A hot spot in the cylinder (carbon, glowing exhaust valve, overhot plug) before the spark
• C) High MAP on CSU engines only
• D) Water in the fuel

1. D — detonation is end-gas knock. A is pre-ignition.
2. B — hot spot before the spark. Octane/MAP drive detonation, not the definition of pre-ignition.