A CFI is explaining to an advanced student the difference between preignition and detonation. What is preignition primarily caused by?
- AHigh manifold pressure combined with low engine RPM
- BAn excessively lean mixture at high power settings
- CA hot spot in the combustion chamber, such as a glowing carbon deposit, igniting the mixture prematurely
- DUse of fuel with a lower octane rating than specified
Show answer & explanationAnswer & explanation
Correct answer: C. A hot spot in the combustion chamber, such as a glowing carbon deposit, igniting the mixture prematurely
Preignition occurs when the fuel-air mixture ignites before the timed spark, typically due to a hot spot such as a glowing carbon deposit or overheated exhaust valve. This differs from detonation, which results from excessive heat and pressure causing spontaneous, uncontrolled combustion after normal ignition. Preignition raises cylinder head temperature and reduces power, and it can occur at any power setting, whereas detonation is closely tied to mixture and power settings.
Why the other options are wrong
- A. High manifold pressure with low RPM is a detonation-related condition, not the primary cause of preignition.
- B. Excessively lean mixture at high power is a classic cause of detonation, not preignition.
- D. Low-octane fuel promotes detonation by lowering the fuel's resistance to knock, not preignition specifically.
Preignition
Ignition of the fuel-air mixture before the scheduled spark timing, usually caused by a hot spot in the combustion chamber such as a glowing carbon deposit or overheated exhaust valve.
- Can occur at any power setting, unlike detonation
- Causes power loss, engine roughness, and elevated CHT
- Can lead to detonation due to added heat and pressure
Memory trick: PREignition = premature spark from a hot spot, not from lean mixture.