Pre-Ignition and Detonation

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Two terms are regularly confused; "pre-ignition" and "detonation".

Simply put, pre-ignition is anytime ignition occurs before the spark plug fires.

Detonation is a different animal. When fuel is ignited with a spark, a flame front and a pressure wave begins to move across the face of the piston. As the fuel and air burn, heat and cylinder pressures increase, and the remaining unburned fuel/air mixture becomes unstable. If the octane is low enough, the remaining fuel can auto ignite. That is, light off BEFORE the first flame front reaches it. Detonation causes a very high, short duration pressure spike. That pressure spike resonates throughout the engine, and the resonation is audible. It's a pinging sound.

Both pre-ignition and detonation can destroy an engine. Pre-ignition is the worse of the two because the damage occurs quickly. The damage can include a holed piston and/or a bent rod.

Engines can usually tolerate a little detonation. Knock sensors help somewhat. Knock sensors are piezoelectric devices that send a signal to the ECU. That signal indicates detonation has occurred. After receiving the signal, the ECU retards the ignition timing for a period of time. After that period of time, the ECU attempts to gradually bring the ignition timing back to factory specs. If detonation is encountered again, the ECU will retard the timing again. This scenario will continue to repeat itself until the underlying reason for the detonation is corrected.

Although knock sensors are great at determining detonation, they aren't a remedy. First, detonation has to occur before the sensor can react. Secondly, there's the problem of driving around with the ignition timing retarded. Engines with retarded ignition have higher than usual cylinder head temperatures. The high cylinder head temperatures have been known to cause the NOx numbers to raise the point where the car fails smog. That's one reason why some people claim they can pass smog by changing gasoline. In addition to high NOx numbers, increased cylinder head temperatures typically cause increased valve guide wear.

After an engine has been disassembled, the effects of detonation are obvious. In a worst case scenario, detonation will result in a broken ring land. It's common for the piston crown and cylinder head to appear as if they were sand blasted. The uneven cylinder head surface will include the valve seats and the valves. This results in a poor seal, requiring a valve job to get things back to normal.

Now that you know what it is and what it does, you may be wondering what causes it.

Pre-ignition is commonly caused by excessive carbon in the combustion chamber, running plugs that are too hot, a broken EGR, or severe overheating. Detonation is ordinarily caused by low octane fuel.

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