“I got pre-detonation in my engine and seized a piston.”

Preignition and detonation are separate and distinct phenomena.

One hundred and ten years ago (1916)English internal combustion engine pioneer Harry Ricardo used a high-speed pressure indicator to better understand combustion. The pressure-vs-volume curve for normal combustion shows compression raising cylinder pressure. The spark occurs as the piston approaches top dead center (TDC) and a short time later the added pressure of combustion sweeps smoothly up to a peak value at around 10-12 degrees after top dead center (ATDC). As the piston continues its downward power stroke the pressure falls as the hot combustion gas does its work, driving the piston down to spin the crankshaft. There is a sharp pressure drop as the exhaust valve opens.
PREIGNITION

Many of us have seen pistons with a hole through the center of dome, looking like someone used a hydraulic press to force a ½-inch rod down through the center of it. This is the result of something glowing red-hot in the combustion chamber, igniting the air-fuel mixture prematurely, long before the normal ignition spark. This compels the rising piston to compress hot, burning charge. This process quickly heats the piston crown far above its normal operating temperature, weakening the material. Very quickly – in one or two combustion cycles, the center of the crown is punched through by extreme pressure. I learned about this from a GM engineer who had studied it in instrumented engines.


The hole through the center of the piston is the signature of preignition. The causes of preignition are the presence of something red-hot in the combustion chamber, such as the center-wire of a spark plug of too hot a heat range, glowing carbon deposits, or possibly a very hot exhaust valve.

Because preignition quickly destroys the piston, there is no continuing knock, tinkle or ping to provide warning, as there is with detonation.

DETONATION


Harry Ricardo was especially interested in this one because he was involved in designing a tank engine in WW I, when gasoline quality control was almost nonexistent. Many engines today are equipped with detonation detection and prevention equipment, so many people have never heard the tinkle of light detonation or the sound, like stones being struck together underwater, of engine knock.

Ricardo also made Pressure-vs-Volume (P-V) curves for detonating engines. You see the pressure rise of compression, then the much greater pressure rise of combustion that follows the spark. All perfectly normal.

Then, just as combustion is finishing, with the flame front nearly at the cylinder wall, come one or more steep pressure spikes. What is happening is that the last bits of air-fuel mixture to burn have been so heated by compression and by the combustion pressure front that they become chemically altered. They are able to auto-ignite, burning at sonic or supersonic speed. This creates the shock waves that make the ping, tinkle, or knock that identifies detonation.

Because detonation occurs out very near the cylinder wall, the first thing we see is a sand-blasted appearance at the edge of the piston crown. If knocking continues, it may erode the edge of the piston enough to expose the piston rings.



The usual causes of detonation are;

Spark timing is too early

Compression ratio is too high

Something is causing unusual heating of the air-fuel mixture

Anti-knock rating (Octane number) of the fuel is insufficient.

Today engine electronics protect against detonation by detecting its presence, then retarding the ignition timing until it stops. You may have seen photos of people peering into the business end of a spark-plug, using a small lighted magnifier. That was how detonation was detected 40 years ago. If you were lucky, you might see what looked like a tiny flake of gray cigarette ash, clinging to a spark plug electrode. It’s not ash, but a particle of aluminum that has just been blasted out of the edge of your piston by the shock wave of detonation.

DFA

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