There are two major forms of abnormal combustion that can damage a spark-ignition piston engine, and they have entirely different causes and effects. People confuse them because both can destroy pistons and wreck engines, but they occur at quite different times in the engine cycle.
Preignition is just what it sounds like: something abnormally hot in the cylinder ignites the fuel-air mixture before the spark. This, by causing your engine to compress hot burning gases, quickly overheats the part of the piston farthest from any source of cooling—the center of its dome. Typically in one or two cycles (which takes maybe a hundredth of a second), the center of the piston dome overheats, loses strength, and collapses downward. Many a humorist, seeing such a piston, has called it “hole in one.”
You cannot have a little preignition, any more than you can be a bit pregnant. If your engine enters preignition, you will hole a piston almost instantly.
Detonation is entirely different because it occurs after the normal spark, and after 95 percent of combustion has been completed. Tiny bits of the last fuel-air charge still unburned, having been heated excessively, are chemically altered to become a sensitive explosive. Late in combustion, these overheated bits autoignite and burn at the local speed of sound—thousands of feet per second—causing shock waves that reverberate from one side of the combustion chamber to the other. The resulting sound has been called variously “tinkle,” “engine knock,” or “rattle.” To me it often sounded like the strange brittle sound made when, swimming underwater, you knock two rocks together. The shock waves physically scour away the low-energy layer of gas, called the boundary layer, that normally provides useful insulation of the metal surfaces of chamber and piston. This allows more heat to flow into those parts, weakening them and allowing continued shock wave action to erode metal from the outer edges of the piston and cylinder head.
In contrast to preignition, detonation is a matter of degree. Chronic light detonation may do little more to your pistons than give their outer edges a sandblast-like roughness. Heavier detonation—larger packets of unburned mixture autoigniting—causes deeper erosion. Heavy detonation, while pounding the engine’s bearings, blasts weakened aluminum off the outer edges of the piston, eventually exposing the piston rings and destroying their seal.
It was British ICE pioneer Harry Ricardo who in 1916 used an engine indicator (a device that graphs cylinder pressure as a function of crank rotation) to discover that preignition and detonation are very different phenomena.
In preignition, the indicator plot showed cylinder pressure as a partial vacuum near the end of the intake stroke, then beginning to rise as the piston reversed direction to begin its compression stroke. Then without warning, pressure shot upward, rising off-scale. Something had obviously ignited the charge close to bottom center, long before the normal ignition spark.
In detonation, by contrast, pressure continues to rise normally through the compression stroke, and combustion is initiated by the ignition spark thirty-something degrees before top dead center (TDC), cylinder pressure rising rapidly as normal combustion begins. Then as the flame front nears the cylinder wall, jagged spikes of extreme pressure appear, indicating ultra-rapid release of energy from the last bits of the charge to burn.
RELATED: The Basics Of Power
In preignition, early ignition (usually by something like the overheated electrode of a spark plug of too hot a heat range) takes place with the piston close to bottom center, between the end of its intake stroke and the beginning of compression.
In detonation, the piston is at the opposite end of the cylinder (near TDC) when overheated small volumes of unburned fuel-air mixture autoignite and burn with explosive speed and violent pressure rise.
Preignition is caused by the presence of something in the cylinder hot enough to act as an ignition source. Detonation is caused by anything that adds heat to the last small volumes of unburned mixture out near the cylinder wall. High intake air temperature, slow combustion or engine lugging, excessive spark advance, too high an engine temperature, excessive compression ratio, or a fuel of insufficient octane number (which measures its resistance to detonation) are the common causes of detonation.
How does autoignition in detonation take place? The longer the last parts of the charge to burn are heated, the more chemical change occurs within them, leading to a growing population of the active radical hydroxyl (OH-). When that population reaches a critical threshold, a chemical chain reaction is triggered, causing combustion that is sustained not by heating the unburned mixture ahead of the flame, but by the sharp pressure rise in the shock wave itself. This pressure step takes place over a thickness of just a few molecules and is therefore a zone of intense molecular activity, well able to initiate combustion.
I’ve held forth on this subject many times before, but the confusion between preignition and detonation persists, so here is yet another dose.