Pre-Ignition Theory - Cause & Effect

What is the difference between detonation and pre-ignition?


Detonation and Pre-ignition have been around since the combustion engine was first invented. Pre-ignition was discovered first but it was probably detonation that was taking place. The idea of detonation was around for a long time before it was proved to exist. The main differences to remember between detonation and pre-ignition are as follows, detonation takes place after the spark plug has ignited the air and fuel in the combustion chamber. Pre-ignition on the other hand occurs before the spark plug has ignited the air and fuel in the combustion chamber. The causes of these two phenomenons are different and we will look into them both a little closer. The results of these occurring in the engine are similar, both can cause catastrophic engine failure. Please read my previous article on detonation theory as that will help you to understand the differences with pre-ignition. Spark timing must be correctly set up, high compression ratio should be taken into account, inlet air temperature can get too hot, advancing the timing too far are to name just a few detonation promoters.


Typical damage from detonation

Piston showing detonation damage.
Piston showing detonation damage.

Typical damage from pre-ignition.

Pre-Ignition burnt straight through the piston crown.
Pre-Ignition burnt straight through the piston crown.

When does pre-ignition occur?


Pre-ignition occurs before the timed spark, pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. Pre-ignition can be caused by spark plugs with the wrong heat range or a glowing piece of carbon could ignite the air/fuel mixture before the spark plug fires, also but not so common is a burning exhaust valve. On a four-cycle engine we have the intake stroke, compression stroke, work stroke and exhaust stroke, these are our four-cycles. When looking for pre-ignition we need to look around the time that the intake stroke finishes and the compression stroke begins. The air and fuel enters the combustion chamber as the piston reaches Bottom Dead Centre on the intake stroke. The piston then begins to move upwards and starts to compress the air and fuel in the combustion chamber. The spark voltage requirements are determined at the firing tip so to light the air and fuel these requirements increase with the amount of air, fuel compression. So, almost anything can ignite the air/fuel mixture at Bottom Dead Centre, most likely being a burning ember. Around Bottom Dead Centre is the easiest time to light that mixture. It becomes increasingly difficult as the pressure starts to build. A glowing ember somewhere in the combustion chamber is the most likely point for pre-ignition to occur. It is possible, that if you have something glowing like a spark plug tip or a carbon ember, it could ignite the charge while the piston is very early in the compression stoke. For a large portion of the compression stroke, the engine is trying to compress burning air and fuel, also remember that these gasses expand when combusted. When this occurs it puts huge stress on the engine and heats up some engine parts to the point that they begin to melt. If this happens, damage occurs very quickly. You can't hear it because we are too far away from Top Dead Centre for the pressure spike to cause the liner walls to vibrate like detonation. This all occurs well before the spark plug fires.


What causes pre-ignition?

Spark plugs with the incorrect heat range can cause pre-ignition. Using spark plugs that are too hot for the application can cause pre-ignition. A spark plug is said to be "hot" if it is a better heat insulator. Better insulated spark plugs keep more heat in the tip of the spark plug, this causes pre-ignition if used on the wrong application. If you were suffering from detonation the heat from the abnormal combustion could cause the spark plug to glow and start pre-ignition. Now, when we looked at piston damage from detonation we could see melting damage around the edges of the piston crown. With pre-ignition from a glowing spark plug pre-ignition damage will show up as melting in the middle of the piston crown. The typical pre-ignition indicator is a hole in the middle of the piston crown, as seen in the picture above. This occurs because in trying to compress the already burned mixture the parts soak up heat very quickly. The parts that survive are the ones that have high thermal inertia, like the cylinder head or cylinder liner. The piston being aluminium has low thermal inertia (aluminium soaks up heat very rapidly). The crown of the piston is thin and can get very hot. When it can't reject the heat and has large pressure loads against it the result is a hole in the middle of the piston.

Burning valves can also cause pre-ignition. Incorrect valve adjustment whereby the valve doesn’t make proper contact with the seat to transfer heat away could be one reason for a burnt valve. The valve overheats and literally melts in the extreme heat. Melted pieces from the valve drop into the combustion chamber and can cause pre-ignition. Other reasons for burnt valves could be incorrect timing or the wrong air fuel ratio. The intake valve can also suffer from pre-ignition when it occurs early enough in the stroke.



Will pre-ignition damage my engine?

So, we now know that pre-ignition occurs before the timed spark but when in the compression stroke will it occur? It is probably much more dangerous when just starting the compression stroke because the piston will be compressing very hot gasses when they are normally far cooler. The piston will not have time to cool and pre-ignition will burn a whole straight through the piston crown. The other worrying aspect is you wouldn't know about it until its too late. It would only take a few strokes of pre-ignition to severely damage the engine. If pre-ignition occurred just before the timed spark, then we would here a noise like detonation because there would be two flame fronts meeting and colliding together in the combustion chamber. So, in this case of pre-ignition we would be able to identify the problem by sound.

Both pre-ignition and detonation can cause catastrophic damage to an engine, feel free to read my previous article for a more in depth look at detonation and the damage it can do to an engine.

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