Walter Atkinson
Well Known Member
There is much confusion over these two phenomena. They are not the same and do not always go together.
First, Detonation is the explosive auto-ignition of the end gasses AFTER the planned spark ignition event. Small pockets of F:A charge auto ignite ahead of the flame front and create a shock wave in the combustion chamber which travels at the speed of sound in that medium. The shock wave bounces back and forth at the specific frequency that matches that speed.
The cause of detonation is the combination of IAT, CHT, and pressure within the combustion chamber. When those factors combine to initate combustion above the retarding effects of the latency period of the fuel, detonation is the result. Contolling CHTs and pressure are the BIG factors in NA engines. Contolling IAT in a TC'd engine thru the use of intercoolers is a major factor in widening the detonation margin. Also, the octane of the fuel and the presence of lead is a major factor.
Surprisingly, detonation, in and of itself, is not terribly destructive. We have run an engine for 30 HOURS of light detonation, 3-4 hours of medium detonation and 30 minutes of heavy detonation with NO ill effects whatsoever. As a matter of fact, the engine was sent to be inspected by an independent third party and the response was, "Why did you want a tear-down inspection, this looks like a normal, healthy engine."
As it turns out, a little, occasional light detonation can actually be good for an engine as it tends to clean out the debries and carbon build up!
The issue is the effect that detonation has on spark plug ceramic integrity.
That leads us to pre-ignition. Pre-ignition is the initiation of the flame front BEFORE the planned spark ignition event. It is a normal flame front, just started too soon. As opposed to detonation where EGT is little changed, pre-ignition results in rapidly falling EGTs and rapidly rising CHTs. The Peak Internal Cylinder Pressures can get very high, very quickly and this can destroy an engine in seconds.
The most common casues of pre-ignition are a failed spark plug ceramic which over heats and acts like a glow plug, a helicoil tang sticking in tothe combustion chamber and on very. very rare occasions, a particle of glowing carbon deposit. The later is exceedingly rare, but is the source most often cited! Go figure?
The most common progression is for a detonation event (from inadequate octane or high CHT or high IAT) to progress to the point where a spark plug ceramic is damaged resulting in pre-ignition which destoys the engine. Detonation is frequently mis-identified as the cause of the destruction.
The comments above are a sumary of the information learned from detonation studies at the most advanced engine test facility in the world, The Carl Goulet Memorial engine Test Facility. FAA experts have come to see this first hand and are adjusting their understanding of these events as a result.
Walter Atkinson
Advanced Pilot Seminars
First, Detonation is the explosive auto-ignition of the end gasses AFTER the planned spark ignition event. Small pockets of F:A charge auto ignite ahead of the flame front and create a shock wave in the combustion chamber which travels at the speed of sound in that medium. The shock wave bounces back and forth at the specific frequency that matches that speed.
The cause of detonation is the combination of IAT, CHT, and pressure within the combustion chamber. When those factors combine to initate combustion above the retarding effects of the latency period of the fuel, detonation is the result. Contolling CHTs and pressure are the BIG factors in NA engines. Contolling IAT in a TC'd engine thru the use of intercoolers is a major factor in widening the detonation margin. Also, the octane of the fuel and the presence of lead is a major factor.
Surprisingly, detonation, in and of itself, is not terribly destructive. We have run an engine for 30 HOURS of light detonation, 3-4 hours of medium detonation and 30 minutes of heavy detonation with NO ill effects whatsoever. As a matter of fact, the engine was sent to be inspected by an independent third party and the response was, "Why did you want a tear-down inspection, this looks like a normal, healthy engine."
As it turns out, a little, occasional light detonation can actually be good for an engine as it tends to clean out the debries and carbon build up!
The issue is the effect that detonation has on spark plug ceramic integrity.
That leads us to pre-ignition. Pre-ignition is the initiation of the flame front BEFORE the planned spark ignition event. It is a normal flame front, just started too soon. As opposed to detonation where EGT is little changed, pre-ignition results in rapidly falling EGTs and rapidly rising CHTs. The Peak Internal Cylinder Pressures can get very high, very quickly and this can destroy an engine in seconds.
The most common casues of pre-ignition are a failed spark plug ceramic which over heats and acts like a glow plug, a helicoil tang sticking in tothe combustion chamber and on very. very rare occasions, a particle of glowing carbon deposit. The later is exceedingly rare, but is the source most often cited! Go figure?
The most common progression is for a detonation event (from inadequate octane or high CHT or high IAT) to progress to the point where a spark plug ceramic is damaged resulting in pre-ignition which destoys the engine. Detonation is frequently mis-identified as the cause of the destruction.
The comments above are a sumary of the information learned from detonation studies at the most advanced engine test facility in the world, The Carl Goulet Memorial engine Test Facility. FAA experts have come to see this first hand and are adjusting their understanding of these events as a result.
Walter Atkinson
Advanced Pilot Seminars