Rob:

Thank you for the thorough explanation of the event s as you understand them.

**Both problems are commonly lumped together and low octane and high cylinder temps can result in either one. When preignition gets bad, it will destroy the engine faster than detonation. It also is harder to detect preignition before serious damage is done.**

Both are commonly but incorrectly lumped together. They are two very different events. Preignition is always bad! <g> Detonation, on the other hand is seldom serious unless it is allowed to escalate.

The most common cause of preignition seems to actually be failed spark plugs which do not shed heat and become the foci of the event. This is frequently the result of damage done to the spark plug ceramic from detonation! The second most common source of preignition is probably a helicoil tang glowing as you have suggested. While carbon deposits have been blamed for decades, it appears that, in fact, this is unlikely.

Further, the data indicates that it is actually quite likely that damage from detonation as you have listed and is commonly sited is really a result of preignition. We have observed extended periods of detonation in which there is no damage whatsoever to the cylinder, rings, valves, etc. I realize that this is counter to the conventional wisdom, but it is in harmony with the observed data.

One of the things you have exactly as I understand it is the reason the CHT's rise. It's the effects on the thermal boundary layer. We have not observed a falling EGT of any significance during detonation, although it makes some sense that that could be a result. We do see a significant fall in EGTs during preignition. That is, of course, a result of the advancing thetaPP.

Thanks again for your comments.

Walter


On Mar 24, 2005, at 8:03 PM, REHBINC@aol.com wrote:

Walter,
 
Preignition results when a hotspot in the combustion chamber ignites the fuel air mixture in advance of the spark plug. Usually the source is a sharp corner or a carbon deposit. Something with plenty of surface to absorb heat but insufficient area to reject it. The result is similar to grossly over advanced timing. If you are lucky, you might find a hole melted through the top of the piston.
 
Detonation is the result of the increase in temperature during the combustion event interacting with fuel elsewhere in the chamber that is already criticaly close to auto ignition conditions. Sometimes the mild shockwave emminating from the burning fuel is enough to push fuel in a hot part of the chamber to ignite. The result is multiple flame fronts, extremely fast pressure rise and not so mild shockwaves. The shockwaves disturb the boundary layer that insulates the combustion chamber walls and greatly increases heat transfer to the head and piston. Ironically, since more heat is lost to the combustion chamber, the exhaust temperature can actually go down, leading one to think there is no problem. If you are lucky, you might find broken rings or ring lands or possilbly a valve with a pie section missing.
 
Both problems are commonly lumped together and low octane and high cylinder temps can result in either one. When preignition gets bad, it will destroy the engine faster than detonation. It also is harder to detect preignition before serious damage is done.
 
One of the big issues with detonation is time. The longer the fuel spends in its hot and compressed state, the longer it has to decompose into compounds that ignite at lower energy levels. That is why increasing rpm is a effective at preventing detonation. Another approach is to make the distance that the flame front has to travel across the combustion chamber shorter. This has been one of the major advances in engine technology of the past few years from the automakers and is known as the fast burn head.
 
The 4 valve, pent roof design has a much shorter long dimension than than the typical hotrod head of a '60's muscle car and with the spark plug in the middle the flame travel distance is shorter still. These heads can live what at least one point more compression on the same octane fuel.
 
I'm not an A/P mechanic but I suspect that the aircraft engine heads have not kept up in this area. Anybody know? George?
 
This could be a nitch for some entreprenuer to exploit in the experimental market. Cut your fuel costs in half, run premium auto fuel in your TSIO?
 
Rob