At what combustion chamber temperature does detonation occur?

I was reading a book last night that stated that 1075 F was about the limit, but it did state it depended on several things, including combustion chamber shape and timing control via knock sensors. The author (Corky Bell, who is an authority on automotive supercharging)doesn't give any explanation on how that number was reached.

Even so, 1075 F seems awfully low, since several org members have reported exhaust gas temperatures of over 1400 F with no evidence of detonation. Was detonation trying to occur at those temperatures but suppressed by timing retard when the knock sensor started to detect it?

It's interesting that 1075 F is just below the melting point of pure aluminum, though. I wonder if that has anything to do with using 1075 as the limit.

Does anybody have any good references (e.g. engineering textbooks)about this?

 

Detonation will occur at local hotspots. Once that happens, the whole volume detonates. If a combustion chamber has sharp edges, the metal in that location will get hotter than other parts and will cause the mixture at that location to become the source of detonation. If the mixture is stagnant, it will not allow for homegenous temperature (it's not mixing well), and will lead to local hotspots that will detonate. It is therefore impossible to say there is one specific "average" cylinder temperature where detonation will occur. It's dependent on the particular engine (design). It's even dependent upon the condition of that engine. For example, carbon buildup will act to insulate the metal and the inside surface will get hotter than a clean metal surface would, and will detonate at a lower temperature than a clean chamber. Two engines can show the exact same average combustion chamber (what you would read on an EGT gauge) while one detonates and the other does not, because one can have local hotspotting while the other does not.

Keep in mind that the due to the relatively good thermal transfer properties of even aluminum, the cylinder and piston will always be much much cooler than the mixture. The mixture may be at 1400 F while the metal is at only 400 F (just an example - the actual #s could be way different).

 

Hi Keven,

What you say makes a lot of sense, especially about the good thermal conducting properties of aluminum. I'm thinking the 1075F temp is for engines with iron blocks and heads that can't dissipate heat as well as an aluminum engine. If that is the case, though, it behooves the boosted folks who are running 1400F+ egt's to make sure their cooling system is at peak efficiency.

 

Water Weter