What Detonation Looks Like

Last time, I showed what preignition looks like, so this time I thought it might be good to take a look at detonation. In this case, we're seeing the result of mild, occasional detonation (such as you would get by overleaning in cruise-climb). The mildness is evident by the fact that only the outer edges of the piston are involved. (Notice the characteristic "sandblasted" look.) The somewhat peened or smoothed-over appearance of the pitted areas, particularly on the left side of the piston (as viewed in this photo), suggest that detonation was encountered only now and then, but over an extended period of engine service. (Over a sufficient period of time, roughness gets "polished out" by the action of lead-containing combustion particulates.)

The positioning of the detonation zones at the far outermost edges of the piston is consistent with operation right at the beginning of detonation onset (in other words, mixture just lean enough to cause incipient detonation, but not lean enough to cause full-on explosive detonation). The mechanism for this is not hard to understand: The abnormal combustion process associated with detonation occurs when unburned fuel and air (far away from the flame front) are heated and compressed past the point of "thermal cracking." There's a short period of time when the fuel molecules actually begin to decompose (split into radicals) on their own, unassisted by combustion with oxygen. Unfortunately, they all tend to do this at once. The radicals so produced then instantly grab onto the nearest oxygen molecules. Instead of steady, slow conflagration, you get an instant bang.

In normal combustion, you don't see fuel molecules crack apart en masse into radicals (detonation precursors) prior to oxidation. Instead, "cracking" happens only inside a very thin flame front. The flame front progresses steadily across the charge volume, creating radicals (in that thin flame zone) as it goes.

So in the engine from which the above piston came, you can imagine that normal combustion happened until the flame front got pretty far away from the spark plug(s). Then the remaining compressed fuel and air at the very edges of the piston went ka-bang!

The engine from which the above piston was taken was not an aircraft engine, but I've seen piston edges that look like this in torn-down Lycomings and Continentals countless times. The next time you visit an engine shop that keeps a garbage bin of old pistons, take a look and you'll see what I mean.