Sunday, October 21, 2007

What Preignition Looks Like

Preignition doesn't always burn a hole straight through the center of the piston, but sometimes it does, and this is what it looks like. This is what might be called a deposit burn-through (or for want of a better term, an ash-hole). The entire top of the piston is overlaid with crusty deposit buildup. What likely happened here is that a an amalgam of very-high-melting-point calcium (or other) deposits started to accumulate on the very top of the piston, in the center. After a period of high BMEP (perhaps runaway turbo boost), the deposits, heated to 1500 degrees or more, simply caused the aluminum underneath to soften. (Aluminum melts at 1220 degrees Fahrenheit, or 660 Celsius.) This is clearly a melt-hole rather than a detonation failure. In the latter case, there would be sharp cleavage planes under the lip of the hole, on the back side (the "underneath" side), sort of like what you see on the back side of a bullet hole in a glass window. This piston happens to be from a marine racing engine. But the same thing can happen (has happened!) to aircraft engines. Depending on the source of the preignition (spark plug vs. deposits) and the flame pattern in the cylinder, the piston can either melt through the center, or melt at the edges, or both. More often than not, the entire piston overheats and over-expands in the cylinder bore and starts to make rubbing contact with the barrel wall, giving rise to smearing/scoring or vertical streak-marks on the sides of the piston. You can see some of that at 7 o'clock on this piston as viewed here.

Thankfully, this sort of failure is rare in aviation.

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