For the Engine expert.
 

I have done the APS courses, and have pulled apart my fair share of motorbike engines, but this failure is interesting... (not my engine)

http://i.imgur.com/1mA18D8.jpg

http://i.imgur.com/wP9ivox.jpg

http://i.imgur.com/nGArt8t.jpg


Apparently just 5 hours old..

Have I missed something?

no, just saying, i have never seen anything like that before, and was wondering if anyone has seen it before, and know what would be the cause. just for curiosity sake. as for the APS part, just a disclaimer that i know about EGTs, CHT,s and their relevance to operation and possible failures. i simply have no idea what might have caused this, apart from a possible material fault.

My guess is the head has been fretting, and blow by past the head seal has caused the damage.

Clearly been running 50 LOP instead of 50 ROP......

Looks like a failed head gasket with the blow by melting the head. Cause of gasket failure? Could be one of several reasons.

Jabiru heads don't use head gaskets.

Hey UL, sorry my first post was before the pictures appeared. That is a mighty unusual failure, I have been staring at engines since I was a little kid and never seen the likes of it. I would tend to concur with bcgallacher that the head gasket failed leading to a fine jet of hot gas, but why it eroded the way it did is interesting. I gather it is a Jabby head?

I'd be taking that to a metallurgist. The gouging doesn't quite look right to have been caused by exhaust gas.

Some good pics there :}

My shoot from the hip guess as I do not have access to the owner or maintainer.

Cylinder head distortion due to poor cooling and/or poor head bolt tension, this leads to a small leak which ultimately gets bigger and erodes the seal area which then progresses into a blow torch on the rest of the head.

Either that or there was some pretty severe porosity that started the cycle of destruction off.:ooh: But given how they are made that is a remote chance indeed.

I am in the blow-by camp as well. Possible the head was not tensioned down evenly...you can see the effects of secondary blow-by either side.

As discussed elsewhere, possibly head not done up properly.
Much more likely than billet materials fault.

Rest of the story is that engine sat for many years unused, fitted and this happened in first 5 hours

If it were cylinder head leakage (and I think that's pretty likely) it would have been very noisy and pretty easy to detect.

Running it for 5 hours to failure seems a bit cavalier to me.

Andy, it might have been a psht, psht whisper as would occur in the secondary damaged areas for a few hours. Not particularly noticeable to the untrained ear. The main damage could have occurred quite quickly and yes noisily in just the last few minutes of operation. The head would have been glowing in that area at that stage.

How much hotter would the "blow-by / blowtorch" flame be than that combusting in the chamber where it's supposed to be combusting?

abut the same, not higher than what would already be peak EGT, as the gasses exit, they are already consuming the oxygen required to burn all the fuel, as it exits into a higher oxygen atmosphere, then any remaining unburnt fuel would be burnt(if it running rich), temps being no different if all the oxygen and fuel combined inside a sealed cylinder at peak EGT, if it was running leaner, then there would be no excess fuel to burn, and the exhaust would already be oxygen rich,so the exiting gases would be entering into an area of yet more oxygen, but with no extra fuel to combine with it, the temps would cool, as well as expansion cooling of the gases exiting into a lower pressure area.

Gas would be exiting at much higher temperatures than what you'd measure as EGT. Just taking a cylinder full of air to TDC at 8:1 makes over 500°C in gas temperature - that's before you even light the wick. I'd suggest that, with combustion, they'd be north of a thousand °C - possibly 1500-1800°, but I haven't done the calcs.

As to whether you'd hear it or not - you can hear exhaust manifold leaks as plain as day and they happen at much lower gas pressures than what happens during combustion. I'd be surprised if it wasn't very noticeable.

The temperature of combustion being in the 3500-3800dF range makes that a serious blow torch. This is noting like EGT level heat.

I would NOT have expected to hear it. We can't even hear severe detonation in an aircraft engine.

My bet is that a metal porosity which allowed failure and the tiniest of leaks, becoming a really big leak due to the failure of any thermal boundary layer and the blow-torch effect.

These are "educated guesses", since I've not seen the actual part.

i think i need to add some context to my post above, my meaning being that the exhaust gas temps leaking through into the void, would be no different to the gas temperatures in the combustion chamber if the engine was 100% fine and running normally.

Even on auto engines, knock can occur long before it is audible to most people and it is mostly a problem at high loads where the engine is generating a lot of noise anyway.

An exhaust leak on the other hand, even a tiny one, can be pretty obvious especially at idle. This was (presumably) a gas leak with at least an order of magnitude increase in pressure behind it. Being only on one cylinder will make it a distinct ticking/hissing sound, I would have thought.