For the Engine expert.

[Ultralights]

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..

[Aussie Bob]

Have I missed something?

[Ultralights]

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.

[CHAIRMAN]

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

[UnderneathTheRadar]

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

[bcgallacher]

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

[Squawk7700]

Jabiru heads don't use head gaskets.

[Aussie Bob]

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?

[Old Akro]

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

[Jabawocky]

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.

[truthinbeer]

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.

[Jetjr]

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

[Andy_RR]

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.

[truthinbeer]

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.

[Squawk7700]

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

[Ultralights]

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.

[Andy_RR]

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.

[Walter Atkinson]

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.

[Ultralights]

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.

[Andy_RR]

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.

[Jabawocky]

I have seen some pretty good exhaust leaks on aircraft engines…..pilots and pax blissfully unaware.

Standing beside it even, might be hard to hear with all the prop noise.

Cars, yes much easier, less competing noise.

[Ultralights]

i thought a rough running engine at idle would have been a decent indicator.

[boltz]

Looking at the exhaust it appears the damage is located in the aft/lower corner. Is that correct?

Also, you said the engine had been sitting around for a long time prior to being used.
Do these heads corrode easily?

It's interesting to note that the burn marks on the cooling fin stops at both of the bolt holes and the breach is nearly right in the middle of these holes.

[Jetjr]

These older Jabiru heads use exhaust manifold gaskets, they are a regular replacement item. Yes you can hear them when they blow out.
The burnt oil between other bolts is another sign of escaping oil past joint and pretty high temps
Id have thought an inside inspection tension of heads and valve clearances a wise move on an engine thats sat around for many years, even a new one.

[Walter Atkinson]

Ultralights:

***
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.
***

Are you sure?

The combustion temp early in the power stroke will be 3500+dF. As the piston moves down, the temp will decrease with combustion chamber expansion until at the time of exhaust valve opening, the combustion temp and exhaust temp will be closer to the same, with the combustion temp being slightly higher. So, if the leak is occurring high in the power stroke, the temp will be MUCH higher. Why would it not leak there when pressure is highest?

[Ultralights]

i see what you mean about it being hard to describe theories online, ill have another crack at it, i am assuming, that some think that a leak such as the one imaged, will cause higher temps due to the leaking gasses, but what i am trying to say, is that the leaking gas temps will be no higher than the maximum temps that can be achieved during a normal cycle, at its highest point on the power stroke. i cant see how the escaping gasses through the leak point could increase temps from what they would already be at the maximum pressure point with combustion as well.

I think i introduced confusion by mentioning EGT, my meaning being the temps inside the cylinder at their maximum peak pressure and temp, the actual temp inside the chamber at that point, not what would be measured on a probe anywhere in the exhaust system downstream of the exhaust valve.

from my understanding from a maximum temperature and pressure in the combustion process, then any leaking gasses would loose temperature rapidly as they expand in to the area beyond the leak, but the gasses passing through the leak should? not increase beyond what would already be their maximum at peak pressure and temperature inside a non compromised cylinder? am i getting this right?

[Andy_RR]

The boundary layer in the combustion chamber acts as a very good insulator to the combustion chamber wall. This is why aluminium, which normally melts at 660°C-ish can contain gas at well over 1000°C

If the boundary layer is disturbed in some way, say by violent gas motion such as knock, the heat transfer rate skyrockets. That's one reason why knock causes heat damage to pistons and cylinder heads. Heat reduces the tensile stress of the material and the high frequency pressure loads do the rest of the fatigue damage to the surface, causing pitting etc.

With a high velocity gas leak through the cylinder head joint, there will be no boundary layer to speak of and the heat transfer to the surrounding metal will be enormous! Yes, very blow-torchy!

[Jetjr]

Maybe plasma cutter ish?

[Jabawocky]

The boundary layer in the combustion chamber acts as a very good insulator to the combustion chamber wall. This is why aluminium, which normally melts at 660°C-ish can contain gas at well over 1000°C

If the boundary layer is disturbed in some way, say by violent gas motion such as knock, the heat transfer rate skyrockets. That's one reason why knock causes heat damage to pistons and cylinder heads. Heat reduces the tensile stress of the material and the high frequency pressure loads do the rest of the fatigue damage to the surface, causing pitting etc.

With a high velocity gas leak through the cylinder head joint, there will be no boundary layer to speak of and the heat transfer to the surrounding metal will be enormous! Yes, very blow-torchy!

Much of what Andy says here, in the hidden detail is so important to many other things…..APS graduates will understand exactly.

Great Post Andrew Whoeveryouare :ok:

[Walter Atkinson]

The gases are not 1000 degrees.

They are 3500-3800dF and being pushed by about 800-1000 psi of combustion pressure. (on average)

Now think about it. :ok:

[Andy_RR]

Alt-248 is what you want, Walter.

And our degrees are bigger than yours, even in Texas.

[youngmic]

Looks like a classic case of a poor seal, being only recently installed would lead me straight back to how it was torqued down or how clean the mating surfaces were.

Interesting choice of placement for the cylinders head bolts, the way their holes cut into the sealing flange seems like a good way to induce distortion. But I'm not an expert.

Jabiru engines look so pretty on the outside.....

[Walter Atkinson]

Andy:

1000dC = 1832dF if my calculations are correct. That's still about half of the combustion temp.

FWIW, and who cares?? <g>

[Andy_RR]

Technically the combustion temperature varies as the pressure rises, but yeah, who cares ;) It's still enough to burn the barn down and destroy any cylinder heads in the way...