premature engine failure at 400 hrs

The Jabiru Engine which is used in many other aircraft.

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heisan
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Re: premature engine failure at 400 hrs

Unread post by heisan » Mon Oct 08, 2018 10:54 am

Chalkie wrote:
Mon Oct 08, 2018 8:05 am
Maybe some metallurgist can comment here...

As far as I know, all valves are made from two bits of metal, the valve stem and head are then friction welded together. IE the two parts forced together and spun to generate enough heat to melt the metal in the area of the joint. This will explain why occasionally an engine 'drops a valve.'

This could be for a number of reasons, such as a valve stuck open which the piston then hits. Or a failed valve spring allowing the collets and 'keeper' (Spring retainer) being dislodged, or the weld simply failing, etc.

Jabi use automotive parts, I wonder if the valves are made as described above, or not?
I am pretty sure the Jabiru valves are single piece. Two failure mechanisms have been identified and addressed through different ADs:
1) Carbon build up on valve stem, which results in a sticky valve, which in turn gets hit by the piston (resolved through improved oil draining and venting from cylinder heads).
2) Stretching and eventual failure of the stem immediately behind the head. Results from too high EGT. Was resolved be revised jetting and temperature requirements.
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Re: premature engine failure at 400 hrs

Unread post by IWG » Mon Oct 08, 2018 11:14 am

Hi Chalkie

You are quite correct with the valve construction, but even in motor vehicle engines
the dropping of valves is most common in the air cooled ones.
Keeping cylinder head temps low and mixture rich when operating at high manifold pressure seems to be the golden rule.
Works on my Jabi, the pic was taken after climb out before leaning mixture for cruise.
Jab CHT.jpg
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Re: premature engine failure at 400 hrs

Unread post by heisan » Mon Oct 08, 2018 11:36 am

This is one of the best write-ups on over-heat failure of Jabiru valves that you are likely to find:

https://assets.publishing.service.gov.u ... _05-10.pdf
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Re: premature engine failure at 400 hrs

Unread post by Chalkie » Mon Oct 08, 2018 5:24 pm

heisan wrote:
Mon Oct 08, 2018 11:36 am
This is one of the best write-ups on over-heat failure of Jabiru valves that you are likely to find:

https://assets.publishing.service.gov.u ... _05-10.pdf
Thanks Justin, for posting such an informative article.

I see that I was wrong in this case: "The microstructure appeared typical of an as-forged 21-4N component."
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Re: premature engine failure at 400 hrs

Unread post by IWG » Tue Oct 09, 2018 9:33 am

You hit the nail on the head Justin,

That is why the flock of Jabis at our field never had this problem, (mine is the newest, 2003).
The only issue was running rich when cruising in hot and high conditions, hence the mixture control.

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Re: premature engine failure at 400 hrs

Unread post by foxdog » Sat Dec 29, 2018 10:32 am

Hi Guys,
Sorry it took so long for me to get back to you.
Please see the attached pics of the valves.
It appears that the exhaust valve jammed open due to carbon buildup on the stem.
The valve stem is perfectly straight.
I have done the following modifications,
1) Double valve springs all round.
2) Forged pistons with a valve cut away,
3)Oil cooling spray on bottom of pistons for additional cooling
My question is should the mixture be altered in order to prevent the carbon build up again or wood the additional valve springs force the valve to close.
So far I have done 8 hours with the mods and all appears to be fine.
Regards,
Foxdog
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Re: premature engine failure at 400 hrs

Unread post by Flooi » Sat Dec 29, 2018 10:40 am

foxdog wrote:
Sat Dec 29, 2018 10:32 am
Hi Guys,
Sorry it took so long for me to get back to you.
Please see the attached pics of the valves.
It appears that the exhaust valve jammed open due to carbon buildup on the stem.
The valve stem is perfectly straight.
I have done the following modifications,
1) Double valve springs all round.
2) Forged pistons with a valve cut away,
3)Oil cooling spray on bottom of pistons for additional cooling
My question is should the mixture be altered in order to prevent the carbon build up again or wood the additional valve springs force the valve to close.
So far I have done 8 hours with the mods and all appears to be fine.
Regards,
Foxdog
All due to carbon buildup.... so few hours? Nou worry ek....Should we then not consider mogas??? :oops:
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Re: premature engine failure at 400 hrs

Unread post by heisan » Sat Dec 29, 2018 10:45 am

Flooi wrote:
Sat Dec 29, 2018 10:40 am
All due to carbon buildup.... so few hours? Nou worry ek....Should we then not consider mogas??? :oops:
The carbon build up is usually due to oil coming through the valve guides - not fuel residue. Changing fuel should not make much difference.
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Re: premature engine failure at 400 hrs

Unread post by foxdog » Sat Dec 29, 2018 10:50 am

Hi Flooi
It has run on mogas for the last 200 hours.
Regards Foxdog
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Re: premature engine failure at 400 hrs

Unread post by Flooi » Sat Dec 29, 2018 11:54 am

heisan wrote:
Sat Dec 29, 2018 10:45 am
Flooi wrote:
Sat Dec 29, 2018 10:40 am
All due to carbon buildup.... so few hours? Nou worry ek....Should we then not consider mogas??? :oops:
The carbon build up is usually due to oil coming through the valve guides - not fuel residue. Changing fuel should not make much difference.
Tx Justin. Any suggestions? Can this be because valve stem seals are worn?
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Re: premature engine failure at 400 hrs

Unread post by heisan » Sat Dec 29, 2018 12:12 pm

There are no valve stem seals...

The problem was largely fixed with the oil drain manifolds (have not heard of many stuck valves after this mod). Would be interested to know if this engine had the manifolds fitted.

Jabiru also recommends 6 month oil change intervals (to reduce oil contamination), and to avoid shock cooling (prolonged glides with a hot engine).

All recent engines also have valve recesses on the piston crowns, so if the valves do stick, it is not catastrophic.
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Re: premature engine failure at 400 hrs

Unread post by Flooi » Sat Dec 29, 2018 12:23 pm

Tx Justin for that most informative response. I have a later model... 2015. So hopefully
that mod was done. Regards. Hennie
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Re: premature engine failure at 400 hrs

Unread post by bslick » Sun Dec 30, 2018 2:23 pm

After 10 years of flying behind a Jabiru engine I got tired of flying with a knot in my gut and sold it. The extremely poor service from the factory, my main source of technical support for the product, was the final straw.

My advice would be to make sure you use an engine monitor and review the output regularly. I would also make sure the aircraft is used regularly and appropriately maintained by somebody competent.

They are good aircraft but a more reliable power plant would be the best solution. I see some folks in Oz and NZ have started using Rotax engines...
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Re: premature engine failure at 400 hrs

Unread post by foxdog » Sun Dec 30, 2018 4:41 pm

Hi Heisin,
How and where do I look to see if the engine has the oil drain manifolds.
It is a 2010 manufactured with the hydraulic lifters
Regards,
Foxdog
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Re: premature engine failure at 400 hrs

Unread post by heisan » Sun Dec 30, 2018 6:46 pm

foxdog wrote:
Sun Dec 30, 2018 4:41 pm
Hi Heisin,
How and where do I look to see if the engine has the oil drain manifolds.
It is a 2010 manufactured with the hydraulic lifters
Regards,
Foxdog
Have a look where the push rod tubes go into the block. If they enter through a machined bolt-on block, that is the oil drain mod. If they go straight into the block, then it is not fitted.
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