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djaflyact
Joined: 14 Oct 2013
Posts: 27
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 Posted: Tue Jan 24, 2017 8:13 pm Post subject: Rotax 914 Exhaust Leak |
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I have a motor glider with the 914 engine. Just bought a good CO detector and was surprised that I have been flying for years with low level CO in the cockpit. I did the classic shop vac blowing in the exhaust to see if I could determine any leaks. To my huge surprise, I didn't need soapy water to find a leak. All four slip joints are blowing air like crazy. The fit seems very loose. I did the same test on a friend's newer 914 and there was no leak at all.
Anyone have any experience with this. I loaded up the joint with anti seize compound as a test and the leak was reduced significantly. The cockpit readings at idle lowered from 20 to 5 on the ground. I am considering using a pipe expanding tool to slightly increase the smaller pipe to make it a tight fit. The metal is stainless steel and I don't know if that can be done. It won't take much to provide the increased tightness.
A big concern is the cost off the exhaust parts if I need to replace them. Rotax is extremely proud of their parts. the 4 pipes and the gatherer total approximately $4700. That is crazy.
Any isights would be appreciated.
Dave A
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davidjoyce(at)doctors.org
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| Posted: Wed Jan 25, 2017 1:25 am Post subject: Rotax 914 Exhaust Leak |
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Dave, I had a somewhat similar experience having invested in a new electronic CO detector instead of one of those useless cardboard dots. I found my leak was through the slots in the join where the exhaust stub fits onto the silencer. Filling that with exhaust cement totally fixed my problem, although naturally that is not going to help you if your remaining problem is still with the exhaust down pipes, but worth checking anyway.
Regards, David Joyce, GXSDJ (914 XS)
On 2017-01-25 04:13, djaflyact wrote: | Quote: | | Quote: | --> Europa-List message posted by: "djaflyact" <djaflyact(at)gmail.com (djaflyact(at)gmail.com)>
I have a motor glider with the 914 engine. Just bought a good CO detector and was surprised that I have been flying for years with low level CO in the cockpit. I did the classic shop vac blowing in the exhaust to see if I could determine any leaks. To my huge surprise, I didn't need soapy water to find a leak. All four slip joints are blowing air like crazy. The fit seems very loose. I did the same test on a friend's newer 914 and there was no leak at all.
Anyone have any experience with this. I loaded up the joint with anti seize compound as a test and the leak was reduced significantly. The cockpit readings at idle lowered from 20 to 5 on the ground. I am considering using a pipe expanding tool to slightly increase the smaller pipe to make it a tight fit. The metal is stainless steel and I don't know if that can be done. It won't take much to provide the increased tightness.
A big concern is the cost off the exhaust parts if I need to replace them. Rotax is extremely proud of their parts. the 4 pipes and the gatherer total approximately $4700. That is crazy.
Any isights would be appreciated.
Dave A
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Remi Guerner
Joined: 14 Dec 2010
Posts: 284
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| Posted: Wed Jan 25, 2017 5:29 am Post subject: Re: Rotax 914 Exhaust Leak |
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I think it is illusory to try to get a 100% sealed exhaust system. Metal to metal joints are not perfect, especially when they are subjected to thermal expansion and vibrations. They can be improved using some sort of sealant but that will not last for long due to the huge temperatures involved. Leaks are more likely on the 914 due to the higher back pressure caused by the turbo when it is working at full boost with the waste gate fully closed. Leaks are also likely on the 912 with the CKT exhaust when the springs age and become weaker.
One solution to prevent fumes to get into the cockpit is to make sure cockpit pressure is always higher than both cowling pressure and outside pressure. To achieve that, the cowling should be well vented with small air intakes and an exit area big enough to suck the airflow. Fresh and/or heated air vents should be at least partially open to create a pressure build-up inside the cockpit. I still have to perform some CO measurements to determine if I was successful at achieving that.
Remi Guerner
F-PGKL
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gilles.thesee(at)free.fr
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| Posted: Fri Jan 27, 2017 9:24 am Post subject: Rotax 914 Exhaust Leak |
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Le 25/01/2017 à 05:13, djaflyact a écrit :
| Quote: | The cockpit readings at idle lowered from 20 to 5 on the ground.
David,
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The direct correlation between ball joint tightness and cockpit CO
readings seems to indicate the problem may lie with your firewall.
Of course your exhaust line should be reasonably tight, but it is
impossible to avoid fumes in an engine compartment.
The firewall is there to prevent flames and engine gases from getting
into the cockpit. The firewall should be perfectly gas tight.
*Any* aperture or pass-through in the firewall should be suitably sealed
with fire resistant grommets, putty, bellows, tape, etc.
Not the tiniest hole, slit, crack should be tolerated.
Some info here (only in French I'm afraid 
http://contrails.free.fr/engine_cpf.php
--
Best regards,
Gilles
http://contrails.free.fr
http://lapierre.skunkworks.free.fr
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willydewey
Joined: 24 Sep 2014
Posts: 53
Location: Guildford Surrey United Kingdom
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| Posted: Mon Jan 30, 2017 1:37 am Post subject: Re: Rotax 914 Exhaust Leak |
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I had a similar problem on my Europa with the 914. Previously the exhaust stub was angled back streamlining I suppose one could call it. Turning it to face 90 deg out cured the problem. The exhaust stream seams to go over the wing and enters the fuselage at the slots near the trailing edge of the wing and then forwards down the tunnel. It then leaks out at the front of the fuselage. The aerodynamic shape of the fuselage seams to aid this. It also does nothing for those who have instruments relying on static pressure within the fuselage.
These are my theories others may disagree
Bill
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