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Cylinder Head Temps not even Rotax 912ULS


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13 hours ago, facthunter said:

.................................  I'm not convinced that a normally operating motor should vent into the bottle at all. Your pressure cap should seal to allow the temp of the coolant to be above 100 C.  ..........................................

 

Nev; a by product of combustion is heat (even in a normally operating engine ). When more fuel is burnt  eg engine under load when climbing, more heat will be generated and must be managed. Less fuel during cruise, descent, taxi and shut down - less heat. These varying heat loads, must be managed/accommodated by the cooling system (in this case a liquid). If you accept heating a liquid causes it to expand, then you must also accept this expansion (increased volume/pressure) must be accommodated somehow. You could just vent it overboard or do what modern cooling systems do, contain it in a bottle/expansion tank (for reuse). If you dont allow the expansion then you risk damaging the cooling system. Alternatively you build a very much robust/heavier system (as in the past) to withstand the pressures generated - not so good for an aircraft.

 

"Your pressure cap should seal to allow the temp of the coolant to be above 100 C" - my understanding is that it does. In climb out, I  see approximately 110C in my system - reducing to the low 90C in cruise.

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My point is you have to exceed the cap pressure to put fluid in the overflow bottle. IF the cap doesn't seal it won't pull the overflow fluid back later.. Nev

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IF you go to 23,000 feet you'd need some other pressure cap than the normal one.. Unpressurised you can't even have a hot coffee at that level. Nev

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Posted (edited)
9 hours ago, facthunter said:

My point is you have to exceed the cap pressure to put fluid in the overflow bottle. IF the cap doesn't seal it won't pull the overflow fluid back later.. Nev

Exceed? - the cap is designed to release ,coolant into the expansion tank, at a predetermined pressure

 

9 hours ago, facthunter said:

IF you go to 23,000 feet you'd need some other pressure cap than the normal one.. Unpressurised you can't even have a hot coffee at that level. Nev

Hummm - Not sure of the practical application of your comment. These days I am an Australian RAA pilot - no intention of going above 9,500 ft (10,000ft VFR limit). My Rotax 912ULS seemed to handle this modest altitude without any evidence of misbehaviour from any of its systems.

 

 

Edited by skippydiesel
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Yes and when that pressure DIFFERENCE is exceeded it lets things into the OVERFLOW container AND the 23,000 feet thing was brought up by another poster as the ceiling for another version of liquid cooled  Rotax. so I used that to point out that the pressure  difference  affected the temp outcome achieved at height..  that IS at ANY height but more so at higher levels.  10,000 ft is where you need to carry oxygen . That is another effect of the lowered atmospheric pressure .Nev

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1 hour ago, facthunter said:

Yes and when that pressure DIFFERENCE is exceeded it lets things into the OVERFLOW container AND the 23,000 feet thing was brought up by another poster as the ceiling for another version of liquid cooled  Rotax. so I used that to point out that the pressure  difference  affected the temp outcome achieved at height..  that IS at ANY height but more so at higher levels.  10,000 ft is where you need to carry oxygen . That is another effect of the lowered atmospheric pressure .Nev

The rotax 915 with its 23,000 foot service ceiling appears to have a standard type radiator cap, if it was rated 1bar or 15psi it would still have 10psi at 20,000 feet if my maths is correct. If the engine is not overheated 10psi is fine.

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3 hours ago, facthunter said:

Yes and when that pressure DIFFERENCE is exceeded it lets things into the OVERFLOW container AND the 23,000 feet thing was brought up by another poster as the ceiling for another version of liquid cooled  Rotax. so I used that to point out that the pressure  difference  affected the temp outcome achieved at height..  that IS at ANY height but more so at higher levels.  10,000 ft is where you need to carry oxygen . That is another effect of the lowered atmospheric pressure .Nev

No offence, Nev but the word "exceed" has, for me, connotations of going beyond/past a limit/margin - often used where limits on safety or structural mechanical integrity may be passed. In this case the opening/closing of a pressure sensitive valve (the cap mechanism) has no such connotations being a normal part of the cooling system operation.

 

Not sure why, in this context, you bring up the supplementation Oxygen  subject - its along time since I did my GA study, if dodgy memory serves me right, O2 for flight crew is mandatory from 12, 000 ft up and for passengers 14, 000ft up. Some people may require O2 below these  levels.

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O2 is  required to be carried abv 10 000 in Australia It's 12,000 in the USA .  These figures affect where you level out after a decompression event also. (Emergency Descent)  The Oxygen is not sufficiently available to a human because of the LOWER total pressure and our physiology.  If you are very fit, Conditioned to altitude and generally young and don't smoke, you are less affected. It's still the same % of the air.

    FL 210 is the LEVEL above which any flying is not considered VFR for Flight planning purposes, regardless of how  far you can see. Nev

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  • 2 months later...

Ok. Some further info lads. 
 

I have been flat out and so only just managed to get on to this now. 
 

I still have the CHT issue. So I swapped the sensors and the problem remain unchanged. 
 

Pulled the plugs and found passenger front (closest to prop) both top and bottom are black.  Rear passenger are a nice grey. Swapped plugs around and problem remained. 
 

Did full leak downs. All perfect.  


All cables are good. 
 

The sensor on the side that has the black plugs is reading high temp.  
 

Only other choice now is to check floats and sync carbs. 
 

But it’s got me beat.  
 

Any ideas?
 

 

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3 hours ago, facthunter said:

The 912 doesn't have an even suck from each carb.. Length is uneven and pulsing is wrong. .Nev

?????????

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The biggest problem for 912's carburettor's  is not the inlet manifold length(s) but the difficulty in getting even operation, in both carb's, using cables. It is not unusual to have a sticking cable, the carb opening closing going slightly out of sink - both may cause uneven fuel delivery. What matters most is that both carbs are delivering the expected fuel/air mixture in the 4800- 5800 rpm range

 

Plug colour, in 912's  is not a reliable indicator of "health" - think about it - this is an engine optimised to run in a fairly narrow RPM range. Taxying and descent power is outside this range. On top of this will be sub optimal engine temperatures at this time. If all else has been adjusted appropriately and engine running as expected, the only time I would worry about plug colour, is when there is an indication of excessive oil consumption - usually black sticky/wet looking deposits.

 

Your CH temp variation could be an unusual  cowling air flow (in/out of prevailing wind), momentary coolant issue or instrument reliability (yes you have swapped the sensor lines but what about the read out itself?)

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 Diagnostic is  best guess, my best guess would be that the cylinder is actually quite rich & some fuel is being burnt in the exhaust port. That is the only way I could imagine a rich mixture would show a high temp.

The return spring is at its weakest when the throttle is near fully open. Any binding in cable would most likely show near  wide open throttle.  I had problems balancing my carbs, I found that the return spring did not have the strength to return with the  very flexible multi strand cable. It just occasionally would slow the throttle. It was difficult to find as the vibration from the engine would ensure it corrected its self. I found a slightly more rigid cable & replaced it. That cured the problem with the slight stiffness just helping the spring to maintain tension  After that it was easy to complete the carb balance.

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Posted (edited)

Update - SOLVED. 


So sometimes you can’t see the forest for the trees. The problem was staring me in the face and I could not see it. 

I decided to just go back to basics and look at what in the system can cause a mixture issue. The very first thing that came to mind was the choke. So I tested between full choke and no choke and found that the choke was binding and not returning to min.  
 

It would appear that the Lame that did the last 100 did not lubricate the throttle components.  I also found the idle mix screws to be way out.  Once I fixed that and lubricated all linkages, the problem is all but gone. The CHT are within 10c of each other. 
 

I have now synced carbs and once the weather clears will test fly. 4000rpm runs shows no black plugs anymore.  
 

one thing this taught me.  When I do my daily I always check throttle travel and choke engagement, but realised I never checked the chokes had returned to minimum. It’s now on the list. 
 

Problem solved and lesson learned. Win win. 

DC822B2E-2F9E-4864-9C24-F29008497BF5.jpeg

Edited by BirdDog
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10 hours ago, BirdDog said:

Update - SOLVED. 


So sometimes you can’t see the forest for the trees. The problem was staring me in the face and I could not see it. 
  
 

It would appear that the Lame that did the last 100 did not lubricate the throttle components.  I also found the idle mix screws to be way out.  Once I fixed that and lubricated all linkages, the problem is all but gone. The CHT are within 10c of each other. 
 

 

one thing this taught me.  When I do my daily I always check throttle travel and choke engagement, but realised I never checked the chokes had returned to minimum. It’s now on the list. 
 

Problem solved and lesson learned. Win win. 

 

IMG_1377.MOV 36.34 MB · 0 downloads

DC822B2E-2F9E-4864-9C24-F29008497BF5.jpeg

Great outcome - its a cautionary story for all of us - humans have atendency to focus in on the complex solution, bypassing the simple.

 

Word of caution -" Lubricating throttle components" - in general, not a good idea can lead to further problems as lubricant becomes contaminated - accelerated wear/sticking/etc.

 

(could only get voice on your video)

 

Throttle "return" springs should be replaced from time to time - may be worth checking in to.

Edited by skippydiesel
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Posted (edited)

Interesting you say don’t lubricate Skippy, when it’s actually a requirement in the Rotax Maintenance Manual. It states to use engine oil. I personally didn’t use oil. I used a marine based lanolin spray 

 

 

 

Edited by BirdDog
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22 minutes ago, BirdDog said:

Interesting you say don’t lubricate Skippy, when it’s actually a requirement in the Rotax Maintenance Manual. It states to use engine oil. I personally didn’t use oil. I used a marine based lanolin spray 

 

 

 

Go for it my friend - For the record I said "not a good idea" but its always "do as I say not as I do"  and I also have succumbed to the temptation and I use a very light smear of Molykote 111 - high quality silicon grease. Attracts very little contamination, great adherence & persistence. I would never use engine oil or traditional grease in this area.

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If the cable has a nylon or any type of "plastic" liner it should not be lubricated. I do not lubricate the throttle or choke cables on the thruster. Cables would be a responsibility of the airframe manufacturer re maintenance.  

Edited by Thruster88
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8 minutes ago, Thruster88 said:

If the cable has a nylon or any type of "plastic" liner it should not be lubricated. I do not lubricate the throttle or choke cables on the thruster. Cables would be a responsibility of the airframe manufacturer re maintenance.  

I’m talking more the linkages and moving parts of both the throttle and the choke, as per the rotax manual. 

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29 minutes ago, Thruster88 said:

Cables would be a responsibility of the airframe manufacturer re maintenance.  

Well that may be "by the book" but i replaced my old cables with custom made jobs a few years back. Of course my Zephyr is a 19/kit, so there is very little by the book but I cant imagine waiting weeks, even months, for a distant manufacturer to send a simple cable that you can have made locally.

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