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ndibiase
Joined: 19 Oct 2008
Posts: 8
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 Posted: Tue Nov 02, 2010 2:16 am Post subject: 912 uls exhaust gas analysis |
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For diagnostic reasons after an over speed of unknown entity on my Savannah LSA plane (consequence : big power loss -500 rpm with perfect carbs , push rods and compression and leak down tests ok. Growing oil consumption, little misfiring) i had the suggestion to do an exhaust gas analysis on my rotax 912 uls (100 hrs from new).
Infact after a boroscope analysis (with a very cheap but low res instrument) there is evidence of some partial burned oil in the combustion chambers (little spots dark orange caramel like appareance).
Leakage can be suspected from valve stem guides.
Someone has done an exhaust gas analysis on rotax 912 uls ?
Is there experience on witch are the normal range value of Co, Co2 , HC ?
If there is an oil leakage is it shown in altered HC reading values ?
Thanks in advance , i am not an expert and i am very far from nearest rotax service center.
Nicola di Biase
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flyadive(at)gmail.com
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| Posted: Tue Nov 02, 2010 4:36 am Post subject: 912 uls exhaust gas analysis |
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Nicola:
I think someone is pulling your leg... Exhaust Gas Analyzation!!!
Just look at your spark plugs. They should be DRY - GRAYISH in color, sometimes with a brownish tinge and evenly eroding. Eroding, should be ZERO in 100 Hours.
Check the internet - Look for a CHAMPION SPARK PLUG INSPECTION CHART. It will give you spark plug color Vs cause.
What color is your engine oil?
When was the last time you changed the oil?
Double Check the compression readings. Sure reads like you have a bad ring or two.
Look at the exhaust pipe... Is is heavy with BLACK SOOT? It should be light gray to dark gray or maybe with a slight brownish tinge. It should also feel dry.
Do you see any oil leaks around the engine? Is there oil spray under the cowl?
Check your radiator - Any oil in the water?
Barry
On Tue, Nov 2, 2010 at 6:16 AM, ndibiase <ndibiase2(at)alice.it (ndibiase2(at)alice.it)> wrote:
| Quote: | --> RotaxEngines-List message posted by: "ndibiase" <ndibiase2(at)alice.it (ndibiase2(at)alice.it)>
For diagnostic reasons after an over speed of unknown entity on my Savannah LSA plane (consequence : big power loss -500 rpm with perfect carbs , push rods and compression and leak down tests ok. Growing oil consumption, little misfiring) i had the suggestion to do an exhaust gas analysis on my rotax 912 uls (100 hrs from new).
Infact after a boroscope analysis (with a very cheap but low res instrument) there is evidence of some partial burned oil in the combustion chambers (little spots dark orange caramel like appareance).
Leakage can be suspected from valve stem guides.
Someone has done an exhaust gas analysis on rotax 912 uls ?
Is there experience on witch are the normal range value of Co, Co2 , HC ?
If there is an oil leakage is it shown in altered HC reading values ?
Thanks in advance , i am not an expert and i am very far from nearest rotax service center.
Nicola di Biase
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=317834#317834
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rlborger(at)mac.com
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| Posted: Tue Nov 02, 2010 5:55 am Post subject: 912 uls exhaust gas analysis |
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Nicola,
One of the biggest issues with an overspeed in a 9xx is a piston contacting a valve. This can result in damage to the piston, a bent valve stem, damaged valve guide or damage to other parts of the valve train. The engine probably needs to be checked by a ROTAX repair shop to ensure everything in the valve train is OK. They will also have a better knowledge of what other things may go wrong as a result of an overspeed.
Good luck!
Bob Borger
Europa Kit #A221 N914XL, XS Tri-Gear, Intercooled 914, Airmaster C/S Prop
http://www.europaowners.org/forums/gallery2.php?g2_itemId=60232
http://www.biplaneforumgallery.com/index.php?cat=10046
Europa Flying!
3705 Lynchburg Dr.
Corinth, TX 76208
Home: 940-497-2123
Cel: 817-992-1117
On Nov 2, 2010, at 5:16, ndibiase wrote:
| Quote: |
For diagnostic reasons after an over speed of unknown entity on my Savannah LSA plane (consequence : big power loss -500 rpm with perfect carbs , push rods and compression and leak down tests ok. Growing oil consumption, little misfiring) i had the suggestion to do an exhaust gas analysis on my rotax 912 uls (100 hrs from new).
Infact after a boroscope analysis (with a very cheap but low res instrument) there is evidence of some partial burned oil in the combustion chambers (little spots dark orange caramel like appareance).
Leakage can be suspected from valve stem guides.
Someone has done an exhaust gas analysis on rotax 912 uls ?
Is there experience on witch are the normal range value of Co, Co2 , HC ?
If there is an oil leakage is it shown in altered HC reading values ?
Thanks in advance , i am not an expert and i am very far from nearest rotax service center.
Nicola di Biase
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Tue Nov 02, 2010 10:02 am Post subject: Re: 912 uls exhaust gas analysis |
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I agree with Bob.
The engine manuals are free for the taking on the web and tell
you exactly what to do in case of an engine overspeed or other
issue.
If you are working in isolation, read the books and don't bother
asking advise from people who are not very familiar with the Rotax.
It is not like other aircraft engines in many ways.
Gas analysis is not a routine procedure, is not recommended by
Rotax, and would not be the best way to diagnose bent lifters and
bad, floated valves from an overspeed.
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Tue Nov 02, 2010 12:59 pm Post subject: Re: 912 uls exhaust gas analysis |
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Hi Nicola,
How bad was the over speed? With prop or without? How long? Do all of the engine temps and pressures look good and do you have all the read outs? (EGT, CHT ect...)
You said now that oil is constantly used? What does the inside of the exhaust pipe look like? (Oily, dry, black soot, gray soot ect...)
There is a procedure in the maint. manual for the over speed check, but these guys are right that a Rotax shop should do the check.
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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ndibiase
Joined: 19 Oct 2008
Posts: 8
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| Posted: Tue Nov 02, 2010 2:41 pm Post subject: Re: 912 uls exhaust gas analysis |
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| Roger Lee wrote: | Hi Nicola,
How bad was the over speed? With prop or without? How long? Do all of the engine temps and pressures look good and do you have all the read outs? (EGT, CHT ect...)
You said now that oil is constantly used? What does the inside of the exhaust pipe look like? (Oily, dry, black soot, gray soot ect...)
There is a procedure in the maint. manual for the over speed check, but these guys are right that a Rotax shop should do the check. |
Over speed was of unknown entity , perhaps > 6200 rpm (i was forced to do a sudden collision avoidance maneuver in the traffic circuit with full throttle, high airplane speed and bad -minimum pitch- variable propeller setting) but it last for only 2-3 seconds.
After that ,there was huge power loss (550 rpm) , CHT normal, EGT with no abnormal sign (max 30° Celsius spread between cylinder couple and 90° between front and rear). Very little misfiring at low load/rpm. Oil consumption has gone from almost negligible to 250 gr. in 2-3 hour.
Leak down test and compression test perfect. carbs and leverages are ok.
spark plug seems ok.
Boroscope analysis (with a very cheap and low res instrument) showed evidence of some partial burned oil in the combustion chambers (little spots dark orange caramel like appearance) but i am not an expert and this is my first time of using this kind of instrument in an engine.
There was also a very subtle sign. Oil recovery procedure (the "gurgle") takes much more propeller turns than before. I am not a mechanic but, in my humble opinion, this can be sign of valve stem guide problems (crankcase pressure escape from the valve guides- heads in 912 are pressurized ). Is it a reasonable hypothesis ?
There is some black soot in the exhaust.
i know that in case over speed > 6200 rpm Rotax maint. manual states "the whole engine must be inspected......and i will contact nearest rotax heavy maintenance center. But i'd like to do a logical analysis before to open the engine.
My summary :
If is it a valve train problem it is strange that leak down and compression tests are perfect.
if is it a crankshaft problem it seems very strange to have not a lot of engine vibrations. i'd like to know how common and what are the usual signs of crankshaft problems in 912 ULS?.
thanks
Nicola di Biase
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Tue Nov 02, 2010 7:50 pm Post subject: Re: 912 uls exhaust gas analysis |
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I sent Nicola's comments in for a second opinion and here is what was said. This come from way up the food chain and should be considered very good information to follow.
_________________________________________________________________________________________________________________________-
Roger
First, what was the differential pressure test result per cylinder, to just say its ok is not good enough in this case. While the book says you can accept 25% loss I would not personally, anything over 10 would send an alarm bell ringing for me.
Indeed, if the RPM was high enough the only way you would do anything is if you kissed a valve and perhaps bent a push rod or two. If the report is correct higher oil consumption might indicate damage to the oil ring(s) and that might have happened in the case of cracking the piston ring lands if the piston hit the head(s)
For sure since he reports performance loss treat it as requiring more in-depth inspection. Remove the heads, check all the push rods and pistons. With the heads off you will know right away if you have a sticking valve. If you have any bent push rods pull all the valves and check for bent valve faces.
On the good side, if no valves hit the pistons it is extremely unlikely to have caused any crankshaft problems, so this should be fine.
The report that it is harder to get it to “gurgle” can be a sign of perhaps damaged rings, or leaking valves, both of which would cause the engine case pressures to be slightly lower and affect the case pressure.
My recommendation, bring it to someone who can do a good evaluation of the top end.
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_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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