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hot EGT's

H

HansLab

Well Known Member
Yesterday was the hottest day ever measured in the Netherlands: in the 93's F.
I still flew my '12 - and enjoyed!!
However the EGT's got high, both of them the same. So high that the alarm went off and I reduced throttle.
Today I searched internet and learned a lot from causes, but I'm curious whether one of you '12 pilots encountered this problem as well - and how you coped?
High EGT means lean mixture, so when in the shop, I will
- pull plugs for evidence of leanness
- check carb's working perfectly (both EGT the same so I don't expect too much of this)
- check carb rubber for leaks (idem)
- choke lever range complete (idem)
- fuel filter for debris
- idle screw for 2 turns open
Some site recalled the wrapping of the exhaust pipes to reduce heat under the cowl. that might be a solution in the future.
To think I just (20 hrs ago) added an oil thermostat to quicker heat up oil during run-up...
All ideas highly appriciated!!
 
Thank you, Stu! But, CHT's aren't lower than usual. Speaking of which: I have some 100 hrs on this plane and nothing serious changed lately for these temps to get that high....
 
Fundamentals...

"High" EGT is useless. How many degrees warmer than previously typical?

The highest EGT's are recorded with mixture near stoichiometric. It may be leaner than previous, but with further leaning, EGT will begin to drop.

It's a 912..timing is unlikely to have changed.
 
There is no such thing as a maximum EGT limit or redline

Below is quoted from an ARTICLE, EGT Myths Debunked, by Mike Busch, an engine expert.
"High EGTs do not represent a threat to cylinder longevity the way high CHTs do. Therefore, limiting EGTs in an attempt to be
“kind to the engine” is simply misguided. There is no such thing as a maximum EGT limit or redline, and trying to keep absolute
EGTs below some particular value—or even worse, leaning to a particular absolute EGT value—is simply wrongheaded."
 
Never seen a rotax, but EGT behavior is somewhat similar for all 4 stroke engines. True that high egts are no danger in and of themselves. HOWEVER, if you normally get X degrees in any given RPM/mixture setting and one day get X + 150 degrees at the same setting, that is your engine telling you something and suggest you listen to it, regardless of what Mike bush says. EGTs are very sensitive to many variables in the engine and are mostly there as a tool for understanding where those variables are at any moment. Whenever your engine starts behaving differently than it normally does under the same conditions, you should ALWAYS investigate why that is. I do agree that comparing your EGT values to another guys EGT values is pointless.

Larry
 
Last edited:
problem: solved

Now the temps are more normal in these area's again, I flew the heck out of the 12. No hot EGT's what so ever. Nice and clean in the 760-770 range.
One pilot on the club said it might have to do with the hot weather, though: warm air is thinner, and the engine ' must work harder' to perform. Hence it'll run lean. Rotax 912 is unable to adjust that - hot EGT's.
I keep you posted whenever another hot day passes, but I guess this ticket can be closed.
Thanks for thinking with me, y'all!
 
760 degrees C is well below the maximum EGT permitted by Rotax (Operator manual, Ed 4, Rev 0, indicates maximum EGT of 880 C). If it has changed suddenly, then it makes sense to seek an explanation. But, the value itself does not represent an anomaly that needs correction.

Regarding the air density issue raised by another pilot in the club, that may not be as relevant as you think. With some imperfection, the Bing carburetors are designed to compensate for air density (being constant velocity, aka, constant vacuum) in design. So, while there is no manual mixture adjustment, the carbs should be compensating automatically. 93 degrees F may be warm for your climate, but plenty of us fly 912 ULS equipped aircraft in weather warmer than that.
 
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