Home » Public Forums » GMCnet » Re: [GMCnet] Non-GMC engine question
| Re: [GMCnet] Non-GMC engine question [message #89090] |
Fri, 18 June 2010 07:24  |
Gary Casey
Messages: 448
Registered: September 2009
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My apoligies for continuing this somewhat off-topic thread, but I think Ken is "hanging by a thread", so to speak :-(
The high EGT being caused by an air leak is highly unlikely - maybe impossible. As Ken said, the highest EGT previously seen was about 1500 regardless of mixture. Second, compared to the air flow through the essentially open throttle, any "normal" air leak caused by a bad gasket or crack in the intake tube would have to be miniscule. It could make a noticeable change in idle characteristic, but in flight? I don't think so.
The only problem with an EGT probe that I can think of is if the probe shorted to ground. It would still read temperature, but with an error. Some aircraft systems use grounded probes and some ungrounded - I don't remember who uses which.
Compression test? I wouldn't bother. A simple way to perform a surprising accurate compression test is to simply crank the engine with the fuel turned off. Are the compression "lumps" uniform. You'll easily detect a difference between cylinders that way. I do that test occasionally, usually inadvertently during a hot start :-). Any leakage in a cylinder that could even slightly effect performance at a high engine speed will be easily detectable during cranking.
There is a natural limit, but I wouldn't call it a theoretical limit, on EGT. It depends primarily on compression ratio and ignition timing. Running on one plug effectively slows combustion (one flame front instead of two), raising the EGT above the maximum that could be obtained with both firing.
The temperature of one cylinder going up by 100 degrees pretty much correlates to a spark plug that ceases firing. The cylinder will continue to put out about 90% of normal power (may or may not be noticeable) and all other temperatures will remain close to normal. Only the higher EGT will indicate the problem, which Ken reported. Did Ken do a mag check in the air after he saw the problem? I know I would not have done that at night at 3500 feet (why so low? At night?), but there are those more "metallically endowed" than myself that routinely do mag checks in the air. In this case it would have immediately identified the problem if it were indeed an ignition-related failure.
My bets are on either a spark plug going shorted or an ignition wire doing the same. Problem is, it might not show up on the ground, so another, "diagnostic", flight (my wife hates the phrase "test" flight) might be in order.
Good luck,
Gary Casey
N422SG
mcolie wrote on Thu, 17 June 2010 20:19
> Ken Burton wrote on Thu, 17 June 2010 12:54
> > Last night on the way home I noticed one of my airplane cylinders was running 1642 degrees EGT. It got my attention to say the least as I was about 1 hour from home well after dark which limits your landing options in case of an emergency.
> >
> > I have never seen an EGT over 1520 or so. Adding carb heat decreased the reading by about 75 degrees but that was still higher than I have ever seen before. The interesting thing is the CHT was the same on that cylinder as all of the rest which was around 340 degrees. Oil temperature was also in the green.
> >
> > I was running full rich at about 3500 feet with an OAT of 72 degrees. This is a carburetored non-turbocharged 87 octane O-320 engine.
> >
> > I have always thought from past leaning experiences that 1500 degrees or slightly higher was the theoretical limit on 100% non-turbo gasoline powered engines.
> >
> > I'm thinking and hoping that this is simply a bad probe.
> > Note: I also intend to run a differential compression test on the cylinder in question in the next couple of days.
> >
> > Is there anyone out there that has any knowledge on this subject?
>
> Ken,
> Let's start with the bad side. . .
> In my decades of lab work, T/couples failed reading low or open. (our systems could detect open)
> Let's hope for the good side...
> You were at cruise - probably 20-24" map????
> If you have developed and intake runner leak, that cylinder would go lean. The lean mixture will burn more slowly - hence high EGT. CHT might go up a little, but that is more dependent of BMEP.
>
> Let's hope it is one of those gaskets where the intake goes into the head.
>
> Matt
Matt Here is my point. On an airplane we have control of the mixture and we lean out the engine when flying over 5000 feet. We lean the mixture until we see peak EGT. If we continue lean the engine past peak EGT very much the engine starts to miss due to an over lean condition. We richen (if that is a word) the mixture so the EGT on the highest cylinder goes down by 50 degrees. (some people go 75 degrees) I have never seen a peak EGT higher than 1520. The reading of 1642 really confuses me. The probes are indeed a thermocouple. I thought thermocouples fail by reading low and not high. This is why I am questioning what I think I know about this whole thing.
Some aircraft have only one EGT probe and some like mine have probes on all cylinders. My past experience has been the peak EGT reading never exceeds 1520 degrees and usually it is around 1500 so I can not understand a reading of 1642 degrees with no change in CHT or engine running smoothness. I would buy your idea of an intake leak on one cylinder if the EGT reading peaked around 1500 and or the engine ran roughly.
I'll still check for an intake leak on that cylinder.
Please tell me more.
--
Ken Burton - N9KB
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