EGT Reading

[sharpie_29]

Hi everyone, im currently flying charters and scenics in C172 RG Cutless its a wondereful aircraft and runs great, BUT when ever i go to lean the aircraft using the EGT Gauge i get a decrease in EGT initially of about 2-3 stages and then and increase back to what one would call "normal" indications once leaning is continued...
Anyone have any suggestions, the boss and i have called around LAME's and they cant answer our question :bored:

Cheers...

[sharpie_29]

A heap of views with nil reply's, obviously everyone is as clueless as we all are haha!

[eckhard]

Mis-rigging of the mixture control/cable?
Mis-reading EGT gauge?
Fouling of the carb heat cable? (Not sure if you have one fitted)

OK, I guess I'm clueless as well!

[Pilot DAR]

First of all, don't worry, as long as it will lean, you're not going to hurt it. However,

This is speculation, but the result of past experimentation...

The EGT probe itself really measures at the tip only. The EGT manufacturers are rather specific as to the placement of the probe in the exhaust path in all three axis, and distance down from the cylinder port. Often times, for a number of reasons, the probe cannot be installed exactly where it should be. Sometimes, a lack of care.

When you lean, you are changing the flame front in the combustion chamber a little, so it is possible that the path out the port, of the exhaust stream itself shifts a little. Thus, hotter or cooler portions of the stream can follow a different path in the first part of the exhaust stack. Possibly, with leaning, you are changing the flow of the exhaust slightly, and thus the tip of the EGT probe is "seeing" a hotter, then cooler, then hotter again exhaust flow.

If you are able to influence the maintenance of the aircraft, ask the mechanic if he can either withdraw, or insert the probe itself in the stack. Many probes are actually adjustable in this way, with two "C" clips. 5 minute job, once the cowls are off. This may produce a different leaning result. If it does, tell us about it!

[piggybank]

CFI Insights - Engine Leaning (http://www.principalair.ca/article%20-%20engineleaning.htm)

The subject is well covered on Google.

[172_driver]

piggybank,

If I read the thread correctly the OP is describing a scenario other than what should theoretically happen. He's telling us the EGT is initially decreasing as he starts to lean. Not very well covered on Google...

[piggybank]

I agree you have not got good replies to your question. I was trained on piston engines some 45 years back but after turbines I am not in a rush to work on them again.

Thats digressing.

As to the remark of the EGT being a variabe in as much as what route the exhaust gases travel down the exhaust tube with leaning. I can't comment on that bit.

Leaning is getting as close as you can to a chemically perfect burn, stoichiometric combustion I think the word is.

The mix burns hotter and will be seen as a rise in cylinder head temperature. Assuming the lag is small between the temperature change and the reading showing on the gauge this is more reliable than whats in the exhaust pipe.

Get the cylinder head too hot and you have an expensive repair coming up.

Surely any results you get will be monitored closely as you lean off and any odd changes are just that, something perhaps due to rigging or characteristic to that specific aircraft.

[sharpie_29]

Thanks for the replys everyone, we are waiting for the 100hrly to come up as DAR said its not a massive issue and i do monitor the CHT obviously as i lean and afterwards.
The aircraft runs fine, the fuel consumption is great once leaned. Everything seems to operational so im happy :)
Back to what DAR said about placement of the probe, the thing is the boss said it hasnt always done it which is strange. (I may of said in my original post that it did though)....

[Mechta]

Having just been reading this thread, I wonder if it contains the answer.
http://www.pprune.org/tech-log/468775-why-do-turbine-engines-require-compressor-section.html

There seems to be a consensus in the above thread that as the compression ratio of an engine is increased, the temperature of the exhaust gas decreases.

In that case, if some of the incoming charge is fuel, which would be displacing what would otherwise be air, then when the air content is increased by leaning the mixture, more air will be admitted. Also if the air is entraining the fuel mixture from the carb jet, this must be using the incoming air's kinetic energy to accelerate the fuel. If less fuel is to be entrained by leaning the mixture, then less work is being done by the incoming air so more of air will reach the cylinder and the air pressure in the cylinder by the time the inlet port shuts will be closer to ambient. Therefore a greater pressure will be achieved at top dead centre.

I am taking about the difference in pressure between ambient pressure outside the engine and the pressure at top dead centre, as a greater pressure difference may actually occur inside the engine the lower the pressure inside the cylinder is below ambient, but that won't help the performance.

Parachute ready, helmet on and prepared for incoming...