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EGT Myths Explained


Louie
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Another example of "useless information" (or should I say, "information of little use) creating worries for pilots. Always a good way to make a quid - sell the idea that a new device gives information that has suddenly become essential to operating a machine, but older machines continue to work happily without it.

 

Perhaps the lesson from this article is that it is more important to understand what an instrument is measuring than it is to fixate on the measurements themselves.

 

On the subject of mixture control, I wonder if we should be so anal about altering the mixture during flight. After all, for most members here, the majority of flights are carried out below 5000', and over relatively short distances. Under these circumstances, it is probably kinder to the engine to hang the expense and run close to rich. If, however, you were doing a long distance flight in legs approaching maximum fuel range, then yanking back on the mixture knob might be advantageous.

 

After all, who of us fiddles with the mixture on our (pre engine management computer) cars when we drive from the coast to the mountains?

 

Old Man Emu

 

 

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i wouldnt call it useless information, if it was useless, why do modern engines rely so heavily on it? as for sell, not so sure, plenty of different branded EGS monitors out there, i have one on my 912..

 

The key is interpreting the information given!!

 

A mag check at cruise RPM will show up a slowly dying plug, well before it shows up on a mag test at Runup. and as there are 2 mags, (ignition systems) and knowing which plugs are fired by which system, i can determine exactly which plug is fouling, or slowly degrading, saving huge amounts of time trouble shooting when a dead plug shows up in a ground runup.

 

they are a great diagnostic tool if you know how to read the data and interpret it... example, if you turn one mag off, and get a small rpm drop, what does that tell you?

 

what if you have an EGT gauge on every cylinder, you do your mag check, and note a small rpm drop as before, but this time, all but 1 EGT rises? whats going on? ,

 

 

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Interesting article, how does the cht apply to water cooled heads on 2 stroke engines. Why do engine manufacturers specify max egt temps? Why do they specify optimal temp operating ranges etc?..This article raises more questions than it answers... My 2 stroke has no temp or mixture control that can be operated from the cabin yet I regularly fly over 5000 ft. This subject of chasing figures does my head in.

 

 

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Thank you very much for posting this thread. I'd agree fully with the article; I started out using an ALCOR gauge in exactly the manner described - lean to peak EGT and then back it off two to three divisions. It worked very well indeed in my PA 28. Trouble was, I had only one of them, whereas to see the GAMI spread, you need a gauge on each cylinder AND manual mixture control. The debate on another thread about EGT spread indicating mixture mal-distribution is a load of nonsense; it needs the ability to measure the GAMI spread.

 

This comes very timely indeed, for the running about to be done in my engine test cell. I'll have to make provision to alter the mixture on the Bing CD carbie, so it can be used to get this information, as a research tool.

 

However, provided the jetting and carbie-bowl venting is correctly set at the factory, a CD carbie like the Bing as fitted to Rotax 912 and Jabiru engines in RAA aircraft, does not need mixture control over the density altitude range allowed for RAA aircraft; arguably, it's better without it.

 

 

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you do have some form of mixture control with rotax and Jab engines, carby heat has a significant difference in mixture levels when used. and with EGT on each cylinder, its noticeable, not only that, it helps with improving the GAMI spread, as warmer air helps with the fuel atomisation meaning the mixture evens out a little across all cylinders.

 

for Unleaded fuel, the best temp for atomisation is 20 deg C apparently. its the reason ICP and other companies using 912's and other engines have an Intake temperature probe, so using the carby heat, i can keep airbox intake temps as close to 20 deg as possible.

 

 

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you do have some form of mixture control with rotax and Jab engines, carby heat has a significant difference in mixture levels when used. and with EGT on each cylinder, its noticeable, not only that, it helps with improving the GAMI spread, as warmer air helps with the fuel atomisation meaning the mixture evens out a little across all cylinders. for Unleaded fuel, the best temp for atomisation is 20 deg C apparently. its the reason ICP and other companies using 912's and other engines have an Intake temperature probe, so using the carby heat, i can keep airbox intake temps as close to 20 deg as possible.

To measure the GAMI spread you need to be able to plot the EGT versus fuel flow across the mixture range that gives peak EGT for all cylinders. I agree it would be possible to do that given an EMS that also measured fuel flow - IF the mixture control means has sufficient authority. The EGT spread at a constant fuel flow is NOT the GAMI spread - as the article for which the link was given in the first post explained.

 

 

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i know its not exactly a gami spread without metered fuel use per cylinder, but a more evenly atomised air fuel mixture, of whatever ratio is decided by jet sizes or carby setup, will result in a more even distribution to each individual cylinder, which will in effect, bring the peak EGT of each cylinder a little bit closer to each other, similarly described by the GAMI spread example. though it obviously wont be as close, or accurate as a separately adjusted mixture, with metered fuel flow readings and matched Injectors, but its a little bit towards achieving that outcome with inflight mixture control limited to only the use of Carby heat.

 

 

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Another example of "useless information" (or should I say, "information of little use) creating worries for pilots. Always a good way to make a quid - sell the idea that a new device gives information that has suddenly become essential to operating a machine, but older machines continue to work happily without it.Perhaps the lesson from this article is that it is more important to understand what an instrument is measuring than it is to fixate on the measurements themselves.

 

On the subject of mixture control, I wonder if we should be so anal about altering the mixture during flight. After all, for most members here, the majority of flights are carried out below 5000', and over relatively short distances. Under these circumstances, it is probably kinder to the engine to hang the expense and run close to rich. If, however, you were doing a long distance flight in legs approaching maximum fuel range, then yanking back on the mixture knob might be advantageous.

 

After all, who of us fiddles with the mixture on our (pre engine management computer) cars when we drive from the coast to the mountains?

 

Old Man Emu

OME, with respect I think you have missed the point of this article. It's not trying to sell anything, quite the contrary, the writer is explaining the history and misunderstanding of trying to chase EGT readings. The reason I posted this, and as alluded to by Daffyd, is the misinformation that exists regarding understanding these readings. The GAMI spread is the key factor.

 

 

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No, I don't think I missed the point of the article. Perhaps I should have put that comment about selling whiz bang stuff should have been put in parentheses as I meant that comment to be an aside.

 

My second paragraph is more to the point.

 

OME

 

 

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Yes, it is indeed. And spot-on.

 

However, I'm looking at this subject from a research point of view; I'm looking for a means of getting a realistic measurement of the mixture distribution between cylinders, and what affects it. What the article makes clear is that merely getting an EGT readout on every cylinder is NOT a valid measurement, due to the various factors that the article explains so lucidly. The absolute EGT value is practically meaningless; a whole generation of pilots use EGT scanners etc as an indication of the healthy operation of their engines, under the mis-apprehension of what these readings actually mean. I've been guilty of that myself, on occasion, and I'm very glad to have been reminded of the error of this. The value of instruments that monitor EGT - unless one uses them as a means of making sure the highest one is still at least 50 C on the rich side of peak EGT, which is not really possible without mixture control - is to allow one to spot when any one EGT changes significantly, out of pattern.

 

i know its not exactly a gami spread without metered fuel use per cylinder, but a more evenly atomised air fuel mixture, of whatever ratio is decided by jet sizes or carby setup, will result in a more even distribution to each individual cylinder, which will in effect, bring the peak EGT of each cylinder a little bit closer to each other, similarly described by the GAMI spread example. though it obviously wont be as close, or accurate as a separately adjusted mixture, with metered fuel flow readings and matched Injectors, but its a little bit towards achieving that outcome with inflight mixture control limited to only the use of Carby heat.

I'd agree with you IF we can establish that the change is in fact due to the increased proportion of the fuel spray that is converted to fuel vapour (i.e. from liquid droplets to gas). I'm NOT talking about finer atomisation; I doubt a small change in induction air temperature can alter that. Do not confuse the two terms.

Unfortunately, the design of the Jabiru airbox means that altering the source of the incoming air by selecting some hot air, also affects the flow pattern within the airbox and thus the amount of swirl going "down the plughole" to the carburettor. So using this as a means of modifying the EGT distribution cannot tell you what is happening to the GAMI spread; to do that you would have to do a GAMI spread measurement via a means of mixture control AT THE CARBURETTOR, firstly with cold air selected, and then with some hot air selected.

 

What this says to me is that to do anything useful in the test cell by way of research into this, I need a means of adjusting the mixture strength at the carburettor. There are ways to do that in a research context on the ground, primarily by modifying the air pressure above the fuel in the float bowl, but I do not consider them very suitable for use in flight, unless the pressure sources they use are not affected by the throttle setting.

 

 

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I suspect the position of the actual egt in relation to the exhaust port and flow pulses in the extractor is quite critical. When we were developing extractors for our racing engines, we used a pyrometer to determine the pulse positions along the extractor so as to place the extractor joins in the correct position so that we had each pipe entering the junction at a low-pressure point for a specific rev range (to best suit the cam being used) and at any given revs the positions of pulses changes, so one gets different positions for hot spots along the length of the extractor depending on the revs. Having the egts set at the same distance from the port is a good step along the way but any difference in the curve of the extractor tube will make variations. The Jab exhaust tubes have different bends for different cylinders and it's not possible to get an exactly uniform position for the egts - at least clamp-type egts - and have the clamp work effectively; welded-nut and screw-in type egts would be easier here, but you'd still need to determine the correct location for each tube for a given rev range (which will be small).

 

 

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I'd agree with you IF we can establish that the change is in fact due to the increased proportion of the fuel spray that is converted to fuel vapour (i.e. from liquid droplets to gas). I'm NOT talking about finer atomisation; .

yes, i agree that this is whats changing, and it is noticeable on an EGT monitor.

 

I know without correct setup and equipment, that getting an accurate GAMI spread is impossible, but what i was trying to say, is that with multiple EGT's , and the use of carby heat, effecting the atomisation, or vaporisation levels of the air fuel mix, it can help take out some of the unevenness or rough running caused by differing mixtures getting to different cylinders.

 

and the use of individual EGT's per cylinder, also makes a fantastic diagnostic tool once you know how to interoperate the data provided, and yes, the actual numbers are meaningless.

 

 

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Well, now that we know that, maybe I can get some data; I can't measure the fuel flow to each cylinder, of course (and I don't know of a fuel flow system that does that even for an injected engine) but I can measure the overall fuel flow quite accurately. Now, if I can just get sufficient mixture adjustment range on the carbie . . .

 

 

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Interesting article, how does the cht apply to water cooled heads on 2 stroke engines. Why do engine manufacturers specify max egt temps? Why do they specify optimal temp operating ranges etc?..This article raises more questions than it answers... My 2 stroke has no temp or mixture control that can be operated from the cabin yet I regularly fly over 5000 ft. This subject of chasing figures does my head in.

Two strokes are little different, I found a good article about two strokes a little while ago: http://www.bcchapel.org/pages/0003/pg11.htm

 

 

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