Any one interested in a nice C-150

[tonycondon]

Burn temps are a factor of mixture, we vary the mixture until we see what we want.

As the mix goes from way rich to way lean it will rise to a point where it starts dropping, that is known as peak, if you continue to lean the temps will start to drop unil the engine starts to studder/stumble/vibrate. that is the point we stop and push the mixture toward the rich, until we are rich enough to run smooth.

that is a point near 50 degree lean of peak.

ive studied the theory behind lean and rich of peak engine operation.

my question: does the location of the temperature probe have any effect on determining the peak EGT? if you had 4 or 5 probes on an exhaust pipe, would they all not show a peak temperature at the same mixture setting? yes, the temperature on each probe would be different, but they all should peak together. so for the purpose of referencing peak EGT, probe location shouldnt matter.

 

[Keith Lane]

so for the purpose of referencing peak EGT, probe location shouldnt matter.

For a given single cylinder, this would be true. But, if you want to know the peak temp for all cylinders simultaneously, and relative to each other, all the probes have to be in the same location in each exhaust header pipe relative to the exhaust valve.

 

[tonycondon]

For a given single cylinder, this would be true. But, if you want to know the peak temp for all cylinders simultaneously, and relative to each other, all the probes have to be in the same location in each exhaust header pipe relative to the exhaust valve.

why?

lets say you had 5 probes spaced an inch apart on a given cylinder, would 50deg ROP not be 50 degROP on all 5 probes?

 

[PittsDriver]

why?

lets say you had 5 probes spaced an inch apart on a given cylinder, would 50deg ROP not be 50 degROP on all 5 probes?

It would be but how much rich or lean of peak relative to each other isn't the only reason for wanting the information. If you're trying to balance the fuel flows to the engine to get it to run well LOP, then you're looking for actual EGTs, not relative. The idea is to get every cylinder matched to a norm so that one isn't running at peak when the rest are cooler and I don't know how you'd do that if the probe placements introduced a variation between jugs.

 

[tonycondon]

really? you can't tell which cylinder is rich and which cylinder is too lean by seeing which one is running 100 LOP when the others are 50 LOP, for example?

 

[Keith Lane]

really? you can't tell which cylinder is rich and which cylinder is too lean by seeing which one is running 100 LOP when the others are 50 LOP, for example?

You could. But to do it you would need to document what each individual cylinder's temperature reading was, relative to the other temperature readings.

A hypothetical scenario 1:
Your engine has perfect mixture balance on all cylinders. (which we know won't happen on a carbureted engine) You know that if one cylinder reads 1350 °, that all will read it. But only if the probes are all located the same distance from the exhaust valve. If the probes are sort of randomly spaced, then you'd have to know that Cylinder #1 always reads, for example, 50° cooler than #2, and that #3 read 100° hotter than #4 and so forth, due to the positioning of the probes. Because this engine is "perfect" then lean away. After it peaks, and starts to run cooler, you are LOP.

A hypothetical scenario 2:
You have a real engine, not like #1. You remove the variability of the readings as in scenario #1 by controlling the readings the only way you can, by making the method of measurement consistent. Putting all the probes in the same position relative to the exhaust valve is the best you can do. Now, when you read that #1 is 50° cooler than #2, you can assume it's because #1 is richer than #2 for some reason. Maybe it's because you have an induction leak on #2, causing a leaner mixture. By knowing that the readings are being made consistently on each cylinder, you can actually tell if one cylinder is getting cheated in the mixture department.

Combine the two engines above, making engine #2 have perfect mixture balance would yield temperature indications that showed the same temperature on all cylinders relative to each other ie. they all read 1350° at the same time, and all change the same amount with each mixture adjustment.

 

[Tom-D]

really? you can't tell which cylinder is rich and which cylinder is too lean by seeing which one is running 100 LOP when the others are 50 LOP, for example?

Yes you can, but remember we must be using the same standard to measure each cylinder's EGT, that is why we must place each probe the same distance from the exhaust valve.

Measuring all cylinders by the same standard gives us the ability to see which peaks first, and by how much. We really don't care what that temp is, as long as we know it reached the peak, and is now on the lean side of the temp range.

Most engines/aircraft we see will have only 1 probe installed in the cylinder that engineers have determined to be the one that will run the hottest on the rich side, thus be the first to reach peak. In these aircraft many pilots feel that this can lead to running the engine with 1 cylinder on the lean side and all other cylinders in the red zone on the rich side due to the spread of temps we see on any engine using the multi probe system.

 

[Tom-D]

ive studied the theory behind lean and rich of peak engine operation.

my question: does the location of the temperature probe have any effect on determining the peak EGT?

NO, you will see the rise and fall of the temps, calibration means nothing if all we want to know is when it happens.

if you had 4 or 5 probes on an exhaust pipe, would they all not show a peak temperature at the same mixture setting? yes, the temperature on each probe would be different, but they all should peak together. so for the purpose of referencing peak EGT, probe location shouldnt matter.

Yes, you are right, but when we need to know how one cylinder relates to the others, we need to have all cylinders being read on the same standard.