The science of EGT vs. TIT

R

RobertGerace

Guest
This is a very technical question, but:

I've been told that individual EGT measurements see 1 pulse of hot air for every 360 degree turn of the crankshaft. This means they get hot, and then cool down, 2400 times per minute. The average temperature is then sensed by the EGT probe for that cylinder and fed back to the engine monitor.

In contrast, the standard implementation in Twin Cessna's is to put the TIT probe aft of all three cylinders on the inboard side.

Because the TIT probe sees three times as many pulses, it has 1/3 the time to cool, and therefore reads hotter.

On all the above I have yet to have anyone disagree.

Now: the information that I seek:

How MUCH hotter should TIT be than the hottest EGT on the inboard side be? Is there text or evidence to support 100dF? 200dF?

I've been told 100 and that is what I saw before my new installation. Since my new installation I see 200. I've come to the conclusion that the TIT value is wrong, and I'm looking for supporting ideas (or for someone to tell me they think I'm wrong, and that 200 is reasonable.)

Thanks!
 
RobertGerace said:
This is a very technical question, but:

I've been told that individual EGT measurements see 1 pulse of hot air for every 360 degree turn of the crankshaft. This means they get hot, and then cool down, 2400 times per minute. The average temperature is then sensed by the EGT probe for that cylinder and fed back to the engine monitor.

In contrast, the standard implementation in Twin Cessna's is to put the TIT probe aft of all three cylinders on the inboard side.

Because the TIT probe sees three times as many pulses, it has 1/3 the time to cool, and therefore reads hotter.

On all the above I have yet to have anyone disagree.

Now: the information that I seek:

How MUCH hotter should TIT be than the hottest EGT on the inboard side be? Is there text or evidence to support 100dF? 200dF?

I've been told 100 and that is what I saw before my new installation. Since my new installation I see 200. I've come to the conclusion that the TIT value is wrong, and I'm looking for supporting ideas (or for someone to tell me they think I'm wrong, and that 200 is reasonable.)

Thanks!

Small nit to pick, should it be for every two revolutions of the crank the EGT sees a hot gas, therefore it only sees the pulses 1200 times per minute? Unless you have one of the rare 2 stroke 310's that is.

Now on to your question:
Seems to me that if the TIT is 200 degrees higher now that the EGT where it used to be 100 degrees higher my first question is what are the EGT numbers now vs before and what is the TIT number now vs before and which one has changed? Thermocouples don't seem to drift much, either they work or they don't. What was changed? Are there some exhaust leaks that got repaired that were leaking hot gasses before it could get to the turbine and now you are getting more of the exhaust to the turbine? That is about the only thing I can think of now. You might want to try the CPA board too.

Just trying to think aloud here, hope I made some sense.
Mark
 
RobertGerace said:
This is a very technical question, but:

I've been told that individual EGT measurements see 1 pulse of hot air for every 360 degree turn of the crankshaft.

Actually it's one "pulse" per two revolutions of the crank (4 stroke engine).

RobertGerace said:
This means they get hot, and then cool down, 2400 times per minute.

Well, it's true that the temp of the exhaust gas fluctuates throughout the cycle, but AFaIK the magnitude of the variation is less than you'd think. When the exhaust valve opens, the gasses have already cooled quite a bit from the peak combustion temp due to radiation and conduction into the cylinder barrel and head plus (this is where most of the heat in the cylinders comes from) the substantial adiabatic drop in temp. From the first part of the exhaust stroke to the end of it, I believe the temp is fairly constant since there's an adiabatic heating going on inside the cylinder and there isn't as big a difference between the gas and the cylinder temp. Once the exhaust valve shuts, there is some cooling from the further expansion of the gas within the exhaust plumbing itself, but this expansion is limited by the high flow out the other end (the residual pressure carried into the next cycle is what we commonly refer to as the "back pressure"). During this expansion/cooling, there isn't much flow in the vicinity of the EGT probe (ideally located 2-3 inches from the valve) so even though the gas is cooler, the probe itself stays at a fairly constant temperature.

I suspect you were visualizing a blast of hot gas followed by a longer blast of cool air with the probe responding to an average temp far below the peak, but there is no blast of cool gas, just a slight falling off in temp and pressure accompanied by a large drop in flow. In fact, the pressure (and temp) probably oscillates up and down several times due to the inertia of the gas.


RobertGerace said:
In contrast, the standard implementation in Twin Cessna's is to put the TIT probe aft of all three cylinders on the inboard side.

Because the TIT probe sees three times as many pulses, it has 1/3 the time to cool, and therefore reads hotter.

That much is true, but there are other reasons for the TIT on a turbocharged engine to be higher than the EGT on a normally aspirated engine. For one thing the induction air temp is much higher with a turbo. An intercooler mitigates this somewhat, but not anywhere near completely. Second, the turbine creates a significantly higher exhaust pressure upstream of the turbo and that higher pressure means higher temps by itself (adiabatics again). Third the combustion temps are higher with a turbo whenever the MAP is higher than ambient. Finally, it's common to insulate the exhaust to carry the maximum amount of energy to the turbine.

[/QUOTE] On all the above I have yet to have anyone disagree.

Now: the information that I seek:

How MUCH hotter should TIT be than the hottest EGT on the inboard side be? Is there text or evidence to support 100dF? 200dF?

I've been told 100 and that is what I saw before my new installation. Since my new installation I see 200. I've come to the conclusion that the TIT value is wrong, and I'm looking for supporting ideas (or for someone to tell me they think I'm wrong, and that 200 is reasonable.)

Thanks![/QUOTE]

New installation of what? The TIT probe/gauge can be checked for accuracy by comparing it's temp with a calibrated probe at the same temp. The readout should be linear so you only need to check it at two points to get the full calibration.

If you mean you expect a 100 F difference between the average individual cylinder EGT reading and the TIT, that does sound about right, but I have no idea what the acceptable limits are. I'd think there should be some data on the relationship between fuel flow at a given MAP/RPM/PA and the TIT somewhere if that's what you are concerned about.
 
Robert:

George Braly has written on the Bo webboard many times about this. I completely undstand right after reading it; then go on to other things and couldn't possibly explain it without reviewing it. Deakin has a good article (actually series of four) on AvWeb. You might wish to peruse them.

From a practical standpoint, TIT is the most sensitive trend instrument; therefore, I reference it on takeoff keeping it under 1280 which means all my cylinders are cool. Changes in MP, and mixture are first shown by a TIT change.

After leveling off and established in cruise, I go to my hottest CHT and keep it under 380. Braly will tell you EGTs are pretty meaningless other than for establishing where stociometric combustion occurs (peat egt). Probe placement, mixture at time of combustion, type probe and other variables affect it. CHT directly corrolates to internal cylinder pressure (which is what the engineers design out engine around). Until we can measure that more directly, CHT is the best we have.

Didn't exactly answer your question, but from a practical standpoint, it's what the pilot needs to know.

Best,

Dave
A-36TN ADS
 
Bob:

Since I run WOT, LOP, the relationships will be different in my plane than yours. Under what conditions are you measuring? MP, Mixture, Prop? I assume Rich of Peak. Do you know how rich? I'll get you an answer.

Dave
 
Bob, I have been through this very issue. The way to do this is to (1) uncowl the engine. (2) With someone you TRUST or who trusts you, run the engine up to 65% power and go past lean, while pointing the Infrared gun at the very exhaust bend in question, one at a time. You'll get the true average temp. Compare with the live reading.

Caution: 65% is ROTSA power on the ground and that prop is not far away. But it is the only way to settle the issues. Sigh.
 
And I'll add the following to the excellent advice.

Probe location makes a huge difference.

On my engine, I have both the 100% regulation, grade A, certified primary TIT instrument. It happens to be an EI digital unit because the original Alcor was no longer supported when it burnt out. I also have a JPI system with individual EGT and TIT metering. I routinely see a 50-ish degree difference between the JPI and the EI. Peaks occur on both at the same mixture setting.

Part of the difference may be the probes themselves, and part may be the specific location.

Also, if you had your exhaust system refurbed, the holes may have been drilled in slightly different places - or one or more of the probes may - just may - be starting to go bad. When my exhaust was redone, I had 3 probes crap out a matter of 50 hours after the work was done. I replaced all 5 probes on the engine.

One other thing. Any resistance in the probe lines can affect readings. So make sure the terminals were cleaned to bright metal where they are connected to the wires going into the cabin.
 
bbchien said:
run the engine up to 65% power and go past lean, while pointing the Infrared gun at the very exhaust bend in question, one at a time. You'll get the true average temp. Compare with the live reading.

Caution: 65% is ROTSA power on the ground and that prop is not far away. But it is the only way to settle the issues. Sigh.

Bruce,

After some quick checking, a gun that handles temps near 1,000c (2,000dF) is over $5,000. Is there a place one can rent one of these guns?

Thanks!
 
Large Turbine FBO. I've seen them for $3,500. Byerly had one in 1998 but I don't know if they still have it....
 
A lot of shops have the digital guns and will do the test for you while you run the plane up. Suprised the shop that did the installation didn't offer.

Dave
 
RobertGerace said:
Bruce,

After some quick checking, a gun that handles temps near 1,000c (2,000dF) is over $5,000. Is there a place one can rent one of these guns?

Thanks!

Bob, you have got to shop better.... http://www.omega.com/ppt/pptsc.asp?ref=OS550&Nav=temj11

Hey!!!! I got a great idea!!! You could pay me to be your operations manager and pay me from all the money I save you!
:dance:
 
Back
Top