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Turbine Inlet Temperature


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How is it possible to have a turbine inlet temperature higher that any of the individual EGT’s? It would seem that the gasses entering the turbo would be downstream of the EGT probes, and the temp should be an average of the six EGT’s, but maybe I’m thinking about this wrong.

 

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2 hours ago, jlunseth said:

There is also a secondary combustion event that occurs in the collector after the exhaust gas leaves the cylinder and is allowed to expand.

Secondary combustion event?  I can only envision that if unburned fuel is passing through the exhaust valves with sufficient remaining oxygen to allow for combustion, which is certainly possible ROP, but LOP I wouldn’t think so? But even if that’s the case, I’m having a hard time with this concept.  I would think TIT would be an average of all six combustion events, rather than somehow additive to temperature? I don’t know what I don’t know, so I’m not saying I’m right, just trying to parse this out to make it more understandable.

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32 minutes ago, Brandt said:

Secondary combustion event?  I can only envision that if unburned fuel is passing through the exhaust valves with sufficient remaining oxygen to allow for combustion, which is certainly possible ROP, but LOP I wouldn’t think so? But even if that’s the case, I’m having a hard time with this concept.  I would think TIT would be an average of all six combustion events, rather than somehow additive to temperature? I don’t know what I don’t know, so I’m not saying I’m right, just trying to parse this out to make it more understandable.

This subject has been asked and answered many times on Mooneyspace. Since on every turbo-charged airplane I've owned, the TIT has always been higher than the highest EGT, I just accept that's the way it is. Although the answers I've read over the years seem to make sense, while I don't fully understand it, the reality is that the TIT is always higher than the hottest single EGT reading, so I go with it. 

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42 minutes ago, LANCECASPER said:

This subject has been asked and answered many times on Mooneyspace. Since on every turbo-charged airplane I've owned, the TIT has always been higher than the highest EGT, I just accept that's the way it is. Although the answers I've read over the years seem to make sense, while I don't fully understand it, the reality is that the TIT is always higher than the hottest single EGT reading, so I go with it. 

Ahh, you’re one of those “because I said so” types… I’ll be watching you from now on…

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1 hour ago, LANCECASPER said:

This subject has been asked and answered many times on Mooneyspace. Since on every turbo-charged airplane I've owned, the TIT has always been higher than the highest EGT, I just accept that's the way it is. Although the answers I've read over the years seem to make sense, while I don't fully understand it, the reality is that the TIT is always higher than the hottest single EGT reading, so I go with it. 

The picture I see in my head is a piece of exhaust pipe chucked up in a vice.  Fire up your torch (American torch burning gas, not UK “torch” with batteries) and wave it across the pipe just once.  Count to 5 and you can grab the pipe with your bare hand.  Now hold the torch on the pipe until it glows a nice red.  That’s the difference.

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The EGT probe is an average of all the time by the exhaust valve. While the exhaust gas is the hottest leaving the cylinder, it is relatively quick in duration compared to the rest of the time the exhaust valve is closed so the average reading is lower than what the true exhaust gas temp actually is. Now the TIT probe is getting blasted 6 times (5 more than the egt probe) in that same time thus the average will be higher as the probe does not get as much time shielded from the exhaust like the EGT probe does. So even though the exhaust gases are cooler at the turbo inlet than when first coming out of the cylinder, the average of six cooler exhaust gases is higher temp than the average of one blast followed by time with much lower temps. 
or put another way one blast doesn’t give the probe enough time to react like a probe getting blasted 6 times in the same time length. 

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Where is the TIT probe vs where is the EGT probe?  I don't know - I am asking so this isn't a rhetorical question.

But leading to - what does the volume of space look like where the gas passing across the probe look like?  I see no reason why the gas cannot heat or cool as it passes through pipes of various size.  PV=nRT and all.  Its how refrigerators work.  I always presumed this has something to do with it.

Also, temps vary between EGT probes in part because of variations of where they are placed.

But the outcome is as Lancaster says, TIT measurements are generally a good bit higher than EGT measurements.

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@brandt. To try to answer your question, whether an engine is operated LOP or ROP there is unburned fuel in the exhaust. It burns in the collector of a TCd engine and I am told by engineers that it burns hotter there than in the cylinder. The objective of either LOP or ROP operation is to slow the combustion cycle, which means of course that combustion is occurring later than at peak. So some unburned fuel will always be pushed out. For that matter some unburned fuel would be pushed out even at peak, or at the point where the combustion cycle is quickest which is somewhat after peak, just less of it than if the combustion cycle is elongated. 

There are three things that cause TIT to be higher than individual EGTs. The first is the measurement effect that has been discussed. The second is the secondary combustion event that occurs in the exhaust system. The third is that the exhaust flow is restricted at the turbine (it hits the fan), meaning higher pressure, and higher pressure equals higher temp. Of these, I am told that the measurement effect is the greater. I should hasten to add that I have heard different explanations of the secondary combustion event from engineers, one being the delayed combustion event that I described and the other being some kind of catalyzation of the exhaust gases themselves as opposed to burning of residual fuel. That is over my head, ask an engineer, which I ain’t.

Lance is right this question has come up many times and has been answered many times by people smarter than me. Try searching on “secondary AND combustion AND TIT” and you will see some of the discussions from the past. I found one that I was involved in in 2016 and I know there were discussions even before that.

None of it is very important. Simple is that TIT will be hotter than individual EGTs so you know what to expect on your monitor, but other than that it does not mean much. Just understand it is normal.  Now if yours suddenly isn’t that should get your attention and lead to an immediate emergency landing post haste and a check of the connection between the collector and the turbocharger but that is another story.

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18 minutes ago, aviatoreb said:

Where is the TIT probe vs where is the EGT probe?  I don't know - I am asking so this isn't a rhetorical question.

But leading to - what does the volume of space look like where the gas passing across the probe look like?  I see no reason why the gas cannot heat or cool as it passes through pipes of various size.  PV=nRT and all.  Its how refrigerators work.  I always presumed this has something to do with it.

I think there is likely some pressure rise at the turbine inlet due to multiple sources of exhaust gas piling up there.   The turbine does resist flow (or it wouldn't turn), so the pressure increase is likely to cause some of the temperature increase.    There are various references around as to how much hp it takes to compress the intake air for various boost levels and volume flow, and that heat goes both into the turbine and the intake air.

There are probably multiple effects combining for the temp rise, including the previously mentioned effects on the EGT probes where they may be reading lower than peak, anyway.   Nobody cares about the absolute temp of the EGT being accurate, so the TIT may just be more reflective of the actual temps.

 

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If you look at my numbers, you can see "TIT" is higher.....except........ I don't have a turbine. This is an Ovation. The "TIT" number is the probe on the collector pipe and collector is exactly what it infers. It collects all the exhaust gas on that side with no break as opposed to individual EGT probes.

You can prove the additive aspects in that the "TIT" temp when running full rich is about 30 degrees higher than the highest EGT(#4). When at peak  EGT is it about 90 degrees higher than the highest (#4). At 50 LOP it is about 60 higher than the highest, again, #4. If there were secondary combustion the flame is longest at full rich (we Beech 18 guys you to play with flame length with the mixture when bored) While one could argue that long flame is cooler due to excess fuel it cannot amount for the very large difference at peak, especially when viewed in light of the LOP differential.

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2 hours ago, Will.iam said:

The EGT probe is an average of all the time by the exhaust valve. While the exhaust gas is the hottest leaving the cylinder, it is relatively quick in duration compared to the rest of the time the exhaust valve is closed so the average reading is lower than what the true exhaust gas temp actually is. Now the TIT probe is getting blasted 6 times (5 more than the egt probe) in that same time thus the average will be higher as the probe does not get as much time shielded from the exhaust like the EGT probe does. So even though the exhaust gases are cooler at the turbo inlet than when first coming out of the cylinder, the average of six cooler exhaust gases is higher temp than the average of one blast followed by time with much lower temps. 
or put another way one blast doesn’t give the probe enough time to react like a probe getting blasted 6 times in the same time length. 

You can try to come up with all kinds of complex answers, but this is the correct one.

The probe has mass and isn’t a max recording device, but an averaging device.

‘The actual peak EGT is significantly higher than displayed

Secondary ignition does exist in some instances, like a top fuel dragster that’s so rich all the O2 is consumed so that when the superheated fuel combines with air after exiting the exhaust pipe it re-lights, but not until after exiting the exhaust 

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Semantics…

The turbo is a mass of metal where hot exhaust gasses are forced past a fan blade that drives a compressor on the other side of the shaft which is compressing air.  It also receives every single exhaust gas cycle of the three cylinders (six for the bravo) with no break between, which means the egt probes experience 1 cycle for every three of the turbo.  It is simple thermodynamics 

There is nothing in the assembly that comprises the “turbo” that does not concentrate, and convect heat. It does not surprise me that the TIT would be hotter than an egt. 
 

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The exhaust side of the turbo is cast iron or maybe Inconel or other heat resistant metal, if the exhaust housing were aluminum it probably would melt, depending on alloy aluminums melting temp is about 1400F

The exh temp just prior and just after the turbine is different, it’s cooler on the exhaust side, it’s cooler because energy has been extracted in order to drive the turbine that drives the compressor.

The compressor adds energy to the intake air and energy is heat, not sure about a turbo but turbine bleed air runs about 400F. It heats it because energy that extracted from the exhaust side is added to the compressed air.

So a turbo cools the exhaust and heats intake air, because it’s moving energy (heat)

A turbo does not create heat, it does extract and move it

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17 minutes ago, A64Pilot said:

A turbo does not create heat, it does extract and move it

Well, I agree with your statement, but just to be clear what drives the turbo is not heat from the exhaust, it is the kinetic force. You could aim a radiant heat source all day at the fan on the inlet side and not much would happen. But you could take it to Antarctica and blow -50 air through it and you bet it would spin. But your statement that the turbo extracts heat is certainly correct, that is one of the reasons the turbo runs hot, it absorbs heat from the exhaust. But the heat is not the force driving the fan, at least that is what the engineers tell me.

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5 hours ago, aviatoreb said:

Where is the TIT probe vs where is the EGT probe?  I don't know - I am asking so this isn't a rhetorical question.

But leading to - what does the volume of space look like where the gas passing across the probe look like?  I see no reason why the gas cannot heat or cool as it passes through pipes of various size.  PV=nRT and all.  Its how refrigerators work.  I always presumed this has something to do with it.

Also, temps vary between EGT probes in part because of variations of where they are placed.

But the outcome is as Lancaster says, TIT measurements are generally a good bit higher than EGT measurements.

Like well over 3x higher! Nominally 380 degrees vs 1550!

I am not mechanic enough to have a clue as to how all that works, but I figure either temperature is hot enough to raise a blister.

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40 minutes ago, jlunseth said:

Well, I agree with your statement, but just to be clear what drives the turbo is not heat from the exhaust, it is the kinetic force. You could aim a radiant heat source all day at the fan on the inlet side and not much would happen. But you could take it to Antarctica and blow -50 air through it and you bet it would spin. But your statement that the turbo extracts heat is certainly correct, that is one of the reasons the turbo runs hot, it absorbs heat from the exhaust. But the heat is not the force driving the fan, at least that is what the engineers tell me.

It's kinda both.   Energy in the form of both heat and kinetic mass goes into the turbo.    That energy is used to compress the intake air, which heats the intake air up quite a bit (by PV=nRT as previously mentioned), so the energy level of the exhaust is lowered by what is extracted to do the work of the intake compression.    So there's energy into the turbo, both heat and kinetic, and there's energy out in several different directions:   compression of the intake air, heat to the turbo itself (some of which is radiated and some of which heats the bearing oil due to friction and conduction),  and heat and kinetic energy out the exhaust.    The exhaust temp is lower than the input temp since some of the heat energy has gone to do work compressing the intake air, and some has been radiated or been absorbed by the oil.   The lower exhaust temp indicates that intake heat energy from the input went somewhere else to do something else before it went out the tailpipe.

Energy is always conserved, so whatever goes in goes out somewhere.    Since the idea is to use as much of it as possible to compress the intake charge, less will go out the exhaust pipe.  The exhaust air will have both less heat and less kinetic energy.

The intake air temperature increases primarily due to the compression, as it doesn't spend enough time in the turbo to be able to absorb much of that heat.   The intake temperature increases due to the pressure increase regardless of whether the compressor is attached to the hot turbo or separated mechanically to isolate them.   Superchargers driven by mechanical means have the same temperature increases of the intake air charge.

 

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5 hours ago, T. Peterson said:

Like well over 3x higher! Nominally 380 degrees vs 1550!

I am not mechanic enough to have a clue as to how all that works, but I figure either temperature is hot enough to raise a blister.

Torrey 380 is your cylinder head temperature (cht) your exhaust gas temperature (egt) is closer to 1400 to 1500 and is the same exhaust gas that flows down to the turbo cooling somewhat but then rises a little when it gets compressed to the turbo fins to spin the turbo. And reads higher than the individual probes because the tit probe is exposed to all 6 cylinders exhaust gases not just one and gives the tit probe a higher temp because of this.  

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As someone already pointed out take a blow torch and a piece of metal and use a heat gun to record the temp of the metal. Now swipe the flame of the torch over the metal once every 6 seconds and watch how the temp of the metal will rise sharply then slowly start coming down until the next swipe but eventually there will be an average temp the gun reads. Now with the heat gun lower the temp of the torch flame by 100 degrees and then swipe the metal once per second for 6 seconds like what happens at the turbo and the metal will not have as much time to cool between swipes just like at the turbo and thus a higher temp will be read even though the flame was cooler than on the first experiment. 

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