EGT's after the turbo

[Cowboy]

I have to agree with you John on many points.  A part in me really wanted to get all technical about a formula for calculating the post turbine temps.  But after reading your last couple posts, reality hit me like a brick and I realized no one is going to grab a calculator while pulling a hill to find if there temps are acceptable.  And likewise, no one is watching their boost gauge at 5 psi like in my previous example.

 

So I did a little datalogging while bringin' in the hay the last couple days.  Nothing scientific, hard to get nice smooth data on gravel roads.

 

 

Here's one graph, the X axis is time in seconds.  

 

Towards the end of the pull, Pre-EGT's are at 1415, while Post-EGT's are at 981.  That makes for a delta temp of 434°F.  Boost stayed pretty steady at 35 psi so that's 12.4 Degrees per PSI.

 

And again:

 

At 31.4 seconds:

Pre-EGT = 1202°F

Post-EGT = 830°F

Delta = 372°F

Boost ~27.2 psi

Degree per PSI = 13.6

 

And one more.  I shifted down this time to try and get the revs a little higher for increased drive pressure.

 

At 14.8 seconds:

Pre-EGT = 1147°F

Post-EGT = 789°F

Delta = 358°F

Boost ~25.2 psi

Degree per PSI = 14.2

 

So overall 10°F per psi boost is pretty close.  Especially when you're swagging at it with a general rule of thumb.  The number does seem to go up with RPM slightly, which makes sense due to increased drive pressure.  

 

On "measuring the temperature of the flame coming out of the head"  I don't believe that to be necessarily accurate.  Pre-turbine temps are 'artificially' (using the word loosely) inflated by drive pressure.  Post-turbine temps better indicate the air to fuel ratio, when you're trying to make power at an RPM that the turbo isn't happy with, then it takes a lot more fuel to make the same amount of boost thus like you say John, Post temps can be out of hand, while pre-temps (which arn't being inflated much by drive pressure due to the lack of it), don't indicate anything as being wrong.

 

As for my comment about turbine efficiency.  I have always assumed that the inefficiency of the turbine adds heat to the exhaust just like the inefficiency of the compressor adds heat to the intake....  But.  I just tried putting together a calculator for it assuming Adiabatic decompression, and well, either I fricked up somewhere, or that's not really the case.  It will take more thought at a later date.

 

EDIT: Quick test of my formula using it to calculate pre-IC temperatures puts it pretty accurate... I don't know anymore.

[Mopar1973Man]

21 minutes ago, Cowboy said:

reality hit me like a brick and I realized no one is going to grab a calculator while pulling a hill to find if there temps are acceptable.

 

This is well outside the common Joe Cummins owner. For someone that is pushing to grab every last bit of power out of engine, this might be an excellent article to be put together in the article database.

Edited July 1, 2017 by Mopar1973Man

[AH64ID]

Good data!! 

 

I'll bite on the drive pressure bit. It's been a couple years since I ran a drive pressure gauge (it's on the shelf and needs some wires moved since the Touch took up a pillar spot), but I think I generally ran a lower drive pressure ratio than you did and it took quite a bit of fuel and rpms to even get to 1:1 and rarely over 1.2:1 (DP:Boost). That could be where you are getting more than 10° per psi. 

 

I'll be doing some towing over the next week and I'll try to take some notes. I should get my drive pressure gauge up and running. It's literally 3' of wire and done. 

 

On a side note, 1400°+ while towing would scare me :-)

[AH64ID]

So I was 21.3K GCW and towed in temps up to 104° at 3,000' and 99° at 4,500'. 

 

12-14°:1 psi is normal for varying throttle and rpms. Under constant loads longer than 30 seconds it almost always stabilizes at 10-11°:1 psi, and sometimes it took longer than 30 seconds but we already know how slow post is to react. 

 

The more I monitor post the more I want to say that it actually is a better indicator of motor heat soak and thus would be easier to drive. It is nice having both thou. 

[CSM]

nevermind.  posted above.  It is interesting to see the differential above!  

 

I kinda am starting to think that the turbine EGT limitations are not from the turbine wheel itself, but from the heat transfer capacity of the bearings/oil flow.  

 

I agree that burned pistons are more of a concern than a turbine though. Turbines tend to be killed due to hot shutdowns. 

 

On another note, I recall reading in a Northrop A&P text from WW2 that the majority of the heat from a valve travels through the stem, not the seat.  Pistons and burned valves are a bit of a concern... but not a huge one as our engines have decent oiling.  

Edited July 9, 2017 by CSM

[AH64ID]

I'd like to think that the bearing isn't seeing all that heat, also with mine being a water cooled turbo if the bearing was the deciding factor on heat transfer mine should have a bigger delta. 

 

I think drive pressure may be the culprit as more drive pressure will result in a bigger pressure drop which leads to a higher temperature drop.