• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

Anatomy of EGTs

buddy

buddy

Active Member
Messages
6,078
Reaction score
65
Location
California, central coast
So I was making this spreadsheet trying to see how different parameters of the engine affects temperatures in the cylinder, and what would lead to potentially high EGTs as a symptom of an issue.

Attached is my spreadsheet and it shows something quite profound. That IAT has the most significant affect on cylinder temperature. a 200F increase in IAT results in a 700F increase in temperature of the air in the cylinder. However boost increases made no difference in precombustion temperatures. The air itself will reach 2000F in the cylinder even before it combusts to some 3000-4000F temperature.

Someone please do the math themselves and see if it compares. This is using ideal equations, for just an air pump essentially, and not accounting for heat on the cylinders that could get transfered to the air before or during compression as well.

So what causes high EGTs then? I wouldnt say boost, but it does when it causes your IATs to shoot up. This is why intercoolers, efficient turbos and WMI curb EGTs so well and increase your power by increasing air mass in the cylinder.

Another cause of EGTs when accelerating is unburnt fuel. So this unburnt fuel is not contributing to your power stroke, but it is flaming as it gets pushed out the exhaust valve so it shows up as high EGTs and what isnt burned comes out as black smoke or a haze. This type of high EGTs would have no effect on cylinder temps, just added heat transfer to the heads during the exhaust stroke.

So there are two types of high EGTs. Those that are caused by increasing peak cylinder temperature by making it toasty in there with hot air and high compression. And then those that are caused by unutilized fuel. Both can be harmful in different ways. If youre accelerating and EGTs spike and you get a puff of black smoke you dont have much to be concerned about as long as your engine coolant temps are not increasing. You dont want to get your heads and head gaskets really hot, but if the heat is not transferring because conducitivity in the metal is inefficient youre alright or if the cooling water absorbs it and takes it away it has done its job.

No smoke and high EGTs may tell you the cylinder temps are peaking higher and higher and sustaining this is bad on pistons potentially and head gaskets when combustion temps could reach 4000F. Even if ECTs are not increasing sustaining high EGTs can be bad.

So what is too high of EGTs? Good Question, everyone has their thoughts on it. Some manufacturers today say 1300F is the maximum safe EGT to sustain, and momentary peaks above it are of no concern. Others have suggested max safe sustained EGTs in the 1200s. Pistons and cylinders take consistent high temperature with no problem, constantly having 3000F air and fuel pushing on them. Thats why they are made of steel, but the head gaskets may not like heat so much when coupled with high pressure. Although boost pressure didnt have much affect on air temp after compression it does have a large affect on peak cylinder pressure, so 18psi and 1300F EGT is still worse than 10psi and 1300F EGT from the perspective of the head gasket and piston rings.

The cooler the better, but when I romp on it, I wont be too concerned about 1300F EGT for a several seconds when accelerating. Climbing a hill for a few minutes slow down and downshift if you need to keep it under 1300F.
 

Attachments

  • TDC Air Temp.xls
    30.5 KB · Views: 55
Last edited:
buddy said:
Hmm, I do too, is that an issue with the attachment or the site, or our computers or internet connection?
Click to expand...

Or perhaps no data actually exists, and that interesting read was just a waste of my time? :)

Seems like another answer to why the ATT results in better power.

I'd like to see that Spreadsheet. Internal Server Errror here.
 
I agree with Buddy there are other things to look at with high EGT.

As you are pushing things to the limit something will fail first. EGT is a way to tell you how hard you are pushing it. Part of a complete breakfast including boost, trans temps, and for the really fancy: IAT, altitude, engine oil temp, fuel consumption (GPH) ...

The main concern is said to be melting the weak link, the pistons... I do not share this thought as being the main concern.
Can say my failure was not caused by this, but, rather running out of clearance with high ECT and a sudden reduction of ECT. Scuff and crack in the piston resulted. Engine oil failure at these temps was also possible causing piston scuffing/crack. The engine may have run normally a long time with the crack, but, our hard use burned the crack through.

So I would add both engine oil failure and/or piston overheating with lack of clearance in regards to piston scuffing engine failure.

I also suggest that burning/melting the outer shell off the glow plugs and resulting glow plug failure debris is a risk. Esp. after running high EGT's (1300 plus degrees for 7000 miles if the gauge is correct) for extended hill trailer pulls and then removing the glowplugs. They had a clean shiny look but a rusty surface texture. Exhaust leak and too much fuel gave me EGT's that are not believed to be possible with a DB2 IP.

My main point is when you push something to the limit a minor failure, like a defective radiator cap suddenly letting go, can completely ruin the engine that otherwise would have survived the failure if it wasn't being run at it's limits.

Where is the EGT limit and what affects it...
 
Last edited:
View attachment TDC Air Temp.xls
No I swear, the data is there, does this one work for you? trying from a different computer that isnt monitored by the man. Does this new forum software have a problem with excel files?

I will convert to PDF or something and show you a pic of the spreadsheet at least, but the way it was you could modify the input parameters.
 
Here is the PDF of it, second page is just all intermediate steps of the equations. This way you dont get to see my math in the cells, but if someone can try doing the math themselves independently and show me if I missed something that would be great, because it wasn't making sense. The ratios of mass to pressure and temperature all seem to cancel eachother out so that boost does not affect temp.
 

Attachments

  • TDC Air Temp.pdf
    115.6 KB · Views: 33
Haven't checked your math or figures but I will throw out that air that has passed though 8-15 stages of compression in an axial flow compressor does not even approach those temps. Recip's might be a bit different, especially when dealing with 22:1. I can't remember off the top as that learnin' was a looong time ago for me and I primarily deal with axial flow compression now.

If I get time tonight I'll sit down with a few of my old text books and see what I can suss out to confirm or refute your figures as given.

I think there are a few additional things you need to consider when looking at EGT and ECT.

There is an inherent lag time between EGT and ECT rise which makes ECT not as effecient and idicatator of cylinder conditions as EGT.

While you make mention of the inefficiency of cast iron in transferring heat by conduction (the very thing that causes the lag), there needs to be given some consideration to the exposed surface to the flame font and temps. While it may be true that the temp is instantaneous and then moves out of the cylinder, the surface of the component is still exposed to the "temperature shock" of the flame front. This in itself can lead to a cracking situation where the stress of temperature differences uneven throughout the component itself can lead to internal stresses sufficient to cause the parent material to crack. Any flaws in the casting (voids, impurities, etc) can only compound the effect.

Considerations for ECT and piston crown temps are even more salient. Flame front temps exposed to the piston have very little to do with ECT. There is some correlation, but it is so "lagged" that ECT is not effective for in cylinder conditions for the piston crown and 1st ring land. EGT is even more relevant, but peak cylinder temperatures are even higher than that seen on an EGT gauge. However, measuring peak "in cylinder" temps is expensive to the point of being impractical in almost anything besides a research environment. Suffice to say, 1300 F on an EGT gauge is definitely higher in cylinder. But it is probably the most relevant indicator of what is actually happening in cylinder.

Need to dust off those old engineering books.....

Cheers
 
Last edited:
Anyone who runs 1300 deg on one of these engines is nuts. I wouldn't run that on a 300 Mack engine and that is a hell of alot beefier. These don't even run fire rings. Your putting bad info out trying to prove it on paper but in the real world someone is going to read it and think it's ok and then be back here crying when they have catastrophic engine failure. Bill Heath himself told me don't run mine over 1150 and I'll take his word well over your calculations. Unless you think you know more than him too. No diesel should be running those kind of egts especially light duty ones. That is over 23 years of truck driving everything from Mack to Cat to Cummins, Detroit etc.
Most of the high egts are caused by a combination of added fuel along with a tight wastegate to pump more boost to burn the added fuel. It's a viscous cycle. These engines were not designed for it and running it that hot will kill it fast. Even the Dmax guys can melt a motor fast that high.
 
slowing down and downshifting will do what opposite? I have done this and it was effective, and if going slower at higher RPM you can still get the power to go up the hill without all the boost. If I had cruise control on then speed would be maintained, boost would go up, and EGTs increase, so that is opposite of what I said.

I did not even prove anything on paper or in the calculations. The calculations were just to see how air temp increases with compression. To see air temp and CR and boost contribution to peak cylinder temps. I am trying to explain to myself what is actually happening since obviously steel pistons do not melt at 1300F. cylinder temps are well over 3000F all the time, and the corellation between EGT and how the engine could be damaged is what I was looking for.

And obviously EGTs are not peak cylinder temps, so if EGT is 1300F, what do you think peak temps are? Must be 3 times that much at least before the piston allowed the combustion to expand and cool before getting forced out.

I found the diesel combustion temp data from a paper where they did measure the combustion temp over the RPM range. The EGT recommendations were referenced from a diesel magazine that was citing what GM, Ford and Dodge recommend as highest sustained EGT temps, and it was 1200-1300F.

The main point though was to show that more boost is not the problem and can surely lead to lower EGT if the IAT is not what is causing them, but if unburnt fuel is. The boost itself, high cylinder pressures is not increasing the temperature. Lower the IATs for most effective lowering of EGTs when adding power.
 
Yeah and my point as you said over in the the other thread is that 1300 deg egt is ok and that is way wrong. I'm not gonna get into what causes high egts because we all know many things do. My point is you should not be telling people that it is OK. That is bad advice. Downshifting to climb a hill send the rpms up and the fuel and boost. Do you Tow ?? Let me ask again..Do you tow heavy ?? I do. I am better off not downshifting as long as the motor can sustain the hill. If I downshift the pyro goes through the roof and moves faster than the tach. The only way I can control egts it to back waaayyy off when downshifted which I don't do unless the truck is having trouble climbing.
 
Acesneights1 said:
Yeah and my point as you said over in the the other thread is that 1300 deg egt is ok and that is way wrong. I'm not gonna get into what causes high egts because we all know many things do. My point is you should not be telling people that it is OK. That is bad advice.
Click to expand...

In your opinion what I said was bad advice. But most of it wasnt my advice. EGT gauges today are more sensitive and accurate than older generation. They climb faster and will show things the old ones didnt. You can determine whatever max EGT you want that is safe and not go over that limit. Thats everyones choice. Ive readily said multiple times I would not sustain 1300F for any good length of time.
 
buddy said:
In your opinion what I said was bad advice. But most of it wasnt my advice. EGT gauges today are more sensitive and accurate than older generation. They climb faster and will show things the old ones didnt. You can determine whatever max EGT you want that is safe and not go over that limit. Thats everyones choice. Ive readily said multiple times I would not sustain 1300F for any good length of time.
Click to expand...
My Pyro is a 300$ Autometer pro series. I know it's accurate. Gauges IMHO are no place to skimp when pushing an engine hard. I use nothing but Autometer. I have the pyro right under the turbo so I am seeing the temps right there.
 
From a light duty diesel working too hard viewpoint:
Downshift and get off the throttle.

Some engines will 'lug' themselves to death if you allow it. Lug means not enough RPM as used here; not the high vibration low RPM WOT definition. Esp the 2008 Duramax with the 6 speed auto 3.73 rear. The ECT goes through the roof along with high boost towing, 28' heavy loaded, when presented with a long hill. It has the power to hold the gear, but, not enough fan and water pump RPM esp with the high boost heating up the CAC before the radiator. IE going below and operating 2000 RPM or less on a hill loaded will skyrocket ECT quickly. 230+ ECT was something I had to live with on that truck as normal with a 25" factory fan locked in screaming. A good cure is to force 5th gear or even 4th gear with less 'needed' throttle. This extra RPM will prevent the heat up at less than WOT - Getting the fan on faster and providing airflow to cool things off. WOT would downshift, but, power and grade do not demand WOT or a downshift. A waste gate to limit boost on the VGT in software etc. would help along with a ECT triggered automatic downshift under high load. GM does not care to manage how high the ECT gets when used under high load, GM never has GM never will. Limit fuel etc to protect things, but, never solve the 'problem' and now emissions make things worse with DPF, EGR cooler, etc. all adding heat load to the cooling water.

The DB2 6.5 does not have the power to avoid WOT and slowing down with the same hill and loads. Maintain high gear or the turbo chokes you at high RPM. I find 3rd slows you down 1-2 MPH just by shifting. TCC lock up override makes no sense, but, you gain or hold speed with the TCC forced locked up. So the 6.5 would be best to stay in high gear around 2100 RPM. (4.10 rear 4L80E.) Set fuel and boost to never get out of hand at WOT, altitude, and full load both high and low RPM. Towing heavy with a 6.5 WOT is more or less unavoidable.

Perhaps the DS4 makes a difference?

Pistons are generally aluminum alloy esp in the 6.5. Ceramic coating helps but has a limit. So does your engine oil...

I have a Autometer pro that ran $160.
 
The DS4 will give you more fuel than you know what to do with when properly coerced.

I guarantee my 15 year old Banks EGT gauge is not realistic the way it climbs slowly, so would never know what my peaks on acceleration really were.
 
Along the lines of what WarWagon said I think the other big factor is time at temperatures and how is the heat being shed from the piston crown and combustion chamber. Just looking at gas equation alone doesn't do it justice. Temperature alone I don't think explains the heat transfer mechanisms. Part has to go to ECT, part to the oil, and other part through the air going in and out of the engine.

It might be interesting to see if you calculate the equivalent cylinder volumes at different compression ratios/boost numbers to see how long the high temperature is on the piston.

It probably will reveal 18 psi boost at 22:1 creates with high IAT creates and maintains an elevated cylinder temp for an appreciable longer portion of the stroke/cycle vs 14 psi boost, 18:1, and low IAT.

It might show the pumping of air might be creating over half of the heat that has to be dissipated. Because the fuel rate and thus heat of combustion might not be changing as significantly as boost/IAT temperature difference (and thus different time durations on cylinder.

Say look at what point in creates enough compression for a good combustion temperature 900F ???. Once you get volume you could calculate time differences that this temperate and above is on the cylinder.

At what point is the temperature wasted "heat"? Compression heat over 1000F ish ???? Is that why GM seems more concerned about IAT than EGT. As long as you monitor IAT and boost then control it for given fuel rates and conditions EGT may be fairly predicatable.

Change the fuelrate with aftermarket programming and or high loads not expected normally and GM's programming / control mechanisms are now deviated from plan and break down.
 
You must log in or register to reply here.
Back
Top