EGT Question

[KTZed]

Ok so ive read on here many times that it is wise to keep your EGT's under about 1500F pre turbo, or at least no long stints at that temp. Then I got to thinking...well dont gas motors have higher EGT's than diesels? My 280Z turbo engine (and many other turbo engines im sure) doesnt even have piston cooling oil squirters. And it, like most gassers, also has an aluminum head and pistons. Sooo... Is it something to do with the way that diesel burns? Higher peak cyl pressures? Im not learned enough in the specific thermodynamics of the diesel engine yet but I am a mechnical engineer so the more techie of an answer the better, cause im just not satisfied with "stay under this temp or your engine will melt"
Cheers

[jimfoo]

I think it has more to do with melting the turbo than the engine in general. I would be surprised if gassers ran any hotter of an EGT.

[KTZed]

hmm, not sure I buy that turbo argument either...only recently have they started to put VNT turbos on gassers (ie. new porsche 911 turbo *and yes I know about the '80's shelby daytona exception) and I was under the impression that it was because the tolerances requierd in the moving parts of the vanes could not be maintained in the high EGT's of the gas engine. Also, things like ceramic turbine wheels have been around for some time (ie. Nissan RB25det...I have one as well) so its not like the turbo technology isnt there.
Has anyone on here actually cooked a turbo? First hand experience is always better than folklore.

PS. my pyrometer (and I believe you said you had the same glowshift gauge jimfoo) shows up to 2500F...surely thats not meant for a diesel?

[KTZed]

Ok well heres a quote from Banks site to support your turbo theory jimfoo

"Which parts will fail first is a matter of the design and materials used in the various parts of the turbo-diesel, but usually it starts with the turbocharger. Under sustained excessive EGT, the square corners at the outer ends of the vanes, where the material is thinnest on the turbine wheel, can become incandescent and then melt, resulting in a rounding off of the square corners."

This still does nothing to answer my gas vs. diesel question though

[jimfoo]

Maybe since Diesels run very lean, there is always plenty of free oxygen to do damage at higher temps. A gas motor wouldn't have much O2 by comparison.

[KTZed]

A gas engine will have highest EGT's while in cruise mode, leanest A/F ratio, while a diesel will have highest EGT's at full fuel, richest, black-sun-blocking-cloudiest A/F ratio.
Heres something from a turbo dodge page (turbo k-cars and such, not diesel trucks)

"The factory rates the TURBO exhaust components at 1650F at a steady state operation- This means while cruising down the highway at normal throttle, engine in O2 feedback mode with the fuel mixture at 14.7:1 exhaust temps CAN (not will) reach as high as 1650F."
"If someone asked me for a magic number I would say 1500F degrees would be ideal at WOT."

Just trying to debunk myths to be able to push the limits with our little diesels as well as trying to justify running a Garret VNT from a Powerstroke or a Holset VGT from a Cummins on my Datsun.

Anyone else care to join in?

[zukgod1]

Ok well heres a quote from Banks site to support your turbo theory jimfoo

"Which parts will fail first is a matter of the design and materials used in the various parts of the turbo-diesel, but usually it starts with the turbocharger. Under sustained excessive EGT, the square corners at the outer ends of the vanes, where the material is thinnest on the turbine wheel, can become incandescent and then melt, resulting in a rounding off of the square corners."

This still does nothing to answer my gas vs. diesel question though

Well I dont belive ANYTHING that comes from Banks.

That's all I have to say about that.

[jimfoo]

While it says nothing about temps, this bit makes sense.

" Modern diesels are mostly alloy head and block.
With cast iron engines high EGT caused turbo and injector damage.
With modem alloy engines high EGT often leads to head, valve and pre-combustion chamber damage as well.
But how can he be so sure it is high EGT that's causing the damage?
Writers in magazines have come up with other theories, but 99 out of 100 turbo-diesels in for turbo or cylinder head damage are clearly caused by excessive combustion temperatures.
Heat fatigue shows itself by cracks as the metal changes its form.
I was one of the rare 'lucky' ones as my new G seemed to be in good shape.
"It happens very rarely that we see a turbo-diesel vehicle over 50K with no damage", Steve told me.
He explained that many vehicles without particularly high mileage arrive for a conversion of some sort.
Sometimes they even try to talk the customer out of it because a diagnostic test tells a tale that the engine is likely to have a major failure before 100 000 kms.
While the damage has been done it is lying invisible inside the engine but it can't be proven without taking the engine apart.
Should they do a modification, even an intercooler, it is often perceived to have contributed to the failure.
It's a real problem.
Diesel engines are happiest when driven on or close to the revs that produce the highest torque.
At higher revs, torque drops off and while power increases so does the temperature generated.
The result is high fuel consumption and high engine temperatures.
This is why above 140 kph most diesel engines will consume about as much fuel as a similar petrol vehicle.
At this speed the petrol engine is happiest, revving high and burning its fuel efficiently, while a diesel is at high-stress, running hot and burning fuel inefficiently.
So when considering a new or used vehicle, think about what kind of driver you are.
If you are towing, buy a petrol."

[jtanguay]

If you are towing, buy a petrol.

 :?

[KTZed]

Ok some more info

Pics of a melted turbo  :shock:  They were using it as a jet engine



And a quote straight from Honeywell (Garrett)
"To obtain the optimum balance of performance and fuel consumption at high speed, a gasoline engine should run ideally with a stoichiometric air-fuel ratio. For a high specific output engine, this results in a high exhaust temperature that requires a turbo to operate reliably at turbine inlet temperatures up to 1050°C.
To make a turbo that functions well under high temperature is feasible. The challenge is to mass produce them at a reasonable cost.  It requires ingenious solutions in turbo materials, designs and wastegate operation."

My thinking is that maybe the components used in diesels aren't designed for temps as high as in gassers to save costs....I spose all turbos arent created equal?
I guess it depends what the actual limiting factor is, which will obviously vary from engine to engine. It seems to me though that all gassers and diesels have the same materials exposed to the combustion chamber so it would likely be the turbo. Unless theres some thermodynamic reason tied into the differences in combustion (cyl press, high CR, precombustion chambers, burn rates etc.) that makes diesels prone to melting/cracking at lower temps that someone can enlighten me on?

[Gearhead]

I've never thought it was a turbo issue.  I've always been told that it's a piston issue.  I don't run my modified 7.3L Powerstroke above 1250*F.  Why?  Because I was told not to.  The same guy that told me not to, said that he sees 1600*F for a few seconds running low 12 second quarter mile passes in an 7.3L F250.  He runs a race truck, and I'm driving a tow vehicle.  I'm sure my tow vehicle wouldn't appreciate a 2min blast up a mountain at 1600*F.  I'm sure that my pistons and turbo could handle 2500*F for a VERY short period of time.  They wouldn't have time to heat soak and melt in 1ms, but a second or two wouldn't be good.

I've said that to say this.  There is no magic number.  There is an agreed safe general temperature that can be run indefinitely without heat induced failure, and a general consensus that a certain period of time over that number COULD cause damage.  That time period is an unknown quantity.  Run what you want, but know that you're in experimental territory, and let us know what you've found.  We all want better performance, but most of us are a little concerned about longevity as well.  After all, some of us are beating on 25+ year old cars.

[KTZed]

I guess part of the reason im persuing this is because it seems like everyone with a diesel says to get a pyrometer and watch it like the end of the world is coming if it goes above 1xxx F, whereas, it is rare to see a high powered turbo gasser with a pyro *even though they have verifiably higher EGT's* and no one seems to care as it is more important to monitor A/F ratio and avoid knock.
If people are freaked out about melting stuff at 1500F in a diesel, why is it all good to run 1600? 1800? 2000? in a gasser when, as far as I know, the affected components (head, pistons, turbo) are essentially the same?
I just dont like following blindly thats all  :wink:

Just had a thought...for a typical gas engine, of all the available energy in the fuel, about 1/3 goes into powering the vehicle, 1/3 goes into the cooling system and 1/3 goes out the exhaust as hot air (rough estimate)...is it possible that due to the diesel cycles higher efficiency, the combustion temperature exposed to the pistons, valves, and head could actually be higher than a gasser, but that heat is better converted to cyl pressure, more torque for a set fuel amount, and less heat is wasted out the exhaust leading to lower EGT's?
Looking for combustion experts out there?

[jtanguay]

now you're starting to understand diesels i think... ever see a gasser running lean?  too lean (and retarded timing) and you could probably melt some stuff.

diesel probably has what... 20x the btu's of gasoline? maybe that will help visualize the amount of heat these engines can produce.  (ever see a diesel engine melt?  :shock: )

[KTZed]

Diesel and gas actually have very similar heating values, both around 19,000BTU/lb (HHV) Diesel has a slightly higher energy density (BTU/Gal) due to its higher weight density (Lbs/Gal)

[Gearhead]

When tuning rotarys, when we used EGT to adjust mixture.  When it got too high, it was time to add fuel.