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Engine Specific Info and Questions => IDI Engine => Topic started by: KTZed on December 19, 2007, 09:01:51 am
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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
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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.
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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?
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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
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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.
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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?
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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.
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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."
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If you are towing, buy a petrol.
:?
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Ok some more info
Pics of a melted turbo :shock: They were using it as a jet engine
(http://www.students.tut.fi/~kiiskils/stsk/turbo/tur_siv.jpg)
(http://www.students.tut.fi/~kiiskils/stsk/turbo/tur_imu.jpg)
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?
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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.
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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?
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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: )
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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)
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When tuning rotarys, when we used EGT to adjust mixture. When it got too high, it was time to add fuel.
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Added fuel (rich mixture) makes gassers run cooler, and makes diesels run hotter.
I think that egt's are most critical to watch on turbocharged engines, because the exhaust outflow restriction (the turbo) also causes heat to pile up inside the engine. This is never an issue on an N/A, and probably not on a supercharged diesel with a free flowing exhaust either. Just guessing about that- the only supercharged diesel I've driven was a '65 GMC two stroke city bus with a two speed automatic. Ran cold all of the time. Loved the sound of that one too.
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Hey Gearhead, what kind of EGT's were you seeing on those rotarys...cause they run even hotter than piston gassers dont they?
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Spark ingition moters have to esentally run a stoichiometric air-fuel ratio. They can't generate high EGTs with out damageing the pistons and spark plugs first.
If they run significantly leaner than stoichiometric, all kinds of nasty stuff happens: Detonation ( and all the associated joy: broken rings, chunks of piston being blasted loose), spark plug electrodes melting off.
If the run significantly richer than stoichiometric, they belch black stinky smoke, and foul their spark plugs.
Within these limits, I belive the EGT stays relatively low. Faliure befor the turbo will happen befor any bad things happen to the turbo. Unless a piston chunk, or ring makes it out of the engine and nukes the turbo.
Mater of fact, EGT is useless in tuning a gasser. Good and bad happen all within a couple of hundred degrees, and thermocouple accuracy/interchangeability is not good enough to prevent engine damage. I tried it on super modified two strokes, and its a useless measurement.
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What do you think about this theory? They're speaking about SI not CI engines but I imagine that injection timing would have a similar effect to ignition timing in our diesels.
"Now there is a huge misconception in regards to EGTs and what is and isn’t harmful to your motor and this is where many are confused. We regularly see 975-1000C while road racing and typically the car will be run at these temps for 20-25 minutes. OMG your pistons are going to melt is often what comes to peoples minds but this simply isn’t the case. I tend to be conservative with ignition timing as this puts less stress on the rods, the bearings, results in a smoother torque curve, and it runs the engine cooler. Yes, I said cooler. By running less ignition timing you are giving the engine less time to burn the air/fuel mixture which often results in a less complete combustion process therefore the process continues to burn as the exhaust valves open resulting in a higher EGT reading. More ignition timing starts the combustion process earlier putting more stress on the rods (due to more angle), the bearings, the pistons, and retains more heat in the cylinders even though the EGT reading will be lower. By giving the cylinder more time to burn the air/fuel mixture more heat is retained in the cylinder and therefore more heat is transferred to the pistons. While this does result in more power (up to a point) it’s much harder on your internal engine components. My point is that high EGTs typically mean less heat transferred into your pistons, not more. This is seriously simplified but I hope you get the picture."
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I agree with that except the line "My point is that high EGTs typically mean less heat transferred into your pistons, not more." That statement is only true if the fueling is unchanged and the timing is changed.
Agreed. I think thats what he was trying to get across. All else being equal, with timing as the only variable.
Now, in theory, the more retarded the timing, the hotter the turbine inlet temp (TIT), the more efficient the turbo will be up to the point of turbine failure. Conversely, the more advanced the timing, again up to a point, the more BMEP you will get but it will be harder on the bearings, rods, etc. and you will be putting more heat into the head, valves, pistons, putting them at risk.
Im applying this to my Datsun as well cause I really want to try a big VNT/VGT on it and I dont want to cook the turbo.
Does anyone have EGT records before/after water/meth injection with no other changes?
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I've had the pump go a whole tooth off once when bird and grill parts sneaked inside the timing belt cover. Birdstrike...
Instantly Sky high EGT and boost.
I prefere to leave the timing alone (stock) and putz with the waste gate and pump fueling.
If you want to try something and report your findings here, cool.
Oh by the way, I have a turbocharged '73 240Z sitting in my garage.
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Bird in the timing belt eh :shock: thats a new one
I dont imagine its beneficial to play with the static timing on the pump but I imagine the advance curve can be modified throughout the rev range (Im sure thats part of the magic Giles works on these things).
Glad to find a fellow S30 owner on the forum! Mine is fairly stock at this point (susp mods and L28 with SU's) but have been trying to get it together to do the L28ET....or there is always that RB25 in the garage :twisted: :twisted: