Why are allowable EGTs for NA engines higher than the allowable EGTs for turbo

[azblazor]

Why is it that Turbo'ed 6.2L engines start to melt their pistons at 1100 deg and NA engines can run at 1300 to 1400 deg without causing damage.

Question 1 - Are these EGT numbers generally accepted as accurate.

Question 2 - Is the reason for the turbo melting pistons at 1100 deg EGT due to the duration of the combustion event being longer, ie the piston absorbs more heat than a NA engine with a 1300 deg EGT for a "shorter" period of time?

Just to clarify, I understand that the turbo engine has a higher pressure charge that is then compressed, but heat damage to pistons can only be caused by.....well......heat, right?

Thanks
Mark

[CleviteKid]

Hi Mark,

I believe that there are a couple of factors involved, and you are correct about one of them, heat. Heat (calories, BTU's, Joules, watt-seconds, etc. etc.) is a combination of temperature and amount of material at that temperature. Would you rather be hit in the forehead by one spark from a grinding wheel at 3000 F, or ten pounds of molten lead at 600 F ??

The amount of hot material does matter, and there is more gas at higher pressure in the turbocharged cylinder. The temperature of the pistons is variable from the top, near combustion temperature, to the bottom, near lube oil temperature. With less heat flowing into the top of the pistons in the N/A engine, the oil below can keep the overall temperature lower than in the turboed engine. And a higher pressure acting on a hotter piston will mean earlier failure in the turboed engine.

The other factor is the turbo itself. The exhaust turbine is temperature limited, and there is a maximum T.I.T. or turbine inlet temperature that the metal can stand before it weakens and 100,000 rpm tears it apart by centrifugal force.

A little known fact is that there is a tendency for diesel exhaust temperature to INCREASE going from the exhaust valve to the turbine, because the carbon monoxide in the exhaust continues to react with excess oxygen in the exhaust to burn to carbon dioxide, releasing heat. T.I.T. can be 100F to 200F hotter than E.G.T. measured near an exhaust valve.

Casey, and I, and other N/A operators have observed E.G.T.'s above 1300 F with no known bad symptoms, because as you point out, and the analysis supports, the N/A engine is more tolerant of these high temps. That does not mean that they are good, in fact high exhaust temp indicates a loss of efficiency of the engine, but when you have to get the load over the mountain, speed has some advantages over efficiency.

As others have often pointed out, the head gaskets will usually let go before the pistons, since our clamping system of heads-to-block was not designed for the hot rods we are all trying to build. But if the head gasket holds, piston failure is a real risk.

Keep the questions coming, Mark. Forces us all to think and learn.

Dr. Lee :cool:

[dieseldummy]

Thanks for clairifying that Dr. Lee. I have always wondered why people say that EGT's can be higher for a n/a than turboes, I think that you sumed it up perfectly. I have one question though; When our pistons are treated with the anodized coating, how much if any does that raise the number for max EGT's, turbo or n/a?

Justin

[BC Clark]

BTW, for political correctness, the aircraft industy now refers to it as Interstage Turbine Temperature or I.T.T. I always thought the other was more intuitive, big sigh.

[CleviteKid]

The anodized coating is SUPPOSED to be good for another 100F E.G.T., but as a metallurgist I am skeptical. I think you need a thicker ceramic coating, like the optional coating applied at The Diesel Depot and other shops to really make a difference in heat flux into the piston.

As a traditionalist, I will stick with the T.I.T. acronym, but if you don't include the periods, the language police might replace the letters *** with *** .

Dr. Lee :rolleyes:

[dieseldummy]

So how much is the ceramic coating good for? The same 100 degrees or more?

[azblazor]

Originally posted by CleviteKid:
Heat ......is a combination of temperature and amount of material at that temperature. ...The amount of hot material does matter, and there is more gas at higher pressure in the turbocharged cylinder...
Dr. LeeDr. Lee

Thanks for that explanation. It has been something I have wondered about for quite some time.

My theory was in error then. So to clarify, the primary factor in the higher transference of heat into the piston of a turbo'ed engine is due to a larger mass of material (denser charge) causing more heat to be transferred into the piston, than a higher temp but lower mass NA charge.

It seems kind of obvious now that you have clearly explained it. But I had talked myself into thinking the primary factor was that the turbo combustion event was longer somehow, and therefore the piston was absorbing more heat from a lower temp.

Thanks,

Mark

[ 03-28-2004, 06:58 AM: Message edited by: azblazor ]