Why allowable EGTs for NA engines are higher than allowable EGTs for turbos

Discussion in '1982-Present GM Diesel' started by azblazor, Mar 28, 2004.

  1. azblazor

    azblazor 1/2 ton status Premium Member

    May 5, 2001
    Likes Received:
    Arizona, Phoenix area
    My post on TheDieselPage:
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    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?

    <font color="green">
    Dr. Lee Swanger (Clevitekid) response:

    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 </font>

    In case you don't know Dr. Lee, here is a bit of info about him:

    Dr. Lee Swanger is a consulting engineer trained in mechanical engineering and metallurgy. Currently he is a principal engineer with a major engineering consulting firm, and a significant part of his work deals with large diesel engines in stationary power and marine propulsion.

    Prior to his current position, Lee was Director of Research and Development for Clevite Bearing Company, and received his engineering degrees from Case Institute of Technology in Cleveland, Ohio and Stanford University in Palo Alto, California. Lee is a Registered Professional Engineer in Ohio, California, Virginia and Florida.

    <font color="purple"> I thought it would be a good idea to start a new topic on this subject for searching on. I encourage any diesel guys who haven't to check out www.thedieselpage.com - it is a great resource!</font>


  2. BlueBlazer

    BlueBlazer 1/2 ton status

    Feb 15, 2001
    Likes Received:
    Very interesting points he makes there. /forums/images/graemlins/thumb.gif

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