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Thermocouple..Pre or Post?

Discussion in 'Diesel Tech / 24 volts' started by Towpack, Aug 21, 2006.

  1. Towpack

    Towpack

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    While I'm waiting for my gauges to arrive from I've been chewing over the pre/post turbo thermocouple debate and I'm leaning towards post turbo and here is my reasoning:


    The thermocouple is only measuring the exhaust gas temperature which as we know increases with load and
    boost so it will cool when the engine is left to idle as the exhaust gasses are then much cooler.
    However in pre-turbo position it will cool much quicker than the turbo giving the false impression that the
    turbo is cool enough to safely shut off the engine.
    Siting it post turbo means the relatively cool gasses coming from the engine at idle will be heated by the still extremely hot turbo housing so it will register a higher temperature for longer giving a more realistic indication of the true turbo temperature?


    I'm certainly no diesel expert but just applying the laws of physics here.....the small amount of metal in the thermocouple itself will cool down much quicker than the large turbo housing.....or am I missing something here?
     
  2. Stone

    Stone Moderator

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    Firstly, your turbo is not your only concern when you are monitoring EGT's. Your primary concern should be what is going on inside your combustion chamber, and that you are not pumping so much fuel and air in there to cause so much heat enough to start melting things down. So, the most accurate picture that you can get of this temperature inside your cylinders is definitely pre-turbo.

    <EDIT: Re-read your post about the turbo and I'll rephrase.> I don't know what exactly is a "safe" temperature for a turbo to be shut off completely, but I've been told that waiting until the EGT pre-turbo hits below 400*F is best practice, so this is what I've been doing.

    I ain't no expert either, but this is what all of the turbo diesel experts have told me...that's why I went with a pre-turbo installation.
     
    Last edited: Aug 21, 2006
  3. Greg_B

    Greg_B

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    Yup...

    All that said I have gone the lazy route and am putting mine post turbo. I simply take 250 or off, and run at max 1000 (but try for 950). Poo will be getting a pyro soon, and it will be post too, so both are the same.

    If I had everything apart, I would drill and tap pre...

    gb
     
  4. Greg_B

    Greg_B

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    I think you are meaning the mass of the turbo would cool down slower, heating up the air? Dunno if that is a real concern. Pre-turbo really is best for all the stated reasons (closer to the flame front, quicker reaction, truest temps). You could get really anal and put on probe at each exhaust port...then you really have an idea what is happening in each cylinder :D

    hth's

    gb
     
  5. Stone

    Stone Moderator

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    Greg:

    I think that his second point is that the turbo itself cools down much slower than the EGT's coming out of the cylinders, therefore when the pyro readings go down quickly when you idle, it doesn't necessarily reflect the temperature that the turbo is at. He's saying that maybe a post-turbo pyrometer will be slightly more accurate in terms of getting an idea as to what the turbo temps really are before shutting down.

    I don't know...maybe it's not necessary for the turbo's health to really know what temperature it's at before shut down as long as you let it idle long enough to be below 400*F pre-turbo prior to shutting down? Really, it's not going to be red hot at that point...what's it going to hurt?

    I still think that the pre-turbo route is the way to go. Come on Greg, you were there when we drilled and tapped into the elbow on my turbo flange without taking the whole thing apart and I've got no issues with my turbo despite your flying debris theory. :D You've got no excuse buddy. :D :D

    Stone
     
  6. denis

    denis (O) toyota nut (O) SILVER Star

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    Greg_B, there is no "flame front" so to speak in a diesel :flipoff2:

    From my experience When idling at rest after a hard run the thermocouple is being heated by the manifold and hot parts in the exhaust sytem, and cooled down by the colder exhaust temps. If you slightly rev the engine you will see the reading go down quicker, if you stop the engine you will see the readings go up again.
     
  7. jcolvin

    jcolvin

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    Pre-turbo. Much less lag. Post-turbo, you may be spiking very high on the EGT's without realising it since the turbo has lots of thermal inertia.
     
  8. rick_d

    rick_d

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    I'm 1" post and my outlet (HDJ7X) has a lot of cast steel to hold the temps, equal to hot side of turbo weight wise.

    1,050 is my top adjusted for what is happening in the turbo.

    if the probe rots off, down stream is nothing unlike "pre" which downstream is the turbo. exhaust gas as we know is not a freindly environment.
     
  9. Towpack

    Towpack

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    If you're running a standard truck as regards fuelling and boost then damage to pistons or combustion chambers shouldn't be an issue under any conditions as these settings were adjusted conservatively at the factory for reliability and longevity.If you have tweaked the fuelling and/or boost for more power then that is another matter.

    What is an issue even with a standard truck (in fact with any turbo motor) is turbo spool down time and, more importantly, cooling before shut down.Even the owner's manual mentions this.

    My thinking was that if you're relying on a pyro to give you an indication of when the turbo is cool enough to shut off the engine then it is best positioned post turbo to measure the gas temp coming out of it rather than the gas temp going into it which will be somewhat cooler at idle.
     
  10. Greg_B

    Greg_B

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    Ok, so who is going to put a probe at each exhaust port, at the manifold where the turbo bolts on, and then post turbo! Then we will all know!

    :D

    gb
     
  11. nickw

    nickw

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    Greg,
    Well....you do own a cruiser parts place. :)
    You will probably pay less then we will.
    And your current one is post turbo. and I bet your are dying to see pre turbo..... :) :D :)
    What do you say?

    I am sure could borrow Sheldons (the one he has not installed yet) and put it in preturbo.
    Then just put a bolt in your manifold or exhaust after to plug it (depending on where you want to keep the sensor).

    Cheers,
    Nick
     
  12. Stone

    Stone Moderator

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    That is definitely not true. As I already mentioned, my 13B-T has not been fueled up, and by shimming my wastegate, I've brought max boost back up to the factory spec of 8 psi. Even then, you would be surprised at how easily I can surpass 1250*F on a small incline with my foot on the throttle.

    I know that Toyota diesels are reputed to be tough, and I bet lots and lots of Toyota diesel owners don't have pyrometers and have regularly exceeded 1250*F during the life of their vehicles...and many of those engines are probably doing fine. But if you're one who doesn't like to leave things to chance, and would like to treat your engine for maximum longevity, then I would still say that a pre-turbo pyrometer would be the way to go.

    The bottom line is...how much does a turbo rebuild cost...and how much does an engine rebuild cost? Here in Canada, one is definitely many times more than the other...;) That's my thinking, anyway.
     
  13. Gold Boy

    Gold Boy

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    Post pyro.

    A friend just installed my pyrometer.

    The sensor is very close to the turbo.

    What info could I post regarding exhaust temperature?

    :beer:

    GB
    py1.jpg
     
    Last edited: Aug 24, 2006
  14. Greg_B

    Greg_B

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    Nick, open a Cruiser company and then talk to me in a few years.

    :D

    That, and I've a list a few miles long to complete 1st...

    It would be very interesting though.

    gb
     
  15. Greg_B

    Greg_B

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    denis...

    I welcome education, and correction if I am using a term incorrectly, or use the incorrect term (and I do quite often) to explain what I mean. I am not shy at opening a discussion where someone has made a statement that conflicts my knowledge, or thought process, nor would expect others to shy away. This is a Tech forum afterall, where we should be able to discuss ideas, educate one another and not fall into a "pat your back", "raise no conflict" myopic, sick, and sad environment. With that disclaimer, can you please expand on your comment.

    Thanks!

    gb
     
  16. Stone

    Stone Moderator

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    Hey Greg:

    You understand way more about fire and combustion than I do...here's some really heady reading about combustion that I found on the net:
    http://powerlab.mech.okayama-u.ac.jp/~esd/comodia2001/contents01.htm

    It's a collection of scientific papers pertaining to all sorts of mechanical engineering stuff including this paper on "Two-Dimensional Imaging of Reaction Zone Structure in Diesel Spray Flame by LIF Technique":
    http://powerlab.mech.okayama-u.ac.jp/~esd/comodia2001/3-22.pdf

    A portion of the introduction to that paper:
    Should make some interesting reading during those early morning hours at work. :D
     
  17. nickw

    nickw

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    Geez,
    Stone, your on time off....
    Greg, a friend is a city firefighter too, he has lots of free time....
    My girlfriend works for Ministry of Forestry. What she calls work, I call play time. Helicopter rides, sledding in winter, quads in summer.......

    Am I the only one that has to work around here.....? :) :)

    Cheers,
    Nick
     
  18. denis

    denis (O) toyota nut (O) SILVER Star

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    OK Greg sorry for the :flipoff2: I guess that's how I like to pick on words and split hairs. The flame front is a spark ignition (SI) term. You have a pretty much homogenous mixture that gets ingnited at one time, one place by the spark, and the combustion propagates from there. In a diesel, long story short the fuel is being injected in a spray of tiny droplets, which spontaneously burn because of the high temperature in the cylinder. Some start burning quicker than some others, locally raising the temps and putting those nearby in adequate conditions for burning as well. At the very begining of the injection the burning zone is somewhat local to the spray but these engines are designed to make the mixture somewhat homogenous as fast as possible putting swirl and/or turbulence in the airflow. That way in an ideal diesel engine a "flame front" would be everywhere in the cylinder and pushing in every direction. At least that's the way I understand it. :doh:
     
  19. denis

    denis (O) toyota nut (O) SILVER Star

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    Sir Ricardo’s Description of Combustion in Diesel Engines

    Quote from a lecture on diesel combustion by Sir Harry Ricardo (1885 – 1974)

    “Iam going to take the rather unconventional course of asking you to accompany me, in imagination, inside the cylinder of a diesel engine. Let us imagine ourselves seated comfortably on the top of the piston, at or near the end of the compression stroke. We are in complete darkness, the atmosphere is a trifle oppressive, for the shade temperature is well over 500 celsius - almost a dull red heat - and the density of the air is such that the contents of an average sitting-room would weigh about a ton; also it is very draughty, in fact, the draught is such that, in reality, we should be blown off our perch and hurled about like autumn leaves in a gale. Suddenly, above our heads, a valve opens and a rainstorm of fuel begins to descend. I have called it a rainstorm, but the velocity of droplets approaches much more nearly that of rifle bullets than of raindrops.

    For a while nothing startling happens, the rain continues to fall, the darkness remains intense. Then suddenly, away to our right perhaps, a brilliant gleam of light appears, moving swiftly and purposefully; in an instant this is followed by a myriad others all around us, some large and some small, until on all sides of us the space is filled with a merry blaze of moving lights; from time to time the smaller lights wink and go out, while the larger ones develop fiery tails like comets; occasionally these strike the walls, but, being surrounded by an envelope of burning vapour, they merely bounce off like drops of water spilt on a red hot plate. Right overhead all is darkness still, the rainstorm continues, and the heat is becoming intense; and now we shall notice that a change is taking place. Many of the smaller lights around us have gone out, but new ones are beginning to appear, more overhead, and to form themselves into definite streams shooting rapidly downwards or outwards from the direction of the injector nozzles.

    Looking round again we see that the lights around are growing yellower; they no longer move in a definite direction, but appear to be drifting listlessly hither and thither; here and there they are crowding together in dense nebulae, and these are burning now with a sickly, smoky flame, half suffocated for want of oxygen. Now we are attracted by a dazzle, and looking up we see that what at first was cold rain falling through utter darkness, has given place to a cascade of fire as from a rocket. For a little while this continues, then ceases abruptly as the fuel valve closes. Above and all around us are still some lingering fire balls, now trailing long tails of sparks and smoke and wandering aimlessly in search of the last dregs of oxygen which will consume them finally and set their souls at rest. If so, well and good; if not, some unromantic engineer outside will merely grumble that the exhaust is dirty and will set the fuel valve to close a trifle earlier.

    So ends the scene, or rather my conception of the scene, and I will ask you to realise that what has taken me nearly five minutes to describe may all be enacted in one five hundredth of a second or even less”.
     
  20. Stone

    Stone Moderator

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    Denis:

    Thanks for sharing that illustration. Just wondering though...in a turbo diesel engine, what is your opinion about the placement of the thermocouple? Pre or Post turbo?

    Oh and Nick...I am back to work now. ;)
     
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