EGT thermocouples

[Tonk]

looking into different egt guages and dont know some info about the sender probe, anyone know if all type 'K' probes are 2.5 ohms?

[LandyManLuke]

2.5 Ohms at what?

All K type thermocouples should be interchangable, as it's a standard combination of metals.

[Tonk]

hmmmmm i kneqw this wasn't gonna be easy

ok, i have a thermocouple which the spec says 2.5 ohms res, 0-2100f, 32.9 mv @1500f

i was just wondering that if i lashed a different egt guage to it whether it would be accurate

[LandyManLuke]

1500 deg F is 815.6 deg C, which according to a K type look up table, should produce 33.930 mV (816degC), the difference may be due to the gauge's cold junction compensation, or just a slightly different thermocouple.

32.906 mV is 791 deg C (on the table i found on Google), a 3% difference, probably well within the tolerances of a K type i suspect.

K type thermcouples are in-accurate, due to the metals used in their construction (originally, when it was not possible to manufacture them accurately), modern K types actually have impurities added to maintain calibration.

I've got a look up table from a better source at home, i can check against that tonight.

[Guest diesel_jim]

1500 deg F is 815.6 deg C, which according to a K type look up table, should produce 33.930 mV (816degC), the difference may be due to the gauge's cold junction compensation, or just a different thermocouple.

32.906 mV is 791 deg C (on the table i found on Google), a 3% difference, probably well within the tolerances of a K type i suspect.

K type thermcouples are in-accurate, due to the metals used in their construction (originally, when it was not possible to manufacture them accurately), modern K types actually have impurities added to maintain calibration.

I've got a look up table from a better source at home, i can check against that tonight.

Looks like the rain has cleared here now....

[Tonk]

1500 deg F is 815.6 deg C, which according to a K type look up table, should produce 33.930 mV (816degC), the difference may be due to the gauge's cold junction compensation, or just a slightly different thermocouple.

32.906 mV is 791 deg C (on the table i found on Google), a 3% difference, probably well within the tolerances of a K type i suspect.

K type thermcouples are in-accurate, due to the metals used in their construction (originally, when it was not possible to manufacture them accurately), modern K types actually have impurities added to maintain calibration.

I've got a look up table from a better source at home, i can check against that tonight.

some of that made sense, so would depends on when my thermocouple was originally produced?

i'm guessing it might be a better bet to order a new one...........

[LandyManLuke]

The gauge and thermocouple that you've got/had may well have been matched, so as to make any inaccuracy less significant.

It's possible that a new gauge & probe pair are matched.

Pairing a new gauge and a new thermocouple will not result in a perfect match, but i would expect, one that is certainly good enough.

The 3% difference mentioned above pails into insignificance when you start considering response lag/heat soak, heat loss between the cylinder and manifold/downpipe etc.

I'm sorry I can only talk in the generic sense, I know people on here have fitted EGT t'couples, maybe they've got contacts with suppliers who can offer a more application specific answer?

[Tonk]

i'm looking at a 'kit' to buy which comes with probe, but they also do the guage separately, i'll just buy the kit

[Ex Member]

OK....

Thermocouples use a bimetallic bond to produce voltage which varies with temperature.

Thermistors are devices that have a varying resistance with temperature. An example is a coolant temperature sensor on any car. Two completely different things.

For an EGT application, I would suggest a 3 mm, Inconel sheathed, ungrounded, type K thermocouple. Mount through a 3 mm stainless compression fitting drilled through to allow the TC all of the way through the fitting.

Other people use 6 mm TCs, but the thermal inertia is too high for this application.

There is nothing special that will need to be done. The TC will be good to within 5 C without any calibration which is more than adequate for this app.

[Turbocharger]

Since I have an EGT-ometer, and K-type thermocouples are relatively cheap, is there a way to switch the inputs from thermocouples to the meter so I can look at more than one temperature?

As far as I can see the official answer is no, but if both junctions on the switch are at the same temperature then the generated voltage will cancel? Or will this not work because a tiny resistance in the switch contacts will screw up my readings?

I'd like to put TCs in the inlet tract too, to see how much work the intercooler is doing etc.

[Daan]

Thats all interesting tech boys, but With the risk of being called a cynical dibnah bobbelhat wearer: Has any of you actually found a need to know the turbotemp? Ie have any of you reached a temperature were you were worried the impeller wheel would indeed fall of?

Reason for asking is that exhaust temp on a diesel isnt that high, I never expected it to be a problem. On the dieselmax landspeedrecord turbos it never was a problem, and the boost on that was more than most of us would ever dream of.

Daan

[Turbocharger]

On the dieselmax it was the limiting factor, they were running 4 bar manifold pressure and the pistons were special units to take the beating. Remember, the BMEP of that engine was over 25bar and the expected life was minutes.

In practice, the Tdi will put out well over 700deg EGT at turbine inlet and, since this is the concensus limit for the standard turbo, I've backed off my fuelling settings (or my right foot, in the shorter term!) as a result.

[Daan]

On the dieselmax it was the limiting factor, they were running 4 bar manifold pressure and the pistons were special units to take the beating. Remember, the BMEP of that engine was over 25bar and the expected life was minutes.

In practice, the Tdi will put out well over 700deg EGT at turbine inlet and, since this is the concensus limit for the standard turbo, I've backed off my fuelling settings (or my right foot, in the shorter term!) as a result.

The limiting factor was not temp but pressure, resulting in blowby. The manifold pressure was 6 bar.

Since more intercooling also results in lower exhausttemp, I wasn't expecting it too big a prob. How does the egt fare in standard form?

Daan

[David Sparkes]

Isn't a guy allowed to play with his toy?

If he want's to know the temperatures, and do original research, why shouldn't he be encouraged, instead of being shouted down?

Dieselmax is a hell of a lot more complicated than just one limiting factor. I'd suggest that Ricardo's expertise cannot be reduced to a single sentance in a Land Rover forum.

In other words, trying to use Dieselmax as justification, or not, for a hobby interest modification on a Land Rover engine is extremely misleading, and of no practical benefit to the original poster. Let's stick to the question, eh?

For Turbocharger, the makers of multimeters that we can buy with plug-in K type thermocouples obviously think that any error generated by the intermediate contacts in the plug and socket are either of no significance, or can be calibrated out, within sensible limits for any application that would use such a cheap and cheerful meter.

As you will know, you can buy K type thermocouples with 'any' length of lead attached, and the correct plugs to terminate the leads. Thus you could buy, say, 6 thermocouples and install them anywhere you like in the engine, intercooler, etc, and have multiple plug ended leads in the cab, then see if you can establish a pattern of readings under your test circumstances.

Remove the plugs, rewiring to a switch of your choice, and see if the results change significantly.

Regarding testing the effectiveness of the intercooler, you can put two thermocouples in, one at the hot point, the other at the cold point. Join the leads together, but reverse them. The output is then the temperature gradient across the intercooler, rather than the specific temperature. This is just an alternative view, I'm not saying it's better or worse.

If you want to be a little more precise about measuring multiple temperatures, then Autospeed, the online magazine, did a circuit some time ago where each thermocouple fed into it's own termination circuit. The output of this circuit was something like 200mV per degree centigrade. The actual figure is not really important at the moment, but it meant that you can switch your display voltmeter to any of the thermocouples without affecting the TC termination, which was your original concern.

I have built this circuit, for 8 thermocouples IIRC, and it works fine. More than an evenings work though :-)

Jeremy Fearn, to drop a name, looks for 30C as a maximum air temperature in the inlet manifold, although this is in competition, a road vehicle will be higher when in a traffic jam, etc.

BTW, I've seen well over 800C at the turbocharger inlet, and the turbocharger has lived to tell the tale.

Cheers

[MogLite]

Thats all interesting tech boys, but With the risk of being called a cynical dibnah bobbelhat wearer: Has any of you actually found a need to know the turbotemp? Ie have any of you reached a temperature were you were worried the impeller wheel would indeed fall of?

Reason for asking is that exhaust temp on a diesel isnt that high, I never expected it to be a problem. On the dieselmax landspeedrecord turbos it never was a problem, and the boost on that was more than most of us would ever dream of.

Daan

Daan - you are a cynical dinah bobblehat wearer

We were tuning the fueling on my Ibex with 300TDI lump. The theory was turn up the fuel - check the EGT, turn up the fuel some more, and check again. We did this a couple of times. I cried enough when the engine got really hot, not the water temp, but the engine. My co-driver looked out of the window at the side exit exhaust, and claimed it was like a bunsen burner. EGT's were still high but sensible, but I reckon we were seconds from a melt down.

Moral of the story - if the thermocouple isn't in deep enough to the gas flow, you'll get low readings

Pulls like a train now though

[white90]

Daan

another forumeer had 2 cracked manifolds before fitting the EGT gauge

mine has run up to 735 and that was time to lift off IMHO.

adjusting FIP without an EGT is a total lottery In my eyes

[David Sparkes]

"Moral of the story - if the thermocouple isn't in deep enough to the gas flow, you'll get low readings".

I'd certainly agree with that.

Which is why I think it's so misleading to say this or that number is the target to either get or not exceed, when different engines are being discussed.

I think I get high numbers because the thermocouple tip is wiped by every slug of hot gas leaving the cylinder head. That's 6 hits per 720 degrees of crankshaft rotation, thus there's 'no' cool down period between hot pulses, and the response to either throttle opening or closing is faster than the multimeter can really display.

Cheers.

[Turbocharger]

Thanks for that - I like the idea of differential measurement using two t/c's.

I have to say Tonk (if you're still reading after the o/t tech!) that if I was starting again, I wouldn't buy a full lights'n'noise EGT setup. I'd use the multimeter I bought recently - Maplin multimeter with temperature test - £15.99 which uses a K-type thermocouple. You still need to install a K-type thermocouple on your engine, then run the lead into the cab and read the temps using the meter. It's not lit and won't fit in a 52mm hole, but once the fuelling's set up and stable, you don't need to know any more. Mine seems to max around 650degC in 5th gear around 85mph, I can push on to 95mph and 745+ but by that point you know you're taking the p!ss and have to remember that the figures could be +/- 50deg depending on thermocouple position, accuracy, no cold-junction compensation etc. My point is, once you're in the right ball park you know when you're being silly.

David - Six slugs of gas - what engine are you running?

[Daan]

Isn't a guy allowed to play with his toy?

If he want's to know the temperatures, and do original research, why shouldn't he be encouraged, instead of being shouted down?

Dieselmax is a hell of a lot more complicated than just one limiting factor. I'd suggest that Ricardo's expertise cannot be reduced to a single sentance in a Land Rover forum.

In other words, trying to use Dieselmax as justification, or not, for a hobby interest modification on a Land Rover engine is extremely misleading, and of no practical benefit to the original poster. Let's stick to the question, eh?

Everyone is allowed to play with his toy. In fact I wasn't shouting at it, and asked a question, because i was wondering whether it is critical, in which case I might have to go playing with it myself.

As far as the dieselmax being more complicated- yes- me talking about it being misleading -no- since I was one of the designers of dieselmax.

Daan

[Tonk]

at present i'm using the set up which came from moglites 200tdi when i had it off him, it mores like a rev counter than a temp guage, it reads from cylinder number one, would i be better reading from a common point to all cylinders? at present i've hit 1350f (which i know is too high) and would just keep climbing if i didn't lift off, but if i back the fuelling off then its sluggish as hell but will still get upto the 1300 deg mark given time, which makes me think i need to change the way its monitored to be sure the readings are right. i'm running one bar boost measured at the inlet manifold.

all suggestions welcome.

[white90]

1350f=732c

mine is fitted in the EGR boss using a mount (made by Scrumps)

has gas flow from all 4 pots approx 6-10" from the valves.

proper length bolts have been fitted since

[white90]

If anyone want a free copy of the Temp control handbook

it is available FOC

here:

http://www.labfacility.co.uk/temperature_handbook_pdf.html

[David Sparkes]

Daan:

"me talking about it being misleading -no- since I was one of the designers of dieselmax".

Firstly, Congratulations for being part of a very successful team.

Secondly, if you go back to what I wrote, in the extract you quoted, I think (hope) you will see that what I was saying is that Dieselmax is so far away from a Land Rover diesel that it's misleading to say that the solutions that worked for Dieselmax can simply crossover to a Land Rover cooking diesel.

I was not intending to say that any facts you quoted were misleading.

Turbocharger:

"David - Six slugs of gas - what engine are you running?"

BMW 2.5 6 cylinder in a 38A Range Rover.

Checking the heat loss over the intercooler is interesting, and might help if you've found a stock intercooler to be ineffective, but I think the most important task is to measure the air temperature in the inlet manifold. If, when you want power, this temperature is less than 30 / 35C then I'd say there is no need to do more work on the intercooling.

Apart from being a good figure to quote at the bar, of course :-)

Cheers.