Jump to content

EGTs


muddy
 Share

Recommended Posts

More out of curiosity than anything, I'm now on my second and seemingly far more accurate egt gauge ( glow shift with 3mm probe) and can quite happily exceed the '750*c' limit I have seen banded around. Sat at 80 with fairish of load for minutes can see 800*c and happily get the whole turbo glowing in the dark, I have peaked at around 900*c for approx 10 seconds pulling hard up hill with a trailer, engine is a 300tdi with twiddled pump, I think a Brunel standard size intercooler and td5 tailpipe with centre box removed and gates valve giving around 23psi at the inlet manifold ( I think this is all the turbo will give and is probably inefficient) in a 90.

Whilst I appreciate its not going to last forever it has run this tune for about 4 years and ~30k my question is; has anyone actually melted/broken/blown up anything at a certain temperature and or run at similer or indeed higher temps?

Will.

Link to comment
Share on other sites

I have just come back from a race in south Wales, I towed a cattle box full of spares and tools down at the top end of 60 all the way. On uphill pulls (it was windy too) I was sometimes reaching temperatures of 850c plus. I tried to keep temperature under 700 as a minimum without dropping away from the convoy. This was quite frustrating as the engine has loads more pull, it just gets hit EGT's. I have ordered a larger intercooler now.

I have run the same tune for around 5 years now and only recently fitted an EGT gauge when I found that I get a flame from my exhaust on a hard pull.

Before I had the gauge it never bothered me and I never had problems. I looked into temperatures and there doesn't seem to be any definitive maximum temperature, however the number 750 is thrown around a lot.

Link to comment
Share on other sites

Interestingly (and contrary to my initial understanding) dropping down a gear and working it less on higher revs actually increases EGT's as opposed to keeping in 5th and labouring the engine more.

I believe that this is due to the turbo being able to keep a sufficient air supply at pressure in order to keep the engine cool (as a diesel is opposite to a petrol, I.e. running rich means hot as opposed to a petrol running hotter when lean)

I did wander whether installing a larger or variable vane turbo in conjunction with a decent charge air cooling system would actually keep it cooler than a normal turbo

Link to comment
Share on other sites

I'm kind of wondering whether I ought to put one in mine now.....

Am I right in thinking your motor is in fairly standard tune? And you don't tend to work it too hard too often? Remember mine is pretty much as turned up as can reliably be done and I have to get it to really work to raise the EGT's to over 650c.

Link to comment
Share on other sites

The other thing to remember ..., or perhaps that should be 'what I came to believe', while running a tuned turbo diesel, was that the EGT meter was essentially an 'economy' meter for diesels, in that if you could get the same performance with lower temperatures you were burning less fuel.

The same performance for the least fuel is a good thing, in my view.

By performance, in this context, I mean maintaining speed on an upgrade, for instance. That speed may be 50 mph with a heavy load, or 70 with a small load, the target performance varies with the circumstances. Effectively you have to start by experimenting, comparing low gear with high revs, against high gear with low revs, while aiming for the same target speed, to see which combination makes the most efficient use of fuel. The lower the EGT the lower the fuel usage.

I agree that the actual temperature readings quoted here (around 750C maximum) were what I saw recommended. Perhaps I was in the wrong forums, but I saw no justification for this number, it was merely repeated ad-infinitum.

No, I never "actually melted/broken/blown up anything at a certain temperature", and I saw much higher temperatures, but I was running a 2.5 BMW engine, so can hardly set a precedent for the 300 TDi engine.

Regards.

Link to comment
Share on other sites

The temperatures indicated on your EGT guage very much depends on where the gauge thermocouple is located.

If you have the thermocouple located in the plate (a common location, as mine is) and just before the turbo it will be at least 50*C higher than if you have drilled and tapped the manifold and have the probe located on the centre line of the turbo spindle.

If you think that this temperature drop is large consider this, the actual combustion temperature of diesel with a boosted air supply is around 1300-1700*C, this is dependant on many factors, engine design, fuel injection volumes, boost air pressure and the altitude where the tests were carried out etc. - this extreme temperature is for milliseconds only and that by the time it comes to the exhaust stroke and it is discharged into the exhaust pipe/manifold it's dropped to around 850-900*C. You start to get the picture that once the energy has been converted into power/work it cools down quickly as it simply a hot gas. You can confirm this easily by driving full power up a hill and then feeling the exhaust temperature at the tail pipe with your hand - it's hot but not so much so that it will burn your hand but try to hand feel the manifold temperature and you can expect some lasting and agonising pain!

Running at over 750*C for more than 5 minutes isn't advised by Garratt as the titanium blades on the turbo can warp.

My 300Tdi runs at an INDICATED maximum temperature of around 820*C full power going up a local steep hill in 3rd gear, with 15 psi boost - however the actual turbo temp as measured by a borrowed school laboratory thermocouple on the C/L of the turbo spindle was actually around 760*C.

Due to the location/proximity of the alternator and the tightest radius I could bend the thermocouple of my EGT gauge unit this location simply wasn't possible but drilling a 1/32" hole for a flexible thermocouple to check the actual turbo temperatures was comparatively easy as was blanking off this small hole after the test was carried out.

Consequently if I keep my EGT gauge below an indicated 800*C I know that I'm not going to do any permanent damage.

Link to comment
Share on other sites

Am I right in thinking your motor is in fairly standard tune? And you don't tend to work it too hard too often? Remember mine is pretty much as turned up as can reliably be done and I have to get it to really work to raise the EGT's to over 650c.

The 88 is at the moment, The 109's had a tweak and the wife drives it......

Kettle will in due course get an LT77/LT230 and I may then think about doing some tuning, she'll be getting an inercooler around then too, if I don't get it fitted before. She does have to work for a living sometimes, though at the moment she's definitely poorly

Link to comment
Share on other sites

Vulcan that's a tad harsh, however if, as you say you are getting flames coming out of your exhaust then either your injection timing is either too far advanced - only by a degree or so -- or too much fuel is being injected and consequently the unburnt fuel (i.e. too much fuel for the entrained air supply) is combusting once it is discharged into the exhaust system.

Either way, your turbo temperature would be cooler due to the excess fuel burning in the exhaust system. Flame-outs are at a lower combustion temperature than the exploding diesel fuel under extreme pressure in the cylinder at TDC, hard to believe but true --- as an example Daihatsu used to set the computer to "over fuel" the factory Charade 3 cylinder turbo GTI rally car at over 10500 rpm to aid in cooling the exhaust valves and piston crowns and we had similar flame outs whenever we changed gear or the turbo relief valve popped off. I know, petrol powered but the same thing applies.

Link to comment
Share on other sites

I must disagree with Discomikey, post #4. When you change down a gear at high EGT, at the nominally same road speed, the increased engine speed means that more air is being fed to the cylinders. This increased air flow at the same fuel rate, must reduce the EGT. In fact, Discomikey agrees with this in his second paragraph: "......... (as a diesel is opposite to a petrol, I.e. running rich means hot as opposed to a petrol running hotter when lean)"

Yes, change down to reduce/limit your EGT.

Mike

Link to comment
Share on other sites

mmm, and petrol running hotter when lean.... well sort of, when lean you get fuel burning slowly which means it burns in the exhaust rather than the cylinders, so it is more that it burns in a different place, moving the heat from one place to another.

But with less oxygen available the amount of heat given out can only really be less than a proper efficient burn.

Link to comment
Share on other sites

I must disagree with Discomikey, post #4. When you change down a gear at high EGT, at the nominally same road speed, the increased engine speed means that more air is being fed to the cylinders. This increased air flow at the same fuel rate, must reduce the EGT. In fact, Discomikey agrees with this in his second paragraph: "......... (as a diesel is opposite to a petrol, I.e. running rich means hot as opposed to a petrol running hotter when lean)"

Yes, change down to reduce/limit your EGT.

Mike

The fuel rate goes up with the engine speed. Whether or not EGTs increase or decrease with engine speed depsnds on how the pump is setup

Link to comment
Share on other sites

The fuel rate goes up with the engine speed. Whether or not EGTs increase or decrease with engine speed depsnds on how the pump is setup

Yes, but I did say "at the nominally same road speed", so the engine is still doing about the same amount of work, but is at higher revolutions and hence more air is being passed through the engine. Same amount of fuel and more air should give a reduced EGT. It does on my 300Tdi.

Mike

Link to comment
Share on other sites

Been thinking about this. There is probably three limits:

1/. Turbine blades soften/ deform - your really o et doing it

2/. Creep rupture failure - gas turbines suffer from this. A 1970's gas turbine I'm about to overhaul has a power turbine inlet temp limit of 820 c. You can run at 820 for 100,000 hours - then blades need changing. If you run hotter you rapidly reduce service life (an rolls Royce RB211 has a reduced turbine blade life of 30% for a 10c increase in operating temp). So run it to hot and you are reducing the turbo life span. Failure mode will be brittle failure of the blade.

3/. Over heating of bearing oil from heat convection - coming of oil, laquering on bearing reducing clearances until bearing runs hot and fails (assumes it is a journal bearing)

Your probably OK firshort bursts, but you are reducing the time to turbo failure - but that might not be significant for you

Link to comment
Share on other sites

I must disagree with Discomikey, post #4. When you change down a gear at high EGT, at the nominally same road speed, the increased engine speed means that more air is being fed to the cylinders. This increased air flow at the same fuel rate, must reduce the EGT. In fact, Discomikey agrees with this in his second paragraph: "......... (as a diesel is opposite to a petrol, I.e. running rich means hot as opposed to a petrol running hotter when lean)"

Yes, change down to reduce/limit your EGT.

Mike

As my previous post states, initially i agreed with your logic, however my EGT gauge disproves this. Sat on the motorway with a load on changing down increased EGT!!!

I can only theorise about whats happening but as I said above, i reckon the turbo is too small to supply the required volumes of air at higher revs. one of the reasons a TDi is known for being gutsy at the lower end but anything over 3500rpm is useless for power

Link to comment
Share on other sites

My 300 tdi with VNT fitted , front mount and solid pipe work very rarely goes over 500 degrees on road running 1.4 bar boost and various internal pump mods.

Interesting to read, and supports my theorey, do you ever tow heavy trailers, i.e. make it really work for long periods of time?

Link to comment
Share on other sites

My experience, Defender 200tdi, standard tune.

Giving it some beans along a flat road EGTs rise in each gear and decrease when changing up a gear.

On a long hill in 5th gear at say 55mph EGTs rise, changing down to 4th and maintaining speed reduces EGTs. ( 1.2 box )

Link to comment
Share on other sites

To me this only shows that going to high on turbo pressure means things get very inefficient very soon. It is potentially the result of intake temps going sky high due to the compression of gasses in the pump side, followed by the exhaust side. Interesting to see the VNT turbo not getting anywhere near these temps. Shows that the efficiency is much better of these.

Daan

Link to comment
Share on other sites

I am not sure I would necessaryly put it that way daan, at least from my experience. My wastegate have been taken of, and have been for the last 3 or so years (300 tdi approx 50k km). My experience is that when running a 1.4 gearbox the car would happily sit at 85 all day long going some 370 degrees with 3500 kg on the hook. Now running a 1,2 gearbox it will similar run at 85 all day long in 5'th just aswell, but now looking at 400 degrees and climbing rapidly when seeing a hill. Going into 4'th gear reduces the egt right away.

I think it is because the turbo isn't flowing as good around 2000 rpm where it sits with the 1,2 and it therefore overfuels and heats more.

Just my thoughts.

Mads

Link to comment
Share on other sites

Change down a gear and you will drop EGT in my view.

Why?

Assume a non turbo engine to make things simpler.

Assume a constant load (steady hill) and constant speed. Therefore it takes the same amount of energy (diesel) what ever the gear (not quite true as engine efficiency will vary with speed, but ignoring that).

A non turbo diesel draws in a fixed amount of air per air intake stroke. The injector pump adds fuel depending on the throttle position. If the load is low then not much fuel is injected. The burn heats the 'spare' air in the cylinder - the exhaust gas is hence an average of the combustion gases and spare air. If you add more fuel you eventually get to full load - assume no spare air, hence all exhaust gases are combustion gases - in this scenario the exhaust gas is hence hotter.

Going back to the steady hill and constant speed:

Your in 4th at 2000 rpm. You change down to 3rd and keep the same speed. The energy needed to move the car has not changed.

The but the engine revs have increased. So you are putting in less fuel per ignition stroke, but the same amount over time as the engine is rev'img faster. However the air intake volume is approx the same per Rev. So you are getting more ignition strokes with less fuel in them - the excess air in the cylinder 'cools' the combustion gas (by being heated). So EGT drops. However exhaust gas volume goes up.

You are burning the same amount of energy, just in smaller chunks more quickly and mixing the resulting gas with more air.

Link to comment
Share on other sites

Change down a gear and you will drop EGT in my view.

Why?

Assume a non turbo engine to make things simpler.

Assume a constant load (steady hill) and constant speed. Therefore it takes the same amount of energy (diesel) what ever the gear (not quite true as engine efficiency will vary with speed, but ignoring that).

A non turbo diesel draws in a fixed amount of air per air intake stroke. The injector pump adds fuel depending on the throttle position. If the load is low then not much fuel is injected. The burn heats the 'spare' air in the cylinder - the exhaust gas is hence an average of the combustion gases and spare air. If you add more fuel you eventually get to full load - assume no spare air, hence all exhaust gases are combustion gases - in this scenario the exhaust gas is hence hotter.

Going back to the steady hill and constant speed:

Your in 4th at 2000 rpm. You change down to 3rd and keep the same speed. The energy needed to move the car has not changed.

The but the engine revs have increased. So you are putting in less fuel per ignition stroke, but the same amount over time as the engine is rev'img faster. However the air intake volume is approx the same per Rev. So you are getting more ignition strokes with less fuel in them - the excess air in the cylinder 'cools' the combustion gas (by being heated). So EGT drops. However exhaust gas volume goes up.

You are burning the same amount of energy, just in smaller chunks more quickly and mixing the resulting gas with more air.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy