light/mid load EGT 3

[partsguy22]

What is an acceptable EGT range for light/mid cruise?

Right now they hover in the 1350-1400*f range while at cruise (50-60 kpa @ 3000rpm)

the engine is a 1755cc mitsu 4cyl
9.5:1 compression
Ive tried AFRs from 14.5 to 17:1
and ign lead from 36 to 42* btdc

No mater what Ive done they wont seem to go down

 

[patprimmer]

Have you checked your gauge. Those are very high temps for part load, although the mixture is in the highest temp range.

Are you sure your timing marks are correct. If you have a dial in timing light throw it away and get a good non adjustable timing light and put accurate marks on the pulley or flywheel if you have access to it.

Regards
Pat
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[partsguy22]

What would be the best way to check a gauge?

The timing is correct. (I would never even own a dial-in timing light) I had the same thought though so checked my base timing from idle to 3000 rpm , it doesn't fluctuate from the 5* its set at and the programmed advance is spot on.

I also forgot to mention that the Engine is turbocharged, if that makes any difference

 

[patprimmer]

Use it to measure something of known temperature or compare it to another or better several other gauges when they are measuring the same thing.

Regards
Pat
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[patprimmer]

Being turboed can make a huge difference. The back pressure betwee the port and the turbo will increase heat. The more the back pressure, the higher the temperature.

Regards
Pat
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[BrianGar]

Pat, taking your last reply into account I want to ask you a quick question if you don't mind.

All 1.8Turbo cars of German manufacture I have driven seem to have a lot of back pressure taking your above known statement into account,

That is, any time I take one for a moderate run and return to base, the exhaust manifold glows red hot, Im talking red now, like a poker in a blacksmiths forge (I have witnessed this on 20 plus cars)

Could this be the fact that a small turbo was fitted in an attempt to reduce lag at low rpm, but at the cost of flow through the unit at higher rpm, thus creating excessive back pressure within the manifold?

Appreciate your answer Pat, you havent failed me yet in the years of reading Ive done on here.

Brian.

 

[partsguy22]

Well that makes sense my turbo is small and its primary application was on a smaller (1597cc) engine

Then how much back-pressure is to much?

 

[MikeHalloran]

The pressure in the exhaust manifold is what _drives_ the turbine in the turbo.

The red heat is an artifact of using the engine's hot exhaust gas, instead of some room temperature gas supply, to provide the pressure.

Both are perfectly normal when the turbo is spooled up and working. They track each other because the turbo is sort of a bootstrap device.



Mike Halloran
Pembroke Pines, FL, USA

 

[BrianGar]

''The pressure in the exhaust manifold is what _drives_ the turbine in the turbo''

I understand that Mike, but surely you can have an excess of pressure too pending flow resistance through turbo?

A pretty large red hot part in a 1.8T engine bay always amazes me everytime I see it.

Thanks for the reply,

Brian.

 

[patprimmer]

There is a balancing act, the smaller the orifice in the turbo housing just before the exhaust gas hits the turbine, the greater the back pressure, the higher the gas velocity, the quicker the response and the lower the potential power.

You trade power for response and can only make the compromise that suits you best. The size of the restriction relative to other dimensions is the AR ratio. I would need to refresh my memory to know exactly how it is calculated, but for a given turbo, I think low AR equals quick response and high IR equalspoor response but big top end.

To much restriction is when the exhaust gets so hot that things fail or the power is unacceptably low.

Regards
Pat
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[BrianGar]

Thanks Pat.

Brian.

 

[MikeHalloran]

The mechanical simplicity of a turbocharger belies the number of variables that it adds to the problem of predicting what any given change might do to the system.

The exhaust manifold pressure is a measure of the load on the engine, or of the engine's response to the load, depending on your perspective, but in either case, you don't particularly want to minimize it.

The backpressure at the exit of the turbine is one pressure that you do want to minimize, because the engine always responds happily to doing so.

I don't think it's correct to call the exhaust manifold pressure 'backpressure'. Sure, that's what it would be in a normally aspirated engine, but in a turbo engine, that flow is not at the back end of anything; it's right in the middle of a strong feedback loop of energy flow.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Mike

I was trying to keep it more simple than cylinder blow down pressure.

It is pressure that backs up behind the nozzle orifice in the turbine housing.

There is an obvious relationship between the load on the compressor wheel due to volume of air pumped and the head of pressure it is pumping against.

There is an obvious relationship between load on the compressor wheel and load on the turbine wheel.

There is an obvious relationship between load on the turbine wheel and exhaust gas flow through the turbine.

There is an obvious relationship between power developed and mass of exhaust gas per unit of time.

There is an obvious relationship between exhaust gas pressure and exhaust gas temperature.

Regards
Pat
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[BrianGar]

I think an attempt was made on the 1.8T power plant to perform like a n/a engine but of bigger cc, some of you may know that it doesn't feel like a turbo'd unit at all, more so a larger cc n/a engine.
No lag, or ''boost coming on kick'' can be felt when driving it, it behaves pretty much like say a 2.5n/a engine when driven.
Maybe this characteristic> the fact that the turbo spools and starts to make boost very low in the rpm range has an effect on manifold temperatures due to the fact that higher up the rpm range a lot of the gas is being directed electronically out the waste-gate in order to prevent the turbo over spooling?

The bottom line is , to Pat and Mike I dont know why the 1.8T reddens its manifold so much from the factory, everytime I see one It makes me wonder, surely all the energy used to do this could be made better use of?

One thing I do know(or think), and keep in mind Im more a n/a guy that a turbo guy, is that I feel compressor maps are sometimes too looked into or over studied taking into account all the rest of the variables the turbo can encounter throughout the load/rpm range.
Dont shoot me down for saying that but Ive seen folk analysing the map, wondering, worrying, calculating, re-calculating, simulating, etc, for days on end before making the final choice and still not being happy how it performs in real life.

To Pat and Mike, I want to say thanks again for your fantastic replys, I value your opinions as Im sure your aware that each reply is very valuable given the fact that real life testing can be a costly route and often a waste of time when direct answers can rule out some ideas or ways of thinking immediately.

Ill always be learning.


Brian.

 

[partsguy22]

I understand all that

However I'm still curious why the EGTs are higher at low load and not during high load .If it were due to a restrictive turbine housing (small AR) or a restrictive manifold the EGT should higher all the time or at least during high airflow

The temperature issue only seems to occur at low load constant speed .I could understand if the problem was at time of high airflow but this is at low throttle, less than 20% .I know that a restriction will cause temperature to rise but why would it rise more when its lower ?

Pat and Mike I do appreciate your incite and thanks to all of you

Aaron

 

[pedalharder]

Its natural to see higher EGT's at low loads. This is caused by your AFR's. A lean mixture burns hotter and slower than a rich mixture. At low loads it is safe to lean the fuel mixture in order to save fuel. This produces a hot slow burning mixture and to account for the burn rate, the ignition timing is advanced.

Im not familiar with your engine but the temperature range seems reasonable. I have seen cruise EGT's as high as 1600f.

 

[BrianGar]

partsguy22, I forgot to apologise for stepping into your post above, sorry if I sent it off course but It seemed like a good time to bring my points up also.

Brian.

 

[fsmyth]

Check out:

http://dodgeram.org/tech/dsl/FAQ/turbo_faq.htm

About half-way down the page, it lists effects
and causes. High EGT is associated with a too-
rich mixture and mis-matched boost levels.

<als>

 

[partsguy22]

If that were the case my EGT at cruise would be much lower

the EGT problem only occurs at lean AFR and Low manifold pressures

...The problem has gotten better since I added more timing to my spark map in the problem areas but there still in the 1300*f range

 

[MikeHalloran]

If you retard the timing enough, all the burning takes place downstream of the exhaust valve.



Mike Halloran
Pembroke Pines, FL, USA