Nissan Patrol 4x4 (Africa)

Herrie wrote:When you are driving in sand you must keep the revs up to keep the turbo cool - with a petrol you can keep the revs lower.

I'm really confused by this statement! My understanding is as follows:

1. Turbos heat up due to the hot exhaust gases that enter the turbine side and also due to the compression of air occurring on the compressor side (Gay-Lussac's 2nd gas law). This is absolutely normal however, and turbos are designed to cope with this heat. The problem comes in when you turn the engine off without allowing sufficient time for the turbo to cool off. Engine oil that is trapped inside a hot turbo will coke ("burn") and degrade, causing rapid bearing damage to the turbo. This does not happen when the oil is circulating, so a hot turbo in a running motor is never a problem.

2. Turbos have a "boost threshold" - it's a combination of revs (i.e. airflow through the turbine side), throttle opening, etc from where they start to work. Below this threshold they do not spin fast enough to generate boost and and therefore the compressor side does not heat up as much as when they are fully spooled up. Lower revs also mean lower amounts of hot exhaust gases, reducing input temperature of the turbo. Lower throttle positions also mean less combustion heat/EGT - even more heat reduction! So... all of this means that turbos actually heat up more when you keep the revs up, not the other way around...

3. Diesel engines generate their maximum torque not very high above idle speed (Chuck's 4.2 engine does this at 2100 RPM), whilst petrol engines typically need about 3-4k RPM to deliver their max torque.

What am I missing here?

Voor ek n of my andwoord gee.Hier is n kopkrapper.
Die persoon wat dit ondervind het is deel van die forum en ek is seker hy sal sy input gee as hy dit raak sien.Ek noem nie n naam nie.
Sy kar het oor 900 grade egt geloop vir n lang tyd.Nou ja ek sal nie n diesel tune naby aan dit nie.Sy engine het nooit gesmelt nie.Hy het n piston gekraak na n lang tyd,maar het nie gesmelt nie.Hy het nie geweet van sy hoe egt nie en die kar gery soos ek en jy.

As I understand it:

Keeping revs up to cool down the EGT is applicable on the overrun, where your diesel flow is minimalised, and your airflow is maximum. When you change over too soon (vehicle is under the torque band, operating without meaningful boost) stepping on the pedal chase EGT up very fast. This normally gives a big smoke cloud. By stepping on it there is a lot of diesel, and not enough air you cause overfueling. Lots of diesel little air. If you change down later the air fuel mixture is more optimal and EGT will in fact stay a bit lower. I think that misht be what Herrie was getting at.

Next point is sand driving. Power and momentum is the key. That that is at the top end.

Like I said I said that is how I understand it. The info is what I have gathered with little experience and some reading. I do stand to be corrected, and will abide by the superior knowledge of someone like Frans . Having said that, 900 Deg ? Dit is darem maar baie

Julle Manne is almal spot on.Wat julle beskryf met n diesel.Dis hoe hulle werk.
If I may add.Plain said or the way I try to explain someting to someone.I some time get carried away,but here goes.
A diesel,the more the fuel the more horses.Turbo or not.A Petrol,the more fuel the less horsepower and waste off fuel and washing the bore and and...
A old school diesel with a injector pump,do not have a throttlebody.The intake is open.They will only fuel to what your right foot are asking.The pump has a mechanical advance system built in the pump to advance and retart the timeing.Fuel pressure play a big role in the mechanical side off things.If the fuel filter are blocked,it wil cause a drop in fuel pressure and the pump wont advance the timeing.To slow advance,no power bad fuel consumption,and smoking.The smoke came from to slow timeing and not more fuel.There are a very fine line in the diffirance between those two.The same engine fitted with a turbo,factory or not, use the same pump.There are a small differance.The pump has a acumelator on the main fuel side.Plunjer.
Timeing,the most important thing on any engine.
The faster the timeing the cooler the egt.The slower the timeing the hotter the egt.Note,it is timeing not fuel settings.A faster timeing will have the burning still in the cylinder,and a slower setting,you will have the flame run into the exhaust,witch will up the egt's.So at a certain temp in the exhaust,the cylinder can be cooler.(dis die kopkrapper wat ek gehad het met 900 grade)
Egt's dont mean a lot for a petrol engine.
The same for a commonrail or computered engine.It uses sensors to do the job.The bad thing thou,they(computers) can cut a engine for over boost and not for over fuel.One thing is the new commonrail measure fuel flow and if it get to much it will cut the fuel.But they still can not see egt's wich in my opinion are importand on a diesel.So make sure you are running the right timeing and then make sure off the boost and then get use to the egt reading.If it changes something happend to the boost or timeing and not to the fuel setting.
The smoke at lower rpm on full throttle,happens before the turbo is on song.If you measure the egt it will never be to high.The engine or ecu are setup to fuel the engine more to get it and the turbo going.
Hope this help
Rdgs
Frans

Hi may I just add this.
Thx Frans now I understand it more clearly.
But EGT , I have read up a lot about it . Once I have alo read a report by Peter Bilt trucks re. EGT.

Firstly, why EGT is so important to keep down is the following.
Above 720 deg C the metalurgical properties of some metals start to undergo a change, this why the 720 -750 deg C as upperlimit is very important.
Operate above that for considerable time and BANG!!!!!!!!!!!!!!!!!!!!! the alloys and metal start to show stress and you have turbo or engine part failure.

Remember there is a difference of approx 180 deg C between pre and post turbo. Pre turbo its hotter and after the whirl in the turbo it cools down. Always install EGT probe as close as possible to TURBO if post installation. So the upperlimit for post turbo is approximately 550 deg C.Pre turbo anywhere in the exhaust manifold is fine.

Causes of very high EGT is mainly overfueling.

My experience with especially the zd30 is that you cannot drive this engine without an EGT gauge fitted. I have driven mine strict according to never exceed the upper limit, and boy old boy to get over that limit was no effort at all.
I also drove this vehicle towing with a light foot.

My current Discovery 3 TDV6 , I havnt got an EGT gauge fitted, I was informed the ECU monitor the EGT anyway. But I am still going to fit my EGT gauge.I will feel much relaxed knowing I can see what temp. my engine is operating at.

EGT gauge DIY is also very easy and cheap, all you need is to look for instrumentation guys in your area , ask for a digital 12 v gauge according the size (square or rectangular)it can fit in an a thermocouple, the probe can be tap and died into the manifold or dump pipe and voila!!!!The ultimate off course is the Madman.

Nuttige thread hierdie!

I forgot to post Madman's EGT setup info, some useful info regarding air restriction & EGT:

Dag Koos,ek wil ook nog iets bylas.
Ek wil eerste se ek het my inligting of kennis of ondervinding nie uit n boek gekry nie,baie van dit kom met die fisiese werk op baie rygoed.Ja en dan die boek of twee om na n spesifikasie te kyk.In elkgeval die mods wat ek al gedoen het en wat baie ander doen,staan nerens in n boek geskryf nie.So die bietjie wat ek weet van n kar is maar om self te krap.
Ons almal weet, te hoe egt breek n diesel,ek het probeer verduidelik hoeveel faktore egt aanhelp.
Sover my kennis strek oor n tdv6,het hy nie n egt meter wat die ecu monitor nie.Hy maak gebruik van ander sensors om te bepaal of die fueling reg is of nie.Hy het lamda.En so pas hy fueling aan.Maar ek wil noem,bou in jou egt meter,en ek dink jy sal verbaas wees waar daai egt gaan draai.Dit sal hoer wees as wat die inligting jou vertel van metale.
Elkgeval dis nie n baie aangename job om n egt voor die turbo in te bou op daai 2.7 nie.
Voorspoed.
Frans

Dankie Frans, ek wil dit doen. Ek kry nerens iemand wat dit al op n TDV6 gedoen het nie, weet jy dalk van so iemand.
Groete
Koos
NS. Dankie Frans het al baie van jou geleer.

Stirring here with a HUGE wooden spoon...

I do not know nearly as much about engines as Frans, and I know just enough about metals to be a danger to me and all those around me. but the melting temperature of most aluminium alloys is between 600 and 670 degrees C. Obviously, pistons are made from alloys with relatively high melting points but still.

Off course, the pistons etc. are not necessarily actually at EGT, but how close they are depends on a multitude of factors, including the efficiency of the piston cooling system (if fitted), and the properties of the combustion flame, and I beleive it is these factors that makes certain engines capable of handling higher EGT's than other engines?