Diesel Fundamentals

[Anonymous]

I just gotta ask, is there such a thing as a Diesel running lean?

The guys at work seem to think so, and that its detramental to most diesels. They reckon that they blow white smoke if it runs lean, and black smoke if it runs rich. I can understand the rich part, but not the white when lean - to me that sounds like its burning oil.

Case in point is a 1KZ-TE, Hilux Surf 3.0 turbo diesel 4 cylinder. Basically an alloy head 5L. The spray pattern out of the (cracked) pre ignition chambers has pitted the surface of the combustion chamber, the head section crew all seem to think its because the japanese diesels run lean.

I reckon that a diesel is meant to run lean by nature of design, since most don't have a throttle, power is controlled by the amount of fuel injected into the cylinder. From a minimal amount to sustain idle, to peak fuel consumption during peak power, in between its from lean to rich..

Anyone out there got any ideas that'll be of use?

 

[Lazy JW]

You are right, they are wrong. A diesel needs to take in a full charge of air at each intake stroke in order to achieve the necessary temperature from compressing the air (this is also why diesels don't develop manifold vacuum like a gasoline engine). As you noted, the speed (and power) is then controlled by the amount of fuel injected into the chamber. Therefore, at idle the ratio MUST be quite lean, and at full power it will get rather rich. At some point there simply isn't enough air for the fuel to burn well and black smoke is the result. A turbo allows for more air and more fuel, the new computerized engines aren't supposed to allow a smoky engine. But there is truly no such thing as too lean at idle as long as it runs. If there isn't enough fuel it simply won't make power.
Joe

 

[Anonymous]

The only thing that burns a diesel engine down is is improper injector timing. the earlier you time it the more fuel gets in the cylinder and increases Heat and also increases the chance of the mixture igniting too early. ALL diesel engines are "lean burn" engines, you control the engine speed with the fuel not the air. The Air intake on a diesel is wide open and there is no throttle plate like a gas engine has. You can lean a diesel out a bit by retarding it to much but you also loose power.

 

[Anonymous]

A bad or poor working injector pump can cause the fuel not to vaporize all the way. Instead of a fine mist of fuel you'll get big drops of fuel which won't burn very well at all.

 

[aussie7mains]

Disco, have seen you for a while, ive moved house and am now living in Ballajura. Did you finish your studies? Im working at AEC doing natural gas engines now.
You guys have to stop thinking of diesel(compression ignition) in terms of petrol engine combustion, they are not the same.
Diesel fuel will burn over a wider range of air fuel ratios than petrol, its doesn't have octane, and doesnt knock.
Compression ignition engines generally run without any control of air entry (no throttle, although some crappy ones with indirect injection like the smaller end of the market do!) so they are always operating with "excess air" in other words there is ALWAYS more oxygen available than what is strictly needed for combustion. Control of power is done by the quantity of fuel being injected, small amount just enough to keep the engine idling against its own friction, large amount to give the required power and torque. The diesel is always limited to a smoke and gas emission level by the fuel pump governor, which controls the injection quantity and the engines speed, modern engines have electronic controls of this. generall more fuel will give more power but emissions become excessive. the design of the combustion chamber is very important here.
If the engine runs into thermal problems like pre chambers falling out its usually because of overheating or other problems.
The small Jap (et al) are to my mind rubbish, and shouldnt really be looked on as Diesel engines at all, the larger heavy duty units which can pull full power all day are proper engines.
So forget "running lean, or rich' isnt true, the amount of heat in the combustion chamber only varies with the amount of fuel going in, so a t low loads, little fuel little heat, EG stick your head up the exhaust on an idle Diesel an its cool as, full power, roast the turkey!

This is why turbos form a diesel to a petrol wont work, excess air is the difference.

 

[goinbroke2]

Good post, I'd like to add that if a diesel has white smoke it can be 1) cold (or no/low compression in a cyl) 2) quill shaft broke (blower drive on 2 stroke detroits) 3) coolant from a crack etc.
Oh, something else, toyota land cruiser diesels have a throttle plate! Very weird, but when you step on the throttle, it lets in more air and a vac port above and below the throttle plate (in conjuntion with a spring) determines how much fuel is added. Very mickey mouse in my opinion and a major pain in the ass to adjust. One way has no power, the other has lots of black smoke and no power. :x
Just thought I'd throw that in there

(21 years heavy diesel mechanic, mostly 10 cyl MTU (mercedes) and 6v-53, 6-71, 8-71, and now perkins and the worst piece of shit of all time bar none, fiat!)
Oh yeah, and 6.9-7.3 ford, cummins,6.2-6.5 gm

 

[Anonymous]

G'day fellas,

Thanks for the tips.. Had another conversation with the blokes in head section today, didn't get any sense out of them. I guess they ain't engineers, or they dont know their stuff very well.

Yeah, I noticed the throttle plate on both a Ford/Mazda T4600 truck engine, and a Landbruiser 2H motor. I thought the 2H was something else originally, till I took a better look.. I saw the fuel pump sitting at the side, and the throttle body, and had a WTF moment, lol. Any idea why they'd put that there? is it to improve compression braking?

A7M, dropped out of uni, after having had a look at the job market for computer work. There is none, its who you know, such as things are in this town. Moved into Belmont, and currently working as a driver / workshop hands at The Engine Exchange. Gonna toss that job as soon, I wanna get myself a spot on a drill crew offsiding. About to get my B class, which is the only thing stopping me, apparently. Got the L's for it, just gotta take an afternoon off from work to take the test.. lol, "Hey boss, can I have half a day off?" "What For?" "To get me B Class.." "What do you need that for?" lol. Oh well. Hows Ballajura compared to Nollamara?

 

[aussie7mains]

Sorry to hear about dropping out, but only you know whats best for you.
The new house is twice the size of the old one and has AC and 3 car garage, so much bigger etc.
As to the throttle on the 2h i believe that the manufactures for smaller light duty diesels use throttles to restict the cylinder pressure at low power in an effort to reduce combustion noise, they also use MM fuel pumps running at lower pressures to achieve the same thing.
Ive recently got my learners permit for thr MR license and will expect to do the test soon, my employer has paid for this.
A7M