Fuel Quantity vs. Heat

[dosma]

I keep seeing posted that more fuel generates more heat in a diesel.  How would that compare to fuel enrichment in a gasser?  I.E. - If you lean out a gasser too much, less fuel/air mixture creates more heat and can burn valves.

I've read in a few FAQs about adjusting the fuel quantity in a diesel, generally looking at smoke and power as the indicators of proper adjustment.  Can one lean out a diesel and burn valves as in a gasser?  This is assuming that the intake piping remains the same and that if the engine is turbo charged, the boost is adusted to factory specs.

I'm going to adjust fuel quantity on my 1.9 AAZ and want to understand the principle before proceeding.  Other than not running well, can adjusting the fuel quantity too low damage an engine or are your worries in this regards only with too high a fuel quantity?

[QuickTD]

I.E. - If you lean out a gasser too much, less fuel/air mixture creates more heat and can burn valves.

This is not technically true. What is happening in the gasser is that you are coming closer to stoichiometric (chemically correct) mixture. A chemically correct mixture will produce the most heat and therefore the most power, but often takes the engine beyond its thermal and detonation limits and is therefore avoided. If you continue to lean a gasoline engine you can actually go beyond stoichiometric and the EGT and combustion temperature will begin to cool. With a diesel you are actually approaching stoichiometric from the lean side rather than the rich side as in a gasser.    

I've read in a few FAQs about adjusting the fuel quantity in a diesel, generally looking at smoke and power as the indicators of proper adjustment.  Can one lean out a diesel and burn valves as in a gasser?

In a word, no. Forget what you know about gassers, it does not really apply here. As I said previously, a diesel is always lean. The cylinder is filled with a full, unthrottled charge of air with every intake stroke and a quantity of fuel consistant with the desired power level is injected near TDC. Fuel quantity is the variable, reduce the quantity of fuel injected and the engine slows down and produces less power. Increase the injected quantity it and it revs up and produces more power. The mixture is extremely variable, often leaner than 100:1 at idle and as rich as 20:1 at full power.

are your worries in this regards only with too high a fuel quantity?

Yes. Reduce the fuel quantity and the engine simply produces less power. Too much fuel for a given amount of air will produce high EGT's.

[QuickTD]

Indeed, the term "mixture" really has no place in discussions concerning stratified charge engines. I'll strive to find more appropriate terms in the future.

[VWRacer]

Lighten up, will you two...!?

Diesel engines "mix" the fuel and air as thoroughly as gassers, otherwise they wouldn't even run, so let's not get too hung up on terminology. We understand that gassers generally lean from rich towards stoich while diesels fatten towards it from waaaaay lean. Yeah, guys sometimes use the terms a little casually, but there's no need to spaz over it.

Besides, if we're going to eliminate 'mixture', we'll have to eliminate other technically incorrect words, starting with 'gasser' (spark ignition engine), diesel (compression ignition engine), motor (engine) and well, you get the idea. Let's not go too far 'round the bend...  :roll:  :wink:

[dosma]

Thanks a lot QuickTD and for the discussion that followed.  Sorry I haven't replied earlier.  Had some stuff that kept me away from the computer.

That's a big word "stoichiometric"  but it makes perfect sense to me now, that diesels approach peak performance mixture (ultimately in the chamber) from a lean mixture by definition, I.E.- A combustion chamber (or two) that starts out as compressed air.  That's pretty lean.

So I think I can go out and tinker with the fuel quantity mechinism with a better understanding of whats going on.

Anyone have a Vanagon and especially a Westfalia camper with a 1.6td or 1.9td that I can compare mpg with?  I'm pretty sure that I'm on the rich side, my mpg is around 22 mpg which isn't too bad I'd feel much better with 25 or 27 on the highway at 60 mph.  But I am pushing a brick that weighs 4500 lb plus through air so don't know that the higher numbers are realistic.  I don't see smoke out the rear window under heavy load but the tailpipe is way down from the window so may be getting more than I can see in the rear window.

Again, thanks for the explainations.