Engine/Transmission (1994 - 1998) Technical Question for diesel experts

[pwerwagn]

I have a question I've wondered about for a long time that nobody has ever been able to answer. On a gas motor, running more fuel cools the combustion process, less fuel (or more air), otherwise known as lean, creates heat that can melt a piston. As we make more and more power with our diesels, it obviously requires more fuel and air. But why when we inject more fuel (i. e. larger injectors, fuel plate, fueling boxes, etc) do our EGT's increase instead of decrease as it would in a gasoline motor? I know that they run rich because of the smoke, and it seems like in general, the more smoke the higher EGT (that is a generalization, not true to every case, i know). Can anyone explain why they run hotter with more fuel and cooler with more air?

 

[RustyJC]

The air/fuel ratio required for theoretically perfect combustion where every fuel molecule is exactly paired with the correct number of oxygen molecules is called a stoichiometric mixture.



Gas engines operate on the rich side of stoichiometric - their air is throttled. As they approach stoichiometric combustion (get leaner), their combustion temperatures rise. As they move away from stoichiometric combustion (get richer), their combustion temperatures drop.



Diesel engines operate on the lean side of stoichiometric - their air is unthrottled. As they approach stoichiometric combustion (get richer), their combustion temperatures rise. As they move away from stoichiometric combustion (get leaner), their combustion temperatures drop.



So, it just depends which side of stoichiometric combustion the engine is operating insofar as air/fuel ratio is concerned.



Rusty

 

[pwerwagn]

thanks

thats the kind of answer I was lookin for.

I know the stoich of a gas motor is 14. 7, what is stoich for diesel?

 

[ronco]

I don't think there is a stoich for a diesel. Mainly because there is no A/F ratio to be concerned with. Anyone else?



Ronco

 

[bmoeller]

Stock, that is.

Stock, what is that? I forgot already. :-laf

 

[mechanic]

I have a question I've wondered about for a long time that nobody has ever been able to answer. On a gas motor, running more fuel cools the combustion process, less fuel (or more air), otherwise known as lean, creates heat that can melt a piston. As we make more and more power with our diesels, it obviously requires more fuel and air. But why when we inject more fuel (i. e. larger injectors, fuel plate, fueling boxes, etc) do our EGT's increase instead of decrease as it would in a gasoline motor? I know that they run rich because of the smoke, and it seems like in general, the more smoke the higher EGT (that is a generalization, not true to every case, i know). Can anyone explain why they run hotter with more fuel and cooler with more air?

All good answers, so let me add this additional curve. A diesel engine runs on compression ingition, and fuel is injected directly into the combustion chamber. The fuel is burned as it is injected. The longer the pulse the longer the burn. The burn stops when the pulse stops or very shortly after. In a gas engine the fuel is injected into the intake port or mixed with the air at the carburator and enters the cylinder as an emulsion that is ready to burn when the proper condition is met. The combuston will not take place unless the mixture is rich enough to burn. Because it is mixed outside the cylinder, the entire cylinder volume must be at that A/F ratio, it has more fuel per cylinder volume (grams of fuel per cubic inch volume of cylinder). If a gas engine were designed to run in the same manor as a diesel (which engineers are still working on and indeed a few are being produced), it would behave much like the diesel. The fuel is injected at or near the point of peak pressure and compression heat, and is concentrated in a small area. So the effect is that the A/F ratio in that small area inside the confined combustion area is actually much richer then the A/F ratio of the entire cylinder. I hope I said that clear enough. Thanks for a question that makes us think.

 

[RustyJC]

Actually, when talking about spark ignited engines, they can be designed to run on either side of stoichiometric combustion. As an example, let's look at methane fueled spark ignited engines used in industrial service (yes, this is on topic - even Cummins builds spark gas engines ). These industrial engines are generally classified as "rich burn" and "lean burn" engines. Insofar as EGT's, detonation and misfire limits are concerned, the rich burn engine moves toward detonation and has higher EGT's as the mixture leans out (more air). The lean burn engine moves toward detonation and has higher EGT's as the mixture is richened (more fuel). It all depends what the engine designer is after.



Rusty

 

[pwerwagn]

Thanks guys. Now I can stop wondering.

by any chance, do any of you have figures such as:

lb/hr fuel flow and CFM at a certain rpm/hp/torque? Altitude and Temperature would help a little too. Not anything to exact or specific, just general figures that have been derived during any of your testing.

 

[pwerwagn]

Nobody has any of these figures?