Can a diesel run too lean, or doesn't it matter?

[JMerola]

This may be a stupid question, so I apologize in advance. I have been try to conceptualize diesel combusion for some time now (my lack of mechanical background doesn't take me very far).



The amount of air taken into the combustion chamber is NOT regulated (ie. as much air as can be forced into the cylinder by the turbo at any given time IS what's compressed) right?



Why do we not have the problem of / or not have to worry about lean conditions (too little fuel/too much air) like the gasser people do?



- had a long drive yesterday, during which I was thinking about turbo's and fueling and timing and... . Couldn't even resolve this 'simple' question in my mind.



- Jason

 

[BHolm]

Diesels are a lean burn engine, hence the efficiency. They operate in an excess air condition all the time and no you can not lean it out and melt it down like a gas engine. If you take fuel away from a diesel you simply lose power. You burn up a diesel by adding too much fuel. Since there is always excess air, the more fuel you add the hotter the fire.



Somebody else hopefully has a more technically correct answer but, I think those are the basics.

 

[Viani]

i don't have the ability to get technical on this question, however, from what i've read and have been told, the more air a diesel engine recieves the lower the exhaust gas temps get and the better fuel mileage it gets. this is why the only mods i will make to my truck, 05 2500 ctd, will be a free flowing air intake and a muffler elimination kit to help the engine breathe better, it already has plenty of power for me.

 

[BigGunZ]

Diesels technically cannot be lean (in a perfect world). Whenever you have a fuel fired engine, whether a diesel or a turbo jet, the combustion process needs air. The air serves 2 functions... . to support combustion of the mix, and provide cooling with the remaining air.

Combustion comes first, meaning that the combustion process will use the air it needs to burn the mix. If the engine has poor air flow, high EGT's will result and likely smoke due to unburned mix.

Gasoline engines, since they rely spark and not spontaneous cumbustion, need exactly the correct mix to run efficiently. Too little fuel causes extreme heat, too much fuel, while causing a cooler burn, is bad for emissions, power production, and even cylinder wall lubrication in extreme cases. EGT's in a gasser are largely controlled by firing the correct mix.

This is an extreme oversimplification and there's more to it but essentially... a diesel can't have too much air because air doesn't control the mix, but a gasser can have too much air because it does, coupled with fuel, control the mix.

 

[SThorud]

And the fact that you can cram as much air into the combustion chamber as possible is why CAT went with twin-turbos on their C11-C15 ACERT truck engines. The high boost keeps the EGTs down and the NOx levels low.

 

[JMerola]

Thanks for the insight guys. I really appreciate the expertise this site has available for the asking.



Can anyone recommend a "layman" book(s) about the diesel engine that will help broaden my knowledge?



Thanks again,



- Jason

 

[Alan Reagan]

The amount of air cannot be regulated. The amount of fuel supplied for the amount of air can be regulated. That's why the air temperature and manifold pressure measurements are taken. Calculations using those two inputs can determine the number of pounds of air being delivered and thus optimize the amount of fuel delivered to the cylinders.

 

[Gene Arnold]

Jason,



This site may be of help in your quest to understand how a deisel works.



http://auto.howstuffworks.com/diesel.htm



Gene