question about gas vs. diesel turbocharged egts
 

Well, this is my first post eventhough i've been reading just about every thread on this site and TDR for the last year or so. Diesels amaze me at their simplicity to be modified and how you can have a 800hp cummins and still run "pump gas".

Anyways...on to my question. Can anyone explain exactly why on diesels when you richen the air/fuel ratio, the egts (and smoke) go up, and you need to run more boost/more efficient turbo to keep the smoke and egts under control. This seems to be to complete opposite of gas motors. I used to be real into toyota supras, for some of the same reasons I like the cummins, you could make huge hp pretty easily on mostly stock parts (at least engine internals). On those motors, and any gas motor for that matter, leaning out the fuel mixture (with a bigger turbo, more boost, NOS or whatever) you had to add more fuel. Some ppl ran egt gauges in their cars as a kinda cheap way to monitor your air/fuel ratio....hot egts=motor was running lean. And when trying to run high boost on pump gas, they would set it up to run extra rich to keep detonation under control. So basically i'm asking if anyone can explain in thermodynamic terms why gas and diesel turbocharged motors seem to be opposites in regard to egts vs. air fuel ratio.

Diesels, since they have no throttle plate, are excess air engines that operate on the lean side of the stoichiometric point (theoretically perfect combustion). Gassers are rich burn engines that operate on the rich side of stoichiometric. As you approach stoichiometry, EGTs increase. As you move away from stoichiometry, EGTs decrease. Since the diesel and gasser are on opposite sides of the stoichiometric point, they behave as opposites.

Rusty

Diesels suck in air always reguardless of throttle position, gassers rely on throttle position. the more diesel that goes in the more power is produced. The more air you can shove in (turbo) the cooler the exhaust temps. Then you can put more fuel in. there will come a point of you cant get enough air in to burn the fuel going in (black smoke) then its time for twins (turbos). [eyecrazy]

RustyJC: that seems to make since, somewhat. i understand a diesel is a lean burn motor since it has no throttle plate, and adding more fuel: richens the mixture, brings it closer to a stoichiometric ratio, and increases egts. However, it seems that if you continue to richen to mixture (so it smokes like a train) the egts continue to rise. same with a gas motor, the closer you get to a stoichiometric mix, the hotter it gets, but doesn't it continue to get hotter the leaner you ran? (not totally sure, as i can't remember any facts/tests of this, but I know on a gas turbo motor, if you get a sudden boost spike, where it's leaned out to possibly 20:1 air:fuel ratio, it can quickly burn up pistons). Can anyone verify or clarify any of this?

Also, does anyone know what the stoichiometric air/fuel ratio is for diesel? and can you put a wide band o2 sensor in the exhaust and monitor the air/fuel ratio. I know that's not necessary at all, i was just curious.

On a diesel, the more air you push in at a set fuel rate, the lower the egts will get. The more fuel you push at a set boost level, the hotter it will get. Its as simple as that. You cant run a diesel too lean. Now, pushing 50 psi on a stock diesel will get you no were. When you add more fuel to along with boost (ALOT more[laugh] ) then you will be able to create more power. If you added alot more fuel and did nothing to the turbo to get more boost, you would create more power, but EGTs would have been increased, probly too high.

DZLPWR: yeah, i totally understand that, just curious why it's the total opposite for gas motors.

hotpaco17,

It is not very useful to think of diesels in tems of rich and lean like we do with gassers.

Look at the "Diesel theory" thread under Gen Diesel Discussion and see my post (Wetspirit) about that subject. Maybe that will help with your question. It is a couple of pages back. Tell me what you think.

Wetspirit

wetspirit: yeah, i read your post just after i posted my question. And that does explain a lot of what I was curious about. Basically gas motors are a closed system, and diesels are an open system. makes sense.
However, IS there stoichiometric point with air and diesel, there's gotta be, just wandering what it is...I guess it's irrelevent in a compression ignition motor for the reasons you stated in your post. I'm sure i could look that up in a chemistry book or something, but does anyone know the answer to that?

Of course there is a ratio of fuel to air that is theoretically perfect. I don't know what that ratio is. In the engine where there is an excess of air, a nozzle spraying pure fuel, a limited amount of time to vaporize completely, widely varying temperatures from the heart of the burn area to the cylinder wall, and a lot of turbulence from intake velocity, combustion chamber design, and a flame front, it seems like the ratio discussion doesn't matter except as a matter of curiosity. The ratios are all over the map depending where you look. Only the smoke signals, the cleanliness, and the temperature of the exhaust really matter. And even if the exhaust is smoking it doesn't mean the engine is too rich. If the smoke is black it means the engine is at or near full power or the air is restricted or it has a bad injector, etc. It could be quite lean in general and be smoking black. If you want to figure the ratio at total air in vs. total fuel in, it will always look too lean unless the engine is at nearly full power (full power means what it's capable of, not what Dodge rates it at).
It makes gassers look so simple, doesn't it? But even they could be running too lean because of improper atomization and an ox sensor that sees unburnt fuel and leans the mix. The engine acts lean while wasting fuel. And driveability suffers. Sheesh. It's amazing to me that fuel injection works as good as it does on gas engines. The main thing it has going for it is that it's not a carburetor. I think one of the keys to efficiency is thorough atomization and it's hard to imagine the fuel becomes completely mixed with the air in the conditions present in engines.

Wetspirit

get stoich and air/fuel out of your head when it comes to diesels...

all you need to know is: more fuel = more power, more fuel = more heat... as long as your EGT's are alright, keep on rolling... diesels will continue to make more power beyond the point where smoke is visible from the tailpipe. on a gas motor, you're losing power if you're seeing smoke.

more fuel = more power... more boost = lower EGT's

when you look at it that way, diesels are actually MUCH simpler than gassers...

Forrest

Thanks for the explanation. It's amazing how versatile the compression ignition engine is. It can run with way too much fuel or barely enough (maybe not to the best of it's ability, but still) and can run on almost anything from kerosene to jet fuel. If ya want more power, just add more fuel and add air as needed to control temps. Gas engines just seem so delicate in comparison, and have many more important variables that affect it's performance much more so than in a diesel, things like valve timing, ignition timing, air/fuel ratio, boost pressure, and type of gas all have to be carefully matched just to keep the motor in one piece, nevermind making max power. Anyways, thanks again for the explanation.

diesels are the greatest thing since sliced bread, but they cost a lot more... and their components cost a lot more... now, they'll generally LAST a lot longer, but you pay for it up front! :(

took me a few years before I could get behind the wheel of a 215 pumped 12v!! :( , but now I'm all smiles! :D

oh, and also realize that valve timing and injection timing are very important w/ a diesel, but the stock cam works great, and dialing in injection timing isn't too tough. once it's set, you're in business. you can go with a bigger cam, but they cost an arm and a leg, and you'll probably want to stick with the recomended installed ICL. and I'm willing to bet that the ICL and LSA are pretty similar to stock. you'll find more affordable tuning ability in playing with injection timing and turbocharger/turbochargers ;)

Forrest

ahmen! I've always liked diesels since my dad got a 96 ford E350 van with a powerstroke, just because they sounded cool, but then he put a chip in it, and I was blown away by how fast that 15 seater "church van" was. I always wanted to drive it and race anything and everything at stop lights...hardly anyone actually tried to race, but i'm sure they were shocked when a big *** van lit up it's tires. He finally sold that just a few months ago with 175000 miles on it because he kept having transmission, injector, and glow plug problems. Luckily I was able to convince him to order a 2005 cummins truck with the 6-speed. I'm still working on him to get a tst or some kind of box and straight pipe it, though.

don't get crazy w/ the fuel... the stock turbos on the new trucks are pretty dang small and easy to kill w/ more fuel!

Forrest