Fuel air ratio vs EGT's vs boost

[IWeiny]

OK, First I must preface this with the fact I am not a mechanic. I do have some Engineering back ground but for the past 6 years I have been a computer geek.



1) I know in a gas engine if you run lean you run hotter. But I see a lot of posts about bigger injectors and boxes increasing EGT's. Why is this?



2) I know in a gas engine you often have to get the engine up to a higher RPM before a turbo will give you boost. But it seems in my truck I get boost whenever I pour more fuel in... (ie more wasted energy comming off the engine so more for the turbo to use. )



Both of these seem "more correct" from a laymans point of view. More fuel means more energy so more heat. But then why is that not true in a gaser?



Just wondered if anyone knew.

Thanks,

 

[Rman]

comparing diesels and gassers is like apples and oranges. Totally different animals.



Gassers run hot when lean... diesels already run incredibly lean by comparison, but this is because of the type of combustion taking place. It has to be that hot. When adding more fuel to this combination it adds much more energy which in-turn is released mostly in heat.



This increase in exhaust temp is the bad thing, but can be controlled by adding more boost to cool the turbo. This increase in exhaust mass also helps turbo spooling, which helps make more boost... you probably already understand this.



Gassers use very different turbos, lower boost turbos in the 10-15 psi range, your typical diesel makes like 20-30, with stock turbos. Gassers spool at higher rpms because gassers make power at much higher rpm's than diesels do. Diesels extraordinary long strokes and high static compression ratio's are what limit rpm's. Though in my opinion this is a good thing.



High rpm engines require intricate balancing or damage will occur, I think this is another one of the plus sides of owning a diesel and what helps longevity.



There are some folks on here that can explain this much more eloquently than myself, I was just trying to answer your questions as simply as I could put it.



Bottom line though, gas engines and their principles are vastly different than diesel technology. Not in a bad way, just different operational theory.



A lot also has to do with the different fuels used as well. This is a very complex subject that is very hard to teach without classroom instruction.



*edit*



I forgot your last question, or perhaps THE question.



The air fuel ratio in a diesel is almost irrelevant, where in a gasser it is crucial. Like I stated before, diesels run incredibly lean and as long as you have oxygen in there you can dump all the fuel you want to make power. Gassers are not this way, it's a complex variable with air fuel ratio. It is limited to this because gasoline will detonate or cause preignition. Since diesel injection occurs "when needed" at or near TDC, this is not a problem. Gassers are finicky and touchy, where a diesel is much more forgiving.



Hope this helps a little.

 

[IWeiny]

Thanks for the reply

What you say makes a lot of sence.

Thanks,

 

[MCummings]

A diesel wil just about melt down at 25:1 A/F ratio, and will idle at 75+:1 (some cars idle up to 200:1 A/F)



When you are off the fuel and above idle (coasting in gear) you are at ~:1 (infinity:1) There is no fuel being injected.



A Diesel has No throttle body, or any other way to limit air intake.



Every intake stroke brings in a full charge of air. From the air filter, to the intake valve, there is NO Vacuum, only Atmosphere (14. 7PSI) or Boost. Even at 0 throttle, but High RPM there is boost. At 3,000RPM a stock truck will still make about 5PSI boost with no throttle input



At idle, your truck will flow about 750CFM of air, but as boost comes up, the engine size will "grow", and with enough boost, your little 359 CI motor will flow 2,000+CFM, and still remain under 3,500RPM.



The leaner your diesel runs, the less fuel it is using to make power, and the cooler your exhaust will be. (both are good things)



The richer you run a Diesel, the hotter it will run, and you will get worse fuel mileage.



The sooner you can spool the turbo, the better response will be, and the better MPG wil be (to a point).



The question about the turbo,, I don't know how a gasser turbo works, but a diesel turbo requires XX amount of CFM before it will spool. If you can meet that CFM for that particular turbo,, it'll spool. Some turbo's require the CFM your motor will produce at 1,500RPM, and some require 3,000RPM worth of CFM before they spool.



Since there is nothing limiting air intake,, the turbo spools as soon as it can, to cool the motor off.





The primary reason for a turbo on a diesel is to cool it down. . not to nessacarily to make more power.



Merrick

 

[rbattelle]

Originally posted by MCummings

The primary reason for a turbo on a diesel is to cool it down. . not to nessacarily to make more power.

[Very respectfully] I'm not sure I can agree with this statement. An engine (be it a reciprocating engine or a jet) is really just a massivly complex air pump. Force-feeding the inlet to that air pump with something like a turbo, which recuperates otherwise wasted energy in the exhaust stream, will improve efficiency (power and economy).



Again, I don't mean to imply you don't know what you're talking about, Merrick, I just don't think I agree that a turbo's primary purpose is cooling.

 

[IWeiny]

Thanks again for the reply, I am learning.



I wish I could add more but I am just reading and soaking it all in.



Thanks,

 

[Vaughn MacKenzie]

Originally posted by MCummings



At idle, your truck will flow about 750CFM of air, but as boost comes up, the engine size will "grow", and with enough boost, your little 359 CI motor will flow 2,000+CFM, and still remain under 3,500RPM.

MCummings you might want to check your math. . . at 750 RPM a Cummins will flow about 78 CFM:



1 cubic foot of air = 1728 cubic inches

a 359 cid engine draws 359 cubes of air every 2 revolutions (due to 4-stroke engine)

For 1 minute: 359 x 750 divided by 2 = 134,625 (cubic inches of air total for 1 min. ). Divide 134,625 by 1728 = 77. 9 CFM.



At 3000 RPM and 30psi boost, not correcting for parasitic losses:



Boost increased 2 atmospheres, so 359 cubes acts as 1077 cubes.

Plug this into the same math. . .

1077 x 3000 divide by 2 = 1,615,500 then divide 1728 to get Cubic Feet gives you 935 CFM.



Then you go with HVAC style power (3000rpm and 60lbs boost):

4 atmospheres, 1436 cid equivalent

Plug in the numbers gives you 1247CFM



Vaughn

 

[MCummings]

Boy,, I got slapped around on this one ! LOL



Should have thunk harder before posting. .



I will readily admit, I was off on the CFM at idle,, some information I passed on without research (aka, starting a rumour)



Now, CFM at WOT, I was talkin' "MCummings" style 3,500RPM @ 90PSI. (and I wonder why I'm rebuilding my motor )



The turbo provides pressure,,, which in turn will provide more oxygen. When you provide more oxygen, you can add more fuel. I feel that if you add oxygen, without adding anymore fuel, you will get a leaner burn, but not nessacrily a power increase.



I guess you would add a little power by adding a turbo to a diesel, but I'm not sure how much.

I've never done this test before, but I guess you could take a N/A diesel, and without touching the injection pump, add a turbo, I think it will cool off your Exhaust, (and heat the intake air), but wil it add more power? I'm not sure,, I feel at best, maybe a couple HP.



I'm willing to learn... .



Merrick

 

[tbrudder]

Originally posted by MCummings

I guess you would add a little power by adding a turbo to a diesel, but I'm not sure how much.

I hesitate to chime in here, since I drive a stocker, but I do have some understanding of turbomachinery and thermodynamics, so I'll throw a few cents in.



As I understand it, extra boost from a turbocharger will never provide more power. It may even take a bit away, all things being equal--the compressor is powered by the turbine in the exhaust stream, so by restricting the exhaust with the turbine wheel, power goes down.



Anyway, the reason you need a turbo for more power isn't really for cooling (though that is an important part), but for the air charge. Fuel is what makes more power, and fuel alone.



Think of it this way--when you burn diesel, energy is released, which makes HP. The more fuel, the more energy, the bigger the grin when you leave the Ford far behind. And what does fuel need to burn? Yup--oxygen. If there's not enough oxygen, all that extra fuel your big injectors are squirting in goes to waste, since the oxygen runs out before the fuel does Cram more oxygen in with a compressor, you can fully utilize all the energy in a bigger squirt of fuel.



Well, maybe not completely to waste--after all, lots of fuel + no boost = lots of smoke...



This is my understanding, anyway, but I've got a bit to learn yet in the world of diesel performance.



--Ty

 

[MCummings]

Originally posted by tbrudder

Anyway, the reason you need a turbo for more power isn't really for cooling (though that is an important part), but for the air charge. Fuel is what makes more power, and fuel alone.

When you add oxygen (Air), and don't add any fuel, the result is cooler Exhaust temps because of the leaner burn.



Great post Ty.



Merrick

 

[tbrudder]

Originally posted by MCummings

When you add oxygen (Air), and don't add any fuel, the result is cooler Exhaust temps because of the leaner burn.

Kinda hard to get used to that--like Ira, I'm more used to gas engines, which burn hotter when they're lean. Piston aircraft engines, for example (at least the 4-cyl-opposed Lycomings I'm used to) have a feature in the carb called the Economizer. I think that's the name--been a while since I was in a Cessna. Name doesn't make much sense, since the thing basically squirts more fuel in, enriching the mixture, when the throttle is firewalled. Keeps the engine cool on takeoff, etc.



Hard to keep the theories straight!

Great post Ty.

Thanks!



--Ty

 

[E7mack96]

Just a thought,



What if I injected liquid oxygen from - say - a 120 CF Oxy tank off a torch set just before the turbo. there would be plenty of oxy for those showerheads.



What do you think.



p. s. I've done this to a gas motor before. Just not on a diesel- YET!

 

[rbattelle]

Originally posted by E7mack96

Just a thought,



What if I injected liquid oxygen from - say - a 120 CF Oxy tank off a torch set just before the turbo. there would be plenty of oxy for those showerheads.



What do you think.



p. s. I've done this to a gas motor before. Just not on a diesel- YET!

Wouldn't the combustion temps go through the roof?

 

[tbrudder]

Honestly, I'm not sure what that would do. I'd imagine the result would be you could run a lot of fuel in there, and your NOx emissions would bottom out (since you're feeding pure O2 instead of the 70+% nitrogen, ~20% O2 atmospheric air.



Only problem I can think of would be the cold. Actually, I'm pretty sure an oxy-acetylene torch uses compressed gaseous oxygen, not LOX. LOX is very, very cold. A quick reference says 90 Kelvin--that's about negative 300 degrees Fahrenheit.



Anyway, even with gaseous O2, when it expands coming out of the tank, it's going to cool off a lot. Not sure how much, but our engines seem to be pretty cold-blooded already--may cause trouble...



Interesting experiment, though.



--Ty