Air:fuel ratio

So... doing a little math trying to check some things on my turbo sizing (which by the way I believe the compressor is dead on) and an interesting question pops up. I’ve seen a lot of info on air:fuel ratio. Basically anything from 18-200:1 are considered acceptable. It seems like anything less than 18 is excessively Smokey. Most factory tunes are 20-22:1 to keep down smoke and EGTs. So my question to the collective is what Air:Fuel ratio does the IDI like to keep EGTs down. I know it’s a little bit of a loaded question because the amount of fuel matters but I’m curious if it’s been discussed here.

Is that really a meaningful question for a diesel? As you already noted, over-rich is smoky (and hot). But don't you want to run a diesel as lean as possible (i.e. the most air) to ensure all the fuel is combusted?

Our engines are variable ratio. The IP is always trying to "lean out" the mixture, reducing fueling to just enough to maintain the rpm. The throttle lever is pushing back against the flyweights, increasing the chosen RPM... more or less.
This means that the engine is always at the "lean point" compared to the commanded rpm/power level; you get smoke when commanding a higher rpm/power level than the engine has air to burn, AND the IP's fuel limit is higher than that point as well.

This is why changing the fuel screw doesn't affect mileage or drivability... it /just/ increases the maximum fuel that the IP can provide... when you tell it to give it that much with your right foot.

Ehem, well I actually have an afr gauge in my truck, and did a thread awhile ago about my findings don’t remember how long ago that was. I offered to upload some data logs with my old stock pump vs the moose pump but people didn’t seem really interested in my nerdyness

So I don’t get any visible smoke until 15-14:1depending on the light conditions, and the smoke isn’t thick and rolling until 9:1 if you take it that far. I’ve set my truck up so I get about 13.5:1 at wot starting off idle.

Interestingly, the stoichiometric ratio (ideal) for diesels is 14.5:1 and I have seen smoke before that mixture (leaner or less fuel) so black smoke is not necessarily unburned fuel, a light haze means you are actually burning at perfect efficiency

As far as your question, the lower the better, low 20s equals 2-300*

At WOT I get up to 1000* if I stay in it for awhile with a load and that’s at 13.5:1

As far as a goal for what afr to aim for it’s always best to aim for the stoichiometric mixture, when I built a turbo mustang for a friend it was tuned for 6:1 afr because it was on full alcohol and that is the stoichiometric mixture for that fuel. Your engine will be happiest at a stoichiometric mixture. If you want to keep egts down go for the leanest mixture as you can, but after 17:1 it seems to be dimishing returns.

Thewespaul said:

Interestingly, the stoichiometric ratio (ideal) for diesels is 14.5:1 and I have seen smoke before that mixture (leaner or less fuel) so black smoke is not necessarily unburned fuel, a light haze means you are actually burning at perfect efficiency

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But wait... that's under ideal conditions, with perfect mixing. A diesel(at least ours) is a striated charge compression ignition engine. Which means that you are shoving a bunch of fuel in in one spot(so that spot is extremely rich), vs the rest of the cylinder. Depending on how well that diesel gets sprayed/distributed, you will get rich and lean spots.

IDIs are... OK at distributing the diesel; poor because of the injector, but good in that the fuel gets sprayed against a hot precup and evaporated(at least with a warm engine) and then swirled into the rest of the cylinder.

This is also why more modern common rail engines are more efficient - finer diesel droplets = more consistent burning and less rich/lean spots.

More than likely, if you run a more 'dirty' injector like a Stage 1, you would have to run a leaner ratio before smoking. A 'cleaner' injector like a stock one with a higher than stock pop pressure would let you run 'richer' before smoking, due to better dispersion.

Personally, I don't really care - run it a bit lean and just shove in enough air that you can run it "lean" and still make a ton of power.

This is true, it could be that one part of the cylinder is 17:1 and another is 10:1, it never is a perfect mixture even on gasoline engines, however you still shoot for the stoichiometric rating for your fuel since that is were you have peak efficiency. In tuning that is your reference that is used to figure out your fuel maps etc. Being that this is being applied to diesels you have to take a few things into account into your tuning, such as your “boost up” fuel or pre boost fuel. Although you want to shoot for the stoichiometric mixture, with a diesel you get a lot better response by having a much richer or Smokey mixture before you hit your target boost because the extra thermal energy helps to move the turbine up to it’s target speed.

I REALLY like this discussion...
Tuning is fun.

@Thewespaul I wish i would have seen that thread. I would love to even see some youtube videos on what afr looks like under various driving conditions! But datalogging would be awesome!

If y’all would like to see it I’ll make it happen

Thewespaul said:

If y’all would like to see it I’ll make it happen

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Honestly I think we could benefit from this info. I would love to see the difference cause I would like more understanding on this topic.

DrCharles said:

Is that really a meaningful question for a diesel? As you already noted, over-rich is smoky (and hot). But don't you want to run a diesel as lean as possible (i.e. the most air) to ensure all the fuel is combusted?

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You can't really lean out a diesel too much. Idle will just drop to the point the engine dies.

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Thewespaul said:

This is true, it could be that one part of the cylinder is 17:1 and another is 10:1, it never is a perfect mixture even on gasoline engines, however you still shoot for the stoichiometric rating for your fuel since that is were you have peak efficiency. In tuning that is your reference that is used to figure out your fuel maps etc. Being that this is being applied to diesels you have to take a few things into account into your tuning, such as your “boost up” fuel or pre boost fuel. Although you want to shoot for the stoichiometric mixture, with a diesel you get a lot better response by having a much richer or Smokey mixture before you hit your target boost because the extra thermal energy helps to move the turbine up to it’s target speed.

I REALLY like this discussion...
Tuning is fun.

Click to expand...

Sure wish I lived close enough to yall and have you tune my truck!!! A screamin rust bucket Prius eater would be awesome!!!

Thanks for the info Wes.

Okay so a little more info. I’m working my way through a model on my turbo with my pump basically so I can use the model for other setups. I only have data for mine right now. With the math I have done, if I use an AFR of 18 as is usually suggested I come up with a single digit boost number to digest that. From my experience, single digit boost numbers may bring the smoke to a haze but it will get hot real quick and in a hurry. To fully digest my pump and not get excessively hot I need to see almost 20 psi at the manifold (which is where my gauge is in the intake hat). If I run the AFR up to 27-30 my boost numbers come to something believable and pretty closely match what I have observed. So my question is at what AFR is the IDI happy and running reasonable EGTs.

Disclaimer: I know there are a lot of things I’m not accounting for yet such as air temperature and IC efficiency etc. I’m working on it. [emoji6]

Unlike a gas engine, the more fuel = more power for the most part. Less fuel = less power. You can go to the extremes of smoke (over fueling)

laserjock said:

Thanks for the info Wes.

Okay so a little more info. I’m working my way through a model on my turbo with my pump basically so I can use the model for other setups. I only have data for mine right now. With the math I have done, if I use an AFR of 18 as is usually suggested I come up with a single digit boost number to digest that. From my experience, single digit boost numbers may bring the smoke to a haze but it will get hot real quick and in a hurry. To fully digest my pump and not get excessively hot I need to see almost 20 psi at the manifold (which is where my gauge is in the intake hat). If I run the AFR up to 27-30 my boost numbers come to something believable and pretty closely match what I have observed. So my question is at what AFR is the IDI happy and running reasonable EGTs.

Disclaimer: I know there are a lot of things I’m not accounting for yet such as air temperature and IC efficiency etc. I’m working on it. [emoji6]

Click to expand...

Ideally I would shoot for from 15-16:1 as far as efficiency and making the most power, but with your setup you probably want some wiggle room as far as egts go that way it is a stress free drive when hauling the camper. That in mind I think 18:1 is too lean and you are leaving some power on the table, I would shoot for 17:1 with your setup. That’s around the mixture I’m going with on the ambulance, and it’s worked well to be a good setup that doesn’t require you to be glued to the gauges when towing, and don’t even need them when empty. Where are you getting your calculations from? Any particular online calculator or did you pick up some combustion theory literature?