Lean and Rich on a diesel

virgil

virgil

still spoolin
Joined
Jan 4, 2007
Messages
291
Since I saw this brought up on the melted down common rail thread with only a few answers which really didn't make sense.so heres my logic

gasoline motor: obvious that its combustion is started with a spark plug and if your too rich you will foul plugs. not enough fuel and you will have high egts. what you want to be is stoich.which is why efi and o2 sensors rule in this day of age. also utilize some type of throttle body to control incoming air, carbs cannot adjust for elevation,temp changes etc.

diesel motor: obvious that fuel is ignited by the high air temps when the air is compressed. can handle a substantial amount of fuel before the fire goes completlely out and you stop making power, i.e rolling black smoke,turbo not staying lit.lean and clean with small injectors in my truck and it can't smoke, egts are very low. larger injectors and there is a haze and the egts are fairly high. but since Iam now running richer with more fuel shouldn't it have gotten my truck near Its "stoich"(being 1250*)

Just saying that applying gasoline tuning logic to a diesel just doesn't work.goal on a diesel is to clear up the smoke and keep your egts stabilized full throttle.
 
virgil said:
Since I saw this brought up on the melted down common rail thread with only a few answers which really didn't make sense.so heres my logic

gasoline motor: obvious that its combustion is started with a spark plug and if your too rich you will foul plugs. not enough fuel and you will have high egts. what you want to be is stoich.which is why efi and o2 sensors rule in this day of age. also utilize some type of throttle body to control incoming air, carbs cannot adjust for elevation,temp changes etc.

diesel motor: obvious that fuel is ignited by the high air temps when the air is compressed. can handle a substantial amount of fuel before the fire goes completlely out and you stop making power, i.e rolling black smoke,turbo not staying lit.lean and clean with small injectors in my truck and it can't smoke, egts are very low. larger injectors and there is a haze and the egts are fairly high. but since Iam now running richer with more fuel shouldn't it have gotten my truck near Its "stoich"(being 1250*)

Just saying that applying gasoline tuning logic to a diesel just doesn't work.goal on a diesel is to clear up the smoke and keep your egts stabilized full throttle.
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Obviously Exhaust Gas Temp is not cylinder temp.. Hense why people that use data-loggers have their pyro probes right at the head, to get as accurate of a cylinder temp reading as they can. black smoke indicates fuel that is 'burning' but not 'combusted'.. you will generally have Black smoke with high egt's and Gray smoke with lower egt's.. a diesel obviously has to run rich, because engine speed is controlled with fuel.. The banks drag truck is runnnin somewhere in the region on 22-26:1 i believe..This is very lean. They are also leaning it out with n2o, which is bad in most cases. 18:1 is ideal, but not always achievable.. my head hurts im going to bed
 
"a diesel obviously has to run rich, because engine speed is controlled with fuel"

running rich reffering to wot, high egts due in part of being overfueled, in the sense that this is a street truck I have to eliminate some of the fuel to clear things up and bring my egts to a safe area.If this were a competition truck I would probably be doing the opposite. but since I can't drive around town bellowing smoke I need to tune a bit differently.
 
a diesel obviously has to run rich, because engine speed is controlled with fuel..
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Actually, the Diesel cycle operates with an excess of air, which is different than the Otto Cycle
 
XTrEmE9156 said:
overfueled doesnt mean more power
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no, but in some cases(sled pullling) a little extra fuel can be used to control temps when the motor is loaded.a diesel running near 14.6:1 is optimum for efficiency...but obviously isn't always easy to achieve at certain power levels.
 
virgil said:
no, but in some cases(sled pullling) a little extra fuel can be used to control temps when the motor is loaded.a diesel running near 14.6:1 is optimum for efficiency...but obviously isn't always easy to achieve at certain power levels.
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14.6:1 is gas ratio... If adding more fuel cools egt's that means that all that extra fuel in there that isn't burning is taking up room that could be occupied by air.. Like i said EGT is not cylinder temp...
 
Gasoline 14.7 : 1 — 6.8%
Natural gas 17.2 : 1 9.7 : 1 7.9%
Propane (LP) 15.5 : 1 23.9 : 1 6.45%
Ethanol 9 : 1 — 11.1%
Methanol 6.4 : 1 — 15.6%
Hydrogen 34 : 1 2.39 : 1 2.9%
Diesel 14.6 : 1 — 6.8%
 
What somebody needs to do is install probes in the cylinders to measure temp. I have my probe in the back half of the outlet on the manifold. Now I'm not saying you should try this but when I was running full cuts with stock injectors I would see egts in the 2100 range. To date I have not had any problems with it knock on wood. But what I want to know is if you ran a fuel to air ratio like that in a common rail how much would it smoke and what would your egts look like?
 
In a diesel engine, the combustion chamber is always full of pure air before the fuel is injected. Because the amount (weight) of air in the combustion chamber is relatively constant (I'm not considering boost here...) and the amount of fuel is variable, diesel engines run at varying air/fuel ratios. At idle, with no load, it is not uncommon to have a diesel engine running at an air/fuel ratio of 60 or 100:1. Under full power, most diesel engines need to run lean of stoichiometric. (BTW: the stoichiometric ratio for diesel fuel is NOT 14.7:1 and it varies slightly depending upon the composition of the fuel.)

For a number of reasons, most diesel engines will emit visible hydrocarbons (i.e.: smoke) if run near or over their stoichiometric fuel ratio. Diesel engines thus always need to be run lean of stoichiometric for emissions reasons.

there is no way diesel can have the same optimum a/f ratio as gasolie...

BTU of one gallon of Gas 147,000 BTU
BTU of one gallon of Diesel Fuel 125,000 BTU
 
A diesel will make the most torque at stoich, and that is really usually a pretty smoky burn. If I remember right the highest cylinder temp as well.
Keep pouring the fuel too it, and cylinder temp comes down from excess unburned fuel removing heat with it.
Call it "Rich" "Overfueled" less than stoich, I don't care, its still something that needs to be addressed.
No one but the half wits are comparing it to a gas engine.
 
XTrEmE9156 said:
In a diesel engine, the combustion chamber is always full of pure air before the fuel is injected. Because the amount (weight) of air in the combustion chamber is relatively constant (I'm not considering boost here...) and the amount of fuel is variable, diesel engines run at varying air/fuel ratios. At idle, with no load, it is not uncommon to have a diesel engine running at an air/fuel ratio of 60 or 100:1. Under full power, most diesel engines need to run lean of stoichiometric. (BTW: the stoichiometric ratio for diesel fuel is NOT 14.7:1 and it varies slightly depending upon the composition of the fuel.)

For a number of reasons, most diesel engines will emit visible hydrocarbons (i.e.: smoke) if run near or over their stoichiometric fuel ratio. Diesel engines thus always need to be run lean of stoichiometric for emissions reasons.

there is no way diesel can have the same optimum a/f ratio as gasolie...

BTU of one gallon of Gas 125,000 BTU
BTU of one gallon of Diesel Fuel 147,000 BTU
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fixed ti for you.

16-18:! is ideal for diesel

these were posted on mcrats site talking about the very thing..air fuel ratio of diesel
Originally Posted by mytmousemalibu
As much as I LOVE coal!, Nathans right! Its sinking our own ship.

You know, the more and more we see fast and faster trucks coming of age, the more prevalent were seein the big numbers put down with low smoke output! A light haze seems to be the sweet spot! I remember reading a statment from Banks a while ago about ideal diesel air/fuel ratio. Diesels start to produce visible smoke @ 18:1 A/F ratio but optimum A/F is 16:1...............There ya go, light haze

Quote:
Originally Posted by JOHNBOY
Well said Nathan. I totally agree with this from a Drag racing stand point and a street stand point. I think that a low to not smoke rule for drag racing is a good thing and can be done. For pulling I am not sure it can be done. With pulling classes setup the way they are. Basicly by turbo size I see no way around it. I know what Gale Banks say about being smoke being wasted power but I call BS. To get rid of the smoke you need and AFR of over 20. That does not make the power that and AFR of 16 does. Yes it smokes but it makes more power. A lot more power. I dont have a problem with my truck smoking the way it does. But it is not a street truck. My street truck is cleaner running than it was stock. FWIW turbo gasser smoke to. My old GN would puff black on WOT shifts with an AFR of low 11s. Most max effort turbo gassers a least haze.
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and soem other info that is relavant to this.

http://www.greenlightmotorsports.com...emart&Itemid=1

Just found this one
http://www.fuelairspark.com/Products...ter/diesel.asp
 
XTrEmE9156 said:
In a diesel engine, the combustion chamber is always full of pure air before the fuel is injected. Because the amount (weight) of air in the combustion chamber is relatively constant (I'm not considering boost here...) and the amount of fuel is variable, diesel engines run at varying air/fuel ratios. At idle, with no load, it is not uncommon to have a diesel engine running at an air/fuel ratio of 60 or 100:1. Under full power, most diesel engines need to run lean of stoichiometric. (BTW: the stoichiometric ratio for diesel fuel is NOT 14.7:1 and it varies slightly depending upon the composition of the fuel.)

For a number of reasons, most diesel engines will emit visible hydrocarbons (i.e.: smoke) if run near or over their stoichiometric fuel ratio. Diesel engines thus always need to be run lean of stoichiometric for emissions reasons.

there is no way diesel can have the same optimum a/f ratio as gasolie...

BTU of one gallon of Gas 147,000 BTU
BTU of one gallon of Diesel Fuel 125,000 BTU
Click to expand...


like you said, these engines operate at a very wide range of air fuel ratios.at wide open throttle 15:1-18:1 would be around stoich for a diesel. some would argue as low as 12:1 but I guess it all depends on exact setup.either way lean and mean on gasoline, lean clean, cool on a diesel.
 
XTrEmE9156 said:
For a number of reasons, most diesel engines will emit visible hydrocarbons (i.e.: smoke) if run near or over their stoichiometric fuel ratio. Diesel engines thus always need to be run lean of stoichiometric for emissions reasons.
Click to expand...

Exactly what i taught in school that a diesel running at peak power will almost always have a haze of smoke from the exhaust. They also run at about 1200* to take advantage of all the fuel in the cylinder.

We never really talked about being stoich in a diesel because of the fact that a diesel uses heat of combustion and with all the varibles of a diesel engine that make it run such as inj. timing, duration and lift on the pintle of the nozzle and amount of fuel being delivered to the nozzle each time it fires and add in the fact that you need the fuel to light the turbo but not have to much drive pressure so the pyro still goes up after the turbo is at max performance its very hard to get the engine to be completely balanced air/fuel wise. I dont worry about air/fuel ratio i use the pyro, boost and exhaust to tell me when its at peak performance. Sorry for the long post just had to get it out there.
 
Gasoline molecules will burn efficiently or not depending on the overall volumetric air-fuel ratio inside the cylinder, because the gasoline is really well dispersed amongst the air (and perhaps because of some other chemistry-reasons too). That's why you get carbon monoxide when the ratio is rich; there's not enough oxygen for each gasoline molecule, so some of the molecules burn "half-way".
Injected Diesel fuel either burns all the way or not at all; that's why you get little CO, but instead get unburned hydrocarbons. And the percentage of injected fuel that does burn is dependent on the injection system's efficiency instead of the overall air-fuel ratio inside the cylinder.

An inefficient injection system (lower pressure, narrow spray pattern, worse atomization) will cause incomplete burning; it will need to inject more fuel to take advantage of a given quantity of air, and the not-burned fuel will act as a coolant and make smoke. An efficient injection system will burn a greater quantity of fuel it injects, more rapidly, and so it tends to burn hotter with less smoke.

Also, the poorly atomized fuel of the inefficient injection system tends to "afterburn" and continue to diffuse and react with available oxygen after the exhuast stroke, which causes higher EGT readings on injection systems that actually produce a cooler burn than more efficient injection systems.

So the actual combustion reaction is never "rich" or "lean", and just delivering an amount of fuel into the chamber of a certain ratio to the amount of air in the chamber may cause a black cloud or a clear cloud depending on how well it's delivered. But incomplete combustion is analogous to "rich" combustion because it causes an inefficient, cool burn, and complete combustion is akin to "lean" combustion in that it causes an efficient, "hot" burn.
 
Even with the most efficient injector, you can only fuel to the point all the oxygen is consumed, fuel after that point, what do we want to call it??

As far as I'm concerned any point I can't see smoke, its lean on fuel. When I'm blasting a 50 foot stack of black......its a little rich.....
 
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