teej9685

I am new to the diesel world and have a question. If these trucks do not have an oxygen sensor, how does the computer know how much air is in the cylinder to deliver the correct amount of fuel? Is it based solely on the boost pressure?

BigBlue

Solely off of boost pressure.

wannadiesel

It's not based solely on boost pressure, it's more complex than that.

teej, the RPM of a diesel engine is controlled by how much fuel is injected, there is no set a/f ratio that the engine management system is trying to attain. Idle to accelerator flat on the floor, the a/f ratio changes all along the way. The more power the driver asks for, the more fuel is injected into the cylinder.

annabelle

Not really as relavent with diesel as compared to gas. You can run much more air in a diesel and not worry about running lean.

74dart

I was told if you add fuel the turbo will proudec more air till it makes hot air.

jrs_dodge_diesel

Somewhat correct. Any turbo has a designed range of operation (RPMs, pressure, etc), usually shown on a graph called a compressor map. When you exceed the designed parameters, you do two things, 1 - overspeed the turbo, which can cause the turbines to fragment into a bunch of pieces due the extreme forces placed on it. 2- you exceed its efficient range of operation. Meaning you are making the turbo work too hard to create boost pressure and creating very hot air at the same time since you have exceeded its operation design. Remember any time you compress air (or anything really) it heats up.

Normally this won't ever be an issue on an unmodified engine, as the turbo is within its designed limits. Only when you mod the engine for more power does this become an issue, and larger turbos/twin turbos become necessary.

As for the original question about A/F ratios. A/F ratios don't really matter in a diesel engine, unlike a gas engine which requires a stoichiometric (stoic) ratio of 14.7:1.

mhuppertz

Seems like black magic to me too. I hear diesels can run at 100:1 in a cruise condition. I still don't understand how a diesel can run so lean without melting the pistons..... I stull haven't heard an explaination of how a diesel can run a wide open air intake and control with fuel only without melting down. In a gasser, combustion temps go down when the mixture get's "richer", and just the opposite in a diesel.
Makes me wonder how they plan to "direct inject" gassers....

bdewey

Have been "direct injecting" gassers for some time now. First one I have heard of was the Isuzu Axiom used the direct injection technology. There maybe been one before that, but first one I have heard of.

PoorMansCat

On a gasser, when you run rich, the exhaust temperatures go UP... due to excessive fuel being loaded into the cylinder.
Wannadiesel explains it the simplest, on a modern ECM controlled engine, the ECM is basically the speed/load governor, if your throttle position sensor is is sending a signal for a desired speed of 700RPM and the engine isn't feeling any load (no boost) then it monitors the speed and tries to inject an equal amount of fuel into each cylinder to achieve and maintain 700RPM, which is very little! If the load goes up, the ECM is still trying to maintain 700RPM, so it will inject more fuel to try and maintain the desired speed, when the extra fuel goes in, a turbocharged engine will also start making boost, when the boost comes up, if you are monitoring the right parameters, you can actually see the fuel cut back slightly since the boost has come in to help maintain RPM.
There are a vast number of parameters that contribute to fuel injected. Engine speed, load, boost pressure, inlet air temperature, coolant temperature...etc... Some Cat engines DO have an FRC fuel limit and it is basically a constantly varying parameter that helps with troubleshooting.
If you know a Cat field technician, get him to show you a status screen snapshot of an engine running under load, you'd be surprised what all plays a part in a modern diesel engine!

mhuppertz

Maintaining a stoicimetric ratio of between 12:1 and 14:1 is the goal of the ECU or a carburetor. The O2 sensor in the exhaust monitors that. The rest of the computer controlls are for fine tuning this process as well as the all important emissions control.
I know (exactly) how a gasser works, I just can't relate it to a diesel. I think gasser is to diesel what physics is to quantum mechanics, largely irreconcilable, at least in my mind.....

PChouinard

Just going out on a limb here. Gassers rely on IGNITION and a relatively slow and steady burn of the fuel thru the power stroke,thus requireing the right f/a mixture(stoich). Meanwhile, the diesel works by DETONATING the actual amount of fuel delivered due to the compression (that makes the heat) of who cares how much air..
The increased pressure delivered to the cyl. plus the thermal expansion= more pressure delivered to the piston. Or something like that.. Bottom line is diesels produce tremendous heat before ignition so lean / rich mixtures are not a factor in temps.. It's the evaporation in a gasser that causes a cooling effect. Diesels atomize... I should have at least one correct point here..

mhuppertz

Instinct is telling me that the fuel is the key, but I still can't grab it.
Weather spark or detonation, it's all ignition. I don't think that's the answer. My 'old '69 Plymouth Fury could run all day as a diesel if I had the the idle set too high. Because of the high compression ratio I could shut off the spark ignition and it would "diesel" for quite a while if I let it. It's not the ignition method per say. Before I understood how the VE was lubricated I ran a 5 gallons of gasoline in my truck ( I ran out fairly far from a diesel station and my wife grought me the wrong fuel can) and I found that my engine ran very smoothly at idle and just above, but crappy at higher advance. I only ran a half a gallon straight gasoline before I made it to a diesel pump, but the damage was done. My VE later seized from running with no lubrication, and I learned an expensive lesson.

Point is I think the truck would have run on gasoline if I could keep the pump timing retarded (and I had a pump that had it's own lubrication). Yes, I know it wouldn't last long, not the point.

I suspect that if you setup a gasser at about 16:1, port injected diesel and lit with a spark that you could (in theory) get it to work. The timing would have to be set ar close to TDC because of the fast burn rate of diesel, and I think it would be a nightmare to keep the A/F ratio adjusted correctly to keep the engine from melting down.

It all points back to the fuel.

I have puzzled and puzzled, and am no closer to an answer..

It's got to be the properties of the fuel.......

wannadiesel

You can run fuel oil/kerosene in a spark ignition engine, you just can't run much compression and it's hard to get it started without some heat to the fuel.

The tight mixture control is needed because of the spark ignition. You have to have an easily burnable mixture because you only get one shot at lighting it. With a diesel you are spraying fuel into an ignition source - air that is hotter than the flash point of diesel fuel. It's gonna light regardless of the mixture.

The reason a lean mixture holes pistons on a gasser is detonation, not a lean mixture causing too much heat.

PoorMansCat

MAN!!!....You guys are using way too many big words for me to comprehend!... This is making my head hurt... You guys sound like you've graduated from a mechanical engineering school... I'm gonna drink a beer (or two) so my head will quit spinning...jk

Raspy

First you have to stop comparing it to a gasser to understand it.

A diesel intakes only air. No fuel. Then an injection event happens with the piston near top dead center that starts the burn. At the injection nozzle there is pure fuel, way too rich. A small distance out the mixture is about right and starts to burn. Still further out it is all air and way to lean to burn. The mixture is only relevant with respect to burning, but not to the overall ratio of fuel to air. Talking about rich and lean is pretty much meaningless. Diesels always have an excess of air unless they are at full power and are smoking. The burn travels out into the combustion chamber until the fuel is used up. Then the exhaust stroke starts.

Imagine a bucket of fuel sitting out in the yard and you light it on fire. It is way lean compared to all the air in the atmosphere, but it burns just fine. Now look closely at what is happening. The puddle is not on fire but vapors just above the puddle are. Out a foot or so there are very active flames. Then out 10 feet or so there are no flames. All in an atmosphere that is way too lean.

Wetspirit