Air Fuel Ratios/mixture

[99350]

This is a question that may be kinda dumb. But when tunning how do you know what the air fuel ratios are when tunningg. We use a wideband for tunning gas motors. Do you just base it off of EGT's or what. Will a wideband O2 be able to work at all with a diesel, i know some of the gasses output by a diesel are different. This is just new to me and I would like to have some ideas before I do anything. Another thing why is it that some tuners like tto have diesels bellowing out a ton of smoke where as others can make similar power levels without as much smoke, is there a benifit to a whole lotta fuel.

I will be purchasing the tuner soon and I have access to a dyno and "open roads" to du some live tuning/testing.

[Power Hungry]

Here's the short version...

Diesels to not care about AFR. If you spray it, it'll burn it. If it runs out of air, what's left comes out as black smoke. No huge deal, but black smoke is simply a waste of good fuel.

EGTs are an indication of several things, but high EGTs can almost always be found on trucks that bellow black smoke because the fuel is still burning outside of the combustion chamber. To make power you need more fuel, but then you need more air to allow the fuel to burn completely in the cylinder. Turbos use heat energy to spin so when you improve the combustion process, the residual heat energy falls off and the turbo slows down causing the fuel to not burn completely and raising the EGTs again which spins the turbo up which reduces EGTs... and so on and so on.

The idea is to keep just enough fuel in the system to keep the turbo operating in its peak pressure map without introducing too much fuel and generating excessive EGTs. It's definitely a balancing act.

Hope this helps.

[cleatus12r]

Just a little bit of extra information.......

If you do weld a bung in your exhaust pipe and put an oxygen sensor in it, you just wasted the oxygen sensor.

If you've ever tried making an oxygen sensor live in the exhaust pipe of a two-cycle engine, you'll be hard pressed to get one to live in a diesel. Cut the already short life expectancy of a wideband O2 down some more.

Diesel engines run on widely varying air/fuel ratios. That's the nature of the beast. There is already 100% of the available air inside the combustion chamber when the piston hits TDC compression but the amount of fuel -ONLY the amount of fuel and when it's injected- decides how much force is transferred to the crankshaft.

[theSLEEPER]

Just wondering, but it seems to me that my truck spools fastest, sounds best, and runs all out when there is just a slight haze... maybe a little more. So does that mean that a diesel runs best with a little richness? Or does it mean that a little added fuel burning in the up-pipes helps spooling?

[cleatus12r]

Yep.

Just a little extra fuel is all it takes to make more power.

[Jackpine]

Great thread, you guys! You just added immensely to my meager store of diesel facts.

- Jack

[99350]

thats what I was looking for guys.

Nope I have not welded in a bung for a wideband I was just wondering what you look for in getting the most optimum A/F ratio.

The only expierence I have with diesels is repairing the broken ones, and that is getting too easy with all the common failures.

My buddies keep telling me to leave the truck well enough alone but I cann't do that I haven't since I bought my 1st car and now I got my 98 accord getting close to 40 mpg's with some tunning.

Next question would it be a waste of $$$$ to have an EGT gauge for each bank?

[cleatus12r]

Basically. I mean, if you WANT to then it's not going to hurt anything. The only time you would really benefit to having one in each back would be to diagnose a driveability or mechanical concern causing a misfire.