Air/Fuel ratio

[Pthomas]

Im pretty new to the diesel world but have some expierence with the GAS burners and at times i use oxygen sensors to (in a way) calculate the fuel/air ratio. the sensor is not much more than a millivolt producer that changes with heat but it does give a good starting point to diagnose and tune. 200mv lean/999mv rich .

My question is : Do any diesels have this and is it possible with such high EGT's. I think (may be wrong ) but some newer diesels have O2 sensors.

 

[Joe G.]

The concept of fuel/air mixtures like in a gasser does not apply. With a gasser the mixture is always something like 1 to 14 or so. Not so in a diesel. You always get a full charge of air. At idle you have a fuel/air mixture over 1 to 100. It will never get as rich as a gasser, but some of these guys are trying. When you get more fuel than you can burn efficiently you get black smoke. In that case you turn up the turbo or cool the air or both.

 

[SS]

I don't think an air-fuel ratio would tell you much on a diesel (I'm not sure if anything exists to measure it). Generally speaking, a diesel takes in the same amount of air for a given RPM and boost pressure. The thing that varies is the amount of fuel. It is ignited not by a spark plug, but by the compression of the air in the cylinders (known as compression ignition). Thus, the amount of air must be relatively constant to generate the pressure in the cylinders. Added boost means more air (oxygen) which means more fuel can be added and still burn. Too much fuel means lots of black smoke, but the engine still runs and creates power (some may say LOTS of power. There is no "throttle plate" like a gasser has in a carb or throttle body. The air-fuel ratio can not vary as much with gasoline as with diesel.



I may have taken a few liberties with this explanation - consider it the readers digest version. If you know this already, I'm sorry. I suppose air-fuel ratio may be useful for idle problem diagnosis or some other specific engine condition?

 

[Pthomas]

thanks

After i posted this i sat back and thought about it. The Black smoke! theres the gauge! hehe.

And thank you much for the info! Ive gained alot of respect for the diesel and the ones who know how this compression ignition power plant works.

now i see why there is twin and even tri turbo setups to get more air in those cylinders.

now im thinking of a scavenge type exhaust