leathermang,

I think that the part of this that I am not explaining properly is the stoichiometry. In a combustion process there is always a particular optimum relationship of fuel and oxygen that provides the highest percentage of usable pressure vs. wasted energy. This is the stoichiometric ratio. At this particular ratio you are wasting the least amount of heat, which must be dissipated via the circulating oil and the circulating coolant. On a diesel if the amount of air remains relatively the same, the waste heat is not on the excessive side until you introduce too much fuel. So basically, to use a gas engine term, lean condition in the diesel does not produce excessive waste heat.

If an acceptable ratio is not reached, you will be making less cylinder pressure to be converted into torque and at the same time more heat that must be taken away by the cooling system.

So, if you are introducing more air, the heat generated by the compression of the air is neglibible as compared to the heat generated by combustion. So as the air flow increases, you can then introduce more fuel to the combustion process and if you do not go on the "rich" side of stoichiometric in a diesel, the combustion will develop more cylinder pressure with minimal waste energy(heat.)

In a gasoline engine, as the fuel/oxygen ratio gets too lean, the combustion process makes less pressure and more waste heat. This is why a really lean condition burns holes in pistons, burns valves, etc. Again in a gas engine, excessive fuel does not create excess waste heat, it's certainly not a good thing because it wastes fuel and washes oil off of the cylinder walls. In addition in a catalytic converter engine, it plugs up the converter.

In a diesel engine things are different, all of the reasons why are not clear to me, but as there is MORE fuel added with the amount of air remaining the same, the cylinder pressure indeed increases, but even with the "lean" condition there is not excessive waste energy. Once you reach a point of too much fuel and not enough air, this condition creates more combustion pressure but creates an inordinant amount of waste heat.

The diesel engine is so different to a gas engine in so many ways, it confuses many people. In the agricultural community where I live, the diesel 1 ton pickups and flat beds are becoming very common place. I get a giggle out of the number that I see with dual exhaust systems modified onto them. Outside of reducing pumping losses, these cowboys are seeing no power or fuel mileage gain whatever unless they are turning up fuel delivery in some way.

Please excuse my rambling,