Historically all diesels have run on excess air meaning that they can have as much air as they want(not metered)
The engine speed/load has always been controlled by the amount of fuel that we feed in to the engine.
This means that the AFR is always changing, depending on the engine speed (Air drawn in) and engine load (fueling demand)
However there comes a speed/engine load point when it is possible that there may be too much fuel for the available air and this would cause black smoke (soot)
As you know the accelerator pedal on modern vehicles is not connected directly to the engine (fly by wire), so when you push the loud pedal into the shag pile the ECU sees the driver demand but it decides how much fuel to feed into the engine.
One of the biggest inputs the ECU looks at is the AMM reading to make sure that the vehicle will not produce any soot.
I Hope that Helps.
Regards mark
PS> it can also be used for EGR feedback but that is another story.
Modern cars, Its all wizardry and witchcraft
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