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A diesel engine usualy has no throttle.
It always takes in as much air as possible.
Power is adjusted by injecting more or less fuel.
Less fuel = less power
More fuel = more power (up to a point)
Modern diesels use EGR to make the engine run richer without adding more fuel.
This results in lower NOx raw emissions, wich need to get converted to nitrogen and water by the SCR catalythic converter.
The SCR cat needs an urea solution to do that, so by needing to convert less NOx into N2 and H2O, it saves you DEF.
Downside of running close to stochiometric is that the engine produces more soot particles.
These particles get trapped and burned in the particle filter.
To not clog up the filter over time, it needs to burn the particles, wich happens when driving fast on the highway for a while or with a regeneration cycle.
In said cycle the engine injects additional fuel after the main combustion to get the exaust gas temperature up and burn off particles.
To avoid unnessesary regeneration cycles, the ECU avoids running stochiometric or even rich.
Diesels have a point where their BSFC is best, but as they have no throttle, they are more efficient under low load than gasoiline engines.
This results in great fuel economy.
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