NY wrote:

I'm not sure what is is about compression-ignition that produces higher NOx
than spark ignition.

Im pretty sure its just down to cylinder pressure and consequent
combustion temperature.

Nitrogen is pretty inert but the hotter you heat it the more NOx you get,
petrol or diesel. Diesels just have a bit of a head start on the
combustion pressure/temperature.

Maybe its simply that they are very lean burn, whereas
petrol engines are stoichiometric - they have an accurate petrol:air ratio
rather than a great excess of air relative to fuel with diesel.

That may be a factor too but I think it stems from the high cylinder temp
primarily.

Tim

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