Question for you engine people about Diesels and NOx

[Cnrb]

Hi guys, my first post here and one about a problem that is cropping up more in the news and its got me curious.

Diesel engines are their significantly higher NOx emissions over Petrols.

Am I right in believing that diesels produce more NOx because they run lean and the excess oxygen is why they produce this NOx. Whereas the fuel and air is precisely controlled in petrols.

My question is if this is the case, why cant the amount of air in a diesel be precisely controlled to reduce the NOx?

Thanks

[GavinPearson]

If you throttle the air you reduce the presence of oxygen but with a fuel economy penalty.
Adding EGR reduces available oxygen and therefore lowers NOx.

[Ive]

Petrol engines eliminate NOx with their catalyst requiring a lambda 1 mix.
Diesel needs excess oxygen, otherwise they produce a LOT of soot.