[fnj]

> So this is ... diesel without the NOx

NOx is not peculiar to diesel engines. Gasoline engines also generate NOx. All internal engines make NOx. As long as you burn fuel + atmospheric air you get NOx. The atmosphere is 78% nitrogen. Some of it gets oxidized.

[teraflop]

This makes me wonder whether it would be theoretically possible to use some of the engine's power output to extract nitrogen from the incoming air. You'd effectively be trading more CO2 pollution (because of the reduced efficiency) for less NOx pollution.

My back-of-the-envelope calculations suggest that off-the-shelf oxygen concentrators aren't efficient enough to make this practical, but I don't know whether there are fundamental thermodynamic limits that would prevent the efficiency from being improved.

[maxerickson]

A typical NOx reduction is to feed some exhaust gas into the air intake to reduce the amount of oxygen in the chamber.

https://en.wikipedia.org/wiki/Exhaust_gas_recirculation

[coredog64]

The catalytic converter helps to manage this in a gasoline engine. Cats are more difficult for diesel, which led to the ammonia fluids that current diesels use to minimize NOx emissions.

[londons_explore]

I don't understand why not simply cool the exhaust gasses down with a big heat exchanger enough to condense the water vapour. That water would then react with the NOx to form dilute nitric acid, dilute enough to just drip onto the road with little environmental harm.

The same condensation process would also trap all the carbon particulates.

[Govindae]

Yes, but stratified charge combustion produces significantly more NOx in cylinder because of the excess oxygen.

[skgoa]

Which is exactly what most gasoline engines do, too.

[Govindae]

Most gasoline engines are homegenous charge, and run within a few percent of the stoichiometric air fuel ratio. Diesels will run at 6x the stoichiometric ideal amount of air.