[phkahler]

>> since catalytic converts could also improve fuel efficiency.

For curious, I think this is because fuel economy and complete/proper combustion are directly at odds with each other. Better MPG means nastier exhaust, so a better catalytic converter will help clean that up. OP?

[Cogito]

Sorry, why do you think that?

More complete combustion would result in more CO2, H2O, and energy. The by products from incomplete combustion are the ones we want to limit - CO, C, various N and S compounds etc.

[refurb]

Not always. Nitrogen oxides are due to higher temps which is more efficient. In order to meet emissions standards, you often take a hit in efficiency (or at least drive-ability).

[Cogito]

Makes a lot of sense, thanks.

[selectodude]

The more efficient the combustion, the hotter it is, and the more NOx is formed. Nothing to do with incomplete combustion. Nitrogen doesn't combust.

[Cogito]

Makes sense - it's not incomplete combustion but rather atmospheric contamination that allows for NOx products, and the amount of those is dependent on heat of combustion. Thanks!

[selectodude]

There's a not insignificant amount of nitrogen in the fuel itself, as well.