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> If you drive the engine with parameters tuned for low emissions, you'll get less efficiency etc.

Can you explain that further? Given a specific power output, lower emission levels would to me indicate that the engine is doing more with a smaller amount of fuel, indicating better efficiency.

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Your argument mainly applies to CO2. Generally, Diesel engine reach better efficiency at higher temperatures, but higher temperatures also lead to more NOX being created. So in the case of NOX, it really is a trade-off.

Unless you use ammonia (from heated urea solution) in a catalyzed reaction, to react with the NOx to produce N2 and H2O. In that case, the trade-off is that you need to add a separate urea tank to your vehicle, and distribution infrastructure at all diesel fueling stations.

NOx is produced by dissociating N2 (primary component of air) which then recombines with O2 (secondary component of air). The engine does do more with a smaller amount of fuel, but a side-effect is to crack N2 and have it recombined with the leftover O2. Furthermore, dissociating N2 occurs at high temperature and pressure (which is why that happens significantly more in diesel engines than in gasoline engines), and the higher a diesel engine's efficiency... the hotter it runs, increasing the dissociation rate of N2 and thus the production of NOx.

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