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In addition to power and performance, one of the reasons VW cheated their emissions results was to improve the fuel economy of their cars. Why exactly does this happen?

I'm familiar with cars and how cars work so feel free to get technical.

And a follow-up question: Where is the "sweet spot" between burning more gas per mile but having that burned gas release fewer emissions, and burning less gas per mile but releasing more emissions per gallon burned?

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  • $\begingroup$ This IEEE Spectrum article explains this. Essentially, it comes down to needing additional fuel to keep the catalytic converter hot enough so it can do its job properly. $\endgroup$ – user5108_Dan Oct 31 '15 at 14:54
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Not all of the pollutants come directly from the fuel. Some (e.g. NOx) come from the air itself, based on the combustion temperature in the cylinder. Running a cylinder at a higher temperature (by leaning the mixture) results in both greater efficiency and greater NOx emissions.

On the other hand, running the cylinder at a lower temperature reduces NOx emissions, but at some point, CO and HC emissions go up because of incomplete combusion (which is why catalytic converters are used).

The "sweet spot" you ask about in your followup question is very much dependent on the specific design of the engine, and it is, of course, the target of ongoing (and proprietary) research.

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Because most controls reduce efficiency, since they are aimed at protecting the catalyst - not reducing pollution. An air/fuel ratio of 14.7/1 keeps the catalyst happy. Engines can and do run cleaner and better without the catalyst, but the EPA is not only decades behind chemistry - it is not concerned with the environment.

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