This is a very important, oft-overlooked point. You can't have your cake and eat it, too. Reducing NOx emissions basically means reducing the engine's efficien

This is a very important, oft-overlooked point. You can't have your cake and eat it, too. Reducing NOx emissions basically means reducing the engine's efficiency, increasing consumption and greenhouse gas emissions. To reduce NOx, you'll need to reduce the temperature in the combustion chamber and reduce the excess oxygen levels in the exhaust (Oxygen in the exhaust being a requirement for generating NOx). One common, cheap way of doing this, is to use EGR (exhaust gas recirculation, feeding some of the exhaust gas back to the inlet). This cools the combustion chamber AND ensures less oxygen is available for NOx generation. A more complicated way is to add urea to the exhaust before feeding the mix through a catalytic converter; the hot exhaust converts urea to ammonia, and ammonia reacts with NOx to create nitrogen and water. This latter approach, if I've understood it correctly, is what VW tried to avoid by cheating - presumably to not have to pass on the added cost and complexity to their customers, making their offerings less competitive.