> I believe, as you said, diesels also have lower CO2 emissions per mile than gasoline engines. There was a massive push following the Kyoto Treaty to reduce C

> I believe, as you said, diesels also have lower CO2 emissions per mile than gasoline engines. There was a massive push following the Kyoto Treaty to reduce CO2 emissions. From 1996, when separate emissions standards for diesel/gasoline engines were introduced in the EU, diesels initially were allowed less than half the emissions of gasoline engines, though since 2005 it's been exactly half. Combined with this, many countries moved vehicle taxation to being based on CO2 emissions of the vehicle, providing a financial incentive for diesel powered cars. As for the cause of the various emissions: NO_x emissions are mostly a function of combustion temperature, AFAIK, whereas PM is indeed caused by incomplete combustion.