bwoodahl has some good comments on diesel efficiency at the crude oil level. Given those comments, and the US government website calculations, it would appear that in terms of reducing the number of barrels of imported oil, adopting diesel engines will hinder progress, not help. Interesting. I don't have experience calculating the barrel impact, but I have done a lot of certified fuel consumption tests on diesels, in industrial applications, and we always took the calorific value of the fuel (by weight) into account, we never worked in litres and gallons.
XXX555, you provide official BMW figures for CO2, but they include models not available or certified in the US. That suggests to me that those figures are not determined using the US or Canadian emission test cycles, and that the engines tested are not in fact certified for the US. Outside North America, engines are tuned to meet local emissions regulations. That makes those figures irrelevant in North America, unless the 35i and 35d figures are NA test cycle figures, and only the 40d is calculated differently. It is not correct to say that diesels are cleaner, only that these figures show lower CO2 emissions. There is a lot more to it than CO2, and to focus only on that figure is cherry-picking.
SkipSauls, you have touched on a significant issue with diesels vs gasoline engines in terms of environmental impact. There are many ways of calculating environmental impact (really, we should consider manufacturing and disposal impacts, ie full life cycle impacts). Addressing just the carbon footprint issue, it is not correct that the site you link measures carbon footprint based on CO2 emissions, although the short caption does say that. The Info button says the following:
The carbon footprint measures greenhouse gas emissions expressed in CO2 equivalents. The estimates presented here are "full fuel-cycle estimates" and include the three major greenhouse gases emitted by motor vehicles: carbon dioxide, nitrous oxide, and methane. Full fuel-cycle estimates consider all steps in the use of a fuel, from production and refining to distribution and final use. Vehicle manufacture is excluded. (U.S. Department of Energy, GREET Model 1.8, Argonne National Laboratory)
That gets to a real issue, which is that while diesels can do well on CO2 emissions (which is what the EU focuses primarily on) they don't traditionally do as well on NOx emissions (which are responsible for smog, and which is what the US and Canada focus primarily on). Urea injection is designed to lower NOx (which is why North American vehicles have urea injection systems such as Blu Tech, and EU passenger vehicles don't have it yet). The Carbon Footprint score considers the NOx emissions, expressed as CO2 equivalents, as well as calculating them over the full fuel cycle (including refining).
It isn't surprising to me that diesels are not as green by some measures. By many consumer measures, they are wonderful, but the above illustrates a more complete picture when considering their impact.
Now, to extend the discussion, look at the X6 Hybrid on the same site. It is worse than either the 35i or 35d in carbon footprint, even without considering the life cycle impacts like battery production and disposal. Wonder who is going to tell the hybrid fans?
Good discussion, thanks for posting.