Another question- since diesel engines get better mileage than gas engines, shouldn't the numbers for US fleet consumption be adjusted to take into account a change-over to the use of biodiesel?
Thanks to everyone who posts knowledgeably on these topics, I'm getting a great education and appreciate the energy (pun intended) you folks put into collecting info and sharing same.
Diesel cars get better mileage because of two reasons. First the Diesel cycle is more efficient in the thermodyanamic sense because it uses higher pressure. In modern high speed diesel engines the differentiation between the Diesel and Otto cycles is blurred, so this is not as strong an effect as it might be.
Second, diesel fuel contains more energy than the same amount of gasoline. So you need more crude oil to make it, and the Miles Per Gallon (or Liters Per Kilometer) metric is misleading. You need to de-rate diesel fuel economy numbers by about 20% to do a good comparison of the two fuels on an energy basis.
Really what should happen is that government standards, and all discussions, should be held in terms of energy use rather than miles per gallon. Doing so would make it possible to make sensible comparisons between electric cars, gasoline and diesel internal combusion engine cars, steam (external combusion) cars, hydrogen, fuel cell, etc.
