Gas v. Diesel and Energy Content

May 21, 2008

Anyone who pays attention to the car world from an efficiency perspective has no doubt heard of the popularity that diesel engines have in Europe, but lack in the United States. Diesels post significantly better distance per volume of fuel (miles per gallon, furlongs per peck) than their gasoline counterparts. I’ve always wondered how much of this disparity can be accounted for by diesel fuel’s higher energy content. A quick search turned up some statistics (See table B-4) produced by Oak Ridge National Laboratory’s Center for Transportation Analysis which lists a gallon of diesel as containing 38.6 MJ per liter and gasoline as posesing 34.8 MJ/L. Not that big of a difference; gasoline has 90% the energy content of diesel.

How is this reflected in the mileage of a real car? Well, let’s look at a Volkswagen Jetta which, until recently in the US, could be had with a 1.8 liter gasoline engine or a 1.9 liter diesel. The gas engine gives 19 MPG city, 27 MPG highway for an EPA combined cycle of 22 MPG (VW Jetta 1.8L gas). The diesel gets 28 MPG city, 39 MPG highway and 32 MPG combined (VW Jetta 1.9L diesel). Working from the combined numbers, the diesel powerplant achieves 45% better mileage with fuel that contains just over 11% more energy content. Not a bad deal at all.

The next question is how this all works out for the environment. Historically, diesels have produced much more environmentally destructive exhaust containing more NOx and particulates. Recently, however, diesel fuel in the United States has been switched to an ultra-low sulfur variety which, in turn, allows for particulate emission control technologies to be employed, reducing the airborne matter released by diesel engines. Do the changes mean significantly cleaner emissions? I’m not sure, because the stats are difficult to find. Perhaps I’ll find them later.