Tuesday, July 17, 2012

A Brief Comment about the Relative Efficiency of Diesel Vehicles

It is true that diesel-powered vehicles get more miles per gallon than comparable gasoline-powered vehicles.  But is this the correct way to compare the two fuels?

By way of background, I have been thinking about this question both because I own a diesel car and because of a fact I ran across long ago at work.

In the late 1970s, I worked in the Treasury's Office of Balance of Payments.  One of my assignments was to maintain a model used to predict U.S. oil imports.  Because the model used data involving refined petroleum products, I became aware of a phenomenon called "refinery gain."  Refinery gain refers to the fact that more barrels of refined product leave a refinery than barrels of crude come into the refinery.  The reason for this is that refined products are less dense than oil and therefore take up more volume for a given weight.

A gallon of diesel is around 17 to 19 percent heavier than a gallon of gasoline and there is more energy to be extracted from a gallon of diesel than a gallon of gasoline.  If we were comparing miles per pound rather than miles per gallon, the advantage of diesel fuel over gasoline in terms of mileage would be much less.  While it is not practical to sell fuel at the retail level by weight, thinking about mileage in this way would make for a different analysis.

At the retail level, those who say that, in the U.S., diesel is more expensive than gasoline are thinking in terms of volume not weight.  Moreover, at the moment, the price of diesel in terms of gallons is about the same as midgrade gasoline at the gas stations I frequent in the Washington, D.C. area.  Diesel is obviously cheaper by weight here. For those wanting to save on fuel costs, buying a diesel car is worth considering, though cars with diesel engines are more expensive than those with gasoline engines. (In my case, this price differential was at least partly offset by a tax credit that was available to the purchasers of particular diesel and hybrid engine models, which is no longer available for the model of car I bought.)

Also, it is interesting that in Europe, volumes of crude oil are usually measured in metric tonnes, while in the U.S. we measure crude oil in terms of barrels (42 gallons), though it has been a long time since oil was transported in 42 gallon barrels.  Thinking about oil in terms of weight rather than volume would seem to be more appropriate.

As far as how green diesel cars are compared to conventional gasoline cars, I have not come across any good analyses.  Part of the problem is that the composition of the emissions from the two fuels is different.  There are also differences in the refining process, if one wants to take a broader view of how green each fuel is.

In Europe, about half of the cars are diesel, while in the U.S. it is a very small percentage. European car purchasers are responding to economic incentives partly due to government tax policies. Whether diesel should be more encouraged in the U.S. is an issue worth studying. The analysis required is more complicated than it might first appear.

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