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In North America today, diesel engines are beginning to make a comeback after years of bad reputation for being dirty and polluting. While 20 years ago, this was largely true, today's diesel is much cleaner - in some cases, cleaner than the gasoline equivalent - in burn and diesel is now the most popular motor fuel in Europe.

If knowing what the costs of ownership and potential savings are what you want, then see our Diesel Engines vs Gasoline Engines page for more information. In this article, we're going to compare the fuels themselves.

Fuel Costs

We touched on this in that previous article, but will get a little more detailed here. In general, at the pump, diesel costs slightly more than gasoline. This has two reasons: the price of oil and taxes.

Diesel requires more oil per gallon to refine than does gasoline, but gasoline requires more energy to refine than diesel. Since the energy in the refining process is not always oil (most of it is natural gas, which is given off as part of the process), the price difference between diesel and gasoline is largely due to the oil:gallon ratio.

Taxes are another matter. The U.S. government and many states tax diesel at a somewhat higher rate than they do gasoline. Usually because diesel is most often used in heavy highway vehicles that cause more wear-and-tear on roads than do cars.

Combined, the two figures mean that diesel is often several cents more per gallon than gasoline is.

Energy Densities

On the flip side, diesel is much more energy dense than gasoline is. Using British Thermal Units (BTUs), this is easy to see:
Gasoline: 125,000 BTU
Diesel: 147,000 BTU
(per gallon)

That's about an 8.5% difference in energy potential between the two fuels. Add to that the way that diesel is burned in the combustion chamber and it's clear that diesel is the more efficient petroleum fuel.

Biofuels

Finally, we come to biologically-based fuels. These renewables are proliferating around the continent as more and more consumers demand "green" alternatives.

In North America, ethanol (the gasoline equivalent) is made from corn or imported. Most imports are sugar cane based (Brazil being the largest country of origin).

Biodiesel, on the other hand, is usually domestic and is usually from soybean oil.

Of the two, the debate over efficiency is ongoing, but most generally agree that biodiesel is the more efficient of the two fuel choices for several reasons. One is that biodiesel requires less processing and refinement versus ethanol. Another is that biodiesel has the same energy properties (within a small percentage) of its petroleum kin whereas ethanol is less energy dense (by about 14%) than is gasoline.

Both, however, are sourced from food crops - a point of contention for many.

Biodiesel, it should be noted, can also be sourced from non-food or even recycled food leftovers whereas ethanol cannot (thought butanol, a more direct gasoline equivalent, can).