Chances are good that your car has an internal combustion engine that burns gasoline. Less than 5 percent of all U.S. passenger vehicles are powered by diesel.

Long regarded as bulky, noisy, smelly, and smoky except by a few Mercedes Benz enthusiasts, diesel engines were relegated to uses where high torque and low R.P.M trumped these objections, such as heavy trucks, buses, trains, barges, ships, and military vehicles. In all, these uses account for about 25 percent of U.S. transportation fuel.

In Europe, where fuel taxes are dramatically higher than in the U.S., motorists have opted for diesel cars because of the fuel’s higher energy content 18 percent more than gasoline and the engine’s increased compression ratio a factor of 2. Those attributes provide a mileage efficiency increase of 25 to 40 percent over comparable gasoline vehicles. Furthermore, manufacturers have improved diesel engines and refiners have reduced the fuel’s sulfur content so that performance is now smooth, powerful, and environmentally clean. Thanks to lower diesel fuel taxes, diesel cars now comprise about 50 percent of European passenger vehicles, and sales this year outnumber gasoline vehicles.

But before you rush out and buy a diesel car, it may be wise to consider all the relevant facts. The biggest concern is the recent increases in diesel fuel prices. Traditionally, they increase in the winter months due to competition with heating oil by an amount determined by winter’s severity. There is usually a decline in late winter followed by an increase in late spring, as refiners adjust the gasoline/diesel mix for summer. From August through November of last year, the prices were about on par. In February, however, diesel prices jumped to about 15 percent over gasoline and it continues to sell at a premium over gasoline. As of July 10, 2008, the national averages for gasoline and diesel were $4.10 per gallon and $4.70 per gallon, respectively. Note, however, that this price is slightly less than equivalent on a Btu basis. We will return to this subject after further discussion of technical differences.

In the gasoline cycle, a nearstoichiometric mixture of air and vaporized fuel is injected into a cylinder during the first expansion stroke, and compressed by a piston in the second stroke to a compression ratio of 10:1. A high voltage spark ignites the mixture and expands against the piston, driving a crankshaft during the third stroke. Finally, the fourth stroke empties the cylinder and the cycle repeats. The original fuel was ethanol, but, although any combustible gas or vaporized liquid is acceptable, attention soon focused on an inexpensive liquid petroleum fraction containing C6 to C12 hydrocarbons. Engine design and petroleum refining advanced in concert to give us today’s high performing, octanetailored gasoline comprised of isoalkanes, cyclicalkanes, and alkylbenzenes. Other components include combustion enhancers oxygenated compounds such as ethanol, sootremoving and deicing promoters, etc. In the U.S. the maximum amount of sulfur in gasoline is 30 ppm.

Today’s diesel fuel is a C10 to C15 distillate fraction containing nalkanes, cycloparaffins, and polyaromatics. Other components include “cetane” boosters organic nitrates and detergents to reduce carbon deposits. The engine operates lean air/fuel > 22 with a combustion temperature of 650o C, far less than the gasoline cycle’s 1,100o C. These conditions provide lower amounts of CO, unburned hydrocarbons, and NOx.

For NOx removal, gasoline methods do not work under these lean conditions rather, selective catalytic reduction, developed to treat power plant exhausts, is used. This involves reacting NOxcontaining streams with urea in the presence of a catalyst to convert NOx to N2 and O2. Bulky equipment and a tank of urea are required. This is no problem for large vehicles but is challenging for light cars.

Gasoline C6C12 is lighter than diesel C10C15 and more volatile. Although some of the components of gasoline and diesel come from straightrun direct from crude distillation, the major part is refined from cracking heavier C13C40+ components. This occurs in the fluidized catalytic cracker assisted by the hydrocracker. Each stream passes through a hydrotreater to remove sulfur and other impurities and is harder to process as the molecular weight increases. Fractions that lead to diesel are harder to crack and desulfurize than for gasoline. The sulfur requirement for diesel is so low 15 ppm that the refiner may even hydrotreat or otherwise desulfurize the diesel product. By changing the operating conditions in the catalytic cracker, different ratios of gasoline/diesel are produced to satisfy seasonal demands for gasoline or diesel, but always one at the expense of the other.

Replacing gasoline with diesel merely decreases petroleum consumption due to higher efficiencies. In the near term, switching to diesel cars will likely decrease consumption somewhat, but those savings are likely to be swamped by the increasing overall number of vehicles. In short, when it comes to looking at the big picture, increased adoption of diesel engines in the U.S. auto fleet will ease, but not eliminate, America’s dependence on oil imports.