Will Dr. Diesel's Turn Ever Come?

The Steering Column


Other than big-rig drivers, Americans haven't worked up much affection for the engines without spark plugs that Rudolf Diesel invented in 1897. Except for a brief burst of popularity during the fuel crisis in the early '80s, when gasoline prices suddenly doubled to about $1.30 a gallon, we've been indifferent to the diesel's ability to squeeze about 30-percent more mileage from a gallon of fuel over a gasoline engine. When fuel is cheap, Americans have had little use for the diesel's traditionally rougher and noisier operation, its smoky exhaust, sluggish cold starts, and smelly, messy fuel.

Environmentalists don't much like them, either. Even though diesel engines achieve about the same fuel-economy bump as gasoline-electric hybrid powertrains, the tree huggers are upset by the greater emissions of oxides of nitrogen (NOx) and particulates from diesel engines.

NOx is one of the major precursor emissions to smog. The federal Tier 2 emissions standards, which begin their five-year phase-in in 2004, will cut allowable NOx emissions by 75 percent to 0.05 gram per mile per vehicle. NOx is created during peak combustion temperatures and pressures, and since diesels run compression ratios roughly double those of gasoline engines, they have trouble meeting this standard. This lofty compression ratio, however, is also one of the key factors behind the diesel's high efficiency. Another component of diesel efficiency is lean combustion, which precludes the use of the three-way catalysts that control NOx so effectively in gasoline engines.

Particulate emissions are the black haze that we see in diesel exhaust, especially from heavy trucks operating at full power. The color comes from tiny particles of soot produced during combustion. It's a problem inherent with diesels because the fuel is injected directly into the combustion chamber at the end of the compression stroke and has very little time to mix with the air in the cylinder. By contrast, in a gasoline engine, the fuel is injected while the air is rushing into the cylinder during the intake stroke and can disperse even farther during the compression stroke. Even though diesels burn lean overall, without this opportunity for thorough mixing, there are invariably tiny pockets of rich mixtures, which generate soot.

Modern diesel injection systems-similar to gasoline electronic fuel injection except that they operate at a fuel pressure of 20,000 psi rather than 50 psi-have greatly reduced this soot generation. Still, the Tier 2 standards mandate a maximum particulate emissions level of 0.01 gram per mile, which no current diesel can meet. To solve this problem, the industry has been fooling around with particulate traps for years. These devices sift the soot from the exhaust and periodically burn it off. But getting these filters to operate for 100,000 miles has been a major challenge.

Furthermore, environmentalists have been calling for even stricter particulate standards. The current Tier 2 limit applies to particles of soot larger than 10 microns. That's four ten-thousandths of an inch, or about one-fifth the thickness of human hair. But there's talk of including particles as small as 2.5 microns. This would exacerbate the particulate problem exponentially.

This utter rejection of diesels by U.S. environmentalists is in stark contrast to attitudes in Europe, where the diesel has been accepted with open arms. About one-third of the new cars and light trucks sold in Europe are purchased with diesel engines-a choice undoubtedly motivated by the fuel costs in Europe of $4 to $5 a gallon. Moreover, upcoming European NOx and particulate limits are four to ten times higher than ours will be. European diesel fuel is also better, with significantly lower sulfur content and higher cetane (a measure of a fuel's affinity for ignition, it's the opposite of octane).

European environmentalists, who are numerous enough to support powerful Green parties in several countries, seem to accept these concessions for diesels. They feel the diesel's benefits in reduced fuel consumption, and the associated CO2 emissions, outweigh the potential health effects of their particulate emissions. Then again, the Europeans have barely discovered no-smoking sections in restaurants.