Modern diesel engines not only provide stellar fuel economy but are quiet and peppy. Still, despite these improvements in performance, diesels remain nasty polluters. Now some of the black clouds that seem to follow diesel vehicles around could be about to disappear.

More-effective diesel-exhaust-cleaning devices are getting tantalizingly close to market. The problem is that diesel exhaust can’t be cleaned with conventional catalytic converters because it contains too much oxygen, which destroys the catalysts. Enter plasmas-electrically charged gases. In a chamber attached to the exhaust pipe, rapidly pulsing electrical fields would convert oxygen molecules into ions that help trigger a first round of chemical changes in the exhaust. Then, a bit further along in the exhaust system, a catalytic converter could finish the clean-up job.

Several companies are betting such plasma devices can be installed in diesel vehicles to burn off smog-causing nitrogen oxides and unhealthy particulates by 2007. That’s when new U.S. diesel emission standards will take effect; the standards require diesel engines to run as cleanly as gasoline engines. “This is a subject of intensive research worldwide right now, to become the first out of the gate to reduce diesel emissions,” says Barry Bhatt, manager of plasma systems at Irvine, CA-based NoxTech.

In December, NoxTech says, the company managed a 94 percent reduction in nitrogen oxides in a laboratory engine test. NoxTech is planning to road-test scaled-up plasma systems this spring. The firm has plenty of competition, though, including a research consortium of federal labs; DaimlerChrysler, General Motors and Ford Motor; and Troy, MI-based Delphi Automotive Systems, which has partnered with French carmaker Peugeot Citron to bring plasma treatment to market in 2005.

The plasma approach “stands a damn good chance of being the diesel exhaust technology selected by vehicle makers for 2007 for light trucks, including SUVs, pickups and minivans,” says John Fairbanks, manager of a U.S. Department of Energy clean-diesel program. There’s one potential roadblock, though: plasma-based exhaust treatment methods are expensive.

Still, Daniel Cohn, a physicist at MIT’s Plasma Science and Fusion Center, says costs could drop under a different plasma scheme that uses a smaller chamber and a simpler power supply. The “plasmatron” he coinvented produces a hydrogen-rich gas from a diesel-fuel and air mixture; this gas is then injected into a catalyst that traps nitrogen oxides. The hydrogen gas reacts with and removes the nitrogen oxides.

Late last year, auto-parts maker ArvinMeritor, also based in Troy, MI, inked a deal with MIT to further develop the plasmatron. Whichever companies ultimately capture the market, though, the real winners are likely to be both drivers and the environment, as clean diesels roar, albeit quietly, onto the roads.