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.