Driving behind a bus or truck as it belches smelly exhaust fumes is enough to convince anyone that emissions from diesel engines should be cleaned up. But while gasoline-engine manufacturers have reduced harmful emissions in their new vehicles by some 90 percent over the past 25 years, makers of diesel engines have thus far managed to reduce noxious exhaust fumes by only about half that percentage.

Now, however, researchers at the University of Southern California are developing a device that uses high-energy electrons to zap noxious diesel exhaust, reducing it to water vapor, carbon dioxide, and air. Within a few years, the technique may prove to be a cost-effective way to bring diesel engines into compliance with ever more stringent emissions standards.

According to the Environmental Protection Agency (EPA), heavy-duty diesel engines produce 25 percent of all vehicle-generated nitrogen oxides (NO_x)-the source of nitric acid (HNO₃), a main component of acid rain and a major source of urban smog. Though the first comprehensive federal regulations controlling the emissions of motorized vehicles were initiated under the Clean Air Act of 1970, the first NO_x standards specifically for diesel emissions did not go into effect until 1984. Then, when air quality continued to worsen from increased traffic, Congress passed the Clean Air Act of 1990, which forced manufacturers of trucks and buses to reduce NO_x emissions by 50 percent relative to the 1984 standard. Finally, in 1995, the EPA, in concert with the California Air Resources Board and producers of heavy-duty engines, drafted a statement of principles that led the EPA to set the latest standards aimed at cutting diesel NO_x emissions another 50 percent by 2004.

Diesel engineers have thus far reduced emissions largely by replacing mechanical air-intake and fuel-delivery controls with more effective electronic systems. Now researchers, including Martin Gundersen and Victor Puchkarev, a pair of electrical engineers at the University of Southern California, are exploring exhaust after-treatment systems. Based on a “pulsed-power” electronic circuit that originated in the 1950s and was refined during work on the Strategic Defense Initiative in the 1980s, their device uses short bursts of high-energy electrons to break down NO_x and other sooty particulates in diesel exhaust before they make their way to the atmosphere.

The USC group has developed a chamber that can be incorporated into a diesel-exhaust system much like a catalytic converter is used in gasoline-powered vehicles. Inside the chamber, a special electronic switch produces a short electrical pulse, like the burst from a camera’s electronic flash attachment, at the rate of thousands of electrical discharges per second. When energetic electrons from these rapid-fire high-voltage sparks are injected into the exhaust stream, similar to the way an electron beam is fired into a fluorescent tube, they strike air and water vapor molecules present in the exhaust. The collisions create an ion plasma-an assortment of electrons and charged particles of nitrogen, oxygen, and hydroxide-which, in turn, reacts with NOx and particulate hydrocarbons to produce carbon dioxide, air, and water vapor.

Early prototypes proved the concept but were inefficient, requiring as much as 50 percent of the engine’s power for their operation. However, Gundersen claims that the team has recently improved the efficiency of the device “by an order of magnitude,” bringing it into a range that manufacturers of diesel-powered vehicles would find appealing.

To achieve the dramatic improvement, Gundersen says his team tweaked the device to shorten the duration of each pulse from about 200 nanoseconds (billionths of a second) to about 50 nanoseconds. Because electron acceleration occurs only in the initial stages of the pulse, shortening the bursts raised the energy of the electrons in the plasma while consuming much less energy overall.

According to Bernard M. Penetrante, a physicist who heads the Environmental Plasma Technologies group at Lawrence Livermore National Laboratory, a dozen or more groups worldwide are now engaged in similar R&D work that may eventually compete for a share of a huge potential market. Indeed, the EPA estimates that some 5 million heavy-duty and 2 million light-duty diesel vehicles now roll along U.S. roads, and that several hundred thousand new diesel vehicles are sold in the country each year.

Gundersen estimates that a pulsed-power after-treatment device would cost on the order of $1,000. If that proves to be the case, the market could easily be worth several billion dollars. And the market may not be limited to diesel engines. Penetrante notes that any device that can reduce NO_x to benign products in a diesel exhaust pipe can also be applied to gasoline engines. He explains that similar conditions exist in new lean-burn (high air-to-fuel ratio) gasoline engines, and that new technologies such as pulsed power will be critical for allowing the next-generation vehicles to meet ever-stricter emissions requirements.