Catalytic converter gets the pollution out of diesel engines

In the near future the usual summer ozone peaks exceeding the allowed threshold may be a thing of the past: the Paul Scherrer Institute (PSI) in Switzerland has developed a new type of catalytic conversion system, which filters nearly all nitrogen oxides out of diesel exhaust gases using a refined control technology. This eliminates the main cause of summer ozone build-up. The process requires a non-toxic urea solution, which future diesel engine commercial vehicles can take with them in a separate refillable tank.

Diesel engines are looked upon as relatively economic and environment-friendly, because they have a better fuel efficiency than gasoline engines. But burning diesel also has a grave disadvantage: it produces nitrogen oxides, which enhance the build-up of hazardous ozone during periods of high solar radiation. “In the end, diesel engines today are the main cause for high ozone values during summer”, says Oliver Kroecher, Exhaust Gas Aftertreatment Group Manager at PSI. Already by 2005, exhaust gas standards for diesel engines are to be tightened massively throughout Europe. And further steps reducing the threshold are planned.

To comply with the new threshold values engine manufacturers are now focussing on the so-called SCR (Selective Catalytic Reduction) technology. Here nitrogen oxides are transformed into nitrogen and water vapour using a catalytic converter and by adding a harmless urea solution. This compelling principle could establish itself in the foreseeable future in all commercial diesel-powered commercial vehicles. In future drivers should get used to refilling an additional urea tank.

Zero emission for combustion engines is long term objective PSI scientists have now developed a practicable SCR catalytic converter that disposes over 90% of the nitrogen oxides in exhaust gases. Kroecher describes the PSI advantages over other SCR prototypes: “Our converter has minimal dimensions and can prevent the escape of ammonia generated during the reaction, thanks to an ingenious regulating system.” To optimise the nitrogen oxide dismantling, the amount of urea added adapts continuously to different drive phases. “Our regulator system all but anticipates the engine activity and can therefore react fast enough to changes”, says Kroecher.

With the nitrogen oxide output going down the problem with harmful soot particles from diesel engines will also be defused. While diesel engines could be tuned to a low soot production, this will however increase the nitrogen oxides. The new SCR catalytic converter strongly reduces this drawback. PSI researchers want to achieve even more. Kroecher: “In the long run, we’re working on zero-emission concepts, in order to develop combustion types creating no other pollutants than carbon dioxide.”

The work on the SCR catalytic converter was carried out together with industry partners and with the support of the Swiss Federal Office of Energy. The Measurement and Control Laboratory at the Swiss Federal Institute of Technology (ETH) in Zurich developed the advanced catalytic converter process strategy. Commercial products based on the PSI know-how will be introduced to the market in the near future.