Stainless steel reduces diesel engine emissions

NIROSTA® 4539, a high-alloy austenitic stainless steel made by Krupp Thyssen Nirosta GmbH, is playing a decisive part in reducing diesel vehicle emissions. The material is used to make the central component of a cooled exhaust gas recirculation system developed by Behr GmbH & Co. The Stuttgart-based company supplies the system for both diesel cars and trucks.

Cooled exhaust gas recirculation lowers emissions of nitrous oxides in particular by reducing the combustion temperature in the diesel engine cylinders. Part of the exhaust gas is passed through a cooler and then added to the fresh air required for the combustion process. This addition lowers the oxygen concentration in the cylinders and with it the combustion temperature. Cooling the exhaust gas reinforces this effect.

Stainless steel from Krupp Thyssen Nirosta is used to make the exhaust gas cooler, which consists of a bundle of laser-welded thin-walled tubes through which the hot exhaust gas is directed. Engine coolant is passed through gaps between the tubes. The exhaust gas cooler is subject to extreme operating conditions and therefore has to be made of a material that resists heat and corrosion as well as displaying high mechanical strength. In addition, the cooler must take up only little space. Exhaust gas temperatures in cars can reach up to 450 degrees Celsius and in trucks up to 700 degrees. The average exhaust gas pressure in car engines is two bars and in truck engines three bars. Because diesel fuel always contains a small amount of sulfur and when the exhaust gas cools an aggressive condensate containing sulfur and other elements is formed, the material must be extremely resistant to corrosion. NIROSTA® 4539, a stainless steel already proven in heavily stressed chemical equipment parts, is qualified to meet these requirements.

The NIROSTA® tube bundle design was made production-ready in close collaboration between Krupp Thyssen Nirosta, Behr and Fischer Edelstahlrohre GmbH. While Krupp Thyssen Nirosta GmbH has developed a special production route for the material at its cold strip plant in Krefeld, Fischer Edelstahlrohre GmbH is responsible for making the laser-welded tubes. The Achern-based company also fits so-called winglets inside the tubes to create turbulence and so reduce contamination and improve heat transfer.

Whereas cooled exhaust gas recirculation is an established technology in cars, it still has great growth potential in the truck market, especially in view of the new regulations due to come into force in the European Union in 2005 which will impose a nitrous oxide limit of 3.5 grams per kilowatt hour for trucks above 3.5 tons. Cooled exhaust gas recirculation systems using NIROSTA® already meet this standard.

For further reference: ThyssenKrupp AG, Trade Press, Bernd Overmaat
Tel: +49-211-824-36012, Fax: +49-211-824-36035
e-mail: overmaat@tk.thyssenkrupp.com