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Du Pont Engineering Polymers : 06 April, 2004  (Company News)
Even under extreme conditions, bushings and guide sleeves of DuPont
The compact EEGR valves are installed in Daimler Chrysler’s CDI engines and in the 2.2 l GM-Saab diesels; the return valves are used in Volkswagen diesels, for example.

Exhaust gas return systems lower the combustion temperature in the cylinders of diesel motors and direct-injection petrol motors. Cooler combustion reduces the generation of harmful nitrogen oxides (NOx) so effectively those motors can meet the European Union regulations for 2005 (Euro 4). The NOx reduction is reinforced when the returned exhaust gases are cooled before being returned to the cylinder.

Excellent resistance to corrosive exhaust gases, the ability to withstand high temperatures, low abrasion and very good dimensional stability were the decisive criteria for Wahler’s choice of material. Dr Bernd Bareis, head of basic research at Wahler, explains in these terms. “A cylindrical operating rod lifts and lowers the valve flap of our EGR valves so that the flap either cuts off the exhaust gas stream, or lets it flow freely, or partially throttles it.

The valves must work reliably in a motor’s warm-up phase as well as when exhaust gas temperatures at the valve entry may reach 500°C or more. This requires the sleeve through which the operating rod passes always to allow the right amount of play and always to present a low-friction surface, although unlubricated. This is the only way we can achieve the delicate adjustment that is needed, with actuating speeds of less than 50 ms even after a minimum of 3.6 million cycles.”

To meet these requirements, Wahler, working with DuPont, developed a sliding bearing made of DuPont’s new thermoplastic Vespel® TP polyimide. The starting point was type TP-8311. With heat deflection temperature (HDT) of 244C according to ASTM D–648, it is specially tailored for use in applications needing high thermal resistance; it also offers good mechanical properties, very high dimensional stability, abrasion resistance, both lubricated and unlubricated, and chemical resistance. The special type, which Wahler picked, meets these requirements and in fact offers an even higher thermal resistance than specified.

“We were able to manufacture the sleeve with its difficult geometry only because this polyimide type, in spite of its high thermal resistance, can be processed as a thermoplastic resin,” Dr Bareis states. “In particular, the material shows such a low thermal relaxation that the required tolerances are respected over the whole temperature range.”

Wahler’s Type 7308 return valves, which are used in combination with the EEGR valves, guide the exhaust gases during a motor’s start-up phase directly into the combustion chamber, bypassing the exhaust-gas cooler. The bushings in this valve are exposed to temperatures of over 550C. This exceeds even the very high thermal resistance offered by Vespel® TP-8311, so here Wahler uses bushings of sintered Vespel® SP-22. Bareis comments: “DuPont delivers these made-to-measure parts ready for use: we simply press them into place without any finishing operation, and they have a lifetime of more than 400 000 operating cycles.”

Gustav Wahler GmbH u. Co. KG, was established in 1902 as a fabricator of metal parts. Today this family-owned company is a leading supplier of temperature- and emission-management solutions to the global automotive industry. With 1100 employees in two factories in Germany and one in Brazil, the company had sales of 160 million Euro in 2002. The main product ranges are thermostats, emission-control solutions (EGR valves and valve flaps, butterfly valves and EGR pipes) and flexible metal pipes.
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