Novel polyimide bearings handle hot exhaust gases

Edited by the Engineeringtalk editorial team Apr 13, 2004

Bushings and guide sleeves made from Vespel polyimide ensure reliable, low-wear, low-friction operation in return valves made by Gustav Wahler of Esslingen, near Stuttgart.

Even under extreme conditions, bushings and guide sleeves of DuPont Vespel polyimide ensure reliable, low-wear, low-friction operation in the electrically operated exhaust gas return valves (EEGR) and pneumatic return valves made by Gustav Wahler of Esslingen, near Stuttgart.

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: "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 500C 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 50ms 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 244C 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 550C.

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".