Loading, Damage, Wear, Friction

The piston rings are loaded by outward stresses when they are stretched to pass over the cylinder and, when installed, by the inward bending stresses imparted when the ring is compressed so it can enter the cylinder. When the rings are properly engineered, the outward stress is equal to the strain imparted when the ring is passed over the cylinder.

Dynamic loading occurs in addition, namely, an axial motion of the piston ring caused by the interactions of gas, mass, and friction forces. In critical situations this leads to ring chatter and vibrations, which in turn can cause ring failure. Extraordinarily high loading on the ring can arise from soot collecting in the piston ring groove, which can cause sticking and ring failure. In addition to the axial motion, the ring also rotates around the circumference. Additional ring damage includes bum traces and seizure.

The service life of the seal made at the piston rings is determined to a large degree by the amount of wear. This includes radial wear (wear on the running surface), axial wear (wear at the flanks, “microwelding,” piston groove wear), and secondary wear at oil control rings (wear between the ring and the tubular spring and between the rails and the spacer spring). The tribologic system surrounding the seal created by the piston ring is extremely complex since virtually all the normal types of wear— abrasive, adhesive, and corrosive—occur to a greater or lesser extent and effect.

The piston group accounts for about 40% of all the friction in the engine. The piston rings cause a bit more than half of this friction.

The factors that influence piston ring friction include the surface pressure, ring thickness (width of the running surface), the rail height in oil control rings, the shape (crowning) of the contact surface, the coefficient of friction for the running surface layer (only in mixed friction areas at TDC and BDC, where the piston speed is very slow), and the number of rings per piston. Measures taken to reduce piston ring friction must not interfere with ring functioning. The sealing effect of the ring set for both gases and the lubricating oil have to be maintained undiminished.

Bibliography:
1. N.N., Kolbenringhandbuch, AE Goetze GmbH, Burscheid, 1995.
2. Jakobs, R., “Ein Beitrag zur Wirkungsweise von negativ vertwistenden Minutenringen in der zweiten Nut von Pkw-Dieselmotoren,” Faehschrift К 41, der Goetze AG, 1988.

3. Furuhama, S., and H. Ichikawa, “L-Ring Effect on Air-Cooled Two- Stroke Gasoline Engines,” SAE Paper 730188, 1973.
4. McLean, D.H., et al, “Development of Headland Ring and Piston for a Four-Stroke Direct Injection Diesel Engine,” SAE Paper 860164, 1986.

5. Arnold, H., and F. Florin, “Zur Berechnung selbstspannender Kolbenringe von konstanter Starke,” Konstruktion, Vol. 1, 1949, No. 9.
6. Gintsburg, B.I., “Splittles-type Piston Rings,” Russian Engineering Journal, Vol. XLVIII, No. 7.

7. Mierbach, A., “Radialdruekverteilung und Spannbandform eines Kolbenringes,” in MTZ, Vol. 55, 1994, No. 2.
8. Wiemann, L., “Die Bildung von Brandspuren auf den Laufflachen der Paarung Kolbenring-Zylinder in Verbrennungsmotoren,” in MTZ, Vol. 32, 1971, No. 2.

9. Buran, U., Chr. Mader, and M. Morsbach, “Plasmaspritzschichten fur Kolbenringe: Stand und Einsatzmoglichkeiten,” Faehschrift К 35, Goetze AG, Burscheid, 1983.
10. Jakobs, R., “EinflussgroBen beim Zylinder(Zwickel)-VerschleiВ von Pkw-Dieselmotoren,” in MTZ, Vol. 44, 1983, No. 12.