GEOMETRY

Large-bore cast iron and vermicular iron piston rings are normally shaped into an oval after casting, as shown schematically in Figure 1.

Figure 1 – Oval shaping of piston rings.

By controlling the circumferential shape of the ring, the optimum sealing can be achieved when in running condition. The contact pressure distribution of a piston ring in operation is not only affected by its free geometric shape, but also by the operating condition of the engine. By using advanced simulation tools, Daros ensures that the ring shape has the optimum design for each engine.

Thermally induced stress leading to hard ring-liner contact and/or unintended blow-by in large-bore engines has been a well-known problem for many years. An effective way to avoid such problems is to adapt the free shape of a straight-cut top ring toward the ‘negative ovality’ shape, as shown in Figure 1. Another way to resolve the problem is to use a gas-tight lock on the piston ring, while controlling the blow-by leakage through grooves on the ring’s periphery, as shown in Figure 2.

Figure 2 – Top piston ring with controlled pressure-release grooves and gas-tight joint and a computation model of the gas-tight joint.

Figure 3 – Temperature distribution in a straight-cut top ring calculated through computer simulations by Daros.

An important function of the running face profile of a piston ring is to aid the build-up of a stable oil film against the cylinder liner, preventing the ring from coming into contact with the liner as much as possible. The load-bearing property of the piston ring package is greatly influenced by the running face geometry. Simulation tools and field tests have ensured that Daros piston rings are optimised to provide the maximum time between overhaul.

Daros Piston Rings P.O. Box 138, SE-435 23 Mölnlycke, Sweden. Phone +46 31 33 84 000 info.gothenburg@federalmogul.com