Modern SI (spark ignition) engines are continuing to evolve for improved fuel economy by utilizing increased peak cylinder pressure, reduced section size piston rings, reduced cylinder bore surface finish, reduced lubricating oil viscosity, usage of alternative fuels, and with potentially increased fuel dilution. These changes all increase the potential for asperity contact between the top ring outer periphery and the cylinder wall, thereby creating a more challenging operational environment for the top ring.
To evaluate the resistance of new advanced top ring coatings, a new aggressive test cycle promoting ring face fatigue and scuffing was developed. The test cycle has been shown to be successful in failing a production GNS (gas nitride steel) coating in 10 hrs of run time.
This paper also presents relative performance data for different top ring face materials and ring features. The ring coatings evaluated include traditional coatings such as GNS and PVD (physical vapor deposition) coating as well as thermal spray coatings consisting of conventional plasma, HDAP (high density air plasma), and HVOF (high velocity oxygen fuel).