The influence of piston ring lubrication on internal combustion engine performance has received considerable attention for over half-century. Studies show that hydrodynamic lubrication prevails through most of engine cycles, and asperity contact only occurs near the vicinity of dead centers [1][2]. In order to solve the governing equation of lubricant, appropriate velocity and pressure boundary conditions should be incorporated into the lubrication equation - Reynolds equation. While lubricant obeys the no-slip velocity boundary condition, the pressure boundaries at the leading and trail edge of piston ring are related to the chamber and the inter-ring crevice gas pressures. A complete lubrication analysis of piston ring requires an inter-ring gas flow model. In most of existing lubrication models, an isentropic orifice flow model is adopted and the gas flow is assumed to an ideal gas passing through the piston ring end gaps with a constant discharge coefficient [2][3][4][5]. In additional to the flow path of piston ring end gaps, gas also flows through the side-clearance between piston ring and flank groove [6][7][8][9][10]. In this paper, a quasi-Rayleigh narrow-channel gas flow model is proposed by consideration of temperature gradient along radial direction of piston assembly. Piston ring friction force is estimated by a test-rig verified mixed lubrication model [11]. Numerical simulation shows that piston ring friction force and ring axial motion are sensitive to inter-ring gas flow model. The instantaneous indicated mean effective pressure (IMEP) method was adopted here to measure the piston friction during motoring condition. Experimental and numerical results indicate that top ring could contribute about 10% of total power cylinder friction loss.

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