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Tham khảo tài liệu 'internal combustion engines fundamentals episode 2 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 730 INTERNAL COMBUSTION ENGINE FUNDAMENTALS second ring can be made more flexible to give better oil control. The objective is to compensate for the torsional deflection of the ring under load so that top-edge contact with the cylinder liner is avoided. Top-edge contact tends to pump oil toward the combustion chamber detracting from the performance of the oil control ring. Bottom-edge contact provides an oil scraping action on the downstroke. The oil control ring meters and distributes the oil directed onto the cylinder liner by the crankshaft system returning excess oil to the crankcase sump. It must exert sufficient pressure against the cylinder possess suitably shaped wiping edges usually two thin steel rings and provide adequate oil drainage. Slotted or composite rings are normally used.14 The tension in all the piston rings holds them out against the cylinder wall and hence contributes to friction. The gas pressure behind the compression rings increases this radial force. The gas pressures behind the second ring are substantially lower than behind the first ring. The gas pressures behind the rings are a function of speed and load. An approximate rule for estimating ring friction is that each compression ring contributes about 7 kPa 1 lb in2 mep.5 Oil rings due to their substantially higher ring tension operate under boundary lubrication they contribute about twice the friction of each compression ring.15 The piston assembly is the dominant source of engine rubbing friction. The components that contribute to friction are compression rings oil control rings piston skirt and piston pin. The forces acting on the piston assembly include static ring tension which depends on ring design and materials the gas pressure forces which depend on engine load the inertia forces which are related to component mass and engine speed . The major design factors which influence piston assembly friction are the following ring width ring face profile ring tension ring gap which .
