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Theoretical and experimental study on frictional losses of coated piston rings with symmetric and asymmetric geometry

An increase in the efficiency of internal combustion engines is a key challenge for engineers today. Mechanical losses contribute significantly to the engine inefficiency, and the piston assembly has the largest share in these losses. Various measures are therefore taken to reduce friction between the piston and the rings against the cylinder. However, the undertaken changes most frequently generate new challenges. For instance, lowering the viscosity of the engine oil or increasing engine load may lead to accelerated wear of the mating surfaces. In order to resolve this problem, more and more complex materials and anti-wear coatings have to be used. Furthermore, under these conditions, the shape of the ring sliding surface becomes more important. This paper presents the results of theoretical and experimental research on the influence of the geometry of the sliding surface of the compression rings and the use of various anti-wear coatings. Motoring tests were carried out on a specially adapted engine over a wide range of rotational speeds and temperatures. Nine sets of rings with three different sliding surface profiles, along with chrome, molybdenum and titanium-based coatings were tested. The piston rings and cylinder liner co-operation was also numerically simulated using input data corresponding to the experimental tests. Models of fluid and mixed lubrications were used in the simulations. Comparison of simulations and measurements indicated that mixed lubrication occurs even at high rotational speeds and low oil temperatures. Generally, the obtained results confirmed that the friction of compression rings can be significantly reduced through the appropriate geometry of the sliding surface and the use of anti-wear coatings.
Topic: Fuels and lubricants
Author: Grzegorz Koszałka
Co-authors: Piotr Wróblewski