An 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 engine ineffciency, 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 the 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’s sliding surface becomes more important. This article presents the results of experimental research on the influence of the geometry of the sliding surface 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 (Mo), and titanium-based coatings, were tested. 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. Of all the tested variants, the best results were achieved for the rings with moderate asymmetry and with titanium nitride (TiN) coatings.