An experimental and theoretical investigation on the effects of piston clearance and oil viscosity on Cranktrain friction

Abstract

In this paper, the effects of clearance between piston and cylinder and oil viscosity index on the friction of area between the piston and cylinder of an internal combustion (IC) engine were investigated experimentally and theoretically. To this end, two values of 33 and 66 µm for the piston clearance, and two types of engine oil namely SAE10W40 and SAE20W50 were considered. To carry out the experiments, the engine was run under motored condition. The experiments were implemented at different oil temperatures and engine speeds. Based on the experimental results, although reduction in the piston clearance and the use of 20W50 oil caused an increase in the friction, but the effects of these two parameters became less with an increase in oil temperature and engine speed. The experimental results showed that the effect of clearance on the friction was more noticeable for low speeds. The maximum influence of the piston clearance decrease on the friction was 16% with the use of 20W50 oil at 1000 r/min when the oil temperature was 35 °C. The minimum influence of the piston clearance on the friction was 0.5% when the oil temperature was 90 °C and the engine speed was set on 6000 r/min. The theoretical results showed that the most contribution of the friction of piston/ring/cylinder assembly belonged to the piston skirt. Moreover, the effect of the piston skirt on the friction at high speeds was more than that of the piston rings. The friction value of the piston rings was not significantly altered by changing the engine speed, but it was considerably changed by the oil temperature variation. Whereas the piston skirt friction had an increasing trend with the speed rise, the change in the engine oil temperature, especially for low temperatures, had no significant effect on the piston skirt friction.