This document covers the mechanisms from the power cylinder which contribute to the mechanical friction of an internal combustion engine. It will not discuss in detail the influence of other engine components or engine driven accessories on friction. In internal combustion engines, the ability of an engine to generate its power potential to its fullest is hindered by the inherent internal friction of the sliding components and the friction of the engine driven accessories. In an era where power and fuel economy are key performance attributes, any operating condition which hinders the ability of an engine to achieve these attributes is of primary interest to the engine designer. Any sliding surface in the engine contributes to the friction of an engine. According to Taylor, (Reference a), the friction of an internal combustion engine is partitioned between the piston and ring assembly and the bearing, valve, and gear trains with the piston assembly accounting for 75% of the friction. Since the power cylinder friction is a major contributor to the over-all mechanical friction of the engine, any friction reduction goes directly to brake power with no increase in emissions, will add thermal efficiency at no cost to the customer, and can potentially improve durability. Any attempt to minimize the friction of an engine logically starts with decreasing the friction of the piston assembly. This document focuses on the friction of the piston and ring assembly sliding in a lubricated bore, in an attempt to communicate the current best thinking on the subject of power cylinder friction. With this understanding, the engine designer or engine development engineer will be able to minimize the friction of the power cylinder assembly thereby contributing to the power output or fuel economy of an internal combustion engine.