a piston 39 s travel at which it changes from an upward stroke i e away from a crankshaft of the engine 101 to a downward stroke i e towards the crankshaft of the engine 101 the term u201c bottom dead center u201d refers to the furthest point of a piston 39 s travel at which it changes from a downward to an upward stroke in a similar manner the term u201c top u201d end of the cylinder 103 refers to an end of the cylinder 103 at which the piston 109 reaches top dead center and the term u201c bottom u201d end of the cylinder 103 refers to an end of the cylinder 103 at which the piston 109 reaches bottom dead center during the operation of the engine 101 the linear speed of the piston 109 varies between a minimum speed for example a zero speed when the piston is stationary relative to cylinder 103 at top dead center or bottom dead center and a maximum speed as the piston 109 moves between top center and bottom dead center as a result of the change in speed of the piston 109 the coefficient of friction between the piston rings 107 and the inner surface 105 of the cylinder varies as the piston 109 travels within the cylinder bore in order to reduce the friction between the sliding components of the engine 101 such as the piston rings 107 and the inner surface 105 of the cylinder a lubricant may be used the frictional coefficient between sliding components may be determined using the stribeck curve which is used to categorize the frictional properties between two surfaces as a function of the viscosity of the lubricant and the relative.