Kinematic of marine piston-crankshaft system

Two-stroke, slow speed main engines are often installed on merchant ships, because of its very high efficiency. That kind of engine has an output of about 5500 kW per cylinder. The mass of piston-crankshaft system reaches over a dozen tons. That reciprocating masses are source of high level of dynamic inertia forces (mass forces). Those forces have big influence on engines working parameters and characteristics. One of them is instability of crankshaft rotational speed, which leads to dangerous torsional vibrations of propulsion system. Some inconsistency can be observed during analysis of piston-crankshaft system kinematic. In the theoretical engine books, the piston speed and acceleration has only two harmonic components, the inertia forces are depended on engine rotation speed and they doubled rotation speed. However, empirical formulas, published by engines producers, give us at least five harmonic components of mass forces. The author tries to find out the theoretical reason of existing (measured) higher harmonic orders of engines inertia forces. It is a first step for developing monitoring system of propulsion system’s torsional vibrations coupled with axial vibration, dynamic shaft line alignment and crankshaft springing. In the paper two analytical methods of piston displacement, speed and acceleration are presented. Well-known (from literature) equations are compared with more-detailed analytical procedures. The analysis was performed for one of the biggest marine MAN B&W engine, type 7K98MC. A discussion about the analysis results was included in the final part of the paper.