Modeling the Inertial Torque Imbalance Within an Internal Combustion Engine: Quantifying the Equivalent Mass Approximation

The objectives of this research are to explore the inertial-torque characteristics of an inline, internal combustion engine with connecting-rod joints that are evenly spaced about the centerline of the crankshaft, and to evaluate the goodness of a mass approximation that is customarily used in machine design textbooks. In this research, the number of pistons within the internal combustion engine is varied from 1 to 8. In order to generalize the results, the inertial-torque equations are nondimensionalized and shown to depend upon only four nondimensional groups, all related to the mass and geometry properties of the connecting rod. As shown in this research, the inertial-torque imbalance is greatest for an engine with two pistons, and that a dramatic reduction in the torque imbalance may be obtained for engine designs that use four or more pistons. It is also shown in this paper that the customary mass approximations for the connecting rod may be used to simplify the analysis for all engine designs without a significant loss of modeling accuracy.