Abstract：For a single cylinder engine, the total unbalanced inertial forces occur in the engine block, which results in engine's vibration and deteriorated noise. In order to eliminate the unbalanced forces, counterweight and primary balance shaft should be attached to the cylinder block so that engine durability and ride comfortability may be further improved. Traditionally one third of connecting rod assembly's mass is treated as reciprocating mass, and two thirds as rotating mass when designing balance mechanism. In this paper, a new method based on the multibody dynamics simulation is introduced to separate the reciprocating mass and rotating mass of connecting rod assembly. The model consists of crankshaft, connecting rod, piston and the simulation is performed subsequently. According to the simulation results of the main bearing loads, the reciprocating mass and rotating mass are separated. Finally a new balance mechanism is designed and simulation results show that it completely balances inertial forces to improve the engine's noise vibration and harshness performance. Keywords：multibody dynamics；reciprocating inertial force；rotating inertial force；primary balance shaft Recently efforts to reduce engine vibration and radiated noise while improving durability and reliability have become increasingly important to engine design due to more stringent requirements for higher NVH (noise, vibration and harshness) performance, low weight and low cost [1 -4] . The noise and vibration in the engine are generated by the combustion pressures within the cylinders and the inertial forces due to the motions of the piston and crankshaft. Generally, the elastic vibration of the engine structure and the dynamic properties of the crankshaft deteriorate the vibration level and sound quality [ 1 , 5 -9 ] . Accordingly, these characteristics are important factors in the design of a low-noise engine.Suh's study on the balance of the three-cylinder engine and balshafts was to reduce the vibrations at the engine mounts [10] . Stanley's study focused on the influence of lowering the counterweight on bearing load, which demonstrated that the average bearing load increased with engine speed and the maximum bearing load initially decreased with engine speed, reached a minimum, and then increased quickly with engine speed [11] . Liu's study showed that it was necessary to apply equivalent ring-weights to each crankpin of the crankshaft when balancing it on a dynamic balancing machine, and the formulas for calculating the accurate ring weights were given [12] .In this paper, a 4-stroke single cylinder engine is modeled, and the separation of connecting rod's equivalent rotating mass and equivalent reciprocating mass is analytically studied by multibody dynamics. Then the crankshaft counterweight and primary balance shafts are designed.
