The Analysis of Bending Moment in Connecting Rod Caps for Internal Combustion Engines

Abstract

The gas pressure force due to combustion and inertial force of moving parts acts on the lower connecting rod head cover. As the crankshaft rotates, the inertial force is always directed away from the center of the crankshaft. The component of the inertial force acting on the connecting rod head cover decreases as the piston approaches top dead center (TDC) and increases as the piston approaches bottom dead center (BDC), but maintains the load of the same sign. Thus, the cap is subjected to a pulsating cycle, constant sign load. The maximum load of the inertial force acting on the cover is generated at the BDC. When calculating the strength of the cap, it was considered that it is a curved beam with a bending load acting in the middle. The size of this load depends on the geometrical parameters of engine, gas pressure at the TDC, the speed of the crankshaft rotation and mass of the parts of cranktrain. Two spark ignition engines (SIE) and two compression ignition engines (CIE) researched with wide range of swept volume, engine speed and geometrical parameters. It was concluded that the bending moment caused by the distributed load in the curved circular beam is smaller than in the straight beam. Moreover, at the real engine the fixed with two bolts connecting rod cap is a statically intractable beam whose bending moment will further decrease.