Engine vibrations of large amplitude are caused by inertial effects of the piston-crankshaft mechanism and gas forces, and are transmitted to the chassis of a vehicle in the form of periodically varying forces through the engine mounts. Both the engine vibrations and mount forces may be minimized by designing proper engine components and mounts, which requires a dynamic simulation of the engine. In this study, a three degree of freedom dynamic model, enabling the simultaneous treatment of the piston-crankshaft mechanism and engine block, was devised for a two-cylinder four-stroke engine. Periodic and temporary variations of crankshaft speed, the variation of torque and power with speed, torsional and translational vibrations of the engine block, and the variation of mount forces with respect to the damping and stiffness coefficients of the mounts were studied. The torsional and translational vibrations of the engine block were found to be mainly affected by the combustion gas force and inertia force of the reciprocating masses, respectively. A simple relation has been obtained to determine the position and mass of counterweights used for eliminating the vertical vibration of the block.