The transient performance of engines is investigated in this paper. A mathematical model of single-cylinder engines is established and the friction torques produced by crankshaft bearing, piston assembly, pumping loss and other major factors are modeled. A dynamic simulation system is developed based on the MATLAB Simulink environment. Friction torques and instantaneous motion characteristics of the engine are predicted. The results show that crankshaft bearing, piston assembly, and pumping loss have a major effect on the transient performance of the engine, and the models described in this paper provide an effective tool to simulate engine processes.