The parallel hybrid-vehicle system discussed here features two mechanisms: an internal-combustion-engine energy-distribution mechanism and dual energy-integration mechanism. The former comprises a first pulley set and a second pulley set, whereby it is possible to adjust its radius ratio and change the road surface oriented output load, output speed and required load, to maintain an optimal operating state for the internal-combustion engine at a given generator rotational speed. In this way, the engine can be maintained in an optimal state. For the dual energy-integration mechanism, any power source can be individually actuated by an electric motor and the power transmitted from the internal-combustion engine energy-distribution mechanism. Moreover, a one-way clutch can prevent the actuated power source from reversion, so any output power source will not be affected by another inactive power source. Also, the two input power-sources can be integrated into a bigger power source via a dual energy-integration mechanism, thus resulting in twice the output energy and obtaining the necessary tractive power to reach the road surface. A dynamic equation is therefore derived for this system to obtain the flow direction of the power source. Furthermore, dynamic equations of various system components can be established by modularized software Matlab/simulink and fuzzy logic are used to control and develop this system's dual energy-integration mechanism as a control strategy. After the engine's energy is distributed by the controller of the dual energy-integration mechanism, decelerated by the reduction ratio of the first pulley set of internal combustion engine distribution mechanism and added to the generator torque energy transmitted from second pulley set, the engine can maintain an optimum state under various operating conditions. (c) 2004 Elsevier Ltd. All rights reserved.