The main goals of this thesis research were to provide a simple concept and to simulate the energy requirements of a hybrid electric vehicle, i.e. to design a very efficient power assist hybrid electric natural gas vehicle without compromising the vehicle space, weight, and comforts. It is recognized that HEV's represent new areas of research, where demonstration projects are also required to find out which modes of energy conversion would best suit the society.  A simple simulation procedure is introduced that is practical, can be used as an initial analytical approach for hybrid vehicle energy analysis, and can be easily validated. Two scenarios consisting of a 0-60-0 km/h and a 0-100-0 km/h cycle were established to demonstrate the highway and city mode driving situations. The energy efficiency and equivalent fuel consumption were determined for the specific hybrid powertrains. A comparison with established SAE drive cycles was effected to illustrate the feasibility of the proposed simulation processes. (Abstract shortened by UMI.)