The performances of experimental and small scale DCW, DIW, HCW, HIW and Faraday MHD generators driven by a scramjet engine are compared by three-dimensional numerical simulation. Numerical results show that the maximum power output is obtained in the DCW generator. The electrodes on the side walls in the DCW and the HCW generator suppress the loss of electrode voltage drop compared with the DIW and the HIW generator. Since the volume-averaged Hall parameter of plasma is less than unity for all the generators, the HCW and the HIW generator have less electric power output than other generators. The power output extracted by the Faraday generator is comparable to that extracted by the DCW generator. The Faraday generator, however, requires the load and the inverter to each number of electrode pairs, so that the complication of external circuits and the increase in cost may be apprehended. The authors conclude that the DCW generator is suitable for the experiment of scramjet engine driven MHD generators.