One of important problems of closed-cycle diesel engines is the drop in combustion pressure and engine power. In the present work the solution in a typical engine using cfd has been investigated. This is achieved by injecting helium into inlet gas mixture. This mixture involves recycling CO2 from the products and injecting oxygen. The basis of closed-cycle engines is EGR. The higher the %EGR, the smaller amount of gases such as nitrogen, which are less effective in chemical reactions. Therefore, the strategy of present work is to use the highest possible %EGR. Results indicate that in the case where helium has a mass fraction of 4.6 to 11.7% and CO2 has a mass fraction of 73 to 66%, after combustion, pressure and power drops occur, and to eliminate them, it is necessary to change o/f ratio. In general, as the amount of helium increases (up to a mass fraction of 20%), the peak pressure increases but does not reach the desired value, and the heat release rate decreases; however, at a mass fraction of 39%, the peak pressure exceeds the desired value, but combustion does not occur, this increase in pressure is the result of piston movement. By changing the o/f ratio from 16.5 to 11.5 and helium with a mass fraction of 9.5% and 67% EGR, the pressure variation matches the desired state. In this case, sfc is lower than in all cases, even the desired case, and the power output is higher than the desired case.