The injected CNG fuel and intake air mixing modelling results of the original nozzle injector is based on variations in injection timing. The original injector nozzle air-fuel mixing and fuel flow velocity was set as a reference for injected CNG and intake air mixing for the improvement of injector nozzles with multi-holes. The diameter of the original injector nozzle is 3.5 mm. In the SPI CNG engine, the fuel-air mixing of the original injector was the best if the CNG fuel is injected at 2 Bar and the intake air flow is at a pressure of 1 atm1 . Based on Czerwinski et al.1 , the discussion focused on 2 bar of injected CNG fuel pressure and 1 atm of intake air flow pressure.