The flow of injected CNG fuel and intake air was differentiated by fixing the colour of the mixing flows. In the results, the injected CNG is in red and the intake air is in blue. By fixing the fluid colour, the flow of injected CNG fuel and intake air can be analysed in the fuel-air mixing. The fuel flow was differentiated by the fixing in valve lift. Without modification to the injector nozzle holes, injected CNG fuel is not mixed perfectly in the CNG engine combustion chamber.