This study was carried out in a transparent engine. In this work we have compared results obtained from a natural gas engine fueled with a standard continuous gas injection with results obtained from a gasoline engine fueled with electronic sequential injection. To compare performance between engines of both types, we have carried out quantitative measurements of fuel/air ratio before ignition and flame image recording. Laser induced fluorescence was used with an excimer KrF laser. As the natural gas and gasoline were not fluorescent at the laser wavelength (248nm), a tracer has been mixed to the fuel. Furthermore, as this tracer is also a fuel, the fresh charge in the cylinder was fluorescent, whereas the burnt gas was not, which enable detection of flame fronts. The images recorded at different crank-angles allowed determination of parameters such as inhomogeneity, overall flame width and spatial propagation speed of flame. Those parameters yielded a characterization of the cycle-to-cycle fluctuations of mixture preparation and flame position and propagation.