Nowadays an elevated number of two, three and four wheels vehicles circulating in the world-wide urban areas is equipped with Port Fuel Injection Spark Ignition (PFI SI) engines. Their technological level is high, but a further optimization is still possible, especially at low engine speed and high load. To this purpose, the scientific community is now focused on deepening the understanding of thermo fluid dynamic phenomena that takes place in this kind of engine: the final purpose is to find key points for the reduction in engine specific fuel consumption and exhaust emissions without a decrease in performance.In this work, the combustion process was investigated in an optically accessible single cylinder PFI SI engine. It was equipped with the head, injection device and exhaust line of a commercial small engine for two-wheel vehicles, it had the same geometrical characteristics in terms of bore, stroke and compression ratio. The combustion chamber was optically accessible through a quartz ring under the engine head and a sapphire window on the piston top. Cycle-resolved visualization was carried out to follow the flame propagation from the spark ignition until the late combustion phase. The visualization was performed from the bottom of the engine and from the lateral side with different camera inclinations. High spatial resolution visualization of the combustion process was carried out. Moreover two color pyrometry method was applied to evaluate the spatial distribution of diffusion flames temperature and soot concentration in the combustion chamber. UV-visible emission spectroscopy was used to identify the chemical species that feature the combustion process and to study the pollutants formation. The cyclic variability and the fuel deposits formation on the combustion chamber surfaces were also investigated. All the measurements carried out in the combustion chamber were correlated with the main engine parameters and exhaust emissions. The effect of the fuel injection phasing was tested. Two fuel injection strategies were considered: in the first one, the fuel injection started when the intake valves were open; in the second one the fuel injection occurred at closed intake valves.