The use of direct injection (DI) engines allows a more precise control of the air-fuel ratio, an improvement of fuel economy, and a reduction of exhaust emissions thanks to the ultra-lean combustion due to the charge stratification. These effects can be partially obtained also with an optimized Air Direct Injection that permits to increase the turbulence at low speed and load increasing the combustion stability especially in lean condition. In this paper, a gasoline PFI (named G-PFI), gasoline PFI-methane DI dual fuel (named G-MDF) lean combustion were analyzed. The G-MDF configuration was also compared with a gasoline PFI - air DI (named G-A) configuration in order to distinguish the chemical effect of methane from the direct injection physical effect. The tests were carried out in a small displacement PFI/DI SI engine. The experimental investigation was carried out in a transparent small single-cylinder, spark ignition four-stroke engine. It was equipped with the cylinder head of a Direct Injection 244 cc engine. The in-cylinder pressure was measured and the indicated mean effective pressure, IMEP, and its CoV were evaluated. 2D-digital imaging optical measurements were performed to analyze the combustion process with high spatial and temporal resolution. In particular, it allows to follow the flame evolution and evaluate the flame front propagation speed. The CO, CO2, HC and NOx emissions were characterized at the exhaust by means of gaseous analyzers. The measurements were performed at 2000 rpm PL in steady state condition. The G-MDF as well as the G-A configurations allow improving the vaporization of the heavy gasoline compound and the homogenization of the charge. Moreover, for the G-MDF, the presence of the methane and its chemical interaction with gasoline heavy hydrocarbon enhances a more efficient combustion.