As can be observed in Figure 2, the asymmetry was less evident for BU40, which showed a more regular evolution of the flame front with slight border wrinkling. This was a first marker of a lower amount of fuel deposits near the valves for the blend if compared to pure gasoline. Bright spots were observed in the burned gas before the flame front reached the chamber wall. The bright spots were due to the fuel deposits on the optical window. During the injection of fuel at closed intake valve, when the gas flow passes through the valves, the fuel droplets are stripped from the fuel film layer. After reaching the combustion chamber, the fuel droplets stuck on the piston surfaces. These fuel deposits also created fuel-rich zones that ignited when reached by the normal flame front. In CV condition, the spots are bigger but less in number than in OV condition. When the injection occurred in open-valve condition, the fuel droplets' sticking was enhanced by the partial carrying of the injected fuel droplets directly into the combustion chamber due to the gas flow. The bright spots have a random nature; during the combustion process, they decreased in size and number, and then, they disappeared before the exhaust valve opening [29]. The evidence of the bright spots decreased with the butanol percentage increasing. This means that the chemical composition of the blends helped the vaporization of the low-volatile component and then the fuel deposits burning. The presence of the fuel deposits, as squeezed film or impinged droplets, had direct effect on the flame radius evolution in terms of kernel cyclic variability and flame stability [25, 34]. Figure 4 reports the trend of the flame radius for gasoline and BU40 evaluated on 100 consecutive cycles. Figure 5 shows the outline of the flame front evaluated for two CADs from the sequences of Figure 2. From its inception until around 6 CAD BTDC, the flame kernels had the same trends for both the fuels. Then, a little difference was observed; this was due to the reduced fuel amount deposited on the intake valves and on the piston surface that influenced the flame propagation. At around 4 to 6 CAD ATDC, flame radius evolution changed dramatically as shown in Figure 5 that shows the outline of the flame front evaluated from the sequences of Figure 2. This was due to approaching the intake valve region. In fact, the heat exchange between the intake ports and the surrounding gas led to the fuel film deposit evaporation. It influenced the composition of the mixture, creating locally fuel-rich zones. The higher fuel amount near the intake valves for gasoline in CV condition induced fuel-richer zones that slowed down the flame front more than in the other conditions.