Characteristics of combustion and soot formation of ethanol-gasoline blends injected by a hole-type nozzle for direct-injection spark-ignition engines

In this study, the combustion and soot formation processes of mid-low level ethanol-gasoline blends (E0, E20 and E50) sprays injected by a hole-type nozzle were examined in a quiescently high-temperature and high-pressure constant volume vessel by various ignition strategies (e.g. ignition position, ignition timing and spark energy). High-speed imaging of OH* chemiluminescence and two-color pyrometry were employed to clarify the flame propagation and soot formation. The results show that a relatively favorable flame area is potential to achieve for both E20 and E50 at the EOI or slightly after EOI timing. Higher ethanol content blends show more sensitivity to the ignition energy due to the physical properties. The soot level tends to decrease with increasing ethanol content at EOI middle ignition, possibly attributed to the dilution effect of ethanol and the increase of oxygen content. At EOI downstream ignition, however, the soot formation increases in both E20 and E50, possibly as a consequence of the downstream fuel heterogeneity consisting of the inhomogeneous mixture and liquid droplets of highly sooting hydrocarbons. Retarding ignition timing can help to decrease the soot formation. Moreover, E20 shows not only comparably intense combustion to E0 but also less soot formation at EOI middle ignition.