Effect of Ethanol Addition on Soot Formation of Gasoline in Laminar Diffusion Flames

Soot emission, known as PM (particulate matter), is becoming a big issue for GDI engines as the emission regulations being increasingly stricter. It is found that ethanol, as an oxygenated bio-fuel, can reduce the soot emission when added to gasoline. In order to fully understand the effect of ethanol on soot reducing, the soot characteristics of ethanol/gasoline blends were studied on laminar diffusion flames. In this experiment, the blending ratio of ethanol/gasoline was set as E0/20/40/60/80. Considering the carbon content decreasing due to ethanol addition, carbon mass flow rate was remained constant. The two-dimensional distributions of soot volume fraction were measured quantitatively by using two-color laser induced incandescence technique. The results showed that ethanol is able to decrease the soot significantly, but the effect of ethanol on soot reduction is weakened with the increasing ethanol ratio. Compared with pure gasoline, the average soot volume fraction in the E20, E40, E60, and E80 flames decrease by 41.5%, 66.9%, 81.0%, and 93.8%, and the peak soot volume fraction decrease by 34.7%, 61.7%, 74.0% and 87.8% respectively. For the whole flame, the flame lift-off height and the soot lift-off height increase with increasing ethanol ratio. On flame center line, as the height above burner increases, the soot volume fraction varies in a trend of three stages: first maintain a constant value of 0, then increase to peak value, and finally decrease to 0. The initial height of soot formation on flame center line also increases with increasing ethanol ratio. The radial soot volume fraction decreases with increasing ethanol ratio at different heights above burner. As the flame height increases, the radial peak soot volume fraction moves from the flame outside towards the flame center gradually, and the peak soot increases first and then decreases.