Soot formation as a function of flow, flame and mixing field above evaporating fuel films in an optically accessible engine

Abstract

Liquid fuel wall films are a known source of hydrocarbon and soot emissions in direct-injection spark-ignition (DISI) engines. Therefore, a comprehensive understanding of the evaporation, mixing, and combustion processes above wall films is desirable. In this study, laser-induced fluorescence (LIF) of acetone excited at 315nm is used to measure the fuel mole fraction in the gas phase above a wall film in an optically accessible DISI engine. To this end, acetone and 3-pentanone are characterized at excitation wavelengths from 305 to 316nm in a heated jet experiment under atmospheric conditions. It is shown that the excitation of acetone at 315nm results in a signal that is sufficiently temperature-independent under engine-relevant conditions. In addition, simultaneous high-speed particle image velocimetry (PIV) and Mie-scattering capture the flow field and cross-sectional flame development. The formation of soot is characterized by natural luminosity. A late injection of acetone during the compression stroke from a single-hole Spray M injector is used to add approximately 8% of the fuel to the homogeneously premixed isooctane-air mixture and form a fuel film on the piston surface. Heavy soot formation occurs when the engine is operated under cold start conditions. After combustion, incandescent soot structures form and persist until the exhaust phase. These soot structures are attributed to the pyrolysis of the fuel as it evaporates into the oxygen-depleted, high-temperature burnt gas. Increasing wall temperatures during cold-start cycles significantly reduces soot formation. However, even at similar temperature levels, strong variations occur. A multi-parameter analysis revealed a strong correlation of the projected soot area with the flow field at ignition and the acetone mole fraction above the film. It is shown that delayed flame-film contact reduces soot formation since it increases the time for evaporation and promotes mixing of acetone-rich regions. Acetone mole fractions in the bulk flow indicate strong turbulent mixing, with fuel-rich regions contributing to soot formation during combustion being typically limited to within 3 mm of the wall.