For better understanding, model development and its validation of in-cylinder soot formation processes of Gasoline Direct Injection (GDI) engines, crank-angle-resolved mass and size distribution of in-cylinder soot during a GDI combustion cycle were investigated via optical measurements and total cylinder sampling technique in an optically accessible Rapid Compression and Expansion Machine (RCEM). A direct-injection, spark-ignited and single-shot combustion event was achieved in the RCEM operated with engine speed 600 rpm, compression ratio 9.0, equivalence ratio 0.9 and natural aspiration. A three-component (iso-octane 65%, n-heptane 10%, toluene 25%) gasoline surrogate fuel and a multi-hole injector shared within the Japanese SIP Innovative Combustion Technology research program were used. As for the optical measurements, two-color method and laser/LED-based Diffused Back Illumination (DBI) high-speed imaging through sapphire windows on the cylinder head and the flat-top piston provided time-sequential in-cylinder soot mass. As for the total cylinder sampling, filter gravimetry and Portable Aerosol Mobility Spectrometer (PAMS) measurements of total-cylinder soot-laden gas provided crank-angle-resolved in-cylinder soot mass and size distribution. The total cylinder sampling was realized by replacing the cylinder head window with a stainless-steel diaphragm, rupturing the diaphragm at an arbitrary crank angle during combustion and rapidly expanding the total-cylinder soot-laden gas to effectively freeze secondary reactions and agglomeration of soot particles. The in-cylinder soot mass obtained by the above mentioned four different methods showed reasonable agreement each other both in increasing trend during combustion and quantitative soot mass. Measured variation of soot size distribution during combustion indicates that formation and agglomeration of soot are simultaneously occurring during combustion. Observation and morphology analysis of sampled soot via High-Resolution Transmission Electron Microscopy (HR-TEM) are also in progress.
