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Thin films of liquid fuel can form on the piston surface in spark-ignited direct-injection (SIDI) engines. These fuel films can result in pool fires that lead to deposit formation and increased hydrocarbon (HC) and smoke emissions. Previous investigations of the effects of piston fuel films on engine-out HC and smoke emissions have been hampered by their inability to measure the fuel-film mass in operating direct-injection engines. In this paper, a recently developed high-speed refractive-index-matching imaging technique is used for quantitative time- and space-resolved measurements of fuel-film mass on a quartz piston window of an optically-accessible direct-injection engine operating over a range of fully-warmed-up stratified-charge conditions with both a high-pressure hollow-cone swirl-type injector and with a high-pressure multihole injector.

High-speed (12 kHz) imaging of combustion luminosity (enhanced by using a sodium fuel additive) has been analyzed and compared to crank angle resolved heat release rates and mass fraction burn profiles in a spray-guided spark-ignited direct-injection (SG-SIDI) optical single-cylinder engine. The addition of a sodium-containing additive to gasoline greatly increases the combustion luminosity, which allows unintensified high-speed (12 kHz) imaging of early partially premixed flame kernel growth and overall flame propagation with excellent signal-to-noise ratio for hundreds of consecutive engine cycles. Ignition and early flame kernel growth are known to be key to understanding and eliminating poor burn cycles in SG-SIDI engines.