Fuel Composition Effects on Particulate Formation from GDI Spray Collapse During Engine Warm Up: Article No. 141304

Understanding the mechanisms that control soot formation in modern direct-injection spark-ignition engines is necessary to mitigate engine tailpipe particulate matter (PM) emissions, particularly at cold-start conditions and during the engine warm-up period. Past work has shown that elevated PM emissions are correlated with the presence of sooting flames both in the bulk gas and as strong pool fires on the walls of the combustion chamber. Understanding the role that fuel properties, fuel composition, and engine operating conditions play for spray-wall interactions, film formation and soot formation is critical to develop new fuel-engine solutions. In this work, experiments were performed in an optical engine and spray vessel to assess the impact of fuel properties, fuel composition, and engine operating conditions on spray morphology and soot formation via Mie scattering and natural luminosity imaging. Engine-out particulate number (PN) and particulate mass (PM) emissions measurements were made simultaneous to the optical measurements. A baseline regular-grade E10 (10%vol ethanol) research gasoline was blended with variable amounts of cyclopentane and with a light, low-soot gasoline to vary both soot propensity and physical properties, and coolant temperature sweeps were performed at different injection timings and with single and multi-injection strategies. Results indicated that soot formation was controlled by two different mechanisms: pool fires resulting from strong fuel-wall interactions, which dominated at early injection timings, and fuel-rich flames near the spark plug, which dominated at late injection timings. When soot formation was controlled by pool fires, increasing coolant temperature or fuel volatility both led to lower soot emissions. However, starting from conditions where soot formation was controlled by rich regions near the spark plug, increasing coolant temperature or fuel volatility led to higher soot emissions because the spray tended to collapse more, reducing the vaporization rate, increasing the liquid penetration and, therefore, increasing the formation of fuel films and soot. Multi-injection strategies have shown to be an effective approach to decreasing soot emissions at all conditions tested. A 1-D spray model modified to account for spray collapse effects on liquid penetration was used to understand the engine results and to develop a phenomenological model of soot formation for DISI engine combustion at spray-collapsing conditions.