Liquid-Phase Penetration under Unsteady In-Cylinder Conditions: Soy- and Cuphea-Derived Biodiesel Fuels Versus Conventional Diesel

Accelerated dilution of engine lubrication oil with unburned fuel is a significant potential issue when utilizing fuels that contain biodiesel. Biodiesel produced from some feedstocks is less volatile than conventional diesel, which makes wall impingement of liquid fuel more likely and reduces the ability of the fuel to evaporate out of engine oil once dilution has occurred. These are growing concerns, particularly with the emergence of strategies that involve injection of fuel into relatively cool, low-density, in-cylinder gases. Examples of these strategies include early direct-injection and late-cycle post-injection, which facilitate partially premixed low-temperature combustion and aftertreatment system regeneration, respectively. A quantitative understanding of liquid-phase penetration for biodiesel fuels is needed to help mitigate these potential issues. This work reports liquid penetration lengths measured in an optical engine under time-varying in-cylinder conditions for soy- and cuphea-derived biodiesel fuels (soy methyl esters = SME and cuphea methyl esters = CuME, respectively) and a commercial ultralow-sulfur diesel (ULSD). Experiments included laser light scattering for measurement of the liquid length and cylinder-pressure data acquisition for heat-release analysis. Data were acquired for multiple injection pressures, intake-manifold pressures, and injection timings for each fuel. SME and CuME both were found to have similar to 20-30% longer liquid lengths than U LSD, despite a large concentration of higher-volatility components in CuME. Compared to ULSD and SME, however, CuME liquid lengths were more dependent on injection timing. Results suggest that early direct-injection and late-cycle post-injection strategies may benefit from the use of CuME vs SME.