Dilution sensitivity of particulate matter emissions from reactivity controlled compression ignition combustion

Dual-fuel reactivity-controlled compression ignition (RCCI) combustion can yield high thermal efficiency and simultaneously low NOX and soot emissions. Although soot emissions from RCCI is very low, hydrocarbon emissions are high, potentially resulting in higher than desired total particulate matter (PM) mass and number caused by semivolatile species converting the particle phase upon primary dilution in the exhaust plume. Such high organic fraction PM is known to be highly sensitive to the dilution conditions used when collecting samples on a filter or when measuring particle number using particle sizing instruments. In this study, PM emissions from a modified single-cylinder diesel engine operating in RCCI and conventional diesel combustion modes were investigated under different dilution conditions. To investigate the effect of the fumigated fuel on the PM emissions, 150 proof hydrous ethanol and gasoline were used as low reactivity fuels to study the relative contribution of fumigant versus directly injected fuel on the PM emissions. Our study found that PM from RCCI combustion is more sensitive to the variation of dilution conditions than PM from single fuel conventional diesel combustion. RCCI PM primarily consisted of semi-volatile organic compounds and a smaller amount of solid carbonaceous particles. The fumigated fuel had a significant effect on the PM emissions characteristics for RCCI combustion. Hydrous ethanol fueled RCCI PM contained a larger fraction of volatile materials and were more sensitive to the variation of dilution conditions compared to the gasoline fueled RCCI mode.