Evaluation of Diesel Oxidation Catalyst Conversion of Hydrocarbons and Particulate Matter from Premixed Low Temperature Combustion of Biodiesel

Premixed low temperature combustion (LTC) in diesel engines simultaneously reduces soot and NO_x at the expense of increased hydrocarbon (HC) and CO emissions. The use of biodiesel in the LTC regime has been shown to produce lower HC emissions than petroleum diesel; however, unburned methyl esters from biodiesel are more susceptible to particulate matter (PM) formation following atmospheric dilution due to their low volatility. In this study, the efficacy of a production-type diesel oxidation catalyst (DOC) for the conversion of light hydrocarbons species and heavier, semi-volatile species like those in unburned fuel is examined. Experimental data were taken from a high speed direct-injection diesel engine operating in a mid-load, late injection partially premixed LTC mode on ultra-low sulfur diesel (ULSD) and neat soy-based biodiesel (B100). Gaseous emissions were recorded using a conventional suite of analyzers and individual light HCs were measured using an FT-IR analyzer. PM emissions data were collected from filter samples taken before and after the DOC using gravimetric analysis, Soxhlet extraction with speciation of extracted HCs, and total organic versus elemental carbon (OCEC) analysis. The previous results were confirmed with LTC of B100 resulting in over an order of magnitude increase in engine-out PM. It was found in the experimental study that 76% of the unburned fuel species responsible for this increase can be converted by a production-type DOC with an inlet temperature of 240°C and gas hourly space velocity (GHSV) of 85,000 hr^-₁. Unfortunately, the remaining unburned biodiesel left unconverted by the DOC still contributed significantly to the PM following dilution. Methyl esters from biodiesel were found to be the primary species contributing to the higher total organic fraction (≻95%) on the PM for biodiesel LTC following a DOC.