Particulate and gas phase hydrocarbon emissions from partially premixed low temperature compression ignition combustion of biodiesel

The research presented in this document examines the results of melding three diesel engine emissions reduction methodologies: partially premixed low temperature combustion (LTC) the use of alternative, biodiesel fuel and aftertreatment using a diesel oxidation catalyst (DOC). It shows how alternative fuels and novel combustion strategies complement each other on one hand and create new emissions challenges on the other.Partially premixed LTC simultaneously reduces soot and NOx emissions for both biodiesel and petroleum diesel fuels. The use of biodiesel in LTC has added benefits of lowering total hydrocarbon (THC) and CO emissions and reducing soot emissions to near undetectable levels. Light hydrocarbon species like ethylene emitted from biodiesel LTC as a fraction of THC are higher independent of ignition delay indicating that biodiesel burns more completely and results in less unburned hydrocarbon (UHC) emissions than petroleum diesel. However, the generally higher gas-phase UHC emissions from LTC compared to conventional combustion results in excessive particulate matter (PM) for biodiesel due to heterogeneous condensation of methyl esters onto soot particles after dilution with atmospheric air. In the work presented here, this condensation process resulted in over an order of magnitude increase in PM emissions for B100 in a late injection LTC condition (LLTC) compared to petroleum-derived fuels. For an early injection LTC (ELTC) condition, PM emissions were almost 100 times higher than the diesel fuels tested. Low vapor pressure methyl esters making up biodiesel have a near 95% conversion from the gas to the particle phase with an undiluted exhaust UHC concentration of 1000 ppm for a 10:1 dilution ratio and 47°C collection temperature.Although the use of biodiesel in LTC increases PM emissions significantly following dilution of the raw exhaust, the results of this work indicate that 80% of UHC in the exhaust is oxidized by a standard DOC with inlet temperature of 240°C. Unfortunately, the remaining unburned biodiesel left unconverted still significantly contributes to the PM following dilution. Methyl esters were found to be the primary species contributing to the higher total organic fraction (>90%) on the PM for biodiesel compared with diesel LLTC following a DOC.