Condensational growth is not typically assumed to be significant compared with adsorption for conversion of unburned hydrocarbons in the exhaust of diesel engines to the particulate phase. However, when partially premixed low temperature combustion (LTC) modes designed to simultaneously reduce soot and NOX emissions are implemented, unburned hydrocarbon (UHC) concentrations in the exhaust are an order of magnitude higher than for conventional combustion modes, increasing the likelihood of gas to particle conversion by condensation. In this work, two LTC operating conditions are compared with conventional diesel combustion using a multi-cylinder direct-injection diesel engine using low-sulfur fuel, a soy-based biodiesel and a 50% by volume biodiesel blend. Gaseous emissions of unburned hydrocarbons were measured and particulate samples were taken using a partial-flow dilution tunnel. Gravimetric analysis of the collected filters, Soxhlet extraction of particulate and speciation using GC-FID was performed for all operating conditions. Elemental carbon (EC) emissions were measured using a thermal optical analyzer and particle size distribution was analyzed using a differential mobility spectrometer. For increasing biodiesel concentration in the fuel, mass emissions of both EC and UHC decreased for all combustion modes compared with petroleum diesel. However, for biodiesel use in LTC modes of operation, particulate mass significantly increased following exhaust dilution. Low vapor pressure methyl esters found in the exhaust of biodiesel LTC increases heterogeneous condensation onto soot particles in the exhaust compared with unburned species from petroleum diesel fuel operation. A model estimating this condensation mechanism accurately predicts the experimental findings of increased mass of particulate for biodiesel operation.
Bibliographical noteFunding Information:
This research was primarily funded by the General Motors, University of Michigan Collaborative Research Laboratory. The authors wish to thank P. Szymkowicz and the Diesel Engine Systems Group at General Motors Global Research and Development for their support of the work. The authors also wish to acknowledge the US-EPA National Vehicle and Fuel Emissions Laboratory for allowing access to particulate filter weighing facilities. S. Batterman and S. Chernyak from the School of Public Health at the University of Michigan were instrumental in conducting OCEC analyses and concentrating extracted samples in support of the research.