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 NO X 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 exper-

Partially premixed combustion strategies offer many advantages for compression ignition engines. One such advantage for those operating on diesel fuels is the simultaneous reduction in soot and NOx achievable over a wide range of equivalence ratios. Though often not measured in engine experiments, gaseous H2 is a byproduct of incomplete combustion and can be useful for the regeneration of aftertreatment devices. Correlations for the exhaust concentration of H2, mostly derived from experiments with homogeneous spark ignition engines, indicate that it is emitted either in proportion to CO directly or as a function of a pseudowater gas shift equilibrium constant. In this work, H2 is measured over a range of equivalence ratios in a multicylinder diesel engine operating in a partially premixed low temperature combustion (LTC) mode using both low sulfur diesel fuel and soy-based biodiesel. Biodiesel was found to have the same bulk gas emissions of major species including H2 over the range of equivalence ratio in LTC for a constant load and combustion phasing. It also was found that the experimental H2 concentration was near the value predicted by the equilibrium constant for equivalence ratios greater that 0.85 but was increasingly lower for leaner points.

Partially premixed combustion strategies offer many advantages for compression ignition engines. One such advantage for engines operating on diesel fuels is the simultaneous reduction of soot and NOX achievable over a wide range of equivalence ratios. Though often not measured in engine experiments, gaseous H2 is a byproduct of incomplete combustion and can be useful for the regeneration of aftertreatment devices. Correlations for the exhaust concentration of H2, mostly derived from experiments with homogeneous spark ignition engines, indicate that it is emitted either in proportion to CO directly or as a function of a pseudo-water gas shift equilibrium constant. In this work, H2 is measured over a range of equivalence ratios in a multi-cylinder diesel engine operating in a partially premixed low temperature combustion (LTC) mode using both low sulfur diesel fuel and soy-based biodiesel. Biodiesel was found to have the same bulk gas emissions of major species including H2 over the range of equivalence ratio in LTC for a constant load and combustion phasing. It also was found that the experimental H2 concentration was near the value predicted by the equilibrium constant for equivalence ratios greater that 0.9 but was increasingly lower for leaner points.