Assessment of particulate matter in exhaust gas for biodiesel and diesel under conventional and low temperature combustion in a compression ignition engine

Particulate matter (PM) from the exhaust gas of a single-cylinder direct-injection compression-ignition engine was investigated by thermogravimetric analysis (TGA), elemental analysis, and transmission electron microscopy (TEM). Two fuels were used: biodiesel derived from waste cooking oil and commercial diesel fuel. Exhaust gas recirculation was applied to implement low temperature combustion (LTC), and the PM emissions of LTC were compared to those of conventional compression ignition combustion. TGA showed that significant mass reduction occurred at a temperature range of 200-420 °C for biodiesel PM in the LTC mode due to desorption of the volatile organic fraction; diesel PM from the conventional combustion mode shows the highest resistance to the desorption within the entire temperature range. Elemental analysis revealed that the weight fractions of hydrogen and oxygen content, of which the volatiles are comprised, are much larger in the LTC mode than the conventional mode. The exposed surface area after the desorption of volatiles and the oxygen group may result in the fast oxidation of biodiesel PM. Particulate matter in the conventional combustion mode contains a large portion of carbon species, in contrast to the LTC mode. The carbon content in diesel PM from conventional combustion could be due to carbonaceous soot particles, because TEM images appeared to be of a highly ordered structure. Using a scanning mobility particle sizer, fewer particles were found to be of the accumulation mode with LTC engine operation than in the conventional combustion mode, which is consistent with the observed low level of smoke emission.