Diesel particulate matter (PM) is classified by the EPA as carcinogenic, with the transportation sector largely responsible these emissions within the United States. Biodiesel (B100) is derived from renewable sources, providing similar chemical composition to diesel fuel and is in the current diesel supply up to 5% across the nation. However, biodiesel has an inherent oxidation issue due to the unique mixture of fatty acid methyl ester (FAME) molecules present in the biodiesel that are not in diesel. Biodiesel oxidation can only be delayed, and the inevitable process results in changes to the original fuel composition that may alter emissions profiles. There have been limited studies on the effect of oxidized biodiesel fuel on PM emissions, and with increasing biodiesel production volumes, it is important to assess due to possible adverse human health effects. In this study, it was hypothesized that the change in fuel composition due to oxidation would lead to lower PM emissions because the presence of more fuel oxygen molecules and secondary oxidation products would enhance self-combustion characteristics.
In this study, PM mass generated from a light-duty diesel engine running on three different fuel types—pure (“neat”) B100 biodiesel, pure B0 diesel, and B20 (20% v/v biodiesel blend with diesel)—was quantified and compared to the PM mass (and concentrations) from repeated emissions testing using artificially oxidized B100 and B20 biodiesel as the fuel source. B100 fuel was heated at 110oC for 5, 10, and 20 hours (“oxidation states” 3, 2, and 1, respectively), verifying the extent of fuel oxidation by building an apparatus (Biodiesel Oxidation Stability Surveyor, BOSS) that quantified the biodiesel fuel’s oxidative stability using a method equivalent to standard methods for determining the biofuel’s induction period. Induction period increased linearly with time spent under the artificial oxidation conditions. A custom, load-based steady-state modal drive cycle was specially developed for emissions testing each neat and oxidized B100 and B20 fuel type in a light-duty diesel engine dynamometer. Observed changes in PM mass with increased fuel oxidation time occurred only for B20 fuel with a 51 ±13% decrease. Fuel properties such as cetane number, biodiesel content, density, and total aromatics were compared between neat and oxidized B20 and B100 samples. Cetane number increased 7% from 66.8 to 71.7 from B100 neat to B100 OX1 (20hrs) and density increased from 0.709g/cm3 to 0.723g/cm3. Chemical analysis of the biodiesel fuels by gas chromatography mass spectrometry (GCMS) quantified individual FAME compounds to determine key species involved in fuel oxidation. B100 FAME concentration widely varied, however, the B20 fuel blend showed that 20 hour artificial oxidation treatment decreased concentrations of the unsaturated FAMEs for C18:3n3, C18:2 cis-9,12, C18:1 (both cis- and trans- isomers) by 41.7 ±3.5%, 33.25 ±8.8%, and 21.9 ±6.9% relative to their initial concentration in the unoxidized fuel, respectively, in general agreement with literature values.
The findings of this study help contribute a better understanding of oxidation effects on biodiesel fuel and link together fuel properties, chemical composition, and particulate emissions whereas most literature excludes detailed analysis of biodiesel fuel composition and associated emissions effects.