Isotopic tracing of fuel carbon in the emissions of a compression-ignition engine fueled with biodiesel blends

Experimental tests were conducted on a Cummins B5.9 direct-injected diesel engine fueled with biodiesel blends. 20% and 50% blend levels were tested, as was 100% (neat) biodiesel. Emissions of particulate matter (PM), nitrogen oxides (NO_x), hydrocarbons (HC) and CO were measured under steady-state operating conditions. The effect of biodiesel on total PM emissions was mixed; however, the contribution of the volatile organic fraction to total PM was greater for higher biodiesel blend levels. When only non-volatile PM mass was considered, reductions were observed for the biodiesel blends as well as for neat biodiesel. The biodiesel test fuels increased NO_x, while HC and CO emissions were reduced. PM collected on quartz filters during the experimental runs were analyzed for carbon-14 content using accelerator mass spectrometry (AMS). These measurements revealed that carbon from the biodiesel portion of the blended fuel was marginally less likely to contribute to PM, compared to the carbon from the diesel portion of the fuel. The results are unlike those obtained in previous tests with the oxygenate ethanol, which was observed to be far less likely contribute to PM than the diesel component of the blended fuel. Analysis of the data suggests that the chemical structure of oxygen-carbon bonds in an oxygenate affects the PM formation process.