Investigating the Effects of Biodiesel Composition on Emissions from a Compression Ignition Engine

For biodiesels to be considered as replacements for conventional fossil fuels, they must be shown to produce cleaner emission profiles than existing fuels. When evaluating the environmental footprint of alternative fuels, the production of harmful pollutants, such as NO_x, vapor-phase hydrocarbons, and particulate matter (PM) are considered. However, a biodiesel’s exhaust composition depends strongly on its fatty acid methyl ester (FAME) content. Depending on the feed stocks used, FAME content can vary significantly among biodiesels, resulting in different physical properties that affect combustion and pollutant production.

To examine the effects of FAME content on exhaust composition, six fuel blends were prepared by mixing coconut and canola biodiesels in various proportions. The blended fuels were then combusted using a single cylinder, Yanmar compression ignition engine, while emission data was recorded. The chemical make-up of coconut and canola biodiesels are quite different; while coconut biodiesel contains mostly short, saturated FAMEs, canola biodiesel contains longer, unsaturated FAMEs, allowing the effects of FAME content and associated physical properties on fuel performance and emissions to be observed. Ultra-low-sulfur diesel (ULSD) was tested as a control to normalize the injection timing of each fuel blend to that of ULSD, and to compare performance and emission results with those of a conventional fuel.

The emission results were very nonlinear. At both 50% and 100% engine loads, NO_x levels peaked at around 80% - 90% canola biodiesel. Vapor-phase hydrocarbon emissions were lowest using a blend of 50:50 coconut to canola biodiesel at 50% load, while no change between blends was observed at 100% load. Particulate matter levels were unusually low across all fuel blends for both loads. The non-linear behavior of emission production with respect to biodiesel blending suggests the possibility of using coconut biodiesel as an additive to improve the emission profiles of better-performing biodiesels.