Effects of long-term aging on neat soybean biodiesel combustion and emissions

Biodiesel is widely used in the transportation industry, with up to 20% blends with Ultra Low Sulfur Diesel (ULSD) being commonplace. However, its molecular structure can lead to aging issues due to oxidation and related effects. This study examines approximately seven-year aged biodiesel in a high compression ratio (21.2) engine. Compared to ULSD, aged biodiesel combustion had a significantly muted premixed burn event with a resulting increase in diffusion-controlled combustion to achieve the required load. Typical biodiesel combustion trends relative to ULSD were found, and a zero-dimensional heat release analysis demonstrated equivalent in-cylinder temperatures with a reduced rate of temperature rise. Degradation of the aged biodiesel increased brake specific fuel consumption by an average of 8.4% over fresh biodiesel. For aged biodiesel, both nitrogen oxides (NOx) and particulate matter (PM) decreased with load initially before adhering to the NOx-PM tradeoff. On average, NOx emissions of aged biodiesel were 16.7% lower than fresh biodiesel, reflecting its more gradual combustion process. The oxidation of aged biodiesel also reduced carbon monoxide and total hydrocarbon emissions relative to fresh biodiesel. The relatively high compression ratio of the engine promoted complete combustion even with the degraded fuel.