The Effect of Timing and Oxidation on Emissions from Biodiesel-Fueled Engines

Abstract

The alkyl monoesters of fatty acids derived from vegetable oils or animal fats, known as biodiesel, are attracting considerable interest as an alternative fuel for diesel engines. Biodiesel-fueled engines produce less carbon monoxide, unburned hydrocarbons, and particulate emissions than diesel-fueled engines. However, biodiesel has different chemical and physical properties than diesel fuel, including a larger bulk modulus and a higher cetane number. Some of these properties can be affected by oxidation of the fuel during storage. These changes can affect the timing of the combustion process and potentially cause increases in emissions of oxides of nitrogen. The objective of this study was to evaluate the effect of injection and combustion timing on biodiesel combustion and exhaust emissions. A John Deere diesel engine was fueled with two different biodiesel fuels, one of which had been deliberately oxidized, and with their 20% blends with No. 2 diesel fuel. The engine was operated at three different timings and two loads at a single engine speed of 1400 rpm. The engine performance of the biodiesel was similar to that of No. 2 diesel fuel with nearly the same thermal efficiency. The range of injection timings studied produced changes of 50% and 34% in the CO and HC emissions, respectively. A reduction in NO<emph subscript=yes>x</emph> emissions of 35% to 43% was observed for a 3° retarded injection timing compared with a 3° advanced injection timing. A common linear relationship was found between the start of injection and the NO<emph subscript=yes>x</emph> emissions for all the fuels studied. When compared at the same start of combustion, the neat biodiesel produced lower NO<emph subscript=yes>x</emph> emissions than the No. 2 diesel fuel.