Biodiesel, an alternative diesel fuel consisting of the alkyl monoesters of fatty acids from vegetable oils and animal fats, can be used in existing diesel engines without modification. However, property changes associated with the differences in chemical structure between biodiesel and petroleumbased diesel fuel may change the engine's injection timing. These injection timing changes can change the exhaust emissions and performance from the optimized settings chosen by the engine manufacturer. This study presents the results of measurements of the speed of sound and the isentropic bulk modulus for methyl and ethyl esters of fatty acids from soybean oil and compares them with No. 1 and No. 2 diesel fuel. Data are presented at 21 ± 1°C and for pressures from atmospheric to 34.74 MPa. The results indicate that the speed of sound and bulk modulus of the monoesters of soybean oil are higher than those for diesel fuel and these can cause changes in the fuel injection timing of diesel engines. Linear equations were used to fit the data as a function of pressure, and the correlation constants are given.Biodiesel has come to be defined as the alkyl monoesters of fatty acids from vegetable oils or animal fats. It can be used as an alternative fuel for diesel engines. Many researchers have reported that oxides of nitrogen (NOx) increase, and particulate matter, carbon monoxide, and unburned hydrocarbons decrease when biodiesel is used in heavy-duty diesel engines (1,2). The reason for the higher NOx emissions is not currently understood but could be due to changes in the chemical composition and the physical properties of the fuel. A major difference between petroleum-based diesel fuel and biodiesel is the 10-11% oxygen content of the biodiesel. The oxygen could change the stoichiometry of the combustion process in a way that produces more NOx. Biodiesel also generally has a higher cetane number than petroleum-based diesel fuel. The cetane number is a diesel fuel property that quantifies the fuel's readiness to autoignite. Higher cetane number corresponds to a shorter ignition delay after the fuel is injected into the cylinder. This results in earlier combustion timing, which tends to increase NOx, and less of the fuel participating in the rapid combustion of the fuel that has premixed during the ignition delay period, which will reduce NOx. These two effects partially offset each other, but higher cetane number in petroleum-based diesel fuels is generally observed to reduce NOx (3). The contribution of biodiesel's higher cetane number to NOx production is still not understood. Changes in physical properties such as viscosity, speed of sound, and bulk modulus also may contribute to higher NOx levels. Injection system anomalies, such as longer injection duration, higher injection pressure, and early injection have been reported by other researchers (4-6).Diesel engines operate by compressing fuel to a high pressure and injecting it into the cylinder where the fuel spontaneously ignites. The fuel is compressed with a piston ...
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