Influence of physical and chemical properties of biodiesel fuels on injection, combustin and exhaust emission characteristics in a direct injection compression ignition engine

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Abstract
This paper describes two topics. One is a study of the effects of biodiesel fuel physical properties on injection characteristics and spray behaviour. The study was done via numerical simulation of the injection system and via laser-sheet imaging of the spray. The second topic is a study of the effects of the constituents of biodiesel fuel on combustion and exhaust emission characteristics in a retrofitted compression ignition engine, as well as combustion under high injection pressure. A critical difference between the gas oil and biodiesel fuel properties is the dependence of the bulk modulus on pressure and temperature. Computation results show that at lower fuel temperatures, the injection timing of biodiesel is effectively advanced relative to that of gas oil. This is because at lower liquid temperatures the bulk modulus of biodiesel fuel is higher than that of gas oil; then the rate of liquid pressure rise goes up and the injection timing is advanced. At higher fuel temperatures, there is no difference in the injection timing and the injection pressure between biodiesel fuel and gas oil. Engine measurements showed that at low engine load, the exhaust particulate matter (PM) using biodiesel contained a higher level of soluble oragnic fraction (SOF) than that using gas oil. This result was related to the results of spray visualization. It was found that the spray penetration for biodiesel was shorter than that for gas oil. Thus the air-fuel mixing process was relatively poor for the biodiesel fuel. Substantial fuel-rich mixture was formed at the centre of the combustion chamber, and a portion of this mixture that failed to burn was emitted as SOF. It was found that both the SOF level and the ignition delay increase with a decrease in the fraction of methyl oleate ester in biodiesel fuel.

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