The gradual depletion of world petroleum reserves, increases in prices of petroleum based fuels and environmental pollution due to exhaust emissions have encouraged studies to search for alternative fuels. Biodiesel is an alternative diesel fuel consisting of alkyl monoesters of fatty acids derived from vegetable oils. It has been the focus of considerable amount of recent research because it is renewable and reduces the emission of some pollutants. The desirability of developing biodiesel from different tree borne oil seeds and decreasing the dependency on petroleum based fuels has been discussed by many over the last few decades. However some of the important issues like compatibility of biodiesel with the crankcase lubricating oil, thermal stability of lubricating oil with biodiesel usage, changes in physical and chemical properties of lubricating oil with biodiesel etc. have not been sufficiently investigated. This needs to be addressed in order to ensure the long term acceptability of biodiesel in an existing family of diesel engines. In the present work these issues have been addressed. For this purpose engine endurance tests were conducted on CI engines. Two new single cylinder four stroke CI engines were operated for 512 hours each for diesel and 100% biodiesel fuel. The endurance tests were conducted as per BIS 10000 part IX norms. Biodiesel from Jatropha oil was prepared in-house using transesterification process. The sample of lubricating oil was collected through a one way valve connected to the crankcase sump after every 128 hours intervals. Thermograviometric analysis (TGA) was used to evaluate the thermal stability of lubricating oil samples obtained from both the engines. The thermal decomposition of lubricating oil samples were measured as a function of various reaction parameters such as temperature, time and heating rates. This TGA test involves a weight change as the oil was heated. The weight loss data of the sample was logged using the in situ computer. Early decomposition of biodiesel fueled engine lubricating oil was observed as compared to diesel fueled engine lubricating oil. The changes in viscosity of lubricating oil were also monitored during the endurance test and discussed in detail. A higher level of crank case dilution was observed in case of biodiesel as compared to diesel.

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