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Investigations were carried out to determine exhaust emissions and combustion characteristics of a conventional diesel engine with different operating conditions [normal temperature and pre-heated temperature] of rice bran oil in crude form and in biodiesel form with varied injection timing and injector opening pressure. Exhaust emissions [smoke and oxides of nitrogen] and combustion characteristics [peak pressure, time of occurrence of peak pressure and maximum rate of pressure] were determined at full load operation of the engine fuelled with diesel, crude rice bran oil and rice bran oil based biodiesel. Comparative studies on exhaust emissions and combustion characteristics were made among diesel, crude vegetable oil and biodiesel operation on diesel engine with varied engine parameters. Smoke levels decreased and NOx levels increased with biodiesel operation. Exhaust emissions and combustion characteristics improved with increase of injector opening pressure, advanced injection timing and preheating of vegetable oil(s).

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It has been found that the vegetable oils are promising substitute, because of their properties are similar to those of diesel fuel and they are renewable and can be easily produced. However, drawbacks associated with crude vegetable oils are high viscosity, low volatility call for low heat rejection combustion chamber, with its significance characteristics of higher operating temperature, maximum heat release, and ability to handle lower calorific value (CV) fuel etc. Experiments were carried out to evaluate the performance of an engine consisting of different low heat rejection (LHR) combustion chambers such as ceramic coated cylinder head-LHR-1, air gap insulated piston with superni (an alloy of nickel) crown and air gap insulated liner with superni insert - LHR-2; and ceramic coated cylinder head, air gap insulated piston and air gap insulated liner - LHR-3 with normal temperature condition of crude rice bran oil (CRBO) with varied injector opening pressure. Performance parameters (brake thermal efficiency, brake specific energy consumption, exhaust gas temperature, coolant load, and volumetric efficiency) and exhaust emissions [smoke levels and oxides of nitrogen [NOx]] were determined at various values of brake mean effective pressure of the engine. Combustion characteristics [peak pressure, time of occurrence of peak pressure, maximum rate of pressure rise] were determined at full load operation of the engine. Conventional engine (CE) showed compatible performance and LHR combustion chambers showed improved performance at recommended injection timing of 27°bTDC and recommend injector opening pressure of 190 bar with CRBO operation, when compared with CE with pure diesel operation. Peak brake thermal efficiencyincreased relatively by 7%, brake specific energy consumption at full load operation decreased relatively by 3.5%, smoke levels at full load decreased relatively by 11% and NOx levels increased relatively by 58% with LHR-3 combustion chamber with CRBO at an injector opening pressure of 190 bar when compared with pure diesel operation on CE