Vegetable oils are a promising substitute for diesel fuel because their properties are similar to those of diesel. They are renewable and can be easily produced. Rudolph Diesel, the inventor of the diesel engine that bears his name, experimented with fuels ranging from powdered coal to peanut oil. Investigations were carried out to evaluate the performance of a conventional diesel engine with different operating conditions [normal temperature and pre-heated temperature] of waste fried vegetable oil in crude form and biodiesel form with varied injection timing and injector opening pressure. Raw oil, containing higher amount of Free Fatty Acids (FFA) (greater than 10 wt%) collected from local restaurants of Hyderabad was converted to biodiesel in two-stage method so as to reduce FFA to 0.52% wt%. Performance parameters (brake thermal efficiency, exhaust gas temperature and coolant load), exhaust emissions (particulate matter (PM) and oxides of nitrogen (NOx)) were determined at various values of brake mean effective pressure of the engine, while combustion characteristics (peak pressure (PP), time of occurrence of peak pressure (TOPP) and maximum rate of pressure (MRPR)) were determined at full load operation of the engine fuelled with diesel, crude vegetable oil and biodiesel. Comparative studies on performance were made among diesel, crude vegetable oil and biodiesel operation on compression ignition engine with varied injector opening pressure and injection timing. The performance of the engine improved with advanced injection timing and at higher injector opening pressure with test fuels. The optimum injection timing was 31°bTDC with conventional engine with diesel and biodiesel operations, while with crude vegetable oil, the optimum injection timing was found to be 32°bTDC.This paper was divided into i) Introduction, ii) Materials and Methods, iii) Results and Discussion, iv) Summary, Research Findings, Future Scope of Studies, followed by list of References consulted.