In this study, a compression ignition (CI) engine designed for diesel fuel was operated using biodiesel. No alteration was made to the fuel system or settings of the Cl engine during tests. The changes in engine performance, emissions, and cycle by cycle (CBQ variations were observed, and their causes were studied. When biodiesel was used as the fuel, acceptable changes occurred in the performance values. The maximum brake mean effective pressure (BMEP) obtained with the biodiesel was 16% lower than that obtained with the diesel fuel, with the difference being 7.5% under maximum power. While biodiesel reduced the maximum engine power by 8.6%, it increased the brake specific fuel consumption by 9.6%. A comparison of exhaust emissions showed that CO emissions of biodiesel are lower than those of diesel fuel. The difference between the obtained minimum values was around 70%. In terms of hydrocarbon (HQ emissions, diesel fuel has produced better results than the biodiesel fuel. Biodiesel resulted in higher NOx emissions than diesel fuel when the engine operation range was considered. The difference was about 13-15% in the maximum power region. The coefficient of variation of ignition delay of both fuels had similar characteristics. However, due to the difference in the cetane numbers, the ignition delay period of biodiesel was longer. Changes of maximum cylinder pressure have occurred at the same magnitude for both fuels for the same engine speeds. The coefficient of variation of maximum cylinder pressure for both fuels had similar characteristics and considerably increased under maximum power conditions.