Vegetable oils demonstrate several problems as fuel for compression ignition engines due to their high viscosity and low volatility characteristics. Viscosity can be brought down by heating or blending them with mineral diesel. In the first part of the present investigations, a novel method of preheating straight vegetable oils with the waste heat of the engine exhaust is presented. Experimental investigations were carried out for combustion, performance, and emission properties of preheated and unhealed jatropha oil in a direct injection engine at different loads at constant engine speed (1500 r/min). Brake specific fuel consumption and exhaust gas temperatures for preheated jatropha oil were found to be higher compared to mineral diesel. Thermal efficiency was lower for preheated jatropha oil compared to mineral diesel. The vegetable oil preheating reduced the CO₂, CO, and HC emissions but increased the NOx emissions. Analysis of cylinder pressure rise, heat release rate, and cumulative heat release was carried out. Preheated jatropha oil shows lower combustion delay but slower heat release rate compared to mineral diesel. Combustion duration for both preheated and unheated jatropha oils is higher than mineral diesel, but heating was found to increase the combustion duration of jatropha oil. Maximum incylinder pressure was observed for mineral diesel for higher load conditions, but at lower load, relatively higher peak cylinder pressure was obtained with preheated jatropha oil. Detailed combustion analysis suggests that preheated jatropha oil gives identical combustion as that of mineral diesel. Hence, waste heat of exhaust gas can be effectively utilized to preheat the jatropha oil for improving its combustion properties as a diesel engine fuel.