The concerns about clean environment, high oil prices, and stringent emission norms are the driving forces for the research on alternative fuels. Extensive investigations have been carried out in recent years on replacing diesel fuel by vegetable-oil-derived biodiesel fuel in compression ignition engines, mainly owing to its renewable source, reduced emissions, and direct usage without any engine modifications. The experimental work reported here has been carried out on a turbocharged, direct injection, multi-cylinder truck diesel engine using diesel fuel and neat karanja-oil-derived biodiesel under varying speed and load conditions. The results of the experimental investigation indicate that a maximum of 2.3°CA (crank angle) advance in dynamic fuel injection timing is observed with biodiesel compared with the diesel fuel. The ignition delay is lower for biodiesel compared with diesel fuel at all engine speeds. The peak cylinder pressure is higher at most of the operating conditions and the maximum rate of pressure rise is lower at low speeds and higher at high speeds for biodiesel compared with diesel fuel. In general an earlier start of combustion and lower combustion duration are observed for biodiesel relative to diesel fuel. The maximum thermal efficiency occurring at the maximum torque speed of 1400 r/min is observed to be 40.7 per cent and 40.5 per cent for diesel and neat biodiesel respectively. A significant reduction in unburnt hydrocarbon and smoke emissions, comparable carbon monoxide emissions, and higher nitric oxide emissions are observed with biodiesel compared with diesel fuel operation. In general, an improvement in combustion characteristics and exhaust emissions, except nitric oxide, are observed for biodiesel compared with diesel fuel, with a slight penalty in brake thermal efficiency.