An experimental investigation was carried out in an automotive diesel engine with 5.59 kW rated power in order to analyse the effect of soya soap stock based acid oil biodiesel on engine's performance, combustion, and emissions characteristics at maximum torque and power conditions. The measured physico-chemical properties show the acid oil biodiesel is mainly composed of 72.3% unsaturated fatty acids and it meets the desired fuel quality for automotive diesel engine. Carbon mono oxide, unburnt hydrocarbon and Smoke emissions with acid oil biodiesel decreased significantly however oxides of nitrogen emission increased in the range from 12% to 38%. In addition to this, power output also dropped marginally and thermal efficiency decreased. The peak in-line fuel pressure and in-cylinder temperature are higher with acid oil biodiesel than base diesel. The results emerged from this study indicate the acid oil biodiesel can be a potential fuel for automotive compression ignition engines. (C) 2017 Elsevier Ltd. All rights reserved.