The purpose of this research was to investigate the effects of water concentration in a biodiesel nanoemulsion fuel on engine performance and exhaust emissions of a 4-cylinder diesel engine. Biodiesel nanoemulsions containing 5%, 10% and 15% water were used for the engine tests and the results were compared with B5, B20 and certified #2 diesel fuels. Biodiesel nanoemulsions produced lower NOx emissions and soot opacity than B5, B20 and certified #2 diesel fuels. Biodiesel nanoemulsion with 5% water concentration produced engine power and torque values that were similar to the values measured for B5 fuel. Increasing water concentrations in biodiesel nanoemulsions increased the engine brake specific fuel consumption and CO emissions. The rate of NOx reduction was greater than the rate of CO increase when the water concentration in biodiesel nanoemulsions increased from 10% to 15%. These results provided strong evidences on the effects of increasing water concentration in biodiesel emulsions on reducing NOx and soot from a 4-cylinder diesel engine. Emulsified biodiesel fuel is a promising alternative method for reducing harmful emissions from diesel engines without requiring significant engine modifications. (c) 2012 Elsevier B.V. All rights reserved.