The rapid depletion of fossil fuels and environmental concerns make alternative fuels such as biodiesel more attractive. Biodiesel has properties comparable to ultra low sulfur diesel (ULSD), however certain properties of biodiesel such as viscosity, calorific value, density and volatility differ from ULSD. These properties strongly affect injection, air-fuel mixing and thereby combustion and performance characteristics of biodiesel in a diesel engine. In order to realize the full potential of biodiesel use in diesel engine certain modifications to engine design and injection system are required. This experimental study aims to optimize the combination of injection timing and combustion chamber geometry to achieve higher performance and lower emissions from biodiesel fueled diesel engine. Experiments were performed using a blend of 20% Pongamia Oil Methyl Ester (POME) by volume in ULSD (B20), in a single cylinder Direct Injection (DI) diesel engine equipped with pistons having Hemispherical and Toroidal Reentrant Combustion Chamber (TRCC) geometries. The test results showed an improvement of 5.64% in brake thermal efficiency, a reduction of 4.6% in brake specific fuel consumption and a 11% increase of oxides of nitrogen (NOx) level for TRCC compared to baseline engine operated with ULSD due to better air-fuel mixing and retarded injection timing. (C) 2012 Elsevier Ltd. All rights reserved.