The effects of injection angle on the combustion characteristics were studied.Two different fuels,diesel and biodiesel,were tested on a DIATA optical engine by performing numerical simulation using the modified KIVA Ⅲ V2.Simulations under different injection conditions were performed and the model was verified by comparing the simulation results with the experimental measurements.The numerical simulations show that lower injection pressure strengthens spray operation,and longer ignition delay and lower peak combustion pressure and peak heat release rate are observed.Injection angle of 110° produces the highest peak combustion pressure and heat release rate for diesel fuel.Soot located in the squish or the regions above the piston bowl are readily oxidized due to abundance of oxygen.Portions of fuel are burnt in the region about the piston bowl or squish for both spray angle of 150° and 70°.Soot located within the piston bowl is oxidized at a much slower rate due to deficient oxygen after combustion.Soot emission mainly due to soot within in the piston bowl at the end of combustion.Any strategy that pushes soot out of piston can improve the oxidization process,thus,reducing soot emission.Extra oxygen in biodiesel also helps in reducing the emission.Soot emissions are reduced from biodiesel blends and this is consistent with the general observation.