Performance and emission characteristics of compression ignition engines depend strongly on inner nozzle flow and spray behavior. These processes control the fuel air mixing, which in turn is critical for the combustion process. The differences in the physical properties of petrodiesel and biodiesel are expected to significantly alter the inner nozzle flow and spray structure and, thus, the performance and emission characteristics of the engine. In this study, the inner nozzle flow dynamics of these fuels are characterized by using the mixture-based cavitation model in FLUENT v6.3. Because of its lower vapor pressure, biodiesel was observed to cavitate less than petrodiesel. Higher viscosity of biodiesel resulted in loss of flow efficiency and reduction in injection velocity. Turbulence levels at the nozzle orifice exit were also lower for biodiesel. Using the recently developed KH-ACT model, which incorporates the effects of cavitation and turbulence in addition to aerodynamic breakup, the inner nozzle flow simulations are coupled with the spray simulations in a "quasi-dynamic" fashion. Thus, the influence of inner nozzle flow differences on spray development of these fuels could be captured, in addition to the effects of their physical properties. Spray penetration was marginally higher for biodiesel, while cone angle was lower, which was attributed to its poor atomization characteristics. The computed liquid lengths of petrodiesel and biodiesel were compared with data from Sandia National Laboratories. Liquid lengths were higher for biodiesel due to its higher boiling temperature and heat of vaporization. Though the simulations captured this trend well, the liquid lengths were underpredicted, which was attributed to uncertainty about the properties of biodiesel used in the experiments. Parametric studies were performed to determine a single parameter that could be used to account for the observed differences in the fuel injection and spray behavior of petrodiesel and biodiesel; fuel temperature seems to be the best parameter to tune. (C) 2010 Elsevier Ltd. All rights reserved.