<p>This work aims at developing a general model for heat release rate of blends of straight soybean oil and diesel oil to be used in a zero-dimensional modeling of compression ignition engines. This modeling can support the design and analysis phases of electricity generation from straight vegetable oils in internal combustion engines. The difficulties in simulation of these systems are associated with the different combustion behavior of the heavy oils when compared to current diesel engine models. A model for heat release rate is developed and validated with measurements in a compression ignition engine. The model employs a combination of two Wiebe functions modeling the premixed, diffusion and residual phases. The Wiebe function parameters were correlated to Sauter mean diameter, the gas Weber number and the volume fraction of soybean oil, all calculated using the thermophysical properties of the straight soybean and diesel oils. Simulated results concerning to performance parameters, in-cylinder pressure data and heat release rate were compared to measurements in a 14.7&#xa0;kW/2200&#xa0;rpm single-cylinder compression ignition engine fueled with straight soybean oil blends in commercial diesel oil in volume proportions of 50% and 80%. An agreement with a relative deviation between the measured and predicted pressure curves smaller than 10% was found for the compression and expansion strokes. Concerning the heat release rate, the simulation reproduces the experimental data with differences in the range of –5% to 20% for most of the cycle.</p>