The performance of an organic Rankine cycle (ORC) used to recover waste heat from the exhaust of a diesel and a spark ignition engine for electric power generation was modeled. The design elements of the ORC incorporated into the thermodynamic model were based on an experimental study performed at Oak Ridge National Laboratory in which a regenerative organic Rankine cycle system was designed, assembled and integrated into the exhaust of a 1.9 liter 4-cylinder automotive turbo-diesel. This engine was operated at a single fixed-load point at which Rankine cycle state point temperatures as well as the electrical power output of an electric generator coupled to a turbine that expanded R245fa refrigerant were measured. These data were used for model calibration.The model was used to predict the steady-state power output, thermal efficiency, and state point temperatures of the ORC as a function of refrigerant flow rate and engine speed/load for the diesel engine used in the experiments and for a spark ignition engine based on experimental exhaust flow data.Net power of the ORC was predicted to increase linearly with engine speed and load, and the performance characteristics were very similar for the two engines despite the large differences in exhaust temperatures and exhaust mass flow rates characteristic of diesel versus S.I. engines. While the maximum predicted net power output of the ORC was about 11.5 kW for the S.I. engine at high speed and load, for light loads in the range of 2-4 bmep and engine speeds of 2000 rpm and lower, the predicted net power was in the range of 1 kW or less, calling into question its practicality for light-duty vehicles.