The performance of an organic Rankine cycle (ORC) that recovers heat from the exhaust of a heavy-duty diesel engine was simulated. The work was an extension of a prior study that simulated the performance of an experimental ORC system developed and tested at Oak Ridge National laboratory (ORNL). The experimental data were used to set model parameters and validate the results of that simulation. For the current study the model was adapted to consider a 15 liter turbocharged engine versus the original 1.9 liter light-duty automotive turbodiesel studied by ORNL. Exhaust flow rate and temperature data for the heavy-duty engine were obtained from Southwest Research Institute (SwRI) for a range of steady-state engine speeds and loads without EGR. Because of the considerably higher exhaust gas flow rates of the heavy-duty engine, relative to the engine tested by ORNL, a different heat exchanger type was considered in order to keep exhaust pressure drop within practical bounds. The effectiveness-NTU based model of the shell-and-tube heat exchanger in the original simulation was replaced with direct finite-difference modeling of a counter-flow finned heat exchanger. A full-scale version of the finned heat exchanger was built from aluminum extrusions and tested for pressure drop in the exhaust of a 6.7 liter Cummins turbo diesel, for comparison with model predictions. In the simulation the engine exhaust heated the refrigerant R245fa working fluid which was expanded in an integrated turbo-generator. Optimizing heat exchanger pressure drops with heat transfer rates was found to be essential. The predicted increase in overall engine/ORC thermal efficiency was approximately 5% across the entire speed/load range of the engine. Refrigerant mass flow rates for peak ORC system power were in the range of 0.7 to 1.45 times the engine exhaust flow rate.