A regenerative organic Rankine cycle (RORC) system is designed to recover the exhaust heat of a diesel engine, and the influence of the intermediate pressure (the pressures at which the steam is extracted from the expander) on performance parameters such as net power output, thermal efficiency and mass flow rate of the working fluid are analyzed. The organic working fluids under investigation are R245fa and the zeotropic mixture isopentane/R245fa (in a 0.7/0.3 mol fraction). Based on initial calculations of RORC system performance, the intermediate pressure is set to 1.15 MPa for the RORC system when using isopentane/R245fa (in a 0.7/0.3 mol fraction) as the working fluid, and 1.2 MPa when using R245fa as the working fluid. A performance analysis of the RORC system using the two different working fluids is then conducted over the whole operating range of a diesel engine. The results show that the zeotropic mixture isopentane/R245fa (in a 0.7/0.3 mol fraction) performs better. Finally, a combined diesel engine and RORC system is defined to evaluate the performance improvement of such a combined system over the whole operating range. Results show that, for the combined system, a 10.54% improvement in power output and a 9.55% improvement in fuel economy can be achieved at the engine's rated condition. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.