An important objective in combustion engine research is to develop strategies for recovering waste heat and thereby increasing the efficiency of the propulsion system. Waste-heat recovery systems based on the Rankine cycle are the most efficient tools for recovering energy from the exhaust gas and the Exhaust Gas Recirculation (EGR) system.The properties of the working fluid and the expansion machine have significant effects on Rankine cycle efficiency. The expansion machine is particularly important because it is the interface at which recovered heat energy is ultimately converted into power. Parameters such as the pressure, temperature and mass-flow conditions in the cycle can be derived for a given waste-heat source and expressed as dimensionless numbers that can be used to determine whether displacement expanders or turbo expanders would be preferable under the circumstances considered.The goal of this theoretical study was to use this approach to analyze waste-heat recovery systems for a heavy-duty diesel engine and a light-duty gasoline engine. Given the different waste-heat rates of these two engines, the relationships between Rankine cycle performance and design aspects such as the expansion ratio and the locations of pinch points in the heat exchanger were evaluated. The calculated values of these parameters were used as inputs in a dimensionless analysis to identify an optimal expansion machine for each case. The impact of varying the working fluid used was investigated, since it had a large impact on the results obtained and provided insights into design dependencies in these systems.