The Organic Rankine Cycle (ORC) is a thermodynamic cycle for the conversion of heat into electricity. ORC technology is applied to waste heat, biomass and natural heat sources, there is currently over 2000 MW installed capacity worldwide. Triogen is a Dutch company producing standardized ORC modules for the application with gaseous (waste) heat sources with at least 1 MW heat available, at a temperature of ca. 450°C. In this work the application of ORC technology for efficiency enhancement of modern diesel generators (flue gas: 350°C, 5 kg/s) is studied. It is pursued to identify all relevant phenomena possibly influencing the system performance and cost and to assess the significance of their influence on system performance. This is done in order to decide whether or not a phenomenon should be taken into account when selecting a fluid. A steady state off design model is developed for the optimization of a sub-critical ORC system for minimal specific cost. Turbine isentropic efficiency is estimated based on in and outflow conditions using efficiency charts by Astolfi and Macchi. Pressure drop, effectivity and cost of evaporator, recuperator and condenser are evaluated using the Triogen evaporator design model and a shell and tube heat exchanger recuperator and condenser model respectively. The influence of ambient temperature, water temperature and water mass flow on the cooling system cost and power consumption is estimated based on supplier figures. In addition limitations on the diesel engine back-pressure and limitations related to flue-gas condensation are taken into account. A genetic algorithm is applied to optimize the process parameters and heat exchanger geometry for minimum specific costs. From the simulations acetone appears to be the optimal working fluid, which can be explained by its good thermodynamic behaviour and good heat transfer properties in recuperator and condenser due to the relatively high condensation pressure. Turbine inlet pressure appears to be relatively low due to the negative effect of high pressure ratios on turbine efficiency. Ambient temperature as well as evaporator wall- and flue-gas temperature limitations appear to have a diverging influence on the specific cost per working fluid, proving the necessity to evaluate these phenomena properly when selecting a working fluid for a specific application.