Organic Rankine cycle (ORC) engines in waste-heat recovery applications experience variable heat-source conditions (i.e. temperature and mass flow rate variations). Therefore maximising the ORC system performance under off-design conditions is of key importance, for the financial viability and wider adoption of these systems. In this paper, the off-design performance of an ORC engine with screw expander and two heat exchanger (HEXs) architectures is investigated, while recovering heat from an internal combustion engine (ICE). Firstly, nominal system sizing results indicate that the screw expander isentropic efficiency exceeds 80%, while the plate HEXs (PHEXs) heat transfer area requirements are 50% lower, than the respective ones for double pipe (DPHEX) design. Next, the ORC engine operation is optimised at part-load (PL) ICE conditions. Although, the HEXs heat transfer coefficients decrease with part-load, the total HEX effectiveness increases, due to higher temperature difference across the working fluids. Findings also reveal that the PHEX performance is less sensitive to the off-design operation. Off-design power output maps indicate that the optimised ORC engine PL reduces to 72%, for ICE PL of 60%, while ORC engines with PHEXs generate slightly more power, for the same heat source conditions. Overall, the modelling tool developed can predict ORC performance over an operating envelope and allows the selection of optimal designs and sizes of ORC HEXs and expanders
