Variable geometry turbine design for off-highway vehicle organic rankine cycle waste heat recovery

Although modern ultra-efficient heavy duty diesel engines exhibit thermal efficiencies of well over 40%, a substantial part of fuel energy will continue being wasted as heat in the exhaust system due to the Diesel cycle limitations. Recovering this potential source of energy could increase the overall thermal efficiency of the engine as well as reduce the exhaust gas emissions and the operational cost of the heavy duty diesel engine. Organic Rankine Cycle (ORC) is regarded as a promising candidate technology for transforming exhaust gas waste heat into electricity (or direct power) due to the nature of the untapped source of energy that it allows to be capitalised on, its relative simplicity and small back pressure impact on engine performance and fuel consumption. This study was carried out for an off-highway engine project but its application is generic to heavy duty applications. An ORC model with a radial expander submodel is implemented in a heavy duty diesel engine powertrain, to evaluate the impact of the ORC on fuel consumption and exhaust gas emissions under various engine operating conditions. Finally, the potential benefit of utilizing a variable geometry expander (VGE) in the ORC model is investigated in this study. Compared to its fixed geometry expander equivalent, the VGE shows a wider range of high efficiency operation within the engine operating window and is therefore a promising addition for consideration in ORC application of this type. Notation bsfc Break specific fuel consumption CO2 Carbon dioxide FGE Fixed Geometry Expander ORC Organic Rankine Cycle SCR Selective Catalytic Reduction TC Turbocompounding TEG Thermo-electric generation VGE Variable Geometry Expander WHR Waste Heat Recovery