Waste heat recovery using fluid bottoming cycles for heavy duty diesel engines

A typical long-haul heavy duty Diesel engine currently rejects up to 50% of the total fuel energy in the form of heat. Due to increasing CO2 emissions and fuel costs, there is a growing interest in techniques that can even partially utilise this wasted resource to improve the overall system efficiency. Fluid Bottoming Cycles (FBC) including Rankine and organic Rankine cycles offer one means towards converting waste heat into usable power. This thesis investigates the potential of FBCs to improve the net power of two computationally modelled (Ricardo WAVE V8.1) 10 litre engine platforms operating at Euro 6 emission levels. The heat to power conversion potential of a FBC largely depends on the selected working fluid, its associated cycle operating mode and the system architecture. Firstly, a detailed systematic methodology for the selection and evaluation of pure working fluids was developed and applied using an advanced chemical process modelling tool (Aspen HYSYS V7.3). Using cycle and fluid fundamentals, screening criteria, and ranking indices, the methodology identified ethyl iodide, methanol, R30, acetone, R152 and E152a as the most suitable fluids amongst the 1800 synthetic, organic and inorganic fluids. Secondly, by varying the expansion inlet parameters, simulations were conducted using 10 pure, dry, isentropic and wet working fluids. The aim was to reduce cycle irreversibilities, highlight the significant sensitivity and performance results, provide directions for practical implementation, and offer new opportunities in energy conversion. For the low, medium and high thermal boundary conditions respectively, liquid expansion (E152a), low pressure limited superheat expansion (methanol, R30, acetone) or dry supercritical expansion (R152), and high pressure limited superheat expansion (using the high temperature organic fluids) were identified as techno-economic optimum. These optimal ORC operating modes achieved efficiencies 65-77% of the theoretical cycle limits. Finally, 13 combinations of thermal and sub-system architectures were methodically analysed and classified in terms of their level of complexity, average system power and relative size. To provide tailored solutions, the pure working fluid methodology was additionally adapted to examine over 750 water blends and 700 organic blends. Aqueous blends of 3-Methyl-1-Butanol and 1-propanol were found to be best suited to the dual pressure and the dual cycle systems. Furthermore, the ethanol-toluene blend was preferred for the high temperature recuperated cycle. The dual cycle system (aqueous blend and E152a combination) showed the maximum potential and produced an average of 7.5% of additional engine crankshaft power.