A conceptual framework for waste heat recovery from compression ignition engines: Technologies, working fluids & heat exchangers

In recent years, the awareness about sustainable energy production using renewable energy sources has increased drastically due to the depletion of fossil fuels and the implementation of environmental protection measures. The effective energy conversion rate of conventional combustion engines stands at 35% whereas the rest of the energy gets dissipated as waste heat to cool the engine. Several Waste Heat Recovery (WHR) methods have been developed so far such as the Organic Rankine Cycle (ORC), thermoelectric generator (TEG), Stirling engine (SE) and the Electric turbo-compounding (ETC). Various research works have been conducted so far to enhance the efficiency of combustion engines through innovative heat recovery systems and by reducing the energy losses, especially in automotive applications. The novelty of the current review article lies in analyzing different aspects of the WHR technologies in terms of increasing the fuel economy and environmental compliance in diesel engines from the perspectives of technology and application feasibility. Further, the current study is also an initiative to present a comprehensive review of different criteria regarding working fluid selection, heat exchanger parameters and their optimization. The results of this article infer that waste heat can be used in an attractive way to produce additional power efficiently, which in turn can result in maximum overall efficiency and optimal usage of the waste energy through the implementation of an exhaust heat recovery system. The current study also recommends that the fuel economy of compression ignition engines can be significantly improved by paying special attention to innovative heat exchanger designs, different parameters of the heat exchangers and working fluid selection.