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Prediction of dynamic Rankine Cycle waste heat recovery performance and fuel saving potential in passenger car applications considering interactions with vehicles’ energy management

Horst, Tilmann Abbe, Tegethoff, Wilhelm, Eilts, Peter, Koehler, Juergen
Energy conversion and management 2014 v.78 pp. 438-451
automobiles, cooling, cooling systems, dynamic models, electric power, emissions, energy, energy use and consumption, fuels, fuels (fire ecology), heat, highways, prediction, weight loss
Waste heat recovery (WHR) by means of a Rankine Cycle is a promising approach for achieving reductions in fuel consumption and, as a result, exhaust emissions of passenger car engines. To find the best compromise between complexity and fuel saving potential, methods for predicting the WHR performance for different system configurations and stationary as well as dynamic driving scenarios are needed. Since WHR systems are usually not included in today’s car concepts, they are mostly designed as add-on systems. As a result their integration may lead to negative interactions due to increased vehicle weight, engine backpressure and cooling demand. These effects have to be considered when evaluating the fuel saving potential.A new approach for predicting WHR performance and fuel saving potential was developed and is presented in this paper. It is based on simple dynamic models of a system for recovering exhaust gas waste heat and its interfaces with the vehicle: the exhaust system for heat input, the on-board electric system for power delivery and the engine cooling system for heat rejection. The models are validated with test bench measurements of the cycle components.A study of fuel saving potential in an exemplary dynamic motorway driving scenario shows the effect of vehicle integration: while the WHR system could improve fuel economy by 3.4%, restrictions in power output due to the architecture of the on-board electric system, package considerations, increased weight, cooling demand and exhaust gas backpressure lead to a reduction of fuel saving potential by 60% to 1.3%.A parameter study reveals that, in addition to weight reduction and efficiency optimization, combining the WHR system with enhanced electrification of engine peripherals is the most effective approach to improve fuel saving potential. When assuming an increase in power demand of the on-board electric system from 750 to 1500W, a fuel saving potential of 4% – referring to a 3.6% higher reference fuel consumption – is reached. WHR could therefore play an important role to overcome the challenges of increased electric power demand in future vehicles.