Analytical Investigation On Exhaust Waste Heat Recovery System Of Vehicle Diesel Engine Based On Organic Rankine Cycle

With the increasing of automobile population and the growing of energy crises, the wasteheat energy recovery of vehicle engine is becoming an effective way to improve enginethermal efficiency, to reduce pollutant emissions and to save energy. In recent years, theorganic Rankine cycle (ORC) technology for waste heat recovery of vehicle internalcombustion engine has been widely studied, and has a broad application prospect.This paper focuses on the waste heat recovery of an internal combustion engine, compareand analyze the performance of three different configurations of ORC system, including: abasic ORC system, an ORC system with a reheater and an ORC system with an internal heatexchanger (IHE). Based on the characteristics of the diesel engine exhaust, the effects ofevaporating temperature and expander expansion ratio on system performance are studied.The results show that, the ORC system with a reheater and the ORC system with an IHE havehigher net power output, but more complicated structure. Therefore, the configuration of ORCsystem should be selected depending on the specific situations.According to the characteristics of vehicle diesel engine under various operatingconditions, a set of ORC system is designed to recover exhaust energy of a diesel engine. Theeffects of evaporating pressure, superheat degree of organic working fluid and the engineoperating conditions on the performance of the ORC system are studied. The net power outputof the system and the thermal efficiency are selected as objectives to optimize the evaporatingpressure under various operating conditions of the diesel engine. The results show that, whenthe evaporating pressure is1.8MPa, the ORC waste heat recovery system has a maximum netpower output and thermal efficiency and lower exergy destruction rate. Heating the working fluid to a superheated state cannot increase the net power output and thermal efficiency of thewaste heat recovery system, but increase the exergy destruction rate dramatically, and increasethe difficulties of manufacturing of the heat exchangers in the system.Based on the distribution characteristics of the waste heat from a vehicle diesel engineunder various operating conditions, the thermodynamic model of the dual loop organicRankine cycle system was established to recover exhaust energy, waste heat from the coolantsystem, and released heat from turbocharged air in the intercooler. The dual loop ORC systemconsists of a high temperature (HT) loop ORC system and a low temperature (LT) loop ORCsystem. R245fa was selected as the working fluid for both loops. The results show that, thenet power output of the LT loop is higher than that of HT loop under the majority of engineoperating conditions. At the engine rated condition, the net power output of the LT loop is17.85kW, and the net power output of the HT loop is10kW. The waste heat recoveryefficiency of the diesel engine-dual loop ORC combined system can reach up to5.47%overthe whole operating range.Finally, a set of vehicle diesel engine waste heat recovery system based on ORC isdesigned and built up aiming at recovering the waste heat from an existing vehicle dieselengine in our laboratory. Through selecting system components, building and examining thetest bench, a preliminary experimental study on vehicle diesel engine ORC system iscompleted.