| Reducing CO2emission is a critical issue for Chinese energy conservation policy.Improving the thermal efficiency of internal combustion engine is an effective solution.At present, using an organic Rankine cycle (ORC) to recover engine waste heat isconsidered the most feasible technology. However, it is found that there are manytheoretical and practical problems required to be solved. In this thesis, waste heatrecovery of internal combustion engine has been investigated, which included selectionof the working fluid, system structure design of organic Rankine cycle, application oforganic Rankine cycle for automotive engine. An experimental platform of organicRankine cycle was established with a finned-tube evaporator and a single screwexpander, and then a preliminary experiment was conducted. These efforts provide somebasic experiences and data for the subsequent theoretical investigation and the practicalapplication on vehicle. The main research works are summarized as follows:Taking into account the operation conditions of organic Rankine cycle assembled ona running vehicle, the thermodynamic performances of nine pure working fluids withhigh normal boiling points, which were R113, R141b, R123, R245ca, R11, R245fa,R236ea, R114, R600(butane), were compared and analyzed. The results indicate that thethermal efficiencies are very close among these working fluids and the efficiencies ofR11and R141b are a bit higher than that of others. However, when the evaporationpressure and the condensation temperature vary, the feasible working regions of thesenine working fluids are quite different. The feasible working regions of R11, R141b,R123, R245fa, and R245ca are better than others. Subsequently, the secure andenvironmental properties were evaluated. As a result, R245fa is the most suitable one forengine waste heat recovery.The operating characteristics of five various ORCs, which included a simple ORC,an ORC with an internal heat exchanger (IHE), an ORC with an open feed organic fluidheater (OFOH), an ORC with a closed feed organic fluid heater (CFOH), and an ORCwith a reheater, were analyzed theoretically for engine waste heat recovery. Theoptimization mathematical model for each ORC was obtained using a genetic algorithm.The influences of the expander inlet pressure, the condenser outlet temperature, theworking fluid superheated temperature, and the expander isentropic efficiency on themaximum thermal efficiency were estimated. The results show that the ORC with anIHE has a higher thermal efficiency and a lower exergy destruction rates, and is the bestchoice for vehicular application.The performance MAP of a gasoline engine was measured on an engine test benchand the heat quantities wasted by the exhaust and coolant systems were obtained and compared within the engine’s entire operating region. Based on these data, thecharacteristics of a novel system combining a gasoline engine with a dual loop ORCwhich recovered the waste heat from both the exhaust and coolant systems was analyzed.A high temperature loop recovers the exhaust heat while a low temperature loop recoversboth the residual high temperature loop heat and the coolant heat. The workingparameters of a dual loop ORC were defined, and the performance of a combinedengine-ORC system was evaluated across this entire region. The results show that the netpower of the low temperature loop is higher than that of the high temperature loop, andthe relative output power improves by from14%to16%in the peak effective thermalefficiency region and from30%to50%in the small load region, and the absoluteeffective thermal efficiency increases by3~6%throughout the engine’s operating region.The performance MAP of a light-duty diesel engine was created using an engine testbench. The heat waste from the exhaust, the intake air, and the coolant were calculatedand compared throughout the engine’s entire operating region. Based on these data, thecharacteristic of a novel system combining a vehicular light-duty diesel engine with adual loop ORC, which recovered waste heat from the engine exhaust, intake air, andcoolant, was analyzed. A high temperature loop recovers the exhaust heat, whereas a lowtemperature loop recovers the residual heat from the high temperature loop and the wasteheat from both the intake air and the coolant. The working parameters of the dual loopORC were defined, and the performance of the combined engine–ORC system wasevaluated across this entire region. The results show that the net power of the lowtemperature loop is higher than that of the high temperature loop, and the relative outputpower improves from14%to16%in the peak effective thermal efficiency region andfrom38%to43%in the small load region. In addition, the brake specific fuelconsumption (bsfc) of the combined system decreases significantly throughout theengine’s operating region.A finned-tube evaporator was designed to recover the exhaust waste heat of a dieselengine, which could work under the high temperature and pressure conditions. Amathematical model of the evaporator was determined based on the detailed geometryand the specific ORC working conditions. Accordingly, a program was created toanalyze the evaporator performance as the diesel engine running through all of itsoperating regions defined by the engine speed and engine load. The results show that theexhaust temperature at the evaporator outlet increases with engine speed and engine load.Although the convective heat transfer coefficient of the organic working fluid issignificantly larger than that of the exhaust gas, the overall heat transfer coefficient isslightly greater than that of the exhaust gas. Furthermore, the heat transfer rate is thegreatest in the preheated zone and least in the superheated zone. Consequently, the heattransfer area for the preheated zone is nearly half of the total area. In addition, the area ofthe superheated zone is slightly greater than that of the two-phase zone. It is concluded that the heat transfer area for a finned tube evaporator should be selected carefully basedon the engine’s most typical operating region.The experimental platform of ORC system for engine exhaust heat recovery wasbuilt up using a10kW single screw expander and the designed finned-tube evaporator.Subsequently, a preliminary test and discussion were carried out. |
PDF Full Text Request