| Carbon dioxide emissions from road transport account for 38.2%of the total emissions of China’s transportation industry.Diesel engines as one of the major power devices of road transportation,face significant carbon reduction pressure.Further improving the thermal efficiency of heavy-duty diesel engines has become an important method to reduce carbon in the short and medium term.To achieve ultra-high thermal efficiency,improving the process of turbocharging,combustion,valve timing,and aftertreatment,can further increase the thermal efficiency of the engine to more than 52%.Meanwhile,the exhaust temperature is also significantly reduced,which brings challenges to the design of waste heat recovery(WHR)systems utilizing the organic Rankine cycle(ORC).Therefore,it is required to explore the ORC system design for high-efficiency and low-temperature diesel engines and the comprehensive optimization of diesel engines.Taking the Weichai WP10H diesel engine as the research object,the exhaust temperature at the lowest fuel consumption point dropped from 391℃ to 310℃ after applying a variety of technologies to improve thermal efficiency.Under the same working conditions,the circulating temperature of the coolant increased from 80℃ to 95℃,and the circulating heat flow decreased by 25.2%due to the application of the electronic water pump and some heat-insulating cylinder liners.According to the change of waste heat source characteristics of the WP10H diesel engine,the thermodynamic model of the WHR system was constructed,and the key influencing factors of recovery potential after the overall decrease of waste heat flow and quality were analyzed.Aiming at the high-temperature heat exchanger that affects the efficiency and economy of the WHR system at the same time,a thermodynamic model of fin-tube heat exchanger was constructed.Based on the heat transfer coefficient calculated by 3D fluid simulation,the correction parameters of heat transfer coefficient in the design of heat exchanger with small fin distance were proposed,the results of empirical formula calculation were corrected,and the error of the heat exchanger model was less than 5%.Using the high-efficiency diesel engine exhaust and coolant as the heat source of the WHR system,a variety of working fluids suitable for 310℃ exhaust temperature and 95℃ coolant circulating temperature,were selected for comparative analysis.The high-temperature WHR system with exhaust gas as the heat source,adopts the cyclopentane recuperation scheme.The scheme can effectively recuperate the working fluid waste heat and achieve a 27%efficiency improvement compared to the traditional ethanol WHR system.The low-temperature WHR system with engine coolant as the heat source selected R1233zd,which has a 2.6%efficiency increase than the R245fa WHR system.In addition,cyclopentane systems with low condensing pressures can achieve expansion ratios of up to 30 with piston expanders,while R1233zd,with expansion ratios limited by evaporation and condensation parameters,can be applied with scroll expanders with an efficiency of about 85%.R1233zd has a low evaporation temperature,and when used as a high-temperature working fluid of high-efficiency diesel engine WHR system,the coolant preheating scheme can achieve slightly higher output than that of cyclopentane recuperation scheme,but the total output of the WHR system with the high-temperature loop and low-temperature loop subsystem were applied in parallel,and the total output of cyclopentane recuperation scheme was about 25%higher than that of R1233zd preheating scheme.Aiming at the characteristics of exhaust temperature decrease and coolant circulating temperature increase of high-efficiency diesel engines,a serial WHR scheme was designed with coolant as the priority heat source and exhaust as the supporting heat source,to achieve higher output at a lower cost.The serial WHR scheme with R1233zd as the working fluid has a significantly higher power output when recovering the waste heat of the high-efficiency diesel engine than other single-loop schemes,and can also reach 85.1%of the maximum power output by the cascade heat exchange scheme.To consider the system efficiency and system cost,the energy production cost(EPC)model was constructed based on the bare module cost method,and the parameter optimization of the high-temperature heat exchanger and the economic optimization of the system scheme were both carried out.The thermodynamic model analyzed the power recovery capacity of five refrigerants as working fluid in high-temperature or low-temperature subsystems,and the cost model analysis of four system schemes with complete exhaust and engine coolant energy recovery.The result showed that for a high-efficiency diesel engine,the R1233zd serial WHR scheme was significantly better than other schemes in terms of economy.The power recovery cost is only 70.1%of the cascade heat exchange scheme.According to the influence of the engine aftertreatment system upstream of the WHR system,the steady-state heat management strategy was optimized.The analysis of the catalyst activity of the aftertreatment system shows that in the exhaust temperature range of 300-350℃,the catalyst of the aftertreatment system works with high efficiency.Meanwhile,the exergy efficiency of the high-temperature waste heat recovery system calculated by the thermodynamic model of the waste heat recovery system also peaked in this temperature range.The analysis of exhaust temperature changes under different intake throttle valve opening and post-injection strategies showed that with 30-40%intake throttle valve opening and thermal management strategy of appropriate post-injection quantity,the exhaust temperature at 250-300℃could be increased to or close to this temperature range,with the fuel consumption rate increase controlled within 1%.For working points in this exhaust temperature range,the optimized thermal management strategy significantly increased the power output of the cyclopentane recuperation exhaust WHR system,with limited fuel consumption increased,and the total thermal efficiency of the diesel engine,engine aftertreatment system,and WHR system,was improved by 1.9%.The thermal management strategy of the aftertreatment system has little influence on the coolant waste heat flow and quality,and the thermal management strategy that can effectively improve the power output capacity of the coolant WHR subsystem is to increase the coolant circulation temperature.The optimization of the above thermal management strategy significantly reduced the capital cost of the parallel and cascade WHR system scheme,and the EPC was decreased by about 13%on average.The improvement of the R1233zd serial WHR scheme was limited.However,after the optimization of the thermal management strategy,the R1233zd serial WHR scheme was still the economically optimal scheme for a high-efficiency diesel engine WHR system. |
