Abstract
This study analyzed the variation law of engine exhaust energy under various operating conditions to improve the thermal efficiency and fuel economy of diesel engines. An organic Rankine cycle (ORC) waste heat recovery system with internal heat exchanger (IHE) was designed to recover waste heat from the diesel engine exhaust. The zeotropic mixture R416A was used as the working fluid for the ORC. Three evaluation indexes were presented as follows: waste heat recovery efficiency (WHRE), engine thermal efficiency increasing ratio (ETEIR), and output energy density of working fluid (OEDWF). In terms of various operating conditions of the diesel engine, this study investigated the variation tendencies of the running performances of the ORC waste heat recovery system and the effects of the degree of superheat on the running performance of the ORC waste heat recovery system through theoretical calculations. The research findings showed that the net power output, WHRE, and ETEIR of the ORC waste heat recovery system reach their maxima when the degree of superheat is 40 K, engine speed is 2200 r/min, and engine torque is 1200 N·m. OEDWF gradually increases with the increase in the degree of superheat, which indicates that the required mass flow rate of R416A decreases for a certain net power output, thereby significantly decreasing the risk of environmental pollution.
Highlights
Waste heat recovery has recently become a more efficient and effective method by which to save energy and reduce emissions
When the heat transfer temperature difference between Texh_in and T4 is higher than 10 K, the PPTD occurrence position between engine exhaust and zeotropic mixture scarcely appears at State Point 4 in Figure 2
When the engine operating condition is determined, the heat transfer temperature difference between Texh_in and T4 decreases with the increase of degree of superheat, which means that the probability of PPTD occurrence position between the engine exhaust and zeotropic mixture being at State Point 4 in Figure 2 increases with the increase of degree of superheat
Summary
Waste heat recovery has recently become a more efficient and effective method by which to save energy and reduce emissions. From the previously mentioned analysis, we can conclude that the running performance of the ORC system can be improved when a zeotropic mixture is used as working fluid or when an internal heat exchanger (IHE) is used. The use of ORC is an effective method by which to recover waste heat from IC engines and is helpful for improving the thermal efficiency of such engines. Based on the first and second laws of thermodynamics, in terms of various operating conditions of the diesel engine, the variation tendencies of the running performances of the ORC waste heat recovery system were studied, variation of occurrence position of the pinch point temperature difference (PPTD, ∆Tpp) between diesel engine exhaust and zeotropic mixture R416A was investigated. The effects of the degree of superheat (Tsup) on the running performances of the ORC waste heat recovery system were analyzed
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