Abstract
One way to increase the thermal efficiency of vehicle diesel engines is to recover waste heat by using an organic Rankine cycle (ORC) system. Tests were conducted to study the running performances of diesel engines in the whole operating range. The law of variation of the exhaust energy rate under various engine operating conditions was also analyzed. A diesel engine-ORC combined system was designed, and relevant evaluation indexes proposed. The variation of the running performances of the combined system under various engine operating conditions was investigated. R245fa and R152a were selected as the components of the mixed working fluid. Thereafter, six kinds of mixed working fluids with different compositions were presented. The effects of mixed working fluids with different compositions on the running performances of the combined system were revealed. Results show that the running performances of the combined system can be improved effectively when mass fraction R152a in the mixed working fluid is high and the engine operates with high power. For the mixed working fluid M1 (R245fa/R152a, 0.1/0.9, by mass fraction), the net power output of the combined system reaches the maximum of 34.61 kW. Output energy density of working fluid (OEDWF), waste heat recovery efficiency (WHRE), and engine thermal efficiency increasing ratio (ETEIR) all reach their maximum values at 42.7 kJ/kg, 10.90%, and 11.29%, respectively.
Highlights
Internal combustion (IC) engines consume considerable amounts of petroleum resources
The following three evaluation indexes were proposed for the combined system: the engine thermal efficiency increasing ratio (ETEIR), waste heat recovery efficiency (WHRE), and output energy density of working fluid (OEDWF)
The graph shows that the decrease of the mass fraction of R152a causes the mixed working fluids to change from wet fluids to dry fluids and increases the critical temperature gradually, M4 approximates to the isentropic working fluid
Summary
Internal combustion (IC) engines consume considerable amounts of petroleum resources. The selection of working fluid has an effect on the running performances of the ORC system [19,20]. Wang et al [22] analyzed nine different pure organic working fluids and claimed that R245ca and R245fa are suitable working fluids for an engine waste heat recovery system. Zeotropic mixtures are superior to pure fluids in ORC system performance, but some researchers have considered applying zeotropic mixtures to an ORC system in recovering waste heat from vehicle engines, but we still have a lot of research work to do on selecting suitable zeotropic mixtures for ORC system. We hope that we can provide a useful reference for researchers interested in recovering waste heat from vehicle engine exhaust
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