Operating Boundary And Application Evaluation For Exhaust Energy Recovery System Of Internal Combustion Engine Based On Rankine Cycle

Exhaust energy recovery(EER) from internal combustion engine(ICE) based on Rankine cycle(RC) shows particular characteristics such as high temperature, large temperature difference, as well as various operating conditions. Therefore, in this paper, researches were focused on the operating boundary and application evaluation on RC-EER system of ICE. The main conclusions are shown as follows.Distribution law of gradient increase was observed by bench test for exhaust sensible heat ratio under the whole engine operating range, and 3%-8% optimal system efficiency would be contributed for light-duty vehicle engine, based on model prediction. According to heat exchanger experiment using different working fluids, it was found that increasing working fluid mass flow rate would improve evaporator efficiency. Nevertheless, there exists flow rate boundary for overheat available, which would decrease as pressure increases, while the pressure limit would be broader under higher engine load. For the condenser, increasing cooling water flow rate would enhance evaporator efficiency. On the contrary, system would operate in unstable state and evaporator efficiency would deteriorate under low engine load, if the condenser failed to provide enough cooling capacity. Besides, obvious fluctuation of flow rate would be caused by even subtle wave of pressure, with the working fluid temperature in the outlet showing an opposite variation. Stability for flow rate and temperature would be improved when pressure went to stabilization.Investigation was also carried out to explore the combined effect of thermo-physical properties of working fluid and its working parameters on system performance. It could be demonstrated that Ja for specific working fluid would increase with the ratio of evaporation and condensation temperature, and working fluid with low Ja would produce more power output. Working fluid with high boiling point(Tb) would have a lower volume flow rate ratio, higher turbine power density and exergy efficiency. However, working fluid with low Tb had more appropriate size parameter for expander and a boarder pressure range for efficient expansion, while that with high Tb had a better match with heat source temperature profile. Evaporator and expander would account for the main proportion of system exergy loss, and the dominative part would also change with pressure. Among the total evaporator area, the part at preheating stage would take the absolute high percentage, so that turbulence at the side of working fluid with high latent heat in this stage would need to be enhanced, while the heat transfer of exhaust gas side at evaporation stage would require to be intensified. In addition, the heat absorbed by working fluid would be released largely to condenser, leading to much bigger condenser size than evaporator, which would be improved by using working fluid with high Tb.Basically, little difference for system benefits would be found between working fluids with high and low Tb, however, the former shows better thermo-economics, among which water is the only one that could realize net profits in the base payback period. Higher the engine speed would contribute larger power increase rate, as well as more pumping loss, also the exergy efficiency would decrease, and expander size parameter would increase linearly.Finally, application feasibility was discussed on various engine operating conditions for different power levels of conventional vehicle ICE. Alternatively, the heavy-duty diesel engine system would have wider economic region and more potential to extend further through system optimization. In conclusion, Rankine cycle technology would exhibit applicable potential to recover exhaust energy of ICE, when both the benefits and cost-effectiveness are optimized under desired rang.