| The Rankine cycle system(RCS)for waste heat recovery of vehicle engines has been regarded as a promising technique to reduce fuel consumption.However,with the implement of the RCS on vehicle engines,where the interactions are strong,complicated and volatile,it is extremely difficult for the overall efficiency optimization during the driving cycles.In this paper,a comprehensive Rankine cycle experimental prototype and simulation model is developed at first.From this,the interactions,called dynamic effect and coupling effect,are discovered,with full consideration of which a Rankine cycle efficiency model is completed.Taking the overall efficiency as the optimizing objective,a two layer control model and corresponding strategy is proposed,aiming at providing a reference for Rankine cycle modeling and control.Firstly,based on the test bench,the Rankine cycle efficiency feature is investigated during engine dynamic variable working conditions,which suggests that the Rankine cycle fuel saving potential should be deteriorated significantly if the engine working condition are volatile,because the Rankine cycle effective working time,the control performance of evaporating pressure and vapor temperature are strongly corrected with the driving cycle features,especially the volatility level of the driving cycle.Higher vehicle speed and less idle condition should have a good performance.Secondly,the power consumption for ORC cooling was discussed,which rises rapidly with the increment of ORC power generation for the maintenance of cooling water temperature.The ORC power generation and cooling power consumption form a profitloss characteristic curve.The curve shows a break-even point and a peak point of net power output.The profit-loss relationship are mainly affected by the condensing pressure.The priority is to increase condensing pressure at high speed and heavy load to arise the heat rejection temperature difference,in order to keep the cooing power consumption from rising too fast.Afterwards,a study is conducted to investigate the possibility and potential to improve the net power output by the exothermic process and endothermic process optimization,also the working fluid selection.Thirdly,in order to obtain a good overall efficiency during driving cycles,a control oriented RCS efficiency model is completed: Based on a foundation model of the first law of thermodynamics,a heat absorption dynamic model and operating modes corrected factor are proposed to describe Rankine cycle dynamic effect;Besides,an empirical cooling power consumption mode is established using a simplified mechanical model to represent the Rankine cycle coupling effect.The approximation accuracy of the simplified efficiency model and detail simulation model is checked under ESC working conditions.The absolute deviation is under 0.2%.Finally,based on the RCS efficiency model,an efficiency optimizing control model and corresponding strategy is proposed,taking the evaporating pressure and condensing pressure as decision variables,and overall efficiency as the optimization objective.According to the more rapid dynamic response of evaporating pressure,the dual optimization problem could be simplified to two single optimization problems sequentially.Besides,a penalty factor of the superheat degree is added to the optimization objective,to prevent the RCS from entering protection mode during the temporary lack of exhaust energy.The control model and strategy is validated under HWFET driving cycle.Results demonstrate that the effective working time percentage could be increased to 94%,the superheat degree could controlled between 5K-15 K,cooling power consumption could be maintained less than 20% of the Rankine cycle power generation,the fuel consumption could be reduced by 2.2%-2.8%.If the Rankine cycle technology is combined with engine smart thermal management,5.7% fuel consumption reduction could be obtained. |
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