| With the rapid development of automobile industry,the consumption of petroleum energy is increasing day by day.But the energy conversion efficiency of traditional automobile internal combustion engine is limited,and a part of energy is lost to the environment in the form of heat.The goal of vehicle energy saving and emission reduction can be realized by the recovery and utilization of engine waste heat energy.In this paper,the organic Rankine cycle waste heat recovery technology was presented for a heavy-duty diesel engines of commercial trucks.A compound cycle system of waste heat recovery and air conditioning refrigeration was proposed to explore the performance of different cycle methods.The laboratory prototype system of organic Rankine cycle,with a piston expander employed as the heat-work conversion and power output device,was independently constructed to verify the feasibility of the system and evaluate the working state and energy recovery effect of the system under different operating parameters.In order to further study the internal working process of the system,the influence rules of working medium state parameters to the performance of core components were discussed and the correlations among system parameters were analyzed by combining experiment and simulation.For real vehicle application scenarios,the system components and recycling methods were optimized and designed.The system performance and fuel saving ability under different exhaust energy conditions were explored to identify the key factors and control strategies of the system.Detailed contents and conclusions are as follow:(1)The characteristics of heavy commercial truck diesel engines were measured and the changes of exhaust temperature,exhaust flow and exhaust components were analyzed.The exhaust energy accounted for 26.1%to 48.1%of the total fuel energy which was sufficient for recycling.However,the fluctuation was strong.(2)The compound cycle system of waste heat recovery and air conditioning refrigeration was proposed to realize the functions of waste heat recovery and air conditioning refrigeration simultaneously.The system can be switched between independent air-conditioning refrigeration mode,independent organic Rankine cycle waste heat recovery mode and compound cycle mode.The performances of subcritical and supercritical cycles,compound and independent systems of different working medium were studied by using Aspen Plus software.The results showed that the increase of evaporation pressure can improve the output power,but the evaporator of supercritical cycle needed a larger heat transfer area.When the same working medium R134a and R1234yf were selected,the compound system can effectively reduce the total heat transfer area and improve the work capacity compared with the independent system.However,the system output power was higher when R245fa working medium was adopted.(3)The laboratory prototype system of organic Rankine cycle was independently constructed consist of plate evaporator,fin type condenser,plunger type working fluid pump,radial piston type expander,related accessories and various test equipment.R245fa was selected as the circulating working fluid of the system.The system worked well and continued to output power during the test.The influence of different operating parameters on the power capacity of system was analyzed by using the test platform.The working medium pump speed was one of the key operating parameters of the system.The optimum system performance can be achieved with appropriate pump speed.When the exhaust gas energy increased,the pump speed corresponding to the maximum power point of the expander increased accordingly and the work capacity of the expander chanced correspondingly,up to 279W.In the test condition,the maximum output power of expander can be obtained by the moderate initial charging amount of working fluid.According to the analysis of the first and second laws of thermodynamics,the maximum thermal efficiency and exergy efficiency of the system output power can reach up to 2.02%and 10.5%,respectively.Most of the energy loss was caused by the evaporator and condenser.Although the energy loss of the expander and the working medium pump was small,the quality was much higher,so their proportion of the total exergy loss in the system was significantly increased.(4)The changing rules of the component performance with the working medium state were studied based on the test results and the GT-SUITE one-dimensional simulation platform.The effect of working fluid temperature on the actual working capacity of unit working fluid was obvious.The maximum value was6.03kJ/kg at the highest temperature point in the test range.With the increase of working medium pressure,the friction torque of the expander increased from2.44Nm to 6.38Nm,and the mechanical efficiency decreased,which limited the power output of the expander.The pressure of working medium in the heat exchangers showed a strong correlation with the heat transfer and the working quality,and the heat transfer process in the heat exchanger has an interaction with the working medium state,so the actual heat transfer process can be better reflected by iterative calculation.(5)Based on the performance of organic Rankine cycle and the correlation of parameters,further research on the optimization of key components for real vehicle applications was carried out to analyze the performance improvement effect of component size parameters and working medium state.The results showed that the flow rate of working medium was an effective method to adjust the energy recovery rate of exhaust gas.With the increase of working medium flow rate,heat absorption and system pressure of working medium increased continually.The increase of the heat transfer area of evaporator can effectively improve the heat transfer power,but the increase rate gradually slowed down and was not conducive to the miniaturization of the system.When the heat transfer area is 6.9m~2,the system can recover 92.7%of the available exhaust energy.Working fluid mass in cycle was a sensitive factor of system pressure,but it was no obvious to heat absorption of working fluid.An axial seven cylinder piston expander was proposed to increase the displacement and expansion ratio of the expander.Timing of gas distribution showed a significant effect on the expansion state of working fluid in the cylinder of the expansion.Too late intake and too long intake angle would cause incomplete expansion and unable to make full use of working fluid energy.On the contrary,there would be overexpansion,resulting in increased exhaust negative work.At the same time,due to the influence of working medium inertia on exhaust,it was necessary to ensure a certain exhaust advance angle.The expansion rate of working medium in cylinder can be used to evaluate the expansion effect of working medium.The expander performed better when the value was in the range of 80-90%.In the working medium state parameters,the increase of flow rate and inlet temperature can improve the work capacity of the expander.Proper inlet pressure was beneficial to the expansion machine for increasing output power,but too high pressure would increase the friction loss.(6)The recovery effect of waste heat under different exhaust energy of engine ESC working conditions was studied and the change rule of optimal operating parameters of the system with exhaust energy was clarified.The basic organic Rankine cycle could reduce the engine fuel consumption by 3.2%.In the C100condition with sufficient exhaust energy,the maximum net output work of organic Rankine cycle reached up to 8.52kW.The energy loss of the system was mainly caused by the heat dissipation of the condenser,while the biggest exergy loss was caused by the heat transfer of the temperature difference between the exhaust gas and the working medium in the evaporator.The recovery effect of waste heat can be improved by means of regenerative organic Rankine cycle which reduced the fuel consumption rate by 3.5%.Besides,the recovery range of exhaust gas energy can be extended to B25 condition.In addition,the heat absorption of working fluid in evaporator and regenerator were complementary,which can reduce the sensitivity of system performance to working medium flow rate.The optimal operating parameters of the system correspond well to the exhaust energy of the engine.With the increase of exhaust gas energy,the flow rate of working fluid should be increased to ensure the heat absorption capacity of working fluid.At the same time,due to it was easy for the system to set up enough evaporation pressure,the demand for working fluid mass in cycle was correspondingly reduced. |
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