Simulation And Analysis Of Tarnscritical CO₂Automotive Air Conditioning System Based On Fuzzy Control

With the society increasingly concerned about environmental and energy issues andthe rapid growth of automobile production, the requirements of automotive properties suchas powerful performance, economy, security and comfort is also greatly increased.Automotive air conditioning is one of the key devices, which is effect the comfort mostly.It is subject to greatly attention of many automotive manufacturers, research workers andconsumers. Refrigerant R134a used in automotive air conditioning faces tremendousalternative pressure. However, natural working medium CO₂is considered to be one of themost promising alternate refrigerant materials based on its excellent thermal properties andenvironmental characteristics. As a result, automotive air conditioning system used CO₂asrefrigerant has become a new hotspot in the automotive technological field. The researchand development of automotive air conditioning systems used CO₂must be speeded up inorder to avoid the passive situation of the technology resulted from the new alternativerefrigerants. Especially, enhance the research and development of control system willintroduce the CAN bus technology and accelerate body integration process so as to increasethe content and grade of automotive technical will have extremely important significance.At first, the composition of CO₂automotive air conditioning and refrigeration cycleprinciple was introduced in this thesis, and then describes the mathematical model of thecomponents that consist of gas cooler, evaporator and the internal heat exchanger, whichlays the foundation for the simulation analysis of the subsequent chapters.By using the MATLAB/Simulink, the steady-state parameter model of thecomponents of transcritical CO₂automotive air conditioning was established in this thesis,and then a simulation mode of refrigeration cycle of the transcritical CO₂automotive airconditioning system which consist of the models of components were established. Theentire model is composed of five subsystem model, which are compressor, gas cooler,evaporator, intermediate heat exchanger and expansion valve. Reasonably considered thecoupling relation between each parameter, and the whole model linked with the refrigerantflow, import and export of enthalpy and pressure parameters organically, thus the methodresponse to the performance of the refrigeration cycle system appropriately.The correctness of the established system model was verified by comparing thesimulation value of the established model in several typical conditions with theexperimental data in the test conditions of the society of automotive engineering of America （SAE）. Base on the correct model, the thesis analysis the system performanceimpacted on the compressor speed, gas cooler inlet air side temperature and evaporator inletair side temperature, and also lay the theoretical foundation for the study of the transcriticalCO₂automotive air conditioning parts model.With the rapid development of automotive technology, automotive function becomeincreasing, and accompany with the control system become more complex and so did thecontrol parameters. All of this need the control system was put forward higher requirements.The control effect of traditional control theory is not very satisfactory with thecharacteristics of non-linearity and hysteresis of the CO₂automotive air conditioningprocess. However, fuzzy control does not depend on a precise mathematical model, it hasthe expertise for nonlinear, multi-objective and multi-parameter control system which cangreatly increase the degree of freedom and accuracy and it has the ideal control effect andstrong robustness. The study shows that the fuzzy control is a ideal control strategy forautomotive air-conditioning control system. Considering the characteristics of CO₂automotive air conditioning, fuzzy control theory is applied to the control system. From theperspective of fuzzy controller, the thesis study on control strategy of four technicalparameters that are compressor speed, high-pressure side pressure, expansion valve openingand blower speed, and introduced the realization method of the four parameters of fuzzycontrol detailedly, and designed fuzzy controller of four main technical parameters above.Through the fuzzy control of compressor speed, the system have the best refrigerationcapacity and discharge pressure under different conditions; the system reaches itsmaximum value of COP and the high-pressure side of the CO₂air conditioning system isimproved by adopting different control strategies in different conditions; the high-pressureside of the system reach the target value through the fuzzy control of the opening ofexpansion valve that regulate the refrigerant flow reasonably; the wind speed was regulatedby the fuzzy control of blower speed so that the cabin temperature reaches the set value in ashort period of time and meet the requirements of comfort and economy.At last, the simulation model of the CO₂refrigeration cycle system was built based onMATLAB/Simulink. Compared the simulation with experimental data in MATLAB andthe result shows that the control effect has been greatly improved. But there are somesteady-state errors in the thesis because of the selection of fuzzy control rule with greatsubjective speculation. Subsequent improvement work can apply the PID controller of theclassical theory into the system, so as to compose of fuzzy PID control system and thus theeffect of control can be further improved.