| In appendix J of "Limits and Measurements of Pollutant Emission from Light Vehicles(Sixth Stage of China)"(hereinafter referred to as "Chinese ?"),a new regulation for leak detection of evaporative system(hereinafter referred to as EVAP system)has been added.Leak detection of EVAP system can control the part of vapor which Onboard Refueling Vapor Recovery device cannot control,therefore leading to the reduction of the fuel evaporation emission.Therefore,the leak detection of EVAP system is significant.Firstly,this paper analyzed the existing leak detection methods,modules and systems,and summarized some influencing factors which need to be verified.Based on the aforementioned analysis,some assumptions for improving the existing leakage monitoring process and system were put forward.Secondly,the vaccum decay method was chosen to be investigated.By simplifying the vapor convective diffusion process in the EVAP system vapor space,the theoretical model of total pressure change in the vaccum decay stage of the EVAP system vapor space was established based on the principle of molecular diffusion and mass transfer.Total pressure change in the vapor space was calculated by Simulink,and the corresponding mass flow rates of air and vapor in the vapor space were also obtained.Thirdly,Fluent simulation was carried out to analyze the influence of the location of leak hole on the pressure field and flow field in the EVAP system vapor space under different vapor space volume and initial pressure.The preferred leak detection period and sensor location were obtained after analyzing the simulation results.Fourthly,the experimental benches and testing flow were designed based on the Chinese VI standard.These experimental benches and testing flow were utilized to verify the collaborative simulation results and the impact of the influencing factors on the pressure decay in the EVAP system vapor space.Based on the experimental results,the rule of judging the diameter of leak hole in the EVAP system was summarized,and the preferred value setting range of the influencing factors in leak detection was given.Finally,based on the collaborative simulation and experimental results,the leak detection process,leak detection system and module were improved.A multi-period leak detection system was designed by combining the vaccum decay method and the engine-off natural vaccum method.The simulation results show that when the leak hole is located on the upper wall of the EVAP system,the decay rate of the pressure slope is higher than that when the leak hole situates on the side wall of the system;the preferred leak detection time is from 30 s to 60 s,and the pressure sensor should be located on the upper wall of the central fuel tank and near the side wall of the tank respectively.The experimental results show that the diameter of leak hole,the vapor space volume of EVAP system,the initial pressure and the amount of vapor adsorbed in the carbon canister have great influence on leak detection;the maximum pressure-decay rate and the proportion of the points inthe specific decay-rate area to all points in the decay time from 0 to 60 s can be used to judge the leak hole diameter.The followings are innovations of this thesis:(1)Fluent simulation was used to analyze the effect of the location of leak hole on the pressure field and flow field in the EVAP system vapor space during the vaccum decay stage.Preferred leakage monitoring time and the location of pressure sensor were obtained.(2)To validate the effectiveness of simulation and the impact of influencing factors on pressure-decay in EVAP system vapor space,a test procedure based on the "Chinese?" Standard was designed.Based on the analysis of the aforementioned factors,the judgment criteria of the leak hole diameter were summarized for the EVAP system used in this thesis.(3)A multi-period leak detection system was designed by combining the vaccum pressure decay method and the engine-off natural vaccun method,which can be used in idle and parking stages respectively. |
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