Research On The Influence Of Pollutants On The Measurement Accuracy Of Hot Film Flow Sensors

Due to its quick response and high measurement accuracy,the hot-film flow sensor is widely used in the automotive engine EFI system.Although the air is filtered before it flows into the sensor,the particulate matter in the air will be deposited on the surface of the chip after being used for a period of time,which will affect the measurement accuracy of the sensor and cause the error of air-fuel ratio.Studying the influence of contamination on the measurement accuracy of the sensors is of great significance for ensuring the normal operation of the engine,improving fuel economy,saving energy and reducing emissions,and protecting the environment.At present there is no research about the influence of contamination on the measurement output of the sensor.Studying the distribution of contamination on the surface of the chip,analyzing the influence of pollutant shape,size,position and other parameters on the measurement output of the sensor and finding out the main influencing factors have practical significance for improving the measurement output and reducing the emission.This paper introduces the internal structure and principle of the hot-film flow sensor,and deduces the theoretical model of the hot-film flow sensor based on the basic principles of thermodynamics.The PFM725 flow sensor was used to carry out a natural deposition experiment.The distribution of contamination on the surface of the chip was analyzed and the input-output of the sensors in different deposition stages was obtained.Based on the experimental results,the contamination model is simplified,and the simulation model of contamination is established.The influence of contamination's shape,size and position on the flow field and temperature field of the chip surface is studied by numerical simulation.According to the changes of flow field and temperature field to identify the main factors.Different types of contamination models have been established at different locations on the surface of the chip by micro-jet molding.By comparing the experimental results of four contamination models and the numerical simulation results,the influence of the height,position and length of contamination on the output of the sensor is analyzed.The research content of this subject helps to understand the influence mechanism of contamination on the measurement accuracy of the sensor and lays a theoretical foundation for improving the working stability and measurement accuracy of the sensor.