| The temperature data of the piston is an indispensable basic data for the study of combustion and heat transfer in the cylinder of IC engine,but it’s very difficult to get it.First of all,the temperature of piston surface is change rapidly.The response speed of the temperature sensor must be fast enough to measure the temperature change on the piston surface,but the traditional temperature sensor doesn’t meet the requirements.Therefore,based on the study of the temperature measurement principle,dynamic characteristics and processing technology of the thermocouple,considering the special requirements of vibration resistance,high temperature resistance,easy to package and install in the piston,this paper designs an axial thin film thermocouple.The experimental results show that the Seebeck coefficient is about 35μV /℃,which shows a good linear relationship with the temperature difference between the cold-end and the hot-end junctions.Secondly,the material and the thermal diffusion coefficient of the axial thin film thermocouple are different from that of the piston.Therefore,embedding the sensor in the piston will change the original heat transfer characteristics of the piston inevitably,resulting in errors.In order to evaluate the temperature measurement performance of the axial thin film thermocouple,response time and measurement accuracy are defined.The heat transfer simulation model of axial thin film thermocouple embedded in the piston is established by ABAQUS software.The influence of material,Structural parameters and thermo-physical parameters of the substrate,the thermocouple wires and the thin film on response time and measurement accuracy is studied systematically.The results show that the substrate material,thermocouple wires material,thin film density and specific heat capacity have little effect on the response time,but the response time increases with the increase of thickness and decrease of thermal conductivity of the thin film.When measuring 42 Cr Mo material piston,J-type thermocouple wire is the optimum choice.In addition,the thermal conductivity of the insulating layer and thermocouple wire material is smaller than that of the piston,which leads to the temperature of the hot-end junction higher.At the same time,the thin film has heat insulation effect on the hot-end junction.The greater the thickness is,the greater the heat insulation effect is.Finding the balance between them can determine the optimum thin film thickness.In this paper,the optimal thickness of the thin film is between 20μm and 50μm.The static error decreases with the increase of the thermal conductivity of the thin film,but has little correlation with the density and specific heat capacity.The dynamic error decreases with the increase of the density of the thin film,and the dynamic error is the smallest when the specific heat capacity and thermal conductivity of the thin film are the same as the piston to be measured.The results can provide theoretical basis for the structural design and optimization of axial thin film thermocouple.Moreover,the piston moves rapidly in a closed cylinder,therefor the transmission of temperature signal is greatly limited.This paper developed a set of piston temperature telemetry system based on Wi-Fi wireless technology.The working temperature can reach 120℃,and the acquisition frequency is 10 kHz.The system can transmit the temperature signal measured to the host computer in real time,and control the slave computer to switch between sleep,wake-up,acquisition and other modes through the host computer,so as to reduce the power consumption of the system. |
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