Study On Wetted Fuel Film Characteristics Based On Constant Volume Test System

The technology of in-cylinder direct injection has potential advantages in improving the fuel economy of gasoline engines and reducing exhaust emissions.It can cover the entire gasoline engine working area by using a homogeneous mixed gas combustion mode and a stratified lean combustion mode.The research on the spray characteristics of the direct injection gasoline engine is one of the key steps in the design of the gasoline direct injection engine.The spray impingement affects the formation of the mixture in the cylinder,combustion and emission.On the one hand,the spray impingement is beneficial to the secondary crushing of the fuel droplets and improving the evaporation and atomization of the fuel;on the other hand,the spray impingement produces the wetted fuel film,and the part of the fuel film is difficult to participate in the combustion,thereby increasing the hydrocarbon emission.At the same time,the fuel film may lead to pool fire and increases the amount of particles.Therefore,this paper will study the characteristics of the fuel film produced by the GDI injector spray impingement process.Design and build a constant volume test system.It mainly includes the design of constant volume device,injector control and high speed camera control system,high pressure fuel supply system,ambient temperature and wall temperature adjustment system.Through dSPACE,precise control of injection pulse width and control of high speed camera are realized.The construction and commissioning of the entire experimental platform ensure the stability of the experimental platform.The thickness of the fuel film was measured by the diffuse reflection method.The principle is that when the angle of incident light is constant,diffuse reflection will occur on the rough metal surface.When the oil film exists on the metal surface,a part of the incident light will be specularly reflected.A change in the intensity of the reflected light enables measurement of the thickness of the oil film.The calibration process selects n-dodecane as the calibration solution,and thecalibration solution is added to the fixed area by the micro-sampler,and the image is recorded by the high-speed camera.After the diffusion of the calibration liquid is completed,a uniform film is formed,and the recording is finished.In order to reduce the error of the experiment,the volume of each calibration solution was repeated 5 times,and the final average was taken to calculate the final result.The calibration curve function was obtained using the Curve Fitting Tool toolbox in MATLAB.The image processing program was written by using Python-based Opencv software.The oil film area and fuel film thickness information in the image were extracted by reading the image,removing the image background,perspective exchange and oil film characteristic parameter extraction.The calibration image was extracted by Canny algorithm.At the edge,calculate the spread area of the calibration solution.The spray impingement test was carried out using the constant volume test system designed in this paper.The effects of fuel injection quantity,fuel injection pressure,distance of impingement,ambient temperature and wall temperature on the characteristics of the wetted fuel film were studied.From the analysis of the thickness and distribution of the fuel film and the fuel rate,the following conclusions are drawn:(1)The influence of the amount of fuel injection on the characteristics of the wetted fuel film shows that the thickness of fuel film,the fuel film area and the fuel rate increase with the increase of the fuel injection amount.(2)The influence of the injection pressure on the characteristics of the wetted fuel film shows that as the injection pressure increases,the thickness of fuel film,fuel film area and fuel rate decrease gradually.(3)The influence of the impingement distance on the characteristics of the wetted fuel film shows that as the impingement distance increases,the thickness of fuel film and the fuel rate decrease gradually,and the fuel film area does not change much.(4)From the influence of temperature on the characteristics of the wetted fuel film,it can be seen that with the increase of ambient temperature and wall temperature,the thickness of fuel film,fuel film area and fuel rate are significantly reduced.