Investigation On The Measurement System Of Fuel Injection Rate Applied To GDI Injector

In order to reduce vehicle emissions and improve fuel economy,more and more automobile manufacturers began to choose direct injection technology.Compared with traditional PFI engine,the gasoline direct injection(GDI)engine can achieve lower fuel consumption,higher power output,and faster response speed.However,GDI engine also has its disadvantages,such as difficulty in air-fuel mixing and high requirement for injection control technology.The injection rate of direct injection injector directly affect the speed of atomization,evaporation,air-fuel mixing and subsequent emission of combustion.Therefore,the accurate measurement of fuel injection rate is of great significance for the design of direct injection combustion system.Most of the existing measuring methods rate are applied to the diesel injector,and the equipment suitable for the measurement of the injection rate of the direct injection gasoline engine is relatively rare.Therefore,in this thesis,a measurement system of injection rate for GDI injector was designed and built by taking the Zeuch’s method as the measuring principle.The system was composed of fuel supply system,control system of fuel injection,main body of measurement and system of signal acquisition and processing,and finally realized integration and software.The system has the following features: small size of the measuring body,good filtering effect,adjustability of fuel temperature,etc.Therefore it’s showed that system was suitable for different types of GDI injector.In the measurement of fuel injection rate applied to the GDI injector,a repeatability experiment was first carried out to ensure the reliability of the measurement system.The result showed that the dispersion of injection rate was less than 4.2%,while the full opening stage of needle valve was less than3%,which indicating the reliability of the system.Secondly,taking into account the measurement accuracy of the system,three affecting factors,including gas content,container stiffness and volume,were experimentally and numerically investigated and the influence of the three factors on the measurement of the injection rate is systematically studied.And the following conclusions can be achieved: 1)Gas content can make the injection rate smaller than the actual value,and the curve fluctuate larger,decay faster or even distort.2)When the injection mass was the same,the low-stiffness container’s volume was more susceptible to pressure rise,resulting in the decrease of the actual fuel compression,the smaller increment of pressure,the relative low curve of the injection rate;3)The injection rate measured in container with small volume rose faster in the early stage of injection,and the maximum instantaneous injection rate was increased.But the larger the injection mass,the greater the fluctuation.Therefore,in terms of different influence laws,this thesis separately put forward prevention and correction methods to reduce the impact on accuracy measurement and improve the accuracy of measurement.Finally,the measurement system of fuel injection rate was applied to the traditional low pressure GDI injector(20MPa)and a new high pressure GDI injector(35MPa)respectively.The thesis analyzed the stability of fuel injection under short pulse width,variations of injection rate under different conditions(injection pressure,pulse width and fuel temperature).And compared the new injector with the traditional GDI injector.The results were as follows: 1)The decrease of injection pulse width led the reduction of the stability of fuel injection,and when the pulse width is less than 0.5ms,the stability decreased more rapidly;2)As the injection pressure got higher,the injection rate was also higher.And the opening and closing speed of the injector’s needle valve was faster.However,the high pressure injection increased the Reynolds number of fuel flow in the injector,which increased the volatility of injection rate;3)The pulse width of injection mainly affected injection’s stability;4)The increase of fuel temperature decreased the corresponding injection rate,and the injection rate fluctuated greatly due to the unstable flow in the injector;5)The new injector has improved the response characteristics of the needle valve.In addition,when the fuel injection is the same,the injection pressure of the new injector is higher,which was beneficial to speed up the atomization and evaporation of fuel,the air-fuel mixing process and improved the combustion efficiency of the engine.