Research On High-pressure Gas Jet Characteristics Of Direct Injection Natural Gas Engine Based On Visualization Technology

In order to respond the increasingly stringent emission regulations and the challenge of energy structure transformation,natural gas is widely used as alternative fuel for internal combustion engines.However,widely used natural gas engine technology cannot conform the strict emission regulations while having better dynamic and economical,therefore,the high-pressure natural gas direct injection technology has attracted widespread attention.In order to use the HPDI technology to achieve in-cylinder stratified combustion to improve dynamic and economical and reduce emissions,it is necessary to achieve precise injection timing-ignition advance angle control,similarly fuel stratification in the cylinder,the distribution in the fuel cylinder and the quality of air-fuel mixture are must to ensure excellent state.So,it must be comprehensive grasp of macro-development features and characteristics of the concentration distribution of the high-pressure gas jets.However,related simulation studies faced diffculties due to the lack of boundary condition setting and results comparison reference data,at present,it must rely on visual testing technology.This article first built a jet system with a pressure of upto 31 MPa,then facilitates the use of PLIF and schlieren method to carry out a comprehensive study on the characteristics of high-pressure natural gas jets,and combines the micro-concentration and macro-development characteristics to analyze the jet development.First,according to the test requirements,a high-pressure jet and supply system,a PLIF test system and a calibration system,and a shading method test system are established in this paper.The calculation and calibration method based on mass flow rate was selected as the PLIF calibration method for this test,and the system accuracy was ensured in many aspects such as parameter setting,optical equipment position relationship,and test operation.In the study of the development characteristics of the high-pressure gas injection concentration field,the development law of the concentration field is divided into three periods: the initial period,the concentration fluctuation period,and the concentration stabilization period.And the jet concentration field developed into the stable period is divided into four regions: the core shockwave region,the concentration expansion-decline region,the concentration fluctuation region and the low concentration diffusion region.Then the researchsummarizes the influence of the jet pressure and respective development laws of the average concentration of jet fluid,the core shockwave region and the concentration fluctuation region.And it was found that the delay of the pressure compensation caused by the choking phenomenon has a great impact on the characteristics of jet concentration,and its effect is proportional to the jet pressure.And due to the high-pressure expansion of the compressible fluid,the turbulence of the jet fluid and the ambient air is weak,and only obviously exists in the low-concentration diffusion zone,therefore,reducing the intake delay and enhancing the turbulence of the jet are of great significance to improve the performance of the direct injection natural gas engine.In the research on concentration gradients,it was found that the concentration gradient of the core shockwave region increased from 1.53 mg/(m L·mm)to 1.89 mg/(m L·mm),the concentration gradient of the concentration expansion-decline region increased from 0.0435mg/(m L·mm)to 0.0497 mg/(m L·mm),the overall jet concentration gradient increased from0.0752 mg/(m L·mm)to 0.1468 mg/(m L·mm).In the concentration fluctuation zone,the axial concentration gradient is still fluctuating after 0.6 ms and decreases significantly at 1.0 ms,the lateral concentration gradient shows an overall upward trend after 0.6 ms,and there is no obvious relationship between the two and the jet pressure.The above analysis points out that Increasing the jet pressure can effectively enhance the mixing efficiency of air,which is beneficial to the formation of mixed gas,and that mastering the high-pressure jet concentration gradient distribution characteristics has practical value for related simulation research and in-cylinder combustion control.In the study of the macroscopic characteristics of high-pressure gas injection,the penetration distance,the estimated value of the initial velocity,the decline rate of the development speed,the development law of the height of the Mach disc,and the influence of the jet pressure on them are analyzed and summarized.Finally,it is verified that the schlieren method has good similarity with the data obtained by PLIF.By comparing the development law of the jet fluid average concentration with the penetration distance,it is concluded that the working-pressure gas in the initial stage directly affects the development of the penetration distance.By comparing the height development data of the Mach disk with the averageconcentration development data of the shock core region,it is found that after the shock core macrostructure is stable,the core area gas continues to be replenished until the inflow and outflow gas flow reaches a dynamic equilibrium.The above conclusions have revealed the concentration field and macroscopic development rules and characteristics of high-pressure gas jets,expounded the effects of jet pressure and various phenomena,and are helpful for related simulation research and accurate control in the cylinder.