| The high pressure direct injection(HPDI)technology of natural gas realizes the diffusion combustion of natural gas.Compared with premixed combustion,the fuel combustion efficiency is significantly improved and the engine has good economic and emission performance on the premise of maintaining the same power performance as the diesel engine.Therefore,HPDI technology is currently the key to fully exploit the advantages of natural gas as a low-carbon fuel.However,in small cylinder diameter engines,the dual fuel concentric biaxial needle injector is the final actuator of diesel and natural gas injection system,and its internal complex hydraulic,pneumatic and electromagnetic coupling influences lead to the uncertainty of its operating characteristics.The coupling influences between parameters have a significant influence on the circulation injection mass and the influence mechanism is complex and variable.The complex injection strategy of HPDI engine further influences the circulation injection volume fluctuation significantly.The fluctuation of cycle injection gas mass not only reduces the economy of natural gas engine and the stability of vibration and noise control,but also reduces the consistency of emission.Therefore,accurate measurement of the transient mass flow rate of the concentric biaxial needle,ensuring the consistency of the injector and reducing the fluctuation of the circulation injection gas volume are urgent problems to be solved by the current high pressure direct injection gas technology.The main research work in this paper is as follows:To accurately measure the coupled injection characteristics of diesel and natural gas in a dual-fuel concentric biaxial needle injector,a method of the same field measurement of the injection law and the injection law is proposed.Based on the principle of momentum method,the injection law is measured.The injection pattern and single injection quantity are measured simultaneously by the momentum method and volume method to obtain the gas injection law with high signal-to-noise ratio,and the accuracy of the proposed test method is verified.The test error of the initial time of oil injection and air injection and the end time of oil injection and air injection is less than 0.113 ms,and the volatility of oil injection and air injection is 1.4% and2.9%,respectively.To study the pressure fluctuation characteristics of the injector caused by diesel and natural gas injection,the pressure variation of the injector caused by single injection,single injection and diesel-natural gas injection mode was explored.The influence of diesel injection on the gas injection rate is fully studied,and the correlation between the injection volume and the characteristic value of diesel inlet pressure is explored.Aiming at the fluctuation characteristics of multiple gas injection cyclic injection volume under the influence of pressure fluctuation,the fluctuation regularity of post injection and previous injection gas volume during main postinjection and pre-main injection was studied respectively,and the fluctuation mechanism of cyclic injection mass under the two injection modes was revealed.The calculation model of water hammer pressure wave of HPDI injector is established,and the model of water hammer compression wave is calculated,which has a good fit with the experimental results.Changing the hydraulic capacitance of the rail pressure part of the model reduces the oscillation frequency and amplitude,which is conducive to reducing the influence of pressure fluctuation.By studying the influence of the length of high-pressure tubing and the volume of the pressure regulating chamber,it is proved that lengthening the length of highpressure tubing will delay the time for the expansion wave generated by pressure relief of the control chamber to reach the rail end,thus increasing the maximum lift of the needle valve and increasing the air injection volume.The peak value of pressure fluctuation can be effectively reduced by setting a pressure regulator chamber at the entrance of the injector,thus reducing the influence of fuel injection on the gas injection mass.When the mass of the pressure regulator chamber is 15 m L,the gas injection mass fluctuation decreases from 33.1% to 11.8% under the condition of short pulse width,and the fluctuation value is only 2.6mg. |
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