| In recent years,with the exhausted of global energy and environmental pollution problems becoming increasingly serious,it is extremely urgent for the engine industry to find new ways to meet the fuel economy while reducing emission standards.The fuel injection law has a direct impact on the combustion process and emissions of the engine.For this reason,research and application of new fuel injection laws has become a breakthrough in the direction of engine development and energy-saving measures.With the development of electronic control technology,new fuel injection laws such as segmented injection have been increasingly studied and applied to diesel engines.The results show that multiple injections can optimize the combustion process,reduce PM and NO_x emissions,that is improving its trade-off relationship,and it is also an effective means to reduce combustion noise.However,for natural gas engines,especially diesel direct-injection natural gas engines,there has been little research on multiple fuel injections.The study of a small amount of its fuel injection laws is focused on the single injection of diesel and natural gas,by adjusting the mass ratio of diesel and natural gas,diesel injection timing,injection pulse width,EGR rate and other parameters,to study its the impact of overall performance.Based on 6L190 diesel/natural gas engine made in Shandong Shengli Power Machinery Limited company,this paper applies AVL-FIRE software,namely the numerical simulation method,proposing to carry out the principle of split injection of natural gas(including natural gas main-pre injection laws and main-post injection laws)for research on the influence of in-cylinder combustion process in high-pressure diesel ignition direct injection natural gas engine,focusing on the influence of natural gas pre-injection quantities,natural gas main-pre injection intervals,natural gas post-injection quantities,and natural gas main-post injection intervals on the combustion process and emissions of the engine cylinder.A general rule is drawn to provide theoretical guidance for prototype optimization tests to meet higher emission standards.The conclusions are as follows:1)Natural gas pre-injection improves the quality of the mixture gas,and it does shorten the ignition delay of the main-injection,reduce the maximum burst pressure and the maximum combustion temperature in the cylinder,as well as reducing NO_x emissions and optimizing the combustion.2)When the pre-main injection interval is between 9°CA and 12°CA,the pre-injection quantity should not be less than 10%;when the pre-main injection interval is between 6°CA and 9°CA,the pre-injection quantity should be between 5%~10%;when the pre-main injection interval is less than 3°CA,any proportion of pre-injection increases NO_x emissions.And then the conclusions can be got that when the pre-injection interval is relatively bigger,more natural gas pre-injection is better;when the pre-injection interval becomes smaller,less natural gas pre-injection is better.And the pre-injection interval should not be too small,otherwise the effect will be counterproductive;3)When the pre-injection quantity is fixed,the pre-main injection interval of 6°CA to9°CA is beneficial to the reduction of NO_x emission;when the pre-injection quantity is less than 5%,the discharge is relatively insensitive to the changes of the pre-main injecton interval.And then the conclusions can be got that for fixed pre-injection conditions,the longer interval of pre-injection is better to improve emissions,and emissions is relatively insensitive to the changes of the interval when the quantity of pre-injection is small;4)The jet effects of post-injection natural gas can promote the turbulent disturbance of the mixed gas in the cylinder,and the combustion of post-injection natural gas increases the temperature in the cylinder,thereby increasing the oxidation rate of HC and CO,and reducing the emission of HC and CO;5)When the main-post injection interval is between 10°CA to 20°CA,natural gas injection quantity of between 7%to 9%is better to reduce the emission of HC and CO,and has a good inhibitory effect on NO_x emissions.When the main post-injection interval is between 5°CA to 10°CA,the post-injection quantity should be between 3%to 7%.And the main-post injection interval should not be less than 5°CA,otherwise the effect will be counterproductive.And then the conclusions can be got that a larger post-injection quantity plays a greater role in reducing HC and CO emissions and inhibiting NOx emissions when the main-post injection intervals are greater.When the main-post injection interval decreases,the post-injection quantity should be reduced.Furthermore,the main-post injection interval should not be too small,or the effect will be counterproductive;6)When the main-post injection quantity is between 7%to 9%,the main-post injection interval that is between 10°CA to 20°CA is beneficial to improve the emission;when the post-injection quantity is less than 7%,the emission is relatively insensitive to the changes in the main-post injection interval.And then the conclusions can be got that when the post-injection quantity is greater,greater main-post injection intervals are beneficial to improving emissions.When the post-injection quantity is small,the emissions are relatively insensitive to the change of main-post injection intervals.7)A main-post injection interval that is between 15°CA to 20°CA significantly improves the HC and CO emissions,and an appropriate increase in the quantity of post-injection is more favorable for NO_x emissions.And then the conclusions can be got that the use of a relatively large range of main-post injection intervals has a significant effect on the improvement of HC and CO emissions,and an appropriate increase in the quantity of post-injection is more conducive to NO_x emissions. |
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