Influence And Optimization Of EGR On Combustion Stability Of Natural Gas Engine

After the promulgation of the National VI emission regulations,natural gas engines mostly adopt the combustion mode of stoichiometric combustion combined with EGR,and ultra-low pollutant emissions can be achieved by adding a three-way catalytic conversion device.Compared with lean combustion,natural gas stoichiometric combustion has lower thermal efficiency.In order to improve the thermal efficiency of natural gas stoichiometric combustion and the economy of medium-load gas,the EGR rate should be increased as much as possible.Too high EGR rate leads to difficulty in ignition,slow combustion rate and in-cylinder mixture concentration.The above problems greatly increase the cyclic variation of combustion and reduce the power and economy of the engine.Therefore,it is necessary to conduct a detailed study on the combustion stability of natural gas stoichiometric combustion combined with EGR.In this paper,taking the natural gas stoichiometric combustion stability as the research object,based on a single-cylinder optical engine,the influence mechanism of the EGR rate on the natural gas stoichiometric combustion characteristics was deeply studied,and the influence mechanism of the ignition mode on the natural gas stoichiometric combustion characteristics was explored.Based on this,a strategy to improve combustion stability by optimizing in-cylinder turbulent motion is proposed.The conclusions of this study are as follows:First,the effect of EGR rate on natural gas stoichiometric combustion characteristics was studied.The EGR rate increases,the variation of the combustion cycle increases,the average indicated effective pressure decreases,the brightness of the flame image decreases,and the in-cylinder combustion temperature decreases.In the process of increasing the EGR rate,the average indicated effective pressure and the cyclic variation show nonlinear changes.When the EGR rate is higher than the "nonlinear value point",the combustion phase is delayed,the after-combustion phenomenon is serious,and the power and economy decline sharply.In addition,when the EGR rate is higher than the "non-linear value point",the duration of the early combustion stage is sharply prolonged,the cycle variation is rapidly increased,and the average indicated effective pressure is greatly reduced.Stability is closely related to the conclusion.Secondly,the research on the effect of ignition mode on the combustion characteristics of natural gas stoichiometric was carried out.Increasing the ignition energy can promote the phenomenon of residual ignition energy on the spark plug,which is beneficial to the formation of the early flame,but only a single fire nucleus can be formed,which has certain limitations on the later propagation of the flame.The influence of the uniformity of the mixture in the passive pre-combustion chamber on the indicated thermal efficiency is more important than the jet flame speed of the nozzle hole.If the passive pre-combustion chamber is ignited stably and effectively,a jet flame distributed around the cylinder can be formed,which promotes the main combustion chamber.The flames burn quickly.The ignition method of micro-ignition can form a widely distributed fire nucleus,which leads to a faster and more stable early flame development rate,which promotes the spread of the later flame.Finally,based on the above test results,an experimental study was carried out to optimize the in-cylinder turbulent motion to improve the combustion stability of natural gas stoichiometric.The eddy current controller can form a clockwise vortex motion and improve the flow field strength of the non-reactive turbulent velocity field in the cylinder.The advance of the micro-injection timing and the increase of the microinjection amount will both improve the indicated thermal efficiency and combustion stability.The increase of turbulence intensity will improve the uniformity of the concentration distribution of the mixture in the cylinder,reduce the emission of unburned hydrocarbons,reduce the generation of yellow flames and the emission of soot.The enhanced turbulence intensity will also increase the overall flame stretch rate in the cylinder,expand the contact area between the flame surface and the unburned mixture,which is beneficial to the spread of the flame and improves the combustion stability.