Simulation Study Of Combustion Emission Characteristics Of Pre-Combustion Chamber Pilot-Ignition Main Combustion Chamber Hydrogen-Rich Natural Gas Engine

The acceleration of the industrialization process has led to the massive consumption of fossil fuels and the increasing emissions of greenhouse gases,resulting in a serious greenhouse effect.In order to effectively control greenhouse gas emissions and achieve the short-term decarbonization goals of the international community,the use of hydrogen-rich natural gas as a new alternative fuel for internal combustion engines is the best choice.In this paper,simulation calculations were carried out for a large-bore active pre-chamber natural gas engine,and the effects of different operating parameters and hydrogen-rich natural gas on the lean combustion and emission characteristics of the engine were analyzed,and the structural parameters of the hydrogen-rich pre-chamber were optimized.The main research content is as follows:（1）A three-dimensional simulation model of a large-bore active pre-combustion chamber natural gas engine was constructed by combining relevant parameters,and the combustion mechanism and model were verified by combining experimental data.The effects of different gas injection strategies,main combustion chamber excess air coefficients and ignition timing and other operating parameters on engine mixture formation as well as combustion and emission processes were investigated.The results show that the gas injection strategy and the excess air coefficient of the main combustion chamber have a greater effect on the overall mixture concentration distribution in the pre-combustion chamber,but a smaller effect at the spark plug,and that with the delay of the gas injection moment,the overall combustion process in the cylinder is delayed,the indicated power and thermal efficiency of the engine are reduced,NOx emissions are reduced,and HC,CO and total CH₄ escape are increased,but a moderate delay of 100°CA has a smaller effect on the above parameters,while a further further delay has a greater effect;with the increase of gas injection amount,the combustion process in the cylinder first accelerates and then slows down,which makes the indicated power and thermal efficiency of the engine first increase and then decrease,and the NOx emission changes the same trend,while the HC,CO and the total CH₄ escapes are opposite,while the influence of the gas injection moment on the engine performance is higher than the gas injection amount in comparison;an increase in the excess air coefficient in the main combustion chamber has a small effect on the mixture concentration in the pre-combustion chamber and its combustion process,but it decreases the mixture concentration in the main combustion chamber,leading to a decrease in flame propagation speed and causing a deterioration in the combustion process,resulting in a decrease in the indicated engine power,an increase in HC,CO and total CH₄ escapes and a decrease in NOx emissions;the advancement of ignition timing will advance the whole combustion process and increase the cycle isovolume,resulting in higher in-cylinder pressure and temperature,increased indicated power and thermal efficiency,and increased NOx emissions,but significantly reduced HC,CO and total CH₄ escapes.（2）The engine performance calculations with different volume hydrogen enrichment ratios were carried out using manifold hydrogen injection while maintaining the above-mentioned operating parameters constant,and the effects of the variation of the main combustion chamber mixture concentration on the engine combustion and emission characteristics were investigated on the basis of the optimum hydrogen enrichment ratio.The results show that the natural gas hydrogen enrichment affects the mixture concentration distribution throughout the combustion chamber,but less at the spark plug,where the mixture concentration distribution in the pre-combustion and main combustion chambers does not change significantly before the 20%hydrogen enrichment ratio,but after the 20%hydrogen enrichment ratio,the non-uniformity of the mixture concentration distribution within them increases significantly;hydrogen-rich improves the combustion rate of the lean mixture in the combustion chamber by significantly increasing the concentration of free radicals such as H and OH in the cylinder,and mainly makes the rapid combustion period in the second half of the pre-combustion chamber and the main combustion chamber significantly shorter,thus accelerating the whole combustion process to proceed,which in turn makes the in-cylinder pressure and temperature rise significantly and the peak phase advance significantly,and finally makes the indicated power and combustion efficiency of the engine increase,and the HC,CO and total CH₄ escapes decrease,but the NOx emission and the pressure rise rate of the main combustion chamber increase,and under a comprehensive comparison,the 20%hydrogen-rich ratio scheme is optimal,and the combustion promotion effect is not obvious by further increasing the hydrogen-rich ratio;hydrogen enrichment helps to improve the combustion process in the cylinder under lean combustion conditions,and the leaner the mixture,the more obvious the improvement effect,especially under ultra lean conditions,hydrogen enrichment greatly reduces the amount of HC,CO and total CH₄ escape,while only slightly increasing the NOx emission.（3）Based on the 20%hydrogen-rich ratio,the key structural parameters of the pre-combustion chamber were optimized using a single-factor variation method.The results show that a moderate increase of 0.5 mm in the diameter of the pre-combustion chamber nozzle can effectively reduce the pressure and temperature of the pre-combustion chamber and the pressure rise rate of the main combustion chamber and improve the engine performance,but an excessive increase or decrease in the diameter of the nozzle can lead to deterioration in performance;increasing the number of orifices from the original machine will result in higher in-cylinder pressure,higher indicated power and thermal efficiency,lower HC,CO and total CH₄ escape,but higher NOx emissions,while excessively increasing the number of orifices to 8 will also cause the pressure rise rate to exceed the limit;the increase of throat diameter in the range of 2mm can make the pressure and temperature of the pre-combustion chamber significantly lower,and the pressure rise rate of the main combustion chamber will also be reduced,which is beneficial to reduce the mechanical load and heat load of the engine,but it will lead to a small increase of pollutant emissions in the cylinder.Taking into account the pressure and temperature of the pre-combustion chamber and the pressure rise rate of the main combustion chamber,the optimal pre-combustion chamber parameters for a hydrogen-rich engine are 5.5 mm diameter of the injection holes,6 number of injection holes and 17 mm diameter of the throat.This paper mainly studies the technical approach and optimization scheme of hydrogen-enriched natural gas engine with large bore precombustion chamber.The research results can provide reference for the development and control of its combustion system.