High Compression Ratio Of Natural Gas Engine Combustion System Optimization Research

With the increasingly serious problem of atmospheric pollution and the oil energy crisis is becoming more and more serious,all over the world are looking for clean energy that can replace oil.As one of the clean fuels to replace oil,natural gas has the advantages of abundant resources,low cost and clean emission and broad market prospect,is the first choice for automotive alternative fuels.At present,most of the domestic natural gas engines are refitted from diesel engines or gasoline engines and their power and economic performance are much worse than those of the original model.It can improve the compression ratio of natural gas engine to improve its power and economy.In this paper,the high compression ratio of gas engine test research shows that:with the increase of compression ratio,gas engine combustion rate is accelerated,the cylinder combustion pressure and temperature,thermal efficiency is obviously improved,but the increase of compression ratio is limited by knocking and other abnormal combustion.So it is necessary to optimized the combustion system of natural gas engine,at the same time of improving the compression ratio,to reduce the tendency to knockThis article uses the CFD simulation software FIRE,established the 3D simulation model of the nature gas engine combustion system to study the combustion process and emission performance of high compression ratio nature gas engine.The basic parameters of the engine bench test is used to determine the initial conditions and boundary conditions,and the cylinder pressure data as the main parameter of model validation for the compression ratio of 14.6 nature gas engine.Through the simulation for knock occurring working condition,introducing the concept of knock predictive parameters-knock react rate and knock factor,which can be used to judge the probability of the knock occurrence and occur area and intensity.The combustion process of three kinds of combustion chambers under knock condition was studied,research show that the wide mouth combustion chamber has small squish area,and the squish strength is weak,which lead the turbulent kinetic energy in cylinder weaker.Besides the combustion speed is slower,the maximum explosion pressure and the rate of pressure rise are reduced,the time of knock is delayed,and the knock strength is reduced,at the same time,the NOx emissions of the main pollutants of natural gas engines have declined.The influence of ignition time and excess air coefficient on combustion and emission performance in cylinder,based on the open combustion chamber,was studied.As the ignition time moves forward,the combustion speed increases,the combustion temperature and pressure rise,and the knock tendency increases.When the ignition time is too late,the combustion lag,the combustion temperature and pressure rise too slowly,and reduce the output power.Reasonable ignition time can help engine obtain the best power,comfort and emission performance.Through the study of excess air coefficient found that when using equivalence ratio and concentrated mixture,the combustion temperature is higher and unburned CH4 increases.When the mixture is too lean,the combustion was lag,the combustion duration is prolonged,the post combustion loss is more,and the output power is reduced.Reasonable excess air coefficient can take good account of both dynamic and emission characteristics.