Effect Of Water Injection On Combustion And Emission Characteristics Of Natural Gas Engine With Stoichiometric Operation

With the increasingly strict emission regulations,equivalence ratio combustion has become the mainstream combustion technology of China VI natural gas engines.However,the combustion temperature of natural gas is high,and the equivalence ratio combustion will cause the heat load in the cylinder and the exhaust temperature to be high.Higher in-cylinder heat load will increase the probability of natural gas engine knocking,and high exhaust temperature will also cause harm to its exhaust aftertreatment system.Therefore,it is necessary to take measures to eliminate the above-mentioned problems caused by the equivalence ratio combustion.In this paper,first,through the constant volume combustion bomb experiment and the CHEMKIN numerical calculation method,the effect of different water dilution,different preheating temperature,and different dilution components on the flame characteristics of the natural gas equivalent ratio combustion laminar flame is studied.and the influence of steam physical and chemical effects on flame speed is compared,which provides basic combustion speed data and combustion mechanism model for natural gas engine model.After that,based on the 1.5TGDI natural gas engine model,the AVL Fire software was used to calculate the influence of the water injection of the intake port on the combustion and emissions of the natural gas engine.The main researches are the impact of in-cylinder temperature,in-cylinder pressure,heat release rate,in-cylinder turbulent kinetic energy,NO emission,CO emission and particulate emissions of natural gas engine under different water dilution,different ignition advance angle,different water injection duration and different water injection cone angle.Among them,single-point water spray is used for different water dilutions and different ignition advance angles,and double-point water spray is used for different spray durations and different spray cone angles.Based on the experimental study of constant volume combustion bomb,it is obtained that the laminar flame velocity decreases linearly with the increase of water dilution.The reduction of the flame velocity by water vapor can be divided into physical and chemical effects,and the physical effect is greater than the chemical effect.Different dilution components have different effects on the flame propagation speed of natural gas.At the same dilution,the effect of different dilution gases on the flame speed is CO₂>H₂O>N₂.The smaller the absolute value of the sensitivity coefficients of the elementary reactions R41,R1 and R43,the greater the laminar flame propagation velocity.The higher the peak concentration of H and OH radicals,the greater the value of laminar flame velocity.The higher the preheat temperature at the same water dilution,the higher the adiabatic flame temperature and the greater the laminar flame speed.Based on the natural gas engine model:when the same ignition advance angle and different water dilutions,as the water dilution increases,the water vapor content in the cylinder increases,the workability of the engine decreases,and the heat load in the cylinder decreases.The average turbulent kinetic energy in the cylinder after water spray is lower than that without water spray.The higher the water dilution,the lower the NO emissions and the higher the CO emissions.The particulate emissions increase first and then decrease.When the same water dilution and different ignition advance angles,the ignition advance angle of 12°CA can maintain the engine workability and effectively reduce the engine heat load.NO emission is the lowest,particulate emissions are low,and CO emissions are the highest.It is necessary to further reduce CO emissions.With the same water dilution,the same ignition advance angle and different water injection durations,the average turbulent kinetic energy in the cylinder with a water injection duration of 5ms is higher than 3ms,and the CO emissions and particulate emissions are low.When the same water dilution,the same ignition advance angle and different water spray cone angles:the water spray cone angle of 30°keeps the engine workability better.The average turbulent kinetic energy in the cylinder with a water spray cone angle of 30°and 45°is lower,and CO emissions are lower.Under the same conditions,the CO emission of double-point spraying is significantly lower than that of single-point spraying.