Simulation Study On Knock Characteristics Of Natural Gas Engine

In recent years,the world’s energy and environmental problems are becoming more and more prominent,the traditional fossil energy is increasingly exhausted and emission regulations are becoming more stringent.With the gradual implementation of the sixth stage motor vehicle emission standards in various provinces and cities,higher requirements are put forward for the engine emission performance.Natural gas is rich in reserves and relatively clean,so the development of natural gas engine has attracted much attention.At present,in order to meet the requirements of emission regulations,natural gas engine adopts theoretical air-fuel ratio combustion technology.The results show that the heat load in cylinder is high and the interaction of pressure wave is enhanced,which increases the probability and intensity of detonation.Therefore,the research on the knock characteristics of natural gas engine is helpful to improve the engine power and economy.In this paper,the numerical simulation model of natural gas engine combustion in cylinder is established based on wp4.6nng200e60 as the research object.Then,the boundary conditions and initial conditions of the model are set,and the accuracy of the simulation model is verified.Finally,through setting 7 monitoring points in the cylinder,the pressure of the monitoring point is carried out fast Fourier transform(FFT),and then the band-pass filter is processed to obtain the band-pass filter pressure curve by means of Fourier inverse transform(IFFT).The maximum pressure oscillation amplitude of the filter pressure is taken as the evaluation index of detonation.The specific research contents are as follows:Firstly,the effects of ignition advance angle,compression ratio and core size on in cylinder deflagration were studied.With the advance of ignition time,the cylinder pressure curve at the monitoring point fluctuates more violently,and the knock index increases;With the increase of compression ratio,the knock intensity increases;The ignition energy and the volume of the initial ignition region are changed with the increase of the core radius,which has a significant effect on the in cylinder detonation index.Secondly,due to the different fuel components of various natural gas sources in China,they contain different proportions of ethane and propane.Therefore,in this study,methane was doped with 5%,10% and 15% ethane and 5% and 10% propane respectively to compare the effects of different natural gas fuel components on engine knock characteristics.The average pressure,average temperature,cumulative heat release rate,temperature distribution and knock index of each monitoring point were compared and analyzed.Finally,taking the fuel consumption ISFC,knock index and tumble ratio as the optimization objectives,the height of combustion chamber h,the distance L from the center of circular arc at the bottom of combustion chamber to the center line,the necking radius r of combustion chamber and the ignition advance angle are optimized by using genetic algorithm.By comparing the optimized combustor structure,the influence of combustor structure parameters on fuel consumption rate and knock index was analyzed.