Research On Combustion Characteristics In Cylinder Of CNG Engine With Large Diameter And Dual-Spark Plug

Natural gas as a clean energy has become the preferred alternative fuel for vehicles.Research and development of natural gas engine is of great significance for alleviating energy crisis and improving environmental problems.However,due to the characteristics of natural gas fuel itself,there are still some problems in large cylinder diameter lean-burn natural gas engine,such as long flame propagation distance,slow combustion speed,high misfire rate and low thermal efficiency.The key problem is to improve the flame propagation speed.Therefore,it has become an important subject to study the combustion characteristics of natural gas and realize the fast combustion of natural gas engine.The dual spark plug ignition fast combustion technology can shorten the flame propagation distance,accelerate the combustion speed and improve the overall performance of the engine.Therefore,this paper adopts the dual spark plug ignition technology,by means of experimental test and computer simulation research,through theoretical analysis,bench test,digital modeling,simulation optimization and other methods,makes a thorough study on the combustion characteristics and emission control measures of large-bore natural gas engine,aiming at improving the power,economy and emission performance of engine.In order to study the combustion characteristics of natural gas fuels,a visual constant volume combustion system suitable for dual spark plug ignition was designed by using constant volume combustion bomb to simulate high temperature and high pressure combustion environment in cylinder.By changing the parameters of fuel-air equivalence ratio,ignition mode,initial pressure and temperature,the combustion image and pressure data were collected by high-speed camera and pressure sensor.Combining with digital image processing technology,the image processing program of MATLAB was compiled,and the two-color method was proposed to characterize the distribution of combustion temperature field.The effects of dual ignition on the combustion characteristics of natural gas,such as flame propagation,combustion pressure,pressure rise rate,heat release rate,combustion period and temperature field,are experimentally studied under a single initial parameter.The results show that:(1)Under different ignition modes,compared with single ignition,synchronous dual ignition shortens the flame propagation distance,speeds up the combustion rate,increases the combustion pressure and heat release rate,shortens the flame development period and the main combustion period,and significantly improves the combustion thermal efficiency of natural gas,especially for lean mixture.Asynchronous dual ignition is more active to realize rapid combustion of dilute mixture.(2)Under different initial pressures,when the initial pressure increases,the mixture concentration increases,the flame propagation speed decreases,the flame radius growth rate slows down,the peak combustion pressure and heat release rate increase,the flame development period and the main combustion increase.The increase of initial pressure slows down the combustion speed.(3)Under different initial temperatures,when the initial temperature increases,the flame propagation speed accelerates,the speed of flame propagation area increases,the maximum combustion pressure and heat release rate increase,the combustion duration shorten,and the increase of temperature promotes combustion and increases thermal efficiency,but too high will also lead to deflagration.(4)Under different equivalence ratios,in chemical equivalence ratio,the flame propagation speed is the fastest,the peak combustion pressure and heat release rate are the highest,the flame development period and the main combustion period are the shortest.The flame propagation speed will be reduced if the flame is too thick or too thin.And the growth rate of radius decreases,the combustion duration prolongs.In order to study the effect of dual ignition on combustion process of large cylinder diameter lean-burn natural gas engine comprehensively,the simulation model of the engine is established by using three-dimensional software such as CATIA and AVL FIRE.The initial and boundary conditions are set accurately,and the model verification is completed.Under the conditions of 1 400 r/min speed,equivalent ratio of 0.7 and full load,the optimization strategies of parameters such as position of dual-spark plug,ignition mode,ignition energy and ignition strategy are put forward.Through simulation calculation,the effects of these parameters on combustion and emission characteristics of dual-ignition natural gas engine cylinder are studied and analyzed,so as to provide a reference for future dual-ignition natural gas engine.The results show that:(1)For different layout schemes of double spark plugs,D2 layout scheme is the best,because of the faster propagation speed of flame front,relatively higher combustion pressure in cylinder,shorter combustion duration and lower NO_X emission in later combustion period.(2)For different ignition energy,when the ignition energy is 60 mJ,the combustion speed of working medium in cylinder is the fastest,the heat release rate of combustion is the largest,the power and fuel economy are the best,so the optimum ignition energy is determined.(3)For synchronous and asynchronous ignition at different ignition time,the maximum combustion pressure occurs in the range of 12-15°CA after the top dead point when the synchronous ignition is706-706°CA and the asynchronous ignition is 706-704°CA.While the indicated power and thermal efficiency are higher,and the power and economy of the asynchronous ignition are better.And the asynchronous ignition 706-704°CA shows larger power,economy and low fuel consumption,which is the best ignition strategies.In order to effectively control the emission of natural gas engine with dual-spark plug and large-bore,this paper presents a NOx emission control strategy based on dual spark plug combined with EGR technology,and simulates and analyses the effects of different EGR rates on combustion characteristics and emission characteristics in cylinder.The results show that,when the ERG ratio is between 0%and 10%,the variation range of in-cylinder pressure,heat release rate and average reaction rate of the engine are small,but the average temperature and NOx emission decrease obviously.When the EGR ratio is more than 10%,the in-cylinder pressure,heat release rate,average reaction rate,average temperature and NOx emission of the engine are all decrease,the indicative power and thermal efficiency also decrease,and fuel consumption increases.When EGR is 10.5%,the target value of NOx emission is basically the same as that of single ignition,and other performances are better than that of single ignition.When EGR is 11.8%,the power,thermal efficiency and fuel consumption of engine are basically the same as that of single ignition,but the target value of NOx is 26.3%lower than that of single ignition.Therefore,When the EGR rate is between 10.5%and 11.8%,the performance of dual ignition engine is better than that of single ignition.This control strategy provides basic research data and technical reserves for meeting more stringent emission regulations in the future.If the goal is to maximize the power and economy of the engine,the optimum EGR rate is 10.5%.At this time,double ignition and single ignition have the same target value of NOx emission.The engine performance under single and double ignition modes is compared and analyzed.The results show that,compared with single ignition,the power and economy of the engine under the dual spark plug ignition mode are significantly improved.In conclusion,the dual-spark plug technology can effectively improve the power,economy and emission performance of natural gas engine with large cylinder bore.