Study On The Performance Simulation Of Ignition-chamber Nature Gas Engine

Nowadays,with energy shortage and environmental pollution has become increasingly serious,the sustainable development of engines is facing a major test.Seeking an ideal alternative fuel is one of the effective ways to solve this problem.Natural gas has gradually become a research hotspot and has broad application prospects because of its large storage,clean combustion products and relatively low price in the world.The application of lean combustion technology in natural gas engines can reduce combustion temperature,reduce NO_x emissions,improve thermal efficiency and reduce gas consumption,but at the same time,it also has problems such as unstable combustion and large cycle changes,so it is necessary to improve the reliability of ignition.The natural gas engine in this paper adopts ignition chamber combustion system.The denser mixture in the ignition chamber is ignited by spark plug discharge.The high temperature and high pressure flame generates a jet through the nozzle connecting the ignition chamber and the main combustion chamber,and ignites the rarer mixture after entering the main combustion chamber,which produces a strong disturbance.Thus,the burning speed of natural gas can be increased and the purpose of full combustion can be achieved.In this paper,a natural gas engine with a bore of 320 mm was simulated by using CFD software CONVERGE.The 3D model includes ignition chamber,main combustion chamber,inlet and exhaust.Suitable models have been selected to simulate a working cycle of the engine and the engine model is verified and calibrated based on experimental data.The effects of the number of orifices in the ignition chamber,conical orifices,central orifices,layered orifices,ignition chamber equivalent ratio,ignition advance angle and ignition chamber heat insulation on the combustion and emission performance of the natural gas engine were designed and analyzed.The conclusions are as follows:(1)The study of the number of orifices in the ignition chamber shows that when the number of orifices is small,the coverage area of the jet flame in the main combustion chamber is small and the combustion is insufficient and when the number of orifices is large,interference will occur between the flames,which is not conducive to flame propagation.The N7 scheme makes the distribution of jet flame more uniform and reasonable,and is superior to the original engine in terms of power and emission.The N9 scheme improves the power compared with the original engine,but the NO_x emission increases.(2)Conical orifice scheme is beneficial to improve combustion efficiency,increases power and reduce NO_x emission.With the increase of cone angle,the engine power increases at first and then decreases,and the NO_x emission shows a downward trend.Compared with the original machine,the cone2 scheme has the most obvious power increase,while the cone3scheme has the largest NO_x reduction.(3)The effect of the central orifice scheme is related to the diameter of the central hole.The power increase of center2 scheme is the most obvious,while the NO_x emission increases at the same time.The center1.5 and center2.5 scheme not only improve power,but also reduce NO_x emission.(4)The layered orifice scheme reduces the drop point of the jet flame in the lower orifice.If the angle between the axis of the lower orifice and the horizontal axis is too large,the velocity will drop rapidly after the flame touches the piston wall,which is not conducive to further outward diffusion,reduces the uniformity of combustion,and reduces the combustion efficiency.The indicated power of layer1 and layer3 scheme is lower than that of the original machine,while the indicated power of layer2 scheme is higher than that of the original machine,but the NO_x emission is increased.(5)The ratio of ignition space is an important parameter that affects the performance of natural gas engine.When the equivalence ratio is too small,the mixture is too thin,which is not conducive to spark plug ignition and the rapid propagation of flame.And excessive equivalence ratio will lead to too little air,which is also not conducive to ignition.With the increase of the average equivalence ratio of the ignition chamber,the indicated power of the engine first increases and then decreases,and the NO_x emission also shows a trend of first rising and then falling.When the average equivalence ratio of the ignition chamber was 1.1 at the time of ignition,the indicated power of the engine was the highest,which increased by3.3%compared with the original engine,but its NO_x emission increased by 5.6%at the same time.(6)With the increase of the ignition advance angle,the detonation pressure of the engine gradually increases.Excessive ignition advance angle will increase the risk of detonation,aggravate engine wear,and NO_x emission will be worsen.Too little ignition advance angle will lead to engine power reduction.Compared with the original machine,18°CA ignition advance angle scheme realized the indicated power increased by 0.7%,and the NO_x emission was reduced by 13%.(7)The heat insulation design of the ignition chamber reduces the heat loss of the ignition chamber,improves the average temperature of the ignition chamber,improves the temperature of the jet flame,improves the ignition energy,and effectively improves the power and thermal efficiency of the engine,but NO_x emission has increased.