Influence Of The In-cylinder Tumble Flow On The Performance Of Spark Ignition Natural Gas Engines

At present,the main technical route of natural gas（NG）engine is spark ignition（SI）equivalent combustion matched with efficient EGR and three-way catalytic converter.Considering the flame propagation and methane combustion hysteresis in SI engine,this paper studies the effect of tumble on the performance of SI NG engine by changing the flow mode in cylinder,in order to speed up the combustion rate of NG in cylinder and improve the thermal efficiency of engine.Based on a SI equivalent combustion NG engine,this study first collected the data of each operating point,and then established the numerical simulation platform of the original engine according to the original three-dimensional model and the original test data,and then verified the simulation platform by the test data of the operating point.The decoupling analysis and research of swirl and tumble show that high SR₀or high TR₀ flow field can obtain strong turbulence,and the turbulence in tumble flow field is much stronger than that in swirl flow field at the same level.In addition,the formation and development of spark core in swirl flow field has a decisive influence on the combustion process of SI NG engine,which makes the variation of NG combustion process with SR₀ present a two-stage characteristic with SR₀=2 as the boundary.When SR₀ is not less than 2,it can give full play to the gain effect of swirl on the combustion process of natural gas,and the indicated thermal efficiency can reach up to 43.18%.The condition of low velocity and high TKE in tumble flow field near TDC is not only beneficial to the formation and development of spark core,but also can enhance the turbulent combustion of NG,which makes the variation of NG combustion process with TR₀ present a two-stage characteristic with TR₀=1.25 as the boundary.When TR₀ is not less than 1.25,tumble can give full play to the gain effect of natural gas combustion process,and the indicated thermal efficiency can reach up to 44.39%.The analysis shows that the tumble flow field can improve the combustion process of SI NG engine more significantly.Three kinds of intake and combustion systems with differentαwere designed and simulated.The results show that the tumble ratio of tumble motion experienced a double peak evolution process,and the first peak value in the intake stroke determines the overall tumble level to a certain extent,and the largerαis,the larger the first peak value is.The analysis shows that the intake port mainly affects the inertia direction of the air flow,and then affects the proportion of the mixture and the intensity of the jet from the upper and lower parts of the valve,and finally affects the scale and intensity of the primary and secondary tumble,and affects the first peak height of the tumble ratio.Taking into account the flow coefficient,the shape of the inlet should be improved to increase the proportion of the mixture injected into the cylinder from the upper part of the valve,so as to obtain high tumble.