Study On The Interaction Mechanism Between In-cylinder Turbulence And Combustion Of Natural Gas Engines

The contradiction between fuel supply and demand and emission pollution caused by the automobile industry has become increasingly prominent.Under the vision of "carbon emission peak and carbon neutrality",natural gas,as a clean and low-carbon alternative fuel,has broad emission reduction space in the field of internal combustion engines.The development of natural gas engine has become one of the major national strategies for energy conservation and emission reduction.It is very important to reasonably organize the air flow in cylinder to achieve efficient and clean combustion of natural gas engine.At present,the research on the interaction between in-cylinder flow and combustion is still in its infancy.This paper starts with the accurate description and evaluation of turbulent flow in cylinder,establishes the mapping relationship between turbulence and combustion parameters,and deeply analyzes the interaction mechanism between turbulence and combustion under the conditions of multi-physical field and multi-parameter field,so as to provide theoretical guidance for further realizing efficient and clean combustion of natural gas engines.For commercial natural gas engines,an innovative evaluation method based on the time and space dynamic distribution of unsteady turbulent flow in cylinder was proposed.Based on the characteristic points of flame front,the evolution law and mapping relationship between turbulence and combustion parameters were obtained.Based on the proposed evaluation method and mapping relationship,the influence law between turbulent flow and combustion process under different combustor shapes and injection strategies was explored.The innovative research work and conclusions in this paper are as follows:(1)Based on the research platform of spark ignition natural gas engine,aiming at the limitation that the flow process in the engine cylinder is difficult to accurately describe and evaluate,an innovative evaluation method for the spatial-temporal dynamic distribution of turbulent kinetic energy in the cylinder was proposed.In this method,the volume proportion of different turbulence intensities in the cylinder with the crank angle was statistically analyzed,and the centroid space distribution of turbulence field with different intensities with the crank angle was statistically analyzed,so as to construct the evaluation system of unsteady turbulent flow in cylinder dynamic space.With this method,the temporal and spatial variations of different turbulent kinetic energy fields in the cylinder with and without combustion process and different combustion chamber shapes are analyzed.The results show that the dynamic change law of the volume proportion of turbulent kinetic energy in the cylinder is continuous.The volume proportion of high turbulent kinetic energy gradually decreases until it disappears with the piston ascending,while the volume proportion of low turbulent kinetic energy gradually increases.With the introduction of combustion process,the volume proportion of high turbulent kinetic energy increases at the same crank angle,and then the change law converges.According to the centroid position statistics,the trajectories of turbulent kinetic energy fields with different intensities in the cylinder were obtained.The combustion chamber with strong swirl and squeeze effects can delay the decay rate of the volume proportion of high turbulent kinetic energy.(2)In view of the limitation that it is difficult to accurately evaluate the interaction law of flow and combustion in cylinder of natural gas engine,the mapping relationship between turbulence and combustion parameters was innovatively constructed based on the characteristic points of flame front.In this study,the flame front is defined as the corresponding area between 90%(flame leading edge)and 10%(flame trailing edge)of methane mass concentration in the combustion reaction zone.At the same time,the central area of the flame along the piston axis was selected as the axial characteristic point,and the central area of the flame along the direction passing through the four valve centerlines was selected as the radial characteristic point.And the corresponding relationship between incylinder flow and combustion parameters was constructed based on the selected five characteristic points.The results show that the combustion process has a certain influence on the flow process.The statistical results of five characteristic points show that the intensity of turbulent kinetic energy in the flame front area is significantly increased by the involvement of combustion.The intensity of turbulent kinetic energy corresponding to the flame leading edge is significantly higher than that of the flame trailing edge.The flow process also affects the combustion process.The intensity of turbulent kinetic energy determines the turbulent combustion speed of flame.The original velocity field near the flame front and the turbulent combustion velocity after the flame propagates to this area jointly determine the moving velocity of the flame front.(3)In order to deeply explore the influence law between turbulent flow and combustion process under different physical field conditions,based on the evaluation method of the spatial-temporal dynamic distribution of in-cylinder turbulent kinetic energy and the mapping relationship between turbulence and combustion parameters proposed in this study,the influence mechanism of combustion chamber shape on turbulent flow and combustion process in engine cylinder was analyzed.The results show that there is a good relationship between the flame front volume and the combustion heat release rate.The shape of combustion chamber has a great influence on the evolution of the in cylinder turbulent flow,and the heat release rate is positively related to the flame front volume and the moving speed of flame front.The volume proportion of different turbulence intensity in the flame front directly affects the combustion heat release process.Higher volume proportion of high turbulence intensity corresponds to the higher peak heat release rate and the earlier combustion phase.(4)In order to deeply explore the influence law between turbulent flow and combustion process under different parameter fields,also based on the evaluation method of the spatialtemporal dynamic distribution of in-cylinder turbulent kinetic energy and the mapping relationship between turbulence and combustion parameters,the influence mechanism of injection strategy on turbulent flow and combustion process in engine cylinder under diesel pilot mode was analyzed.The results show that with the increase of percentage energy substitution,the standard deviation and cyclic variation of in-cylinder pressure at each crank angle and peak pressure gradually increase,and the combustion mode changes from diffusion combustion to premixed combustion.The turbulent kinetic energy near the fuel jet is greatly increased instantaneously during the injection process,but the turbulent kinetic energy dissipation process in the cylinder is also accelerated.With the advance of the injection time,the effect of the instantaneous turbulent kinetic energy change introduced by the injection on the combustion process decreases.