| Vehicles powered by petroleum resources continue to accelerate the consumption of non renewable resources,but also pose a huge challenge to the natural environment.Environmental pollution and energy shortages have prompted the promulgation of stricter regulations.In the face of increasingly stringent regulations,natural gas as a clean and low-carbon alternative fuel has a broad prospect in the field of internal combustion engines.However,natural gas fuel is always limited by the physical characteristics of slow combustion speed,so the reasonable organization of air flow in the cylinder is crucial for natural gas engines.This paper builds a numerical simulation platform based on a spark ignition equivalent combustion natural gas engine to investigate the effects of swirl and tumble on the flow and combustion in the cylinder;With the goal of obtaining weak swirl and strong tumble flow,the combustion chamber head is first ridged to become a dome type combustion chamber,and then the inlet structure is optimized.Finally,the angle between the valve axis and the inlet centerline is adjusted for the tumble flow channel,thereby improving the turbulence intensity of the natural gas engine as a whole,accelerating the fuel combustion rate,and ultimately achieving thermal efficiency improvement.Firstly,the impact of swirl and tumble on the flow and combustion in the cylinder was analyzed.The study found that low initial tumble ratio and swirl ratio had a small impact on the flow and combustion in the cylinder.The initial swirl ratio was bounded by SR=2,and the initial tumble ratio was bounded by TR=1.75.Continuing to increase the initial value could obtain higher turbulent kinetic energy.The peak cylinder pressure increased,and the phase of the peak shifted forward.At the same intensity,the effect of high tumble ratio on improving turbulent kinetic energy was more obvious.During the compression stroke stage,the high turbulent kinetic energy corresponding to the high roll flow organization attenuates faster than the eddy current.Under the same swirl ratio,different tumble ratios,and the same tumble ratio,different swirl ratios,the effects of the initial tumble ratio and swirl ratio on flow and combustion are the same as when acting alone.The starting angle height is designed based on the original machine α= Two groups of domed combustion chambers,10 ° and 15 °,were used to study and analyze the effects of different angle heights on the flow and combustion in the cylinder.The study found that compared to the original engine,the tumble ratio in the cylinder of the dome type combustion chamber was improved overall,and the turbulent kinetic energy increased synchronously.Compared to the low angle dome combustion chamber,the high roll ratio obtained by the high angle dome combustion chamber model increases the turbulent kinetic energy at the ignition time,promotes flame propagation,increases the peak cylinder pressure,moves the peak phase forward,and shortens the combustion duration.However,the roll flow structure in the cylinder of the low angle dome combustion chamber is gradually compressed and crushed in the late stage of the compression stroke,making it difficult to significantly improve the turbulent kinetic energy,so the combustion performance has not significantly improved compared to the original engine.Based on the domed combustion chamber,the original engine air ports were optimized to obtain dual tangential and tumble flow ports.The effects of different corner heights of the domed combustion chamber and the coordinated optimization of the inlet ports on the in-cylinder flow and combustion were studied.The study found that,at the same starting angle height of the dome type combustion chamber,the double tangential port has a higher tumble ratio compared to the tumble port during the intake stroke stage,but the tumble ratio decreases significantly during the compression stroke,gradually lower than the tumble port.Therefore,the overall level of turbulent kinetic energy at the ignition time is lower,and the combustion speed is slower.The dome type combustion chamber with high lift angle and the tumble flow passage have the maximum cylinder pressure and heat release rate peaks,and the combustion duration is the shortest.The angle between the valve axis and the inlet centerline was studied based on a high lift angle dome combustor with a tumble flow port α The effect of tumble flow intensity on cylinder flow and combustion.Research has found that as α As the angle increases,the included angle between the mixture and the cylinder wall decreases,which is more conducive to the injection of mixture from the intake valve side to the exhaust valve side during the intake phase,while reducing the proportion of mixture entering the cylinder from the underside of the valve,thereby forming a strong tumble structure,improving the turbulence intensity in the cylinder at the ignition time,and promoting combustion;Continue to increase the included angle,greatly improving the combustion effect in the cylinder α= Compared to 0 °,α= The 40 ° cylinder pressure peak value increased by 7 MPa,the peak phase advanced by 10 ° CA,the combustion duration shortened by 15.3 ° CA,and the indicated thermal efficiency increased by2.1%. |
