| The quality enhancement measures of small single cylinder agricultural diesel engines are restricted by using object and purpose.The optimization must benefit to in-cylinder combustion and do not increase costs to much.In this paper,six kinds of combustion chamber were described.They were the original ω combustion chamber,contracting-open combustion chamber A,straight-open combustion chamber B,expanding-open combustion chamber C, center-crown combustion chamber D and dumbbell-shaped combustion chamber E.Then,by using the CFD software, physical model establishment, HyperMesh, dynamic meshing definition, the initial boundary conditions for the above combustion chambers were carried out respectively.From the burning essence, we can understand the influences upon in-cylinder flow, mixture formation and combustion process by different combustion chamber structure through numerical simulation of three dimensional microscopic level.Propose the key factors of combustion process optimization through combustion chamber structure improvement.Improve overall performance of small agricultural diesel engine and simplify the test research.The simulation study showed, combustion chamber offset to left had a great influence on in-cylinder flow field.The combustion chamber flow field structure was asymmetry.In-cylinder turbulent kinetic energy was unevenly distributed.The swirl in the piston pit was offset to left with greater speed.The swirl speed of piston top on the upper left was superior to the right.Without contracting restraint, Expanding-open combustion chamber made better use of fresh air on piston top in the initial stage of combustion.The delayed ignition period and rapid combustion period was influenced obviously. The ignition was advanced and the rise rate of pressure and temperature became larger.However, the whole combustion chamber flow intensity was low and the combustion duration was the shortest, which gave a bad effect to the oxidation of Soot,so the Soot emissions were increased.Compared with expanding-open combustion chamber,contracting-open combustion chamber overcame its shortcomings, was beneficial to improve the swirl flow speed in whole combustion chamber. At the same time,as the contracting rate rose, the swirl flow speed in later burning became higher, making the later burning sufficient,improving work capacity in the expansion stroke. While long duration of high temperature lead to NOx emissions larger.Straight-open combustion chamber could satisfy the original machine performance and easyily processing.It could replace the existing diesel engine combustion chamber while considering the generation cost. Increasing the central boss of piston pit, the swirl flow speed inside the piston pit became weaken.With the piston moving upward in compression stroke, the swirl intensity on the upper piston enhanced and area increased,ensuring the mixed gas combust rapidly and completely in the slow burning period near TDC,enhancing dynamic performance.Howeve, the airflow of low-speed region above the central boss increased,which would be easy to form the high-temperature region, NOx generation were increased significantly. Visiblely, it was limited that improving the combustion emission performance only by changing the form of contracting or the size of the central boss.The combustion chamber E was a copy of dumbbell-shaped combustion chamber.It obviously had time domain characteristics of concentrated combustion to lean combustion.The strong flow field in the expansion stroke was kept.The ignition was advanced and The diffusion combustion became rapidly.Compared with the original machine, combustion chamber E could basically reach the original machine power and the Soot emission level, and significantly reduced NOx emissions, improved the NOx and Soot Trade-off relationship.If giving further adjustments to the fuel injection time,Soot emissions could be slightly reduced; if increasing fuel oxygen content, the carbon deposit generation in combustion chamber pit would be reduced markedly. |
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