燃烧室缩口尺寸对柴油机燃烧过程的影响

摘要

The simulations of combustion and emission for an agricultural high-power diesel engine were studied under differ-ent necking sizes of combustion chamber .The results show that decreasing the necking size can result in the increase of in-cylin-der turbulent kinetic energy and help the gas mixing ,which promotes the flow within the cylinder .The cylinder pressure de-creases with the decline of necking size ,while the maximum pressure changes little .The temperature inside the cylinder chan-ges slightly with the change of necking size ,the necking can improve the combustion process in the cylinder ,but the excessive necking size is not beneficial .For the chambers with different necking sizes ,the in-cylinder velocity fields are almost the same at ignition timing ,but the velocity field at top dead center slightly strengthens with the decrease of necking diameter .During the retarded combustion period ,the combustion rate decreases rapidly in the combustion chamber with smaller necking diame-ter .As the necking size decreases ,the maximum generation of NO xfirst increases and then decreases ,while the tendency for the maximum generation of soot is just opposite to that of NO x.At the opening moment of exhaust valve ,the NO xemission first increases and then decreases while the soot emission shows a trend of zigzag rise .The thermal efficiency first decreases and then increases with the decrease of necking diameter .%基于某大功率农用柴油机,对燃烧室不同缩口尺寸下柴油机缸内燃烧和排放规律进行了仿真计算,结果发现:减小缩口尺寸可以增大缸内湍动能,有利于缸内燃气混合,促进缸内流动;减小缩口尺寸,缸内压力减小,但缸内最大压力变化不大;缸内温度则随缩口尺寸的不同而略有变化,在燃烧室结构中设置缩口可以在一定程度上改善燃烧过程,但缩口尺寸并非越大越好;对于不同缩口的燃烧室,发火时刻缸内速度场基本相同,上止点处速度场随着缩口直径的减小略有增强,当燃烧进入到缓燃期,对于缩口直径较小的燃烧室,燃烧速率迅速下降;随着缩口直径的减小,缸内NO x最大生成量先增大后减小,而炭烟最大生成量先减小后增大;排气门打开时刻,NO x排放先增大后减小,炭烟排放则呈锯齿形上升;热效率随缩口直径的减小先增大后减小.