过浓系数与对置活塞发动机的燃烧效率

Coefficient of richness and combustion efficiency inopposed-piston engines

针对空气利用率用于描述对置活塞(OP2S)二冲程压燃式发动机油气相互作用规律时局限性较大的问题,给出过浓系数的定义并以之作为对置活塞二冲程压燃式发动机缸内油气混合效果的评价指标。在此基础上通过基于CONVERGE软件的数值模拟研究了初始涡流比和喷油器倾角对OP2S燃烧效率的影响规律。结果表明:过浓系数可以排除平均当量比的影响并有效反映喷油参数对油气混合效果的影响。提高涡流比可以强化喷油过程中的油气混合,同时改变喷油过程形成的燃油径向分布。过小或过大的涡流比使燃油过度集中于气缸轴线附近或气缸壁附近,造成局部过浓,喷油结束后的燃烧效率下降。整个循环的燃烧效率由喷油过程中及喷油结束后的燃烧放热过程共同决定。在所研究的范围内,涡流比对燃油径向分布的影响可以使燃烧效率在0.6~1.0的范围内变化。喷油器倾角同样可以改变燃油径向分布位置及燃烧效率,且这一影响可以与涡流对燃油径向分布规律的影响相叠加。

Opposed-piston 2-stroke(OP2S)compress ignition engines have different fuel-air interactions compared with ordinary CI engines,while air utilization has proved inadequate in measuring fuel-air mixing in CI engines.In this study,the concept of coefficient of richness was proposed as an index of fuel-air mixing in OP2S engines to replace air utilization.Numerical simulation was carried out with CONVERGE software to study the effect of swirl ratio and injector yaw on combustion efficiency in OP2SCI.Results showed that the coefficient of richness was effective in representing fuel-air mixing regardless of average equivalence ratio.Larger swirl ratio could improve fuel-air mixing during injection while affecting radial distribution of fuel at end of injection.Fuel concentration near either cylinder axis or cylinder wall could hinder fuel-air mixing after injection,decreasing combustion efficiency.Combustion efficiency could vary between the range 0.6 and 1.0 of swirl ratio.Injector yaw may also impact radial distribution of fuel and in turn the combustion efficiency.This effect was taken into account jointly with that from swirl.