高海拔模拟环境下柴油机燃烧粗暴可视化试验研究

Optical Experiments on Diesel Knock Under Simulated High-Altitude Conditions

基于一台可视化快速压缩机试验平台,结合高速摄影和瞬态压力测试等手段,通过调控燃烧边界条件开展了高海拔(4500m)模拟环境下柴油机燃烧粗暴可视化试验研究.选择4种不同十六烷值(CN)的柴油燃料,研究了喷油压力和燃料十六烷值对柴油喷雾撞壁过程和燃烧粗暴强度的影响.研究结果表明:燃油喷雾撞壁和燃料十六烷值对燃烧粗暴特性影响显著.在喷雾撞壁工况下,提高喷油压力会使自燃时刻提前,自燃反应前锋的传播速度加快,燃烧粗暴强度增加;在相同喷油压力情况下,延长喷雾撞壁距离却使峰值压力显著下降,同时降低了燃烧粗暴倾向.同时,低十六烷值燃料的燃烧粗暴倾向明显高于高十六烷值燃料.然而,对于CN<45的柴油而言,降低十六烷值并不会使燃烧粗暴强度发生显著变化,说明低十六烷值燃料的燃烧粗暴特性对喷油压力和撞壁距离更加敏感.通过分析高速摄影图像发现,燃烧粗暴起源于近壁面混合气自燃,超音速自燃反应前锋在封闭燃烧室中的传播可诱发压力振荡,从而有可能会加速燃烧室核心部件损坏和烧蚀.研究结果对于高海拔重型柴油机燃烧过程优化和燃烧粗暴控制具有重要现实意义.

Based on a rapid compression machine,this work investigated optical experiments on diesel knock under simulated high-altitude(4500m)conditions through adjusting combustion boundary conditions and synchronically performing high-speed direct photography and simultaneous pressure acquisition.Four diesel fuels with different cetane numbers were selected to study the effects of fuel injection pressure and cetane number(CN)on the spray impingement process and diesel knocking intensity.Results show that spray impingement and cetane number have significant effects on the diesel knock.Under spray impingement conditions,autoignition is advanced with the increase of the fuel injection pressure,and the propagation speed of the autoignition reaction front is accelerated,thus enhancing the diesel knocking intensity.For the given injection pressure,a longer wall distance decreases the peak pressure significantly and reduces the tendency to diesel knock;meanwhile,diesel knock is more likely to be encountered with a low cetane number fuel.However,for the diesel of CN<45,the reduction of cetane number does not significantly change the diesel knocking intensity,which indicates that the knock intensity of the low cetane number fuel is generally sensitive to the injection pressure and wall distance.Further analysis of visualization images shows that the diesel knock originated from the near-wall autoignition.The propagation of supersonic reaction fronts in a confined combustion chamber results in a destructive pressure oscillation,which may accelerate the damage and ab-lation of core components in the combustion chamber.These research results have important practical significance for combustion process optimization and the control for diesel knock of high-altitude heavy-duty diesel engines.