双燃料燃烧是一种实现内燃机高效清洁燃烧的新型燃烧方式,国内外对燃用双燃料的内燃机性能和排放开展了较为广泛的研究,但对双燃料缸内燃烧过程的认识有待深入.本文搭建了一套光学发动机缸内燃烧中间产物激光测试系统,该系统可以实现甲醛和羟基(OH)的二维同时定性测量.为了验证该激光诊断系统的可行性,首先在甲烷层流预混火焰上对甲醛和羟基的激光诱导荧光(LIF)光谱和图像进行采集,确定甲醛和OH激发波长分别为355和282.95 nm.随后在光学发动机上对双燃料缸内燃烧过程中甲醛和羟基进行了非同时测量,分析了双燃料燃烧双阶段放热过程中甲醛和OH分布区域.光学发动机转速为1200 r·min-1,循环当量总油量为30 mg正庚烷.进气冲程初期气道喷射异辛烷,上止点前10°曲轴转角在缸内直喷9 mg正庚烷.激光诱导荧光成像表明,甲醛生成于低温放热阶段,主要分布在缸内直喷燃油油束附近区域,之后甲醛充满整个燃烧室空间;高温放热过程中燃烧室壁面附近区域的甲醛首先消耗,伴随甲醛消耗OH首先出现于燃烧室边缘,高温放热阶段过后,甲醛基本消失, OH逐渐充满整个燃烧室.最后对双燃料缸内燃烧过程甲醛和OH同时测量发现,甲醛消耗伴随OH的产生,甲醛和OH分布区域总体而言在空间上是分开存在的,但在局部区域甲醛和OH可能并存.
Dual-fuel combustion is a promising method for achieving high-efficiency clean combustion in internal combustion engines. Most current research focuses on the effects of dual-fuel injection on engine performance and emissions. Our understanding of dual-fuel in-cylinder combustion processes needs further investigation. In this study, an optical diagnostic system was established to determine the intermediate products during in-cylinder combustion; the system enabled simultaneously qualitative two-dimensional measurements of formaldehyde and OH radicals. To confirm the feasibility of using this laser diagnostic system, laser-induced fluorescence (LIF) spectra and images of formaldehyde and OH radicals in a laminar premixed methane flame were acquired; the excitation laser wavelengths for formaldehyde and OH radicals were verified to be 355 and 292.85 nm, respectively. Non-simultaneous determination of formaldehyde and OH radicals in the combustion chamber was performed to analyze the two-stage heat release process and distribution regions of formaldehyde and OH radicals during dual-fuel combustion. In this investigation, the engine speed was kept at 1200 r·min-1 and the total equivalent fuel quality was 30 mg of n-heptane. Isooctane was injected in intake manifold at the beginning of the intake stroke and n-heptane (9 mg) was directly injected into the cylinder at 10° crank angle before compression top dead center. The results indicate that formaldehyde is formed in the low-temperature heat-release stage and is mainly located in the region near the spray jet; formaldehyde then fills most of the combustion chamber. When the high-temperature heat-release stage is initiated, formaldehyde located at the edge of the combustion chamber is consumed first, followed by consumption of formaldehyde in the center region. Accompanied with the disappearance of formaldehyde during the high-temperature heat-release stage, OH radicals first emerge at the edge of the combustion chamber and later fill the whole combustion chamber.