燃料敏感性对内燃机部分预混燃烧(PPC)光谱特性的影响研究

在一台光学发动机上，利用火焰高速成像技术和自发光光谱分析法，研究了燃料敏感性(S)为0和6时对发动机缸内火焰发展和燃烧发光光谱的影响。试验过程中，通过改变喷油时刻 (SOI=-25，-15和-5°CA ATDC) 使燃烧模式从部分预混燃烧过渡到传统柴油燃烧模式。通过使用正庚烷、异辛烷、乙醇混合燃料来改变燃料敏感性。结果表明，在PPC模式下(-25°CA ATDC)，火焰发展过程是从近壁面区域开始着火，而后向燃烧室中心发展，即存在类似火焰传播过程，同时在燃烧室下部未燃区域也形成新的着火自燃点。敏感性对燃烧相位影响较大，对缸内燃烧火焰发展历程影响较小；高敏感性燃料OH和CH带状光谱出现的时刻推迟，表明高敏感性燃料高温反应过程推迟，且光谱强度更低，表明碳烟辐射强度减弱。在PPC到CDC之间的过渡区域(-15°CA ATDC)，燃烧火焰发光更亮，燃烧反应速率比-25°CA ATDC时刻的反应速率更快。高、低敏感性燃料对缸压放热率的影响规律与-25°CA ATDC相近，此时的燃烧反应更剧烈，放热率更高，碳烟出现时刻更早。该喷油时刻下的光谱强度高于PPC模式下的光谱强度，说明此时的CO氧化反应与碳烟辐射更强。在CDC模式下(-5°CA ATDC)，由于使用的燃料活性较低，燃烧放热时刻过于推迟，放热量很小，缸内燃烧压力低，因此燃料敏感性对缸压和放热率的影响不明显，但从燃烧着火图像中可以看到高敏感性燃料的火焰出现时刻较低敏感性燃料推迟。低敏感性燃料的燃烧初期蓝色火焰首先出现在燃烧室中心，着火火焰出现时刻更早，之后蓝色火焰从中心向周围扩散，呈现火焰传播为主导的燃烧过程；燃烧后期，局部混合气过浓区导致亮黄色火焰面积逐渐增大并向周围扩散。高敏感性燃料的火焰发展趋势与低敏感性燃料类似，黄色火焰的亮度与面积更小。尽管高、低敏感性燃料的OH和CH带状光谱的出现时间相近，但高敏感性燃料的光谱强度仍更低。综合分析，火焰发展结构与自发光光谱特征主要受喷油时刻的影响，燃料的敏感性主要影响着火时刻和火焰自发光光谱强度，且高敏感性燃料的光谱强度更低。

Study on the Influence of Fuel Sensitivity on the Spectral Characteristics of Partially Premixed Combustion in Internal Combustion Engines

In this paper, the effect of fuel sensitivity (S) of 0 and 6 on the flame development and combustion luminescence spectrum in the engine cylinder are studied in an optical engine by using flame high-speed imaging technology and self-luminescence spectroscopy. The combustion model was transitioned from the new partially premixed combustion to diesel combustion mode by changing the injection timing (SOI= -25, -15, -5°CA ATDC), and the fuel sensitivity is changed by using n-heptane, iso-octane, ethanol mixed fuel. The results show that in the PPC model(-25°CA ATDC), the flame development process starts from the area near the wall and then develops toward the center of the combustion chamber and it is a similar flame propagation process, and a new unburned area is formed in the lower part of the combustion chamber. Sensitivity has a more significant impact on the combustion phase and less on the development history of the combustion flame in the cylinder. The high-sensitivity fuel suppresses when the OH and CH band spectra appear, and the spectral intensity is lower. With the change of fuel injection time, the spectral change trend of the two sensitive fuels is the same. The high-sensitivity fuel suppresses the high-temperature reaction process, weakens the soot radiation and reduces the spectral intensity. In the transition area between PPC and CDC (-15°CA ATDC), the combustion flame glows brighter than -25°CA ATDC, and the combustion reaction rate is faster than the reaction rate at -25°CA ATDC. The influence of high and low sensitivity fuels on the cylinder pressure heat release rate is similar to -25°CA ATDC. The combustion reaction is more intense, the heat release rate is high, and the soot appears earlier. The intensity of the spectrum at this time of fuel injection is higher than that in the PPC model, indicating that the CO oxidation reaction and soot radiation are stronger. In CDC mode, due to the low fuel activity used, the time of combustion heat release is too delayed, the heat release is small, and the combustion pressure in the cylinder is low, which is close to the misfire condition. Therefore, the effect of fuel sensitivity on cylinder pressure and heat release rate is not significant. The blue flame of low-sensitivity fuel first appears in the center of the combustion chamber at the beginning of the combustion, and the ignition flame appears earlier, and then the blue flame spreads from the center to the surroundings, showing a combustion process dominated by flame propagation. In the later stage of the combustion, the local mixed gas passes the dense zone causes the bright yellow flame area to increase and spread to the surroundings gradually. The flame development trend of high-sensitivity fuel is similar to that of low-sensitivity fuel, and the brightness and area of yellow flame are small. The appearance time of OH and CH band spectra of high-sensitivity and low-sensitivity fuels is similar, and the spectral intensity of high-sensitivity fuels is low. It may be because at the time of fuel injection, the flame retardation period is long enough, and the oxidation of ethanol in the susceptible fuel is the dominant factor. Comprehensive analysis shows that the flame development structure and spectrum development process are mainly affected by fuel injection time. The fuel sensitivity mainly affects the ignition time and flame self-luminescence spectrum intensity, and the spectrum intensity of low-sensitivity fuels is great.