Research On Soot Formation Characteristics In Combustion Process Using Laser Induced Incandescence

The reduction of particulate matter(PM) emissions has become an important issue for both diesel and gasoline direct injection(GDI) engines nowadays. Laser induced incandescence(LII), as an optical diagnostic technique, is a powerful tool to study the soot formation characteristics of the fuels and in-cylinder soot concentration distributions. The LII test results can not only provide validation for soot models, but also help to understand more about the soot formation and oxidation process, which is an important step for the reduction of engine-out PM emissions.This work studies the soot formation characteristics in combustion processes by quantitative measurements of soot concentration using LII and laser extinction method(LEM). A LII-LEM measurement system that is able to measure the soot concentration in high-temperature high-pressure environments was designed and built. The problems of significant beam steering and real-time correction of background noise were solved when performing LEM test in high-temperature high pressure environment. The problem of frequency fluctuation of the laser synchronizing signal when performing in-cylinder LII test was also sovled. To examine the validity of the technique, it was used to measure the soot concentration distribution in the ethylene laminar diffusion flame and also in a high-temperature high-pressure constant volume vessel. A linear correlation between LII and LEM test results was shown and this can be used to examine if a cerntain LII test was seriously affected by signal trapping effect.The auto-ignition and soot formation characteristics of naphtha and wide distillation fuel jets were studied using LII-LEM and OH chemiluminescence imaging in a constant volume vessel. For naphtha, results show that compared with diesel, naphtha has longer flame lift-off length, first-soot distance and soot inception time, and lower peak soot concentration. With decreasing ambient temperature, the differences between these two fuels increase. For the wide distillation fuel, results show that with increasing gasoline proportion in the fuel, the liquid spray length decreases linearly, the flame lift-off length, first-soot distance and soot inception time increase, the peak soot concentration decreases, the location of initial soot formation and the peak soot concentration region move from the edge to the center of the flame. The trends of first-soot distance and soot inception time are closely related to the trend of the lift-off length.Finally, quantitative measurement of in-cylinder soot concentration distribution of a GDI engine was achieved using LII-LEM. The effects of air-fuel ratio and injection strategies on soot formation characteristics were studied. Results show that when the piston fuel film exists, the peak soot concentration in the pool fire region can be several times higher than that in the free space of the combustion chamber. By using double injections(the first one in intake stroke, the second one in compression stroke) and optimizing the injection timing of the second injection, the pool-fire-soot can be reduced effectively. In homogeneous charge condition, the in-cylinder soot level increases slightly when the excess air ratio decreases from 0.9 to 0.8, while the soot level increases significantly when the excess air ratio decreases from 0.8 to 0.7.