Improvement Of Engine Combustion By Oxidizer Design

With the increase of vehicle population and the stringency of emission regulation, one key problem in energy and environmental fields is to decrease emissions from internal combustion (IC) engines. In order to improve combustion and decrease emissions, this paper studies oxidizer design of IC engines. Based on the researches on the characteristics of steady-state combustion flame of LPG and lightweight diesel fuel which are mainly used for IC engines as gas and liquid fuel in different oxidizer environment, the paper emphatically studys the oxygen-enriched combustion and performance of LPG engine during cold-phase and diesel engine during both cold-phase and steady-state with composite intake air.A cup burner equipment was designed and built. With the burner, the characteristics of steady-state combustion flame of LPG and lightweight diesel fuel in different oxidizer environment were studied. For LPG, the effects of oxidizer composition on LPG partial premixing and diffusion flame were observed. When the flowrate of O2/N2 oxidizer is fixed, with the decrease of O2 concentration, the length and brightness of the diffusion flame first increase and then decrease, the distance between the bottom of the visible flame and the rim of the combustion cup increases till the flame disappears. When the flowrate of O2/N2 oxidizer is fixed, with the increase of N2 flowrate, the height of LPG diffusion flame increases, the visible length firstly increases and then decreases, the brightness gradually weakens, peak flame temperature drops and the high temperature area reduces. Through the numerical simulation of LPG diffusion flame, it is shown that with the increase of oxygen concentration of O2/N2 oxidizer, the combustion area reduces and its center moves to the rim of the cup. At the same time, the high temperature layer becomes thicker and the area of high temperature centralizes. For lightweight diesel, the effects of oxygen concentration of oxidizer on the diffusion flame and smoke of fluid level combustion were investigated and the effects of composition and flowrate of oxidizer on the flame of wick combustion were emphatically studied. The results show that the diesel diffusion flame of both fluid level combustion and wick combustion can be divided into two main parts: the lower taper flame and the upper brush-shaped flame. For diesel fluid level combustion, with constant O2/N2 oxidizer flowrate, the increase of oxygen concentration in oxidizer makes the height of diffusion flame increases. when the oxygen concentration is low, the smoke is much; when the oxygen concentration rises, the smoke disappears and the further increase of oxygen concentration makes smoke re-appear. For wick combustion, with the increase of oxygen concentration, the height of flame first increases to the peak value and then drops. Compared with N2, CO₂ has more evident effect on the combustion of both LPG and diesel.The intake air oxygen-enrichment systems for cold-start study of LPG engine and diesel engine were built based on gas membrane seperation technology. For LPG engine, the characteristics of combustion and emission in first cycle were studied based on cycle-controlled method and the running behaviors during start/warm-up and hot idle stages with different oxygen concentration intake air were compared. For the diesel engine, the effects of oxygen concentration in intake air at borderline temperature on combustion and emissions were emphatically studied. The results show that for both LPG engine and diesel engine, OEA makes start speed rise. At the same time, with the increase of oxygen concentration in intake air, soot, HC and CO emissions significantly decrease while NOx emissions increase. Through the analyses of in-cylinder combustion in first cycle of LPG engine during cold start, it shows that with OEA, the peak of in-cylinder pressure increases and the corresponding phase advances. At the same time, the heat release becomes quicker and the phase corresponding to the maximum heat release rate advances too. The analyses of in-cylinder combustion of diesel engine during cold start show that with OEA, the ignition timing obviously advances and the in-cylinder pressure rises compared with ambient air.A system which can supply composite intake air was built. With the same fuel supply timing, the characteristics of combustion and emission were studied when the diesel engine was supplied with oxygen-enriched intake air (OEA) and the intake air diluted with inert gas Ar,N2 and CO₂. The results show that with OEA, the soot, HC and CO emissions all decrease significantly while NOx emissions increase obviously. When the ambient air is diluted with Ar, N2 or CO₂, the soot emissions increase while NOx emissions decrease. With the same addition amount into intake air, the use of CO₂ leads to the smallest increase of soot and largest decrease of NOx emissions. Based on the obvious differences of OEA and inert gas on combustion, the characteristics of combustion and emission of diesel engine with composite intake air including O2, CO₂ and N2 were studied and obtained the composition-optimized intake air with which the soot and NOx emissions can be reduced simultaneously. Then composition-optimized intake air was applied to diesel engine for further study. The results show that with the above composition-optimized intake air, the NOx and soot emissions were reduced by 40% and 50%, respectively, compared with ambient air. At the same time, the indicated thermal efficiency just dropped a little bit.When the composition-optimized intake air is supplied to the diesel engine, with the simultaneous increase of O2 and CO₂ concentration in intake air, the adiabatic and polytropic exponent gradually decrease, ignition delay lengthens and mean indicated pressure drops. Soot and NOx emissions both reduce and HC and CO emissions scarcely vary.The characteristics of combustion and emission of the diesel engine using OEA was compared to that using oxygenated blend fuel. For OEA, the oxygen concentration was 22.1%. The oxygenated blend fuel included 10%DMC and 90%diesel. The fuel supply timing was fixed at 28℃A. The research results show that compared to oxygenated blend fuel, OEA makes longer combustion duration and lower indicated thermal efficiency and soot emissions as well as higher NOx emissions. With OEA, the HC and CO emissions are lower than those with oxygenated blend fuel and the diesel engine can tolerate more amount of EGR.The in-cylinder combustion and emissions with composite-optimized intake air were simulated using KIVA3V code coupled with the turbulence chemistry interaction model. At the same time, the effects of CO₂ and O2 in intake air on combustion and emission were analyzed. The results show that, with the simultaneous increase of O2 and CO₂ concentrations in composite-optimized intake air, in-cylinder temperature, NO and O atom concentrations all decrease and the areas of high temperature and concentration reduce. When the CO₂ concentration in intake air is relatively low, thermal effect on reducing NO is weak. But after the CO₂ concentration is more than 15%, thermal effect becomes the mail factor for NO. Compared with the replacement of N2 with CO₂, the decrease of in-cylinder temperature, NO, O2 and OH concentration is more obvious than CO₂ is directly added into intake air. The in-cylinder combustions of diesel engine were respectively simulated with 21% and 25% oxygen concentration intake air. With the increase of oxygen concentration, the in-cylinder O2 amount increase, OH and O concentration rises. The area of the high OH and O concentration expand too.