Study On The Atomization And Ignition Process Of Methanol/biodiesel Blend Fuel

Oxygenated fuels are are promising alternative fuels of diesel engine to realize clean combustion. Methanol and biodiesel, attracting more and more attention recent years, are both oxygenated alternative fuels of diesel engine. The application of methanol/biodiesel blend in diesel engine can reduce the use of fossil fuels and also reduce emission pollutants. The molecular structure and physical-chemical properties of methanol and biodiesel are different from diesel. So, the atomization and ignition process of the fuels in the combustion chamber are different, either.This paper carried out studies on the small agricultural diesel engine fuelled with methanol/biodiesel blend, and the paper were divided into seven chapters. The blend fuel structure, droplet evaporation characteristics, spray characteristics, ignition process and methods to improve the blend ignition process were carried out by theoretical analysis and experimental test in this research work. Based on the studies, a new explanation on the secondary atomization process of methanol/biodiesel blend fuel was put forward, and the ignition mode of diesel engine fuelled with the methanol/biodiesel blend fuel was presented. In addition, methanol mixing ratio range in the blend fuel was determined.To study the physical and chemical properties of methanol/biodiesel blend, the mutual solubility of the blend fuel was investigated using the contact angle test. Through the small angle X-ray scattering test, the methanol droplet diameter in the blend fuel was analyzed. According to the chemical structure characteristics of the fuels, the mixed mode of methanol and biodiesel was studied by analyzing intermolecular force, and the relationship between molecule structure and physical-chemical properties was studied. The results show that, compared with diesel, methanol was easily dissolved in biodiesel. But on the micro level, it was proved that nano methanol droplets existed in the blend fuel. If 10% or 20% methanol was mixed in biodiesel, the average radius of methanol droplets in the blend was 11.00 nm or 9.88 nm. Based on molecular structure characteristics and active atomic mechanism, the formulas for estimating cetane number of alcohols and esters were put forward, and the cetane number of methanol/biodiesel blends with different mixing ratios was calculated.In order to investigate the atomization process of the blend fuel, the droplet evaporation characteristics and spray process were studied.(1) According to the evaporation characteristics of methanol/biodiesel droplet in diesel engine, the blend droplet evaporation model in convective thermal air environment was established and verified. And the effect of the environmental state parameters, mixing ratio, droplet geometry dimension on the droplet evaporation regulation were analyzed. The analysis results show that, during the blend droplet evaporation process, methanol evaporation finished earlier than biodiesel evaporation. In addition, the droplet life was shortened along with the increase of environmental temperature, flow intensity, droplet initial radius and methanol mixing ratio, extended with the increase of environmental pressure. The addition of methanol accelerated droplet evaporation.(2) Through the simulation of in-cylinder spray process and spray test at normal temperature and pressure conditions, the spray penetration, spray cone angle, Sauter mean diameter(SMD), cylinder concentration field, turbulent velocity field, fuel evaporation rate at different injection pressures and methanol mixing ratio were studied. The studied results show that, with the increase of methanol mixing ratio, the spray penetration of the blend was shortened, the spray cone angle was increased, and the fuel evaporation quality at the same crank angle was increased. Combined with the analysis of the blend structure, the addition of methanol could promote secondary atomization. Secondary atomization process of the blend droplet could be divided into three stages: evaporation of fuel on the surface, vaporization of internal methanol, droplet expansion and broken up, and the second evaporation of methanol/biodiesel blend in the injection was explained.Focused on the ignition process of diesel engine fuelled with methanol/biodiesel blend, the combustion process model of 186 F diesel engine was established. According to the characteristics of oxygenated fuels, temperature and oxygen to fuel ratio were proposed as the criterions for judging ignition of the blend in the cylinder. The results show that, if less methanol was mixed in biodiesel, biodiesel was self-ignited earlier than methanol. With the increase of methanol mixing ratio, the ignition location point of biodiesel was gradually transferred from the edge to the center of the oil beam. In the combustion chamber of diesel engine, the ignition process of methanol/biodiesel blend included three stages: self-ignition of biodiesel, spark ignition of methanol, mutual promoting ignition of methanol and biodiesel. The ignition process of methanol/biodiesel blend was the mutual promotion process of self-ignition and spark ignition. Thus, the self-ignition-spark ignition mode for methanol/biodiesel blend was proposed. In addition, with no change of the 186 F diesel engine structure, the methanol mixing ratio should be less than 14%, 16%, 24% respectively, based on the analysis of fuel ignition characteristics, engine power and rough work.As the cetane number of methanol is quite low, and high mixing ratio of methanol will result in poor self-ignition problem for the blend. The addition of cetane improvers, adjusting compression ratio and injection parameters were studied to improve the ignition process of the blend. Through the infrared spectrum and electron paramagnetic test, the reason why improvers could advance ignition of the blend was analyzed from the point of view of chemical bond cracking energy, fuel component and free radical concentration. By the 186 F diesel engine bench test, and the regulation of combustion characteristics and emission pollutants of diesel engine fuelled with the blends was investigated. The results show that, the improvers could increase the free radical concentration. Besides, the of the improvers could start decomposition reaction at lower temperature and produce chain initiators. So, the blend ignited earlier. Among the three cetane number improvers, cyclohexyl nitrate showed the best effect on shortening ignition delay, followed by isooctyl nitrate and diethylene glycol dimethyl ether.