Research Of The Diesel Engine Quasi-Conical Spray System Based On Near-Wall Impingement

Simultaneously reducing PM and NO_X emissions has been one of the most important tasks for diesel engine researchers. For the purpose of developing high efficiency and low emission diesel engines, a great deal of work has been done on new diesel combustion research. Spray impingement, a usually happening phenomenon in small and medium diesel engines, is always one of the focuses of this kind of research. Effective utilization of spray impingement could result in better fuel atomization and higher fuel-air mixture quality, and accordingly lower fuel consumption and harmful emissions. The research in this dissertation just takes this aim. Based on Diesel Hot Premixed Combustion concept and the application of Conical Spray systems, this dissertation investigates measures to improve spray structure, fuel-air distribution, as well as combustion in cylinder with the application of near-wall impingement spray. The following research work has been performed:(1) With the application of AVL FIRE 3D CFD package, impingement sprays' characteristics and development process were simulated. The results indicate that Fuel-air distribution characteristics of impingement spray is largely affected by impingement angle, distance and orientation surface shape. In the condition of short distance, limited impingement area and appropriate impingement angle, spray penetration can be adjusted in a certain extent. The penetration increases as the consequence of smaller impingement angle. Compared with free spray, short distance impingement spray performs with bigger spread angle and width, shorter liquid kernel length, and earlier vaporization's happening. Variation in the percentage of impinged fuel leads to further change of fuel-air distribution characteristics.(2) According to the principle of secondary atomization led by spray impingement, near-wall impingement quasi-conical spray combustion system was developed, in which multi-hole nozzle and impingement orientation part take main roles. Fuel jets injected from multi-hole nozzle impinge on inclined orientation surface and spread along the surface as shape of sectors, by which a quasi-cone shape spray is formed. The spray penetration can be regulated according to the adjustment of impingement angle in design. Impingement spray systems are classified into three kinds according to different orientation surface shapes, which are outer cone-surface, inner cone-surface and multi-piece impingement spray system respectively. Based on the percentage of impinged fuel, the spray is categorized as whole-impingement spray and half-impingement spray. (3) Optical measurement system was set up to study spray process. The system employed high speed camera and digital image processing program to investigate spray distribution characteristics as well as spray parameters such as spray penetration, spray angle, spray diffusion area, etc. the investigation reveals that the three impingement sprays have similar distribution characteristics. With the increase of ambient pressure, spray penetration and spread area decreases, but spray angle, maximum width, and thickness increase. Under the condition of higher needle valve opening pressure, spray penetration and angle increase. Under the same condition, comparing whole-impingement spray and half-impingement spray, the former has bigger spray angle, maximum width and spread area but shorter penetration than the latter. Half-impingement spray has advantages of impingement spray and free spray. Half-impingement spray has bigger spray spread area and longer spray penetration, which result in reasonable distribution of fuel in combustion chamber.(4) Fuel-air mixture formation and combustion process were simulated. The calculation results demonstrate that whole-impingement spray mode has quite short penetration, which means most part of fuel as well as ignition points concentrate in the central part of combustion chamber, and the air in outer part of combustion chamber cannot be utilized effectively, moreover, combustion performance deteriorates greatly. On the contrary, half-impingement spray mode has much longer penetration than the former mode. Accordingly, in this mode air utilization, fuel-air mixture distribution and combustion performance are considerably improved. In any case, fuel impingement angle should be reasonable. Otherwise, too much momentum loss would result in stagnation of fuel near impingement location. With the help of squish and anti-squish flow in re-entrant chamber, the amount of such stagnant fuel is largely decreased and then immoderate rich fuel-air mixture is reduced and even diminished. As for the size of impingement device, as small as possible size should be chosen to reduce its negative effect on cylinder flow and the amount of stagnant fuel. From the perspective of in-cylinder mixture formation process and combustion efficiency, half-impingement mode is better than whole-impingement mode, multi-piece impingement mode is better than inner conical-face impingement mode, and inner conical-face impingement mode is better than outer conical-face impingement mode.(5) Combustion performance of close-impingement quasi-conical spray system was tested on a single cylinder 135 diesel test engine. In order to intensify fuel-air mixing process and decrease local immoderate rich fuel-air mixture around impingement location, E20G15 blend fuel was used to defer ignition time, improve fuel-air mixture quality and promote burning rate. The test results indicate that the combustion performance of half-impingement mode is better than that of whole-impingement mode. Compared with free spray, as the fuel consumption decreasing, the soot emission increase slightly with E0 blends fuel. With the combination of close impingement and E20G15 diesohol, ignition delay period is prolonged remarkably, mixture speed and the degree of homogeneity of fuel-air mixture are improved obviously. In the mid and low operation range, soot and NO_X emission are decreased simultaneously for the lower in-cylinder pressure and temperature. Ethanol-diesel-gasoline blend fuel has the characteristics of good atomization, good mixture formation quality, long ignition delay period, high combustion speed rate, and so on; with the combination of close impingement Quasi-Conical Spray System and this blend fuel, diesel combustion performance is improved.