| The development of internal combustion engine has to be faced with the huge stress of energy and environment protection. HCCI (Homogeneous Charge Compression Ignition) combustion could break through the low PM and NOX limit of conventional diesel engines, so it has become an attractive technology for diesel engines. But diesel fuel has high distillation temperatures, diesel homogeneous charge preparation is one of the key challenges for diesel HCCI. Focused on diesel homogeneous charge preparation, the fuel film evaporation characteristics and spray hot-impingement process were experimentally investigated. Based on the results of the above experiments, diesel quasi-homogeneous charge was prepared by spray hot-impingement, premixed HCCI and early direct-injection HCCI were fulfilled. Electrostatic charging atomization of diesel spray was also investigated to explore another diesel atomization technology.By employing TGA (Thermogravimetric analysis), the effects of heating rate, fuel film thickness and environmental flux were investigated. Compared with petrochemical distillation measurement method and apparatus developed in papers, states of fuel film in TG analyzer are closer to the fuel film existing conditions in internal combustion engines, film mass and temperature could be simultaneously precisely measured and recorded, the environmental flux also could be precisely controlled. Hence, TGA has more advantages in fuel film evaporation investigation, and TGA is especially suitable to study the relationship between the wall temperature and the film evaporation. It was revealed that, the evaporation sequence could not be strictly divided by the boiling points of each component for multi-component dissolved mixture during a quick evaporation process, and the heavier components could vaporize before reaching their boiling points. Films of 0# diesel fuel would fully evaporate when film temperature was beyond 250℃. The increase of heating rate would shorten fuel film evaporation time distinctly, but the end-point temperatures were scarcely affected. The thinner initial film thickness would result in a lower temperature for the same evaporated ratios. The slight change of environmental flux would evidently affect fuel film evaporation process.A diesel spray hot-impingement experimental system was set up, and the effects of wall temperature and heating power on the behavior of impinging spray were investigated at atmospheric pressure by employing high-speed camera video. It was found that, the thermal radiation of hot metal board could accelerate the atomization and evaporation of the spray front before impingement. When the wall temperature was higher than 200℃, the atomization of impinging spray became evident, the spray projection area began to hugely increase. When the wall temperature was at 260~360℃, the spray projection area changed a little, but spray uniformity could improve with wall temperature increase. If the wall temperature exceeded 360℃, the spray projection area would decrease dramatically with wall temperature increase, but spray uniformity would be further improved. The effect of wall heating power was displayed by the change of wall temperature. This experiment supplied reference data for the following design of spray hot-impingement apparatus used in engine tests.By employing the spray hot-impingement apparatus designed in this paper, premixed diesel HCCI was fulfilled, and the effects of cooled external EGR, and intake heating were investigated. This apparatus is compact and can be fixed at the inlet of intake port. Only the metal wall of the equipment was heated, and it could be heated to be 250°C with about 4 minutes (power supply is 600 W, no air flow), then the engine could be started. The intake mixture temperature was only about 40°C higher than that of the intake air before the equipment, so the heat load of the engine would not be improved distinctly. It was revealed that the fuel-air mixture made by spray hot-impingement was quasi-homogeneous, and the limit of HCCI combustion was PM emission. Without other means, HCCI combustion run in the range ofλ=4 ~ 5 at the engine speed of 700 ~ 1000 r/min, when the hot surface temperature was about 370°C. Cooled external EGR could extend the high load limit of this system toλ≤2.6. Intake heating could extend the low load limit toλ≥6.8.By employing the spray hot-impingement method, the atomization and evaporation of early direct-injection fuel would be promoted, then the early direct-injection diesel HCCI could be realized. The effects of compression ratio and fuel supply advance angle were evaluated. An air-gap-insulted piston was designed, and the nozzle hole cone angle was reduced. So wall temperature of certain part on the piston top was improved, and spray hot-impingement would also be realized. The atomization and evaporation of early direct-injection fuel were improve by hot-impingement, and the engine could run atλ=5.5~7.5 (600r/min) with HCCI combustion. When compression ratio was increased, the two heat release phases all advanced. HC, CO and exhaust opacity were reduced, but NOX emission increased dramatically. If fuel supply timing was retarded, peak value of the first heat release phase would decrease with nearly unchanged combustion timing, and peak value of the second heat release phase would be increased with the advanced combustion timing. Along with the fuel supply delay, the in-cylinder combustion mode would be gradually transferred from HCCI to conventional spray diffusion combustion , so HC and CO emissions could effectively reduce, but NOX would increase sharply.The diesel electrostatic charging atomization was realized under high injection pressure on an applied fuel supply system, and this technology was employed to improve the diesel atomization. The model of break-up critical charge mass ratio for diesel droplet was constituted based on Rayleigh model, and the critical charge mass ratio was deduced to be 10-6~10-4C/g. In this paper, the designed high-pressure injection charger could realize the electrostatic atomization under the maximum injection pressure of 45~67MPa, and the atomization was improved after electrostatic charging. But the obtained charge mass ratio was relatively low. |
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