| Gasoline direct injection(GDI) is an important means to improve spark-ignition engine performance. The spray characteristics of the air-assisted direct injector are different from the common high pressure direct injector. Because it has high quality atomization effect in the lower fuel-injection pressure, and its spray is the mixture of fuel and air. There are few domestic and foreign researches about this kind of injector applied to GDI engine. So it was investigated that the macroscopic and microscopic characteristics of spray of an air-assisted direct injector by the visual experiments with high-speed photography and laser diffraction in this paper. And the forming process of mixture in-cylinder was simulated and studied by using the simulation model of GDI engine which was established base on test data. It was analyzed that the effect of injection parameters and airflow motion in-cylinder on the spray characteristics and mixing process.The visual experiments were completed by using a constant volume bomb and an optical engine which had a completely transparent cylinder. The constant volume bomb can provide the same temperature and pressure condition like engine cylinder in the time of fuel injection, but it is very difficult to imitate airflow motion in-cylinder perfectly. And it cannot be reflected that the influence of fuel injection time, the shape of piston head and engine speed on the spray characteristics of GDI engine. To cover this shortage the optical engine was developed which had a completely transparent cylinder. It can be researched more accurately and completely that the effect of various parameters on spray characteristics by using two experimental platform. The spray fuel-air ratio(γ) was defined according to the feature of air-assisted direct injector. To illustrate the influence of airflow motion in-cylinder on the spray, some new concepts were introduced first like the left semi-cone angle of spray(α), the right semi-cone angle of spray(β) and the deflection of spray angle(Φ). In addition, it was also defined that the spray penetration(L) and the spray angle(θ) which were common parameters. The spray characteristics were described better by these five parameters. Base on these, it was analyzed that the effect of spray fuel-air ratio, fuel-injection pressure, ambient pressure, fuel injection time, the shape of piston head and engine speed on the spray characteristics of the air-assisted direct injector. And it was measured and researched that the sauter mean diameter(SMD) of this nozzle by using laser particle size analyzer.The simulation model of GDI engine was established base on the computational fluid dynamics software(AVL-FIRE). To guarantee the simulation result credibility and accuracy, the intake port model and spray model of GDI engine simulation model were validated by comparing with experimental data. Afterwards it was simulated and analyzed that the influence of SMD, fuel injection time and the shape of piston head on the mixing process and fuel concentration distribution.The research results of microscopic characteristics of spray show that the SMD of the spray from the air-assisted direct injector was proportional to the spray fuel-air ratio(γ). Especially the SMD becomes smaller obviously when the spray fuel-air ratio(γ) is less than 0.8. And the SMD reduced and tended to be stable with prolonging the injection interval. The performance of atomization is improved as the injection pressure increase.The research results of macroscopic characteristics of spray show that in the constant volume bomb the spray angle decreased with the value of the spray fuel-air ratio(γ) reducing(air injection quantity increase or fuel injection quantity decrease). But the effect of injection quantity on spray penetration is no significance. Under the same experiment conditions, the spray penetration is longer and the spray angle is smaller with injecting aviation kerosene than using gasoline. The spray angle and penetration of using two different fuel both increase as the fuel injection pressure increase. With the ambient pressure rise up, the spray penetration of using two different fuel both reduces, and the spray angle with injecting aviation kerosene decrease, but the spray angle with injecting gasoline is no significant variation.The research results of mixing process in-cylinder show that in the optical engine because of the influence of intake airflow, spray angle enlarges and spray penetration reduces as the air injection quantity increases. In addition, the variation tendency of spray angle is markedly different to the experimental results under the condition of constant volume bomb. Spray is deflected by the influence of airflow. With delaying injection timing,the impact of airflow on spray is weakening,and deflection of spray angle is decreased and spray penetration distance is increased. Spray angle,semi-cone angle of spray and deflection of spray angle are decreased with decrease of engine speed with no significant variation in spray penetration distance. The spray characteristics were changed by the effect of in-cylinder air flow field with different fuel injection time, engine speed and the shape of piston head. In addition, the spray space distribution in the cylinder and the collision location with piston were modified by the impact of these factors. Then it was also influenced that the fuel evaporation and the concentration distribution of mixture in-cylinder. |
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