멀티홀 인젝터의 플래시 분열 모델링 및 직접분사 불꽃점화 가솔린 기관의 중부하 해석에의 적용

Abstract

In common driving situation, the engine operates at middle load. In this condition, flash boiling occurs for Direct-Injection Spark-Ignition (DISI) gasoline engines. To analyze mixture formation inside engines in middle-load conditions with computational fluid dynamics (CFD) code, a model of the flash breakup is necessary. In this study, a flash breakup model was developed and applied to investigate mixture formation and the combustion process in DISI gasoline engines. The flash breakup model was developed based on bubble dynamics. The Rayleigh-Plesset equation was employed to predict bubble growth inside droplets and a single bubble was assumed for each droplet to simplify the problem. When the void fraction exceeded the critical value, flash breakup was assumed. When flash breakup was anticipated, two droplets were generated and the droplet radius was predicted using mass conservation for the droplet. A normal velocity was predicted using energy conservation, the pressure gradient between inside and outside of the droplet, and the bubble growth rate. A thermodynamic-mechanical breakup module was created using the flash breakup model developed in this study and the Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) breakup model, which predicts the breakup induced by aerodynamic forces. Depending on the condition, either the flash breakup model or the KH-RT breakup model predicted the breakup of the droplet. The initial bubble radius, surface dilatational viscosity and critical void fraction were set as model constants and the effects of these constants on penetration and bubble growth were investigated. Single-component fuel was used for validation of the flash breakup model. When the flash breakup model was used to analyze the flash boiling spray for various fuels and fuel temperatures, it over-predicted a reduction in spray tip penetration as increasing the fuel temperature for low superheat conditions. When a normal velocity was predicted, the flash breakup model captured the expansion of spray plume and structural deformation. In highly superheated conditions, deformation of the spray target was predicted. For gasoline which is a multi-component fuel, the vapor pressure of various types of fuel was selected as the fuel vapor pressure for the Rayleigh-Plesset equation. When using gasoline vapor pressure, the spray morphology was captured by simulation. Compared to the simulation without the flash breakup model, prediction of structural deformation was improved when the flash breakup model was used for low ambient pressure condition spray simulation and analysis of mixture formation in engines. Using the flash breakup model, the mixture formation and combustion processes were analyzed to investigate the effects of injection timing and injection pressure. As injection timing was retarded, the mixture homogeneity decreased. When the fuel was injected at an early stage of the intake process, the turbulent kinetic energy at BTDC 60° decreased. In case of late injection, turbulent kinetic energy at BTDC 60° increased because the positive tumble flow induced by fuel injection reduced the turbulence dissipation. When the injection pressure was increased, the mixture homogeneity increased. Turbulent kinetic energy increased when the fuel was injected at a late stage of the intake process. The spray pattern was optimized for high-load conditions and simulations for high-load and middle-load conditions were carried out using the flash breakup model. The spray pattern was optimized with a reversed U-shape and the target was raised to generate positive tumble. Since the turbulent kinetic energy was higher for the optimized case than for the reference case, the initial combustion speed increased.