Abstract
Predictions of the mixture formation process inside gasoline DI engines are strongly required to improve both the fuel consumption rate and the exhaust emissions. Swirl-type injectors are commonly used for gasoline DI engines, as its spray characteristics are favorable for gasoline DI engines. The spray formation of a swirl-type injector consists of a liquid sheet and droplets that arise from the breakup of the liquid sheet. Numerical simulations of free sprays formed by a swirl-type injector are carried out on the basis of a method of DDM (Discrete Droplet Model) with employing different models for the breakup of the liquid sheet and that of droplets downstream. The breakup of the liquid sheet is modeled by Reitz's wave breakup model while that of droplets downstream adopts TAB (Taylor Analogy Breakup) model. Schematic diagram is illustrated in Figure 1. In this study, the boundary condition of the ambient pressure is set at two values ; a negative pressure and a high pressure. The droplet deformation calculated by the breakup model is incorporated into the drag force term to take the influence of the drag variations into account. The results of calculated parcels on centered vertical cross-section are shown in Figure 2. Figure 2(a) is the result assuming that the drag force is of a rigid sphere (without droplet deformation), while Figure 2(b) is the result assuming that the drag force is of an ellipsoid (with droplet deformation). As a result, by taking the drag force variation due to deformation of droplets into account, calculations under high ambient pressure can reproduce the change of spray shape. The drag force of droplets and the pressure difference of spray inside and outside cone contribute to the change of the spray shape. This pressure difference is small when the ambient pressure is low.
