Modeling of air-assisted and flash boiling sprays in gasoline direct injection engines

Gasoline Direct Injection (GDI) engines for automobiles have shown 10 to 15% better fuel economy than conventional port-injected engines as well as improved performance, but production GDI engines produce hydrocarbon (HC) emissions that are too high to meet U.S. emissions standards. A major contributor to HC emissions is the impingement of liquid fuel onto the piston and cylinder walls during engine operation. In fact, most production GDI engines are designed to utilize impingement for distribution and vaporization of the fuel because current GDI fuel injectors produce large droplets at high velocities, and there is not enough time for the fuel spray to vaporize before it impacts on the piston and cylinder walls.; In order to reduce impingement and HC emissions, a “soft” fuel spray must be produced that penetrates slowly and vaporizes quickly. Two potential methods for producing soft sprays are air-assisted injection and flash boiling. Air-assisted fuel injection uses highly pressurized air to atomize the fuel before it leaves the injector. Flash boiling uses bubble nucleation and growth in superheated fuel to further shatter the fuel spray as it exits the injector.; A simplified bubble growth model was developed to capture the bubble size distribution in a flash boiling fuel spray. The low computational cost of the model makes it possible to simulate the growth of thousands of bubble parcels, which are numerical entities that consist of a number of bubbles assumed to have identical properties.; Impingement simulations were made for air-assisted sprays and flash boiling sprays. Both the air-assisted sprays and flash boiling sprays reduced fuel impingement substantially compared with a conventional GDI fuel injector. The air-assisted fuel injector produced the softest sprays in this study, but flash boiling may be more cost-effective for the automotive industry. The modeling methods developed by this work may be useful for the optimization and design of air-assisted and flash boiling fuel injectors in future GDI engines. Moreover, the simplified bubble growth model can be applied to the simulation of bubbly flows in many applications ranging from nuclear reactor coolant safety to refrigeration systems.