Break-up of liquid fuel films from the surfaces of the intake port and valve in port-fuel-injected engines

Abstract

The atomization of liquid fuel films from the intake port and valve surfaces was investigated. The start-up mixture preparation condition for a port-fuel-injected (PFI) engine was considered in which fuel spray from the injector forms liquid films and puddles upon impact of the cold port and valve surfaces. These fuel films are then re-atomized as a result of aerodynamic and other forces as the intake valve opens. Little information is available on this re-atomization process, which can lead to large droplets and ligaments entering the combustion chamber, resulting in poor combustion. A steady flow axisymmetric valve chamber was used in conjunction with an optically accessible research engine to study this fuel film break-up in detail. Two general atomization forces were observed: Gravitationally (or mechanically) controlled and aero-dynamically controlled. Aerodynamic forces contributed to both ligament break-up and film shear break-up. The different modes of break-up were clearly correlated to air flow separation from the valve and port surfaces. A conceptual model of fuel film break-up from valve and port surfaces is given. The modes of film break-up in the engine were quite similar to those observed in the steady flow experiments. The exception was the presence of break-up due to valve motion. The intake manifold pressure strongly affected the amount of fuel build-up in the port, which in turn affected the fuel pooling and break-up. Injector targeting was also determined to affect the accumulation and atomization of liquid fuel.