Multi-hole Injectors for DISI engines: Nozzle Hole Configuration Influence on Spray Formation

High-pressure multi-hole injectors are one candidate injector type for closed-spaced direct injection (DI) gasoline engines. In such a system, the spark plug must be located close to the spray and, during stratified operation, the spray is ignited very soon after the fuel droplets have been vaporized. Thus there are very high demands on the sprays used in such a system. An additional challenge is the positioning of the spark plug relative to the spray; both consistent ignitability and the absence of liquid fuel droplets must be achieved. Many injector parameters influence spray formation; for example, hole diameter, length to hole diameter ratio, nozzle hole configuration etc. This paper investigates the spray formation and spray induced air movement associated with rotational symmetrical and asymmetrical nozzle hole configurations. Four different nozzles with different hole configurations and umbrella angle were investigated both experimentally and numerically in a heated/ pressurized spray chamber. Their influence on spray formation, spray induced air motion, cross-flow velocity, fuel/air ratio, turbulence and cycle-to-cycle variations were studied. It was found that rotational symmetrical configurations produce non-coherent isolated clouds of fuel. If an asymmetrical configuration is used instead (holes positioned along a horseshoe-shaped arc) then, by choosing the injector configuration carefully, it is easier to obtain a coherent fuel cloud; this also facilitates better control over the conditions at the spark plug, for example the fuel/air ratio, cross-flow velocity and turbulence. Furthermore, asymmetrical nozzles benefit from smaller fuel gradients and enhanced mixing between spray plumes as a result of shorter spray plume distance and improved spray-induced air motion. All the nozzles tested produced partially premixed vapor clouds, with cycle-to-cycle variations. These variations may be an important issue for ignition stability in a closed-spaced combustion system