|In spark ignition engine, mixture formation process is critical issue to reduce emission from combustion, such as particulate matter. Smaller liquid droplet, higher homogeneity, and higher vaporization are pursued as target of direct injection technologies. But, due to limitation of plume direction and high vapor pressure of fuels, especially in wall-guided system, spray structure collapse under flashboiling conditions are rising as a supreme question to be avoided. Flashboiling is a spray expansion with drastic pressure drop of ambient across the vapor pressure. In spark ignition engine, cone-like-shaped spray were developed to achieve wallguided mixture formation, so that the injector nozzles have been optimized to the representative fuel; gasoline. By following the gasoline engine technologies, Liquefied Petroleum Gas (LPG) spray were also developed, but the spray shapes are significantly different due to higher vapor pressure and boiling characteristics. Typical conical sprays under flashboiling circumstances show a contracted shape with expanding shape after nozzle outlet, accompanying large penetration and recirculation vortex. But, the LPG, primarily composed with propane and n-butane, have quite different from gasoline spray, because it has different physical characteristics and it would be continuously exposed to boiling ambient condition. Each plume of LPG fuel spray was wider compare to gasoline spray, due to higher volatility and larger vapor pressure, so that the interaction between plumes was severe compare to typical liquid fuels. Accordingly, optimization processes of nozzle designs were necessary to achieve better mixture characteristics. In this paper, single-, double-, tripleand 6-hole nozzle were examined, and the influences of counter-bore on n-butane spray structure were identified. Under flashboiling conditions, the number of nozzles had impact on inter-plume interactions, more severe spray collapse with higher numbers. In addition to the number effects, counter-bore also make the spray structure sensitive to flashboiling, which could deteriorate mixing processes. But, in an aspect of penetration of the sprays, due to large momentum exchange of fuel droplet with evaporating n-butane, higher vaporizing rate under flashboiling could reduce spray penetration with less nozzle injectors. In case of 6 nozzle injector, the tendency followed that of liquid fuel spray, but with less number of nozzle than 3 the flaring tendency of penetrations was not observed. This transition was originated from high volatility of nbutane, which causes not only expansion of bubble in liquid core, also in each droplet to evaporate faster. To establish certain model for n-butane spray, the models in preceding researches are not sufficient, so that it is necessary to evaluate the evaporating character of the fuel in ambient and the design of injector nozzles in terms of spray structures.