직접분사식 액체 LPG 분무특성에 관한 연구

Abstract

Liquefied petroleum gas (LPG) has been used as motor fuel due to its low emissions and low cost. The fuel feeding system has been improved with stringent requirement for exhaust emissions. LPG carburetion system was first introduced, then the system has been changed to a precisely controlled gas injection system, but this gas feeding system has a limitation on improving power output. In order to improve an engine performance, a multi-point port injection system was introduced recently, and a liquid direct injection system into a cylinder was suggested as a next generation system to maximize a fuel economy as well as a power.

This study addresses the analysis of the LPG spray from single hole injector. The spray images are visualized and compared with diesel or gasoline sprays in a wide injection and ambient gas pressure ranges. A short arc strobo having flashing duration of 20micro-s and CCD camera of Pulnix are used to take the images. The injection pressure is generated up to 150MPa by Haskel air driven pump. And the ambient pressure is adjusted by pumping air into a constant volume chamber. Three different test conditions are given, which are the continuous injection case with various injection pressures, the spray development case with various injection pressures and the spray development case with ambient pressure variation. The LPG spray photographs are compared with sprays of gasoline and diesel fuel at the same conditions, and the spray angles and penetration lengths are also compared, then the spray behavior is analyzed.

The LPG spray photos show that the dispersion characteristics depends very sensuously on the ambient pressure soon after injection. The spray angle is very wide in the low ambient pressure conditions until 0.3MPa, but the angle value is quickly reduced at the condition over the pressure. However the down stream of the LPG spray still shows much wider dispersion and less penetration than those of gasoline and diesel sprays due to fast evaporation. That says the LPG fuel in liquid phase by high injection rail pressure at the nozzle tip is quickly evaporated into gas phase in the low ambient pressure cases, but the spray leaving the nozzle is remaining as liquid phase in the high ambient pressure cases. Therefore the spray is dispersed widely with low penetration like LPG gas injection in the low ambient pressure case, and the spray is less dispersed and longer penetrated like liquid gasoline spray in the high ambient pressure cases, but the behavior at the spray tip is much wider and less penetrated than those of gasoline and diesel. It indicates that the behavior of LPG spray might be more dependent on the gas flow motion in cylinder than the spray injection velocity. Therefore, the chamber and intake port shape might be careful to get a suitable flow motion for designing direct injection LPG engines.