LPG 연료의 분무특성과 확산연소에 관한 연구

Abstract

The need for more fuel efficient and less emission vehicles has driven the development of alternative fuels such as Liquefied Petroleum Gas which is capable of meeting the limits of better emission levels without many modifications to current engine design. 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 the fuel economy as well as power.

This study addresses firstly the analysis of the LPG spray from single hole injector. The spray images are visualized and compared with diesel and gasoline sprays in a wide injection and ambient gas pressure ranges. The injection pressure is generated up to 150MPa by Haskel air driven pump. And the ambient pressure is adjusted by pumping air into constant volume chamber. The LPG spray photos show that the dispersion characteristic of the spray soon after injection depends very sensitively on the ambient pressure. The spray angle is very wide in a low ambient pressure condition until the saturated pressure of 0.252MPa, but the angle is quickly reduced at the condition over the pressure. However, the down stream of the LPG spray shows much wider dispersion and less penetration than those of gasoline and diesel sprays due to fast evaporation.

Secondly this study addresses the diffusion flame of LPG spray in a constant volume combustion chamber having an impinging plate. The high speed digital camera is used to take the flame images. The LPG spray photos show that the dispersion characteristics are dependent sensitively on the ambient pressure. In a low trap pressure LPG fuel in liquid phase evaporates quickly and does not reach down easily to the impinging plate having a hot coil for ignition. The quick evaporation makes the temperature low and the spray penetration low, which makes the equivalence ratio very low near the ignition coil and then makes the ignition hard. However, in a high trap pressure the spray leaving the nozzle is remaining as liquid phase like a diesel fuel spray. The LPG gathers around the ignition site on the plate, which makes an intensive flame near the plate.

It is considered that the behavior of LPG spray might be more dependent on the gas flow motion than the spray injection velocity in the cylinder with low trap pressure. That phenomenon will be found easily on the early injection of a direct injection LPG engine. Therefore, the chamber and intake port shape might be careful to get a suitable flow motion for designing direct injection LPG engines.