1 ILASS-Europe’99 Toulouse 5-7, July 1999 STRUCTURE OF SPRAYS GENERATED BY PRESSURE SWIRL INJECTORS FOR DIRECT-INJECTION GASOLINE ENGINES E. Abo-Serie, C. Arcoumanis and M. Gavaises Thermofluids Section, Department of Mechanical Engineering Imperial College of Science, Technology and Medicine London, UK and B. Argueyrolles and F. Galzin Research Division Renault SA, France ABSTRACT The transient spray generated by a high pressure swirl injector, used in emerging direct-injection gasoline engines, was visualised under atmospheric conditions by a high magnification and resolution CCD- based imaging system. Following a delay period between signal initiation and first appearance of fuel at the nozzle exit, four stages have been identified during the spray development: a very early asymmetric poorly atomised jet penetrating in the central part of the nozzle hole with a tip velocity proportional to injection pressure, an asymmetric non-hollow spray, a swirl-developing hollow-cone spray with a multi-layer structure, and a fully-developed and well atomised hollow-cone spray with a cone angle nearly independent of injection pressure. The first two stages correspond to the period before the liquid film is formed in the nozzle hole and the last two to the period of film development. Due to the transient nature of the spray and its varying structure with time it has been necessary to estimate the penetration of the spray in two different ways; in terms of its leading edge during the first two stages and in terms of the distance to its maximum width at the last two. High magnification images of the annular liquid sheet at the nozzle exit also revealed the co-existence of two modes of droplet formation, through ligaments in the direction of injection and through wave crest stripping in the radial direction. INTRODUCTION As research into direct-injection spark-ignition (DISI) gasoline engines intensifies in anticipation of their increasing penetration into the passenger car market, it becomes clear that a prerequisite for optimisation of their performance and emissions is the understanding of the spray characteristics as a function of injection and chamber pressure. This becomes even more important in view of the dual strategy of DISI engines where for full load operation the fuel is injected during the induction stroke, in order to generate a homogeneous stoichiometric or slightly rich mixture, whereas for part load operation the fuel is injected during late compression towards a piston cavity in order to form a stratified mixture at the spark plug at the time of ignition at overall air/fuel ratios in excess of 40. This mode of mixture preparation depends on optimisation of the piston cavity, the air motion and the spray structure and is critical to the stability of the engine and the NOx and hydrocarbon emissions. This project focuses on the spray characteristics from a swirl pressure atomiser driven by a common-rail system; the fuel is injected at various injection pressures into the atmosphere, which can be considered representative of injection during the induction stroke corresponding to full load conditions. The spray is visualised with a non-intensified CCD camera in order to identify its structure at various times during the injection period. The imaging work described here represents an extension to the computational work of [1] where a combination of 1-D and two-phase CFD models has allowed the internal nozzle flow to be identified and initial conditions at the nozzle exit to be estimated as inputs to CFD spray models. Furthermore, it will provide useful information for the modelling of the atomisation process of the injected liquid, since a number of recent investigations, for example see[2-5] have employed the linear instability analysis to account for this flow phenomenon. EXPERIMENTAL SYSTEM