ILASS Europe, 21 st Annual Conference on Liquid Atomization and Spray Systems, Mugla, Turkey, 2007. PIV MEASUREMENT OF INJECTION PRESSURE INFLUENCE ON GAS ENTRAINMENT IN GDI-ENGINES B. Prosperi 1,2 , J. Hélie 2 and R. Bazile 1 1 Institut de Mécanique des Fluides de Toulouse, 31400 Toulouse, France. prosperi@imft.fr 2 SIEMENS VDO Automotive, BP1149,1 av. Paul Ourliac, 31036 Toulouse cedex 1, France ABSTRACT : The gas entrainment induced by a hollow-cone spray under various injection conditions is studied experimentally in order to evaluate the effects on mixture formation. Particle Image Velocimetry with Fluorescent tracers (F-PIV has been applied to allow measurements in the close vicinity of the spray-edge. Next, the obtained data (instantaneous velocity fields) have been processed in a specific way. In the "quasi-steady" region of the air-flow surrounding the spray, the injection pressure - and back pressure effects on the mass-flow rate of entrained gas can be predicted by a semi-empirical model that takes the injection parameters into account. Keywords: GDI, piezo-electric injector, spray, Fluorescent PIV, injection pressure, gas entrainment 1. INTRODUCTION To achieve fuel economy and to reduce pollutant emissions, a new generation of injectors has allowed to implement injection-strategies like lean-stratified combustion modes [1][2]. Based on stability and reliability criteria for the mixture formation, the "spray-guided" direct injection strategy has been chosen to optimize the combustion process. In a recent study, the benefits of Gasoline Direct Injection (GDI) with piezo-injectors has been shown by Siemens VDO Automotive [3]. These results are used in the present study. The main advantages of these injectors are their wide operating range and their highly reproducible injection behaviour. In a previous article [4], the influence of air-density on gas entrainment was studied under constant injection pressure. The present work deals with the influence of injection pressure on air entrainment in case of hollow-cone dense sprays (figure 1). The aim of the present work is to establish a link between mixture formation [5] and injection-conditions. Rottenkolber et al. [6] have proposed an adaptation of the Particles Image Velocimetry using Laser-Induced Fluorescence (FPIV). This technique is used in the present study to measure air entrainment in dense two-phase air-flows. Under realistic engine conditions, this method allows the measurement of the gas-velocity flow-field all around and very close to the spray edge. Moreover, when low-pressure injections are performed under atmospheric conditions, this method allows monitoring the gas velocity flow field inside the spray. A typical case is given as an illustration. From the measurements performed under various conditions, the axial evolution of the air-entrained cumulative mass-flow rate is computed. Next, these results are compared with those obtained with an integral model [7] developed for an axi-symmetrical full-cone spray. Although this model shows good correspondence for the global flow-rate over the Z-axis with experimental results over the region of interest [8], the model does not correctly predict the local air velocity in a direction perpendicular to that of the surface of the cone. Whenever, a basic axial-evolution of the air-entrained velocity is proposed and fitted with a reference case. The injection global parameters dependence of the integral-model is used in combination with the empirical fit for the air-entrained velocity to enable a semi-empirical model to predict the effects under various injection conditions. Figure 1: Gasoline injection spray. 2. EXPERIMENTALAPPARATUS 2.1 Injection test bench and spray characteristics GDI automotive fuel system is used to pressurize gasoline up to 200 bars. The gasoline direct injector is an outward-opening piezoelectric driven one generating a 80° conical liquid sheet due to its annular shaped orifice (4.2 mm diameter). The atomization induced by this kind of injector produces very fine droplets that have been characterized under various injection pressures with Malvern Spraytech granulometer. As the spray is dense, measurements could be performed between 30 mm and 75 mm far from the nozzle under various injection pressures in atmospheric condition. As integrated measurements along laser path, results for droplet mean diameter should be taken with care and will be taken as rough estimations in the present study (Table 1). Finer PDA measurements [9] are on course to characterize precisely the droplet-size poly-dispersion distribution. R (mm) Z (mm)