Effects of Fuel Temperature and Ambient Pressure on a GDI Swirled Injector Spray

Abstract

The effects of fuel temperature on both the geometry and the droplet size and velocity of a GDI swirled injector spray were investigated by means of visualizations and PDA measurements. Isooctane was used as model fuel and was injected in a quiescent bomb at injection pressure of 7 MPa. Bomb pressure ranged from 40 kPa to 800 kPa with injector nozzle temperature ranging from 293 K to 393 K. A drastic change in spray geometry was observed when conditions above the vaporization curve were reached. The temperature increase has two macroscopic effects on the spray geometry: at the nozzle exit the liquid flash boiling strongly enlarges the spray angle, at a certain distance from the nozzle the air entrainment collapses the spray. Raising the fuel temperature up to flash boiling conditions causes a significant decrease of the average droplet size. INTRODUCTION The potential advantages of gasoline direct injection (GDI) in terms of both fuel economy and efficient transient control are almost universally recognized. However, these advantages are dependent on a correct mixture preparation and distribution inside the cylinder. This means that both a proper atomizer and a thorough understanding of the processes involved in spray formation, vaporization and fuel distribution are needed [1]. High pressure swirl atomizers are so far the most common design proposed to fulfil the requirements of a real engine. At relatively moderate injection pressure they present a well atomized spray with a fairly limited penetration. At standard ambient conditions a pressure swirled injector produces a typical hollow cone spray structure [2]. However the spray characteristics of these injectors are strongly dependent on both internal and external operating conditions. In fact the hollow cone structure can disappears because of rail pressure variations [3]. Moreover the spray structure observed under cold conditions can change at high fuel temperature [3, 4]. In this work pure isooctane was injected (by a prototype Magneti Marelli injector) in a quiescent bomb. Injector tip temperature and bomb pressure were parametrically changed at constant injection pressure. Spray geometry and penetration were observed by simple visualization methods, droplet size and velocity were measured by Phase Doppler Anemometry (PDA). EXPERIMENTAL SETUP AND PROCEDURE The experimental apparatus usually employed at CNR-TeMPE for unsteady spray optical diagnostics [5-7] was slightly modified for this study. The stainless steel injection bomb (sketched in Figure 1) has internal diameter 206 mm and height 300 mm. It has four 100 mm diameter 40 mm thick glass windows positioned at 0°, 110°, 180° and 270° angles. It can be pressurized up to 1 MPa and electrically heated up to 473 K. The injector injected downward on the bomb axis and was placed in a holder presenting a cavity for the heat exchange fluid circulation. SAE 15W-40 oil heated in a thermostatic bath was forced by a circulation pump in the injector holder. A K-type thermocouple, placed in contact with the injector tip wall, was used to control the injector temperature. Due to the low injection frequency ( <1 Hz) and the low fuel flow rate, the fuel present inside the injector nozzle was in thermal equilibrium with the wall. A 0.5 mm diameter thermocouple was placed inside the injector pipe at the position of the internal filter and was used to control the fuel temperature stability. A piston accumulator was used to pressurize the fuel at 7 MPa by means of pressurized nitrogen. A membrane-type accumulator was placed near the injector to dampen pressure oscillations during injector opening. Pressure variations of ±150 kPa around a steady value were measured by means of a Kistler 601A transducer placed at the end of the injector hose. The spray bomb was pressurized by nitrogen and for tests below atmospheric pressure a compressed air vacuum generator was used. The bomb pressure was measured by means of a Kistler 4075A10 transducer. The injection opening pulse duration was set at 3 ms by the control unit.