Citation: Marchi, A., Nouri, J. M., Yan, Y. & Arcoumanis, C. (2010). Spray stability of outwards opening pintle injectors for stratified direct injection spark ignition engine operation. International Journal of Engine Research, 11(6), pp. 413-437. doi: 10.1243/14680874JER605 This is the accepted version of the paper.This version of the publication may differ from the final published version. Abstract: The spray characteristics of three prototype piezo-electric pintle-type injectors was investigated under different operating conditions in an optical engine equipped with direct injection system and designed for spray-guided combustion concept. The pintle-type outward opening has the potential to overcome many of the typical problems related to spray-guided injection. Its hollow cone spray presents a better air utilization than the multi-hole with good penetration performance during early injection and a negligible dependence of the spray angle from the backpressure; the latter is essential for spray-guided approach to be successful as it fully depends on spray stability. The three prototype injectors are designed with different nozzle exit geometries for optimisation of the injectors for the most stable spray at all engine conditions. The emeringing fuel sprays, for single and double injection, were visualised using Mie scattering and a high speed CCD camera. The injectors' performance were assessed by constructing mean and RMS images at different operating conditions of injection pressures, back pressure, injector needle lift, and engine speed. From these images a through angle analysis were performed by comparing the mean, standard deviation, maximum and minimum spray cone angle at different conditions; the spray stability could be quantified by analysising the mean and RMS images and mean and RMS spray cone angles. Overall, the classification of the three prototypes has shown that the Inward Seal Band positive step design produces the most robust spray angle, which is ideally suited for stratified fuel mixture formation in spray-guided configurations for DISI engines. Permanent

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ABSTRACT

The internal nozzle and near the nozzle exit flows of an enlarged transparent model of an outwards opening injector were investigated for different flow rates and needle lifts under steady state flow conditions. A high resolution CCD camera, high speed video camera and an LDV system were employed to visualize the nozzle flow and quantify the tangential velocity characteristics. The images of the internal flow between the valve seat and the square cross-section end of the needle guide revealed the presence of four separated jet flows and four pairs of counterrotating vortices with each pair bounded in-between two adjacent jets. The counterrotating vortices are highly unstable with a circumferential oscillatory motion which was transmitted to the spray outside the nozzle with almost the same frequency. The dominant circumferential frequencies at the nozzle exit were identified by FFT analysis of the tangential velocities. A linear relationship exists between the dominant frequencies and the flow Reynolds number based on injection velocity and needle lift. Magnified images of the flow just outside the nozzle exit showed formation of interconnecting streamwise strings on the liquid film as soon as it emerges from the annular exit passage. The interspacing between the strings was found to be linearly related to injection velocity and almost independent of the needle lift.

Computational fluid dynamics (CFD) methods have been used to investigate the internal nozzle flow of an outwards-opening piezo-driven pintle injector designed for sprayguided direct injection gasoline engines. The internal nozzle flow has been investigated for various nozzle designs, with emphasis placed on the effect of manufacturing tolerances on the internal and near-nozzle flow characteristics. The methods employed include features such as moving wall boundaries and time-dependent pressure or flowrate inlet conditions, cavitation as well as Eulerian and Lagrangian near-nozzle and spray models. The results reveal that not only the nozzle internal geometric details and manufacturing tolerances influence significantly the flow conditions at the exit of the pintle injector, but the actual spray characteristics are significantly influenced by the external geometry of the nozzle housing and the pintle shape. The atomization process of the liquid emerging from the pintle nozzle seems to be different from that realized in other nozzle designs used in direct injection gasoline engines; a so-called string-type of spray is formed at the nozzle exit, as confirmed by near-nozzle CCD spray images. The mechanism of string formation is attributed to the limited liquid volume passing through the needle seat area and partly occupying the available volume, while the details of the geometry in this location may enhance local air entrainment. The velocity differences along the circumference of the nozzle exit magnify those flow instabilities and have a predictable effect on the dispersion of the liquid droplets near the nozzle exit.