Wall Impingement Process of a Multi-Hole GDI Spray: Experimental and Numerical Investigation

The Direct Injection (DI) of gasoline in Spark Ignition (SI) engines is very attractive for fuel economy and performance improvements in spark ignition engines. Gasoline direct injection (GDI) offers the possibility of multi-mode operation, homogeneous and stratified charge, with benefits respect to conventional SI engines as higher compression ratio, zero pumping losses, control of the ignition process at very lean air-fuel mixture and good cold starting.The impingement of liquid fuel on the combustion chamber wall is generally one of the major drawbacks of GDI engines because its increasing of HC emissions and effects on the combustion process; in the wall guided engines an increasing attention is focusing on the fuel film deposits evolution and their role in the soot formation. Hence, the necessity of a detailed understanding of the spray-wall impingement process and its effects on the fuel distribution. The experimental results provide a fundamental data base for CFD predictions.In this paper investigations have been performed using a 7-hole injector, 0.179 mm in hole diameter, spraying in a constant volume vessel with optical accesses. To examine the effects of various factors on development of the spray impinging on the wall, experiments have been conducted at different injection pressures, diverse wall inclination angles and at atmospheric pressure. The acquired images have been processed for extracting the characteristic parameters of the impinging fuel at the different operative conditions.The multi-hole spray has been simulated by Star-CD code taking into account the commercial gasoline properties and the real mass flow rate derived from experimental measurements. In order to correctly reproduce spray impingement and fuel film evolution, a numerical methodology has been defined. Lagrangian sub-models and numerical parameters have been validated against experimental results

Optical Characterization of Biodiesel and Diesel Fuel Sprays from a CR Injection Apparatus

The fuels from renewable resources have obtained an increasing interest for transport application in the last decade because of their biodegradability, potential improvements on exhaust emissions and benefits on the virtuous CO2 cycle of the earth. The different physic-chemical characteristics of the biofuel, respect to diesel fuel, affect the combustion phenomenon in diesel engines being different the droplets distribution in the combustion chamber and, consequently, the air-fuel mixture preparation in the ignition delay ready to be burned. The recent trend to enhance the spray atomisation, increasing the injection pressure and the hole number in the nozzle to better distribute the fuel, imposes a deep understatement of the spray characteristics in terms of tip penetration, cone-angle, droplet velocity, fragmentation and vaporization. The modern Common Rail (CR) injection apparatus enable a management of injection strategy both in terms of injection pressure and injection number and timing per cycle. They allow to exploit all the potentiality of modulated combustion in engine for NOx and noise reduction, acting on pilot and pre-injections, and matter particulate using post and late injections. In this paper a study of overall behaviour of spray from rapeseed methylester (RME) biofuel and diesel fuel has been carried out in an optically accessible vessel filled with inert gas (N2) with pressure ranging between 0.1 to 5.0 MPa. The injections were obtained by a CR apparatus driven by a Programmable Electronic Control Unit (PECU) enabling different strategies performances. The injector mounted an axially disposed cylindrical single hole nozzle (0.18 mm in diameter and 1.0 mm in thickness) and the investigated injection pressures have been 60, 90 and 120 MPa. The sprays have been lightened by a pulsed sheet (100 μm in thickness and 10 ns in duration) generated by the second harmonic of a Nd-YAG laser and matched at different instant from the start of injection. The jet images have been captured by a CCD camera collecting the light scattered at right angle and synchronized with the light sheet. A digital pulser/delayer has allowed finely investigating the entire spray duration (1.0 ms). The spray characteristics have been extracted by a digital image processing software. Tip penetration and spray cone-angle have been strictly measured while droplet velocities and nozzle discharge coefficient have been derived from the data. The diverse characteristics of the investigated fuels have produced quite differences in sprays global performances in terms of penetrations and cone-angles. These differences have shown a no-monotonic behaviour during the spray duration and their relationships with the injection pressure have been observed

Desktop X-ray tomography for low contrast samples

Based on the experience in the use of polycapillary optical systems, recently XLab Frascati LNF and IM CNR have been strongly involved in studying the techniques for high resolution X-ray Imaging and micro-tomography that intends in the development of a new imaging instrument to examine low con-trast samples complicated by fast developing processes. In order to get the reliable signal to noise ratio, typically available via synchrotron radiation (SR) dedicated X-ray optical devices, for the desktop solu- tions we have to increase the radiation fluxes from conventional sources. As known, manipulated through polycapillary optics beams result in getting higher fluxes with respect to a pin-hole (with a gain factor of 102–103). Moreover, polycapillary semi lenses can provide low divergent beams comparable by the divergence with synchrotron radiation beams (mrad order). These features make possible the realization of high resolution imaging of low contrast samples in the transmission mode without various algorithmic processes as typically done, for instance, for phase contrast imaging. This work presents the results on X-ray micro tomography for both static biological and fast dynamic samples as well as a possible future development of a polycapillary-based experimental layout for bio-medical imaging diagnostics, for the studies in material and environmental sciences, for diagnostics ofhi-tech samples, etc

GDI spray structure analysis by polycapillary s-ray μ-tomography

A X-ray l-tomography technique, using a Cu Ka source at 8.048 keV coupled with both polycapillary optics and CCD detector, has been developed to reconstruct the composition of a transient gasoline spray generated by a high-pressure GDI injector for automotive applications. The polycapillary elements enable shaping the divergent beams and getting high-contrast images due to the suppression of radiation multiple scattering. A pressure-tight device permits the 360 rotation of a six-hole nozzle, with a step of 0.1, at injection pressures up to 20 MPa, while the spray plume develops in a vented Plexiglas chamber at the atmospheric backpressure. The entire system is configured as a table-top experiment. The extinction images acquired along the X-ray source-spray-detector line-of-sight have permitted the reconstruction of a 3D structure together with a morphology of the jets within a 3 mm region downstream the nozzle. The spray shape as well as the propagation direction can be clearly identified in the tomographic reconstruction for all the six jets. Quantitative measurements of the fuel mass density in the near nozzle region have been performed. Typical Gaussian-shape distribution of the intensities appears for the cross sections revealing the more dense jet regions in the core, while slight longitudinal asymmetries indicate an interaction between the jet plumes

Experimental And Numerical Investigation On Mixture Formation In A HDDI Diesel Engine With Different Combustion Chamber Geometries

One of the most important phases in thedevelopment of direct-injected Diesel engines is theoptimisation of the fuel spray evolution within thecombustion chamber, since it strongly influences boththe engine performance and the pollutant emissions.Aim of the present paper is to provide information aboutmixture formation within the combustion chamber of aheavy duty direct injection (HDDI) diesel engine formarine applications. Spray evolution, in terms of tippenetration, is at first investigated under quiescentconditions, both experimentally and numerically,injecting the fuel in a vessel under ambient temperatureand controlled gas back-pressure. Results of penetrationand images of the spray from the optically accessiblehigh pressure vessel are used to investigate thecapabilities of some state of the art spray models withinthe STAR-CD software in correctly capturing sprayshape and propagation.The experimental investigation is carried outusing a mechanical injection pump equipping a heavyduty eight cylinder engine. Only one of its plungers isactivated, and the fuel is discharged through a sevenhole nozzle, 0.40 mm in diameter, mounted on amechanical injector. Tests are carried out at two differentload fuel amounts, representing 50%, and 100%respectively, and results are used as data base for theCFD setup. CFD analyses of the intake andcompression strokes are at first performed in order tocompare two different combustion chambers anddifferent jet orientations with respect to the combustionchamber cavities, running the engine under motoredconditions and injecting 50% load fuel amount. Both thetwo tested pistons show two-stage deep valve pocketshollowed under the valve seats projections, but somerelevant differences exist in the piston outer region andin the squish area.Subsequently, full CFD analyses of the intake,compression and combustion processes are performedfor the two different combustion chambers and thepreviously optimised jet orientation, operating the engineat full load, maximum revving speed. Numericalpredictions are used to assess the influence of bothcombustion chamber shapes on the mixture formationeffectiveness and the engine-out emissions

3D structure of liquid sprays: x-ray µ-radiography and tomography by polycapillary based technique

This work reports the results of X-ray μ-tomographic investigation on the inner structure of high pressure fuel sprays. X-ray imaging is widely used in industry where non-destructive and high accuracy measurements of the samples morphology are required. A high flux beam can overcome the problems related to the low absorption of hydrocarbon chains as fossil fuels, therefore synchrotron X-ray sources are generally used for fuel sprays investigation. A desktop facility has successfully been used to characterize high pressure gasoline sprays for automotive applications. A X-ray tube coupled with polycapillary optics has been used providing a high flux beam with low divergence. In this paper the last improvements concerning quantitative measurements carried out on fuel sprays are reported