Gasoline Direct Injection Sprays Research Articles

Impact of the Primary Break-Up Strategy on the Morphology of GDI Sprays in 3D-CFD Simulations of Multi-Hole Injectors

The scientific literature focusing on the numerical simulation of fuel sprays is rich in atomization and secondary break-up models. However, it is well known that the predictive capability of even the most diffused models is affected by the combination of injection parameters and operating conditions, especially backpressure. In this paper, an alternative atomization strategy is proposed for the 3D-Computational Fluid Dynamics (CFD) simulation of Gasoline Direct Injection (GDI) sprays, aiming at extending simulation predictive capabilities over a wider range of operating conditions. In particular, attention is focused on the effects of back pressure, which has a remarkable impact on both the morphology and the sizing of GDI sprays. 3D-CFD Lagrangian simulations of two different multi-hole injectors are presented. The first injector is a 5-hole GDI prototype unit operated at ambient conditions. The second one is the well-known Spray G, characterized by a higher back pressure (up to 0.6 MPa). Numerical results are compared against experiments in terms of liquid penetration and Phase Doppler Anemometry (PDA) data of droplet sizing/velocity and imaging. CFD results are demonstrated to be highly sensitive to spray vessel pressure, mainly because of the atomization strategy. The proposed alternative approach proves to strongly reduce such dependency. Moreover, in order to further validate the alternative primary break-up strategy adopted for the initialization of the droplets, an internal nozzle flow simulation is carried out on the Spray G injector, able to provide information on the characteristic diameter of the liquid column exiting from the nozzle.

Macroscopic non-reacting spray characterization of gasoline compression ignition fuels in a constant volume chamber

Recently, gasoline compression ignition (GCI) engines have become a topic of interest due to its benefits in high thermal efficiency and low emissions. Combustion in GCI engines is highly governed by the fuel stratification which is strongly associated with the spray characteristics like penetration length. Researchers have proposed both theoretical, and regression models for liquid penetration length of diesel and gasoline direct injection (GDI) spray at non-evaporating conditions. However, there are no models for gasoline sprays at elevated ambient gas temperatures, pressures, and injection pressures in literature. This research gap needs to be bridged, as it is crucial for GCI engine technology. In this study, penetration length was investigated using a high-pressure custom-made multi-hole gasoline injector. High reactivity low carbon fuel (RON 77), designed explicitly for GCI engines, and E10 certification fuel (RON 91) were used and compared. Diffused back illumination (DBI) and shadowgraph were implemented for liquid and vapor phase penetration measurement, respectively. It was found that the spray characteristics of high reactivity fuel were similar to E10 certification fuel under non-reacting conditions. Statistical analysis was performed for both liquid and vapor phase penetration lengths, and empirical models were developed with good agreements to the experimental data under high ambient gas pressure and temperature relevant to GCI engine operating conditions using GCI fuels at higher injection pressures. A ‘separation point’ was defined for the liquid phase after which it reaches a steady state, and it was demonstrated to be different from the ‘breakup time’ found in the literature.

Evaluation of a Scale-Resolving Methodology for the Multidimensional Simulation of GDI Sprays

The introduction of new emissions tests in real driving conditions (Real Driving Emissions—RDE) as well as of improved harmonized laboratory tests (World Harmonised Light Vehicle Test Procedure—WLTP) is going to dramatically cut down NOx and particulate matter emissions for new car models that are intended to be fully Euro 6d compliant from 2020 onwards. Due to the technical challenges related to exhaust gases’ aftertreatment in small-size diesel engines, the current powertrain development trend for light passenger cars is shifted towards the application of different degrees of electrification to highly optimized gasoline direct injection (GDI) engines. As such, the importance of reliable multidimensional computational tools for GDI engine optimization is rapidly increasing. In the present paper, we assess a hybrid scale-resolving turbulence modeling technique for GDI fuel spray simulation, based on the Engine Combustion Network “Spray G” standard test case. Aspects such as the comparison with Reynolds-averaged methods and the sensitivity to the spray model parameters are discussed, and strengths and uncertainties of the analyzed hybrid approach are pointed out. The outcomes of this study serve as a basis for the evaluation of scale-resolving turbulence modeling options for the development of next-generation directly injected thermal engines.

Multi-pulse shadowgraphic RGB illumination and detection for flow tracking

This work demonstrates the application of a multi-color LED and a consumer color camera for visualizing phase boundaries in two-phase flows, in particular for particle tracking velocimetry. The LED emits a sequence of short light pulses, red, green, then blue (RGB), and through its color-filter array, the camera captures all three pulses on a single RGB frame. In a backlit configuration, liquid droplets appear as shadows in each color channel. Color reversal and color cross-talk correction yield a series of three frozen-flow images that can be used for further analysis, e.g., determining the droplet velocity by particle tracking. Three example flows are presented, solid particles suspended in water, the penetrating front of a gasoline direct-injection spray, and the liquid break-up region of an “air-assisted” nozzle. Because of the shadowgraphic arrangement, long path lengths through scattering media lower image contrast, while visualization of phase boundaries with high resolution is a strength of this method. Apart from a pulse-and-delay generator, the overall system cost is very low.

Light sheet fluorescence microscopic imaging for the primary breakup of diesel and gasoline sprays with real-world fuels.

This paper describes the adaptation of the laser-induced fluorescence measurement technique for the investigation of the primary breakup of modern diesel and gasoline direct injection sprays. To investigate the primary breakup, a microscopic technique is required, and with the help of special tracer dyes, a high fluorescence signal can be achieved in the visible range of the electromagnetic spectrum, resulting in good image quality with a nonintensified camera. Besides the optimization of the optical setup for the microscopic field of view, different tracer dyes are compared, and their solubility and fluorescence are tested in the desired surrogate and real-world fuels. As a tracer, the phenoxazine dye Nile Red was found to provide sufficient solubility in alkanes as well as suitable emission and excitation spectrum for the use of the second-harmonic frequency of a Nd:YAG laser (532 nm). The good quantum efficiency delivered by Nile Red also meant that single-shot images clearly showing spray structures in regions measuring up to 3 mm by 3 mm around the nozzle outlet could be recorded. Compared to relatively easy shadowgraph techniques and complex and costly x-ray synchrotron measurements, light sheet fluorescence microscopic imaging is not overly complex yet delivers excellent data on spray structures as well as qualitative fuel distribution.

INTERPLUME VELOCITY AND EXTINCTION IMAGING MEASUREMENTS TO UNDERSTAND SPRAY COLLAPSE WHEN VARYING INJECTION DURATION OR NUMBER OF INJECTIONS

The collapse or merging of individual plumes of direct-injection gasoline injectors is of fundamental importance to engine performance because of its impact on fuel–air mixing. However, the mechanisms of spray collapse are not fully understood. The purpose of this work is to study the effects of injection duration and multiple injections on the interaction and/or collapse of multiplume gasoline direct injection sprays. High-speed (100 kHz) particle image velocimetry is applied along a plane between plumes to observe the full temporal evolution of plume interaction and potential collapse, resolved for individual injection events. Supporting information along a line of sight is obtained using diffused back illumination. Experiments are performed under simulated engine conditions using a symmetric 8-hole injector in a high-temperature, high-pressure vessel at the "Spray G" operating conditions of the Engine Combustion Network. Longer injection duration is found to promote plume collapse, while staging fuel delivery with multiple, shorter injections is resistant to plume collapse.

Internal and near nozzle measurements of Engine Combustion Network “Spray G” gasoline direct injectors

Gasoline direct injection (GDI) sprays are complex multiphase flows. When compared to multi-hole diesel sprays, the plumes are closely spaced, and the sprays are more likely to interact. The effects of multi-jet interaction on entrainment and spray targeting can be influenced by small variations in the mass fluxes from the holes, which in turn depend on transients in the needle movement and small-scale details of the internal geometry. In this paper, we present a comprehensive overview of a multi-institutional effort to experimentally characterize the internal geometry and near-nozzle flow of the Engine Combustion Network (ECN) Spray G gasoline injector. In order to develop a complete picture of the near-nozzle flow, a standardized setup was shared between facilities. A wide range of techniques were employed, including both X-ray and visible-light diagnostics. The novel aspects of this work include both new experimental measurements, and a comparison of the results across different techniques and facilities. The breadth and depth of the data reveal phenomena which were not apparent from analysis of the individual data sets. We show that plume-to-plume variations in the mass fluxes from the holes can cause large-scale asymmetries in the entrainment field and spray structure. Both internal flow transients and small-scale geometric features can have an effect on the external flow. The sharp turning angle of the flow into the holes also causes an inward vectoring of the plumes relative to the hole drill angle, which increases with time due to entrainment of gas into a low-pressure region between the plumes. These factors increase the likelihood of spray collapse with longer injection durations.

Air Entrainment in High Pressure Multihole Gasoline Direct Injection Sprays

Air Entrainment in High Pressure Multihole Gasoline Direct Injection Sprays

Experimental and numerical study of flash boiling in gasoline direct injection sprays

Multi-hole gasoline direct injection (GDI) sprays under flash boiling conditions are investigated experimentally and numerically. GDI injectors produce multiple spray plumes that expand and eventually collapse under flash boiling conditions. The experimental studies provide limited information regarding the collapsing sprays. The numerical investigations provide further insight to these complex flow fields. However, the spray injection model that is generally used in spray simulation is incapable of modelling flash boiling effects. Therefore, a new spray injection model is introduced that can capture the rapid expansion of the flash boiling sprays in the near nozzle region. The spray penetrations obtained from the simulations compare reasonably well with the experimental results. The effects of air entrainment on the spray collapse are also investigated. In the end a parametric study has been performed in order to identify the important control parameters of flash boiling in GDI injectors.

Experimental characterization of high pressure gasoline direct injection sprays

An experimental investigation of multihole gasoline direct injection sprays at high injection pressures in a quiescent test chamber is carried out. A multihole injector with six injection holes and 60° spray cone angle is used for this purpose. High-speed Mie-scattering and high-speed Schlieren visualisations are used to characterise vapour penetration, liquid penetration, spray angle and velocity under typical engine high load and low load conditions. Variations of chamber conditions are also investigated in order to assess the effects of chamber temperature, density and pressure on the spray characteristics. The important chamber parameters are identified which affect the performance of multihole injectors in spark ignition engines.

A Comparison of Experimental and Modeled Velocity in Gasoline Direct-Injection Sprays with Plume Interaction and Collapse

A Comparison of Experimental and Modeled Velocity in Gasoline Direct-Injection Sprays with Plume Interaction and Collapse

REMOVED: Correlations between particulate matter emissions and gasoline direct injection spray characteristics

REMOVED: Correlations between particulate matter emissions and gasoline direct injection spray characteristics

Differential infrared thermography of gasoline direct injection sprays

Advances in experimental techniques and simulations are necessary building blocks for the overall effort to reduce the CO2 and pollutant emissions of future gasoline direct injection engines. Wall wetting and inhomogeneous fuel distribution are two known causes of particulate emissions whose prediction within three dimensional computational fluid dynamics can be made more reliable based on accurate spray temperature data. Results from differential infrared thermography (DIT) of a hollow cone and multi-hole gasoline direct injector are presented. Compared to previous applications of DIT, the data processing and temperature calculation methods have been refined. Interpretation of the results is supported by simulations based on the light scattering and absorption properties of small droplets given by Mie theory.

Flashboiling-induced targeting changes in gasoline direct injection sprays

By definition, flashboiling is referred to as superheated injections. The sudden occurrence of boiling inside the fuel can change the spray structure dramatically. Up to 99% of all injection processes during the New European Driving Cycle and 95% during ‘Real Driving Emissions’ tests are, with respect to mid-range cars, in a state of thermodynamic non-equilibrium below the specific vapor pressure of gasoline. Considering this fact, the scientific question is not the appearance of flashboiling during the operation of stoichiometric homogeneous charge direct injection gasoline engines but the intensity of occurring spray processes and their influence on nominal spray designs. As a consequence of induced targeting changes, the positive influence of flashboiling on the droplet size distribution and the penetration depth can be counteracted. As main driving factors for targeting changes, jet-to-jet interactions can be identified. By applying appropriate nozzle design features, the potential of flashboiling can be exploited and the targeting changes of the nominal spray designs, considered negatively, are avoided mostly. This work focuses on flashboiling-induced targeting changes, the so-called phenomenon of “spray collapse”: its root cause, development and avoidance.

Time-resolved X-ray Tomography of Gasoline Direct Injection Sprays

Time-resolved X-ray Tomography of Gasoline Direct Injection Sprays

Quantitative mixing measurements and stochastic variability of a vaporizing gasoline direct-injection spray

Spark-ignition direct-injection engines operating in a stratified, lean-burn regime offer improved engine efficiency; however, seemingly random fluctuations in stratified combustion that result in partial-burn or misfire prevent widespread implementation. Eliminating these poor combustion events requires detailed understanding of engine flow, fuel delivery, and ignition, but knowing the dominant cause is difficult because they occur simultaneously in an engine. This study investigated the variability in fuel–air mixture linked to fuel injection hardware in a near-quiescent pressure vessel at high-temperature conditions representative of late, stratified-charge injection. An eight-hole spark-ignition direct-injection spray was interrogated using high-speed schlieren and Mie-scatter imaging from multiple, simultaneous views to acquire the vapor and liquid envelopes of the spray. The mixture fraction of vaporized sections of the spray was then quantified at a plane between plumes using Rayleigh scattering. Probability contours of the line-of-sight vapor envelope showed little variability between injections, whereas probability contours derived from planar, quantitative mixing measurements exhibit greater amounts of variability for lean-combustion-limit charge. The mixture field between plumes was characterized by multi-hole and end-of-injection dynamics that attract the plumes to each other and toward the injection axis, resulting in a liquid-fuel-droplet-dense merged central jet in the planar measurements. Supplemental long-working distance microscopy imaging showed the existence of fuel droplets far downstream in the region of the planar laser measurements.

CFD optimization for GDI spray model tuning and enhancement of engine performance

► Two examples of CFD based optimization in the design of a GDI SI engine are given. ► The Simplex algorithm is used to automatically validate a GDI spray model. ► Portability of the model to three different GDI multi-hole injectors is proven. ► Optimization of the mixture formation to minimize a GDI engine consumption is also realized. ► Optimal double injection reduces fuel consumption more than single injections. Coupling a 3D Computational Fluid Dynamics (CFD) tool with a rigorous method of decision making is becoming indispensable in the design process of complex systems, as internal combustion engines. CFD based optimization (CFD-O) is here carried out on a single cylinder, four-valve, four-stroke gasoline direct injection (GDI) engine, to enhance mixture formation under stratified charge operation, hence to choose between the single or double injection strategy maximizing the engine power output. A 3D engine model is coupled with the Simplex algorithm to find the optimal synchronization of both injection and spark timing within the working cycle. CFD-O is also addressed to perform the validation of the gasoline spray model, that otherwise reveals tedious and time-consuming. The Simplex algorithm is used to tune the constants entering a model developed by authors, as applied to three different high pressure GDI injectors, preliminary experimentally characterized. Fully automatic procedures are assessed to be exploited in the phase of engine design, whose contribution may be of great importance to reduce development costs and time-to-market of new technologies.

Fuel spray dynamic characteristics of GDI high pressure injection system

In order to improve the fuel consumption and exhaust emission for gasoline engines, gasoline direct injection (GDI) system is spotlighted to solve these requirements. Thus, many researchers focus on the investigation of spray characteristics and the fuel formation of GDI injector. This paper presents a complete numerical and experimental characterization of transient gasoline spray from a high pressure injection system equipped with a modern single-hole electric controlled injector in a pressurized constant volume vessel. The numerical analysis is carried out in a one-dimensional model of fuel injection system which is developed in the AVL HYDSIM environment. The experimental analyses are implemented through a self-developed injection rate measurement device and spray evolution visualization system. The experimental results of injection rate and spray dynamics are taken to tune and validate the built model. The visualization system synchronize a high speed CMOS camera to obtain the spray structure, moreover, the captured images are taken to validate the injector needle lift process which is simulated in the model. The reliability of the built model is demonstrated by comparing the numerical results with the experimental data. The formed vortex structure at 0.8 ms is effectively disintegrated at 6.2 ms and the spray dynamics become rather chaotic. The fuel flow characteristics within injector nozzle extremely influence the subsequent spray evolution, and therefore this point should be reconsidered when building hybrid breakup GDI spray model. The spray tip speed reach the maximum at 1.18 ms regardless of the operation conditions and this is only determined by the injector itself. Furthermore, an empirical equation for the spray tip penetration is obtained and good agreement with the measured results is reached at a certain extent. This paper provides a methodology for the investigation of spray behavior and fuel distribution of GDI engine design.

Numerical and experimental study of the vaporization cooling in gasoline direct injection sprays

The spray of a high-pressure gasoline direct injection (GDI) injector has been studied experimentally at engine-like conditions and by numerical simulations. The high-pressure injection of isooctane fuel into a chamber leads to an evaporation of the injected liquid and to the formation of an ignitable fuel–air mixture at particular locations in the spray. The energy needed for this evaporation is provided by the background gas which in turn experiences a temperature drop. Experiments were conducted using the pure rotational coherent anti-Stokes Raman scattering (RCARS) technique in order to measure the temperature of the gas phase. The numerical simulation were done with a mixed Eulerian–Lagrangian approach. For varying injection pressures, a clear influence on the vaporization cooling has been found in the numerical results which is in contrast to the experiments. However, the experimental results for varying fuel temperatures are well reproduced.

SINGLE-DROP EMPIRICAL MODELS FOR SPRAY IMPACT ON SOLID WALLS: A REVIEW

Simplified impingement models are largely used in numerical simulation of spray impingement on solid walls, since the direct simulation of multiple drop impact is outside of the present computer capabilities. These models are usually semiempirical, based on single-drop impact experimental data and some reasonable constraints to allow extrapolation to different impact conditions. This article presents a comparison of the most commonly used drop impingement models in the present scientific literature and in commercial codes. A classification of the models and a unified scheme is proposed in terms of driving variables and outcomes. Every model is then analyzed and its peculiarities and possible weaknesses are pointed out, especially those related to the physical consistency. Finally, a comparison among the various models for a possible use in Gasoline Direct Injection (GDI) and diesel spray simulation is presented.