G. M. Bianchi

Modeling Atomization of High-Pressure Diesel Sprays

This paper deals with a numerical and experimental characterization of a high-pressure diesel spray injected by a common-rail injection system. The experiments considered a free non-evaporating spray and they were performed in a vessel reproducing the practical density that characterizes a D.I. diesel engine at injection time. The fuel was supplied at high pressure by a common-rail injection system with a single hole tip. The computations have been carried out by using both the TAB model and a hybrid model that allows one to describe both liquid jet atomization and droplet breakup. In order to validate the breakup model, an extensive comparison between data and numerical predictions has been carried out in terms of spray penetration, Sauter mean diameter, near and far spray cone angles, and spray structure.

Advanced Modeling of Common Rail Injector Dynamics and Comparison with Experiments

The aim of this work is to set up a methodology for simulating Common Rail high-pressure injectors based on coupling a lump-model with CFD two-phase multi-dimensional computations. The unit simulated is the Bosch injector. The injector lump-model resulted in the definition of the three sub-models for hydraulics, mechanics and electromagnetics. The second-order differential governing equations have been solved in Matlab/Simulink environment and are properly coupled together with the one-dimensional partial differential equations that describe the unsteady pipe flow. A detailed library of thermo-mechanical properties for ISO-4113 oil and diesel fuel is included. Cavitation effects on discharge coefficient in the main orifices were accounted for by using results from CFD steady two-phase flow simulations. The evaluation of the model capability was assessed by using detailed experiments carried out at different practical injector operating conditions. Instantaneous and integrated injected flow rate, and injector needle lift were measured and collected for comparison with simulation. CFD steady computations revealed to be unavoidable in driving the lump-model toward a high reliability of injector performances over the whole range of injection pressures and energizing times.

On the Applications of Low-Reynolds Cubic k-εTurbulence Models in 3D Simulations of ICE Intake Flows

The evaluation of the steady-flow discharge coefficient of ICE port assemble is known to be very sensitive to the capability of the turbulence sub-models in capturing the boundary layer dynamics. Despite the fact that theintrinsically unsteady phenomena related to flow separation claim for LES approach, the present paper aims to demonstrate that RANS simulation can provide reliable design-oriented results by using low-Reynolds cubic k-e turbulence models. Different engine intake port assemblies and pressure drops have been simulated by using the CFD STAR-CD code and numerical results have been compared versus experiments in terms of both global parameters, i.e. the discharge coefficient, and local parameters, by means of static pressure measurements along the intake port just upstream of the valve seat. Computations have been performed by comparing two turbulence models: Low-Reynolds cubic k-e and High-Reynolds cubic k-e. The analysis leaded to remarkable assessments in the definition of a correct and reliable methodology for the evaluation of engine port breathing capabilities. Comparison between numerical results and experiments showed that the low-Reynolds cubic k-e model is unavoidable to correctly capture the influence of port feature variations on engine permeability. In particular, the deficiencies demonstrated by High-Reynolds cubic k-e turbulence model in resolving the influence of near-wall shear and adverse pressure gradient effect on boundary layer dynamics are completely overcome by the use of the Low-Reynolds formulation.

Combined Experimental and Numerical Analysis of Knock in Spark Ignition Engines

A detailed analysis of knocking event can help improving engine performance and diagnosis strategies. The paper aim is a better understanding of the phenomena involved in knocking combustion through the combination of CFD and signals analysis tools. CFD simulations have been used in order to reproduce knock effect on the in-cylinder pressure trace. In fact, the in-cylinder pressure signal holds information about waves propagation and heat losses: for the sake of the diagnosis it is important to relate knock severity to knock indexes values. For this purpose, a CFD model has been implemented, able to predict the combustion evolution with respect to Spark Advance, from non-knocking up to heavy knocking condition. The CFD model validation phase is crucial for a correct representation of both regular and knocking combustions: the operation has been carried out by means of an accurate statistical analysis of experimental in-cylinder pressure data. The simulation results allow relating the combustion characteristics to their effect on the in-cylinder pressure signal. It is then possible to highlight critical issues regarding typical knock detection methodologies, while disclosing novel approaches. One of the main results is the validation of knock detection strategies based on the low-frequency content of the pressure signal. These strategies can be used together with standard high-frequency based techniques in order to improve the detection robustness.Copyright

Multi-dimensional modeling of the air/fuel mixture formation process in a PFI engine for motorcycle applications

The preparation of the air-fuel mixture represents one of the most critical tasks in the definition of a clean and efficient SI engine. Therefore it becomes necessary to consolidate the numerical methods which are able to describe such a complex physical process. Within this context, the authors developed a CFD methodology into an open-source code to investigate the air-fuel mixture formation process in PFI engines. Attention is focused on moving mesh algorithms, Lagrangian spray modeling and spray-wall interaction modeling. Since moving grids are involved and the mesh quality during motion strongly influences the computed in-cylinder flow-field, a FEM-based automatic mesh motion solver combined with topological changes was adopted to preserve the grid quality in presence of high boundary deformations like the interaction between the piston bowl and the valves during the valve-overlap period. The fuel spray was modeled by using the Lagrangian approach, and the spray sub-models (atomization and breakup) were tuned according to experimental validations carried out in previous works. Specific submodels were implemented to describe the impingement of fuel spray with the engine walls. The evolution of the resulting liquid film was also taken into account by solving the mass and momentum equations with the Finite-Area discretization method. The proposed methodology was applied to simulate a single-cylinder SI engine for motor-scooter applications at a low load operating condition. This operating point was chosen since these engines often run very close to idle conditions when they are used in the urban areas.

Advanced Modelling of a New Diesel Fast Solenoid Injector and Comparison with Experiments

Upcoming Euro 4 and Euro 5 emission standards are increasing efforts on injection system developments in order to improve mixture quality and combustion efficiency. The target features of advanced injection systems are related to their capability of operating multiple injection with a precise control of the amount of injected fuel, low cycle-by-cycle variability and life drift, within flexible strategies. In order to accomplish this task, injector performance must be optimised by acting on: optimisation of electronic, driving circuit, detailed investigation of different nozzle hole diameter configurations, assessment of the influence of manufacturing errors on hole diameter and inlet rounding on injector performance.

Numerical Analysis of High-Pressure Fast-Response Common Rail Injector Dynamics

Managing the injection rate profile is a powerful tool to control engine performance and emission levels. In particular, Common Rail (C.R.) injection systems allow an almost completely flexible fuel injection event in DI-diesel engines by permitting a free mapping of the start of injection, injection pressure, rate of injection and, in the near future, multiple injections. This research deals with the development of a network-based numerical tool for understanding operating condition limits of the Common Rail injector. The models simulate the electro-fluid-mechanical behavior of the injector accounting for cavitation in the nozzle holes. Validation against experiments has been performed. The model has been used to provide insight into the operating conditions of the injector and in order to highlight the application to injection system design.

Numerical Comparative Analysis of In-Cylinder Tumble Flow Structures in Small PFI Engines Equipped by Heads Having Different Shapes and Squish Areas

For increasing the thermal engine efficiency, faster combustion and low cycle-to-cycle variation are required. In PFI engines the organization of in-cylinder flow structure is thus mandatory for achieving increased efficiency. In particular the formation of a coherent tumble vortex with dimensions comparable to engine stroke largely promotes proper turbulence production extending the engine tolerance to dilute/lean mixture. For motorbike and scooter applications, tumble has been considered as an effective way to further improve combustion system efficiency and to achieve emission reduction since layout and weight constraints limit the adoption of more advanced concepts. In literature chamber geometry was found to have a significant influence on bulk motion and turbulence levels at ignition time, while intake system influences mainly the formation of tumble vortices during suction phase. The most common engine parameters believed to affect in-cylinder flow structure are:1. Intake duct angle;2. Inlet valve shape and lift;3. Piston shape;4. Pent-roof angle.The present paper deals with the computational analysis of three different head shapes equipping a scooter/motorcycle engine and their influence on the tumble flow formation and breakdown, up to the final turbulent kinetic energy distribution at spark plug. The engine in analysis is a 3-valves pent-roof motorcycle engine. The three dimensional CFD simulations were run at 6500 rpm with AVL FIRE code on the three engines characterised by the same piston, valve lift, pent-roof angle and compression ratio. They differ only in head shape and squish areas. The aim of the present paper is to demonstrate the influence of different head shapes on in-cylinder flow motion, with particular care to tumble motion and turbulence level at ignition time. Moreover, an analysis of the mutual influence between tumble motion and squish motion was carried out in order to assess the role of both these motions in promoting a proper level of turbulence at ignition time close to spark plug in small 3-valves engines.Copyright

Validation of a Lagrangian Ignition Model in SI Engine Simulations

Ignition process plays a key role in flame kernel formation and heavily affects further combustion development. The paper aim is to present a 1D lagrangian ignition model and to validate it against real engine configurations. A lump model for the electrical circuit of the spark plug is used to compute breakdown and glow energy. At the end of shock wave and very first plasma expansion, a spherical kernel is deposited inside the gas flow at spark plug location. A simple model allows computing initial flame kernel radius and temperature based on physical mixture properties and spark plug characteristics. The sphere surface of the kernel is discretized by triangular elements which move radially according to a lagrangian approach. Expansion velocity is computed accounting for both heat conduction effect at the highest temperatures and thermodynamic energy balance at relatively lower temperatures. Turbulence effects and thermodynamic properties of the air-fuel mixture are accounted for. Restrikes are possible depending on gas flow velocity and mixture quality at spark location. CFD solver and 1D/lagrangian ignition model are closely coupled at each time step. The model proves to strongly reduce the grid sensitivity. The CFD model validation phase is crucial for a correct representation of both kernel formation and combustion development: the operation has been carried out by means of an accurate statistical analysis of experimental in-cylinder pressure data in real engine configurations.Copyright

A Quasi-Direct 3D Simulation of the Atomization of High-Speed Liquid Jets

In this paper a quasi-direct solution of transient three-dimensional CFD calculations based on a finite volume approach has been adopted to simulate the atomization process of high velocity liquid jets issuing an injector-like nozzle. An accurate Volume-of-Fluid (VOF) method is used to reconstruct and advect the interface between the liquid and gas phases. An extended mesh which includes the injector nozzle and the upstream plenum has been considered in order to investigate accurately the effect of nozzle flow conditions on the liquid jet atomization. Cavitation modeling has not been included in the present computations. Two different mean injection velocities, 150 m/s and 270 m/s, respectively, have been considered in the calculations as representative of semi-turbulent and fully-turbulent nozzle flow conditions. The liquid-to-gas density ratio is kept fixed at 57. The calculations show that atomisation is directly linked to the temporally and spatially correlated turbulence of the liquid jet. The bulk flow perturbation and the relaxation of the boundary layer have been found to be the basic mechanisms that generate surface perturbations of the liquid jet.© 2005 ASME