1,216 research outputs found

    Numerical Analysis of In-cylinder Tumble Flow Structures – Parametric 0D Model Development

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    Abstract Both in the automotive and in the motorcycle fields the requirement of step-by-step improvements for optimizing the engine cycle is still present. In particular the focus of the optimization process is to reduce the raw emissions and at the same time to not penalize the engine performance. In this research field the engine modeling is of great importance because the application field of the experimental measurements is very narrow, time-consuming and expensive. Hence the modeling technique is a wide used and a wide recognized instrument for helping in the design process. Another important function of the modeling is to provide the engine designers with the most important guidelines. The main focus is to fast provide designers with some fundamentals during the first designing stage which, if not the conclusive, is close to the final project. The present paper deals with the development of a theoretic-interpretative 0D model which could highlight the most significant parameters in the engine design process and in particular in the determination of: • The tumble velocity at IVC and its residual value at TDC; • The squish velocity at TDC; • Their mutual interaction. These parameters are well recognized to be especially meaningful because they determine, at different times of the combustion process, the combustion velocity. The faster the combustion velocity, the lower the engine cycle-by-cycle variability

    Development of a Emission Compliant, High Efficiency, Two-valve DI Diesel Engine for Off-road Application

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    Nowadays, environmental concerns are posing a great challenge to DI Diesel engines. Increasingly tightening emission limits require a higher attention on combustion efficiency. A high efficiency Diesel engine can be developed only mastering all the parameters that can affect the combustion and, therefore, NOx and soot emissions. In this scenario, computational fluid-dynamics can prove its power guaranteeing a deeper understanding of mixture formation process and combustion. In this work, the development of an engine in order to fulfill Tier 4i emission standard will be presented, the Tier 4i compliance must be reached without an excessive increase of the final cost of the engine. Originally, the engine was a two-valve engine supplied with a DPF, since no SCR aftertreatment is supplied, NOx emission target are achieved through external exhaust gas recirculation and retarding the start of injection. Through combustion process simulations, performed with the CFD code KIVA3D, varying different geometric parameters and the intensity of the swirl ratio, the interaction between the swirl flow field, generated by the intake duct, the reverse squish motion, and motions aerodynamically generated by spray has been investigated leading to a better interaction between the flow field, the fuel spray and the piston bowl geometry and to the definition of a new engine lay-out. The study shows how, given the need of retarded injection for limiting NOx emission, the decrease of swirl ratio, when combined with a proper piston bowl design, allows a significant decrease of soot emissions and the achievement of Tier 4i emission standard. The study has been validated comparing the intake phase simulations, performed with the CFD code Fire v2009 v3, followed by the combustion process performed with the KIVA3D code, with the experimental result obtained from the engine assembled following the developed design. © 2013 The Authors

    Development of a chemical-kinetic database for the laminar flame speed under GDI and water injection engine conditions

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    Abstract The use of direct injection, supercharging, stoichiometric operation and reduction of the engine displacement, necessary to limit the specific consumption without reducing the power, makes the current spark ignition engines sensible to both the detonation and the increase of the inlet turbine temperature. The current research has therefore focused on the study of strategies aimed at reducing the risk of detonation using traditional and innovative solutions such as water injection. The application and optimization of these strategies can not ignore the knowledge of physical quantities characterizing the combustion such as the laminar flame speed. The laminar burning speed is an intrinsic property of the fuel and it is function of the mixture composition (mixture fraction and dilution) and of the thermodynamic conditions. The experimental measurements of the laminar flame speed available in the literature, besides not being representative of the pressure and temperature conditions characteristic of GDI engines, rarely report the effects of dilution by EGR or water vapor. To overcome the limitations of the experimental campaign it is possible to predict the value of the laminar flame speed resorting to numerical combustion models based on chemical kinetics. The increased performance of computing systems makes affordable the use of chemical schemes with a high number of species and reactions without facing an excessive temporal cost. In this work it is presented a methodology for the construction of a laminar flame speed database based on a non-reduced kinetic scheme and an open source solver (Cantera) for a commercial gasoline surrogate under the typical conditions of GDI engines with the addition of the effects of dilution with water and EGR

    Evaluation of the effects of a Twin Spark ignition system on combustion stability of a high performance PFI engine

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    The continuous demand for high performances and low emissions engines leads the engine manufactures to set the operating range of combustion devices near to their stability limit. Combustion stability is closely related to the formation of the first ignition kernel: an effective way of lowering Cycle-by-Cycle Variation (CCV) is to enhance the start of combustion by means of multiple sparks. A Ducati engine was equipped with a Twin Spark ignition system and a consistent improvement in combustion stability arised for both part load and full load conditions. At part load a sensible reduction of cycle-by-cycle variability of indicated mean effective pressure was found, while at full load condition the twin spark configuration showed an increase of power, but with higher knocking tendency. The aim of this work is to better understand the root causes of the increased level of knock and to make a critical evaluation of most used knock indexes, by means of an accurate analysis of the experimental and simulated pressure signals. The numerical methodology based on a perturbation of the initial kernel by a statistical evaluation of mixture condition at ignition location. A lagrangian ignition model developed at University of Bologna was used, here modified to take into account the statistical distribution of mixture around the spark plugs. The RANS simulations proved to be accurate in representing all the main information related to combustion efficiency and knocking events. © 2015 The Authors. Published by Elsevier Ltd

    basics on water injection process for gasoline engines

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    Abstract Actual and future limits to the global CO2 emissions and the necessity of a further reduction of the fossil non-renewable fuels have moved the automotive engine research toward new solutions. With focus on reciprocating internal combustion engines, the mass of CO2 emitted in the atmosphere is a function of the fuel consumption. Therefore, the designers are focusing their attention on both the drop of passive resistances and the improvement of the engine efficiency. As far as the latter is concerned, the reduction of in-cylinder temperature and the adoption of stoichiometric combustion on the full range of engine operation map are the most investigated solutions. Water injection is thought to help in fulfilling these goals thus contributing towards more efficient engines. The aim of the present work is to understand the basic thermophysical and chemical fundamentals governing the water injection application in modern downsized spark ignited engines. The investigation has been carried out with aid of CFD simulation by using AVL FIRE v.2017 solver

    Assessment of the Cavitation Models Implemented in OpenFOAM® Under DI-like Conditions

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    Abstract Direct injection engine performance is strictly correlated to the fluid dynamic characteristics of the injection system. Actual DI engines, both Diesel and gasoline, employ injector characterized by high injection pressure that, associated to micro-orifice design, result in cavitation flow conditions inside injector holes. The cavitation has a beneficial effect on the atomization process and a negative one on the physical erosion generated by the vapor bubble collapse. In order to quantify both effects with a numerical approach, the reduced dimension and the complex flow structures reduce the efficacy of an experimental approach, thus the cavitation model used is of primary importance. The present work addresses the validation of the mixture model-based cavitation models that are implemented in OpenFOAM®, with particular focus on the Schnerr and Sauer model, using the experimental results, available in literature, for a two-phase flow in an optically accessible nozzle under diesel-like conditions

    Large Eddy Simulation of a Steady Flow Test Bench Using OpenFOAM®

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    Abstract Stationary flow bench testing is a standard experimental methodology used by the automotive industry to characterize a cylinder head. In order to reduce the development time, the use of a CFD-based virtual test bench is nowadays a standard practice too. The use of a conventional \RANS\ methodology for the simulation of the flow through the ducts of an engine head allows to get only the mean flow variables distributions because the time average of the generic flow variable fluctuation is zero by definition, but the fluid-dynamics of a stationary flow bench is not really stationary due to the flow instability induced by the duct design and the interaction between valve jets in a multi-duct head. In order to obtain an in-depth knowledge of the fluid-dynamics of a stationary flow bench test rig a \LES\ simulation of a heavy duty \DI\ diesel engine head with two intake ducts, for which experimental data was available, has been carried out using OpenFOAM®. The comparison between LES, experimental and conventional \RANS\ results widened the understanding of the test-bench fluid-dynamics and of the swirl generation process. Due to the high computational cost of the \LES\ approach, the outcomes of this latter have been also used to evaluate potential accuracy improvements of the \RANS\ simulation, namely using a model sensible to flow anisotropies and curvatures such as a \RSTM\ model. The simulation with the new turbulence model has been carried out and compared with the previous results demonstrating predictive improvements with an affordable computational cost for industrial routine usage

    Comparison between Conventional and Non-Conventional Computer Methods to Define Antiknock Properties of Fuel Mixtures

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    Research Octane Number (RON) is one of the primary indicators for the determination of the resistance of gasoline fuels to autoignition. This parameter is usually determined with a test procedure involving a standardized engine that requires expensive hardware and time-consuming tests. In this work, a set of different methods with which to determine the RON of gasoline fuel surrogates is presented, considering only computer simulations, which allows to reduce both cost and time for the evaluation. A palette of 11 chemical species has been chosen as the basis for the surrogates’ database, which will be investigated in the work, allowing the representation of the complex chemical formulation of fuels in an easier way. A simplified zero-dimensional engine model of the standard variable compression ratio is used to provide pressure and temperature, then employed to calculate RON. This is done first by means of existing methods, and then by introducing new processes concerning a simplified chemical reactor built on kinetic schemes. Finally, these different methodologies are tested against a molar weighted sum of RONs of each chemical specie, allowing to have a criterion for comparison and evaluating their real prediction capabilities

    numerical evaluation of the applicability of steady test bench swirl ratios to diesel engine dynamic conditions

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    Engine coherent flow structures such as swirl and tumble motions are key factors for the combustion process due to their capability to rise turbulence levels and enhance mixing which, in turns, severely influence both fuel efficiency and pollutant emissions. Automotive industry has therefore put great efforts over the last decades in evaluating air flow during induction stroke and air flow within the cylinder. Nowadays swirl and tumble motion characterizing a specific cylinder head are evaluated experimentally at design stage mainly using stationary flow benches. Such tests allow characterizing each head prototype using non-dimensional parameters like swirl and tumble ratios and, finally, to compare the different designs. In the present work the authors focused their attention on the swirl ratio characterization, firstly reviewing the two main methodologies for evaluating such parameter and more precisely the AVL and the Ricardo ones. A numerical method is then proposed in order to reproduce the stationary test bench with the final goal to develop a fast and accurate virtual test bench for cylinder head design. Simulations have been carried out on different VM Motori engine heads for which experimental data were available. The comparison between computational and experimental swirl ratios allowed to evaluate the suitability of using a virtual test bench as alternative or complementary to experiments. These results widened the understanding of the swirl fluid-dynamics and suggested that care must be taken when comparing duct designs having no geometrical similarity. Finally dynamic simulations have been performed for the head prototypes in order to compute the engine swirl in realistic conditions and to compare it with the steady bench results. This allowed evaluating the capability of the two different "static" swirl ratio definition (AVL/Ricardo) in correctly estimating real engine swirl. © 2015 The Authors. Published by Elsevier Ltd

    Untangling the extracellular matrix of idiopathic epiretinal membrane: a path winding among structure, interactomics and translational medicine

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    Idiopathic epiretinal membranes (iERMs) are fibrocellular sheets of tissue that develop at the vit-reoretinal interface. iERMs consist of cells and extracellular matrix (ECM) formed by a complex array of structural proteins and a large number of proteins that regulate cell-matrix interaction, matrix deposition and remodelling. Many components of the ECM tend to produce a layered pat-tern that can influence the tractional properties of the membranes. We applied a bioinformatics approach on a list of proteins previously identified with an MS-based proteomic analysis on sam-ples of iERM to report the interactome of some key proteins. The performed pathway analysis highlights interactions occurring among ECM molecules, their cell receptors, and intra or extra-cellular proteins that may play a role in matrix biology, in this special context. In particular, integ-rin β1, cathepsin B, epidermal growth factor receptor, protein-glutamine gam-ma-glutamyltransferase 2, and prolow-density lipoprotein receptor-related protein 1 are key hubs in the outlined protein-protein cross-talks. A section on the biomarkers that can be found in the vitreous humor of patients affected by iERM and that can modulate matrix deposition is also pre-sented. Finally, translational medicine in iERM treatment has been summed up taking stock of the techniques that have been proposed for pharmacologic vitreolysi
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