1,721,036 research outputs found
Extraordinary renditions e obblighi positivi derivanti dall’articolo 3 CEDU: il segreto di Stato nel caso “Abu Omar”
No full textIdle Stalling Phenomena in High Performance Spark Ignition PFI Engines: an Experimental Analysis
No full textHigh performance Spark Ignition (SI) Port-Fuel Injected (PFI) internal combustion engines are usually optimized to deliver high power output at high speed in Wide Open Throttle (WOT) conditions. However, they also have to run consistently at idle, possibly with stoichiometric Air-Fuel Ratio (AFR), in order to limit tailpipe emissions. The two requirements are sometimes conflicting, as it is difficult to match high-speed volumetric efficiency with lowspeed turbulence: the intake runner size and shape are often designed for performance, meaning that usually they do not guarantee a satisfying air-fuel mixing at idle. The
consequence of poor mixture formation may be high cycle-tocycle
variation or misfiring, with obvious consequences on pollutant emissions and driveability. In the worst cases, however, the consequence could be even more serious: stalling phenomena have been observed on the test bench. While running at idle, the engine suddenly stops: the event is so quick that the idle controller is not able to react. The paper shows a detailed experimental analysis of stalling phenomena, based on engine speed, intake pressure, incylinder pressure, ion current information. Intake and incylinder pressure data show that stalling phenomena are related to anomalous combustions taking place
during the compression stroke: the negative torque generated by
such combustions is able to stop the engine. Further analysis show
that these phenomena are triggered by defined conditions: a
partial combustion releasing little heat and leading to a
constant pressure exhaust stroke seems to be a necessary
condition to ignite the undesired combustion. Ion current
signals show that the combustion extends during the exhaust
stroke, and continues throughout the following intake and
compression strokes. The sensitivity of the phenomenon to changes in the injection layout suggests that its origin is related to the process of mixture formation. The presence of a large amount of liquid fuel in the cylinder could lead to diffusive combustions,
maintained throughout the exhaust stroke and the subsequent
intake stroke, thus resulting in a backfire. The Rate Of Heat Release (ROHR) analysis based on incylinder pressure confirms that the frequency of the phenomenon is higher in the cylinder where more liquid fuel is likely to be accumulated
Development of a Novel Machine Learning Methodology for the Generation of a Gasoline Surrogate Laminar Flame Speed Database under Water Injection Engine Conditions
Full text linkThe water injection is one of the technologies assessed in the development of new internal combustion engines fulfilling new emission regulation and policy on Auxiliary Emission Strategy assessment. Besides all the positive aspects about the reduction of mixture temperature at top dead center and exhaust gases temperature at turbine inlet, it is well known that the water vapor acts as a mixture diluter, thus diminishing the reactants burning rate. A common methodology employed for the Reynolds-Averaged Navier-Stokes Computational Fluid Dynamics (RANS CFD) simulation of the reciprocating internal combustion engines’ turbulent combustion relies on the flamelet approach, which requires knowledge of the Laminar Flame Speed (LFS) and thickness. Typically, these properties are calculated by means of correlation laws, but they do not keep into account the presence of water mass fraction. A more precise methodology for the definition of both the LFS and thickness is thus required. The interrogation of a previously computed look-up table of such properties during run time seems to be a suitable and more accurate method than using correlations. In order to generate a database with all the possible combinations of chemical and physical properties that can be reached during the simulation of internal combustion engines, including the presence of a given mass fraction of water vapor and exhaust gases, a very high number of detailed chemical kinetics simulations need to be performed. The present work aims to introduce a new methodology for the fast generation of laminar flame characteristics look-up tables that account also for the presence of water vapor in the reacting mixture. By using this new approach, engine designers will have the possibility to generate look-up tables of laminar flame characteristics for different fuels with the same computational cost that is currently required to generate a single table
Multicycle Simulation of the Mixture Formation Process of a PFI Gasoline Engine
No full textThe mixture composition heavily influences the combustion process of Port Fuel Injection (PFI) engines. The local mixture air-index at the spark plug is closely related to combustion instabilities and the cycle-by-cycle Indicated Mean Effective Pressure (IMEP) Coefficient of Variation (CoV) well correlates with the variability of the flame kernel development. The needs of reducing the engine emissions and consumption push the engine manufactures to implement techniques providing a better control of the mixture quality in terms of homogeneity and variability. Simulating the mixture formation of a PFI engine by means of CFD techniques is a critical issue, since involved phenomena are highly heterogeneous and a two phase flow must be considered. The aim of the paper is to present a multi-cycle methodology for the simulation of the injection and the mixture formation processes of high performance PFI engine, based on the validation of all the main physical sub-models involved. A semi-empirical methodology for the correct initialization of the Lagrangian spray is presented in the paper. The spray-wall interaction sub-models are usually based on semi-empirical correlation. In this paper the Kuhnke model was tuned by means of experimental data, chosen coherently with the spray phenomena taking place in the considered engine. Since the liquid wall film plays a key role in the mixture formation of PFI engines, an accurate representation of the wall film dynamics was enforced by the solution of the liquid film momentum equation. The gas flow dynamics in the intake port strongly interact with the liquid fuel evolution and droplet breakup, thus in this work a multi-cycle methodology for the evaluation of the mixture inside the cylinder was proposed. In order to validate the simulation results, an optical access has been created on the engine airbox, allowing to use a fast camera to capture images of the actual injection process. The comparison of simulated and acquired images confirmed that both the gas and liquid fuel dynamics have been correctly reproduced. The evaluation of the injection timing influence on the engine performance was finally accomplished. Copyright © 2012 SAE International
A Numerical Methodology for the Multi-Objective Optimization of an Automotive DI Diesel Engine
No full textNowadays, an automotive DI Diesel engine is demanded to provide an adequate power output together with limit-complying NOx and soot emissions so that the development of a specific combustion concept is the result of a trade-off between conflicting objectives. In other words, the development of a low-emission DI diesel combustion concept could be mathematically represented as a multi-objective optimization problem. In recent years, genetic algorithm and CFD simulations were successfully applied to this kind of problem. However, combining GA optimization with actual CFD-3D combustion simulations can be too onerous since a large number of simulations is usually required, resulting in a high computational cost and, thus, limiting the suitability of this method for industrial processes. In order to make the optimization process less time-consuming, CFD simulations can be more conveniently used to build a training set for the learning process of an artificial neural network which, once correctly trained, can be used to forecast the engine outputs as a function of the design parameters during a GA optimization performing a so-called virtual optimization. In this paper, a numerical methodology for the multi-objective virtual optimization of the combustion inside an automotive DI Diesel engine, based on artificial neural networks combined with genetic algorithms, is presented. Copyright © 2013 SAE International
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Automatic Combustion Phase Calibration With Extremum Seeking Approach
No full textOne of the most effective factors influencing performance, efficiency, and pollutant emissions of internal combustion engines is the combustion phasing: in gasoline engines electronic control units (ECUs) manage the spark advance (SA) in order to set the optimal combustion phase. Combustion control is assuming a crucial role in reducing engine tailpipe emissions and maximizing performance. The number of actuations influencing the combustion is increasing, and as a consequence, the calibration of control parameters is becoming challenging. One of the most effective factors influencing performance and efficiency is the combustion phasing: for gasoline engines, control variables such as SA, air-to-fuel ratio (AFR), variable valve timing (VVT), and exhaust gas recirculation (EGR) are mostly used to set the combustion phasing. The optimal control setting can be chosen according to a target function (cost or merit function), taking into account performance indicators, such as indicated mean effective pressure (IMEP), brake-specific fuel consumption (BSFC), pollutant emissions, or other indexes inherent to reliability issues, such as exhaust gas temperature or knock intensity. Many different approaches can be used to reach the best calibration settings: design of experiment (DOE) is a common option when many parameters influence the results, but other methodologies are in use: some of them are based on the knowledge of the controlled system behavior by means of models that are identified during the calibration process. The paper proposes the use of a different concept, based on the extremum seeking approach. The main idea consists in changing the values of each control parameter at the same time, identifying its effect on the monitored target function, and allowing to shift automatically the control setting towards the optimum solution throughout the calibration procedure. An original technique for the recognition of control parameters variations effect on the target function is introduced, based on spectral analysis. The methodology has been applied to data referring to different engines and operating conditions, using IMEP, exhaust temperature, and knock intensity for the definition of the target function and using SA and AFR as control variables. The approach proved to be efficient in reaching the optimum control setting, showing that the optimal setting can be achieved rapidly and consistently. © 2014 by ASME
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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