1,720,986 research outputs found
Understanding the role of soot oxidation in gasoline combustion: A numerical study on the effects of oxygen enrichment on particulate mass and number emissions in a spark-ignition engine
Full text linkThe production of increasingly clean engines has become imperative. More stringent regulations for internal combustion engines are constantly proposed, and recent number-based regulations have become a new challenge, since historically only a mass-based regulation needed to be met. It is known that soot particles detectable at the exhaust of an engine are the results of the competition between the formation of soot precursor species and their oxidation. However, the attention is mainly focused on inhibiting soot precursors formation, and much less research is dedicated at elucidating the benefits achievable from enhancing soot oxidation rates. Soot oxidation can be enhanced by increasing the in-cylinder oxygen content. Oxygenated fuels, which are often added to gasoline in order to achieve more efficient combustion, can represent a possible way in the pursuit of this goal. However, chemical mechanisms are still uncertain for practical fuels, and ambiguous results can be produced when the effect of oxygenated fuels on gasoline engine combustion and soot emissions is considered. In the present study, 3-D Computational Fluid Dynamics simulations were performed and the numerical results were compared with existing experimental data, in which load increases were achieved by pure oxygen addition within the intake manifold of a single-cylinder Spark-Ignition (SI) engine. Studying the effects that an addition of 5% and 10% by volume (with respect to air) of additional oxygen produces on the combustion process, allowed to provide basic additional information on soot formation and oxidation, avoiding the uncertainties associated with chemistry models. A semi-detailed soot model and a chemical kinetic model, including poly-aromatic hydrocarbon formation, were coupled with the G-equation flame propagation model for the SI engine simulations and for predicting soot mass and particulate number density. Improvements in the modeling of gasoline premixed combustion were achieved, as well. Specifically, different approaches in the evaluation of the laminar flame speed of gasoline (which is a key factor for obtaining reliable SI engine simulations) were critically compared and analyzed. The numerical results showed aspects that were not possible to appreciate by only referring to the experimental results on which this work was based. It was possible to observe that the higher soot concentrations were located in regions characterized by lower temperatures and lower OH concentrations. Oxygen addition favored a faster burning velocity and produced higher in-cylinder temperatures. However, the production rates of both OH radicals and soot precursor species resulted enhanced. The analysis of these concurrent phenomena allowed to explain why in the experiments the soot mass per kg of fuel was lower for the oxygenated combustion cases
Steady-state Characterization of Particle Number Emissions from a Heavy-Duty Euro VI Engine Fueled with Compressed Natural Gas
Full text linkThe objective of the present work is to provide an exhaustive characterization of size distributions and number density of the particles emitted from a modern EURO-VI heavy-duty 4-cylinder engine, fueled with compressed natural gas. To achieve this goal, a wide range of operating conditions (for a total of 60 operating points) were investigated during the experimental campaign. Namely, the engine speed was varied from 800 to 3500 rpm and the engine load ranged from 20 to 100% of the full-load condition. Steady-state and stoichiometric conditions were ensured during the tests. The data were collected by using two particle sample devices, located at two distinct sampling points. In particular, samples were simultaneously collected directly from the exhaust pipe, upstream of the Three-Way Catalyst (TWC) and from an exhaust gas dilution system (CVS). In the first case, a fast-response particle size spectrometer (DMS500) was employed, while a condensation particle counter (APC489) was used in the second case. The experimental approach used in the present work allowed the identification of the correlations linking the main engine working parameters with the emitted particle levels of the tested natural gas engine. Furthermore, the use of two sampling devices located in two different positions along the exhaust stream, allowed to highlight the effects that the TWC and the dilution tunnel can produce on particulate emissions. The reported results provide more insight on the particle emission process related to natural gas engines
Comprehensive Numerical Analysis of a Four-Way Two-Position (4/2) High-Frequency Switching Digital Hydraulic Valve Driven by a Ring Stack Actuator
No full textThis paper presents a feasibility study using a commercially available ring stack actuator to develop a four way-two position (4/2) high frequency switching digital hydraulic valve. The excellent characteristics of multilayer piezoelectric actuators, such as a simple design, reduced moving parts, high reliability, and fast response, make them ideal for constructing this type of digital hydraulic valve. High frequency switching digital hydraulic valves (HFSVs), indeed, must be able to switch from fully open to fully closed positions in less than 5 ms, while maintaining minimal pressure losses and delivering large flows. The proposed valve architecture is assessed using well-established equations implemented in a Simulink model, allowing the hydraulic, mechanical, and electrical parts of the valve to be accurately simulated. The paper first provides a detailed description of the numerical model. Next, the hysteresis model of the ring stack actuator is validated against the data provided by the manufacturers on their website. Finally, the numerical results obtained with both open-loop and closed-loop control systems are presented. The simulations show that at a switching frequency of 200 Hz with maximum amplitude and duty cycle of the input pulse digital signal, the valve exhibits high average flow rates (~60 L/min), low average power consumption (~1500 W), and maintains a pressure drop of only 15 bar. Moreover, the simulations reveal that the control system is very effective since the valve switching time is within 1 ms
Novel, cost-effective configurations of combined power plants for small-scale cogeneration from biomass: Design of the immersed particle heat exchanger
Full text linkThis paper aims at proposing a general design procedure for the application of the Immersed Particle Heat Exchanger to a novel, small scale, externally fired combined cycle capable of generating electrical and thermal power from carbon-neutral biomass. The Immersed Particle Heat Exchanger serves as the high temperature heat exchanger needed to couple the Brayton cycle with an external combustor of biomass; it is composed of either one module or more modules, with each module being constituted by two heat exchange columns. The combustion gases and the working fluid (clean air) flow separately in the two columns, and ceramic particles are employed as solid intermediate medium to transfer heat between the two columns. Three particle-handling systems, mainly composed of rotary valves coupled with guillotine valves, are conceived in this paper to move the particles within the heat exchanger; analytical models are developed for the design of these systems and for the evaluation of the associated energy losses. A new architecture employing internal cooling channels is also proposed for the two heat exchange columns. In addition, an optimization design procedure, based on the coupling between a computational fluid dynamic model and a genetic algorithm, is developed to correctly select the number of heat exchanger modules as well as the main project parameters, with application to a combined cycle of 110Â kWe with an overall efficiency of about 70%. The numerical results show that the project is highly viable. Only a negligible part of the compressed air is lost because of the particle handling systems (about 0.2%); furthermore, the size of the heat exchanger results to be compact, as the selected optimum is characterized by two heat exchanger modules, with each having an overall height, necessary for the heat exchange, of about 4Â m and a maximum diameter of 1.2Â m
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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