1,720,956 research outputs found
Design, Construction and Testing of Hypocycloid Machines
No full textThe hypocycloid mechanism, in its basic or modified version, can be used in reciprocating machines as an alternative solution to the conventional crankshaft-connecting rod system. This kinematic device is quite old in concept, but still gives rise to some research since it has some positive mechanical features, and it allows several functions to be easily combined. In the hypocycloid mechanism in fact the connecting rod has a perfectly sinusoidal straight-line motion, a characteristic that theoretically reduces the piston side thrust to zero and permits a perfect balance of inertia forces without auxiliary rotating shafts. Moreover, by means of a second head with suitable seals that prevent leakage to the crankcase, the lower portion of the cylinder can be used as a second working chamber without using the crosshead thus reducing the dimensions of the double acting machine. The authors have undertaken theoretical and experimental research in order to evaluate the positive features, in particular those concerning the possibility of the combination of functions, and to compare these characteristics with the evident shortcomings. This has led to the design and construction of a double acting compressor, whose performances have been compared with those of a traditional compressor. A perfectly balanced two-stroke engine, with the scavenging pump separated from the crankcase, has also been constructed and teste
Influenza del Grado di Spazio Morto del Carter-Pompa sulle Prestazioni di un Motore a Due Tempi ad Accensione Comandata
No full textInfluenza dell'iniezione diretta di benzina sulle prestazioni e sulle emissioni di un motore a due tempi
No full textL'iniezione diretta di benzina nel cilindro rappresenta attualmente per i motori ad accensione comandata una nuova possibilità per migliorarne le prestazioni, in modo particolare per quanto riguarda le emissioni di inquinanti allo scarico e il consumo di combustibile. Già si è potuto constatare che nei motori a quattro tempi i nuovi apparati di alimentazione hanno prodotto sensibili miglioramenti per quanto concerne questi aspetti, in particolare se utilizzati contestualmente alla stratificazione della carica ed ai nuovi convertitori catalitici, che presentano elevata efficienza anche con miscele magre. Anche nei motori a due tempi l'iniezione diretta è stata però utilizzata con evidenti vantaggi per quanto concerne le emissioni di idrocarburi incombusti e il consumo di carburante. Per valutare le possibilità di una tale applicazione si è installato pertanto un apparato di iniezione elettronico, di recente realizzazione, su di un motore a due tempi a carter-pompa dotato anche di un sistema di alimentazione ad iniezione indiretta che permette di effettuare un confronto in termini di emissioni e consumi. Questo nuovo apparato di iniezione diretta, grazie alle sue elevate prestazioni di ripetibilità, di qualità dei getto, di controllo della quantità di massa iniettata e della fasatura, ha permesso di ottimizzare il funzionamento dei motore per quanto riguarda la caratteristica meccanica a piena ammissione. Il confronto tra i due sistemi di alimentazione ha portato infine ad evidenziare le notevoli possibilità dei sistema di iniezione diretta nei riguardi dell'emissione di inquinanti e dei consum
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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