1,720,959 research outputs found
Carburatore controllato ettronicamente con sistema di retroazione: sperimentazione su un piccolo motore 4T per uso stazionario
No full textNell’ambito di una ricerca svolta in collaborazione tra università ed industria, concernente sistemi d’alimentazione
innovativi a basso costo atti a ridurre le emissioni nei motori di piccola cilindrata, destinati prevalentemente all’uso
industriale ed agricolo, è stata valutata l’applicazione su un piccolo motore a 4T di un carburatore controllato
elettronicamente mediante retroazione.
Il sistema d’abbattimento delle emissioni inquinanti più utilizzato è il catalizzatore allo scarico. In presenza di
sistemi di alimentazione ad iniezione vengono in genere usati catalizzatori trivalenti, mentre nei motori di piccola
cilindrata non stradali, alimentati a carburatore per ragioni di carattere economico, si utilizzano catalizzatori solo
ossidanti e per limitare la formazione degli ossidi di azoto si opera lontano dai valori stechiometrici del titolo
(spostandosi o nel ricco o nel povero). La precisione richiesta al sistema d’alimentazione è relativamente bassa quando
si operi nel ricco, ma diviene più elevata, per non incorrere in instabilità di funzionamento, operando nel povero. In
quest’ultimo caso è pertanto desiderabile un controllo in retroazione in base al titolo di miscela effettivamente
realizzato, che può essere rilevato in vario modo.
Il prototipo di carburatore controllato elettronicamente deriva da quello di serie ed è in grado di variare il titolo
della miscela modulando le portate delle arie antagoniste del massimo e del minimo. A tal fine sono presenti sui
carburatori opportuni condotti ausiliari (“arie pilotate”) controllati mediante solenoidi pulsanti. Il mantenimento del
titolo desiderato è ottenuto grazie ad una centralina elettronica che pilota i solenoidi in DUTY-CYCLE
Behaviour of a Small Two-Stroke Engine with Direct Liquid Injection and stratified Charge
No full textHigh-pressure liquid fuel injection is a suitable means to get either stratified charge or homogeneous charge for two-stroke engines. This paper shows the development of this solution for a small 50 cm3 engine for light motorcycles. By means of computational fluid dynamics, a combustion chamber suitable for proper fuel distribution in every engine operating condition has been designed. It has been realized, and experimental results confirm its fairly satisfactory behaviour, with good fuel economy, low exhaust emissions and small cycle-to-cycle variation even at light loads. Recent CFD studies indicate how to improve engine geometry to achieve a better stratification stability at partial loads independently on engine speed
Studio fluidodinamico della formazione della miscela nel condotto di aspirazione di un motore alimentato con iniettore-pompa pilotato in frequenza
No full textMediante il codice fluidodinamico KIVA3v è stato simulato il comportamento di un motore monocilindrico per usi stazionari alimentato con un sistema d’iniezione indiretta pilotato in frequenza.
Obiettivo dello studio è stato di valutare le discrepanze tra il titolo realizzabile e quello desiderato ed in particolare l’entità delle fluttuazioni di titolo tra un ciclo e l’altro, problemi peculiari di tale sistema d’iniezione e dovuti essenzialmente alla limitazione a poche unità del numero massimo di iniettate per ciclo del motore.
Sono stati indagati l’interazione tra spray e parete del condotto di aspirazione, la formazione del film liquido, l’evaporazione del film e delle gocce, la miscelazione tra l’aria e il vapore per diffusione locale e per trasporto convettivo. Sono state simulate differenti lunghezze del condotto di aspirazione e posizionamenti dell’iniettore ed è stata considerata l’ipotesi di adottare materiali porosi o dispositivi mediante i quali incrementare la superficie bagnata del condotto, al fine di ottenere un vantaggioso “effetto spugna” (o “effetto polmone”)
Numerical and Experimental Study of Direct Injection for Charge Stratification in a 200 cc Two-Stroke Motorcycle Engine
No full textThis paper shows a numerical and
experimental study concerning liquid highpressure
direct injection with charge stratification
in a 200 cm3 two-stroke motorcycle engine.
Numerical predictions have been performed
using KIVA-3v code with different combustionchamber
shapes and spray characteristics.
Experimental outcomes have substantially
confirmed the predictions of computation and
proved that injector characteristics are of the
utmost importance to achieve satisfactory results
Optimisation of a Stratified Charge Strategy for a Direct Injected Two-Stroke Engine
No full textDirect fuel injection is becoming mandatory in two-stroke S.I. engines, since it prevents one of the major problems of these engines, that is fuel loss from the exhaust port. Another important problem is combustion irregularity at light loads, due to excessive presence of residual gas in the charge, and can be solved by charge stratification.
High-pressure liquid fuel injection is able to control the mixing process inside the cylinder for getting either stratified charge at partial loads or quasi-stoichiometric conditions, as it is required at full load. This paper shows the development of this solution for a small engine for moped and light scooter, using numeric and experimental tools. In order to obtain the best charge characteristics at every load and engine speed, different combustion chambers have been conceived and studied, examining the effects of combustion chamber geometry, together with injector position and injection timing
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
- …
