1,721,065 research outputs found
What is the Future of the Two-Stroke S.I. Engine?
No full textThe conventional two-stroke S.I. engine is simple and low-cost, however its scavenging process is unfavourably characterised by the loss of a great part of the fresh charge from the exhaust port. Besides, bad combustion and/or misfire occur at light loads, because of the excessive ratio of residual gas to fresh gas within the cylinder. These drawbacks, because of the oncoming new limits for exhaust emission, might cause the death of this engine in few years, at least in Europe and in the other developed countries.
However, innovative solutions are expected saving this engine. The main one is direct fuel injection, which allows to avoid fuel short circuiting to the exhaust port during scavenging, even in two stroke engines of simple construction with loop scavenging, since the cylinder is scavenged with air alone. Moreover, some fuel injection systems can generate charge stratification, thus solving the problems occurring at light loads or, in the case of air-assisted fuel injection which is not able to perform late injection, the adoption of ATAC combustion should be the solution for light loads problems.
Thanks to direct fuel injection and to their intrinsic internal EGR (which gives rise to low CO and NOX emissions), the small two-stroke engines might satisfy the regulation ECE 2002 even without catalytic converter (the four-stroke ones need the converter).
The increasing diffusion of direct-injection, stratified-charge induces to consider evolved two-stroke engines in the automobile field too, since, on the one hand, direct injection and charge stratification equalise four-stroke and two-stroke-engines as regards exhaust emissions and, on the other hand, two-stroke engines have remarkable advantages of torque regularity and availability at low engine speeds. Of course, in this case, valved two-stroke engines without cylinder ports should be considered, to reduce cylinder wear and thermal deformation and to decrease lubricant
Alcune osservazioni di anatomia funzionale del compartimento posteriore del ginocchio umano (legamento popliteo obliquo)
No full textNumerical and Experimental Analysis on a Small GDI, Stratified Charge, Motorcycle Engine
No full textIn the field of engines for light motorcycles, two-stroke
cycle survival is submitted to the application of direct fuel
injection and charge stratification, even in the case of low-cost
small engines. However, charge stratification is a difficult
target in two-stroke engines, chiefly because timings of late
injection (necessary for charge stratification) and of early
injection (necessary for homogeneous charge) are much closer
than in four-stroke engines. The compatibility between
stratified and homogeneous charge operations needs a thorough
CFD study of injection and mixing processes, with the support
of techniques of spray visualization. Results strongly depend on
the possibility of optimising the interaction between in-cylinder
gas-dynamic field and spray; experimental activity is necessary
as data source and verification of computational prediction.
This paper shows the latest CFD investigation, experimental
tests and results concerning a 50 cm3 engine for light
motorcycles. The injection is of the liquid type with wall-andair
guided spray produced by a swirl injector. The research has
been focused on the attainment of charge stratification at every
engine speed. Spray actual characteristics have been
investigated, attesting suitable repeatability and proper
variation versus backpressure. Engine satisfactory behaviour
even at light loads in unthrottled condition is proved by good
fuel economy and engine stability in dynamometric bench tests.
Exhaust gas analysis and indicated pressure behaviour confirm
stratification and combustion correctness
Progress in Two-Stroke S.I. Engines
No full textThanks to its lightness, small dimensions and attractive simplicity, the two-stroke, S.I. engine may be potentially preferable to the four-stroke one for city cars and other small vehicles.
However, the loss of fresh mixture from the exhaust port, together with bad combustion and/or misfire at light loads constitute great limitations.
Our researches directed to solve these problems concern the development of an air-assisted, low-pressure, pumpless injection systems, which bypasses the trapping losses, and the solution of combustion problems at light loads by means of charge stratification and innovative ignition systems
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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