1,720,980 research outputs found
Estudio de la emulsificación difusión como método para la preparación de partículas lipídicas
Full text linkEn la presente investigación se estudia el método emulsificación-difusión con el propósito de determinar la influencia tanto de los materiales de partida como de las condiciones de preparación, sobre el tamaño y el potencial zeta de partículas lipídicas. En términos generales, la naturaleza y la concentración del lípido utilizado determinan el tamaño y el potencial zeta de las partículas. Así, los mejores resultados se presentan al utilizar como lípido diestearato de glicerilo, cuyas partículas exhiben tamaños inferiores a 200 nm y potenciales zeta cercanos a los -12.7 mV. Otros lípidos, como el alcohol estearílico y el ácido esteárico, aunque permiten la formación de las partículas, presentan índices de polidispersidad elevados. Por el contrario, la naturaleza de los solventes orgánicos tiene influencia significativa en el tamaño pero no en el potencial zeta, de partículas preparadas a partir de diestearato de glicerilo. Al evaluar otros materiales lipídicos, la naturaleza del solvente orgánico no determina las propiedades de las partículas. El estudio con diferentes agentes estabilizantes evidencia que los tensioactivos no iónicos son los adecuados para la preparación de las partículas y que la concentración de dicho agente estabilizante puede afectar el tamaño de la partícula sin lograr cambios significativos en el potencial zeta. Los tensioactivos iónicos no permiten la formación de partículas lipídicas por este método. En conjunto, los resultados obtenidos en este estudio son analizados considerando los parámetros de solubilidad de Hildebrand y algunas propiedades fisicoquímicas de los materiales de partida. Desde este punto de vista, la afinidad química entre los compuestos parece determinar el tamaño de las partículas y la tensión superficial del solvente orgánico, así como su solubilidad en agua se identifican como las propiedades fisicoquímicas que podrían determinar la formación de las partículas. De otro lado, según los resultados relacionados con la influencia de las condiciones operacionales del proceso de preparación, la formación de las partículas parece estar gobernada por las variables que determinan la eficiencia de la emulsificación y no por aquellas relacionadas con la etapa de difusión. Si se tiene en cuenta que en la literatura científica proponen dos mecanismos de formación de las partículas por este método, uno basado en el efecto Gibbs-Marangoni y otro en el que cada gota de emulsión da origen a una partícula, la evidencia experimental obtenida en la presente tesis, sugiere que este último podría ser el mecanismo que predomina cuando se separan partículas sólidas lipídicas por el método de emulsificación-difusión.Abstract. This research work investigates the emulsification-diffusion as method for preparing lipid nanoparticles. In this way, the influence of the starting materials and of the operating conditions related to the preparation on the size and the zeta potential of the particles are systematically evaluated. In general terms, the nature and the concentration of the lipid are the main factors determining the size and the zeta potential of the particles. Thus, the best results are from glyceryl distearate rendering particle sizes less than 200 nm and zeta potential around of -12.7 mV. Other lipidic materials investigated such as stearyl alcohol and stearic acid lead to the formation of particles, however high polydispersity indices are obtained. In contrast, the nature of the organic solvents has a significant influence regarding the size of the particles obtained from glyceryl distearate, but not on their zeta potential. The organic solvent nature has not influence on size and zeta potential of particles prepared by using other lipidic starting materials. Results obtained when different stabilizing agents are used evidence that no-ionic surfactants are appropriate for the particle preparation and the used concentration governs the particle size. Ionic surfactants do not lead to the particle formation. One explanation of these results is proposed from the Hildebrand solubility parameters and some physicochemical properties of the starting materials. Then, it seems that the chemical affinity determinates the particle size and the surface tension of the organic solvent and its water solubility are the most important physicochemical properties influencing the particle formation. On the other hand, according to the results from the study of the operating conditions, the particle formation is perhaps governed by the operating variables influencing the emulsification efficiency but not by those related to the diffusion stage. Bear in mind that reported research works propose to mechanistic approaches regarding the particle formation via this technique, the first one based on the Gibbs–Marangoni phenomenon and the second one considering that each particle is obtained from a droplet of the emulsion, the experimental evidence obtained in this work suggests that the last mechanistic approach predominates when solid lipid particles are prepared by the emulsification-difussion method.Maestrí
Reduction of Cold-Start Emissions through Valve Timing in a GDI Engine
Full text linkThis work examines the effect of valve timing during cold crank-start and cold fast-idle (1200 rpm, 2 bar NIMEP) on the emissions of hydrocarbons (HC) and particulate mass and number (PM/PN). Four different cam-phaser configurations are studied in detail: 1. Baseline stock valve timing. 2. Late intake opening/closing. 3. Early exhaust opening/closing. 4. Late intake phasing combined with early exhaust phasing. Delaying the intake valve opening improves the mixture formation process and results in more than 25% reduction of the HC and of the PM/PN emissions during cold crank-start. Early exhaust valve phasing results in a deterioration of the HC and PM/PN emissions performance during cold crank-start. Nevertheless, early exhaust valve phasing slightly improves the HC emissions and substantially reduces the particulate emissions at cold fast-idle. The combined strategy consisting of late intake and early exhaust phasing shows a considerable reduction in both the cold crank-start HC and PM/PN emissions of 30%. In fast idle, the HC and PM emissions respond differently to the different valve timing strategy. The combined late IVO and early EVC configuration with moderate combustion phasing retard gives the best HC and PM emissions and reasonable engine stability.Consortium on Engine and Fuels Researc
Cycle-by-Cycle Analysis of Cold Crank-Start in a GDI Engine
Full text linkThe first 3 cycles in the cold crank-start process at 20°C are studied in a GDI engine. The focus is on the dependence of the HC and PM/PN emissions of each cycle on the injection strategy and combustion phasing of the current and previous cycles. The PM/PN emissions per cycle decrease by more than an order of magnitude as the crank-start progresses from the 1st to the 3rd cycle, while the HC emissions stay relatively constant. The wall heat transfer, as controlled by the combustion phasing, during the previous cycles has a more significant influence on the mixture formation process for the current cycle than the amount of residual fuel. The results show that the rise in HC emissions caused by the injection spray interacting with the intake valves and piston crown is reduced as the cranking process progresses. Combustion phasing retard significantly reduces the PM emission. The HC emissions, however, are relatively not sensitive to combustion phasing in the range of interest
Effect of Operation Strategy on First Cycle CO, HC, and PM/PN Emissions in a GDI Engine
Full text linkThe impact of the operating strategy on emissions from the first combustion cycle during cranking was studied quantitatively in a production gasoline direct injection engine. A single injection early in the compression cycle after IVC gives the best tradeoff between HC, particulate mass (PM) and number (PN) emissions and net indicated effective pressure (NIMEP). Retarding the spark timing, it does not materially affect the HC emissions, but lowers the PM/PN emissions substantially. Increasing the injection pressure (at constant fuel mass) increases the NIMEP but also the PM/PN emissions.Borg-Warner CorporationChrysler CorporationFord Motor CompanyGeneral Motors Corporatio
Fuel carbon pathway in the first cranking cycle of a gasoline direct injection engine
No full textThe fuel carbon pathway for the cold-start first cranking cycle in a gasoline direct injection engine is characterized quantitatively. The engine is fired for a single cycle in one cylinder at a specified cranking speed and at a coolant temperature of 20 °C. The fuel carbon is accounted for from measurements of the exhaust carbon (CO[subscript 2], CO, and hydrocarbon). The remaining carbon is assumed to go into the oil and crankcase. The parameters studied are the amount of injected fuel, the injection timing, the intake pressure, the injection pressure, and the cranking speed. Substantial fuel enrichment is needed to produce stable combustion in the first cycle, with significant residual fuel that goes into preparing the mixture of the second cycle and into the oil and crankcase. The first cycle hydrocarbon emissions as a fraction of the fuel are not sensitive to the fuel enrichment, the manifold absolute pressure, and the injection pressure.Consortium on Engine and Fuels Researc
Potential of Negative Valve Overlap for Part Load Efficiency Improvement in Gasoline Engines
No full textThis paper reports on the potential of negative valve overlap for improving the net indicated thermal efficiency of gasoline engines during part load. Three fixed fuel flow rates, resulting in indicated mean effective pressures of up to 6 bar, were investigated. At low load, negative valve overlap (NVO) significantly reduces the pumping loses during the gas exchange loop, achieving up to 7% improvement in indicated efficiency compared to the baseline. Similar efficiency improvements are achieved by positive valve overlap (PVO), with the disadvantage of worse combustion stability from a higher residual gas fraction. As the load increases, achieving the wide-open throttle limit, the benefits of NVO for reducing the pumping losses diminish, while the blow-down losses from early exhaust valve opening increase.
However, a symmetric NVO strategy combined with a shorter exhaust duration has a higher potential for reduction in part-load fuel consumption, as the exhaust valve opening timing can be optimized to minimize the blow-down losses.Borg-Warner CorporationFiat Chrysler Automobiles (Firm)General Motors Corporatio
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
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