1,721,001 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
Analytical Correlations for Modeling the Laminar Flame Speed of Natural Gas Surrogate Mixtures
Full text linkNatural gas is increasingly used as an alternative to petroleum fuels in internal combustion engines and industrial power plants because of its smaller environmental effects, as well as for economic reasons. Many applications, such as the spark-ignition engine simulations and the design of burners, require an accurate calculation of its laminar flame speed. Encouraging progress has been made in developing detailed chemical kinetic models for its prediction, but such models are still extremely complex and require significant computational effort. The laminar flame speed is an intrinsic property that is a function of the unburnt mixture composition, temperature, and pressure, therefore it is possible to develop analytical correlations based on experimental measurements, without losing accuracy, and that are more easily implemented in CFD codes than tabulated data. The purpose of this study is to provide a simple, but accurate expression for modeling the laminar flame speed of natural gas as a function of its composition and over a wide range of operating conditions. In particular, a correlation valid for a natural gas ternary surrogate mixture of methane, ethane and propane is proposed. To achieve this aim, correlations for pure methane, as well as for binary methane/ethane and methane/propane mixtures were derived and combined to obtain a formulation suitable for different compositions of natural gas. It must be highlighted that some empirical correlations are already available in the literature, but they are usually based on a limited set of experimental measurements, thus they can fail outside the range in which they have been validated against experiments. In this study, measurements of laminar flame speeds obtained by several research teams are collected, compared, and critically analysed with the aim to develop more accurate empirical correlations. A comparison with available correlations in the literature shows the improvement in accuracy obtainable with the approach proposed in the present work
Measured and Predicted Soot Particle Emissions from Natural Gas Engines
No full textDue to the new challenge of meeting number-based regulations for
particulate matter (PM), a numerical and experimental study has been
conducted to better understand particulate formation in engines
fuelled with compressed natural gas. The study has been conducted
on a Heavy-Duty, Euro VI, 4-cylinder, spark ignited engine, with
multipoint sequential phased injection and stoichiometric
combustion. For the experimental measurements two different
instruments were used: a condensation particle counter (CPC) and a
fast-response particle size spectrometer (DMS) the latter able also to
provide a particle size distribution of the measured particles in the
range from 5 to 1000 nm. Experimental measurements in both
stationary and transient conditions were carried out. The data using
the World Harmonized Transient Cycle (WHTC) were useful to
detect which operating conditions lead to high numbers of particles.
Then a further transient test was used for a more detailed and deeper
analysis. Finally 3-D Computational Fluid Dynamics (CFD)
simulations were performed and the numerical results obtained were
compared to particle size distributions (PSDs) derived from the
experimental measurements carried out in stationary conditions. In
this way the influences of engine load and regime on particle size
distribution (PSD) were determined. A semi-detailed soot model and
a chemical kinetic model, including poly-aromatic hydrocarbon
(PAH) formation, were coupled with a spark ignition model and the
G equation flame propagation model for the SI engine simulations
and for predictions of soot mass and particulate number density.
Qualitative agreements of in-cylinder particle distributions were
obtained and results are helpful to understand particulate formation
processes
Computational study of a two-stroke direct-injection reactivity-controlled compression ignition engine
No full textReactivity-controlled compression ignition combustion has proved to be effective in reducing the pollutant emissions and the fuel consumption in four-stroke internal-combustion engines. The application of this combustion mode to port-controlled crankcase-scavenged two-stroke engines also seems promising to avoid short-circuiting of fresh charge and to take advantage of the intrinsic residual exhaust gas. Accordingly, a computational study of a small-bore two-stroke dual-fuel direct-injection reactivity-controlled compression ignition engine was made including computational fluid dynamics simulations and zero-dimensional modeling. The zero-dimensional model is used to supply suitable initial conditions for the computational fluid dynamics simulations and to generate useful operating maps. These maps predict the engine behavior, highlighting the conditions where combustion would be controllable by means of in-cylinder reactivity stratificarion. The computational fluid dynamics simulations were validated against experimental data under motored and fired conditions, and the spray model was calibrated against dedicated bench tests. The in-cylinder behavior was explored to understand the effect and the importance on the engine operation of several types of stratification, including thermal stratification and reactivity stratification caused by the scavenging process and fuel injections. The models emphasize the importance of the exhaust gas thermal content which can promote combustion. Furthermore, its stratification in the combustion chamber due to the scavenging process, together with the reactivity stratification caused by the dual-fuel injection is able to change both the combustion phasing and the combution duration, thereby increasing the efficiency and reducing the combustion roughness
Predicting lubricant oil induced pre-ignition phenomena in modern gasoline engines: The reduced GasLube reaction mechanism
No full textRecent research highlights the influence of the presence of lubricant oil droplets on the combustion process in Direct Injection Spark Ignition (DISI) engines. Lubricant oil is considered to be the main responsible agent for the onset of pre-ignition phenomena, which can escalate highly undesired super-knock events. Moreover, lubricant oil plays a primary role in the generation of very fine soot particle emissions. In the present work, a reduced reaction mechanism is developed for modeling the combustion of gasoline-oil mixtures, allowing one to simulate the variation in ignitability of gasoline-like fuels induced by the presence of lubricant oil. In this study, a single hydrocarbon species, namely n-Hexadecane (n-C16H34), is shown to reproduce lubricant oil chemical and physical characteristics. Great effort has been performed to identify the most significant reaction pathways to reduce the complexity of the chemistry mechanism and the number of variables, while maintaining the important features of detailed mechanisms, for the highest computational efficiency. The proposed reduced mechanism has been validated for a wide range of operating conditions. It is employed for 3D simulations of experimental measurements in which iso-Octane was blended with different percentages of lubricant oil and its surrogates. Operating conditions representative of those of a typical turbocharged DISI engine are considered. The very good agreement obtained in the comparison with the experimental data confirms the effectiveness of the proposed “GasLube” mechanism in reproducing lubricant oil's influence on ignition propensity of gasoline-like fuels. Furthermore, the 3D numerical simulations allowed a detailed analysis of the ignition phenomenon, providing more insight into the basic processes of lubricant oil induced pre-ignition events in DISI gasoline engines
Investigation of Lubricant Oil influence on Ignition of Gasoline-like Fuels by a Detailed Reaction Mechanism
Full text linkA reaction mechanism, suitable for simulating the lubricant oil influence on the combustion process of gasoline-like fuels, is developed. The proposed work is motivated by evidences reported in the literature highlighting that lubricant oil droplets can be the most likely inducer of pre-ignition phenomena, as well as the fact that lubricant oil can represent the main source of very fine soot particles emitted from the engine. In other words, the mixture of fuel and lubricant oil, through complex physical and chemical reactions involving long hydrocarbons chains, can auto-ignite before the spark timing or, if not the case, can become one of the major candidates in the generation of soot precursors. Therefore, developing a reliable reaction mechanism able to simulate the oil-fuel mixture behavior and contribution in terms of pre-ignition and soot formation is fundamental for predicting the onset of knocking phenomena and particle size distributions of soot emissions. In this study, surrogate components reproducing the lubricant oil propensity to ignition were first identified. A detailed reaction mechanism was then developed and validated starting from existing mechanisms proposed for the single species. In particular, alkanes ranging from C16 to C18 were selected as oil surrogates. Zero-dimensional numerical simulations were conducted in order to validate the proposed mechanism versus literature experimental data aimed at reproducing the effect of commercial lubricants on ignition propensity of gasoline-like fuels. From this analysis it was possible to define a suitable composition of the surrogate mixture proposed to model lubricant oil effects
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
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