A digital simulation of the exhaust nitric oxide and soot formation histories in the combustion chambers of a swirl chamber diesel engine

This work presents a computer simulation of the exhaust nitric oxide and soot emission histories from a four stroke, naturally aspirated, Diesel engine with a swirl prechamber (divided chamber). The simulation is based on a thermodynamic analysis, which was validated successfully concerning the performance of the engine (load, fuel consumption, maximum pressures, etc). The analysis includes the calculation of the heat exchange between gas and walls in both the main chamber and (swirl) prechamber, after computing the relevant characteristic velocities and lengths, while combustion in both the main chamber and the swirl prechamber is attacked by proposing a two-zone combustion model. The concentration of the constituents in the exhaust gases is calculated by incorporating a complete chemical equilibrium scheme for the C−H−O system of the eleven species considered, together with chemical rate equations for the calculation of nitric oxide (NO). A model for the evaluation of soot formation and oxidation rates is also included, in order to compute the net soot concentration. The contribution of each chamber to the formation of NO and soot is given by presenting time (crank angle) diagrams of the net NO and soot formation inside each chamber (histories of formation). To validate the analysis, an extensive experimental investigation is undertaken at the authors’ laboratory on a Diesel engine of this type by evaluating its exhaust emission characteristics in a wide range of operating conditions. The experimental results are found to be in good agreement with the theoretical results obtained from the computer program implementing the analysis, while the detailed NO and soot net formation histories provide insight into the mechanisms involved.     

Direct observation of atomic scale graphitic layer growth

The demand for better understanding of the mechanism of soot formation is driven by the negative environmental and health impact brought about by the burning of fossil fuels. While soot particles accumulate most of their mass from surface reactions, the mechanism for surface growth has so far been characterized primarily by measurements of the kinetics. Here we provide atomic-scale scanning tunneling microscope images of carbon growth by chemistry similar to that of importance in soot formation. At a temperature of 625 K, exposure of the surface of highly ordered pyrolytic graphite to 1 Langmuir of acetylene leads to the formation of both graphitic and amorphous carbonaceous material at the edges of nanoscale pits. Given the similarity of the electronic structure at these graphite defect sites to that of soot material growing in flames at higher temperatures, the present studies shed light on the mechanism for soot growth. These experiments also suggest that healing of defect sites in graphene nanostructures, which are of considerable interest as novel electronic devices, should be possible at modest surface temperatures by exposure of defected graphene to unsaturated hydrocarbons.     

New alternative (partially homogeneous) combustion process as a method for reduction of concentrations of nitric oxides and soot in combustion products of diesel

A new alternative (partially homogeneous) combustion process in diesel engines is proposed that provides changes in local values of the excess air coefficient and the temperature of the working substance in ranges lying beyond the area of the formation of nitric oxides and soot. It is important that the process is realized without significant changes in the serial construction. Modeling of the alternative process and the traditional diesel engine process is carried out on the basis of fundamental equations of three-dimensional nonstationary transfer in combination with models of combustion and formation of harmful substances: nitric oxides and soot. The set of equations for the model of the diesel cylinder with moving boundaries (valves, piston) is used in the Reynolds form and is closed by means of the k-ξ-f turbulence model considering the near-wall anisotropy of turbulence. The developed 3D models of the diesel engine work process are realized in the CFD code of the AVL-FIRE software complex. It is found that transition from the classic diesel engine process to partially homogeneous combustion makes it possible to block practically completely the formation of nitric oxides and to decrease soot emission substantially.     

Reactor length-scale modeling of chemical vapor deposition of carbon nanotubes

Chemical Vapor Deposition (CVD) of carbon nanotubes from a gas mixture consisting of methane (carbon precursor) and hydrogen (a carrier gas) in the presence of cobalt, nickel or iron catalytic particles in a cylindrical reactor is modeled at the reactor length-scale by solving a continuum-based coupled boundary-layer laminar-flow hydrodynamics, heat-transfer, gas-phase chemistry and surface chemistry problem. The model allows determination of the gas-phase fields for temperature, velocity, and various species as well as the surface-species coverages and the carbon deposition rate. Various available experimental and theoretical assessments are used to construct the necessary database for gas-phase and surface chemistry and gas-phase transport parameters. A reasonably good agreement is found between the model predicted and the experimentally measured carbon nanotubes deposition rates over a relatively large range of processing conditions.     

Effects of ambient pressure and precursors on soot formation in spray flames

The effects of ambient pressure on soot formation in spray jet flames are investigated by means of a two-dimensional direct numerical simulation (DNS). In addition, the effects of precursors on soot formation are also discussed. The inception models considering acetylene and polycyclic aromatic hydrocarbon (PAH) precursors are employed and compared in this study. The extended flamelet/progress-variable approach (EFPV) in which heat transfer between a droplet and the surrounding fluid can be considered is employed as a combustion model. Jet A is used as a liquid fuel with considering the detailed chemistry including 274 chemical species and 1537 elemental reactions by the flamelet library. The evaporating droplets’ motions are tracked by the Lagrangian method and the non-equilibrium Langmuir–Knudsen model is used as an evaporation model. Results show that the spray jet flame structure and the soot formation characteristics are considerably affected by the ambient pressure condition. The soot volume fraction increases with increasing the ambient pressure. It is also revealed that the dominant process to promote the soot formation is different between the acetylene and PAH precursor models. It is essential to model a soot inception process with appropriate precursors in order to understand a detail mechanism of soot formation, since the precursor which plays the important role depends on the type of fuel and the combustion conditions.     

Nanostructure and reactivity of nascent carbon particles from 2,5-dimethylfuran/n-heptane swirling inverse diffusion flames

Carbon particles (Soot) have been the one of primary pollutants inevitably with the combustion of fossil fuels. A better understanding on incipient soot was useful to build the models of soot formation and even control soot emissions. The present work focuses on the nanostructure and oxidation reactivity of nascent carbon particles (soot) formed from 2,5-dimethylfuran (DMF)/n-heptane non-swirling and swirling inverse diffusion flames (IDFs). The nascent soot samples were derived from three different fuels: 100% n-heptane, 50% n-heptane/50% DMF and 100% DMF. In addition, the effects of swirling combustion and collection time on characteristics of nascent soot were investigated in detail using the high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Results demonstrated that nascent soot from pure n-heptane flames presented the film-like morphology and nanostructure of amorphous nature, while the nearly primary particles with more well-organized nanostructure were found in pure DMF soot. Swirling combustion increased the entire flame front areas and diameters due to the existence of tangential velocity at the outlet of burner. Moreover, swirling combustion could enhance the mixability of the oxidizer stream and fuel stream, leading to more young soot within the fuel stream being oxidized in the inception stage. However, the collection time on soot characteristics exhibited quite negligible impacts in comparison to the swirling effects.     

Control of bio-MEMS surface chemical properties in micro fluidic devices for biological applications

Surface chemistry of silicon/glass based bio-MEMS was controlled by depositing plasma polymerized acrylic acid (ppAc) films at two different electrode positions in a two-stage plasma reactor. AFM and XPS were used to characterize the surface roughness and surface chemistry of the films, respectively. The surface of bio-MEMS was highly functionalized with carboxylic/ester functionalities with a very good surface uniformity. The proportion of carbon atoms as C-OX, C(==O)OX functionalities was decreased and an increase in C==O functionalities was observed when the electrode position was increased from the mesh. These functionalized bio-MEMS devices have advantages in fabrication of reusable micro fluidic devices and the variation of fluid velocity by changing the surface properties may be used to develop a micro-mixing system to control the mixing ratio of different fluids for different biological and chemical applications.     

Surface transformations of carbon (graphene, graphite, diamond, carbide), deposited on polycrystalline nickel by hot filaments chemical vapour deposition

The deposition of carbon has been studied at high temperature on polycrystalline nickel by hot filaments activated chemical vapor deposition (HFCVD). The sequences of carbon deposition are studied by surface analyses: Auger electron spectroscopy (AES), electron loss spectroscopy (ELS), X-ray photoelectron spectroscopy (XPS) in a chamber directly connected to the growth chamber. A general scale law of the (C/Ni) intensity lines is obtained with a reduced time. Both, shape analysis of the AES C KVV line and the C1s relative intensity suggest a three-step process: first formation of graphene and a highly graphitic layer, then multiphase formation with graphitic, carbidic and diamond-like carbon and finally at a critical temperature that strongly depends on the pretreatment of the polycrystalline nickel surface, a rapid transition to diamond island formation. Whatever the substrate diamond is always the final product and some graphene layers the initial product. Moreover it is possible to stabilize a few graphene layers at the initial sequences of carbon deposition. The duration of this stabilization step is strongly depending however on the pre-treatment of the Ni surface.     

Combustion engine optimization: a multiobjective approach

To simulate the physical and chemical processes inside combustion engines is possible by appropriate software and high performance computers. For combustion engines a good design is such that it combines a low fuel consumption with low emissions of soot and nitrogen oxides. These are however partly conflicting requirements. In this paper we approach this problem in a multi-criteria setting which has the advantage that it is possible to estimate the trade off between the different objectives and the decision of the optimal solution is postponed until all possibilities and limitations are known. The optimization algorithm is based on surrogate models and is here applied to optimize the design of a diesel combustion engine.     

Characterization of carbon surface chemistry by combined temperature programmed desorption with in situ X-ray photoelectron spectrometry and temperature programmed desorption with mass spectrometry analysis

The analysis of the surface chemistry of carbon materials is of prime importance in numerous applications, but it is still a challenge to identify and quantify the surface functional groups which are present on a given carbon. Temperature programmed desorption with mass spectrometry analysis (TPD-MS) and X-ray photoelectron spectroscopy with an in situ heating device (TPD-XPS) were combined in order to improve the characterization of carbon surface chemistry. TPD-MS analysis allowed the quantitative analysis of the released gases as a function of temperature, while the use of a TPD device inside the XPS setup enabled the determination of the functional groups that remain on the surface at the same temperatures. TPD-MS results were then used to add constraints on the deconvolution of the O1s envelope of the XPS spectra. Furthermore, a better knowledge of the evolution of oxygen functional groups with temperature during a thermal treatment could be obtained. Hence, we show here that the combination of these two methods allows to increase the reliability of the analysis of the surface chemistry of carbon materials.     

Hydrophilicity of soot particles formed in the combustion chamber of a jet engine

The particles of soot, formed in the combustion chamber of a jet engine under conditions typical of a cruising power mode, are capable of adsorbing a considerable amount of water vapor owing to their microcrystalline structure, microporosity, and chemically heterogeneous surface.     

Oxidation treatment of diesel soot particulate on CexZr1-xO2

Catalytic oxidation of diesel soot particulate on Ce(x)Zr(1-x)O(2) catalysts was investigated. Results indicated that Ce/Zr ratios had a significant influence on the catalytic activities. Compared with the ignition temperature (T(i)) of uncatalyzed soot combustion, T(i) of Ce(0.5)Zr(0.5)O(2) with the best catalytic behavior decreased by 80 degrees C. The reactant gas compositions (O(2), H(2)O and NO) affected the catalytic activities too. O(2)-TPD, TG-DTA and XPS characterization results showed that Ce(x)Zr(1-x)O(2) released lattice oxygen continuously to promote the soot combustion even no gas oxygen occurred in the reaction atmosphere. The mechanisms of spill-over and reduction/oxidation functioned synergistically for soot catalytic combustion.     

Diesel emission control: Catalytic filters for particulate removal

The European diesel engine industry represents a vital sector across the Continent, with more than 2 million direct work positions and a turnover of over 400 billion Euro. Diesel engines provide large paybacks to society since they are extensively used to transport goods, services and people. In recent years increasing attention has been paid to the emissions from diesel engines which, like gasoline engine emissions, include carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NO_x). Diesel engines also produce significant levels of particulate matter (PM), which consists mostly of carbonaceous soot and a soluble organic fraction (SOF) of hydrocarbons that have condensed on the soot.

Meeting the emission levels imposed for NO_x and PM by legislation (Euro IV in 2005 and, in the 2008 perspective, Euro V) requires the development of a number of critical technologies to fulfill these very stringent emission limits (e.g. 0.005 g/km for PM). This review is focused on these innovative technologies with special reference to catalytic traps for diesel particulate removal.     

RECENT DISCOVERIES IN CARBON BLACK FORMATION AND MORPHOLOGY AND THEIR IMPLICATIONS ON THE STRUCTURE OF INTERSTELLAR CARBON DUST

In this review we discuss about the presence of carbon under the form of dust grains and organic molecules in the interstellar and circumstellar medium. The spectral evidences about the presence of these carbon-based dust and molecules in the space is also reviewed. We would like to emphasise that the molecules present in space, namely polyyne chains and rings, polycyclic aromatic hydrocarbons and fullerenes which are the precursors of the formation of interstellar carbon dust have been also detected by in situ mass spectrometry in ordinary flames and in the carbon arc formed between graphite electrodes under special conditions. Thus the mechanism which lead to soot formation in a flame should be similar to that leading to carbon dust in space. In fact, the morphology of the carbon black produced industrially by furnace process or by thermal decomposition appears very close to the morphology of carbon dust produced under conditions which should simulate the circumstellar conditions. In both cases curved graphene sheets can be observed together with onion-like particles and defective structures which we have defined fullerene-like structures. The presence of these structures may affect the surface chemistry of the carbon dust grains by enhancing the electron affinity in comparison to graphite.     

Developing improved models of oxidatively treated carbon fibre surfaces,using molecular simulation

In this work computer modelling investigations are reported into the chemical functional groups that are believed to contribute to composite fibre/matrix adhesion. The CERIUS [Compos A: Appl Sci Manuf 29 (1998) 1291] and MOPAC computational packages are employed to simulate the sorption interactions of small molecules at a range of functionalised surfaces. The results of these calculations are compared with fibre surface energetic data reported previously, and models of the fibre surface chemistry with varying degrees of surface treatment are developed. There is agreement that surface treatment of carbon fibres improves adhesion and adsorption characteristics, yet the factors controlling this have still to be determined fully. The empirical study using the computational model developed in this work attempts to resolve the changes in fibre adsorption phenomena into contributions arising from specific surface oxygen and nitrogen functional groups. The general trend in surface concentration of these atoms is similar to XPS analyses of the surface treated carbon fibres and is in agreement with previously published work.     

Numerical simulation of soot formation in pulverized coal combustion with detailed chemical reaction mechanism

A two-dimensional unsteady numerical simulation with a detailed chemical reaction mechanism that considers 158 species and 1804 reactions is applied to pulverized coal combustion in a mixing layer and the soot formation behavior is investigated in detail. The computational conditions and ignition process are the same as those in our previous work (Muto et al., 2017). The results show that the peak of the mass density of the soot is distributed in the region where the gas temperature is higher than the unburned gas temperature of the mixture of volatile matter and air (1300–1400?K) and lower than the flame temperature (2000?K $～$). This is due to the fact that soot formation from the precursors (C₂H₂ and C₆H₆) is enhanced as the gas temperature increases, whereas the quantities of the precursors and the produced soot are reduced due to oxidation at the higher gas temperature condition that exists close to the flame. The peak value of the mass density of the soot is also distributed in the region between the peak values of the gas temperature and the probability density function of the number of coal particles.     

Contribution to prediction of soft and hard failure occurrence in combustion engine using oil tribo data

When assessing technical systems, we apply different condition indicators. If we take into account indirect diagnostic indicators, lubricant seems to be a very good source of different information. Where the oil is part of a system, it is possible to get the information about the oil condition as well as the system itself. In this article we focus on the system of piston combustion engine and tribodiagnostic data. The indirect diagnostic information is the concentration level of contaminating particles in the oil. We observe specific particles of oil contamination - soot which is formed in the operation as a by-product during fuel combustion. The soot contaminates the oil significantly and the soot concentration shows under what conditions the system was operated. It also indicates that the system condition gradually deteriorates. We have a statistically very interesting set of data from the operation of heavy off-road vehicles. The recorded soot operation data depend on a few operation values: kilometres [km], days [day] and moto-hours [Mh]. Modelling a soft and a hard failure which might occur the moment the concentration reaches its critical value is performed with selected stochastic diffusion processes, namely the Wiener process with drift and the Ornstein-Uhlnbeck process. The main aim and contribution of our article is to estimate and study the distribution of the first hitting time of a critical threshold and to determine the moment when the soft and hard failure occurs in a vehicle. We also try to estimate the engine useful residual life. We are aware, however, that there are numerous possibilities of using these results.     

柴油机油烟的燃烧:其可溶有机组分含量对动力学参数的影响

论述了两种可溶有机组分含量(8w %,25w %)的柴油机油烟的燃烧机理。采用加热速率5℃ min～20℃ min非等温条件下的热重分析(TGA),考察了其在空气中的燃烧。应用Ozawa Flynn Wall方法,研究了析出物质的数据资料,并由复变非线性回归方法获得了各动力学参数。两种油烟的析出可根据三段机理予以模定,即:(1)可溶有机组分(SOF)的蒸发,(2)SOF热裂解,(3)炭基质燃烧。由于两种油烟所得的动力学参数非常近似,表明SOF含量对基质燃烧的影响极其有限。     

Semi-empirical method for the prediction of soot fomation in gas turbine combustors

Soot formation in gas turbine combustors has serious effects on liner durability, carbon deposition and smoke emission. In this work, an analytical study is made to derive quantitative relationships for calculating temperature, unburned hydrocarbon and oxygen concentration from previous detailed experimental data . The quantities of the derived parameters are used in a soot formation model to predict soot concentration along the primary zone.

The soot concentration predicted compares favourably with experiment, especially at full load conditions. This supports the capability of the model with the derived quantitati ve relationships in predicting soot concentration for different gas turbine combustors.     

Modern combustion analysis techniques using lasers

Principall laser techniques for the analysis of elementary combustion processes are outlined, including the laser Doppler velocimeter for measuring flow velocities, the correlation spectro scopy method for measuring soot diameters and the coherent antiStokes Raman scattering method for temperature measurement. Research activities in these fields are describ ed, based on recent reports in the literature, and practical applications centring on engine combustion are tabulated.