Late Cycle Soot Oxidation in Diesel Engines

Diesel engines are the most common engine technology used in transportation. Its widespread use comes from its inherent high efficiency and its relative low cost. It suffers from pollutant emission issues, however, that need detailed understanding of their origins to meet the strictest current and upcoming emission standards. The work presented in this thesis focuses on the study of the mechanism at the origin of soot particle emissions and gives indications on which parameters can effectively reduce them.There are two competing in-cylinder processes, soot formation and soot oxidation, governing soot emissions from conventional combustion in diesel engines. To this day, it is still often believed that inhibiting the formation of soot reduces its emissions, despite several studies showing poor relationship between soot formation and emission in most conditions encountered in diesel engines. The results presented in this thesis aims at highlighting the importance of soot oxidation in the late cycle over the soot formation process. An effort was realized to identify the parameters governing its oxidation and how it results in lower emission levels. The study of those parameters has been realised through the use of in-cylinder sampling of gases and optical measurements. The optical measurements were realised in a heavy-duty diesel engine using a Bowditch design with a specifically designed three-valve cylinder head. This cylinder head had an optical access located at the top of the combustion chamber, allowing the study of processes occurring in the late cycle, when the piston is moving down into the cylinder. The diagnostic chosen for this study is called laser extinction, a line of sight technique based on the absorption of light by the soot particles present in the flame. The data collected gave information on the evolution of the soot concentration and could be used to characterise the soot oxidation rate in the late cycle. The in-cylinder sampling measurements were realized in order to gather information on the soot particle characteristics.The results showed a strong correlation between the soot oxidation rates and the emission levels, underlining the importance of improving the oxidation rate rather than the soot formation in order to reduce soot emissions. It was shown that increasing the mixing of gases in the cylinder by means of increased turbulences could lead to a negative impact on the soot oxidation rates that were passed on to the soot levels in the exhaust. A variation of temperature of gases showed little to no impact on the soot oxidation rates and did not seem to effectively impact the emission levels. An increase of the gas density in the cylinder of gases lead to improved oxidation rate that also reduced emission levels. Of the injection related parameters studied, the injection pressure and the injector hole size had a strong impact on the soot oxidation rates even long after the end of the injection process. The most important parameter identified that affected the soot oxidation rates was the oxygen concentration. This parameter was studied in detail and it showed that a reduction of oxygen concentration lead to a slower maturation of soot during the combustion. While this feature produced more reactive soot particles prone to be more easily oxidized, it was revealed to be concerning a limited amount of particles in the cylinder. Moreover, it was largely overruled by the reduction of OH produced in the flame. Using simulation tools, it was shown that lowering the oxygen concentration reduced the flame temperature, which in turn reduced the OH concentration. The reduction of OH correlated strongly with the reduction in oxidation rates observed in the optical measurements, to a greater extent than just the drop of oxygen concentration

Evolution of In-Cylinder Diesel Engine Soot and Emission Characteristics Investigated with Online Aerosol Mass Spectrometry

To design diesel engines with low environmental impact, it is important to link health and climate-relevant soot (black carbon) emission characteristics to specific combustion conditions. The in-cylinder evolution of soot properties over the combustion cycle and as a function of exhaust gas recirculation (EGR) was investigated in a modern heavy-duty diesel engine. A novel combination of a fast gas-sampling valve and a soot particle aerosol mass spectrometer (SP-AMS) enabled online measurements of the in-cylinder soot chemistry. The results show that EGR reduced the soot formation rate. However, the late cycle soot oxidation rate (soot removal) was reduced even more, and the net effect was increased soot emissions. EGR resulted in an accumulation of polycyclic aromatic hydrocarbons (PAHs) during combustion, and led to increased PAH emissions. We show that mass spectral and optical signatures of the in-cylinder soot and associated low volatility organics change dramatically from the soot formation dominated phase to the soot oxidation dominated phase. These signatures include a class of fullerene carbon clusters that we hypothesize represent less graphitized, C5-containing fullerenic (high tortuosity or curved) soot nanostructures arising from decreased combustion temperatures and increased premixing of air and fuel with EGR. Altered soot properties are of key importance when designing emission control strategies such as diesel particulate filters and when introducing novel biofuels

Quantitative in-cylinder fuel measurements in a heavy duty diesel engine using structured laser illumination planar imaging (SLIPI)

Laser-induced fluorescence (LIF) for quantitative fuel concentration measurements in a combustion engine is a challenging task. Measuring close to the walls of the combustion chamber is even more challenging as both the incident laser light and the signal are strongly reflected on the walls of the combustion chamber. By using a new technique called Structured Laser Illumination Planar Imaging (SLIPI) such background effects, as well as unwanted multiply scattered light, can be suppressed allowing for higher measurement accuracy. In this article we demonstrate, for the first time, the feasibility of the SLIPI technique for gas phase LIF and in-cylinder measurements. Results from regular LIF and SLIPI measurements are also compared. The measurements were made on a non-reacting fuel-jet with acetone as a fuel tracer in a heavy duty diesel engine (Scania D12). It is observed that the equivalence ratio measured by SLIPI in the free part of the jet is only two thirds of that measured by regular LIF during the early jet development

METSTOR: A GIS to look for potential CO2 storage zones in France

International audienceThe METSTOR project offers a methodology to look for potentially interesting CO2 storage areas in France at the initial stage, before the "site selection" step. Our tool, embodied in a Geographic Information System, is based on an interactive map of CO2 storage capacities. Other relevant information layers are included. The geographic layers are complemented with a series of online technical notices. It seems to be the first open online GIS that offers policy makers, businesses and the public at large an integrated access to that necessary information. Our prototype, limited mainly to the Paris Basin, is released online at www.metstor.fr

Investigation of late-cycle soot oxidation using laser extinction and in-cylinder gas sampling at varying inlet oxygen concentrations in diesel engines

[EN] This study focuses on the relative importance of O-2 and OH as oxidizers of soot during the late cycle in diesel engines, where the soot oxidation is characterized in an optically accessible engine using laser extinction measurements. These are combined with in-cylinder gas sampling data from a single cylinder engine fitted with a fast gas-sampling valve. Both measurements confirm that the in-cylinder soot oxidation slows down when the inlet concentration of O-2 is reduced. A 38% decrease in intake O-2 concentration reduces the soot oxidation rate by 83%, a non-linearity suggesting that O-2 in itself is not the main soot oxidizing species. Chemical kinetics simulations of OH concentrations in the oxidation zone and estimates of the OH-soot oxidation rates point towards OH being the dominant oxidizer.The authors gratefully acknowledge the Swedish Energy Agency, the Competence Center for Combustion Processes KCFP (Project number 22485-3), and the competence center METALUND funded by FORTE for financially supporting this research. The authors acknowledge Volvo AB for providing the gas-sampling valve and personally Jan Eismark (Volvo AB) and Mats Bengtsson (Lund University) for their technical support.Gallo, Y.; Malmborg, VB.; Simonsson, J.; Svensson, E.; Shen, M.; Bengtsson, P.; Pagels, J.... (2017). Investigation of late-cycle soot oxidation using laser extinction and in-cylinder gas sampling at varying inlet oxygen concentrations in diesel engines. Fuel. 193:308-314. https://doi.org/10.1016/j.fuel.2016.12.013S30831419

METSTOR: A GIS to look for potential CO2 storage zones in France

The METSTOR project offers a methodology to look for potentially interesting CO2 storage areas in France at the initial stage, before the "site selection" step. Our tool, embodied in a Geographic Information System, is based on an interactive map of CO2 storage capacities. Other relevant information layers are included. The geographic layers are complemented with a series of online technical notices. It seems to be the first open online GIS that offers policy makers, businesses and the public at large an integrated access to that necessary information. Our prototype, limited mainly to the Paris Basin, is released online at www.metstor.fr.CO2 storage; Carbone capture and storage; Methodology; France; Site selection; Geographic information system; Storage capacities; Aquifers; Coal deposits; Depleted fields; Risk assessment

Effect of Geological Heterogeneities on Reservoir Storage Capacity and migration of CO 2 Plume in a Deep Saline Fractured Carbonate Aquifer

In a reservoir characterization study of the Hontomín deep saline aquifer, the impact of geological heterogeneities on reservoir storage capacity and the migration of the CO2 plume is explored. This work presents, for the first time, very long-term (up to 200 years) simulations of CO2 injection into the naturally fractured Sopeña Formation, of the lower Jurassic age, at Hontomín. CO2 injection was simulated as a dual permeability case with Eclipse compositional software. The matrix permeability of the carbonate reservoir is quite low (0.5 mD) and thus fluid flow through the fractures dominates. The reservoir is dissected by eight normal faults which limited its southeast extension and divided it into several segments. The effect of geological heterogeneities was tested through scenario-based modeling and variation of parameters characterizing heterogeneity within realistic limits based on other similar formations. This modeling approach worked well in Hontomín where the database is completely scarce. The plume migration, the reservoir storage capacity, and pressure, were each influenced in diverse ways by incorporating particular types of heterogeneities. The effect of matrix heterogeneities on reservoir storage capacity was substantial (by factors up to ~2.8×), compared to the plume migration. As the reservoir matrix permeability heterogeneity increased, the reservoir storage capacity markedly decreased, whilst an increase in porosity heterogeneity significantly increased it. The vertical gas migration in the homogeneous base case was relatively larger compared to the heterogeneous cases, and gas accumulated underneath the caprock via hydrodynamic trapping. It was also observed that, in heterogeneous cases, gas saturation in rock layers from top to bottom was relatively high compared to the base case, for which most of the gas was stored in the topmost layer. In contrast, the impact on storage capacity and plume movement of matrix vertical to horizontal permeability ratio in the fractured carbonate reservoir was small. The impact of the transmissibility of faults on reservoir pressure was only observed when the CO2 plume reached their vicinity

Geological Model of a Storage Complex for a CO2 Storage Operation in a Naturally-Fractured Carbonate Formation

Investigation into geological storage of CO2 is underway at Hontomín (Spain). The storage reservoir is a deep saline aquifer formed by naturally fractured carbonates with low matrix permeability. Understanding the processes that are involved in CO2 migration within these formations is key to ensure safe operation and reliable plume prediction. A geological model encompassing the whole storage complex was established based upon newly-drilled and legacy wells. The matrix characteristics were mainly obtained from the newly drilled wells with a complete suite of log acquisitions, laboratory works and hydraulic tests. The model major improvement is the integration of the natural fractures. Following a methodology that was developed for naturally fractured hydrocarbon reservoirs, the advanced characterization workflow identified the main sets of fractures and their main characteristics, such as apertures, orientations, and dips. Two main sets of fracture are identified based upon their mean orientation: North-South and East-West with different fracture density for each the facies. The flow capacity of the fracture sets are calibrated on interpreted injection tests by matching their permeability and aperture at the Discrete Fracture Network scale and are subsequently upscaled to the geological model scale. A key new feature of the model is estimated permeability anisotropy induced by the fracture sets

Parameters Influencing Soot Oxidation Rates in an Optical Diesel Engine

Soot emissions from diesel engines are the net result of two competing processes: soot formation and soot oxidation. Previous studies have shown poor correlation between soot formation rates and the soot emissions. This article presents a systematic study of a number of parameters affecting soot oxidation rate and how it correlates with the soot emissions. An optical heavy-duty engine is used in conjunction with a laser extinction setup in order to collect time resolved data of the soot concentration in the cylinder during the expansion stroke. Laser extinction is measured using a red (685 nm) laser beam, which is sent vertically through the cylinder and modulated to produce 10 pulses per crank angle degree. Information is obtained about the amount of soot formed and the soot oxidation rate. The parameters studied are the motored density at top dead center (TDC), motored temperature at TDC, injection pressure, engine speed, swirl level and injector orifice diameter. A central composite design is employed to assess the importance of the parameters as well as identifying potential interaction effects. A single exponential decay function is fit to the extinction data to describe the oxidation process and the half-life of the decay is used as a measure of the oxidation rate. The half-lives are compared with engine out emissions and the importance of each parameter is studied using regression analysis. The results suggest that the injection pressure has the strongest effect on the late-cycle soot oxidation rate, while the temperature at TDC has the weakest effect of the parameters studied

Comparison of Laser-Extinction and Natural Luminosity Measurements for Soot Probing in Diesel Optical Engines

Soot emissions from diesel internal combustion engines are strictly regulated nowadays. Laser extinction measurement (LEM) and natural luminosity (NL) of sooty flames are commonly applied to study soot. LEM measures soot along the laser beam path and it can probe soot regardless of temperature. NL integrates the whole field of view and relies on soot temperature. In this work, a comparison of simultaneously recorded LEM and NL data has been performed in a heavy-duty optical engine. A 685 nm laser beam is used for LEM. The laser was modulated at 63 kHz, which facilitated subtraction of the background NL signal from the raw LEM data. By Beer-Lambert’s law, KL factor can be calculated and used as a metric to describe soot measurements. A compensation of transmitted laser intensity fluctuation and soot deposits on optical windows has been performed in this work. The data compensation successfully reduced the transmitted laser intensity fluctuation and made it possible to study in-cylinder low temperature soot residual. The KL curves were compared with NL curve in this work. In the late cycle the KL curve can successfully show the low temperature soot which is not detected by NL. The KL curve is found to rise about 2 CAD ahead of the corresponding NL curve due to liquid fuel spray disturbance. In this case, LEM is not a suitable method to calculate KL for analyzing the early soot formation if there are liquid phase fuel droplets crossing the probing laser beam