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Journal Article

Transient simulation of NOx reduction over a Fe-Zeolite catalyst in an NH3-SCR system and study of the performance under different operating conditions

2011-08-30
2011-01-2084
The NO reduction in an ammonia SCR converter has been simulated by a 1D+1D model for a single representative channel to parametrically study the characteristics of the system under typical operating conditions. An appropriate model has been selected interpreting the chemical behavior of the system and the parameters are calibrated based on a comprehensive set of experiments with an Fe-Zeolite washcoated monolith for different feed concentrations, temperatures and flow rates. Physical and chemical properties are determined as well as kinetics and rate parameters and the model has been verified by experimental data at different operating conditions. Three different mechanisms for the surface kinetics to model NO reduction have been assessed and the results have been compared in the cases of steady DeNO performance and transient response of the system. Ammonia inhibition is considered in the model since it has a major effect specifically under transient operating conditions.
Journal Article

The Effect of Cycle-to-Cycle Variations on the NOx-SFC Tradeoff in Diesel Engines under Long Ignition Delay Conditions

2017-09-04
2017-24-0100
Cycle-to-cycle variations in internal combustion engines are known to lead to limitations in engine load and efficiency, as well as increases in emissions. Recent research has led to the identification of the source of cyclic variations of pressure, soot and NO emissions in direct injection common rail diesel engines, when employing a single block injection and operating under long ignition delay conditions. The variations in peak pressure arise from changes in the diffusion combustion rate, caused by randomly occurring in-cylinder pressure fluctuations. These fluctuations result from the excitation of the first radial mode of vibration of the cylinder gases which arises from the rapid premixed combustion after the long ignition delay period. Cycles with high-intensity fluctuations present faster diffusion combustion, resulting in higher cycle peak pressure, as well as higher measured exhaust NO concentrations.
Technical Paper

THE Post Injection: Coalescence of 3D CFD-CMC Simulation, 2D Visualizations in a Constant Volume Chamber and Application in a Modern Passenger Car Diesel Engine

2015-09-06
2015-24-2515
Past research has shown that post injections have the potential to reduce Diesel engine exhaust PM concentration without any significant influence in NOx emissions. However, an accurate, widely applicable rule of how to parameterize a post injection such that it provides a maximum reduction of PM emissions does not exist. Moreover, the underlying mechanisms are not thoroughly understood. In past research, the underlying mechanisms have been investigated in engine experiments, in constant volume chambers and also using detailed 3D CFD-CMC simulations. It has been observed that soot reduction due to a post injection is mainly due to two reasons: increased turbulence from the post injection during soot oxidation and lower soot formation due to lower amount of fuel in the main combustion at similar load conditions. Those studies do not show a significant temperature rise caused by the post injection.
Journal Article

Soot Emission Measurements and Validation of a Mean Value Soot Model for Common-Rail Diesel Engines during Transient Operation

2009-06-15
2009-01-1904
Measurements of the soot emissions and engine operating parameters from a diesel engine during transient operation were used to investigate the influence of transient operation on the soot emissions, as well as to validate a realtime mean value soot model (MVSM, [1]) for transient operation. To maximize the temporal resolution of the soot emission and engine parameter measurements (in particular EGR), fast instruments were used and their dynamic responses characterized and corrected. During tip-in transients, an increase in the soot emissions was observed due to a short term oxygen deficit compared to steady-state operation. No significant difference was seen between steady-state and transient operation for acceleration transients. When the MVSM was provided with inputs of sufficient temporal resolution, it was capable of reproducing the qualitative and, in part, quantitative soot emission trends.
Technical Paper

Simulations of In-Cylinder Processes in a Diesel Engine Operated with Post-Injections Using an Extended CMC Model

2014-10-13
2014-01-2571
In this study, numerical simulations of in-cylinder processes associated to fuel post-injection in a diesel engine operated at Low Temperature Combustion (LTC) have been performed. An extended Conditional Moment Closure (CMC) model capable of accounting for an arbitrary number of subsequent injections has been employed: instead of a three-feed system, the problem has been described as a sequential two-feed system, using the total mixture fraction as the conditioning scalar. A reduced n-heptane chemical mechanism coupled with a two-equation soot model is employed. Numerical results have been validated with measurements from the optically accessible heavy-duty diesel engine installed at Sandia National Laboratories by comparing apparent heat release rate (AHRR) and in-cylinder soot mass evolutions for three different start of main injection, and a wide range of post injection dwell times.
Technical Paper

Reduction of NOx Emissions of D. I. Diesel Engines by Application of the Miller-System: An Experimental and Numerical Investigation

1996-02-01
960844
Emissions and performance parameters of a medium size, medium speed D.I. diesel engine with increased charge air pressure and reduced but fixed inlet valve opening period have been measured and compared to the standard engine. While power output and fuel consumption are slightly improved, nitric oxide emissions can be reduced by up to 20%. The measurements confirm the results of simulations for both performance and emissions, for which a quasidimensional model including detailed chemistry for nitric oxide prediction has been developed.
Journal Article

Predicting In-Cylinder Soot in a Heavy-Duty Diesel Engine for Variations in SOI and TDC Temperature Using the Conditional Moment Closure Model

2013-09-08
2013-24-0016
Numerical simulations of in-cylinder soot evolution in the optically accessible heavy-duty diesel engine of Sandia National Laboratories have been performed with the multidimensional conditional moment closure (CMC) model using a reduced n-heptane chemical mechanism coupled with a two-equation soot model. Simulation results are compared to the high-fidelity experimental data by means of pressure traces, apparent heat release rate (AHRR) and time-resolved in-cylinder soot mass derived from optical soot luminosity and multiple wavelength pyrometry in conjunction with high speed soot cloud imaging. In addition, spatial distributions of soot relevant quantities are given for several operating conditions.
Technical Paper

Oxygenated Fuels for Particulate Emissions Reduction in Heavy-Duty DI-Diesel Engines with Common-Rail Fuel Injection

2000-10-16
2000-01-2885
Oxygenated fuel additives are currently an important research topic for particulate emissions reduction in diesel engines with direct injection (DI) to meet future emission regulations. In this work more than twenty oxygenated hydrocarbons from the literature were considered as diesel fuel additives. Butylal (an acetal compound, chemical formula C9H20O2) offers significant advantages over most other oxygenates in that its physical properties are very close to those of common diesel fuel. Wear scar measurements were conducted to evaluate the lubricity characteristics of diglyme (C6H14O3), ethyldiglyme (C8H18O3), butylal and different diesel-butylal mixtures. The results reveal the low lubricity of all oxygenated compounds. Thus, for the engine tests, a lubricity improver has been added to the diesel-butylal mixtures.
Technical Paper

Optical Investigations of Soot Reduction Mechanisms using Post-Injections in a Cylindrical Constant Volume Chamber (CCVC)

2014-10-13
2014-01-2839
Past research has shown that post injections have the potential to reduce Diesel engine exhaust PM concentration without any significant influence in the NOx emissions. In earlier research it was observed that soot reduction due to a post injection is based on three reasons: increased turbulence (1) and heat (2) from the post injection during soot oxidation and lower soot formation due to smaller main injection for similar load conditions (3). The second effect of heat addition during the soot oxidation is debated in the literature. The experimental investigation presented in the current work provides insight into the underlying mechanisms of soot formation and reduction using post injections under different operating conditions. The experimental data have been obtained using a cylindrical constant volume chamber with high optical access. The soot evolution has been obtained using 2-color-pyrometry.
Journal Article

Optical Investigation of Sooting Propensity of n-Dodecane Pilot/Lean-Premixed Methane Dual-Fuel Combustion in a Rapid Compression-Expansion Machine

2018-04-03
2018-01-0258
The sooting propensity of dual-fuel combustion with n-dodecane pilot injection in a lean-premixed methane-air charge has been investigated using an optically accessible Rapid Compression-Expansion Machine (RCEM) to achieve engine-relevant pressure and temperature conditions at the start of pilot injection. A Diesel injector with a 100 μm single-hole coaxial nozzle, mounted at the cylinder periphery, has been employed to admit the pilot fuel. The aim of this study was to enhance the fundamental understanding of soot formation and oxidation processes of n-dodecane in the presence of methane in the air charge by parametric variation of methane equivalence ratio, charge temperature, and pilot fuel injection duration. The influence of methane on ignition delay and flame extent of the pilot fuel jet has been determined by simultaneous excited-state hydroxyl radical (OH*) chemiluminescence and Schlieren imaging.
Journal Article

Numerical Study of the Influence of EGR on In-Cylinder Soot Characteristics in a Heavy-Duty Diesel Engine using CMC

2014-04-01
2014-01-1134
This paper presents numerical simulations of in-cylinder soot evolution in the optically accessible heavy-duty diesel engine of Sandia Laboratories performed with the conditional moment closure (CMC) model employing a reduced n-heptane chemical mechanism coupled with a two-equation soot model. The influence of exhaust gas recirculation (EGR) on in-cylinder processes is studied considering different ambient oxygen volume fractions (8 - 21 percent), while maintaining intake pressure and temperature as well as the injection configuration unchanged. This corresponds to EGR rates between 0 and 65 percent. Simulation results are first compared with experimental data by means of apparent heat release rate (AHRR) and temporally resolved in-cylinder soot mass, where a quantitative comparison is presented. The model was found to fairly well reproduce ignition delays as well as AHRR traces along the EGR variation with a slight underestimation of the diffusion burn portion.
Technical Paper

Numerical Study of Turbulence and Fuel-Air Mixing within a Scavenged Pre-Chamber Using RANS and LES

2019-04-02
2019-01-0198
It is well-known that the spatial distribution of turbulence intensity and fuel concentration at spark-time play a pivotal role on the flame development within the pre-chamber in gas engines equipped with a scavenged pre-chamber. The combustion within the pre-chamber is in turn a determining factor in terms of combustion behaviour in the main chamber, and accordingly it influences the engine efficiency as well as pollutant emissions such as NOx and unburned hydrocarbons. This paper presents a numerical analysis of fuel concentration and turbulence distribution at spark time for an automotive-sized scavenged pre-chamber mounted at the head of a rapid compression-expansion machine (RCEM). Two different pre-chamber orifice orientations are considered: straight and tilted nozzles. The latter introduce a swirling flow within the pre-chamber. Simulations have been carried out using with two different turbulence models: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES).
Journal Article

Numerical Modelling and Experimental Characterization of a Pressure-Assisted Multi-Stream Injector for SCR Exhaust Gas After-Treatment

2014-10-13
2014-01-2822
Simulations for a pressure-assisted multi-stream injector designed for urea-dosing in a selective catalytic reduction (SCR) exhaust gas system have been carried out and compared to measurements taken in an optically accessible high-fidelity flow test rig. The experimental data comprises four different combinations of mass flow rate and temperature for the gas stream with unchanged injection parameters for the spray. First, a parametric study is carried out to determine the importance of various spray sub-models, including atomization, spray-wall interaction, buoyancy as well as droplet coalescence. Optimal parameters are determined using experimental data for one reference operating condition.
Technical Paper

Numerical Investigation of Soot Dynamics at Engine-Relevant Conditions

2018-04-03
2018-01-0204
Formation of soot in an auto-igniting n-dodecane spray under diesel engine relevant conditions has been investigated numerically. As opposed to research addressing turbulence-chemistry interaction (TCI) by coupling diffusive turbulence models with more sophisticated models in the context of Reynolds-Averaged Navier-Stokes equations (RANS), this study employs the advanced sub-grid scale k-equation model in the framework of a Large Eddy Simulation (LES) together with the uninvolved Direct Integration (DI) approach. A reduced n-heptane chemical mechanism has been employed and artificially accelerated in order to predict the ignition for n-dodecane accurately. Soot processes have been modelled with an extended version of the semi-empirical, two-equation model of Leung, which considers C2H2 as the soot precursor and accounts for particle inception, surface growth by C2H2 addition, oxidation by O2, oxidation by OH and particle coagulation.
Technical Paper

Numerical Investigation of Nozzle-Geometry Variations and Back-Pressure Changes on High Pressure Gas Injections under Application-Relevant Conditions

2018-04-03
2018-01-1138
In the present work numerical simulations were carried out investigating the effect of fuel type, nozzle-geometry variations and back-pressure changes on high-pressure gas injections under application-relevant conditions. Methane, hydrogen and nitrogen with a total pressure of 500 bar served as high-pressure fuels and were injected into air at rest at 200 bar and 100 bar. Different nozzle shapes were simulated and the analysis of the results lead to a recommendation for the most advantageous geometry regarding jet penetration, volumetric growth, mixing enhancement and discharge coefficient. Additionally an artificial inlet boundary conditions was tested for the use with real-gas thermodynamics and was shown to be capable of reducing the simulation time significantly.
Technical Paper

Near-Wall Unsteady Premixed Flame Propagation in S.I. Engines

1995-02-01
951001
A computational study of the near-wall premixed flame propagation in homogeneous charge spark ignited engines is presented on the basis of a spectral concept accounting for flow-chemistry interaction in the flamelet regime. Flame surface enhancement due to wrinkling and modification of the local laminar flame speed due to flame stretch are the main phenomena described by the model. A high pass filter in the turbulent kinetic energy spectrum associated with the distance between the ensemble-averaged flame front location and the solid surface has been also introduced. In addition a probability density function of instantaneous flamelet positions around the above mean flame front location allows to consider statistical effects in a simplified way. Issues of temperature distribution within the boundary layer and associated heat losses, except for the concept of a thermal quenching distance, are thereby not explicitly taken into account.
Technical Paper

Natural Gas Engines for Cogeneration: Highest Efficiency and Near-Zero-Emissions through Turbocharging, EGR and 3-Way Catalytic Converter

2000-10-16
2000-01-2825
Combustion engines for decentralized power generation or cogeneration in general, are subject to increasingly stringent pollutant emissions regulations. Motivated by the Europe-;wide lowest allowable NOx levels in Switzerland - particularly in the Zurich metropolitan area with 50 mg/Nm3 at 5% O2 - and in close cooperation with industry, the I.C. Engines and Combustion Laboratory (LVV) of the Swiss Federal Institute of Technology Zurich (ETHZ) has investigated some new operating concepts and engine processes in order to overcome the dilemma between low emissions and high efficiency, which is usually encountered in engine optimization. Our final approach thereby involves the Exhaust Gas Recirculation (EGR) combined with stoichiometric mixture (λ = 1) and a 3-way catalytic converter. The engine is supercharged and the intake mixture aftercooled for high power density and thermal efficiency.
Journal Article

Modeling Split Injections of ECN “Spray A” Using a Conditional Moment Closure Combustion Model with RANS and LES

2016-10-17
2016-01-2237
This study investigates n-dodecane split injections of “Spray A” from the Engine Combustion Network (ECN) using two different turbulence treatments (RANS and LES) in conjunction with a Conditional Moment Closure combustion model (CMC). The two modeling approaches are first assessed in terms of vapor spray penetration evolutions of non-reacting split injections showing a clearly superior performance of the LES compared to RANS: while the former successfully reproduces the experimental results for both first and second injection events, the slipstream effect in the wake of the first injection jet is not accurately captured by RANS leading to an over-predicted spray tip penetration of the second pulse. In a second step, two reactive operating conditions with the same ambient density were investigated, namely one at a diesel-like condition (900K, 60bar) and one at a lower temperature (750K, 50bar).
Technical Paper

Integration of a Cool-Flame Heat Release Rate Model into a 3-Stage Ignition Model for HCCI Applications and Different Fuels

2014-04-01
2014-01-1268
The heat release of the low temperature reactions (LTR or cool-flame) under Homogeneous Charge Compression Ignition (HCCI) combustion has been quantified for five candidate fuels in an optically accessible Rapid Compression Expansion Machine (RCEM). Two technical fuels (Naphthas) and three primary reference fuels (PRF), (n-heptane, PRF25 and PRF50) were examined. The Cetane Numbers (CN) of the fuels range from 35 to 56. Variation of the operating parameters has been performed, in regard to initial charge temperature of 383, 408, and 433K, exhaust gas recirculation (EGR) rate of 0%, 25%, and 50%, and equivalence ratio of 0.29, 0.38, 0.4, 0.53, 0.57, and 0.8. Pressure indication measurements, OH-chemiluminescence imaging, and passive spectroscopy were simultaneously implemented. In our previous work, an empirical, three-stage, Arrhenius-type ignition delay model, parameterized on shock tube data, was found to be applicable also in a transient, engine-relevant environment.
Technical Paper

Influence of Water-Diesel Fuel Emulsions and EGR on Combustion and Exhaust Emissions of Heavy Duty DI-Diesel Engines equipped with Common-Rail Injection System

2003-10-27
2003-01-3146
In this paper we investigate the effect of the introduction of water in the combustion chamber of a DI-diesel engine on combustion characteristics and pollutant formation, by using water-diesel fuel emulsions with three distinct water amounts (13%, 21% and 30%). For the measurements we use a modern 4-cylinder DI-diesel engine with high-pressure common rail fuel injection and EGR system. The engine investigations are conducted at constant speed in different operating points of the engine map with wide variations of injection setting parameters and EGR rate. The main concern refers to the interpretation of both measured values and relevant thermodynamic variables, which are computed with analytical instruments (heat release rate, ignition delay, reciprocal characteristic mixing time, etc). The analysis of the measured and computed data shows clear trends and detailed evaluations on the behavior of water-diesel fuel emulsions in the engine process are possible.
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