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Frojd K.,LOGE AB | Mauss F.,TU Brandenburg
SAE International Journal of Engines | Year: 2011

Interactions between in-cylinder combustion and emission aftertreatment need to be understood for optimizing the overall powertrain system. Numerical investigations can aid this process. For this purpose, simple and numerically fast, but still accurate models are needed for in-cylinder combustion and exhaust aftertreatment. The chemical processes must be represented in sufficient detail to predict engine power, fuel consumption, and tailpipe emission levels of NOx, soot, CO and unburned hydrocarbons. This paper reports on a new transient one-dimensional catalyst model. This model makes use of a detailed kinetic mechanism to describe the catalytic reactions. A single-channel or a set of representative channels are used in the presented approach. Each channel is discretized into a number of cells. Each cell is treated as a perfectly stirred reactor (PSR) with a thin film layer for washcoat treatment. Heat and mass transport coefficients are calculated from Nusselt and Sherwood laws. Either detailed or global surface chemistry is applied in the thin film layer. Three global parameters are used to align the detailed chemistry model with a given catalyst topology and composition; one parameter for heat transfer, one for mass transfer and one for overall reaction efficiency. This allows considering detailed surface chemistry, molecular diffusion and heat conductivity while maintaining affordable CPU time. Detailed, usually unknown, specifications of the catalyst material are insignificant for the presented approach. The models' applicability is demonstrated for a single-channel of a NOx-storage catalyst (NSC). The detailed surface chemistry by Koop and Deutschmann is utilized. Good agreement between experimental data and model results is achieved. The investigation of surface site fractions shows, that CO and C3H6 from exhaust gases inhibit NO oxidation by the same process; in both cases surface bound CO blocks the sites for NO oxidation. The inhibition effect is mainly determined by the total concentration of carbon atoms contained in CO and HC in the exhaust stream. Oxidation by surface bound oxygen was further found to be the major pathway for HC conversion. The lasting inhibition effect of unburned hydrocarbons on NO oxidation was studied by a transient calculation. In this test a sudden cutoff of unburned hydrocarbons in the exhaust stream was assumed. The response time for NO oxidation was found to be 5.5 seconds. The high response time proofs the necessity of using a transient model of sufficient detail to simulate catalytic oxidation during transient engine processes or under cyclic variations. © 2011 SAE International. Source

Bjerkborn S.,LOGE AB | Frojd K.,LOGE AB | Perlman C.,LOGE AB | Mauss F.,TU Brandenburg
SAE International Journal of Engines | Year: 2012

This paper reports on a turbulent flame propagation model combined with a zero-dimensional two-zone stochastic reactor model (SRM) for efficient predictive SI in-cylinder combustion calculations. The SRM is a probability density function based model utilizing detailed chemistry, which allows for accurate knock prediction. The new model makes it possible to - in addition - study the effects of fuel chemistry on flame propagation, yielding a predictive tool for efficient SI in-cylinder calculations with all benefits of detailed kinetics. The turbulent flame propagation model is based on a recent analytically derived formula by Kolla et al. It was simplified to better suit SI engine modelling, while retaining the features allowing for general application. Parameters which could be assumed constant for a large spectrum of situations were replaced with a small number of user parameters, for which assumed default values were found to provide a good fit to a range of cases. Only one parameter, the turbulence intensity, needed tuning to obtain excellent agreement for various cases. The laminar flame speed is obtained from a laminar flame speed library generated using detailed chemistry. The flame development was calculated from the turbulent flame speed under the assumption of a spherical flame. A Monte Carlo geometry calculation was applied to cater for arbitrary cylinder geometries and spark plug positions, modelling the geometrical properties of the flame with high precision. In later stages of the project, a polygon based description of the flame surface was used, to achieve faster computational times than those of the Monte Carlo model. Copyright © 2012 SAE International. Source

Lundgren M.,Lund University | Tuner M.,Lund University | Johansson B.,Lund University | Bjerkborn S.,LOGE AB | And 4 more authors.
SAE Technical Papers | Year: 2013

The relatively new combustion concept known as partially premixed combustion (PPC) has high efficiency and low emissions. However, there are still challenges when it comes to fully understanding and implementing PPC. Thus a predictive combustion tool was used to gain further insight into the combustion process in late cycle mixing. The modeling tool is a stochastic reactor model (SRM) based on probability density functions (PDF). The model requires less computational time than a similar study using computational fluid dynamics (CFD). A novel approach with a two-zone SRM was used to capture the behavior of the partially premixed or stratified zones prior to ignition. This study focuses on PPC mixing conditions and the use of an efficient analysis approach. It was done in three steps: a validation of the two-zone SRM against CFD and experimental data, a parametric study using a design of experiment (DOE) approach to late cycle mixing conditions, and analyses of fuel mass distribution with time-resolved probability density functions (TPDF). Results from the investigation show that the two-zone SRM is suitable for prediction of the PPC conditions and is able to run simulations at an average of 25 min/cycle. The findings of the parametric study showed, that a higher mixing intensity is preferable to longer mixing duration before the start of combustion as it decreases pressure rise rate without penalizing combustion efficiency. The TPDF plots offer a good alternative when presenting mixture fraction distributions. However, they may be more suited to smaller amounts of data than are presented in this investigation. Source

Perlman C.,LOGE AB | Frojd K.,LOGE AB | Seidel L.,LOGE AB | Tuner M.,Lund University | Mauss F.,TU Brandenburg
SAE Technical Papers | Year: 2012

This paper reports on a fast predictive combustion tool employing detailed chemistry. The model is a stochastic reactor based, discretised probability density function model, without spatial resolution. Employing detailed chemistry has the potential of predicting emissions, but generally results in very high CPU costs. Here it is shown that CPU times of a couple of minutes per cycle can be reached when applying detailed chemistry, and CPU times below 10 seconds per cycle can be reached when using reduced chemistry while still catching in-cylinder in-homogeneities. This makes the tool usable for efficient engine performance mapping and optimisation. To meet CPU time requirements, automatically load balancing parallelisation was included in the model. This allowed for an almost linear CPU speed-up with number of cores available. As the number of cores increased, temporarily idle CPU's and computer cluster overhead cost was found to start affecting the overall CPU cost, but speed-up was observed up to 200 cores. A clustering algorithm allowing for any number of controlling parameters was further utilised. The algorithm clusters the different particles based on user provided parameters and dispersion thresholds. After finishing the chemistry step, the clustered solutions are mapped back to the individual particles while preserving each individual particle's distance from its cluster mean. The clustering algorithm was found to give a larger CPU speed-up the more particles were used and also to be effective both for detailed and reduced chemical mechanisms. Copyright © 2012 SAE International. Source

Lehtiniemi H.,LOGE AB | Borg A.,LOGE AB | Mauss F.,TU Brandenburg
SAE Technical Papers | Year: 2016

Several models for ignition, combustion and emission formation under diesel engine conditions for multi-dimensional computational fluid dynamics have been proposed in the past. It has been recognized that the use of a reasonably detailed chemistry model improves the combustion and emission prediction especially under low temperature and high exhaust gas recirculation conditions. The coupling of the combustion chemistry and the turbulent flow can be achieved with different assumptions. In this paper we investigate a selection of n-heptane spray experiments published by the Engine Combustion Network (ECN spray H) with three different combustion models: well-stirred reactor model, transient interactive flamelet model and progress variable based conditional moment closure. All models cater for the use of detailed chemistry, while the turbulence-chemistry interaction modeling and the ability to consider local effects differ. The same chemical mechanism is used by all combustion models, which allows a comparison of ignition delay, flame stabilization and flame lift-off length between the experiments and the results from simulations using the different combustion models. The investigated parameters influence the predictions of computational fluid dynamics simulations of diesel engines. This study indicates that the most reasonable behavior with respect to ignition, flame stabilization and flame structure is predicted by the progress variable based conditional moment closure model. © Copyright 2016 SAE International. Source

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