1,721,010 research outputs found
Large-Eddy simulation analysis of spark configuration effect on cycle-to-cycle variability of combustion and knock
No full textCycle-to-cycle variability is numerically simulated for high-speed, full-load operation of a turbocharged gasoline direct injection engine. Large-Eddy simulation is adopted to replicate the fluctuations of the flow field affecting the turbulent combustion. Experimental data were provided at knock onset, and large-Eddy simulation was validated for the same condition. In the original engine configuration, the spark plug is displaced toward the exhaust side, while the electrodes orientation is arbitrary. A 90 rotation is imposed to evaluate the effects of the aerodynamic obstruction caused by the electrode with respect to the flow field and the flame kernel growth. A second speculative analysis is performed modifying the position of the spark plug. The electrodes are shifted 2mm toward the intake side since this variation is compatible with the cylinder head layout. For both variations in orientation and position, the effects on the flow field around the spark plug are investigated. Statistical analysis is carried out on early flame kernel formation and knock tendency. The results highlight that the orientation of the electrodes affects the flow field for each cycle but plays a negligible role on the statistical cyclic variability, indirectly justifying the lack of an imposed orientation. As for the spark plug position, the numerical analysis indicate that the shifting of the electrodes toward the intake side slightly improves the knock limit mainly because of a reduction in in-cylinder peak pressure. In general, it is inferred that improvements may be achieved only through a simultaneous modification of the fuel jet orientation and phasing
Analysis and Simulation of Non-Flamelet Turbulent Combustion in a Research Optical Engine
Full text linkIn recent years, the research community devoted many resources to define accurate methodologies to model the real physics behind turbulent combustion. Such effort aims at reducing the need for case-by-case calibration in internal combustion engine simulations. In the present work two of the most widespread combustion models in the engine modelling community are compared, namely ECFM-3Z and G-equation. The interaction of turbulent flows with combustion chemistry is investigated and understood. In particular, the heat release rate characterizing combustion, and therefore the identification of a flame front, is analysed based on flame surface density concept rather than algebraic correlations for turbulent burn rate. In the first part, spark-ignition (S.I.) combustion is simulated in an optically accessible GDI single-cylinder research engine in firing conditions. The turbulent combustion regime is mapped on the Borghi-Peters diagram for all the conditions experienced by the engine flame, and the consistency of the two combustion models is critically analysed. In the second part, a simple test case is defined to test the two combustion models in an ideally turbulence-controlled environment: this allows to fully understand the main differences between the two combustion models under well-monitored conditions. and results are compared against experimental databases of turbulent burn rate for wide ranges of Damkohler (Da) and Karlovitz (Ka) numbers. The joint experimental and numerical study presented in this paper evaluates different approaches within the unified flamelet/non-flamelet framework for modelling turbulent combustion in SI engines. It also indicates guidelines for reduced calibration effort in widespread combustion models
Investigating the Impact of Varied C-Rates on Lithium-Ion Batteries: A 1D Simulation Study
Full text linkWith the advancement of powertrain technology and progressive vehicle electrification, one of the solutions for the growing need for energy storage is batteries. A secondary battery is a device that stores electrical energy in chemical form and delivers it as electrical energy when needed (discharging), with the possibility to revert the process converting electrical to chemical energy (charging). The lithium-ion battery type used in the study offers increased energy and power density with a cell voltage of approximately 3.6 V, making it suitable for use in portable electronic devices like mobile phones and laptops. In this research, a 1D model (through-electrolyte direction) of lithium-ion battery was analysed, in which the effect of different C-rates was investigated using the battery and design module of COMSOL Multiphysics software for 0.1C, 0.5C, 1C, 2C, and 3C rates, relevant for automotive applications. The simulation results of the lithium-ion battery model constitute an important step towards the development of battery technology, allowing an understanding of the transport processes in the electrodes and electrolyte. The results revealed that under higher C-rates of operation, differences emerge in electrolyte and electrode voltage ranges, salt concentration profiles in the electrolyte, surface and center electrode particle lithium concentrations
Understanding the origin of cycle-to-cycle variation using large-eddy simulation: Similarities and differences between a homogeneous low-revving speed research engine and a production DI turbocharged engine
Full text linkA numerical study using large-eddy simulations (LES) to reproduce and understand sources of cycle-to-cycle variation (CCV) in spark-initiated internal combustion engines (ICEs) is presented. Two relevantly different spark-ignition (SI) units, that is, a homogeneous-charge slow-speed singlecylinder research unit (the transparent combustion chamber (TCC)-III, Engine 1) and a stratifiedcharge high-revving speed gasoline direct injection (GDI) (Engine 2) one, are analyzed in fired operations. Multiple-cycle simulations are carried out for both engines and LES results well reproduce the experimentally measured combustion CCV. A correlation study is carried out, emphasizing the decisive influence of the early flame period variability (1% of mass fraction burnt (MFB1)) on the entire combustion event in both ICEs. The focus is moved onto the early flame characteristics, and the crucial task to determine the dominant causes of its variability (if any) is undertaken. A two-level analysis is carried out: the influence of global parameters is assessed at first; second, local details in the ignition region are analyzed. A comparison of conditions at combustion onset is carried out and case-specific leading factors for combustion CCV are identified and ranked. Finally, comparative simulations are presented using a simpler flame deposition ignition model: the simulation flaws are evident due to modeling assumptions in the flame/flow interaction at ignition. The relevance of this study is the knowledge extension of turbulence-driven phenomena in ICEs allowed by advanced CFD (Computational Fluid Dynamics) simulations. The application to different engine types proves the soundness of the used models and it confirms that CCV is based on enginespecific factors. Simulations show how CCV originates from the interplay of small- and large-scale factors in Engine 1, due to the lack of coherent flows, whereas in Engine 2 the dominant CCV promoters are local air-to-fuel ratio (AFR) and flow velocity at ignition. This confirms the absence of a generally valid ranking, and it demonstrates the use of LES as a development and designorienting tool for next-generation engines
A RANS-Based CFD Model to Predict the Statistical Occurrence of Knock in Spark-Ignition Engines
No full textEngine knock is emerging as the main limiting factor for modern spark-ignition (SI) engines, facing increasing thermal loads and seeking demanding efficiency targets. To fulfill these requirements, the engine operating point must be moved as close as possible to the onset of abnormal combustion events. The turbulent regime characterizing in-cylinder flows and SI combustion leads to serious fluctuations between consecutive engine cycles. This forces the engine designer to further distance the target condition from its theoretical optimum, in order to prevent abnormal combustion to severely damage the engine components just because of few individual heavy-knocking cycles. A RANS-based model is presented in this study, which is able to predict not only the ensemble average knock occurrence but also a knock probability. This improves the knock tendency characterization, since the mean knock onset alone is a poorly meaningful indication in a stochastic event such as engine knock. The model is based on a look-up table approach from detailed chemistry, coupled with the transport of the variance of both mixture fraction and enthalpy. These perturbations around the ensemble average value are originated by the turbulent time scale. A multivariate cell-based Gaussian-PDF model is proposed for the unburnt mixture, resulting in a statistical distribution for the in-cell reaction rate. An average knock precursor and its variance are independently calculated and transported, and the earliest knock probability is always preceding the ensemble average knock onset, as confirmed by the experimental evidence. This allows to identify not only the regions where the average knock first occurs, but also where the first knock probability is more likely to be encountered. The application of the model to a RANS simulation of a modern turbocharged direct injection (DI) SI engine is presented and a small percentage of knocking cycles is predicted by the model although the average behavior is knock-free, in agreement with the experiments. The estimate of the knocking probability improves the consolidated “average knock” RANS analysis and gives an indication of the statistical knock tendency of the engin
A Methodology to Improve Knock Tendency Prediction in High Performance Engines
Full text linkThe paper presents a comprehensive numerical methodology for the estimation of knock tendency in SI engines, based on the synergic use of different frameworks [1]. 3D-CFD in-cylinder analyses are used to simulate the combustion and to estimate the
point-wise heat flux acting on engine components. The resulting heat fluxes are used in a conjugate heat transfer model in order to reconstruct the actual point-wise wall temperature distribution. An iterative loop is established between the two simulation realms. In order to evaluate the effect of temperature on knock, in-cylinder analyses are integrated with an accurate chemical description of the actual fuel
Large-eddy simulation of cycle-resolved knock in a turbocharged SI engine
No full textThe paper presents a numerical study of cycle-to-cycle variability in a turbocharged GDI engine. The Large-Eddy Simulation technique is adopted in this study in conjunction with the recent ISSIM-LES model for spark-ignition, allowing a dedicated treatment of both the flame kernel formation and flame development phases. Numerical results are compared with an extended dataset of experimental test-bed acquisitions, where the engine is operated at knock-limited spark advance. The agreement of both ensemble averaged combustion pressure history and of its standard deviation confirm the validity of the adopted numerical framework able to correctly quantify the degree of CCV measured by the experiments. Knock tendency is evaluated by means of an in-house developed knock model, based on a tabulation technique for AI delays of the same RON98 gasoline as the one used in experiments. The results confirm the knock-free condition of the experimental KLSA, for which the cycle-resolved knock signature is extremely weak just for the cycles in the highest band of the CCV-affected combustion. The visualization of the pressure wave allows to identify the exhaust side as the most knock-prone region. Finally, spark-advance is increased by 3 CA with respect to the experimental edge-of knock limit, in order to simulate an experimentally prevented operating condition. Local pressure measurements mimicking flush-mounted transducers confirm the severe knock damage related to this condition. The predictive capability of the combustion CCV and of the adopted knock model confirm the heavy and recurrent cycle-resolved knock damage
Study of les Quality Criteria in a Motored Internal Combustion Engine
No full textIn recent years, Large-Eddy Simulation (LES) is spotlighted as an engineering tool and severe research efforts are carried out on its applicability to Internal Combustion Engines (ICEs). However, there is a general lack of definitive conclusions on LES quality criteria for ICE. This paper focuses on the application of LES quality criteria to ICE and to their correlation, in order to draw a solid background on future LES quality assessments for ICE. In this paper, TCC-III single-cylinder optical engine from University of Michigan is investigated and the analysis is conducted under motored condition. LES quality is mainly affected by grid size and type, sub-grid scale (SGS) model, numeric schemes. In this study, the same grid size and type are used in order to focus on the effect on LES quality of SGS models and blending factors of numeric scheme only. In the first section of the study, single grid estimators are used to compare two sub-filter models which are static Smagorinsky model and dynamic Smagorinsky model. Also, two cases which are simulated with different blending factors for numeric schemes and same SGS model are compared. In the second section, the in-cylinder gas-dynamics and flow structures are analyzed by comparing experimental results (pressure transducers and Particle Image Velocimetry (PIV) velocity fields) with a dataset of consecutive LES cycles. The flow analysis focuses at four different crank angle positions (bottom dead center (BDC), middle of exhaust and intake valve opening timing and mid-compression stroke) on the same section plane as PIV visualizations. Finally, the connection between the LES quality criteria and the accuracy of simulation results with experiments is discussed and conclusions are drawn to outline a best practice in LES quality for ICE
Integrated In-Cylinder/CHT Analysis for the Prediction of Abnormal Combustion Occurrence in Gasoline Engines
No full textIn order to improve fuel conversion efficiency, currently made spark-ignited engines are characterized by the adoption of gasoline direct injection, supercharging and/or turbocharging, complex variable valve actuation strategies. The resulting increase in power/size ratios is responsible for substantially higher average thermal loads on the engine components, which in turn result in increased risks of both thermo-mechanical failures and abnormal combustion events such as surface ignition or knock. The paper presents a comprehensive numerical methodology for the accurate estimation of knock tendency of SI engines, based on the integration of different modeling frameworks and tools. Full-cycle in-cylinder analyses are used to estimate the point-wise heat flux acting on the engine components facing the combustion chamber. The resulting cycle-averaged heat fluxes are then used in a conjugate heat transfer model of the whole engine in order to reconstruct the actual point-wise temperature distribution of the combustion chamber walls. The two simulation realms iteratively exchange information until convergence is met. Particularly, the effect of point-wise temperature distribution on the onset of abnormal combustion events is evaluated. In-cylinder analyses account for the actual autoignition behavior of the air/fuel mixture through a look-up table approach: the combustion chamber is treated as a two-zone region (burnt/unburnt), where ignition delay tabulation, generated off-line using a constant pressure reactor, is applied to the unburnt region to estimate cell-wise knock proximity. The methodology is applied to a high performance engine and the importance of an accurate representation of the combustion chamber thermal boundary conditions when aiming at precisely evaluating the surface ignition/knock tendency is highlighted
LES Multi-cycle Analysis of a High Performance GDI Engine
No full textThe paper reports the application of LES multi-cycle analysis for the characterization of cycle to cycle variability (hereafter CCV) of a highly downsized DISI engine for sport car applications. The analysis covers several subsequent engine cycles operating the engine at full load, peak power engine speed. Despite the chosen engine operation is usually considered relatively stable, relevant fluctuations were experimentally measured in terms of in-cylinder pressure evolution and combustion phasing.
On one hand, despite the complex architecture of the V-8 engine, the origin of such CCV is considered to be poorly related to cyclic fluctuations of the gas-dynamics within the intake and exhaust pipes, since acquisitions of the instantaneous pressure traces at both the intake port entrance and exhaust port junction by fast-response pressure measurements over 250 subsequent engine cycles showed almost negligible differences in both amplitude and phasing compared to those within the cylinder.
On the other hand, being the combustion affected by a complex chain of preceding factors (air admission during the intake stroke, variations in the residual gas fraction, generation of complex turbulent flow structures, fuel injection and dispersion in the combustion chamber and subsequent mixing, interaction between the spark discharge and the surrounding local flow pattern, etc.) a clear understanding of the actual origin of cyclic variability is far from being trivial.
LES CFD simulations can therefore become a very powerful tool to help investigating the possible causes of such cyclic variations, since detailed analyses of both global and local parameters can be carried out on an almost unlimited set of available virtual measurements.
In the first part of the paper, the modeling framework is presented and considerations on the adopted numerical strategy are presented, with particular emphasis on grid size, grid distribution and numerical parameters. Subsequently, LES results are analyzed and discussed in order to understand the cycle-to-cycle variations through the use of correlation coefficients between global/local flow variables in order to highlight the major causes of CCV and establish a possible hierarchy among the analyzed quantities.
Finally, criticalities of the currently adopted approach and possible enhancements are briefly discussed at the end of the paper.
The results presented in the paper clearly highlight the potential of the modeling methodology to help understanding the origin of CCV as well as to address possible engine optimizations to limit the cyclic dispersion
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