Auto-ignition Model Research Articles

Uncertainty quantification in the ab initio rate-coefficient calculation for the [formula omitted] reaction

Theoretical methods to obtain rate coefficients are essential to fundamental combustion chemistry research, yet the associated uncertainties are largely unexplored in a systematic manner. In this paper we focus on the study of parametric uncertainties for a hydrogen-atom-abstraction reaction, CH3CH(OH)CH3+OH→CH3C(OH)CH3+H2O, which bears significant importance in low-temperature alcohol combustion and especially in autoignition models. After identifying the parameters causing significant uncertainty in the rate-coefficient calculations, Bayesian inference is employed to determine the joint probability density function (PDF) thereof using the experimental data of Dunlop and Tully (1993) [6] on isopropanol+OH. The inferred PDFs are compared to the various parameter values obtained from high-level electronic-structure calculations in order to assess the limitations of current methodologies. To gain insight on modeling the kinetic isotope effect (KIE), the reaction of the hydroxyl radical with deuterated isopropanol is also investigated.

Evaluation of an unsteady flamelet progress variable model for autoignition and flame development in compositionally stratified mixtures

The unsteady flamelet progress variable (UFPV) model has been proposed by Pitsch and Ihme [“An unsteady/flamelet progress variable method for LES of nonpremixed turbulent combustion,” AIAA Paper No. 2005-557, 2005] for modeling the averaged/filtered chemistry source terms in Reynolds averaged simulations and large eddy simulations of reacting non-premixed combustion. In the UFPV model, a look-up table of source terms is generated as a function of mixture fraction Z, scalar dissipation rate χ, and progress variable C by solving the unsteady flamelet equations. The assumption is that the unsteady flamelet represents the evolution of the reacting mixing layer in the non-premixed flame. We assess the accuracy of the model in predicting autoignition and flame development in compositionally stratified n-heptane/air mixtures using direct numerical simulations (DNS). The focus in this work is primarily on the assessment of accuracy of the probability density functions (PDFs) employed for obtaining averaged source terms. The performance of commonly employed presumed functions, such as the dirac-delta distribution function, the β distribution function, and statistically most likely distribution (SMLD) approach in approximating the shapes of the PDFs of the reactive and the conserved scalars is evaluated. For unimodal distributions, it is observed that functions that need two-moment information, e.g., the β distribution function and the SMLD approach with two-moment closure, are able to reasonably approximate the actual PDF. As the distribution becomes multimodal, higher moment information is required. Differences are observed between the ignition trends obtained from DNS and those predicted by the look-up table, especially for smaller gradients where the flamelet assumption becomes less applicable. The formulation assumes that the shape of the χ(Z) profile can be modeled by an error function which remains unchanged in the presence of heat release. We show that this assumption is not accurate.

Computational Fluid Dynamics Simulation of Diesel Engines with Sophisticated Injection Strategies for In-Cylinder Pollutant Controls

This work presents a computational fluid dynamics (CFD) methodology to simulate diesel combustion in high-pressure common-rail engines, which employ sophisticated injection strategies with rate shaping to reduce the in-cylinder pollutant formation and achieve the emission legislation EU6 and US Tier2 Bin 5. The physics of the mixture formation and its effects on combustion and, hence, on emission formation needs to be correctly described by the numerical tool. At the same time, in order to be used in a design and optimization stage of a new engine, the CFD methodology must be very efficient in terms of time requirements. For these purposes, the authors propose and assess the capabilities of the characteristic time-scale combustion model (CTC). The Shell autoignition model is used to predict autoignition, while the standard CTC model was improved by introducing a new expression to compute the characteristic time scale to be applied also in the presence of very high EGR rates. Soot emissions were predicted by the eight reaction step mechanism proposed by Fusco et al., and the extended Zeldovich mechanism was used to estimate NOx concentration. The proposed combustion model was implemented into the Lib-ICE code, which is a set of libraries and applications for internal combustion engine modeling developed by the authors under the OpenFOAM technology. Validation considers two different sets of operating modes, which are representative of common driving conditions and considered to be challenging for the proposed CFD methodology.

Direct numerical simulation of autoigniting mixing layers in MILD combustion

MILD combustion is a new combustion technology which promises an enhanced efficiency and reduced emission of pollutants. It is characterized by a high degree of preheating and dilution of the reactants. Since the temperature of the reactants is higher than that of autoignition, a complex interplay between turbulent mixing, molecular transport and chemical kinetics occurs. In order to reveal the fundamental reaction structures of MILD combustion, the process of a cold methane–hydrogen fuel jet issuing in a hot diluted coflow and the subsequent ignition process is modeled by direct numerical simulation of autoigniting mixing layers using detailed chemistry and transport models. Detailed analysis of one-dimensional laminar mixing layers shows that the ignition process is dominated by hydrogen chemistry and that non-unity Lewis number effects are of the utmost importance for modeling of autoignition. High scalar dissipation rates in mixing layers delay the autoignition time, but have a negligible effect on the chemical pathway followed during ignition. This supports the idea of using homogeneous reactor simulations for the construction of chemistry look-up tables. Simulations of two-dimensional turbulent mixing layers confirm the effect of scalar dissipation rate on autoignition time. The turbulence–chemistry interaction is limited under the investigated conditions, because the reaction layer lies at the edge of the mixing layer due to the very small value of the stoichiometric mixture fraction. When the oxidizer stream is more diluted, the autoignition time is delayed, allowing the developing turbulence to interact more with the ignition chemistry. The results of these direct numerical simulations employing a detailed reaction mechanism are expected to be used for the development of tabulated chemistry models and sub-grid scale models for large-eddy simulations of MILD combustion.

Methodology to define gasoline surrogates dedicated to auto-ignition in engines

A methodology to define proportions of compounds in gasoline surrogates dedicated to auto-ignition modelling in engines is described. Based on auto-ignition properties and global atom content of the reference gasoline, this methodology provides surrogates nearing gasoline auto-ignition in engine but also gasoline density or lower heating value. Increasing the number of surrogate compounds induces a convergence of the mixture defined according to the methodology towards the classification into chemical families of the gasoline to emulate, which is a key for pollutant emission representativity. Applied to a European unleaded gasoline with an octane rating of 95, a ternary surrogate made of 13.7% of n-heptane (by mole fraction), 42.8% of isooctane and 43.5% of toluene is obtained. The direct experimental comparison of this surrogate and of the reference gasoline in an engine operated in Controlled Auto-Ignition (CAI) combustion mode confirms the global validity of the approach (auto-ignition timing but also power output, efficiency and pollutant emissions). The selected operating point for this comparison is also modelled with 3D CFD to demonstrate that the final aim of gasoline emulation in engine codes with such a surrogate can be successfully reached.

CFD modelling of cyclohexane auto-ignition in an RCM

The kinetic modelling of cyclohexane auto-ignition in a rapid compression machine is addressed, based on the comprehensive kinetic mechanism, comprising 499 species and 2323 reactions, then reduced to 56 species involved in 196 reactions and 50 species involved in 143 reactions. The purpose is to explore the merits of reduced kinetic models that can be incorporated into Computational Fluid Dynamic codes for the numerical investigation of the performance of fuels in engines, with specific reference to Homogeneous Charge Compression Ignition (HCCI) or Controlled Auto-Ignition (CAI).Calculations of ignition delay, using the SPRINT zero-dimensional code assuming adiabatic reaction, have been performed for the stoichiometric mixture of cyclohexane in air, comprising C6H12+9O2+33.86N2, at an initial pressure of 0.48bar, yielding compressed gas pressures of 7.2–9.7bar over the compressed gas temperature range 650–925K. There is reasonable agreement between the predicted delays of each scheme over the whole range of compressed gas temperatures. The common feature for alkane fuels, of negative temperature dependent ignition delays in an intermediate range, is predicted. However, the agreement with experimental measurements, especially in the negative temperature dependent region, is not very satisfactory. Non-adiabatic reaction as a cause of the discrepancy is addressed, initially via simple tests using SPRINT.The reduced mechanism comprising 50 species has then been implemented into the multidimensional CFD code FLUENT, which is the maximum number of species that can be incorporated in this code. The predictions of FLUENT are in excellent agreement with those from SPRINT under closed, constant volume adiabatic conditions. When non-adiabaticity at the wall is assumed, the FLUENT calculations show how temperature gradients evolve, leading to spatial differences in the rates of development of reaction. In particular, reaction is able to evolve more rapidly in the boundary layer region at compressed gas temperatures that correspond to the region of negative temperature dependence of ignition delay, so causing a reduction in the overall ignition delay relative to that predicted under adiabatic conditions. Ignition itself is initiated in the boundary layer under these circumstances.

Modeling of Auto-Ignition and Combustion Processes for Dual-Component Fuel Spray

Modeling of Auto-Ignition and Combustion Processes for Dual-Component Fuel Spray

Kinetic modelling of extinction and autoignition of condensed hydrocarbon fuels in non-premixed flows with comparison to experiment

A detailed chemical–kinetic mechanism is used to predict critical conditions of extinction and autoignition of condensed hydrocarbon fuels in non-premixed flows. The mechanism includes reactions that describe “low temperature chemistry” as well as “high temperature chemistry” for many high molecular weight fuels. The fuels considered here are n-heptane, n-decane, n-dodecane, n-hexadecane, and iso-octane. The kinetic model is validated by comparing its predictions of critical conditions of extinction and autoignition for these fuels with experimental data obtained in a counterflow configuration where a steady laminar flow of an oxidizer is directed over the vaporizing surface of a condensed fuel. The residence time in this configuration is given by the strain rate. The kinetic model predicts that n-heptane is most difficult to extinguish followed by n-decane, n-dodecane, and n-hexadecane. This is in agreement with experimental data. Computations show that the influence of low temperature chemistry on critical conditions of extinction is small. The kinetic model predicts that at low strain rates n-hexadecane is most easy to ignite followed by n-dodecane, n-decane, and n-heptane. At high values of strain rate, n-heptane is more easy to ignite in comparison to n-decane. This is again in agreement with experimental data, including the “cross-over” in relative reactivities of n-heptane and n-decane. Sensitivity analysis shows that at low strain rates autoignition is promoted by low temperature chemistry for all fuels. At high strain rates, autoignition for n-heptane is promoted by high temperature chemistry, whilst low temperature chemistry continues to play a significant role in promoting autoignition for the other straight-chain hydrocarbon fuels. The “cross-over” in relative reactivities of n-heptane and n-decane is attributed to competition between the rates of low temperature chemistry, rates of high temperature chemistry, and rates of molecular transport for these fuels.

Control of highly diluted combustion in Diesel engines

In this paper, an open-loop control strategy to improve the stability of highly diluted Diesel combustion during sharp transients is proposed. The main advantage of this open-loop method is that no additional in-cylinder sensor is required. The approach coordinates existing airpath and fuelpath controllers, and aims at accurately controlling the end of the cool-flame phenomenon. For this purpose, the injection time is adjusted based on an auto-ignition model and a cool-flame model. Both models are expressed under integral forms, named knock integral models. Experimental results obtained on a test bench and on-board a vehicle are presented. The results highlight the relevance of the approach in terms of combustion stability improvement, pollutant emissions and noise.

On the use of a tabulation approach to model auto-ignition during flame propagation in SI engines

The direct solving of all chemical reactions and transport phenomena responsible for the progress of combustion in internal combustion engines is today unreachable in an industrial context. Therefore, models are introduced that largely reduce the number of quantities to solve but are most often restricted to a specific combustion mode. Flame propagation and auto-ignition are two such combustion modes that interact during knocking in spark-ignition engines. The auto-ignition process may be described based on the tabulation of auto-ignition delays and reaction rates, while the flame propagation can be modelled with a coherent flame model. Because of the construction hypotheses of each model, their coupling for reactive mixtures combining both combustion modes is not straightforward. A proper treatment of the combustion progress description is necessary to avoid any bias in the individual reaction rates. The present paper proposes such a coupling approach through the definition of a specific progress variable for the auto-ignition modelling. The influence of the proper accounting for the interaction between combustion modes is demonstrated based on an academic test case and engine calculations at trace-knocking conditions.

Auto-ignition model based on tabulated detailed kinetics and presumed temperature PDF – Application to internal combustion engine controlled by thermal stratifications

The prediction of the auto-ignition sensitivity to the temperature in new engine combustors is a challenge in the community of numerical combustion. This paper is devoted to the modeling of temperature fluctuations for the simulation of reactive flows in real internal engine configurations. It aims at validating the temperature fluctuation equation model Truffin and Benkenida and its coupling with the ECFM3Z combustion model of Colin and Benkenida. Especially, this study focuses on the auto-ignition process which is described by the TKI (Tabulated Kinetics for Ignition) model of Knop and Jay. TKI is based on a tabulation method for reaction rates and its coupling with the temperature fluctuation is achieved through a presumed PDF approach. The integral limits for the PDF integration are determined locally through transport equations with appropriate closures on isothermal walls. The resulting model, called TKI– υ T , is applied on Homogeneous Charge Compression Ignition (HCCI) combustion mode engine for which the only source of thermal stratification is wall heat loss. Comparisons with experiments demonstrate the impact of temperature fluctuations and the ability of the model to improve the prediction of the auto-ignition model.

A Phenomenological Model for Prediction Auto-Ignition and Soot Formation of Turbulent Diffusion Combustion in a High Pressure Common Rail Diesel Engine

A new phenomenological model, the TP (Temperature Phase) model, is presented to carry out optimization calculations for turbulent diffusion combustion in a high-pressure common rail diesel engine. Temperature is the most important parameter in the TP model, which includes two parts: an auto-ignition and a soot model. In the auto-ignition phase, different reaction mechanisms are built for different zones. For the soot model, different methods are used for different temperatures. The TP model is then implemented in KIVA code instead of original model to carry out optimization. The results of cylinder pressures, the corresponding heat release rates, and soot with variation of injection time, variation of rail pressure and variation of speed among TP model, KIVA standard model and experimental data are analyzed. The results indicate that the TP model can carry out optimization and CFD (computational fluid dynamics) and can be a useful tool to study turbulent diffusion combustion.

Numerical modelling of diesel spray auto-ignition and combustion

This work presents the development and implementation of a spray combustion model based on the spray size distribution moments approach to spray modelling. The governing equations for both the gas and liquid phases are solved by the finite volume method based on an Eulerian framework. The model used in the auto-ignition and combustion analysis is a combination of the probability density function and eddy break-up approaches. This scheme is developed in order to effectively validate both ignition and combustion phases. It is designed to compromise between the chemical reaction rate when dealing with the auto-ignition mode, and the turbulence reaction rate which dominates the combustion mode using a reaction progress variable which represents the reaction level. These average reaction rate source terms in transport equations for species are evaluated by a probability density function averaging approach. The developed ignition and combustion model is shown to have significant potential as a numerical tool for predicting the phenomena involved in spray combustion in diesel engines.

Calculation and Experiment of Soot Emission for a Directed Injection Engine

To calculate soot for a high-pressure common rail diesel engine a new soot model (TP model) is presented.. In soot source term phase, when zone temperature T<1500K, gas-phase kinetics is considered and the soot precursors –PAHs (Polycyclic aromatic hydrocarbons) is calculated. When zone temperature T ≥ 1500K, the gas-phase chemistry and soot source terms are calculated. The soot model integrated with the new auto-ignition models is then applied in multidimensional simulations. The TP model is then implemented in KIVA code instead of original model to carry out optimizing. The results of soot with variation of injection time, variation of rail pressure and variation of speed among TP model, KIVA standard model and experimental data are analyzed. The results indicate that the TP model can carry out optimization and CFD (computational fluid dynamics) and can be a tool to calculate for a high-pressure common rail directed injection diesel engine.

A New Functional Global Auto-ignition Model for Hydrocarbon Fuels - Part 2 of 2: Model Formulation, Development and Performance Assessment

A New Functional Global Auto-ignition Model for Hydrocarbon Fuels - Part 2 of 2: Model Formulation, Development and Performance Assessment

A New Functional Global Auto-ignition Model for Hydrocarbon Fuels - Part 1 of 2: An Investigation of Fuel Auto-Ignition Behaviour and Existing Global Models

A New Functional Global Auto-ignition Model for Hydrocarbon Fuels - Part 1 of 2: An Investigation of Fuel Auto-Ignition Behaviour and Existing Global Models

A new LES model coupling flame surface density and tabulated kinetics approaches to investigate knock and pre-ignition in piston engines

Downsizing currently appears as a promising technique for reducing the CO 2 emissions of spark-ignition engines. Nevertheless boost levels are limited by the occurrence of uncontrolled auto-ignitions of fresh gases. These latter are sporadic and can have localized origins. The large-eddy simulation (LES) theoretically seems to be a promising numerical approach to study these abnormal combustion processes as a part of the turbulence spectrum is resolved and the effects of cycle-to-cycle variations are taken into account. In this context, a dedicated model integrating the auto-ignition model TKI (Tabulated Kinetics for Ignition) and ECFM-LES (Extended Coherent Flame for LES) is first detailed. It is then applied to the modeling of simple test cases representative of phenomena encountered during abnormal combustions. Finally, a qualitative study is performed to verify that well-known experimental trends related to knock can be retrieved numerically in a LES context. To this purpose, several operating points for a spark-ignition engine are computed. Signals of in-cylinder pressures that have the same appearance as those found in the literature are obtained. Retarding spark timing enables numerically to limit the occurrence of knock, as commonly used in engine control. A pre-ignition case is also regarded and shows the possibility to use the proposed model for further quantitative studies of this phenomenon.

A methodology for analysis of diesel engine in-cylinder flow and combustion

A methodology is developed for the analysis of diesel engine in-cylinder processes and combustion. Beginning from CAD data of the engine geometry, the methodology involves use of a commercial code AVL FIRE for simulation of suction stroke, and an open-source code KIVA-3V for simulation of the closed-valve part of the diesel cycle. For this, an algorithm is first developed to map a generalised three-dimensional Computational Fluid Dynamics (CFD) solution from an unstructured mesh in AVL FIRE to a structured mesh in KIVA-3V to provide initial conditions for the closed-valve simulations. For simulation of diesel combustion process, an integrated KIVA-3V code is developed by incorporating two well-validated models into the standard code: the Shell hydrocarbon auto-ignition model for simulation of diesel auto-ignition under conditions of high temperature and pressure, and the Characteristic-time model for simulation of subsequent diesel combustion. The integrated code is validated and calibrated against experimental pressure measurements in a naturally aspirated direct injection diesel engine. These tools are then used for exploring the potential of a constant-speed, turbocharged diesel engine towards emission reduction. The case study involves combustion simulations for exploring multiple injection strategy for the engine concerned.

Modeling the Auto-Ignition of Oxygenated Fuels using a Multistep Model

The research presented here describes the application of a multistep (8-step) autoignition model to oxygenated fuels such as alcohols and esters in a rapid compression machine. This modeling concept is aimed at capturing the ignition behavior of new oxygenated fuel blends, where detailed or reduced chemical kinetics data are not available. The predicted ignition delays from the multistep autoignition model using the biodiesel surrogate fuel methyl butanoate are validated against results attained using a detailed chemical kinetic mechanism (Dooley, S.; Curran, H.J.; Simmie, J.M. Combust. Flame 2008, 153 (1−2), 2−32.) in conjunction with CHEMKIN. Once the multistep model constants were calibrated for methyl butanoate, the model showed good agreement with the detailed mechanism ignition delays, but with significantly reduced computational time. The multistep model was tested over a compressed temperature range of 750−925 K, compressed pressures from 10 to 46 atm and equivalence ratios from 0.5 to stoichiomet...

Self-ignition of diesel spray combustion

This work presents the development and implementation of auto-ignition modelling for DI diesel engines by using the probability density function-eddy break-up (PDF-EBU) model. The key concept of this approach is to combine the chemical reaction rate dealing with low-temperature mode, and the turbulence reaction rate governing the high-temperature part by a reaction progress variable coupling function which represents the level of reaction. The average reaction rate here is evaluated by a PDF averaging approach. In order to assess the potential of this developed model, the well-known Shell ignition model is chosen to compare in auto-ignition analysis. In comparison, the PDF-EBU ignition model yields the ignition delay time in good agreement with the Shell ignition model prediction. However, the ignition kernel location predicted by the Shell model is slightly nearer injector than that by the PDF-EBU model leading to shorter lift-off length. As a result, the PDF-EBU ignition model developed here are fairly satisfactory in predicting the auto-ignition of diesel engines with the Shell ignition model.