2019

Corinna Netzer

Thema: Simulation and assessment of engine knock events

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Abstract:

Sophisticated engine knock modeling supports the optimization of the thermal efficiency of spark ignition engines. For this purpose the presented work introduces the resonance theory (Bradley and co-workers, 2002) for three-dimensional Reynolds-Averaged Navier-Stokes (RANS) and for the zero-dimensional Spark Ignition Stochastic Reactor Model (SI-SRM) simulations. Hereby, the auto-ignition in the unburnt gases is investigated directly instead of the resulting pressure fluctuations. Based on the detonation diagram auto-ignition events can be classified to be in acceptable deflagration regime or possibly turn to a harmful developing detonation. Combustion is modeled using detailed chemistry and formulations for turbulent flame propagation. The use of detailed chemistry caters for the prediction of physical and chemical properties, such as the octane rating, C:H:O-ratio or dilution. For both models, the laminar flame speed is retrieved from surrogate specific look-up tables compiled using the reaction mechanism for Ethanol containing Toluene Reference Fuels by Seidel (2017). In the fresh gas zone, the scheme is used for auto-ignition prediction. For this purpose, the G-equation coupled with a Well-Stirred-Reactor model is applied in RANS. In analogy, in the SI-SRM the combustion is modeled using a two zone model with stochastic mixing between the particles. RANS is used to develop the knock classification methodology and to analyze in detail location, size and shape of the auto-ignition kernels. RANS estimates the ensemble average of the process and therefore cannot reproduce a developing detonation. Hence, Large Eddy Simulation (LES) is used to verify the methodology. Studies using wide ranges of surrogates with different octane rating and cycle-to-cycle variations are carried out using the computationally efficient SI-SRM. Cyclic variations are predicted based on stochastic mixing, stochastic heat transfer to the wall, varying exhaust gas recirculation composition and imposed probability density functions for the inflammation time and the scaling of the mixing time retrieved from RANS. The methodology is verified for spark timing and octane rating. It is shown that the surrogate formulation has an important impact on knock prediction. RANS is suitable to predict the mean strength of auto-ignition in the unburnt gas if the thermodynamic and chemical state of the ignition kernel is analyzed instead of the pressure gradients. The probability of the transition to knocking combustion can be determined. Good agreement between RANS and SI-SRM are obtained. The combination of both tools gives insights of local effects using RANS and the distribution of auto-ignition in the whole pressure range of an operating point using SI-SRM with reasonable computationally cost for development purposes.

2018

Amruta Nawdiyal

Thema: The influence of allylic site abstraction reactions of olefin on cyclo paraffin formation

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Abstract:

This thesis is a combined work of understanding the high temperature oxidation chemistry of cycloalkanes viz. methylcyclohexane based on previously developed cyclohexane and extending it to generate the larger n-propylcyclohexane chemical kinetic mechanism. The detailed kinetic reaction mechanism model for the oxidation of 1-hexene previously developed has been added to account for the ring opening of cyclohexane forming 1-hexene. As an update to the publication, preference of allylic H-abstractions from 1-hexene has been taken into account and retro-ene reaction producing propene has been added. The complete model is composed of 329 species and 2065 reactions with 3796 reversible elementary reactions. Further, these models have been validated against different experiments such as shock tubes, jet stirred reactors and laminar flames to cover full range of temperatures, pressures and equivalence ratios making the models comprehensive and was found to be adequate to satisfactorily reproduce the experimental data. The allylic radicals (C6H11-D1R3) preferred abstractions from 1-hexene improves the C6H11 profiles in the 1-hexene model. But it also influences the otherwise isomerization path of C6H11-D1R6 to CYC6H11 (Cyclohexyl radical) which would further form cyclohexene (CYC6H10). It is observed that CYC6H10 profiles in 1-hexene flames and cyclohexane speciation are over-predicted. The major decomposition pathway of the cycloalkanes is through H-abstractions on the ring. The path which leads towards ring opening to form olefin is observed for cyclohexane and methylcyclohexane but is very low. The fulvene pathway influence on benzene profiles of 1-hexene is obvious but do not seem to affect the cycloalkanes. This infers there are other benzene formation pathways in cycloalkanes. Some possible pathways would be the dehydrogenation of dienes and dehydrogenation of cyclo-olefins.

2017

Xiaoxiao Wang

Thema: Kinetic Mechanism of Surrogates for Bio-Fuels

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Lars Seidel

Thema: Development and Reduction of a Multicomponent Reference Fuel for Gasoline

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Abstract:

Within this thesis, a detailed multicomponent gasoline surrogate reaction scheme was developed
and reduced to a four component scheme of skeletal size. The main target is to
cover the most important features for typical spark ignited (SI) combustion - flame propagation,
emission formation and the tendency to auto ignite and subsequently cause engine
knock. To achieve this a variable mechanism concept was developed to include sub models
for different fuels as needed. Using this approach a detailed mechanism describing the oxidation
of n-heptane, iso-octane, toluene and ethanol was compiled and compared against
various experiments published in literature. Furthermore, correlations were developed to
suggest four component gasoline surrogates based on typical fuel data sheets. The correlation
method is validated against measurements in the Cooperative Fuel Research (CFR) engine
from various groups and further compared against correlations between octane numbers
(ON) and predicted 0D ignition delay times. These correlations are used to identify
and discuss the impact of the uncertainty of two reactions on ignition delay time of a multicomponent
fuel. To be able to reduce the detailed scheme in a time efficient way existing
reduction concepts where improved and applied to different schemes and targets. Since
various reduction techniques are available, an optimal sequence of those was worked out.
Using this sequence of reduction steps two multicomponent schemes were compiled: one
scheme for the prediction of laminar flame speeds and one for the prediction of major emissions
and auto-ignition. To underline that the suggested reduction procedure is universal
it was also applied to n-heptane as single fuel surrogate for diesel fuel and to a large two
component fuel from another work group.

2016

Andreas Manz

Thema: Modeling of End-Gas Autoignition for Knock Prediction in Gasoline Engines

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Abstract:

Downsizing of modern gasoline engines with direct injection is a key concept for achieving future CO22 emission targets. However, high power densities and optimum efficiency are limited by an uncontrolled autoignition of the unburned air-fuel mixture, the so-called spark knock phenomena. By a combination of three-dimensional Computational Fluid Dynamics (3D-CFD) and experiments incorporating optical diagnostics, this work presents an integral approach for predicting combustion and autoignition in Spark Ignition (SI) engines. The turbulent premixed combustion and flame front propagation in 3D-CFD is modeled with the G-equation combustion model, i.e. a laminar flamelet approach, in combination with the level set method. Autoignition in the unburned gas zone is modeled with the Shell model based on reduced chemical reactions using optimized reaction rate coefficients for different octane numbers (ON) as well as engine relevant pressures, temperatures and EGR rates. The basic functionality and sensitivities of improved sub-models, e.g. laminar flame speed, are proven in simplified test cases followed by adequate engine test cases. It is shown that the G-equation combustion model performs well even on unstructured grids with polyhedral cells and coarse grid resolution. The validation of the knock model with respect to temporal and spatial knock onset is done with fiber optical spark plug measurements and statistical evaluation of individual knocking cycles with a frequency based pressure analysis. The results show a good correlation with the Shell autoignition relevant species in the simulation. The combined model approach with G-equation and Shell autoignition in an active formulation enables a realistic representation of thin flame fronts and hence the thermodynamic conditions prior to knocking by taking into account the ignition chemistry in unburned gas, temperature fluctuations and self-acceleration effects due to pre-reactions. By the modeling approach and simulation methodology presented in this work the overall predictive capability for the virtual development of future knockproof SI engines is improved.

2015

Michal Pasternak

Thema: Simulation of the Diesel Engine Combustion Process Using the Stochastic Reactor Model

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Abstract:

The present work is concerned with the simulation of combustion, emission formation and fuel effects in Diesel engines. The simulation process is built around a zero-dimensional (0D) direct injection stochastic reactor model (DI-SRM), which is based on a probability density function (PDF) approach. An emphasis is put on the modelling of mixing time to improve the representation of turbulence-chemistry interactions in the 0D DI-SRM. The mixing time model describes the intensity of mixing in the gas-phase for scalars such as enthalpy and species mass fraction. On a crank angle basis, it governs the composition of the gas mixture that is described by PDF distributions for the scalars. The derivation of the mixing time is based on an extended heat release analysis that has been fully automated using a genetic algorithm. The predictive nature of simulations is achieved through the parametrisation of the mixing time model with known engine operating parameters such as speed, load and fuel injection strategy. It is shown that crank angle dependency of the mixing time improves the modelling of local inhomogeneity in the gas-phase for species mass fraction and temperature. In combination with an exact treatment of the non-linearity of reaction kinetics, it enables an accurate prediction of the rate of heat release, in-cylinder pressure and exhaust emissions, such as nitrogen oxides, unburned hydrocarbons and soot, from differently composed fuels. The method developed is particularly tailored for computationally efficient applications that focus on the details of reaction kinetics and the locality of combustion and emission formation in Diesel engines.

2014

Galin Nakov

Thema: Rohemissionsmodellierung auf Basis Detaillierter Chemie

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Carlo Locci

Thema: Large Eddy Simulations Modelling of Flameless Combustion

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Abstract:

The environmental emergency has led to the development of new combustion technologies. In this context, flameless combustion (FC in this manuscript) offers the prospect of a less polluting and more efficient technology. In FC, combustion is strongly diluted with recirculated burnt gases. Consequently the oxygen content is reduced and temperature peaks are smoothed, yielding reduced heat release. These conditions dramatically reduce the conditions of NO pollutant formation and increase the efficiency of the combustion process. Being FC a relatively new technology, it still needs optimization and R&D, which can be expensive and time consuming. Potentially, CFD can reduce both the financial costs as well as the R&D projects length. The context in which this thesis is inserted is exactly the numerical modeling of FC, by using Large Eddy Smulations for its better prediction of the turbulent ternary mixing (fuel - burnt gases -air), compared to RANS. This work has been divided into two main parts. In the first, combustion in FC has been investigated by means of a new tabulated combustion model initially written in the context of the EC-KIAI project and developed and adapted to FC in this thesis. The model uses diluted homogeneous reactors DHR to simulate FC and it was developed to account for under adiabatic enthalpy losses and the ternary mixing typical of FC. The model was firstly validated on a non-premixed flame academical configuration called Flame D and subsequently on a real FC combustor from the work of Verissimo et al. The results obtained for these configurations are quite correct although some discrepancies in CO prediction are observed. In the second part of the thesis, the NO pollutant modeling in FC is investigated. With this aim, the Diffusion Flame - NO relaxation approach DF-NORA was developed. It consists in tabulating the NO relaxation towards equilibrium of the NO source term in a flamelet structure. As done in the first part, the model was first validated on Flame D and then employed in a real FC configuration. Results are quite satisfactory in both configurations. The encouraging results obtained in this work open the possibility of applying the proposed developments to real industrial configurations in the future.

2013

Ngozi Ebenezer

Thema: Adaptive Polynomial Tabulation: A Computationally Efficient Strategy For Complex Kinetics

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Abstract:

A new approach to solve the initial value chemical rate equation system is presented. In this approach zero, first and second order polynomials are used in real-time to approximate the solution of the initial value chemical rate equation system. The sizes of the local regions valid for the different orders of polynomial approximation are calculated in real-time. To improve accuracy the chemical state space is partitioned into hypercubes. During calculations the hypercubes accessed by the reactive mixture are divided into PRISM hypercubes depending on the accuracy of the local solution. Mixture initial conditions are stored in the PRISM hypercubes. Around each stored initial condition two concentric ellipsoids of accuracy (EOA) are defined. These include the ISAT and identical EOAs. The time evolution of mixture initial conditions which encounter an identical and ISAT EOA are approximated by zero and first order polynomials respectively. With a certain number of stored initial condition within a PRISM hypercube, its second order polynomial coefficients are constructed from the stored initial conditions. The time evolution of additional mixture initial conditions which encounter this PRISM hypercube are approximated with second order polynomials. The APT model is simplified by the replacement of the entire set of species mass fractions with a progress variable based on the enthalpy of formation evaluated at 298 K. APT has 3 degrees of freedom which include the progress variable, total enthalpy and pressure. The APT model is tested with a zero dimensional Stochastic Reactor Model (SRM) for HCCI engine combustion. A skeletal n-heptane/toluene mechanism with 148 chemical species and 1281 reactions is used. In the test, the HCCi engine simulations using APT are in very good agreement with the model calculations using the ODE solver. The cool flame and main ignition events are accurately captures. The major and minor species are also accurately captured by APT. In SRM-HCCI calculations without cyclic variations, I obtained a computational speed up factor greater than 1000 when APT was used in all operating points considered without significant loss in accuracy. For the SRM-HCCI engine calculations with cyclic variations, APT demonstrated a computational speed up exceeding 12 without significant loss in accuracy. These tests demonstrate that APT possesses adaptive control of tabulation errors, and the frequency of identical, ISAT and PRISM calls increases with error tolerance. The APT algorithm was further modified such that only 15 initial conditions are stored per time step for the first 5 cycles. After the 5th cycle initial conditions are added per time step if the cumulative sum of stored initial condition for that time step for the 5th cycle and all the subsequent cycles is less than 15. These modifications give rise to a minimal size APT library, spread of initial conditions within the PRISM hypercube (more accurate second order polynomial coefficients) and removal of redundant ODE integration calculations for the construction of mapping gradients. This modified version of APT was tested with the n-heptane/toluene fueld SRM-HCCI engine model. A computational speed close to 20 was obtained without any significant loss in accuracy.

Linda Maria Beck

Thema: Multiphysical Modelling of Regular and Irregular Combustion on Spark ignition Engines using an Integrated/Interactive Flamelet Approach

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Abstract:

The virtual development of future Spark Ignition (SI) engine combustion processes in three-dimensional Computational Fluid Dynamics (3D-CFD) demands for the integration of detailed chemistry, enabling - additionally to the 3D-CFD modelling of flow and mixture formation - the prediction of fuel-dependent SI engine combustion in all of its complexity. This work presents an approach, which constitutes a coupled solution for flame propagation, auto-ignition, and emission formation modelling incorporating detailed chemistry, while exhibiting low computational costs.

For modelling the regular flame propagation, a laminar flamelet approach, the G-equation is used. Auto-ignition phenomena are addressed using an integrated flamelet approach, which bases on the tabulation of fuel-dependent reaction kinetics. By introducing a progress variable for the auto-ignition - the Ignition Progress Variable (IPV) - detailed chemistry is integrated in 3D-CFD. The modelling of emission formation bases on an interactively coupled flamelet approach, the Transient Interactive Flamelet (TIF) model.

The functionality of the combined approach to model the variety of SI engine combustion phenomena is proved first in terms of fundamentals and standalone sub-model functionality studies by introducing a simplified test case, which represents an adiabatic pressure vessel without moving meshes. Following the basic functionality studies, the sub-model functionalities are investigated and validated in adequate engine test cases. It is shown, that the approach allows to detect locally occurring auto-ignition phenomena in the combustion chamber, and to model their interaction with regular flame propagation. Moreover, the approach enables the prediction of emission formation on cell level.

2012

René Thygesen

Thema: Entwicklung eines partiell vorgemischten Verbrennungsmodells für die Mehrfacheinspritzstrategie am Dieselmotor

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Henrik Hoffmeyer

Thema: Combustion characteristics of turbo-charged DISI-engines

Betreuer: Prof. Dr.-Ing. Fabian Mauß

Abstract:

In spite of progress in the development of alternative powertrain systems and energy sources, the internal combustion and all its derivates still are and will be the main powertrain for automobiles. In SI-engines, several approaches compete with each other like the controlled auto ignition (CAI or HCCI), throttle-free load control using variable valvetrains, stratified mixture formation with lean engine operation or highly turbo charged downsizing concepts all combined with gasoline direct injection. The presented work makes a contribution for a deeper understanding of the combustion process of a turbo charged direct injection engine operating with external EGR as well as lean stratified mixture. Using detailed test bench investigations and introducing a new optical measurement tool, the combustion process is described in detail focusing on the occurrence of non-premixed combustion phenomena. The influence of engine parameters like global and local air-/fuel ratio, external EGR and fuel rail pressure as well as the influence of fuel parameters are discussed giving a characterization of the combustion process of stratified engine operation. Furthermore, the influences of non-inert exhaust gas components on engine knock tendency are investigated using external EGR with an EGR catalyst. Opposing the results to numerical analysis, combustion characteristics of turbo charged DISI-engines are presented.

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