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A chemical sub-model for realistic CFD simulations of Diesel engines is developed and demonstrated by application to some test cases. The model uses a newly developed progress variable approach to incorporate a realistic treatment of chemical reactions into the description of the reactive flow. The progress variable model is based on defining variables that represent the onset and temporal development of chemical reactions before and during self ignition, as well as the stage of the actual combustion. Fundamental aspects of the model, especially its physical motivation and finding a proper progress variable, are discussed, as well as issues of practical implementation. Sample calculations of Diesel-typical combustion scenarios are presented which are based on the progress-variable model, showing the capability of the model to realistically describe the ignition-and combustion phase.

To address the growing need for detailed chemistry in engine simulations, new software tools and validated data sets are being developed under an industry-funded consortium involving members from the automotive and fuels industry. The results described here include systematic comparison and validation of detailed chemistry models using a wide range of fundamental experimental data, and the development of software tools that support the use of detailed mechanisms in engineering simulations. Such tools include the automated reduction of reaction mechanisms for targeted simulation conditions. Selected results are presented and discussed.

Auto-Ignition is a phenomenon that occurs in many practical combustion processes (engine knock, ignition in rapid compression machines or shock tubes etc). For many purposes, the combustible mixture can be treated as uniform in space, allowing zero-dimensional modelling. Sometimes, however, non-uniformities in temperature or in pressure cause “hot spot” formation, having increased temperature with respect to the surrounding. Since ignition delay is highly temperature dependent, the hot spot will ignite much earlier than its surrounding, leading to space- and time-dependent processes governed by the superposition of chemistry, gas-dynamics, and transport. This paper presents mathematical models to simulate homogeneous and hot spot ignition in one-dimensional geometries by solution of the conservation equations using detailed chemistry and a multi-species transport model.

In this work, the potential of laser-induced ignition to improve combustion initiation and heat release in a direct-injection engine is investigated by a combined experimental and numerical investigation. Laser ignition is studied in fuel/air mixtures with homogeneous equivalence ratio fields. The results provide knowledge about minimum ignition energies and the ignition limits of laser-induced ignition. Furthermore, in mixtures with nominally identical conditions, statistical variations of the ignition success are observed experimentally. These variations can be explained, based on numerical simulations, by fluctuations in the strain rate in the turbulent in-cylinder flow. Additionally, laser ignition in engines with a spray-guided combustion mode, with strongly inhomogeneous fuel/air mixtures, was investigated.

A progress variable approach for the 3D-CFD simulation of DI-Diesel combustion is introduced. Considering the Diesel-typical combustion phases of auto-ignition, premixed and diffusion combustion, for each phase, a limited number of characteristic progress variables is defined. By spatial-temporal balancing of these progress variables, the combustion process is described. Embarking on this concept, it is possible to simulate the reaction processes with detailed chemistry schemes. The combustion model is coupled with a mesh-independent Eulerian-spray model in combination with orifice resolving meshes. The comparison between experiment and simulation for various Diesel engines shows good agreement for pressure traces, heat releases and flame structures.

In this work, constant volume combustion is studied using a zero-dimensional FORTRAN code, which is a wide-ranging chemical kinetic simulation that allows a closed system of gases to be described on the basis of a set of initial conditions. The model provides an engine- or reactor-like environment in which the engine simulations allow for a variable system volume and heat transfer both to and from the system. The combustion chamber is divided into two zones as burned and unburned ones, which are separated by an assumed thin flame front in the combustion model used for this work. Equilibrium assumptions have been adopted for the modeling of the thermal ionization, where Saha's equation was derived for singly ionized molecules. The investigation is focused on the thermal ionization of NO as well as for other species. The outputs generated by the model are temperature profiles, species concentration profiles, ionization degree and an electron density for each zone.

The present paper concentrates on the option of catalytic autothermal reforming of gasoline for fuel cell applications. Major parameters of this process are the “Steam to Carbon Ratio” S/C and the air to fuel ratio λ. Computations assuming thermodynamic equilibrium in the autothermal reactor outlet (ATR) were carried out to attain information about their proper choice, as failure in adjusting the parameters within narrow limits has severe consequences on the reforming process. In order to quantify velocity distribution just ahead the catalyst and to evaluate mixing uniformity we designed an ATR featuring an optical access: Thus flow visualization using PIV (Particle Image Velocimetry) and LIF (Laser Induced Fluorescence) technique is possible. Preliminary PIV-results are presented and compared with CFD computations (Computational Fluid D ynamics).

In this work, the spark-ignition (SI) of a methane/air mixture contained in a constant-volume chamber is investigated by numerical simulations. A cylinder-shaped vessel filled with a methane/air mixture containing two electrodes is used as simulation model. The impact of an electrical discharge at the electrodes on the surrounding gas is simulated, with detailed treatment of the ignition process involvig chemical kinetics, transport phenomena in the gas-phase and electrodynamical modeling of the interaction between spark and fuel/air mixture. For the calculations, a 2D-code to simulate the early stages of flame development, shortly after the breakdown discharge, has been developed. Computational results are shown for ignition of a methane air-mixture.

We apply a method for the visualization and semi-quantitative estimation of small spatial temperature fluctuations in internal combustion engines with premixed loads. It is based on laser-induced fluorescence (LIF) of formaldehyde (CH2O), which is formed in the unburned gas near the end of the compression stroke. The chemical reactions leading to formaldehyde formation during the phase before auto-ignition are strongly temperature-dependent. The concentration of CH2O therefore acts as a natural, very sensitive tag for local gas temperature variations. A correlation between temperature fluctuation and formaldehyde concentration fluctuation is assessed by using numerical simulations involving a detailed treatment of chemical reactions leading to formaldehyde formation in the unburned gas. Formaldehyde is detected in the unburned gas of an optically accessible test SI engine by laser-induced fluorescence (LIF) along a line.

Basic optical properties have been investigated in order to characterize the HCCI-combustion process. Basic optical properties of a Homogeneous Charge Compression Ignition (HCCI) engine have been investigated in order to characterize the combustion process. The absorption of light propagating through the combustion chamber has been spectrally resolved for four different fuels. Significant differences between the fuels could be detected. Complementary information could be obtained by recording spontaneous emission of radiation during combustion. Raman point measurements were used to quantify cycle-to-cycle variations of the equivalence ratio. The homogeneity of the charge was monitored by the use of two-dimensional tracer LIF. That method was also utilized to investigate the flame development. The experiments were performed in a six-cylinder, truck-sized engine with one cylinder modified to allow for optical access.

A detailed understanding of the complex chemistry-turbulence interaction is gaining an increasing importance for further improvement of IC engine performance. Multidimensional optical diagnostic techniques have become a versatile tool for engine development. Sophisticated automatic data post-processing will achieve an increasing significance for efficient data reduction in such optical experiments. The focus of this paper is the detection of flame fronts using different image processing algorithms. In a further step of the data reduction, the extraction of the length of the flame front and the area of the burnt gases is presented. A strategy relying on a sensitivity analysis is discussed which allows an objective choice of parameters necessary for the application of the mathematical algorithms.

Growing interest in pollutant emission reduction has increased the importance of numerical simulations of spark ignition as a first step in IC engine combustion. In this work, we present simulations involving the coupling of flow, chemical reactions and molecular transport with the discharge processes. The main focus hereby is to investigate the early stages of the formation of a flame kernel in a two-dimensional, cylindrical geometry with electrodes. The computational results shown here include the initial shock-determined phase after the breakdown of the channel, but also the transition to flame propagation for a methane-air mixture.

This paper reports on the development of novel time resolved spectroscopic imaging techniques for the study of spark ignition phenomena in combustion cells and an SI-engine. The techniques are based on planar laser induced fluorescence imaging (PLIF) of OH radicals, on fuel tracer PLIF, and on chemiluminescence. The techniques could be achieved at repetition rates reaching several hundreds of kilo-Hz and were cycle resolved. These techniques offer a new path along which engine related diagnostics can be undertaken, providing a wealth of information on turbulent spark ignition.

In-cylinder crank-angle resolved imaging of fuel and OH was obtained using planar laser induced fluorescence (PLIF) in a Homogenous Charge Compression Ignition (HCCI) engine. Investigations were carried out to ascertain the extent to which the combustion process in an HCCI engine is affected by the charge homogeneity. In the experiments, the heterogeneity of the charge was varied and the effect on the combustion process was monitored. The result shows a heterogeneous combustion with large spatial and temporal variations, even with a homogeneously premixed charge. It is therefore concluded that the charge inhomogeneity has a modest effect on the combustion process.