Search Results

This paper presents the results of experimental and numerical investigations on pre-ignition in a series-production turbocharged DISI engine. Previous studies led to the conclusion that pre-ignition can be triggered by auto-ignition of oil droplets generated in the combustion chamber. Analysis of more recent experiments shows that a modification of the engine operation parameters that promotes spray/lubricant interaction also increases pre-ignition frequency, while modifications that enhance the speed of chemical reactions (thereby favoring auto-ignition) have little or no influence. The experimental and numerical findings can be explained if we assume the existence of a substance (originating from lubricant/fuel interaction) that displays extremely short ignition delay times.

This paper presents the results of a study on reasons for the occurrence of pre-ignition in highly supercharged spark ignition engines. During the study, the phenomena to be taken into account were foremost structured into a decision tree according to their physical working principles. Using this decision tree all conceivable single mechanisms to be considered as reasons for pre-ignition could be derived. In order to judge each of them with respect to their ability to promote pre-ignition in a test engine, experimental investigations as well as numerical simulations were carried out. The interdependence between engine operating conditions and pre-ignition frequency was examined experimentally by varying specific parameters. Additionally, optical measurements using an UV sensitive high-speed camera system were performed to obtain information about the spatial distribution of pre-ignition origins and their progress.

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 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.

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.

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, 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.

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).

We present the effect of EGR, at a set fuel flow rate and intake temperature, on the operating parameters of timing of combustion, duration of combustion, power output, thermal efficiency, and NOx emission; which is remarkably low. We find that addition of EGR at constant inlet temperature and constant fuel flow rate has little effect on HCCI parameter of start of combustion (SOC). However, burn duration is highly dependent on the amount of EGR inducted. The experimental setup at UC Berkeley uses a 1.9-liter 4-cylinder diesel engine with a compression ratio of 18.8:1 (offered on a 1995 VW Passat TDI). The engine was converted to run in HCCI mode by addition of an 18kW air pre-heater installed in the intake system. Pressure traces were obtained using four water-cooled quartz pressure transducers, which replaced the Diesel fuel injectors. Gaseous fuel (propane or butane) flowed steadily into the intake manifold.

In this study, the occurrence of auto-ignition centers in a two-stroke SI engine was investigated using planar laser-induced fluorescence (PLIF). An experimental SI engine equipped with glass windows to enable full optical access into the combustion chamber was operated under knocking conditions. The pulsed output of two XeCl excimer lasers was formed into planar light sheets (300 μm × 4 cm), which were spatially overlapped and directed into the combustion chamber of the operating engine. Unburned fuel components fluoresce strongly when illuminated with XeCl laser radiation; burned regions display no fluorescence. Self-ignited regions therefore show up as dark sites in the fluorescence images, indicating local consumption of the fuel. The resulting PLIF images were recorded using fast-gated ICCD cameras. By delaying the second laser pulse a specific time (100 ns-600 μs), image pairs were acquired which allowed the temporal development and mutual influence of hot-spots to be studied.

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.