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Subject of this work is a simulation model for PCCI combustion that can be used in closed-loop control development. A detailed multi-zone chemistry model for the high-pressure part of the engine cycle is extended by a mean value model accounting for the gas exchange losses. The resulting model is capable of describing PCCI combustion with stationary excactness. It is at the same time very economic with respect to computational costs. The model is further extended by identified system dynamics influencing the stationary inputs. For this, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created.

A Flamelet formulation for premixed turbulent combustion has been developed based on a scalar field equation. Local effects like flame stretch and flame front curvature were introduced into the equation. Direct numerical simulations of a flame propagating in a turbulent flow field revealed characteristic flame structures as they are experimentally observed. The calculated turbulent flame speed is in good agreement with correlations of experimental data. Experimental verification of the model has been carried out by Laser tomographic experiments in a VW transparent engine. Flame front structures were visualized resolving the highest possible length scale range. The spectral properties of the flame structures were investigated and compared with the model predictions. A good agreement was found between the characteristic power spectrum of the spatial flame front fluctuations and the scalar fluctuations in the model.

An interactive approach to Diesel spray simulation is proposed in which a one-dimensional cross-sectionally averaged system of equations for the two-phase flow is solved independently of the CFD-code in order to provide the source terms accounting for the mass, momentum and heat exchanges between the gas and liquid phase. The transport equations are solved in the multi-dimensional CFD-code only for the gas phase. Validation is performed by comparing the computed spray penetration lengths and the distribution of fuel mass fraction to experimental data and with the computational results of KIVA-II. The computational results agree well with the measurements and are shown to be grid-independent, while the spray model in KIVA-II leads to a strongly grid-dependent result.

Combustion and pollutant formation in a new recently introduced Common-Rail DI Diesel engine concept with roof-shaped combustion chamber and tumble charge motion are numerically investigated using the Representative Interactive Flamelet concept (RIF). A reference case with a cup shaped piston bowl for full load operating conditions is considered in detail. In addition to the reference case, three more cases are investigated with a variation of start of injection (SOI). A surrogate fuel consisting of n-decane (70% liquid volume fraction) and α-methylnaphthalene (30% liquid volume fraction) is used in the simulation. The underlying complete reaction mechanism comprises 506 elementary reactions and 118 chemical species. Special emphasis is put on pollutant formation, in particular on the formation of NOx, where a new technique based on a three-dimensional transport equation within the flamelet framework is applied.

Subject of this work is the recently introduced extended Representative Interactive Flamelet (RIF) model for multiple injections. First, the two-dimensional laminar flamelet equations, which can describe the transfer of heat and mass between two-interacting mixture fields, are presented. This is followed by a description of the various mixture fraction and mixture fraction variance equations that are required for the RIF model extension accounting for multiple injection events. Finally, the modeling strategy for multiple injection events is described: Different phases of combustion and interaction between the mixture fields resulting from different injections are identified. Based on this, the extension of the RIF model to describe any number of injections is explained. Simulation results using the extended RIF model are compared against experimental data for a Common-Rail DI Diesel engine that was operated with three injection pulses.

Recently, a consistent mixing model for the two-way coupling of a CFD code and a zero-dimensional multi-zone code was developed. This work allowed for building an interactively coupled CFD-multi-zone approach that can be used to model HCCI combustion. In this study, the interactively coupled CFD-multi-zone approach is applied to PCCI combustion in a 1.9l FIAT GM Diesel engine. The physical domain in the CFD code is subdivided into multiple zones based on one phase variable (fuel mixture fraction). The fuel mixture fraction is the dominant quantity for the description of nonpremixed combustion. Each zone in the CFD code is represented by a corresponding zone in the zero-dimensional multi-zone code. The zero-dimensional multi-zone code solves the chemistry for each zone, and the heat release is fed back into the CFD code. The thermodynamic state of each zone, and thereby the phase variable, changes in time due to mixing and source terms (e.g., vaporization of fuel, wall heat transfer).

The flame radiant emission from internal combustion engines is investigated experimentally by means of a fast spectroscopic measuring technique. This technique enables cycle resolved emission spectroscopy within a single combustion process and was applied to a DI diesel engine. The ‘cycle resolved emission spectroscopy’ (CRES) measuring technique with its optical components: optical probe, light-guide, spectrograph and ICCD-Camera is described in detail. These components were carefully designed for highest spectral transmittance for maximum signal/noise ratio in the UV and VIS wavelength region. The investigations focus on two different aspects: Broadband spectral radiation caused by thermal excited soot particles and narrowband spectral radiation caused by chemoluminescence from electronically excited OH radicals are measured. The soot radiation measurements in the visible enable calculations of the mean soot particle temperature and the mean soot volume fraction.

The non-premixed turbulent combustion and emission formation in a modern DI diesel engine relies mostly on the mixture formation process induced by the diesel fuel spray. Therefore the numerical simulation of this process has to incorporate accurate spray modeling which captures the physics of the spray formation, propagation and vaporization. A widely used framework for spray modeling is the Discrete Droplet Model (DDM) which also is applied in the present work. In the DDM framework, separate submodels account for droplet breakup, droplet-droplet interaction and evaporation. Due to the empirical nature of these submodels (particularly droplet breakup and collision) necessitated by an incomplete representation of the physics, and by the inability to isolate each process under diesel engine relevant conditions, some of the constants controlling the outcomes of these submodels require calibration.

Quenching of premixed flames at cold walls is investigated to study the importance of the model fuel choice for combustion modeling. Detailed chemical mechanisms for two different fuels, namely the low-molecular-weight fuel methane, and the more complex fuel iso-octane are employed. For both fuels the response of the flame to the very rapid heat loss at the cold wall is studied. The most important and significant difference between methane and iso-octane for this problem is the postquench oxidation of unburned hydrocarbons. Methane shows fast oxidation of unburned fuel and intermediate hydrocarbons whereas postquench oxidation for iso-octane is slow especially for the intermediate hydrocarbons. Furthermore, the Soret effect which is usually considered to be of minor importance appears to be important in modeling the rate limiting diffusion process. This is caused by different directions of the thermal diffusive transport for certain species.

Ethanol is frequently used as a blending component in standard gasoline, with blend rates up to 10%vol liq . n-Butanol has received recent interest as an alternative fuel instead of ethanol for use in spark ignition engines. Similar to ethanol, n-butanol can be produced via the fermentation of sugars, starches, and lignocelluloses obtained from agricultural feedstock. It is of great interest to modern engine development to understand the effect of ethanol and n-butanol as blending components on the laminar burning velocity of standard gasoline. The laminar burning velocity is one key parameter for the numerical simulation of gasoline engine combustion processes. Tested fuel components are ethanol, n-butanol, and standard marked gasoline without any oxygen content. Fuel blends consist of standard-marked gasoline containing ethanol and butanol. The maximum blend rate of oxygenates is 10%vol liq . Experiments were done at different equivalence ratios between 0.7 and 1.3.

Representative Interactive Flamelets (RIF) have proven successful in predicting Diesel engine combustion. The RIF concept is based on the assumption that chemistry is fast compared to the smallest turbulent time scales, associated with the turnover time of a Kolmogorov eddy. The assumption of fast chemistry may become questionable with respect to the prediction of pollutant formation; the formation of NOx, for example, is a rather slow process. For this reason, three different approaches to account for NOx emissions within the flamelet approach are presented and discussed in this study. This includes taking the pollutant mass fractions directly from the flamelet equations, a technique based on a three-dimensional transport equation as well as the extended Zeldovich mechanism. Combustion and pollutant emissions in a Common-Rail DI Diesel engine are numerically investigated using the RIF concept. Special emphasis is put on NOx emissions.

In this work, surrogate fuels composed of n-decane and alpha-methylnaphthalene (AMNL) with different compositions according to the reference cetane numbers 53, 45, 38, and 23 are investigated. In addition to the two-component mixtures, we examine a three-component mixture composed of n-decane, AMNL, and di-n-butyl ether (DNBE) corresponding to a reference cetane number of 53. Spray characteristics of liquid and fuel vapor phase and the relationship between ignition quality and lift-off length are investigated. The experimental results show, first of all, that for these mixtures, the cetane number is a good indicator for the ignition delay. Diesel and surrogate fuels have different liquid penetration lengths, which depend on the evaporation rate, and hence vapor pressure and boiling point of the fuels.

In Common-Rail DI Diesel Engines, multiple injection strategies are considered as one of the methodologies to achieve optimum performance and emission reduction. However, multiple injections open a whole new horizon of parameters which affect the combustion process. These parameters include the number of injection events, the duration between the starts of each injection event, the splitting of the total fuel mass on the different injection events, etc. In the present work, the influence of the number of injection events and the influence of the duration between the starts of each injection event on emission levels are investigated. Combustion and pollutant formation were experimentally investigated in a Common-Rail DI Diesel engine. The engine was operated at conventional part-load conditions with 2000 rpm, no external EGR, and an injected fuel mass of 15 mg/cycle.

Increasing concern for the environment and the impending scarcity of fossil fuels requires continued development in hydrocarbon combustion science. For compression-ignition engines, adding oxygenated compounds to the fuel can reduce noise, soot formation, and unburned hydrocarbons while simultaneously increasing thermal efficiency. In order to reliably model and design compression-ignition engines to use new fuel blends, accurate spray characteristic data is required. In this study, the spray characteristics of various blends of the oxygenated compound di-n-butyl ether (DNBE) with standard EN590 Diesel fuel are presented, including spray cone angle and spray penetration length for both liquid and gas phases. The experiments were conducted in a spray chamber at ambient conditions of 50 bar and 800 K, simulating TDC conditions in a Diesel engine. Injection pressures were varied from 700-1600 bar.

Diesel engine modeling by means of CFD (computational fluid dynamics) has become a more and more important tool in the development process for new engine design. An adequate and reliable Diesel engine model relies on many features. Beside the combustion and spray modeling, the question what model fuel should be used is discussed and in the past, a mixture of n-decane and α-methylnaphthalene, denoted as IDEA fuel, was found to be a good surrogate fuel for Diesel for the conventional Diesel combustion mode. New combustion designs such as PCCI (premixed charged compression ignition) are a possible solution for the strict upcoming emission limits. Due to a shift to lower temperatures and better homogenization, less NOx and soot is formed. To model these combustion designs, a re-evaluation of the model fuel that is to be used is required when the benefit of a detailed chemical reaction mechanism is favored in the combustion modeling.

The laminar burning velocity is one key parameter for the numerical simulation of gasoline engine combustion processes. In order to understand the effect of the laminar burning velocity of different fuel components on modern engine development it is of great interest to conduct experiments under high initial pressure and temperature. Initial conditions in this publication are a pressure of p = 10bar and a temperature of T = 373K. Special focus has been laid on the common C1 and C2 alcohols, methanol and ethanol, which are frequently used for blending components in standard gasoline. The experimental setup consists of a spherical closed pressurized combustion vessel with optical access. Schlieren measurements coupled with a high speed camera are used for image acquisition to track the expanding flame front. Finally, a post processing tool is used to extrapolate the measurements to zero stretch. Experiments were done at different fuel-air ratios between Φ = 0.8 and up to Φ = 1.2.

In Common-Rail DI Diesel Engines, a low combustion temperature process is considered as one of the most important possibilities to achieve very small emissions and optimum performance. To reduce NOx and Soot strongly, it is necessary to achieve a homogenization of the mixture in order to avoid the higher local temperatures which are responsible for the NOx formation [1]. Through the homogenization it is also possible to obtain a stoichiometric air-fuel ratio in order to significantly reduce the Soot emissions. One way to achieve this homogeneous condition is to start injection very early together with the use of higher EGR rates. The direct effect of these conditions cause a longer ignition delay (this is the time between start of the injection and auto-ignition during physical and chemical sub processes such as fuel atomization, evaporation, fuel air mixing and chemical pre-reactions take place) so that the mixture formation has more time to achieve a homogeneous state.

Engine noise pollution is as harmful as other forms of pollution to human health. Apart from the health effects, noise also has an adverse effect on the engine structure, thus requiring a sturdier construction to maintain long engine life. In a conventional direct injection diesel engine the fuel ignites spontaneously shortly after the beginning of injection. The Combustion process causes fluctuations in heat release and therefore, fluctuations in combustion chamber pressure. Combustion generated noise can be lowered by lowering the fluctuations in heat release or pressure. Which can be achieved by separating the fuel evaporation and fuel-air mixing from start of ignition in space and in time. The noise is mainly affected by the early part of the combustion process due to higher rates of heat release. Combustion noise generation in the early stage of combustion is not yet entirely understood.

Auto-ignition in Diesel engines, occurring essentially under non-premixed and partially premixed conditions, is considerably different to homogeneous ignition. In order to study the relevant chemistry--mixing interactions, it is assumed that the ignition of Diesel fuel can be described by using the single component model fuel n-heptane. Starting from a detailed chemical reaction scheme with about 1000 elementary reactions among 168 chemical components, a skeletal mechanism consisting of 98 reactions and 40 components is derived, which is still capable of describing the auto-ignition process under Diesel engine conditions and concentrations of NO, relevant intermediate components. Introducing steady state assumptions for intermediate species which are consumed rapidly leads to a reduced 14-step mechanism. The mechanism is validated with auto-ignition delay times from shock tube experiments by Adomeit for different temperatures, pressures, and equivalence ratios.

Oxygenates, such as methanol or ethanol, are frequently used as blending components in standard gasoline. One oxygenate, dimethyl ether (DME), is also used as a fuel component in some regions of the world, for example in Asia. In addition, patent reviews show the potential of DME as a blending component in liquefied petroleum gas (LPG) or mixed with propane. The laminar burning velocity is one key parameter for the numerical simulation of gasoline engine combustion processes. Therefore, it is of great interest for modern engine development to understand the effect of oxygenates on the laminar burning velocity. The experimental results have been conducted under engine-like conditions with elevated initial pressures of up to 20 bar and initial temperatures of 373 K. Experiments were done at equivalence ratios between 0.8 and 1.3. The experimental setup consists of a spherical closed pressurized combustion vessel with optical access.