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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).

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 autoignition delay time and location of a n-heptane fueled high pressure and high temperature spray combustion chamber under Diesel engine conditions has been investigated numerically. The conservation equations for the fluid dynamics of sprays have been solved using the KIVA-II code with its standard spray models. A detailed chemical mechanism of 81 elementary reactions and 37 chemical species has been applied to describe the ignition and combustion of n-heptane. The coupling between complex chemistry and turbulence is treated by employing the Representative Interactive Flamelet (RIF) concept. Unsteady flamelets are computed using a separate flamelet code that interacts with the CFD solver at each time step. The scalar dissipation rate, which is an important parameter for the flamelet, has been studied numerically under different conditions.

Early injection strategies in the case of part-load conditions are offering the possibility to enhance mixing and evaporation. Due to the early injection, ignition and evaporation are separated in time and space for that less rich pockets from where soot is formed are occurring. For reducing NOx, cooled EGR is a method to dilute the intake charge. The combustion is shifted to lower temperatures and less NOx is formed. More, the cooling of the intake charge and the higher heat capacity enhance the evaporation time for that ignition starts at later times and combustion is retarded. For the simulation of such engine cases using high rates of EGR with an early fuel injection, a CFD (Computational Fluid Dynamics) code is coupled interactively with the flamelet model that will be applied here as combustion model. That approach, known as RIF (Representative Interactive Flamelet) model, requires a re-evaluation of the chemical reaction mechanism.

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.

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.

Flamelet modeling allows the application of comprehensive chemical mechanisms, which. include all relevant chemical combustion processes that occur in a DI Diesel engine during autoignition, the burnout in the partially premixed phase, the transition to diffusive burning and formation of pollutants like NO, and soot. The highly nonlinear dependencies of the chemistry need not to be simplified, and the complete structure of the flame is preserved. Using the Representative Interactive Flamelet (RIF) model the one-dimensional unsteady set of partial differential equations is solved online with the 3-D CFD code. The flamelet solution is coupled to the flow and mixture field by the current boundary conditions (enthalpy, pressure, scalar dissipation rate). In return, the flamelet code yields the species concentrations, which are then used by the 3-D CFD code to compute the temperature field.

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.

Numerical simulations for an n-heptane fueled high pressure and high temperature chamber under Diesel engine conditions have been performed to study soot formation and oxidation processes. A kinetically based soot model has been applied, which accounts for the pyrolysis and oxidation of fuel and formation of polycyclic aromatic hydrocarbons (PAHs) by the use of a detailed kinetic mechanism. PAH growth and oxidation is modeled by a fast polymerization process, coagulation of PAHs leads to particle inception. The soot particles are allowed to coagulate with other particles and PAHs. The interaction of soot particles with the gas phase is modeled by heterogeneous surface reactions leading to particle growth due to acetylene addition and particle oxidation by hydroxyl radicals and molecular oxygen. The conservation equations for the fluid dynamics are solved with the KIVA II code and the coupling with chemistry is treated by employing the flamelet concept.

In Diesel engines combustion proceeds essentially under partially premixed and non-premixed conditions. In this study the flamelet model for non-premixed combustion is derived and its implementation into 3-D codes is discussed. The model is capable of describing auto-ignition, the following burnout of the partially premixed phase, and the transition to diffusive burning. Flamelet modeling has the advantage of separating the numerical effort associated with the resolution of fast chemical time scales from the fluid dynamics' scales occuring in the 3-D computation of the engine combustion cycle. Three additional scalar field equations have to be solved in the 3-D engine code, while the entire chemistry consisting of up to 1000 or more chemical reactions is simultaneously treated in a separate 1-D code describing the flamelet structure. A new aspect proposed here is to use so-called RIFs (Representative Interactive Flamelets), which are solved on-line with the 3D-code.

Correct prediction of mixture formation in fuel sprays is a prerequisite in the framework of 3D CFD engine simulations using a reliable combustion model. To understand the process of fuel evaporation and mixture formation in dense atomized sprays a simultaneous Mie/Shadow imaging technique and a 1D-linear Raman scattering technique are applied to investigate spray formation of high pressure direct injection. Mixture composition is one of the most important parameters in fuel sprays and is difficult to measure because liquid and vapor phases appear simultaneously. Ethanol is used as a model fuel since the phase-dependent spectral shift of the OH stretching vibration allows the Raman signal separation of liquid and vapor phase. The investigations are carried out in a high temperature high pressure injection chamber, where pressures and temperatures can be set up to 5MPa and 800 K.

Combustion and pollutant formation in a DI-Diesel engine are numerically investigated using the Eulerian Particle Flamelet Model (EPFM). A baseline case for part load operating conditions is considered as well as an EGR variation. The surrogate fuel consisting of n-decane (70% liquid volume fraction) and α-methylnaphthalene (30% liquid volume fraction) is used in the simulation. Results are compared to experimental data that has been obtained using real diesel fuel. The effect of multiple flamelets on the simulation of the auto-ignition process and the pollutant formation is discussed and a converging behavior of the model with respect to the number of flamelets is found. The effect of homogenization of the three-dimensional mixture field is investigated and it has been included in the formulation of the scalar dissipation rate.