Applying a Tabulated Chemistry Approach for the Calculation of Combustion and Emissions in Diesel Engines

It is generally acknowledged, that more details of the chemical reactions occurring in the flame front should be accounted for in the CFD simulations, but with increasing the number of species and reactions involved the associated CPU cost grows quickly beyond practical engineering time limits. Aim of this work is to increase computation efficiency by using a tabulation technique, without losing any accuracy. In order to achieve these goals, dedicated software solution for the generation of CFD look-up tables for advanced combustion models, is applied. Simulations were run for real life Diesel engine, for 5 different EGR levels. FGM results are showing very good match with measurements and direct calculation of the chemical reactions. The runtime for CFD simulations, including chemistry pre-processing, does only mildly increase with the number of species used in the reaction mechanism; simulations with 1000+ species have been realized within 20 hrs on 8 CPU cores

Implementation of the Semi Empirical Kinetic Soot Model within Chemistry Tabulation Framework for Efficient Emissions Predictions in Diesel Engines

Soot prediction for diesel engines is a very important aspect of internal combustion engine emissions research, especially nowadays with very strict emission norms. Computational Fluid Dynamics (CFD) is often used in this research and optimisation of CFD models in terms of a trade-off between accuracy and computational efficiency is essential. This is especially true in the industrial environment where good predictivity is necessary for engine optimisation, but computational power is limited. To investigate soot emissions for Diesel engines, in this work CFD is coupled with chemistry tabulation framework and semi-empirical soot model. The Flamelet Generated Manifold (FGM) combustion model precomputes chemistry using detailed calculations of the 0D homogeneous reactor and then stores the species mass fractions in the table, based on six look-up variables: pressure, temperature, mixture fraction, mixture fraction variance, progress variable and progress variable variance. Data is then retrieved during online CFD simulation, enabling fast execution times while keeping the accuracy of the direct chemistry calculation. In this work, the theory behind the model is discussed as well as implementation in commercial CFD code. Also, soot modelling in the framework of tabulated chemistry is investigated: mathematical model and implementation of the kinetic soot model on the tabulation side is described, and 0D simulation results are used for verification. Then, the model is validated using real-life engine geometry under different operating conditions, where better agreement with experimental measurements is achieved, compared to the standard implementation of the kinetic soot model on the CFD side

LES of the flow in a DISI engine: analysis of turbulent scalar-velocity correlations

The underlying correlations between cyclic variability in the velocity field, spray boundary conditions and the spatial distribution of equivalence ratio in a realistic direct injection spark ignition engine have been investigated by means of Proper Orthogonal Decomposition (POD). The method of snapshots has been employed to perform both phase-dependent and phase-independent decomposition of the scalar-velocity correlations. LES based simulation of 30 engine cycles has been used for POD analysis

Large Eddy Simulation of the Flow and Mixing Field in an Internal Combustion Engine

The call for environmentally compatible and economical vehicles, still satisfying demands for high performance, necessitates immense efforts to develop innovative engine concepts. Whereas direct injection gasoline engines promise considerable fuel savings, they are prone to large variations in the flow and mixing field which may lead to incomplete combustion. Modern internal combustion (IC) engine concepts like the Gasoline Direct Injection offer a great chance to meet current and future emission standards. Especially air-guided direct injection systems used to instantiate stratified charge at part load allow for an optimised fuel consumption and a low level of emissions. During this crucial process, the engine is very sensitive to cycle-to-cycle variations of the flow and mixing field. While numerous experimental and RANS-based numerical investigations concentrated on the way to gain insight into the behavior of the spray in IC-Engines, LES may help in delivering detailed unsteady information needed to understand better the strongly transient phenomena ongoing in the combustor. The present study is dedicated to the detailed investigation of the phenomena of cycle-to-cycle variations in a realistic IC-engine using LES in order to achieve a better understanding of their nature, origin and their influence on the flow and mixing field in a combustion chamber, and also in order to create a base for future improvements. The configuration investigated represents the “BMBF” generic four-stroke direct fuel injection engine with variable charge motion system. This is a realistic IC-Engine with four canted valves with asymmetric cylinder head and asymmetric bowl. The well-known KIVA-3V code extended to LES which is capable of simulating two-phase engine flows was used. A relatively fine computational mesh reflecting all geometrical features of the ”BMBF” IC-engine has been created and tested. In order to characterize the cycle-to-cycle variations LES calculations coupled with a suitable parallelization strategy have been used to simulate for 50 full engine cycles. Phase-averaged statistics have been presented for characteristic crank angles. Investigations of the cyclic fluctuations have shown that the cycle-to-cycle phenomena are directly linked to the turbulence and can not be considered separately from each other. In the case of single-phase flow, the maximal intensity of cycle-to-cycle velocity variations in the combustion chamber is reached during the intake, mainly at the tip of the intake jet, and during compression, mainly at the center of the tumble motion. At the end of compression stroke the highest intensity of cyclic fluctuations is found at the center of the in-cylinder tumble motion which is roughly located near the spark plug close to the ignition point. At the same time examination of the expansion and exhaust strokes shows relatively low intensity of the cycle-to-cycle velocity fluctuations. The quality of the LES simulations applied to complex engine configurations has been assessed. Numerical and statistical errors have been analyzed. As a general guideline a mesh with grid size seems to be the minimum requirement to represent the flow field with reasonable accuracy. In order to control statistical errors it can be recommended to perform roughly 25 engine cycles in order to get mean velocities right and 50 cycles to ensure a good prediction of cyclic fluctuations. In the case of two-phase flow the flow field in the combustion chamber is defined by a superposition of in-cylinder charge motion and injected fuel spray jet. This interaction results in a considerable increase of the intensity of cyclic velocity fluctuations at the center of the tumble motion. The analysis has shown a great impact of velocity cyclic variations on the air-fuel mixing processes as well as fuel jet penetration and forming of fuel vapor cloud in the area near the spark plug. A lean fuel mixture is mostly found at the spark plug location for the considered ignition points under the given operating condition. Inflammable air-fuel mixtures lead to engine misfires which directly affect the work output and the vehicle driveability. These effects have to be considered in the development of modern DISI IC-engines