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2003 | Book

Modeling Engine Spray and Combustion Processes

Author: Dr.-Ing. Gunnar Stiesch

Publisher: Springer Berlin Heidelberg

Book Series: Heat and Mass Transfer


About this book

The utilization of mathematical models to numerically describe the performance of internal combustion engines is of great significance in the development of new and improved engines. Today, such simulation models can already be viewed as standard tools, and their importance is likely to increase further as available com­ puter power is expected to increase and the predictive quality of the models is constantly enhanced. This book describes and discusses the most widely used mathematical models for in-cylinder spray and combustion processes, which are the most important subprocesses affecting engine fuel consumption and pollutant emissions. The relevant thermodynamic, fluid dynamic and chemical principles are summarized, and then the application of these principles to the in-cylinder processes is ex­ plained. Different modeling approaches for the each subprocesses are compared and discussed with respect to the governing model assumptions and simplifica­ tions. Conclusions are drawn as to which model approach is appropriate for a specific type of problem in the development process of an engine. Hence, this book may serve both as a graduate level textbook for combustion engineering stu­ dents and as a reference for professionals employed in the field of combustion en­ gine modeling. The research necessary for this book was carried out during my employment as a postdoctoral scientist at the Institute of Technical Combustion (ITV) at the Uni­ versity of Hannover, Germany and at the Engine Research Center (ERC) at the University of Wisconsin-Madison, USA.

Table of Contents

1. Introduction
This complex task of improving on combustion engines, that have already reached a very high level of sophistication during their more than 100 year long history, can nowadays be achieved only by a combination of advanced experimental and computational studies. Despite the quantitative uncertainties of numerical simulations that are often greater than those of experiments, the modeling of combustion engine processes has some significant advantages that make its utilization in today’s engine development a necessity. In this regard, it is obvious that numerical simulations are especially suited to carry out extensive parametric studies, since they are much more time and cost effective than the alternative construction and investigation of numerous prototypes.
Gunnar Stiesch
2. Thermodynamic Models
The models described in this chapter are called thermodynamic models since they are based on the first law of thermodynamics and mass balances only. The principles of momentum conservation are not considered in this model type and spatial variations of composition and thermodynamic properties are neglected. Thus, the entire combustion chamber of an internal combustion engine is typically treated as a single, homogeneously mixed zone. These assumptions obviously represent a significant abstraction of the problem and prohibit the usage of thermodynamic models in order to study locally resolved subprocesses such as detailed spray processes or reaction chemistry. However, the great advantage of these models is that they are both easy to handle and computationally very efficient. Therefore, they are still widely used in applications where there is only interest in spatially and sometimes even temporally averaged information and where computational time is crucial.
Gunnar Stiesch
3. Phenomenological Models
While the thermodynamic combustion models described in Chap. 2 are relatively easy to handle and are characterized by a low computational effort, they are lacking the ability to make predictions of the effects of important engine parameters on combustion without prior measurements. The main reasons for this deficiency are that major subprocesses are either not modeled at all or described by solely empirical correlations and that the assumption of an ideally mixed combustion chamber makes it impossible to estimate pollutant formation rates that are strongly affected by local temperatures and mixture compositions. On the other hand, the multidimensional CFD models that are based on the locally resolved solutions of mass-, energy- and momentum-conservation and that include detailed submodels for spray and combustion phenomena, are computationally expensive, and they demand that the user has a much deeper understanding of the governing physical and chemical processes in order to correctly interpret the simulation results. Moreover, the predictive quality with respect to global quantities such as pressure traces and apparent heat release rates is not necessarily better than with simpler models. This is because the many subprocesses taking place inside a combustion chamber are often interacting with each other such that relatively small errors encountered within particular submodels may add up to a considerable error in the overall result of the computation.
Gunnar Stiesch
4. Fundamentals of Multidimensional CFD-Codes
The abbreviation CFD stands for computational fluid dynamics which indicates the numerical solution of multidimensional flow problems that may be of unsteady and turbulent nature. In general, multidimensional flow problems are governed by conservation principles for mass energy and momentum. The application of these principles results in a set of partial differential equations in terms of time and space that need to be integrated numerically as they are too complex to be solved analytically.
Gunnar Stiesch
5. Multidimensional Models of Spray Processes
Spray processes play an important role in many technical systems and industrial applications. Examples are spray cooling, spray painting, crop spraying, humidification and spray combustion in furnaces, gas turbines, rockets, as well as diesel and gasoline engines, to name only a few. Typical drop sizes in sprays vary over several orders of magnitude for different applications. Figure 5.1 gives a qualitative classification of broad spray classes.
Gunnar Stiesch
6. Multidimensional Combustion Models
A chemical reaction between the reactant species A a , A b , etc., that forms the product species A c , A d , etc., is often written as
$${v_a}{A_a} + {v_b}{A_b} + \ldots \to {v_c}{A_c} + {v_d}{A_d} + \ldots $$
where the v i are termed the stoichiometric coefficients of the reaction. Since every chemical reaction can generally proceed in both directions, the arrow in Eq. 6.1 may be replaced by an equal sign. The general form of the reaction equation now becomes
$$ \sum\limits_i {{v_i}{A_i}} = 0 $$
where, by convention, the stoichiometric coefficients v i are positive for the product species and negative for the reactant species.
Gunnar Stiesch
7. Pollutant Formation
In the ideal case of complete combustion of a hydrocarbon fuel with stoichiometric air, the exhaust gas would be composed of the chemical species carbon dioxide (CO2), water (H2O) and molecular nitrogen (N2) only. For lean equivalence ratios, molecular oxygen (O2) could be observed among the products as well. However, in real combustion systems there are two reasons that inhibit complete combustion: (i) Elementary chemical reactions never proceed completely into one direction, but they always approach an equilibrium state between products and reactants. Thus, at least a small amount of reactants will remain. (ii) Local boundary conditions such as mixture distribution, temperature and turbulence level are often non-ideal. Therefore, flame extinction, accompanied with unburned or partially burned species, or the formation of entirely new products, e.g. soot or nitrogen oxides, may occur. Consequently, additional components are present in the exhaust gases of combustion engines. These components are carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter which is often approximated as soot. Depending on the quality of the fuel there may also be traces of sulfur oxides (SOx) within the exhaust gas.
Gunnar Stiesch
8. Conclusions
In the above text the state of the art in modeling in-cylinder processes of internal combustion engines has been presented and discussed. The development and application of such mathematical formulations is of great importance in today’s research and development of combustion engines for several reasons. Firstly, simulation models, that have been properly adjusted to a specific range of boundary conditions, can be utilized to execute extensive parametric studies. In this context, simulation models are much more time and cost efficient than the alternative execution of experiments. Secondly, and maybe most importantly, numerical simulation tools can provide detailed information about any process variable at any point in time and space, that would be impossible to obtain with the sole execution of experiments. Consequently, a much better basis for the interpretation of complex results will be available, if both numerical and experimental studies are conducted in parallel. This aspect is of special importance, as combustion engines become more and more sophisticated and the task of further improving their performance becomes more and more complex. Last but not least, numerical simulations allow to perform conceptual studies with extreme boundary conditions, that could not be realized in experiments because of either too large or too small length and time scales, or because a dangerous outcome prohibits the execution of the respective experiment.
Gunnar Stiesch
Modeling Engine Spray and Combustion Processes
Dr.-Ing. Gunnar Stiesch
Copyright Year
Springer Berlin Heidelberg
Electronic ISBN
Print ISBN