Mixture Formation in Internal Combustion Engines

This book covers the various approaches to modelling and optimising the spray and mixture formation processes in modern internal combustion engines. Due to their complexity and importance in predicting the temporal and spatial distribution of liquid and gaseous fuel inside the cylinder, special emphasis is put on the detailed description of multi-dimensional CFD-models. The book describes and discusses the most widely used mathematical models for in-cylinder spray and mixture formation processes. These processes are the most important sub-processes today affecting engine fuel consumption and pollutant emissions. The relevant thermodynamic and fluid dynamic processes are summarized, and then the application to the in-cylinder processes is described. Although the combustion and formation of emissions are not part of this book, the effects of the different mixture formation concepts on the combustion process are always included.

After the introduction (chapter 1), the fundamentals of mixture formation like injection systems and nozzle types, break-up regimes of liquid jets and droplets, semi-empirical relations and basic experimental investigations are summarized in chapter 2. Hence, this chapter provides the reader with the basic knowledge in order to facilitate the comprehension of the following parts. In the third chapter the basic conservation equations which are necessary for the three-dimensional simulation of mixture formation are derived. This chapter includes the relevant equations for the description of the continuous phase (Eulerian description, conservation equations, turbulence modelling) and the dispersed phase (Lagrangian description, spray equation, Monte-Carlo-Method and stochastic-parcel-method).

The three-dimensional CFD modelling of primary and secondary spray break-up, evaporation, turbulent dispersion, collision, wall impingement and ignition is presented in detail in chapter four. Example calculations are compared with semi-empirical relations and corresponding experimental data - represented in diagrams as well as in images resulting from modern optical measurement techniques - in order to discuss the capabilities of today's simulation models as well as their shortcomings.

Chapter five is about the influence of grid resolution on the numerical computations of jets. The Problem of defining an optimal grid size as well as approaches to reduce the grid dependency are discussed.

The last chapter deals with modern strategies for mixture formation and combustion processes. Today's and future requirements concerning the development of injection systems capable of optimising the combustion process in terms of reduced fuel consumption and emissions are discussed. Mixture formation concepts like gasoline direct wall-guided, air-guided and spray-guided injection techniques, variable rate shaping, variable needle lift, modulation of injection pressure as well as the application of homogeneous charge compression ignition concepts are described and compared with experimental data.

This book may serve both as a graduate level textbook for combustion engineering students and as a reference for professionals in the field of in-cylinder mixture formation and combustion engine modelling.

Résumé
A systematic control of mixture formation with modern high-pressure injection systems enables us to achieve considerable improvements of the combustion pr- ess in terms of reduced fuel consumption and engine-out raw emissions. However, because of the growing number of free parameters due to more flexible injection systems, variable valve trains, the application of different combustion concepts within different regions of the engine map, etc., the prediction of spray and m- ture formation becomes increasingly complex. For this reason, the optimization of the in-cylinder processes using 3D computational fluid dynamics (CFD) becomes increasingly important. In these CFD codes, the detailed modeling of spray and mixture formation is a prerequisite for the correct calculation of the subsequent processes like ignition, combustion and formation of emissions. Although such simulation tools can be viewed as standard tools today, the predictive quality of the sub-models is c- stantly enhanced by a more accurate and detailed modeling of the relevant pr- esses, and by the inclusion of new important mechanisms and effects that come along with the development of new injection systems and have not been cons- ered so far. In this book the most widely used mathematical models for the simulation of spray and mixture formation in 3D CFD calculations are described and discussed. In order to give the reader an introduction into the complex processes, the book starts with a description of the fundamental mechanisms and categories of fuel - jection, spray break-up, and mixture formation in internal combustion engines.

Contenu
Fundamentals of Mixture Formation in Engines.- Basic Equations.- Modeling Spray and Mixture Formation.- Grid Dependencies.- Modern Concepts.- Conclusions.