Examples of technical systems

Reciprocating piston engines are the most common type of engines: the well-known cycle of a four-stroke engine (intake-compression-combustion-exhaust) is estimated to take place ca. 10^14 (100000000000000) times a day on earth.

It is clear that a ‘product’ made so widely, should be optimised using all available means. Once again, the principal questions arising here, namely those relating to efficiency and pollutant formation, can ultimately be answered only through deeper insight into the underlying processes of combustion.

The investigation of an internal combustion engine using laser diagnostic methods is one of the research areas pursued at the Institute of Technical Thermodynamics. At the heart of the experiment lies a simple internal combustion engine provided with glass windows, allowing a view from above into the combustion chamber and the introduction of a horizontal laser light section from the side (Fig. 11, left). The experiment is used e.g. for an investigation of chemical species that provide precise information about the processes taking place during ignition and combustion.  



Drawing of an engine with a glass cylinder head.

Laser-induced fluorescence signal from the engine’s combustion chamber in the presence of knocking. The direction of observation is from the top along the cylinder’s central axis. The colour scale indicates the fluorescence intensity of intermediate species (here e.g. formaldehyde), formed during the fuel’s chemical decomposition prior to ignition and broken down after ignition during the combustion itself.



Fig. 11 shows the spatial distribution of a typical intermediate product (formaldehyde, CH2O), formed during the long sequence of chemical reactions (cf. Fig. 1 on the left) and then broken down again. The phase of formaldehyde formation coincides with the fuel’s breakdown and the start of self-ignition; during the combustion itself, formaldehyde is broken down again very rapidly. Dark zones in the image correspond to burned waste gas, since no intermediate product is present here any longer. Bright zones are created immediately prior to combustion, where the fuel is undergoing chemical decomposition and reactive intermediate products are being formed for a short time.

The presence of hot spots, in which self-ignition takes place independently of the normal flame (which propagates into the combustion chamber from the spark plug, located at bottom right), becomes visible through dark spots in the unburned zone. Such investigations provide information about self-ignition processes in engines which in the near future could form the basis for new operating modes, namely for the HCCI (homogeneous charge compression ignition) mode and for the CAI (compression auto ignition) mode.



Summary and outlook

Combustion has been and remains one of the most important processes for the progress of our technological world. Whereas its essential utility, namely the generated heat, can be regarded from purely thermodynamic aspects, the causes of the well-known problems such as e.g. pollutant formation, can be understood only through detailed consideration of the underlying chemical and physical processes. These are mainly the complex course of elementary chemical reactions, and the interaction of these reactions with physical processes such as turbulent mixing of substance flows and transport phenomena. Whilst the fundamental physical and chemical equations are known in principle, their complete numerical analysis for the purpose of predictive simulations of technical combustion processes is impracticable. In addition, these equations depend on many numerical parameters, which often are not yet known with sufficient accuracy.

Nonetheless, ever more precise simulation methods are needed now, let alone in the future, for the development of clean combustion systems. The most pressing tasks of combustion research, therefore, include the development of efficient but still sufficiently accurate methods for describing and modelling combustion processes. Experience shows that an efficient and accurate description of combustion processes is possible despite its intrinsic complexity. Paradoxically, it is precisely a property that adds substantially to the complexity of combustion, namely the existence of processes with highly differing time-scales, that makes possible a significantly simplified description. Therefore, hierarchical modelling concepts are the method of choice where technical combustion systems are to be described by means of models with predictive power. Through a combination of laser diagnostic methods and modern mathematical modelling strategies, the Institute of Technical Thermodynamics is contributing to the development of such hierarchical models.  



Literature: Books for combustion
  • J. Warnatz, U. Maas, R. W. Dibble, Combustion, Springer, ISBN 3-540-42128-9, (2006)
  • N. Peters, Turbulent Combustion, Cambridge University Press, ISBN 0-521-66082-3, (2000)
  • S. R. Turns, An Introduction to Combustion, McGraw-Hill, 2nd ed., ISBN 978-0071169103, (2000)






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