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Gas/surface interactions and damaging mechanisms in knocking combustion


Clarify the interactions between knocking combustion and the combustion chamber walls; mechanisms for wall erosion by knock will be derived. Thus a quantitative basis will be developed for improved design of combustion chambers of S.I. engines which are less sensitive to knock and which exhibit a wider tolerance for varying fuel qualities (e.g. for alternative less polluting fuels).
In a first phase the required experimental facilities and diagnostic techniques have been installed and developed: these included 4 optical research engines at 4 different sites, a knock damage simulator, 2 rapid compression machines, a high pressure/temperature chemical flow reactor, a novel LIF technique for identification of formaldehyde, ultra high speed schlieren photography (up to 2 million fps), ultra fast surface temperature measurement, a modeless CARS system and a particle tracking technique.

Effective simultaneous experimental and theoretical work was established as the basis for improved understanding of the physical and reaction chemical process of knock initiation, propagation and knock induced gas/surface interactions. Improved detailed reaction kinetic mechanism and a global autoignition scheme have been generated. Based on these results computer simulation programs have been adapted and developed for: a 3D engine cycle simulation, a 1D reactive flow modelling using detailed chemistry, a 2D reactive flow in-cylinder pressure wave interaction model and a detailed 3D model for simulating reactive crevice flow and boundary layer interactions as well as the associated wall surface loading. The codes provided valuable numerical results.

Based on the concept of exothermic centres a unified theory of knock has been developed which gives now for the first time a consistent explanation of all physical and chemical observations. Knock is initiated by autoignition at localized 'exothermic centres', which may arise due to appreciable temperature and/or compositional inhomogeneities. The severity of knock is controlled by the mode of reactive gas dynamic coupling, which itself is determined by the thermochemical state of the end-gas, especially the rate of energy release, the degree of end-gas inhomogeneity and particularly the exothermic centre interactions by pressure waves. Under critical conditions this leads to a developing detonative combustion. Also, for this most severe mode of knock, the mechanisms of the relevant interactions between flow and wall surfaces causing excessive material loading and erosive knock damage have been clarified by well defined theoretical and experimental investigations.

The results allow now a full understanding of the knocking process and form a solid basis for later derivation of measures to avoid knock. For practical improvement of modern engines, such analysis has to go along with specific engine design features. Some generally valid conclusions to extend the knock limit of SI engines have already been derived.
Work in this project will be divided as follows:

DB will build a knock simulator and a standardized optically accessible single cylinder engine for use with an ultra fast Schlieren visualization technique. Similar single cylinder engines will be modified for test studies of flow field diagnostics (University Leeds) and double pulse laser induced fluorescence techniques (University Stuttgart). SHELL, Thornton will study knock with a Hydra single cylinder engine using laser sheet techniques. The University of Trondheim will test and select a suitable engine combustion code from existing programmes (SALES, KIVA) in which reaction kinetics of the University of Stuttgart will be incorporated. The University of Lille, France will set up a rapid compression machine and a well stirred reactor to determine reaction rates of higher hydrocarbons which are needed for realistic modelling of knocking combustion.

The group will carry out the main tasks of this project by applying all experimental, diagnostics, theoretical and numerical tools in a systematic analysis of gas phase reactions and interactions with the combustion chamber walls. Operating conditions and details of research topics will be chosen for all participants after careful selection of relevant processes and full agreement in the working group. This will allow for an optimum coordination of work at the various laboratories.

A continuous evaluation process will be maintained throughout the second half of the project in order to come to a consistent and complete analysis of the results and a data base which is as complete as possible. The early start of evaluation will also aid to promote development and improvement of computer codes. This task will be carried out by all members of the working group.


Daimler-Benz AG
Epplestraße 225
70567 Stuttgart

Participants (6)

Shell Research Ltd
United Kingdom
Thornton Research Centre
CH1 3SH Chester
Pfaffenwaldring 12
70569 Stuttgart
Rue Paul Duez 42
59800 Lille
University of Leeds
United Kingdom
Woodhouse Lane
LS2 9JT Leeds
University of Trondheim
18,Kolbjörn Hejes Vei
7034 Trondheim
Université de Lille I (Université des Sciences et Technologies de Lille Flandres Artois)
Domaine Universitaire Scientifique
59655 Villeneuve D'ascq