Chemical modelling of autoignition

Current limitations in our understanding preclude a completely realistic and comprehensive mathematical model of engine autoignition and knock. Even if one were to be possible, the computing resource required would 

Cinefilms of knocking combustion by Withrow and Rassweiler [49] showed, as early as 1936, that in a knocking cycle the end gas autoignites 

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In 1985 Leppard [156] reported engine measurements, for stoichiometric ethane-air, of pressure and end gas temperature, the latter derived from the energy equation. The occurrence of autoignition agreed closely with prediction based on an earlier chemical model of Westbrook and Dryer [52]. From their engine experiments, Cowart et al. [59] also compared, for iso-octane and -pentane, the predictions of the simplified models of Hu and Keck [75] and Chun et al. [157], and the more detailed kinetic predictions of Westbrook et al. [158]. These were found to simulate the time of knock occurrence if the kinetic data were re-calibrated. This, and the subsequent work of Brussovansky et al. [76], showed the need for accurate allowances for heat transfer and piston blow-by, because of their important effect on the derived end gas temperature. Where end gas temperature can be measured directly this problem is circumvented. 

Soyhan, H. (2000). Chemical Kinetic Modelling of Autoignition Under Conditions Relevant to Knock in Spark Ignition Engines, Ph.D. Thesis, Istanbul Technical University. 

As has already been seen, these a priori lumped kinetic models account for the macroscopic properties of the reactions, such as cool flames, the delays of autoignition... but without being able to relate them to the chemical structure of the reactants. In other words, they are incapable of describing both qualitatively and quantitatively the formation of individual molecules, such as the toxic substances (carbon monoxide, aldehydes, butadiene, aromatics, PAHs, soot) or the tropospheric pollutants (nitrogen and sulphur oxides, unburnt hydrocarbons, various oxygenated compounds), produced by the burning of fuels. There is therefore a strong requirement to develop detailed reaction mechanisms, likely to predict both the kinetic and chemical characteristics of these reactions. 

Machrafi, H., Lombaert, K., Cavadias, S., Guibert, P., Amoraoux, J. Reduced chemical reaction mechanisms experimental and HCCT modelling investigations of autoignition processes of wo-octane in internal combustion engines. Fuel 84, 2330-2340 (2005)... 

Because of its large number of empirical parameters, this model can be fitted to a variety of fuels and can reproduce the important autoignition phenomena. This is accomplished with only a modest computational requirement and, consequently, it has been used in a number of engine modelling studies over the past 20 years. Its main deficiencies are the difficulty of extending it to new fuels (which has been done only by the original authors) and its still limited representation of the real chemistry. Because of the generic formulation, the species are rather ill-defined. It would be difficult to use, for instance, in a study of chemical octane requirement increase, discussed in Section 7.5.2. 

H. Li, D.H. Miller and N.P. Cernansky, Development of a Reduced Chemical Kinetic Model for Prediction of Preignition Reactivity and Autoignition of Primary Reference fuels, SAE Technical Paper 960498 (1996). 

Wamatz, J., Maas, U., Dibble, R.W. Combustion. Physical and Chemical Fundamentals, Modeling and Simulation, Experiments, Pollutant Formation, 4th edn. Springer, Berlin (2006) Zddor, J., Taatjes, C.A., Fernandes, R.X. Kinetics of elementary reactions in autoignition chemistry. Prog. Energy Combust. Sci. 37, 371 (2011)... 

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