C.A.R.S. temperature measurements and chemical kinetic modelling of autoignition in a methanol-fuelled internal combustion engine

Doctoral Thesis


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University of Cape Town

The temperature inside the cylinder of a methanol-fuelled single-cylinder Ricardo E6 research engine running under knocking conditions, is measured by means of Coherent Anti-Stokes Raman Spectroscopy (CARS), and the pressure is measured with a pressure transducer. In order to obviate any errors arising from deficiencies in the spectral scaling laws which are commonly used to represent nitrogen Q-branch spectra at high pressure, a purely experimental technique is employed to derive temperatures from CARS spectra by cross-correlation with a reference library of spectra recorded in an accurately calibrated high-pressure high-temperature optical cell. The temperature and pressure profiles obtained from the engine running under knocking conditions, are then used as input data for chemical kinetic modelling of end-gas autoignition. Five published mechanisms (Grotheer et al 1992, Grotheer and Kelm 1989, Norton and Dryer 1989, Dove and Warnatz 1983, .and Esser and Warnatz 1987) are used in the autoignition study, and the results for the different mechanisms are compared. A good qualitative understanding of the mechanism underlying end-gas autoignition in the engine is obtained, although the calculated autoignition points occur slightly earlier than the observed point. A sensitivity analysis of the methanol autoignition system is undertaken, and the importance of the decomposition of hydrogen peroxide and the hydroperoxyl chemistry is demonstrated. The discrepancies between the predicted results of the different mechanisms is shown to be caused by a small number of sensitive reactions for which there are conflicting data. Finally, a linear mode analysis from the geometric qualitative theory of differential equations is performed on the non-linear chemical rate equations. The equilibrium points in the generalised phase space of the non-linear chemical system are shown to be defined in terms of three regions. The equilibrium points are unstable improper nodes in the first region (T < ll00K), unstable focii in the second region ( 1100K<T<1170K), and stable focii in the third region (T> 1170K).

Bibliography: p. 258-268.