1996-10-01

An Assessment of Predictivity of CFD Computations of Combustion and Pollutants Formation in D.I. Diesel Engines 962055

In the present paper the status of development of diesel combustion and pollutants formation modelling at Diesel Engines and Fuels Research Division of Istituto Motori is pointed out. The main features and performances of the model are discussed comparing the numerical results with some experimental data.
For the experiments a single cylinder direct injection diesel engine was used. In the head of the engine two small quartz windows have been mounted, in order to obtain pictures of the injection and combustion processes by high speed cinematography, and to apply the two colour technique for soot temperature and soot loading measurements. The soot loading was measured by the two colour technique and the a priori and the experimental uncertainties of the measurement technique were carefully evaluated. In addition, the engine may be also equipped with a second head, in which a fast acting valve allows the direct sampling of the combustion products.
To simulate the diesel engine combustion, the Kiva II code, with some improvements, was used. In particular a hybrid model based on both WAVE and TAB models was adopted for jet break up modelling, and a spray wall impaction model was added to the original code.
New combustion and soot models were also set-up. The main features of the models are the following:
The liquid fuel injection vaporisation and dispersion is computed as provided by the Kiva II code routines; the fuel vapour undergoes conversion to acetylene by a single step reaction. The acetylene formed is oxidised at high temperature by a single step stochiometric reaction with mean rate defined as in standard E.B.U. combustion models.
Simultaneously with the acetylene combustion a three-step soot formation and oxidation model performs the soot loading computations for each computational cell jointly with a modified Zel'dovich mechanism for NO computations. The computational results demonstrate that the proposed model, represents the main features of the diesel combustion, despite the strong simplifications introduced. However, further work is required to obtain a satisfactory predictivity of multidimensional models of combustion and exhaust emission.

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