The use of CNG as a fuel for SI engines represents a feasible solution to improve engine fuel economy (so reducing also CO2 emissions) and to face with short term urban pollution. To full exploit its potential is, however, necessary to tailor engine characteristics to this gaseous fuel in order to optimize its performances. Numerical simulation of these engines plays an important role in the design procedure, but specific problems related to CNG combustion have to be solved. In the present paper this task is performed by means of a modified version of KIVA-3 with respect to the combustion model formulation. The modified combustion model formulation is based on flamelet hypothesis, and constitutes an extension of the classical BML concept. Its formulation avoids any explicit dependence on turbulent time (different from the traditional approach based on “characteristic times”), by modeling length scales on the basis of laminar flames properties. The numerical results have been compared with the experimental ones carried out at the CNR Istituto Motori of Naples under different engine loads, engine speeds and air/fuel mixture conditions. The results obtained are really encouraging, even under lean conditions when the interaction between turbulent flow field and the involved combustion phenomena becomes more complex.

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