Development and Experimental Validation of a Combustion Model with Detailed Chemistry for Knock Predictions

Aim of this work is to develop a general purpose model for combustion and knocking prediction in SI engines, by coupling a thermo-fluid dynamic model for engine simulation with a general detailed kinetic scheme, including the low-temperature oxidation mechanism, for the prediction of the auto-ignition behavior of hydrocarbons. A quasi-D approach is used to describe the in-cylinder thermodynamic processes, applying the conservation of mass and energy over the cylinder volume, modeled as a single open system. The complex chemistry model has been embedded into the code, by using the same integration algorithm for the conservation equations and the reacting species, and taking into account their mutual interaction in the energy balance. A flame area evolution predictive approach is used to evaluate the turbulent flame front propagation as function of the engine operating parameters.

The experimental activity was carried out in the combustion chamber of an optically accessible, single-cylinder S.I., P.F.I. engine, equipped with a commercial head. Experimental data basically consisted of optical measurements correlated to the combustion and auto-ignition processes within the cylinder. Optical measurements were based on 2D digital imaging, UV-visible natural emission spectroscopy and chemiluminescence of radical species (OH and HCO). Optical investigations were used to characterize the combustion process both in normal and knocking conditions. Theoretical and experimental analyses allowed to fully characterize the flame structure and the propagation speed as well. In order to estimate the prediction accuracy and reliability of the numerical procedure, different spark timings and fuels (pure iso-octane and mixture of n-heptane with iso-octane and toluene) were considered.