Development of Zonal Models for Analysis of Engine Knock

University dissertation from Shahrokh Hajireza, Dösvägen 17, II 226 54 Lund, tel.+46-46-307656

Abstract: Autoignition in SI engines is an abnormal combustion mode and may lead to engine knock which is an undesired phenomenon in SI engines. It may cause damage and it is a source of noise in engines. Knock limits the compression ratio of the engine and a low compression ratio means low fuel conversion efficiency of the engine. This thesis presents a theoretical investigation about the modeling of the end gas behavior in an SI engine as it is affected by the ongoing physical and chemical processes. The modeling establishes a combination of thermodynamics, chemical kinetics, heat and mass transfer and gas dynamics. The modeling has been concentrated on two directions: for spatially homogeneous and inhomogeneous gas mixtures, respectively. Zero-dimensional zonal models have been developed for spatially homogeneous mixtures of primary reference fuels (PRF) of n-heptane and iso-octane. These models enable studies of transient behavior of the gas with detailed chemistry for such complex fuels. In zonal models, the gas volume in the combustion chamber is divided into a number of control volumes or zones in order to compensate for the spatial variation of quantities. Each zone is assumed to be a homogeneous mixture with a uniform temperature, mole fraction and mass fraction of species. The spatial variation of the pressure is neglected, i.e., it is assumed to be the same in the whole combustion chamber at every instant of time. The ordinary differential equations of energy and species were derived and solved. The occurrence of homogeneous autoignition was studied in each zone as function of different engine and fuel parameters. The calculated temperature profiles were in a good agreement with corresponding experimental results. After developing the first kind of zonal models, a two-dimensional model was developed with a simpler chemical kinetic mechanism in the compressed end gas region. The preliminary results for studies of inhomogeneity were obtained and as a more sophisticated model, the second group of zonal model was developed for engine related conditions. Another kind of zonal models was also developed in which the gas in the combustion chamber was divided into three zones. In that model, the burned gas and the unburned gas in the core region were assumed as homogeneous mixtures and the unburned gas in the region close to the wall was modeled in the way that inhomogeneity was considered in one dimension. The laminar, compressible, unsteady flow and thermal fields were solved by an implicit method. A detailed chemical kinetic mechanism for the mixture of n-heptane and iso-octane was included in the model. The autoignition history, development of hot spots and the propagation of reaction fronts were simulated. The results showed that the inhomogeneous initial fields of temperature, oxidizer and fuel caused the hot-spot autoignition or development of exothermic centers. In addition, ignition of the hot spots lead the propagation of reaction fronts to the surroundings. The propagation velocities of the reaction fronts were estimated.

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