Development of a New Multi-Zone Model for the Description of Physical Processes in HCCI Engines

Homogeneous Charge Compression Ignition (HCCI) engines have the potential of reducing NOx emissions as compared to conventional Diesel or SI engines. Soot emissions are also very low due to the premixed nature of combustion. However, the unburned hydrocarbon emissions are relatively high and the same holds for CO emissions. The formation of these pollutants, for a given fuel, is strongly affected by the temperature distribution as well as by the charge motion within the engine cylinder. The foregoing physical mechanisms determine the local ignition timing and burning rate of the charge affecting engine efficiency, performance and stability. Obviously the success of any model describing HCCI combustion depends on its ability to describe adequately both the chemistry of combustion and the physical phenomena, i.e. heat and mass transfer within the cylinder charge. In the present study a multi-zone model is developed to describe the heat and mass transfer mechanism within the cylinder. The main objective is to determine the temperature of each zone during the processes of compression, combustion and expansion. This will provide in-cylinder temperature distribution that obviously determines the ignition angle and combustion rate of the charge. The position and configuration of each zone relative to the others and to the engine cylinder is determined by geometrical considerations. The zones are allowed to exchange heat based on the local gas temperature difference. Mass transfer between zones is also considered as imposed by the motion of the piston. For the prediction of charge ignition and combustion rate a simplified model is used, to evaluate the behavior of the present model under firing conditions. Moreover the effect of equivalence ratio, initial charge temperature and compression ratio on the temperature distribution and mass transfer is examined. Thus the goal of the present work is not to concentrate on the chemical kinetics but to develop a phenomenological model capable of providing a realistic distribution of the charge temperature. This model can then be coupled to models describing accurately the reaction mechanism.