Investigation of the prechamber geometrical configuration of a natural gas spark ignition engine for cogeneration: part. I, numerical simulation
The effects and the consequences of transferring the ignition point of the conventional combustion chamber into a small unscavenged prechamber were evaluted through a numerical simulation based on the CFD-code KIVA- 3V. The prechamber flow characteristics at the location of the gap between the spark plug electrodes and in the crank angle period where the ignition is expected to occur were compared to the corresponding conditions in the conventional combustion chamber. The influence of the prechamber geometrical configuration was evaluted through variations of the nozzle orifice diameter, number and orientation, as well as prechamber volume and internal shape. The results show that the velocity magnitude is mainly dependent on the prechamber shape, and that it is generally of the same order as with direct ignition. Further, the turbulence intensity varies strongly with the geometrical configuration and reaches, in most cases, a much higher value in the prechamber. The partial dilution of the unburnt mixture with unscavenged prechamber residual gas generally leads to a slightly lower fuel to air equivalence ratio. However, nozzle orifices imparting a swirl motion or a prechamber shape with an almost uniform cross section can result in fuel concentration very close to or beyond the flammability limit. The prechamber charge and thereby the amount of energy for the main chamber ignition depends on the prechamber volume and on the pressure drop across the nozzle orifices.