Computational assessment towards understanding the energy conversion and combustion process of lean mixtures in passive pre-chamber ignited engines

In this paper, a computational study was performed using a combination of several numerical tools to better understand the limiting aspects of combustion in a passive pre-chamber ignition system when operating at lean conditions. A specific methodology was developed to analyze in detail the scavenging and combustion processes of this ignition concept. Results show how the scavenging of passive pre-chambers is primarily dependent on the force that the piston makes on the gas during the compression stroke, being independent of the pre-chamber geometry as along as the ratio between the total cross sectional area of the pre-chamber holes and the prechamber volume is kept within a suitable range. Moreover, a successful lean combustion, with an air-to-fuel ratio around 2, cannot be achieved as the burning rates inside the pre-chamber significantly decrease due to the low laminar flame speeds, that results in low quality jets. Further results show that increasing the flow temperature can help to recover competitive combustion rates when knocking combustion is not a limiting factor. The contribution of the heat losses through the pre-chamber walls to the overall energy balance of the pre-chamber has been estimated, showing that their impact is negligible (< 5%). Alternatives for increasing the laminar flame speed were proposed in order to improve combustion inside the pre-chamber. Although the pre-chamber combustion profile was successfully improved, none of the proposed solutions were able to completely burn the main chamber charge with the current pre-chamber design.