The Impact of Pre-Chamber Design on Part Load Efficiency and Emissions of a Miller Cycle Light Duty Gasoline Engine

The efficiency and emission potential of pre-chamber combustion in a Miller cycle light duty gasoline engine operated under part load was evaluated. Several pre-chamber designs that examine the engine performance tradeoffs with nozzle diameter, pre-chamber volume, number of nozzles, and pre-chamber fuel enrichment were investigated for both excess air and cooled external EGR dilution strategies. The introduction of pre-chamber jet ignition was observed to significantly reduce the main-chamber combustion duration while reducing cyclic variability under dilute conditions, benefiting from the long-reach ignition jets and enhanced turbulence. However, the pre-chamber design that provided the fastest combustion led to reduced brake efficiency primarily due to increased wall heat loss. Maintaining the total nozzle area while increasing the number of nozzles was identified as a means to minimize the additional heat loss and maintain fast burn rates. In addition, fuel enrichment within the pre-chamber was observed to extend the lean limit while greatly reducing engine-out NOx emission dependent on specific pre-chamber geometry. Specifically, for the cooled EGR calibration strategy, the engine’s dilution tolerance was determined to be primarily affected by the scavenging performance of the passive pre-chamber. For all designs evaluated, the inferior scavenging and increased heat loss associated with passive pre-chamber resulted in similar efficiency when compared to a well-designed spark-ignition engine operating under part-load conditions and with cooled EGR dilution.