A numerical study on effects of pre-chamber syngas reactivity on hot jet ignition

Ignition by a pre-chamber generated hot jet is a promising technology to reduce NOx emissions through extending lean burn limits. However, the role of the pre-chamber fuel reactivity in the main chamber ignition still remains unclear. In the present study, effects of the pre-chamber fuel reactivity on ignition of mixture in a main chamber by the hot jet, which is generated by the combustion of syngas in the pre-chamber, are numerically investigated. In the investigation, different ratios of CO/H₂ of syngas are considered under a thermally-equal condition. CFD simulations are performed using the code based on the KIVA-3V release 2 program coupled with an in-house developed chemical solver. A detailed chemical kinetics mechanism with 15 species and 41 reactions is adopted for the hydrogen and syngas oxidation. The hot jet ignition delay time is characterized by the maximum rate-of-change of the pressure in the main chamber. For the combustion initiation process, three stages are identified: ignition stage (stage I), flame development stage (stage II), and flame propagation stage (stage III). Significant effects of fuel reactivity are only observed on stage I that the ignition delay time considerably increases with increasing CO/H₂ ratio. Further analyses of the local temperature of the hot gas and some important elementary reactions indicate that the low reaction rate of carbon monoxide oxidation (R22: CO + OH = H + CO₂) causes incomplete fuel conversion in the orifice due to insufficiently available time for the reactions. The fuel conversion analysis confirms that the small-diameter orifice effect is the main reason for hot jet temperature drops under low pre-chamber syngas reactivity conditions, exhibiting as inhibiting effects on stage I.