Effects of H2/CO addition on knock tendency and lean limit in a natural gas SI engine

Dedicated exhaust gas recirculation is an attractive way to improve the performances of internal combustion engines. Hydrogen (H-2) and carbon monoxide (CO) are primary combustible components in the reforming cylinder. Detailed knowledge of the effects of H-2/CO on knock tendency and intensity and even lean limit is essential for the better utilization of this sort of technologies. In this paper, the influences of H-2/CO on knock characteristics and lean limit are experimentally investigated. The experimental results indicate that the lean limit could be extended by 5-10% with 1% and 2% H-2/CO addition, respectively and meanwhile their addition significantly decreases THC and CO emissions despite of the slight increase in NOx emission. The knock-limited spark advance angles are determined using a pressure-based method. H-2/CO addition significantly increases knock tendency and intensity due to the increased energy density and faster flame propagation speed. Knock intensity increases linearly as knock onset angle is advanced towards to top dead center (TDC) due to higher end-gas temperature. However, the increase in knock intensity caused by H-2/CO addition is closely related to faster flame propagation speed. The sensitivity analysis reveals the crucial reaction in promoting flame propagation with H-2/CO addition is the branching reaction H+O-2=O+OH. And their addition also increases the reaction rates of another two dominating reactions CH3+HO2=CH3O+OH and CO+OH=CO2+H. 1% and 2% H-2/CO addition extend lower equivalence ratio limits by 14% and 26% which is beneficial for further efficiency improvement and emission reduction.