Hydrogen enrichment of compressed natural gas (HCNG) was investigated in a spark-ignition engine using an experimental framework to examine the combined effects of blend composition, load, knock limitation, and lean operability. The study comprised a baseline CNG spark sweep, stoichiometric hydrogen-fraction sweeps from 19.5 to 100 vol% at 3 bar, 6 bar, and full load, and lean-limit exploration. The baseline sweep assessed spark-timing effects on efficiency, peak pressure, and NO x emissions, while the parametric tests were conducted at CA50 ≈ 8 ° aTDCf. Under stoichiometric operation, hydrogen shortened flame development by up to 73.1% and combustion duration by up to 72.9%. Carbon-based emissions decreased, whereas pressure-rise rates and knock propensity increased; full load became knock-limited above 68% H 2 . In lean sweeps, hydrogen extended the stable λ range, and the results show that the most favorable HCNG blend depends on operating condition, supporting variable dual-fuel metering. • High load requires more CNG to suppress knock and maintain volumetric energy. • The optimum H2 fraction depends strongly on load rather than a fixed blend ratio. • NOx can drop to approximately zero with hydrogen enrichment in lean burn. • Efficiency optimum occurs near the λ where NOx approaches zero. • Hydrogen extends the practical lean operating window of CNG combustion.
MARTINS et al. (Mon,) studied this question.