Combustion phasing and emissions trade-offs of hydrogen-enriched methane in a spark-ignition engine: Fixed spark timing versus MBT-optimized ignition

This study experimentally investigated how hydrogen enrichment and spark timing jointly govern combustion, performance, and emissions in a spark-ignition engine fueled by different methane–hydrogen blends under fixed boundary conditions (11:1 compression ratio, 1500 rpm, 25% load). Methane was blended with hydrogen at different hydrogen fractions (HF) ranging from 20% to 80% by volume, and two ignition strategies were compared: (a) fixed spark timing of 30 °CA before top dead center (BTDC) for all fuel blends and (b) blend-specific optimized ignition timing (OPT-IG), selected by progressively retarding the spark until the crank angle at 50% mass fraction burned (CA50) fell within the adopted 5–10 °CA after top dead center (ATDC) near the maximum brake torque (MBT) window. The cycle-resolved in-cylinder pressure was measured and used to compute the heat-release rate (HRR) and mass fraction burned (MFB) profiles, from which the combustion phasing metrics CA10, CA50, and CA90 were extracted. Under a fixed 30 °CA BTDC timing, increasing HF accelerated flame development and shortened the main burn, advancing CA50 and shifting the peak pressure and heat release toward top dead center (TDC), which reduced brake efficiency and elevated NOx emissions. Additionally, volumetric efficiency decreased from 72% at HF20 to 65% at HF80, indicating a distinct charge-displacement penalty at high hydrogen content. The OPT-IG timing required progressively less spark advance with increasing HF, decreasing from 24 °CA BTDC at HF20 to 12 °CA BTDC at HF80, while maintaining CA50 at approximately 6–7 °CA ATDC within the adopted 5–10 °CA ATDC near-MBT window. At HF80, OPT-IG increased mean BMEP from about 249 to 263 kPa and mean BTE from 21.3% to 22.5%; both within-HF contrasts remained significant after Holm multiplicity adjustment (adjusted P < 0.001). It also reduced NOx from 4350 to 2530 ppm, corresponding to an approximately 42% reduction, although CO and HC increased because later combustion reduced the post-flame oxidation window. Overall, the results quantify the growing sensitivity of hydrogen-methane operation to the ignition strategy at higher hydrogen fractions and demonstrate that MBT calibration is essential to realize the benefits of hydrogen combustion without disproportionate efficiency or emission penalties.