Hydrogen-Assisted Extension of the Lean Operating Limit of a Small-Bore Air-Cooled Gasoline Direct-Injection Engine: Combustion Stability, Efficiency Optimization, and Emissions

Extending the lean operating limit of small-bore gasoline direct-injection (GDI) engines offers a practical pathway to improve fuel conversion efficiency in the light-duty transportation segment; however, deteriorated flame propagation in lean gasoline limits stable operation. The present investigation experimentally examines green hydrogen enrichment as an enabler of extended lean operation in a 390 cm3 single-cylinder air-cooled GDI engine at excess-air ratios (λ) of 1.15 and 1.22, with hydrogen energy fractions of 0–5.8% inducted through the intake manifold, at 3000 rpm, wide-open throttle, and maximum-brake-torque spark timing. Under pure-gasoline operation at λ = 1.22, the coefficient of variation of indicated mean effective pressure reached 5.23%, surpassing the 5% limit for stable combustion, while adding just 1.5% hydrogen was sufficient to bring the engine back to stable operating conditions. When hydrogen enrichment was raised to its upper limit of 5.8% at the same lean condition, peak in-cylinder pressure rose by 15%, the flame-development duration shortened by 20%, and combustion phasing returned to the target window of 8–12 °CA aTDC. In addition, brake thermal efficiency climbed from 26.55% to 29.25%. In terms of emissions, CO and HC decreased by 53.6% and 21.4%, respectively, while NOx increased moderately by 8.3%. However, the brake-specific CO, HC, and NOx decreased by 65.4%, 41.4%, and 19.3%, respectively; the NOx reduction arose despite a modest rise in the measured concentration. Therefore, the 5.8% hydrogen addition at λ = 1.22 is the best-performing operating condition, demonstrating the balanced nature of hydrogen-assisted lean-burn combustion, where hydrogen restores combustion quality while extended leaning controls the thermal and emission penalties.