Hydrogen-assisted lean combustion represents a promising approach for improving the environmental performance of spark-ignition engines while maintaining high energy efficiency. This study presents an environmental assessment and operating window identification of a gasoline direct injection engine equipped with hydrogen port fuel injection under lean and ultra-lean operating conditions. Hydrogen energy shares ranging from 0% to 40% were investigated with respect to brake thermal efficiency, combustion stability expressed by the coefficient of variation of indicated mean effective pressure (COV IMEP), and gaseous emissions including carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides (NOₓ). The results show that hydrogen enrichment improves thermal efficiency, significantly reduces CO and HC emissions, and extends the stable lean-burn operating range beyond λ = 2.0. Furthermore, ultra-lean operation contributed to the suppression of NOₓ formation while maintaining stable combustion. An integrated environmental evaluation identified a favourable operating window corresponding to hydrogen energy shares of approximately 30–40%, where the best balance between efficiency, emission reduction, and combustion stability was achieved. The identified operating conditions provide a valuable basis for the future development of advanced combustion control and fuel injection management strategies aimed at minimizing environmental impacts.
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