Decarbonising the automotive sector requires powertrains capable of delivering cleaner and more efficient energy conversion processes. Low‑carbon gaseous fuels such as natural gas and hydrogen represent promising alternatives for spark‑ignition engines, as they facilitate more sustainable combustion while maintaining operational feasibility. Hydrogen, in particular, enables CO₂‑free combustion and can achieve net‑zero lifecycle emissions when produced from renewable sources. However, its inherently high reactivity and flame speed pose operational challenges, including an increased risk of backfiring, difficulties in controlling the combustion rate, and a higher propensity for engine knock, especially in stoichiometric mixtures. Blending hydrogen with natural gas can mitigate these limitations by reducing mixture reactivity and improving combustion stability, while preserving the low‑carbon characteristics of both fuels.

Despite their potential to lower CO₂ emissions, the combustion of natural‑gas–hydrogen mixtures may still generate pollutants such as CO, NOₓ and unburned hydrocarbons. Controlling these emissions requires a detailed understanding of how the combustion process governs pollutant formation, especially under varying fuel compositions and operating conditions. Furthermore, cleaner in‑cylinder conversion processes can simplify the design and improve the performance of after‑treatment systems, which must be tailored to the specific characteristics of hydrogen‑assisted combustion. However, the literature still lacks comprehensive studies describing the interplay between combustion dynamics and pollutant formation when using hydrogen–natural‑gas blends.

The main objective of this work is to characterise the combustion process in relation to the formation of CO and NOₓ emissions in a spark‑ignition engine operating with various hydrogen–natural‑gas mixtures. Experimental tests were carried out at three engine speeds (1000, 1750 and 2000 rpm), varying both the fuel–air equivalence ratio and the hydrogen fraction in the mixture. The engine was evaluated under three fuelling strategies: pure natural gas (ϕ = 0.7–1.0), pure hydrogen (ϕ = 0.3–0.7) and binary mixtures of natural gas and hydrogen. The experimental data were processed using a two‑zone thermodynamic diagnostic model and a kinetic diagnostic model, enabling a detailed analysis of both the combustion process and the formation of pollutant emissions.

The results show that, although adding hydrogen reduces overall carbon emissions, CO formation persists under stoichiometric conditions when the engine operates on natural gas. Increasing the hydrogen fraction leads to higher NO formation, primarily due to the faster energy release associated with hydrogen’s high laminar flame speed. Conversely, when the engine runs on 100% hydrogen, the use of lean mixtures with equivalence ratios below 0.5 almost completely suppresses NO emissions while maintaining stable engine operation. These findings provide valuable insight into the relationship between combustion evolution and pollutant formation, supporting the development of cleaner, more efficient energy‑conversion systems based on hydrogen‑enriched gaseous fuels.