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Hydrogen as potential engine fuel can appear either as a single gas or as a component in processing gases e.g. syngas, hythane and coke gas. The research in this paper investigates impact of combustible mixture dilution on abnormal combustion called knock in the reciprocating internal combustion engine. Dilution can be realized by either exhaust gas recirculation (EGR) or making the combustible mixture lean. Novelty of this work is a new metrics defined as dilution ratio, which makes it possible to compare knock reduction caused by either EGR or leaning the air-gas mixture to the engine. Two gaseous fuels were investigated: hydrogen and coke gas with 65% hydrogen. Conclusion based on the proposed dilution ratio states that, for hydrogen as the fuel, applying EGR is more effective in knock reduction than making the mixture lean. It was found that EGR strategy in the hydrogen fueled engine can reduce knock intensity from initial 40 kPa–20 kPa, whereas by leaning the mixture to the same dilution ratio, the knock is reduced to approximately 28 kPa. With respect to coke gas, it is proved that both EGR and lean mixtures influence on knock reduction at the same strength. • Combustible mixture dilution impact on combustion knock intensity. • Dilution ratio as a metrics for both exhaust gas recirculation and lambda. • Engine knock reduced more effectively by applying exhaust gases recirculation. • The correlation between dilution ratio and knock intensity is almost linear. [ABSTRACT FROM AUTHOR]