Effect of Hydrogen Enrichment on Flame Morphology and Combustion Evolution in a SI Engine Under Lean Burn Conditions

Uncertainty of fuel supply in the energy sector and environmental protection concerns have motivated studies on clean and renewable alternative fuels for vehicles as well as stationary applications. Among all fuel candidates, hydrogen is generally believed to be a promising alternative, with significant potential for a wide range of operating conditions. In this study, a comparison was carried out between CH4, two CH4/H2 blends and two mixtures of CO and H2, the last one taken as a reference composition representative of syngas. It is imperative to fully understand and characterize how these fuels behave in various conditions. In particular, a deep knowledge of how hydrogen concentrations affect the combustion process is necessary, given that it represents a fundamental issue for the optimization of internal combustion engines. To this aim, flame morphology and combustion stability were studied in a SI engine under lean burn conditions. The engine was fuelled with CH4, CH4/H2 (75-25%vol and 50-50%vol) and H2/CO (50-50%vol and 75-25%vol). The engine was operated at fixed rotational speed and wide open throttle. Lean operation was studied in detail through combined methodologies based on thermodynamic analysis and optical diagnostics. Specifically, cycle resolved UV-visible digital imaging was applied to follow flame front propagation. Image processing was applied to evaluate flame speed and other morphology parameters, including flame displacement and centroid motion. Moreover, a detailed study of local curvature was presented. The excess air ratio was raised from 1.4, to values close to the flammability limit for each fuel. In order to maintain roughly the same fluid dynamic conditions (swirl, tumble, turbulence intensity, among others) spark timing was set according to the maximum brake torque of the baseline case (CH4) in the condition of lambda (?) 1.4.