Thermodynamic Analysis of SI Engine Operation on Variable Composition Biogas-Hydrogen Blends Using a Quasi-Dimensional, Multi-Zone Combustion Model 2009-01-0931

In this work, a quasi-dimensional, multi-zone combustion model is analytically presented, for the prediction of performance and nitric oxide (NO) emissions of a homogeneous charge spark ignition (SI) engine, fueled with biogas-H₂ blends of variable composition. The combustion model is incorporated into a closed cycle simulation code, which is also fully described. Combustion is modeled on the basis of turbulent entrainment theory and flame stretch concepts. In this context, the entrainment speed, by which unburned gas enters the flame region, is simulated by the turbulent burning velocity of a flamelet model. A flame stretch submodel is also included, in order to assess the flame response on the combined effects of curvature, turbulent strain and nonunity Lewis number mixture. As far as the burned gas is concerned, this is treated using a multi-zone thermodynamic formulation, to account for the spatial distribution of temperature and NO concentration inside the burned volume.

The simulation code is applied to published experimental data of a SI engine operated on variable composition biogas-H₂ mixtures. It is revealed that the model is able to capture the effect of the fuel parameter satisfactorily in terms of both engine performance and NO emissions. It is concluded that the addition of increasing amounts of H₂ in biogas results in a decrease of the flame-development period, while the increase in the combustion velocities seems to be effective almost up to the point of 10% mass fraction burned (MFB). However, the course of heat release seems to be independent of the fuel case, being univocally determined by the geometry of the combustion chamber. Additionally, due to the very high mass diffusivity of H₂ and the positively stretched flames, it results that these always take advantage of stretch effects, enhancing their burning intensity. At the same time, the positioning of the investigated flames into the thin reaction zones regime provides insight into their phenomenology. Finally, detailed information is gained concerning the effect of H₂ enrichment of biogas on the temporal and spatial formation of NO inside the engine cylinder.