Development of Chemistry-Based Laminar Flame Speed Correlation for Part-Load SI Conditions and Validation in a GDI Research Engine

The detailed study of part-load conditions is essential to characterize engine-out emissions in key operating conditions. The relevance of part-load operation is further emphasized by the recent regulation such as the new WLTP standard. The combustion development at part-load operations depends on a complex interplay between moderate turbulence levels (low engine speed and tumble ratio), low in-cylinder pressure and temperature and stoichiometric-to-lean mixture quality (to maximize fuel efficiency at partial loads). From a modelling standpoint, the reduced turbulence intensity compared to full-load operations complicates the interaction between different sub-models (e.g. re-consideration of the flamelet hypothesis adopted by common combustion models). In this paper, the authors focus on chemistry-based simulations for laminar flame speed of gasoline surrogates at conditions typical of part-load operations. The analysis is an extension of a previous study focused on full-load operations of a methodology based on detailed chemistry 1D simulations of the flame structure. The comparison with the previous research reveals that flames at partial loads experience analogous temperature levels, despite the generally lower pressure. Therefore, particular attention will be devoted to the temperature scaling of flame speed, as well as to the extension to lean mixtures. The proposed correlation is applied to simulate the combustion development on a single-cylinder research engine operated at a 0.7 bar absolute pressure part-load condition provided with an optical access to the combustion chamber. The experimental data derived by the aforementioned kind of equipment allows a detailed description of the flame development since early flame kernel growth and, therefore, the role of an accurate laminar flame speed modelling is discussed in details. The correlation for laminar flame speed proposed by the authors constitutes a useful reference for similar studies and it can be used in conjunction with the most common CFD combustion models.