Abstract
Nowadays, soot emissions are one of the major concerns in Direct Injection Spark Ignition engines. Soot prediction models can be computationally expensive, especially when particle mass, number, and size distribution are to be forecast. While soot formation heavily depends on the chemical and physical characteristics of the fuel, the simulation of the exact composition of a real gasoline is computationally unfeasible. Thus, it is essential to find simplified yet representative pathways to reduce the computational cost of the simulations. On the one hand, the a-priori investigation of the factors influencing particulate onset can be a simplified approach to compare different solutions and strategies with much cheaper costs than the modelling of soot formation and oxidation mechanisms. On the other hand, the use of surrogate fuels is a practical approach to cope with the fuel chemical nature. Although they poorly mimic the evaporation properties of a real gasoline, Toluene Reference Fuels are broadly adopted to match combustion relevant properties of the real fuels. In this study, the spatial distribution of the Threshold Soot Index in the fluid domain is investigated for three surrogates characterized by an increasing content of toluene (0 mol%, 30 mol%, 60 mol%). The correlation between the sooting tendency and the fuel distribution in the combustion chamber before spark ignition time can provide useful preliminary indications, without spending the computational effort of the whole soot model multicycle resolution. In particular, two approaches for the lagrangian description of the injected fuel are investigated: a multicomponent approach and a single component one, this last driven by a high-fidelity lumped modelling of the surrogate properties for both liquid and vapor phase.
Highlights
Alternative fuels, innovative combustion processes and aftertreatment systems are the predominant research fields to further develop Internal Combustion Engines (ICEs) amid the propulsion system innovation process
Emissions in ICEs can be investigated with both experiments at the test bench and Computational Fluid Dynamics (CFD): accurate description of flow field [5], spray [6][7], mixing [8], combustion [9][10], knock [11][12], and emission formation [13] can be achieved with advanced models
The three investigated blends are characterized by Research Octane Number (RON), Motor Octane Number (MON), and αst values very similar to those of a commercial gasoline, while the sooting tendency is characterized by largely different Threshold Soot Index (TSI), which are increased by ~10 points from one surrogate to another by varying the toluene content
Summary
Alternative fuels, innovative combustion processes and aftertreatment systems are the predominant research fields to further develop Internal Combustion Engines (ICEs) amid the propulsion system innovation process. The three investigated blends are characterized by RON, MON, and αst values very similar to those of a commercial gasoline, while the sooting tendency is characterized by largely different TSIs, which are increased by ~10 points from one surrogate to another by varying the toluene content. In this way, the TSI distribution in the combustion domain can be tested for a wide range of aromatic contents. The effectiveness of a lumped fuel-modelling approach is critically investigated by evaluating the predictions of fuel and TSI distributions at spark time, to spot to what extent the lumped-single component approach is reliable and able to predict charge stratification and TSI distribution compared to a more refined, yet more expensive, multicomponent fuel modelling approach
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