Large-Eddy simulation of lean and ultra-lean combustion using advanced ignition modelling in a transparent combustion chamber engine

The need for Internal Combustion Engines (ICEs) to face near-future challenges of higher efficiency and reduced emissions is leading to a renewed interest towards lean-combustion. Several operational issues are associated to lean combustion, such as an abrupt increase of combustion cycle-by-cycle variability (CCV), leading to unbearable levels of Indicated Mean Effective Pressure (IMEP) variation and to misfiring cycles. Significant potential in the wide-scale establishment of lean combustion might come from Large Eddy Simulation (LES), which is able to elucidate the relationships between local physical processes (e.g. velocity magnitude, Air to Fuel Ratio (AFR), etc.) and early combustion progress (e.g. 1%) in unprecedented manners. To this aim, an improved ignition model for LES is proposed in the paper. Two premixed propane-air lean strategies are selected from the wide TCC-III database. A lean-stable (lambda = 1.10, also named lean) and lean-unstable (lambda = 1.43, also named ultra lean) conditions are simulated, highlighting the model capability to well reproduce the sudden rise in CCV for increased mixture dilution. Explanations are given for the observed behaviour and a hierarchical quest for CCV dominant factors is presented. Finally, the different role of local flow field is highlighted for the two cases, and the comparison of optical acquisitions of OH* emission against simulated flame iso-surface up to 1% burn duration reinforce the simulation fidelity. The study shows the investigation possibilities of innovative combustion strategies given by advanced LES simulations. The understanding of turbulent combustion dynamics and the knowledge of the related lean-burn instabilities are key enabler for the exploration of new efficient lean-burn operations.