Detailed prediction of HRR and NOx emissions in CI engines via a novel thermodynamic model with constant equivalence ratio zones

Abstract

This paper presents a novel, quasi dimensional-model for the simulation of the combustion process in compression-ignition engines. The model discretizes with a multiple number of zones the in-cylinder air-fuel mass on fixed values of local equivalence ratio, with the charge stratification determined from a 2D reconstruction via a one-dimensional control-volume-based spray approach. Reacting multi-zones are further split into three conceptual sub-zones: liquid, unburnt vapor and burnt vapor, in which chemical reactions proceed according to a tabulated kinetics of ignition (TKI) model. This approach provides a simple methodological framework for the combustion of direct injection of virtually every kind of liquid fuel and relies on a detailed phenomenological chemical/physical link of jet’s reacting phenomena. To account for engine geometry, a spray-wall interaction sub-model has been added to the axial spray. The model has been validated against experimental data and detailed CFD simulation results. First, the direct injection model (as a free jet) has been assessed with respect to ECN sprays A, C, and D experiments. For both the long injection and split injection cases, all the combustion phases are well predicted: premixed peak, mixing-controlled combustion, burn-out are seamlessly described and in good agreement with experimental and detailed CFD data. The wall interaction sub-model was validated with a suitable experiment where a reacting jet impinges on a mock-up wall inside a combustion vessel. Finally, the model has been validated against real heavy-duty engine experimental observations of 151 points of a complete engine map. The tuning of the model consists in two parameters, that are engine-specific, hence constant for the whole map. With these assumptions, the AHRR traces are well described in all simulated conditions. Mean predicted BSFC is only slightly underestimated (−2.3%), as it is the NOx production (−1.6%).