Modeling multi-component fuel evaporation, flame propagation, and chemical kinetics processes for GDI engines

Yang, S. Modeling Multi-Component Fuel Evaporation, Flame Propagation, and Chemical Kinetics Processes for GDI Engines. University of Wisconsin-Madison, 2010.

The present work focused on improving combustion sub-models for modeling gasoline engines and on developing a new approach for modeling realistic fuel vaporization processes.

The present improved combustion sub-models are more precise and fundamentally based: (1) A transport equation residual model was introduced that differentiates between CO2 and H2O from the previous engine cycle or EGR and that which is from the combustion products of the current engine cycle. (2) A Damkohler criterion model was introduced to evaluate whether the combustion is locally controlled by flame propagation or by volumetric heat release for each flame containing cell. (3) An improved “primary heat release” calculation model that more precisely considers the chemical kinetics heat release in unburned regions of flame-containing cells was formulated. (4) A model for flame quench processes in lean mixtures was developed, and finally, (5) An integrated model was developed and used to simulate the combustion process in a Gasoline Turbocharged Direct Injection (GTDI) engine.

An existing continuous multi-component (CMC) fuel evaporation model was integrated with the improved combustion sub-models. However, the current continuous multi-component fuel model considers the source terms contributed by chemistry in the mean and the second moment transport equations and is shown to be more accurate than previous models that neglect this coupling. A “PRF adaptive” method was proposed that formulates a relationship between the fuel vapor mixture PRF number and the fuel vapor mixture composition.

A discrete multi-component (DMC) fuel evaporation model formalism was integrated with the improved combustion sub-models. In the integration, a blending cetane number approach was introduced where the cetane number is assumed to be a linear combination of the cetane numbers of the components.

A new DCMC fuel vaporization model was developed, and was implemented into a multi-dimensional CFD code. In the model, each family of hydrocarbons in a real hydrocarbon fuel was modeled as a PDF function of molecular weight, and the DCMC model can thus successfully distinguish the evaporation of components in different hydrocarbon families that have similar molecular weights. Some new features of the vaporization of realistic fuels were revealed using the DCMC model.