(2-18) A Refined Two-Zone Heat Release Model for Combustion Analysis in SI Engines((SI-6)S. I. Engine Combustion 6-Modeling)

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The present work gives a further contribution to the heat release analysis of the measured cylinder pressure time-history, that is the most commonly used combustion diagnostic for determining the actual burning rate in spark-ignition engines. A critical examination was made of the thermodynamic models applied for such an analysis, paying specific attention to their calibration techniques, to the simplifications adopted and to the correlations used for evaluating the bulk gas-wall heat transfer. With respect to these latter, the convective surface-averaged heat flux is usually determined by a quasi-steady application of Newton's law, regarding the heat transfer coefficient as a function of the instantaneous flow properties as well as of geometric and operating engine variables. In a previous paper, the authors proposed and applied a more general complex-variable formulation of Newton's law of convection for modeling the instantaneous surface-averaged heat flux so as to take the unsteadiness effect of the gas-wall temperature difference into account. A refined two-zone heat release model for cylinder pressure-data reduction was developed and assessed. As in the former version of the model, the thermodynamic properties of both reactants and products were evaluated from JANAF tables with a multiple species equilibrium composition calculation performed for the burned zone. The novelties of the present model include the following improvements and their combined implementation. In addition to the new unsteady convection model, a CAD procedure was introduced in order to estimate the burned- and unburned-zone heat-transfer wall areas for assigned geometric features of the flame front, whereas the burned- and unburned-gas volumes are often used as weighting factors of their related heat fluxes in the global heat-transfer calculation. Furthermore, the energy conservation equation was applied to the unburned-gas zone instead of the isentropic relation that is commonly used for evaluating the temperature of the unbumed gas. A refinement was also included for the calibration procedure of the cumulative mass-fraction burned, or of the fuel-energy released, at the end of the flame propagation process. Usually, the predicted mass-fraction burned after combustion is completed is matched to the measured combustion efficiency by adjusting the heat-transfer correlation constants. More specifically, the amount by which the theoretical maximum mass-fraction burned is taken to be less than unity is set equal the unreleased energy fraction determined from either the measured HC or the measured HC, CO and H_2 in the engine exhaust. This lost energy is evaluated as a fraction of the total fuel energy available. In the proposed heat-release model, the heat-transfer calibration is made through an overall energy balance of the whole cylinder charge during the combustion process. The unreleased energy predicted at the end of the flame propagation is correlated to the combustion efficiency determined from the exhaust gas composition. The new heat-release model was applied to the analysis of the combustion parameters in a multivalve SI engine fueled by either gasoline or CNG under a significant sample of operating conditions. The model appeared to be an accurate tool of combustion diagnostic for SI engines, being also relevant to diesel engine application.
一般社団法人日本機械学会の論文
2001-07-01

(2-18) A Refined Two-Zone Heat Release Model for Combustion Analysis in SI Engines((SI-6)S. I. Engine Combustion 6-Modeling)

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