A Fundamental Model for Predicting Fuel Consumption, NO_x and HC Emissions of the Conventional Spark-Ignited Engine

Abstract

A model of the four-stroke S.I. engine cycle has been developed which predicts fuel consumption, NO_x and HC emissions as a function of engine design and operating conditions. The model is primarily thermodynamic in nature containing no formal spatial dependence. The major new features of the model are: first, a treatment of heat transfer which confines heat losses to a boundary layer region surrounding a central adiabatic core; second, an integral boundary-layer analysis of in-cylinder burnup of quenched hydrocarbons; and third, a calculation of exhaust port HC oxidation which considers the temperature history of each element of gas leaving the cylinder. The main adjustable parameters of the model relate to the rate of heat transfer and the ratio of the two-plate quench to the single-wall thickness. An extensive comparison of model predictions with experimental CFR engine data is presented. The results show excellent agreement between predicted and experimental fuel consumption and NO_x emissions. Predicted trends of HC emissions were excellent for variations in spark timing, EGR, and A/F ratio for stoichiometric and lean conditions, while under rich conditions, predicted HC levels fell below experiment. Only fair agreement with experiment was obtained for HC predictions under conditions of varying speed und load. The adjusted ratio of initial one-wall to two-plate quench thickness in the present calculations was found to be six times the expected laminar value, suggesting that the actual process of quenching and in-cylinder burnup is more complex than the essentially laminar, single-step kinetics treatment employed or that additional sources of HC are present which have not been taken into account in the model.