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Empirical combustion modelling of a qualitatively governed spark ignition (SI) engine incorporating Exhaust Gas Recirculation (EGR) was studied using AVL Boost. SI engines are quantitatively governed by controlling the quantity of charge inducted, leading to high throttling losses, and reduction in thermal efficiency. In this article, the charge purity was reduced by incorporating EGR, thereby reducing the amount of available oxygen in the charge inducted, to qualitatively govern the engine. The effect of charge dilution is also to slow down the flame development and propagation, increasing the combustion duration. A combustion model was developed, which estimates the laminar and turbulent burning velocities using the predictions of the in-cylinder traces and the turbulence intensity, using published empirical correlations. This algorithm resulted in a curve fit equation, to predict the combustion duration and the start of combustion as a function of turbulent burning velocity. Using qualitative governing, the brake mean effective pressure was reduced by 20% using a maximum of ∼20% EGR. This also led to 20% reduction in both NO x and CO at full load, with only a 2.5% increase in brake specific fuel consumption, however the unburned HC emissions have also increased by 50%. |
