Application of detailed chemistry and CFD for predicting direct injection HCCI engine combustion and emissions

The present study applied a detailed chemical kinetic mechanism to simulate the combustion and emissions in a direct-injection homogeneous charge compression ignition (HCCI) engine. The engine is a heavy-duty diesel engine equipped with a pressure-swirl injector using gasoline as the fuel. The intake air was heated to help fuel vaporization to achieve HCCI conditions. The CHEMKIN code was implemented into KIVA-3V so that the chemistry is solved in the context of the engine computational fluid dynamics simulation. The effects of turbulent mixing on the reaction rate were considered. The computations started from intake valve closure with the initial conditions provided by a one-dimensional cycle simulation code that can simulate the gas-exchange process. Good levels of agreement in combustion and emissions were obtained using the present model. Predicted cylinder pressures and heat release rates agreed well with measurements. The computational results showed that a lean and fairly homogeneous mixture was obtained under the current engine configurations. Relatively low gas temperature (with peak value only about 2000 K) was observed in the present HCCI combustion that produced low NOx emissions. The model also predicted the correct trends in unburned hydrocarbon and carbon monoxide emissions as the start-of-injection timing and engine load were varied. It was also found that the unburned hydrocarbons and carbon monoxide emissions increased drastically if the overall equivalence ratio was less than a certain limit, for example, 0.15, due to poor combustion.