Investigation of real gas effects on combustion and emissions in internal combustion engines and implications for development of chemical kinetics mechanisms

Abstract

Real gas effects on combustion and emissions in internal combustion engines are investigated using three-dimensional computational fluid dynamics. The Peng–Robinson equation of state is implemented to describe pressure–volume–temperature relationships and to calculate thermodynamic properties and relevant partial derivatives. Four facilities are modeled, including non-reacting compression in a motoring engine, combustion in a conventional diesel combustion engine and in a reactivity controlled compression ignition engine, as well as for a non-reacting reflected wave in a shock tube. It is found that the real gas effects of gas mixtures in practical internal combustion engine operation are sensitive to the operating load and the amount of premixed fuel. Excellent agreement against experiments was found for engine simulations with the Peng–Robinson equation of state in terms of cylinder pressure and apparent heat release rate. However, discrepancies with predictions from the ideal gas law grow with increased load and larger amounts of premixed fuel. In particular, the predicted emissions of soot, NOx, CO and unburnt hydrocarbons show increasing sensitivity to real gas effects as a result of changes in combustion phasing. Fuel condensation is also modeled using a vapor–liquid phase equilibrium solver and significant dependency on the equation of state used is found. Therefore, it is recommended to include real gas effects in internal combustion engine modeling to capture combustion and emissions characteristics accurately. Additionally, the results emphasize the role of real gas effects on reaction rates. Shock tube simulations are used to demonstrate the importance of using the real gas equation of state in the interpretation of chemical kinetic measurements. Significantly different compressed gas temperatures behind the reflected shock are predicted when real gas effects are considered. This needs to be realized when developing chemical kinetic models and rate constants for engine applications from shock tube data.