Reduction of kinetic mechanisms for fuel oxidation through genetic algorithms

Abstract

New automotive engine combustion concepts, such as the Homogeneous Charge Compression Ignition (HCCI) or the Controlled Autoignition (CAI), have been developed in the last years in order to fulfil the very stringent European regulations limiting pollutant emissions (mainly nitrogen oxides and particulate matter). These combustion modes are not controlled by the physical phenomena prior to combustion, as usual in traditional Compression Ignition (CI) and Spark Ignition (SI) engines, but by the fuel chemical kinetics leading to autoignition, which is described by the corresponding reaction mechanism. Since the detailed reaction mechanism of surrogate fuels (which are used instead of the original fuels in order to simulate their oxidation process) consists of hundreds of species and thousands of reactions, it cannot be used for engine simulation purposes due to the very excessive computational time requirements. Thus, reduced reaction mechanisms, which simulate properly some characteristics of the detailed one (autoignition time, combustion temperature, etc.), are used instead. Among the typical reduction techniques (Quasi-Steady-State Assumption (QSSA), Reaction Analysis, etc.), Genetic Algorithms (GA) allow for both an efficient reduction of small/medium size reaction mechanisms and, when combined with other techniques, a more effective reduction of large reaction mechanisms. In this work, a general and innovative GA methodology for the reduction of kinetic mechanisms describing the hydrocarbon oxidation processes has been proposed, and its application to the reduction of two different mechanism sizes (a medium one describing the methane autoignition and a larger one derived from the oxidation of a diesel fuel surrogate) has provided better results when compared to other GA methodologies or reduction techniques. The reduced mechanisms provide very similar autoignition time and combustion temperature values to those obtained from the detailed mechanism under different engine operating conditions (intake pressure and temperature, equivalence fuel/air ratio and exhaust gas recirculation rate).