Frozen Equilibrium and EGR Effects on Radical-Initiated H2 Combustion Kinetics in Low-Compression D.I. Engines Using Pistons with Micro-Chambers

Using hydrogen as a fuel, this chemical-kinetics study qualitatively examines the phenomenon of “frozen equilibrium” in Stratified Charge Radical Ignition (SCRI) engines with direct injection (DI) and exhaust gas recirculation (EGR). In such engines, this phenomenon is believed to preserve select highly reactive species formed in the side chambers (called micro-chambers) embedded inside the piston bowl so that these species can be carried-over to enhance autoignition in the next engine cycle. In turn this enhancement makes possible ignition and combustion at compression ratios that are markedly lower than those considered “standard” (for a given fuel), resulting in reduced emissions. Analysis is based on a detailed chemical-kinetics mechanism that includes NO_x production and makes use of up to 19 species and 58 reactions. Along with detailed versions of the momentum and energy equations, this mechanism is simultaneously solved within the two separate but interactively connected open systems representing the individual main chamber and micro-chamber combustion processes. Part of the aim of the present study is to identify several of the engine processes and dominant chemical-kinetics sub-mechanisms responsible for the much lower compression ratios and reduced NO_x emissions in SCRI-DI engines with EGR.