NOx Reduction Kinetics Mechanisms and Radical-Induced Autoignition Potential of EGR in I.C. Engines Using Methanol and Hydrogen

This numerical study examines the chemical-kinetics mechanism responsible for EGR NO_x reduction in standard engines. Also, it investigates the feasibility of using EGR alone in hydrogen-air and methanol-air combustion to help generate and retain the same radicals previously found to be responsible for the inducement of the autoignition (in such mixtures) in IC engines with the SONEX Combustion System (SCS) piston micro-chamber. The analysis is based on a detailed chemical kinetics mechanism (for each fuel) that includes NO_x production. The mechanism for H-air-NO_x combustion makes use of 19 species and 58 reactions while the methanol-air-NO_x mechanism is based on the use of 49 species and 227 reactions.

It was earlier postulated that the combination of thermal control and charge dilution provided by the EGR produces an alteration in the combustion mechanisms (for both the hydrogen and methanol cases) that lowers peak cycle temperatures-thus greatly reducing the production of NO_x. Through the present numerical calculations, the actual chemical-kinetics mechanisms responsible for these lower peak temperatures and emissions have been identified. The calculations clearly show the effect of EGR in lowering peak temperatures and pinpoint how this in-turn results in lower NO_x emissions (for both fuels). Further, for stoichiometric mixtures, the study is able to determine whether (or not) EGR by itself is capable of providing the intermediate species and radicals previously found to induce autoignition (and TO enhance combustion) in hydrogen and methanol IC engines using the SCS technology (H₂O₂ CH₂O and HO₂ for the case of methanol). For both fuels the results are negative. The same findings are argued to be true for 2-stroke engines using exhaust gas carryover to provide charge dilution and thermal heating control.