A novel NOx reduction approach for 4-stroke direct-injection spark-ignition natural gas engines is examined. Secondary chambers are fitted into the cylinder peripheries as radical ignition (RI) species generation sites and equipped to enable fuel-insertion control and rapid heat addition. These chambers can thus regulate the production and transfer (into the main chamber) of RI species to augment combustion for reduced NOx and increased combustion stability. The analysis uses experimental data and full chemical-kinetics. The formulation governing equations are solved within multiple zones in both the secondary and main chambers, as the gas mixtures interact thermo-chemically and hydro-dynamically among themselves, with the internal cylinder boundaries and with the manifold (exchanging energy, momentum, mass and chemical species). Results suggest the potential of this technology for simultaneous NOx and CO reduction.