Exhaust Aftertreatment Systems for Gasoline and Alternative-Fueled Engines with Reduction of HC, CO, NOX and PM

An exhaust gas emissions aftertreatment system for spark-ignition engines, which simultaneously reduces the particulate matter, HC, CO, and NOx content of the exhaust. Various embodiments of the system have both a closely-coupled TWC device, and an under-floor treatment device. The under-floor device has either TWC or NOx reduction functionality, depending on whether the engine is run under stiochiometric or lean burn operating conditions.