Since air is about 70% nitrogen, it's supposed to pass through the engine without reacting at all. If combustion temps get above a certain point, the oxygen left over from a lean mixture after all the fuel burns starts reacting with Nitrogen to form Nitrous Oxide, Nitric Oxide, and other nasty smog-producing nitrogen compounds. The front section of a dual-bed catalytic converter is a "reducing catalyst" that breaks down the NOX compounds and separates the Oxygen from the Nitrogen. The N2 goes on through the oxidixing section (back half of the converter) without reacting with anything. The Oxygen released by the reducing catalysy then combines with the Carbon Monoxide (CO) to form CO2. Sometimes it's necessary to add a little oxygen via air injection to the center of the converter to complete the CO/CO2 conversion in the rear of the catalytic converter. The resulting exhaust gas is nearly stoichiometric, which is a fancy word for being nothing but CO2 and water vapor. Exhaust gas recirculation (EGR) is used to displace a little oxygen in the intake charge, reducing combustion temperatures and minimizing the formation of NOX in the first place.
Jerry
