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How does exhaust-gas recirculation reduce nitrogen oxide emissions?

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Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

The exhaust gas recirculation (EGR) system reduces nitrogen oxide emissions by directing a portion of the exhaust gases back into the combustion chamber of the internal-combustion engine.1

These recirculated gases are combined with the fuel-air mixture before entering the cylinder head. By introducing these inert gases, the system effectively lowers the peak temperature of combustion. Because nitrogen oxides are primarily formed when combustion temperatures are high enough to cause molecular nitrogen in the air to react with oxygen, this reduction in temperature directly favors lower production of these pollutants. While this process is an effective emission control system, it typically results in a slight loss of overall engine efficiency.12