Exhaust gas recirculation (EGR) is an NOx (Nitrogen oxide and Nitrogen Dioxide) reduction technique used in most Gasoline and Diesel engines.
EGR works by recirculating a 5-10% of an engine's exhaust gas back to the engine cylinders. Intermixing the incoming air with recirculated exhaust gas dilutes the mix with inert gas which slows the combustion, and lowers the peak temperatures. Because NOx formation progresses much faster at high temperatures, EGR serves to limit the generation of NOx. EGR valves remain closed at engine idle since the inert gas received from the EGR would not provide necessary power to keep an engine running at low RPM.
Recirculation is usually achieved by piping a route from the exhaust manifold to the inlet manifold, which is called external EGR. A control valve (EGR Valve) within the circuit regulates and times the gas flow. Some engine designs perform EGR by trapping exhaust gas within the cylinder by not fully expelling it during the exhaust stroke, which is called internal EGR.
In modern diesel engines, the EGR gas is cooled through a heat exchanger to allow the introduction of a greater mass of recirculated gas.