Exhaust Gas Recirculation

Exhaust gas recirculation, or EGR, is a nitrogen oxide emissions reduction technique used in petrol, diesel and some hydrogen internal combustion engines. It works by recirculating a portion of the engine exhaust gas back into the cylinders, where it displaces atmospheric air and lowers the oxygen available for combustion, which in turn lowers peak in cylinder temperatures and reduces the high temperature reaction between atmospheric nitrogen and oxygen that produces nitrogen oxides. In spark ignition engines, typically five to fifteen percent of exhaust gas is routed back through the intake as EGR, and in gasoline engines the technique can also improve efficiency by allowing a larger throttle opening, reducing pumping losses. EGR is generally not used at high engine loads, since it would reduce peak power output, and it is omitted at idle to avoid rough combustion.