In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide () emissions reduction technique used in petrol/gasoline and diesel engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. This dilutes the in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat to reduce peak in-cylinder temperatures. is produced in high temperature mixtures of atmospheric nitrogen and oxygen that occur in the combustion cylinder, and this usually occurs at cylinder peak pressure. Another primary benefit of external EGR valves on a spark ignition engine is an increase in efficiency, as charge dilution allows a larger throttle position and reduces associated pumping losses.