The EGR is the Exhaust Gas Recirculation unit in any given automobile. This valve controls the amount of Nitrogen Oxide (NOx) released in the combustion process, helping to reduce the temperature of the engine.
Turning off EGR "reduces the volume of the combustion chambers." This is necessary during hard acceleration to elicit the most power from the engine.
The EGR also is turned off when the car is idle. Because the engine is sensitive to the swirling mixture of air and fuel, the EGR is not needed to pump exhaust out of the engine. If it were turned on during this time, the car would most likely stall.
EGR is most useful when the car is at a cruising limit. The air to fuel ration should be as "lean" as possible to get the most energy out of the car. During this time, the car also generates the most heat, making it necessary for the EGR to act as a cooling agent.
