EGR was developed to reduce combustion temps by injecting inert gases into the combustion chamber....obviously we don't want to `reduce' the volume of the combustion chambers (effectively reduced engine displacement) during hard acceleration, so EGR is turned off when you need full power (WOT conditions). At idle, the engine is very sensitive to air/fuel mixture ratios and swirl in the combustion chamber, so introducing EGR at idle is not on either. However at cruise the Fuel/Air mixture is set as lean as possible for maximum economy and this in turn generates the highest temperatures, and so the EEC-V uses these conditions to inject exhaust gas into the inlet manifolds to reduce emissions.
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