It's a system which is used to reduce combustion temperatures (good for emissions reduction but also good for engine longevity). It just routes a small amount of spent gasses back into the engine to pull down engine temps. Most cars have it (despite popular belief) whether it's accomplished using variable valve timing or an external valve system... The use of an extremely aggressive EGR system is how Honda gets such outrageous mileage and is able to run an engine WAY leaner than what's considered a safe and practical level.
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