The purpose of the exhaust gas recirculation process was to simply put a small, carefully metered amount of exhaust gas back into the combustion chamber. Exhaust gas has already spent its fuel, so to speak, and is considered an inert gas and will not support combustion. This precise amount of recirculated inert gas slows the burning process in the combustion chamber where NOx is produced. Slowing the burning process lowers the temperature during combustion, thereby reducing or eliminating nitrogen oxide production. This action is most needed during cruise conditions and not at idle or heavy throttle conditions. Some may still incorrectly believe that the EGR is supposed to recycle the exhaust gas - burning it twice must be enough to reduce emissions, right? Wrong.

We just said the exhaust gas won't support combustion - it's already been burned. So, the main purpose? Reduce NOx. The side benefits? Better fuel economy for one, and maybe performance as well. Consider some of the advanced strategies for EGR flow control on some of the later engine applications. The inert exhaust gas actually displaces a small amount of space in the combustion chamber, which in turn reduces a small amount of volume that the air/fuel charge occupies. This can have an effect on fuel economy, albeit slightly, if the EGR valve is malfunctioning.