"EGR stands for 'Exhaust Gas Recirculation.' It is a strategy for reducing the temperature in the combustion chamber by diverting some exhaust gas back into the intake manifold. The already-burned gas is cooler, and absorbs heat from the new combustion event, which helps reduce NOx emissions. More NOx is produced when the peak cylinder temperature is higher. It turns out that engine pumping losses can also be reduced by using EGR. Because part of the charge is inert, the throttle can be opened more to admit fresh air. The closer you can operate to wide-open throttle, the better for pumping losses and fuel efficiency. Years ago, the amount of EGR was controlled by a valve that opened and closed in response to manifold vacuum. Another development was valve control based on a backpressure sensor. But nowadays, with most engines having cam phasing, most EGR is internal EGR generated from exhaust and intake valve overlap."