What is the main purpose of the exhaust gas recirculation (EGR) system?

The main purpose of the exhaust gas recirculation (EGR) system is to reduce nitrogen oxide (NOx) emissions. The EGR system works by recirculating a portion of the engine's exhaust back into the intake manifold, mixing it with fresh air and fuel. This process lowers the combustion temperature within the engine cylinders, which significantly reduces the formation of NOx gases. High temperatures during combustion promote the formation of NOx, so by lowering these temperatures, the EGR system effectively minimizes harmful emissions, making it a crucial component for meeting environmental regulations and standards.

In the context of the other choices, while improving fuel efficiency, providing additional cooling, or enhancing power output may be desirable aspects of engine operation, they are not the primary function of the EGR system. Fuel efficiency improvements may arise incidental to the operation of EGR, and while it may help in managing engine temperatures, its main role remains focused on emission control. The enhancement of power output is not a direct function of the EGR system either, as it is primarily designed with emissions reduction as its core objective.