Exhaust Gas Recirculation Coolers (EGR coolers) are devices that aid in reducing nitrogen oxide emissions (NOx) for internal combustion engines. Recirculating nitrogen oxide emission can substantially aid in the reduction of heat and pressure in the engine. In total it provides an excellent reduction in emissions, and most modern engines must use a system of recirculation to meet current emissions standards set by the US Environmental Protection Agency.
The general process is very simple. Exhaust Gas Recirculation systems typically operate by taking some of the exhaust gas coming from recent combustion in the engine, passing it through filters to remove soot/particles, and then passing the gas back into the combustion cycle. The amount of gas can vary by EGR system, and in some more complex systems the amount of gas is varied based on engine load. In all methods the gas coming from the exhaust is always set to mix with some air before being passed back into the combustion chamber to ensure a solid fire.
In diesel engines EGR cooling systems operate in the same method, they take lower oxygen exhaust and funnel it back into the intake of the engine. This actually sacrifices power and economy for better quality emissions and reduced engine temperature. Diesel engines use particulate filters designed specifically to remove soot, ash, and other debris from the exhaust of the engine, and in advanced implementations of ERG coolers these filter remove these particles before passing the exhaust gas back into the combustion. In less advanced systems without filters this is known to cause some buildup of extra matter inside the areas of the engine where the gas is routed. Over time this can cause different problems in an engine like misfires, hesitation, and surging.
Although the process does cause a loss of power in the engine, modern methods of EGR cooling are able to mitigate this loss substantially resulting in both better emissions quality for only a minor tax on efficiency and power of the engine.