Exhaust gas recirculation (EGR) is a NOx (nitrogen oxide and nitrogen dioxide) reduction technique used in most gasoline and diesel engines.

EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. Intermixing the incoming air with recirculated exhaust gas dilutes the mix with inert gas, lowering the peak combustion temperatures and (in diesel engines) reducing the amount of excess oxygen. Because NOx formation progresses much faster at high temperatures, EGR serves to limit the generation of NOx. NOx is primarily formed when a mix of nitrogen and oxygen is subjected to high temperatures.

EGR in Diesel Engines

In modern diesel engines, the EGR gas is cooled through a heat exchanger to allow the introduction of a greater mass of recirculated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since diesels always operate with excess air, they benefit from EGR rates as high as 50% (at idle, where there is otherwise a very large amount of excess air).

Since diesel engines are unthrottled, EGR does not lower throttling losses in the way that it does for SI engines (see above). However, exhaust gas (largely carbon dioxide and water vapor) has a higher specific heat than air, and so it still serves to lower peak combustion temperatures; the diesel engine's efficiency is still improved by reduced heat rejection and dissociation.

There are only two reasons for you to keep it: if your legislation requires it, or the emmissions require it.

Blank it off (to bluff casual inspection) or remove all the bits. The diesel definitely runs better without it.

EGR systems have been used to reduce emissions of nitrogen oxides (NOx) from gasoline engines for almost 20 years. Basically, they work by recirculating exhaust gases back into the intake stream, which cools the combustion process and, thereby, reduces NOx formation. Because of tightening NOx standards, EGR systems are being developed for use in diesel engines, as well.

However, the application of EGR to diesel engines presents several challenges, including: insufficient differential pressure across the EGR line, which leads to a low flow rate of recirculated gases; contamination of the engine with particulate matter present in the exhaust; and increased emissions of unburned fuel components and particulate matter.