Bringing fuel and air together is the basis of the combustion process. However, if the level of oxygen, and therefore the temperature, of combustion is too high, then harmful nitrogen oxide (NOx) emissions are produced. Exhaust gas recirculation (EGR) valves play a key part in controlling this oxygen density and therefore the production of NOx.
The EGR valve is located in a small gap between the intake and exhaust manifolds. Here, it adjusts the amount of exhaust gas that is recirculated back into the intake manifold. This means that when an engine is idling, the EGR valve remains closed until the engine is warm and working under load. As this load and the combustion temperature increases, the EGR valve opens and sends exhaust gas back into the intake manifold. This is then mixed with the intake air to reduce oxygen levels and combustion speed according to driving conditions. This process also lowers the amount of NOx emissions produced.