Exhaust gas recirculation (EGR) Is an efficient NOX (Nitrogen oxide and Nitrogen Dioxide) reduction technique used in most Gasoline and Diesel engines. EGR works by recirculating a quantity of exhaust gas back to the engine cylinders. The mixing of recirculated exhaust gas with incoming air reduces the amount of available oxygen to the combustion process and reduces the peak temperature of the combustion process. This limits the generation of NOX.

EGR is usually achieved by a piping route from the exhaust manifold to the inlet manifold. A control valve (EGR Valve) will be placed within the circuit to regulate and time the gas flow. Some engine designs achieve EGR by trapping exhaust gas (not fully expelling during the exhaust stroke) within the cylinder.

In a modern diesel engine it is likely that the EGR gas is also cooled through a heat exchanger to further cool the gas, and thus the combustion, and to allow the introduction of a greater mass of recirculated gas.

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EGR valves