its called exhaust gas recirculation (EGR)
Reduces the amount of nitrogen oxide in the exhaust stream
Lowers combustion temperatures and improves engine efficiency
EGR mixtures reduce the overall amount of energy transferred to the piston due to a reduction in the specific heat ratio in gasoline engines
Lower combustion temperatures cause an incomplete burn of fuel, increasing particulate matter in diesel engines
An EGR system reduces the formation of NOx by recirculating small amounts of exhaust gases into the intake manifold, where they mix with the incoming fuel-air mixture. The dilution of the incoming fuel-air mixture reduces peak combustion temperatures and pressures, resulting in a reduction of NOx.