To reduce nitrogen oxide (NOx) emissions even further, the OM 651 engine of the A 220 CDI is equipped with what is known as multiple exhaust gas recirculation (EGR). In addition to high-pressure EGR, where hot exhaust gases are taken from the manifold and reintroduced on the fresh air side, downstream of the intercooler, exhaust gases are diverted at a lower pressure level. This low-pressure EGR diverts the filtered exhaust gases downstream of the diesel particulate filter, cools them and uses a valve to return them to the fresh air flow upstream of the turbocharger.
NOx generation is primarily influenced by the oxygen concentration in the combustion chamber (= proportion of exhaust gases).
This leads to disadvantages with respect to particulate emissions and fuel consumption. Low-pressure EGR solves these problems, as it does not reduce the drive energy of the turbocharger while at the same time considerably reducing the throttling requirement of a high-pressure EGR system.