Exhaust-gas recirculationConceptExhaust-Gas Recirculation (EGR) is a highly effectiveinternal engine measure to lower NOx emissions on diesel engines.
A distinctionis made between:· InternalEGR, which is determined by valve timing and residual gas.· ExternalEGR, which is routed to the combustion chamber through additional lines and acontrol valve.The NOx-reducing effect is mainly due to the followingcauses:· Reductionin exhaust-gas mass flow.· Dropin the rate of combustion, and thus local peak temperatures due to an increasein the inert-gas component in the combustion chamber.· Reductionin partial oxygen pressure or local excess-air factor.
The NOx-reduction is possible in three ways – · Dilutioneffect – It is the reduction in the inlet charge oxygen concentration (mainreason for NOx reduction).· Thermaleffect – It is the increase in inlet charge heat capacity, and· Chemicaleffect – Modification in combustion process as a result of dissociation of CO2and water vapourSince high local temperatures (> 2,000 K) and asufficiently high partial oxygen pressure are required to form NOx, themeasures listed above result in a drastic reduction in the formation of NOx asthe EGR rate rises. Reducing the reactive components in the combustion chamberalso leads to a rise in black smoke, which limits the quantity of recirculatedexhaust gas. The quantity of recirculated exhaust gas also affects the periodof ignition lag.
If EGR rates are sufficiently long in the lower part loadrange, ignition lag is so great that the diffusive combustion component, thatis so typical of diesel engines, is strongly diminished, and combustion onlystarts after a large percentage of the air and fuel has been mixed. Thispartial homogenization is used in new or future (p) HCCI combustion processesto achieve extremely low-NOx and low-particulate combustion in the lowpart-load ranges.