A Study of EGR Stratification in an Engine Cylinder

One strategy to decrease the amount of oxides of nitrogen formed and emitted from certain combustion devices, such as the internal combustion engine, is to recirculate and introduce a portion of the unburned product gases with the air and fuel. The effect of the re-circulated gases is to decrease the maximum temperatures in the flame zone, and hence decrease NOx formation. However, dilution of the air-fuel mixture in an engine using stratified EGR could offer significant fuel economy saving comparable to lean burn or stratified charge direct-injection SI engines. The most critical challenge is to keep the EGR and air-fuel mixture separated, or to minimize the mixing between the two zones to an acceptable level for stable and complete combustion. Swirl-type stratified EGR and fuel-air flow structure is considered desirable for this purpose, because the circular shape of the cylinder tends to preserve the swirl motion. Moreover, the axial piston motion has minimal effect on the swirling motion of the fluid in the cylinder. In this study, we consider intake system design in order to generate a two-zone combustion system, where EGR is maintained in a layer on the periphery of the cylinder, and the fuelair mixture is maintained in the center of the cylinder. KIVA-3V was used to perform numerical simulations on an engine with a central intake and peripheral EGR ports. The simulations were performed to determine if the twozones can be generated in the cylinder, and to what extent mixing between the two zones occurs. For the engine geometry used in this study, the results showed that it is possible to generate the two zones, but mixing is difficult to control.