In the last years automotive researchers and manufacturers are focusing a large attention on the development and the optimisation of aftertreatment systems able to meet the ever more severe regulations on exhaust gas emissions. The scientific literature highlights that all the emission control systems require proper operating temperatures and an accurate flow control to guarantee reliable and effective processes. In particular, to assure the suitable thermal level for efficient treatments, the addition of supplemental fuel is often necessary, with a not negligible penalty on the global engine efficiency. To reduce this effect, innovative reversed flow converters have been proposed over the past few years. They are based on the cyclic inversion of the exhaust gas between the two system ends (active flow control). Conversely, unidirectional flow within the aftertreatment system represents the technical solution largely adopted in practice (passive flow control). The aim of the present work is to analyse and compare the energetic performances and the emissions conversion capability of active and passive aftertreatment systems for lean burn engines. To this purpose, a computational one-dimensional unsteady model has been developed and validated. The code permits to assess the heat exchange between the solid and the exhaust gas, to evaluate the conversion of the main engine pollutants, and to estimate the energy effectiveness and the potential fuel saving capability of active emission control systems. The response of the systems to variations in engine operating conditions have been investigated. Specifically, the influence of the temperature and mass flow rate of the exhaust gases and the effect of the temperature of solid phase have been analysed. Furthermore, the effects of the geometric and physical characteristics of the devices have been illustrated and thematic maps have been built in order to define the proper configuration of the emission control systems. The analysis highlighted that the active flow control tends to increase the thermal inertia of the apparatus and then it appears more suitable to maintain higher temperature level and to guarantee higher pollutants conversion at low engine loads after long full load operation. Conversely, the unidirectional flow is preferable when a rapid heating (i.e., cold start, warm up phase, etc.) is required.