## 1 Introduction  
New emission regulations applied to the ground transport in major countries are focused on the reduction of both greenhouse gases and pollutant emissions [1]. In the particular case of Europe, an agreement was reached to further reduce the exhaust CO\({}_{2}\) targets beyond 2020. Thus, the New European Driving Cycle (NEDC)-based CO\({}_{2}\) limit in 2020 (95 g/km for passenger cars) will be reduced by a 15% in 2025 and 37.5% in 2030 [2]. The new regulation also defines zero- and low-emission vehicles (ZLEV) as those with CO\({}_{2}\) emission below 50 g/km [3]. This ZLEV category includes battery full electric vehicles (BEV) and plug-in hybrid electric vehicles (PHEV). This strategy opens the door to original engine manufacturers to meet CO\({}_{2}\) fleet requirements offering PHEV powered by internal combustion engines (ICEs), without the need to resort to BEV extensively [4]. In this context of hybridization, several advanced technologies for ICEs are available to reach near-zero CO\({}_{2}\) emissions while allowing for the sustenance of the automotive industry and market [5]. ICE improvements promote benefits in CO\({}_{2}\)[6] and pollutant emissions [7], being the exhaust aftertreatment systems (ATS) key to meet the limits on pollutant emissions in a context of highly dynamic operating conditions [8]. However, the early activation of the catalytic converters is becoming a critical challenge. On the one hand, the optimization of the engine thermal efficiency has resulted in the reduction of the exhaust gas temperature [9]. On the other hand, the electric propulsion periods make the engine to face long switch-off phases during which the ATS is cooled down [10]. Specific warm-up strategies must be applied to reach early catalyst light-off, what comes at the expense of fuel consumption and subsequently (CO\({}_{2}\)) penalty. These strategies comprise active techniques, such as fuel post-injection [11] and electric heating [12], or passive techniques, as those related to thermal insulation of the combustion chamber [13] and the exhaust line [14] or management of the exhaust valve timing [15; 16]. The availability on demand of electric power is leading the focus to the use of pre-turbine ATS [4]. This layout of the exhaust line has been studied in the past and is presently revisited as a passive way to increase the conversion efficiency of the catalytic converters. The most evident advantage of the pre-turbine ATS location is the increase of the catalyst temperature. However, besides the earlier thermal catalyst activation, the potential to reduce the fuel consumption exists when the pre-turbine ATS substitutes some of the post-turbine elements. If any ATS device is moved from downstream to upstream of the turbine, its pressure drop is reduced due to the higher gas density (only from certain boosting pressure so that the turbine inlet pressure offsets the higher temperature) and the fact that it is not multiplied by the turbine expansion ratio to set the engine backpressure (exhaust manifold pressure) [17]. This is particularly relevant for wall-flow monoliths, whose baseline pressure drop is high and increases as the soot is collected (while lower soot accumulation would occur in pre-turbine placement due to higher passive oxidation). As a drawback, a loss of enthalpy is found at the turbine inlet, mainly during accelerations, caused by the thermal inertia and heat losses of the ATS prior to the turbine [18]. Consequently, the turbocharger lag is increased damaging the drivability. A balanced solution consists of the pre-turbine ATS downsizing looking for an equilibrium between fuel consumption benefit while decreasing the thermal inertia (both decreased as the ATS size is smaller) [19]. As a result, the downsized ATS is able to keep high conversion efficiency of gaseous pollutants [20] and filtration efficiency in wall-flow particulate filters [21], but the turbocharger lag cannot be fully recovery to the traditional post-turbine ATS performance without additional measures.