# Impact of Exhaust Gas Recirculation on Gaseous Emissions of Turbocharged Spark-Ignition Engines
## 1 Introduction  
The great majority of research regarding internal combustion engines over the last few decades has been focused on efficiency increase and pollutant emission reduction as a consequence of the progressively more stringent regulations and the need to address the problem of climate change [1]. In this sense, for the last decades, compression ignition engines (CI) have shown important benefits in terms of fuel consumption compared to spark-ignition (SI), whereas the control of exhaust gas emissions requires more expensive and complex aftertreatment systems [2, 3]. Therefore, new technologies arised in spark-ignition engines to help reduce the gap in fuel consumption while maintaining the advantages in exhaust emissions [4]. These new technologies such as direct injection, turbocharging and downsizing, variable valvetrain, or exhaust gas recirculation (EGR) became a hot spot to achieve efficiency levels more similar to CI engines, while keeping traditional SI engine-out emission advantages [5,6]. Electric powertrains also come into play as a good solution to displace emissions from urban areas owing to a good promotion from public opinion, governments, and associations [7]. However, their lower power densities, higher production costs, as well as the real impact that these may have on ambience have limited up to now their market share [8,9]. For this reason, hybrid powertrains combining electrification with SI engines is currently seen as the most spread solution in the upcoming years. Nevertheless, their implementation implies new challenges for other emission control, since the vehicle operation in electrical mode can imply a delay in the engine and aftertreatment warm-up. [PERSON] et al. [10] found that the temperature drop in the exhaust line linked to engine shutdown in electrical mode is in the order of 150 \({}^{\circ}\)C, so the catalyst may drop below the light-off temperature for the next engine start. This supports the increased emission observed in several other studies. In this way, [PERSON] et al. [11] demonstrated high regulated emissions during high-powered cold starts. The results exceeded more than 9 times Federal Test Procedure (FTP) gaseous emissions for cold starts. As far as particulate matter is concerned, its emission from plug-in hybrid electric vehicles (PHEV) can be of the same order of magnitude as these from vehicles without any electrification [12]. The reason is found in the high number of high-powered engine starts through the driving cycle, as also concluded by [PERSON] et al. [13]. In this context, engine development for future electrified powertrains may need to take particular attention to emission control not only from the aftertreatment system perspective but also from the point of view of their formation inside the cylinder. In this sense, it is necessary to analyze the influence of the aforementioned technologies aimed at reducing engine fuel consumption, also looking at their impact on engine-out emissions. Particularly, the application of EGR can significantly modify the characteristics of the combustion process and the exhaust gas composition as well as further reduce the exhaust gas temperature. Some of these aspects are discussed next. First, as it is well known, EGR was adopted in CI engines due to its capability to reduce raw NO\({}_{x}\) emissions. This capability is well explained by the simplified Zeldovich mechanism [14,15], in which it is assumed that NO formation is in equilibrium. The basics are described in Equation (1),  
\[\frac{d[NO]}{dt}=\left(\frac{6\times 10^{16}}{T^{0.5}}\right)\cdot exp\left( \frac{-69096}{T}\right)\cdot[O_{2}]e^{0.5}\cdot[N_{2}]_{e} \tag{1}\]  
where \([NO]\) refers to the instantaneous molar concentration of NO, and \([O_{2}]_{e}\) and \([N_{2}]_{e}\) denote the equilibrium concentration of molecular oxygen and nitrogen, respectively. According to this expression, NO\({}_{x}\) emissions, mainly composed of NO, depend on oxygen concentration and temperature.