# Impact of Exhaust Gas Recirculation on Gaseous Emissions of Turbocharged Spark-Ignition Engines
## 1 Introduction
Therefore, a double effect occurs when introducing EGR into the cylinder:  
1. **Thermal**: The exhaust gases are composed of a high proportion of H\({}_{2}\)O and CO\({}_{2}\), which have a greater heat capacity than other air constituents, acting as a heat sink and reducing the temperature inside the cylinder during compression and combustion. 2. **Dilution**: The oxygen concentration is reduced as a result of the higher amount of inert gases inside the chamber, reducing the reactivity related to NO formation. These effects have been studied in depth [16, 17, 18] resulting in a clear demonstration of NO\({}_{x}\) reduction with O\({}_{2}\) concentration decay. However, the benefits of EGR implementation in SI engines can go beyond being only a NO\({}_{x}\) inhibitor. As stated, the presence of exhaust gases with higher heat capacities acts as a heat sink, hence reducing in-cylinder temperature and heat transfer losses [19,20]. Furthermore, benefits at both partial and high loads are found. The need to increase the intake pressure to bring in exhaust gases along with fresh air is reflected in more open throttle positions and reduced pumping losses during throttled operation. In addition, using EGR allows better combustion phasing at higher loads, which is typically limited due to knock combustion phenomena [21]. Thanks the lower reactivity of the mixture knock probability is reduced, so the spark control can be optimized, thus achieving better efficiencies [22]. Additionally, the temperature of the exhaust gases is also reduced as a combination of both effects (dilution plus more advanced phasing). Therefore, fuel enrichment commonly done at these high loads to protect the turbine from thermal fatigue [2; 19; 23] can be limited or completely suppressed, achieving further benefits from fuel consumption and emissions perspective. Nevertheless, apart from the benefits achieved from fuel consumption perspective, EGR addition could negatively affect other emissions such as CO, Particle matter (PM), and Unburnt hydrocarbon (HC), as discussed by [PERSON] et al. [24], or critically slow down flame development, producing unstable combustion and large cyclic variations, as shown by [PERSON] et al. [25]. [PERSON] et al. [26] also proved the trend of HC increase with EGR use for all tested points due to reduction in combustion temperature and a greater quenching area. [PERSON] et al. [27] studied the effects of combining EGR with stratified combustion as well as injection timing. The results showed the greater potential of EGR compared to retarded ignition timing on reducing harmful NO\({}_{x}\) and HC emissions. Other studies have focused on analyzing and comparing cooled and uncooled EGR, finding out that cooled EGR normally shows more benefits than hot EGR from efficiency perspective [28], mainly thanks to an improved volumetric efficiency. However, cooled EGR results in a further reduction of NO\({}_{x}\) emissions, whereas HC emissions and cycle-to-cycle variations deteriorate [15]. Furthermore, the usage of cooled EGR can be significantly limited by condensation issues either in the EGR cooler [29] or in the intercooler [30], reducing the potential impact of the EGR circuit, especially in hybrid electric vehicle applications, where temperature would significantly decrease when the powertrain runs in electric mode. In summary, previous works highlight that EGR usage in spark-ignition engines can significantly alter the traditional composition of exhaust gases, at least reducing NO\({}_{x}\) and increasing HC concentrations, and can reduce the exhaust gas temperature. This may further stress the operation of aftertreatment systems used in such applications, since traditional three-way catalysts depend on NO\({}_{x}\) for HC and CO oxidation. Therefore, the current work intends to further assess the impact of EGR addition on engine-out emissions and to provide a preliminary evaluation about the potential oxidation performance of a purpose-made catalyst from a theoretical perspective.