# Impact of Exhaust Gas Recirculation on Gaseous Emissions of Turbocharged Spark-Ignition Engines
## 4 Conclusions  
In the current paper, an investigation of the effects of usage of exhaust gas recirculation on a spark-ignition engine-out emissions is presented. For this purpose, a 4-cylinder turbocharged Gasoline direct injection (GDI) engine equipped with VVT, VGT, and a low-pressure EGR line was used. In order to have a meaningful assessment of these emissions, the engine had been previously calibrated for 6 engine operating points varying speed and load and for different levels of EGR using the DoE technique, trying to always maintain stoichiometric conditions. Then, the found calibrations were run on the engine to assess the effect of increasing EGR quantity on emissions. Finally, a stoichiometric-based model of the reactions in the catalyst was used to evaluate the potential impact of the NO\({}_{x}\) reduction and the HC increase with EGR on the catalyst performance. Doing so, the following conclusions have been found:  
* In general, an approximately linear decay of NO\({}_{x}\) and increase of HC is found as a function of the addition of external EGR, according to the expectations. On the one hand, NO\({}_{x}\) is driven by the reduction of both the maximum combustion temperature as well as the oxygen availability in the unburned zone. On the other hand, the reduction of the flame propagation speed linked to this lower oxygen concentration, together with the lower rate of heat transfer to the cylinder walls, results in a higher quenching distance, increasing unburned HC. * In terms of the oxygen concentration in the exhaust gases, which can be of interest for the catalyst operation, different trends are found depending on how the engine is controlled in terms of the VVT system. When the amount of internal residuals trapped in the cylinder is small, the lower oxygen concentration in the unburned zone (which will be transfered in the exhaust gases through quenching in the walls and crevices) when increasing EGR dominates over the increase in the quenching distance, inducing a reduction of the oxygen content seen in the exhaust. Contrarily, when the amount of these internal residuals increases (at low speed and load with high valve overlap), the effect of external EGR on the oxygen concentration in the unburned zone is smaller, since the addition of external EGR results in lower internal residuals due to the increase in intake manifold pressure, partially compensating the presence of external EGR and inducing an almost flat trend in CO and oxygen. However, when the amount of internal residuals plus external EGR is excessive, combustion instability appears, inducing cycles with low combustion efficiency and a significant increase of oxygen concentration in the exhaust. * Regarding CO, the general trend is to reduce them when increasing EGR as a consequence of kinetic effects on the dissociation reaction of CO\({}_{2}\) but to a lesser extent than the increase in HC emissions previously observed. * In general, despite the reduction of oxygen concentration and nitrogen oxides in the exhaust gases when increasing EGR, the oxygen available is still theoretically enough to produce a complete oxidation in the catalyst. This is in part thanks to the reduction of CO induced as EGR increases. However, for very high levels of EGR, the margin available in the oxygen amount is very small, especially when combustion stability deteriorates. Therefore, strategies with slightly lean operation may be needed in such conditions in order to ensure a robust response of the aftertreatment if the EGR rate is high enough to tolerate a small deterioration of NO\({}_{x}\) conversion efficiency. Conceptualization, J.D.I.M. ; formal analysis, E.J.S. and R.P. ; funding acquisition, J.D.I.M. and P.P. ; investigation, P.P., J.D.I.M., E.J.S., and R.P. ; methodology, J.D.I.M. and P.P. ; project administration, J.D.I.M., software, P.P. and E.J.S. ; supervision, P.P. and J.D.I.M. ; writing--original draft, J.D.I.M. and R.P. ; writing--review and editing, P.P., J.D.I.M., E.J.S., and R.P.