# Impact of Exhaust Gas Recirculation on Gaseous Emissions of Turbocharged Spark-Ignition Engines
## 3 Results and Discussion
### Medium Load: 3000 rpm - 12 bar BMEP  
Figure 3 shows the calibration produced by the DoE methodology as a function of the EGR rate. As previously mentioned, VVT settings at 0-0 represent the minimum overlap condition, which is in this case the optimal before EGR is introduced. Instead, when EGR is introduced, the exhaust valve timing is maintained but the intake valve opening is anticipated. Additionally, the spark timing is severely anticipated as a consequence of two phenomena: the reduction of knock probability and the slower flame propagation characteristics. The results obtained in terms of emissions are depicted in Figure 4. The data are presented in terms of the variation produced with respect to the calibration without EGR using the data extracted from the gas analyzer measurement in mole fraction. The most direct and clear result is the one corresponding to NO\({}_{x}\) emissions (upper left chart). As stated in the introduction, the combination of lower oxygen concentration and higher heat capacity in the unburned region as a consequence of EGR introduction results in a reduction of NO formation according to the Zeldovich mechanism. The unburned hydrocarbon (lower left graph) trend instead is mostly related to the quenching phenomena. As the EGR rate increases, laminar flame speed is deteriorated, which can be confirmed by the increase in the induction time reflected in the same graph. This induction time is defined as the lapse between the spark activation and 10% of the cumulative heat released, related to the initial kernel growth which is known to be greatly influenced by laminar flame speed as well as flow characteristics around the spark plug [34]. Regarding the oxygen content in the exhaust gases (upper right figure), it has to be highlighted that, since the engine is always running at stoichiometric  
Figure 3: Engine calibration achieved by the DoE for 3000 rpm and 12 bar BMEP. conditions, this oxygen is significantly influenced by the aforementioned quenching phenomena. On the one hand, as quenching distance increases, the volume of fresh mixture existing in the exhaust gases is higher, so higher oxygen levels could be expected. However, with the increase of the EGR rate, the oxygen concentration inside this fresh mixture is lower, driving the continuous reduction of oxygen concentration found in the results. More complex is the evolution of the CO emissions (lower right graph). In general, CO concentration in the exhaust is governed by the kinetics of the oxidation reaction of CO to CO\({}_{2}\) in the last stages of the combustion process, as it can be seen from the inverse trend shown by the CO\({}_{2}\) in the same plot. In principle, the reduction of oxygen concentration when increasing the EGR rate implies that thermal equilibrium of that reaction would be shifted towards a higher concentration of CO. However, there are two phenomena opposite to that trend. On the one hand, reduction of the combustion temperature reduces the dissociation of CO\({}_{2}\) once it is formed. On the other hand, the kinetics of the reaction are slowed down. Nevertheless, it has to be considered that there is a severe interaction between these aspects and the evolution of the combustion process due to the instantaneous temperature evolution inside the cylinder, so changes in the spark timing and/or the variable valve timing settings may also influence the CO trend. Finally, it has to be considered that, at very high EGR rates, the combustion stability is deteriorated, as it can be observed in the COV of the IMEP, which is plotted with the oxygen concentration (upper right side), producing an increase of CO emissions compared to the general trend achieved with EGR. As a conclusion from the previous results, high external EGR operation induces exhaust gases with very low oxygen and NO\({}_{x}\) while unburned hydrocarbons increase and CO decrease but to a lesser extent.