# Impact of Exhaust Gas Recirculation on Gaseous Emissions of Turbocharged Spark-Ignition Engines
## 3 Results and Discussion
### Low Load: 1500 rpm - 6 bar BMEP  
In the case of 1500 rpm 6 bar BMEP, the engine performance is determined by the fact that the load is controlled by the throttle operation. Therefore, when using the same methodology previously explained, optimization of the DoE tends to produce calibrations with the highest possible overlap (VVT 40-40) to reduce the pumping losses. This reduction is produced by the higher amount of internal residuals trapped at the valve closures and greater throttle positions, as it can be seen in Figure 6, where maximum values of residuals up to 25% can be achieved when the engine operates in this condition compared to around 5% when minimum overlap is set (VVT 0-0). The reduction of the maximum amount of internal residuals achieved when the external EGR rate increases is related to the lower pressure difference across the engine (between intake and exhaust manifolds), since a higher amount of EGR is linked to higher intake manifold pressure to achieve the same engine load. Figure 5: Catalyst performance extrapolation for 3000 rpm and 12 bar BMEP. The lower residual concentration found with smallest possible overlap (VVT 0-0) is also translated into the possibility of increasing the EGR rate compared to higher overlap, and the benefit is seen in gross indicated efficiency probably due to lower heat losses. This can be observed in Figure 7, where two EGR sweeps are shown, one at maximum overlap (blue) and the other at minimum overlap (orange). The results of the EGR variation for the maximum overlap condition in terms of emissions are depicted in Figure 8. The data are presented using the same organization as already described for the medium load operating point. Again, NO\({}_{x}\) emissions (upper left chart) are directly driven by the lower temperature and oxygen content of the unburned zone inside the cylinder. Unburned hydrocarbon (lower left graph) results are mostly linked to the impact of EGR on flame propagation characteristics, as highlighted by the higher induction time. Regarding the oxygen content available in the exhaust gases, different results are achieved depending on the EGR range. Initially, up to 10% EGR, the oxygen content was almost independent on the EGR rate, contrary to the continuous reduction seen in the medium load operating condition. This can be partially explained by the interaction with the internal residuals. While in the medium load conditions these residuals are not very significant and therefore the oxygen content is directly linked to the external EGR, in the case of the low load operation, the oxygen content is severely affected by these residuals. As previously seen, as the EGR rate increases, the amount of internal residuals is reduced due to the effect on the pressure differential  
Figure 6: Residuals prediction as a function of variable valve timing system (VVT) settings for 1500 rpm and 6 bar BMEP. **left**: no EGR; **right**: 20% EGR. Figure 7: EGR sweeps with different VVT settings for 1500 rpm and 6 bar BMEP. Maximum overlap: blue; minimum overlap: orange. across the engine, compensating the trend in terms of oxygen concentration in the unburned mixture. Therefore, it is not until the external EGR is increased up to 15% that the decrease of the oxygen content can be observed. However, from this point on, there is a severe combustion stability deterioration, more critical than the one observed at higher load due to the combination of high internal residuals and external EGR, leading to a new increase of O\({}_{2}\) concentration. This evolution affects the CO-CO\({}_{2}\) kinetics, summarized in the lower right plot of Figure 8.  
Due to the interaction of the emissions result with the internal residuals, it is also interesting to analyze what would be the result if the engine was run at minimum overlap condition. This information is depicted as continuous lines in Figure 9 and compared to the previous results with maximum overlap, represented in dashed lines.