# 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 all cases, the datum to calculate the variations of emissions is the condition with no external EGR and maximum valve overlap (i.e., the no EGR condition from the previous graph). The most evident difference between both datasets is the range itself of external EGR that can be added by maintaining stable combustion. While previously a deterioration in the coefficient of variation of the IMEP could be observed from 15% EGR, a reduction of the internal residuals allows the EGR to increase up to 25% before any impact on combustion instability can be detected. In terms of NO\({}_{x}\), it is clear how running the engine without external EGR or internal residuals results in a huge increase of these emissions. Actually, it can be seen how the level of NO\({}_{x}\) only with internal residuals is not reached until adding around 25% EGR, which is consistent with the residual prediction achieved with GT-Power for the maximum overlap condition, previously depicted in Figure 6. Regarding unburned hydrocarbons, the reduction of internal residuals results in a faster flame propagation (lower induction time) and consequently lower emissions, but the slope of increase with the addition of external EGR is similar for both VVT settings. Moreover, the reduced amount of residuals implies that the oxygen concentration achieves a monotonous reduction as a function of EGR until unstable combustion operation is reached, similar to what was already discussed for medium load conditions, compared to the almost flat behavior seen with maximum overlap. Furthermore, the oxygen availability is always lower than in the case of the maximum overlap condition, as a consequence of the lower quenching distance and therefore lower volume in the unburned zone at the end of combustion. Adding external EGR with minimum overlap induces a reduction of CO, and an increase of CO\({}_{2}\) is also observed, while the results are similar in CO and slightly lower in CO\({}_{2}\) compared to the high overlap operation at similar total residuals. Therefore, it can be concluded that  
Figure 8: Engine-out emissions as a function of the EGR rate for 1500 rpm and 6 bar BMEP for the maximum valve overlap condition. the effect of EGR on exhaust emissions is not so much affected by the different engine operating point but more about the interaction with the internal residuals through the VVT. Figures 10 and 11 show the prediction of the required O\({}_{2}\) to completely abate CO and HC emissions in the catalyst for the high and low valve overlap conditions, respectively. As it can be seen, in both cases, the cumulated oxygen available in the exhaust is higher than the one required theoretically to completely oxidize HC and CO. However, slight differences are observed depending on the valve overlap conditions. In the case of the low overlap condition, the margin between available and required oxygen is reduced as the EGR rate increases, which is similar to the trend already described in the 3000 rpm medium load keypoint. The main difference is that, in this case, the equivalence ratio control results in a slightly lean mixture for all conditions, while in the medium load case, the mixture was slightly rich for the highest EGR point, affecting oxygen availability. Instead, for the high overlap condition, the margin is similar for the whole EGR range considered. This behavior is mostly due to the low sensitivity previously seen for CO and oxygen content in the exhaust gases when increasing EGR in the maximum valve overlap case. Figure 10: Catalyst performance estimation for 1500 rpm and 6 bar BMEP: high valve overlap conditions. Figure 9: Engine-out emissions as a function of the EGR rate for 1500 rpm and 6 bar BMEP. Continuous lines: minimum overlap; dashed lines: maximum overlap.