*5.2. Simulation Results*

In Figure 9, comparisons of experimental and simulated in-cylinder pressure histories, rate of heat release, combustion phasing parameters and normalized emissions are presented for a low-speed mid-load (SI01), a mid-speed high-load (SI05) and a high-speed low-load (SI06) operating points. Extended comparisons by means of pressure, RoHR and crank angle resolved emissions for all the operating conditions listed in Table 6 can be found in Appendix B. Overall, the SRM simulation results show a close match with experiment by means of in-cylinder pressure for di fferent operating conditions. Slight deviations can be seen for the operating points SI05 (2000 rpm and 20 bar IMEP) and SI06 (2500 rpm and 5 bar IMEP) in Figure 9b,c by means of start of combustion and peak cylinder pressure. However, the overall agreemen<sup>t</sup> is considered to be acceptable, especially considering the typical cycle-to-cycle variability. Compared to the Diesel engine campaign, a much closer match between online and CPV solver can be seen in the SI cases.

**Figure 9.** In-cylinder pressure history and apparent rate of heat release (left), combustion phasing parameters (top right) and normalized engine-out emissions (bottom right) comparisons between experimental data and SRM simulations for operating points SI01 (**a**), SI05 (**b**) and SI06 (**c**).

This is explained by the fact that in SI mode, the dominant phenomenon is the flame propagation rather than mixing controlled combustion, where particles reach fully burned state (C = 1) much faster and are moved to the burned zone. In addition, given the early start of injection, the mixture is assumed to be homogeneous and close to stoichiometry. The homogeneity in lambda, together with the particles quickly reaching C = 1, makes the interpolation particularly accurate. Stochastic effects are still present due to the SRM treatment of the heat transfer; however, hardly any difference can be seen in terms of pressure and rate of heat release histories as well as in terms of combustion phasing parameters and peak cylinder pressure location.

In Figure 10, engine-out emissions and major combustion phasing parameters are summarized for all the investigated operating conditions. All values are normalized to experimental data. By means of engine-out emissions, while simulations and experiments agree acceptably well, for most of the cases, noticeable differences can be seen for CO and NO between online and tabulated chemistry solver. Regarding NO, the differences are related to the fact that in the detailed scheme a more advanced prompt and thermal formation mechanism for NOx is accounted for, while in the tabulated chemistry solver only thermal NO source terms are considered. As for CO, a similar discrepancy as in the Diesel simulation campaign can be seen. The operating point SI06 (at 2500 rpm and 2 bar IMEP) shows the largest difference (approximately 17%) for NO emissions against experimental data. This may be explained by the noticeable in predicted peak cylinder pressure which results in a different in-cylinder temperature. The comparison of the tool chain accuracy and the tabulated chemistry solver accuracy is leading to similar conclusions as for the Diesel engine test case in Section 4.

**Figure 10.** Experimental and simulated engine-out emissions as well as performance parameters (CA50 and PCPCAD location) for all operating conditions of the SI engine simulation campaign. Data have been normalized with respect to experimental values.
