*4.2. Simulation Results*

In Figure 7, comparisons of experimental and simulated in-cylinder pressure histories, rate of heat release, combustion phasing parameters and normalized emissions are presented for a low-load (HD04), a mid-load (HD06) and a high-load (HD07) operating points. The quantities noted in the following figures as CA05, CA10, CA50 and CA90 represent the crank angle location at which 5, 10, 50 and 90 per cent of the total heat has been released, respectively.

Detailed comparisons by means of pressure, RoHR and crank angle resolved emissions for all the operating conditions listed in Table 2 can be found in Appendix A. In Figure 8, comparisons of the CA50, peak cylinder pressure location in CAD (PCPCAD), as well as CO, CO2, unburned hydrocarbons (uHC) and NO at EVO are shown for all operating points. To comply with data confidentiality restrictions, all the results shown in this simulation campaign are presented in a normalized fashion. With respect to engine out emissions, different normalization strategies have been applied to ensure meaningfulness of the shown comparisons. More in detail, for CO2 and NO, the simulated ppm values have been normalized with respect to the experimental measurements. For CO and uHC instead, the normalization has been computed based on the difference in ppm between simulated and experimental data with a threshold value set to approximately 100 ppm. In other words, if simulated uHC or CO presents a normalized factor of 2.0, it means that the absolute difference between experimental and simulated values is approximately 200 ppm.

**Figure 7.** 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 (**a**) HD04, (**b**) HD06 and (**c**) HD07.

**Figure 8.** Experimental and simulated engine-out emissions as well as performance parameters (CA50 and PCPCAD location) for all operating conditions of the heavy-duty engine simulation campaign. Data have been normalized with respect to experimental values.

On the other hand, if simulated and experimental HC or CO differ by less than 100 ppm, then the factor is set to 1.0, so to underline an acceptable agreement. Such formulation was considered necessary to cope with the fact that the measured CO and uHC are in absolute terms very low. Hence a standard normalization would have resulted in a set of misleadingly high factors for CO and uHC from the engineering standpoint. For uHC, in particular, the difference between experimental and simulated engine-out ppm values never exceeded 30 ppm across all the operating points, and therefore, the set of comparison factors for uHC in Figure 8 is homogeneously set to 1.0.

By means of combustion phasing, the SRM results are in good agreemen<sup>t</sup> with experimental data for the majority of the analyzed operating conditions. Visible discrepancies can be observed by means of peak cylinder pressure predictions in HD08 and HD09. However, such result was considered acceptable, considering that on the experimental side, these operating points showed a strong cycle-to-cycle variability, as it can be noticed via the large fluctuations of the heat release rate between 0 and 40 crank angle degrees after TDC (see Figures A8 and A9 in Appendix A).

As for the online versus CPV simulation results, both simulations resulted to be in close agreemen<sup>t</sup> with each other across the whole range of simulated data. The tabulated chemistry solver predicted a combustion phasing within less than 0.5 CAD difference with the detailed online chemistry solver at the mid and high loads. For the low load points (HD04 and HD05), a slightly more noticeable difference (≈2.0 CAD) between online and tabulated chemistry solutions can be seen when comparing the predicted start of combustion (See Figures A4 and A5 in Appendix A). At low loads, combustion initiates while the mixture is the NTC region, which, as discussed in Section 3.2, is the most challenging regime for progress-variable-based models. It is therefore likely to happen that under these conditions the interpolation error starts to play a visible role. Nevertheless, a 2 CAD discrepancy in start of combustion is well within a range typically considered acceptable for engine performance studies and considering accuracy of the sensors used during the experimental campaigns.

With respect to engine-out emissions, both solvers showed good agreemen<sup>t</sup> with experimental data for CO2, uHC and NO. Di fferences in CO2 are explained by di fferences in the C/H ratio of the real fuel blend and the surrogated fuel blend. These di fferences also influence the CO emission calculation. With respect to NO, the di fferences between tabulated and detailed chemistry are lower than di fferences between the SRM predictions and the engine measurements. For these species, it can be stated that the accuracy of the tabulated chemistry solver is not influencing the accuracy of the tool chain. This statement is also true for the prediction of CA50 and PCP. For carbon monoxide emissions, the online chemistry solver showed a noticeably closer match (less than 100 ppm di fference) with experimental data. While the CO predictions from the CPV solver lie within a more than acceptable range from the engineering point of view, it is important to note that correct tabulation of CO during the expansion phase is another challenging area when progress variable models are concerned. Unlike methods proposed in [23,24], the present method does not account for a time-scale dependent retrieval of the CO emissions from the table. This means that the accuracy of the final CO yield depends on how the close to (or far from) equilibrium the value stored in the table at progress 1 is. In the present study, the presented level of accuracy between online and tabulated chemistry solver-based CO values (± 200 ppm) was considered to be acceptable. In future studies, however, a time-scale dependent CO retrieval strategy will be considered.

#### **5. Gasoline Engine Simulations**

In this section, results of a spark-ignited engine simulation campaign are shown and discussed. In the first sub-section, a brief description of the experimental data and computational setups are presented. Secondly, engine performance parameters and engine-out emissions are compared between experimental data, online and tabulated chemistry solver. All the simulation results presented in this section were obtained using the commercial software LOGEengine version 3.2.1 [53].

#### *5.1. Engine Data and Simulation Setups*

The experimental data were measured on a single cylinder research engine at the TU Berlin [48]. Cylinder bore and stroke are 82.0 and 71.9 mm, respectively, while the compression ratio is 10.75:1. The single cylinder engine is specifically designed for combustion investigations and features both port and direct fuel injection systems. The present engine experiments were conducted using the centrally mounted direct fuel injector. The start of fuel injection is at −270 CAD aTDC. Eight fired operating points were selected and are summarized in Table 6. For each operating condition, in-cylinder as well as manifold pressure were recorded for 250 consecutive cycles by means of a low- and high-pressure sensors. More details on the experimental setup and measuring equipment used can be found in Kauf et al. [59].


**Table 6.** Spark-ignition engine operating conditions.

The fuel used during the experimental campaign was a RON95 E10 commercial gasoline, at 150 bar injection pressure. A four-component mixture comprising mole percentages of 31.9% *iso*-octane, 11.4% *n*-heptane 35.6% toluene and 20.8% Ethanol was used in the simulation campaign instead. Comparisons of the major fuel properties are listed in Table 7. The adopted reaction mechanism is

based on the detailed ETRF scheme developed by Seidel consisting of 475 species and 5160 reactions. The detailed reaction scheme was validated for di fferent experiments and engine relevant conditions for both auto-ignition and laminar flame speed in several previous works [39,60].

**Table 7.** Liquid properties of the experimental and surrogate fuel mixture used in the SI engine simulation campaign.


The SRM model calibration for the presented operating conditions was carried out using the procedure described in [61]. In Tables 8 and 9, the main SRM model parameters and the calibrated K-k model constants [47] are presented, respectively. As done for the compression ignition engine campaign, the same set of model parameters and constants were applied to both the online and tabulated chemistry solver runs without any re-calibration.

**Table 8.** SRM main model settings for the spark-ignition engine simulation campaign.


**Table 9.** Calibrated constants for the K-k turbulence model.

