## 2 Methods
### Numerical Simulations  
Figure 2 shows the numerical framework that was used in [23] for the numerical analysis of the PC-HCCI combustion concept as the preceding activity of the initial measurements performed in [22]. The same numerical framework is used in this study to further enhance the experimental measurements by giving insight into certain results that cannot be measured experimentally. The numerical framework consists of a reduced 1D/0D model made in AVL BoostTM, extensively validated based on experimental measurements and 3D-CFD validations in previous studies [13; 23; 24]. The existing 0D PCSI Combustion Model does not include chemical kinetics in the unburned zone; however, a multi-zone HCCI combustion model was developed at the Faculty of Mechanical Engineering and Naval Architecture and then integrated into AVL BoostTM v2013.2 [25]. This model was validated with an operating point taken from the previous experimental investigation of the HCCI combustion mode [19] and then used to simulate the HCCI combustion with the compression ratio of 17.5 to obtain the reference results for the evaluation of PC-HCCI combustion. Boundary conditions imposed in numerical simulations are the intake pressure profile and intake temperature, exhaust pressure and temperature, cylinder head temperature along with the operating parameters such as spark timing, injected fuel mass, etc. Additional information about the entire numerical framework can be found in [23]. As mentioned before, the pre-chamber excess air ratio cannot be easily determined because of the mixing process of different air and fuel mixtures from both the pre-chamber and main chamber through pre-chamber oifices. This can, however, be obtained by simulation, assuming the good accuracy of the model. In Figure 3, the comparison of pressure profiles between the experiment, 3D-CFD, and 0D PCSI combustion model at the highest efficiency operating point at \(\lambda=1.6\) is shown. For the same operating point, the comparisons of PC fuel energy and PC air excess ratio profiles between the 3D-CFD and 0D PCSI combustion model at \(\lambda=1.6\) are shown in Figure 4. It can be concluded that the 0D model can accurately predict the values of both PC fuel energy and PC air excess ratio around the expected spark timing range. Figure 2: The depiction of the numerical framework. Finally, as it is hard to define from pure measurements of cylinder pressure whether a measured point achieves the HCCI or PCSI combustion modes, the verification of the type of combustion mode is thus once again performed by 0D/1D numerical simulations. Since the 0D PCSI combustion model does not include chemical kinetics in the unburned zone, the combustion will be governed by flame propagation even at intake temperatures sufficiently high for HCCI combustion. That is why, by comparing the experimentally obtained pressure profiles with those obtained by numerical simulations of PCSI (flame propagation only) at identical boundary conditions, it can be indicated whether the HCCI combustion in the main chamber is triggered or not. If a good match between pressure traces from the experiment and the simulation is achieved, it can be assumed that pure flame propagation governs the combustion process. If the experimentally obtained pressure curve starts to deviate from that obtained by simulation, the triggering of the HCCI combustion is indicated. An example of such an analysis is performed on reference operating points taken from the initial measurements of PC-HCCI [22], whose operating parameters are given in Table 4.