## 4 Conclusions  
An experimental and numerical evaluation of the pre-chamber induced HCCI combustion concept (PC-HCCI) is performed to investigate the flexibility in inducing HCCI combustion in the main chamber and to show the overall engine performance and emissions. It was shown that:  
* A very similar performance is obtained with significant variations in intake temperature, proving the reliability and adaptability of this combustion concept.
* The combustion phasing can be reliably controlled by adjusting either the spark timing or the reactivity of the pre-chamber mixture. The optimal combustion phasing achieved with the combination of earlier spark timing and lower mixture reactivity in the pre-chamber leads to both higher efficiency and lower NOx emissions.
* Unlike in conventional HCCI combustion, the variations in boundary conditions during engine operation can be easily compensated for, ensuring adequate combustion stability, and eliminating potential misfires.
* A similar indicated efficiency can be obtained as with the conventional HCCI combustion, with up to 30 \({}^{\circ}\)C lower intake temperatures. The NOx emission levels are slightly elevated but still insignificant.  
* Compared to the PCSI combustion at the same engine load, an increase of 4 percentage points in indicated efficiency and two times lower NOx emissions were achieved. Compared to the most efficient PCSI operating point, the 1 percentage point lower indicated efficiency was obtained, but the specific NOx emissions are reduced by approximately 70%. This means that the PC-HCCI combustion can achieve ultra-low NOx emissions, while at the same time maintaining the indicated efficiency at a much higher level, which was not possible with the ultra-lean PCSI combustion.
* Overall, some major individual challenges of both PCSI and HCCI combustion modes can be resolved, without some new limitations or challenges identified. This makes the PC-HCCI a feasible technology for achieving reliable, stable, and efficient operation with ultra-low NOx emissions in both new and retrofit engines.  
In the following research activities, it is planned to repeat the same Design of Experiments with the initially used pre-chamber design to investigate the effect of pre-chamber geometry, followed by further investigations at different engine loads and engine speeds and a more targeted optimization of the intake temperature at each operating point.  
Conceptualization, J.K. and D.K.; methodology, J.K. and D.K.; software, J.K.; validation, J.K., S.U. and V.D.; formal analysis, J.K., S.U. and V.D.; investigation, J.K.; resources, J.K. and D.K.; data curation, J.K. and D.K.; writing--original draft preparation, J.K., S.U., V.D. and D.K.; writing--review and editing, J.K. and D.K.; visualization, J.K., S.U. and V.D.; supervision, D.K.; project administration, D.K.; funding acquisition, D.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Croatian Science Foundation (HRZZ), grant number IP-2019-04-4900 under the project \"Research of More Efficient and Environment-Friendly Pre-Chamber Spark Ignition Combustion\" (EF-PRECOM).  
Not applicable.  
The data presented in this study are available on request from the corresponding author due to the size of the datasets.  
The authors declare no conflicts of interest.