## 2 Methods
### Experimental Measurements  
The experiments conducted in this study were performed using an experimental setup at the Laboratory of IC Engines and Motor Vehicles of the Faculty of Mechanical Engineering and Naval Architecture at the University of Zagreb. The testbed consists of an AC dynamometer with a maximum brake power of 30 kW at 4000 rpm and a research engine made from Hatz 1D81Z. The research engine is an air-cooled single-cylinder engine modified to operate in different combustion modes. The compression ratio of the engine can be varied by using cylinder support rings and gaskets of different thicknesses, while for different combustion modes, different modifications of the engine head and of the intake system are made. In this study, the compression ratio is set to 17.5 and the head was modified to house the pre-chamber. The pre-chamber system is an active system that consists of a spark plug and a pre-chamber injector. Pre-chamber system design enables changing the pre-chamber tip, which enables easy change of pre-chamber geometry. The air-cooling system's temperature depends on environmental temperature inside the testbed room, while cooling air mass flow is defined by crankshaft rotational speed. The wall temperature of the combustion chamber depends on the engine load and testbed room temperature and is similar at the same engine loads, but it is not directly controlled. However, the pre-chamber and high-pressure injector are additionally cooled by compressed air to prevent overheating, especially at high loads. Engine specifications are given in Table 1.  
As previously mentioned, the engine was naturally aspirated without intake throttling. Intake air is heated by an inline air heater to achieve constant predefined intake temperatures. During the experiment, intake, exhaust, cylinder, and pre-chamber pressures are measured, as well as temperatures. For emissions, the carbon monoxide (CO), carbon dioxide (CO\({}_{2}\)), total hydrocarbon (THC), and nitrogen oxide (NO\({}_{\text{X}}\)) emissions are measured. A detailed description of the experimental setup can be found in [7]. The pre-chamber geometry used in this experiment is the geometry with a small volume and 6 orifices. The geometry parameters of the pre-chamber are given in Table 2.  
Figure 1 shows the previously described experimental analysis approach on some of the results from previous investigations by the authors, published in [6], that were used as a reference for the design of experiments for the PC-HCC combustion. Figure 1a shows the results of PCSI combustion at a global air excess ratio of 1.6, which represents the dilution level after which the highest indicated efficiency is no longer limited by knocking combustion, as well as the minimal dilution level where the NOx emissions can be maintained under the legal limit of 0.4 g/kWh. By advancing the spark timing, an increase in indicated efficiency can be observed, but the combustion phasing is limited by the occurrence of knock (operating point marked red). By retarding the spark timing, it is possible to reduce the NOx emissions below 0.4 g/kWh (operating points marked green) at the expense of the indicated efficiency of around 2% of points when compared to the operating point with the highest efficiency (marked blue) which satisfies the knock limit. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Engine type & 4 stroke, air cooled \\ Displacement & 667 cm\({}^{3}\) \\ Stroke & 85 mm \\ Bore & 100 mm \\ Connecting Rod length & 127 mm \\ Compression ratio & 17.5 \\ Number of Valves & 2 \\ Inlet Valve Opens/Closes & 36 \({}^{\circ}\)CA BTDC/60 \({}^{\circ}\)CA ABDC \\ Exhaust Valve Opens/Closes & 54 \({}^{\circ}\)CA BBDC/21 \({}^{\circ}\)CA ATDC \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Engine specifications.