## 1 Introduction
However, the pre-chamber with intermediate cross-sectional area achieves the greatest combustion rate at the early combustion phase, and consequently, the most stable combustion is shown by the lowest coefficient of variation of indicated mean effective pressure (CoV IMEP). With all that said, the best performance in terms of pre-chamber geometry, influenced by lean limit, short combustion duration, low emissions, and high efficiency [12], can be obtained by optimizing the pre-chamber for different engine speeds, loads, and spark timings [13]. By using simulations, it has been concluded that a larger pre-chamber volume is more favorable in most operating conditions, while orifices with smaller diameters ensure better performance in lean mixtures because of the increased cross-flow velocities, which increase combustion speed. Despite the benefits of an active pre-chamber spark-ignited system, it still has challenges at very lean mixtures, low engine speeds, and high loads [14], and conditions at which the highest efficiency is obtained do not satisfy low-NOX-emission regulations. Another promising low-temperature lean burn concept is a homogeneous charge compression ignition (HCCI), which combines the advantages of compression ignition (CI) and spark ignition (SI) combustion. In HCCI, the premixed homogeneous charge is autoignited due to a temperature rise during compression stroke [15]. This enables operating with very lean mixtures and consequently results in reduced heat losses and low pumping losses. The form of combustion can be used on many different fuels, i.e., fuels that can form a premixed air/fuel mixture; however, because of their different chemical pathways, different fuels show different requirements for operating conditions and different performances. The HCCI combustion fueled by gasoline shows higher indicated efficiency compared to methane, and higher indicated efficiency than the conventional SI combustion mode [16] at the same engine load. At the same time, NO\(\chi\) and soot emissions are very low. To enable HCCI combustion, intake temperatures must be increased, but in [17], it was observed that the increase in compression ratio results in lowering the intake temperature and higher indicated power and torque. The increase in load, represented by the indicated mean effective pressure (IMEP), can be obtained by increasing intake pressure while high indicated efficiency and stable operation are maintained [18]. The HCCI combustion has some unresolved challenges that prevent its wide use, and among them, the lack of autoignition timing control is one of the most important. Control of combustion timing is important because if the start of ignition is too early, it leads to a high ringing intensity, while combustion phasing that is too late leads to incomplete combustion or misfire [15]. Small variations in the intake air temperature result in significant changes in combustion timing, which can lead to unstable engine operation and lack of ignition [19]. Some new combustion concepts that are based on HCCI and aim to provide more control have been researched, e.g., partial fuel stratification [20], spark-assisted compression ignition (SACI) [21], etc. In the partial fuel stratification concept, the reactivity of the mixture is varied by changes in fuel stratification, but the concept relies on the ratio of the sensitivity of fuel reactivity to excess air. This sensitivity in gasoline changes as the system pressure changes with increased sensitivity at higher pressures and is not uniform throughout the entire operating conditions. In spark-assisted compression ignition, the spark discharge is used to initiate an autoignition process similar to HCCI combustion while ensuring controllability of combustion phasing similar to conventional SI engines. This provides stable ignition and combustion that is less dependent on transient and environmental conditions while keeping high efficiency and low emissions, similar to the HCCI combustion engine.