**1. Introduction**

The development of internal combustion engines focused on reducing fuel consumption is primarily leading to improvements in lean-burn technology. Gaseous fuels are mainly becoming attractive as an energy factor due to their lower carbon content, effectively reducing carbon dioxide and particulate emissions into the atmosphere [1].

The combustion of methane in spark-ignition engines in stoichiometric mode is a relatively common solution applied in many types of propulsion systems. An expanded opportunity to improve engine ecology and reduce fuel consumption is provided using a two-stage combustion system—TJI [2–5]. This system can be classified into passive and active prechambers [6]. The passive prechamber is filled with homogeneous fuel–air mixtures from the main chamber during the compression stroke. The active prechamber system is integrated with an auxiliary fuel-metering device to accurately control the equivalence ratio of the stratified mixture. Thus, the passive prechamber and active prechamber systems are also named homogeneous prechamber and stratified prechamber systems, respectively [6]. The above shows much greater benefits when using an active prechamber system. Ignition mechanisms [7–9] and inter-chamber flows have been well described and explained.

Combustion with turbulent jet ignition systems is carried out over a wide range of excess air ratios. In the range of excess air ratio 1–1.5, the authors of the article [10]

**Citation:** Pielecha, I.; Szwajca, F. Combustion of Lean Methane/Propane Mixtures with an Active Prechamber Engine in Terms of Various Fuel Distribution. *Energies* **2023**, *16*, 3608. https://doi.org/ 10.3390/en16083608

Academic Editors: Tomasz Czakiert and Monika Kosowska-Golachowska

Received: 13 March 2023 Revised: 18 April 2023 Accepted: 19 April 2023 Published: 21 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

investigated the combustion of various fuels in a prechamber using a single-cylinder test engine. Additionally, in the range of excess air ratio from 1.5 to 1.9, the effect of the crosssectional area of the prechamber discharge holes was studied. The tests were conducted on an engine fueled with natural gas [11]. Studies on the flammability of the air–fuel mixture using a constant-volume chamber and the TJI system were conducted. In addition to base natural gas, hydrogen was also burned [12]. On another model test stand, specifically a rapid compression machine, different configurations of the ignition system, such as PC spark plug, TJI, and a three-stage system, were compared for an excess air ratio of 1.5 [13]. In another study [14], Nanosecond Repetitively Pulsed Discharge (NRPD) ignition systems and turbulent jet ignition (TJI) were tested in a constant volume chamber with an excess air ratio of up to 1.8.

The effects of co-combustion of methane and propane are not well recognized, as evidenced by the small number of published research results. Studies of these fuels as an additive to diesel fuel have been conducted. Their use reduced fuel consumption by 21% or 15%, respectively [15]. The same reduction in NOx emissions (by 56–57%) was obtained regardless of the additive used.

The much higher density of propane than methane (1.964 versus 0.715 kg/m3) and a boiling point of −42 ◦C (methane −162 ◦C) cause difficulties in getting propane into the cylinder in gaseous form [16]. It is necessary to keep appropriate conditions such as low pressure.

One of the concepts for burning lean mixtures is the so-called hybrid combustion [17]. It uses a micro-flame-ignited (MFI) in dimethyl ether (DME) direct injection system and gasoline at a lambda excess air ratio of 2.0. Li at el. [17] conducted studies of DME combustion with the hybrid combustion of DME and gasoline. As a result, the indicated mean effective pressure (IMEP) and cyclic variation are reduced in the double direct injection conditions.

Modeling of the methane combustion process with the TJI system was conducted by Distaso et al. [18]. The research was carried out using an active prechamber at λ = 1.3. Such a value was found to be the limiting value in a standard engine. The prechamber (cylindrical in shape) was placed angular to the cylinder axis. As a result of the conducted exhaust emission analyses, it was determined that in both chambers at the exhaust valve open EVO, the mass fraction of CO2 produced is almost the same. The production of CO and HC in the prechamber is significantly higher (by two orders of magnitude). The mass fraction share of NOx in the prechamber is an order of magnitude smaller than in the main chamber.

A future direction for fueling internal combustion engines may be using ammonia. Liu et al. [19] report that using a TJI system for ammonia combustion improves the stability of engine operation and makes it possible to obtain higher IMEP values with respect to a reference engine. However, Vinod et al. list many barriers to the development of this fuel, such as long ignition delay, low flame development rate, and low reactivity [20]. The quality of the combustion process can be improved by adding methane to ammonia [21] or by using a reactivity controlled turbulent jet ignition (RCTJI) system [22]. Research by Zhang et al. [21] indicates that a 10 to 20% methane addition value leads to an increase in combustion pressure and an increase in the average rate of pressure rise.
