*3.1. Test Stand*

A single-cylinder AVL 5804 research engine with Eddy current engine dynamometers was used to study the combustion process. The engine has an ignition system that allows real-time control of ignition advance angle and coil charging time. The cylinder head of the engine was adapted to a two-stage combustion system (Figure 1). The adaptation involved expanding the bore to fit the prechamber along with the direct fuel delivery system and pressure sensor. An active prechamber system with fuel delivery through a check valve was used. A prechamber with a 1.7 mm diameter straight 6-hole and an M10 spark plug was employed. Prechamber volume is 5.93% of clearance volume Vc. Due to the different properties of the fuels, methane was supplied to the main chamber at a pressure of 9 bar regardless of the engine mode. In single-fuel mode, methane was supplied to the prechamber at a pressure of 3 bar, while in dual-fuel mode the propane pressure was 1 bar. Fuel injection simultaneously occurs into the intake manifold and the prechamber during the intake stroke 300 ◦CA before TDC. An injector located in the intake manifold delivers fuel to the main chamber (this chamber is determined by the volume of the cylinder after the intake valves are closed)—Figure 1. Under these conditions, the use of higher injection pressure is not necessary. Other engine specifications are shown in Table 1 and Figure 2.

**Figure 1.** Scheme of cylinder head: (**a**) model of active prechamber; (**b**) view of main and prechamber; (**c**) view of intake duct and combustion chamber (main and prechamber).


**Figure 2.** Test stand layout with a 2-stage combustion system fueled by methane and propane.

	- Excess air ratio (λ = 1.3; 1.5; 1.8);
	- Dose of fuel to prechamber: energy value of fuel for prechamber: 10; 20; 30; 40; 45; 50; and 60 J (while keeping the total energy value of fuel supplied to the engine constant).
	- Engine speed: n = 1500 rpm;
	- Total fuel dose: qo = 13.5 mg (energy = 675 J).

In view of the different calorific values of the fuels (methane and propane—Table 2), the energy content of the fuel dose was determined, rather than the mass directly.



Studies of thermodynamic parameters of combustion were conducted using combustion pressure sensors for the main chamber, AVL GH14D (0–250 bar), and in the prechamber, Kistler 6081 (0–250 bar), whose signals were recorded using AVL IndiSmart (8-channel + IFEM amplifiers) together with AVL crank angle (364C01; 0.1 deg). Gas flow rates: air (Sensycon Sensyflow P; 0–400 kg/h; error < ±0.8%); combustible gases into the prechamber (Bronkhorst 111B; 0.1–100 g/h; accuracy ±0.5% RD plus ±0.1% FS) and into the main chamber (Micro Motion ELITE CMFS010M; 0.1–2 kg/h; accuracy ±0.25%). Gas feed settings were adjusted using a system for controlling the timing and start of injection (Mechatronics Control Gas Injectors). Exhaust gas analysis was carried out using an Axion RS+

analyzer (CO: 0–10% accuracy ±0.02% abs, HC: 0–4000 ppm accuracy ±4 ppm abs, NO: 0–4000 ppm accuracy ±5 ppm abs), a typical portable emissions measurement system (PEMS—Portable Emissions Measurement System) from Global MRV. Exhaust gases were measured using the following methods: CO and HC—spectrometric via analyzer (NDIR) and NO—electrochemical.

## *3.2. Method of Analyzing Research Results*

Methane was chosen as the primary fuel supplied to the cylinder due to its lower carbon content in the molecule. The research work included feeding methane or propane into the prechamber.

By changing the value of the excess air ratio and the type of fuel, engine operating conditions change even within the range of the same test point. Controlling engine operation by keeping the ignition angle constant, the maximum pressure angle constant, or the combustion center constant is possible. The last indicator, defined as the angle at which 50% of the heat is released (its value was set at 8 deg aTDC), was chosen:

$$\text{CoC} = \alpha \text{ at } 0.5 \times \int\_{\text{SOC}}^{\text{EOC}} \frac{\text{dQ}\_{\text{net}}}{\text{d}\alpha} \text{d}\alpha,\tag{1}$$

where SOC—start of combustion; EOC—end of combustion. In a similar way, the beginning of combustion (the angle at which 5% of the heat is exerted) and the end of combustion (the angle at which 90% of the heat is exerted) were determined.

A criterion for the stability of engine operation has also been defined as the unevenness of operation determined by the value of the coefficient of variation CoV(IMEP) < 3.0% [25]. Older sources give this indicator a value of 10% [26] or this value is given in a range [27]. This indicator was defined as

$$\text{CoV(IMEP)} = 100 \times \frac{\sigma(\text{IMEP})}{\mu(\text{IMEP})} \,\text{}\tag{2}$$

where σ and μ are the standard deviation and the mean value, respectively, over a number of consecutive combustion cycles (analysis applies to 100 consecutive cycles).

Other thermodynamic indicators were determined as follows:

1. Heat release rate dQnet(α) dα

$$\frac{\mathrm{d}\mathrm{Q}\_{\mathrm{net}}(\alpha)}{\mathrm{d}\alpha} = \frac{\gamma}{\gamma - 1} \mathrm{P}(\alpha) \frac{\mathrm{d}\mathrm{V}(\alpha)}{\mathrm{d}\alpha} + \frac{1}{\gamma - 1} \mathrm{V}(\alpha) \frac{\mathrm{d}\mathrm{P}(\alpha)}{\mathrm{d}\alpha} \,\mathrm{}\tag{3}$$

where P is the instantaneous cylinder pressure, α is the crank angle, γ is the ratio of the specific heats, and V is the instantaneous cylinder volume.

2. Indicative power (Ni):

$$\mathbf{N}\_{\mathbf{i}} = \frac{\mathbf{V}\_{\mathbf{s}} \times \text{IMEP} \times \mathbf{n}}{\boldsymbol{\pi}}\,\tag{4}$$

where Vs—engine displacement, n—engine speed, and τ—cyclicality of engine operation. 3. Specific fuel consumption (gi):

$$\mathbf{g}\_{\rm i} = \frac{\mathbf{G\_{MC}} + \mathbf{G\_{PC}}}{N\_{\rm i}},\tag{5}$$

where G—fuel consumption in the main chamber (MC) and prechamber (PC), respectively. 4. Indicative efficiency (ηi):

<sup>η</sup><sup>i</sup> <sup>=</sup> <sup>1</sup> gi × LHV , (6)

where LHV—heating value of methane.

5. Specific emissions of exhaust components (ei):

$$\mathbf{e}\_{\mathbf{i}} = \frac{\mathbf{a}\_{\mathbf{i}} \times \mathbf{C}\_{\mathbf{i}} \times (\mathbf{G}\_{\mathbf{a}} + \mathbf{G}\_{\text{MC}} + \mathbf{G}\_{\text{PC}})}{\mathbf{N}\_{\mathbf{i}}},\tag{7}$$

where i = CO, THC, NO, Ci—concentration; CO, THC, NO, Ga—air consumption; GMC, GPC—fuel consumption in both chambers; ai—density ratios (CO = 0.000966; CTHC = 0.000479, CNO = 0.001587) [28].

An illustrative curve of the measured quantities recorded during the tests is shown in Figure 3. In addition to the cylinder and prechamber pressures, the duration of the pulse controlling the injectors and ignition coil is also shown. Fuel was injected into both combustion chambers at different pressures. As can be seen from the figure, the dose injection time into the prechamber is significantly shorter than the fuel injection time into the main chamber.

**Figure 3.** An example of the waveform of the recorded signals with the description of the fuels fed to both combustion chambers (start of fuel feeding to both chambers at the angle α = 260 deg bTDC).
