## 2 Experimental Setup and Data Acquisition  
The research was conducted on a single-cylinder research engine type SB 3.1, constructed based on the technical solutions of the multicylinder SW 680 engine. The engine'sdesign includes a connecting rod, a piston with rings, a cylinder liner, timing valves, a valve control system, an injector, and a modified cylinder head directly derived from the SW 680 engine. This solution provides extensive capabilities for analysing the combustion process and selected operational parameters characteristic of the SW 680 engine series. The SB 3.1 engine enables cylinder pressure measurement, compression ratio (\(\epsilon\)) adjustment in the range from 14 to 20, and smooth variation of fuel injection timing and valve timing phases, making it a versatile research stand. The input parameters adopted were combustion pressure and rotational speed. Engine parameters are presented in the table (Table 1). To evaluate combustion quality based on vibration signals and operational parameters, the engine was mounted on a dynamometer stand equipped with the following elements:  
* AMX 210 eddy-current brake for torque measurement;
* AMX 210 photoelectric sensor for rotational speed measurement;
* AVL 733 gravimetric meter for hourly fuel consumption recording;
* AVL 553 temperature stabilization system to maintain a constant cooling fluid temperature at 75 \({}^{\circ}\)C.  
The stand was also equipped with a vibration measurement system, comprising a Bruel&Kjaer type 4391 piezoelectric transducers mounted on the engine head (tri-axial adapter), a Bruel&Kjaer 3050-A-060 measurement cassette (Bruel & Kjaer, Naerum, Denmark), and an AVL 364 crankshaft angle marker for synchronization with the engine's operational cycle. A schematic diagram of the system is presented in Figure 1a. and the vibration signal acquisition location in Figure 1b. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Ignition type & Compression ignition \\ Working cycle & Four-stroke \\ Maximum useful power (N\({}_{\text{e}}\)) & 23.2 kW at 2200 rpm \\ Maximum torque (M\({}_{\text{o}}\)) & 109.8 Nm at 1600 rpm \\ Cylinder bore diameter (D) & 0.127 m \\ Piston stroke (S) & 0.146 m \\ Compression ratio (\(\epsilon\)) & 15.75 \\ Connecting rod to crank radius ratio (r/L) & 0.262 \\ Engine displacement (V\({}_{\text{ss}}\)) & 1.852 \(\times\) 10\({}^{-3}\) m\({}^{3}\) \\ Number of cylinders (i) & 1 \\ Combustion system & Direct fuel injection into the cylinder \\ Injection pump type & Inline plunger, type P 51T2–10.03 II RVO \\ Number of valves per cylinder & 2 \\ Intake valve opening & 4\({}^{\circ}\) CA BTDC \\ Intake valve closing & 57\({}^{\circ}\) CA ABDC \\ Exhaust valve opening & 42\({}^{\circ}\) CA BBDC \\ Exhaust valve closing & 24\({}^{\circ}\) CA ATDC \\ Injector type & W1B-01 with D1 LMK 140/2 nozzle \\ Injector opening pressure & 17.5 \(\pm\) 0.5 MPa \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters of the SB 3.1 engine. During the tests, the signals were recorded within a frequency band ranging from 0.1 Hz to 10 kHz, with a sampling frequency of 12 kHz, thus enabling spectral analysis up to 6 kHz (in accordance with [PERSON]'s criterion). Calibration of the measurement channel was carried out before each measurement series to minimize errors. Controlled quantities were torque (measured and set using the eddy-current brake) and crankshaft rotational speed. The maximum achievable torque on the stand was 90 Nm; thus, five load levels were applied: 0%, 22.5 Nm (25%), 45 Nm (50%), 67.5 Nm (75%), and 90 Nm (100% of maximum torque). The range of rotational speeds included values of 700, 1000, 1200, 1500, and 1700 rpm. At the lowest speed (700 rpm), measurements were conducted exclusively at idle (0% load), since in practical diesel engine operation, significant loads are rarely applied at such low speeds (Table 2).