## 1 Introduction
During the operation of internal combustion engines, some components gradually degrade, leading to damage that, at a particular instant, is invisible to the user. In the early stages of damage development, there are changes in the frequency structure of the signal. The sensor's location is important for measuring vibroacoustic signals because the signal carries many of the processes in the engine. Therefore, the choice of sensor location significantly impacts the quality of information in a given signal and the accuracy of diagnostic decisions [9; 10]. For the case of the cracking of internal combustion engine components (i.e., shafts and nozzles), durability was defined as the number of cycles of stress changes, the number of repetitions of a specific load block, or the working (operation) time necessary for stable growth in cracking from an assumed initial dimension to a specified final extent [3; 4; 10]. In addition, cracks occurring in the mechanical elements of internal combustion engines cause vibration energy dissipation [3]. [PERSON] et al. [11] used noninvasive methods of vibroacoustic phenomena to diagnose a combustion engine fault. The noninvasive methods were essential in planning studies based on vibration and acoustic signals. The authors applied discrete wavelet transforms and probabilistic neural networks for signal analysis. Moreover, in another study [12], [PERSON] presented trials of using vibration signals generated with a four-cylinder car combustion engine to diagnose a fuel injector state. One of the diagnostic methods of combustion engines is dilatometry, which deals with methods of measuring the thermal expansion of materials [13]. The authors investigated the dilatometric methods for damage diagnosis and engine damage formation. Research reveals that two cases of malfunction relate to needle-nozzle holder precision pairs of self-ignition engines. Different thermal expansions of these pairs cause changes in clearances and friction energy [14]. In another study, many injector nozzles used in self-ignition engines cracked [15]. The damaged nozzles were composed of 38 CrMoAl steel and were nitride. The investigations revealed that the main damage modes are longitudinal and circumferential cracking on the external surface of the head region and the dent at the head tip. Metallurgical examinations showed that the present cracking failure of the nozzles is not related to the metallurgical defects and the strike on the heads by other objects. In addition, [PERSON] and [PERSON] examined cracked injector nozzles composed of 18 CrNi8 carburized steel [16]. Fractographic studies indicated that the origin of the crack was in an oil-filled groove within a thin wall, and a brittle intergranular fracture was the cause of the damage to the injector nozzles. Metallurgical studies performed with a spectroscope showed that brittle Al\({}_{2}\)O\({}_{3}\) inclusions were present in crack areas, and the stress concentration around the inclusions promoted crack initiation. [PERSON] [17] conducted a damage examination of a self-ignition engine injector nozzle used in a truck. The nozzle was produced from 18 CrNi8 case-hardening steel, and it cracked after about 400 h operation. Integrity assessment was performed on the injector nozzle via photo documentation, chemical analysis, microhardness measurement, metallographic investigation, and scanning electron microscopy. The research results showed that the damage occurred because of cavitation wear in the internal surface of the nozzle body, followed by fatigue cracking. [PERSON] et al. [18] studied injectors for bio-oil supply that suffered premature damage from 50 to 1500 h operation. They found plastic deformation, channel obstruction, microcracking, erosion, and cavitation damage. The authors concluded that the chemical and physical composition of the fuel caused damage to the injectors. Changes in the technical state of the engine, which are caused by the early stages of wear and damage, are difficult to detect.