**1. Introduction**

Nowadays, reducing energy consumption and harmful emissions of internal combustion engines is a very important issue addressed by both research institutions and manufacturers of internal combustion engines. Development of control technology allows us to customize the operating mode of the device according to the currently required output parameters, while often the tuning of mechanical systems in terms of torsional vibration is ignored [1–6]. Tuning current mechanical drives in terms of torsional vibration by conventional (passive) vibroisolation methods is increasingly problematic. This is mainly due to operation in a wide range of speeds, uneven operation cylinders (deactivated cylinders, uneven fuel supply to the cylinders) and also the increased value of excitation amplitudes.

Vibroisolation methods can be divided according to what extent there is a controlled change of system parameters and according to energy supply requirements during operation of [7,8]: *passive vibroisolation*, where the dynamic parameters of the mechanical system cannot be actively changed and these solutions do not require additional energy for their function; *semi-active vibroisolation*, also called as *adaptive vibroisolation*, where it is possible to change basic parameters such as torsional sti ffness, damping coe fficient and mass moment of inertia, and these vibroisolation systems need energy during the change of their parameters; and *active vibroisolation*, where an additional dynamic torque component is introduced into the system, and these vibroisolation systems need constant power supply during the operation of the mechanical system.

In the case of passive vibroisolation, the system is pre-tuned in terms of torsional vibration in such a way that the system parameters such as torsional sti ffness of shafts (depending on their diameter) or mass moment of inertia (e.g., by adding a flywheel) are suitably adjusted. Another way is to use special devices such as flexible shaft couplings, vibration absorbers and dampers. In this method of torsional oscillating mechanical systems tuning, the properties of the system are predetermined. However, it should be noted that the properties of these elements (apart from flywheels and pendulum vibration absorbers) may change over time, causing the mechanical system to be not tuned properly. Viscous dampers contain fluids such as silicon oils whose properties change over time. Due to high temperatures, the used liquid may solidify, causing the damper to lose its function completely [9]. Flexible couplings with rubber elastic elements can significantly change their properties depending on the static torque, operating temperature, the number of operating cycles during operation and also due to the aging of the used rubber material [10–12].

Semi-active vibroisolation systems use devices with the possibility to change their mechanical parameters a ffecting the size of torsional vibration (torsional sti ffness, damping coe fficient and moment of inertia). Devices using a change of torsional sti ffness include some types of already manufactured shaft couplings such as pneumatic flexible shaft couplings [13–15] and magnetic shaft couplings [16,17]. Moreover, in the field of robotics, attention is currently paid to elements with variable torsional sti ffness (variable sti ffness actuators, variable sti ffness joints) and variable damping devices too. Although the use of these devices is expected mainly where there may be an unexpected collision with surrounding objects or humans (e.g., household robots), there is a possibility to utilize their principles in the field of mechanical drives too. A fairly comprehensive overview of these elements can be found in [18]. Devices with variable mass moment of inertia include flywheels containing weights, whose center of gravity in the radial direction relative to the axis of rotation can be shifted [19]. Alternatively, they contain a fluid mechanism [20].

Active vibroisolation systems are developed mostly in the form of design concepts (patents), though some were verified theoretically by dynamic model simulations or by simplified physical models in laboratory conditions, but gradually they are already beginning to be verified in real ship propulsion systems as well [8]. To eliminate torsional vibrations in real time, it is proposed to use devices such as electrodynamic brakes [21–23], piezoelectric dampers [24] and inertial mass actuators [8], which introduce additional torque to the system. The time course of the additional torque is proposed mostly as harmonic [7,8] or as periodic pulses [21,23].

This article deals with a semi-active vibroisolation system using pneumatic flexible shaft coupling with constant twist angle control. This system is suitable, as it is specially designed, for tuning mechanical systems where the load torque has a fan characteristic when the load torque is approximately proportional to the square of the rotational speed (fans, ship propellers, centrifugal pumps) [25].

The main goal of this research is to verify the ability of an electronic constant twist angle control system (ECTACS), developed by us, to maintain a pre-set constant twist angle of the used pneumatic flexible shaft coupling during operation of an experimental torsional oscillating mechanical system (TOMS) in laboratory conditions.
