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DEVELOPMENT OF A MULTILAYER STRUCTURE FOR POWER UNIT ACOUSTIC SHIELDING

The article presents the theoretical aspects of noise protection device development for the power unit of trucks, as well as the practical research and experimental development of materials for the production of acoustic capsules. The studies of physical-mechanical, heatconducting, sound-absorbing, vibration-absorbing properties of various polyurethane materials were carried out. The prospect of polymeric composite materials use as the acoustic protection of trucks has been revealed. One of the main problems of modern ecology is the noise pollution of the environment. More and more people feel the negative impact of noise on general health condition. Transport noise, as the main source, prevails in industrial environment [1]. The mechanisms and the systems of vehicle power units are the main source of both external and internal noise of transport. The requirements for the noise of cars throughout the history of vehicles are tightened constantly. Nowadays, the noise of vehicles is normalized by the national standard of Russian Federation GOST R 41.51 (UNECE Regulation No. 51). According to the document, the maximum value of the total noise level is in the range of 74-80 dBA.


INTRODUCTION
Despite the world experience in the design of vehicle noise reduction tools, there is a need to find new and more effective solutions to reduce vehicle noise. The problem of noise reduction is technically conditioned by the characteristics of power units and the layout solutions of a car as a whole [2,3,4,5].
The reduction of the noise emission of trucks, thereby the significant reduction of the overall levels of internal and external noise will allow the introduction and the improvement of an integrated system of design and technological measures.
Let's analyze the methods of automobile engine noise reduction. The classification of noise protection means and methods is described in GOST 12.1.029. Noise protection means that do not use an additional energy source are considered passive, and the means in which an additional energy source is used are considered to be active. The active means of protection work on the principle of attenuating interference between acoustic fields, which are formed by "primary" noise sources, whose sound field must be reduced, and by "secondary" sources, whose sound field is superimposed on the mutable sound field [6,7]. It should be noted that active noise protection systems are ineffective for noise process muffling in road transport, unreliable in operation (especially under conditions of high dynamic and temperature loads) and relatively expensive. Therefore, they are not widely used in mass production. In parallel with this, the means of collective and individual protection are distinguished. The consideration of individual means of protection against noise in relation to a car driver and passengers is not advisable.
The means of collective protection in relation to a source of noise excitation are subdivided: reducing noise in the source of its occurrence and reducing noise on the way of its propagation from a source to a protected object. The reduction of noise in a source of its occurrence is achieved by the reduction of noise excitation or the sound-emitting ability of a noise source.
A possible method of combat airborne noise is the soundproofing of a power unit by its shielding. This method is also called an acoustic encapsulation of an internal combustion engine body and a gearbox. An acoustic capsule can be made of a dense sound-reflecting metallic or a polymeric shell layer, including the one, covered (lined) on the inside by a sound-absorbing material. The effectiveness of this method depends on the properties of the used sound-absorbing materials and the design (the configuration) of an acoustic capsule body. The problems of noise emission of power unit isolation have been and still are the subject of research by many scientists [8,9,10,11,12].

MATERIALS AND METHODS
Acoustic materials are divided into vibration-damping, sound-absorbing, sound-proof and combined ones [13,14,15]. A lot of attention is paid to the development of new vibrationand sound-absorbing, vibration-and sound-insulating, combined materials, which is proved by the number of registered patent documents.
The disadvantages of soundproofing method are that the mass of a vehicle and its cost increases, and in the low-frequency region this method is not effective enough. Most materials used for noise suppression are fire hazardous. The indicator of car maintainability is decreased, since after the repair work performance, the properties of the soundproofing structure deteriorate inevitably. Besides, the shielding (encapsulation) of power units of vehicles complicates the process of heat removal from the thermally loaded surfaces of its housing and makes it difficult to ventilate an engine compartment.

RESULTS AND DISCUSSION
The All materials for acoustic shielding should have a minimum specific gravity, resistance to water, dirt, ozone aging, the effects of diesel fuel and motor oils, and be non-combustible and well suited to processing.
Fasteners should provide an easy installation and the removal of screens for prompt repair and inspection.
The work on the improvement of acoustic screens of trucks was started with the analysis of the acoustic characteristics of internal combustion engines of trucks [16].
The results of measurements concerning engine noise levels of different models are shown on During the analysis of these noise-absorbing materials, they found that the materials under study do not correspond to the set of established norms and requirements for noise-proof materials used for trucks (operating temperature range from -70 C° to +1000 C°, thermal conductivity coefficient 0.37 ÷ 0,41, the coefficient of sound absorption is 0.65 ÷ 90).
In this regard, it is required to search for and develop new composite materials that can be used as more effective materials for noise-proof screening of trucks.
Fiberglass based on a polyester matrix can be successfully replaced by the fiberglass based on a rigid polyurethane matrix with a lower specific gravity, which makes it possible to reduce the total weight of a power unit acoustic capsule.
They performed the studies of a rigid glass-filled PPU manufactured through innovative mass parts per 100 mass parts of the matrix component [17].
In order to assess the physical and the mechanical properties of fiberglass based on rigid polyurethane foam, it was tested to determine the breaking stress at bending and Shore D hardness. The results of the measurements are shown in Table 1.  The results of sound absorption, thermal conductivity and dynamic modulus of elasticity study of the developed composite materials based on an elastic polyurethane foam matrix with a polyol-isocyanate ratio of 125:100, and fillers-aerosil and aluminum hydroxide are presented in Table 2. It is the best of the studied filled composite materials on the basis of an elastic foam polyurethane matrix with dispersed filler the volumetric content of which is equal to thirty percent because it showed consistently high values of the sound absorption coefficient and increased thermal conductivity over the entire frequency range from 800 to 1600 Hz [18,19,20]. The problem of vibration isolation for acoustic capsules is preferable to be solved by the introduction of flexible dampers in the fastening units of screen frameworks to the bearing elements of a vehicle structure [21].
It is also noted that the use of aluminum foil as a working layer increases both the sound absorption coefficient (the increase occurs unevenly over the entire frequency range) and the coefficient of thermal conductivity (by about 5%). Aluminum foil together with the composite material acts as an additional source of thermal energy and sound wave dissipation. The use of corrugated aluminum foil enhances this effect.

SUMMARY
The analysis of the performed work shows the possibility of operational performance characteristic improvement concerning the acoustic screening of a truck power unit. The use of a lighter frame material made of rigid glass-filled polyurethane foam, manufactured using the "Fiber Composite Spraying" technology, reduces the total weight of an acoustic capsule.
The use of the developed noise-absorbing dispersion-filled composite material based on elastic polyurethane foam and aluminum hydroxide reduces the sound pressure level in the range from 800 to 2000 Hz, increases the heat transfer from the heated elements of an engine.
The use of dampers in the mounting area helps to reduce the transmission of vibrations, replacing the brackets of fasteners for quick-release locks and reduces the complexity of assembly and repair operations for acoustic screens.