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Possibilities of design and construction are presented by which the noise radiated by the surface of an engine can be reduced. Vibration isolation, stiffening, or sound-reducing shells, can be applied within comparatively short time; however, their effect on reducing the total noise is restricted to about 5 dBA. Total enclosure of the engine is highly effective. In this particular design, the space requirement is comparatively small, the weight increase is not too excessive, sound-absorbing material in the clearance is not required, and the noise reduction achieved is as high as 15-20 dBA. New design concepts, taking into account acoustical principles right from the start, will be the most economical approach for the future. Knowledge of the structure-borne sound within the engine is essential in this respect; particularly, further knowledge gained through tests on the nonrunning engine, using a “banger rig” for simulating combustion excitation.

In accordance with the main efforts of research and development in noise control of diesel engines measures which are essential for a noise reduction of more than 10 dB(A) and results achieved with an advanced direct injection combustion system are described. In general the noise of external engine parts can be reduced to a large extent only by vibration isolation or sound reducing shells. Considering economic aspects the attenuation of the total engine noise by more than 10 dB(A) requires a complete encapsulation of the engine which can with newly designed engines at least be partly integrated into the engine structure. Outgoing from some fundamentals about sound reducing shells the different approaches for low noise engines based on these principles and developed in the last decade are discussed.

This paper describes a newly constructed research facility which was specifically designed for noise reduction work on internal combustion engines. Various approaches for reducing engine noise are discussed, and a method which permits locating individual sources of structure-borne sound is reviewed. A measuring system for airborne sound, radiated from the engine surfaces, is described. Some new findings and new problems encountered in noise reduction work are discussed.

An efficient method for the reduction of vehicle noise is to close the engine compartment in a soundtight manner. By this means, the sound radiation from the power unit, which has main influence on the vehicle's exterior noise, can be reduced in an economic way. Furthermore, the increased suppression on the impulsive high frequency noise is subjectively recognized as an additional advantage. However, a large scale use of an encapsulation demands for proper cooling of the engine and, with respect to interior noise, a deterioration of passenger comfort must be avoided. In two different vehicles three kinds of engine enclosures have been tested. It is demonstrated that thermal problems within the enclosure can be avoided by an appropriate design. In addition, with some development of the cooling system, sufficient cooling rate can be provided.

Investigations have been carried out into the suitability of radial flow fans as a replacement for axial flow fans. Project objectives were to reduce cooling system noise without increasing bulk volume or impairing efficiency. These considerations apply particularly to vehicles with engines of high output. The application of tangential flow fans was also investigated and is covered in this paper. This type of fan may be better suited for radiators of low height and large width, typical of passenger cars with very low hoods. As the radial flow fan satisfied requirements, further testing was carried out to acquire data for the layout of such cooling systems. All significant design characteristics and ambient conditions were taken into account. Commercially available tangential flow fans did not provide improvements in either efficiency or noise. It is considered however, that development of rotor and air guiding walls may lead to the achievement of a satisfactory solution.

The noise and vibration properties of diesel engines call for increased efforts in manufacturing passenger cars to achieve a level of comfort comparable to gasoline cars. Severe problems arise at low engine speed when high compression forces give rise to vehicle body vibrations. Together with the vibration behavior at mid and high engine speed this fact demands a careful selection of engine mount position and mount properties. This selection must be made with respect to both the engine vibration excitation and to the vibration response of the vehicle body. Noise within the vehicle is influenced by the transfer of vibration energy from the engine into the vehicle structure and its radiation into air, as well as by the noise characteristics of the engine and the sound transmission properties of the vehicle cabin.

Future noise regulations for trucks require a 1 m engine noise level of 95 dBA or below at rated speed and load. To achieve this goal, noise reductions at source - lowering the excitation forces and the structure response - or secondary measures such as engine mounted shielding or encapsulation have proved to be effective. Since encapsulation technology is already well advanced, although rarely applied by vehicle manufacturers, the reduction of noise at source has become an important technique. In this paper it will be reported that the required noise level of 95 dBA can be achieved by the combined effect of reducing combustion excitation, optimising the engine structural response and using low noise materials for noisy engine parts. For all three subjects different approaches for noise reduction and their effect on overall engine noise, performance and reliability will be discussed in detail.

Investigations have been carried out as to whether cooling systems with tangential flow fans can be designed with the same noise, efficiency and package size as acoustically optimized cooling systems using axial flow fans. Previous work had shown that cooling systems with commercially available tangential flow fans could not meet these requirements. In the case of low and wide radiator cores, however, the application of tangential flow fans appeared advantageous since the tangential flow fan is fully adaptable to the width of the radiator core. For the main part of the development work an experimental tangential flow fan was used which permitted a wide range of modifications to be made and enabled the air flow to be observed. Extensive development resulted in a compact cooling system with a radiator core located downstream close to the impeller and with characteristics coming up to expectations.