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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.

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.