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. Concerning the passenger compartment, it is an intrinsic advantage of the enclosure that there is almost no noise to be radiated downwards onto the road and reflected back into the vehicle. Thus, it is not only possible to avoid disadvantages with respect to interior noise, but with some effort even the noise levels within the passenger compartment can be reduced