The typical air conditioning unit found in most automobiles comprises of a heating and/or ventilating fan in the dashboard region and includes a manifold for directing hot or cold air out through various openings above and/or below the dashboard. Examples of air conditioning units providing an additional second fan are disclosed and described in DE-OS 33 04 575 and DE-OS 30 43 934.
The air conditioning unit described in DE-OS 33 04 575 places an additional fan in the rear region Of the passenger compartment and is mounted to the existing rear vent opening located therein. The additional fan is provided to achieve a faster air exchange by exhausting interior air from the passenger compartment to the outside ambient atmosphere.
DE-OS 30 43 934 teaches to provide a second fan (in addition to a first fan housed in a conventional manner in the dash board) behind the back passenger seat to provide a uniform air circulation within the passenger compartment.
The addition of a second fan for the improvement of the operation of automotive air conditioning systems typically involves a trade off in driver (and passenger) comfort. For an air conditioning system employing two fans to perform adequately at higher vehicular speeds, the fans must be operated at their performance limits. This results in an undesirable increase in the amount of fan noise perceived by the driver and passengers. The undesirable fan noise is only compounded by the addition of a second fan in the rearward regions of the passenger compartment.
Therefore, there is a definite need in the art to provide a quiet air conditioning system having the advantages of quick air exchange between exterior and interior air and which provides uniform air circulation in the passenger compartment. There is also a need in the art to provide an improved dual fan air conditioning system whereby the noise generated by the fans is greatly reduced, and where the system is intelligent such that the fans recognize a sudden change in the passenger compartment air pressure (such as a sudden pressure drop in response to an opened window) and do not attempt to operate near their performance limits in order to keep the passenger compartment pressurized.