Abstract:
A reduced noise automotive ventilation system uses two blowers, a front mounted blower that pulls outside air in, and a rear mounted fan that exhausts interior air from the cabin space. By pulling and pushing in series, the speed of each blower is reduced, and the attendant blower noise is considerably reduced. In addition, a closed floor duct routes recirculated air from the rear to the front blower, eliminating the conventional open blower noise path to the vehicle interior in recirculation mode.

Description:
TECHNICAL FIELD 
     This invention relates to automotive ventilation systems in general, and specifically to such a system which uses a pair of in series blowers to reduce system noise. 
     BACKGROUND OF THE INVENTION 
     Typical vehicle bodies have what may be termed an interior or cabin space within which driver and passengers sit, and within which it is necessary to provide constant ventilation air flow and, preferably, to temper the air flow, as well. The cabin space is broadly comprised by a front bulkhead (structure such as instrument panels, etc, that wall off the engine compartment), a roof, floor, and rear bulkhead, defined generally as structure that walls off the rear seating area from the trunk. Conventional vehicle heating, air conditioning and ventilation systems (HVAC systems) consist basically of a box like housing, containing blower, heat exchangers, and air valves through which air is either pulled from outside the vehicle and blown into the cabin space, slightly pressurizing it, or recirculated within the vehicle. 
     In the “fresh” or outside air mode, the outside air that is pulled in by the blower is blown toward the rear of the cabin space and eventually, under the slight cabin pressure, flow through rear vents into the trunk space and passively exhausted back to the outside through conventional, passively acting vent valves. These are typically a one way acting rubber flap that can be blown slightly outward under pressure, but which close to prevent reverse flow. In recirculation mode, recirculated cabin air is pulled back up through a large under dash opening and into the HVAC housing before being blown back out. Blower noise is a concern with such a system. The single blower, since it provides all of the air moving power both for air introduction and air exhaustion, must turn at high RPMs, and noise goes up severely with higher blower speeds. Another cause of noise is the large under dash opening through which recirculated air is pulled. It represents a very direct blower noise path from the HVAC housing into the cabin space. 
     Multi blower ventilation systems are known. More than one blower would not, intuitively, seem to provide a noise reduction, and just the opposite in fact. Such systems do appear to be primarily directed at issues other than noise, and primarily at a desire to separate the fresh air introduction and interior air recirculation functions by providing a blower and ductwork dedicated independently to each function. For example, a farm tractor ventilation system disclosed in U.S. Pat. No. 4,344,356 shows a two blower system in which the two blowers operate in parallel to separate the fresh air introduction and in side air recirculation functions . The primary purpose of the system is to create a constant and high interior cabin pressurization to assure that exterior dust is not pulled in, a concern that is more pronounced with agricultural and earth moving equipment. 
     SUMMARY OF THE INVENTION 
     The subject invention provides a ventilation system directed primarily at on road vehicles in which a pair of blowers operate cooperatively and in series, rather than in parallel, to both create a front to rear, outside air ventilating flow, and to create an interior air, recirculating air flow, with reduced blower noise in each case, despite the extra blower. 
     In the embodiment disclosed, two blower housings are incorporated, a ventilation housing at the front of the cabin space and an exhaust housing at the rear. A continuous duct runs from the exhaust housing back to the ventilation housing, taking the place of the under dash opening into the ventilation housing for recirculated air. The ventilation housing has a valved blower which, in a ventilation mode, pulls in outside air into the cabin space, or, in a recirculation mode, pulls air from the front of the common duct. The exhaust housing has a blower which, in an exhaust mode, pulls in cabin air and exhausts it, under power, outside the vehicle, or instead blows air into rear of the common duct and back toward the ventilation housing. 
     The two blowers thus work in series, when each is in its ventilation mode, to pull outside air in at the front of the cabin space and exhaust it, actively, through the rear of the cabin space. The same air flow can be achieved with each blower turning at significantly less speed than a conventional single blower. 
     The two blowers also operate in series in recirculation mode, but recirculate interior cabin air from the front of the cabin space, to the rear, and then back to the front through the common duct. In addition, since, in recirculation mode, the return leg of the flow is through a continuous, closed common duct, there is no direct, open noise path either blower to the cabin space. Both the reduced fan speed and the closed duct serve to reduce perceived blower noise within the cabin space. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the invention will appear from the following written description, and from the drawings, in which: 
     FIG. 1 is a side view of a typical vehicle partially broken away to reveal the cabin space and showing a conventional ventilation system in ventilation mode; 
     FIG. 2 is a view like FIG. 1, but in recirculation mode; 
     FIG. 3 is a side view of a typical vehicle and its cabin space, similar to FIG. 1, but incorporating a preferred embodiment of the ventilation system of the subject invention, in ventilation mode; 
     FIG. 4 is a view like FIG. 3, but in recirculation mode. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a typical vehicle, indicated generally at  10 , has an interior cabin space, indicated generally at C, having a front area indicated generally at F, and a rear area indicated generally at R. The terms “front” and “rear” are intended to be very general, and the structures that define and delimit the front and rear will, in any particular case, differ. Typically, the front would be defined by a those structures in front of a front seat passenger or driver, such as windshield  12 , instrument panel  14 , and the like. The rear would be defined by structures such as the shelf  16  behind the rear passengers and whatever structure separates the rear seating from the trunk space  16 . Above, the cabin space C would be bounded by the roof structure  18 , and below by the floor  20 . Terms such as “front” and “rear” are intended here to be inclusive, rather than exclusive, and relate more to the air movement inside the space C, than to its structural features. That is, air will move generally from front to rear, in a ventilation mode, and will circulate back again generally from rear to front in a recirculation mode. This will be generally true regardless of the physical structures that bound and define the cabin space C. 
     Referring next to FIGS. 1 and 2, in a conventional ventilation system, a housing  22  at the front of the cabin space C contains, in addition to one or more non illustrated heat exchangers, a blower  24  that forces air out of housing  22  and through a ventilation outlet  26  into the cabin space C. Blower  24  pulls air in through two alternate sources, a “fresh” or outside air inlet  28 , which opens outside the cabin space C and a recirculation inlet  30  located below the outside air inlet  28 , generally beneath the instrument panel  14 . Blower  24  is located generally between outside air inlet  28  and recirculation air inlet  30 . Located in front of blower  24  is a flapper door type air valve  32  which can be selectively swung back and forth between a ventilation mode, shown in FIG. 1, and a recirculation mode, shown in FIG.  2 . 
     Referring to FIG. 1, in the ventilation mode, door  32  closes off the recirculation inlet  30  and opens the outside air inlet  28  to blower  24  and the inside of ventilation housing  22 . Air is blown out of housing  22 , through the ventilation outlet  26  into the cabin space C, and forced generally to the rear. Specifically, air flow is along the inside of windshield  12 , along the roof  18  and, ultimately at the rear R of cabin space C, an exhaust vent  34  allows air to pass to the trunk space  16 . From trunk face  16 , air finally flows out of a conventional, passive one way vent valve  35  to the outside. Vent L valves like  36  are typically simple rubber flaps or membranes that work unidirectionally to let air out, but not back in. This keeps a continual flow of outside air through the cabin space C. Because all of the power for ventilation is provided by the single front mounted blower  24 , it must turn fast enough to provide enough power to send the ventilation air all the way to the rear and exhaust it through vent valve  35 . Since blower noise goes up dramatically with blower speed, high noise levels are almost inevitable. Proposals exist to dampen out such noise with insulation and baffles, but these muffle, rather than directly reduce, the noise, and also inevitably retard air flow. 
     Referring to FIG. 2, in the recirculation mode, door  32  swings up to close off outside air inlet  28  and open up recirculation air inlet to cabin space C. Now, blower  24  pulls interior air in through the recirculation air inlet  30 , up through housing  22  and back through outlet  26  into the cabin space C. Air moves front to back and returns to the front in a more or less circular pattern. The blower  24  must turn just as fast, and now a very direct noise path is opened up from blower to the cabin space C through the wide open recirculation inlet  30 . 
     Referring next to FIGS. 3 and 4, a preferred embodiment of the ventilation system of the invention can be and is used in the same vehicle, with the same cabin space, trunk, etc, and all common elements are given the same number primed (′). One significant difference is that the recirculation Inlet  30  no longer represents a direct opening to cabin space C′, and instead serves as a front duct port  36 , described in more detail below. Several additional features are added, which cooperate with existing features and components, to create an improved ventilation operation and significantly reduced noise. Generally, a second, rear mounted housing and blower, as well as a closed common duct between the front and rear, act in series to boost ventilation power but, counter intuitively, to reduce blower noise. Specifically, an exhaust housing  38  is provided at the rear, generally between the rear seating and the trunk space  16 ′, into which the exhaust vent  34 ′ serves as a cabin air inlet, rather than venting into the trunk space  16 ′ directly. Exhaust housing  38  contains a blower  40  similar to the front blower  24 ′, which pulls air into exhaust housing  38  and expels it selectively through either an exhaust outlet  42  into trunk space  16 ′, or through a rear duct port  44 , as directed by a flapper door type air valve  46 . In addition, in the embodiment disclosed, a second cabin air inlet  48  running beneath the rear seating. Extending continuously between the front and rear duct ports  36  and  44  is a closed, common duct  50 , which runs generally along the floor  20 ′, beneath the seating. These additional features work as described next. 
     Referring next to FIG. 3, when rear flapper door  46  is situated as shown, in what can be termed an exhaust mode, the front flapper door is concurrently moved to its ventilation mode. Now, rather than passively entering the trunk space  16 ′, air from the rear of cabin space C′, is actively exhausted by rear blower  40  into trunk space  16 ′. Interior air initially pushed toward the rear by front blower  24 ′ is now actively pulled into exhaust housing  38 , by rear blower  40 , through exhaust vent  34 ′ and additional inlet  48 , and pushed through exhaust outlet  42  into trunk space  16 ′. Since the exhaust action is now active, rather than passive, the additional interior air inlet  48  prevents the conventionally sized vent  34 ′ from limiting flow. Once in trunk space  16 ′, air is passively vented to the outside through the vent valve  36 . The rear blower  40  acts in concert or in series with the front blower  24 ′, actively pushing out the outside ventilation air that it actively pulled in. Therefore, neither blower  24 ′ nor  40  need turn as rapidly as conventional blower  24  would be required to turn to achieve the same ventilation air flow. Noise in the ventilation mode is thus significantly reduced, due to that factor alone. 
     Referring next to FIG. 4, when rear flapper door  46  is situated as shown, in a recirculation mode, the exhaust outlet  42  is closed off and the rear duct port  44  is opened. Concurrently, at the front, the front flapper door is situated so as to close off the outside air inlet  28 ′, and open the front duct port  36 . Now, air pulled into the exhaust housing through exhaust vent  34 ′, by rear blower  40 , is forced through rear duct port  44  and into the common duct  50 . Front blower  24 ′ pulls air from common duct  50 , through front duct port  36 , and ultimately blows it back into cabin space C′ through ventilation outlet  35 ′. Just as it the ventilation mode, the two blowers  24 ′ and  40  work in series, so the power needed for the recirculating flow is shared. Neither blower need turn as rapidly as a conventional single blower would, thereby inherently reducing blower noise. Furthermore, the closed, continuous common duct  50  prevents the direct, open noise path to the cabin space C′ that exists with the conventional recirculation mode described above. 
     Variations in the disclosed embodiment could be made. The flapper doors could be replaced with film belts or any other valve mechanism that would selectively open up or block off the respective openings. The common duct  50  could be one or more ducts or the equivalent, and could extend elsewhere than along the floor  20 . However, that is the most convenient location, and it is unlikely that one would ever wish to reverse the air flow pattern, that is, vent air to flow nearer the floor and return air to run nearer the roof. The exhaust air could be exhausted even more directly from the cabin space C, that is, directly outside, rather than to the trunk space  16 . However, the trunk space  16  is a convenient exhaust air site in vehicles that have one. Therefore, it will be understood that it is not intended to limit the invention to just the embodiment disclosed.