Abstract:
A corrosion-resistant heat exchanger for use in cooling a mixture of exhaust gas and charged air is provided comprising an air inlet chamber, an air outlet chamber, and a plurality of flat tubes through which the mixture passes and is cooled by ambient air. The flat tubes include turbulators to promote cooling of the exhaust gas/air mixture, and the turbulators are formed from a first aluminum alloy that is cladded by a second aluminum alloy, which combination of alloys create a sacrificial “brown band” layer upon brazing. In a highly preferred embodiment, both the tubes and the turbulators are formed of the same material.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a heat exchanger in general, and turbulator for a charge air cooler in particular, that is resistant to corrosion, and a method for making such a heat exchanger in an efficient manner. A heat exchanger including a turbulator made according to the present invention may be particularly advantageous for use with vehicles in which low-pressure exhaust gas is fed into the charge air cooler.  
       BACKGROUND OF THE INVENTION  
       [0002]     Automobile and other motor vehicle manufacturers face a constantly increasing demand to produce cleaner and more fuel-efficient vehicles. These manufacturers must produce internal combustion engines that meet power demands while simultaneously satisfying the growing emissions requirements of various governmental and industry standards.  
         [0003]     Engine technologies that address this demand include electronic engine controls and associated sensors, turbochargers and exhaust/engine gas recirculation (“EGR”) methods. Where charged air technologies are deployed, e.g., use of a turbocharger, it has long been recognized that a properly implemented heat exchanger—also known as an aftercooler or intercooler—can increase engine efficiency while simultaneously decreasing undesirable emissions.  
         [0004]     The process of charging air through a turbocharger results in an increase in temperature of the charged air. Aftercoolers are heat exchangers, typically air-to-air heat exchangers, that use ambient or outside air to cool engine intake air after it has passed through a charging device but before it enters an engine&#39;s combustion chambers. Cooler fuel/air mixtures burn more efficiently with significantly reduced emissions than charged air that is not cooled. Appropriate air charging, when coupled to air-to-air cooling devices, can substantially increase an engine&#39;s power density with little or no increase in its physical dimensions or reduction in its life-before-overhaul expectancy.  
         [0005]     Exhaust gas recirculation (EGR) is a complementary technology for reducing harmful emissions, particularly in diesel engines. Diesel engines compress large volumes of air, which when mixed with fuel and ignited combusts to create power. When a hydrocarbon fuel burns, the fuel is converted to carbon dioxide and water vapor. Additional byproducts of combustion include acids, particulate impurities as well as oxides of nitrogen (NOx). In an EGR system, some of the exhaust gases are returned to the intake manifold for additional combustion to increase the amount of fuel that is burned before entering the atmosphere. EGR systems also typically result in a lower combustion temperature and reduced NOx exhaust byproducts than non-EGR systems.  
         [0006]     In many EGR systems, exhaust gas is returned directly to the intake manifold of an engine. It is possibly, however, to also route exhaust gas through an air charger aftercooler or intercooler (for convenience, this patent refers to all such heat exchangers as “aftercoolers”) to further cool the exhaust gas before it enters the engine&#39;s intake manifold. Most aftercoolers, however, are designed to pass only air (not exhaust gas) through the cooling tubes of the heat exchanger. Although air typically does not include constituent elements that attack the components of an aftercooler, recirculated engine gas does. Exhaust gas includes, among other things, harmful acid condensates. These corrosive acid condensates, such as sulfuric, acetic and formic acids, increase the possibility of corrosion within the aftercooler.  
         [0007]     A need therefore exists for an aftercooler that can withstand the corrosive effects of the exhaust gas condensate. Delicate elements within the aftercooler, and “turbulator” components in the cooling tubes of the heat exchanger in particular, must be made corrosion resistant. Although corrosion resistant alloys for heat exchangers are known, such as those described in U.S. Pat. No. 6,921,584, which is incorporated by reference, no such alloys have been used on the turbulator components.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention is generally directed to a heat exchanger for a motor vehicle, such as an aftercooler, that includes turbulator components within the heat exchanger cooling tubes. The turbulator components are comprised of a core and cladding, which, when brazed in a brazing oven, create a “brown band” or sacrificial diffusion layer that resists corrosion in a superior manner when compared with previously known turbulator components. The cladding is typically an Al—Si alloy that has a lower melting point relative to the core. As is known in the art, during the brazing process, silicon from the cladding diffuses into the core, thereby forming a sacrificial layer, which is useful in resisting corrosion of the core components. This sacrificial layer is also known in the art as a “brown band” layer.  
         [0009]     In a highly preferred embodiment, the turbulator components are formed from a modified AA 3003 aluminum core or similar alloy (the “AA” designation refers to the Aluminum Association Inc., which specifies the composition of standard aluminum alloys), which, in turn, is cladded with a relatively thin layer (e.g., 5% of the core size) of AA 4045 aluminum on both sides. The turbulator components are then brazed to a tube component in a brazing oven. In a highly preferred embodiment, the turbulator components are formed by downgauging from the tube components. Alternatively, turbulator components may comprise the same or similar chemistry as used in the tube, such that a suitable brown band may be formed.  
         [0010]     Further objects, features and advantages of the invention, will become apparent from the detailed description of the preferred embodiments that follows, when considered in conjunction with the attached figures of drawing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Exemplary embodiments of the invention are given below with reference to the drawing, in which:  
         [0012]      FIG. 1  is a drawing that schematically illustrates air flow within an engine designed to route EGR exhaust gas through an aftercooler;  
         [0013]      FIG. 2  is a perspective view of an exemplary aftercooler heat exchanger that includes turbulator components;  
         [0014]      FIG. 3  is an exploded perspective view of a cooling tube within an aftercooler heat exchanger further illustrating the turbulator components;  
         [0015]      FIG. 4  is a perspective drawing that illustrates the core and cladding on an aluminum alloy useful in forming turbulator components according to the present invention;  
         [0016]      FIG. 5  is a cross sectional view of the aluminum alloy of  FIG. 4 ;  
         [0017]      FIG. 6  is a cross sectional view of a cooling tube within a heat exchanger made according to the present invention;  
         [0018]      FIG. 7A  is an exploded view of the region in which a turbulator component of the present invention is placed next to the side wall of a cooling tube prior to brazing; and  
         [0019]      FIG. 7B  is an exploded view of the region in which a turbulator component of the present invention is brazed to the side wall of a cooling tube; and  
         [0020]      FIG. 8  is provided for comparison purposes and is an exploded view of the region in which a prior art turbulator component is brazed to the side wall of a cooling tube. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]     One possible example for the application of the present invention involves an EGR turbocharged diesel engine system.  FIG. 1  schematically illustrates the “plumbing” of such a system. The diesel engine itself is schematically represented by reference numeral  8 . In this system, ambient air  1  enters a turbocharger  2 , which is powered by exhaust gas  7  exiting the diesel engine  8 . Exhaust gas  7 , after powering the turbine of the turbocharger  2 , is subsequently vented to the atmosphere  9 . Charged air  3  exiting from turbocharger  2  is thereafter mixed with a different stream of (low pressure) exhaust gas  5 . Although this mixing may occur in a variety of ways, it is illustrated in  FIG. 1  as occurring in a mixer element  4 , which may be a suitable valve or manifold.  
         [0022]     In this exemplary environment, after the charged air  3  is mixed with exhaust gas  5 , the mixture  6  enters an air-to-air aftercooler  10 . Air mixture  6  passes through cooling tubes (described below) within the aftercooler  10 , which are maintained in fluid contact with a forced stream of ambient air (not shown). The interaction of ambient air with the cooling tubes causes the charged air and exhaust gas mixture  6  to cool. The now-relatively cooler charged air and exhaust gas mixture  11  is supplied to the intake manifold of the diesel engine  8 , where it is subsequently combusted.  
         [0023]     The exhaust gas  5  that mixes with charged air  3  is at a relatively lower pressure in comparison with the exhaust gas  7  used to power the turbine within turbocharger  2 . The process of sampling exhaust gas  5  to reroute back into the intake manifold of a diesel engine is known to a person of skill in the art and is described, for example, in U.S. Pat. No. 5,802,846, which is owned by Caterpillar Inc. of Peoria, Ill., and which is hereby incorporated by reference in its entirety.  
         [0024]      FIG. 2  is a perspective, sectional view of a heat exchanger, and more particularly, a heat exchanger that may be used as an air-to-air aftercooler, such as the aftercooler  10  of  FIG. 1 . This type of heat exchanger is described in more detail in U.S. Pat. No. 6,729,388 B2, which is owned by Behr GmbH &amp; Co. of Stuttgart, Germany, and which is hereby incorporated by reference in its entirety.  
         [0025]     Charge air cooler  10  comprises a finned-tube block  12 , which is connected to a charge air inlet chamber  14  and to a charge air outlet chamber (not shown). The finned-tube block  12  comprises flat tubes  16 , between which fins  18  are arranged in the form of webbed fins or corrugated fins. The fins  18  are brazed to the flat tubes  16 . The charged air and exhaust gas fuel mixture  6  ( FIG. 1 ) flows from the charge air inlet chamber  14  through the flat tubes  16  to the charge air outlet chamber. Perpendicular to this charge air flow, the fins  18  are subjected to the action of ambient air  20 . Because the charge air has a significantly higher temperature than the ambient air  20 , heat transfer takes place from the charge air to the ambient air  20 .  
         [0026]     Reference numeral  22  identifies internal fin members, which are also known as “turbulators,” that are arranged within the flat tubes  16  and brazed thereto. The turbulators promote improved mixing of charged air within flat tubes  16 .  FIG. 3  is a cutaway view of a flat tube  16 , whose end wall  24  is also illustrated in  FIG. 2 . In  FIG. 3 , the orientation and arrangement of turbulator components  22  is more readily visible. The charged air/exhaust gas mixture  6  generally flows through the flat tube  16  in the direction illustrated by arrow  26 . By means of turbulators  22 , an improved mixing of the hot core flow with the boundary layer flow is achieved. Thus, the heat of the core flow is no longer guided by the internal fin member and the boundary layer flow in isolation by the wall. That is, the boundary layer flow is deliberately broken up and mixed. This results in an increase in the heat transfer performance of the charge air cooler  10 .  
         [0027]     Although  FIGS. 2 and 3  illustrate a particularly advantageous type of turbulator, it will be appreciated by a person of skill in the art that different structures and arrangement of structures may be used as the turbulator elements and still be within the scope of the invention.  
         [0028]      FIG. 4  illustrates aluminum cladding  32  being placed on an aluminum core  30 . As described in the above-referenced U.S. Pat. No. 6,921,584, which is incorporated by reference, the cladding is typically an Al—Si alloy that has a lower melting point relative to the core. As is known in the art, during the brazing process, the cladding becomes a sacrificial layer, which is useful in resisting corrosion of the core components. This sacrificial layer is also known in the art as a “brown band” layer.  
         [0029]     A cross-sectional view of a brazing sheet suitable useful in corrosive-resistant air-to-air heat exchanger components is illustrated in  FIG. 5 . This figure shows cladding  42  and  44 , which has been rolled onto core  40 . In a highly preferred embodiment of the invention, the cladding is comprised of an AA 4045 alloy and the core is comprised of an AA 3003 alloy or similar. Another suitable cladding alloy for controlled-atmosphere brazing (CAB) techniques includes AA 4343 or 4047. In addition, for vacuum brazing (VB) techniques, AA 4004, 4104, or 4047 cladding may be used. In a highly preferred embodiment, the thickness of cladding  42  and cladding  44  comprises no more than 15% of the thickness of the core  40 . Due to the diffusion of silicon during the brazing process, however, applicants have empirically determined that a cladding of approximately 5% of the thickness of core  40  will result in an optimal sacrificial layer. A composition suitable for creating such a “brown band” sacrificial layer, as shown in the rolled assembly of  FIG. 5 , may be used to form end walls  24  of flat tubes  16  of the heat exchanger ( FIG. 2 ) as well as the turbulator components  22  of tubes  16 .  
         [0030]      FIG. 6  illustrates brown band turbulators  22  within a tube  16 , and more particularly between end walls  24 . In the preferred embodiment of the invention, each end wall  24  is comprised of a flat tube that has a core  50 ,  60  and cladding on both sides  52 ,  55  and  62 ,  64  respectively, thereby forming the sacrificial brown band layer upon brazing. In this embodiment, both the tubes and the turbulator components are formed of the same brown band material, thereby promoting a maximum anti-corrosive effect.  
         [0031]      FIG. 7A  is an exploded view of the turbulator component  22  and the end wall  24  prior to a brazing process. Cladding  56   a  and  58   ba  surrounds core  55   a  of a turbulator component  22 . Likewise, cladding  62   a  and  64   a  surrounds core  60   a  of an end wall  24 . At this point in the process of manufacturing the heat exchanger, the brown band sacrificial layer has not yet been created.  
         [0032]      FIG. 7B , in turn, illustrates an exploded view of the same region illustrated in  FIG. 7A , but after the a turbulator component  22  of the present invention has been brazed to the side wall  24  of flat tube  16 . As is evident in this figure, the turbulator  22  itself includes a core  55  and sacrificial layers  56 ,  58 . Likewise, end wall  24  of the flat tube  16  is comprised of a core  60  and sacrificial layer  62 ,  64 . The brazing process converts the surface of the turbulator core by silicon diffusion from the cladding layer into the core, i.e.,  56   a ,  58   a , into sacrificial layers  56  and  58 . The same is true for the cladding on the end walls. In this highly preferred embodiment, the alloy used to form the turbulator components is identical to the alloy used to form the tube wall, although a person of skill in the art will recognize that this need not always be the case. In this regard, the material used to form the turbulator components may be “downgauged” from the material used to form the tube components. The chemical interaction of the various layers during a controlled-atmosphere brazing (CAB) and/or vacuum brazed (VB) process resulted in a sacrificial brown band layer being formed on the entire turbulator and tube assembly. As illustrated in  FIG. 7B , fillets  70 ,  72  represent the bond between the turbulator  22  and tube  24 , which fillets  70 ,  72  themselves include a brown band sacrificial layer.  
         [0033]     In contrast to the structure illustrated in  FIG. 7 ,  FIG. 8  illustrates a prior art turbulator/tube junction. In this prior art embodiment, neither the turbulator nor the tube includes a sacrificial brown band layer. Due to the lack of such layer, this turbulator component will not have the same anti-corrosive effects in connection with recirculated engine gas than will the more protected embodiment of  FIG. 7 . In addition, it has been found through experimental methods that the fillets  70 ,  72  created in the embodiment of  FIG. 7  are greater in size than the fillets  82 ,  84  of the embodiment of  FIG. 8 . It is noted, however, that the sacrificial layer in the preferred embodiment is formed independently of the shape of the tubulator components.  
         [0034]     While this invention has been described with an emphasis upon particular embodiments, it should be understood that the foregoing description has been limited to the presently contemplated best modes for practicing the invention. For example, the precise form of the turbulators and/or the flat tubes may be modified in accordance with the invention. It will be apparent that further modifications may be made to the invention, and that some or all of the advantages of the invention may be obtained. Also, the invention is not intended to require each of the above-described features and aspects or combinations thereof. In many instances, certain features and aspects are not essential for practicing other features and aspects. The invention should only be limited by the appended claims and equivalents thereof, since the claims are intended to cover other variations and modifications even though not within their literal scope.