Abstract:
A heat exchanger includes a housing with an exhaust inlet, an exhaust outlet, a fluid inlet, and a fluid outlet. A plurality of heat exchange conduits has a first surface in fluid communication with the exhaust inlet and the exhaust outlet and has a second surface in fluid communication with the fluid inlet and the fluid outlet, wherein the first surface is coated with a material including Teflon®.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/794,796, filed on Apr. 25, 2006. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to heat exchangers, and more particularly to heat exchangers for an exhaust gas recirculation system of a vehicle. 
     BACKGROUND OF THE INVENTION 
     Vehicle engines produce oxides of nitrogen (NOx) as a component of vehicle emissions. In an effort to reduce NOx levels in vehicle emissions, manufacturers typically employ an exhaust gas recirculation (EGR) system. The EGR system reduces NOx levels by recirculating exhaust gas into the intake manifold where the exhaust gas mixes with incoming air and fuel. NOx forms in high concentrations when combustion temperatures exceed a predetermined temperature. By diluting the air/fuel ratio, peak combustion temperatures are reduced. 
     Combustion temperatures can be further reduced by cooling the re-circulated exhaust gas. Therefore, some EGR systems include a heat exchanger that cools the exhaust gas before injection into the intake manifold. A valve or other metering device may be used to regulate the flow of exhaust into the intake manifold. 
     In the heat exchanger, the exhaust travels through a plurality of heat exchange conduits that are made from a thermally conductive material. One surface of the heat exchange conduits is in contact with the exhaust gas and another surface is in contact with a fluid (coolant or air) that absorbs heat from the exhaust gas. The heat transfer efficiency may be reduced due to fouling or coagulation of exhaust particles in the heat exchange conduits. As a result, the heat exchanger is usually oversized to compensate for fouling. This compromises packaging space, heat exchanger design, and/or vehicle weight. 
     SUMMARY OF THE INVENTION 
     Accordingly, a heat exchanger includes a housing with an exhaust inlet, an exhaust outlet, a fluid inlet, and a fluid outlet. A plurality of heat exchange conduits has a first surface in fluid communication with the exhaust inlet and the exhaust outlet and has a second surface in fluid communication with the fluid inlet and the fluid outlet, wherein the first surface is coated with a material including polytetrafluoroethylene, known by the trademark Teflon®. The trademark Teflon® will be used in the description. 
     In one feature, the housing includes an inlet plate and an outlet plate where first ends of the plurality of heat exchange conduits mate with the inlet plate and second ends of the plurality of heat exchange conduits mate with the outlet plate. 
     In another feature, heat exchange conduits include a plurality of elongate tubes that extend between the inlet plate and the outlet plate. 
     In still another feature, the material including Teflon® further includes bronze. 
     In yet another feature, an exhaust gas recirculation system includes the heat exchanger and an exhaust gas recirculation valve that directs exhaust gas from an exhaust to an engine. 
     In an alternate embodiment, a heat exchanger includes a housing including an exhaust inlet, an exhaust outlet, a fluid inlet, and a fluid outlet. An exhaust conduit in fluid communication with the exhaust outlet and the exhaust inlet, comprises an area between facing surfaces of a first plate and a second plate and the facing surfaces are coated with a material including Teflon®. A fluid conduit in fluid communication with the fluid inlet and the fluid outlet, comprises an area between facing surfaces of the second plate and a third plate, and the second plate transfers heat from exhaust gas flowing through the exhaust conduit to fluid flowing through the fluid conduit. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram illustrating a vehicle with an exhaust gas recirculation system according to the present invention; 
         FIG. 2A  is a cross-sectional side view of a tube-type heat exchanger according to the present invention; 
         FIG. 2B  is a cross-sectional end view of a tube-type heat exchanger according to the present invention; 
         FIG. 3A  illustrates plate-type heat exchanger according to an alternate embodiment; 
         FIG. 3B  is a cross-sectional side view of a plate-type heat exchanger according to an alternate embodiment; and 
         FIG. 3C  is a cross-sectional side view of a plate-type heat exchanger according to the alternate embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group), and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     According to the present invention, surfaces of heat exchange conduits in a heat exchanger are coated with a material including Teflon® to reduce fouling. 
     Referring now to  FIG. 1 , a vehicle  10  includes an exhaust gas recirculation (EGR) system  14  that selectively supplies re-circulated exhaust gas  16  to an engine  17 . The EGR system  14  includes the EGR valve  18  and a heat exchanger  19 . A control module  20  selectively opens and closes the EGR valve  18  during engine operation to allow the re-circulated exhaust gas  16  into an intake manifold  24 . The EGR valve  18  may be positioned between partially open and partially closed positions. Exhaust gas  30  from the engine  17  flows into an exhaust manifold  32 . A recirculation exhaust conduit  34  directs some of the exhaust gas  30  into the EGR system  14 . The heat exchanger  19  cools the re-circulated exhaust gas  16 . 
     Skilled artisans will appreciate that the present invention applies to various heat exchanger configurations. For example, the heat exchanger may have a tube-type, plate-type, shell-type, or any other suitable design. 
     Referring now to  FIGS. 2A and 2B , an exemplary embodiment of a tube-type heat exchanger  28  includes a plurality of tubes  50  that are located in a housing  52 . An exhaust inlet opening  53  and an exhaust outlet opening  54  are located at opposite ends of the heat exchanger  28 . The housing  52  also includes a fluid inlet opening  55  and a fluid outlet opening  56 . An inlet plate  58  and outlet plate  59  may be positioned between the exhaust inlet opening  53  and housing  52  and between the housing  52  and the exhaust outlet opening  54 , respectively. 
     The re-circulated exhaust gas  16  enters the heat exchanger  28  through the exhaust inlet opening  53 , flows through the plurality of tubes  50 , and exits through the exhaust outlet opening  54 . The re-circulated exhaust gas  16  is cooled as it flows through the plurality of tubes  50 . For example, a fluid  60  such as coolant or air surrounds the tubes  50 . Since the tubes  50  are made from a highly conductive material, the fluid  60  surrounding the tubes  50  absorbs heat as the re-circulated exhaust gas  16  flows through the tubes  50 . 
     The fluid inlet opening  55  and fluid outlet opening  56  define a pathway through the cylindrical housing  52  for the fluid  60 . More specifically, the fluid  60  enters the fluid inlet opening  55 , flows between the tubes  50 , and exits through the fluid outlet opening  56 . The inlet and outlet plates  58 , 59  contain the fluid  60  within the housing  52 . 
     A material  64  that includes Teflon® is applied to the inner surfaces of the tubes  50 . The material  64  may include a thermally conductive material since Teflon® impedes heat transfer. The thermally conductive material may include bronze. The material  64 , when applied to the heat exchange conduits in the heat exchanger  28 , reduces fouling. 
     Referring now to  FIG. 3A , an alternate embodiment of the present invention is a plate-type heat exchanger  100 . The plate-type heat exchanger  100  includes a plurality of plates, shown in conjunction with  FIGS. 3B  and  3 C, within a housing  102 . An exhaust inlet  104 , an exhaust outlet  106 , a fluid inlet  108 , and a fluid outlet  109  are in fluid communication with the plates within the housing  102 . 
     Referring now to  FIG. 3B , a cross-sectional side view of the plate-type heat exchanger  100  illustrates the flow of exhaust gas  1   10  through the plate-type heat exchanger  100 . According to an exemplary embodiment of the present invention, exhaust gas  110  enters the plate-type heat exchanger  100  through the exhaust inlet  104 , flows through a plurality of exhaust conduits  111 , and out of the exhaust outlet  106 . The plates  112  are placed in a parallel arrangement with respect to each other within the housing  102 . The plates  112  are separated from each other to create exhaust conduits  111  and fluid conduits  113  for the exhaust gas  110  and fluid  114  to flow through, respectively. The plates  112  will have one side in fluid communication with the exhaust gas  110  and the opposite side in fluid communication with the fluid  114 . A Teflon®-based material  115  is used to coat the exhaust conduits  111 , the exhaust inlet  104 , and the exhaust outlet  106  to reduce fouling. 
     Referring now to  FIG. 3C , a cross-sectional side view of the plate-type heat exchanger  100  illustrates the flow of fluid  114  through the plate-type heat exchanger  100 . According to an exemplary embodiment of the present invention, fluid  114  enters through the fluid inlet  108 , flows through a plurality of fluid conduits  113  located between the plates  112 , and out of the fluid outlet  109 . The plates  112  transfer heat from the exhaust gas  110  to the fluid  114 . In some implementations, the exhaust gas  110  and fluid  114  may flow in the same direction (not shown) and/or the plates  112  may have depressions (not shown) to create a more efficient heat transfer. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.