Abstract:
Disclosed is a heat exchanger for a combustion engine, comprising a first connection zone ( 1, 102 ) for delivering a fluid that is to be cooled, at least some of said fluid being composed of exhaust gas of the combustion engine, a second connection zone ( 3, 103 ) for discharging the fluid, and an exchanger zone ( 2, 101, 104, 105 ) which is arranged between the first and the second connection zone relative to a flow path of the fluid. A coolant can flow around the exchanger zone ( 2, 101, 104, 105 ) while at least part of the heat exchanger is made of ferritic steel.

Description:
The invention relates to a heat exchanger for an internal combustion engine. 
     BACKGROUND 
     Heat exchangers for cooling recirculated exhaust gas are known from the prior art. In general, in the case of exhaust-gas cooling, there is the problem of the high chemical aggressiveness of the exhaust gas and the low pH value of its condensates. For this reason, only exhaust-gas heat exchangers produced from austenitic steels with high corrosion resistance have existed previously. Steels of said type result in high material costs and often further follow-up costs on account of the more complex machining operations. Furthermore, austenitic steels are usually poor heat conductors, such that heat exchangers with a predefined cooling power are of relatively large and heavy construction. 
     SUMMARY 
     It is the object of the invention to specify a heat exchanger for cooling exhaust gas or exhaust-gas/air mixture of an internal combustion engine, which heat exchanger can be produced at low cost. 
     As a result of at least a part of the heat exchanger being composed of ferritic steel, it is possible for costs to be saved on account of the usually relatively low prices for said steels. 
     In a preferred embodiment, the often better heat capacity of ferritic steels than austenitic steels is utilized to a particular extent in that the ferritic part of the heat exchanger is in contact with the fluid. Overall, therefore, a material-saving, weight-saving and cost-saving embodiment, of small construction, of a heat exchanger for exhaust-gas cooling is made possible as a result of the higher thermal conductivity of the ferritic steel. 
     The fluid is particularly preferably an in particular recirculated exhaust gas or exhaust-gas/air mixture of the internal combustion engine, with the fluid temperature in the first connecting region being more then 300° C., in particular more than 500° C., during normal operation. The risk of condensation of acidic condensate out of the exhaust gas is thereby reduced in the region of the entire heat exchanger. 
     In one preferred embodiment, the ferritic part of the heat exchanger corresponds substantially to the first connecting region and is welded to the exchanger region. The temperatures are particularly high specifically in the first connecting region, for which reason ferritic steels can be used in a relatively problem-free manner. In addition, ferritic steels usually have a lower coefficient of thermal expansion than austenitic steels, for which reason the combination of a ferritic connecting region with a subsequent austenitic exchanger region is particularly favorable with regard to expansion-related material stresses. In this context in particular, the first connecting region preferably has a flaring of a throughflow cross section in the direction of the exchanger region. An adjustable flap can also preferably be arranged in the connecting region. A distribution of the exhaust gas between a cooler region of a bypass duct can for example take place by means of the flap. 
     In a further preferred embodiment, the exchanger region has a plurality of exchanger tubes. Tube coolers are mechanically very stable and are expedient in particular in connection with a liquid coolant. For this purpose, the exchanger region expediently has an exchanger housing through which the liquid coolant can flow. Since the exchanger housing is often not in contact with the exhaust gas, it is particularly expedient for the exchanger housing to be composed of the ferritic steel, since even in the event of said exchanger housing rusting through, no liquid coolant passes into the combustion chambers of the engine. 
     In order to improve a heat exchanger power, the exchanger tubes can advantageously be composed of the ferritic steel, since said material has good thermal conductivity. 
     It can particularly advantageously be provided that a further part of the heat exchanger is composed of a further ferritic steel. There are ferritic steels with different levels of corrosion resistance and mechanical properties, which is often reflected in the material price. Depending on the extent to which the relevant part of the heat exchanger is subjected to corrosion or is involved in heat conduction, the different parts of a heat exchanger can be composed of different ferritic steels in order to optimize costs. 
     In a further preferred embodiment, the heat exchanger comprises a plurality of plate elements which are connected to one another in a stacked manner. A heat exchanger of said type is suitable in a particularly favorable manner as an exhaust-gas heat exchanger. Here, a fin element is advantageously arranged between the plate elements, which fin element is composed of the ferritic steel. On account of the design, corrosion of the fin element often does not result in the risk of a breakthrough of cooling liquid into the fluid region, which would otherwise lead to engine damage as a result of water shock. In particular separately insertable fin elements are therefore particularly predestined to be formed from ferritic steel. A fin element of said type can be arranged in the fluid to be cooled and/or in the coolant. If a fin element is arranged both in the fluid and also in the coolant, then said fin elements often differ in terms of design. 
     Here, a housing which encompasses the plate elements is particularly preferably provided, which housing is composed of the ferritic steel. Corrosion of the housing as a result of a long service life would not lead to a connection between the coolant and the exhaust gas, as a result of which the risk of engine damage is reduced. A housing of said type is a component of considerable size, in the case of which considerable costs can be saved by using ferritic steel. When using a sufficiently corrosion-resistant ferritic steel, the plate elements can however preferably also be composed of ferritic steel, which promotes the heat conduction and therefore the overall exchanger power for a given installation size. 
     A further part of the heat exchanger is generally preferably composed of an austenitic steel, as a result of which a material with a high level of corrosion resistance is used at least at critical points. The austenitic steel is preferably a steel from the group  1 . 4301  and  1 . 4404 . These material designations correspond to the DIN EN 100 88-2 standard, to which all of the numbered material designations specified within the context of the present invention relate. 
     The part composed of ferritic steel and the part composed of austenitic steel are particularly preferably directly cohesively connected to one another by means of welding or soldering. A cohesive connection of said type, in particular a direct or autogenous welded connection or by means of a soldered connection, ensures a particular secure connection. Tests have shown that at least the ferritic and austenitic steels preferred for the heat exchanger construction can generally be cohesively connected to one another, in particular welded or soldered or adhesively bonded, without problems. 
     The ferritic steel is preferably a steel from the group  1 . 4006  and  1 . 4016 . In the event of relatively low demands on corrosion resistance, the ferritic steel can preferably be a steel from the group  1 . 1169 .  1 . 0461 ,  1 . 0462  and  1 . 0463 , these being low-alloyed steels and fine-grain steels. Suitable higher-alloyed ferritic steels with at least 12% Cr content are preferable from the group  1 . 4000 ,  1 . 4002 ,  1 . 4006  and  1 . 4113 . Higher-alloyed and stabilized steels (with titanium and niobium) are preferable from the group  1 . 4509 ,  1 . 4513 ,  1 . 4512  and  1 . 4520 . 
     In a further preferred heat exchanger, the coolant is gaseous, in particular air. Exchangers of said type do not harbor the risk of water shock in the event of corrosion, and have particularly high demands with regard to heat conduction of the materials in order to obtain a suitable cooling capacity. The use of ferritic steels is therefore suitable. 
     A heat exchanger according to the invention can be arranged in a low-pressure branch downstream of an exhaust-gas turbine (low-pressure EGR). Lower mechanical loads and temperature differences occur in said arrangement. Heat exchangers can however also alternatively be arranged in a high-pressure branch upstream of an exhaust-gas turbine. 
     Further advantages and features of a heat exchanger according to the invention can be gathered from the exemplary embodiments described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Below, two preferred exemplary embodiments of a heat exchanger according to the invention are described and explained in more detail on the basis of the appended drawings. 
         FIG. 1  shows a three-dimensional, partially cut-away view of a first exemplary embodiment of a heat exchanger according to the invention. 
         FIG. 2  shows a three-dimensional exploded illustration of a second exemplary embodiment of a heat exchanger. 
         FIG. 3  shows a schematic section view through a fully-assembled heat exchanger according to  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The exhaust-gas heat exchanger according to  FIG. 1  is constructed according to the principle of a tube bundle exchanger. Said exhaust-gas heat exchanger has a first connecting region  1  for the supply of the exhaust gas (or exhaust-gas/air mixture), an exchanger region  2  in which the major part of the heat exchange takes place, and a second connecting region  3  for the discharge of the exhaust gas. An adjusting flap  6  which can be driven by means of an actuator  4  via a mechanism  5  is rotatably mounted in the first connecting region  1 , by means of which adjusting flap  6  the exhaust-gas flow can be deflected in an adjustable manner between a bypass duct  7  and a bundle of heat exchanger tubes  8 . 
     The bypass duct  7  and the exchanger tubes  8  are welded to one another by means of head elements  9 , with an exchanger housing through which liquid coolant can flow also being formed by means of a housing casing  10  by welding to the head elements  9 . Two connecting pipes  11  for conducting the liquid coolant through the exchanger housing are provided on the housing casing  10 . 
     In the described heat exchanger, at least the first connecting region  1 , which is composed of a housing which widens in the direction of the exchanger region  2 , is composed of a ferritic steel, in particular the steel  1 . 4006  according to the DIN EN 100 27-2 standard. The housing casing  10  is expediently also formed from this steel. 
     Depending on the temperature range of the exhaust-gas flow, the latter being dependent inter alia on whether the cooler is inserted into a low-pressure or high-pressure exhaust-gas recirculation system, it is also possible for the exchanger tubes  8 , the head elements  9  and also the second connecting region  3  to be composed of a ferritic steel. On account of the relatively high risk of condensation in the relatively cool region of the gas outlet, the second connecting region  3  is preferably produced from a ferritic steel rust-of resistant and stabilized quality, in particular  1 . 4512  or  1 . 4509 . The exchanger tubes  8  and/or the bypass duct  7  and/or the head elements  9 , in the event that these are composed of ferritic steel, are preferably produced so as to be of rust-resistant and stabilized quality (in particular  1 . 4512  and/or  1 . 4509 ). 
     In order to save costs, it is possible in particular for outer add-on parts such as for example retaining plates etc. to be composed of ferritic steel, in particular  1 . 1169 ,  1 . 0461 ,  1 . 0462  or  1 . 0463 . 
     The heat exchanger of the second exemplary embodiment ( FIG. 2 ) is embodied as a plate-type heat exchanger. A number of plate elements  104  are arranged in an outer housing  101  which has a first connecting region  102  for the connection of a supply for the exhaust gas and a second connecting region  103  for the connection of a discharge for the exhaust gas. The housing  101  also comprises a closure cover  105  on which are provided connections  106 ,  107  for the connection of supply lines and discharge lines for a coolant. The plate elements  104  and regions of the housing  101  and cover  105  together form the exchanger region of the heat exchanger. 
     Each of the plate elements  104  is constructed from two plates  104   a ,  104   b , with a fin element  108  being provided between the plates  104 ,  104   b . The in each case upper plate  104   a  has a pipe-like arched portion  104   c  which adjoins the edge of an aperture of the lower plate of the subsequent plate element. The individual pipes  104   c  of the plate elements are aligned with one another and with the connections  106 ,  107  of the cover  105 . The plate element  104  which is furthest remote from the cover has a lower plate  104   b  which has no apertures. In this way, a cavity through which the liquid coolant can flow is formed by the number of intermediate spaces between the in each case upper plate  104   a  and lower plate  104   b , with edge-side delimitations of the cavities being formed by welding the turned-up edges  104   d  of the plates  104   a ,  104   b  to one another. 
     The coolant flows in each of the plate elements between the one pipe which is assigned to the connection  106  and the other pipe which is assigned to the connection  107 . Here, the fins  108  around which the coolant flows ensure an additionally improved exchange of heat between the coolant and the plates, with turbulence of the coolant being generated in particular. 
     The intermediate space, defined primarily by the height of the pipe  104   c , between two adjacent plate elements  104  is in each case open at the end side of the plate elements to the connecting regions  102 ,  103  of the housing  101  of the heat exchanger. The exhaust gas flows through said intermediate spaces, with said exhaust gas being cooled on the large-area plate elements  104   a  which are cooled by the coolant. 
     For the mechanical stability and for the cooling the housing  101 , the longitudinal-side edge regions  104   d  of the plate elements  104  are turned up and bear flat against the inner wall of the housing  101  in regions (see in particular  FIG. 3 ). In particular, a welded or soldered connection of the plate elements  104  to the inner wall of the housing  101  is provided so as to cover as large an area as possible, such that the housing  101  is provided with a sufficient cooling capacity. 
     The housing  101  is preferably produced from a ferritic steel. The latter can in particular be a cost-effective steel such as for example  1 . 1169 ,  1 . 0461 ,  1 . 0462  and  1 . 0463 . In the event of corrosion of the housing part  101 , there would be no discharge of liquid coolant into the exhaust gas, for which reason the use of a cheaper material is permitted in the interests of a cost-risk trade-off. 
     In order to improve the exchanger capacity, and therefore also in order to reduce the installation size for a given exchanger capacity, the plate stack  104  and also the cover  105  can be composed of a ferritic steel. Since said elements generate a separation between the exhaust gas and the liquid coolant, the ferritic steel is preferably a particularly corrosion-resistant type, for example  1 . 4000 ,  1 . 4002  or  1 . 4113  or else a high-value ferritic steel such as  1 . 4513  or  1 . 4520 . 
     As shown in  FIG. 3 , fin elements  109  can also be arranged between the plate elements  104 , around which fin elements  109  the exhaust gas flows and which fin elements  109  therefore provide an enlarged exchanger surface. Said fin elements  109  can also be composed of ferritic steel.