TECHNICAL FIELD

[0001] This invention relates to an EGR cooler attached to an EGR apparatus, which recirculates exhaust gas from an engine to suppress generation of nitrogen oxides, so as to cool the exhaust gas for recirculation.

BACKGROUND ART

[0002] Known is an EGR apparatus which recirculates part of exhaust gas from an engine in a vehicle or the like to the engine to suppress generation of nitrogen oxides. In such an EGR apparatus, cooling the exhaust gas to be recirculated to the engine will drop the temperature of and reduce the volume of the exhaust gas to lower the combustion temperature in the engine without substantial decrease of output thereof, thereby effectively suppressing generation of nitrogen oxides. To this end, some EGR apparatuses are equipped with, midway of exhaust gas recirculation lines to the engines, EGR coolers for cooling the exhaust gas.

[0003] FIG. 1 is a sectional view showing an example of the above-described EGR cooler wherein reference numeral 1 denotes a cylindrical shell with axial opposite ends to which plates 2 are respectively fixed to close the ends of the shell 1. Penetratingly fixed to the respective plates 2 are opposite ends of a number of tubes 3 which extend axially within the shell 1.

[0004] A cooling water inlet 4 is attached from outside to the shell 1 near one end thereof and a cooling water outlet 5 is attached from outside to the shell 1 near the other end thereof so that cooling water 9 is supplied via the cooling water inlet 4 into the shell 1, flows outside of the tubes 3 and is discharged via the cooling water outlet 5 out of the shell 1.

[0005] The respective plates 2 have, on their sides away from the shell 1, bowl-shaped hoods 6 fixed to the plates 2 so as to enclose end faces of the plates 2. The one and the other hoods 6 provide central exhaust gas inlet and outlet 7 and 8, respectively, so that the exhaust gas 10 from the engine enters via the exhaust gas inlet 7 into the one hood 6, is cooled, during passage through the tubes 3, by heat exchange with the cooling water 9 flowing outside of the tubes 3 and is discharged to the other hood 6 to be recirculated to the engine via the exhaust gas outlet 8.

[0006] In such conventional EGR cooler, however, the end of the tube 3 penetrates into and is fixed to the plate 2 via a brazed portion 11 as shown in FIG. 2 in an enlarged scale. Especially on an outlet side of the exhaust gas 10 as shown, cooling the exhaust gas 10 in the tube 3 will generate condensate containing a vitriolic component which may flow out via the outlet of the tube 3 and corrode the brazing metal (generally nickel brazing metal) constituting the brazed portion 11. If the cooling water 9 should leak therethrough, it may be guided to the engine, causing a trouble.

[0007] Contemplated nowadays is welding the end of the tube 3 to the plate 2 by laser radiation L as shown in FIG. 3. Use of this kind of laser radiation will advantageously bring about laser weld 12 highly resistive against corrosion by the condensate; on the other hand, the weld penetration by the laser radiation L does not reach deep and sufficient weld depth D is hard to obtain, resulting in the laser weld 12 with low bonding strength. If the tube 3 should have undergone excessive thermal expansion, the laser weld 12 may be damaged to cause leakage of the cooling water 9.

[0008] More specifically, upon laser-welding the end of the tube 3 to the plate 2, generally effected is laser radiation L on the side of the plate 2 away from the shell 1 and in parallel with the axis of the tube 3, and welded is a boundary between the plate 2 and the end face of the tube 3 with weld depth D of the order of a wall thickness of the tube 3. As a result, only obtainable is the laser weld 12 having strength lower than that of the tube 3 itself.

[0009] In an actual operation, laser radiation L is effected from directly above, with the tube 3 being stood upright. Therefore, to merely increase the laser intensity for the purpose of increasing the weld depth will disadvantageously result in an increased possibility that the molten portion may flow into the tube 3 to narrow the flow channel. Thus, to increase the laser intensity is inherently limitative.

[0010] Furthermore, as described above, with the laser weld 12 having the shallow weld depth D, the tube 3 is welded to a through-hole 13 of the plate 2 over only a small area on the side away from the shell 1. Therefore, minute crevice may be formed over a major part of the boundary between the through-hole 13 and the tube 3. In this crevice, cavitation may occur due to variation of hydraulic pressure derived from minute vibrations of the tube 3, resulting in generation of crevice corrosion in a deepest portion of the crevice (a portion abutting on the laser weld 12). As a result, the end of the tube 3 may be damaged to cause leakage of the cooling water 9.

[0011] The present invention was made in view of the above facts and has its object to prevent the cooling water from leaking out to the flow channels of the exhaust gas, thereby preventing engine trouble from occurring.

SUMMARY OF THE INVENTION

[0012] An EGR cooler according to claim 1 of the invention comprises a cylindrical shell, plates fixed to axial opposite ends of said shell so as to close the ends of the shell, hoods fixed to sides of the plates away from said shell so as to enclose end faces of the plates, tubes extending axially within the shell and having opposite ends penetratingly fixed to the respective plates, cooling water being supplied into and discharged from said shell, exhaust gas being passed through said tubes from one of the hoods to the other hood for thermal exchange of said exhaust gas with said cooling water, and is characterized in that an end of the tube penetrating the plate is formed as a tapered portion with diameter gradually increased toward the side away from the shell, the tapered portion being wholly welded to the plate by laser radiation from the side away from the shell.

[0013] In this manner, when the end of the tube is formed as the tapered portion with diameter gradually increased toward the side away from the shell, the inner periphery of the tapered portion has a shape divergent to the side away from the shell to have a bevel in the form of mortar, so that laser radiation from the side away from the shell can be readily carried out throughout the inner periphery of the tapered portion.

[0014] Then, a resulting laser weld has a high bonding strength, the weld depth being increased to an extent corresponding to the thickness of the plate. Moreover, formation of minute crevice between the tube and the through-hole of the plate is avoided so that no crevice corrosion occurs.

[0015] An EGR cooler according to claim 2 of the invention comprises a cylindrical shell, plates fixed to axial opposite ends of said shell so as to close the ends of the shell, hoods fixed to sides of the plates away from said shell so as to enclose end faces of the plates, tubes extending axially within the shell and having opposite ends penetratingly fixed to the respective plates, cooling water being supplied into and discharged from said shell, exhaust gas being passed through said tubes from one of the hoods to the other hood for thermal exchange of said exhaust gas with said cooling water, and is characterized in that an end of the tube penetrates into a through-hole of the plate which is formed with a notch on the side toward the shell, and is welded to the plate by laser radiation from the side away from the shell such that a laser weld reaches the notch.

[0016] In this manner, with the through-hole of the plate being formed with the notch on the side toward the shell, an unwelded portion is left as the notch widely opened to the shell when the end of the tube is welded to the plate by means of laser radiation from the side away from the shell such that the laser weld reaches the notch. As a result, no minute crevice is formed between the tube and the through-hole of the plate and no crevice corrosion occurs. It is therefore possible to have a structure which, under the condition that no crevice corrosion occurs, has the laser intensity increased to such a degree that no portion melted by the laser radiation will flow into the tube to narrow the channel, and the weld depth increased as much as possible to increase the bonding strength of the laser weld as high as possible.

[0017] An EGR cooler according to claim 3 of the invention comprises a cylindrical shell, plates fixed to axial opposite ends of said shell so as to close the ends of the shell, hoods fixed to sides of the plates away from said shell so as to enclose end faces of the plates, tubes extending axially within the shell and having opposite ends penetratingly fixed to the respective plates, cooling water being supplied into and discharged from said shell, exhaust gas being passed through said tubes from one of the hoods to the other hood for thermal exchange of said exhaust gas with said cooling water, and is characterized in that the tubes penetrate into and are fixed to the plate via brazed portions such that an end of the tube extends out from the plate by a predetermined length and the extending end of the tube penetrates into and is fixed to a sub plate by laser weld, whereby said sub plate covers said brazed portions.

[0018] Thus, such covering of the plate with the sub plate will cause any condensate containing a vitriolic component, which may be generated by cooling the exhaust gas in the tube and may flow out via the outlet of the tube, to be isolated by the sub plate to which the tubes penetratingly fixed via the laser welds having high resistance against corrosion such that no condensate contacts the brazed portions of the plate. As a result, corrosion of the brazing filler metal constituting the brazed portions due to the condensate is positively avoided while the bonding strength of the tubes to the plate is kept high by the brazed portions. Even if water should leak due to any crack created in the brazed portions, the cooling water is dammed by the sub plate to stay between the sub plate and the plate.

[0019] An EGR cooler according to claim 4 of the invention comprises a shell in the form of a cylindrical container, tubes extending axially within the shell and having opposite ends penetratingly fixed to axial opposite ends of said shell, cooling water being supplied into and discharged from said shell, exhaust gas being passed through said tubes for thermal exchange of said exhaust gas with said cooling water, and is characterized in that the tubes have increased diameter and thickness so as to increase cross sectional areas and strength of flow channels, a gas flange being fitted over tips of the respective tubes extruded out of the shell.

[0020] This allows the number of tubes to be reduced to a required minimum and line for recirculation of exhaust gas may be properly branched and directly connected to the gas flange at the tips of the respective tubes extruded out of the shell. Therefore, even if condensate containing a vitriolic component is generated due to cooling of the exhaust gas in the tubes, avoided is its adverse effect such as corrosion on the penetrating fixed portions of the tubes to the shell. If a crack should be generated on the penetrating fixed portions of the tubes to the shell to cause water leakage, the leaked, cooling water leaks out only outside of the shell and is prevented from intruding into the flow channels of the exhaust gas.