Patent Publication Number: US-6698392-B1

Title: Single-part cooling channel piston for a combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a single-part cooling channel piston for a combustion engine with a piston head of forged steel, comprising a combustion bowl in the piston crown, a ring wall with a ring belt and an all-round closed cooling channel level with the ring belt. The piston skirt is connected to the pin bosses attached to the piston head. 
     2. The Prior Art 
     Generic single-part cooling channel pistons are known for example from European Patent No. EP 0 799 373 B1 or DE 100 13 395 C1. In the cooling channel pistons described therein, a piston blank is manufactured by forging an annular recess and the cooling channel, open to the bottom, is provided by metal-cutting machining methods, and then the outer contour of the piston is finish-machined. The axial height of the annular recess corresponds in EP 0 799 373 B1 at least to the axial height of the cooling channel. This is necessary since a hook-like turning tool is inserted into the recess for manufacture of the cooling channel open to the bottom and the cooling channel must be hollowed out into its required form by appropriate axial and radial infeed. 
     The drawback with these pistons is that the height of the hook-like turning tool determines the achievable cooling channel height and hence the quantity of heat to be dissipated from the piston crown as a consequence of high wall thicknesses. To increase the cooling channel height or to reduce the wall thickness between the cooling channel and the piston crown, the recess for insertion of the turning tool would have to be increased, which would however entail an unwelcome increase in the height of the piston. 
     On the other hand, the piston stability would decrease as a result of the aforementioned reduced wall thickness. Therefore, the above manufacturing methods and piston designs are not suitable for improvement of the piston with respect to its height or its stability for the high ignition pressures and temperatures as encountered in modern diesel engines. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved piston concept for a single-part cooling channel piston with which inexpensive manufacture of a low-height piston is assured and with which piston deformation due to the effects of high gas pressures and temperatures can be effectively countered. 
     This object is achieved by piston having a cooling channel formed in the piston head with holes spread over its circumference towards the piston crown, with the holes spaced such that the piston material present between such holes forms supporting ribs for the piston crown. 
     With a piston manufactured in this way, at least part of the cooling channel can be formed closer towards the piston crown or combustion bowl and nevertheless has excellent form stability plus a low piston height. In addition, the arrangement of the supporting ribs effects a kind of chamber formation inside the cooling channel, i.e. creates shaker areas, whereby a prolongation of the dwell time of the cooling oil is achieved and hence an improved heat dissipation of the piston areas to be cooled. In particular, the cooling effect can be further improved by a higher number of holes in those areas of the cooling channel in which the combustion radiation impacts the piston crown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention. 
     In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
     FIG. 1 shows a piston in accordance with the invention in a cross-section in the pin direction; 
     FIG. 2 shows a piston in accordance with the invention from below, in a section along the line II in accordance with FIG. 1; 
     FIG. 3 shows a piston in accordance with the invention, in a cross-section transverse to the piston pin direction; 
     FIG. 4 shows a piston in accordance with the invention, in a section along the line IV—IV in FIG. 1; 
     FIG. 5 shows a piston in accordance with the invention, in a perspective view; and 
     FIG. 6 shows a piston in accordance with the invention, in a section along the line IV—IV in FIG. 1, in a further embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in detail to the drawings, the single-part cooling channel piston in accordance with the invention comprises a piston head  1  of forged steel or forgeable aluminum alloy with a combustion bowl  3  in its piston crown  2 , a ring wall  4  with ring belt  11 , and an all-round closed cooling channel  7  level with the ring belt  11 , where a piston skirt  9  is connected to the pin bosses  12  attached to the piston head  1 . The manufacture of the piston is in accordance with EP 0 799 373 B1, where prior to the closure of the cooling channel  7  by means of a cover  13 , holes  14  are provided in accordance with the embodiment in the cooling channel  7  that are arranged symmetrically spread over the circumference and are positioned in the direction of the piston crown, i.e. parallel to the longitudinal piston axis K. The depth h B  of the holes  14  is not more than a quarter of the total height H of the cooling channel  7 , so that an unhindered circulation of cooling oil remains assured. Due to this design, shaker areas are created for the cooling oil which increase the cooling effect. 
     The cooling oil inlet  5  and the cooling oil outlet  10  are arranged opposite to one another in a cooling channel cover  13  comprising a two-part spring element. The cooling channel  7  is closed at its end open to the piston crown  9  by the cooling channel cover  13 . 
     As shown in FIGS. 3 and 4, the material areas between the holes  14  form supporting ribs  8  whose width is determined by the spacing of the holes  14 . The holes  14  are spaced in the cooling channel  7  on the circumferential side such that the spacing corresponds to at least half the diameter of the holes  14 , with the hole diameters all being identical. It is of course at the discretion of the person skilled in the art to use different hole diameters too, with the spacing then corresponding to half the largest hole diameter in order to counter any piston deformation during engine operation. 
     The holes  14  and hence the supporting ribs  8  are, as shown in FIG. 4, arranged radially symmetrically over the circumference of the cooling channel  7 . In a further embodiment (not shown), the number of holes  14  and hence the spread of the supporting ribs  8  can be such that in the major/minor thrust direction D or GD a larger number of supporting ribs  8  is arranged than transversely thereto, i.e., a non-symmetrical spread in the cooling channel  7  is achieved on the circumferential side. In this way, the spread of the holes  14  and hence of the supporting ribs  8  in the cooling channel  7  can, if the latter is characterized by the quadrants I-IV formed by the main piston axes K H , be such that within a quadrant a symmetrical (FIG. 4) or non-symmetrical or partially symmetrical spread (FIG. 6) is achieved, or the spread is dependent on the local temperature distribution in the piston crown. In particular, the cooling effect can be further improved by a higher number of holes in those areas of the cooling channel  7  in which the combustion radiation from the ignited fuel impacts the piston crown  2 . As a result, in the event of loading the tension strains of the piston can also be better countered. 
     The holes  14  can be designed as round holes, as shown in FIGS. 4 and 6, or as elongated holes (not shown) whose long sides face radially outwards from the piston center to the piston wall. With these respective embodiments of the hole arrangements, it is achieved that the holes have different spacings and hence the material designed as supporting ribs  8  is increased. Crucial for the spacing of the holes is that the distance from hole axis to hole axis corresponds on the circumferential side to at least half the largest hole diameter used. 
     For further influence on the heat dissipation from the combustion bowl  3 , the ends of the holes  14  can be round or, as shown in FIG. 1, angular. 
     The axes of the holes  14  can, as shown in FIG. 1, be arranged parallel to the longitudinal piston axis K and/or, i.e. in combination, at an acute angle thereto, with the holes preferably pointing in the direction of the piston crown  2  or combustion bowl  3 . 
     Accordingly, while only a few embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.