EP0758715A1

EP0758715A1 - A piston for diesel engines

Info

Publication number: EP0758715A1
Application number: EP96610030A
Authority: EP
Inventors: Jorn Dragsted
Original assignee: Dampskibsselskabet AF 1912 AS; Dampskibsselskabet Svendborg AS
Current assignee: AP Moller Maersk AS
Priority date: 1995-08-11
Filing date: 1996-08-12
Publication date: 1997-02-19
Also published as: DK172217B1; DK90295A

Abstract

A piston for diesel engines comprises several parts (1, 2) inter-connected by way of welding and being adapted to transfer combustion forces to a piston rod (20). The piston (1) comprises a central integrally shaped part (2) which is adapted to be permanently connected to the piston rod (20)) and which comprises a cylindrical central wall (4) extending coaxially with the central axis (6) of the piston. In addition, the integrally shaped part (2) comprises ribs (5) projecting radially from the central wall and extending in substantially radial planes including the central axis (6) of the piston (1). The piston comprises furthermore a piston top (7) extending as an end wall for the cylindrical central wall (4) and the spaces between the radial ribs (5) to the area adjacent the outer ends of the ribs (5). The piston (1) comprises an outermost cylindrical part (3) provided with outer circumferential annular grooves (12-15) for receiving sealing rings (16-18). At an upper end the outermost cylindrical part (3) is connected to the outer periphery of the piston top (7) by way of welding.

Description

The invention relates to a piston for diesel engines and comprising several parts interconnected by means of welding, and which is adapted to transfer combustion forces to a piston rod.
It is known to reduce the thickness of the material in the top of a piston and consequently the temperatures in the material by using ribs for supporting said top. Here it is a question of structures cast integral with radial ribs or combinations of radial and circular ribs. A characteristic feature of such structures is that the inner surface of the piston cannot be processed or only processed at heavy expenses because such a processing necessitates tools capable of entering the tight cavities. In addition, it is difficult to remove the chips resulting from the processing. Furthermore, it is very difficult to check the quality of the material due to the casting skin and the poor access conditions.
Non-processed inner surfaces suffize for a number of purposes, but as a demand arises for an increased performance of the engine, the requirements to the nature of the surface and the quality of the material are intensified too so as to ensure the best possible heat transmission and fatigue strength. In other words the piston tops cast integral with rib supports will be expensive to manufacture. In connection with small pistons for four-stroke engines it is known to manufacture the piston top of several parts which are processed individually and subsequently interconnected by way of electron beam welding.
The object of the invention is to provide a piston which is suited for use especially for large two-stroke diesel engines, and which is relatively inexpensive to manufacture.
The piston according to the invention comprises a central integrally shaped part which is adapted to be permanently connected to the piston rod, and which comprises a cylindrical central wall extending coaxially with the central axis of the piston, said integrally shaped part further comprising ribs projecting radially from the central wall and extending in substantially radial planes including the central axis of the piston, as well as a piston top extending as an end wall for the cylindrical central wall and the spaces between the radial ribs to the area adjacent the outer ends of said ribs, and that the piston comprises an outermost cylindrical part provided with outer circumferential annular grooves for receiving sealing rings, said outermost cylindrical part at the upper end being connected to the outer periphery of the piston top by way of welding.
The resulting piston is structured in a simple manner and with parts which can be processed to a desired extent and subsequently interconnected by way of welding, such as by way of electron beam welding. Alone the central part transfers the combustion forces to the piston rod partly through the cylindrical support adjacent the centre and partly through the ribs projecting radially therefrom and associated with the piston top. The extent, the thickness, and the number of the radial ribs are adjusted to the expected load on the engine. The acceptable thickness of the piston top can be obtained by a processing by means of conventional cutting tools, and the chips can be easily removed. In addition, it is easy to check the quality of the material everywhere because the access conditions are fine. As a result, considerable cost advantages are obtained compared to conventional pistons shaped with ribs, and furthermore it has been highly ensured that the quality of the material meets the provisions. The number of ribs can be relatively high because it is not necessary to consider minimum requirements to the thickness of the cores which is necessary in connection with conventionally cast piston tops. Therefore, the thickness of the piston top can be further reduced though the same stress level is maintained.
As the piston is made of an integrally shaped central part and an outer cylindrical part which are subsequently welded together by way of for instance electron beam welding, it is possible to use various types of materials for the manufacture of the individual members. The outermost cylindrical part can be made of a curable material, such as unalloyed steel with a high content of carbon, while the central part can be made of a comparatively more heat--resistant material, such as a particular alloy, for instance Cr-Mo-steel. When the outermost cylindrical part is not made of a sufficiently wear-resisting material, the surface in the circumferential annular grooves are suitably coated with chromium. The two parts can, however, also be made of the same material and consequently be welded together by way of a conventional welding.
It is also known to manufacture pistons with inner channels for a circulation of a coolant. Unlike the conventional, non-processed ribbed pistons with an integrally cast piston top it is by means of the piston according to the invention possible to carry out an efficient cooling of the top when a guide plate according to the invention is secured in each rib space, said guide plate defining an upper cooling chamber adjacent the piston top, and the inner end of each of these cooling chambers communicate openly with the interior of the central wall by means of through bores therein, and the lower end of the outermost cylindrical part comprises closing means for sealingly closing the area around the central part to the surroundings. Such guide plates ensure an efficient cooling of the rear side of the piston top. The guide plates are suitably secured to the ribs by way of welding or by means of screws. The combination of radial ribs primarily being carrying, but also heat-conducting ensures optimum flow conditions for the heat-transferring coolant and allows a manufacture of a the piston top with minimum thicknesses in the material. The closing means is suitably formed by a piston skirt serving to control the piston in a surrounding lining.
According to the invention the ribs may be provided with bores extending from the interior of the central wall to the outer ends of the ribs and being adapted to carry coolant to or away from the outer ends of the upper cooling chambers. As a result, an efficient guiding of the flow of coolant is ensured in such a manner that said coolant is highly efficient everywhere on the rear side of the piston top and the adjacent parts of the piston. Further, bores may according to the invention be provided in the piston rod for guiding the coolant away from the interior of the piston.
According to the invention, the piston rod is particularly advantageously provided with a central, axial opening, in which a guiding means is mounted for guiding the coolant to and away from the respective sets of bores.
An efficient guiding of the flow of coolant to and from the upper cooling chamber is obtained by a circumferential cylindrical ring according to the invention being mounted at the outer ends of the ribs, the upper end of said circumferential cylindrical ring flushing with the outer ends of the end plates and being sealingly connected thereto as well as to the intermediary parts of the ribs, and by the ring being provided with an opening opposite each bore in the ribs.
According to a particularly simple embodiment the guiding means may according to the invention comprise a pipe arranged coaxially within the piston rod and being provided with a circumferential flange, said guiding means abutting by said flange the inner side of the cylindrical central wall of the central part between the respective sets of bores.
The invention is explained in greater detail below with reference to the accompanying drawing, in which

Figure 1 illustrates a portion of an axial, sectional view through an embodiment of a piston according to the invention mounted at the end of a piston rod, only a portion of said piston rod appearing,
Figure 2 is a sectional view taken along the line II-II of Figure 1, and
Figure 3 is a portion of an axial section of another embodiment of the central part of the piston according to the invention mounted at the end of a piston rod.

The piston shown in Figures 1 and 2 is designated the general reference numeral 1 and comprises a central part designated the general reference numeral 2, and an outermost cylindrical part designated the general reference numeral 3. The central part 2 comprises a cylindrical central wall 4, from which radial ribs 5 extend substantially along radial planes including the central axis 6 of the piston. At the top, cf. Figure 1, the central part 2 comprises a piston top 7 forming an end wall for the cylindrical central wall 4 and the spaces between the ribs 5. The piston top 7 extends to the area adjacent the outer ends of the ribs 5. The piston top is almost cup-shaped in a manner known per se with a recess at the centre towards the piston rod side.
The cylindrical central wall 4 is shaped integral with the radial ribs 5 and the piston top 7,as said parts are manufactured by way of casting or forging of a suitable material and processed according to desired. Guiding plates 8 are secured between the ribs 5, said guiding plates extending substantially parallel to the piston top 7. These guiding plates 8 are secured in a manner not described in greater detail by way of welding or screwing and define an upper cooling chamber 9 accommodated between the piston top 7 and the individual guiding plates 8. Each cooling chamber 9 is connected to the inner side of the cylindrical central wall by means of a through bore 10 and is besides open to the area around the ribs at the outer ends of the guiding plates 8, when seen in the radial direction. As illustrated by means of dotted lines, a bore 11 is furthermore shaped through each rib and extends from the inner side of the cylindrical central wall 4 at the end farthest away from the piston top 7 to an area adjacent the guiding plates 8, but on a level therebelow when seen in a direction away from said piston top 7.
The central part 2 is secured to the outermost cylindrical part by way of electron beam welding at 12. On the outer side the outermost cylindrical part 3 comprises circumferential grooves 12, 13, 14, 15, and 15', where sealing rings 16, 17, 18 and 18' are arranged in the latter four grooves in a manner known per se. The uppermost groove 12 serves to be engaged by a lifting device. Immediately opposite the outer ends of the guiding plates 8, the outermost cylindrical part comprises a circumferential recess 19 in the inner wall.
The piston 1 is secured to the end of a piston rod 20 through the central part 2 in such a manner that the central cylindrical wall 4 and the ribs 5 directly abut said piston rod 20 and are supported thereof. In addition, the ribs 5 are shaped with an axially projecting protrusion 21 at their outer ends for the abutment against the external periphery of the piston rod 20. The securing to the piston rod can be carried out in a conventionally known manner by means of bolts, which is diagrammatically shown at 22.
At the lower end farthest away from the piston top 7, the outermost cylindrical part is provided with an annular piston skirt 23. This piston skirt 23 serves furthermore as a closing means secured by means of bolts 24 and sealingly abutting the external periphery of the piston rod 20. A sealing ring 25 is provided between the closing means 23 and the piston rod 20. The closing ring 23 is dimensioned such that in use it guidingly abuts the inner side of the cylinder.
The piston rod 20 is provided with a central, axial opening 26, in which a tubular guiding means 27 is arranged. The tubular guiding means 27 extends coaxially to the piston rod, and at the upper end this guiding means 27 is provided with a flange 28 welded thereon for the securing of said guiding means 27 onto a number of projections 29 shaped at the end of the piston rod 20 around and along the rim of the opening 26. These projections 29 are arranged such that a free passageway exists between said projections 29 into the bores 11 through the ribs 5. The guiding means 27 is secured to the piston rod by means of bolts 30 and sealingly abuts the inner side of the central cylindrical wall 4 in the area between the openings into the two sets of bores 10 and 11 mutually separated in axial direction. In order to facilitate the securing of the guiding means 27, the flange 28 comprises a circumferential recess 31 for receiving the projections 29.
When the piston is used, coolant is carried to said piston through the piston rod 20 along the outer side of the guiding means 27, such as indicated by means of an arrow 32. Subsequently the coolant flows as indicated by means of an arrow 33 through the bores 11 to the inner side of the outermost cylindrical part 3 at the recess 19. Then the coolant flows as indicated by means of an arrow 34 upwards and towards the piston top 7 and then backwards and radially inwards as indicated by means of the arrow 35 through the cooling chambers 9 so as finally to flow through the bores 10 and into the interior of the central part 2. The coolant flows out of the central part 2 through the interior of the guiding means 27, cf. the arrow 36. The described guiding of the flow of coolant ensures that the parts of the piston being subjected to the highest temperatures, i.e. the area around the periphery of the piston top 7, are quickly supplied with fresh coolant, which in turn is quick removed again in such a manner that a good cooling and consequently a possibility of using a particularly thin piston top 7 is ensured.
Instead of the bores 11, it is possible to use bores 37 indicated by means of dotted lines through the piston rod 20 with the result that the coolant is guided centrally upwards through the guiding means 27 and radially outwards through the cooling chambers 9. Subsequently, the coolant is guided downwards into the bottom of the piston and again into the interior of the piston rod 20 through the bores 37.
As illustrated in Figure 3, where parts corresponding to the parts shown in Figures 1 and 2 are designated the same reference numeral, a circumferential cylindrical ring 38 can be mounted along the outer periphery of the ribs. This ring 38 is sealingly connected to the guide plates 8 at its upper end adjacent the piston top 7. The ring 38 is provided with openings 39 opposite the bores 11 through the ribs 5. This ring 38 ensures both an additionally efficient guiding of the flow of coolant to the cooling chambers 9 immediately below the piston top 7, and a simple mounting of the guide plates because said plates can be connected to said ring 38 for the mounting between the ribs 5.
The invention has been described with reference to preferred embodiments. Many modifications can be carried out without thereby deviating from the scope of the invention.

Claims

A piston for diesel engines and comprising several parts (1, 2) interconnected by way of welding, and which is adapted to transfer combustion forces to a piston rod (20), characterised in that the piston (1) comprises a central integrally shaped part (2) adapted to be permanently connected to the piston rod (20) and comprising a cylindrical central wall (4), which extends coaxially with the central axis (6) of the piston, said integrally shaped part (2) further comprising ribs (5) projecting radially from the central wall and extending in substantially radial planes including the central axis (6) of the piston (1), as well as a piston top (7) extending as an end wall for the cylindrical central wall (4) and the spaces between the radial ribs (5) to the area adjacent the outer ends of said ribs (5), and that the piston (1) comprises an outermost cylindrical part (3) provided with outer circumferential annular grooves (13, 14, 15 and 15') for receiving sealing rings (16, 17, 18 and 18'), said outermost cylindrical part (3) at the upper end being connected to the outer periphery of the piston top (7) by way of welding.
A piston as claimed in claim 1, and in which channels are provided for circulation of coolant, characterised in that a guide plate (8) is secured in each rib space, said guide plate defining an upper cooling chamber (9) adjacent the piston top (7), and that the inner end of these cooling chambers (9) communicate openly with the interior of the central wall (4) by means of through bores (10) therein, and that the lower end of the outermost cylindrical part (3) comprises closing means for sealingly closing the area around the central part (2) to the surroundings.
A piston as claimed in claim 2, characterised in that bores (11) are provided in the ribs (5), said bores (11) extending from the interior of the central wall (4) to the outer ends of the ribs (5) and being adapted to guide coolant to or away from the outer ends of the upper cooling chamber (9).
A piston as claimed in claim 2 or 3, characterised in that bores (37) are provided in the piston rod for guiding coolant away from the interior of the piston (1).
A piston as claimed in claim 2, 3 or 4, characterised in that the piston rod (20) comprises a central axial opening (26), in which guiding means (27) is mounted for carrying coolant to and away from the respective sets of bores (10 and 11, respectively).
A piston as claimed in claim 2, 3 or 4, characterised in that the a circumferential cylindrical ring (38) is mounted at the outer ends of the ribs (5), the upper end of said ring flushing with the outer ends of the guide plates (8) and being sealingly connected thereto as well as to the intermediate portions of the ribs, and that an opening (39) is provided in the ring (38) opposite each bore (11) in the ribs (5).
A piston as claimed in claim 5, characterised in that the guiding means (27) comprises a pipe arranged coaxially with the piston rod and comprising a circumferential flange (28), by which it abuts the inner side of the cylindrical central wall (4) of the central part (2) between the respective sets of bores (10 and 11, respectively).