A PISTON FOR AN INTERNAL COMBUSTION ENGINE, IN PARTICULAR A BIG MARINE DIESEL ENGINE

Abstract: A piston for an internal combustion engine, in particular a big marine diesel engine, and of the type which as power-transferring member between piston and piston rod has an annular rib integral with the piston head, in which, in an annular area between the side wall of the piston, which is facing downwards from the piston head and which is provided with external piston rings and the rib, supporting elements are provided which end downwards in a conical wall which is penetrated by bores extending substantially in parallel with the axis of the piston, through which bores cooling oil may pass, and in which the conical wall, when viewed in axial section of the piston, is connected with the side wall by means of a rounded wall portion. With a view to reducing the sensitivity towards dynamic stresses in the area at the transition between the supporting elements and the side wall, the side wall has at the rounded wall portion a reduced wall thickness.

Description: A PISTON FOR AN INTERNAL COMBUSTION ENGINE, IN PARTICULAR A BIG MARINE DIESEL ENGINE The present invention relates to a piston for an internal combustion engine, in particular a big marine diesel engine, and of the type which as power-transferring member between piston and piston rod has an annular rib integral with the piston head, in which in an annular area between the side wall of the piston, which is facing downwards from the piston head and which is provided with external piston rings, and the rib supporting elements are provided which end downwards in a conical wall which is penetrated by bores extending substantially in parallel with the axis of the piston, through which bores cooling oil may pass, and in which the conical wall, when viewed in axial section of the piston, is connected with the side wall by means of a rounded wall portion. Pistons of this type are used in connection with slow-speed two-stroke crosshead diesel engines. Over the years it has been possible to increase the power output of these engines by increasing the effective mean pressure. This increase of the power output brings about a corresponding load on the piston head and consequently on the parts of the piston which are to transfer the load to the piston rod. As the load is to a very high degree dynamic, the supporting elements of the piston must be constructed with a view to withstanding formation of cracks. The higher load also makes big demands on the cooling of the piston, for which reason a strengthening of the piston cannot be done solely by increasing the wall thickness in the exposed places. A particularly exposed area is the transition between the side wall of the piston and the supporting elements transferring the forces from the exterior areas of the piston head to the power transferring rib. The object of the present invention is to provide a piston of the type mentioned by way of introduction, in which the risk of fractures in the critical area has been reduced. According to the invention this object is met by the piston being characterized by the subject matter of the characterizing clause of claim 1. It has surprisingly turned out that a construction, in which the wall thickness of the side wall of the piston in the critical area has been reduced, results in smaller dynamic tensional loads on the piston. In practice the reduced wall thickness is made as an undercutting of the side wall in the area, in which the supporting elements with a rounding meets the side wall. The undercutting and the rounding entail a separation of the dynamically most heavily loaded areas of the construction, which, in spite of the fact that the absolute load is not reduced, results in a substantially reduced risk of fracture. According to the invention it is in particular advantageous that the rounded wall portion constitutes one of the delimitations of the side wall area with reduced wall thickness, By this design the radius of the rounding may be increased compared to the embodiments known up till now and the sensitivity towards dynamic load is reduced. A sufficient separation between the loaded areas is according to the invention obtained thereby that an axial section through the rounded portion has substantially the shape of part of a circle and that the centre of this circle is positioned such that it is not closer to the axis of the piston than the generatrix positioned farthest away from this axis for each of the cooling bores. According to the invention it is preferable that the angle between the conical wall and the piston axis is between 30 and 60°. With such an angle the cooling bores will obtain such an approximately elliptical intersection surface with the conical surface that a suitable distance between the dynamically heavily loaded areas is optimally obtainable. The arrangement according to the invention is in particular suited for pistons with a big top section, i.e. that the side wall portion, in which the piston rings are positioned, is close to the lower end of the side wall. According to the invention the area with reduced wall thickness of the side wall is placed closer to the piston head than the piston rings. It is according to the invention preferred that the area of the side wall with reduced thickness extends from approx. 1/2 of the height of a piston ring above the uppermost piston ring to the rounded wall section. The area with reduced wall thickness is made in a simple way by turning, if the area with reduced thickness is defined by the rounded wall portion and a conical surface, the angle between the axis of the piston and the guide of the conical surface being 5- 25°. The invention will now be described in detail in the following with reference to the drawing, in which Fig. 1 is a half axial section through a first embodiment of a top section for a piston according to the invention, and Fig. 2 is a half axial section through a second embodiment of a top section for a piston according to the invention. The piston top section shown in Fig. 1 comprises a piston head 1 which is integral with a downwards facing annular rib 2, which at its downwards facing end surface 3 is adapted to be fastened to a flange on a piston rod by means of a number of bolts which is inserted in the holes 4. The rib 2 thus serves the purpose of transferring the forces acting on the piston head to the piston rod. Integral with the piston head 1 is, moreover, a downwards facing side wall 5, which on part of its height is provided with a number of grooves 6 for receiving piston rings. At the lower end surface 7 of the side wall 5 a number of threaded bolt holes 8 is provided, said holes serving the purpose of fastening a bottom part which constitutes the underside of the piston and which at the bottom closes the interior hollow space of the piston. In the area between the side wall 5 and the ring wall 2 a circumferential interspace 9 is provided, which at the top is lengthened by means of a circle of bores 10. The bores 10 are connected with the interspace 11 within the ring wall 2 by means of ducts 12. Through the ducts 12 cooling oil, which through a duct in the piston rod is conveyed up into the central hollow space 11, may be injected through the ducts 12 into the bores 10, from where the oil through the interspace 9 is conveyed to a return duct in the piston rod. Between the bores 10 walls 13 are provided which serve as supporting elements between the side wall 5 and the annular rib 2. The walls 13 end at the bottom in a conical wall 14, which in a soft rounding 15 joins the annular rib 2. At the upper end the conical wall joins the side wall 5 by means of a rounding 16 which constitutes the upper delimitation of a portion 17 of the side wall 5 with reduced thickness. At the bottom the portion 17 is defined by a conical wall 18. The rounding 15 has preferably, when viewed axially, a circular profile with a radius of between 5 and 24 mm depending on the diameter of the piston. The centre of the circle is preferably placed on or outside of the extension of the guide for the bore 10 placed farthest away from the piston axis. The area of the side wall 5 with reduced wall thickness serves the purpose of removing the dynamically most heavily loaded areas in the transition between the conical wall 14 and in the transition between the walls 13 and the conical wall 14. Though the side wall 5 is weakened in the area with reduced wall thickness, such a reduction of the dynamic load is obtained that the risk of fracture in the critical area is minimized, partly through an increased flexibility of the construction, partly through a more appropriate positioning of the most heavily loaded areas relative to one another. The embodiment according to the invention is in particular suited for pistons with a big top section, i.e. where the piston rings are placed far away from the piston head 1. It is preferred that the transition 19 between the portion with reduced wall thickness and the remaining part of the side wall with the piston rings are placed approx. 1/2 ring height above the upper piston ring groove 6. The angle between the piston axis and the conical wall 18 is preferably 5 to 25°. The angle between the conical wall 14 and the piston axis is normally between 30 and 60°. Fig. 2 shows another type of piston, in which the bores 10 have been replaced by a circumferential hollow space 20. The hollow space 20 is defined by a support- ing wall 13*, which at the bottom has a conical wall 14. The supporting wall 13' is provided with spaced bores 10, through which cooling oil, which through the ducts 12 flows into the hollow space 20, may be returned to the return duct in the piston rod. The piston shown in Fig. 2 is like the piston according to Fig. 1 provided with a portion 17 of the side wall 5 with reduced thickness. In Fig. 2 the same reference numerals as in Fig. 1 are used for analogous parts, and these parts need no further description. This portion is in the same way defined upwards by a rounding with substantially circular cross-section, when the piston is axially viewed, and with a centre for the radius of curvature positioned radially outside the extension of the, when radially viewed, outermost guide for the bore 13 ■ and preferably on or outside the guide for the inner surface of the side wall 5. The dimensions for the portion with reduced wall thickness are besides as stated in connection with Fig. 1.

Claims: P A T E N T C L A I M S 1. A piston for an internal combustion engine, in particular a big marine diesel engine, and of the type which as power-transferring member between piston and piston rod has an annular rib integral with the piston head, in which in an annular area between the side wall of the piston, which is facing downwards from the piston head and which is provided with external piston rings, and the rib supporting elements are provided which end downwards in a conical wall which is penetrated by bores extending substantially in parallel with the axis of the piston, through which bores cooling oil may pass, and in which the conical wall, when viewed in axial section of the piston, is con- nected with the side wall by means of a rounded wall portion, c h a r a c t e r i z e d in that the side wall at the rounded wall portion has a reduced wall thickness. 2. A piston according to claim 1, c h a r a c t e r i z e d in that the rounded wall portion constitutes one of the delimitations of the side wall area with reduced wall thickness. 3. A piston according to claim 1 or 2, c h a r a c t e r i z e d in that an axial section through the rounded portion has substantially the shape of part of a circle and that the centre of this circle is positioned such that it is not closer to the axis of the piston than the generatrix positioned farthest away from this axis for each of the cooling bores. 4. A piston according to claim 1, 2 or 3, c h a r a c t e r i z e d in that the angle between the conical wall and the piston axis is between 30 and 60°. 5. A piston according to any of the claims 1-4, c h a r a c t e r i z e d in that the area with reduced wall thickness of the side wall is placed closer to the head of the piston than the piston rings. 6. A piston according to claim 5, c h a r a c t e r i z e d in that the area of the side wall with reduced thickness extends from approx. 1/2 the height of a piston ring above the uppermost piston ring to the rounded wall section. 7. A piston according to any of the claims 1-6, c h a r a c t e r i z e d in that the area with reduced thickness is defined by the rounded wall portion and a conical surface, the angle between the axis of the piston and the guide of the conical surface being 5-25°. AMENDED CLAIMS [received by the International bureau on 2 March 1998 (02.03.98); original claims 1-7 replaced by amended claims 1-3 (1 page)] 1. A piston for an internal combustion engine, in particular a big marine diesel engine, and of the type which as a power-transferring member between piston and piston rod has an annular rib integral with the piston head and provided with rib supporting elements in an annular area between the rib and the piston's side wall, which is facing downwards from the piston head and which is provided with external piston rings, the rib supporting elements ending downwards in a conical wall, which for passage of cooling oil is penetrated by bores extending substantially in parallel with the axis of the piston, and which conical wall, when viewed in axial section of the piston, is connected with the side wall by means of a rounded wall portion, through which an axial section substantially has the shape of a circle, c h a r a c t e r i z e d in that the rounded wall portion has an extension as known per se into a portion of the side wall with reduced wall thickness, which extension enables the centre of said circle to be positioned such that it is not closer to the axis of the piston than the generatrix positioned farthest away from this axis for each of the cooling bores. 2. A piston according to claim 1, c h a r a c t e r i z e d in that the area with reduced wall thickness of the side wall is placed closer to the head of the piston than the piston rings. 3. A piston according to claim 2, c h a r a c t e r i z e d in that the area of the side wall with reduced thickness extends from approx. 1/2 the height of a piston ring above the uppermost piston ring to the rounded wall section. STATEMENT UNDER ARTICLE 19(1) PCT In view of the prior art disclosed in the references an amended claim 1 has been drafted on basis of the original claim 3. Attention is drawn to the fact that the piston according to the invention comprises a piston head which differs substantially from the structures disclosed in the references .

Inventors: CRONE ERIK, CRONE, ERIK
Applicants: CRONE ERIK, MAN B & W DIESEL GMBH, CRONE, ERIK, MAN B & W DIESEL A/S
Application year: 1997
Main classification: F02F3
IPC codes: F01P3, F02B3, F02B75, F02F3, F16J1