WO2000022290A1

WO2000022290A1 - Arrangement for counteracting the origination of tensile stress in at least one area of an engine block and method for counteracting the origination of such tensile stress

Info

Publication number: WO2000022290A1
Application number: PCT/SE1999/001838
Authority: WO
Inventors: Owe SALVÉN
Original assignee: Scania Cv Aktiebolag (Publ)
Priority date: 1998-10-14
Filing date: 1999-10-13
Publication date: 2000-04-20
Also published as: SE521019C2; SE9803499D0; SE9803499L

Abstract

The present invention relates to an arrangement and a method for counteracting the occurrence of tensile stresses in at least one region (12-15) of an engine block (2), which engine block (2) incorporates at least one removable element. Such an element may be a main-bearing cap (9). Said element is intended, in a state whereby it is connected to the engine block (2), to create compressive stresses in said region (12-15). These compressive stresses will thus counteract tensile stresses which occur in the region during operation of the engine.

Description

Arrangement for counteracting the origination of tensile stress in at least one area of an engine block and method for counteracting the origination of such tensile stress

BACKGROUND TO THE INVENTION, AND STATE OF THE ART

The invention relates to an arrangement which is intended to counteract the occurrence of tensile stresses in at least one region of an engine block according to the preamble to patent claim 1, and a method for counteracting the occurrence of such tensile stresses according to the preamble to patent claim 10.

Engine blocks for combustion engines, at least large engines, are usually made of cast iron which contains graphite lamellae. A problem with such engine blocks is that the graphite lamellae may act as preferential fracture points in the engine block boundary walls. This means that there is risk of crack formation in regions of the engine block when it is subject to tensile stresses. Especially in engine blocks for combustion engines of the V type there are regions which are particularly subject to tensile stresses which may lead to cracks in the engine block. Such regions are primarily parts of the engine block main bearing in which the engine's crankshaft is supported. The cause of the occurrence of such cracks is that the gas pressure in the engine's combustion chamber gives rise to a force which, via the engine's pistons and piston rods, loads the crankshaft and its main bearing. Particularly when the piston is in a position where combustion causes high pressure on the piston, the main bearing is acted upon by a force which has a direction which substantially corresponds to the longitudinal direction of the cylinder. In this position of the piston, said force acts upon he main bearing so that it is deformed elastically and its circular shape becomes slightly oval. This involves the occurrence of compressive stresses in the main bearing in a first region of the main bearing where said force acts and in a second region which is situated substantially centrally opposite the first region. Said deformation leads at the same time to third and fourth regions, which are situated substantially centrally between said first and second regions, being elongated with the result that tensile stresses occur in these regions. It is thus in said third and fourth regions that there is risk of crack formation. SUMMARY OF THE INVENTION

The object of the present invention is to provide an arrangement and a method which prevent or at least counteract the occurrence of tensile stresses in at least one region of the engine block so that the risk of cracks in that region of the engine block is eliminated.

This object is achieved by the invention with the arrangement mentioned in the introduction by its incorporating the features indicated in the characterising part of patent claim 1. A basic idea of the invention is therefore that the engine block in a connected state should be subjected to compressive stresses in the regions where there is risk of crack formation. Prior initiation of compressive stresses in said sensitive regions will thus cause these compressive stresses to counteract the tensile stresses which occur during the operation of the engine, the value of which can thereby be kept within acceptable limits so that the risk of crack formation is eliminated and/or that it is possible to increase the force to which the piston is subject without increasing the risk of crack formation. This latter possibility thus means that the compression pressure in the cylinder can be increased.

According to a preferred embodiment of the invention, said element exhibits a surface which can be placed in contact with a surface of the engine block, and means which are intended to press said surfaces to bear upon one another with a compressive force in said connected state. Clamping such an element firmly in the engine block by means which, for example, take the form of bolts will create compressive stresses in the engine block. The possibility thus arises of initiating compressive stresses in regions which are subject to tensile stresses during the operation of the engine. In order to impart compressive stresses to said regions, said surfaces may, in a state of assembly in which they are not connected to one another, exhibit in at least one section a shape differing from one another. Such a differing shape may for example be a heightened portion of one of said surfaces. This means that the compressive force which causes said element to abut against the engine block in the connected state will be distributed so that the largest compressive stress occurs particularly in the region which is situated adjacent to or in the vicinity of said heightened portion. Appropriate placing of said differing shape of said abutting surfaces will make it possible to impart compressive stresses to such regions which are subject to tensile stresses during the operation of the engine. Said surfaces may with advantage form, in a state of assembly in which they are not connected to one another, an angle with one another in said section. One of said surfaces may thereby be slightly inclined relative to the other surface. In a connected state, compressive stresses in the engine block are thereby initiated at a value which substantially continuously decreases from the highest point of said surface to its lowest point. Such an inclined surface may be formed either on said element or on the engine block.

According to another preferred embodiment of the invention, the engine block incorporates cast iron. Engine blocks made of cast iron with graphite lamellae are particularly sensitive to tensile stresses which may lead to crack formation in the boundary walls of the engine block. Graphite lamellae in the engine block boundary walls may constitute preferential fracture points there. The present invention is nevertheless also applicable with advantage to engine blocks made of other materials.

According to another preferred embodiment of the invention, the engine block forms part of a combustion engine. The movement of the pistons in the engine's cylinders results in the transmission to the engine block of forces which in certain regions lead to tensile stresses in the engine block. The invention is particularly applicable with advantage to combustion engines which include at least one cylinder which has an extent which forms an angle within the range 30° to 60°, preferably about 45°, with a plane through said surfaces of said element and the engine block in a connected state. Said sensitive region may include a wall section of a main bearing in which the crankshaft is supported. Said removable element is here a main-bearing cap. In so-called V-engines, there occurs during the downward movement of the piston a force which acts upon the crankshaft's main bearing so that the latter' s circular shape is deformed elastically to a slightly oval shape. The fact that the cylinder here forms an angle within the range 30° to 60° with a plane through said mutually abutting surfaces of said element and engine block means that said elastic deformation causes elongation of the main bearing in two regions at a distance from said plane. One of these regions is in the engine block and the other in the bearing cap. The same problem does not usually occur in in-line engines, since the extent of their cylinders and the consequent force direction form substantially a right-angle with said plane. This means that said elongation of the main bearing and initiation of tensile stress occur substantially in said plane. Tensile stresses arising can be eliminated there at least partly by said abutting surfaces of the engine block and the bearing cap being placed a small distance away from one another.

The invention also includes a method for counteracting the occurrence of tensile stresses in at least one region of an engine block, which engine block includes at least one removable element. The method is characterised by the features indicated in the characterising part of patent claim 10. The tensile stresses in the region which occur during the operation of the engine are thereby eliminated or the value of the tensile stresses is at least limited in said region during the operation of the engine, and the risk of cracks occurring in the engine block is eliminated and/or the load on the piston can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described below by way of example with reference to the attached drawings, in which:

Fig.1 depicts schematically a cross-section of a combustion engine of the V type with a bearing cap in a state in which it is connected to the engine block.

Fig. 2 depicts a bearing cap according to the present invention, in a state of assembly in which it is not connected to the engine block.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Fig.1 depicts a multi-cylinder combustion engine 1 of the V type. The engine may for example take the form of a diesel engine used for driving a heavy-duty vehicle such as a truck or a bus. The combustion engine 1 incorporates an engine block 2 which substantially encloses the moving parts of the engine 1. The combustion engine 1 incorporates a number of cylinders 3,3', two of which appear in Fig. l. These cylinders 3,3' are arranged in two rows and are so arranged that the extent of the cylinders form a V shape. The cylinders 3,3' each contain a combustion chamber 4 in which combustion takes place. During combustion, a piston 5 is pushed downwards by the gas pressure in the combustion chamber 4, and the piston 5 by means of a connecting-rod 6 and a crank pin transmits the force to a crankshaft 7 which is caused to rotate. The crankshaft 7 is supported for rotation in a main bearing 8. The main bearing 8 is composed of a semicircular recess in the engine block 2 and a corresponding semicircular recess in a bearing cap 9. The bearing cap 9 is fastenable by means of bolts 10a, 10b, 1 la, 1 lb to the engine block 2. Tightening primarily of bolts 10a, 10b causes the bearing cap 9 to abut with compressive force against the engine block 2.

During operation of the combustion engine 1 , the gas pressure in the combustion chamber 4 provides a force so that the piston 5 and the connecting rod 6 move in the direction of the arrows in Fig.1. The connecting rod 6 is supported for rotation on a crank pin so that a rotary movement is transmitted to the crankshaft 7. Said force is substantially imparted in the longitudinal direction of the cylinder 3, i.e. in the direction of the arrows shown in Fig.l, and the crankshaft 7 will transmit at least part of said force to the region 12 of the main bearing 8 when the piston 5 is close to its lower dead centre position and the combustion starts and initiates a powerful combustion pressure which urges the piston downwards with great force. The action of said force causes slight elastic deformation of the main bearing 8 to a slightly oval shape. At the same time, the boundary walls in the regions 12,13 of the main bearing 8 are pushed inwards under the influence of compressive stresses, while the regions 14,15 of the main bearing 8 are pulled outwards under the influence of tensile stresses. Particularly when the engine block 2 is made of cast iron with graphite lamellae, there is risk of graphite lamellae in the regions 14,15 acting as preferential fracture points for cracks when these regions 14,15 are subjected to tensile stresses.

The combustion engine 1 thus incorporates a multiplicity of cylinders, usually four or six in number, and a multiplicity of main bearings 8. The cylinder 3' becomes elongated and transmits a force which acts in a different direction from the force from the cylinder 3. A main bearing 8 which is situated between the cylinders 3,3' is thereby deformed elastically also by the force from cylinder 3' when the latter' s piston is at its dead centre position, to a slightly oval shape which extends in the longitudinal direction of the cylinder 3'. At the same time, compressive stresses will occur in the regions 14,15 of the main bearing, while tensile stresses occur in the regions 12,13. In V-engines, tensile stresses thus occur, during the operation of the engine, in two regions 13,14 of the engine block 2.

Fig.2 shows a bearing cap according to the present invention, in a state of assembly in which it is not connected to the engine block 2. It shows the bearing cap 9 exhibiting a first surface 16 and a second surface 17 which are intended to abut against corresponding surfaces 18,19 of the engine block 2. The surfaces 16,17 of the bearing cap 9 form, in said unconnected state of assembly, an angle with corresponding surfaces 18,19 of the engine block 2. The surfaces 16,17 each exhibit a highest portion 16', 17' adjacent to the main bearing 8, after which the surfaces 16,17 are inclined slightly downwards to a lowest portion 16", 17". The difference between the highest portions 16', 17' and the lowest portions 16", 17" of the surfaces 16,17 is in this example of the order of 0.1 to 0.3 mm. In alternative embodiments this difference may be considerably greater, e.g. up to 0.6 mm or even more. The diagram in Fig. 2 is only intended to illustrate the invention schematically and is therefore not drawn to scale. When the bearing cap 9 is screwed firmly to the engine block 2 by means of bolts 10a, 10b, the surfaces 16,17 of the bearing cap 9 are pressed against the surfaces 18,19 of the engine block 2. The portions of said surfaces 16,17,18,19 which adjoin the main bearing 8 thus first come into contact with one another during the process of fitting the main-bearing cap 9. Progressively as the main-bearing cap 9 is pressed against the engine block 2, the surfaces 16,17 are pressed harder against the surfaces 18,19 until they abut substantially parallel with one another. The portions 16', 17' situated highest of the surfaces 16,17 will thus have been elastically deformed, thereby initiating compressive stresses from said highest sections substantially to regions in and in the vicinity of the main bearing 8. The sensitive regions 12,13,14,15 of the main bearing 8 thus receive a built-in compressive stress which, during operation of the engine 1, counteracts and substantially reduces the tensile stresses described above which occur in these regions 12,13,14,15 during operation of the engine 1.

The invention is in no way limited to the embodiment depicted in Fig.2 but may be varied freely within the scopes of the patent claims. Alternatively, the surfaces 18,19 of the engine block 2 may be inclined instead of the surfaces 16,17 of the bearing cap. Other shapes differing from one another of said surfaces 16,17,18,19 may also be used with advantage to cause appropriate imparting of compressive stresses to sensitive regions. The invention is also applicable to other types of engines and engine blocks which are made of other materials than cast iron, e.g. aluminium.

Claims

PATENT CLAIMS

1. Arrangement intended to counteract the occurrence of tensile stresses in at least one region (12-15) of an engine block (2), which engine block (2) incorporates at least one removable element, characterised in that said element, in a state in which it is connected to the engine block (2), is intended to provide compressive stresses in said region (12-15).

2. Arrangement according to claim 1, characterised in that said element exhibits a surface (16,17) which may be placed in contact with a surface (18,19) of the engine block (2), and means which are intended to press said surfaces to abut against one another with a compressive force in said connected state.

3. Arrangement according to claim 2, characterised in that said surfaces (16,17,18,19), in a state of assembly in which they are not connected to one another, exhibit in at least one section a shape differing from one another.

4. Arrangement according to claim 2 or 3, characterised in that said surfaces (16,17,18,19), in a state of assembly in which they are not connected to one another, form an angle with one another in said section.

5. Arrangement according to claim 4, characterised in that the angle which the surfaces (16,17,18,19) form with one another is matched by a difference between their highest and lowest parts of 0.1 to 0.6 mm, advantageously between 0.1 and 0.3 mm.

6. Arrangement according to any one of the foregoing claims, characterised in that the engine block (2) incorporates cast iron and that the engine block (2) forms part of a combustion engine (1).

7. Arrangement according to claim 2 or 6, characterised in that said combustion engine (1) incorporates at least one cylinder (3) which has an extent which forms an angle within the range 30°-60°, preferably about 45°, with a plane through said surfaces (16, 16', 17, 17') of said element and the engine block (2) in a connected state.

8. Arrangement according to claim 7, characterised in that said region incorporates a wall section of a space which encloses the crankshaft (7) of the engine (1), that said wall section incorporates part of a main bearing (8) in which the crankshaft (7) is supported, and that said element is a main-bearing cap (9).

9. Arrangement according to claims 2-8, characterised in that the means intended to press said surfaces (16,17,18,19) to abut against one another with a compressive force in said connected state take the form of bolts (10a, 10b).

10. Method for counteracting the occurrence of tensile stresses in at least one region of an engine block (2), which engine block (2) incorporates at least one removable element, characterised in that said element is connected to the engine block (2) in such a way as to cause compressive stresses in said region.