GB2343933A

GB2343933A - Combustion engine piston

Info

Publication number: GB2343933A
Application number: GB9927319A
Authority: GB
Inventors: Sakari Pisilae; Tommy Roennskog
Original assignee: WECOMETAL Oy
Current assignee: WECOMETAL Oy
Priority date: 1998-11-19
Filing date: 1999-11-18
Publication date: 2000-05-24
Anticipated expiration: 2019-11-18
Also published as: GB9927319D0; GB2343933B; GB2343933A9; FI106396B; FI982507A0; DE19955809B4; DE19955809A1; FI982507A

Abstract

The piston comprises a piston crown (1) and a skirt (3) surrounding a piston pin, which, at a joint surface, fit together on at least one axial support surface (11, 12) while the piston crown and the skirt are joined by at least one screw (2) extending through the support surface. Further, the top of the piston has a circular outer cooling gallery (9) and, inside it, an inner cooling gallery (10). In the piston of the invention, at least one recess (19), which is lower than the rest of the bottom (18) of the cooling gallery, is arranged to its outer cooling gallery. During the stroke of the piston, the recess causes a tangential movement in the cooling oil in the piston, thus improving the cooling of the piston.

Description

2343933 COMBUSTION ENGINE PISTON The present invention relates to a

combustion engine piston according to the preamble of claim 1.

Large diesel engine pistons are usually constructed multi-part pistons which currently typically comprise two interconnected parts, a piston crown and a bottom part, a piston skirt. These parts are joined by a screw connection comprising one or more screws. The piston crown forms a part of a combustion chamber and also comprises ring grooves of the piston. The purpose of the piston skirt is to transmit the gas pressure forces directed to the piston crown to a conrod through the piston pin. From the conrod, the forces move on to a crankshaft. The piston skirt is also arranged to receive any side thrust against the cylinder sleeve.

Because the running of an engine causes the piston to heat up considerably, the piston parts need to be cooled. The excess heat is usually transferred away from the piston using oil. The most advanced cooling method for large diesel engine pistons is a method known as the Shaker cooling. In this method, the piston crown typically has two cooling galleries, an inner cooling gallery and an outer cooling gallery surrounding it, to which galleries the cooling oil is led. Such cooling galleries are usually rotationally symmetrical with the inner cooling gallery being either open or partly closed at the bottom in the direction of the piston pin. The outer cooling gallery is circular or cyclically symmetrical at the top.

However, heavy-oil driven diesel engines, for instance, have the drawback that the cooling oil in the piston cooling areas cannot, to a sufficient extent, transfer the heat away from the most heavily heating piston components. To avoid this problem, the shape of the cooling gallery has been altered with drilled recesses, for instance, to improve the penetration of the cooling oil as disclosed in EP publication 0 464 626. The recesses may also be elongated recesses produced by machining as disclosed in DE publication 41 18 400. Different types of cooling gallery shapes have also been disclosed in DE publications 38 42 321 and 39 19 872.

When analysing the piston load, it can be noted that in the operating environment the piston components change shape under temperature load and gas pressure and, because of the changes, wearing stress amplitudes occur in the screws of the screw connection which joins the piston parts to each other. The size and number of the stress amplitudes affect the life expectancy of the screws. When the stress amplitudes are high, screw connection fatigue increases, thus also increasing the probability of failure of the entire piston.

Therefore, it can be noted that there are deficiencies related not only to the cooling of the present pistons, but especially to the correct aiming of the cooling.

It is an object of the present invention to reduce the impact of the deficiencies in prior art and to create an entirely new solution to improve and better aim the cooling of a piston and, thus, reduce the stress amplitudes of the screws in the screw connection of the piston.

This object is achieved by a combustion engine piston having the characteristics specified in the claims of the present invention. More specifically, the apparatus of the invention is primarily characterised by what is stated in the characterising part of claim 1.

The shape of the cooling gallery of the invention provides a cooling effect which is more advantageous than the conventional effect. When the piston descends and its speed slows, inertial force presses the cooling oil in the outer cooling gallery of the piston into the recesses of the invention in the bottom part of the cooling gallery. Next, when the piston starts its upward stroke and its speed increases, inertial force keeps the oil pressed to the recesses. According to one preferred embodiment of the invention, the shape of the bottom part of the cooling gallery is such that the oil collects momentarily into two recesses in the bottom part of the cooling gallery. When the piston continues upward, it reaches its maximum speed, after which the speed starts to slow down again. At this time, the oil continues its movement at unaltered speed and is released from the bottom part of the cooling gallery.

After a certain time and while it is still unevenly spread along the periphery of the piston, the oil hits the top part of the cooling gallery. Due to the high acceleration of the piston, the free surface of the oil evens out momentarily in the top part of the gallery. Due to its uneven spread, the oil can, however, only even out by means of a peripheral flow. Thus, the oil, which had collected into the recesses, starts to flow tangentially while it spreads into the piston crown.

Owing to the solution of the invention, the cooling oil in the cooling galleries of the piston not only moves back and forth between the top part and the bottom part of the cooling gallery in the direction of the longitudinal axis of the piston 3 during the reciprocal movement of the piston, but, due to the new design of the bottom part of the outer cooling gallery, a peripheral flow of the oil can also be achieved as well as an improved convection. This way, greater quantities of heat can be transferred away from the hot walls of the piston crown by means 5 of the cooling oil.

In the solution of the invention, in which the screws of the screw connection joining the piston crown and skirt are arranged to the inner axial support surface of the piston against the piston pin substantially perpendicular to it, the recesses of the outer cooling gallery are arranged substantially at the piston pin. This way, the bottom of the cooling gallery at the location of the screws is at its highest, whereby there is less cooling surface in the area of the screw connection. Thus, the temperature difference caused by thermal load can be made as small as possible between the screws and the displacement of the piston in the screw connection and, thus, the stress amplitude in the screw can be reduced. This makes the screws considerably more durable. It is well known that the stretching part in a screw connection is the screw. A screw creates a screw force which presses together the parts being joined. In the following, the space to which this screw force is directed is called a pressure piece. A big and varying temperature difference between the screw and the pressure piece surrounding it causes significant stress amplitudes in the screw.

A significant temperature difference is often generated in the material of the screw connection in a constructed piston and in the material surrounding the screws. This happens, because the threads of the screw, from which heat is transferred to the screws, are close to the combustion chamber. The screw is otherwise surrounded by a clearance hole and there is air, which is a good thermal insulator, around it. Thus, the screw heats up uniformly to a high temperature. The heat is conducted from the material surrounding the screw elsewhere in the structure and the conduction is efficient if the distance to the cooling gallery is short.

The stress amplitudes affect considerably the life expectancy of the screw. The amplitudes and their impact can, however, be significantly reduced with the solution of the present invention.

With the solution, in which the screws of the screw connection joining the piston crown and skirt are arranged to the inner axial support surface of the piston substantially perpendicularly against the piston pin, the 4 piston skirt can be shaped in such a manner that the flow of the gas pressure force moves through the pressure piece of the screw connection as little as possible. This way, the skirt can be made dome-like and the pressure piece is primarily formed by bolt hole shanks surrounding the screws. Owing to this structure, the alternating load caused by gas pressure causes as low stress amplitudes as possible in the screws.

In the following, the invention is described with reference to the attached drawing which shows a longitudinal section of a combustion engine piston, showing only the parts that are essential for the understanding of the invention, with the section being partly in the direction of the longitudinal axis of the piston and partly perpendicular to it.

The figure shows a preferred embodiment of the solution of the invention. In the figure, the piston comprises a piston crown 1 and a bottom part 3 of the piston preferably joined to the piston crown with screws 2. The piston crown comprises a plate 4 and a substantially cylindrical wall 5 which has at least one ring groove 6 for the piston curl. The plate receives the gas pressure force directed to the piston. The top surface of the plate is shaped in such a manner that it forms a combustion bowl 7 and the bottom surface 8 of the plate forms the top part of a circular outer cooling gallery 9 and the top part of an inner cooling gallery 10 in the middle of the piston. This inner cooling gallery is connected to the outer cooling gallery of the piston through oil channels.

The parts of the piston of the invention are joined together at a joint surface which has at least one support surface. The embodiment of the invention shows a solution which has two support surfaces, i.e. an inner axial support surface 11 and an outer axial support surface 12. The annular inner axial support surface is located between the cooling galleries 9 and 10. The outer axial support surface is arranged substantially to the cylindrical wall 5. The top part of the outer cooling gallery of the piston is arranged between the support surfaces in the piston crown.

The bottom part 3 of the piston, i.e. the skirt, also comprises an inner and outer axial support surface 11 and 12 which have the bottom part 9 of the outer cooling gallery of the piston between them. The skirt further comprises sliding surfaces 13, which are against the cylinder sleeve surrounding the piston, and a piston pin boss 14 with bearing drillings.

The piston crown 1 and the skirt 3 are arranged to support themselves on the annular axial support surfaces 11 and 12 located on one or more joint surfaces. The inner axial support surface 11 of the skirt forms a part of an annular shoulder 16 which is substantially parallel to the assumed centre axis of the piston, the annular shoulder being arranged to receive the screws 2 joining the piston parts to each other. The annular shoulder surrounds the inner cooling gallery 10 of the piston together with the bottom surface 8 of the piston crown plate while the outer cooling gallery 9 of the piston is arranged to surround the annular shoulder on the outside. The inside of the skirt is, in the area of the annular shoulder, arranged in a dome-like shape below the inner axial support surface. Bolt hole shanks 17 arranged to extend down from the dome form a significant part of the pressure piece surrounding the screw connection.

Cooling oil used in Shaker cooling is fed into both cooling galleries 9 and 10 of the piston. The cooling oil is led to the first cooling gallery through at least one feeding duct from which the oil flows through oil channels into the second cooling gallery.

In the piston of the invention, the flow of the cooling oil in the outer cooling gallery 9 is improved by making the bottom 18 of the cooling gallery uneven preferably by milling. One or more recesses 19 which are lower than the rest of the bottom are arranged in the bottom of the cooling gallery. When the piston moves, the cooling oil in the cooling galleries can thus be led to collect in the recess(es) while it is in the bottom part of the gallery. Due to the movement of the piston, the oil hits the top part 20 of the plate in the cooling gallery before the top dead centre of the movement. When hitting the top part of the gallery, the oil is unevenly spread along the periphery of the piston due to the recesses in the bottom part of the gallery. Inertial force presses the oil into the top part of the cooling gallery and the oil evens out momentarily through a tangential flow. This produces a peripheral flow in the oil, which conducts large volumes of heat away from the hot walls of the piston crown.

In another preferred embodiment of the invention, the screws of the screw connection joining the piston crown I and the piston skirt 3 are arranged to the inner axial support surface 11 of the piston substantially perpendicularly against the piston pin. In this case, the bottom 18 of the outer cooling gallery in connection with the pressure piece is higher than other parts of the cooling gallery bottom, since the bottom is deeper in the direction of the piston pin than perpendicular to it. Because the outer cooling gallery 9 is low at the 6 location of the screws, and the radial cross-sectional area of the cooling gallery at the same time is at its smallest, the cooling effect'achieved with cooling oil is also smaller in the area of the pressure piece of the screw connection than in other parts of the cooling gallery. This way, a smaller temperature difference is achieved between the screws and the pressure pieces surrounding them.

When the annular shoulder 16 of the skirt 3 is arranged to be domelike below the inner axial support surface 11, and the highest part of the dome 21 arch is substantially arranged to be at the location of the screws 2, the bolt hole shanks 17, which extend downwards from the dome and form a significant part of the pressure piece of the screw connection, produce a piston skirt structure in which the gas pressure force flow moves through the pressure piece of the screw connection as little as possible. This structure reduces the stress amplitudes caused by the gas pressure load in the screws.

It will be understood that the above description and the related figure are only intended to illustrate the present invention. The invention is thus not limited to what is stated above or to the embodiment specified in the claims, but it will be obvious to those skilled in the art that the invention can be modified in many ways within the scope of the inventive idea disclosed in the attached claims.

In the attached claims, reference numerals have been included for an easier understanding of the claims, but they are not intended in any way to limited the scope of the claims.

7

Claims

1. A combustion engine piston comprising a piston crown (1) and a skirt (3) surrounding the piston pin, which, at a joint surface, support themselves on at least one axial support surface (11, 12) while the piston crown and the skirt are joined by at least two screws (2) extending through the support surface with one end of the screws being arranged close to the combustion chamber in the top part and being thus thermally loaded, and the piston having an outer cooling gallery (9) circularly surrounding it and, inside it, an inner cooling gallery (10), and the outer cooling gallery (9) has in the bottom (18) formed on the top surface of the skirt (3) recesses (19) which are lower than the rest of the bottom of the cooling gallery, c h a r a c t e r i s e d in that there are at least two recesses (19) and they are arranged substantially in line with the piston pin, whereby the cooling oil in the cooling galleries is arranged to primarily collect into the recesses in the bottom (18) while the oil is in the bottom part of the outer cooling gallery (9), and the cooling oil which is unevenly spread along the periphery of the piston is arranged, at the highest point of the piston stroke, to hit the piston crown (20) and this action is arranged to make the cooling oil flow tangentially to spread the cooling oil evenly in the piston crown.

2. A piston as claimed in claim 1, c h a r a c t e r i s e d in that the recess (19) is made by milling.

3. A piston asclaimed in claim 1 or2, characterised inthat there are two recesses (19).

4. A piston as claimed in any one of the preceding claims, c h a r a c It e r I s e d in that the bottom (18) of the outer cooling gallery (9) is, at the location of the pressure piece surrounding the screws (2), arranged to be at its highest in relation to the rest of the bottom of the cooling gallery while the radial cross-sectional area of the cooling gallery is at its smallest.

5. A piston as claimed in claim 4, c h a r a c It e r i s e d in that the piston skirt (3) is arranged to form a pressure piece surrounding the screw (2), while the pressure piece extends, at the location of the screw, like a dome close to the bottom (18) of the outer cooling gallery (9) and forms a bolt hole shank (17) extending to the skirt around the screw.

6. A piston as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the screw (2) is arranged to or in connection with a section plane substantially perpendicular against the piston pin.

7. A combustion engine piston, substantially as hereinbefore described with reference to the accompanying drawing.