GB2112498A - Piston head and skirt assembly - Google Patents

Abstract

A crosshead piston assembly 10 comprises a head member 18 and a skirt member 20, each of which can be secured independently to a gudgeon pin 14. The skirt member 20 has a reduced diameter upper end portion 66 which fits within a lower end portion 24 of the ring belt portion 23 of the head member 18, thereby exposing maximum head member surface area to a cylinder wall or cylinder sleeve 99 for cooling. The skirt end portion 66 has a seal groove 32 which contains a ring 67 for sealing the gap 62 between the head member and the skirt member. <IMAGE>

Description

SPECIFICATION A piston This invention relates to pistons for internal combustion engines and more particularly to two-piece or crosshead-type pistons.

Crosshead type pistons comprising separate head and skirt members which are independently connected to a gudgeon pin are well known and are preferred in many applications, such as in heavy duty two stroke diesel engines. One advantage of this type of piston is that the skirt member generally bears all of the side thrust applied to the piston through the connecting rod, leaving the head member to transmit only axially directed forces from the combustion chamber. Another advantage is that the skirt member may be made of less expensive materials because it is not exposed to the forces of the combustion chamber.

One problem associated with crosshead type pistons is excessive heat build-up, particularly in the head member which is exposed to the combustion chamber. In one-piece pistons, heat from the piston crown can be transferred directly to the piston skirt for dissipation. However, in crosshead pistons with the head member separate from the skirt, heat which otherwise could be directly transferred to the skirt can be dispersed no further than the ring belt - the cylindrical portion of the head just below the crown which usually contains piston rings. The exterior surface of the ring belt is exposed to the cylinder wall (typically a cylinder liner in heavy duty engines) which draws heat from the head member.

The head member heat build-up problem becomes more pronounced as the size of the head is reduced. There is a tendency to make the head member as small as possible because more expensive materials and/or processing is used in the head member.

The problem is further complicated because the head member, more particularly the ring belt, usually has a reduced diameter section which fits into the skirt member, thereby reducing the area of the ring belt which is exposed to the cylinder wall.

According to the present invention there is provided a piston comprising a head member and a skirt member which is formed separately from the head member, the head member having a crown for exposure to a combustion chamber and comprising a radially outer cylindrical portion for close sliding association with a cylinder wall, and means for operatively engaging a gudgeon pin, and the skirt member having a radially outer cylindrical portion, which is coaxial with the cylindrical portion of the head member, for close sliding association with the cylinder wall, and means for operatively engaging the gudgeon pin, the skirt member also having a reduced diameter cylindrical end portion which is received within the head member, an annular seal being provided between the cylindrical end portion and the head member.

An embodiment in accordance with the present invention helps to alleviate the heat build-up problem in crosshead pistons by exposing more of the head member surface area to the cylinder wall without increasing the size of the head member or the amount of material used in it. A unique sealing arrangement is also provided at the junction of the head and skirt members, in which, preferably, the reduced diameter cylindrical end portion of the skirt member has an annular groove which contains a seal ring in engagement with the lower end portion of the head member.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a partially exploded cross-sectional view of a piston; Figure 2 is an enlarged cross-sectional view of part of the piston of Figure 1.

Figure 1 illustrates a crosshead piston 10 for connection to a connecting rod 12 by a gudgeon pin 14. The piston comprises a head member 18 and a skirt member, or crosshead, 20, which are separately formed, preferably of cast iron. The skirt 20 is preferably formed of ductile cast iron while the head 18 is preferably formed of malleable cast iron, which is more expensive to process, but which is more suitable for exposure to a combustion chamber.

The upper portion of the piston head 18 comprises a crown 22 which forms a combustion chamber defining wall. Extending from the outer edge of the crown is an outer cylindrical ring belt 23 in which are formed three compression ring grooves.

The piston head 18 further comprises a body portion 33 having a pair of spaced downwardly extending (as viewed in Figure 1) gudgeon pin encircling arms 34 having gudgeon pin openings 42.

The arms 34 are interconnected at their upper ends by a saddle 36 which is in turn connected to the crown 22 buy a plurality of axially and radially extending struts or columns 37. Columns 37 include radially outwardly extending rib portions 38 which integrally connect the columns 37 to the ring belt 23 and with the outer portions of the crown 22.

The saddle 36 defines a downwardly opening semicylindrical recess 40. The recess 40 terminates circumferentially in a pair of radial shoulders 43 which retain a semicylindrical bearing insert 44 having an inwardly facing bearing surface preferably containing a plurality of grooves (not shown) for the distribution of lubricant thereover. Oil supply openings 54 and 55 are provided in the centres of the saddle 36 and the bearing insert 44 repectively.

The skirt 20 comprises generally a cylindrical wall 57 having a pair of centrally located oppositely disposed, and inwardly extending bosses 58. Each boss 58 defines a cylindrical gudgeon pin receiving opening 60 which, when the head member and skirt member are assembled, is axially aligned with the opening 42 of the body portion 33. At its lower end, the skirt 20 has two oil ring grooves 72 for suitable oil scraper rings. Below each groove 72 there is a plurality of oil drain holes 73 to carry away oil from the oil rings in a conventional manner.

The head and skirt members 18 and 20 of the piston 10 arre retained together by the cylindrical gudgeon pin 14which is received in the openings 60 and 42 of the skirt and head members 18 and 20 respectively and engages the bearing insert 44 across a substantial portion of its upper surface. The connecting rod 12 is secured to the gudgeon pin 14 by bolts (not shown) which extend through openings 84 in the connecting rod and openings 78 in the gudgeon pin.

The gudgeon pin 14 is hollow, having a slightly thickened wall at its centre where oppositely disposed upper and lower openings 76 and 77, respectively, are provided. A tube 88 is aligned with openings 76 and 77 to provide an oil passage through the gudgeon pin. The tube 88 is aligned with a drilled oil passage 90 extending lengthwise through the connecting rod 12. Metal closure plates 59 seal each end of the hollow gudgeon pin 14.

As is more clearly shown in Figure 2, the upper end of the skirt 20 has an enlarged wall 66 which is approximately three times as thick as the other areas of the skirt wall. The radially outer portion of the wall 66 is machined away to provide a reduced diameter end portion having a cylindrical surface 64. The ring belt lower wall 24 which received the upper end of the skirt wall 66 is also machined at its lower end portion to create an enlarged diameter interior cylindrical surface 63. The head member 18 and the skirt member 20 are designed to operate without contacting each other near the interface area shown in Figure 2. Each member 18 and 20 is secured to the gudgeon pin independently, as previously described.

A narrow annular gap 62 exists between the inner cylindrical surface 63 of the head member 18 and the outer reduced diameter cylindrical surface 64 of the skirt member 20. This gap permits a slight rocking of the skirt 20 under the angular piston rod forces but prevents these angular forces from being transferred to the head member 18. Bevelled annular surfaces 71 and 72 on the skirt and head members, respectively, aid in assembly and in the limited movement of the skirt.

The gap 62 is sealed by a cast metal, such as iron, sealing ring 67 which allows relative sliding or rocking movement between the head and skirt members without deterioration of the ring 67 and without breaking of the seal between the members.

The ring 67 has a tight end gap in the range of .003 inches, which is possible because the ring experiences little heat expansion due to the piston design.

The seal ring is received in a seal groove 32 which is cut into the circumferential surface 64 of the skirt member 20. It is widely preferred that the seal groove 32 be cut into a circumferential surface such as 64 to allow the ring 67 to engage a cylindrical surface such as 63. This sealing arrangement is believed to be more effective during any sliding or rocking between the head and skirt members than previously known designs which used elastomeric sealing rings, or which relied upon seals between radial or conical surfaces.

It can be seen that the ring belt 23, by virtue of the present invention in which the head member extends over the skirt member, is lengthened and has an overlap or receiving portion 24, the external surface of which is exposed to a cylinder or cylinder sleeve wall 99, thereby allowing an associated cylinder or cylinder sleeve wall to draw more heat from the head member. The additional exposed area is created without increasing the size or the material used in the head, as compared to similar designs in which the head member is received within the skirt.

Although the previously described embodiment of the invention is preferred presently, numerous mod ifications or variations are within the scope of this invention as defined in the following claims.

Claims (6)

1. A piston comprising a head member and a skirt member which is formed separately from the head member, the head member having a crown for exposure to a combustion chamber and comprising a radially outer cylindrical portion for close sliding association with a cylinder wall, and means for operatively engaging a gudgeon pin, and the skirt member having a radially outer cylindrical portion, which is coaxial with the cylindrical portion of the head member, for close sliding association with the cylinder wall, and means for operatively engaging the gudgeon pin, the skirt member also having a reduced diameter cylindrical end portion which is received within the head member, an annular seal being provided between the cylindrical end portion and the head member.

2. A piston as claimed in claim 1, in which an annular seal groove is provided in either the cylindrical end portion of the skirt member or the head member, the annular seal being positioned in the groove and engaging the head member or the cylindrical end portion, respectively.

3. A piston as claimed in claim 2, in which the seal groove is in the cylindrical end portion of the skirt member.

4. A piston as claimed in claim 3, in which the cylindrical portion of the head member has an enlarged diameter inner cylindrical surface for receiving the cylindrical end portion of the skirt member, the annular seal engaging the enlarged diameter inner cylindrical surface.

5. A piston as claimed in any one of the preceding claims, in which the seal is a cast metal ring.

6. A piston substantially as described herein with reference to, and as shown in, the accompanying drawings.