Piston crown for an internal combustion engine

Abstract

 The invention concerns a piston crown for an i.c. engine piston of the kind having an annular support skirt which projects from an inner surface of the head wall of said crown with its free edge providing a butt surface for engaging the associated piston rod.
A disadvantage with known crowns of this kind is that there is a tendency for fretting, and hence wear, to take place at the point where said butt surface engages the piston rod, and an object of the invention is to overcome this disadvantage.
The invention is characterised in that the inner diameter (D,) of said annular support skirt (02a) of the crown (01) at a point adjacent the inner surface (06) of said head wall (05) is larger than the inner diameter (D,) thereof at a point where its butt surface (07) engages the head surface (3a) of said piston rod (03).
If appropriate at least two annular support skirts (02a, 014) may be provided in radially juxtaposed relationship.

Description

[0001] The present invention relates to piston crowns for the pistons of internal combustion engines.

[0002] One known construction of piston crown is illustrated in Figure 1. In this Figure, the piston crown 01 is formed with an annular support skirt 02, which projects from the inner surface 06 of the top (head) wall 05 of said crown, the free edge of the skirt providing an annular butt surface 07 which is supported by the periphery of the upper (head) surface 03a of its associated piston rod 03. The annular skirt 02 holds the crown spaced above the upper surface 03a of the piston rod, and the skirt 02 divides this space into coolant liquid chambers 08 and 09 which are connected by a coolant liquid passageway 04 which extends through said skirt 02. In such a piston crown, gas pressure exerted upon the piston crown 01 is transmitted via the support skirt 02 to the piston rod 03. However, there is a disadvantage with such a crown in that,3ue to deformation of the crown caused by the gas pressure, an uneven surface pressure and/or relative displacement in the radial direction tends to arise between the butt surface 07 of the support skirt 02 and the piston ring 03, and so, fretting may sometimes occur on said butt surface.

[0003] An object of the present invention is to provide a piston crown, in which occurrence of fretting on the butt surface of the support skirt and/or the head surface of its associated piston rod can be prevented.

[0004] According to the present invention a piston crown for an internal combustion engine piston of the kind having an annular support skirt which projects from the inwardly directed surface of the top (head) wall of said crown with its free edge providing a butt surface which is supported on the head surface of its associated piston rod, is characterised in that the inner diameter of said annular support skirt at a point adjacent said inner surface of the head wall of the crown is larger than the inner diameter thereof at a point adjacent said head surface of the piston rod.

[0005] Thus, the invention provides for the surface pressure on said butt surface to be kept constant against a varying load of gas pressure, so that displacement in the radial direction of said butt surface may be restricted to zero, or at least to a minute value, thereby virtually eliminating wear at this point, and consequently prolonging the life of the crown/piston rod assembly.

[0006] The present invention can be widely applied to internal combustion engines.

[0007] The above-mentioned and other features and objects of : the present invention will become more apparent by reference to the following description of two preferred embodiments of the invention taken in conjunction with the accompanying drawings, in which:-

Figure 1 is a cross-sectional view showing one known form of piston crown,

Figure 2 is a cross-sectional view showing a piston crown according to a first preferred embodiment of the present invention, and,

Figure 3 is a cross sectional view showing a piston crown according to a second preferred embodiment of the . present invention.

[0008] Referring now to Figure 2, in said first embodiment, the piston crown 01 is basically similar to the known form described in Figure 1 and like parts.have the same reference numbers. However, in accordance with the invention, the annular support skirt 02a is angled (or tilted), radially, as shown, so that its inner diameter D₁ at the point adjacent the inner surface 06 of the head wall 05 of the crown is larger than its inner diameter D₂ at the point adjacent the head surface 03a of the piston rod 03. The particular shape and dimensions of the above-described annular support skirt 02a for any particular application can be determined by a calculation method, such as a finite element method, or the like. The annular butt surface 07 of the support skirt 02a butts against the peripheral part of the head surface 03a of the piston rod 03, and they are fastened together by means of a plurality of circumferentially spaced bolts 010.

[0009] In use of a piston crown constructed as described above, upon operating the internal combustion engine equipped therewith, gas pressure exerted upon the outer surface 011 of the head wall 05 of the piston crown 01 is transmitted through the annular support skirt 02a to the piston rod 03.

[0010] In such instance, if the heretofore known support skirt 02 as shown in Figure 1 is used, then by a resultant force of the gas pressure exerted substantially in the vertical direction upon the outer surface O11 of the piston crown 01 and gas pressure exerted substantially in the horizontal direction upon the side surface 013 above the piston ring grooves 012 of the piston crown 01, there would be a tendency for the butt surface 07 of the support skirt 02 to produce a gap clearance from the head surface 03 around its outer periphery, whereas around the inner periphery of said butt surface, it would press strongly against said head surface 03a, resulting in unevenness of pressure produced by the butt surface 07, and hence a tendency for relative displacement in the radial direction between the support skirt 02 and the piston rod 03 to occur.

[0011] On the other hand, in the piston crown having the free edge of the support rib 02 displaced towards the center of the piston crown, as is the case with the angled annular support skirt 02a according to the present invention, due to the inner diameter D_l being larger than the inner diameter D₂ as described above, at the butt surface 07, gas pressure exerted upon the outer surface O11 of the crown generates a clockwise bending moment M1 as viewed in Figure 2, while the gas pressure exerted upon the side surface generates an anticlockwise bending moment M₂ as viewed in Figure 2, and these bending moments M1 and M₂ can be made to offset each other, so that the pressure produced by the butt surface 07 is made uniform and, consequently, relative displacement in the radial direction between the annular support skirt 02a and the piston rod 03 is reduced to zero of virtually eliminated.

[0012] By employing a piston crown constructed as described above in accordance with the invention, the following advantage can be obtained. That is, since the surface pressure unevenness as well as relative displacement in the radial direction at the butt surface 07 between the support skirt 02a and the piston rod 03y are reduced to zero or minimized, generation of fretting on that butt surface can be prevented. Thereby, wear of the piston crown 01 and the piston rod 03 can be precluded.

[0013] In the second preferred embodiment of the present invention illustrated in cross-section in Figure 3 the piston crown 01 is basically similar to the first embodiment, except that it is provided with two annular support skirts as shown, although in general any appropriate number of support skirts could be provided. In Figure 3, in addition to the support skirt 02a as described in the first embodiment, a further support skirt 015 of smaller diameter is provided and is fixed to the piston rod 03 by circumferentially spaced bolts 015.

[0014] The further skirt 014 divides the chamber 08 into two smaller chambers 08a and 08b and these are connected by a further coolant passageway 4a. The inner diameters D₁ and D₃ adjacent the surface 06 of the head wall 05 of the crown 01 of the larger diameter support skirt 02a and the smaller diameter support skirt 014, respectively, are formed larger than their respective inner diameters D₂ and D₄ adjacent the head surface 03a of the piston rod 03. The fixing holts 015 could be omitted, if necessary.

[0015] The effects and advantages of the embodiment shown in Figure 3 are similar to those of the first preferred embodiment shown in Figure 2.

[0016] While the present invention has been described above in connection with two preferred embodiments, as a matter of course it is intended that all matter contained in the above description, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

Claims

1. A piston crown for an internal combustion engine piston of the kind having an annular support skirt which projects from an inner surface of the head wall of said crown with its free edge providing a butt surface which is supported on the head surface of its associated piston rod, characterized in that the inner diameter (D₁) of said annular support skirt (02a) at a point adjacent the inner surface (06) of said head wall (05) is larger than the inner diameter (D_l) thereof at a point where its butt surface (07) engages said head surface (3a) of the piston rod (03).

2. A piston crown in an internal combustion engine as claimed in Claim 1, characterized in that at least two annular support skirts (02a, 014) are provided in radially juxtaposed relationship, each being of larger inner diameter (D₁, D₃ respectively) where they project from said inner surface of the head wall than their respective inner diameters (D₁, D₂) adjacent their said butt surfaces.

Drawing