Engine piston assembly and forged piston member therefor having a cooling recess

Abstract

Present day diesel engines having aluminum piston assemblies are limited to combustion chamber pressures of approximately 12,410 kPa (1,800 psi) whereas the desire is to increase such pressures up to the 15,170 kPa (2,200 psi) range. To reach such levels the instant piston assembly includes a steel piston member having an upper cylindrical portion defining a top surface, a depending tubular wall and an annular cooling recess having one or more machined surfaces of revolution about a central axis. The cooling recess is located beneath the top surface and juxtaposed to the top ring groove for removing heat away therefrom in use. The piston member is preferably forged and subsequently machined to precisely controllable dimensions. Moreover, the piston assembly is preferably of the articulated type and includes a forged aluminum piston skirt connected to the piston member through a common wrist pin.

Description

This is a continuation-op top of the cooling recess 132 is in juxtaposed elevational relationship with the top of the ring groove 112. It is also elevationally disposed directly underneath the peripheral top surface 98 of the piston member 78, and within an elevational distance therefrom identified by the letter E. In one embodiment the diameter D was 124 mm, and the distance E was about 5 to 5 mm 5.5 mm. Thus, relatively thin, substantially constant wall thicknesses are created for substantially even heat distribution and for maximum cooling. The inner wall surface 126 is a machined surface of revolution about the central axis 66 which permits precise dimensional control and concentricity between the bottom of the ring groove 112, 116, and 120 and the wall surface. Dimensional control and concentricity between the bottoms of the ring grooves and the surface 126 and especially the bottom of the closest ring groove 116 to the surface 126 is extremely critical because any deviation can materially weaken the tubular wall 78 resulting in cracking, uneven heat distribution and/or differential thermal distortion. The inwardly facing wall surface 126, the outwardly facing wall surface 128 the downwardly facing portion 130 defining the cooling recess 132 are all machined surfaces of revolution about a central axis 66 eliminate any imperfections that could cause the propagation of cracks and differential thermal distortion. By machining the surfaces 126 and 128 and the downwardly facing wall 130, wall thicknesses, concentricity and surface finishes can all be precisely controlled. Alternatively, with the arrangement shown in FIG. 8 with a deep forged recess 132, it may only be necessary that the inwardly facing wall surface 126 be a machined surface of revolution for dimensional control and concentricity with relation to the bottoms of the ring grooves 112, 116, and 120, and specifically the closest ring groove 116.

In addition to the dimensional constraints mentioned above, it is to be appreciated that the articulated piston assembly 76 is preferably manufactured in a particular way devoid of complex shapes and by using certain materials. Specifically, the upper steel piston member 78 is preferably forged from a chrome-moly alloy steel material such as a basically 4140 modified steel material. The lower aluminum piston skirt 80 is likewise preferably forged an alloy aluminum material such as a basically SAE 321-T6 modified aluminum material.

The aforementioned alloy steel is particularly adaptable to Class II forging procedures, and can provide an austenitic grain size 5 or finer which is highly desirable to resist the high compression pressures above about 13,790 kPa (2,000 psi), and preferably above about 15,170 kPa (2,200 psi). Etched cross sectional samples of the forged steel piston member have indicated that the grain flow lines therein are generally or broadly oriented in an inverted U-shaped configuration that roughly approximates the shape of the piston member portion shown in FIGS. 3, 6 and 7 and/or roughly aligns the grain flow lines with the web 166 and the tubular wall 108, and this contributes substantially to the cross sectional strength thereof.

The aforementioned forged aluminum alloy has a high hardness, excellent wear resistance, and a relatively low coefficient of thermal expansion.

Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.

Claims

1. AΘ An articulated piston assembly comprising an upper forged one piece steel piston member, a lower aluminum skirt member, and a pin articulately mounting the members for reciprocating movement in an engine, the upper piston member comprising:

an upper portion of substantially cylindrical shape and having a central axis, a top surface, a tubular wall depending from the top surface and forged integral with the upper portion, the tubular wall having a peripheral groove having a bottom surface and adapted to receive a sealing ring, the peripheral groove being spaced a preselected minimal elevational distance trh from the top surface, a lower end surface, and an inwardly facing wall surface extending upwardly from the lower end surface;

the upper portion further including an outwardly facing wall surface spaced radially inwardly from the inwardly facing wall surface and a downwardly facing transition portion blendingly associated with the inwardly and outwardly facing wall surfaces to collectively define an annular cooling recess, the transition portion being elevational elevationally spaced a relatively short distance "E" from the top surface, the top of the cooling recess being in juxtaposed elevational relation with the peripheral groove to provide for removing heat from around the peripheral groove; and

the inwardly facing wall surface being a machined surface of revolution about the central axis integral or one piece forging to maintain dimensional control and concentricity between the bottom surface of the peripheral groove and the inwardly facing wall surface.

2. The forged steel articulated piston member assembly of claim 1 wherein the outwardly facing wall surface and the downwardly facing transition portion are machined surfaces of revolution about the central axis.

3. The forged steel articulated piston member assembly of claim 1 wherein the upper piston member further including includes a lower portion having a pair of depending pin bosses blendingly associated with the cooling recess and individually defining a bore, and the bores being aligned along a common axis.

4. The forged steel articulated piston member assembly of claim 3 wherein the upper portion and lower portion are an integral or one-piece forging.

5. The forged steel articulated piston member assembly of claim 4 wherein the material used for the forging upper piston member is a chromium-molybdenum steel.

6. The forged steel piston member of claim 1 wherein the elevational distance "E" between the top surface and the top of the cooling recess is approximately 6 mm.

7. The forged steel articulated piston member assembly of claim 1 wherein the upper portion defines a recessed crown surface which contains machined surfaces of revolution about the central axis so that a relatively uniform web is defined between the crown surface and the cooling recess.

8. The forged steel articulated piston member assembly of claim 1 wherein the minimum radial thickness between the cooling recess and the innermost portion of the peripheral groove is about 1.74 mm.

9. The piston assembly of claim 7 wherein the upper and lower portions are an integrally formed forging.

10. The piston assembly of claim 7 including an aluminum piston skirt, and a wrist pin connecting the piston member to the piston skirt.

11. The piston assembly of claim 10 wherein the outwardly facing wall surface and the downwardly facing transition portion are machined surfaces of revolution about the central axis.

12. An engine articulated piston assembly for an engine of the type having a block defining a bore, a cylinder liner supported in the block and defining a piston bore, and a cylinder head connected to the block, wherein the improvement comprises:

a forged steel piston member piston having an upper portion of substantially cylindrical shape and having a central axis, a peripheral top surface, a tubular wall depending from the top surface and defining an outwardly facing top land, a top ring groove spaced a preselected minimal elevational distance trh of about 5 mm from the top surface, a lower end surface, and an annular inwardly facing wall surface extending upwardly from the lower end surface;

the upper portion further including an annular outwardly facing wall surface spaced radially inward from the inwardly facing wall surface and a downwardly facing transition portion blendingly associated with the inwardly and outwardly facing wall surfaces to collectively define an annular cooling recess located in juxtaposed relation with to the top ring groove, the inwardly facing wall surface being a machined surface of revolution about the central axis; and

a lower portion including a pair of depending pin bosses blendingly associated with the cooling recess and individually defining a bore and with the bores being aligned on a common axis;

a piston skirt defining a pair of wrist pin receiving bores aligned with the bores of the pin bosses; and

a wrist pin in the aligned bores for articulately connecting the piston skirt and the piston member.