GB2563026A

GB2563026A - Crankpin and crankshaft

Info

Publication number: GB2563026A
Application number: GB1708587.9A
Authority: GB
Inventors: Kalogiannis Konstantinos; Merritt David; Omar Mian Abdullah
Original assignee: Mahle International GmbH; Mahle Engine Systems UK Ltd
Current assignee: Mahle International GmbH; Mahle Engine Systems UK Ltd
Priority date: 2017-05-30
Filing date: 2017-05-30
Publication date: 2018-12-05
Anticipated expiration: 2037-05-30
Also published as: GB201708587D0; GB2563026B

Abstract

A crankshaft 2 comprising a crankpin 4, suitable for mounting two big-end bearings 6 of two connecting rods of an engine, is disclosed. The radius of the crankpin 4 varies along its length such that the crankpin comprises two convex portions 12 engageable with respective big-end bearings 6, which may be adjacent along the length of crankpin 4. Each convex portion comprises a first end radius 14, a central radius 16, and a second end radius 18 with central radius 16 positioned between, and greater in size than, first and second end radii 14, 16; typically, central radius 18 may be 0.5 µm to 4 µm larger than the end radii. Each convex portion 12 may have a parabolic variation in radius or may alternatively have a central flat portion. Convex portions 12 may be identical or may be mirror-images of each other.

Description

Field of the Invention

The invention relates to a crankpin for use in a V-type engine, and a crankshaft comprising such a crankpin. In particular, the invention relates to an improved crankpin for use in any engine in which a single shared crankpin is configured to engage with respective big-end bearings of two connecting rods.

Background of the Invention

Crankpins, or crank journals, for use in internal combustion engines form the journals of the big-end bearings from which connecting rods drive the pistons. The surface of a crankpin forms the journal surface which, in use, runs against a co-operating bearing surface of the big-end bearing. In engines containing crankshafts, the crankpins are arranged so that their axes are offset from the central axis of the crankshaft so as to transform the linear reciprocating motion of the pistons into rotational motion of the crankshaft.

Some types of engine, such as V-type engines and certain flat engines, are configured so that two connecting rods are driven from the same crankpin. Such an arrangement requires two big end bearings to be mounted on the same crankpin, which may be termed a shared crankpin.

Summary of the Invention

The present invention provides a crankpin and a crankshaft as defined in the appended independent claims, to which reference should now be made. Some preferred or advantageous features of the invention are set out in dependent subclaims.

In a first aspect, there is provided a crankpin for a V-type engine, wherein the radius of the crankpin varies along its length such that the crankpin comprises two convex portions engageable with respective big-end bearings of two connecting rods of the engine, each convex portion comprising a first end radius, a central radius, and a second end radius, the central radius being positioned between, and greater in size than, the first and second end radii.

The crankpin may alternatively be termed a crank journal, and may be advantageously suitable for use in any application requiring a shared crankpin for engaging with two plain bearings. A particularly preferred application of the invention is as a shared crankpin, or a double crankpin, in a “V-type” internal combustion engine, particularly in a V8 diesel engine. A V-type engine is an example of an internal combustion engine in which the crankshaft is configured such that two respective big-end bearings of two connecting rods are mounted on a single shared crankpin. However, the skilled person will understand that the present invention may be equally applicable to any other type of engine that is configured to require a shared crankpin. This may be the case, for example, in certain “flat” engine configurations. References herein to a V-type engine should therefore be understood to encompass any other form of engine containing a shared crankpin.

The “convex portions” of the crankpins are portions of the crankpin which curve or bulge outwards, or otherwise protrude radially from the generally cylindrical shape of the crankpin. When viewed side-on or in cross-section, the outer surface of each convex portion may have a smoothly curved profile. Alternatively, the profile of each convex portion may comprise one or more outwardly projecting sections and one or more flat sections, as long as the convex portions project outwardly with respect to the underlying cylindrical shape of the crankpin.

The inventors’ tests have demonstrated that the crankpin of the present invention, comprising two convex portions, may advantageously provide a lower level of edge contact with the surfaces of bearings mounted on the crankpin. Furthermore, the crankpin of the present invention was found to provide improved performance with respect to bearing lubrication. The inventors’ tests have shown that crankpins embodying the present invention may advantageously demonstrate improved characteristics with respect to oil film intensity, wear depth and total power loss, compared to conventional cylindrical crankpins.

Thus, the crankpin of the present invention may exhibit advantageously reduced edge contact, and highly desirable lubrication characteristics.

In a preferred embodiment, the two convex portions are identical to one another. Alternatively, the two convex portions may be mirror images of one another, or the two convex portions may be shaped differently.

In a preferred embodiment the first end radius of each convex portion is equal to the second end radius. Alternatively, the first end radius of each convex portion may be different to the second end radius. For example, the radii of the convex portions at their outermost ends (towards the ends of the crankpin) may be smaller than the radii of the convex portions at their innermost ends (towards the centre of the crankpin).

In a preferred embodiment the central radius of each convex portion is between 0.5 pm and 4 pm greater, or between 1 pm and 3.5 pm greater, or between 2 pm and 3 pm greater, than the first end radius and/or the second end radius. Preferably the central radius of each convex portion is 2 pm greater, or 3 pm greater, than the first end radius and/or the second end radius.

The difference in radius between the central radius and the first end radius and/or the second end radius may advantageously relate to the lubrication performance exhibited by the crankpin. The inventors have found that compared to a conventional cylindrical crankpin, the lubrication performance of a crankpin according to the present invention varies depending on the “height” of the convex portion (the greatest difference between the central radius and the first end radius and/or the second end radius). In particular, the lubrication performance was found to improve as the height of the convex portions increased, up to an optimum performance at a certain height, after which further increases in height caused the performance of the crankpin to deteriorate, resulting in increased wear of the big-end bearing surface.

In a preferred embodiment the radius of the crankpin varies non-linearly along the axial, or longitudinal, length of each convex portion. That is, when viewed side-on or in crosssection, the outer surface of each convex portion may have a curved profile. The radius of each convex sections may vary smoothly along its length, or the radius may vary more steeply in some positions than others. For example the rate of change in radius at either end of the convex sections may be greater than the rate of change of the radius nearer or at the middle of the convex portion.

As the crankpin is always symmetrical about its longitudinal, or rotational, axis, the nonlinearly-varying radius may lead to a “barreled” or barrel-shaped convex portion. Thus, the crankpin may comprise two barrel-shaped convex portions.

In a preferred embodiment the variation of the radius along each convex portion is parabolic in shape. That is, the varying radius defining one of the convex portions may be expressed as a parabolic function, and the side-on profile of the convex portion may be a parabola. This embodiment may advantageously form two barrel-shaped convex portions.

In a preferred embodiment of the crankpin, each convex portion may comprise a flat central section with a constant radius equal to the central radius. That is, the central section is a cylindrical portion with radius equal to the central radius. The term “flat” in this respect relates to a side-on view of the crankpin, as when viewed from the side the constant radius of this central section makes this section appear flat relative to the “curved” sections of non-linearly-varying radii.

Preferably each convex portion may comprise first and second curved end sections arranged on either side of the flat central sections. As above, the term “curved” in this case refers to a section whose radius varies non-linearly along the axis of the crankpin. The curved end sections may have parabolic profiles.

In a preferred embodiment, each flat central section may comprise less than 40%, or less than 30%, or less than 25% of the length of the crankpin. Thus, where both convex portions comprise flat central sections, the two flat central sections combined may comprise less than 80%, or less than 60%, or less than 50% of the total length of the crankpin.

The two convex portions may be separated by a central shaft portion, or a shaft portion, with a constant radius. Alternatively, the two convex portions may be directly adjacent to one another.

Big-end bearings are sized to engage with crankpins having a nominal radius that is slightly smaller than the internal radius of the bearing surface, so that a clearance is provided between the two surfaces. Typical clearances between crankpins and bearing surfaces are in the order of tens of micrometres. In a preferred embodiment of the present invention, the central radii of the convex portions are greater in size than the nominal radius of the crankpin, while the first and second end radii are smaller than the nominal radius of the crankpin. Preferably the height of the convex portions may be symmetric about the nominal width of the crankpin. That is, when the crankpin has nominal radius “x”, the central radius may be “x+y”, and the first and second end radii may be “x-y” (corresponding to convex portions of height “2y”).

As the height of the convex portions is far smaller than the nominal radius of the crankpin, a double-convex crankpin of nominal radius “x” and actual radius “x+y” may advantageously be engageable with a big-end bearing that is configured for use with crankpins of nominal radius “x”.

In a particularly preferred embodiment, the crankpin is configured to engage with and support two connecting-rod big-end bearings arranged adjacent to one another along the length of the crankpin. Thus preferably the width of each convex portion substantially corresponds to or is greater than the width of a big end bearing. The crankpin is sufficiently long to allow clearances between the big ends and the big-end bearings, and the convex portions are positioned to interact with the two bearings.

In a second aspect there is provided a crankshaft comprising a crankpin as described in relation to the first aspect.

Specific Embodiments and Best Mode of Invention

Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which;

Figure 1 is a side-on partial cross-section of a portion of a crankshaft comprising a shared crankpin according to a first embodiment of the present invention;

Figure 2A is a schematic partial cross-section of a crankpin according to the first embodiment of the present invention;

Figure 2B is a schematic partial cross-section of a crankpin according to a second embodiment of the present invention; and

Figure 2C is a schematic partial cross-section of a crankpin according to a third embodiment of the present invention.

Figure 1 is a side-on view showing a portion of a crankshaft 2, including a crankpin 4 on which two big-end bearings 6, serving two corresponding connecting rods (not shown) are mounted. The big-end bearings each comprise a cylindrical bearing shell which circumferentially surrounds the crankpin, and are shown in Figure 1 in cross-section to reveal the crankpin. The inner surface 8 of the big-end bearings forms a running surface against which the outer surface 10 of the crankpin runs, in use. During use, lubricating oil forms a film between the surfaces of the crankpin and the running surface of the bearing. The oil film preferably provides hydrodynamic lubrication during normal running, but this will not normally be the case on start up.

The crankpin 4 is generally cylindrical in shape, and is symmetrical about a central longitudinal axis. As shown in cross-section in Figure 1, the crankpin comprises two convex portions 12 arranged end-to-end along the axis of the crankpin. Each convex portion 12 comprises a first end radius 14, a central radius 16, and a second end radius 18, the central radius being positioned between, and greater in size than, the first and second end radii. In the embodiment shown in Figure 1 the convex portions are identical to one another, with each convex portion taking up one half of the total length of the crankpin. The convex portions are configured so that each convex portion engages with one of the two bearings 6. The radii of the convex portions vary along the axis of the crankpin according to parabolic functions, so that the side-on profiles of the convex portions are parabolas, and the convex portions have smoothly curved outer surfaces 10 which create “barrel” shapes.

The central radii 16 of the convex portions 12 are slightly smaller than the internal radii of the inner surfaces 8 of the big-end bearings 6, while there is a greater separation between the ends of the convex portions and the edges of the respective bearings 6. This greater separation at the bearing edges may advantageously reduce edge-wear on the bearing surfaces 8.

The central radius 16 of each convex portion is 2 pm greater than the first end radius 14 and the second end radius 18. Particularly advantageously, the central radius 16 is 1 pm greater than the nominal radius of an equivalent cylindrical (non-convex) crankpin, while the first and second end radii are 1 pm smaller than the nominal radius of such a crankpin. Thus the clearance between the centre of the convex portions 12 and the bearing surfaces 8 is 1 pm less than it would be with an equivalent cylindrical crankpin, while the clearance at the edges of the convex portions is 1 pm more. The inventors have found that this arrangement advantageously exhibits improvements in bearing edge wear, and improved oil film intensity, wear depth and total power loss, compared to an equivalent cylindrical crankpin.

Figure 2a, 2B and 2C show schematic edge profiles of three crankpins according to three different embodiments of the present invention, each crankpin having a different profile.

Figure 2A shows the profile of an upper edge of a “double-barreled” crankpin 4, as described above in relation to Figure 1.

Figure 2B shows a crankpin 40 according to a second embodiment of the present invention, the crankpin having two convex portions 120 separated by a central shaft portion 130 with a constant radius equal to the first end radius and the second end radius. The length of the central shaft portion 130 may advantageously be chosen to correspond to the separation between the edges of the two big end bearings mounted on the crankpin, so that the bearings are mounted centrally with respect to the convex portions 120.

The central radius of each convex portion is 3 pm greater than the first end radius and the second end radius. Particularly advantageously, the central radius is 1.5 pm greater than the nominal radius of an equivalent cylindrical (non-convex) crankpin, while the first and second end radii are 1.5 pm smaller than the nominal radius of such a crankpin. Similarly to the crankpin of Figure 1, this arrangement may provide improvements in bearing edge wear, and improved oil film intensity, wear depth and total power loss, compared to an equivalent cylindrical crankpin.

Figure 2C shows a crankpin 400 according to a third embodiment of the present invention, the crankpin having two convex portions 1200. Each convex portion comprises a flat (cylindrical) central section 1300 with a constant radius equal to the central radius. The lengths of the central sections 1300 are each equivalent to 50 % of the length of the convex portion 1200, or 25 % of the total length of the crankpin 400. On either side of the flat central section 1300 are arranged first and second curved end sections 1500, the profiles of which are shaped as portions of a parabola. This arrangement may advantageously combine the characteristics of a flat journal surface with reduced edge wear on the bearing surfaces.

The radius of each central section 1300 is 2 pm greater than the first end radius and the second end radius. Particularly advantageously, the central radius is 1 pm greater than the nominal radius of an equivalent cylindrical (non-convex) crankpin, while the first and second end radii are 1 pm smaller than the nominal radius of such a crankpin. Similarly to the crankpin of Figure 1, this arrangement may provide improvements in bearing edge wear, and improved oil film intensity, wear depth and total power loss, compared to an equivalent cylindrical crankpin.

Claims

1. A crankpin for a V-type engine, wherein the radius of the crankpin varies along its length such that the crankpin comprises two convex portions engageable with respective big-end bearings of two connecting rods of the engine, each convex portion comprising a first end radius, a central radius, and a second end radius, the central radius being positioned between, and greater in size than, the first and second end radii.

2. A crankpin according to claim 1, in which the two convex portions are identical, or are mirror images of one another.

3. A crankpin according to claim 1 or 2, in which the first end radius of each convex portion is equal to the second end radius.

4. A crankpin according to claim 1,2 or 3, in which the central radius of each convex portion is between 0.5 pm and 4 pm greater, or between 1 pm and 3.5 pm greater, or between 2 pm and 3 pm greater, than the first end radius and/or the second end radius.

5. A crankpin according to any of claims 1 to 4, in which the variation of the radius along each convex portion is parabolic in shape.

6. A crankpin according to any of claims 1 to 4, in which each convex portion comprises a flat central section with a constant radius equal to the central radius, preferably in which each flat central section comprises less than 40%, or less than 30%, or less than 25% of the length of the crankpin.

7. A crankpin according to claim 6, each convex portion comprising first and second curved, optionally parabolic, end sections arranged on either side of the flat central sections.

8. A crankpin according to any preceding claim, in which the two convex portions are separated by a central shaft portion with a constant radius.

9. A crankpin according to any preceding claim, in which the crankpin is configured to support two connecting rod big end bearings arranged adjacent to one another along the length of the crankpin.

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10. A crankshaft comprising a crankpin as defined in any preceding claim.