Method for machining the connecting rod passage in a cylinder crankcase, cylinder crankcase and engine obtained using the method

A method for machining a passage for a connecting rod in an internal combustion engine crank case upper half, including producing, at a base of the cylinder barrel, a straight cut of cylindrical shape capable of allowing the connecting rod a lateral excursion as a piston moves up and down in the cylinder, using a rotary milling tool which is plunge-operated from underneath the cylinder, and a direction of plunging of the milling tool is oriented at an oblique angle to the axis of the cylinder.

BACKGROUND

The present invention relates to the machining of cylinder crankcases of an internal combustion engine.

More specifically, the invention relates to a method for machining a connecting rod passage in a cylinder crankcase of an internal combustion engine, the crankcase and the engine produced in accordance with this method.

The method involves producing at the base of the cylinder shaft a straight cut of cylindrical form, which is capable of allowing the lateral travel of the connecting rod when the piston moves up and down in the cylinder, with a rotary milling tool which operates by means of travel below the cylinder.

A connecting rod of an internal combustion engine generally comprises a connecting rod little end, which carries the drive shaft of the piston, and a connecting rod big end which is connected to the crankshaft. The connecting rod little end has a bore which receives the drive shaft of the piston. The connecting rod big end also has a bore which receives a crankpin which is offset relative to the rotation axis of the crankshaft.

The connecting rod serves to connect the crankshaft to the piston in order to convert the rotation movement of the crankshaft into an alternating vertical displacement of the piston inside the cylinder. The big end thereof rotates with the crankshaft, whilst the little end thereof is displaced vertically with the piston between the positions of top dead centre and bottom dead centre, shown inFIG. 1. Between these two extreme positions, the connecting rod is raised and lowered with the piston, moving towards and moving away from the wall of the cylinder.

The openings which allow the travel of the connecting rod at the cylinder base, or connecting rod passages, are generally machined at the base of the cylinder shaft, using a rotating milling cutter which moves by means of vertical downward movements. The milling cutter acts on the base of the shaft from the front and the cut, or recess, formed in this manner is a straight cut in the lower portion thereof.

The frontal action on the cylinder shaft by the milling cutter involves a combined risk of rapid wear of the milling tool and of the skirt of the piston when it moves downward into contact with the recess.

BRIEF SUMMARY

An object of the present invention is to improve the quality of the machining carried out for the connecting rod passages of a cylinder, whilst limiting the risks of premature wear of the piston skirt and that of the milling tool.

To this end, the invention proposes that the direction of travel of the milling tool be orientated obliquely relative to the axis of the cylinder.

The cut of the connecting rod passage machined in the cylinder shaft is therefore orientated obliquely relative to the axis thereof.

DETAILED DESCRIPTION

FIG. 1is a cross section of a cylinder crankcase1of an internal combustion engine, and the connecting rod2associated with the piston (not illustrated) close to the top dead centre and bottom dead centre positions thereof. The connecting rod big end3has a bore3awhich receives a crankpin (not illustrated) of the crankshaft. The connecting rod little end4has a bore4a, which receives the drive shaft of the piston (also not illustrated). The shaft of the cylinder6is covered internally with a lining7which is normally inserted during the casting operation. The crankshaft passage8also appears in the plane of section between the two illustrations of the connecting rod2. Behind this, it is possible to see the base of a vertical cylinder9having a smaller diameter than the previous one, which allows the release of the honing tool (not illustrated) used to carry out the finishing of the surface state of the shaft, and the through-hole11for the passage of gases extending horizontally through the cylinder9.

The enlarged view ofFIGS. 2 and 3, in which the connecting rod is no longer shown, schematically illustrates the milling tool13in an operating position at the base of the shaft6(FIG. 2) and the recess12produced thereby in accordance with the known machining method, by means of vertical downward movements below the cylinder. The recess12formed in this manner is a straight cut which forms a sharp edge12aat the base of the shaft6. As indicated above, the frontal action on the base of the shaft by the milling tool leads to rapid wear thereof. Furthermore, the skirt of the piston (not illustrated) is also at risk of damage if it comes into contact with this edge and the lateral travel of the connecting rod is strictly limited by the height of the cut12at the base of the shaft, which has a sharp edge, as shown inFIG. 3.

FIG. 4shows a cylindrical milling tool13carrying out a downward movement which is no longer vertical but instead oblique in the cylinder. Such a machining operation is carried out with a less violent impact of the tool on the shaft6. The surface of the recess or cut14is cylindrical. It is orientated in an oblique manner, in accordance with the axis of the downward movement of the tool. This surface is smaller than with the known method.

FIGS. 5 and 6show the connecting rod2and illustrate the space16inside which the connecting rod moves around the crankshaft. This space is referred to by specialists as a “mandolin” as a result of its resemblance to the musical instrument of the same name. The shape thereof is imposed by the dimensions and the kinematics of the connecting rod.

The drawings show that the recess14, produced according to the invention, allows sufficient space to be provided between the cylinder and the path of the connecting rod3, when the piston is raised and lowered, in order to prevent any risk of impact in this region.

The connecting rod and the base of the cylinder shaft are thus protected from mutual impacts which may occur during operation as a result of the wear of components, or the increase of their operational plays during the life of the engine.

As indicated above, the invention further affords other advantages. In particular the skirt of the piston is no longer at risk of meeting the sharp edge when it moves in the lower portion of the shaft, close to the bottom dead centre. Finally, the milling tool is also protected from frontal impacts against the base of the shaft.