US2801889A - Light metal piston for internal combustion engines - Google Patents

Description

1957 c. BUCKEN ETAL 2,801,889

LIGHT METAL PISTON FOR INTERNAL coususnon ENGINES Filed Feb. 17, 1956 United States Patent LIGHT METAL rIsToN non INTERNAL COMBUSTION ENGINES Curt B ii cken, Heilhronn, arid Max Kuhm, Neckarsulm,

Germany, assignor's to Karl Schmidt G.

Neckarsulm, Germany Application February 17, 1956, Serial No. 566,300 Claims priority, application Germany February 19, 1955 4 Claims. (Cl. 30 9-9) This invention relates to pistdiis or light metal alloys, particularly aluminum alloys, for internal combustion engines. The invention is directed to a piston having a unitary metallic piston skirt, and in which regulating members of materials having a lower thermal expansion, such as iron alloys, are provided.

It is a known requirement that all pistons that are fitted in engines for passenger vehicles should run silently. To accomplish this, pistons, which have, for example, diameters up to 80 mm. have been so constructed that they have the smallest possible working clearance in the cylinders. In pistons of such size the working clearances are usually less than 0.04 mm. However, a piston acquires a greater diameter at operating temperature than the cylinder, because the light metal of the piston expands to a greater extent under the action of heat than the cylinder made of iron. In order to prevent the piston from rubbing against the cylinder wall, or even seizing against the cylinder wall, the piston initially is made oval in section. These conditions apply, however, only to the skirt of the piston which is open at the bottom, because only this part is sufficiently elastic to assume the necessary deformations. The upper part of the piston skirt, on the other hand, cannot be correspondingly deformed, because it is too rigid. Pistons are therefore so machined that they have, in 1ongitudinal section, a substantially conically or slightly convex outer surface wall. In this way, the piston is given sufficient clearance from the cylinder Wall at the top part of its skirt, so that the above-mentioned difficulties at the operating temperature are suppressed from the beginning.

A piston which is equipped with expansion restricting or expansion regulating inserts of metal having lower thermal expansion than that of the light metal alloys, and the skirt of which is made cylindrical, but oval in cross-section, or at least almost cylindrical-oval, has a control in the upper part of the skirt so that between the cylinder and the piston a uniform play is ensured under all conditions of operation. All pistons in modern engines are accordingly so fitted that they have sufiicient running clearance relative to the cylinder at their upper skirt part under all conditions of operation, while the bottom part of the skirt is still so constructed that during operation, the diameter of the piston is greater than the bore of the cylinder. The bottom part of the skirt must therefore be deformed.

Light metal alloys, particularly aluminum alloys, have however only limited elastic properties. All pistons of the type described will therefore be bent in at the bottom end of the skirt after running for a relatively short time, the amount of the bending in corresponding to the increase of the piston diameter as compared to the cylinder bore.

As a rule, the diameter on the bearing surface sides of the piston, after running 2,000 to 8,000 kilometers, becomes about 0.04 to 0.66 mm. smaller as compared with its condition when fitted. After a running time in. b. H.,.

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corresponding to 2,000 to 8,000 kilometers, therefore, the original clearance from the cylinder wall at the bottom part of the skirt is increased from 0.04 to 0.08 or 0.1 mm., the piston begin to slap. In order to obviate this disadvantage, the bottom part of the piston skirt is reinforced with relatively strong horizontal thickened walls or ribs, running therearound, so that the recession of the bottom part of the skirt is prevented. These thickened walls or ribs running around the skirt, however, necessitate the use of cores comprising five or seven parts for the casting apparatus. Although these multi-part cores can be withdrawn from a solidified casting' of the type described without having to destroy or damage the casting, nevertheless for reasons of economy, and in view of the large number of pistons produced at the present time, it is an extremely great advantage to be able to work with only three-part cores, because these can be handled in automatic or semi-automatic casting and multiple casting machines, which is not the case with fiveand seven-part cores.

The objects of the invention are to produce a piston which can be cast with a triple-part core suitable for automatic or semi-automatic casting machines, while having the bottom part of the skirt so constructed that a body is formed which is still as rigid as possible for resisting deformation.

To this end, according to the invention, the walls of the piston are given a greatly varying thickness, so that adjacent the wrist pin bosses the wall has a minimum thickness, and the opposed bearing surface walls have a maximum thickness, and with a part of the piston hearing surface wall further thickened by an inner chordlike thickened area. This is accomplished by making the triple mold core bundle oval, the major axis of the oval being in the direction of the axis of the wrist pin bosses and the minor axis at right angles thereto.

In order to stiffen the open piston skirt end adequately beneath the wrist pin bosses, strong ribs are provided which are transverse of the axis of the bosses. These ribs have their maximum cross-section directly beneath the bosses, and decrease in thickness towards both sides, so that they fair in the body of the skirt wall at the inner ends of the mold side cheeks.

In order to enable the invention to be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically an embodiment of the invention, and in which:

Figure 1 is a cross-sectional view through the piston head and skirt; and

Figure 2 is a cross-sectional view through the piston skirt, and showing the three-part mold.

In Figure 1, the light metal piston has a piston head 2 separated from the skirt 4 by slots 6 and 8, respectively. An expansion restricting or regulating member 10 is inserted in the upper portion of the skirt, immediately beneath the slots 6 and 8. However, the invention is also applicable to a non-slotted piston.

As shown in Figure 2, the skirt wall is of varying thickness. The bearing wall portions 12 have the greatest thickness, with the skirt tapering to the thinnest wail portion 14 on the sides of the piston containing the wrist pin bosses 16. Beneath each boss are crescent-shaped ribs 18, each of which has the greatest cross-section beneath the boss, and faired into the skirt wall adjacent the thick skirt wall portions 12. Between the respective ends of rib 18, the skirt wall is reinforced by a chord-like metal filling 20. The three-part mold core is also shown in Figure 2. It is composed of a center part 22 and a side or cheek parts 24 and 26, respectively. The entire core is of oval section, the major diameter lying on the axis through bosses 16, and the minor axis at right angles thereto, and through the center of the thick skirt wall portions 12.

The center core part 22 is made of a length less than that of the inner diameter of the skirt so that the reinforcing 20 is obtained during the casting of the piston.

Hence the piston has a stiff open skirt which is adequately reinforced against the bending in of the lower part of the skirt; and consequently, the piston will run without slapping for a long period.

The piston has the further advantage in that it canbe produced by automatic or semi-automatic casting machines since such machines can operate the three-core parts without difiiculty. The piston construction is fully as useful as pistons requiring five or six cores for their preparation.

Having now described the means by which the objects of the invention are obtained, we claim:

1. A light metal piston for internal combustion engines and having a piston head, a unitary piston skirt, and low expansion regulating members in said skirt, and wrist pin bosses, said skirt comprising a wall thicker on the bearing surface sides of the skirt than on the wrist pin boss sides, and crescent-shaped ribs beneath the wrist pin bosses and centered therebeneath.

2. A light metal piston as' in claim 1, further comprising a chord-like reinforcing in said skirt on the bearing surface sides thereof.

3. A light metal piston as in claim 2, further comprising said reinforcing extending over substantially the full length of said skirt.

4. A light metal piston as in claim 3, said crescentshaped ribs having ends faired into said reinforcing.

References Cited in the file of this patent UNITED STATES PATENTS 2,213,775 Vostrez Sept. 3, 1940 2,321,054. Vostrez June 8, 1943 FOREIGN PATENTS 855,647 Germany Feb. 20, 1942

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