US2882107A - Light alloy piston - Google Patents

Description

Filed Feb. 28. 1958 VEN ' I IN HERMAN By' if United States Patent LIGHT ALLOY PISTON Hermann Mahle, Stuttgart, Germany Application February 28, 1958, Serial No. 718,292

Claims priority, application Germany March 6, 1957 6 Claims. ('Cl. 309-13) This invention relates to pistons. In particular, the invention is directed to a light metal piston having expansion regulating inserts for use in an internal combustion engine. This invention is an improvement over the piston disclosed in my copending application Serial No. 569,009, filed March 2, 1956, now Patent No. 2,827,347, for Light Metal Piston.

In my aforesaid application, it is disclosed that light metal pistons having expansion regulating inserts contain slots between the ring section and skirt. These slots were replaced by thin walls having a cross-section of from 1.2 to 3 percent of the top surface area of the piston head with each wall having a thickness less than 2 percent of the outside skirt diameter. Such connecting walls between the relatively thin skirt and the rigid piston head improve the transfer of heat from the piston head to the skirt and thus improve the heat flow from the ring section to the skirt without any substantially deleterious effect upon the control exerted by the metal inserts upon the expansion of the light metal skirt.

The object of this invention is to produce the same effect by the use of a thicker connecting wall between the ring section and the skirt. It has been found that the connecting wall can have a thickness of more than 2 percent of the outer skirt diameter. This increased thickness enables the piston to take higher mechanical stresses as exist, for example, in a high compression engine.

In general, this is accomplished by increasing the axial height of the wall parts forming the heat and mechanical stress transfer zone in addition to adjusting the radial thickness of the connecting wall with respect to the dimensions of the skirt, both adjustments being dependent upon the size of the expansion regulating metal inserts.

The means by which the objects of the invention are obtained are disclosed more fully with reference to the accompanying drawings, in which:

Figure 1 is a longitudinal cross-sectional view through one half of a piston showing the invention; and

Figure 2 is a partial similar view of a modified form.

In each form of the invention, the piston is composed of a piston head 1, including the piston crown and ring section, and an integral skirt 2. The heat expansion of the skirt is regulated as known to the prior art by sheet metal steel inserts 3 which are cast in the skirt so that they exert a force against the inside surface of the skirt and form therewith a bimetallic unit. The thickness of each insert, not shown in the drawings, is between 1.5 and 4 percent of the outer diameter of the skirt, and for purposes of example, will be taken to be 0.1 inch. The axial height of each insert is at least ten times the thickness and in the example will be larger than 1.0 inch.

The transfer zone 4 joining the crown and ring section to the skirt, and which extends on the thrust or bearing surface of the piston between the piston pin bosses, must now be proportioned to meet the following requirements. Zone 4 must be strong enough to transmit mechanical stresses and also to permit a sufficient heat flow from the piston head to the skirt, and it also must be sufliciently flexible so that it does not prevent the heat expansion control of the inserts 3.

Thus zone 4 is taken to lie between the planes a-a and b-b. Lower plane aa lies on the lower side of the lowest ring groove 5 and plane b-b lies above and closer to the piston crown. The axially extending distance between these two planes corresponds to the axial height of the transfer zone and is at least six times the thickness. of insert 3 or in the present example being at least six times 0.1 inch or 0.6 inch.

Furthermore, the wall thickness for the transfer zone 4 is taken from the bottom of the ring grooves to the inner surface of the ring section, and this wall thickness is larger than disclosed in aforesaid application Serial No. $69,009, but less than three times the thickness of insert 3. Consequently, in the example being given, the wall thickness 2: in Figure 2 and the average wall thickness between the planes a-a and b-b in Figure l is smaller than 0.3 inch. Furthermore, in Figure 1, the requirement is that the maximum value for distance y is four times the thickness of insert 3. Preferably zone 4 has an average thickness of more than 2.7 percent of the outer diameter of the piston skirt. Because of the axial height and thickness of zone 4. it contains more than one ring groove and still retains sufficient flexibility.

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

1. A light metal piston for internal combustion engines in which the piston skirt is continuous and integral with the piston head and in which individual portions of the skirt are connected by reinforcing curved metal inserts cast in the skirt, having a lower coeflicient of expansion than the light metal, which conform substantially to the curvature of the skirt, the said inserts together with the skirt material constituting bimetallic elements which check the radial heat expansion of said skirt along the skirt cylinder wall bearing surfaces between the piston bosses, said inserts having an axial length at a minimum the tenfold of their radial thickness, and in the ring section the wall parts between the inner walls of the ring grooves and the inner surface of the ring section within an area between the plane of the bottom flange of the lowermost ring groove and a closer to the piston crown lying crosssection-al plane, which is distant from said bottom flange of the lowermost ring groove, at a minimum the sixfold of the thickness of the inserts, and having an average thickness of more than 2.0 percent of the outer diameter of the piston skirt, but less than the threefold of the thickness of the inserts.

2. A light metal piston as in claim 1, said ring section wall parts between the inner walls of the ring grooves and the inner surface of the ring section within an area between the plane of the bottom flange of the lowermost ring groove and a closer to the piston crown lying crosssectional plane which is distant from said bottom flange of the lowermost ring groove at a minimum the sixfold of the thickness of the inserts having an average thickness of more than 2.7 percent of the outer diameter of the piston skirt but less than the threefold of the thickness of the inserts.

3. A light metal piston as in claim 1, said ring section wall parts between the inner walls of the ring grooves and the inner surface of the ring section within an area between the plane of the bottom flange of the lowermost ring groove and a closer to the piston crown lying cross-seetional plane which is distant from said 'bottom flange of the lowermost ring groove, at a minimum the sixfold of the thickness of the inserts, having a thickness of at a maximum the double of the thickness of the inserts.

4. A light metal piston as in claim 1, the radial thickness of the ring section between the inner walls of the ring grooves and the inner surface of the ring section in the said plane of the cross-section lying closer to the piston crown at a maximum being the fourfold oi the inserts, wall thickness and decreasing in direction to the plane of the bottom flange of the lowermost piston ring groove.

5. A light metal piston as in claim 1, said area, of the ring section between the plane of the cross-section lying closer to the piston crown and the bottom flange of the piston ring grooves.

4 6. A light metal piston as in claim 1, the radial thickness of the inserts being between 1.5 and 4.0 percent of the outer piston skirt diameter and not larger than the thickness of the skirt walls, which together with the inserts constitute bimetallic elements.

References Cited in the file of this patent UNITED STATES PATENTS Johnston Dec. 11, 1951 2,827,347 Mable Mar. 18, 1958!

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