GB1584087A

GB1584087A - Piston

Info

Publication number: GB1584087A
Application number: GB3403777A
Authority: GB
Original assignee: Karl Schmidt GmbH
Current assignee: Karl Schmidt GmbH
Priority date: 1976-10-18
Filing date: 1977-08-12
Publication date: 1981-02-04
Also published as: DE2646922A1

Description

(54) PISTON (71) We, KARL SCHMIDT GmbH, a body corporate organised under the laws of the German Federal Republic, of Christian Schmidt-Strasse 8/12, 7107 Neckarsulm, German Federal Republic, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a piston having a number of ring grooves formed in the ring area and intended to receive piston rings.

The purpose of the piston rings of a piston for an internal combustion engine is to seal the combustion chamber in relation to the crank drive chamber, to assist the transmission of heat from the piston to the cylinder wall, and to control the oil economy of the engine. A prerequisite is that the rings should lie with their periphery close against the cylinder wall and with a ring flank against the flank of the piston groove.

Radial contact is generally achieved through the spring force of the rings, which is assisted by the gas pressure passing behind the ring. Axial contact is governed by gas pressure and acceleration forces, and can vary as between the flank of a bottom ring and the flank of an upper ring.

The conditions under which piston rings work are very unfavourable. Because of mass and gas forces the rings perform axial movements. During operation of the engine the pistons and cylinders continuously assume different diameters because of the varying load and of the correspondingly changing generation of heat, while in addition cylinder liners have a conical internal shape during operation, even in watercooled engines.

Both types of diameter fluctuations must be compensated for by the piston ring.

which for this purpose contracts and ex pands radially. In addition, "piston gouging" on the change of slide path brings about an inclination of the longitudinal axis of the piston in relation to the axis of the cylinder. The sealing surfaces of'the rings then lie at an angle on the cylinder wall.

When the rings make poor contact or are lifted off, the passage of gases past the rings and consumption of oil increase; local overheating of piston rings, piston skirts, and liners are the consequence of the passage of hot combustion gases past the rings. Carbonisation of lubricating oil, jammed piston rings, and seized pistons are further con sequences.

Furthermore, the amount of combustion gases passing through from the combustion chamber to the crank drive chamber is dependent upon the mounting and running clearance of the piston lands, because excessive clearance of the lands leads to a considerable increase of the leakage of gas.

On the other hand, the lands should be given adequate clearance, since in these parts high temperatures, caused for example by damage to the injection system, may have considerable unforseen effects. Addi tonal consequences are poor lubricating conditions, the influence of the passage of hot combustion gases. and the deposition of combustion residues.

There is therefore a tendency for the clearance of piston rings and that of piston lands to be adjusted to one another in such a manner that the lowest possible oil consumption, with adequate lubrication of sliding parts, and at the same time the lowest possible leakage of gas are achieved.

It has been proposed in the specification of U.S. Patent No. 3,463,057 to make the diameter of the top land of a piston smaller than is necessary in accordance with its thermal loading. By this means the gap between the top land and the wall of the cylinder is enlarged, so that the full pressure of the combustion gases becomes effective at that flank of the first piston ring which is nearer the piston head, as well as behind the piston ring itself, whereby a greater difference in pressure is obtained between the region above and that below the first piston ring. The increased difference in pressure has the consequence that the first piston ring lies firmly against the bottom flank of the first piston ring groove and that the running surface of the piston ring is pressed against the cylinder wall.

Through this action the film of oil present between the sliding parts is subjected to greater pressure, which produces a reduced thickness of the oil film and thus lower consumption of oil. The disadvantage of this prior construction is that the increased pressure at the flank the piston ring lying nearer the piston head subjects the piston ring to increased mechanical loading. The reduction of the diameter of the top land in comparison with the diameter of the piston also results in an increase in temperature in the.region of the piston head.

The specification of German Patent No.

20,26,272 describes a piston in which there is provided beneath the first piston ring groove an additional groove which serves as an oil collecting ring and is open only in the direction of the cylinder wall. Although this is not expressly stated, this oil collecting ring also brings about a variation of the difference in pressure between the first and second piston ring grooves. During the movement of the piston the oil contained in the oil collecting groove is moved to-and-fro over the length of the stroke, that is to say more oil is provided for the first piston ring than in known types of pistons, so that the thickness of the oil film adhering to the cylinder wall in increased, thus resulting in increased oil consumption.Furthermore, with this piston a fluttering of the second piston ring, caused by the oil collecting groove situated above the piston ring and acting as a throttle, has frequently been observed. Ring flutter is the most critical phenomenon in respect of its effect, since it causes the seal in relation to the combustion gases and the oil to be broken.

According to the present invention there is provided a piston wherein the piston has a piston head, a piston face and a plurality of annular grooves disposed in a ring area and intended to receive piston rings, and wherein the piston land lying between the groove nearest the piston face and the next adjacent groove is set back radially inwards at its intersection with said next adjacent groove.

With a preferred embodiment of the present invention there is built up between the first and second piston ring grooves a difference in pressure which in its effect permits an optimum compromise between gas leakage and oil comsumption.

Preferably the end face of the first piston land is set back, at least in the region of the flank of the land beside said next adjacent groove and lying nearer the piston skirt, to the extent of at least 0.325% of the nominal diameter of the piston.

The end face of the piston land may be set back by means of a recess which is of approximately rectangular cross-section and which extends from the flank of the piston land on the piston skirt side over at least 90% of the axial extent of the piston land.

Starting from the edge of the flank of the piston land on the piston head side the recess may however also be triangular in cross-section.

Through this shaping of the end face of the first piston land the volume of the gap between the piston and the cylinder wall facing it is enlarged, and thus the pressure of the combustion gases against the flank of the first piston ring on the piston skirt side is reduced in comparison with the prior art.

This means that with a low pressure level an increased difference in pressure having a sealing effect is achieved. Furthermore, an increase of the temperature of the piston head is avoided by the present construction.

It is also made impossible for oil to be dragged to-and-fro, while on the contrary oil is offered to the piston ring in the second ring groove, this ring, which serves to control oil consumption, lying in contact at the bottom edge of its running surface and being at a distance from the cylinder wall in the direction of the combustion chamber, so that during the movement in the direction of the combustion chamber the ring can slide thereover.

In addition, the reduced pressure of the combustion gases in the region of the first piston land has full effect on the second piston ring, so that for this ring also there is still an adequate difference in pressure between the flanks.

Particularly in pistons having two piston rings, that is to say a compression ring and an oil stripper ring, the conical shape of the end face of the first piston land has given good results.

In order to enable the invention to be more readily understood, reference will now be made to the accompanying drawings, which illustrate diagrammatically and by way of example two embodiments thereof, and in which: Figure 1 is a longitudinal sectional view through part of a piston head, and Figure 2 is a view similar to Figure 1 of another piston.

Referring now to Figure 1, there is shown part of a piston 1 with three piston ring grooves 2, 3 and 4. The piston land 5 lying between the piston ring groove 2 lying nearest the piston face, and the next adjacent groove 3 is set back radially to the extent of 0.325% of the nominal diameter of the piston over an axial extent of 90% starting from the flank of the piston land on the piston skirt side, through the provision of a recess 6.

Figure 2 shows part of another piston 7 having piston ring grooves 8 and 9. The piston land 10 lying between the piston ring grooves 8 and 9 is set back radially by the formation of a triangular section recess 11 extending from the edge of the flank of the piston land on the piston head side, the amount by which the flank of the piston land on the piston skirt side is set back amounting to 0.325% of the nominal diameter of the piston.

WHAT WE CLAIM IS: 1. A piston, wherein the piston has a piston head, a piston face and a plurality of annular grooves disposed in a ring area and intended to receive piston rings, and wherein the piston land lying between the groove nearest the piston face and the next adjacent groove is set back radially inwards at its intersection with said next adjacent groove.

2. A piston as claimed in Claim 1, wherein, at least in the region of the flank of the piston land beside said next adjacent groove, the setting-back is at least 0.325% of the nominal diameter of the piston.

3. A piston as claimed in Claim 1 or 2, wherein, the setting-back extends over at least 90% of the axial extent of the piston land.

4. A piston as claimed in Claim 1 or 2 wherein the setting-back is triangular in cross-section, starting from the edge of the flank of the piston land on the piston head side.

5. A piston substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. adjacent groove 3 is set back radially to the extent of 0.325% of the nominal diameter of the piston over an axial extent of 90% starting from the flank of the piston land on the piston skirt side, through the provision of a recess 6. Figure 2 shows part of another piston 7 having piston ring grooves 8 and 9. The piston land 10 lying between the piston ring grooves 8 and 9 is set back radially by the formation of a triangular section recess 11 extending from the edge of the flank of the piston land on the piston head side, the amount by which the flank of the piston land on the piston skirt side is set back amounting to 0.325% of the nominal diameter of the piston. WHAT WE CLAIM IS:

1. A piston, wherein the piston has a piston head, a piston face and a plurality of annular grooves disposed in a ring area and intended to receive piston rings, and wherein the piston land lying between the groove nearest the piston face and the next adjacent groove is set back radially inwards at its intersection with said next adjacent groove.