EP0030399A1 - Light metal piston - Google Patents

Abstract

In order to achieve a separate regulation of the upper and lower edge of the shaft in a light metal control piston and to avoid a direct transfer of deformation between the upper and lower edge of the shaft and to reduce the shading area between the upper and lower edge of the shaft, the piston shaft must be at the level of the pin bosses (4) Provide the circumference with a completely or partially circumferential transverse slot (5).

Description

The invention relates to a light metal control piston.

The light metal piston as a transmission element of the thermal into mechanical energy has become indispensable in modern internal combustion engine construction. A large number of requirements are placed on such a piston in all operating states. It is always the goal to achieve good straightness with the light alloy piston, despite the very different thermal expansion coefficient of the piston material compared to gray cast iron cylinders.

All such piston designs are based on the idea of preventing the undesired thermal increase in diameter of the piston with the aid of expansion-resistant steel inserts and of adapting the piston play in the pressure and counterpressure directions to the diameter of the cylinder under all possible operating conditions. This process is called the control effect.

The cylinder of the internal combustion engine deforms in the radial and axial direction under the thermal load and the acting gas pressures and hardly maintains its cylindrical shape. The piston must follow these deformations and through play and elasticity allow an adjustment, but too high elasticities have a disadvantageous effect.

When the upper edge is loaded, it deforms and, at the same time, the lower edge also deforms to an increased extent, as a result of which the rigidity of the piston skirt is impaired. Shaft stiffness is understood to mean the resistance to the deformation of the piston skirt under the action of force. When the piston tilts around the upper edge, the operating clearance becomes the lower edge. by this additional deflection amount due to the load on the upper edge. This increases the piston inclination at the same time, so that head striking and increased shaft noise are possible. Conversely, this also applies to tilting around the lower edge of the shaft.

With conventional control pistons, this effect occurs to a greater extent because the deformation is transmitted from the rigid upper edge of the shaft to an increased extent to the more elastic lower edge of the shaft.

The control characteristic of the piston is determined by the coefficient of expansion and the temperature gradient between the piston head and shaft. Expansion coefficient and temperature gradient are different for each piston type and are influenced by design and engine-specific measures.

The interaction of piston play, control action and shaft deformation is decisive for the straight guidance of the piston. It should always be borne in mind that there is a compromise between the requirements of sufficient structural stability, smooth running, seizure protection, oil consumption ₃ Friction power and weight must be closed, because the stiffness and the regulating effect partly influence each other.

The running behavior of a piston is essentially determined by the play behavior in the operating phase. It is therefore a priority to be able to use a piston that has almost the same play behavior in all operating phases. Control pistons are used for this purpose, with which the natural expansion on the shaft is reduced in the running direction. Pistons are known for this, which separate the head and the ring field from the shaft via a transverse slot and which in some cases additionally have a steel insert on the shaft. Such constructions fulfill the control function satisfactorily, but can only be used to a limited extent due to inadequate structural strength.

For reasons of strength and temperature, control pistons with steel inserts are used for motors with higher loads. Disadvantages with these constructions have emerged that either a poor regulating effect is achieved or, with a locally satisfactory regulating effect, there are strong differences in the regulating value between the upper and lower edge of the stem. In particular in the case of ring-shaped or band-shaped inlays in the region of the upper end of the shaft, a reversal of the regulating action is even found on the lower shaft, that is to say that larger expansion values are set than is given by the natural expansion of the base material. With all of these constructions there is one separate control of the control effect of the upper edge of the shaft and the lower edge of the shaft is not possible.

So that such pistons can be designed to be free from seizure at full load, there is usually a stronger local reduction in the grinding curve or a greater installation clearance is provided than is the case with slotted control pistons. This leads to an increase in play during cold start and part load, which has a disadvantageous effect on straight guidance behavior and piston noise. (The task of a good control piston is to have minimal play over the entire speed and load range.) The cheaper play behavior of a control piston with steel inserts leads to additional weight compared to an aluminum single-metal piston. However, the piston weight is increasingly limited due to the need for mass balance. As a further task, therefore, the requirement of lighter control pistons must be met.

In addition, tests have shown that by reducing the area of the piston skirt, friction losses in the engine can be reduced by 3 to 5% of the useful power.

• - eine getrennte Regelung des Ausdehnungskoeffizienten von Kolbenschaftober- und -unterkante mit der Möglichkeit, gezielte Ausdehnungskoeffizienten an Schaftober- und -unterkante einzustellen, geschaffen;
• - eine direkte Verformungsweitergabe zwischen Kolbenschaftober- und -unterkante verhindert;
• - und die Schaftfläche zwischen Schaftober- und -unterkante reduziert wird.

These aspects have led to the task according to which

• - A separate regulation of the expansion coefficient of the upper and lower edges of the piston skirt with the possibility of setting specific expansion coefficients on the upper and lower edges of the skirt has been created;
• - prevents a direct transfer of deformation between the upper and lower edge of the piston skirt;
• - And the shaft area between the upper and lower edge of the shaft is reduced.

The solution to this problem can be seen in the fact that the piston skirt is provided at the level of the pin hubs on the entire circumference with a completely or partially circumferential transverse slot.

According to a further feature of the invention, the transverse slot widens from the load-bearing areas of the shaft surfaces towards the pin bosses into the upper and lower shaft parts.

The lower shaft part is connected to the pin hub, so that the upper and lower parts of the piston shaft are designed as sliding shoe-like tabs in the area of the supporting shaft surfaces. The shaft guide is therefore made in two parts.

So that the side forces can be absorbed by the lower piston skirt part, this is equipped with a stiffening collar on the inside.

In order to achieve optimal locking, it is advisable to arrange a control element in the upper part of the shaft.

In the proposed embodiment, the expansion of the upper edge of the shaft is controlled by a control element and that of the lower edge of the shaft via the temperature control. The direct heat flow to the lower edge of the shaft is interrupted. A heat supply takes place just over the hub. The hub temperature is therefore significantly higher than the shaft temperature of the lower edge. Due to the temperature differences in one plane, the control effect is based on the increased expansion in the direction of the bolt.

The inventive design of the piston skirt does not deform the lower edge when the upper edge is loaded or the upper edge of the piston skirt when the lower edge is loaded. The operating clearance of the lower edge is therefore not additionally increased when the upper edge is loaded, which ensures better shaft guidance and thus cheaper oil consumption.

Since the axial length of the upper part of the piston skirt is relatively short, the height of the control member can also be made shorter than the previous control piston designs, which leads to an additional weight saving.

Furthermore, better control values are achieved at the upper edge of the piston skirt.

Overall, any desired control value can be set on the upper and lower edge by constructively varying the wall thicknesses of the piston skirt parts. A reversal of the expansion coefficient is avoided.

The result is an upright grinding curve with good straightness. The piston skirt designed according to the invention can be realized in segment strips, ring strips, band strips and also in pistons with bimetal strips.

The upper and lower edge forces are recorded separately. The constructive solution is mainly designed for the vertical absorption of forces. This applies to the shaft forces if the friction forces can be neglected. Since the frictional forces with a coefficient of friction of 0.01 are only 1% of the lateral forces, neglect is justified. The support area to the hub can therefore be made narrow.

In the case of a high-strength design of the piston, the upper shaft part is connected directly to the ring field, while the lower shaft part can be separated from the ring field with less load.

• Fig. 1 einen Längsschnitt durch die Druck-/Gegendruck-Ebene des Kolbens.
• Fig. 2 einen Längsschnitt durch die Bolzenebene der beiden Kolbenhälften und einer Seitenansicht der rechten Kolbenhälfte.

The invention is illustrated by way of example in the drawing and is explained below. Show it:

• Fig. 1 shows a longitudinal section through the pressure / back pressure plane of the piston.
• Fig. 2 shows a longitudinal section through the bolt plane of the two piston halves and a side view of the right piston half.

The piston consists of the piston head 1, the upper part 2 and the lower part 3 of the piston skirt. The two piston skirt parts 2 and 3 are separated by a transverse slot 5 which extends approximately at the level of the pin bosses 4 and which extends from the regions 6, 6 'of the supporting shaft surfaces to the pin bosses 4 in an upward and downward arc. The lower shaft part 3 has a stiffening collar 7, which with the Bolt hub 4 is connected. A segment strip 9 is cast in between the pin bosses 4 and the lowest ring groove 8 as a control element. The control element 9 separates the upper edge of the upper shaft part 2 from the ring field by a gap which forms:

Claims (6)

1. Light metal control piston, characterized by a circumferential slot (5) which is wholly or partially encircling the entire circumference of the piston shaft at the level of the pin bosses (4). 2. Light metal control piston according to claim 1, characterized in that the transverse slot (5) from the load-bearing regions (6, 6 ') of the shaft surfaces extends towards the pin bosses into the upper and lower shaft parts (2, 3). 3. Light metal control piston according to claims 1 and 2, characterized in that the lower shaft part (3) is connected to the pin bosses. 4. Light metal control piston according to claims 1 to 3, characterized in that the lower shaft part (3) is provided by a stiffening collar (7) located on its inside. 5. Light metal control piston according to one or more of claims 1 to 4, characterized in that a control member (9) is arranged in the upper shaft part (2). 6. light metal control piston according to one or more of claims 1 to 5, characterized in that each desired control value on the upper and lower edge of the piston skirt is adjustable by constructively varying the wall thicknesses of the piston skirt parts (2, 3).

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