GB1577353A

GB1577353A - Light alloy piston

Info

Publication number: GB1577353A
Application number: GB1494777A
Authority: GB
Original assignee: Karl Schmidt GmbH
Current assignee: Karl Schmidt GmbH
Priority date: 1976-04-08
Filing date: 1977-04-07
Publication date: 1980-10-22
Also published as: DE2615213A1; BR7701805A; IT1080552B; FR2347540A1

Description

(54) LIGHT ALLOY PISTON (71) We, KARL SCHMIDT G.m.b.H. 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 cast light alloy piston made from a single type of metallic material, which is intended for use in a piston machine, preferably in a high-dry internal combustion engine.

Light alloy pistons have become well established for use in the conversion of thermal into mechanical energy in modern internal combustion engines. At the present time, such pistons must meet a large number of requirements under all operating conditions.

The desire to avoid dead spaces and detrimental squeeze spaces in the combustion chamber of a piston engine has resulted in intricately shaped pistons which can be subjected to very high thermal loads.

Since the advent of light alloy pistons, numerous developments for their improvement have been proposed and made. The object has always been to ensure movement of the light alloy piston along a straight line in spite of the fact that its coefficient of expansion differs greatly from that of the cylinder, which consists preferably of gray cast iron.

All these piston designs are based on the concept of preventing the undesired diametral thermal expansion of the piston by the provision of annular or plate-like expansion-resisting steel inserts in the upper portion of the skirt. It has been attempted to cause the piston to expand mainly in the direction of the gudgeon pin axis and ensure that clearance of the piston in the pressure and back-pressure directions is adapted to the cylinder diameter under all operating conditions as far as possible. The term 'controlled-expansion pistons' is applicable to pistons which embody structural features which ensure that such behaviour takes place.

Under the thermal load and the pressure applied, the cylinder of an internal combustion engine deforms in the radial and axial directions and hardly retains its cylindrical shape. The piston must allow for such expansion so as to provide the necessary clearances and must thus have such an elasticity that it can adapt itself to the cylinder in spite of these deformations of the latter.

The necessity to provide expansion-controlled pistons is emphasized by the fact that they facilitate said adaptation.

It is apparent that a controlled-expansion piston will not perform satisfactorily unless certain basic requirements are met in the design of the internal combustion machine and of the piston. Above all, the knowledge of the loads to be imposed on engines of a given type is essential for the design of a piston which is satisfactory in view of the requirements regarding expansion and control material.

The expansion control action depends upon the coefficient of expansion of the piston and upon the temperature gradient between the head and skirt of the piston. This temperature gradient is different for each internal combustion engine and is influenced by design features.

The movement of the piston along a straight line depends upon the cooperation of the piston clearance, expansion control and skirt deformation. In this connection a compromise is always necessary between the requirements for sufficient stiffness, stress concentration factor and expansion control action if the requirements regarding smooth running and seizure-proofness are to be fulfilled.

The requirements to be met by a piston of an internal combustion engine have been increased particularly by the desire for more comfortable riding in conjunction with a higher motor power and a high acceleration of the vehicle. For the economic use of vehicles, compact engines are required. This involves a small displacement in conjunction with a high horsepower-dislacement ratio, also with a view to future legislation concerning exhaust gases.

In addition to the main requirements to be met by a piston regarding the transmission of power, movement along a straight line, seal, and dissipattion of heat, other properties, which were previously considered as being of secondary importance, have progressively gained in significance. The requirements for silent running must be matched with the requirements regarding the stress concentration factor, minimum wear, and seizure-proofness. A satisfactory seal against the passage of oil and combustion gases past the ring-carrying portion can be maintained for long periods of operation only if the piston is properly designed, machined accurately to the required dimensions, and provided with a satisfactory surface finish. Owing to the high speed of internal combustion engines, the weight of the pistons is gaining in importance as a design aspect.Owing to this severe requirement, lightweight pistons are mainly used now. An exact knowledge of the stresses which will be imposed upon the piston material and of its expansion control action and its behaviour in other respects is one of the requirements for a proper design of an expansion-controlled piston.

Expansion-controlled pistons have been proposed in which. the stiff, hot head of the piston tends to enforce an oval deformation of the skirt in the direction of the gudeeon pins bv means of a bridge structure consisting of the ring-carrying portion and the gudgeon pin bosses.

Such expansion-controlled pistons exhibit a more or less pronounced separation between the piston head and the carrying surfaces of the skirt. In this way the control action is promoted, e.g., by transverse slots, which may be located adjacent to the lower part of tht ring-carrying portion. Such pistons have been used with satisfaction in internal combustion engines for low thermal loads (W. D. Bensinger and A. Meier "Kolben, Pleuel und Kurbelwelle bei schnell-laufenden Verbrennungsmotoren, Springer-Verlag 1961, pages 15 to 19).

On the other hand, an unslotted expansion-controlled piston is advantageous in internal combustion engines for high thermal loads because in these pistons the heat transfer from the hot piston head to the relatively thin-walled skirt is not impeded. The stability of the piston is increased so that its deformability in the direction of the tilting plane is reduced.

For this reason, an unslotted light alloy piston provided with expansion control elements has been described in "Aluminium" 45, 1969, page 545. In such a piston the transverse slots previously considered necessary between the skirt and the ring-carrying portion have been omitted and certain design rules have been complied with. In spite of the absence of slots, the thermal expansion is well controlled and is caused to proceed in the direction of the piston axis as a result of the dependence of the stress distribution on temperature.

A light alloy piston has been proposed which consists of a single type of metal and is free from expansion control inserts and in which the piston head and the carrying shaft are separated entirely or in part. In this piston, the gudgeon pin bosses are connected to the piston head by depending supporting ribs and each of said bosses is connected to the skirt by two supporting walls, which extend at an angle of 0 to 45 degress to the vertical plane which contains the axes of the piston pins.

Owing to its transverse slots, this expansion-controlled piston exhibits a very good expansion control and this results in a highly rectilinear movement in the cylinder. On the other hand, this piston is not satisfactory in high-duty internal combustion engines because the transverse slots impede the flow of heat and reduce the strength of the piston.

According to the present invention there is provided a cast light alloy controlled expansion piston which is suitable for use in a piston machine, the piston being made from a single type of metallic material, wherein the piston comprises a full skirt and a pistonhead from which extend ribs terminating in gudgeon pin bosses the ribs being attached to the underside of the piston head, at regions which when the piston is in use, constitute regions of zero moment and extending transversely to the gudgeon pin axis, each gudgeon pin boss being connected to the skirt by two supporting walls, extending from the periphery of the gudgeon pin boss at an angle of 0 to 450 to an axial plane of the gudgeon pin bosses and wherein a rigid top portion comprising the head and a ringcarrying portion is integrally connected throughout its periphery to a thin-walled cylindrical skirt portion the supporting walls being provided at the same position, relative to the longitudinal axis of the skirt as the gudgeon pin bosses and having a height which is equal to 45 to 65% of the length of the skirt.

Because the ribs which attach the gudgeon pin bosses to the underside of the piston head are attached at regions of zero movement, the gudgeon pin bosses are considered to be "freely suspended" from the piston head and these ribs are, when the piston is in use, subjected only to stress or pressure and not bending.

Preferably the supporting walls have a height which is equal to 50 to 60%, of the length of the skirt and extend at an angle of 30 to 45" to said axial plane of the gudgeon pin bosses.

In the present piston, heat can be transferred from the head of the piston preferably along two paths. The main flow of heat proceeds through the gudgeon pin bosses, which are susupended from the inside of the piston head, and the supporting walls to the skirt.

A small part of the heat is transferred by the ring-carrying portion to the skirt. Because the interior of the piston is defined by very large surfaces, heat is also transferred to lubricating oil which is splashed in the internal combustion engine.

Because the piston head and the gudgeon pin bosses are at a high temperature relative to the solid skirt portion, a sufficiently large thermal expansion in the direction of the gudgeon pin axis is exhibited by the piston head and the gudgeon pin bosses and is transmitted by the supporting ribs to the skirt. As a result, the skirt is expanded in the direction of the axis of the gudgeon pins and, because the periphery of the skirt remains virtually unchanged, the diameter in the pressure and back-pressure directions is reduced.

This expansion control results in a suitable ovality and convexity and in the fact that the deformation-resisting, cylindrical skirt exhibits approximately the same actual expansion as the cylinder under all operating conditions of the internal combustion engine, whether the cylinder consists of gray cast iron or aluminium.

Owing to this matching, numerous desirable expansion control effects can be achieved so that the field of application of this unslotted expansion-controlled piston is not restricted, as with conventional expansioncontrolled pistons, by the dependence of the horsepower-displacement ratio on the piston diameter.

With these measures, piston noise caused by changes of the contacting portions in the cylinder can be much better controlled because the control of the shaft movement is improved.

This expansion-controlled piston of high stability may be designed with relatively small wall thickness so that much weight can be saved in comparison with known expansion-controlled pistons provided with steel inserts.

In a preferred embodiment of the invention, the freely susnended gudgeon bosses are connected by the supporting ribs to an asymmetrically shaped head, which has portions extending on different level and defining a recess which forms part of the combustion chamber.

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 an embodiment thereof, and in which: Fig. 1 is a transverse sectional view of a piston, and Fig. 2 is a cross-section taken on the dotdash line in Fig. 1.

Referring now to the drawings there is shown an unslotted controlled-expansion piston having a shaped head, Fig. 1 being a view taken on a vertical plane which contains the axes of the gudgeon pins. The piston has a relatively thick-walled top portion consisting of a shaped asymmetrical head 1 and a ring-carrying portion 2 connected to a thin-walled cylindrical skirt 3, which adjoins the ring-carrying portion 2.

In the piston shown in the drawings, expansion control is ensured because the gudgeon pin bosses 5, which are freely suspended by means of supporting ribs 4 to an asymmetrical head 1, are connected to the cylindrical skirt 3 by supporting walls 6, which extends at an angle of 45" to the gudgeon pin axis 7 and have a height which amounts to 60% of the length of the skirt.

The skirt 3 is formed with apertures 8, which are centred on the axis 7 and aligned with the gudgeon pin bosses 5.

WHAT WE CLAIM IS:- 1. A cast light alloy controlled-expansion piston which is suitable for use in a piston machine, the piston being made from a single type of metallic material, wherein the piston comprises a full skirt and a piston head from which extends ribs terminating in gudgeon bosses, the ribs being attached to the underside of the piston head at regions which when the piston is in use constitute regions of zero movement and extending transversely to the gudgeon pin axis, each gudgeon pin boss being connected to the skirt by two supporting walls, extending from the periphery of the gudgeon pin boss at an angle of 0 to 45" to the axial plane of the gudgeon pin bosses and wherein a rigid top portion comprising the head and a ring-carrying portion is integrally connected throughout its periphery to a thinwalled cylindrical skirt portion the supporting walls being provided at the same position relative to the longitudinal axis of the skirt as the gudgeon pin bosses and having a height which is equal to 45 to 65% of the length of the skirt.

2. A light alloy piston as claimed in claim 1, wherein the height of each supportine wall is equal to 50 to 60% of the length of the skirt.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. to be "freely suspended" from the piston head and these ribs are, when the piston is in use, subjected only to stress or pressure and not bending. Preferably the supporting walls have a height which is equal to 50 to 60%, of the length of the skirt and extend at an angle of 30 to 45" to said axial plane of the gudgeon pin bosses. In the present piston, heat can be transferred from the head of the piston preferably along two paths. The main flow of heat proceeds through the gudgeon pin bosses, which are susupended from the inside of the piston head, and the supporting walls to the skirt. A small part of the heat is transferred by the ring-carrying portion to the skirt. Because the interior of the piston is defined by very large surfaces, heat is also transferred to lubricating oil which is splashed in the internal combustion engine. Because the piston head and the gudgeon pin bosses are at a high temperature relative to the solid skirt portion, a sufficiently large thermal expansion in the direction of the gudgeon pin axis is exhibited by the piston head and the gudgeon pin bosses and is transmitted by the supporting ribs to the skirt. As a result, the skirt is expanded in the direction of the axis of the gudgeon pins and, because the periphery of the skirt remains virtually unchanged, the diameter in the pressure and back-pressure directions is reduced. This expansion control results in a suitable ovality and convexity and in the fact that the deformation-resisting, cylindrical skirt exhibits approximately the same actual expansion as the cylinder under all operating conditions of the internal combustion engine, whether the cylinder consists of gray cast iron or aluminium. Owing to this matching, numerous desirable expansion control effects can be achieved so that the field of application of this unslotted expansion-controlled piston is not restricted, as with conventional expansioncontrolled pistons, by the dependence of the horsepower-displacement ratio on the piston diameter. With these measures, piston noise caused by changes of the contacting portions in the cylinder can be much better controlled because the control of the shaft movement is improved. This expansion-controlled piston of high stability may be designed with relatively small wall thickness so that much weight can be saved in comparison with known expansion-controlled pistons provided with steel inserts. In a preferred embodiment of the invention, the freely susnended gudgeon bosses are connected by the supporting ribs to an asymmetrically shaped head, which has portions extending on different level and defining a recess which forms part of the combustion chamber. 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 an embodiment thereof, and in which: Fig. 1 is a transverse sectional view of a piston, and Fig. 2 is a cross-section taken on the dotdash line in Fig. 1. Referring now to the drawings there is shown an unslotted controlled-expansion piston having a shaped head, Fig. 1 being a view taken on a vertical plane which contains the axes of the gudgeon pins. The piston has a relatively thick-walled top portion consisting of a shaped asymmetrical head 1 and a ring-carrying portion 2 connected to a thin-walled cylindrical skirt 3, which adjoins the ring-carrying portion 2. In the piston shown in the drawings, expansion control is ensured because the gudgeon pin bosses 5, which are freely suspended by means of supporting ribs 4 to an asymmetrical head 1, are connected to the cylindrical skirt 3 by supporting walls 6, which extends at an angle of 45" to the gudgeon pin axis 7 and have a height which amounts to 60% of the length of the skirt. The skirt 3 is formed with apertures 8, which are centred on the axis 7 and aligned with the gudgeon pin bosses 5. WHAT WE CLAIM IS:-

1. A cast light alloy controlled-expansion piston which is suitable for use in a piston machine, the piston being made from a single type of metallic material, wherein the piston comprises a full skirt and a piston head from which extends ribs terminating in gudgeon bosses, the ribs being attached to the underside of the piston head at regions which when the piston is in use constitute regions of zero movement and extending transversely to the gudgeon pin axis, each gudgeon pin boss being connected to the skirt by two supporting walls, extending from the periphery of the gudgeon pin boss at an angle of 0 to 45" to the axial plane of the gudgeon pin bosses and wherein a rigid top portion comprising the head and a ring-carrying portion is integrally connected throughout its periphery to a thinwalled cylindrical skirt portion the supporting walls being provided at the same position relative to the longitudinal axis of the skirt as the gudgeon pin bosses and having a height which is equal to 45 to 65% of the length of the skirt.

3. A light alloy piston as claimed in

claim 1 or 2, wherein the supporting walls extend at an angle of 30 to 45" to said axial plane.

4. A light alloy piston as claimed in any one of claims 1 to 3, wherein the gudgeon pin bosses are connected by the supporting ribs to an asymmetrically shaped piston head, which has portions extending on diffent levels and defining a recess which forms part of the combustion chamber.

5. A light alloy piston substantially as hereinbefore described with reference to the accompanying drawings.