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(11) | EP 0 385 390 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Lightened piston |
| (57) In a lightened piston for internal combustion engines, of the so-called "H" type,
namely of the type in which there are envisaged two opposed portions of skirt (3,
3′) having a limited circumferential extension, connected to a couple of reinforcing
ribs (4), said ribs have a curvilinear and/or broken configuration, at least two sections
of said ribs having portions forming an angle between each other. |
[0001] The present invention concerns a lightened piston for internal combustion reciprocating engines of the so-called "H" type, namely of the type in which a portion of the skirt is missing, and therefore only two opposed portions of skirt with limited circumferential extension are present.
[0002] Said two remaining portions of skirt act as guide shoe and, due to the fact that during engine operation they must mate the cylindrical shape of the barrel, in rest cold conditions their shape does not exactly match the cylindrical barrel, but they are shaped in a way as to allow some clearance with respect to said barrel. The value of said clearance is exactly calculated during designing, in order to take into consideration the thermal expansion of the material at the engine operative temperature and thus to obtain, at said temperature, perfect adhesion of said skirt portions to the cylinder walls.
[0003] "H"-shaped pistons are generally provided with reinforcing or stiffening members, or ribs, which join to each other the opposed portions of the skirt. Said ribs have a rectilinear configuration and extend between the corresponding ends of the opposed portions of skirt, being at the same time joined to piston the pin supports.
[0004] Obviously the ribs as well are subject to thermal expansion and the effects of said expansion have to be summed to those resulting from the thermal expansion of the skirt portions. In fact, because of their rectilinear structure, when they are submitted to thermal expansion, the ribs press on the skirt portions which are then pushed outwards to a much greater extent than it occurs for the thermal expansion of the skirt portions only.
[0005] Said considerable additional effects due to the presence of the ribs must be obviously taken into consideration during the designing stage, and all that results in the need of a more marked clearance between the skirt portions and the barrel and in an oval-shaped configuration of the skirt-ribs assembly.
[0006] The main disadvantage of such a kind of configuration lies in the fact that the skirt-ribs unit results to be very sensitive to temperature variations and assures a perfect coupling between piston and cylinder only within a limited range around the engine operative temperature, that is the temperature on which material expansions have been calculated.
[0007] For temperature values below said range, between barrel and piston there is excessive clearance which prevents good guide along the cylinder, while for higher values, deriving for example from thermal overload of the engine, the risk of seizure exists.
[0008] Another disadvantage is given by the fact that the structure thus obtained involves extreme stiffening of the piston, which, besides its reciprocating movement within the barrel, also presents an oscillatory movement about the piston pin axis. A high stiffness of the structure does not allow the piston to deaden its own "flappings" within the barrel, and reduces its adaptability to the cylinder.
[0009] Furthermore, in the embodiments according to the state of the art, the union point between ribs and pin supports, in the vicinity of the center line of the supports, results to be offset with respect to the actual point of highest stress experienced by the piston pin supports, which point results to be located in correspondence to the area near the internal face of each support.
[0010] There is therefore the need of an "H" piston presenting optimal adhesion of the skirt portions to the cylinder barrel within a wide range of temperatures around the design temperature, having a not extremely rigid structure and in which the union between ribs and piston pin supports is positioned in an optimal way.
[0011] An object of the present invention is therefore to solve the abovesaid problems providing an "H" piston which may assure optimal adhesion between skirt and cylinder walls in a wide range of temperatures around the operative temperature.
[0012] Another object of the present invention is to provide an "H" piston with ribs forming a not extremely rigid structure and joined to the piston pin supports in correspondence to the point of maximum stress.
[0013] To achieve said objects, the invention concerns a piston for internal combustion reciprocating engines, of the so-called "H" type, namely of the type in which two opposed portions of skirt are provided, with limited circumferential extension, connected by a couple of reinforcing members, or ribs, characterized in that said ribs have a curvilinear and/or broken configuration, at least two sections of which present extensions forming an angle between each other.
[0014] As better described later on, thanks to the particular configuration of said ribs, the present invention allows to provide a piston having lower sensitivity to temperature deviations from the design temperature, a less rigid structure and better absorption of the stresses localized on the piston pin supports. Moreover, in the case of two-stroke engines, when the piston according to the invention is in the vicinity of the Bottom Dead Center, it is possible to have a wider surface for the passage of the transfer flow between crankcase and combustion chamber, in that to the space for the main transfers provided in the cylinder, the space allowed by the particular configuration of the ribs is added. The present invention is further described with reference to the accompanying drawings given by way of illustrative and non limiting example, in which:
- figure 1 is a side view of a possible embodiment of the invention;
- figure 2 is a bottom view of the piston of figure 1;
- figures 3 to 6 are top views of possible alternative embodiments to those of figures 1 and 2; and
- figure 7 is a bottom view of a piston according to the invention for a two-stroke engine housed in the relevant cylinder.
[0015] With reference to figure 1, the piston 1, of the so-called "H" type, is provided without the skirt areas placed in correspondence to the pin supports 6, and only presents two opposed portions of skirt 3, 3′, joined by a couple of reinforcing members, or ribs, 4.
[0016] As better shown in figures 2, 3 and 4, the ribs 4 have a curvilinear and/or broken configuration, and at least two sections of said ribs have such directions as to form an angle between each other.
[0017] Preferably, each rib portion 4 comprised between each skirt portion 3, 3′ and the corresponding side of the piston pin support 6 presents a point of direction change of the rib itself. Namely, each portion 4 presents at least a point in which the rib changes direction and forms two sections positioned according two directions forming an angle between each other. As better described later on, said sections can be either rectilinear or curvilinear.
[0018] In this way the thermal expansion of the ribs generates a corresponding thrust, which is oriented according to a curvilinear or broken directrix, which allows part of said thrust to be discharged towards areas of the piston other than the portions 3, 3′. Similarly, also size variations due to a decrease in temperature versus the design temperature have a reduced effect on the desired coupling between the skirt portions 3,3′ and the cylinder walls. A piston is thus obtained which is much less sensitive than the traditional ones to temperature deviations from the design temperature and allows a steady and optimal coupling between skirt portions and cylinder walls within a much wider temperature range than it was possible in a piston of traditional type.
[0019] As already mentioned, it has been noticed that the stress transfer between piston and piston pin mostly takes place in the first millimetres of the support starting from its more internal face. Consequently, as shown in figure 2, in the preferred embodiments the union of the ribs 4 with the supports 6 is positioned in correspondence to said more internal sides of the supports.
[0020] In the embodiment shown in figure 2, the ribs 4 connect the ends of the two opposed portions of skirt with relevant supports specularly between each other and according to a "leaf spring" configuration. Namely, the ribs 4, starting from the ends 7 of a skirt portion 3, first of all bend towards the inside joining with the more internal side of the supports 6 and then go towards the other skirt portion 3′ in a way as to form a second curve, specular to the first one, extending as far as the ends 8 of the skirt portion 3′.
[0021] In this preferred embodiment four flex points are present, namely four points of change of inclination located between each support 6 and each skirt portion 3, 3′, but it is obvious that any curvilinear or broken configuration may be used which may allow to reduce the effects of the temperature deviation from the design temperture in the abovedescribed way and which at the same time reduces the stiffness of the skirt-ribs structure in comparison with the traditional "H" pistons.
[0022] Figures 3 to 6 show alternative embodiments of a piston according to the invention. In figure 3, the rib presents a configuration which follows a broken line formed by three rectilinear portions 9, 9′ and 10, one of which, namely portion 10, lies in correspondence to the pin supports 6 and is essentially perpendicular to their axis. Said portion 10 of the ribs 4 is also present in the embodiment of figure 4, where however the remaining sections of the rib, namely portions 11 and 12′ (corresponding to the portions 9 and 9′ of figure 3) have a curvilinear configuration so as to give the rib a mixed-line configuration.
[0023] Of course, though it is preferable to have the union of the ribs with the supports in the vicinity of the internal face of the supports themselves, the aforedescribed advantages can be achieved with otherwise located rib-support union points, as in the piston shown in figure 5. Furthermore, the ribs may be even not symmetrical (or specular) to the axis AA of the supports 6, as shown in figure 5. Similarly, as shown in figure 6, the rib sections 12 and 13 provided in one half of the piston must not necessarily be specular to similar sections 12′ and 13′ provided in the other half of the piston with respect to the section B-B.
[0024] Figure 7 shows a piston according to the invention, housed in a cylinder 14 for two-stroke engines. As it can be noticed, it allows to add to the space due to transfers 15 a further space which, in the vicinity of the Bottom Dead Center, assures a wider surface for the passage of the transfer flow between crankcase and combustion chamber.
1. A piston for internal combustion reciprocating engines, of the so-called "H" type,
namely of the type in which two opposed portions of skirt are provided, of limited
circumferential extension, connected by a couple of reinforcing members, or ribs,
characterized in that said ribs present a curvilinear and/or broken configuration,
at least two portions of each rib extending in a way as to form an angle between each
other.
2. A piston according to claim 1, characterized in that each portion of rib comprised
between skirt portion and piston pin support presents at least one point of change
of the direction of the rib itself.
3. A piston according to claim 1, characterized in that the union between ribs and
piston pin supports is located in correspondence to the most internal sides of said
supports.
4. A piston according to claim 3, characterized in that said ribs extend from the
ends of one of the two portions of skirt and, forming an inward curve, join with the
internal portion of the pin supports, then leave said internal portion forming a second
curve specular to said first curve, as far as the ends of the second one of said skirt
portions, according to a so-called "leaf-spring" configuration.
5. A piston according to any of the preceding claims, characterized in that, in correspondence
to the internal portions of the supports, said rib presents an essentially rectilinear
portion perpendicular to the axis of said pin supports.
6. A piston according to any one of the preceding claims, characterized in that it
is manufactured in light alloy.