Piston for a heat engine, heat engine with such a piston and method

Description

[0001] The present invention relates to a piston for a heat engine, particularly for a reciprocating internal combustion engine. The invention also relates to a heat engine comprising such a piston. The invention further relates to a method for coating such a piston. Finally, the invention relates to a method for implementing such a heat engine.

[0002] The field of the invention is that of pistons of thermal machines, in particular of reciprocating and internal combustion thermal engines.

[0003] As is known, such a piston comprises a skirt, a crown, and a ring carrier. Within the engine, the piston moves in a reciprocating motion within the cylinder liner of the cylinder block. The skirt guides the piston within the liner. The crown is positioned to be in contact with the combustion gases and to withstand the forces resulting from combustion. The ring carrier is located between the skirt and the crown. The ring carrier consists of alternating grooves and ridges, which are designed to receive the piston rings.

[0004] Current vehicle pollution / fuel consumption measurement cycles, such as the NEDC (New European Driving Cycle), have led to a trend towards engine downsizing. New engines have a smaller displacement while developing the same power output.

[0005] By 2017, the new WLTP ("Worldwide Harmonized Light Vehicles Test Procedure") cycle will favor high-power engines.

[0006] This results in increased power output for the same engine displacement, and therefore increased thermal and mechanical stresses within the combustion chambers. Consequently, pistons, which are currently mostly made of aluminum alloy for light vehicles, are being progressively replaced by steel pistons.

[0007] To minimize the impact of this change on piston weight, their geometry is modified, resulting in shorter pistons. This geometric modification alters the piston / cylinder liner contact, and therefore the bearing surfaces (contact and friction).

[0008] For vehicles equipped with aluminum pistons, the piston / cylinder liner contact is optimized by depositing a graphite coating on the skirt.

[0009] For mass-produced light vehicles equipped with steel pistons, the coated area is still preferentially the skirt. However, it appears increasingly important to reconsider the piston's bearing surfaces on the cylinder liner during its reciprocating motion, which tends to reduce clearance at the piston's bearing surfaces.

[0010] Furthermore, steel pistons are predominantly used in industrial vehicle engines due to combustion pressures that can reach 250 bar. In recent years, there have been no major changes to the piston / cylinder liner contact in these engines.

[0011] DE 41 13 773 and DE 43 10 491 disclose pistons illustrating the state of the art, provided with contact cords whose diameter is equal to the diameter of the skirt.

[0012] In DE 41 13 773, the contact cords have a coating of variable thickness around the central axis.

[0013] In DE 43 10 491, the contact cords have a thick layer of synthetic resin. For example, this synthetic resin is polyamide, incorporating graphite and metallic particles. The resin layer has a thickness of between 15 and 25 µm.

[0014] These pistons are not entirely satisfactory, in terms of reducing wear and the risk of seizing.

[0015] The aim of the present invention is to propose an improved piston, taking into account the above context.

[0016] To this end, the invention relates to a piston for a heat engine, comprising: a skirt which is designed to guide the piston in translation along a central axis in a counter-piece and constitutes a first bearing area for the piston in the counter-piece, a head which extends transversely to the central axis and is designed to be in contact with the combustion gases, and a ring carrier which includes at least two rings and at least two grooves designed to receive rings, including a first ring adjacent to the head and a second ring located between the first ring and the skirt, characterized in that the cords include at least one contact cord having a diameter greater than a minimum diameter of the skirt in order to constitute a second bearing area of ​​the piston in the counter-piece, and in that at least one contact cord has a friction-reducing surface coating, formed at least on a radial sector covering an angle of at least 30 degrees, and up to a single sector covering an angle of 360 degrees.

[0017] The applicant observed that in the new movable coupling designs, at least one of the ring carrier cords is likely to become an area of ​​contact and friction with the cylinder liner. Such friction leads to increased fuel consumption and therefore higher CO2 emissions.

[0018] In this context, the invention optimizes piston / cylinder liner contact within the engine. By providing a larger diameter contact strip, the invention defines a preferred bearing area on the piston ring carrier, complementing the bearing area defined on the skirt. By applying a surface coating to at least this contact strip, the invention reduces the coefficient of friction between the two parts in contact and moving relative to each other. Furthermore, the invention significantly reduces wear and / or the risk of seizing.

[0019] Within the framework of the invention, the diameter of the contact cords is considered without the coating, while the diameter of the skirt is considered without the bores and recesses.

[0020] According to other advantageous features of the invention, taken individually or in combination: At least one contact ring has a diameter greater than the average diameter of the skirt. The piston is a short piston, with a height less than its diameter. The base material of the piston is steel. That is, the skirt, head, and ring carrier of the piston are made of steel. Preferably, this steel is forged steel. The friction-reducing surface coating is amorphous carbon of the DLC (diamond-like carbon) type aC:H or ta-C. At least one contact ring has at least one sublayer formed beneath the friction-reducing surface coating. At least one contact ring lacks a sublayer formed beneath the friction-reducing surface coating. The second contact ring has a diameter greater than the first. Of the ring carrier's contact rings, only the contact ring(s) have a friction-reducing surface coating. The ring carrier comprises two contact rings.The two contact cords are the second cord and a third cord. The two contact cords are the first cord and the second cord. The friction-reducing surface coating is formed on a single radial sector covering an angle of at least 30 degrees. The friction-reducing surface coating is formed on two radial sectors diametrically opposed with respect to the central axis, each covering an angle of at least 30 degrees. The radial sector(s) cover an angle limited to 30 degrees. The radial sector(s) cover an angle limited to 45 degrees. The radial sector(s) cover an angle limited to 60 degrees. The radial sector(s) cover an angle limited to 90 degrees. The radial sector(s) cover an angle limited to 120 degrees. The friction-reducing surface coating is formed over 360 degrees around the central axis.The friction-reducing surface coating has a maximum roughness Rz of 2 µm or less, preferably 0.5 µm or less, obtained, for example, by polishing or sanding. The friction-reducing surface coating has a thickness, defined radially to the central axis, of between 1 and 5 µm, preferably between 2 and 3 µm. The friction-reducing surface coating has a thickness of 10 µm or less. The first bead is uncoated. The grooves are uncoated.

[0021] The invention also relates to a heat engine, comprising a piston as mentioned above, and a counter-piece receiving the piston.

[0022] For example, the counter piece can be made of steel, stainless steel, cast iron, aluminum alloy, etc.

[0023] Preferably, the counter-piece has a friction-reducing surface coating of DLC-type amorphous carbon.

[0024] When the heat engine is a motor, the counter-part is a sleeve.

[0025] The cylinder liner can be a dry liner, attached to a cylinder block casing.

[0026] Alternatively, the liner can be a wet liner, removable from the cylinder block casing, with a coolant interposed between the liner and the casing.

[0027] Preferably, the liner has a friction-reducing surface coating to minimize wear and / or the risk of seizing. Even more preferably, this coating is made of DLC-type amorphous carbon. As a non-limiting example, the liner and its coating may conform to the teachings of document WO2013 / 164690.

[0028] The invention also relates to a method for coating a piston as mentioned above. The method is characterized in that it comprises the following steps: a step of positioning a mask on the segment holder, and a step of localized deposition of a friction-reducing surface coating through the mask, at least on at least one contact bead.

[0029] The invention also relates to a method of implementing a thermal machine as mentioned above, characterized in that the skirt and at least one contact cord comprising the friction-reducing surface coating constitute the bearing areas of the piston in the counter-piece.

[0030] The invention will be better understood upon reading the following description, given solely by way of non-limiting example and made with reference to the accompanying drawings in which: there figure 1 is a side view, with partial section, of a piston according to the invention, of the short piston type, where the second bead constitutes a contact bead comprising a surface coating; the figure 2is a perspective view of the piston of the figure 1 ; there figure 3 is a larger-scale partial cross-section of the segment holder, showing the second cord having a diameter greater than the skirt, as well as the first and third cords; the figure 4 is a larger-scale partial cross-section of the second bead, showing a variant in which the second bead has an underlayer formed beneath the surface coating; the figure 5 is a perspective view showing a mask intended to cover the piston during the deposition of the surface coating; the figure 6 is a cross-section of the mask in the transverse plane defined by the line VI-VI at the figure 5 ; THE figures 7, 8 and 9 are sections similar to the figure 6 showing variations of the mask; the Figure 10 is a view analogous to the figure 5 showing another variant of the mask; the figures 11 to 14 are partial sections analogous to the figure 3showing different piston variants; the figure 15 is a perspective view analogous to the figure 2 showing a variant in which the segment holder has only two cords; and the figure 16 is a view analogous to the figure 2 showing a variant in which the piston is of the long piston type.

[0031] On the figures 1 to 3 A piston 1 conforming to the invention is shown.

[0032] Piston 1 is designed to be used in a heat engine, more specifically a reciprocating internal combustion engine. Piston 1 is housed in a cylinder liner C of a cylinder block B, partially shown only in the diagram. figure 1 for the sake of simplification. Preferably, the liner C has a friction-reducing surface coating R made of amorphous carbon of the DLC type.

[0033] Piston 1 is made of metallic material, preferably steel, aluminum alloy, or formed by assembling parts of different metallic materials.

[0034] Piston 1 has a longitudinal central axis X1, a height H1 defined parallel to the axis X1, and a diameter D1 defined radially to the axis X1. Piston 1 is a short piston, having a height H1 less than its diameter D1.

[0035] Piston 1 includes a skirt 2, a head 3 and a ring carrier 4. Piston 1 is designed to receive rings, which are not shown for the sake of simplicity.

[0036] Within the engine, piston 1 is driven by an alternating translational movement along the axis X1 in the counter-piece formed by the sleeve C.

[0037] More specifically, the piston 1 moves in the sleeve C according to a main translational movement along the axis X1, and secondary movements which can be summarized as a lateral movement perpendicular to the axis X1 and a rotational movement around an axis perpendicular to the axis X1 (tilting movement).

[0038] This results in the piston 1 being guided in the liner C by means of a contact between its skirt 2 and the liner C, a contact which may be more marked between the liner C and the lower end of the skirt 2. The tilting movement and the deformations of the skirt 3 also cause a contact between the liner C and the head 3 and / or the ring carrier 4 of the piston 1 on the tilting side.

[0039] The skirt 2 consists of a generally tubular wall centered on the axis X1, having an external diameter D2. The skirt 2 is designed to guide the piston 1 in the liner C, and constitutes a first support zone Z2 of the piston 1 in the liner C. The skirt 2 preferably has a friction-reducing surface coating.

[0040] Preferably, the coating of skirt 2 is made of DLC type amorphous carbon.

[0041] Alternatively, the skirt 2 coating can be made of graphite or any other material suitable for the intended application.

[0042] Head 3 consists of a wall extending transversely to axis X1. Head 3 is designed to be in contact with the combustion gases and to receive the forces due to said combustion.

[0043] In the example shown in the figures, head 3 is formed by a flat wall. Alternatively, head 3 can be formed by a hollow wall, having a blind cavity open on the outside.

[0044] The segment holder 4 is located between the skirt 2 and the head 3. The segment holder 4 comprises three cords 11, 12, and 13, as well as three grooves 14, 15, and 16 designed to receive segments. Each cord 11, 12, and 13 consists of a cylindrical surface. Each groove 14, 15, and 16 consists of a cylindrical surface recessed relative to the adjacent cords, as well as two annular flat surfaces connected to the adjacent cords.

[0045] The first cord 11 is located in the immediate vicinity of the head 3, in contact with the gas and the fire.

[0046] The second cord 12 is located in the intermediate part of the segment holder 4 between the skirt 2 and the head 3, being slightly closer to the head 3.

[0047] The third cord 13 is located near skirt 2.

[0048] The groove 14 is located between the cords 11 and 12. The groove 14 is designed to receive the fire segment, in contact with the gases and the fire.

[0049] The groove 15 is located between the cords 12 and 13. The groove 15 is designed to receive the sealing segment, ensuring total gas sealing by blocking those that would have passed through the fire segment.

[0050] The groove 16 is located between the cord 13 and the skirt 2. The groove 16 is designed to receive the scraper segment, ensuring the scraping of lubricant on the surface of the liner C.

[0051] In the new concepts of mobile coupling, the ring carrier 4 is likely to become a second support zone Z4 of the piston 1 in the liner C, in other words a second contact and friction zone of the piston 1 with the liner C.

[0052] In the embodiment of the invention illustrated in figures 1 to 3 The bead 12 has a diameter D12 greater than the minimum diameter D2 of the skirt 2. Thus, the bead 12 is a contact bead constituting the second bearing zone Z4 of the piston 1 in the cylinder liner C, in addition to the first bearing zone Z2 formed by the skirt 2. Preferably, the diameter D12 is greater than the average diameter D2 of the skirt 2, which is likely to deform during operation. For example, the diameter D12 can be 10 to 50 µm greater than the diameter D2.

[0053] Furthermore, the cord 12 has a friction-reducing surface coating 20, formed over a sector covering a 360-degree angle around the axis X1. In other words, the coating 20 is deposited over the entire circumference of the cord 12. The coating 20 reduces the coefficient of friction between the cord 12 and the sleeve C, which are in contact and moving relative to each other. Moreover, the coating 20 significantly reduces wear and / or the risk of seizing.

[0054] As shown in the figure 3 , the diameter D12 of the cord 12 is considered without the coating 20. For its part, the diameter D2 of the skirt 2 is considered without the bores and recesses.

[0055] More generally, within the scope of the invention, the piston 1 comprises at least one contact bead having a diameter greater than the minimum external diameter of the skirt and having a friction-reducing coating formed over at least one radial sector covering an angle of at least 30 degrees, and up to a single sector covering an angle of 360 degrees. In other words, the coating extends around the axis X1 over at least one radial sector of 30 degrees, at most over one sector of 360 degrees, and can extend over several radial sectors, each covering an angle of at least 30 degrees.

[0056] In the method of implementation of figures 1 to 3, the coating 20 is formed only on the surface of the cord 12, without spilling into the neighboring grooves 14 and 15. Indeed, for certain applications and / or for certain materials, the coating 20 present in the grooves could be subject to a tearing phenomenon, generating contamination at the contact between the segment holder 4 and the liner C.

[0057] Alternatively, the coating 20 can be formed on the surface of the cord 12 and in the adjacent grooves 14 and 15. In this case, the deposition of the coating 20 is simplified.

[0058] In addition, the coating 20 is formed over the entire height of the cord 12, defined parallel to the axis X1.

[0059] Alternatively, the coating 20 can be formed only over part of the height of the bead 12, specifically in its center. This prevents the coating 20 from overflowing into the grooves 14 and 15 when it is deposited on the bead 12.

[0060] Coating 20 has a maximum roughness Rz less than or equal to 2 µm, preferably less than or equal to 0.5 µm, obtained for example by polishing or sanding.

[0061] The coating 20 has a thickness, defined radially to the X1 axis, preferably equal to 2 µm. Alternatively, this thickness may be between 1 and 5 µm, and preferably between 2 and 3 µm. The thickness of the coating 20 may vary depending on the diameter D1. For example, in the case where the piston 1 is fitted to a heavy-duty vehicle engine, it is conceivable that this thickness could be up to 10 µm. Preferably, the coating 20 has a constant thickness, defined radially to the X1 axis.

[0062] In practice, in the support zone Z4, the bead 12 can rub against the jacket C on only one radial sector, and not around its entire circumference. Thus, it is sufficient to deposit the coating 20 on a single radial sector, covering an angle of at least 30 degrees.

[0063] Alternatively, in the support zone Z4, the bead 12 may rub against the liner C in two diametrically opposed portions. In this case, it is sufficient to deposit the coating 20 on two diametrically opposed radial sectors of the bead 12, each covering an angle of at least 30 degrees.

[0064] According to other alternatives, the coating 20 can be deposited on one or two radial sectors of the cord 12, each covering an angle limited to 45, 60, 90 or 120 degrees, around the axis X1.

[0065] The coating 20 has a lower coefficient of friction than the material constituting the segment holder 4. Thus, the friction between the jacket C and the cord 12 provided with the coating 20 is reduced, compared with a cord 12 without coating 20.

[0066] Preferably, coating 20 is made of amorphous DLC-type carbon. In other words, coating 20 is a layer of carbon hybridized in sp2 or sp3, with or without hydrogen. For example, coating 20 can be composed of ta-C, aC:H, or ta-C:H. Again, preferably, coating 20 is aC:H.

[0067] Alternatively, coating 20 can be made of graphite or any other material suitable for the intended application. Preferably, coating 20 is homogeneous.

[0068] On the figure 4 , the second cord 12 has a sub-layer 22 formed under the surface coating 20. For example, this sub-layer 22 has a Cr and / or W and / or Ni base.

[0069] On the figures 5 and 6 is illustrated an example of a method for coating a piston 1 according to the invention, aimed at depositing the coating 20 on the bead 12.

[0070] The process includes at least one step of positioning a mask 40 on the piston 1, then a step of depositing the coating 20 on the bead 12 through the mask 40. The process may include other steps without departing from the scope of the invention.

[0071] The mask 40 is positioned on the piston 1 according to a translational movement T40 directed along the axis X1, as illustrated by an arrow at the figure 5 . The mask 40 then covers at least part of the piston 1, during the deposition of the coating 20.

[0072] The mask 40 comprises a tubular part 42 and a flat part 43. When the mask 40 is positioned on the piston 1, the part 42 covers the ring holder 4, while the part 43 covers the head 3.

[0073] In the flat section 42, an intermediate portion 44 includes two diametrically opposed slots 45 and 46, as well as two diametrically opposed joining zones 47 and 48. The slots 45 and 46, formed through the mask 40, allow the coating 20 to be deposited onto the bead 12. The zones 47 and 48 allow the sections 42 and 43 to be joined.

[0074] As shown in the figure 6 , when the mask 40 is positioned on the piston 1, the slot 45 defines an angle sector α1 corresponding to the radial sector 21 of the coating 20, the slot 46 defines an angle sector α2 corresponding to the radial sector 22 of the coating 20, the zone 47 defines an angle sector β1 and the zone 48 defines an angle sector β2, around the axis X1.

[0075] The geometry of portion 44 of mask 40 determines the geometry of coating 20. In particular, the geometry of slots 45 and 46 determines the extent of sectors 21 and 22 of coating 20 formed on bead 12.

[0076] For example, the figure 6 Angles α1 and α2 each measure 156 degrees. Alternatively, angles α1 and α2 can have different values, depending on the desired extent for sectors 21 and 22.

[0077] Continuing with the example of the figure 6 , angles α1 and α2 are identical, likewise angles β1 and β2 are identical, nevertheless it is conceivable to provide different angles to define different sectors 21 and 22.

[0078] Thus, the mask 40 makes it possible to isolate the parts of the piston 1 which are not intended to be coated, and to limit the coating 20 deposition to the desired portions of the bead 12 and the skirt 3.

[0079] The material of the mask 40 can be chosen according to the coating deposition technique 20. For example, the mask 40 can be made of steel or aluminum.

[0080] As a non-limiting example, the coating deposition step 20 can be carried out according to the teachings of document WO2012 / 156647.

[0081] Depending on the shape of piston 1, the nature of the materials of piston 1 and liner C, and the nature of the contact, piston 1 / liner C friction represents 20 to 30% of engine friction losses.

[0082] Thus, the application of a coating to skirt 2 and bead 12 can be valued in terms of reducing friction, and therefore fuel consumption and CO2 emissions.

[0083] As a first approximation, the new geometry of the pistons should lead to attributing between 5 and 10% of the engine friction losses to the contact between the bead 12 and the liner C. Under these conditions, the coating 20 applied to the bead 12 should make it possible to reduce the engine friction losses by the order of 2 to 5%.

[0084] Advantageously, the piston coating process 1 can include the following steps, taken individually or in combination.

[0085] Before the mask positioning step 40, the process may include a surface preparation step for the second bead 12, for example by machining or polishing.

[0086] Before the coating 20 deposition step, generally before the mask 40 positioning step, the process may include a piston 1 washing step, and in particular washing of the bead 12 intended to receive the coating 20.

[0087] After the coating 20 deposition step, the process may include a finishing step of the external surface of the coating 20, for example by polishing.

[0088] Variants of masks 40 for the manufacture of a piston 1 according to the invention are shown in figures 7 to 10For the sake of simplicity, the constituent elements of these masks are comparable to mask 40 of the figures 5 and 6 they have the same numerical references, and only the differences are detailed below.

[0089] On the figure 7 The junction zones 47 and 48 are radially offset relative to the rest of the mask 40. Thus, the mask 40 has a single slot 45 allowing the coating 20 to be formed over a 360-degree sector around the axis X1, as with the piston 1 of the figures 1 to 3 .

[0090] On the figure 8 The mask 40 has a single slot 45 with an angle α1 of 30 degrees. Thus, the mask 40 allows the coating to be deposited on a single radial sector 21 limited to 30 degrees.

[0091] On the figure 9 Angle α1 measures 90 degrees, while angle α2 measures 110 degrees. Slots 45 and 46 have different geometries.

[0092] On the Figure 10, the tubular part 42 is more elongated so as to cover the skirt 2. Thus, the skirt 2 can receive a different coating than the cord 12, or be uncoated.

[0093] Alternatively, the tubular part 42 can be more or less elongated, and therefore cover a greater or lesser height of the skirt 2.

[0094] Other embodiments of a piston 1 according to the invention are shown in the figures 11 to 16 For the sake of simplicity, the constituent elements of piston 1 comparable to those of the first embodiment described above bear the same numerical references, and only the differences are detailed below.

[0095] On the figure 11 , the first cord 11 is the contact cord having a diameter D11 greater than the minimum diameter D2 of the skirt 2 and provided with a friction-reducing coating 10.

[0096] On the figure 12, the third cord 13 is the contact cord having a diameter D13 greater than the minimum diameter D2 of the skirt 2 and provided with a friction-reducing coating 30.

[0097] On the figure 13 , the first cord 11 and the second cord 12 are the contact cords having diameters D11 and D12 greater than the minimum diameter D2 of the skirt 2 and provided with friction-reducing coatings 10 and 20.

[0098] In this case, the mask 40 used when depositing the coatings 10 and 20 can comprise two superimposed portions 44, provided with slots 45 and 46. Alternatively, the coatings 10 and 20 can be deposited successively using two different masks 40.

[0099] Preferably, coating 10 is made of the same material as coating 20.

[0100] Optionally, the cord 11 may include at least one underlayer formed under the coating 10, as with the coating 20.

[0101] On the figure 14 , the second cord 12 and the third cord 13 are the contact cords having diameters D12 and D13 greater than the minimum diameter D2 of the skirt 2 and provided with friction-reducing coatings 20 and 30.

[0102] On the figure 15 The ring holder 4 has only two cords, 11 and 12, and no cord 13. Only the second cord, 12, has a friction-reducing coating 20. The ring holder 4 receives a fire and sealing ring in the groove 14 and a scraper ring in the groove 15.

[0103] On the figure 16 , piston 1 is a long piston, with a height H1 greater than its diameter D1.

[0104] Alternatively, the piston 1 can have a height H1 equal to the diameter D1 without going out of the scope of the invention.

[0105] Furthermore, piston 1 and mask 40 can be shaped differently from figures 1 to 16without departing from the scope of the invention. Furthermore, the technical characteristics of the various embodiments and variants mentioned above may be combined, in whole or in part. Thus, piston 1 can be adapted in terms of cost, functionality, and performance.

Claims

1. Combustion engine piston (1), comprising: - a skirt (2) for guiding the piston (1) in translation along a central axis (X1) in a counter-part (C) and consisting of a first contact area (Z2) of the piston (1) in the counter-part (C), - a head (3) which extends transverse to the central axis (X1) and is intended to be arranged in contact with the combustion gases, and - a ring carrier (4) which comprises at least two lands (11, 12, 13; 11, 12) and at least two grooves (14, 15, 16; 14, 15) intended for receiving the rings, including a first land (11) adjoining the head (3) and a second land (12) situated between the first land (11) and the skirt (2), characterized in that the lands (11, 12, 13; 11, 12) include at least one contact land (12; 11; 13; 11, 12; 12, 13) having a diameter (D12; D11; D13; D11, D12; D12, D13) greater than a minimum diameter (D2) of the skirt (2) in order to form a second contact area (Z4) of the piston (1) in the counter-part (C), and in that the at least one contact land (12; 11; 13; 11, 12; 12, 13) comprises a friction-reducing surface coating (20; 10; 30; 10, 20; 20, 30), formed at least on a radial sector (21; 22) covering an angle (α1; α2) of at least 30 degrees, and up to on a single sector covering an angle of 360 degrees, the diameter of the said one contact land being considered without the coating, and the diameter of the skirt being considered without any bore and without any reinforcements.

2. The piston (1) according to claim 1, characterized in that at least one contact land (12; 11; 13; 11, 12; 12, 13) has a diameter (D12; D11; D13; D11, D12; D12, D13) greater than an average diameter (D2) of the skirt (2).

3. The piston (1) according to one of claims 1 or 2, the piston (1) is a short piston, having a height (H1) smaller than its diameter (D1).

4. The piston (1) according to one of claims 1 to 3, characterized in that the base material of the piston (1) is steel.

5. The piston (1) according to one of claims 1 to 4, characterized in that the friction-reducing surface coating (20; 10; 30; 10, 20; 20, 30) is made from DLC-type amorphous carbon a-C:H.

6. The piston (1) according to one of claims 1 to 4, characterized in that the friction-reducing surface coating (20; 10; 30; 10, 20; 20, 30) is made from DLC-type amorphous carbon ta-C.

7. The piston (1) according to one of claims 1 to 6, characterized in that the at least one contact land (12; 11; 13; 11, 12; 12, 13) includes at least one sublayer (22) formed below the friction-reducing surface coating (20; 10; 30; 10, 20; 20, 30).

8. The piston (1) according to one of claims 1 to 7, characterized in that the second land (12) has a diameter larger than the first land (11).

9. The piston (1) according to one of claims 1 to 8, characterized in that among the lands (11, 12, 13; 11, 12) of the ring carrier (4), only the contact land(s) (12; 11; 13; 11, 12; 12, 13) include a friction-reducing surface coating (20; 10; 30; 10, 20; 20, 30).

10. The piston (1) according to one of claims 1 to 9, characterized in that the ring carrier (4) comprises a single contact land (12; 11; 13).

11. The piston (1) according to claim 10, characterized in that the single contact land is the second land (12).

12. The piston (1) according to one of claims 1 to 9, characterized in that the ring carrier (4) comprises two contact lands (11, 12; 12, 13).

13. The piston (1) according to claim 12, characterized in that the two contact lands are the second land (12) and a third land (13).

14. The piston (1) according to claim 12, characterized in that the two contact lands are the first land (11) and the second land (12).

15. A heat engine, comprising: - a piston (1) according to one of claims 1 to 14; and - a counter-part (C) receiving the piston (1).

16. The heat engine according to claim 15, characterized in that the counter-part (C) includes a friction-reducing surface coating (R) made from amorphous carbon of the DLC type.

17. A process for coating a piston (1) according to one of claims 1 to 14, characterized in that the process comprises the following steps: - a step for positioning a mask (40) on the piston (1), and - a step for localized deposition of a friction-reducing surface coating (20; 10; 30; 10, 20; 20, 30) through the mask (40), at least on the at least one contact land (12; 11; 13; 11, 12; 12, 13).

18. A process for implementing a heat engine according to one of claims 15 or 16, characterized in that the skirt (2) and the at least one contact land (12; 11; 13; 11, 12; 12, 13) including the friction-reducing surface coating (10; 30; 10, 20; 20, 30) make up the contact areas (Z2; Z4) of the piston (1) in the counter-part (C).