FR2782748A1 - PISTON SEGMENT AND PISTON SEGMENT COMBINATION FOR DIRECT INJECTION DIESEL ENGINE - Google Patents

Abstract

The invention relates to a piston ring. It relates to a piston ring (7) composed of a steel containing 0.4 to 0.8% by weight of C and 6 to 11% by weight of Cr, and having a nitrided layer on at least one sliding surface of the piston ring. The nitrided layer has a hardness of between 800 and 1,100 Hv. A combination of segments comprises, in addition to the aforementioned segment used as the second segment, a first pressure segment (6) composed of martensitic stainless steel containing 0.4 to 1.2% by weight of C and 11 to 18% by weight of Cr and having a hardened surface layer having a nitrided layer on the sliding surface of the first pressure segment and an optional layer formed by PVD deposition treatment of the upper layer of the nitrided layer. Application to direct injection diesel engines.

Description

The present invention relates to a piston ring which is used in

  an internal combustion engine and, more specifically, a piston ring which can be used in

a diesel engine.

  In diesel engines, fuel is injected into compressed air at high pressure and temperature, so that the fuel is burned and creates energy. Diesel engines are classified into direct injection engines and auxiliary chamber engines, corresponding to the processes of mixing the fuel injected with air. It has been stated that the auxiliary chamber type diesel engine is a diesel engine having an auxiliary chamber in a different cylinder.

  than a main combustion chamber and injects the fuel

  rant in the auxiliary chamber, and that the excess air ratio can be set to a small value since the mixture of fuel with air is good. On the other hand, it was also considered that the diesel engine of the direct injection type

  is a high-pressure diesel fuel injection engine

  sion, in a combustion chamber formed by a cylinder head and the upper part of a piston and has a lower fuel consumption and a smaller amount of exhaust gas containing CO compared to the diesel engine with auxiliary chamber, and it allows exhaust gas regulation. In addition, it is stated that the creation of burnt products, giving carbon deposits and the like,

is reduced.

  A large number of chamber type diesel engines

  have been used because a large motor

  sion can easily give high power.

  In the auxiliary chamber type diesel engine, a large number of burnt products, such as carbon deposits and the like, are formed and carbon deposits and

  analogues adhere to the segment so that abrasion appears

  rait in the segment. In addition, the type diesel engine

  auxiliary chamber can be subjected to abrasion by corro-

  fuel. As a result, the segments used in an auxiliary chamber type diesel engine had to have excellent abrasion resistance, seizure resistance and resistance properties.

the heat.

  As a result of these considerations, a segment formed of martensitic stainless steel containing 11-18% Cr, which is relatively expensive, has been used as the first pressure segment in an auxiliary chamber diesel engine. In addition, a nitriding gas treatment is performed on the surface of the segment to increase the abrasion resistance. Furthermore, as the first segment of

  pressure, a rectangular segment, or a half segment

  keystone or full keystone is often

  used for scraping deposits created.

  In addition, a second pressure segment used in the auxiliary chamber diesel engine is not used in a harsher environment than that of the first pressure segment. Furthermore, since the second pressure segment has a complicated shape, for example of a frustoconical segment,

  of a frusto-conical segment or of a frusto-conical segment

  hooking nique, a cast iron segment is used for machinability reasons. In addition, for improved abrasion resistance, a chrome segment having a peripheral part as well as upper and

  lower surface is often used.

  However, when the second pressure segment is

  made with a thin shape so that the cost of consuma-

  tion of the fuel is reduced, the mechanical resistance of the cast iron segment is insufficient. For example, the examined Japanese patent applications Nos. Sho 58-25 863 and 61-54 103 form the second pressure segment of the same steel as the

first pressure segment.

  In addition, an integral segment is used, the surfaces of which are subjected to a chromium-plating or nitriding treatment with a gas as an oil scraper segment in the

  auxiliary chamber type diesel engine.

  Recently, however, for reasons of fuel cost reduction, exhaust control and endurance, a diesel engine of the direct injection type has been used for both large and small engines. instead of type diesel engine

with auxiliary chamber.

  When a pressure segment used in an auxiliary chamber type diesel engine is used as a pressure segment in a direct injection type diesel engine, the cost of the pressure segment becomes high given the severity of the medium. use so that

  problems of lack of profitability arise.

  The subject of the present invention is the advanced solution

  tagger of the aforementioned problems and therefore relates to a piston ring, in particular suitable as a second pressure segment, which is inexpensive and has an abrasion resistance which is suitable for a diesel engine of the direct injection type, for reasons of profitability and conditions of use of the piston ring in a diesel engine of the direct injection type, as well as a combination of piston rings suitable for an engine

  diesel of the direct injection type.

  To this end, the inventors have studied in depth the abrasion resistance of a piston ring

  used in a diesel engine of the direct injection type.

  Consequently, the inventors considered that resistance

  Sufficient abrasion resistance could be ensured even when a segment formed of low chromium steel was used as the second scraper segment to which a light thermal load is applied. In addition, the inventors have also found that the low chromium steel segment can be used sufficiently as an oil scraper segment. In addition, the inventors have found that a hardened layer harder than the chromium coating could be produced by applying a nitriding treatment.

  to steel containing chromium, and allowed an increase

  abrasion resistance.

  We first describe the results of experiments which

  been used as the basis of the present invention.

  The inventors have produced a second segment of press-

  oil scraper segment for direct injection type diesel engine and chamber type diesel engine

  auxiliary and compared the abrasion amplitudes of the diff

  different piston rings. The results are shown in Figure 2. In this case, a 70 µm nitriding layer was formed on the outer peripheral surface of each piston ring. Only 100 µm chrome coating

  thick was exposed to part of the peripheral surface

  external risk of the piston ring. A direct injection type diesel engine and an auxiliary chamber type diesel engine having four cylinders in line, a bore of 92 mm and a total displacement of the piston of 2000 cm3 were used respectively. The test conditions were a rotation speed of 4,000 rpm for all loads, for 300 h. As shown in FIG. 2, the amplitude of abrasion of a piston segment is large in the case of a second pressure segment coated only with chromium or steel with low chromium content (less or equal content). at 10%

  in Cr) of a diesel engine of the auxiliary chamber type.

  On the other hand, in the case of a second pressure segment

  steel with a high chromium content (13% or more), the ampli

  abrasion study of a piston ring is less than or equal to 7 pm, so that the abrasion resistance is satisfactory. On the other hand, the amplitude of abrasion of a piston ring is small even when a steel pressure segment containing 8% or 10% of Cr is used in a diesel engine of the direct injection type. It can thus be seen that the steel segment with 8% or 10% of Cr has the same sufficient abrasion resistance as the steel segment with high chromium content (13% or more) in an engine of the type with auxiliary room. It has also been found that, in an oil scraper segment of a diesel engine of the direct injection type, a steel segment containing 8 or

  % Cr could be used enough.

  The present invention has been carried out according to the

aforementioned knowledge.

  More specifically, the invention relates to a piston ring for a direct injection diesel engine in which the piston ring is composed of a steel containing 0.4 to 0.8% by weight of C and 6 to 11% by weight of Cr, and has a nitrided layer on the sliding surface of the segment

piston at least.

  According to the invention, the nitrided layer preferably has a hardness of between 800 and 1,100 Hv. Furthermore, the invention relates to a combination of piston rings intended for a diesel engine of the type with

  direct injection in which a first segment of press-

  sion is composed of martensitic stainless steel containing 0.4 to 1.2% by weight of C and 11 to 18% by weight of Cr, and has a surface hardened layer containing a nitrided layer on the sliding surface of the first segment pressure at least and a possible layer having undergone a physical vapor deposition (PVD) treatment in the upper layer of the nitrided layer, and in which a second pressure segment is composed of a steel which contains 0.4 a 0.8% by weight of C and 6 to 11% by weight of

  Cr and has a nitrided layer on the sliding surface

  at least the second pressure segment.

  According to the invention, the hardness of a surface hardened layer formed on the sliding surface of the first pressure segment is preferably greater than or equal to 900 Hv, and the hardness of the hardened layer formed on the sliding surface of the second pressure segment is

  preferably between 800 and 1,100 Hv.

  In addition, a combination of piston rings for a diesel engine of the direct injection type is such that a first pressure segment is composed of martensitic stainless steel containing 0.4 to 1.2% by weight of C and 11 to 18 % by weight of Cr and has a hardened surface layer containing a nitrided layer on the sliding surface of the

  at least first pressure segment and an eventual layer

  trowel treated by PVD deposition in the upper layer of the nitrided layer, and a second pressure segment is composed of a steel containing 0.4 to 0.8% by weight of C and 6 to 11% by weight of Cr and has a nitrided layer on the sliding surface of at least the second pressure segment, and an oil scraper segment is composed of steel containing 0.4 to 0.8% by weight of C and 6 to 11% by weight of

  Cr and has a nitrided layer on the sliding surface

the oil scraper segment.

  Other characteristics and advantages of the invention

  will be better understood on reading the description which will

  follow examples of embodiments, made with reference to the appended drawings in which: FIG. 1 is a longitudinal section of an example of combination of piston rings;

  Figure 2 (a) is a graph showing the influence

  the quantity of chromium in the steel on the abrasion amplitude of a piston ring used as the first pressure segment; and

  Figure 2 (b) is a graph showing the influence

  the amount of chromium in steel on the amplifier

  abrasion study in a piston ring used as

oil scraper segment.

  A piston ring according to the invention is of a type which has an advantageous abrasion resistance for a second pressure segment or an oil scraper segment or combined oil scraper segments comprising the three-piece type or the two-piece type of a direct injection diesel engine. The piston ring is preferably formed of a metal rod and a nitrided layer is

  formed at least on the sliding surface of the segment.

  A basic material of a piston ring intended for a second pressure segment or an oil scraper segment is a steel material containing 0.4 to 0.8% by weight of C and 6 to 11% by weight of Cr and preferably 0.1 to 0.4% by weight of Si and 0.2 to 0.4% by weight of Mn, the remainder being

formed of Fe and impurities.

  Carbon is an important element for increasing the hardness and mechanical strength of the material.

  base and it increases resistance to abrasion by combi-

  naison with chromium for the formation of hard and fine chromium carbide. Such carbon effects can be obtained when the amount of carbon contained is greater than or equal to 0.4% in the base material. On the other hand, if the

  amount of carbon exceeds 0.8%, a large primary carbide

  sier is likely to form, the segment is weakened and reduces the mechanical resistance, and it also has a greater aggressiveness with respect to the opposite surface, so that the abrasion of the internal circumferential surface of a cylinder is increased. Consequently, the quantity of carbon is limited to a value of between 0.4 and 0.8% and of

  preferably between 0.5 and 0.7% by weight.

  Chromium is an element which is soluble in the state

  solid in a matrix which increases the resistance to cha-

  their resistance to corrosion, and it combines with carbon to form a hard and fine carbide which increases seizure resistance, abrasion resistance and resistance to deposits. Such effects of chromium can be

  observed when the amount of chromium contained is greater than

  less than or equal to 0.6%. On the other hand, if the amount of chromium exceeds 11%, the abrasion resistance is further increased, but the amount of alloy added is not the most

  desirable from the point of view of the user environment

  sation in a diesel engine of the direct injection type, and excessive properties are obtained which cause a reduction in profitability and simultaneously a large number

  of chromium carbides precipitate and deteriorate the machinability

  lity. Thus, chromium is limited to the range of 6 to 11% by

weight.

  Silicon is an element which plays the role of a deoxidizer and which increases the resistance to deposition. Such effects of silicon can be observed when the amount of silicon is greater than or equal to 0.1%. On the other hand, if the quantity exceeds 0.4%, the machinability is reduced and the toughness is deteriorated. As a result, silicon is

  limited to the range of 0.1 to 0.4% by weight.

  Manganese is an element that plays the role of a

  deoxidizing and which increases mechanical resistance and wear

  hot stability. We can observe such effects of manga-

  born when manganese is present in an amount at least equal to 0.2%. On the other hand, if the amount of manganese exceeds 0.4%, the cold machinability during the formation of the segments is reduced. As a result, manganese is

  limited to the range of 0.2 to 0.4% by weight.

  The rest is further composed of iron and impurities.

  As impurities, phosphorus may be present in an amount less than or equal to 0.04% and sulfur in an amount

less than or equal to 0.03%.

  In a piston ring formed of a steel material having the above composition, a nitrided layer is formed on at least one periphery which constitutes a sliding surface. The process for forming the nitrided layer is not particularly limited, but the nitrided layer is preferably formed by nitriding treatments, for example in the gas phase, by ions, in a salt bath, with infiltration of sulfur or similar. The nitriding treatment is preferably carried out by nitriding in a gas in an ammonia atmosphere with a mixture of ammonia and nitrogen. In addition, it is obviously possible to form a nitrided layer on all other surfaces.

  than the sliding surface of a piston ring.

  A surface layer having an abrasion resistance greater than chrome plating is formed on a surface of a base material containing chromium by a nitriding treatment. It is desirable to form the layer of

  nitriding with a hardness between 800 and 1100 Hv.

  The hardness of the nitrided layer is modified by the chromium content of the base material. For example, when the chromium content of the base material is 6%, the hardness

  of the surface layer of a scraper segment becomes com-

  taken between 800 and 900 Hv after nitriding treatment with a gas. On the other hand, when the chromium content of the base material is equal to 11%, the hardness of the surface layer of a piston ring becomes between 1000 and 1100 Hv. On the contrary, if only chromium plating is carried out in place of nitriding in a gas, the hardness of the surface layer is at most equal to 820 Hv. In addition, in the case of nitriding in a gas, the hardness of the layer

  nitrided may be modified by nitrura-

  tion, for example the atmosphere ingredients, the harshness of the treatment, and the setting of the treatment time. If the hardness of a nitrided layer is less than 800 Hv, the amplitude of abrasion of the piston ring itself becomes large. On the other hand, if the hardness of a nitrided layer exceeds 1100 Hv, the aggressiveness with respect to the

  inner wall of a cylinder is excessive.

  Although a composite layer, called white chips, is formed on the surface of the nitrided layer, this layer

  composite is hard and has excellent abrasion resistance

  if we. Thus, in a piston segment used as a second pressure segment or an oil scraper segment in a direct injection diesel engine according to the invention, the

  nitrided layer can be used to increase the resistance

  abrasion resistance of the segment without removing the composite layer. Alternatively, after removal of the composite layer, the nitrided layer can also be used with

  exposure of a diffused layer to the surface.

  It is desirable to use the combination of a pre-

  mier piston ring and a second piston ring in a direct injection diesel engine. The first piston ring has the following composition and a hardened surface layer on the sliding layer at least, and it is used as the first pressure segment. The second piston ring has the above composition and a nitrided layer on the sliding layer at least, and it is used

  as second pressure segment or oil scraper segment.

  The combination of the piston rings for a diesel engine of the direct injection type, which is an example of the present invention, is shown in the

  Figure 1. Figure 1 is a longitudinal section shown

  both a state in which a piston is fixed in the bore of a cylinder. A piston 2 is introduced into the bore of the cylinder 1. A first pressure segment 6, a second pressure segment 7 and a combined oil scraper segment 8 are fixed respectively to the grooves 3, 4 and 5 of the housing.

  segments formed at the periphery of the piston 2.

  Although, in FIG. 1, the first pressure segment 6 is represented as a keystone segment on one side, it is not limited to a segment having such a configuration. It can for example be in the form of a

  keystone with two inclined sides or in rectangular form

  laire. A hardened surface layer 10 is formed on the sliding surface and the other external surfaces of the

  first pressure segment 6. The second segment 7 of pres-

  sion of Figure 2 is shown as a segment having a frustoconical face. However, since the pressure segment 7 must have hermetic properties and scraping of the lubricant, it can obviously have a frusto-conical shape, a frusto-conical shape with internal size or the like. A nitrided layer 9 is formed on the sliding surface and the other surfaces of the second pressure segment 7. In addition, the combined oil scraper segment 8 of Figure 1 is an oil scraper segment having a helical expander member. A nitrided layer 9 is formed at the periphery of an oil scraper segment 12 having an I-shape. The first pressure segment (upper segment) for

  diesel engine with direct injection is exposed to a temperature

  high rature as in a diesel engine of the auxiliary chamber type and must have good resistance to heat and seizure in addition to good resistance to abrasion. Consequently, advantage is used for the first pressure segment of a diesel engine of the type with

  direct injection, a martensi- stainless steel material

  tick such as SUS 410J1, SUS 440B or the like, containing 0.4 to 1.2% by weight of C and 11 to 18% by weight of Cr (high chromium content) and preferably containing 0.2 to 0.5 % by weight of Si and 0.2 to 0.5% by weight of Mn, the remainder being formed of Fe and impurities, as base material, as

  in the case of an auxiliary chamber type diesel engine.

  When the amount of carbon is less than 0.4%,

  the carbide content which increases the resistance to abrasion

  is insufficient and this abrasion resistance cannot be obtained. On the other hand, when the quantity of carbon exceeds 1.2%, the carbides become large and the

  mechanical resistance of the piston ring is reduced.

  Thus, the carbon is advantageously limited to the range of 0.4 to 1.2%. Chromium is soluble in the solid state and increases corrosion resistance and resistance to

  heat, and it forms carbides which increase the resistance

  abrasion resistance. When the amount of chromium is less than 11%, these effects are insufficient. On the other hand, if the amount of chromium exceeds 18%, the amount of soluble chromium in the solid state in the matrix increases and the abrasion resistance increases. However, resistance

  segment mechanics also increases and its toughness is weak

  improved. Consequently, the amount of chromium is advantageous-

  sely limited to the range of 11 to 18% by weight.

  Silicon is an element which plays the role of a deoxidizer and which increases the resistance to deposition. Such effects of silicon can be observed when the amount

  of silicon contained is greater than or equal to 0.2%.

  On the other hand, if the amount of silicon exceeds 0.5%, the machinability is reduced and the toughness is deteriorated. The quantity of silicon is therefore preferably limited to the

  range between 0.2 and 0.5% by weight.

  Manganese is an element which plays the role of a deoxidizer and which increases the mechanical resistance and hot machinability. These effects of manganese can be observed when the amount of manganese is higher or

  equal to 0.2% by weight. On the other hand, if the amount of man-

  ganèse exceeds 0.5% by weight, the cold machinability during the formation of the segments is reduced. Consequently, the amount of manganese is preferably limited to the range

from 0.2 to 0.5% by weight.

  In addition, in order to increase the abrasion resistance and the heat resistance, an amount of nickel less than or equal to 0.6% by weight, of

  molybdenum less than or equal to 1.25% by weight and vanana-

  dium in an amount less than or equal to 0.15% by weight.

  The rest is made up of iron and impurities. The impu-

  allowed deductions are phosphorus up to 0.04% and sulfur

  up to 0.09% by weight maximum.

  In a first pressure segment, a super layer

  hardened form is formed at least on the sliding surface

  segment. The hardened surface layer is preferably composed of a nitrided layer and of a possible layer having undergone a PVD deposition treatment formed as the upper layer of the nitrided layer. The process for forming the nitrided layer is not limited within the scope of the present invention. However, nitriding in a gas, ionic nitriding, nitriding in a salt bath or nitriding by sulfur infiltration or the like can advantageously be used as a process. In particular, nitriding in a gaseous atmosphere with a

  ammonia atmosphere or an atmosphere of a mixture of ammonia

  niac and nitrogen or the like is preferable. The nitrided layer can also be formed on all surfaces other than the sliding surface of the piston ring. The

  hardness of the hardened surface layer formed on the surface

  face of the piston ring is preferably between 900 and 1,400 Hv. Since white chips formed by the nitriding treatment on the surface layer are hard and brittle, they are preferably removed by machining, for example by polishing or the like. The thickness of the hardened surface layer is preferably in the form of a nitrided layer of 70 to 110 μm. When a layer is formed by a PVD deposition treatment, the thickness of the hardened surface layer preferably comprises a thickness of from 70 µm to a nitrided layer and from 5 to 100 µm of a

  layer treated with PVD deposition treatment. If the thick

  If a nitrided layer is less than 10 μm, the effects of the layer are insufficient. Alternatively, considering the abrasion of the piston ring, a layer

  nitrided with a thickness greater than 110 µm is not necessary

  to be. If the thickness of the layer formed by treatment of

  PVD deposit is less than 5 pmn, the effects are insufficient

  sants whereas if the thickness of the layer formed by PVD deposition treatment exceeds 100 μm, the layer risks

separate.

Example

  A first pressure segment 6, a second pressure segment 7 and an oil scraper segment 12 having a helical-shaped expander 11 were produced using the steel materials indicated in Table 1. These segments were fixed in a diesel engine of the direct injection type and a diesel engine of the auxiliary chamber type respectively, with the combinations indicated in Table 3, and tests were carried out on real machines. After the tests, the amplitudes of abrasion of the sliding surfaces of the piston rings were examined. The engines used for the tests were a direct injection type diesel engine and an auxiliary chamber diesel engine each having four cylinders

  placed in series with a 92 mm bore and a

  total of 2,000 cm3. The test conditions were a rotation speed of 4,000 rpm for the total load for 300 h, and a cooling water temperature of 95 C and an oil temperature of 130 C (engine oil

used: 10W-30). The dimensions of the piston rings, both for the diesel engine to

  direct injection and the auxiliary chamber diesel engine, were as follows: first pressure segment: width 2 mm, thickness 3.35 mm second pressure segment: width 1.5 mm, thickness 3.3 mmn oil scraper segment: width 3 mm, thickness 2, 35 mm (having a helical expander) The nitriding treatment conditions and the hardness of the peripheries of the respective piston rings

are shown in Table 2.

  The results of tests carried out with machines

  are shown in Table 3. The amplitude of abra-

  the piston ring has been represented by the amplitude

  thickness reduction at the sliding position.

Table 1

  Steel Chemical composition (% by weight) n C Si Mn PS Cr Fe A 0.86 0.34 0.31 0.020 0.006 18.0 rest B 0.58 0.22 0.33 0.018 0.004 7.0 rest C 0.50 0.18 0.33 0.018 0.004 10.4 rest D 0.55 0.23 0.32 0.018 0.004 5.0 rest E 0.71 0.28 0.30 0.016 0.003 8.0 rest Table 2 First segment Second segment Oil pressure scraper segment Treatment conditions- Nitriding by one Nitriding by Nitriding by mentdenitriding gas 550 Cx10h ungaz550 Cx5hungaz 550 Cx5h Layer Thickness 90 pm 65 pm 70 pm nitride Hardness 1 220 Hv 980 Hv 930 Hv PVD treatment yes no no

Table 3

  Combination of segments n Type of diesel engine

Remar-

  Amplitude Properties Evacue Steel n abra- tion of reinforcement sion, pm tability First segment A 6.2 of pressure Second segment Type to According to

  pressure B injection 3.6 O O in-

  direct vention Oil scraper segment 1 B 14, 1 First segment A 12.0 pressure Type to Example Second chamber segment

auxiliary pressure 10.8 campa-

  rative Scraper segment e 34.9 _ oil B 34.9 oil First pressure segment A 11.8 Type a Example

  Second segment C room 9.9 X X class-

9.9X X class-

  auxiliary pressure pressure siqye Oil scraper segment C 31, 0 2 First pressure segment A 5.8 Second segment Type to According to

  pressure C injection 2.8 O O in-

  direct ventio Oil scraper segment C 13.0 First segment A 6.8 pressure Type to Example Second segment TYeaefP

  Second injection segment 6.8 O X compa-

  3 direct pressure rative scraper segment D 20.1 oil First segment A 6, 8 pressure Type to According to Type e to According to Second segment Tp

  Second segentE injection 3.0 O O in-

  4 direct pressure ventio Oil scraper segment E With the combination of the piston rings of the example of the invention, it is possible to obtain a piston ring for

  direct injection diesel engine with sufficient resistance

  resistant to abrasion. The piston rings according to the invention

  tion have sufficient effect when used as second pressure segments or oil scraper segments in a direct injection diesel engine. In the comparative examples in which the piston rings according to the invention are used as second pressure segments or oil scraper segments in a diesel chamber engine

  auxiliary, the abrasion resistance is insufficient.

  According to the invention, a piston ring can be

  made with the necessary resistance to abrasion, using

  A small amount of high chromium stainless steel containing a large amount of chromium is inexpensive. Consequently, the invention has considerable advantages for the industry. In addition, since chromium plating is not necessary, precautions against

  environmental pollution is not necessary.

  Of course, various modifications can be made by those skilled in the art to the segments and combinations which have just been described solely by way of non-illustrative example.

  limiting without departing from the scope of the invention.

Claims (4)

  1. Piston ring for direct injection diesel engine, characterized in that the piston ring (7) is composed of a steel containing 0.4 to 0.8% by weight of C and 6 to 11% by weight of Cr, and has a nitrided layer on a   sliding surface of the piston ring at least.   2. Segment according to claim 1, characterized in that the nitrided layer has a hardness of between 800 and 1,100 Hv.   3. Combination of piston rings for a diesel engine of the direct injection type, characterized in that a first   pressure segment (6) is made of stainless steel   tensitic containing 0.4 to 1.2% by weight of C and 11 to 18% by weight of Cr and has a hardened surface layer comprising a nitrided layer on the sliding surface   at least the first pressure segment and an eventual layer   trowel formed by PVD deposition treatment of the upper layer of the nitrided layer, and a second pressure segment (7) is composed of a steel containing 0.4 to 0.8% by weight of C and 6 to 11% by weight of Cr and has a nitrided layer on the sliding surface of the second segment of pressure at least.   4. Combination according to claim 3, characterized in that the hardness of the hardened surface layer formed on the sliding surface of the first pressure segment is greater than or equal to 900 Hv, and the hardness of the hardened layer formed on a surface of sliding of the second   pressure segment is between 800 and 1,100 Hv.