JPH0330710B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to internal combustion engines, and more particularly to pistons thereof. A piston of an internal combustion engine has a piston body that cooperates with a cylinder and a cylinder head to define a combustion chamber, and a plurality of piston rings fitted into piston ring grooves of the piston body. The main body is generally made of aluminum alloy due to the need for weight reduction, and piston rings are required to have excellent wear resistance, so cast iron such as special cast iron or spheroidal graphite cast iron is generally used for piston rings. It is composed of With the recent increase in the output of internal combustion engines, pistons are being exposed to increasingly harsh conditions.Therefore, in order to improve the heat resistance and wear resistance of the piston, parts of the piston body, In particular, attempts have been made to use reinforcing fibers to compositely strengthen the side outer circumferential surface of the head and the wall surface of the piston ring groove (top ring groove), which are subject to the most severe thermal load and are prone to wear. In order to improve durability, attempts have been made to construct piston rings (compression rings) from steel. In this case, short alumina fibers are generally used as the reinforcing fibers due to their excellent adhesion to aluminum alloys and stability at high temperatures, and the piston ring constituent materials are generally Si- Cr steel is used. However, when a piston body whose wall surface of the piston ring groove is made of an aluminum alloy compositely reinforced with alumina short fibers and a piston ring made of Si-Cr steel are used in combination, the internal combustion engine is particularly susceptible to harsh conditions. When the piston ring is operated under normal conditions (cooling water temperature of 100°C or higher, oil temperature of 130°C or higher), the so-called ring stay occurs, where the piston ring sticks to the lower wall of the piston ring groove (the wall opposite to the cylinder head). As a result, the function of the piston ring may not be fully demonstrated, leading to problems such as an increase in the amount of blow-by gas, a decrease in engine performance, and an increase in oil consumption. As a result of X-ray analysis, it was confirmed that aluminum alloy was adhered to the bottom surface of the piston ring that caused the ring stay, and that the piston ring was caused by the combustion of the air-fuel mixture in the combustion chamber of the internal combustion engine. Since the piston ring groove is strongly pressed against the lower surface of the piston ring groove by the high-temperature and high-pressure gas and is heated to a high temperature, it is presumed that the synergistic action of these causes the ring stay. The inventors of the present application, as a piston for internal combustion engine,
As mentioned above, when a piston body whose wall surface of a piston ring groove is made of aluminum alloy compositely reinforced with alumina short fibers and a piston ring made of Si-Cr steel are used in combination. In view of various problems, we have developed a piston in which the piston ring compression ring is made of various carbon steels and special steels, and the wall surface of the piston ring groove is made of an aluminum alloy compositely reinforced with alumina short fibers. As a result of various experimental studies in combination with the main body, we have found that if the compression ring is made of martensitic stainless steel, especially martensitic stainless steel containing more than a certain amount of carbon, the ring and It has been found that the occurrence of ring stuck can be prevented while suppressing the wear of the ring groove wall surface that receives this. The present invention is based on the knowledge obtained as a result of various experimental studies conducted by the inventors of the present invention, and is based on the knowledge obtained as a result of various experimental studies conducted by the inventors of the present invention. The object of the present invention is to provide a piston that enables an internal combustion engine to be used in normal operating conditions for a long period of time. According to the present invention, a piston for an internal combustion engine is provided, which is made of an aluminum alloy, and the wall surface of the piston ring groove is compositely reinforced with short alumina fibers made of 80 wt% or more of alumina and the balance of silica. piston body and carbon content.
This is achieved by a piston for an internal combustion engine having a compression ring made of martensitic stainless steel of 0.55 wt% or more and fitted into the piston ring groove. According to the present invention, the heat resistance and wear resistance of the piston are improved by compositely reinforcing the wall surface of the piston ring groove with alumina short fibers, and the compression ring is made of martensitic stainless steel. This improves the wear resistance of the piston, thereby improving the durability of the piston.Although the reason for this is not clear, it is possible to reliably prevent the occurrence of ring stays due to adhesion of aluminum alloy. This makes it possible to use the internal combustion engine under normal operating conditions for a long period of time. According to the results of experimental research conducted by the inventors of the present application, regardless of the fiber length and fiber diameter of the alumina short fibers, if the bulk density of the alumina short fibers is 0.08 g/cc or less, the piston body It was not possible to sufficiently improve the heat resistance of the piston ring groove and the wear resistance of the piston ring groove wall surface, and the bulk density of the alumina short fibers could not be sufficiently improved.
Even if the increase exceeds 0.3 g/cc, the wear resistance of the piston ring groove wall does not improve significantly, and when the bulk density of the alumina short fibers exceeds 0.3 g/cc, the amount of wear on the compression ring as a mating material decreases. In addition, due to the difference in thermal expansion coefficient between the aluminum alloy and the alumina short fibers, thermal fatigue cracks may occur at the boundary between the composite reinforced part with the alumina short fibers and the non-composite reinforced part. It has been confirmed that there is. According to one detailed feature of the invention, the bulk density of the short alumina fibers is therefore between 0.08 and 0.3 g/cc, preferably between 0.08 and 0.25.
g/cc. Furthermore, when producing alumina short fibers, it is unavoidable to generate so-called non-fiber particles that cannot completely take the form of fibers due to the manufacturing method, and these non-fibrous particles are extremely hard (Hv (25 g) = 700 or more). , and their size is very large, several tens to hundreds of microns, compared to fibers with a diameter of several microns, so non-fibrous particles, especially non-fibrous particles with a particle diameter of 150 microns or more, are contained in relatively large amounts. In this case, machining of the piston ring groove is extremely difficult, and the amount of wear on the compression ring as a mating member increases, and furthermore, non-fibrous particles fall off from the matrix, resulting in compression failure. It has been experimentally confirmed that abnormal wear such as scuffing may occur on the underside of the ring. Therefore, according to another detailed feature of the present invention, the non-fibrous particles with a particle size of 150μ or more contained in the alumina short fiber aggregate used for composite reinforcement of the piston body are
It can be kept below 7wt%. In general, alumina has various crystal structures, and among these, alpha alumina is known to have the most stable structure and to have high hardness and elastic modulus.
For example, short alumina fibers that are commercially available as heat-resistant materials have an α-alumina content (ratio of the weight of α-alumina to the total weight of alumina in the short alumina fibers) of 60 wt% or more in terms of heat resistance and dimensional stability. There are many things. Judging from the properties of α-alumina and α-alumina-containing short alumina fibers, when an aluminum alloy is composite reinforced with α-alumina-containing short alumina fibers, the higher the α-alumina content, the stronger the composite part. Although the mechanical strength and wear resistance of the material are improved, it is presumed that the amount of wear on the mating member increases and the workability decreases. However, according to the results of experimental research conducted by the present inventors, contrary to the above expectations, the wear resistance and workability of the composite part are improved when the α-alumina content is 5 to 60 wt%, preferably 10 to 50 wt%. It has been recognized that the wear rate of the mating member can be improved and the amount of wear on the mating member can be reduced. According to yet another detailed feature of the invention, therefore, the α-alumina content of the short alumina fibers is selected within the above-mentioned range. Furthermore, according to the results of experimental research conducted by the inventors of the present application, ring stays can be avoided if the compression ring is made of martensitic stainless steel, but martensitic stainless steel Among them, the carbon content is 0.55wt
It has been confirmed that the occurrence of ring stakes cannot be effectively prevented unless the martensitic stainless steel has a martensitic stainless steel of at least 0.60 wt%. Also, as the carbon content increases, carbon and chromium combine to form carbides,
Since the amount of chromium dissolved in the matrix structure is reduced by a corresponding amount, the corrosion resistance of martensitic stainless steel is reduced. Furthermore, if the carbon content increases, the stainless steel becomes too hard, making the piston ring more likely to break out when the compression ring is assembled to the piston body. According to yet another detailed feature of the invention, the martensitic stainless steel constituting the compression ring has a carbon content of 0.55 to 0.55.
It is set at 1.2wt%, preferably 0.60-0.85wt%. Martensitic stainless steels that meet these requirements include JIS standard SUS420J2 (revised to increase carbon content to the above range), SUS440B,
SUS440A (carbon content increased to the above range)
and so on. In addition, piston rings made of these martensitic stainless steels have a fine martensitic structure as a base and have a hardness of Hv (1
It is used after being heat-treated by quenching and tempering so that the weight is 380-450kg. The present invention will be described in detail below with reference to examples in connection with the results of various experimental studies conducted by the inventors of the present application. A piston body made of an aluminum alloy whose piston ring groove wall surface is compositely reinforced with alumina short fibers and a compression ring made of various steels are assembled into a diesel engine, and the diesel engine is put into test operation. As a result, ring stays may not occur if the compression ring is made of martensitic stainless steel, and even in martensitic stainless steel, ring stays may occur depending on the carbon content. Since it was recognized that this phenomenon occurred in some cases and in others it did not occur, the first cylindrical test specimen was formed from various martensitic stainless steels with different carbon contents, and composite reinforced with alumina short fibers. A second cylindrical test piece made of an aluminum alloy was set in a Van der Holft friction and wear tester, and a mock test was conducted to determine whether or not adhesion occurred. First, a cylindrical body with an outer diameter of 10 mm and a length of 15 mm was made of various martensitic stainless steels shown in Table 1 below ( _A1 is Si-Cr steel as a comparative example that has been used conventionally). and the cylindrical body
The first cylindrical test piece was created by heating it to 1050℃ for 40 minutes, then quenching it in air to quench it, then heating it to 480-520℃ for 2 hours, and then performing a tempering process by cooling it with water. did. In addition, the average fiber diameter is 3.1μ, and the α-alumina content is 30wt.
% alumina staple fiber (95.5wt% Al ₂ O ₃ , 4.5wt%
SiO ₂ ) aggregate with a bulk density of 0.15 g/cc and an outer diameter of 86
A cylinder with an inner diameter of 53 mm and a length of 100 mm was molded using silica as an inorganic binder. In this case, the alumina short fiber aggregate has a particle size of
The total amount of non-fibrous particles of 150μ or more was treated to be 0.8wt%, and the short alumina fibers were oriented in a two-dimensional random manner along a plane perpendicular to the axis of the cylinder. Next, the fiber molded body thus formed was
After preheating to ℃, mold of high pressure casting equipment (temperature 270
℃), pour molten aluminum alloy (JIS standard AC8A) into the mold (temperature 750℃),
The molten metal was pressurized with a plunger at a pressure of 1000 kg/cm ² to penetrate between the individual short alumina fibers, and the pressurized state was maintained until the molten metal completely solidified. After the molten metal has completely solidified, a cylindrical solidified body is removed from the mold, and a second cylindrical test piece with an outer diameter of 83 mm and a length of 100 mm is created by machining the solidified body. was subjected to _T7 heat treatment. Next, the first and second cylindrical test pieces prepared as described above were set in a van der Holft friction and wear tester so that their axes were perpendicular to each other, and the side surface of the first cylindrical test piece was subjected to a second cylindrical test. against one side
Press with a pressing force of 3.75 kg, and apply room temperature lubricating oil (cattle motor oil 5W-30) to the contact parts.
An adhesion test was conducted in which the second cylindrical specimen was rotated for 20 minutes at a circumferential speed of 0.9 m/sec while supplying . The test results are shown in the column of Table 1 below. Furthermore, this table 1
Here, "generation" means that a phenomenon has occurred in which aluminum in the matrix of the second cylindrical test piece is transferred to the surface of the first cylindrical test piece, which is a pre-adhesion stage.

[Table] In addition, pistons having a piston body whose top-ring groove wall surface is compositely reinforced with alumina short fibers and a top ring made of martensitic stainless steel with various carbon contents were sequentially prepared. It was installed in a diesel engine and an adhesion test was conducted using an actual machine. First, using silica as an inorganic binder, the same alumina short fiber aggregate as the alumina short fiber aggregate used to create the second cylindrical test piece for the adhesion test using the test piece described above was prepared with an outer diameter of 95 mm. Inner diameter 75
A cylindrical fiber molded body with a length of 21 mm and a height of 21 mm was produced.
In this case, the total amount of non-fibrous particles with a particle size of 150μ or more contained in the alumina short fiber aggregate is 0.8wt%.
The alumina short fibers were oriented in a substantially two-dimensional random manner parallel to the cylindrical outer peripheral surface of the fiber molded body. Next, the fiber molded body was heated to 600℃.
After preheating the mold in high pressure casting equipment (temperature 270℃)
molten aluminum alloy (JIS standard AC8A) as the matrix metal is poured into the mold (temperature 750°C), and the molten metal is poured with a plunger.
The molten metal was applied to a pressure of 1000 Kg/cm ² to penetrate between the individual short alumina fibers, and the pressurized state was maintained until the molten metal completely solidified. After the molten metal has completely solidified, the solidified body is removed from the mold, and the solidified body is made of an aluminum alloy composite reinforced with alumina short fibers, with an outer diameter of 93 mm, an inner diameter of 75 mm, and a height of 21 mm.
A cylindrical body of mm was cut out by machining. Next, the cylindrical body is placed in a piston casting mold, and molten aluminum alloy (JIS standard AC8A) (temperature 750°C) is poured into the mold to form the material for the piston body. After subjecting the material to T ₇ heat treatment, piston ring grooves, etc. were formed by machining, and a piston body with a diameter of 90 mm and a length of 99 mm was formed. The distance from the piston head to the bottom wall of the top ring groove is 18.5 mm, the minimum width of the top ring groove is 2 mm, the depth of the top ring groove is 4.5 mm, and the slope of the top wall of the top ring groove is 18.5 mm. The angle was 7°. In addition, the outer diameter of martensitic stainless steels A ₂ to _{A 9} and Si-Cr steel A ₁ shown in Table 1 above is 90 mm.
Form a C-shaped top ring material with an inner diameter of 82.6 mm, a maximum thickness of 2.2 mm, and an inclination angle of 7° on the top surface.
A half-keystone top ring was formed by heating to 1050°C for 20 to 30 seconds, quenching in air, and then tempering by heating to 580°C for 20 to 30 seconds and cooling. Fig. 1 is a partially cutaway perspective view of a piston having a piston main body and top ring formed as described above, and Fig. 2 is a partial longitudinal section showing an enlarged main part of the piston shown in Fig. 1. It is a diagram. In these figures, 1 indicates the piston body,
The combustion gas flows onto the side outer circumferential surface 2 of the main body and the cylinder bore wall surface 4 of the piston main body 1 and the cylinder block 3.
Two ring grooves 7 and 8 are formed to receive the compression ring 5 that prevents oil from leaking from the combustion chamber to the crank chamber through the gap between the two ring grooves 7 and 8, and a ring groove 10 is formed to receive the oil ring 9 that scrapes off excess oil. has been done. In the illustrated embodiment, a portion along the side outer circumferential surface 2 from the head portion 11 to below the bottom wall 12 of the top ring groove 7 is compositely reinforced with alumina short fibers. The composite reinforcing portion 13 defines the wall surface of the top ring groove 7 that receives the top ring 5, and also includes a top land 14 and a second land 15 in the portion exposed to the side outer peripheral surface 2.
A part of it has been defined. The second ring 6 is made of special cast iron for piston rings, which has been conventionally used as a constituent material for piston rings. The pistons constructed as described above were sequentially assembled into a diesel engine, and an adhesion test was conducted using an actual machine under the test conditions shown in Table 2 below. Table 2 Engine used: 4-cylinder 4-cycle diesel engine Cylinder bore diameter: 90mm Stroke: 86mm Compression ratio: 21.5 Total displacement: 2188c.c. Fuel used: Light oil Engine speed: 4400rpm Engine load: Full load Cooling water temperature: 105℃ Test Time: 50 hours The results of this adhesion test are shown in the column of Table 1 above. In the column of Table 1, "occurrence" indicates that the top ring 5 stuck to the lower wall 12 of the top ring groove 7 and ring sticking occurred. From the columns of Table 1 and more, it can be seen that martensitic stainless steel having a carbon content of 0.55 wt% or more, preferably 0.60 wt% or more is preferable as a constituent material of the top ring. In addition, the bulk density of alumina short fiber is 0.11g/cc,
Adhesion test using test piece and actual machine conducted under the same conditions as each test above, except that it was modified to 0.21g/cc, 0.26g/cc, average fiber diameter as reinforcing fiber.
Each of the above tests except that short alumina fibers (95wt% Al ₂ O ₃ , 5wt% SiO ₂ ) with an α-alumina content of 50wt% and a total amount of 1wt% of non-fibrous particles with a particle size of 150μ or more were used. An adhesion test using a test piece and an actual machine was conducted under the same conditions as above, and an adhesion test was conducted under the same conditions as the above tests, except that JIS standard AC8P and AC4C were used as the matrix metal. In the adhesion test using an actual machine, results similar to those shown in Table 1 above were obtained. From the results of the adhesion test using the above-mentioned test pieces and actual equipment, regardless of the specifications of the short alumina fibers and the composition of the aluminum alloy, it is possible to use carbon content of 0.55wt% or more as a top-spring constituent material, and furthermore, 0.60wt% or more. Since martensitic stainless steel was found to be preferable, cylindrical specimens were formed from martensitic stainless steel that met the above requirements and composite reinforced with alumina short fibers with various α-alumina contents. Block test pieces were formed from the aluminum alloy that had been prepared, and these test pieces were sequentially set in a van der Holft friction and wear tester to minimize the amount of wear on the bottom surface of the top ring and the bottom wall of the top ring groove. In order to suppress this, an abrasion test was conducted to find out what range of α-alumina content of short alumina fibers is appropriate. First, a cylindrical body with an outer diameter of 35 mm, an inner diameter of 31 mm, and a length of 8.7 mm was formed from martensitic stainless steel _A5 shown in Table 1 above, and the cylindrical body was subjected to the adhesion test using the test piece described above. A cylindrical test piece was prepared by performing the same heat treatment. In addition, the alpha alumina content is 2wt% and 8wt, respectively.
%, 20wt%, 34wt%, 43wt%, 61wt%, 81wt
%, 93wt% of alumina short fibers (95.5wt% Al ₂ O ₃ , 4.5wt% SiO ₂ ) with an average fiber diameter of 3.1 μm were made into a rectangular parallelepiped of 80 x 80 x 20 mm at a bulk density of 0.15 g/cc. It was molded using silica as an inorganic binder. In this case, the total amount of non-fibrous particles with a particle size of 150μ or more in the alumina short fiber aggregate is
The alumina short fibers were treated to have a concentration of 0.8 wt%, and the alumina short fibers were oriented in a two-dimensional random manner along a plane of 80 x 80 mm. After preheating the fiber molded body thus formed to 600°C, it is placed in a mold (temperature: 270°C) of a high-pressure casting machine, and a molten metal (temperature: 750°C) of aluminum alloy (JIS standard AC8A) is placed in the mold. was poured, and the molten metal was pressurized with a plunger at a pressure of 1000 kg/cm ² to allow the molten metal to penetrate between the individual short alumina fibers, and the pressurized state was maintained until the molten metal completely solidified. After the molten metal has completely solidified, the solidified body is removed from the mold, and a part made of aluminum alloy reinforced with alumina short fibers is extracted from the solidified body by machining to obtain a shape of 10.16×15.7 mm.
×6.35mm (alumina short fibers are two-dimensional random orientation parallel to the 15.7×6.35mm plane) block test piece
_B1 to _B8 were prepared, and each block test piece was subjected to _T7 heat treatment. Next, the cylindrical test piece and block test piece prepared as described above were sequentially set in the LFW friction and wear tester, and the cylindrical test piece was brought into contact with the block test piece.
While supplying room temperature lubricating oil (castle motor oil 5W-30) to those contact parts, the contact surface pressure is 20
A wear test was conducted by rotating a cylindrical test piece for 1 hour at a sliding ^speed of 0.3 mm/sec and a sliding speed of 0.3 mm/sec. The results of this wear test are shown in FIGS. 3 and 4. In Figures 3 and 4, the upper half shows the wear amount of the block test piece (wear depth μ), and the lower half shows the wear amount of the cylindrical test piece (wear loss mg).
represents. Furthermore, FIG. 4 is a graph showing the relationship between the α-alumina content of short alumina fibers and the wear amount of cylindrical test pieces and block test pieces based on the test results shown in FIG. 3. From these Figures 3 and 4, especially Figure 4, when the alpha alumina content is 5wt% or more, especially 10~
It can be seen that the amount of wear on the block specimen is small when the amount is 70wt%. On the other hand, it can be seen that the wear amount of the cylindrical test piece is small when the α-alumina content of the short alumina fibers is 5 to 60 wt%, especially when it is about 10 to 50 wt%. Therefore, it is preferable that the alpha alumina content of the short alumina fibers for compositely reinforcing the wall surface of the top ring groove of the piston body is 5 to 60 wt%, particularly 10 to 50 wt%. Although the present invention has been described above with reference to several examples in relation to the results of various tests, the present invention is not limited to these examples, and there are many other examples within the scope of the present invention. It will be apparent to those skilled in the art that various embodiments are possible.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway perspective view of one embodiment of a piston for an internal combustion engine according to the present invention, and FIG. 2 is a partial vertical sectional view showing an enlarged main part of the embodiment shown in FIG. 1. , FIGS. 3 and 4 are graphs showing the results of friction and wear tests conducted using the α-alumina content of short alumina fibers as reinforcing fibers as a parameter. DESCRIPTION OF SYMBOLS 1...Piston body, 2...Side outer peripheral surface, 3...Cylinder block, 4...Cylinder bore wall surface, 5...Top ring, 6...Second ring, 7...Top ring groove, 8...Second ring groove, 9...Oil ring, 10 ...Ring groove, 11...Head part, 12...Lower wall, 13...Composite reinforcement part.

Claims (1)

[Scope of Claims] 1. A piston for an internal combustion engine, which is made of aluminum alloy and has a wall portion of a piston ring groove.
The piston body is compositely reinforced with short alumina fibers consisting of 80wt% or more alumina and the remainder silica, and is made of martensitic stainless steel with a carbon content of 0.55wt% or more, and is fitted into the piston ring groove. A piston for an internal combustion engine having a compression ring. 2. The piston for an internal combustion engine according to claim 1, wherein the piston ring groove is a top ring groove, and the compression ring is a top ring.