JPH09170490A - Cast iron piston for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce influency of deformation in applying soft nitride treatment in an iron type piston applied with soft nitride treatment on its surface by forming an annular recessed escape groove at a skirt part of a piston or at a position spaced at a specified interval from a lower end surface thereof. SOLUTION: In a cast iron piston 1 of which a pinton top 3, combustion chamber 4, rim 4a, top ring groove 5, second ring groove 6, oil ring groove 7, skirt 8 and piston hole 9 are applied with soft nitride treatment 2 on their surfaces, an annular recessed escape groove 8a is formed at a skirt part 9 or at a position L spaced at a specified interval from a lower end surface of the skirt 8 in order to reduce influence of deformation caused by heat and cylinder pressure at the time of combustion as well as influence of deformation by the nitride treatment. A sliding surface of the piston 1 at the time of operation is thereby turned into two places of upper half Ca and lower half of the skirt 8, whereby uniform contact of the sliding surface is attained to improve the sliding performance of forming an oil film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cast iron piston for an internal combustion engine, and more particularly to a cast iron piston in which the piston surface of a diesel engine is subjected to nitriding or soft nitriding treatment.

[0002]

2. Description of the Related Art Conventionally, a piston which has been required to have a heat resistance load in a high-power diesel engine for vehicles has recently been subjected to a gas soft nitriding treatment on a required portion of the outer shape forming surface of a piston body after finishing, which is made of ductile cast iron. The actual case is 3-
It is known from Japanese Patent No. 87848 and will be described with reference to FIG. As shown in FIG. 4, a gas soft nitriding furnace having an ammonia gas 22 introduced therein, 23 is a piston body made of ductile cast iron whose finishing is finished, and 24, 25 and 26 are required for the outer surface of the piston body 23. Each outer shape forming surface 28a, 28b, 28 such as a portion, that is, the surface of the outer periphery 23a of the piston body 23, the surface of the piston ring groove 23b, the inner peripheral surface of the piston pin hole 23c, the inner surface of the combustion chamber 23d, and the like.
high-frequency heating coil for heating c, 28d, 27
Is a mount for mounting the piston body. When performing the soft nitriding treatment on the piston body 23, first, the piston body 23 is placed on the pedestal of the soft nitriding furnace 21, and then the high frequency heating coil 24 is placed near the outer circumference 23a of the piston body 23 and the piston ring groove 23b. The high frequency heating coil 25 is installed near the outer forming surface 28c of the piston pin hole 28c, and the high frequency heating coil 26 is installed near the outer forming surface 28d of the combustion chamber 23d. Next, from the outside, ammonia gas 22 is introduced into the furnace 21, and the high frequency heating coil 24,
The outer circumference 2 of the piston body 23 is energized by energizing 25 and 26.
3a, piston ring groove 23b, piston pin hole 23
c, each outer shape forming surface 28a, 28b, 28 of the combustion chamber 23d
c and 28d are heated to a predetermined temperature (about 550 ° C.) by high frequency. Therefore, ammonia gas or the like is heated and decomposed into nitrogen gas and hydrogen gas, and the nitrogen gas is used to form the outer periphery 23a of the piston body 23, the piston ring groove 23b, the piston pin hole 23c, and the outer shape forming surfaces 28a of the combustion chamber 23d.
8b, 28c and 28d are impregnated, and the gas soft nitriding treatment is applied to the hatched portion in FIG. As described above, the outer periphery 23a of the piston body 23 made of ductile cast iron, the piston ring groove 23b, the piston pin hole 23c, the combustion chamber 23.
When the gas soft nitriding treatment is applied to the outer shape forming surfaces 28a, 28b, 28c, 28d of d, the fatigue strength of these portions is about 1.5 times higher than that in the case where the gas soft nitriding treatment is not performed, and seizure resistance is increased. And the wear resistance are further improved as compared with the case of performing hard chrome plating, for example. Therefore, the outer shape forming surfaces 28a, 28b, 28c of the piston body 23,
No cracks are formed in 28d, and seizure resistance and wear resistance of the outer periphery 23a, the piston ring groove 23b, and the piston pin hole 23c, which are the sliding portions, are improved. Also, when the entire piston body 23 is soft-nitrided, the piston body 23 must be heated, and therefore the piston body 2
3 may be deformed, but since only the necessary part is partially heated by high frequency heating, the piston body 2
3 does not deform. Further, since it is not necessary to heat the entire piston main body 23, it is not necessary to heat the entire gas nitrocarburizing furnace 21, energy can be saved, and the processing time is shortened to perform efficient gas nitrocarburizing. be able to. In the embodiment of the present invention,
The case where the gas soft nitriding treatment of the piston body is performed by high frequency heating has been described, but if it is possible to heat the required portion, not only high frequency heating but also various heating means can be adopted. It goes without saying that various changes can be made without departing from the scope of the invention.

Next, examples of pistons in which the sliding area of the skirt of the piston is remarkably reduced to reduce sliding resistance are disclosed in Japanese Utility Model Publication No. 62-8346 and Japanese Utility Model Publication 62-62.
No. 2071 publication. This will be described with reference to FIGS. First, a case of Japanese Utility Model Laid-Open No. 62-8346 will be described with reference to FIGS. In addition, FIG. 6 is A of FIG.
FIG. As shown in FIG. 5, the lower surface is formed into an arcuate surface 3 at two upper and lower positions in the center of both sides of the skirt portion 33 around the axis of the piston pin hole 32 of the piston body 31.
4 with a semicircular outer diameter surface 35a, 35b leaving a recess 3
6 is provided, and a concave curved surface 37 of the lower outer diameter surface is formed. With this configuration, the air and the lubricating oil are mixed with each other in the space of the recess to reduce the viscosity of the lubricating oil. Since the sliding area of the skirt is extremely small, there is little sliding resistance.

A case of Japanese Utility Model Laid-Open No. 62-2071 will be described with reference to FIGS. 7 and 8. In addition, FIG. 8 shows B- of FIG.
The B sectional view is shown. As shown in FIG. 7, the skirt portion 42 of the piston body 41 is provided with recesses 44 in the upper and lower portions of the annular outer diameter surface 43 in the circumferential direction, and at the upper side of the annular outer diameter surface 43. Is a sloping surface 45 toward the recess 44
Are formed. With this configuration, similarly to the above, the air and the lubricating oil are agitated in the space of the recess, and the viscosity of the lubricating oil is reduced. Since the sliding area of the skirt is extremely small, there is little sliding resistance.

[0005]

However, the conventional techniques have the following problems. (1) In the example of Japanese Utility Model Laid-Open No. 3-87848 of the prior art, the combustion chamber of the piston is subjected to gas nitrocarburizing treatment, but the piston top surface is not subjected to gas nitrocarburizing treatment. There is a problem that there is a risk of fatigue fracture starting from a microcrack during long-term operation. In the piston of a high-power diesel engine, the combustion chamber and the top surface of the piston are exposed to high-temperature combustion gas when the engine is operating. Therefore, the temperature of the combustion chamber of the piston rises, especially at the rim portion of the combustion chamber of the reentrant piston with improved combustion, and
There is a problem in that the temperature near the exhaust valve actuation on the top surface of the piston is particularly high, and a piston made of ductile cast iron that has not been subjected to soft nitriding treatment causes fatigue fracture from the origin of microcracks during long-term operation. It is necessary to perform soft nitriding treatment on the portion of the top surface of the piston where the temperature near the exhaust valve operates, where the temperature rises, to increase the high temperature fatigue strength. That is, in a cast iron piston that operates under high temperature conditions, the material strength of the rim portion of the combustion chamber is substantially determined by its high temperature fatigue strength and the strength of the iron-based oxide film generated during operation. Cast iron pistons that operate under high temperature conditions The damage to the rim of a piston is fatigue fracture originating from microcracks in the iron-based oxide film generated on the rim surface. The iron-based oxide film is generally brittle compared to the iron base material, and its strength is lower than that of the iron base material. The strength of the iron-based oxide film decreases as its thickness increases. Further, the thickness thereof increases with the engine operating time or the rise of the piston.

(2) Next, it is desired to nitrid the piston skirt to improve seizure resistance and wear resistance. However, the skirt of the piston is nitrided to change its shape. There is a problem that it is easy to do. In the conventional cast iron piston shown in FIG. 2, before the nitriding treatment, the deformation due to nitriding, the influence due to the heat at combustion and the deformation due to the in-cylinder pressure are not considered, that is, the annular skirt 8a is formed in the skirt portion 8 of the piston. An example will be described in which a conventional piston has a soft nitriding treatment applied to all surfaces. As shown in FIG. 2, the material is ferritic annealed spheroidal graphite cast iron, which is subjected to nitrocarburizing treatment 2 after completion of machining. As shown by the one-dot chain line in the figure, the soft nitriding treatment 2 is applied to the entire surface of the cast iron piston 10. That is, the surfaces of the piston top portion 3, the combustion chamber 4, the rim portion 4a, the top ring groove 5, the second ring groove 6, the oil ring groove 7, the skirt 8 and the piston pin hole 9 are treated. The right side of FIG. 3 shows the height of the cast iron piston 10 on the vertical axis, and corresponds to each part of the piston 10 on the left side of FIG. The horizontal axis represents the piston outer diameter dimension mm on the thrust side in the direction perpendicular to the piston pin hole 9 of the cast iron piston 10. Dotted profile D in the figure, ie, j
1, k1 and l form the profile shape after the completion of machining and before the soft nitriding treatment. When the soft nitriding treatment 2 is performed in such a shape of the profile D, it is transformed into a solid line profile E, that is, a shape of j1, k2, l by nitriding. This deformation is large at the center of the piston pin hole 9, and is deformed by ΔD from k1 to k2. Top j1 of skirt 8
The lower part of the skirt 8 has the rigidity of the cast iron piston 10 and is not deformed. As a result, the solid line profile E after the nitriding treatment has a barrel shape. During piston operation of this solid line profile E, there is deformation ΔE due to heat during combustion and cylinder pressure, which causes deformation from the piston outer diameter j1 to j2,
The sliding surface has a barrel-shaped two-dot chain line profile H.
When the cast iron piston 10 having a profile H of a two-dot chain line having a barrel-shaped sliding surface is incorporated into an engine and operated,
There was a problem that the local surface pressure at the k2 portion of profile E was high, the sliding characteristics of the oil film formation were poor, and seizure was likely to occur.

(3) In addition, the conventional technique of Japanese Utility Model Laid-Open No. 62-834
In the cases of Japanese Patent No. 6 and Japanese Utility Model Laid-Open No. 62-2071, the sliding area of the skirt portion of the piston is made extremely small to reduce sliding resistance. However, as the diesel engine output increases, the piston slap also increases, and the load applied to the skirt of the piston increases.
The load per unit area of the sliding surface of the skirt is significantly increased due to the increase of the load and the significant reduction of the sliding area of the skirt. Further, in the case of Japanese Utility Model Laid-Open No. 62-2071 of FIG. 7, the lower end of the skirt portion 42 has a useless recess 4
It is 4. Due to this useless recess 44, the inclination becomes large when the piston is operated, and the surface pressure is further increased. This increase in surface pressure may increase seizure and wear from the skirt of the piston. Seizure resistance,
A piston with excellent wear resistance and low sliding resistance is required.

The present invention focuses on the above-mentioned conventional problems, and in a cast iron piston for an internal combustion engine, it is excellent in high-temperature oxidation resistance and high-temperature strength due to high output, and is capable of operating a combustion chamber rim portion for a long time. Prevents fatigue fracture starting from microcracks. Further, it is an object of the present invention to provide a cast iron piston having a piston skirt having excellent seizure resistance and wear resistance, and having low sliding resistance, and further having improved wear resistance of the top ring groove and the piston pin hole. Make it the purpose.

[0009]

In order to solve the above-mentioned problems, in the invention of the cast iron piston for an internal combustion engine of the present invention, a surface for improving wear resistance and / or oxidation resistance / and / or high temperature strength is provided. In cast iron or iron-based pistons that are subjected to nitriding or soft nitriding treatment before nitriding or soft nitriding treatment 2 is performed on the piston surface, the influence of nitriding treatment, heat during combustion, and deformation due to cylinder pressure is reduced. For this reason, the annular recessed escape groove 8a is formed at the skirt portion 8 of the piston and at a position separated from the lower end surface by a predetermined distance La. With such a configuration, high temperature oxidation resistance and high temperature resistance are improved, and as a result, fatigue fracture originating from fine cracks can be prevented during long-term operation of the combustion chamber rim portion of the piston. Further, it is possible to obtain a piston in which seizure resistance and wear resistance of the skirt portion of the piston are improved by increasing the output and the sliding resistance is small. Furthermore, the wear resistance of the top ring groove 5 and the piston pin hole 9 can be improved.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described with reference to FIG.
Will be described in detail with reference to. The left side of FIG. 1 is a partial cross-sectional view of the cast iron piston of the present invention, and the right side of FIG. 1 is a drawing of the profile of the skirt portion of the cast iron piston of the present invention. As shown on the left side of FIG. 1, the material is ferritic annealed spheroidal graphite cast iron, which is subjected to soft nitriding treatment after completion of machining. As shown by the chain line in the figure, cast iron piston 1
The soft nitriding treatment 2 is applied to the entire surface of the. That is, the piston top portion 3, the combustion chamber 4, the rim portion 4a, the top ring groove 5,
Second ring groove 6, oil ring groove 7, skirt 8,
The surface of the piston pin hole 9 is treated. Before the nitriding treatment, the skirt portion 8 of the piston 1 is separated from the lower end surface of the skirt portion 8 by a predetermined distance in order to reduce the influence of the deformation due to the nitriding, the heat at the time of combustion, and the deformation due to the pressure inside the cylinder.
An annular Da recess escape groove 8a is formed at the La position. The profile shape of the concave escape groove 8a will be described with reference to FIG. The right side of FIG. 1 shows the height of the cast iron piston 1 on the vertical axis, and corresponds to each part of the left piston 1 of FIG.
The horizontal axis represents the piston outer diameter dimension mm on the thrust side in the direction perpendicular to the piston pin hole 9 of the cast iron piston 1. Dotted profile A in the figure, ie, a1, b1, c1, d1, e
, Has formed the profile shape after the completion of machining and before the soft nitriding treatment. Distance D from outer diameter d1 to outer diameter c1
In order to reduce the influence of a on the deformation of a, the dimension in which the annular concave relief groove 8a is formed in advance is shown. When the soft nitriding treatment 2 is performed in the shape of the dotted line profile A, the solid line profile B, that is, a1, b2, c2,
It transforms into the shape of d2, e. This deformation is concave relief processing 8
It is large in the part where a is marked, and ΔC in the place where the rigidity from c1 to c2 is low.
Only deform. The deformation amounts of b1, b2 and d1, d2 are small because they have rigidity. When the piston of the solid line profile B is operated, it is transformed into a two-dot chain line profile C, that is, a2, b2, c2, d2, e. (It is displayed as a shift because it overlaps with the solid line.) There is a deformation ΔA due to heat during combustion and cylinder pressure, it is deformed from a1 to a2, and the sliding surface during actual operation is the upper half Ca of the skirt 8. , The lower half of Cb.

Next, the operation will be described. Preliminary annular concave relief process 8a considering deformation of piston skirt
By forming the above, the sliding surface at the time of piston actuation becomes two places, the upper half Ca and the lower half Cb of the skirt 8, and the sliding surfaces are evenly hit. As a result, the sliding characteristics of the oil film formation were improved. In addition, there is a problem that the high power output causes fatigue fracture from the origin of fine cracks due to long-term operation of the rim portion of the combustion chamber, but it has been improved by soft nitriding treatment. The effect of the improvement will be described with reference to FIG. In the case of FIG. 3, the material is ferritic annealed spheroidal graphite cast iron, and the effect of improvement is seen by comparing the case where soft nitriding treatment is performed after completion of machining and the case where it is not performed. The vertical axis shows the oxide film thickness, and the horizontal axis shows the operating time for engine durability. The oxide film thickness on the vertical axis represents the deterioration of the piston. The larger the thickness, the larger the deterioration. From the conventional experience, the allowable limit is 70 μm as a standard,
Values above this value are in the dangerous area, and values below this value are in the safe area.
The measurement location is the rim of the combustion chamber where the piston easily cracks. The solid line F in the figure is the deterioration characteristic without soft nitriding treatment,
At the q point, the allowable limit is reached after 950 hours of durable operation. or,
The dotted line G in the figure is the deterioration characteristic with soft nitriding treatment, and is in a safe area with a margin of durability for 1000 hours of operation. Comparing with and without soft nitriding at the endurance operation time of 500 h, the oxide film thickness decreased from the n point to the p point (thick dotted arrow),
It is reduced to about 1/5.

[0012]

As described above, according to the present invention,
Before the nitriding or the soft nitriding treatment 2 is performed on the surface of the piston, in order to reduce the influence of the nitriding treatment 2, the heat at the time of combustion and the deformation due to the in-cylinder pressure, the piston skirt portion 8 has an annular concave escape groove 8a. The sliding surface at the time of piston operation is two places, the upper half Ca and the lower half Cb of the skirt 8. By the synergistic effect of reducing the sliding surface and reducing the friction coefficient by the nitriding treatment 2, The dynamic resistance can be reduced and the oil film can be formed well. As a result, seizure resistance and wear resistance due to high output of the skirt portion of the piston are improved. Also, by subjecting the surface of the piston to nitriding or soft nitriding treatment 2, the high temperature oxidation resistance and high temperature resistance are improved, and as a result, microcracks are generated at the piston combustion chamber rim portion during long-term operation. It is possible to prevent fatigue failure. Further, the top ring groove 5 due to high output, and
The wear resistance of the piston pin hole 9 can be improved.

[Brief description of the drawings]

FIG. 1 is a partial half sectional view showing a cast iron piston of the present invention and a profile view of a skirt portion of the present invention.

FIG. 2 is a half sectional view showing a conventional cast iron piston and a profile view of a conventional skirt portion.

FIG. 3 is an oxide film thickness on the vertical axis of the present invention and a conventional example;
The figure explaining the relationship with the driving time of a horizontal axis is shown.

FIG. 4 is a diagram illustrating a nitriding treatment of a conventional cast iron piston.

FIG. 5 shows a front view of a prior art piston that reduces sliding resistance.

FIG. 6 is a sectional view taken along line AA of FIG. 5;

FIG. 7 shows a front view of another prior art piston that reduces sliding resistance.

FIG. 8 is a sectional view taken along the line BB of FIG. 7;

[Explanation of symbols]

 1 Cast iron piston 2 Soft nitriding 8 Skirt 8a Concave surface relief processing

Claims (1)

[Claims] 1. A cast iron or iron-based piston whose surface is nitrided or soft-nitrided to improve wear resistance and / or oxidation resistance / and / or high temperature strength resistance. Before soft nitriding treatment (2), in order to reduce the effect of nitriding treatment, heat during combustion and deformation due to in-cylinder pressure, at the piston skirt (8) and at a prescribed distance from the lower end surface ( A cast iron piston (1) for an internal combustion engine, characterized in that an annular concave relief groove (8a) is formed at a position (La).