JPH01230479A - Structural member of internal combustion engine - Google Patents

Abstract

PURPOSE:To obtain a structural member of an internal combustion engine effective in preventing abnormal combustion, fully making use of the light-weight of ceramic, by using a sintered silicon nitride having a specific thermal conductivity as the structural member of internal combustion engine. CONSTITUTION:The objective structural member of internal combustion engine is made of a sintered silicon nitride having a thermal conductivity of >=0.05cal/ cm.sec. deg.C between 500 deg.C and 700 deg.C. The thermal conductivity of sintered silicon nitride is dependent upon the kind and amount of a sintering assistant, particle diameter, pore ratio, intergranular phase, etc. Examples of the sintering assistant to achieve such a high thermal conductivity and achieve sufficiently high mechanical strength for a structural member of an internal combustion engine are Y2O3, AlN, ZrO2, rare-earth metal oxide, etc. Preferably, the particle diameter is increased and the pore ratio and the amount of intergranular phase are decreased to increase the thermal conductivity. The increase in the particle diameter and the decrease in the pore ratio can be achieved by raising the sintering temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application and Problems Therewith] The present invention relates to a structural member for an internal combustion engine formed using a silicon nitride sintered body at °C.

[Prior Art] Ceramic materials such as silicon nitride have been used and researched as parts of day cell engines due to their excellent heat resistance and heat insulation properties, and some of them have been put into practical use.

On the other hand, in recent years, high rotation and high output have been required for ignition type internal combustion engines. Attempts are being made to manufacture biscuits, pist shillings, etc.

However, the present inventors have found that it is difficult to directly apply silicon nitride, which is suitable for the above-mentioned diesel engine, to an ignition-type internal combustion engine.

In other words, in a spark-ignition internal combustion engine, in normal combustion, combustion begins with spark ignition and progresses sequentially through heat transfer and diffusion. When used on the inner wall surface of a cylinder, etc., the end gas tends to reach a high temperature, resulting in a rapid temperature rise exceeding the autoignition temperature, resulting in knocking. Due to the occurrence of knocking, the temperature of various parts within the combustion chamber further increases, eventually causing surface ignition, which eventually causes burnout of the engine.

This situation is caused by the fact that silicon nitride, which has traditionally been used mainly in diesel engines, has a low thermal conductivity in terms of thermal efficiency, but in spark-ignition internal combustion engines, the thermal conductivity is low. This is because if the temperature is low, the inner wall surface of the cylinder, etc. tends to reach a high temperature, and this tendency is further accelerated in ceramics whose thermal conductivity decreases as the temperature rises. Thus, the silicon nitride described above was not suitable as a material for ignition type internal combustion engines.

Silicon carbide, which has high thermal conductivity, has been considered as another ceramic material, but it has low strength, toughness, and thermal shock resistance, and cannot be used as structural members for these materials.

The present invention aims to solve the above-mentioned conventional problems,
It was developed by focusing on the thermal conductivity of silicon nitride sintered bodies, and can suppress the temperature rise of structural members of internal combustion engines above a specified temperature, and prevent abnormal combustion such as knocking and pre-ignition. Moreover, it is an object of the present invention to provide a structural member for an internal combustion engine that fully takes advantage of the excellent lightness and other properties of ceramics.

[Means for Solving the Above Problems] The present invention for solving the above-mentioned problems provides that structural members of internal combustion engines have a thermal conductivity of at least 0. The gist is that a silicon nitride sintered body having a temperature of 0.5 cal/cm-sec.degree. C. or higher is used.

In this way, in the present invention, the thermal conductivity is set at 500°C to 700°C.
0 within the range of 05 cal/cm-sec+'c or more, and 0. 04cal/
It is formed at a higher temperature than C...・sec・℃ or less.

Here, the things that influence the thermal conductivity of the silicon nitride sintered body are the type and amount of the sintering aid, as well as the grain size, porosity, grain boundary phase, etc.

Examples of sintering aids that can ensure such high thermal conductivity and mechanical strength as structural members of internal combustion engines include Y2O3, La2'3, and MgO1AQ203.
, A Q N, Z r 02 and rare earth element oxides. In order to obtain the desired thermal conductivity, one or more of these sintering aids may be contained and their ratios may be adjusted as appropriate.

For example, when AQ 203 or 80N is used as a sintering aid, the amount of AQ added is 3% by weight based on the total weight.
The following is desirable. The reason for limiting the AQ amount in this way is as follows. That is, when AQ increases, A0203 etc. react with Si3N4 to form a β-sialon (S1s-zAQzNe-zO2) solid solution, and the solid solubility increases as the amount of A12203, AQN, etc. added increases. growing. The larger the solid solubility, that is, the larger the value of Z in the formula, the lower the thermal conductivity.
This is because if the AQ amount exceeds 3% by weight, it will not be possible to form the film within the above-mentioned range of thermal conductivity.

In addition, when using ZrO2 as a sintering aid, 2r02
As the amount increases, the deterioration due to oxidation resistance at high temperatures becomes more severe, but if it is less than 7% by weight, oxidation will not be a problem at temperatures below the operating environment of structural parts of spark ignition internal combustion engines.
This range is desirable.

In addition to those mentioned above, the sintering aid may be of any type and amount as long as it can produce a silicon nitride sintered body that can withstand the usage conditions of an internal combustion engine.

The thermal conductivity also depends on the grain boundary phase, porosity, and the like.

That is, in order to increase the thermal conductivity, it is desirable that the grain size is large, and that the porosity and the amount of grain boundary phase are small. Thus, in order to increase the particle size and reduce pores, it is better to increase the sintering temperature. Therefore, Si3
The gas pressure sintering method, which can suppress the decomposition of N4 and increase the sintering temperature, is an effective method for producing highly thermally conductive 5L3N4 materials.

Furthermore, structural members of internal combustion engines to which this silicon nitride sintered body can be applied include parts that are exposed to combustion gas and easily reach high temperatures, such as intake and exhaust valves, valve seats, plug insulators, pistons, Examples include piston rings, cylinder heads, cylinders, etc., and the special intake and exhaust valves are the most suitable members for the present invention because they are the most likely to reach high temperatures and have a large area.

In addition, even when this silicon nitride sintered body is used in sliding parts, it lowers the temperature of the sliding surface and makes it difficult for the metal on the other side to adhere, so it can prevent metal scuffing. Not only is wear reduced and the lifespan extended, but also sliding resistance can be reduced, resulting in an internal combustion engine with less friction loss.

[Function] The thermal conductivity of silicon nitride sintered bodies used for structural members of internal combustion engines is at least 0.0 in the temperature range of 500°C to 700°C. The reason why knocking was effectively prevented by increasing the temperature to 05 cal/cm·sec·°C or higher is as follows.

When the temperature of the cylinder inner wall, etc., which transfers heat to the air-fuel mixture in the combustion chamber, reaches 500°C or higher, the air-fuel mixture tends to rise to the self-ignition temperature (approximately 300°C), taking into account the effects of self-heat radiation, etc. It has been found. Therefore, in order to prevent such self-ignition, it is essential to increase the thermal conductivity of the cylinder inner wall to a predetermined value or higher at temperatures of 500° C. or higher to promote heat dissipation.

However, metal materials have a property that their thermal conductivity increases as the temperature increases, while ceramics have a property that the thermal conductivity decreases as the temperature increases, so conventional ceramic materials usually have -・At a lower temperature range than the constant temperature, heat-resistant metals (
For example, the thermal conductivity is higher than that of 5 (JH36: JIS standard), but it decreases as the temperature rises, and becomes lower than that of heat-resistant metals in the high temperature range. 04cal/cm+s
It is below ec◆℃. It has been found that due to such thermal conductivity properties, conventional silicon nitride sintered bodies are susceptible to knocking.

Based on this, the present inventors conducted a detailed study on the value of thermal conductivity at 500° C. or higher, and as a result, it was found that the following range is appropriate.

In other words, Nimonic 80 has the lowest thermal conductivity among the heat-resistant metals conventionally used in internal combustion engines.
A (JIS standard) has a thermal conductivity of 0.700°C.
Although the temperature is slightly below 0.5 cal/cm-sec+°C, even when this heat-resistant steel is used, knocking is not a major problem and it is used. Therefore, in order to suppress the occurrence of knocking to the same level as or lower than that of conventional heat-resistant metals, the thermal conductivity should be set to 0.0 in the range of 500°C to 700°C. It is necessary that the temperature is 0.5 cal/cm·sec·°C or higher.

Note that the high temperature range of 700°C or higher far exceeds the self-ignition temperature of the air-fuel mixture, so the thermal conductivity in this range has little effect on the knocking generation limit, so there is no need to take it into consideration. However, since the thermal conductivity in the temperature range of 7.0°C or higher has an effect on pre-ignition, it is desirable that it be as high as possible.

[Example 7] In the following, examples of the present invention will be described using a silicon nitride sintered body adjusted to a predetermined thermal conductivity characteristic.
A case will be explained in which intake and exhaust valves for a 6-cylinder gasoline engine (compression ratio 9.2) are manufactured.

The intake and exhaust valve of the silicon nitride sintered body according to this example is manufactured through the following steps. That is, Si2N4 powder and sintering aid powder have a specific surface area of S i 3N
4 (12rrr/g), Y2O3 (10tn'/g
), La203 (3tn'/g), Mg0 (20i/g
), AQ203 (10g/g), AQN (3i/g),
Produced with a particle size of Z r 02 (14 tTI'/g).

Next, these powders were mixed with the composition shown in Table 1 (Samples 1 to 3).
...This example), an organic solvent is added and wet mixing is performed using a hot mill or the like. In the next step, this mixture is dried, then molded using an organic binder, and further heated to degrease it.

Subsequently, this molded body was heated to 20 atm and 19 atm for sample 1.
00℃ x 2 hours, sample 2 is 10 atm, 180
0℃×2 hours, 1 atm for sample 3, 1750℃×2
After sintering under certain conditions, it was processed into the shape of a predetermined intake and exhaust valve.

In order to demonstrate the effects of this example, sample 4.0, which is made of the unpublished powder shown in Table 1, was used as a comparative example corresponding to the prior art.
Regarding No. 5, an intake and exhaust valve was manufactured by setting the sintering conditions to 1 atm and 1750° C. for 2 hours, and using the other conditions similar to those of this example. Furthermore, silicon carbide (sample 6), 5UH36 (sample 7) and N imonic 8 shown in FIG.
An intake/exhaust valve was also manufactured using the material of 0 A (sample 8).

Note that the thermal conductivity of these materials is shown in FIG.

Experimental example> Samples 1 to 8 manufactured through each of the above steps were
The system was installed in a 6-cylinder gasoline engine (compression ratio 9.2) and the occurrence of knocking was investigated by advancing the ignition timing, and the results are shown in the second section. The operating conditions of the engine were set at 5600 rpm and full load.

As is clear from Table 2, the intake and exhaust valves using the silicon nitride sintered body according to this example (Samples 1.2.3) are different from the conventionally used intake and exhaust valves made of heat-resistant metal (Sample 7. 8)
It can be seen that sample 1, which has a knocking resistance equal to or higher than that of the heat-resistant steel intake and exhaust valves (sample 7.8), which has particularly good thermal conductivity, has knocking resistance superior to that of the heat-resistant steel intake and exhaust valves (sample 7.8).

In addition, a silicon nitride sintered body with low thermal conductivity (comparative example, sample 4.5) exhibits knocking resistance in a small advance angle range (22°), but as the advance angle increases, knocking occurs once. It can be seen that when this starts to occur, strong knocking occurs.

In general, the silicon carbide suction valve (sample 6) broke during the test when it was raised to 400 Orpm or higher, and the strength suitable for a structural member of an internal combustion engine could not be obtained.

As described above, the silicon nitride sintered body of this example exhibits excellent anti-knocking properties as evidenced by the ignition advance angle experiment, but this means that if the ignition advance angle is the same, the compression ratio will be higher. This means that the compression ratio can be equal to or higher than that of conventional intake and exhaust valves made of heat-resistant metal, and therefore the output characteristics of the engine can be improved.

In addition, the intake and exhaust valve of this example allows the weight of the valve train to be significantly reduced due to the lightweight nature of ceramic without impairing the knocking characteristics. As a result, the maximum rotation speed can be increased by 20% compared to conventional metal valves, and as a result, the maximum output can be increased by 15%.

As a second example, a piston was manufactured using the same material as sample 3 of the first example (sample 3A), and the piston was installed in an engine to test its knocking characteristics. In this test, the occurrence of knocking was investigated by varying the thickness of the top of the piston (=11-) to change the compression ratio.

For comparison, a screw I-N (sample 5) manufactured from the same material as the first comparison example (sample 5) of the silicon nitride sintered body was used.
A) and a general-purpose AQ alloy piston (sample 9) were also tested.

As is clear from Table 3, when the piston according to this example is used, the compression ratio can be increased compared to the comparative example (sample 5A), and the degree of this is almost the same as that of the AC alloy piston. It turned out to be the same.

Although the thermal conductivity of AQ gold alloy is significantly better than that of ceramic, there is almost no difference between the piston and the piston made of AQ alloy because the temperature of the piston is not so high. Rather, it is thought that the knocking characteristics depend on the temperature on the exhaust valve side. On the other hand, in the case of a ceramic piston, the temperature tends to be considerably higher than that of an AQ alloy piston, and it seems that the temperature is sensitively affected by the thermal conductivity near the temperature of the mixed scum immediately before knocking occurs. That is, in the piston of the example,
It seems that the higher thermal conductivity of 500°C to 700°C as compared to the comparative example (sample 5A) worked effectively to increase the compression ratio.

Therefore, the piston of this example does not reduce knocking characteristics compared to pistons made of AQ alloy,
By effectively utilizing the effect of silicon nitride sintered body, which has a very high specific strength compared to F2 alloy, it is possible to reduce the weight by 30% compared to that made of AQ alloy. We were able to improve the arrival time by 8% and obtain an excellent response.

[Effects of the Invention] As explained above, according to the present invention, occurrence of abnormal combustion such as knocking, which is a problem in spark ignition internal combustion engines, can be suppressed,
To provide structural members that make full use of the excellent features of ceramics, such as light weight, high-temperature durability, high strength, and good sliding properties, to enable high-speed, high-output spark-ignition internal combustion engines. Can be done.

In addition, by using this silicon nitride sintered body in sliding parts of internal combustion engines, in addition to the excellent wear resistance inherent to the silicon nitride sintered body, the temperature of the sliding surface is lowered, so It is also possible to prevent adhesion caused by high temperatures on metal sliding surfaces.

f j ○　X

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between temperature and thermal conductivity of silicon nitride sintered bodies and the like.

Claims (1)

[Claims] Thermal conductivity at 500℃ to 700℃ is 0.05cal/c
A structural member for an internal combustion engine, characterized in that a silicon nitride sintered body having a temperature of m·sec·°C or more is used.