COMBUSTION-CHAMBER STRUCTURE OF ENGINE

BACKGROUND OF THE INVENTION

The present invention relates to a combustion-chamber structure of an engine which comprises a heat-barrier layer to suppress heat loss.

A combustion chamber of a gasoline engine or the like for a vehicle is required to reduce heat dissipation (heat loss) through a wall surface of the combustion chamber. A technology that a heat-barrier layer made of a small heat-conductivity material is coated on the combustion-chamber wall surface, such as a crown surface of a piston, for heat-loss reduction is known. A temperature difference between combustion gas generated in the combustion chamber and the combustion-chamber wall surface is made so small by providing the heat-barrier layer that the heat loss can be reduced.

Japanese Patent Laid-Open Publication No. 2018-172997 discloses a combustion-chamber structure in which a heat-insulation layer is provided at a piston crown surface in addition to the heat-barrier layer. The heat-barrier layer covers an entire part of the piston crown surface, thereby suppressing the heat dissipation through a piston body. The heat-insulation layer is provided below the heat-barrier layer and in a central area, in a radial direction, of the piston crown surface, thereby making this central area be the area where the heat does not escape easily. Thereby, a temperature distribution in which the temperature of an central area, in a radial direction, of the combustion chamber is relatively high, whereas the temperature of an outside area, in the radial direction, of the combustion chamber is relatively low is formed. This temperature distribution has a merit that in a case where a homogeneous-charge compression-ignition combustion (in other words, a premixed compression-ignition combustion) is performed, the combustion is made properly slow and thereby a rapid increase of a cylinder internal pressure or heat loss can be properly suppressed.

The combustion chamber is also partitioned by an intake valve and an exhaust valve. Accordingly, it is necessary to suppress the heat dissipation from the intake valve and the exhaust valve as well for reduction of the heat loss of the combustion chamber. Herein, it may be considered that the heat-barrier layer and the heat-insulation layer are also provided at respective valve heads of the intake valve and the exhaust valve, similarly to the structure of the above-described patent document. There is a problem, however, that the heat may be excessively stored at the heat-insulation layer, thereby making the temperature of the valves improperly high. That is, the heat-insulation layer may store the heat which has not been insulated (blocked) by the heat-barrier layer, so that this heat-insulation layer having the high temperature may heat the heat-barrier layer. This heating may cause a temperature increase of the valve itself, thereby increasing the cylinder temperature. Thereby, the air taken in an intake stroke of the engine may be heated excessively, so that improper preignition may occur in a compression stroke of the engine.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a combustion-chamber structure of an engine which can properly suppress the temperature increase of the valve which may cause the preignition, attaining the heat-loss reduction.

A combustion-chamber structure of an engine according to the present invention comprises a cylinder block, a cylinder head, a piston, a valve, and a combustion chamber partitioned by the cylinder block, the cylinder head, the piston, and the valve, wherein the valve is configured to open or close a port opening to the combustion chamber and comprises an umbrella part and a stem part, the umbrella part of the valve includes a valve body which includes a valve head facing the combustion chamber and a valve face positioned on an opposite side to the combustion chamber, a heat-insulation layer which is provided at the valve head and has smaller heat conductivity than the valve body, a heat-barrier layer which is provided to cover the valve head provided with the heat-insulation layer and has smaller heat conductivity than the valve body and the heat-insulation layer, and a heat-diffusion layer which is provided between the heat-insulation layer and the heat-barrier layer and has larger heat conductivity than the heat-insulation layer and the heat-barrier layer, and the heat-diffusion layer comprises a contact portion which is provided to extend up a position of the umbrella part of the valve which contacts with the cylinder head when the valve is closed.

According to the present combustion-chamber structure, the valve head is covered with the heat-barrier layer having the smaller heat conductivity than the valve body and the heat-insulation layer. Accordingly, the temperature difference between the valve head and the combustion chamber is made so small that heat transfer to the valve body can be suppressed. Further, the heat which has passed through the heat-barrier layer is stored at the heat-insulation layer. Accordingly, the high temperature of the heat-barrier layer (valve head) can be maintained. Meanwhile, the heat-diffusion layer is provided between the heat-insulation layer and the heat-barrier layer. This heat-diffusion layer has the larger heat conductivity than both the heat-barrier layer and the heat-insulation layer and comprises the contact portion contacting with the cylinder head. Accordingly, even in a case where the heat-insulation layer has stored the heat excessively, this heat can be made to escape to the cylinder head through the heat-diffusion layer. Consequently, the temperature increase of the valve which may cause the preignition can be prevented properly.

In the above-described combustion-chamber structure of the engine, it is preferable that the cylinder head have the larger heat conductivity than the valve body.

According to this combustion-chamber structure, the heat of the heat-insulation layer which is transferred through can be made to escape to the cylinder head more than the valve body.

In the above-described combustion-chamber structure of the engine, it is preferable that the cylinder head comprise a valve seat which is provided at the port opening and with which a portion of the umbrella part of the valve body contacts, and the contact portion of the heat-diffusion layer be provided at the portion of the umbrella part which contacts with the valve seat.

The valve seat provided at the port opening necessarily contacts with the umbrella part of the valve when the intake or exhaust port is closed. Accordingly, a heat dissipation path (route) from the valve body to the cylinder head can be secured by providing the contact portion of the heat-diffusion layer at the portion of the umbrella part which contacts with the valve seat.

In the above-described combustion-chamber structure of the engine, it is preferable that the valve be an intake valve, and the heat-barrier layer be provided on the valve face of the umbrella part of the valve as well. In this case, it is preferable that the heat-barrier layer be provided on the stem part of the valve as well.

According to this combustion-chamber structure, the heat dissipation from the valve face or the stem part of the valve can be suppressed by the heat-barrier layer even in a case where the temperature of the valve body of the intake valve increases. Accordingly, the air passing through the intake port is suppressed from being heated excessively by the intake valve, so that the preignition can be prevented properly.

In the above-described combustion-chamber structure of the engine, it is preferable that the valve be an exhaust valve, and the heat-barrier layer be provided on the valve face of the umbrella part of the valve as well. In this case, it is preferable that the heat-barrier layer be provided on the stem part of the valve as well.

According to this combustion-chamber structure, the surface temperature of the valve face of the umbrella part and the stem part of the exhaust valve can be maintained at the high temperature by the heat-barrier layer. The exhaust valve provided at the exhaust port is exposed to high temperature by exhaust heat of the combustion gas. Accordingly, the heat transfer to the valve body of the exhaust valve, i.e., the heat loss, can be suppressed properly by providing the heat-barrier layer on the valve face and the stem part of the exhaust valve.

In the above-described combustion-chamber structure of the engine, it is preferable that the heat-diffusion layer include a first portion which is provided between the heat-insulation layer and the heat-barrier layer at the valve head, the contact portion, and a second portion which is an underlayer of the heat-diffusion layer which is provided at the valve face and the stem part.

According to this combustion-chamber structure, the first portion of the heat-diffusion layer receives the heat of the valve head and the second portion receives the heat of the valve face and the stem part of the valve. The heat received by the first portion and the second portion of the heat-diffusion layer is made to escape from the contact portion to the cylinder head. The exhaust valve receives the heat from exhaust gas passing through the exhaust port, so that its temperature increases. Meanwhile, the intake valve receives the heat from EGR gas or blow-back gas of the combustion gas from the combustion chamber which is caused by setting a valve overlap term, so that its temperature possibly increases. Accordingly, by configuring the heat-diffusion layer to comprise the above-described first portion and the above-described second portion, the excessive temperature increase of the exhaust valve and the intake valve can be prevented properly.

In the above-described combustion-chamber structure of the engine, it is preferable that the valve be an exhaust valve, the heat-insulation layer and the heat-diffusion layer be provided to cover an entire part of the umbrella part of the valve, and the heat-barrier layer be provided to cover the entire part of the umbrella part of the valve except the contact portion of the heat-diffusion layer. In this case, it is preferable that the heat-insulation layer, the heat-diffusion layer, and the het-barrier layer be provided to cover at least a section of the stem part of the valve which is continuous to the umbrella part of the valve.

According to this combustion-chamber structure, the umbrella part of the exhaust valve is covered with three layers of the heat-insulation layer, the heat-diffusion layer, and the heat-barrier layer except the above-described contact portion. More preferably, at least a portion of the stem part which is continuous to the umbrella part is covered with these three layers as well. That is, the heat-insulation layer is provided at not only the valve head facing the combustion chamber but the valve face positioned on its opposite side and the stem part. Accordingly, the temperature of the heat-barrier layer provided on the valve face and the stem part can be maintained at the high temperature by means of the heat-insulation layer, so that the heat loss at the exhaust valve can be suppressed properly. Further, the heat dissipation path (route) made by the heat-diffusion layer can be secured so that the heat-insulation layer does not store the heat excessively, so that the excessive temperature increase of the exhaust valve can be prevented properly.

In the above-described combustion-chamber structure of the engine, it is preferable that the valve be an exhaust valve with cooling function in which a coolant sealing portion is formed at the valve body, the heat-insulation layer and the heat-diffusion layer be provided to cover the umbrella part of the valve, the heat-barrier layer be provided to cover the umbrella part of the valve except the contact portion of the heat-diffusion layer, and the heat-insulation layer and the heat-diffusion layer be provided to extend up to a position which overlaps with the coolant sealing portion of the valve body.

According to this combustion-chamber structure, the heat stored at the heat-insulation layer or the heat received by the heat-barrier layer can be carried to the coolant sealing portion via the heat-flowing layers. Accordingly, the excessive temperature increase of the exhaust valve can be prevented properly.

In the above-described combustion-chamber structure of the engine, it is preferable that the heat-barrier layer be made of heat-resistant silicon resin which has the heat conductivity of 0.05-1.50 W/mK, and the heat-diffusion layer be made of copper-based material, Corson alloy, beryllium copper, fiber-reinforced aluminum alloy, or titanium aluminum which have the heat conductivity of 35-600 W/mK.

The present invention will become apparent from the following description which refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing an engine to which a combustion-chamber structure according to an embodiment of the present invention is applied.

FIG. 2 is a sectional view showing details of an intake valve shown in FIG. 1.

FIG. 3 is a partially-sectional side view showing a valve of a comparative example 1.

FIG. 4 is a partially-sectional side view showing a valve of a comparative example 2.

FIG. 5 is an explanatory diagram of preignition which may be generated in a combustion chamber of a comparative example.

FIG. 6 is a partially-sectional side view showing a valve according to a first embodiment of the present invention.

FIG. 7 is a diagram explaining a behavior (operation) of heat in a case where the valve of the first embodiment is used.

FIG. 8 is a chart showing materials which are applicable to respective structural members of the combustion-chamber structure of the engine.

FIG. 9 is a partially-sectional side view showing an intake valve according to a second embodiment.

FIG. 10 is a partially-sectional side view showing an intake valve according to a third embodiment.

FIG. 11 is a partially-sectional side view showing an exhaust valve according to a fourth embodiment.

FIG. 12 is a partially-sectional side view showing an exhaust valve according to a fifth embodiment.

FIG. 13 is a partially-sectional side view showing an exhaust valve according to a sixth embodiment.

DETAILED DESCRIPTION OF THE INVENTION
Entire Structure of Engine

Hereafter, a combustion-chamber structure of an engine according to embodiments of the present invention will be described specifically referring to the drawings. FIG. 1 is a schematic sectional view showing an engine to which the combustion-chamber structure according to the embodiments of the present invention is applied. The engine described here is a multi-cylinder engine which includes cylinders and pistons and is installed to the vehicle as a power source for driving the vehicle, such as an automotive vehicle. The engine includes an engine body 1, intake-exhaust manifolds, not illustrated, which are assembled to the engine body 1, and auxiliary devices, such as various kinds of pump.

The engine body 1 of the present embodiments is capable of performing the spark-ignition combustion (SI combustion) in which the mixture of fuel and air is ignited by spark in the combustion chamber and the homogeneous-charge compression-ignition combustion (HCCI combustion) in which the mixture is self-ignited. A principle ingredient of the fuel supplied to the engine body 1 is gasoline. Generally, the spark-ignition combustion is performed in a high-load high-speed engine operation, whereas the homogeneous-charge compression-ignition combustion is performed in a middle/low-load middle/low-speed engine operation at the engine body 1. Herein, the present invention is applicable to a combustion chamber of the engine which is unable to perform the homogeneous-charge compression-ignition combustion.

The engine body 1 comprises a cylinder block 3, a cylinder head 4, and pistons 5. The cylinder block 3 has plural cylinders 2 (only one of these is illustrated in the figure) which are arranged in a direction perpendicular to a paper plane of FIG. 1. The cylinder head 4 is attached to an upper face of the cylinder block 3 such that it closes respective upper openings of the cylinders 2. The piston 5 is stored in each cylinder 2 such that the piston 5 reciprocates therein, and connected to a crankshaft 7 via a connecting rod 8. The crankshaft 7 rotates around a central axis thereof according to a reciprocating movement of the piston 5. A cavity 5C which is concaved downwardly, in a cylinder axial direction, is formed at a crown surface 5H of the piston 5.

A combustion chamber 6 is partitioned above the piston 5. An intake port 9 and an exhaust port 10 which respectively connect to the combustion chamber 6 are formed at the cylinder head 4. At a bottom surface 4a (ceiling surface 6U) of the cylinder head 4 are formed an intake-side opening portion 41 (port opening) which is a downstream end of the intake port 9 and an exhaust-side opening portion 42 (port opening) which is an upstream end of the exhaust port 10 as an opening to the combustion chamber 6.

An intake valve 11 to open/close the intake-side opening portion 41 and an exhaust valve 12 to open/close the exhaust-side opening portion 42 are assembled to the cylinder head 4. In a case of a doble overhead camshaft (DOHC) type engine, for example, the two intake-side opening portions 41 and the two exhaust-side opening portions 42 are provided at each of the cylinders 2, and the two intake valves 11 and the two exhaust valves 12 are provided as well. Respective structures of the intake valve 11 and the exhaust valve 12 will be described specifically later.

The combustion chamber 6 is partitioned by the cylinder block 3, the cylinder head 4, and the piston 5. More specifically, a combustion-chamber wall surface which partitions the combustion chamber 6 comprises an inner wall surface of the cylinder 2, the piston crown surface 5H (hereafter, referred to as “crown surface 5H” simply) which is the upper surface of the piston 5, the combustion-chamber ceiling surface 6U which is a bottom surface of the cylinder head 4, and respective umbrella parts (valve heads 113, 123) of the intake valve 11 and the exhaust valve 12.

An intake-side valve driving mechanism 13 and an exhaust-side valve driving mechanism 14 which drive the intake valves 11 and the exhaust valve 12, respectively, are provided at the cylinder head 4. The respective stem parts of the intake valves 11 and the exhaust valve 12 are driven linked with the rotation of the crankshaft 7 by these valve driving mechanisms 13, 14. Thus, the valve head of the intake valve 11 opens/closes the intake-side opening portion 41, and the valve head of the exhaust valve 12 opens/closes the exhaust-side opening portion 42.

The intake-side valve driving mechanism 13 comprises an intake-side variable valve timing mechanism (intake-side S-VT) 15. The intake-side S-VT 15 is an electrical type of S-VT which is provided at an intake camshaft and configured to change an opening/closing timing of the intake valve 11 by continuously changing a rotational phase of the intake camshaft relative to the crankshaft 7 within a specified angle range. Likewise, the exhaust-side valve driving mechanism 14 comprises an exhaust-side variable valve timing mechanism (exhaust-side S-VT) 16. The exhaust-side S-VT 16 is an electrical type of S-VT which is provided at an exhaust camshaft and configured to change an opening/closing timing of the exhaust valve 12 by continuously changing a rotational phase of the exhaust camshaft relative to the crankshaft 7 within a specified angle range.

A single spark plug 17 to supply ignition energy to the mixture in the combustion chamber 6 is attached to the cylinder head 4 for each cylinder 2. The spark plug 17 is attached to the cylinder head 4 such that it is arranged at a central space, in a radial direction, of combustion chamber 6 and its ignition point is exposed to an inside space of the combustion chamber 6. The spark plug 17 discharges a spark from its tip according to a power supply from an ignition circuit, not illustrated, thereby igniting the mixture in the combustion chamber 6. The ignition plug 17 of the present embodiments is used to perform the spark-ignition combustion in the high-load high-speed engine operation. Further, this is also used, when the homogeneous-charge compression-ignition combustion is performed, in a case where it is hard to perform the self-ignition right after an engine start during a cold time, the homogeneous-charge compression-ignition combustion is assisted under a specified load or speed conditions (spark assist), or the like.

A single injector 18 to inject the gasoline, as the principle ingredient of the fuel, from its tip portion into the combustion chamber 6 is attached to the cylinder head 4 for each cylinder 2. A fuel supply pipe 19 is coupled to the injector 18. The injector 18 injects the fuel supplied through the fuel supply pipe 19 toward the cavity 5C. A high-pressure fuel pump (not illustrated) which includes a plunger type of pump and the like and is operationally connected to the crankshaft 7 is coupled to an upstream side of the fuel supply pipe 19. A common rail for pressure accumulation which is common to the all cylinders 2 is provided between the high-pressure fuel pump and the fuel supply pipe 19. The fuel pressure-accumulated in the common rail is supplied to the injector 18 of each cylinder 2, and the high-pressure fuel is injected from the injector 18 into the combustion chamber 6.

Specific Structure of Valve

Subsequently, a specific structure of the intake valve 11 (valve) will be described. Herein, a basic structure of the exhaust valve 12 is similar to the intake valve 11. FIG. 2 is a sectional view showing details of the intake valve 11. The intake valve 11 (exhaust valve 12) is a so-called poppet valve and comprises an intake valve body 110 (valve body) which comprises an umbrella part 111 and a stem part 112.

The umbrella part 111 comprises a valve head 113 which faces the combustion chamber 6 and a valve face 114 which is positioned on an opposite side to the combustion chamber 6. As described above, the valve head 113 is a combustion-chamber wall surface which partitions a part of the combustion chamber 6. The stem part 112 comprises a tip section 112A which is connected to the umbrella part 111 and a base end section 112B to which a driving force is applied from the intake-side valve driving mechanism 13. The stem part 112 is held by a valve guide 131 so as to move in an axial direction.

A valve spring 132 is attached around the stem part 112. The valve spring 132 is interposed between the a spring seat 133 which is fixed around the base end section 112B and an upper face of the cylinder head 4. The valve spring 132 presses the spring seat 133, so that the intake valve 11 is biased in a direction in which the umbrella part 111 seals the intake-side opening portion 41 (in an upward direction).

A ring-shaped valve seat 4S is provided at an opening edge of the intake-side opening portion 41. A part of the umbrella part 111 contacts with the valve seat 4S. Specifically, a portion of the valve face 114 around its outer peripheral edge contacts with an inner peripheral wall of the valve seat 4S when the intake valve 11 is closed. This contacting of the umbrella part 111 with the valve seat 4S makes the intake port 9 and the combustion chamber 6 be shut off, so that the combustion chamber 6 is sealed. Meanwhile, when the intake valve 11 is opened, the umbrella part 111 moves separately from the valve seat 4S according to a downward move of the intake valve 11, so that the intake port 9 and the combustion chamber 6 have a connection state.

Explanation of Comparative Examples of Valve

Before describing a valve according to the embodiments of the present invention, valves of the comparative examples will be described. FIG. 3 is a partially-sectional side view showing an intake valve 11P1 (an exhaust valve is similar to this intake valve) of a comparative example 1. The intake valve 11P1 has a heat-barrier layer 720 only. Specifically, the intake valve 11P1 comprises the intake valve body 110 including the umbrella part 111 and the stem part 112, and an entire portion of the umbrella part 111 (the valve head 113 and the valve face 114) is covered with the heat-barrier layer 720. The heat-barrier layer 720 is a coating layer which is made of a material having the sufficiently-smaller heat conductivity than the intake valve body 110, such as heat-resistant silicon resin.

The umbrella part 111 of the intake valve 11P1, especially the valve head 113, faces the combustion chamber 6, so that it is exposed to the high temperature. In a case of an intake/exhaust four-valve type of engine body 1, for example, an area which the four valve heads 113 occupy shows a considerably large rate relative to an entire area of the combustion-chamber wall surface. Accordingly, it is necessary to take some countermeasures for suppressing heat loss through the intake valve 11P1.

The heat-barrier layer 720 covering the umbrella part 111 is the layer having the small heat conductivity, and therefore its temperature changes depending on the temperature of an inside of the combustion chamber 6. Therefore, a difference between the temperature of the combustion gas inside the combustion chamber 6 and the surface temperature of the umbrella part 111 is made so small that heat transfer to the intake valve body 110 can be blocked to a certain degree. Accordingly, the heat loss can be reduced to a certain degree. However, the heat-barrier layer 720 is generally a thin layer which is made of a material having the small volume specific heat. Therefore, the heat-barrier layer 720 has the poor (inferior) heat-storage performance and cannot block the heat transfer to the intake valve body 110 perfectly, so that the heat loss cannot be reduced sufficiently.

FIG. 4 is a partially-sectional side view showing an intake valve 11P2 (an exhaust valve is similar to this intake valve) of a comparative example 2. The intake valve 11P2 has a heat-insulation layer 710 in addition to the heat-barrier layer 720. The intake valve 11P2 of the comparative example 2 is the same as the intake valve 11P1 of the comparative example 1 regarding a structure in which an entire portion of the umbrella part 111 of the intake valve 11P2 is covered with the heat-barrier layer 720. Further, in an area of the intake valve 11P2 which corresponds to the valve head 113, the heat-insulation layer 710 is arranged adjacently to a back-face side of the heat-barrier layer 720. That is, the valve head 113 is covered with two layers of the heat-insulation layer 710 and the heat-barrier layer 720 positioned above the heat-insulation layer 710.

The heat-insulation layer 710 is made of a material having the large volume specific heat and has the heat-storage performance. The heat-insulation layer 710 stores the heat which has passed through the heat-barrier layer 720. Therefore, the heat-insulation layer 710 heats (retains the heat of) the heat-insulation layer 710 provided on the valve head 113. Accordingly, the surface temperature of the valve head 113 is made high, so that a difference between the surface temperature of the valve head 113 and the temperature of the combustion gas in the combustion chamber 6 can be made small. In other words, the heat transfer from the combustion chamber 6 to the intake valve body 110 is blocked, so that the heat dissipation is suppressed. Consequently, the heat loss can be reduced considerably.

Herein, according to the research conducted by the inventors and others, it has been found that the structure of the intake valve 11P2 has the following problems. In a case where the temperature inside the combustion chamber 6 is not made relatively high, for example, when the homogenous-charge compression-ignition combustion using the lean mixture is performed in the low-load engine operation, the intake valve 11P2 of the comparative example 2 works effectively. That is, the heat-insulation layer 710 retains an appropriate stored temperature, thereby heating the heat-barrier layer 720 properly. Accordingly, the surface of the valve head 113 can be made to reach the temperature which is suitable for suppressing the heat loss.

Meanwhile, in a case where the temperature inside the combustion chamber 6 is made relatively high, the heat-insulation layer 710 stored the high temperature heats the heat-barrier layer 720 excessively. The engine body 1 performs the homogenous-charge compression-ignition combustion using the lean mixture in the middle-load engine operation and performs the spark-ignition combustion with the air-fuel ratio: 2=1 in the high-load engine operation, for example. Since the amount of fuel injection becomes relatively large in the middle/high-load engine operation, the temperature of the combustion gas in the combustion chamber 6 becomes relatively high. Therefore, the valve head 113 comes to receive the high temperature as well, so that the heat-insulation layer 710 comes to store the high heat as well. Since the heat-barrier layer 720 is heated by this heat-insulation layer 710, the surface temperature of the valve head 113 becomes considerably high.

FIG. 5 is a diagram showing a phenomenon which may occur in the middle/high-load engine operation in a combustion-chamber structure using the intake valve 11P2 of the comparative example. In FIG. 5, not only the intake valve 11P2 but the exhaust valve 12P2 having the similar structure thereto are shown. The exhaust valve 12P2 comprises an umbrella part 121 and a stem part 122, and the umbrella part 121 comprises the heat-insulation layer 710 and the heat-barrier layer 720 which are similar to those of the intake valve 11P2.

When the heat-insulation layer 710 stores the high-temperature heat and the heat-barrier layer 720 is heated by this heat, the respective valve heads 113, 123 of the intake valve 11P1 and the exhaust valve 12P2 come to have the high temperature. The valve heads 113, 123 which have been excessively heated generate the heat operative to heat the combustion chamber 6 (an arrow H in FIG. 5), so that the cylinder internal temperature is made excessively high. Accordingly, the temperature of the air taken into the combustion chamber 6 in an intake stroke of the engine increases, and when this air having the increased is compressed in a compression stroke of the engine, preignitions PIG may occur. That is, there may occur the phenomenon in which a part of the mixture is ignited at an earlier timing than a normal (appropriate) compression-ignition timing. In this case, some problems, such as an improper torque fluctuation or output decrease of the engine body 1, may be caused.

Description of Embodiments of Valve

The present embodiments provide combustion-chamber structures which can suppress of occurrence of the preignitions PIG shown in FIG. 5, reducing the heat loss through the intake valve 11 and the exhaust valve 12. In the embodiments 1-6 described below, various structures of the intake valve 11 and the exhaust valve 12 which can provide the above-described combustion-chamber structure will be exemplified.

Embodiment 1

FIG. 6 is a partially-sectional side view showing the intake valve 11 according to a first embodiment. FIG. 7 is an enlarged view of a sectional portion of the intake valve 11 of FIG. 6, which shows a positional relationship of the intake valve 11 with the vale seat 4S (cylinder head 4). The structure of the intake valve 11 which is shown here is applicable to the exhaust valve 12. The intake valve 11 comprises the intake valve body 110 which includes the umbrella part 111 and the stem part 112, the heat-insulation layer 71 and the heat-barrier layer 72 which are shown in the comparative example 2 as well, and the heat-diffusion layer 73 which is not provided in the comparative example 2.

The heat-insulation layer 71 is provided at the valve seat 113 of the umbrella part 111. The heat-insulation layer 71 has a specified thickness in a valve shaft (axial) direction and is of a circular shape which is similar to the valve head 113 in a plan view of the valve shaft direction. A circular-shaped outer peripheral edge 711 of the heat-insulation layer 71 extends up to a position near an outer peripheral edge of the valve head 113 (umbrella part 111). Of course, this circular shape, in the plan view, of the heat-insulation layer 71 is just one example, and this layer 71 may have any other shape, such as a polygon. Further, the heat-insulation layer 71 may have a smaller size than the valve head 113, and the heat-insulation layer 71 may be provided only in a central area, in a radial direction, of the valve head 113, for example. The thickness, in the valve shaft direction, of the heat-insulation layer 71 can be selected from a range of 1-6 mm, for example.

It is preferable that the heat conductivity of the heat-insulation layer 71 be as small as possible from viewpoints of suppressing the heat from escaping from the combustion chamber 6 through the intake valve 11 (the exhaust valve 12) (suppression of the heat loss), and at least a material which has the smaller heat conductivity than the intake valve body 110 (an exhaust valve body 120) is used. Further, it is preferable that the heat-insulation layer 71 have the volume specific heat which is as large as possible, i.e., the high heat-storage performance, from viewpoints of maintaining the valve head 113 at the high temperature.

The heat-barrier layer 72 is provided to cover the valve head 113 where the heat-insulation layer 71 is provided for suppression of the heat loss through the intake valve body 110. That is, the heat-insulation layer 72 is exposed to the surface of the valve head 113. The heat conductivity of the heat-barrier layer 72 is set to be smaller than those of the intake valve body 110 and the heat-insulation layer 71 from viewpoints of suppressing the heat from escaping from the valve head 113 to the intake valve body 110. By providing the heat-barrier layer 72, the temperature difference between the combustion gas generated in the combustion chamber 6 and the valve head 113 can be made small, thereby reducing the heat loss. The thickness, in the valve shaft direction, of the heat-barrier layer 72 can be selected from a range of 0.03-0.25 mm, for example.

The heat-diffusion layer 73 is provided between the heat-insulation layer 71 and the heat-barrier layer 72 such that its combustion-chamber-side face contacts with the heat-barrier layer 72 and its opposite-side face contacts with the heat-barrier layer 72. The heat-diffusion layer 73 is the layer which has the function of making the heat stored at the heat-insulation layer 71 escape to the cylinder head 4 so that the temperature of the valve head 113 where the heat-insulation layer 71 is provided does not become too high. It is preferable that the heat conductivity of the heat-diffusion layer 73 be as large as possible from viewpoints of immediate transfer of the heat stored at the heat-insulation layer 71 to the cylinder head 4. Therefore, the heat-diffusion layer 73 is configured to have the larger heat conductivity than the heat-insulation layer 71 and the heat-barrier layer 72. The thickness, in the valve shaft direction, of the heat-diffusion layer 73 can be selected from a range of 1-5 mm, for example. Herein, it is preferable that the heat resistance, which is represented by “heat conductivity/thickness,” of the heat-diffusion layer 73 be as small as possible from viewpoints of appropriate heat diffusion. Therefore, the thickness of the heat-diffusion layer 73 is set properly considering the heat conductivity of the material of which the heat-diffusion layer 73 is made.

Referring to FIG. 7, the heat-diffusion layer 73 comprises a contact portion 731 which is provided to extend up to a position of a part of the valve face 114 from the outer peripheral edge of the valve head 113. A portion around an outer peripheral edge (a portion having the largest diameter) of the valve face 114 becomes a contact face CP (contacting position) of the umbrella part 111 which contacts with the valve seat 4S when the intake valve 11 is closed. The above-described contact portion 731 extends up to the position of the contact face CP. That is, the contact portion 731 is located at a position which directly contacts with a reception face 43 of the valve seat 4S when the intake valve 11 is closed. The heat-diffusion layer 73 receives the heat which is excessively stored at the heat-insulation layer 71 and makes this heat escape from the contact portion 731 to the cylinder head 4 through the valve seat 4S.

An operation (move) of the above-described heat dissipation (heat escaping) will be described referring to arrows D1-D3 show in FIG. 7. As shown by the arrow D1, since the heat-barrier layer 72 has the extremely-low heat conductivity and changes its temperature depending on the chamber temperature of the combustion chamber 6, the heat transfer from the combustion gas in the combustion chamber 6 to the intake valve body 110 can be blocked considerably. That is, the heat can be prevented from escaping from the combustion chamber 6 through the valve head 113. Thereby, the heat loss can be reduced. However, since the heat-barrier layer 72 cannot block the heat transfer perfectly, the heat is made to pass through to a certain degree as shown by the arrow D2. The heat-insulation layer 71 of the present embodiment is made of the material having the large volume specific heat, thereby providing the superior heat-storage performance. Accordingly, the heat passed through the heat-barrier layer 72 (the arrow D2) and the surrounding heat are stored at the heat-insulation layer 71.

Then, the heat-insulation layer 71 which has stored the heat comes to heat the heat-barrier layer 72. Accordingly, the valve head 113 where the heat-insulation layer 71 is provided can be maintained at the high temperature. However, as described regarding the comparative example 2, the heat-insulation layer 71 stores the high-temperature heat in a certain engine operation where the temperature of the combustion gas is relatively high. Accordingly, the heat-insulation layer 72 is excessively heated, so that the preignition is caused. In order to prevent this problem, the heat-diffusion layer 73 is provided between the heat-insulation layer 71 and the heat-barrier layer 72 such that the heat-diffusion layer 73 receives the heat stored at the heat-insulation layer 71. Further, as shown by the arrow D3, when the contact portion 731 contacts with the valve seat 4S, the heat-diffusion layer 73 makes the heat received from the heat-insulation layer 71 escape to the valve seat 4S. This heat is transferred from the valve seat 4S to the cylinder head 4. Accordingly, the excessively high temperature of the valve head 113 is so suppressed that the preignition can be prevented from occurring previously.

Subsequently, examples of the material which can be appropriately used as a structural member of the combustion chamber 6 are shown. A casting of a metal-based material, such as aluminum alloy AC4B (the heat conductivity=96 W/mK, the volume specific heat=2667 kJ/m³K), can be used as respective base materials of the cylinder block 3 and the cylinder head 4. Further, aluminum alloy AC8A (the heat conductivity=125 W/mK, the volume specific heat=2600 kJ/m³K) can be used as a base material of the piston 5 (piston body 50).

Heat-resistant steel which is superior in the heat-resistant performance, the wear-resistant performance, and the corrosion-resistant performance can be used for the intake valve body 110 and the exhaust valve body 120. Martensite-based heat-resistant steel SUH11 based on chrome, silicon, and carbon (the heat conductivity=25 W/mK, the volume specific heat=3850 kJ/m³K) can be used for the intake valve body 110, for example. Martensite-based heat-resistant steel SUH35 based on chrome, silicon, and carbon (the heat conductivity=18 W/mK, the volume specific heat=3565 kJ/m³K) can be used for the exhaust valve body 120, for example.

Like the above-described examples, it is preferable that the cylinder head 4 have the larger heat conductivity than the intake valve body 110 and the exhaust valve body 120. Since the contact portion 731 of the heat-diffusion layer 73 contacts with the valve face 114 of the umbrella part 111, the heat can be made to escape to the intake valve body 110 as well. However, by setting the heat conductivity of the cylinder head 4 to be larger than those of the intake valve body 110 and the exhaust valve body 120, the heat can be made to escape from the contact portion 731 to the valve bodies 110, 120 actively.

The material which has the smallest heat conductivity and the smallest volume specific heat is selected for the heat-barrier layer 72 among the structural members of the intake valve 11 and the exhaust valve 12 (the intake valve body 110 and the exhaust valve body 120, the heat-insulation layer 71, the heat-barrier layer 72, and the heat-diffusion layer 73). That is, the appropriate material of the heat-barrier layer 72 is selected such that this layer 72 does not diffuse the heat easily and does not store the heat easily. A range of the preferable heat conductivity of the heat-barrier layer 72 is about 0.05-1.50 W/mK, and a range of the preferable volume specific heat of the heat-barrier layer 72 is about 500-1500 kJ/m³K.

For example, the heat-resistant silicon resin can be exemplified as the material of the heat-barrier layer 72 which meets the above-described requirements. The silicon resin made of three-dimensional polymer having the high branching degree which is represented by methyl silicon resin and methylphenyl silicon resin can be exemplified as the above-described silicon resin, and polyalkylphenylsiloxane or the like are preferably used, for example. This silicon resin may contain microballoon particles, such as Shirasu balloons. The heat-barrier layer 72 can be formed by a coating process in which the above-described silicon resin is coated on the valve face 114 of the umbrella part 111 where the heat-insulation layer 71 and the heat-diffusion layer 73 are formed, for example.

The heat-insulation layer 71 does not diffuse the heat easily but stores the heat easily. The material which has the larger heat conductivity than the heat-barrier layer 72 and the extremely-smaller heat conductivity than the intake valve body 110 and the exhaust valve body 120 is selected for the heat-insulation layer 71 in order to suppress the heat diffusion. Further, the material which has the larger volume specific heat and the larger heat resistance than the heat-barrier layer 72 is selected for the heat-insulation layer 71 in order to provide the appropriate heat-storage performance A range of the preferable heat conductivity of the heat-insulation layer 71 is about 0.2-10 W/mK, and a range of the preferable volume specific heat of the heat-insulation layer 71 is about 1800-3500 kJ/m³K.

A ceramics material can be exemplified as the material of the heat-insulation layer 71 which meets the above-described requirements, for example. In general, since the ceramics material has the small heat conductivity but has the large volume specific heat and the superior heat resistance, this material is suitable for the heat-insulation layer 71. Specifically, a preferable ceramics material is zirconia (the heat conductivity=3 W/mK, the volume specific heat=2576 kJ/m³K). Alternatively, the ceramics material, such as silicon nitride, silica, cordierite, or mullite, a porous SUS based material, calcium silicate, or the like can be used as the material of the heat-insulation layer 71 as well.

The heat-diffusion layer 73 makes the heat stored at the heat-insulation layer 71 escape to the cylinder head 4 from the contact portion 731, and therefore this layer 73 is the layer which easily diffuses the heat. Thus, the heat-diffusion layer 73 has the largest heat conductivity among the structural members of the intake valve 11 or the exhaust valve 12. A range of the preferable heat conductivity of the heat-diffusion layer 73 is about 35-600 W/mK. Further, it is preferable that the thickness of the heat-diffusion layer 73 be set such that the heat resistance is within a range of 0.002-0.06 m²K/W. A copper-based material (the heat conductivity=400 W/mK, the volume specific heat=3500 kJ/m³K), Corson alloy, beryllium copper, fiber-reinforced aluminum alloy, titanium aluminum, or the like can be used as the material of the heat-diffusion layer 73 which meets the above-described requirements. The above-described copper-based material is particularly preferable because even in a case where the thickness is set at 2 mm, the heat resistance of the heat-diffusion layer 73 can be controlled (suppressed) at a value of 0.005 m²K/W.

FIG. 8 shows a preferred material selection example of the base materials of the intake valve body 110 and the exhaust valve body 120 (valve base materials), the cylinder block 3, the cylinder head 4 and the piston 5, the heat-insulation layer 71, the heat-barrier layer 72, and the heat-diffusion layer 73. FIG. 8 shows the heat conductivity λ, the volume specific heat ρc, the heat diffusivity (λ/ρc), the Z-directional thickness t, the heat resistance (t/λ), and the heat permeability (√λρc) of each of these materials. Herein, a right-side small column of the heat diffusivity shows each value of the respective layers in a case where the heat diffusivity of the heat-barrier layer 72 is considered as “1”.

Embodiment 2

In a second embodiment, a preferable example as the intake valve will be shown. FIG. 9 is a partially-sectional side view showing an intake valve 11A according to the second embodiment. The intake valve 11A comprises the intake valve body 110 including the umbrella part 111 and the stem part 112, the heat-insulation layer 71, the heat-barrier layer 72, and the heat-diffusion layer 73. What is different from the above-described first embodiment is that there is provided a valve-face heat-barrier layer 721 which covers over the valve face 114 of the umbrella part 111.

The valve-face heat-barrier layer 721 covers the valve face 14 over an area from a base end section 111A (a portion connected to the tip section 112A of the stem part 112) of the umbrella part 111 to a position of the umbrella part 111 which partially overlaps with the contact portion 731. That is, the structure in which the contact portion 731 is exposed to the contact face CP of the umbrella part 111 is the same as that of the first embodiment. The valve-face heat-insulation layer 721 can have the same material and thickness as the heat-barrier layer 72. Herein, the heat-barrier layer 721 may be provided to extend up to a position located above a portion of the stem part 112 (a specified length from the tip section 112A toward the base end section 112B).

Even if the heat-insulation layer 71 and the heat-barrier layer 72 are arranged on the side of the valve head 113, the intake valve body 110 has the heat. The higher the temperature of the combustion chamber in the combustion chamber becomes, the higher the temperature of the intake valve body 110 becomes. As shown in FIG. 2, the umbrella part 111 of the intake valve body 110 is positioned in the intake port 9 when the valve 11 is closed. At this moment, the intake air inside the intake port 9 contacts with the valve face 114 of the umbrella part 111. Further, when the valve 11 is opened as well, the intake air flowing from the intake port 9 into the combustion chamber 6 hits against the valve face 114. Therefore, if the temperature of the surface of the valve face 114 becomes high, the temperature of the intake air is increased. If the excessively-heated intake air is introduced into the combustion chamber 6 in the intake stroke of the engine, the preignition occurs in the compression stroke of the engine.

The valve-face heat-barrier layer 721 prevents the heat of the umbrella part 111 from escaping to the outside from the valve face 114. That is, as shown by the arrow D4 in FIG. 9, the valve-face heat-barrier layer 721 performs the function of capturing the heat inside the umbrella part 111. Accordingly, even in a case where the temperature of the umbrella part 111 of the intake valve 11 becomes high, the heat can be prevented from escaping from the valve face 114. Accordingly, the temperature of the intake air passing through the intake port 9 is suppressed from increasing excessively, so that the preignition can be prevented.

Herein, the heat of the umbrella part 111 is made to escape to the cylinder head through the contact portion 731 of the heat-diffusion layer 73 to a certain degree. This point is similar to a case of the intake valve 11 of the first embodiment in which the valve-face heat-barrier layer 721 is not provided. However, since there are many cases where the above-described heat dissipation (escaping) through the contact portion 731 is not enough, it is preferable that the heat dissipation from the valve face 114 be suppressed by providing the valve-face heat-barrier layer 721 like the present embodiment.

Embodiment 3

A preferable example of the intake valve will be shown as a third embodiment as well. FIG. 10 is a partially-sectional side view showing an intake valve 11B according to the third embodiment. The intake valve 11B comprises the intake valve body 110 including the umbrella part 111 and the stem part 112, the heat-insulation layer 71, the heat-barrier layer 72, and the heat-diffusion layer 73. What is different from the above-described second embodiment is that there is provided a stem-part heat-barrier layer 722 which is provided to cover over the stem part 112 in addition to the valve-face heat-barrier layer 721, and the heat-diffusion layer 73 extends from the valve face 114 to the stem part 112.

Similarly to the second embodiment, the valve-face heat-barrier layer 721 is provided on the valve face 114 in an area from an end edge of the contact portion 731 to the base end section 111A. The stem-part heat-barrier layer 722 is provided on the stem part 112 such that it is continuous to the valve-face heat-barrier layer 721. That is, the valve-face heat-barrier layer 721 and the stem-part heat-barrier layer 722 cover the valve face 114 and the stem part 112 except the contact portion 731. It is preferable that the stem-part heat-barrier layer 722 cover around the tip section 112A of the stem part 112, especially a section of the stem part 112 which projects downwardly from the valve guide 131 (FIG. 2).

The heat-diffusion layer 73 comprises a valve-head heat-diffusion layer 730 (first portion), the above-described contact portion 731, a valve-face heat-diffusion layer 732 (second portion), and a stem-part heat-diffusion layer 733 (second portion). The valve-head heat-diffusion layer 730 is provided between the heat-insulation layer 71 and the heat-barrier layer 72 at the valve head 113. The contact portion 731 is exposed to the contact face CP of the stem part 111 and contacts with the valve seat 4S similarly to the first and second embodiments. The valve-face heat-diffusion layer 732 is a base layer of the valve-face heat-barrier layer 721 which is provided on the valve face 114. The stem-part heat-diffusion layer 733 is a base layer of the stem-part heat-barrier layer 722 which is provided on the stem part 112.

The intake valve 11 receives the heat from the exhaust gas in some cases. For example, in a case where a closing timing of the intake valve is delayed by setting a valve overlap term, there may occur blow back of the exhaust gas from the combustion chamber 6 to the intake port 9 after the combustion. In this case, the intake valve 11 is heated by this blow-back exhaust gas. Especially, the umbrella part 111 is heated. Further, the intake valve 11 may be heated by the EGR gas as well. Thus, the intake valve 11 heated by the exhaust gas may increase the temperature of the intake air excessively.

Herein, the intake valve 11B of the third embodiment is configured such that the heat-diffusion layer 73 is provided with the valve-face heat-diffusion layer 732 and the stem-part heat-diffusion layer 733 and thereby the heat received by the umbrella part 111 and the stem part 112 escapes from the contact portion 731 to the cylinder head 4. That is, as shown by the arrow D4 in FIG. 10, the valve-face heat-barrier layer 721 performs the function of capturing the heat inside the umbrella part 111. This is the same as the second embodiment. Further, as shown by an arrow D41, the stem-part heat-barrier layer 722 performs the function of capturing the heat inside the stem part 112. Therefore, even in a case where the temperature of the umbrella part 111 and the stem part 112 of the intake valve 11 is made high, the heat dissipation from the surfaces of the valve face 114 and the stem part 112 can be suppressed.

Meanwhile, in a case where the valve-face heat-barrier layer 721 and the stem-part heat-barrier layer 722 are exposed to the high temperature through contacting with the exhaust gas and the like that, the heat-diffusion layer 73 performs of the function of the heat dissipation. That is, the valve-face heat-diffusion layer 732 receives the heat from the valve-face heat-barrier layer 721, and the stem-part heat-diffusion layer 733 receives the heat from the stem-part heat-barrier layer 722. This heat is made to escape from the contact portion 731 to the cylinder head 4 as shown by the arrow D5. Accordingly, the excessively-high temperature of the intake valve 11B can be prevented.

Embodiment 4

A preferable example of the exhaust valve will be shown as a fourth embodiment as well. FIG. 11 is a partially-sectional side view showing an exhaust valve 12A according to the fourth embodiment. The exhaust valve 12A comprises the intake valve body 120 including the umbrella part 121 and the stem part 122, the heat-insulation layer 71, the heat-barrier layer 72, the heat-diffusion layer 73, and a valve-face heat-barrier layer 721 which is provided to cover over a valve face 124 of the umbrella part 121. A layer structure of the exhaust valve 12 of the fourth embodiment is the same as that of the intake valve 11A of the second embodiment.

The valve-face heat-barrier layer 721 covers the valve face 124 in an area from a base end section 121A to a position which partially overlaps with the contact portion 731 of the heat-diffusion layer 73. Similarly to the above-described embodiments, the contact portion 731 is exposed to the contact face CP of the umbrella part 121 with the valve seat 4S. Herein, the heat-barrier layer 721 may be provided to extend up to a position located above a portion of the stem part 122 (a specified length from the tip section 112A toward the base end section 112B).

The exhaust valve 12A which is provided at the exhaust port 10 is exposed to the high temperature by the exhaust heat of the combustion gas. Differently from the second embodiment, the valve-face heat-barrier layer 721 suppresses the umbrella part 121 from receiving the exhaust heat. The temperature of the valve-face heat-barrier layer 721 increases through contacting with the high-temperature exhaust gas, so that the temperature difference between the exhaust gas and the valve face 124 is made small. Accordingly, as shown by the arrow D6 in FIG. 11, the heat transfer of the exhaust heat to the umbrella part 121 through the valve face 124 can be suppressed by providing the valve-face heat-barrier layer 721. That is, the heat loss can be suppressed.

Embodiment 5

A preferable example of the exhaust valve will be shown as a fifth embodiment as well. FIG. 12 is a partially-sectional side view showing an exhaust valve 12B according to the fifth embodiment. The exhaust valve 12B comprises the structure in which the exhaust valve body 120 is covered with three layers of the heat-insulation layer, the heat-barrier layer, and the heat-diffusion layer except the contact portion 731. While the fifth embodiment shows an example in which a section of the stem part 122 which is continuous to the umbrella part 121 is covered with the above-described three layers, only the umbrella part 121 may be covered with the above-described three layers.

The exhaust valve 12B comprises the heat-insulation layer 71 which is provided to correspond to the valve head 123, a valve-face heat-insulation layer 712 which is provided to extend from the outer peripheral edge 711 of the heat-insulation layer 71 onto the valve face 124, and a stem-part heat-insulation layer 713 which is provided on the stem part 122 as the heat-insulation layers. The exhaust valve 12B further comprises the valve-face heat-barrier layer 721 and the stem-part heat-barrier layer 722 covering the stem part 122 as the heat-barrier layers, in addition to the heat-barrier layer 72 covering the valve head 123.

The heat-diffusion layer 73 comprises the valve-head heat-diffusion layer 730 (first portion), the above-described contact portion 731, the valve-face heat-diffusion layer 732 (second portion), and the stem-part heat-diffusion layer 733 (second portion). The valve-head heat-diffusion layer 730 is provided between the heat-insulation layer 71 and the heat-barrier layer 72 at the valve head 123. The contact portion 731 is exposed to the contact face CP of the umbrella part 121 and contacts with the valve seat 4S. The valve-face heat-diffusion layer 732 is provided between the valve-face heat-insulation layer 712 and the valve-face heat-barrier layer 721 at the valve face 124. The stem-part heat-diffusion layer 733 is provided between the stem-part heat-insulation layer 713 and the stem-part heat-barrier layer 722 at the stem part 122.

The valve-face heat-barrier layer 721 suppresses the umbrella part 121 from receiving the exhaust heat similarly to the fourth embodiment. As shown by the arrow D6 in shown in FIG. 12, it can be suppressed by providing the valve-face heat-barrier layer 721 that the exhaust heat is transferred to the umbrella part 121 through the valve face 124. The stem-part heat-barrier layer 722 suppresses the exhaust heat from being transferred to the stem part 122 as shown by an arrow D61 similarly to the stem-part heat-barrier layer 722. The valve-face heat-insulation layer 712 and the stem-part heat-insulation layer 713 are provided to maintain the respective temperatures of the valve-face heat-insulation layer 721 and the stem-part heat-barrier layer 722 at a high temperature. The valve-face heat-insulation layer 712 and the stem-part heat-insulation layer 713 store the heat which passes through the valve-face heat-insulation layer 721 and the stem-part heat-barrier layer 722 as shown by the arrow D7, and heat the heat-barrier layers 721, 722. Thereby, the heat loss through the exhaust valve 12B can be securely suppressed.

The valve-face heat-diffusion layer 732 and the stem-part heat-diffusion layer 733 are provided to make the heat stored at the valve-face heat-insulation layer 712 and the stem-part heat-insulation layer 713 escape. The valve-face heat-diffusion layer 732 receives the heat from the valve-face heat-insulation layer 712 and the valve-face heat-insulation layer 721. The stem-part heat-diffusion layer 733 receives the heat from the stem-part heat-insulation layer 713 and the stem-part heat-barrier layer 722. This heat is made to escape from the contact portion 731 to the cylinder head 4 as shown by the arrow D8. That is, even in a case where the valve-face heat-insulation layer 712 and the stem-part heat-insulation layer 713 store the heat at the excessively high temperature, a heat dissipation route formed by the heat-diffusion layers 732, 733 is secured. Herein, the valve-head heat-diffusion layer 730 performs the heat-dissipation function of the heat-insulation layer 71 of the valve head 123, which is the same as the other embodiments. Accordingly, the excessive increase of the temperature of the exhaust valve 12B which may cause the preignition can be prevented properly.

Embodiment 6

A preferable example of the exhaust valve will be shown as a sixth embodiment as well. FIG. 13 is a partially-sectional side view showing an exhaust valve 12C according to the sixth embodiment. The exhaust valve 12C is the valve having the cooling function in which a coolant sealing portion 125 is formed at the exhaust valve body 120. The sealing portion 125 extends from the stem part 122 up to an area of the umbrella part 121 which is positioned on a slightly-deep side of the base end section 121A. A coolant which is sealed into the coolant sealing portion 125 is metallic sodium, for example.

The heat-insulation layer 71 and the heat-diffusion layer 73 of the exhaust valve 12C are provided to cover the umbrella part 121. The heat-barrier layer 72 is provided to cover the umbrella part 121 except the contact portion 731. Further, the heat-barrier layer 72 and the heat-diffusion layer 73 extend up to the stem part 122 such that they overlap with the coolant sealing portion 125. That is, similarly to the above-described embodiment 5, the three-layer structure of the heat-insulation layer 71, the heat-barrier layer 72, and the heat-diffusion layer 73 is formed in the area from the umbrella part 121 to an end edge of the coolant sealing portion 125 except the contact portion 731. Meanwhile, a two-layer structure of the heat-barrier layer 72 and the heat-diffusion layer 73 is formed in an area which overlaps with the coolant sealing portion 125.

Specifically, the valve-face heat-diffusion layer 732 is provided between the valve-face heat-insulation layer 712 and the valve-face heat-barrier layer 721 at the valve face 124. Meanwhile, the stem-part heat-diffusion layer 733 contacts with the stem-part heat-barrier layer 722 at its outside face, but its inside face contacts with a surface of the stem part 122. That is, the stem-part heat-diffusion layer 733 faces the coolant sealing portion 125.

The heat transfer to the exhaust valve body 120 can be suppressed by providing the valve-face heat-barrier layer 721 and the stem-part heat-barrier layer 722 as shown by the arrows D6, D61 in FIG. 13. The valve-face heat-insulation layer 712 stores the heat which has passed through the valve-face heat-barrier layer 721 (arrow D7) and maintains the heat-barrier layer 721 at the high temperature. In a case where the valve-face heat-insulation layer 712 is heated at the high temperature excessively, as shown by the arrow D8, this high-temperature heat is transferred to the valve-face heat-diffusion layer 732 and made to escape from the contact portion 731 to the cylinder head 4.

The heat of the heat-insulation layer 71 and the valve-face heat-insulation layer 712 is made to escape to the coolant sealing portion 125 as well by the heat-diffusion layer 73. That is, the heat-diffusion layer 73 is configured such that the valve-head heat-diffusion layer 730 which contacts with the heat-insulation layer 71 and the valve-face heat-diffusion layer 732 which contacts with the valve-face heat-insulation layer 712 are connected by the contact portion 731, and the heat-diffusion layer 73 comprises the stem-part heat-diffusion layer 733 which faces the coolant sealing portion 125. Therefore, the heat of the heat-insulation layers 71, 712 is transferred to the stem-part heat-diffusion layer 733 and made to escape to the coolant sealing portion 125 as shown by the arrow D9. Even in a case where the stem-part heat-barrier layer 722 is exposed to the high temperature, the heat is made to escape to the coolant sealing portion 125. Accordingly, the excessive temperature increase of the exhaust valve 12C which may cause the preignition can be prevented properly.

Operations/Effects

According to the above-described combustion-chamber structure of the engine of the present embodiments, at least the valve heads 113, 123 are covered with the heat-barrier layer 72 having the smaller heat conductivity than the intake valve body 110, the exhaust valve body 120, and the heat-insulation layer 71. Accordingly, the temperature difference between the valve heads 113, 123 and the combustion gas in the combustion chamber 6 is made so small that the heat transfer to the valve heads 113, 123 can be suppressed. Further, the heat which has passed through the heat-barrier layer 72 is stored at the heat-insulation layer 71. Accordingly, the high temperature of the heat-barrier layer 72 (valve heads 113, 123) can be maintained. Thus, the heat loss through the intake valve 11 and the exhaust valve 12 can be reduced properly.

Meanwhile, the heat-diffusion layer 73 is provided between the heat-insulation layer 71 and the heat-barrier layer 72. The heat-diffusion layer 73 has the larger heat conductivity than both the heat-barrier layer 72 and the heat-insulation layer 71 and comprises the contact portion 731 contacting with the valve seat 4S of the cylinder head 4. Accordingly, even in a case where the heat-insulation layer 71 has stored the heat excessively, this heat can be made to escape to the cylinder head 4 through the heat-diffusion layer 73. Consequently, the temperature increase of the intake valve 11 and the exhaust valve 12 which may cause the preignition can be prevented properly.

COMBUSTION-CHAMBER STRUCTURE OF ENGINE

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CPC

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