JP2966129B2 - Engine combustion chamber structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a combustion chamber structure of an engine, and more particularly, to a structure for improving a blow-by of an air-fuel mixture.

[0002]

2. Description of the Related Art Conventionally, engines have been designed to maximize the opening area of intake and exhaust valves in order to improve intake efficiency and scavenging efficiency, and to employ a compact combustion chamber to improve flammability. In some cases, the engine output is improved. With such an engine,
The number of valves has been increased to efficiently secure the opening area of the valve.
The respective intake port openings and the respective exhaust port openings of the combustion chamber are arranged close to each other. Therefore, the valve overlap (hereinafter, referred to as the valve overlap) in which both the intake and exhaust valves are open
During the period of overlap), the fresh air (air-fuel mixture) flowing into the combustion chamber from the intake port easily flows into the exhaust port, so-called fresh air blow-through easily occurs.
Problems such as deterioration of emission performance and reduction of output and fuel efficiency occur. On the other hand, there is a concept of providing a wall around an intake port or an exhaust port. For example, as disclosed in Japanese Patent Application Laid-Open No. 55-104519,
It is known that a wall is formed around the opening of the intake port so as to form a wall extending toward the combustion chamber to determine the direction of the air-fuel mixture flowing from the intake port into the combustion chamber.

[0003]

However, when a wall is provided around the intake port as described in the above publication,
If the gap between the periphery of the large diameter portion of the intake valve head portion and the wall is too large, the wall does not have the effect of restricting the flow path of fresh air to the exhaust side, and it is not possible to effectively prevent blow-by of fresh air. . On the other hand, if the gap is reduced, the intake resistance increases, the charging efficiency decreases, and the output decreases. Also,
Even if the protrusion of the wall portion is too large, the throttle period of the flow passage is lengthened, the intake resistance is increased, the charging efficiency is reduced, the output is reduced, and the mixture in the cylinder is disturbed. There is a problem that it is difficult to generate a vertical swirling flow of the air-fuel mixture, resulting in deterioration of combustibility.

[0004] Further, as one action at the time of overlap, there is a scavenging action in which fresh air pushes exhaust gas in a combustion chamber to an exhaust port to scavenge the gas. This scavenging action can be efficiently obtained to improve scavenging efficiency. There is a request to do. In particular, in an engine equipped with a supercharger, during a supercharging operation, the scavenging action is improved by pressurized intake air (fresh air), the temperature of the air-fuel mixture in the cylinder is reduced due to a reduction in residual gas, and knock resistance is improved. There is also a demand for improved performance.

The present invention has been made in view of the above points, and an object of the present invention is to effectively prevent fresh air from flowing through without increasing intake resistance and to promote a scavenging action. Therefore, it is intended to improve emission performance, output, and fuel efficiency.

[0006]

Means for Solving the Problems In order to achieve the above object, a solution taken by the invention of claim 1 is that a plurality of intake port openings and exhaust port openings opened to a combustion chamber are brought close to each other. It is premised on a combustion chamber structure of an engine that is arranged in such a manner as to form a protruding wall extending in the lift direction of the intake valve from the valve seat at each intake port opening to the combustion chamber side. The protruding wall is provided along a peripheral edge of each intake port opening on the exhaust port side, and a protruding margin of the protruding wall is set to be higher than a valve lift amount of the intake valve at a top dead center and a top dead center. A value (mm) obtained by multiplying the crank angle (°) from the dead point until the exhaust valve substantially closes by 0.15, whichever is greater, is set to be smaller than the larger value. The gap between the large-diameter portion of the umbrella portion and the peripheral edge is set smaller than the protrusion, and the gap between the large-diameter portion of the intake valve umbrella portion and the combustion chamber wall on the bore peripheral side is defined by the intake valve umbrella portion. The gap is set to be larger than the gap between the periphery of the large diameter portion and the protruding wall.

According to a second aspect of the present invention, in the first aspect of the invention, the engine is provided with a supercharger, and the valve overlap period in which the intake and exhaust valves are open before and after the top dead center is set to be long. And

According to a third aspect of the present invention, in the first aspect of the invention, the intake port opening extends in the lift direction of the intake valve from the valve seat to the combustion chamber side along the periphery of the bore peripheral side of the intake port opening. A tumble generating wall having a predetermined gap with the periphery of the large diameter portion of the portion is provided, and a large diameter of the intake valve head portion is provided in a portion between the valve seat of the tumble generating wall and a portion corresponding to the protrusion of the protruding wall. It is configured to form a concave portion that increases the gap with the peripheral edge.

According to a fourth aspect of the present invention, in the first aspect of the present invention, the exhaust-side protruding wall extending in the lift direction of the exhaust valve from the valve seat to the combustion chamber side along the intake-side peripheral portion of each exhaust port opening. The protrusion margin of the exhaust-side protruding wall is provided so that the valve lift amount of the exhaust valve at the top dead center and the crank angle (°) from the time of opening of the intake valve to be opened before the top dead center to the top dead center .
Is multiplied by 0.15 (mm) , whichever is greater, and the gap between the periphery of the large diameter portion of the exhaust valve head and the protruding wall is set to be smaller than the protruding margin. And a gap between the large-diameter portion periphery of the exhaust valve umbrella portion and the combustion chamber wall on the bore peripheral side is set to be larger than the gap between the large-diameter portion periphery of the exhaust valve umbrella portion and the exhaust-side protruding wall. I have.

[0010]

With the above arrangement, according to the first aspect of the present invention, the protrusion of the protruding wall is adjusted so that the valve lift of the intake valve at the top dead center and the exhaust valve from the top dead center until the exhaust valve substantially closes after the top dead center. value obtained by multiplying 0.15 to the crank angle (°) between the (m
m) or less, whichever is greater, the protrusion of the protruding wall does not greatly exceed the valve lift of the intake valve at the end of the overlap period of the intake and exhaust valves. , Do not grow unnecessarily. Therefore, an increase in intake resistance due to the provision of the protruding wall is minimized. Further, since the gap between the projecting wall and the periphery of the large diameter portion of the intake valve head is set smaller than the projecting margin, the effect of restricting the flow passage of fresh air at the projecting wall portion is effectively exhibited, A blow-by of fresh air is effectively prevented. At this time, the gap between the large-diameter portion periphery of the intake valve umbrella portion and the combustion chamber wall on the bore peripheral side is set to be larger than the gap between the large-diameter portion periphery of the intake valve umbrella portion and the protruding wall. The fresh air flows into the combustion chamber from the periphery of the bore of the intake port. The fresh air flowing from the bore peripheral side forms a vertical swirling flow (reverse tumble flow) flowing from the cylinder wall on the intake side to the exhaust side along the top surface of the piston in the cylinder. The scavenging action of pushing out the exhaust gas (residual gas) remaining in the cylinder to the exhaust port is obtained, and the scavenging efficiency is improved.

According to the second aspect of the present invention, when the supercharger is provided and the valve overlap period is set to be long, in the operation of the first aspect of the present invention, particularly, in the scavenging operation, the supercharger is used. The pressurized intake air (fresh air) effectively pushes out residual gas, and the scavenging action is further promoted.

According to the third aspect of the present invention, in the portion having the protruding wall, a recess is formed in the tumble generating wall so as to increase the gap between the large diameter portion and the periphery of the intake valve head portion. The function and effect are obtained, and from the point where the protruding wall is eliminated, that is, from the time when the valve overlap is substantially completed, the flow path of the fresh air on the bore peripheral side of the intake port is narrowed by the tumble generating wall, Fresh air mainly flows into the combustion chamber from the exhaust side of the intake port.
The fresh air flowing from the exhaust side promotes the generation of a vertical swirling flow (tumble flow) flowing from the cylinder wall on the exhaust side to the intake side along the top surface of the piston in the cylinder, thereby improving the combustibility. Can be

According to the fourth aspect of the present invention, since the exhaust-side protruding wall is provided at the periphery of the exhaust port opening on the intake port side in the same manner as the protruding wall provided at the intake port opening, the exhaust resistance is minimized. The blow-by of the fresh air (air-fuel mixture) is prevented while the air-fuel mixture is minimized, and the air-fuel mixture is more likely to be scavenged by a vertical swirling flow (reverse tumble flow) in the cylinder. The function and effect can be further improved.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an overall schematic configuration of an engine according to a first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an engine. The engine 1 includes a cylinder block 3 forming a cylinder 2, a cylinder head 4 joined to an upper surface of the cylinder block 3, and a piston 5 reciprocating in the cylinder 2. A combustion chamber 6 defined in the cylinder 2 by a lower surface of a cylinder head 4 and a top surface of a piston 5.
Are formed.

An intake passage 7 is connected to the combustion chamber 6.
At the opening to the combustion chamber 6, intake valves 8, 8 are provided.
The intake air is introduced into the combustion chamber 6 at a predetermined timing. An air cleaner 9 and an air flow meter 1 for detecting the amount of intake air are arranged in this intake passage 7 in order from the upstream.
0, a supercharger 11 as a mechanical supercharger driven by an output shaft of the engine 1, an intercooler 12 for cooling intake air, a surge tank 13 for absorbing intake pulsation, and an injector for injecting fuel. 1
4 are provided. Further, a bypass passage 15 for bypassing the supercharger 11 and the intercooler 12 is provided in the intake passage 7, and a supercharger for adjusting the supercharging pressure by the supercharger 11 is provided in the bypass passage 15. A bypass valve 16 is provided.

An exhaust passage 17 is connected to the combustion chamber 6, and exhaust valves 18, 18 are provided at the opening to the combustion chamber 6.
Is provided to exhaust the exhaust gas from the combustion chamber 6 at a predetermined timing. An exhaust camshaft 21 that drives the exhaust valves 18, 18 to open and close at a predetermined timing includes:
A variable valve timing mechanism 20 for changing the opening / closing timing of the exhaust valves 18, 18 according to the operation state is provided. As shown in FIG. 2, the engine speed is low (low load) and the engine speed is high (high). Opening / closing timing is changed to delay the closing timing of the exhaust valves 18 at high engine speed (from 5 ° after top dead center to 25 ° after top dead center).
The overlap period is set to be long. That is, in this embodiment, during the overlap period in which the intake valve 8 and the exhaust valve 18 are both open, the top dead center at which the intake valve 8 opens when the engine is at low speed (at low load). From 15 ° before to 5 ° after top dead center where the exhaust valve 18 closes,
When the engine is running at high speed (at high load), it is set to extend from 15 ° before top dead center when the intake valve 8 opens to 25 ° after top dead center when the exhaust valve 18 closes. Reference numeral 19 denotes a catalyst provided in the exhaust passage 17 for purifying exhaust gas.

FIGS. 3 and 4 show details around the combustion chamber. In these figures, the cylinder head 4 has a cylinder bore left side portion that branches from one main intake port 22 on the left side wall side of the cylinder head 4 and guides fresh air to the cylinder 2.
Two independent intake ports 23 and 24 are provided. The independent intake ports 23 and 24 are provided such that one end is opened on the left inclined wall surface of the combustion chamber 6 and the other end is opened on the left side wall (main intake port 22) of the cylinder head. One end of the opening 25 is opened immediately upstream of the opening of the independent intake port 23 on the combustion chamber side.
And a sub-intake port that extends closely below the main intake port 22 and has the other end opened directly below the opening of the main intake port 22 on the left side wall of the cylinder head. The main intake port 22 and the sub intake port 2 on the left side wall of the cylinder head 4
5 is connected to the downstream end of the intake manifold 29 corresponding to each opening, and the downstream end passage corresponding to the opening of the main intake port 22 of the intake manifold 29 includes:
A secondary valve 30 is provided. The secondary valve 30 opens during high load operation to allow the intake air to flow through the main intake port 22, and closes during low load operation to close the main intake port 22.
Is controlled so as to cut off the flow of intake air and to flow the intake air to the sub intake port 25.

Two independent exhaust ports 27 and 28 for guiding exhaust gas of the cylinder 2 outward are provided on the right side of the cylinder bore of the cylinder head 4. One end of each of the independent exhaust ports 27 and 28 is opened on the inclined wall on the right side of the combustion chamber 6, and is connected to the independent intake port 2.
3 and 24 are arranged so as to be opposed to and close to the respective opening portions on the combustion chamber side, and the other ends are gathered in one exhaust port 26 on the right side wall of the cylinder head and open on the right side wall of the cylinder head. Is provided.

Valve seats 31, 31 and valve seats 32, 32 are provided at the respective opening portions on the combustion chamber side of the independent intake ports 23, 24 and the independent intake ports 23, 24, respectively. As shown in FIGS. 4 and 5, a protruding wall 33 extending in the lift direction of the intake valve 8 toward the combustion chamber is provided on the periphery of the exhaust port side of the valve seats 31, 31 of the independent intake ports 23, 24. Are formed. Here, in this embodiment,
As shown in FIG. 12, the valve lift amount Lit of the intake valves 8, 8 at the top dead center is set to 1 mm.
From FIG. 2, the crank angle between the top dead center and the exhaust valves 18, 18 after the top dead center until the exhaust valves 18, 18 are substantially closed is 25 ° (at high engine speed or high load).
The value A multiplied by 5 is 3.75 (mm). At this time,
The protrusion H of the protruding wall 33 from the valve seat 31 is defined as A
Or less, whichever is greater, ie 3.7 in this case.
It is set to 5 mm or less, and specifically, the protrusion H is set to 2 mm. The valve lift Lec of the intake valve 8 when the exhaust valve 18 is closed, that is, at the end of the overlap period, is set to 3 mm. Further, a gap α between the protruding wall 33 and the periphery of the large diameter portion 8a of the umbrella portion of the intake valve 8
Is set smaller than the protrusion H, and the gap β between the periphery of the large diameter portion 8a of the umbrella portion of the intake valve 8 and the combustion chamber wall on the side of the bore is set larger than the gap α. Have been.

With the above construction, in the above embodiment, the flow path from the exhaust port side of each of the openings of the independent intake ports 23 and 24 to the combustion chamber is formed by the protruding wall 33 in the above-described manner.
8 starts to open, and the valve lift is
It will be squeezed until it exceeds the overhang H of 3,
It is possible to prevent blow-by in which fresh air flowing from the independent intake ports 23 and 24 directly flows out to the independent exhaust ports 27 and 28 via the flow passages. In this case, since the protrusion H is smaller than the valve lift Lec of the intake valve 8 at the end of the overlap period, the flow passage is not restricted after the overlap period, and the overlap period is not increased. Shortly before the end, the aperture stops. This is because the valve lift of the exhaust valve 18 is small near the end of the overlap period,
During that period, the amount of fresh air blown through is small and the influence is small, but rather the influence of the intake resistance becomes large, so that this is suppressed. Therefore, the above-mentioned protruding wall 33 can effectively prevent fresh air from flowing through while minimizing an increase in intake air resistance.

At this time, the gap β between the periphery of the large diameter portion 8a of the head portion of the intake valve 8 and the combustion chamber wall on the side of the bore is defined by the protruding wall 33 and the large diameter portion 8a of the head portion of the intake valve 8. Clearance α with the periphery
Therefore, fresh air flows into the combustion chamber 6 from the periphery of the bore of the opening on the combustion chamber side of the independent intake ports 23 and 24. Then, the fresh air flowing from the bore peripheral side forms a vertical swirling flow (reverse tumble flow) flowing from the cylinder wall on the intake side to the exhaust side along the top surface of the piston 4 in the cylinder 2. It is possible to obtain a scavenging effect of pushing out the exhaust gas (residual gas) remaining in the cylinder 2 to the independent exhaust ports 27 and 28 at the time of the overlap, thereby improving the scavenging efficiency. In particular, in a supercharging operation region in which supercharging is performed by the supercharger 11, the pressurized intake air (fresh air) effectively pushes out residual gas, thereby further promoting the scavenging action. Thus, the temperature of the air-fuel mixture can be lowered by reducing the residual gas in the cylinder 2. Therefore, knock resistance in the supercharging operation region can be improved, and the output can be effectively improved. Therefore, in the above embodiment, it is possible to achieve both the prevention of fresh air blow-through and the improvement of scavenging efficiency, and to improve the emission performance, output, and fuel efficiency.

FIGS. 6 to 8 show the vicinity of a combustion chamber of an engine according to a second embodiment of the present invention, which differs from the first embodiment only in that a tumble generating wall is provided. This is the same as the first embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted. That is, in these figures, reference numeral 34 denotes the valve seats 31, 3 of the independent intake ports 23, 24.
A tumble generating wall is formed on the outer periphery of the bore on the outer peripheral side toward the combustion chamber side in the lift direction of the intake valve 8. The tumble generating wall 34 extends to the lower surface of the cylinder head 4, has a predetermined gap with the periphery of the large diameter portion 8 a of the umbrella portion of the intake valve 8, and protrudes from the valve seat 31. White H
The large diameter portion 8a of the umbrella portion of the intake valve 8
The recess 35 is provided so that the gap between the peripheral edge and the protruding wall 33 is larger than the gap α.

With the above arrangement, in this embodiment, when the valve lift of the intake valves 8, 8 is smaller than the protrusion H of the protruding wall 33, fresh air flows through the recess 35. The same effects as in the first embodiment can be obtained. Then, when the valve lift of the intake valves 8, 8 exceeds the protrusion H of the protruding wall 33, that is, from the time when the overlap period is substantially finished, the tumble generating wall 34 causes the independent intake port 23, The flow passage from the periphery of the bore of each opening of the combustion chamber 24 is narrowed, and fresh air mainly flows from the exhaust port side of each opening of the independent intake ports 23 and 24 on the combustion chamber side to the combustion chamber. Will flow in. Therefore, the fresh air flowing from the exhaust port-side flow passage promotes the generation of a vertical swirling flow (tumble flow) flowing in the cylinder 2 from the exhaust-side cylinder wall to the intake side along the top surface of the piston 4. And combustibility can be improved. Therefore, the output and the fuel consumption can be further improved by the improvement of the flammability.

FIGS. 9 to 11 show the surroundings of a combustion chamber of an engine according to a third embodiment of the present invention, which differs from the first embodiment only in that an exhaust-side protruding wall is provided. Are the same as those in the first embodiment, and the same portions are denoted by the same reference numerals and detailed description thereof will be omitted. That is, in these figures, reference numeral 36 denotes a valve seat 32 of the independent exhaust port 27, 28,
The exhaust-side protruding wall is formed to extend in the lift direction of the exhaust valve 18 along the intake port-side peripheral edge of the outer periphery 32. The protrusion F of the exhaust-side protruding wall 36 is determined by the valve lift amount Let of the exhaust valve 18 at the top dead center and the crank angle between the time when the intake valve 8 opens before the top dead center and the top dead center.
(°) multiplied by 0.15 (mm) , whichever is greater. That is, in this embodiment,
As shown in FIG. 12, the valve lift amount Let of the exhaust valve 18 at the top dead center is set to 2 mm (at the time of high engine speed or high load), and from FIG. The crank angle up to the top dead center is 15 °, and the value obtained by multiplying the crank angle by 0.15 is 2.25 (mm). Therefore, the protrusion F of the exhaust-side protrusion wall 36 is 2.
It is set to 25 mm or less, and specifically, the protrusion F is set to 2 mm. The valve lift amount Lio of the exhaust valve 18 when the intake valve 8 is opened, that is, at the start of the overlap period, is set to 3 mm. Also,
The gap γ between the periphery of the large diameter portion 18a of the exhaust valve 18 and the exhaust-side protruding wall 36 is set to be smaller than the projection F, and the large diameter portion 18a of the exhaust valve 18
The gap δ between the peripheral edge and the combustion chamber wall on the bore peripheral side is the above-mentioned gap γ
It is set larger than.

With the above configuration, in this embodiment, the independent exhaust ports 27 and 2 are provided by the exhaust side projecting wall 36.
8, the flow passage from the intake port side of each opening on the combustion chamber side is:
When the exhaust valve 18, 18 is closed and its valve lift becomes less than or equal to the projection F of the exhaust-side protruding wall 36, the valve is throttled.
It is possible to prevent blow-by in which fresh air that has flowed in from the ports 3 and 24 flows out to the independent exhaust ports 27 and 28 through the flow passages as they are. In this case, the protrusion F
Is smaller than the valve lift of the exhaust valve 18 at the beginning of the overlap period, so that the flow passage is not narrowed at the beginning of the overlap period, and the throttle is started a little after the start of the overlap period. Will be done. This is because the intake valve 8 immediately after the start of the overlap period
The valve lift is small and the amount of fresh air inflow is small,
The effect of blow-through is small, and in this case, the flow resistance of the exhaust gas due to the throttle, that is, the exhaust resistance is not increased, and the outflow of the exhaust gas is prioritized. This makes it possible to effectively prevent blow-through while minimizing an increase in exhaust resistance.
Further, since the exhaust gas flows out from the bore peripheral side of each opening of the independent exhaust ports 27 and 28 on the combustion chamber side, the exhaust gas is easily subjected to a scavenging action by fresh air flowing into the cylinder 2, thereby improving scavenging efficiency. Can be achieved. The above is obtained at the same time as the effect of the protruding wall 33 provided at the openings of the independent intake ports 23 and 24, and the synergistic effect thereof can further enhance the effect of the first embodiment.

In each of the above embodiments, the protrusion H of the protrusion wall 33 is set to 2 mm. However, it is usually desirable that the protrusion H is set to 1.5 to 2.5 mm. Similarly, for the projection F of the exhaust-side projection wall 36, 1.5
It is desirable to set it to 2.5 mm. Further, the same effect can be obtained even in a structure in which only the intake port for generating the tumble flow without the auxiliary intake port 25 and the secondary valve 30 is provided.

[0028]

As described above, according to the combustion chamber structure of the engine according to the first aspect of the present invention, the protruding wall provided at the intake port opening allows the fresh air to flow through while minimizing the intake resistance. Can be effectively prevented. At the same time, the fresh air forms a vertical swirling flow (reverse tumble flow) in the cylinder to scavenge the residual gas, thereby improving scavenging efficiency. Therefore, it is possible to achieve both prevention of fresh air blow-through and improvement of scavenging efficiency, and it is possible to improve emission performance, output, and fuel efficiency.

According to the second aspect of the present invention, when the present invention is applied to an engine having a supercharger and having a large valve overlap period, the operation of the first aspect of the present invention is particularly effective in the scavenging operation. The intake air (fresh air) pressurized by the feeder will effectively push out residual gas,
The scavenging action can be further promoted, and the residual gas in the cylinder can be reduced to lower the temperature of the air-fuel mixture. Therefore, knock resistance in the supercharging operation region can be improved, and the output can be effectively improved.

According to the third aspect of the present invention, the tumble generation wall can promote the generation of the tumble flow of the air-fuel mixture in the cylinder, thereby improving the combustibility.

According to the fourth aspect of the present invention, the exhaust-side protruding wall can further prevent the fresh air from flowing through and improve the scavenging action, thereby further improving the effect of the first aspect of the present invention. .

[Brief description of the drawings]

FIG. 1 is an overall schematic configuration diagram of an engine according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing valve timing.

FIG. 3 is a sectional view of a combustion chamber according to the first embodiment of the present invention.

FIG. 4 is a bottom view of the combustion chamber according to the first embodiment of the present invention.

FIG. 5 is an enlarged view of a main part of FIG. 3;

FIG. 6 is a diagram corresponding to FIG. 3 according to a second embodiment of the present invention.

FIG. 7 is a diagram corresponding to FIG. 4 according to a second embodiment of the present invention.

FIG. 8 is an enlarged view of a main part of FIG. 6;

FIG. 9 is a diagram corresponding to FIG. 3 according to a third embodiment of the present invention.

FIG. 10 is a diagram corresponding to FIG. 4 according to a third embodiment of the present invention.

FIG. 11 is an enlarged view of a main part of FIG. 9;

FIG. 12 is a characteristic diagram showing a valve lift amount.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine 4 Cylinder head 6 Combustion chamber 8 Intake valve 8a Head part large diameter part 11 Supercharger (supercharger) 18 Exhaust valve 18a Head part large diameter part 23, 24 Independent intake port 27, 28 Independent exhaust port 31 Valve seat ( Inlet port opening) 32 Valve seat (exhaust port opening) 33 Projection wall 34 Tumble generation wall 35 Recess 36 Exhaust side projection wall

Claims (4)

(57) [Claims] A plurality of intake port openings and exhaust port openings that open to a combustion chamber are arranged close to each other, and a lift of an intake valve is located closer to the combustion chamber than a valve seat at each of the intake port openings. In a combustion chamber structure of an engine having a projecting wall extending in the direction, the projecting wall is provided along a peripheral edge of each intake port opening on the exhaust port side, and a projecting margin of the projecting wall is Either the valve lift amount of the intake valve at the dead center or a value (mm) obtained by multiplying the crank angle (°) between the top dead center and the exhaust valve after the top dead center substantially closes by 0.15 by 0.15. The gap between the protruding wall and the large-diameter portion peripheral edge of the intake valve umbrella portion is set to be smaller than the projecting margin, and the large-diameter portion peripheral edge of the intake valve umbrella portion and the bore peripheral side are set to be smaller than the large value. The gap with the combustion chamber wall is the large diameter of the intake valve head A combustion chamber structure for an engine, wherein the clearance is set to be larger than a gap between a peripheral edge and the protruding wall. 2. The combustion chamber structure of an engine according to claim 1, wherein the engine is provided with a supercharger, and a valve overlap period in which the opening periods of the intake and exhaust valves overlap near the top dead center is set long. 3. An intake valve opening extends in the lift direction of the intake valve from the valve seat to the combustion chamber side along the peripheral edge of the intake port opening on the bore peripheral side, and has a predetermined clearance with the peripheral edge of the large diameter portion of the intake valve head portion. A tumble generating wall is provided, and in a portion between the valve seat of the tumble generating wall and a portion corresponding to the protrusion of the protruding wall, a concave portion that increases a gap between the large diameter portion periphery of the intake valve head portion and a concave portion is provided. The engine combustion chamber structure according to claim 1, wherein the combustion chamber structure is formed. 4. An exhaust-side protruding wall extending in the lift direction of the exhaust valve from the valve seat to the combustion chamber side along a peripheral edge of each exhaust port opening on the intake port side, and a protruding margin of the exhaust-side protruding wall. Is a value (mm ) obtained by multiplying the valve lift amount of the exhaust valve at the top dead center and the crank angle (°) from the opening time of the intake valve that opens before the top dead center to the top dead center by 0.15. ) , The gap between the periphery of the large diameter portion of the exhaust valve head and the protruding wall is set to be smaller than the protrusion, and the large diameter portion of the exhaust valve head. The engine combustion chamber structure according to claim 1, wherein a gap between a peripheral edge and a combustion chamber wall on a bore peripheral side is set to be larger than a gap between a large diameter portion peripheral edge of the exhaust valve head and the exhaust side protruding wall. .