EP2074291A2

EP2074291A2 - Internal combustion engine

Info

Publication number: EP2074291A2
Application number: EP07825226A
Authority: EP
Inventors: Takashi Amano
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2006-10-02
Filing date: 2007-10-01
Publication date: 2009-07-01
Also published as: WO2008041084A2; US20100037840A1; WO2008041084A3; WO2008041084A8

Abstract

When viewed from the upper face of a cylinder head (52A) in which an intake port (10A) communicating with a combustion chamber (54) of an internal combustion engine (50A) is formed, a starting point (Pl) of the port (10A) is defined as a point of intersection of the streamline of the intake port (10A) and an inlet-side opening plane of the intake port (10A) as projected on a horizontal plane (S), and an end point (P2) is defined as a point of intersection of the streamline of the port (10A) and the center -axis of an intake valve (55) as projected on the horizontal plane (S). The starting point (Pl) is located closer to the center of the chamber (54) than a straight line (L2) that contains the end point (P2) and extends in a direction orthogonal to the axis of the crankshaft on the plane (S), and the streamline (F) of the port (10A) projected on the horizontal plane (S) is curved toward the center of the chamber (54), with respect to a straight line (L1) that contains the starting point (P1) and extends in a direction orthogonal to the axis of the crankshaft on the plane (S).

Description

INTERNAL COMBUSTION ENGINE

BACKGROUND OF THE INVENTION 1. Field of the Invention [OOOl] The invention relates to an internal combustion engine, and in particular to an internal combustion engine in which a whirling airflow formed in a combustion chamber can be intensified even when a valve lift of intake valves is in a small to middle range.

2. Description of the Related Art

[0002] Various intake port designs have been proposed for the purpose of improving the manner in which intake air flows into a combustion chamber. For example, it has been proposed in Japanese Patent Application Publication No. 10-169453 (JP-A- 10- 169453) to provide a direct injection type internal combustion engine having intake ports that are inclined inwards, i.e., toward each other, such that the distance between two intake ports becomes gradually smaller as they get closer to the combustion chamber. In the direct injection type engine thus constructed, two tumble flows from the two intake ports join together so that strong turbulence is formed at around the ignition plug while a fuel-air mixture having a rich air fuel ratio is formed at around the ignition plug, whereby the engine can operate in a lean-burn mode with improved reliability. It has also been proposed in Japanese Patent Application Publication No. 09-236043 (JP-A-09-236043) to provide a cylinder head of an internal combustion engine having upright ports that are curved in the axial direction of a camshaft so as to secure spacing between the upright ports and inner pivots for swing arms. In the cylinder head of the engine in which the swing arms of the inner pivot type and the upright ports are both provided, the space available for formation of the upright ports is greatly restricted. Even in this case, deterioration of the intake air efficiency or other problems due to the space restrictions can be minimized. [0003] In the meantime, technologies for producing whirling airflows, such as tumbles or swirls, in cylinders of the engine are known in the art. By causing the engine of this type to produce a whirling airflow having an increased intensity, it may be possible to expand a lean-burn region in which the engine is operable in a lean-burn mode and improve the output performance. In this regard, the manner of introducing the intake air into the cylinder is one of the important factors in producing a high-intensity whirling airflow. With this background, technologies for improving the manner of introducing the intake air into the cylinder have been proposed in Japanese Patent Application Publication No. 07-279751 (JP-A-07- 279751) and Japanese Utility Model Application Publication No. 59-135335 (JP-U-59-135335).

[0004] In order to produce a high-intensity whirling airflow in the cylinder, it is necessary to design intake ports so as to achieve the optimum intake-port shape or arrangement that satisfies this requirement. However, the whirling airflow is intensified typically when the valve lift of the intake valves is mainly in a middle to high range. FIG. 39A and FIG. 39B schematically show typical examples of intake ports 1OX (lOXa and 10Xb in FIG. 39A) and 1OY (1OYa and 10Yb in FIG. 39B) along with a combustion chamber 54, intake valves 55 and exhaust valves 56. In FIG. 39A and FIG. 39B, the intake ports 1OX and 1OY as viewed from the upper face of the cylinder head are projected on a horizontal plane, and a point of intersection of the streamline of each intake port and the plane of the inlet-side opening of the intake port, as projected on the horizontal plane S, is defined as a starting point Pl, while a point of intersection of the streamline of the intake port and the center axis of the intake valve 55, as projected on the horizontal plane S, is defined as an end point P2. Here, the center axis of the intake valve 55 does not mean the axis of a stem of the intake valve, but means the axis that passes the center of an umbrella portion of the intake valve. Also in FIG. 39A and FIG. 39B, straight line Ll represents a straight line that contains the starting point Pl and extends in a direction orthogonal to the axis of the crankshaft on the horizontal plane S, and straight line L2 represents a straight line that contains the end point P2 and extends in a direction orthogonal to the axis of the crankshaft on the horizontal plane S.

[0005] In FIG. 39A, the intake ports 1OX (10Xa and 10Xb) are illustrated which are shaped such that a straight line that connects the starting point Pl with the end point P2 is substantially identical with the streamline F of the intake port projected on the horizontal plane S (which will be simply called "projected streamline of the intake port"), and such that the distance between the two intake ports 10Xa and 10Xb gradually increases in a direction from the starting point Pl to the end point P2. In FIG. 39B, the intake ports 1OY (lOYa and 10Yb) are illustrated which are shaped such that the projected streamline F of the intake port is not located closer to the center of the combustion chamber 54 than the straight line Ll, namely, the projected streamline F is not located on the inner side of the straight line Ll (in other words, is located on the outer side of the straight line Ll). With the intake ports 1OX and 1OY thus shaped, the intake air flows uniformly from the entire area of the downstream-side opening of the intake port 10 into the combustion chamber 54, and therefore, a whirling airflow is not favorably intensified when the valve lift of the intake valve is in a small to middle range. [0006] When the valve lift of the intake valve is in a small to middle range, in particular, a valve stem portion, for example, of the intake valve becomes a major obstacle to flow of the intake air since the mainstream of intake air normally has no particular directional characteristics, namely, the mainstream is not caused to flow in any particular direction. Thus, a whirling airflow formed in the combustion chamber, if any, is not always intensified as desired when the valve lift is in a small to middle range. Therefore, some room for improvements remains in the degree of mixing of the fuel-air mixture required for reducing emissions, such as HC and CO, and lessening deterioration of the fuel economy, frame propagation characteristics required for improvement of combustion during cold start or lean-burn operation, and the combustion speed required for preventing knocking. Nevertheless, no particular guidelines or schemes have been presented for designing the intake ports so that the whirling airflow can be favorably intensified even in the small to middle range of the valve lift. [0007] Another problem encountered when intake air flows into the cylinder is that the intake air interferes with the stem of the intake valve and is thus split into streams, whereby an intended flow of intake air cannot be formed in the combustion chamber. In this respect, when the intake valve is lifted largely, a large amount of intake air is intensely introduced into the cylinder, and therefore, a high-intensity whirling airflow is relatively easily produced in the cylinder even in the presence of the above problem. However, it is difficult to produce a whirling airflow having a sufficiently high intensity solely from the flow of intake air at the time when the intake valve is lifted high. It is thus necessary to improve the manner of introducing the intake air into the cylinder when the valve lift of the intake valve is in a small to middle range, so as to produce a whirling airflow having a sufficiently high intensity.

[0008] With the background as described above, it has been proposed in JP-A-07-279751 as identified above to offset the opening of the intake port, along with the intake valve, to the outer side of the combustion chamber, thereby to form a large quantity of intake airflow directed to the middle of the combustion chamber and draw the intake air toward the middle of the combustion chamber. According to the technology proposed in the above -identified publication, therefore, it may possible to improve the manner of introducing the intake air into the cylinder when the valve lift of the intake valve is in a small to middle range. With this technology, however, the intake air is split into branch streams by the stem of the intake valve, thus still leaving a large amount of intake air that does not flow toward the middle of the combustion chamber, which makes it difficult to provide a whirling airflow having a sufficiently high strength. Also, with the proposed technology, the intake air may be concentrated too much at around the middle of the combustion chamber when the intake valve is in a middle- to high-lift region, and the intake air flowing into the cylinder may hit against the wall of the cylinder at an excessively high velocity, which may result in a reduction of the intensity of the whirling airflow produced in the cylinder.

SUMMARY OF THE INVENTION

[0009] The present invention was developed in view of the above-described problems. Thus, it is an object of the invention to provide an internal combustion engine wherein intake ports are designed so that a whirling airflow produced in a combustion chamber can be intensified even when the valve lift of intake valves is in a small to middle range, and wherein the intake air can be introduced into the cylinder in a favorable manner so as to produce a whirling airflow in the cylinder, from the time when the valve lift of the intake valves is in a small to middle range.

[OOIO] According to one aspect of the invention, there is provided an internal combustion engine including an intake port that communicates with a combustion chamber, and an intake valve having an umbrella portion and a stem connected at one end thereof to the umbrella portion, wherein the intake port has a starting point that is a first point of intersection of a streamline of the intake port and an inlet-side opening plane of the intake port, and an end point that is a second point of intersection of the streamline of the intake port and a center axis of the intake valve, as viewed from an upper face of a cylinder head in which the intake port is formed, the first and second points of intersection being projected on a horizontal plane. In this internal combustion engine, the streamline of the intake port projected on the horizontal plane is curved toward a center of the combustion chamber so as to be at least partially located closer to the center of the combustion chamber than a first straight line that contains the starting point and extends in a direction orthogonal to an axis of a crankshaft on the horizontal plane, and a second straight line that contains the end point and extends in a direction orthogonal to the axis of the crankshaft on the horizontal plane. [0011] While the positional relationship between the starting point and the end point may vary widely in designing the intake port, the intake air drawn into the combustion chamber through the intake port flows outwards, or straight, or inwards, depending upon the design of the intake port. In the internal combustion engine having the above-described intake port design, the intake air flowing into the combustion chamber is given an increased directional characteristic due to the curved shape, so that the intake air is more likely to flow in a particular direction. Thus, even when the valve lift of the intake valve is in a small to middle range, a whirling airflow produced in the combustion chamber can be intensified.

[0012] In the internal combustion engine as described above, the starting point may be located closer to the center of the combustion chamber than the second straight line.

[0013] When the intake port is designed so that the starting point and the end point are positioned as described above, the intake air that flows through the intake port normally tends to be directed outwards as a whole when flowing into the combustion chamber, as shown in FIG. 39A. In the internal combustion engine having the intake port curved in the manner as described above, on the other hand, the mainstream of intake air, which has been deflected by an airflow control valve, or the like, can be directed so as to flow through the inner side of a valve stem portion of the intake valve when flowing into the combustion chamber.

Thus, the mainstream of intake air is prevented from interfering with the valve stem portion. Accordingly, a whirling airflow produced in the combustion chamber can be intensified even when the valve lift of the intake valve is in a small to middle range.

[0014] In the engine as described above, the starting point may lie on the second straight line.

[0015] When the intake port is designed so that the starting point and the end point are positioned as described above, the intake air that flows through the intake port normally tends to flow straight into the combustion chamber. In the internal combustion engine having the curved intake-port design as described above, on the other hand, the mainstream of intake air can be directed in the manner as described above, and therefore, a whirling airflow produced in the combustion chamber can be intensified even when the valve lift of the intake valve is in a small to middle range.

[0016] In the engine as described above, the starting point may not be located closer to the center of the combustion chamber than the second straight line. [0017] When the intake port is designed so that the starting point and the end point are positioned as described above, the intake air that flows through the intake port normally tends to be directed inwards as a whole when flowing into the combustion chamber. In the internal combustion engine having the curved intake-port deign as described above, the directional characteristic of the intake air can be enhanced (i.e., the intake air is further likely to be directed inwards), and therefore, a whirling airflow produced in the combustion chamber can be intensified even when the valve lift of the intake valve is in a small to middle range.

[0018] Thus, in the internal combustion engine as described above, the whirling airflow produced in the combustion chamber can be intensified even when the valve lift of the intake valve is in a small to middle range.

[0019] In a preferred embodiment of the invention, the intake valve is a specific intake valve in which the stem is offset such that an inner passage region located closer to the center of the combustion chamber, out of two intake- air passage regions on the opposite sides of a plane that contains a center axis of the stem, becomes larger, and such that the center axis of the stem does not contain a center of a bottom face of the umbrella portion.

[0020] In the engine according to the preferred embodiment as described above, the intake air that is about to flow into the combustion chamber toward the middle thereof is more likely to be prevented from interfering with the stem of the intake valve, thereby to form an increased quantity of intake-air flow toward the middle of the combustion chamber when the valve lift of the intake valve is in a small to middle range. Thus, the intake air can be introduced into the cylinder in a favorable manner so as to form a whirling airflow in the cylinder, from the time when the valve lift of the intake valve is in a small to middle range.

[0021] The preferred embodiment as described above is different from the technologies proposed in JP-AO7-279751 and JP-U-59-135335 as identified above, in that the stem is offset in the manner as described above, in view of the object of the invention to improve the manner of flowing of intake air from the time when the valve lift of the intake valve is in a small to middle range, and the level of the necessity to accomplish the object. In this respect, the stem of the intake valve is conventionally formed, in view of the strength and the ease in machining, such that the center axis of the stem contains the center of the bottom face of the umbrella portion, and the stem extends in a direction perpendicular to the bottom face. As compared with the case where the location of the intake valve is changed along with the position of the opening of the intake port so as to improve the manner of flowing of intake air without offsetting the stem, for example, it is advantageous or preferable to offset the stem in terms of the increased freedom in changes. In this respect, too, the manner in which the intake air flows into the cylinder can be more favorably improved in the engine as described above.

[0022] While the plane that contains the center axis of the stem is not necessarily limited to a single plane provided that the plane can divide the intake-air passage region into the inner side and outer side of the stem with respect to the combustion chamber, this plane is specified as a plane that divides the intake-air passage region into the inner side and the outer side with respect to the combustion chamber so that the mainstream of intake air that flows into the cylinder so as to produce a whirling airflow in the cylinder is mainly contained in the inner passage region. In this respect, in the engine in which the intake port is formed so as to produce a tumble flow as a whirling airflow in the cylinder, if the plane that contains the center axis of the stem is further made parallel to the axis of the cylinder, the mainstream of intake air is mainly contained in the inner passage region, and therefore, the intake air that is about to flow into the cylinder toward the middle of the combustion chamber is more likely to be prevented, with higher reliability, from interfering with the stem.

[0023] The above statement that "the inner passage region becomes larger" means that the inner passage region becomes larger as compared with the case where a conventional intake valve is provided in which the stem is not offset as in the preferred embodiment. In this respect, where the conventional intake valve in which the stem is not offset is provided in the engine in which the intake port is formed so as to produce a tumble flow in the cylinder, the intake-air passage region is usually supposed to be substantially equally divided into an inner passage region and an outer passage region by a plane parallel to the axis of the cylinder. In the preferred embodiment, therefore, the stem of the specific intake valve is offset such that, more specifically, the inner passage region becomes larger than the other region, i.e., the outer passage region.

[0024] The stem of the specific intake valve as indicated above may be offset to an upstream side with respect to the center of the specific intake valve, in a direction of flow of intake air.

[0025] As a specific method for offsetting the stem of the specific intake valve so as to improve the manner in which the intake air flows into the cylinder, the stem may be offset in a direction perpendicular to the direction of flow of intake air as viewed in a horizontal projection plane, and may be further offset to the downstream side with respect to the center of the specific intake valve in the direction of flow of intake air, or may be further offset to the upstream side with respect to the center of the specific intake valve in the direction of flow of intake air. Namely, the stem of the specific intake valve may be offset away from the plane that contains the center axis of the cylinder and is parallel to the flow of intake air. If the stem of the specific intake valve is further offset to the downstream side with respect to the center of the specific intake valve in the direction of flow of intake air, the flow of intake air may not be smoothly formed right above and downstream of the umbrella portion of the specific intake valve. It is, therefore, preferable that the stem is offset in the manner as described above, namely, is offset to the upstream side with respect to the center of the specific valve, in the direction of flow of intake air.

[0026] The specific intake valve may be formed such that a portion of the umbrella portion of the specific intake valve, which corresponds to the inner passage region, has a smaller volume than a portion of the umbrella portion which corresponds to the outer passage region.

[0027] In the engine as described just above, the inner passage region of intake air located close to the center of the combustion chamber can be made larger than the outer passage region, and therefore, an increased amount of intake air can be caused to flow toward the middle of the combustion chamber when the valve lift of the intake valve is in a small to middle range. Thus, the intake air can be introduced into the cylinder in a favorable manner so as to produce a whirling airflow in the cylinder, from the time when the valve lift of the intake valve is in a small to middle range. Also, even where the degree of offsetting of the stem, i.e., the offset amount of the stem, is reduced to be smaller than that of the intake valve in which the stem is offset without making the volume of the inner passage region larger, an equivalent effect can be provided, and therefore, the strength of the intake valve can be favorably maintained.

[0028] It is preferable that the umbrella portion of the specific intake valve is smoothly formed over the entire circumference thereof so as not to impede flow of intake air. To smoothly form the umbrella portion of the specific intake valve, at least a part of the umbrella portion of the specific intake valve may be formed in the shape of an arc in cross section. In this regard, a portion of the umbrella portion of the specific intake valve corresponding to the inner passage region and a portion corresponding to the outer passage region may be both formed in the shape of arcs in cross section, and the radius of curvature of the portion corresponding to the inner passage region may be made smaller than that of the portion corresponding to the outer passage region, so that the volume of the portion corresponding to the inner passage region can be easily made smaller than the portion corresponding to the outer passage region, as described above.

[0029] The specific intake valve may further include a rotation preventing device that prevents the specific intake valve from rotating about the center axis of the stem of the specific intake valve. [0030] When the specific intake valve rotates about the stem, the intake port may not be properly closed. This problem can be eliminated by providing the specific intake valve with the above-mentioned rotation preventing device.

[0031] In the engine as described above, the intake air can be introduced into the cylinder in a favorable manner so as to produce a whirling airflow in the cylinder, from the time when the valve lift of the intake valve is in a small to middle range.

[0032] In another preferred embodiment of the invention, the intake valve is a specific intake valve in which the stem is inclined, when the intake valve is in a closed state, such that a distal end of the stem is located closer to a plane that contains a center axis of a cylinder and is substantially orthogonal to the axis of the crankshaft, than a center of a bottom face of the umbrella portion, in a direction substantially parallel to the axis of the crankshaft.

[0033] In the engine in which the stem of the specific intake valve is inclined in the manner as described above, an increased flow of intake air is drawn toward the middle of the combustion chamber, so that the mainstream of intake air that flows into the cylinder toward the middle of the combustion chamber can be increased or intensified. Thus, according to the preferred embodiment, the intensity of a whirling airflow produced in the cylinder can be enhanced even when the valve lift of the intake valve is in a small to middle range, and the intake air can be introduced into the cylinder in a favorable manner so as to produce such a whirling airflow in the cylinder, from the time when the valve lift of the intake valve is in a small to middle range.

[0034] In the above^{^}described engine in which the stem of the specific intake valve is inclined, the umbrella portion of the intake valve is also inclined so that the intake air that flows along a portion of the umbrella portion of the specific intake valve which is closer to the center of the combustion chamber than the stem is particularly directed so as to be dispersed toward the periphery of the combustion chamber. Generally, the mainstream of intake air is more likely to be concentrated at around the middle of the combustion chamber as the lift of the intake valve becomes higher, resulting in an increased velocity of flow of intake air that flows toward the middle of the combustion chamber. In the engine according to the preferred embodiment as described above, on the other hand, the intake air that flows into the cylinder when the lift of the intake valve is in a middle to high range is favorably prevented from hitting against the wall of the cylinder at an excessively high velocity, and the intensity of a whirling airflow produced in the cylinder will not be reduced or less likely to be reduced. In the above description of the preferred embodiment, the phrase that "the specific intake valve is in a closed state" is used for denning the specific intake valve as that being in a certain state (i.e., closed state) by way of example. The same phrase wiD be used in the description of the following embodiment.

[0035] The stem of the specific intake valve may be inclined, when the specific intake valve is in a closed state, such that the distal end of the stem is located closer to an exhaust port than the center in a direction of flow of intake air. [0036] With the above arrangement, the intake air that flows into the cylinder along the umbrella portion can be further dispersed toward the bottom dead center of the cylinder. Thus, the intake air flowing into the cylinder is favorably prevented from hitting against the wall of the cylinder at an excessively high flow velocity, which would result in a reduction of the intensity of a whirling airflow produced in the cylinder. Furthermore, in the engine as described above, it may be possible to prevent the mainstream of intake air from being concentrated too much at around the middle of the combustion chamber, and further improve the intensity of the whirling airflow, depending upon the degree by which the stem of the specific intake valve is inclined in the direction of flow of intake air.

[0037] Thus, in the engine as described above, the intake air can be introduced into the cylinder in a favorable manner so as to produce a whirling flow in the cylinder from the time when the valve lift of the intake valve is in a small to middle range, and at the same time the intake air is prevented from hitting against the wall of the cylinder at an excessively high velocity, which would result in a reduction of the intensity of the whirling airflow produced in the cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The features, advantages, and technical and industrial significance of this invention will be better understood by reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawings in which:

FIG. 1 is a view schematically showing an intake port 1OA along with a principal part of an internal combustion engine 5OA; FIG. 2 is a schematic view of the intake ports 1OA that is illustrated three-dimensionally;

FIG. 3 is a view schematically showing the intake port 1OA as viewed in a horizontal plane on which it is projected;

FIG. 4 is a graph showing the relationship between the valve lift and the tumble intensity with respect to the intake port 1OA and a conventional intake port 10X;

FIG. 5 is a view schematically showing an intake port 1OB as viewed in a horizontal plane on which it is projected;

FIG. 6 is a view schematically showing an intake port 1OC as viewed in a horizontal plane on which it is projected;

FIG. 7 is a view schematically showing a principal part of an internal combustion engine IOOA associated with one cylinder, as viewed in a vertical cross- section; FIG. 8 is a view schematically showing a principal part of the internal combustion engine IOOA associated with one cylinder, as viewed in a horizontal plane on which that part of the engine IOOA is projected;

FIG. 9 is a view schematically showing intake valves 155A in a cross-section taken along line A" A as shown in FIG. 8; FIG. 10 is a graph showing the relationship between the tumble intensity and the valve lift;

FIG. 11 is a graph showing the relationship between the offset amount L, and the tumble intensity, valve strength and the flow rate of air,^"

FIG. 12 is a view schematically showing an intake valve 155Ab that is oriented in the same direction as in FIG. 8, as viewed in a direction perpendicular to a bottom face of its umbrella portion ub;

FIG. 13 is a view similar to that of FIG. 8 illustrating the principal part of the engine 10OA, in which the offset amount L is set to be larger than D/4i

FIG. 14 is a graph schematically showing the distribution of the flow velocity measured at the middle of a combustion chamber 154;

FIG. 15 is a graph showing the relationship between the offset amount L and the tumble intensity, with respect to the case where a stem stm is offset to the upstream side in a direction F of flow of intake air, and the case where the stem stm is offset to the downstream side J FIG. 16 is a view schematically showing an intake valve 155Bb in a manner similar to that of FIG. 12;

FIG. 17 is a graph schematically showing the distribution of the flow velocity measured at the middle of the combustion chamber 154;

FIG. 18A, FIG. 18B and FIG. 18C are views showing the fuel consumption characteristic of an internal combustion engine IOOB during lean-burn operation and the output performance thereof during high-load operation.'

FIG. 19 is a view schematically showing intake valves 155C in a cross-section similar to the A-A cross-section shown in FIG. 8; FIG. 20 is a view schematically showing the intake valve 155C in a manner similar to that of FIG. 12;

FIG. 21 is a graph schematically showing the distribution of the flow velocity measured at the middle of the combustion chamber 154;

FIG. 22 is a view schematically showing a principal part of an internal combustion engine IOOD associated with one cylinder, as viewed in a horizontal plane on which that part of the engine IOOD is projected;

FIG. 23 is a view schematically showing a principal part of an internal combustion engine IOOE associated with one cylinder, as viewed in a horizontal plane on which that part of the engine IOOE is projected; FIG. 24 is a graph showing the relationship between the tumble intensity and the valve lift;

FIG. 25 is a view schematically showing a principal part of an internal combustion engine 200A associated with one cylinder, as viewed in a vertical cross-section; FIG. 26 is a view schematically showing a principal part of the internal combustion engine 200A associated with one cylinder, as viewed in a horizontal plane on which that part of the engine 200A is projected;

FIG. 27 is a view schematically showing intake valves 255A in a cross-section taken along line B-B as shown in FIG. 26; FIG. 28A and FIG. 28B are views schematically showing the patterns of flow of intake air that flows into the cylinder;

FIG. 29 is a graph showing the relationship between the tumble intensity and the valve lift;

FIG. 3OA and FIG. 3OB are views schematically showing the patterns of flow of intake air that flows into the cylinder,^'

FIG. 31A and FIG. 3 IB are graphs schematically showing the distribution of the flow velocity measured at the middle of a combustion chamber 254;

FIG. 32 is a view schematically showing a principal part of an internal combustion engine 200B associated with one cylinder, as viewed in a horizontal plane on which that part of the engine 200B is projected, in a manner similar to that of FIG. 26;

FIG. 33 is a view schematically showing an intake valve 255Bb alone, which is oriented in the same direction as in FIG. 32; FIG. 34A and FIG. 34B are graphs schematically showing the distribution of the flow velocity measured at the middle of the combustion chamber 254;

FIG. 35A, FIG. 35B and FIG. 35C are views showing the fuel consumption characteristic of the internal combustion engine 200B during lean-burn operation and the output performance thereof during high-load operation,^" FIG. 36 is a view schematically showing a principal part of an internal combustion engine 200C associated with one cylinder, as viewed in a horizontal plane on which that part of the engine 200C is projected;

FIG. 37 is a view schematically showing a principal part of an internal combustion engine 200D associated with one cylinder, as viewed in a horizontal plane on which that part of the engine 200D is projected;

FIG. 38 is a graph showing the relationship between the tumble intensity and the valve lift; and

FIG. 39A and FIG. 39B are views schematically showing conventional intake ports 1OX and 1OY, along with a combustion chamber 54, intake valves 55 and exhaust valves 56.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] In the following description and the accompanying drawings, the present invention will be described in more detail with reference to exemplary embodiments.

[0040] Initially, a first embodiment of the invention will be described. FIG. 1 schematically shows an intake port 1OA (that represents intake port 10Aa and intake port 10Ab) having an intake port design of an internal combustion engine according to the first embodiment of the invention, along with a principal part of the internal combustion engine 5OA. The engine 5OA is a direct fuel injection type gasoline engine in which the fuel is injected directly into cylinders. It is, however, to be understood that the invention is not limitedly applied to this type of engine, but may be applied to other types of engines, rather than the direct fuel injection type gasoline engine. While the engine 5OA is an in-line four-cylinder engine having four cylinders arranged in a line, the cylinder arrangement and the number of cylinders are not limited to those of the engine 5OA, but may be selected as appropriate. While a principal part of the engine 5OA, more specifically, a principal part of a cylinder 51a as a typical cylinder, is illustrated in this embodiment, the rest of the cylinders are constructed similarly.

[0041] The engine 5OA includes a cylinder block 51, a cylinder head 52A, a piston 53, and other components. The cylinder 51a having a generally cylindrical shape is formed in the cylinder block 51. The piston 53 is received in the cylinder 51a. A cavity 53a that serves to guide tumble flow T is formed in the top face of the piston 53. The cylinder head 52A is fixed to the upper face of the cylinder block 51. A combustion chamber 54 is formed as a space surrounded by the cylinder block 51, cylinder head 52A and the piston 53. The cylinder head 52A is formed with intake ports 1OA (lOAa and 10Ab) through which intake air is drawn to the combustion chamber 54, and exhaust ports 20 (20a and 20b) through which combustion gas is discharged from the combustion chamber 54. Also, an intake valve 55 for opening and closing a channel of each intake port 1OA and an exhaust valve 56 for opening and closing a channel of each exhaust port 20 are mounted in the cylinder head 52A. In addition, an ignition plug and a fuel injection valve (not shown), for example, are mounted in the cylinder head 52A. [0042] The intake air flows into the combustion chamber 54 through the intake port 1OA after being deflected by an airflow control valve (not shown), so as to create a high-intensity tumble flow T in the combustion chamber 54. While an inlet-side opening of the intake port 1OA is formed in a side face of the cylinder head 52A in this embodiment, the intake port 1OA may be an upright port whose inlet-side opening is formed in the upper face of the cylinder head 52A. Also, a whirling airflow produced in the combustion chamber 54 is not limited to the tumble flow T, but may be, for example, a reverse tumble flow that circulates in the direction opposite to that of the tumble flow T as shown in FIG. 1, or a slanting tumble flow as a combination of the tumble flow T and a swirl flow.

[0043] FIG. 2 is a schematic view that illustrates the intake ports 1OA three- dime nsionally. The vertical direction as seen in FIG. 2 denotes a direction parallel to a direction in which the cylinder 51a extends, and the horizontal direction denotes a direction orthogonal to that direction. When viewed from the upper face of the cylinder head 52A, the intake ports 1OA (10Aa, 10Ab) are projected onto a horizontal plane S as shown in FIG. 2. Vertical planes G orthogonal to the axis of the crankshaft divide the horizontal plane S into the inner side and the outer side. In FIG. 2, a vertical plane Gl containing a starting point Pl and a vertical plane G2 containing an end point P2 are respectively illustrated. In this connection, the starting point Pl is a point of intersection of the streamline of the intake port 1OA and the inlet-side opening plane of the intake port 1OA as projected on the horizontal plane S, and the end point P2 is a point of intersection of the streamline of the intake port 1OA and the center axis of the intake valve 55 as projected on the horizontal plane S. In this embodiment, the center axis of the intake valve 55 is contained in the vertical plane G2. It is understood from FIG. 2 that the starting point Pl is located on the inner side of the vertical plane G2. It is also understood that a projected streamline F of the intake port 1OA, which is the streamline projected on the horizontal plane S, is curved inwardly of the vertical plane Gl. The horizontal plane S and the vertical plane Gl cross each other to form a straight line Ll (see FIG. 3), and the horizontal plane S and the vertical plane G2 cross each other to form a straight line L2 (see FIG. 3). In other words, the starting point Pl is located closer to the center of the combustion chamber than the straight line L2 that contains the end point P2 and extends in the direction orthogonal to the axis of the crankshaft on the horizontal plane, and the streamline of the intake port as projected on the horizontal plane is curved to be closer to the center of the combustion chamber than the straight line Ll that contains the starting point Pl and extends in the direction orthogonal to the axis of the crankshaft on the horizontal plane. [0044] FIG 3 schematically shows the intake ports 1OA (10Aa, 10Ab) as projected on the horizontal plane. In FIG. 3, the combustion chamber 54 and the intake and exhaust valves 55, 56 are illustrated along with the intake ports 1OA. As shown in FIG. 3, the starting point Pl is located on the inner side of the straight line L2. The projected streamline F of the intake port 1OA is curved inwardly of the straight line Ll. With this arrangement, a mainstream of intake air, which has been deflected by the airflow control valve, is directed mainly in the first half of the curved portion so as to be introduced into the combustion chamber 54 from between a valve stem portion of the intake valve 55 and the inner wall of the intake port 1OA. Namely, even if the positions of the starting point Pl and end point P2 have the relationship as shown in FIG. 3, the intake port 1OA having the thus curved intake port design is able to direct the mainstream in the manner as described above. The curved portion may be designed, more specifically, the position, length and degree of curvature of the curved portion may be determined, depending upon the specifications, such as an angle of inclination of the intake port 1OA.

[0045] FIG. 4 shows the relationship between the valve lift and the tumble intensity, with respect to the intake port 1OA having the intake port design according to the present embodiment and the conventional intake port 1OX as a comparative example. The intake port 1OX is designed as shown in FIG. 39A. In the present embodiment, the mainstream of intake air is focused on the inner side of the intake port 1OA, so that a flow that conforms to the umbrella shape of the intake valve 55 is formed when the valve lift is in a small to middle range, and the intake air is most smoothly introduced into the combustion chamber 54. As a result, the tumble intensity can be enhanced particularly in the small to middle range of the valve lift, as compared with the intake port 1OX. As is understood from the above description, the intake port 1OA provides an intake port design of the engine which makes it possible to intensify the whirling airflow produced in the combustion chamber, even when the valve lift of the intake valve 55 is in a small to middle range.

[0046] Next, an intake port 1OB having an intake port design according to a second embodiment of the invention will be described. The intake port 1OB is different from the intake port 1OA having the intake port design according to the first embodiment, in that the starting point Pl is located to be contained on a straight line L3 that contains the end point P2 and extends in a direction orthogonal to the axis of the crankshaft on the horizontal plane S, and that the projected streamline F of the intake port 1OB as projected on the horizontal plane S is curved inwardly of the straight line L3. FIG. 5 schematically shows the intake ports 1OB (lOBa, 10Bb) when projected on the horizontal plane. FIG. 5 also shows the combustion chamber 54 and the intake and exhaust valves 55, 56 along with the intake ports 1OB. It is understood from FIG. 5 that the starting point Pl is located on the straight line L3 in the intake port 1OB, and that the projected streamline F of the intake port 1OB is curved inwardly of the straight line L3.

[0047] With the above arrangement, the mainstream of the intake air is directed mainly before passing the curved portion so as to be introduced into the combustion chamber 54 from between the valve stem portion of the intake valve 55 and the inner wall of the intake port 1OB. Namely, even if the positions of the starting point Pl and end point P2 have the relationship as shown in FIG. 5, the intake port 1OB having the thus curved intake port design is able to direct the mainstream in the manner as described above. As is understood from the above description, the intake port 1OB provides an intake port design of the engine which makes it possible to intensify the whirling airflow produced in the combustion chamber, even when the valve lift of the intake valve 55 is in a small to middle range.

[0048] Next, an intake port 1OC having an intake port design according to a third embodiment of the invention will be described. The intake port 1OC is different from the intake port 1OA having the intake port design according to the first embodiment, in that the starting point Pl is not located on the inner side of the straight line L2 (namely, is located on the outer side), and the projected streamline F of the intake port 1OC as projected on the horizontal plane S is curved inwardly of the straight line L2. FIG. 6 schematically shows the intake ports 1OC (lOCa, lOCb) when projected on the horizontal plane. FIG. 6 also shows the combustion chamber 54 and the intake and exhaust valves 55, 56 along with the intake ports 1OC. As is understood from FIG. 6, the starting point Pl is located on the outer side of the straight line L2, and the projected streamline F of the intake port 1OC is curved inwardly of the straight line L2.

[0049] With the above arrangement, the mainstream of the intake air is directed mainly before passing the curved portion so as to be introduced into the combustion chamber 54 from between the valve stem portion of the intake valve 55 and the inner wall of the intake port 1OC. Namely, even if the positions of the starting point Pl and end point P2 have the relationship as shown in FIG. 6, the intake port 1OC having the thus curved intake port design is able to direct the mainstream in the manner as described above. When the starting point Pl is located on the outer side of the straight line L2, an intake port may be formed to provide a projected streamline F that is substantially identical with the straight line connecting the starting line Pl with the end point P2, so that the intake air is directed inwards as a whole and is thus introduced into the combustion chamber 54. However, the intake port 1OC having the intake port design according to the present embodiment can enhance the directional characteristics of the mainstream. Where the starting point Pl is located relatively close to the straight line L2, for example, the curved form of the intake port 1OC becomes particularly effective. As is understood from the above description, the intake port 1OC provides an intake port design of the engine which makes it possible to intensify the whirling airflow produced in the combustion chamber, even when the valve lift of the intake valve 55 is in a small to middle range.

[0050] The illustrated embodiments are preferred embodiments of the . invention. It is, however, to be understood that the invention is not limited to these embodiments, but may be otherwise embodied with various modifications without departing from the principle of the invention. For example, while the intake ports 1OA, 1OB and 1OC are independent ports in the illustrated embodiments, the intake port is not limited to this type, but may be a Siamese port in which the intake passage is divided into two branch passages at the downstream side, which join into a single passage at the upstream side.

[0051] Next, a fourth embodiment of the invention will be described. [0052] FIG. 7 schematically shows a principal part of an internal combustion engine IOOA according to the fourth embodiment of the invention. More particularly, FIG. 7 shows one cylinder of the engine IOOA as viewed in vertical cross- section. The engine IOOA is a direct fuel injection type gasoline engine, and employs a two-intake-valve structure in which each cylinder is provided with two intake valves. It is, however, to be understood that the engine IOOA is not limited to any particular type provided that the invention can be effectively practiced. For example, the engine may be a so-called lean-burn engine, or an engine having, for example, a three -intake -valve structure as described later. The engine IOOA may also have an appropriate number of cylinders and an appropriate cylinder arrangement.

[0053] The internal combustion engine IOOA has a cylinder block 151, a cylinder head 152, a piston 153, and other components. A cylinder 151a having a generally cylindrical shape is formed in the cylinder block 151, and the piston 153 is received in the cylinder 151a. The cylinder head 152 is fixed to the cylinder block 151. A combustion chamber 154 is formed as a space surrounded by the cylinder block 151, cylinder head 152 and the piston 153. The cylinder head 152 is formed with intake ports HOAa and HOAb (which will be simply and generically called "intake port HOA", this way of calling being applied to other components) through which the intake air is introduced into the combustion chamber 154 (or into the cylinder), and exhaust ports 120 (120a and 120b) through which combustion gas is discharged from the combustion chamber 154. Furthermore, an intake valve 155A for opening and closing each intake port HOA and an exhaust valve 156 for opening and closing each exhaust port 120 are respectively mounted in the cylinder head 152. The engine IOOA is provided with a rotation preventing means (not shown). The rotation preventing means may be implemented by, for example, forming a slit that extends in a direction in which a stem stm of the intake valve 155A extends, in the stem stm, and providing the cylinder head 152 with a stem holding part that engages with the slit. In this embodiment, the intake valves 155A (155Aa, 155Ab) are regarded as specific intake valves.

[0054] An ignition plug 157 is mounted in the cylinder head 152 such that its electrode protrudes from above into the combustion chamber 154. A fuel injection valve (not shown) is mounted in the cylinder head 152 such that its injection hole protrudes into the intake port HOA. The fuel injection valve is adapted to inject fuel directly into the cylinder 151a on the intake stroke. The fuel injection valve is not limited to this type or position, but may be mounted in the cylinder head 152 at a position closer to the cylinder block 151 than the intake port HOA such that its injection hole protrudes into the combustion chamber 154, or at a position above the combustion chamber 154. The intake air flowing from the intake port HOA into the cylinder 151a creates a whirling airflow in the cylinder. In this embodiment, the whirling- airflow is, specifically, in the form of a tumble flow T as shown in FIG. 7. A cavity that serves to guide the tumble flow T may be formed in the top face of the piston 153.

[0055] FIG. 8 schematically shows a principal part of the engine IOOA associated with one cylinder, as viewed in a horizontal plane on which that part of the engine IOOA is projected. FIG. 9 schematically shows the intake valves 155A (155Aa, 155Ab) in a cross-section taken along line A-A in FIG. 8. As shown in FIG. 8, the intake port IIOA extends long so as to cause intake air to flow toward the middle of the combustion chamber 154 as viewed in the horizontal projection plane. With this arrangement, the intake air flows through the intake port IIOA toward the middle of the combustion chamber 154, while forming a mainstream of intake air that produces a tumble flow T in the cylinder. Thus, the direction F of flow of intake air is substantially orthogonal to an axis L4 substantially parallel to the axis of the crankshaft, as shown in FIG. 8. In this connection, the direction F of flow of intake air indicates the direction in which the mainstream of the intake air flows so as to produce a tumble flow T in the cylinder, and may be represented by a direction of extension of the intake port IIOA in a portion where the stem stm is placed, depending upon the shape or design of the intake port HOA.

[0056] As shown in FIG. 8 and FIG. 9, the stem stm is offset such that the center axis Cl of the stem stm does not contain the center P2 of the bottom face of an umbrella portion ub of the intake valve 155A. In the present embodiment, the stem stm is offset in a direction substantially perpendicular to the direction F of flow of intake air as viewed in the horizontal projection plane, in other words, in a direction substantially parallel to the axis L4, as shown in FIG. 8. Here, an intake air channel of the intake port IIOA through which intake air passes is divided by a plane Sl containing the center axis Cl of the stem stm is divided into two regions, namely, an inner passage region inr and an outer passage region otr. The stem stm is offset such that the inner passage region inr located closer to the center of the combustion chamber 154 becomes larger than that of the case where the stem stm is not offset, more specifically, such that the inner passage region inr becomes larger than the outer passage region otr, as shown in FIG. 8. Thus, the intake air caused to flow toward the middle of the combustion chamber 154 is more likely to be prevented from interfering with the stem stm, as shown in FIG. 8. Accordingly, flow of intake air directed toward the middle of the combustion chamber 154 is more likely to be formed when the valve lift of the intake valve is in a small to middle range.

[0057] In FIG. 8, the plane Sl is further assumed to be a plane substantially parallel to the center axis C3 of the cylinder, and therefore, the plane Sl is represented by a straight line in FIG. 8. When a cross-section (a suitable section indicating the shape of the intake port IIOA at a certain position in the direction of extension of the intake port HOA, for example, a section perpendicular to the intake port IIOA or a section perpendicular to the direction F of flow of intake air) of the intake port IIOA is taken at a position where the stem stm, which may be received in a stem guide (not shown), is disposed in the intake port IIOA, the cross-section of the intake port IIOA is divided into two portions by the plane Sl, and these two portions respectively correspond to parts of the inner and outer passage regions inr, otr.

[0058] In this respect, it is preferable that a portion corresponding to the inner passage region inr is larger than a portion corresponding to the outer passage region otr in all of the sections of the intake port IIOA, but this is not always true, depending upon the design of the intake port HOA. As described above, the inner and outer passage regions inr, otr are intake-air passage regions taken at the position where the stem stm is disposed in the intake port HOA.

Thus, the inner and outer passage regions inr, otr do not include intake-air passage regions partitioned by the plane Sl at the location where the stem stm is not disposed in the intake port HOA.

[0059] FIG. 10 shows the relationship between the tumble intensity and the valve lift. The tumble intensity is represented by the number of tumble revolutions. FIG. 10 shows the results of comparison in terms of the tumble intensity between the engine IOOA and an internal combustion engine IOOX having intake valves whose steins stm are not offset, in place of the intake valves 155A. The engine IOOX is substantially identical with the engine IOOA except that the engine IOOX has different intake valves from those of the engine IOOA. It is understood from FIG. 10 that the tumble intensity is improved from the time when the valve lift of the intake valve is in a small to middle range in the engine IOOA, as compared with the engine IOOX.

[0060] Next, the offset amount L of the stem stm will be described in detail. FIG. 11 shows the relationship between the offset amount L, and the tumble intensity, valve strength and the flow rate of air. FIG. 12 schematically shows the intake valve 155Ab disposed on the right-hand side in FIG. 8, as viewed in a direction perpendicular to the bottom face of the umbrella portion ub, when the intake valve 115Ab is oriented in the same direction as that in FIG. 8. As shown in FIG. 12, the offset amount L is established which indicates a distance from the center P2 to the stem stm (more specifically, point P5 as a point of intersection of the bottom face of the umbrella portion ub and the center axis Cl of the stem stm). In FIG. 12, D represents a valve outside diameter, which is the outside diameter of the umbrella portion ub of the intake valve 155Ab. Also, the arrow "POSITIVE" associated with the offset amount L indicates a direction in which the stem stm is displaced away from the center axis C3 of the cylinder in a direction substantially orthogonal to the direction F of flow of intake air.

[0061] As shown in FIG. 11, when the offset amount L is in the range of 0 (zero) to D/12, the tumble intensity increases as the offset amount L increases. Even where the offset amount L further increases to be larger than D/12, the tumble intensity increases at a low rate as the offset amount L increases. Thus, if the offset amount L is in the range as indicated by the following expression (l), the tumble intensity can be improved.

0 < L ...(1)

[0062] If the offset amount L further increases to be around D/4, on the other hand, the valve strength of the intake valve 155A starts being largely reduced. If the offset amount L further increases to be larger than D/4, the valve strength is largely reduced, and the flow rate of air starts being largely reduced. The reduction in the flow rate of air is considered as being caused by an extreme reduction in the amount of intake air passing the outer passage region otr. It is thus preferable that the offset amount L is within a permissible range as indicated by the following expression (2). 0 < L ≤ D/4 ...(2)

[0063] Since the tumble intensity is in the middle of largely increasing when the offset amount L is in the range of 0 to D/12, as shown in FIG. 11, it is further preferable that the offset amount L is in a recommended range as indicated by the following expression (3). D/12 < L < D/4 ...(3)

[0064] FIG. 14 schematically shows the distribution of the velocity of flow of air measured at the middle of the combustion chamber 154. FIG. 14 shows the results of comparison in terms of the flow velocity between the engine IOOA and the engine 10OX. When the offset amount L is within the range as indicated by the following expression (4), the flow velocity obtained in the engine IOOA is larger than obtained in the engine 10OX, but is less than a predetermined value α.

0 < L < D/12 ...(4) [0065] If the offset amount L is within the recommended range as indicated by the above expression (3), on the other hand, the flow velocity becomes equal to or larger than the predetermined value α. As is understood from this result, it is further preferable that the offset amount L is within the recommended range as indicated by the expression (3). The range as indicated by the above expression (3) or (4) is considered as a range that can provide a reasonable effect even in the case where the position of the stem stm as viewed in the direction F of flow of intake air is changed. In FIG. 12, area ARl represents an area that corresponds to the range as indicated by the expression (4) and is considered as providing a reasonable effect, and area AR2 represents an area that corresponds to the range as indicated by the expression (3) and is considered as providing a reasonable effect. In the engine IOOA constructed as described above, the intake air can be introduced into the cylinder in a favorable manner so as to produce tumble flow T in the cylinder, from the time when the valve lift of the intake valve is in a small to middle range.

[0066] Next, a fifth embodiment of the invention will be described. An internal combustion engine IOOB according to the fifth embodiment is basically identical with the engine IOOA of the fourth embodiment, except that an intake valve 155B (which represents intake valve 155Ba and intake valve 155Bb) of the engine IOOB replaces the intake valve 155A of the engine IOOA. The intake valve 155B is different from the intake valve 155A in that a stem stm of the intake valve 155B is further offset to the upstream side of the center P2 as viewed in the direction F of flow of intake air. In this embodiment, the intake valve 155B is considered as a specific intake valve. [0067] FIG. 15 shows the relationship between the offset amount L and the tumble intensity with respect to the case where the stem stm is offset to the upstream side in the direction F of flow of intake air and the case where the stem stm is offset to the downstream side in the same direction. FIG. 16, which is similar to FIG. 12, schematically shows the intake valve 155Bb disposed on the right-hand side, like the intake valve 155Ab disposed on the right-hand side in FIG. 8. As shown in FIG. 16, an installation angle θ is established which represents an acute angle formed by a straight line L6 that is substantially orthogonal to the direction F of flow of intake air and contains the center P2 and a straight line L 7 that contains the center P2 and a point P5 that lies on the center axis Cl of the stem stm. The arrow "POSITIVE" associated with the installation angle θ in FIG. 16 indicates that the installation angle θ assumes a positive value when the stem stm is offset to the upstream side in the direction F of flow of intake air, as shown in FIG. 16. In FIG. 16, the offset amount L is set in the same manner as in FIG. 12. [0068] As described above with regard to the fourth embodiment, it is preferable that the offset amount L is within the recommended range as indicated by the above expression (3). If the installation angle θ is further set to be larger than about 70 degrees or smaller than about -70 degrees, the stem stm will be contained in the area ARl. Thus, it is preferable that the installation angle θ is within a permissible range as indicated by the following expression (5).

-70° < θ < 70° ...(5)

It is, however, to be noted that the tumble intensity is more or less improved if the stem stm is located in the area ARl, and therefore, a reasonable effect can be provided if the installation angle θ is equal to or larger than -90° and is equal to or smaller than 90°.

[0069] If the stem stm is offset to the upstream side with the installation angle θ set to 90°, the tumble intensity increases at a low rate as the offset amount

L increases, and then largely increases, as shown in FIG. 16. If the stem stm is offset to the downstream side with the installation angle θ set to -90°, the tumble intensity decreases at a low rate as the offset amount L increases, and then largely decreases. This may be because when the stem stm is offset to the downstream side, it is difficult to smoothly form flow of intake air to the downstream side, right above the umbrella portion ub. Thus, it is further preferable that the installation angle θ is in a recommended range as indicated by the following expression (6).

0° < θ < 70° ...(6)

[0070] Even in the case where the installation angle θ is set to within the above-described range, the valve strength is reduced as shown in FIG. 15 if the offset amount L is larger than D/4. Where the installation angle θ is established, therefore, it is preferable to form the stem stm so that the offset amount L and the installation angle θ are in the ranges of the expression (3) and expression (6), respectively. In FIG. 16, area AR3 represents an area corresponding to the case where the offset amount L is set to within the range as indicated by the expression (3), and the installation angle θ is set to within the range as indicated by the expression (6). In the present embodiment, therefore, the stem stm is offset so as to be contained in the area AR3 as shown in FIG. 16.

[0071] FIG. 17 schematically shows the distribution of the velocity of flow of air measured at the middle of the combustion chamber 154. FIG. 17 shows the results of comparison in terms of the flow velocity between the engine IOOB and the engine 10OX. When the stem stm of the intake valve 155B is located in the area AR3, the engine IOOB provides a flow velocity that is larger than a predetermined value β. The predetermined value β is larger than the above-mentioned predetermined value α. It follows that the engine IOOB provides a higher flow velocity than that of the engine IOOA as described above in the fourth embodiment of the invention.

[0072] FIG. 18A, FIG. 18B and FIG. 18C show the fuel consumption characteristic of the engine IOOB during lean-burn operation, and the output performance during high-load operation. The fuel consumption characteristic during lean-burn operation is specifically indicated in FIG. 18A by the relationship between the fuel consumption rate of the engine IOOB and the air-fuel ratio, and the output performance during high-load operation is specifically indicated in FIG. 18B by the relationship between the engine torque and the engine speed of the engine IOOB. In FIG. 18B, the output performance is indicated by the full-load performance. FIG. 18A and FIG. 18B show the results of comparison between the engine IOOX and the engine IOOB. FIG. 18C quantitatively indicates the fuel consumption reduction rate and output performance improvement rate that are expected to be achieved when the offset amount L is within the ranges of the expression (2) and expression (3), and also indicates the fuel consumption reduction rate and output performance improvement rate that are expected to be achieved when the installation angle θ is within the ranges of the expression (5) and expression (6).

[0073] When the stem stm is offset by an appropriate degree, a high-intensity tumble flow T can be formed in the cylinder and strong turbulence can be produced from the time when the valve lift of the intake valve is in a small to middle range. Therefore, the combustion characteristics are improved during lean-burn operation, resulting in a reduction of the fuel consumption rate, and the output performance can be improved during high-load operation. As shown in FIG. 18A, the fuel consumption rate is reduced in the engine 100B, as compared with the engine 10OX, and a lean-burn region in which the engine is operable at a lean air-fuel ratio is expanded. Also, as shown in FIG. 18B, the engine torque is improved over the entire range of the engine speed in the engine IOOB as compared with the engine 10OX. It will be understood from FIG. 18B that the degree of improvement of the engine torque increases as the engine speed decreases, and that the effect provided by offsetting the stem stm is greater as the engine speed is lower.

[0074] If the offset amount L is set to within the ranges of the expression (2) and the expression (3), the fuel consumption reduction rate and output performance improvement rate as quantitatively indicated in FIG. 18C are expected to be achieved. If the installation angle θ is set to within the ranges of the expression (5) and the expression (6), the fuel consumption reduction rate and output performance improvement rate as quantitatively indicated in FIG. 18C are expected to be achieved. In the engine IOOB constructed as described above, the intake air can be introduced into the cylinder in a favorable manner so as to produce tumble flow T in the cylinder, from the time when the valve lift of the intake valve is in a small to middle range.

[0075] Next, a sixth embodiment of the invention will be described. An internal combustion engine IOOC according to the sixth embodiment is substantially identical with the engine IOOB according to the fifth embodiment, except that an intake valve 155C (which represents intake valve 155Ca and intake valve 155Cb) of the engine IOOC replaces the intake valve 155B of the engine IOOB. The intake valve 155C is different from the intake valve 155B in that the volume of a portion of the umbrella portion ub corresponding to the inner passage region inr is made smaller than that of a portion of the umbrella portion ub corresponding to the outer passage region otr. More specifically, when the portion corresponding to the outer passage region otr is rotated about the center axis Cl of the stem stm, to be superimposed on the portion corresponding to the inner passage region inr, these portions do not coincide with each other, and the portion corresponding to the inner passage region inr is at least partially contained in the portion corresponding to the outer passage region otr. In this embodiment, the intake valve 155C is considered as a specific intake valve.

[0076] FIG. 19 schematically shows the intake valves 155C (155Ca and 155Cb) in cross section similar to the section A-A of FIG. 8. The intake valve 155C is specifically formed such that the portion of the umbrella portion ub corresponding to the inner passage region inr and the portion corresponding to the outer passage region otr are both formed in the shape of an arc as viewed in a cross-section provided by a plane containing the center axis Cl, and such that the portion corresponding to the inner passage region inr has a smaller radius of curvature than the portion corresponding to the outer passage region otr. The portion corresponding to the inner passage region inr, which has a radius R2 of curvature, and the portion corresponding to the outer passage region otr, which has a radius Rl of curvature, are respectively formed so as to be smoothly connected with the stem stm, and the radius R2 of curvature is set to be smaller than the radius Rl of curvature.

[0077] In the sixth embodiment in which the umbrella portion ub is formed in the manner as described above, the portion of the umbrella portion ub corresponding to the inner passage region inr has a smaller volume than the portion corresponding to the outer passage region otr. Thus, the inner passage region inr can be made larger, and therefore, further increased flow of intake air toward the middle of the combustion chamber 154 can be formed when the valve lift of the intake valve is in a small to middle range. The umbrella portion ub of the intake valve 155C is smoothly formed over the entire circumference so as not to impede flow of intake air. In this respect, the umbrella portions ub of the intake valves 155A and 115B are formed in a similar manner.

[0078] FIG. 20 schematically shows the intake valve 155C (lδδCb) disposed on the right-hand side, like the intake valve 155A disposed on the right-hand side in FIG. 8, in a manner similar to FIG. 12 or FIG. 16. In FIG. 20, the offset amount L is set in the same manner as in FIG. 12, and the installation angle θ is set in the same manner as in FIG. 16. In the engine IOOC having the intake valve 155C, further increased flow of intake air toward the middle of the combustion chamber 154 can be formed as described above. Therefore, even if the offset amount L is made smaller in the engine IOOC than in the engine IOOA or

10OB, the engine IOOC can provide an effect equivalent to that provided by the engine IOOA or 10OB. Thus, in the case of the engine IOOC, the recommended range of the offset amount L can be expanded as indicated by the following expression (7), as compared with the recommended range as indicated by the above expression (3).

D/24 < L ≤ D/4 ...(7)

In FIG. 20, area AR4 represents an area corresponding to the recommended range as indicated by the above expression (7), in which a reasonable effect is supposed to be provided. [0079] In the case where the installation angle θ is established, if the installation angle θ is larger than about 80 degrees or smaller than about -80 degrees when the offset amount L is D/4, the stem stm is contained in the above-mentioned area AR4. Thus, in the case of the engine IOOC, the permissible range of the installation angle θ can be expanded as indicated by the following expression (8), as compared with the permissible range as indicated by the above expression (5).

-80° < θ < 80° ...(8)

[0080] As in the case of the engine 10OB, it is preferable in the engine IOOC that the stem stm is offset to the upstream side relative to the center P2, in the direction F of flow of intake air. Thus, in the case of the engine lOOC, the recommended range of the installation angle θ can be expanded as indicated by the following expression (9), as compared with the recommended range as indicated by the above expression (6). 0° < θ < 80° ...(9)

[0081] As the ranges of the offset amount L and the installation angle θ are expanded as described above, the strength of the intake valve 155C can be more favorably maintained. Also, in the case of the engine 100C, the stem stm is preferably formed so as to satisfy both of the ranges indicated by the above expressions (7) and (9). In FIG. 20, area AR5 represents an area corresponding to the case where the offset amount L is set to within the range as indicated by the expression (7) and the installation angle θ is set to within the expression (9).

[0082] FIG. 21 schematically shows the distribution of the velocity of flow of air measured at the middle of the combustion chamber 154. FIG. 21 shows the results of comparison in terms of the flow velocity among the engine lOOC, the engine IOOB and the engine 10OX. In the engine IOOB and the engine lOOC compared with each other in FIG. 21, the offset amount L is set to the same value within the recommended range indicated by the above expression (3), and the installation angle θ is set to the same value within the recommended range indicated by the above expression (6). Also, in FIG. 21, the umbrella portion ub of the intake valve 155B of the engine IOOB is formed such that the portion of the umbrella portion ub corresponding to the inner passage region inr and the portion corresponding to the outer passage region otr have the same radius of curvature, and are smoothly connected with the stem stm. As shown in FIG. 21, the engine lOOC provides a larger flow velocity than the engine lOOC. In the engine lOOC constructed as described above, the intake air can be introduced into the cylinder in a favorable manner so as to form a tumble flow T in the cylinder, from the time when the valve lift of the intake valve is in a small to middle range.

[0083] Next, a seventh embodiment of the invention will be described. An internal combustion engine IOOD according to the seventh embodiment is different from the engines 10OA, IOOB and IOOC of the fourth through sixth embodiments as described above, in that the engine IOOD has a three-intake -valve structure, namely, each cylinder is provided with three intake valves. FIG. 22, which is similar to FIG. 8, schematically shows a principal part of the engine IOOD associated with one cylinder, as viewed in a horizontal plane on which that part of the engine IOOD is projected. In the engine IOOD, stems stm of intake valves 155D (155Da, 155Db) located at the opposite ends are offset in a manner similar to that of the intake valve 155A. Furthermore, the installation angles θ of the intake valves 155Da, 155Db may be set in a manner similar to that of the intake valve 155B, and each of the intake valves 155Da, 155Db may be formed such that a portion of the umbrella portion ub corresponding to the inner passage region inr has a smaller volume than a portion corresponding to the outer passage region otr, as in the intake valve 155 C. [0084] In the engine IOOD as shown in FIG. 22, the intake valves 155Da,

155Db located at the opposite ends with respect to one cylinder are regarded as specific intake valves. With the specific intake valves thus provided, increased flow of intake air toward the middle of the combustion chamber 154 can be formed when the valve lift of the intake valves is in a small to middle range, even where the engine IOOD has the three-intake-valve structure. In the engine IOOD constructed as described above, the intake air can be introduced into the cylinder in a favorable manner so as to produce a tumble flow T in the cylinder, from the time when the valve lift of the intake valve is in a small to middle range.

[0085] Next, an eighth embodiment of the invention will be described. An internal combustion engine IOOE according to the eighth embodiment is substantially identical with the engine IOOA according to the fourth embodiment, except that intake ports HOE (which represent intake port HOEa and intake port HOEb) are further provided with an airflow control valve 160 that deflects intake air in the intake ports HOE so as to create a high-intensity tumble flow T in the cylinder. Namely, the engine IOOE is equivalent to the engine lOOAthat is further equipped with the airflow control valve 160. The engines 10OB, lOOC-and IOOD according to the fifth through seventh embodiments may also be provided with airflow control valves that operate in substantially the same manner and provide substantially the same effect as the airflow control valve 160.

[0086] FIG. 23, which is similar to FIG. 8, schematically shows a principal part of the engine IOOE associated with one cylinder, as viewed in a horizontal plane on which that part of the engine IOOE is projected. One of the opposite edges of the airflow control valve 160 is pivotally supported by a valve stem 161, and the other edge of the airflow control valve 160 is formed with a recessed portion K for directing intake air toward the middle of the combustion chamber 154 when the valve 160 is closed. To cause the recessed portion K to direct the intake air toward the middle of the combustion chamber 154, the other edge of the airflow control valve 160 is recessed such that the depth of the recess increases as it approaches a portion of the valve 160 corresponding to the middle of the combustion chamber 154, and such that the recessed portion K is expanded at the center thereof. When the airflow control valve 160 is fully closed or half open, the intake air flowing through the intake port HOE passes the recessed portion K, thereby to be directed toward the middle of the combustion chamber 154. By directing the intake air toward the middle of the combustion chamber 154 before the intake air flows into the cylinder, the amount of intake air flowing toward the middle of the combustion chamber 154 can be increased. As a result, the amount of intake air that interferes with the stem stm of the intake valve 155E can be further reduced, so that the intake air can be introduced into the cylinder in a favorable manner.

[0087] FIG. 24 shows the relationship between the tumble intensity and the valve lift. FIG. 24 shows the results of comparison in terms of the tumble intensity, between the engine IOOE and an internal combustion engine IOOY having intake valves whose stems are not offset, in place of the intake valves 155E. The engine IOOY is substantially identical with the engine 100E, except that the intake valves of the engine IOOY are different from those of the engine 10OE. It is understood from FIG. 24 that the tumble intensity is improved when the airflow control valve 160 is fully closed, rather than when the valve 160 is fully open. It is also understood from FIG. 24 that in the engine 100E₁ as compared with the engine IOOY, the tumble intensity is improved when the airflow control valve 160 is fully closed from the time when the valve lift of the intake valves is in a small to middle range. In the engine IOOE constructed as described above, the intake air can be introduced into the cylinder in a favorable manner so as to produce a tumble flow T in the cylinder, from the time when the valve lift of the intake valve is in a small to middle range.

[0088] Next, a ninth embodiment of the invention will be described. FIG. 25 schematically shows a principal part of an internal combustion engine 200A according to the ninth embodiment of the invention. More particularly, FIG. 7 shows one cylinder of the engine 200A as viewed in vertical cross section. The engine 200A is a direct fuel injection type gasoline engine, and employs a two -intake -valve structure (i.e., each cylinder is provided with two intake valves). It is, however, to be understood that the engine 200A is not limited to any particular type provided that the invention can be effectively practiced. For example, the engine may be a so-called lean-burn engine, or an engine having, for example, a three -intake -valve structure as described later. The engine 200A may also have an appropriate number of cylinders and an appropriate cylinder arrangement.

[0089] The internal combustion engine 200A has a cylinder block 251, a cylinder head 252, a piston 253, and other components. A cylinder 251a having a generally cylindrical shape is formed in the cylinder block 251, and the piston 253 is received in the cylinder 251a. The cylinder head 252 is fixed to the cylinder block 251. A combustion chamber 254 is formed as a space surrounded by the cylinder block 251, cylinder head 252 and the piston 253. The cylinder head 252 is formed with intake ports 210Aa and 210Ab (which will be simply and generically called "intake port 210A", this way of calling being applied to other components) through which the intake air is introduced into the combustion chamber 254 (or into the cylinder), and exhaust ports 220 (220a and 220b) through which combustion gas is discharged from the combustion chamber 254. Furthermore, an intake valve 255A for opening and closing each intake port 210A and an exhaust valve 256 for opening and closing each exhaust port 220 are respectively mounted in the cylinder head 252. In this embodiment the intake valves 255A (255Aa, 255Ab) are regarded as specific intake valves. [0090] An ignition plug 257 is mounted in the cylinder head 252 such that its electrode protrudes from above into the combustion chamber 254. A fuel injection valve (not shown) is mounted in the cylinder head 252 such that its injection hole protrudes into the intake port 210A. The fuel injection valve is adapted to inject fuel directly into the cylinder 251a on the intake stroke. The fuel injection valve is not limited to this type or position, but may be mounted in the cylinder head 252 at a position closer to the cylinder block 251 than the intake port 210A such that its injection hole protrudes into the combustion chamber 254, or at a position above the combustion chamber 254. The intake air flowing from the intake port 210A into the cylinder 251a creates a whirling airflow in the cylinder. In this embodiment, the whirling airflow is, specifically, in the form of a tumble flow T as shown in FIG. 25. A cavity that serves to guide the tumble flow T may be formed in the top face of the piston 253.

[0091] FIG. 26 schematically shows a principal part of the engine 200A associated with one cylinder, as viewed in a horizontal plane on which that part of the engine 200A is projected. FIG. 27 schematically shows the intake valves 255A (255Aa, 255Ab) in a cross-section taken along line B-B in FIG. 26. As shown in FIG. 26, the intake port 210A extends long so as to cause intake air to flow toward the middle of the combustion chamber 254 as viewed in the horizontal projection plane. With this arrangement, the intake air flows toward the middle of the combustion chamber 254, while forming a mainstream of intake air that produces a tumble flow T in the cylinder. In this embodiment, the mainstream of intake air is formed by flow of intake air that flows in the direction F of flow of intake air as shown in FIG. 26, In this connection, the direction F of flow of intake air indicates the direction in which the mainstream of intake air flows so as to produce a whirling airflow in the cylinder, and may be represented by the direction of extension of the intake port 210A in a portion where the stem stm is placed, depending upon the shape or design of the intake port 210A.

[0092] In FIG. 26, straight line L4 is substantially parallel to the axis of the crankshaft, and plane S2 contains the center axis C3 of the cylinder, and is substantially orthogonal to the straight line L4, namely, is substantially orthogonal to the axis of the crankshaft. As shown in FIG. 26 and FIG. 27, the stem stm of the intake valve 255A is inclined so that, while the intake valve 255A is in the closed state, the distal end PIl of the stem stm is closer to the above-indicated plane S2 than the center P2 of the bottom face of the umbrella portion ub, as viewed in a direction substantially parallel to the axis of the crankshaft. The distal end PlI of the stem stm and the center P2 of the bottom face of the umbrella portion ub are both located on the center axis Cl of the stem stm. In the engine 200A in which the intake valves 255A are inclined as described above, an increased amount of intake air is drawn to the side closer to the middle of the combustion chamber 254, so that the mainstream of intake air that flows into the cylinder toward the middle of the combustion chamber 254 can be increased or intensified. Thus, the engine 200A is arranged to improve the intensity of the tumble flow T even when the valve lift of the intake valves is in a small to middle range.

[0093] In the engine 200A in which the stems stem of the intake valves 255A are inclined, the umbrella portions ub of the intake valves 255A are also inclined as shown in FIG. 27. In the engine 200A thus constructed, the intake air that flows along the umbrella portion ub on the side closer to the center of the combustion chamber 254 than the stem stm is directed in such a manner as to spread toward the periphery of the combustion chamber 254, as shown in FIG. 26. In the engine 200A₁ therefore, the intake air that flows into the cylinder when the valve lift of the intake valves is in a middle to high range is favorably prevented from hitting against the inner wall of the cylinder 251a at an excessively high velocity, which would result in a reduction of the intensity of the tumble flow T produced. Inter-stem angle Θ2 shown in FIG. 27 is an acute angle formed between the center axes Cl of the stems stm of the intake valves 255A (255Aa and 255Ab). The degree of inclination of the intake valves 255A can be changed through setting of this inter-stem angle Θ2.

[0094] In the engine 200A in which the mainstream of intake air is drawn toward the middle of the combustion chamber 254, the distance Lv between the valve seats on which the intake valves 255Aa, 255Ab rest as shown in FIG. 27 can be made larger than that in the case where the angle Θ2 is set to 0°. Namely, in the engine 200A, it is possible to draw the mainstream of intake air toward the middle of the combustion chamber 254 without reducing the distance Lv between the valve seats. Thus, in the engine 200A, as compared with the engine in which the inter-stem angle Θ2 is equal to 0°, it is possible to draw the mainstream of intake air toward the middle of the combustion chamber 254 while assuring sufficient strength of the combustion chamber 254.

[0095] FIG. 28A and FIG. 28B schematically show the patterns of flow of intake air that flows into the cylinder. More specifically, FIG. 28A schematically shows the pattern of flow of intake air in one cylinder of an internal combustion engine 200X as viewed in vertical cross section as in FIG. 25, and FIG. 28B schematically shows the pattern of flow of intake air in one cylinder of the engine 200A as viewed in vertical cross section as in FIG. 25. The engine 200X is substantially identical with the engine 200A except that the inter-stem angle Θ2 is set to 0° in the engine 200X. In the engine 200X as shown in FIG. 28A, the intake air is likely to hit against a wall of the cylinder 251a at an excessively high velocity when the valve lift of the intake valves is in a middle to high range, resulting in noticeable occurrence of flow Fs along the wall of the cylinder 251a, which causes a reduction in the strength of the tumble flow T produced in the cylinder. In the engine 200A as shown in FIG. 28B, on the other hand, the intake air flows into the cylinder while being dispersed by an appropriate degree when the valve lift of the intake valves is in a middle to high range. Therefore, the above-mentioned flow Fs along the wall of the cylinder 251a is less likely to occur, and the otherwise possible reduction in the strength of the tumble flow T produced can be lessened or prevented. [0096] FIG. 29 shows the relationship between the tumble intensity and the valve lift. The tumble intensity is represented by the number of tumble revolutions. FIG. 29 shows the results of comparison in terms of the tumble intensity between the engine 200X and the engine 200A. It is understood from FIG. 29 that the tumble intensity is improved in the engine 200A from the time when the valve lift of the intake valves is in a small to middle range, as compared with that of the engine 200X.

[0097] Next, the inter-stem angle Θ2 will be explained in detail. In a common internal combustion engine, the inter-stem angle Θ2 is set to 0°. If the angle Θ2 is within a range as indicated by the following expression (10), on the other hand, the tumble intensity can be improved. 0° < θ2 ...(10)

[0098] As the inter-stem angle Θ2 increases, however, it becomes physically difficult to appropriately place cams (not shown) for opening and closing the intake valves 255A. Accordingly, it is preferable in view of the placement of the cams that the angle Θ2 is within a permissible range as indicated by the following expression (ll).

0° < θ2 < 10° ...(H)

[0099] When the angle Θ2 is set to be larger than 0°, the intake air flowing into the cylinder is dispersed toward the periphery of the combustion chamber 254. If the dispersion occurs excessively, however, the tumble intensity may be reduced, rather than improved. FIG. 3OA and FIG. 3OB schematically show the patterns of flow of intake air that flows into the cylinder in the engine 200A in which the interstem angle Θ2 is set to be larger than 6° and equal to or smaller than 10°. More specifically, FIG. 30A schematically shows the pattern of flow of intake air in one cylinder of the engine 200A as viewed in vertical cross section as in (the case of) FIG. 25, and FIG. 3OB schematically shows the form of flow (manner of flowing) of intake air in (one cylinder of) the engine 200A as viewed in a cross-section similar to (the) B-B (cross-)section as shown in FIG. 26. [0100] If the inter-stem angle Θ2 is set to be larger than 6° and equal to or smaller than 10°, there may arise a situation where the intake air flowing into the cylinder is excessively dispersed, as shown in FIGS. 3OA and 30B, depending upon the shape of the combustion chamber 254. If the angle Θ2 is set to be larger than 0° and less than 1°, on the other hand, a significant effect cannot be expected. Thus, it is further preferable that the interstem angle Θ2 is in a recommended range as indicated by the following expression (12). 1° < Θ2 < 6° ...(12)

[0101] FIG. 31A and FIG. 31B schematically show the distribution of the velocity of flow of air measured at the middle of the combustion chamber 254. FIG. 31A and FIG. 3 IB show the results of comparison in terms of the flow velocity between the engine 200A in which the inter-stem angle Θ2 is within the range of the above expression (12), and the engine X. More specifically, FIG. 31A shows the distribution of the flow velocity when the valve lift of the intake valves is in a small to middle range, and FIG. 31B shows the distribution of the flow velocity when the valve lift of the intake valves is in a middle to high range. When the inter-stem angle 02 is within the range as indicated by the expression (12), the engine 200A provides a flow velocity that is larger than a predetermined value α2 as shown in FIG. 31A when the valve lift of the intake valves is in a small to middle range. The predetermined value α2 is larger than the maximum flow velocity that can be achieved by the engine 200X. When the valve lift of the intake valves is in a middle to high range, on the other hand, the engine 200A provides a flow velocity that is equal to or larger than a predetermined value β2, as shown in FIG. 3 IB. The predetermined value β2 is larger than the predetermined value α2, and is smaller than a predetermined value γ2. If the flow velocity becomes higher than the predetermined value γ2, flow Fs along the wall of the cylinder 251a is likely to occur, and the tumble intensity is reduced.

[0102] FIG. 3 IB also shows the velocity of flow of air in the engine 200A (hereinafter referred to as "engine 200A- 1") as a comparative example in which the inter-stem angle Θ2 is set to be larger than 6° and equal to or smaller than 10°. It is understood from FIG. 31B that, in the engine 200A-1, the mainstream of intake air is drawn too much to the middle of the combustion chamber 254, and the flow velocity increases excessively to be larger than the predetermined value γ2. In the engine 200A in which the inter-stem angle Θ2 is within the range of the above expression (12), on the other hand, the intake air that flows into the cylinder can be spread out by an appropriate degree to the periphery of the combustion chamber 254. Thus, in the engine 200A, the width of distribution of the flow velocity over which the flow velocity is equal to or larger than the predetermined value β2 can be made larger than a predetermined width W2, and the mainstream of intake air is prevented from being concentrated too much at the middle of the combustion chamber 254, so that the flow velocity becomes equal to or larger than β2 and is smaller than the predetermined value γ2 when the valve lift of the intake valves is in a middle to high range. In the engine 200A constructed as described above, the intake air can be introduced into the cylinder in a favorable manner so as to form a tumble flow T in the cylinder from the time when the valve lift of the intake valve is in a small to middle range, and the intake air flowing into the cylinder is prevented from hitting against the wall of the cylinder 251a at an excessively high velocity, which would result in a reduction in the intensity of the tumble flow T formed in the cylinder. [0103] Next, a tenth embodiment of the invention will be described. An internal combustion engine according to the tenth embodiment is substantially identical with the engine 200A of the ninth embodiment, except that when the intake valve 255A is in the closed state, the stem stm of the intake valve 255A is inclined such that the distal point PlI of the stem stm is located closer to the exhaust port 220 than the center P2 as viewed in the direction F of flow of intake air. The intake valve 255A inclined in this manner will be hereinafter referred to as intake valve 255B (which represents intake valve 255Ba and intake valve 255Bb). In this embodiment, the intake valve 255B is regarded as a specific intake valve. FIG. 32, which is similar to FIG. 26, schematically shows a principal part of the engine 200B associated with one cylinder, as viewed in a horizontal plane on which that part of the engine 200B is projected. FIG. 33 schematically shows the intake valve 255Bb alone, which is disposed on the right-hand side in FIG. 32 and is oriented in the same direction as in FIG. 32. [0104] In FIG. 32 and FIG. 33, installation angle Θ3 is defined as an acute angle formed between a straight line L6 that contains the center P2 and is substantially orthogonal to the direction F of flow of intake air, and the center axis Cl of the stem stm, as viewed in the horizontal projection plane. By suitably setting the installation angle Θ3, the stem stm may also be inclined in the direction F of flow of intake air. The arrow "POSITIVE" associated with the installation angle Θ3 in FIG. 33 indicates that the installation angle Θ3 assumes a positive value when the distal end PlI of the stem stm is located closer to the exhaust port 220 than the center P2 as viewed in the direction F of flow of intake air.

[0105] When the stem stm of the intake valve 255B is inclined with the installation angle Θ3 being set to 90°, it is difficult to draw the intake air toward the middle of the combustion chamber 254. Where the installation angle Θ3 is reduced from 90°, too, a significant effect cannot be expected if the degree of the reduction is small. If the installation angle Θ3 is smaller than 0°, on the other hand, it may be difficult to smoothly form flow of intake air to the downstream side, right above the umbrella portion ub. Thus, it is preferable that the installation angle Θ3 is within in a permissible range as indicated by the following expression (13).

0° < θ3 < 70° ...(13) [0106] If the installation angle θ is set to within the range of the above expression (13), the umbrella portion ub is further inclined, and the intake air flowing along the umbrella portion ub on one side of the stem stm closer to the center of the combustion chamber 254 is also dispersed toward the bottom dead center of the cylinder 251a. As a result, the intake air flowing into the cylinder is more favorably prevented from hitting against the wall of the cylinder 251a, which would result in noticeable occurrence of flow Fs along the wall of the cylinder 251a. Depending upon the installation angle Θ3, which is in the range of the expression (13), the dispersion of the intake air may contribute to production of the tumble flow T, or the intake air may be introduced into the cylinder in a manner suitable for production of the tumble flow T. In this respect, where the tumble intensity is to be further improved, it is further preferable that the installation angle Θ3 is within a recommended range as indicated by the following expression (14). 10° < θ3 < 60° ...(14) [0107] FIG. 34A and FIG. 34B schematically show the distribution of the velocity of flow of air measured at the middle of the combustion chamber 254. FIG. 34A and FIG. 34B show the results of comparison in terms of the flow velocity between the engine 200B in which the installation angle Θ3 is within the range of the above expression (14), and the engine 200X. More specifically, FIG. 34A shows the distribution of the flow velocity when the valve lift of the intake valve is in a small to middle range, and FIG. 34B shows the distribution of the flow velocity when the valve lift is in a middle to high range. The inter-stem angle Θ2 is suitably set within the range of the above expression (10) in accordance with the set value of the installation angle Θ3. When the installation angle Θ3 is within the range of the above expression (14), the engine 200B provides a flow velocity that is larger than the predetermined value α2 as shown in FIG. 34A, when the valve lift of the intake valve is in a small to middle range. The predetermined value cc2 is larger than the maximum flow velocity that can be achieved by the engine 200X. When the valve lift of the intake valve is in a middle to high range, on the other hand, the engine 200B provides a flow velocity that is larger than a predetermined value β2', as shown in FIG. 34B. The predetermined value β2' is larger than the predetermined value β2, and is smaller than the predetermined value γ2.

[0108] FIG. 34B also shows the velocity of flow of air with respect to the engine 200B (hereinafter referred to as "engine 200B- 1") as a comparative example in which the mainstream of intake air is drawn too much to the middle of the combustion chamber 254, and the flow velocity increases excessively to be larger than the predetermined value γ2. In the engine 200B, on the other hand, the intake air that flows into the cylinder can be dispersed by an appropriate degree toward the periphery and bottom of the combustion chamber 254, so that the width or range of distribution of the flow velocity over which the flow velocity is equal to or larger than the predetermined value β2 can be made larger than the predetermined width W2. Thus, the mainstream of intake air is prevented from being concentrated too much at the middle of the combustion chamber 254, so that the flow velocity becomes larger than the predetermined value β2' and smaller than the predetermined value γ2 when the valve lift of the intake valve is in a middle to high range. In FIG. 33, area ARIl represents an area or range of installation angle Θ3 corresponding to the case where the flow velocity becomes equal to or larger than the predetermined value α2 and smaller than the predetermined value β2 in the engine 200B₁ and area AR12 represents an area or range of installation angle Θ3 corresponding to the case where the flow velocity becomes equal to or larger than the predetermined value β2. In the present embodiment, the installation angle Θ3 of the intake valve 255B is set to a value included in the area AR12.

[0109] FIG. 35A, FIG. 35B and FIG. 35C show the fuel consumption characteristic of the engine 200B during lean-burn operation, and the output performance during high-load operation. The fuel consumption characteristic during lean-burn operation is specifically indicated in FIG. 35A by the relationship between the fuel consumption rate of the engine 200B and the air-fuel ratio, and the output performance during high-load operation is specifically indicated in FIG. 35B by the relationship between the engine torque and the engine speed of the engine 200B. In FIG. 35B, the output performance is indicated by the full-load performance. FIG. 35A and FIG. 35B show the results of comparison between the engine 200X and the engine 200B. FIG. 35 C quantitatively indicates the fuel consumption reduction rate and output performance improvement rate that are expected to be achieved when the inter-stem angle Θ2 is within the ranges of the expression (ll) and the expression (12), and also indicates the fuel consumption reduction rate and output performance improvement rate that are expected to be achieved when the installation angle Θ3 is within the ranges of the expression (13) and the expression (14).

[0110] When the stem stm is inclined by an appropriate degree, through setting of the inter-stem angle Θ2 and the installation angle Θ3, a high-intensity tumble flow T can be formed in the cylinder and strong turbulence can be produced from the time when the valve lift of the intake valve is in a small to middle range. Therefore, the combustion characteristics are improved during lean-burn operation, resulting in a reduction of the fuel consumption rate, and the output performance can be improved during high-load operation. As shown in FIG. 35A, the fuel consumption rate is reduced in the engine 200B, as compared with the engine 200X, and a lean-burn region in which the engine is operable at a lean airfuel ratio is expanded. Also, as shown in FIG. 35B, the engine torque is improved over the entire range of the engine speed in the engine 200B as compared with the engine 200X. It will be understood from FIG. 35B that the degree of improvement of the engine torque increases as the engine speed decreases, and that a greater effect is provided by inclining the stem stm through suitable setting of the inter-stem angle Θ2 and the installation angle Θ3 as the engine speed is lower.

[0111] If the inter-stem angle Θ2 is set to within the ranges of the expression (11) and the expression (12), the fuel consumption reduction rate and output performance improvement rate as quantitatively indicated in FIG. 35C are expected to be achieved. If the installation angle Θ3 is set to within the ranges of the expression (13) and the expression (14), the fuel consumption reduction rate and output performance improvement rate as quantitatively indicated in FIG. 35C are expected to be achieved. In the engine 200B constructed as described above, the intake air can be introduced into the cylinder in a favorable manner so as to form a tumble flow T in the cylinder from the time when the valve lift of the intake valve is in a small to middle range, and the intake air flowing into the cylinder is prevented from hitting against the wall of the cylinder 251a at an excessively high velocity, which would result in a reduction in the intensity of the tumble flow T formed in the cylinder. [0112] Next, an eleventh embodiment of the invention will be described.

An internal combustion engine 200C according to the eleventh embodiment is different from the engines 200A and 200B according to the ninth and tenth embodiments in that the engine 200C has a three-intake-valve structure, namely, each cylinder is provided with three intake valves. FIG. 36, which is similar to FIG. 26, schematically shows a principal part of the engine 200C associated with one cylinder, as viewed in a horizontal plane on which that part of the engine 200C is projected. In the engine 200C, the inter-stem angle θ formed between the stems stm of intake valves 255Ca, 255Cb located at the opposite ends is set in a manner similar to that of the intake valves 255A. Furthermore, the installation angle Θ3 of the intake valves 255Ca₁ 255Cb may be set in a manner similar to that of the intake valve 255B.

[0113] In the engine 200C, the intake valves 255Ca, 255Cb located at the opposite ends with respect to one cylinder are regarded as specific intake valves. With the specific intake valves thus provided, the mainstream of intake air flowing toward the middle of the combustion chamber 254 can be increased or intensified when the valve lift of the intake valves is in a small to middle range, and occurrence of flow Fs along the wall of the cylinder 251a can be suppressed when the valve lift of the intake valves is in a middle to high range, even where the engine 200C has the three -intake -valve structure. In the engine 200C constructed as described above, the intake air can be introduced into the cylinder in a favorable manner so as to form a tumble flow T in the cylinder from the time when the valve lift of the intake valve is in a small to middle range, and the intake air flowing into the cylinder is prevented from hitting against the wall of the cylinder 251a at an excessively high velocity, which would result in a reduction in the intensity of the tumble flow T formed in the cylinder.

[0114] Next, a twelfth embodiment of the invention will be described. An internal combustion engine 200D according to the twelfth embodiment is substantially identical with the engine 200A according to the ninth embodiment, except that intake ports 210D (which represent intake port 210Da and intake port 210Db) are further provided with an airflow control valve 260 that deflects intake air in the intake ports 210D so as to create a high-intensity tumble flow T in the cylinder. Namely, the engine 200D is equivalent to the engine 200A that is further equipped with the airflow control valve 260. The engines 200B and 200C according to the tenth and eleventh embodiments may also be provided with airflow control valves that operate in substantially the same manner and provide substantially the same effect as the airflow control valve 260.

[0115] FIG. 37, which is similar to FIG. 26, schematically shows a principal part of the engine 200D associated with one cylinder, as viewed in a horizontal plane on which that part of the engine 200D is projected. One of the opposite edges of the airflow control valve 260 is pivotally supported by a valve stem 261, and the other edge of the airflow control valve 260 is formed with a recessed portion K for directing intake air toward the middle of the combustion chamber 254 when the valve 260 is closed. To cause the recessed portion K to direct the intake air toward the middle of the combustion chamber 254, the other edge of the airflow valve 260 is recessed such that the depth of the recess increases as it approaches a portion of the valve 260 corresponding to the middle of the combustion chamber 254, and such that the recessed portion K is expanded at the center thereof. When the airflow control valve 260 is fully closed or half open, the intake air flowing through the intake ports 210A passes the recessed portion K, thereby to be directed toward the middle of the combustion chamber 254. By directing the intake air toward the middle of the combustion chamber 254 before the intake air flows into the cylinder, the mainstream of intake air flowing toward the middle of the combustion chamber 254 can be increased or intensified.

[0116] FIG. 38 shows the relationship between the tumble intensity and the valve lift. FIG. 38 shows the results of comparison in terms of the tumble intensity, between an internal combustion engine 200Y and the engine 200D of this embodiment. The engine 200Y is substantially identical with the engine 200D, except that the inter-stem angle Θ2 is set to 0° in the engine 200Y. It is understood from FIG. 38 that the tumble intensity is improved when the airflow control valve 260 is fully closed, rather than when the valve 260 is fully open. It is also understood from FIG. 38 that in the engine 200D, as compared with the engine 200Y, the tumble intensity is improved when the airflow control valve 260 is fully closed from the time when the valve lift of the intake valves is in a small to middle range. In the engine 200D constructed as described above, the intake air can be introduced into the cylinder in a favorable manner so as to form a tumble flow T in the cylinder from the time when the valve lift of the intake valve is in a small to middle range, and the intake air flowing into the cylinder is prevented from hitting against the wall of the cylinder 251a at an excessively high velocity, which would result in a reduction in the intensity of the tumble flow T formed in the cylinder.

[0117] The illustrated embodiments are preferable embodiments of the invention. It is, however, to be understood that the invention is not limited to these embodiments, but may be embodied with various modifications or improvements, without departing from the principle of the invention.

Claims

CLAIMS:

1. An internal combustion engine (50A) including an intake port (lOA) that communicates with a combustion chamber (54), and an intake valve (55) having an umbrella portion (ub) and a stem (stm) connected at one end thereof to the umbrella portion (ub), said intake port (lOA) having a starting point (Pl) that is a first point of intersection of a streamline (F) of the intake port (lOA) and an inlet-side opening plane of the intake port (lOA), and an end point (P2) that is a second point of intersection of the streamline (F) of the intake port (lOA) and a center axis of the intake valve (55), as viewed from an upper face of a cylinder head (52A) in which the intake port (lOA) is formed, said first and second points of intersection being projected on a horizontal plane, characterized in that: the streamline (F) of the intake port (lOA) projected on the horizontal plane is curved toward a center of the combustion chamber (54) so as to be at least partially located closer to the center of the combustion chamber (54) than a first straight line (Ll) that contains the starting point (Pl) and extends in a direction orthogonal to an axis of a crankshaft on the horizontal plane, and a second straight line (L2) that contains the end point (P2) and extends in a direction orthogonal to the axis of the crankshaft on the horizontal plane.

2. The internal combustion engine according to claim 1, characterized in that the starting point (Pl) is located closer to the center of the combustion chamber (54) than the second straight line (L2).

3. The internal combustion engine according to claim 1, characterized in that the starting point (Pl) lies on the second straight line (L2).

4. The internal combustion engine according to claim 1, characterized in that the starting point (Pl) is not located closer to the center of the combustion chamber (54) than the second straight line (L2).

5. The internal combustion engine according to any one of claims 1 through 4, characterized in that the intake valve (155A — 155D) comprises a specific intake valve (155A - 155D) in which the stem (stm) is offset such that an inner passage region (inr) located closer to the center of the combustion chamber (154), out of two intake-air passage regions on the opposite sides of a plane that contains a center axis of the stem (stm), becomes larger, and such that the center axis (Cl) of the stem (stm) does not contain a center (P2) of a bottom face of the umbrella portion (ub).

6. The internal combustion engine according to claim 5, characterized in that the stem (stm) of the specific intake valve (155B) is offset to an upstream side with respect to the center (P2) of the specific intake valve (155B), in a direction of flow of intake air.

7. The intake combustion engine according to claim 5 or 6, characterized in that a portion of the umbrella portion (ub) of the specific intake valve (155C), which corresponds to the inner passage region (inr), has a smaller volume than a portion of the umbrella portion (ub) which corresponds to the outer passage region (otr).

8. The internal combustion engine according to any one of claims 5 through 7, characterized in that the specific intake valve (155E) further includes a rotation preventing device that prevents the specific intake valve (155E) from rotating about the center axis (Cl) of the stem (stm) of the specific intake valve (155E).

9. The internal combustion engine according to any one of claims 1 through 8, characterized in that the intake valve (255A - 255D) comprises a specific intake valve (255A - 255D) in which the stem (stm) is inclined, when the intake valve is in a closed state, such that a distal end (PIl) of the stem (stm) is located closer to a plane that contains a center axis (C3) of a cylinder and is substantially orthogonal to the axis of the crankshaft, than a center (P2) of a bottom face of the umbrella portion, in a direction substantially parallel to the axis of the crankshaft.

10. The internal combustion engine according to claim 9, characterized in that the stem (stm) of the specific intake valve (255A — 255D) is inclined, when the specific intake valve (255A - 255D) is in a closed state, such that the distal end (PIl) of the stem (stm) is located closer to an exhaust port (256) than the center (P2) in a direction of flow of intake air.