combustion engine

Abstract

An internal combustion engine (10) for combusting a fuel-air mixture supplied to a combustion chamber (21) of a cylinder bore (14, 15) having a central axis (AX14, AX15) to thereby cause a piston (13) to reciprocate in the cylinder bore (14, 15), the internal combustion engine (10) having an intake port (23) and an exhaust port (25) opening into the combustion chamber (21), the internal combustion engine (10) comprising: a crankshaft (17) rotating based on the reciprocating movement of the piston (13); an intake valve (22) and an exhaust valve (24), each having a stem (26), wherein the intake valve (22) is configured to open or close the intake port (23), and wherein the exhaust valve (24) is configured to open or close the exhaust port (25); a first and a second rocker arm (29) provided for the respective intake valve (22) and exhaust valve (24), wherein the first rocker arm (29) is configured to cause the intake valve (22) to open or close the intake port (23), and wherein the second rocker arm (29) is configured to cause the exhaust valve (24) to open or close the exhaust port (25); a camshaft (31) having an intake cam (32) which transmits drive force to the first rocker arm (29) and an exhaust cam (33) which transmits drive force to the second rocker arm (29); and a power transmission mechanism (40) which transmits the rotation of the crankshaft (17) as a rotational force for the camshaft (31) to one end of the camshaft (31), where: each of the first and second rocker arms (29) comprises: a first end (29b) pivotally supported in the internal combustion engine (10) via a pin (28); a second end (29c) in contact with the stem (26) of a corresponding one of the intake valve (22) and the exhaust valve (24); and a cam contact member (29a) in contact with a surface of a corresponding one of the intake cam (32) and the exhaust cam (33) between the first and second ends (29b, 29c); the shafts (26) of the respective intake valve (22) and exhaust valve (24) are arranged separately from one another on both sides of a virtual reference plane (VP1) of the camshaft (31) and are arranged separately from one another in an axial direction of the camshaft (31), wherein the virtual reference plane (VP1) contains a central axis (AX31) of the camshaft (31) and runs parallel to the central axis (AX14, AX15) of the cylinder bore (14, 15), wherein the shafts (26) of the respective intake valve (22) and exhaust valve (24) are inclined with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15); the stem (26) of the intake valve (22) is arranged on one side of a thrust direction (T) of the piston (13) with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15); the stem (26) of the exhaust valve (24) is arranged on one side of a counter-thrust direction (R) of the piston (13) with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15); ends (26b) of the shafts (26) are arranged in a space, the ends (26b) being in contact with the respective second ends (29c) of the respective first and second rocker arms (29), the space being formed by extending the cylinder bore (14, 15) in a direction of its central axis (AX14, AX15); each of the first and second rocker arms (29) is arranged so that its cam contact element (29a) is closer to the central axis (AX14, AX15) of the cylinder bore (14, 15) than its first and second ends (29b, 29c), the first and second rocker arms (29) being arranged so that the cam contact elements (29a) overlap with each other as viewed from the axial direction of the camshaft (31); the crankshaft (17) is offset by a first displacement amount (OS1) from the central axis (AX14, AX15) of the cylinder bore (14, 15) in the thrust direction (T) of the piston (13); the camshaft (31) and the cam contact element (29a) of the first rocker arm (29) are each offset by a second displacement amount (OS2) from the central axis (AX14, AX15) of the cylinder bore (14, 15) in the thrust direction (T) of the piston (13); and an inclination angle (θ3) by which an extension line (AX26) of the stem (26) of the intake valve (22) is inclined with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15) is greater than an inclination angle (θ4) by which an extension line (AX26) of the stem (26) of the exhaust valve (24) is inclined with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15), wherein the intake port (23) is arranged on one side of the thrust direction (T) of the piston (13) with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15), the intake port (23) having an area that is larger than an area of the exhaust port (25); and the exhaust port (25) is arranged on one side of the counter-thrust direction (R) of the piston (13) with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15).

Description

TECHNICAL FIELD

[0001] The present invention relates to internal combustion engines with a lighter and smaller structure and low fuel consumption. GENERAL STATE OF THE ART

[0002] Internal combustion engines feed liquid or gaseous fuel into their combustion chambers to burn the fuel, causing their pistons to reciprocate. Internal combustion engines are each equipped with a mechanism that converts the piston's reciprocating motion into rotating power from a crankshaft. These internal combustion engines are installed as power sources in motor vehicles.

[0003] Typically, this type of internal combustion engine incorporates a valve mechanism integrated into a cylinder head. The valve mechanism drives intake and exhaust valves mounted on the combustion chamber. The valve mechanism typically transmits the rotational drive force of the crankshaft to a camshaft, thereby driving the camshaft.

[0004] US Pat. No. 4,730,588 A describes a valve actuation mechanism for a motor vehicle internal combustion engine, in which two intake valves of a cylinder are controlled by cams of a camshaft. One of the cams is rotatably and axially displaceably mounted on the camshaft. This cam can be displaced via a hydraulic cylinder to engage the camshaft.

[0005] The document DE 43 18 401 C2 shows an internal combustion engine in which the axis of the crankshaft has a lateral offset relative to the running path of the cylinder of the engine.

[0006] The document DE 10 2006 013 346 A1 discloses a reciprocating piston engine in which the central axis of the crankshaft is offset from the longitudinal axis of the cylinder of the engine.

[0007] Document DE 19 23 782 A shows a rocker arm and spring arrangement for a cam-controlled valve of an internal combustion engine, wherein the valve is controlled by a camshaft in cooperation with a rocker arm and a spring.

[0008] Document DE 196 07 591 B4 describes a valve train for actuating a lift valve with a valve stem guided in a valve guide. To actuate the lift valve, a pressure force acting in the direction of the valve guide is introduced into the valve stem via a cam-operated valve lever.

[0009] In patent publication JP 2000-230607 A, the intake and exhaust valves are opened or closed by a single camshaft and a pair of rocker arms. In the patent publication, the rotation of the crankshaft is transmitted to the camshaft via a pair of bevel gears and an intermediate shaft. This design aims to reduce the size of the internal combustion engine. SUMMARY

[0010] In such an internal combustion engine described in the patent publication, rocker arms are located on either side of a portion of the camshaft, with the portion of the camshaft located above the center axis of the bore of a cylinder of the internal combustion engine. Each of the rocker arm axes passes through a center portion of the corresponding rocker arm such that the rocker arms are pivotally supported on the corresponding rocker arm axes. For example, one inner end of each rocker arm is attached to the lobe of the camshaft, and the other, outer end is connected to a corresponding valve. As the camshaft rotates, each rocker arm pivots, causing the corresponding intake or exhaust valve to open or close.

[0011] However, the design of the internal combustion engine described in the patent publication results in a large included angle between the intake valve and the exhaust valve, which makes it difficult to reduce the size of the combustion chamber.

[0012] Furthermore, in such an internal combustion engine described in the patent publication, the piston in each cylinder is coupled to the crankshaft at one end via a connecting rod, and reciprocates within the cylinder with the rotation of the crankshaft. This can generate a large lateral force, pushing the piston toward the inner cylinder wall during the combustion and power strokes of the internal combustion engine's four-stroke cycle.

[0013] That is, such an internal combustion engine described in the patent publication may have limitations on reducing the size of the combustion chamber and the internal combustion engine, making it difficult to reduce fuel consumption. In addition, such an internal combustion engine described in the patent publication may increase fuel consumption due to the large lateral force exerted on the piston.

[0014] In view of the circumstances set forth above, one aspect of the present invention seeks to provide internal combustion engines designed to address at least one of the problems set forth above.

[0015] In particular, an alternative aspect of the present invention aims to provide internal combustion engines in which a reduction in size and weight is possible while reducing fuel consumption.

[0016] According to a first aspect of the present invention, there is provided an internal combustion engine for combusting a fuel-air mixture supplied to a combustion chamber of a cylinder bore having a central axis, thereby causing a piston to reciprocate within the cylinder bore. The internal combustion engine has an intake port and an exhaust port opening into the combustion chamber. The internal combustion engine includes a crankshaft that rotates based on the reciprocation of the piston, and an intake valve and an exhaust valve each having a stem, the intake valve being configured to open or close the intake port, and the exhaust valve being configured to open or close the exhaust port.The internal combustion engine includes first and second rocker arms provided for the respective intake valve and exhaust valve, the first rocker arm being configured to cause the intake valve to open or close the intake port, and the second rocker arm being configured to cause the exhaust valve to open or close the exhaust port. The internal combustion engine includes a camshaft having an intake cam that transmits drive power to the first rocker arm and an exhaust cam that transmits drive power to the second rocker arm, and a power transmission mechanism that transmits rotation of the crankshaft as rotational power for the camshaft to one end of the camshaft.Each of the first and second rocker arms includes a first end pivotally supported in the internal combustion engine via a pin; a second end in contact with the stem of a corresponding one of the intake valve and the exhaust valve; and a cam contact member in contact with a surface of a corresponding one of the intake cam and the exhaust cam between the first and second ends. The stems of the respective intake valve and exhaust valve are arranged apart from each other on both sides of a virtual reference plane of the camshaft and are arranged apart from each other in an axial direction of the camshaft. The virtual reference plane includes a center axis of the camshaft and is parallel to the center axis of the cylinder bore. The stems of the respective intake valve and exhaust valve are inclined with respect to the center axis of the cylinder bore.The stem of the intake valve is arranged on one side of a piston thrust direction with respect to the center axis of the cylinder bore. The stem of the exhaust valve is arranged on one side of a piston counter-thrust direction with respect to the center axis of the cylinder bore. Ends of the stems are arranged in a space, the ends being in contact with the respective second ends of the respective first and second rocker arms, the space being formed by extending the cylinder bore in a direction of its center axis. Each of the first and second rocker arms is arranged so that its cam contact element is closer to the center axis of the cylinder bore than its first and second ends. The first and second rocker arms are arranged so that the cam contact elements overlap with each other as viewed from the axial direction of the camshaft.The crankshaft is offset by a first displacement amount from the center axis of the cylinder bore in the piston thrust direction. The camshaft and the cam contact element of the first rocker arm are each offset by a second displacement amount from the center axis of the cylinder bore in the piston thrust direction. An inclination angle by which an extension line of the stem of the intake valve is inclined with respect to the center axis of the cylinder bore is greater than an inclination angle by which an extension line of the stem of the exhaust valve is inclined with respect to the center axis of the cylinder bore.

[0017] According to the first aspect of the present invention, the intake port is located on a side of the piston thrust direction with respect to the center axis of the cylinder bore. The intake port has an area larger than an area of the exhaust port. The exhaust port is located on a side of a counter-thrust direction of the piston with respect to the center axis of the cylinder bore.

[0018] According to a second aspect of the present invention, the first rocker arm and the second rocker arm are designed to have the same structural shape.

[0019] According to a third aspect of the present invention, the power transmission mechanism includes a first bevel gear attached to one end of the crankshaft, a second bevel gear attached to one end of the camshaft, a third bevel gear meshing with the first bevel gear, a fourth bevel gear meshing with the second bevel gear, and a two-ended power transmission shaft to which the third and fourth bevel gears are attached. The power transmission shaft is arranged to be orthogonal to extension lines of the respective crankshaft and camshaft. EFFECTS OF THE INVENTION

[0020] According to the first aspect, the cam contact element of each of the first and second rocker arms is in contact with the surface of a corresponding one of the intake cam and the exhaust cam between the journal and the second end. This allows free design of the intake valve and the exhaust valve and the first and second rocker arms, thereby preventing a reduction in the size of the combustion chamber or the like. The first and second rocker arms are arranged so that the cam contact elements overlap with each other as viewed from the axial direction of the camshaft. This reduces an inclination angle of each of the intake valve and the exhaust valve with respect to the center axis of the cylinder bore, thereby reducing the size of the combustion chamber.The crankshaft, camshaft, and cam contact elements of the first and second rocker arms are arranged so that they are offset from the cylinder center axis in a piston thrust direction. This causes the camshaft to be close to the crankshaft and reduces the lateral force that pushes the piston toward an inner wall of the cylinder bore during the combustion and power strokes. For these reasons, it is possible to make the power transmission mechanism between the crankshaft and the camshaft compact in the direction of piston reciprocating motion, thereby reducing the fuel consumption of the internal combustion engine.

[0021] Furthermore, according to the first aspect, an inclination angle by which an extension line of the stem of the intake valve is inclined with respect to the center axis of the cylinder bore is larger than an inclination angle by which an extension line of the stem of the exhaust valve is inclined with respect to the center axis of the cylinder bore. This allows the cam contact element and the camshaft to be largely offset in the piston thrust direction. This allows the size of the intake valve located on the thrust direction side, the size of the power transmission mechanism between the crankshaft and the camshaft, and a strength of the side force to be adjusted as desired.

[0022] According to the second aspect, the intake port is located on the piston thrust direction side with respect to the center axis of the cylinder bore. The intake port has an area larger than the area of the exhaust port. The exhaust port is located on the piston reverse thrust direction side with respect to the center axis of the cylinder bore. This allows the cam contact element and the camshaft to be significantly offset in the piston thrust direction.

[0023] According to the third aspect, the first rocker arm and the second rocker arm are configured to have the same structural shape. This reduces the number of elements required to manufacture the internal combustion engine, thereby reducing the manufacturing cost of the internal combustion engine and improving the machinability of the elements when assembling them into the internal combustion engine.

[0024] According to the fourth aspect, the power is transmitted from the crankshaft to the camshaft via the first to fourth bevel gears. This reduces the size of the power transmission shaft and reduces the weight compared to a drive mechanism using chains and / or belts, making it possible to effectively reduce fuel consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other aspects of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawings, in which Fig. 1 is a partially sectioned and perspective elevational view of an internal combustion engine according to an embodiment of the present invention, illustrating an overall structure of the internal combustion engine as viewed from one of its side directions; Fig. 2 is a partially sectioned and perspective elevational view of the internal combustion engine showing an area around its piston from a direction perpendicular to the one side of Fig. 1, viewed from the axial direction of a crankshaft of the internal combustion engine; Fig. 3 is a partially sectioned and perspective view of the internal combustion engine showing an area around its power transmission mechanism from the perpendicular to the one side direction of Fig. 1, viewed from the axial direction of the crankshaft of the internal combustion engine; Fig. 4 a sectional view along the line AA in Fig. 1, which illustrates a cylinder head viewed from its top side; Fig. 5 is a partially sectioned and perspective view of the internal combustion engine showing an area around its valve drive mechanism from the same direction as in Fig. 2 illustrated; and Fig. 6 is a partially sectioned and perspective bottom view of the internal combustion engine illustrating a configuration of the valve drive mechanism in the cylinder head as viewed from its cylinder bore side. DETAILED DESCRIPTION OF AN EMBODIMENT

[0026] An internal combustion engine according to an embodiment of the present invention will be described below with reference to the accompanying drawings. Fig. 1 to 6 schematically illustrate an internal combustion engine 10 according to an embodiment of the present invention.

[0027] With reference to Fig. 1 to 3, the internal combustion engine 10 is manufactured to have a monoblock structure in which a cylinder block 11 and a cylinder head 12 are integrally formed. The internal combustion engine 10, simply referred to as the engine, is installed in a motor vehicle as a power source for rotating unillustrated drive wheels of the motor vehicle.

[0028] The cylinder block 11 has a double-cylinder structure in which two cylinder bores, i.e., inner cylinders, 14 and 15 are arranged side by side. In each of the cylinder bores 14 and 15, a piston 13 having a substantially circular or elliptical cross-sectional shape is installed so as to be vertically movable, i.e., reciprocally movable in the axial direction of the corresponding cylinder bore. In this embodiment, the axial direction of each cylinder bore corresponds to the vertical direction, i.e., Fig. 1 to 3 and 5 illustrate the up-and-down direction of the engine 10. The cylinder head 12 is located on top of the cylinder block 11 to form a combustion chamber 21 for each of the cylinder bores 14 and 15 between the cylinder head 12 and the corresponding piston 13. The piston 13 has a top surface 13a, and each of the cylinder bores 14 and 15 has a ceiling surface 21a with a curved surface in cross section. The ceiling surface 21a of each of the cylinder bores 14 and 15 is arranged to face the top surface 13a of the corresponding piston 13, thereby forming a combustion chamber 21 between the ceiling surface 21a and the top surface 13a. The ceiling surface 21a comprises an intake opening, ie, an inlet, 23, which opens into the combustion chamber 21, and an exhaust opening, ie, an outlet, 25, which opens into the combustion chamber 21.The engine 10 includes an intake valve 22 for opening or closing the intake port 23 and an exhaust valve 24 for opening or closing the exhaust port 25. The intake and exhaust valves 22 and 24 serve, for example, as first and second valve elements, respectively.

[0029] The engine 10 operates by supplying a mixture of combustion air and fuel to the combustion chamber 21; the combustion air is drawn into the engine 10 from the ambient air. The fuel used may be a liquid fuel, such as gasoline, or a gaseous fuel, such as compressed natural gas (CNG). After supplying the air-fuel mixture, the engine 10 operates by causing a spark plug 30 to generate a spark in the combustion chamber 21, resulting in combustion of the air-fuel mixture. These operations of the engine 10 reciprocate the piston 13 up and down in each of the cylinder bores 14 and 15. In order to supply the air-fuel mixture to the combustion chamber 21, the intake valve 22 operates to repeatedly open and close the intake port 23 with each intake stroke, thereby repeatedly supplying the air-fuel mixture to the combustion chamber 21.In order to discharge exhaust gases after combustion, the exhaust valve 24 operates to repeatedly open and close the exhaust port 25 with each exhaust stroke, thereby repeatedly discharging the exhaust gases from the combustion chamber 21.

[0030] In the cylinder block 11, the lower end of the piston 13 in each of the cylinder bores 14 and 15 is coupled to a crankshaft 17 via a connecting rod 16, so that the reciprocating motion of the piston 13 is converted into a rotational motion of the crankshaft 17, thereby generating a driving force for rotating the drive gears. Note that the crankshaft 17 is housed in a lower case 19, which serves as an oil pan for collecting lubricating oil.

[0031] In the cylinder head 12, the ceiling surface 21a forming part of the combustion chamber 21 for each of the cylinder bores 14 and 15 includes circumferential edges 23a and 25a around the respective intake and exhaust ports 23 and 25.

[0032] The intake and exhaust valves 22 and 24 are arranged as the valve elements so as to face the respective peripheral edges 23a and 25a. Each of the intake and exhaust valves 22 and 24 has a valve disc disposed in the combustion chamber 21 and seatable on a corresponding one of the peripheral edges 23a and 25a. Each of the intake and exhaust valves 22 and 24 also includes a valve stem 26 connected to the valve disc at its lower end as a stem portion, and a coil spring 27 provided around the valve stem 26. The coil spring 27 operates to bias the valve stem 26 so that the valve disc is moved toward a corresponding one of the peripheral edges 23a and 25a. This results in the valve disc sitting tightly against a corresponding one of the circumferential edges 23a and 25a, thereby closing a corresponding one of the intake and exhaust openings 23 and 25.

[0033] The upper end 26b of the valve stem 26 of each of the intake and exhaust valves 22 and 24 is arranged to be in close contact with one end 29c of a rocker arm 29 pivotally supported at its other end, i.e., a base end 29b, by an adjusting pin 28; the base end 29b is also referred to as a first end, and the end 29c is also referred to as a second end. The rocker arm 29 is provided with a contact roller 29a rotatably mounted at its central portion. The contact roller 29a for each of the intake and exhaust valves 22 and 24 serves as a cam contact member. Specifically, an intake cam 32 and an exhaust cam 33, each having a cam surface 32a and 33a, are attached to the camshaft 31 so as to be rotated together with the camshaft 31.A top surface of the contact roller 29a is arranged to contact a corresponding one of the cam surfaces 32a and 33a of each of the intake cam 32 and the exhaust cam 33.

[0034] Specifically, each of the intake and exhaust cams 32 and 33 serves as a drive cam to press the corresponding contact roller 29a through its cam surface 32a or 33a when rotated together with the camshaft 31. This structure of the intake and exhaust cams 32 and 33 and the respective rocker arms 29 serving as first and second rocker arms allows each of the rocker arms 29 to pivot about the base end 29b as a pivot point, with the base end 29b being supported by the upper end 28a of the adjusting pin 28 (see Fig. 5). The pivoting of each of the rocker arms 29 allows the valve stem 26, whose upper end 26b is in contact with the second end 29c of the corresponding rocker arm 29, to be moved against the biasing force of the coil spring 27 toward a corresponding one of the intake and exhaust ports 23 and 25. This results in each of the intake and exhaust valves 22 and 24 being lifted away from the closed contact state at a corresponding one of the peripheral edges 23a and 25a, thereby opening a corresponding one of the intake and exhaust ports 23 and 25.

[0035] In this embodiment, the intake valve 22 and the exhaust valve 24 for each of the cylinder bores 14 and 15 are arranged so that their valve stems 26 are arranged separately from each other on both sides of the camshaft 31 and that they are arranged separately from each other in the axial direction of the camshaft 31 (see Fig. 5 and Fig. 6).

[0036] Note that in this embodiment, the axial direction of the camshaft 31 is defined as the longitudinal direction, i.e., the front-backward direction, of the engine 10, and the forward direction of the engine 10 corresponds, for example, to the forward direction of the motor vehicle. In addition, a width direction, i.e., a right-left direction, of the engine 10 is defined as orthogonal to the longitudinal direction and the vertical direction of the engine 10. Thus, the valve stems 26 of the intake and exhaust valves 22 and 24 are arranged separately from each other on the right and left sides of the camshaft 31, respectively. In addition, the rocker arm 29 for the intake valve 22 and the rocker arm 29 for the exhaust valve 24 are arranged to oppose each other parallel to the axial direction of the camshaft 31 via a gap, and are operable to drive the corresponding intake and exhaust valves 22 and 24, respectively.

[0037] As in Fig. 5, the rocker arms 29 for driving the respective intake and exhaust valves 22, 24, which are provided for each of the cylinder bores 14 and 15, are arranged so that their contact rollers 29a overlap with each other as viewed from the axial direction of the camshaft 31. That is, the contact rollers 29a of the rocker arms 29 for the cylinder bore 14 are located close to a center axis AX14 of the cylinder bore 14, and the contact rollers 29a of the rocker arms 29 for the cylinder bore 15 are located close to a center axis AX15 of the cylinder bore 15. These rocker arms 29 for each of the cylinder bores 14 and 15 have the same shape. Specifically, as shown in Fig. 5 and Fig. 6, common parts are used as the rocker arms 29 for the intake valve 22 and the rocker arms 29 for the exhaust valve 24, and the rocker arms 29 for the intake and exhaust valves 22 and 24 are alternately arranged back to back. Specifically, the rocker arms 29 for each of the intake and exhaust valves 22 and 24 are arranged such that the second end 29c and the first end 29b of one of the rocker arms 29 are opposite to the first end 29b and the second end 29c of the other in the longitudinal direction of the engine 10, respectively.

[0038] As described above, the contact rollers 29a of the rocker arms 29 for the cylinder bore 14 are arranged close to the center axis AX14 of the cylinder bore 14, and the contact rollers 29a of the rocker arms 29 for the cylinder bore 15 are arranged close to the center axis AX15 of the cylinder bore 15. This results in a smaller inclination angle of the valve stem of each of the intake and exhaust valves 22 and 24. It should be noted that in Fig. 4 the intake ports 23 for the respective cylinder bores 14 and 15 communicate with intake manifolds, i.e., intake pipes, 51. The exhaust ports 25 for the respective cylinder bores 14 and 15 communicate with an exhaust pipe 52, and they then communicate with a catalytic converter 53. A starter 55 causes the crankshaft 17 of the engine 10 to begin rotating, thereby starting the engine 10. The rotation of the crankshaft 17 drives an air compressor 56.

[0039] With reference to Fig. 1, the camshaft 31 has two journal bearings, i.e., round plate-like members, 31a, and is rotatably supported by two bearing members 36, each of which is supported on the bearings 31a. The set of a journal bearing 31a and a corresponding bearing member 36 serves as a bearing. The bearing members 36 are integrally formed on corresponding support ribs, which protrude, for example, from an inner surface of the top surface of the cylinder head 12 so as to be spaced apart from each other in the axial direction of the camshaft 31. The camshaft 31 is coupled to the crankshaft 17 via a power transmission mechanism 40.Specifically, rotation of the camshaft 31 synchronized with the rotation of the crankshaft 17 causes the intake cams 32 and the exhaust cams 33 to rotate with the reciprocating movements of the piston 13, which causes the intake and exhaust valves 22 and 24 to open and close the respective intake and exhaust ports 23 and 25 in sequence.

[0040] As in Fig. 1 and Fig. 3, the power transmission mechanism 40 includes a first bevel gear 41, a second bevel gear 42, a third bevel gear 43, a fourth bevel gear 44, and an intermediate shaft, that is, a power transmission shaft, 45. The first bevel gear 41 is fixedly attached to one end, such as a front end, of the crankshaft 17. The second bevel gear 42 is fixedly attached to one end, such as a front end, of the camshaft 31 so as to be substantially opposite to the first bevel gear 41. The third bevel gear 43 is engaged with the first bevel gear 41, and the fourth bevel gear 44 is engaged with the second bevel gear 42. The third and fourth bevel gears 43 and 44 are respectively fixedly attached to the two ends of the intermediate shaft 45 serving as a power transmission shaft. The crankshaft 17 and the camshaft 31 are respectively rotatably supported in the lower housing 19 and the upper housing 20.the cylinder head 12 such that their axial directions are parallel to each other and their extension lines are orthogonal to the intermediate shaft 45. The crankshaft 17 and the camshaft 31 are coupled to each other via the engagement of the respective bevel gears 41 to 44 so that they rotate together.

[0041] By this configuration of the crankshaft 17, the power transmission mechanism 40, and the camshaft 31, the rotation of the crankshaft 17 causes the third bevel gear 43 to rotate, and the rotating third bevel gear 43 transmits the rotation to the intermediate shaft 45. The rotation of the intermediate shaft 45 is transmitted as rotational force to the second bevel gear 42, which meshes with the fourth bevel gear 44, causing the camshaft 31 to rotate. Note that the intermediate shaft 45 is rotatably supported by bearings (not shown) in a front housing 49, which is attached to the front end of the engine 10.

[0042] Thus, the power transmission mechanism 40 transmits the rotation of the crankshaft 17 to the camshaft 31 without any pulleys or sprockets fixed to the front ends of the crankshaft 17 and the camshaft 31, and without a chain or belt looped therearound. Therefore, the power transmission mechanism 40 eliminates excessive tension on the front ends of the crankshaft 17 and the camshaft 31, which leads to a reduction in the strength of the bearings supporting the front ends of the crankshaft 17 and the camshaft 31.

[0043] In addition, the engine 10 according to this embodiment is configured such that the direction of reciprocating motion, that is, an extension line, of the valve stem 26 of each of the intake and exhaust valves 22 and 24 for the cylinder bore 14 is inclined with respect to the center axis AX14 so as to pass through the center axis AX14. Likewise, the engine 10 according to this embodiment is configured such that the direction of reciprocating motion, that is, an extension line, of the valve stem 26 of each of the intake and exhaust valves 22 and 24 for the cylinder bore 15 is inclined with respect to the center axis AX15 so as to pass through the center axis AX15.

[0044] Specifically, extension lines AX26 of the valve stems 26 of the respective intake and exhaust valves 22 and 24 for the cylinder bore 14 in the cylinder head 12 of the engine 10 are inclined by respective angles θ1 and θ2 with respect to the central axis AX14 such that the extension lines AX26 are close to the camshaft 31 and pass through the central axis AX14. In other words, each of the valve stems 26 of the intake and exhaust valves 22 and 24 for the cylinder bore 14 in the cylinder head 12 is arranged to have an inclined shape on a radial extension line from a predetermined point of the central axis AX14, the radial extension line being inclined with respect to the central axis AX14 by the corresponding angle θ1 or θ2.

[0045] Likewise, extension lines AX26 of the valve stems 26 of the intake and exhaust valves 22 and 24 for the cylinder bore 15 in the cylinder head 12 of the engine 10 are inclined by respective angles θ1 and θ2 with respect to the central axis AX15 such that the extension lines AX26 are close to the camshaft 31 and pass through the central axis AX15. In other words, each of the valve stems 26 of the intake and exhaust valves 22 and 24 for the cylinder bore 15 in the cylinder head 12 is arranged to have an inclined shape on a radial extension line from a predetermined point of the central axis AX15, the radial extension line being inclined with respect to the central axis AX15 by the corresponding angle θ1 or θ2.

[0046] It should be noted that the inclination angles θ1 and θ2 of the valve stems 26 of the respective intake and exhaust valves 22 and 24 with respect to the center axis AX14 of the cylinder bore 14 are the inclination angles when viewed from a direction orthogonal to the camshaft 31 and to the vertical direction of the engine 10; as shown in Fig. 5, when viewed from the axial direction of the camshaft 31, inclination angles θ3 and θ4 of the valve stems 26 of the respective intake and exhaust valves 22 and 24 are formed with respect to the center axis AX14 of the cylinder bore 14. Likewise, when viewed from the axial direction of the camshaft 31, inclination angles θ3 and θ4 of the valve stems 26 of the respective intake and exhaust valves 22 and 24 are formed with respect to the center axis AX15 of the cylinder bore 15.

[0047] The structure of the engine 10 provides, compared with a conventional engine with inclination angles θ1 and θ2 of 0°, a wider separation space S between the intake cam 32 and the exhaust cam 33 for each of the cylinder bores 14 and 15. The separation space S between the intake cam 32 and the exhaust cam 33 for the cylinder bore 15 allows one of the shaft bearings 31a and a corresponding bearing member 36 to be arranged on the central axis AX15, thereby supporting the camshaft 31. The separation space S between the intake cam 32 and the exhaust cam 33 for the cylinder bore 14 allows the other of the shaft bearings 31a and a corresponding bearing member 36 to be arranged on a first side, i.e., the front side, of the central axis AX14; the first side of the central axis AX14 is closer to the second bevel gear 42 than a second side of the central axis AX14 opposite to the first side.The other of the shaft bearings 31a and the corresponding bearing element 36 support the front end of the camshaft 31.

[0048] That is, excessive load is not applied to the front end of the camshaft 31 to which the second bevel gear 42 of the power transmission mechanism 40 is fixed, resulting in the elimination of a bearing conventionally required to support the front end of the camshaft 31. This structure therefore supports the front end of the camshaft 31 rotatably without the need for bearings between the intake cam 32 and the second bevel gear 42. This results in a shortening of the length of the camshaft 31 and a reduction in the number of bearings, each consisting of a shaft bearing 31a and a corresponding bearing member 36. This makes it possible to reduce the size and weight of the engine 10, thereby reducing fuel consumption due to the reduction in the size and weight of the engine 10.

[0049] The engine 10 according to this embodiment is configured so that each of the intake and exhaust valves 22 and 24 performs an opening and closing operation during two reciprocating movements, that is, four strokes, of the piston 13. Therefore, the power transmission mechanism 40 of the engine 10 is required to reduce the rotational speed of the crankshaft 17 to half and rotate the camshaft 31 at the reduced rotational speed. For this reason, the power transmission mechanism 17 is configured so that the number of teeth of the second bevel gear 42 is twice the number of teeth of the fourth bevel gear 44, and the diameter of the second bevel gear 42 is larger than that of the fourth bevel gear 44.

[0050] This structure of the power transmission mechanism 40 reduces the rotational speed of the camshaft 31 by the gear ratio between the engagement surface 42a of the second bevel gear 42 and the engagement surface 44a of the fourth bevel gear 44. This makes it possible to reduce the length of the camshaft 31 compared with the case of reducing the rotational speed of the camshaft 31 by the gear ratio between the engagement surface 41a of the first bevel gear 41 and the engagement surface 43a of the third bevel gear 43.

[0051] Specifically, as described above, the diameter of the second bevel gear 42 is larger than that of the fourth bevel gear 44 according to the gear ratio between the engagement surface 42a of the second bevel gear 42 and the engagement surface 44a of the fourth bevel gear 44. As a result, the engagement surface 42a of the second bevel gear 42 and the engagement surface 44a of the fourth bevel gear 44 face the camshaft 31 below the camshaft 31, and a normal direction of the mutual engagement surfaces 42a and 44a is greatly inclined with respect to the opposite direction between the camshaft 31 and the mutual engagement surfaces 42a and 44a. Therefore, the pressure F from the fourth bevel gear 44 on the camshaft 31 acts on the normal direction of the engagement surface 42a, and a component of the pressure in the opposite direction between the camshaft 31 and the mutual engagement surfaces 42a and 44a acts only on the camshaft 31.This results in a reduction in the pressure acting on the camshaft 31. For this reason, the bearings, each consisting of a shaft bearing 31a and a corresponding bearing element 36, located in the respective separation spaces S, hold the camshaft 31 rotatably with high reliability, while bearings for holding the front end of the camshaft 31 close to the second bevel gear 42 are omitted.

[0052] In the engine 10 according to this embodiment, the crankshaft 17 is arranged so that its center axis AX17 is offset from each of the center axes AX14 and AX15 of the cylinder bores 14 and 15 by a displacement amount OS1 to one side, for example, the left side, in the right-left direction of the engine 10 (see Fig. 2 and Fig. 3). In other words, the central axis AX17 is offset by the displacement amount OS1 from a virtual plane VP, for example, to the left, wherein the virtual plane VP contains the central axes AX14 and AX15 of the respective cylinder bores 14 and 15 and is located in a direction parallel to the central axis AX17 of the Crankshaft 17, in other words to the central axis AX31 of the camshaft 31, extending direction (see Fig. 2 and Fig. 3).

[0053] As if by an arrow in Fig. 2, the crankshaft 17 is rotated clockwise, so that the piston 13, which is coupled to the crankshaft 17 by the connecting rod 16, reciprocates in the vertical direction in each of the cylinder bores 14 and 15 by the movement of the connecting rod 16. For this reason, the piston 13 moves in the up-down direction in each of the cylinder bores 14 and 15 while oscillating in the Fig. 2. For this reason, during the combustion and power strokes of the four-stroke cycle of the engine 10, the piston 13 moves downward in each of the cylinder bores 14 and 15 while being subjected to a lateral force that pushes the piston 13 in the plane of movement of the connecting rod 16 toward the left inner wall of a corresponding one of the cylinder bores 14 and 15, as shown in Fig. 2. In this embodiment, the direction in which the piston 13 moves in the plane of movement of the connecting rod 16 is Fig. 2 is subjected to the lateral force, that is, the Fig. 2, is referred to as the thrust direction T of the piston 13. In contrast, a direction opposite to the thrust direction T in the plane of movement of the connecting rod 16, ie, the direction in Fig. 2, is referred to as the counter-thrust direction R of the piston 13.

[0054] Specifically, in this embodiment, the engine 10 is configured such that the crankshaft 17 is offset from the virtual plane VP in the thrust direction T of the piston 13. This configuration makes it possible to move the piston 13 downward while maintaining a posture such that a center axis of the piston 13 is as parallel as possible to, e.g., close to, the center axis AX14 or AX15 of a corresponding one of the cylinder bores 14 and 15. This reduces the lateral force that pushes the piston 13 toward the inner wall of a corresponding one of the cylinder bores 14 and 15. As a result, it is possible to effectively reciprocate the piston 13 up and down in each of the cylinder bores 14 and 15.In other words, it is possible to reduce the fuel consumption required to obtain a predetermined level of driving power from the engine 10 compared with the case of obtaining the same level of driving power from a comparative engine in which there is no offset of the crankshaft 17.

[0055] In addition, as in Fig. 6, the intake port 23 is arranged on the upper surface 21a of the combustion chamber 21 such that the center of the intake port 23 is offset from a virtual reference plane VP1 in the thrust direction T, i.e., the left direction, of the piston 13, wherein the virtual reference plane VP1 includes the central axis AX31 of the camshaft 31 and extends parallel to the central axis AX14, AX15 of each cylinder bore of the engine 10. In contrast, the exhaust port 25 is arranged on the upper surface 21a of the combustion chamber 21 such that the center of the exhaust port 25 is offset from the virtual reference plane VP1 in the reverse thrust direction R, i.e., the right direction, of the piston 13.

[0056] Here, the area surrounded by the circumference 23a of the intake port 23 in the upper surface 21a of the combustion chamber 21 is larger than that surrounded by the circumference 25a of the exhaust port 25 in the upper surface 21a of the combustion chamber 21. This results in an end surface of the valve plate of the intake valve 22 for opening and closing the intake port 23 having a larger diameter corresponding to the diameter of the intake port 23. The larger diameter of the end surface of the valve plate of the intake valve 22 results in the valve stem 26, which is coaxial with a center 22c of the end surface of the valve plate, being displaced from the vertical plane VP1 in the thrust direction T of the piston 13.

[0057] For this reason, the camshaft 31 is arranged so that its central axis AX31 is offset in the same way as the crankshaft 17 in the right-left direction of the engine 10 from each of the central axes AX14 and AX15 of the cylinder bores 14 and 15 by a displacement amount OS2 to the left, that is, to the side of the thrust direction (see Fig. 2 and Fig. 5). In other words, the central axis AX31 is offset from a virtual plane VP by the displacement amount OS2 to the left, i.e., to the side of the thrust direction.

[0058] With reference to Fig. 5, the inclination angle θ3 of the extension line AX26 of the valve stem 26 of the intake valve 22 with respect to the center axis AX14 of the cylinder bore 14 is greater than the inclination angle θ4 of the extension line AX26 of the valve stem 26 of the exhaust valve 24 with respect to the center axis AX15 of the cylinder bore 15. As described above, the rocker arms 29 are attached to the respective intake and exhaust valves 22 and 24 to drive them. For this reason, adjusting the inclination of the valve stem 26 with respect to the center axis AX14 of the cylinder bore 14 allows the contact roller 29a and the camshaft 31 to be offset by the displacement amount OS2 from each of the center axes AX14 and AX15 of the cylinder bores 14 and 15 in the thrust direction (see Fig. 2 and Fig.3). In order to make the combustion chamber 21 compact for each of the cylinder bores 14 and 15, the valve stems 26 of the respective intake and exhaust valves 22 and 24 are inclined with respect to a corresponding one of the center axes AX14 and AX15 so as to be arranged in a space formed by extending a corresponding one of the cylinder bores 14 and 15 in the direction of the center axes AX14 and AX15, that is, in the vertical direction of the engine 10. Note that, in this embodiment, the camshaft 31 is arranged so that its center axis AX31 is offset to approach the crankshaft 17 due to the large-diameter intake port 23, but the present invention is not limited thereto. In particular, it is possible to design the offset of the camshaft 31 so that it is brought closer to the crankshaft 17 when the diameter of the intake port 31 is equal to that of the exhaust port 25.

[0059] Specifically, the structure of the engine 10 results in a shorter distance between the crankshaft 17 and the camshaft 31 compared to a distance between the crankshaft and the camshaft of a conventional engine, which results in a certain degree of shortening of the length of the intermediate shaft 45. Therefore, the engine 10 results in an engine 10 in which the cylinder block 11 and the cylinder head 12 each have a lower height, thereby reducing the size and weight of the engine 10. The reduction in the size and weight of the engine 10 results in a reduction in fuel consumption.

[0060] As described above, the engine 10 according to this embodiment improves fuel combustion efficiency due to the reduced combustion chamber 21 and reduces the lateral force exerted on the piston 13 reciprocating up and down in each of the cylinder bores 14 and 15. In addition, the engine 10, which has a lighter weight, results in reduced fuel consumption. INDUSTRIAL APPLICABILITY

[0061] The present disclosure set forth above has described one embodiment of the present invention, but the present invention is not limited thereto. Specifically, the present invention can be implemented in various embodiments within its scope.

Claims

[1] An internal combustion engine (10) for combusting a fuel-air mixture supplied to a combustion chamber (21) of a cylinder bore (14, 15) having a central axis (AX14, AX15) to thereby cause a piston (13) to reciprocate in the cylinder bore (14, 15), the internal combustion engine (10) having an intake port (23) and an exhaust port (25) opening into the combustion chamber (21), the internal combustion engine (10) comprising: a crankshaft (17) rotating based on the reciprocating movement of the piston (13); an intake valve (22) and an exhaust valve (24), each having a stem (26), wherein the intake valve (22) is configured to open or close the intake port (23), and wherein the exhaust valve (24) is configured to open or close the exhaust port (25); a first and a second rocker arm (29) provided for the respective intake valve (22) and exhaust valve (24), wherein the first rocker arm (29) is configured to cause the intake valve (22) to open or close the intake port (23), and wherein the second rocker arm (29) is configured to cause the exhaust valve (24) to open or close the exhaust port (25); a camshaft (31) having an intake cam (32) which transmits drive force to the first rocker arm (29) and an exhaust cam (33) which transmits drive force to the second rocker arm (29); and a power transmission mechanism (40) which transmits the rotation of the crankshaft (17) as a rotational force for the camshaft (31) to one end of the camshaft (31), where: each of the first and second rocker arms (29) comprises: a first end (29b) pivotally supported in the internal combustion engine (10) via a pin (28); a second end (29c) in contact with the stem (26) of a corresponding one of the intake valve (22) and the exhaust valve (24); and a cam contact member (29a) in contact with a surface of a corresponding one of the intake cam (32) and the exhaust cam (33) between the first and second ends (29b, 29c); the shafts (26) of the respective intake valve (22) and exhaust valve (24) are arranged separately from one another on both sides of a virtual reference plane (VP1) of the camshaft (31) and are arranged separately from one another in an axial direction of the camshaft (31), wherein the virtual reference plane (VP1) contains a central axis (AX31) of the camshaft (31) and runs parallel to the central axis (AX14, AX15) of the cylinder bore (14, 15), wherein the shafts (26) of the respective intake valve (22) and exhaust valve (24) are inclined with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15); the stem (26) of the intake valve (22) is arranged on one side of a thrust direction (T) of the piston (13) with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15); the stem (26) of the exhaust valve (24) is arranged on one side of a counter-thrust direction (R) of the piston (13) with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15); ends (26b) of the shafts (26) are arranged in a space, the ends (26b) being in contact with the respective second ends (29c) of the respective first and second rocker arms (29), the space being formed by extending the cylinder bore (14, 15) in a direction of its central axis (AX14, AX15); each of the first and second rocker arms (29) is arranged so that its cam contact element (29a) is closer to the central axis (AX14, AX15) of the cylinder bore (14, 15) than its first and second ends (29b, 29c), the first and second rocker arms (29) being arranged so that the cam contact elements (29a) overlap with each other as viewed from the axial direction of the camshaft (31); the crankshaft (17) is offset by a first displacement amount (OS1) from the central axis (AX14, AX15) of the cylinder bore (14, 15) in the thrust direction (T) of the piston (13); the camshaft (31) and the cam contact element (29a) of the first rocker arm (29) are each offset by a second displacement amount (OS2) from the central axis (AX14, AX15) of the cylinder bore (14, 15) in the thrust direction (T) of the piston (13); and an inclination angle (θ3) by which an extension line (AX26) of the stem (26) of the intake valve (22) is inclined with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15) is greater than an inclination angle (θ4) by which an extension line (AX26) of the stem (26) of the exhaust valve (24) is inclined with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15), wherein the intake port (23) is arranged on one side of the thrust direction (T) of the piston (13) with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15), the intake port (23) having an area that is larger than an area of the exhaust port (25); and the exhaust port (25) is arranged on one side of the counter-thrust direction (R) of the piston (13) with respect to the central axis (AX14, AX15) of the cylinder bore (14, 15). [2] Internal combustion engine according to claim 1, wherein the first rocker arm (29) and the second rocker arm (29) are designed to have the same structural shape. [3] An internal combustion engine according to claim 1, wherein the power transmission mechanism (40) comprises: a first bevel gear (41) attached to one end of the crankshaft (17); a second bevel gear (42) attached to one end of the camshaft (31); a third bevel gear (43) meshing with the first bevel gear (41); a fourth bevel gear (44) meshing with the second bevel gear (42); and a power transmission shaft (45) having two ends to which the respective third and fourth bevel gears (43, 44) are attached, the power transmission shaft (45) being arranged to be orthogonal to extension lines of the respective crankshaft (17) and camshaft (31). [4] The internal combustion engine according to claim 3, wherein the second bevel gear (42) has a number of teeth greater than a number of teeth of the fourth bevel gear (44) and has a diameter greater than a diameter of the fourth bevel gear (44).

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