Multi-cylinder engine

Abstract

To provide a multi-cylinder engine that features cylinders with different displacements while allowing for a simplified and miniaturized crankshaft structure. [Solution] In a multi-cylinder engine (31) comprising a first cylinder (39a), a second cylinder (39b) having a larger bore diameter than the first cylinder (39a), a first connecting rod (34a) with one end connected to a first piston (35a) sliding inside the first cylinder (39a) and the other end connected to a crank member (32), and a second connecting rod (34b) with one end connected to a second piston (35b) sliding inside the second cylinder (39b) and the other end connected to a crank member (32), the first connecting rod (34a) and the second connecting rod (34b) are connected to the crank member (32) such that the first piston (35a) and the second piston (35b) have the same stroke length (ST).

Description

TECHNICAL FIELD

[0001] The present invention relates to a multi-cylinder engine. BACKGROUND ART

[0002] From the perspectives of reducing environmental load and improving economic efficiency, there is a demand for improving the fuel efficiency performance of engines. As one of the methods for improving fuel efficiency, a multi-cylinder engine including a plurality of cylinders with different bore diameters is known. Patent Document 1 discloses a three-cylinder two-stroke engine in which the bore diameter and stroke of the central cylinder are made larger than those of the cylinders at both ends. By performing all-cylinder operation and operation using only the cylinders at both ends, it is possible to switch between a power mode and an eco-mode, and fuel consumption can be reduced. PRIOR ART DOCUMENTS PATENT DOCUMENTS

[0003] PATENT DOCUMENT 1 Japanese Unexamined Patent Publication No. 2014-238065 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0004] However, in the conventional configuration, since cylinders with different bore diameters also have different strokes, the crank structure is complicated, which is a factor that leads to an increase in the size of the engine. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a multi-cylinder engine that includes cylinders with different displacements, and that allows simplification of the crank structure and downsizing of the engine. MEANS FOR SOLVING THE PROBLEMS

[0005] As one aspect of the present disclosure, a multi-cylinder engine is provided, comprising: a first cylinder; a second cylinder having a larger bore diameter than the first cylinder; a first connecting rod having one end connected to a first piston sliding inside the first cylinder and the other end connected to a crank member; and a second connecting rod having one end connected to a second piston sliding inside the second cylinder and the other end connected to the crank member, wherein the first connecting rod and the second connecting rod are connected to the crank member such that the first piston and the second piston have the same stroke length. [Effects of the Invention]

[0006] According to the present invention, it is possible to provide a multi-cylinder engine that has cylinders with different displacements while simplifying the crank structure and making the engine smaller. [Brief explanation of the drawing]

[0007] [Figure 1] This is a side cross-sectional view of a saddle-type vehicle according to an embodiment of the present invention. [Figure 2] This is a diagram showing the power unit. [Figure 3] This is a perspective view showing the internal structure of the engine body from the output shaft side. [Figure 4] This diagram shows the crankshaft along with its surrounding components. [Figure 5] This is a diagram showing the cross-sectional structure including the first and second cylinders. [Figure 6] This is a perspective view of the valve train mechanism. [Figure 7] This diagram shows the valve train mechanisms on the first cylinder side and the second cylinder side, along with their surrounding components. [Figure 8] This diagram schematically shows the power output characteristics of the engine itself. [Modes for carrying out the invention]

[0008] Embodiments of the present invention will be described below with reference to the drawings. In the description, directions such as front, back, left, right, and up and down refer to directions relative to the vehicle body unless otherwise specified. In each figure, the symbol FR indicates the front of the vehicle body, the symbol UP indicates the top of the vehicle body, and the symbol LH indicates the left side of the vehicle body.

[0009] [Embodiment] [1. Overview of the saddle-type vehicle 10] Figure 1 is a side view of a saddle-type vehicle 10 according to an embodiment of the present invention. The saddle-type vehicle 10 is a vehicle that comprises a body frame 11, a power unit 12 supported by the body frame 11, a front fork 14 that supports the front wheel 13 in a steerable manner, a swing arm 16 that supports the rear wheel 15, and a seat 17 for the rider. The saddle-type vehicle 10 is a vehicle in which the occupant sits straddling a seat 17. The seat 17 is located above the rear of the vehicle frame 11.

[0010] The vehicle frame 11 comprises a head pipe 18 located at the front end of the vehicle frame 11, a front frame 19 located behind the head pipe 18, and a rear frame 20 located behind the front frame 19. The front end of the front frame 19 is connected to the head pipe 18. The seat 17 is supported by the rear frame 20.

[0011] The front fork 14 is supported by a head pipe 18 so that it can be steered left and right. The front wheel 13 is supported by an axle 13a located at the lower end of the front fork 14. A steering handle 21, which is held by the rider, is attached to the upper end of the front fork 14.

[0012] The swingarm 16 is supported by a pivot shaft 22 which is supported by the vehicle frame 11. The pivot shaft 22 is an axis that extends horizontally in the vehicle width direction. The pivot shaft 22 is inserted through the front end of the swingarm 16. The swingarm 16 swings up and down around the pivot shaft 22. The rear wheel 15 is supported by an axle 15a provided at the rear end of the swing arm 16.

[0013] The power unit 12 is arranged between the front wheel 13 and the rear wheel 15, and is supported by the vehicle body frame 11. The power unit 12 is an internal combustion engine. The power unit 12 includes a crankcase 23 and a cylinder portion 24 that accommodates a reciprocating piston. An exhaust device 25 is connected to the exhaust port of the cylinder portion 24. The output of the power unit 12 is transmitted to the rear wheel 15 by a driving force transmission member that connects the power unit 12 and the rear wheel 15.

[0014] In addition, the saddle-type vehicle 10 includes a front fender 26 that covers the front wheel 13 from above, a rear fender 27 that covers the rear wheel 15 from above, a step 28 on which an occupant places their feet, and a fuel tank 29 that stores fuel used by the power unit 12. The front fender 26 is attached to the front fork 14. The rear fender 27 and the step 28 are provided below the seat 17. The fuel tank 29 is supported by the vehicle body frame 11.

[0015] [1.1 Overview of Power Unit 12] Figure 2 is a diagram showing the power unit 12. The power unit 12 is a unit swing power unit in which an engine main body 31 and an arm portion 41 that supports the rear wheel 15 are integrated. The engine main body 31 constitutes an internal combustion engine and includes a crankshaft 32 extending in the vehicle width direction. The crankshaft 32 is rotatably supported by the crankcase 23. A generator 100 is provided at one end portion (the right end portion in the present embodiment) of the crankshaft 32. An output shaft 101 is integrally provided at the other end portion (the left end portion in the present embodiment). The output shaft 101 is arranged inside the arm portion 41.

[0016] The cylinder section 24 comprises, in order from the crankshaft 32 side, a cylinder block 51, a cylinder head 52, and a head cover 53. Figure 2 shows the internal structure of the cylinder block 51, and a portion of the outer shape of the cylinder block 51 is shown by a dashed line. The engine body 31 is a horizontal engine in which the cylinder section 24 extends almost horizontally in the front-rear direction of the vehicle, and in a side view of the vehicle, it has a structure that extends forward with a slight upward slope at the front.

[0017] Above the power unit 12 is the throttle body 61, which constitutes the throttle system. The throttle body 61 is connected to the rear (top) surface of the cylinder head 52 and controls the amount of air supplied to the engine body 31. Fuel from the injectors is also supplied to the engine body 31.

[0018] The engine body 31 is configured as a parallel twin-cylinder engine, and two intake passages 61a are provided spaced apart to the left and right in order to supply air and fuel independently to each cylinder. A throttle valve is independently positioned in each intake passage 61a, and the throttle valve is driven by a drive-by-wire motor. Note that the throttle body 61 is not limited to a drive-by-wire system, and may also be a manual type that drives the throttle valve using a throttle wire.

[0019] The cylinder section 24 houses the valve train mechanism 71, which constitutes the valve train. The valve train mechanism 71 will be described later. Figure 3 is a perspective view showing the internal structure of the engine body 31 from the output shaft 101 side. As shown in Figure 3, connecting rods 34a and 34b are connected to the crankshaft 32 at intervals from left to right, and pistons 35a and 35b are connected to the ends of each connecting rod 34a and 34b.

[0020] An output shaft 101, located at the left end of the crankshaft 32, is provided with a drive pulley 63 positioned within the arm portion 41. The drive pulley 63 constitutes part of the belt-type continuously variable transmission, and other components of the belt-type continuously variable transmission, such as the belt 63v and driven pulleys, are arranged within the arm portion 41. The belt 63v is stretched between the drive pulley 63 and the driven pulley, transmitting the rotation of the drive pulley 63 to the driven pulley. The rotation of the driven pulley is transmitted to the rear wheel 15. By varying the winding diameter of the drive pulley 63, the gear ratio between the drive pulley 63 and the driven pulley changes. Note that the belt-type continuously variable transmission is just one example of a power transmission mechanism that transmits the output of the power unit 12 to the drive wheel, the rear wheel 15, and other power transmission mechanisms may be used.

[0021] As shown in Figure 3, a sprocket 65 that rotates integrally with the crankshaft 32 is positioned to the right of the crankshaft 32, more specifically between the generator 100 and the connecting rod 34a. The rotation of the sprocket 65 is transmitted to the valve train 71 via the cam chain 66 (Figure 2).

[0022] [1.2 Structure around the crankshaft 32] Figure 4 shows the crankshaft 32 along with its surrounding components. The crankshaft 32 has a split structure that divides it into a first crankshaft 32a which constitutes the right side and a second crankshaft 32b which constitutes the left side. A first crank pin 36a is provided on the first crankshaft 32a. The base end of a first connecting rod 34a, which is connected to the right-side first piston 35a, is attached to this first crank pin 36a. Furthermore, a first crank web 37a is provided on the first crankshaft 32a for rotational balancing. The tip of the first connecting rod 34a is connected to the first piston 35a via a first piston pin 38a.

[0023] A second crank pin 36b is provided on the second crankshaft 32b. The base end of the second connecting rod 34b, which is connected to the left second piston 35b, is attached to this second crank pin 36b. Furthermore, a second crank web 37b is provided on the second crankshaft 32b for rotational balancing. The tip of the second connecting rod 34b is connected to the second piston 35b via a second piston pin 38b. The second crankshaft 32b is integrated with the output shaft 101.

[0024] The diameter of the second crankpin 36b is larger than that of the first crankpin 36a. Similarly, the diameter of the second piston pin 38b is also larger than that of the first piston pin 38a.

[0025] Figure 4 schematically shows the crankcase 23. The crankcase 23 has a left-right split structure, with the left case 23L being referred to as the "left crankcase 23L" and the right case 23R as the "right crankcase 23R". The split surface Lc, which marks the division between the left and right cases 23L and 23R, is located in or near the center of the left and right sides of the crankcase 23, engine body 31, power unit 12, and saddle-type vehicle 10, respectively. Hereafter, the position of the split surface Lc will be appropriately referred to as the "center Lc".

[0026] As shown in Figure 4, the first cylinder 31a, located on one side (right side) of the left-right central Lc, has a first cylinder 39a in which the first piston 35a slides. The second cylinder 31b, located on the other side (left side) of the left-right central Lc, has a second cylinder 39b in which the second piston 35b slides. The first cylinder 39a and the second cylinder 39b are formed within the cylinder block 51.

[0027] In this configuration, as shown in Figure 4, the bore diameter d2 of the second cylinder 39b is larger than the bore diameter d1 of the first cylinder 39a, and the first piston 35a and the second piston 35b are formed to have the same stroke length. More specifically, by positioning the central axes of each crank pin 36a and 36b at the same distance from the crank axis J1, the first piston 35a and the second piston 35b are set to have the same stroke length ST. Even though the stroke length ST of the first piston 35a and the second piston 35b are the same, the displacement of the first cylinder 31a and the second cylinder 31b is different because their bore diameters d1 and d2 are different.

[0028] Since the stroke lengths ST of the first piston 35a and the second piston 35b are the same, the same crank structure and manufacturing equipment used in a typical multi-cylinder engine where each cylinder has the same displacement can be applied. Therefore, the crank structure can be simplified while having cylinders 31a and 31b with different displacements.

[0029] In this configuration, by making the diameter of the second crankpin 36b larger than the diameter of the first crankpin 36a, it is possible to ensure rigidity to cope with the high combustion pressure on the second cylinder 31b side, which has a larger displacement, while reducing the weight on the first cylinder 31a side, which has a smaller displacement, thereby achieving weight reduction. Furthermore, by making the diameter of the second piston pin 38b larger than the diameter of the first piston pin 38a, it is possible to ensure rigidity to cope with the high combustion pressure on the second cylinder 31b side, which has a larger displacement, while reducing the weight on the first cylinder 31a side, which has a smaller displacement, thereby achieving weight reduction.

[0030] Figure 5 shows a cross-sectional structure including the first cylinder 39a and the second cylinder 39b. Figures 4 and 5 show the cylinder axis C1 passing through the center of the first cylinder 39a and the cylinder axis C2 passing through the center of the second cylinder 39b. The two cylinder axes C1 and C2 are parallel and at the same height. In this configuration, as shown in Figure 5, the structure of the cylinder block 51 is designed so that the cross-sectional shape on the side of the first cylinder 31a is made smaller and the cross-sectional shape on the side of the second cylinder 31b is made larger, thereby ensuring block rigidity that corresponds to the difference in displacement of each cylinder 31a and 31b, while also reducing weight.

[0031] As shown in Figure 5, each cylinder 31a, 31b is provided with multiple stud bolts 55a, 55b for fastening the cylinder block 51 and cylinder head 52 together. The spacing of the stud bolts 55b on the second cylinder 39b side is wider than the spacing of the stud bolts 55a on the first cylinder 39a side. This ensures connection rigidity corresponding to the difference in displacement between each cylinder 31a, 31b. A cam chain chamber 56 for housing the cam chain 66 is provided on the first cylinder 31a side. However, the cross-sectional shape of the cylinder block 51 and the number and arrangement of the stud bolts 55a and 55b can be changed as appropriate.

[0032] In this configuration, as shown in Figure 4, a recess 32h is formed in the first crankshaft 32a on the small displacement side for press-fitting a portion of the second crankshaft 32b on the large displacement side, and the first crankshaft 32a and the second crankshaft 32b are integrated by press-fitting the second crankshaft 32b into this recess 32h.

[0033] Furthermore, one of the bearings 45a, 45b, and 45c (45b in this configuration) that rotatably support the crankshaft 32 is positioned in the stepped portion 32j created by press-fitting the second crankshaft 32b into the recess 32h. The remaining bearings 45a and 45c are positioned on both sides of the crankshaft 32. This configuration allows the distance between the bearings 45b and 45c supporting the second crankshaft 32b (bearing span) K2 to be narrower than the distance between the bearings 45b and 45a supporting the first crankshaft 32a (bearing span) K1. This increases the support rigidity of the second crankshaft 32b, which is subjected to high combustion pressure, and optimizes the difference in support rigidity with that of the first crankshaft 32a.

[0034] As shown in Figure 5, the intake port 57a and exhaust port 58a of the first cylinder 31a are composed of a single passage. The intake port 57b and exhaust port 58b of the second cylinder 31b, which has a larger displacement, are each composed of two passages, and the two passages that make up the exhaust port 58b merge downstream.

[0035] [1.3 Structure of the valve train mechanism 71] Figure 6 is a perspective view of the valve train mechanism 71 that controls the opening and closing of the intake ports 57a, 57b and the exhaust ports 58a, 58b. Figure 7 is a diagram showing the valve train mechanism 71 on the first cylinder 39a side and the second cylinder 39b side, along with the surrounding configuration. As shown in Figures 6 and 7, the cylinder block 51 is equipped with a first intake valve 71a that opens and closes the intake port 57a on the first cylinder 39a side, a first exhaust valve 72a that opens and closes the exhaust port 58a on the first cylinder 39a side, a second intake valve 71b that opens and closes the two intake ports 57b on the second cylinder 39b side, and a second exhaust valve 72b that opens and closes the two exhaust ports 58b on the second cylinder 39b side.

[0036] Each valve 71a, 72a, 71b, and 72b is biased by a valve spring 73 to close each port 57a, 58a, 57b, and 58b. In Figure 6, reference numeral 74a indicates a plug port communicating with the combustion chamber of the first cylinder 31a, and reference numeral 74b indicates a plug port communicating with the combustion chamber of the second cylinder 31b. Spark plugs 75a and 75b are inserted into each plug port 74a and 74b.

[0037] The valve train 71 comprises a camshaft 76 positioned above the intake valves 71a and 71b, a rocker arm shaft 77 positioned parallel to the camshaft 76, and first rocker arms 78a, 79a on the first cylinder 31a side and second rocker arms 78b, 79b on the second cylinder 31b side, which are pivotably supported by the rocker arm shaft 77. This valve train 71 is a single-cam type valve train.

[0038] The camshaft 76 is equipped with cam groups 80a and 80b for driving the valves 71a, 72a, 71b, and 72b of each cylinder 31a and 31b. A cam sprocket 85 is connected to one end of the camshaft 76. The cam sprocket 85 rotates synchronously with the crankshaft 32 via a cam chain 66, thereby driving the valves 71a, 72a, 71b, and 72b. In this configuration, the first cylinder 31a is formed as a two-valve structure with one intake valve 71a and one exhaust valve 72a, while the second cylinder 31b, which has a larger displacement than the first cylinder 31a, is configured as a four-valve structure with two intake valves 71b and two exhaust valves 72b. Therefore, the first rocker arms 78a, 79a and the second rocker arms 78b, 79b have shapes corresponding to the difference in the number of valves.

[0039] The valve train 71 further includes a variable valve mechanism 86 that slides the cam groups 80a and 80b in the axial direction of the camshaft 76, thereby releasing them from the rocker arms 78a, 79a, 78b, and 79b, thereby suspending the operation of each valve 71a, 72a, 71b, and 72b. The valve train 71 also includes a decompression mechanism 87 that opens the exhaust valve 72b of the second cylinder 31b when the power unit 12 is started, thereby reducing the force required to crank the engine and improving starting performance.

[0040] The variable valve mechanism 86 includes a first slide member 86a that can move a cam group 80a for the first cylinder 31a in the axial direction of the camshaft 76, a first solenoid actuator 86b that can extend and retract a pin toward a groove ma provided in the first slide member 86a, a second slide member 86c that can move a cam group 80b for the second cylinder 31b in the axial direction of the camshaft 76, and a second solenoid actuator 86d that can extend and retract a pin toward a groove mb provided in the second slide member 86c.

[0041] The groove ma of the first slide member 86a is shaped to move the cam group 80a for the first cylinder 31a to a position away from the first rocker arms 78a and 79a by alternately causing two pins protruding from the first solenoid actuator 86b to protrude. As a result, by protruding one pin from the first solenoid actuator 86b, it is possible to put the first cylinder 31a into a deactivation state independently. When the other pin is protruding, the first slide member 86a moves to a position where the cam group 80a contacts the first rocker arms 78a and 79a, and the intake port 57a and exhaust port 58a can be opened and closed according to the profile of the cam group 80a.

[0042] Furthermore, the groove mb of the second slide member 86c is shaped to move the cam group 80b for the second cylinder 31b to a position away from the second rocker arms 78b and 79b by alternately causing two pins protruding from the second solenoid actuator 86d to protrude. This makes it possible to put the second cylinder 31b into a deactivation state independently by protruding one of the pins from the second solenoid actuator 86d. When the other pin is protruding, the second slide member 86c moves the cam group 80b to a position where it contacts the second rocker arms 78b and 79b, and the intake port 57b and exhaust port 58b can be opened and closed according to the profile of the cam group 80b. Note that the shape and structure of each part of the variable valve mechanism 86 may be changed.

[0043] The decompression mechanism 87 includes a decompression cam 87a positioned near the second rocker arm 79b used to operate the exhaust valve 72b of the second cylinder 31b, and a rotation mechanism 87b that rotates the decompression cam 87a in response to the operation of a lever provided on the handle 21. The operation of the lever is transmitted to the rotation mechanism 87b via a wire 87c, and in response to the operation of the lever, the decompression cam 87a is rotated between a retracted position, where it is moved away from the second rocker arm, and a pressed position, where it pushes down the second rocker arm 79b.

[0044] The decompression cam 87a has a cam surface 87a1 that retracts from the second rocker arm 79b in the retracted position and pushes the second rocker arm 79b in the pressed position. In response to the driver's operation of the lever, the exhaust valve 72b of the second cylinder 31b is opened, effectively reducing the force required for cranking.

[0045] Figure 8 is a schematic diagram showing the output characteristics of the engine body 31. In Figure 8, the horizontal axis represents engine speed NE, and the vertical axis represents engine power P. In Figure 8, characteristic P1 shows the state when only the first cylinder 31a is operating, characteristic P2 shows the state when only the second cylinder 31b is operating, and characteristic P3 shows the state when both the first cylinder 31a and the second cylinder 31b are operating. The first cylinder 31a has a two-valve structure, which simplifies the flow of air and fuel compared to a four-valve structure. As a result, the flow velocity of air and fuel in the combustion chamber is easier to increase, and furthermore, because the "stroke (S) / bore (B) ratio" is large, the intake airflow can be enhanced, which improves combustion efficiency.

[0046] The second cylinder, 31b, has a four-valve structure, which increases the total valve area compared to a two-valve structure, resulting in smoother intake and exhaust airflow. As a result, it offers advantages such as improved volumetric efficiency at high RPMs and increased power output. In this configuration, by utilizing the characteristics of both cylinders 31a and 31b, it is possible to improve combustion efficiency in the low-load range and increase output in the high-load range, thereby expanding the cylinder deactivation range.

[0047] As described above, the power unit 12 of this embodiment includes an engine body 31 for a multi-cylinder engine, which includes a first cylinder 39a, a second cylinder 39b having a larger bore diameter than the first cylinder 39a, a first connecting rod 34a with one end connected to a first piston 35a sliding inside the first cylinder 39a and the other end connected to a crank member, and a second connecting rod 34b with one end connected to a second piston 35b sliding inside the second cylinder 39b and the other end connected to a crankshaft 32 that functions as a crank member. The first connecting rod 34a and the second connecting rod 34b are connected to the crankshaft 32 such that the first piston 35a and the second piston 35b have the same stroke length ST. This makes it possible to have cylinders with different displacements while simplifying the crank structure, thus enabling simplification of the crank structure and miniaturization of the engine.

[0048] Furthermore, a generator 100 is provided at one end of the crankshaft 32, and an output shaft 101 is provided at the other end, with the second cylinder 39b positioned closer to the output shaft 101 than the first cylinder 39a. By positioning the second cylinder 39b, which has a larger displacement and generates greater torque, closer to the output shaft 101, the effect of torsional torque on the crankshaft 32 can be reduced. This reduces the rigidity required for the crankshaft 32, making the engine lighter and more compact.

[0049] Furthermore, the system includes a first crankpin 36a that connects the other end of the first connecting rod 34a to the crankshaft 32, and a second crankpin 36b that connects the other end of the second connecting rod 34b to the crankshaft 32, with the diameter of the second crankpin 36b being larger than the diameter of the first crankpin 36a. This configuration ensures rigidity to cope with the high combustion pressure on the second cylinder 39b side, which has a larger displacement, while reducing the weight by making the first crankpin 36a on the first cylinder 39a side, which has a smaller displacement, smaller.

[0050] Furthermore, since the cam chain 66 is installed between the first cylinder 39a and the generator 100, the first cylinder 39a and the second cylinder 39b can be positioned closer to the output shaft 101, thereby enabling further miniaturization and weight reduction of the crankcase.

[0051] Furthermore, the crankshaft 32 comprises a first crankshaft 32a, which functions as a first crank member to which the first connecting rod 34a is connected, and a second crankshaft 32b, which is constructed separately from the first crankshaft 32a and functions as a second crank member to which the second connecting rod 34b is connected. By making the crankshaft 32 a split structure, the machining work of the first crankshaft 32a and the second crankshaft 32b can be simplified.

[0052] Furthermore, the "crank member" in this disclosure may include crank webs (first crank web 37a and second crank web 37b). Also, since the second crankshaft 32b is integrated with the output shaft 101, the output generated on the second cylinder 39b side, which has a larger displacement, can be efficiently transmitted to the output shaft 101, thereby enabling the miniaturization of the engine.

[0053] Furthermore, by providing a crankcase 23 that houses the crankshaft 32, and arranging the split surface Lc of the crankcase 23 to be located between the first cylinder 39a and the second cylinder 39b, it is possible to miniaturize the engine while appropriately maintaining the rigidity of the crankcase 23.

[0054] [2. Other Embodiments] The above embodiments are merely one way of implementing the present invention, and can be modified, improved, and applied at will without departing from the spirit of the invention.

[0055] For example, in this embodiment, the bore diameters d1 and d2 of the two cylinders 31a and 31b are made different, while the stroke is kept the same, thereby making the displacement of each cylinder 31a and 31b different. However, it is also possible to make the displacement different by keeping the bore diameters d1 and d2 the same and making the stroke different. Furthermore, it is also possible to make the bore diameters d1 and d2 and the stroke different. Furthermore, although the present invention has been described in the context of applying it to a two-cylinder engine in the saddle-type vehicle 10 shown in Figure 1, it is not limited to this, and the present invention can be broadly applied to other saddle-type vehicles, mobile vehicles other than saddle-type vehicles, and multi-cylinder engines with three or more cylinders used in vehicles other than mobile vehicles.

[0056] [3. Configurations supported by the above embodiments] The above embodiment supports the following configuration.

[0057] (Configuration 1) A multi-cylinder engine comprising a first cylinder, a second cylinder having a larger bore diameter than the first cylinder, a first connecting rod with one end connected to a first piston sliding inside the first cylinder and the other end connected to a crank member, and a second connecting rod with one end connected to a second piston sliding inside the second cylinder and the other end connected to the crank member, wherein the first connecting rod and the second connecting rod are connected to the crank member such that the first piston and the second piston have the same stroke length. This configuration allows for a simpler crank structure while incorporating cylinders with different displacements, thus enabling a simpler crank structure and a smaller engine.

[0058] (Configuration 2) The multi-cylinder engine according to Configuration 1, wherein a generator is provided at one end of the crank member, an output shaft is provided at the other end, and the second cylinder is positioned closer to the output shaft than the first cylinder. This configuration allows for the placement of the second cylinder, which has a large displacement and generates high torque, on the output shaft side, thereby reducing the effect of torsional torque on the crankshaft. This reduces the required rigidity of the crankshaft, enabling a lighter and more compact engine.

[0059] (Configuration 3) A multi-cylinder engine according to Configuration 1 or 2, comprising a first crank pin connecting the other end of the first connecting rod to the crank member, and a second crank pin connecting the other end of the second connecting rod to the crank member, wherein the diameter of the second crank pin is larger than the diameter of the first crank pin. This configuration ensures rigidity to handle the high combustion pressure on the second cylinder side, which has a larger displacement, while also reducing weight by miniaturizing the diameter of the first crankpin on the first cylinder side, which has a smaller displacement.

[0060] (Configuration 4) The multi-cylinder engine according to Configuration 1 or 2, wherein a cam chain is installed between the first cylinder and the generator. This configuration allows the first and second cylinders to be positioned closer to the output shaft, enabling further miniaturization and weight reduction around the crankshaft.

[0061] (Configuration 5) The multi-cylinder engine according to Configuration 4, wherein the crank member comprises a first crank member to which the first connecting rod is connected, and a second crank member which is configured separately from the first crank member and to which the second connecting rod is connected. This configuration simplifies the machining process for the first and second crankshafts by using a split structure for the crankshaft.

[0062] (Configuration 6) The multi-cylinder engine according to Configuration 5, wherein the second crankshaft is integrated with the output shaft. This configuration allows for efficient transmission of power generated by the larger displacement second cylinder to the output shaft, enabling a smaller engine size.

[0063] (Configuration 7) A multi-cylinder engine according to any one of Configurations 2 to 6, comprising a split crankcase that houses the crank member, wherein the split portion of the crankcase is located between the first cylinder and the second cylinder. This configuration allows for engine miniaturization while maintaining adequate crankcase rigidity. [Explanation of Symbols]

[0064] 10... Saddle-type vehicle 12 Power Units 23 Crankcase 24 Cylinder section 31. Engine body (multi-cylinder engine) 32 Crankshaft (Crank component) 32a First crankshaft (first crank member) 32b Second crankshaft (second crank component) 34a First Connecting Rod 34b Second Connecting Rod 35a First piston 35b Second piston 36a First crankpin 36b Second crankpin 39a First Cylinder 39b Second Cylinder 66 Cam Chain 100 generators 101 Output shaft d1 Bore diameter of the first cylinder d2 Bore diameter of the second cylinder ST Stroke amount of the first and second pistons Lc split surface (crankcase split point)

Claims

1. In a multi-cylinder engine comprising a first cylinder (39a), a second cylinder (39b) having a larger bore diameter than the first cylinder (39a), a first connecting rod (34a) with one end connected to a first piston (35a) sliding inside the first cylinder (39a) and the other end connected to a crank member (32), and a second connecting rod (34b) with one end connected to a second piston (35b) sliding inside the second cylinder (39b) and the other end connected to the crank member (32), The first connecting rod (34a) and the second connecting rod (34b) are connected to the crank member (32) such that the first piston (35a) and the second piston (35b) have the same stroke length (ST). Multi-cylinder engine.

2. A generator (100) is provided at one end of the crank member (32), and an output shaft (101) is provided at the other end. The second cylinder (39b) is positioned closer to the output shaft (101) than the first cylinder (39a). A multi-cylinder engine according to claim 1.

3. A first crank pin (36a) connects the other end of the first connecting rod (34a) to the crank member (32), The second connecting rod (34b) is connected to the crank member (32) by a second crank pin (36b), The diameter of the second crankpin (36b) is larger than the diameter of the first crankpin (36a). A multi-cylinder engine according to claim 1 or 2.

4. A cam chain (66) is installed between the first cylinder (39a) and the generator (100). The multi-cylinder engine according to claim 2.

5. The crank member (32) is connected to a first crank member (32a) to which the first connecting rod (34a) is connected, The system includes a second crank member (32b) which is a separate component from the first crank member (32a) and to which the second connecting rod (34b) is connected. The multi-cylinder engine according to claim 4.

6. The second crank member (32b) is integrated with the output shaft (101). The multi-cylinder engine according to claim 5.

7. The crankcase (23) has a split structure that houses the crank member (32), The division point (Lc) of the crankcase (23) is located between the first cylinder (39a) and the second cylinder (39b). The multi-cylinder engine according to claim 2.

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