Engine unit of motorcycle

A plurality of cylinders are disposed in parallel in a vehicle width direction, a clutch chamber is disposed in either one of the right and the left in the vehicle width direction, an intake manifold, a fuel injection device, a throttle body, and an intake pipe are disposed between a cylinder assembly and an air cleaner in an upper part of a crankcase assembly. The throttle body is disposed on an opposite side in the vehicle width direction of the clutch chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-246533, filed on Nov. 8, 2012, and the Japanese Patent Application No. 2012-281712, filed on Dec. 25, 2012, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an engine unit of a vehicle such as an automobile, and specifically to a neighborhood of an intake system thereof.

2. Description of the Related Art

An intake system of an engine unit of a typical motorcycle has an intake tube connected to an intake port of an engine (internal combustion engine), and a throttle valve adjusting an opening degree of the intake tube. In a motorcycle on which a parallel multi-cylinder engine is mounted, a space in which a throttle body or an intake tube is disposed is generally smaller compared with that of a four-wheel vehicle. Thus, in Patent Documents 1, 2, constitutions in each of which a drive motor driving a throttle valve is disposed on a same side as that of a fuel injection valve are proposed. In these constitutions described in Patent Documents 1, 2, it is possible to dispose a drive motor in a vacant space generated in a case where a fuel injection valve is disposed in an intake tube, so that an increase in a size of the engine unit can be avoided.

Further, since the space in which the intake tube is provided is small, there is used a constitution in which the intake tube is extended to the inside of an air cleaner in order to obtain a necessary intake tube length. However, such a constitution causes a problem that a capacity of the air cleaner is decreased. In particular, in an engine where great importance is put on torque improvement in a low to medium rotation area, an intake tube length is tend to be set longer. In such an engine, the capacity of the air cleaner is decreased further, and output reduction (in particular, transient characteristic reduction) is brought about.

Besides, in an intake system of an engine unit of a motorcycle, in order to prevent output reduction due to intake interference, an independent throttle body in which a throttle valve is provided in each cylinder is adopted. Further, in order to improve an engine response by a throttle operation, a constitution in which a throttle valve is placed to a cylinder of an engine unit as close as possible is generally used.

However, in such an intake system, an intake inertia effect to be obtained becomes small in a slightly open region of the throttle valve. This is because in the slightly open region of the throttle valve, an intake tube length for obtaining the intake inertia effect is not a full length of an actual intake tube but is a length between an intake valve and the throttle valve. As a result, a charge efficiency of the slightly open region being a regular region becomes low. Then, a driver drives increasing a rotation number of an engine in order to obtain a necessary torque, bringing about aggravation of fuel consumption.

Further, the independent throttle body adopted in the motorcycle is manufactured by aluminum die casting. Such a constitution makes the independent throttle body heavier and more expensive, compared with a resin intake manifold for a four-wheel vehicle or the like.

In a conventional engine, in an intake system in particular, an intake tube is extended to the inside of an air cleaner, so that a substantial air cleaner capacity which can be used in an intake process of the engine is decreased by an amount of an inner capacity of the intake tube. An intake tube length is tend to be set longer in an engine aiming at torque improvement in a low and medium speed region in particular, and the air cleaner capacity is further decreased in such an engine, which results in output reduction (transient characteristic, in particular).

Further, in a layout of an independent throttle body type, an inertia effect obtained in a throttle valve subtly open region is small. This is because in the throttle valve subtly open region an actual intake tube full length is not an intake tube full length itself but a length from an intake valve to a throttle valve in a throttle body. As a result, a charging efficiency in the throttle valve subtly open region being a standard region is low, leaving no choice but to drive while increasing a rotation number in order to obtain a necessary torque, so that fuel consumption becomes worse if no measure is taken.

SUMMARY OF THE INVENTION

Under the circumstances, an object of the present invention is to provide an engine unit of a motorcycle, the engine unit securing sufficient intake tube length and air cleaner capacity and realizing compactification or the like effectively.

Further, an object of the present invention is to provide, in a motorcycle having a parallel multi-cylinder engine as an internal combustion engine, an engine unit of a motorcycle enabling a longer path of combustion air in an intake system without bringing about enlargement of a disposition space of the internal combustion engine.

An engine unit of a motorcycle according to the present invention is an engine unit of a motorcycle in which a plurality of cylinders are formed in parallel in a vehicle width direction and a cam chain chamber is formed on either one of sides in the vehicle width direction, the engine unit having: an air cleaner disposed above a main body of the engine unit and taking in and cleaning combustion air; one throttle body controlling a flow amount of the combustion air having been cleaned by the air cleaner; an intake pipe connecting the air cleaner and the throttle body in a manner that the combustion air is able to circulate; and an intake manifold distributing the combustion air whose flow amount is controlled by the one throttle body to the plurality of cylinders, wherein the one throttle body and the intake manifold are disposed above a crankcase assembly of the engine unit and behind a cylinder assembly of the engine unit, and wherein the throttle body is disposed in a position biased to a side on which the cam chain chamber is provided in terms of the vehicle width direction and in the rear diagonally downward of an intake port of the cylinder nearest to the cam chain chamber.

It is preferable that it is constituted so that the throttle body is disposed in a manner that a center axis of a throttle bore is almost in parallel to the vehicle width direction, and that an end part on a downstream side of the combustion air is coupled with the intake manifold and an end part on an upstream side of the combustion air is connected to the intake pipe.

It is preferable that it is constituted so that the intake manifold has the one manifold collecting section formed in a cylinder shape having a bottom and a plurality of intake tubes independent of each other and reaching the intake ports of the plurality of cylinders from the one manifold collecting section, and that the manifold collecting section is disposed coaxially with the center axis of the throttle bore of the throttle body on the downstream side of the combustion air of the throttle body, and is disposed on an inner side of a width of a cylinder head of the cylinder assembly in terms of the vehicle width direction.

It is preferable that it is constituted so that the plurality of intake tubes are bent in an arc shape in side view and are inclined in relation to a front and rear direction and extended in top view.

It is preferable that it is constituted so that a fuel injection device is disposed in a manner to protrude upward on an upper surface of the intake tube, and that the intake tube is constituted to overlap the intake pipe in side view.

It is preferable that it is constituted so that the throttle body is an electronic control type throttle body having a throttle valve and a drive motor driving the throttle valve, and that the drive motor is constituted to be positioned below the intake pipe in side view.

It is preferable that it is constituted so that the intake pipe is not extended to the inside of the air cleaner.

An engine unit of a motorcycle according to the present invention is an engine unit of a motorcycle in which a plurality of cylinders are disposed in parallel in a vehicle width direction and a clutch chamber is disposed in either one of the right and the left in the vehicle width direction, the engine unit having: an intake manifold, a fuel injection device, a throttle body, and an intake pipe disposed between a cylinder assembly and an air cleaner in an upper part of a crankcase assembly, wherein the throttle body is disposed on an opposite side in the vehicle width direction of the clutch chamber.

It is preferable that it is constituted so that a throttle bore center axis of the throttle body is set to be almost parallel to the vehicle width direction and that the throttle body is integrally fixed to the intake manifold.

It is preferable that it is constituted so that the throttle body is disposed on an inner side in the vehicle width direction of the cylinder assembly.

It is preferable that it is constituted so that the intake manifold includes a plurality of separated intake tubes connected to cylinder heads of respective cylinders and a manifold collecting section formed in a cylinder shape having a bottom, the manifold collecting section gathering the intake tubes integrally.

It is preferable that it is constituted so that the respective intake tubes are formed in curved shapes to round rearward in an upper part of the throttle body in side view and are disposed in a manner that inclines in relation to a longitudinal direction of the manifold collecting section are increased gradually or decreased gradually in rear view.

It is preferable that it is constituted so that the intake tube disposed farthest from the clutch chamber overlaps the throttle body in plan view.

It is preferable that it is constituted so that an engine is surrounded by a vehicle frame, that is, by a seat rail with regard to an upper part thereof, a down tube with regard to a front part, and a body tube with regard to a rear part, respectively, and that the fuel injection device, the intake tubes, a throttle valve, and its drive motor, from above in side view, are disposed tandem almost in parallel to a cylinder axis line in a space surrounded by the cylinder assembly, the air cleaner, and the crankcase assembly below the seat rail.

It is preferable that it is constituted so that the air cleaner is disposed in front of the body tube constituting the vehicle frame, and that the intake pipe connecting the throttle body and the air cleaner is connected without being extended to the inside of the air cleaner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a first embodiment of the present invention will be described in detail with reference to the drawings. In the following description, a direction of a motorcycle1is based on a direction of a driver riding on the motorcycle1. In each drawing, an upper side of the motorcycle1is indicated by an arrow Tp, a lower side is indicated by an arrow Bt, a front side is indicated by an arrow Fr, a rear side is indicated by an arrow Rr, a right side is indicated by an arrow R, and a left side is indicated by an arrow L. In the description of the present embodiment, a right and left direction is a vehicle width direction. Further, a vehicle width center of the motorcycle1is indicated by a center line CB.

First, an overall configuration of the motorcycle1having an intake system5according to the present embodiment (hereinafter, referred to simply as the motorcycle1) will be described with reference toFIG. 1andFIG. 2.FIG. 1is a right side view schematically showing the constitution of the motorcycle1.FIG. 2is a left side view schematically showing the constitution of the motorcycle1. As shown inFIG. 1andFIG. 2, the motorcycle1includes a vehicle body frame, a steering gear12, an engine unit3as an internal combustion engine, and a rear wheel suspension system13. An intake system5according to the present embodiment is applied to the engine unit3.

A twin spar frame11is applied to the vehicle body frame. The twin spar frame11includes a steering head pipe111, a right and left pair of side frames112, and body frames113. Each portion of the twin spar frame11is formed of, for example, an iron-based material or an aluminum alloy-based material, and is joined into a single unit by welding or the like.

The steering head pipe111is formed in a tube shape which inclines rearward. The right and left pair of side frames112extend from the steering head pipe111toward the rear diagonally outside respectively. The body frames113are provided on rear sides of the right and left pair of side frames112, and are formed to extend from rear end parts of the right and left pair of side frames112almost downward, in rear view. Then, a space for housing the engine unit3is formed between the right and left pair of side frames112, in top view.

Further, a right and left pair of seat rails114is provided on a rear side of the twin spar frame11. The right and left pair of seat frames114is formed to extend from the body frames113toward the rear, respectively. Further, the right and left pair of seat rails114are apart from each other in a right and left direction (vehicle width direction) with a predetermined distance.

The steering gear12is provided in a front part of the twin spar frame11(vehicle body frames) rotatably in relation to the twin spar frame11. The steering gear12is constituted by including a front wheel121, a steering shaft122, a right and left pair of front forks123, and a handle unit124.

The steering shaft122is rotatably supported by a steering head pipe111. The right and left pair of front forks123is disposed in the right and left of the steering shaft122. The front wheel121is rotatably supported by a lower end part on the right and left pair of front forks123. The front wheel12is provided with a brake disk in a manner to rotate integrally. Besides, the right and left pair of front forks123is provided with a brake rim acting on the brake disk. The handle unit124is provided in upper end parts of the steering shaft122and the right and left pair of front forks123. The handle unit124has a right and left handle grips125. The handle grip125on the right side is provided with a throttle grip and a brake lever operating the brake rim of the front wheel121. The throttle grip is rotatably provided in the handle grip125on the right side. Further, an accelerator position sensor (not shown) for detecting an operation amount (rotation angle) of the throttle grip is provided, for example, in the twin spar frame11. The accelerator position sensor transmits a detected operation amount of the throttle grip to an ECU (described later). The handle grip125on a left side is provided with a clutch lever which operates a clutch. Further, the handle unit124is provided with a meter unit in which meters are unitized and switches for operating lights (neither is shown).

A parallel multi-cylinder engine (here, four-cylinder engine is exemplified) of a side cam chain type is applied to the engine unit3as the internal combustion engine. A main body of the engine unit3includes a cylinder assembly31and a crankcase assembly32. In the cylinder assembly31, a plurality of (here, four) cylinders301(combustion chambers) are formed parallel to each other in a manner to be aligned in the right and left direction. The crankcase assembly32is provided with a crankshaft, a counter shaft, and a driven shaft each in a rotatable manner. Besides, a drive chain sprocket transmitting a rotary motive power to a rear wheel133is provided in a left side rear part of the driven shaft.

The engine unit3is mounted on the twin spar frame11. Then, a part of the engine unit3(here, a part of the cylinder assembly31) is positioned between the right and left pair of side frames112of the twin spar frame11.

Further, the engine unit3is provided with the intake system5which takes in combustion air from the outside and distributes to each cylinder301. The intake system5includes an air cleaner51, an intake pipe52, a throttle body53, and an intake manifold54. The air cleaner51takes in combustion air from the outside and cleans. The throttle body53controls a flow amount of the combustion air. The intake manifold54distributes the combustion air to each cylinder301. The intake pipe52is a flow path of the combustion air, and connects the air cleaner51, the throttle body53, and the intake manifold54.

Note that a detailed constitution of the intake system5of the engine unit3will be described later.

The motorcycle1is provided with an ECU (Engine Control Unit), whose illustration is omitted. The ECU detects an operation of each section by the driver, and controls the engine unit3in correspondence with the detected operation. Note that the ECU is a computer which has an CPU, a ROM, and a RAM. As a result that the CPU reads and executes a computer program stored in the ROM, control of the engine unit3is realized.

The rear wheel suspension system13includes a right and left pair of swing arms131, a shock absorber (hidden and not seen in the drawing), and the rear wheel133. The rear wheel suspension system13is provided on a rear side of the twin spar frame11, and is coupled with the body frames113in a vertically swingable manner.

A shock absorber is provided between the swing arm131and the body frame113or the seat rail114, and absorbs or alleviates a vibration or an impulse transmitted from the swing arm131to the body frame113or the seat rail114.

The rear wheel133is swingably supported by rear end parts of the swing arms131. The driven sprocket is provided on a left side of the rear wheel133in a manner to rotate integrally. A drive chain is wound around a drive sprocket of the engine unit3and the driven sprocket of the rear wheel133. A rotary motive power of the engine unit3is transmitted to the rear wheel133by the drive chain.

An exhaust system14includes a muffler141and an exhaust pipe142. The muffler141is disposed behind the engine unit3, beside the rear wheel133. One end part (front end part) of the exhaust pipe142is connected to an exhaust port303of the cylinder assembly31of the engine unit3. The other end part (rear end part) of the exhaust pipe142is connected to a front side of the muffler141. Besides, the exhaust pipe142goes toward the front from a front side of the engine unit3, bent toward the rear in front of the engine unit3, passes beside or below the engine unit3, and reaches the front side of the muffler141.

A seat201(driver's seat) on which the driver is seated and a seat202(tandem seat) on which a passenger is seated are mounted on an upper side of the seat rail114in a detachable manner. The seats201,202and the seat rail114are provided with rock mechanisms for fixing the seats201,202to the seat rail114. A fuel tank203is provided on an upper side of the right and left pair of side frames112, and on a front side of the seats201,202.

Further, the motorcycle1is provided with a cover member covering the outer side. The cover member includes a front cover204covering a front part of the motor cycle1, a side cover205covering a side part, and a rear cover206covering a rear part. These covering members are mounted on the twin spar frame11, the front fork123, or the like in a detachable manner. These covering members are shelf-shaped members, and are formed, for example, of a synthetic resin material or the like. These cover members form a design of an exterior appearance of the motorcycle1by covering the outer side of the motorcycle1.

Other than the above, the motorcycle1is provided with a front fender207covering an upper side of the front wheel, a rear fender covering an upper side of the rear wheel, a headlight209, a tail light210, a winker211and a driver's mirror212, and so on.

Next, a constitution of the engine unit3will be described with reference toFIG. 3toFIG. 6.FIG. 3toFIG. 5are drawings schematically showing overall constitutions of the engine unit3and its intake system5.FIG. 3is a right side view,FIG. 4is a left side view, andFIG. 5is a rear view.FIG. 6, being a cross-sectional arrow diagram taken along a VI-VI line ofFIG. 4, is a diagram schematically showing an internal constitution of the engine unit.

As described above, the parallel multi-cylinder engine of a side cam chain type is applied to the engine unit3. Note that in the present embodiment, though a parallel four-cylinder engine having four cylinders301(combustion chambers) formed in the engine unit3is exemplified, the number of the cylinders301is not limited thereto. It suffices if the engine unit3is a parallel multi-cylinder engine having a plurality of cylinders301.

As shown inFIG. 3toFIG. 5, the cylinder assembly31has a cylinder block311, a cylinder head312, and a cylinder head cover313.

The plural (four) cylinders301are formed in the cylinder block311. The plural cylinders301, whose axial lines are almost parallel to one another, are formed in a manner to be aligned in the right and left direction (vehicle width direction). Besides, a piston is disposed inside each cylinder301in a manner to be able to reciprocate. Note that the axis lines of the plural cylinders301incline forward.

The cylinder head312is provided on an upper side of the cylinder block311. As shown inFIG. 6, in the cylinder head312are formed, per each cylinder301, an intake port302supplying air-fuel mixture of fuel and combustion air to each cylinder301and an exhaust port303leading out exhaust gas form each cylinder301. Each intake port302is formed on a rear side of the cylinder head312, and can take in the air-fuel mixture from the rear of the cylinder head312. Each exhaust port303is formed on a front side of the cylinder head312, and can lead out the exhaust gas to the front of the cylinder head. Further, in an upper part of the cylinder head312, there is provided a valve moving device opening/closing each cylinder301and each intake port302, and each exhaust port303.

The cylinder head cover313is provided on an upper side of the cylinder head312. The cylinder head cover313covers the valve moving device and so on from the upper side.

Here, the valve moving device provided in the cylinder head312and a drive mechanism thereof will be described briefly.

The valve moving device and its drive mechanism have an intake valve, an exhaust valve, an intake side cam shaft, an exhaust side cam shat, and a cam chain.

The intake valve opens/closes between each cylinder301and each intake port302. The exhaust valve opens/closes between each cylinder301and each exhaust port303. The intake side cam shaft has a cam driving each intake valve. The exhaust side cam shaft has a cam driving each exhaust valve. The intake side cam shaft and the exhaust side cam shaft are rotatably provided in the cylinder head312in a manner that their axis lines become parallel in the right and left direction. Note that conventional known configurations can be applied to all the intake valve, the exhaust valve, the intake side cam shaft, and the exhaust side cam shaft.

The driven sprocket is provided in an end part (in the present embodiment, right side end part) of each of the intake side cam shaft and the exhaust side cam shaft. Meanwhile, a crankshaft (described later) is provided with the drive sprocket. The cam chain is wound around the driven sprocket and the drive sprocket. Thereby, the intake side cam shaft and the exhaust side cam shaft rotate in synchronicity with the crankshaft, and each intake valve and each exhaust valve act in synchronicity with a rotation of the crankshaft.

As shown inFIG. 6, a cam chain chamber304is formed on either one of sides (in the present embodiment, right side) in the right and left direction of the cylinder assembly. The cam chain is housed in the cam chain chamber304.

As described above, since the cam chain chamber304is formed on the one side of the cylinder assembly31, a center CIof the intake ports302is biased from a vehicle width center CBto an opposite side (left side) of the side on which the cam chamber304is formed.

As shown inFIG. 3toFIG. 5, the crankcase assembly32is provided on a lower side of the cylinder assembly31. The crankshaft assembly32is provided with the crankshaft, a counter shaft, a driven shaft, the clutch, and a transmission system (speed change gear). The crankshaft, the counter shaft, and the driven shaft are provided inside the crankcase assembly32rotatably and in a manner to be almost parallel to one another.

The crankshaft is coupled with each piston provided in each cylinder301by a connecting rod. The crankshaft and the counter shaft are connected by the clutch in a manner that a rotary motive power is able to be on and off. The clutch is provided on a right side of the crankcase assembly32. The transmission system is constituted between the counter shaft and the driven shaft.

A left side end part of the driven shaft protrudes to an outer side of the crankcase assembly32, and the drive chain sprocket is provided in this left side end part. Besides, the drive chain is wound around the drive chain sprocket and the driven chain sprocket of the rear wheel133.

The crankshaft, the counter shaft, and the driven shaft are disposed almost in parallel to one another in the right and left direction. The counter shaft is disposed behind the crankshaft, and further, the driven shaft is disposed behind the counter shaft. Thus, the crankcase assembly32projects from an lower side of the cylinder assembly31toward the rear. Therefore, the engine unit3as a whole has an almost L-shaped constitution in side view.

The intake system5has the air cleaner51, the intake pipe52, one throttle body53, and the intake manifold54. The air cleaner51takes in combustion air from the outside and cleans. The intake pipe52is a path for supplying the combustion air to the intake manifold54. The throttle body53controls a flow amount of the combustion air. The intake manifold54distributes the combustion air to each cylinder301.

The air cleaner51is provided above a main body of the engine unit3. In particular, a front part of the air cleaner51is positioned above (directly above) the cylinder head cover313. On the other hand, a rear part of the air cleaner51is positioned more rearward than the cylinder head cover313.

The front part of the air cleaner51is provided with an intake portion511for taking in combustion air from the outside. The intake portion511has a constitution of a tube shape which protrudes from the front part of the air cleaner51toward the front diagonally outer side.

A filter element513cleaning the air having been taken in by filtration is provided inside the air cleaner51.

A lead out portion512for supplying the cleaned combustion air is provided in a rear part of the air cleaner51. More specifically, the lead out portion512is provided in a position from a side surface to a lower surface of a side (in the present embodiment, right side) on which the cam chain chamber304of the engine unit3is provided, in the rear part of the air cleaner51. The lead out portion512has a constitution of a tube protruding downward. Note that the lead out portion512protrudes to the outside of the air cleaner51but does not protrudes to the inside thereof (seeFIG. 5,FIG. 13).

The intake pipe52, the throttle body53, and the intake manifold54are disposed behind the cylinder assembly31and above the crankcase assembly32. Further, since the rear part of the air cleaner51is positioned more rearward than the cylinder head cover313, the throttle body53and the intake manifold54are positioned below the rear part of the air cleaner51. In other words, on a rear side of the engine unit3, a region whose front, upper part, and lower part are surrounded by the cylinder assembly31, the crankcase assembly32, and the air cleaner51is formed. The intake pipe52, the throttle body53, and the intake manifold54are disposed in this region.

Here, detailed constitutions of the intake pipe52, the throttle body53, and the intake manifold54will be further described with reference toFIG. 7toFIG. 9.FIG. 7toFIG. 9are views schematically showing constitutions of the intake pipe52, the throttle body53, and the intake manifold54.FIG. 7is a top view,FIG. 8is a rear view, andFIG. 9is right side view, respectively. Note that in the following description “upstream side” and “downstream side” indicate an upstream side and a down stream side of the flow direction of combustion air in the intake system5, respectively.

The intake pipe52has the tube-shaped constitution. An end part on the upstream side of the intake pipe52is connected to the lead out portion512of the air cleaner51, while an end part on the downstream side is connected to the throttle body53. An axis line of the end part on the upstream side of the intake pipe52is almost parallel to a top and bottom direction. On the other hand, an axis line of the end part on the downstream side of the intake pipe52is almost parallel to the right and left direction. An axis line of the intake pipe52is bent in a middle part. As described above, the intake pipe52makes combustion air supplied downward by the air cleaner51flow into the throttle body53from a right side (side on which the cam chain chamber304of the engine unit3is provided).

An electronic control type throttle body is applied to the throttle body53. The throttle body53has a throttle valve534capable of changing an opening degree of a path533of combustion air, a drive motor531being a driving force source of the throttle valve534, and a driving force transmitting mechanism portion transmitting a driving force of the drive motor531to the throttle valve534. The ECU controls the drive motor531of the throttle body53, in correspondence with an operation amount of the throttle grip detected by the accelerator position sensor. Thereby, a flow amount of the combustion air passing through the throttle body53is controlled.

The intake manifold54has one manifold collecting section541and a plurality, corresponding to the number of the intake ports302, of (here, four) intake tubes542. The intake manifold54is made of a resin material and manufactured by injection molding or the like. Or, the intake manifold54is made of aluminum and manufactured by casting. As just described, a resin molding product or an aluminum casting product leads to reduction in weight or manufacturing cost of the intake manifold54.

The manifold collecting section541has a constitution of a cylinder shape having a bottom, and one end part (end part on the upstream side) in an axial direction is open, while the other end part (end part on the downstream side) is closed. In the drawing, the constitution of the manifold collecting section541formed in the cylinder shape is shown, but a cross-sectional shape of the manifold collecting section541is not limited to a circle. An axis line of the manifold collecting section541is parallel to the right and left direction, and the manifold collecting section541is disposed in a manner that the open end part on the upstream side is positioned on a side on which the cam chain chamber30of the engine unit3is provided and that the closed end part on the downstream side is positioned on a side opposite thereto. The manifold collecting section541is disposed in a position biased from the vehicle width center CBto an opposite side (left side) in the right and left direction of the cam chamber304(seeFIG. 5and so on).

Each intake tube542connects the manifold collecting section541and each intake port302of the cylinder assembly31in a manner that combustion air is flowable. In side view, an axis line of each intake tube542is bent in an arc shape projecting upward. More specifically, in side view, each intake tube542extends upward diagonally forward from an upper surface of the manifold collecting section541being a starting point, being bent in an arc shape to go along an outer peripheral surface of the manifold collecting section541, reaches each intake port302from the rear diagonally upper side.

As described above, the intake manifold54has one manifold collecting section541and the plural intake tubes542, and has a constitution in which the plural intake tubes542branch from the one manifold collecting section541.

Further, center lines CPof at least two of plural intake tubes542incline by a predetermined angle in relation to the front and rear direction in top view. More specifically, as shown inFIG. 7in particular, the three intake tubes542positioned on the upstream side of the manifold collecting section541incline in a manner to be displaced toward a side (here, right side) on which the cam chain chamber304is provided, as going from the manifold collecting section541(upstream side) of the intake manifold54toward each exhaust port303(downstream side). On the other hand, one intake tube542positioned on the most downstream side of the manifold collecting section541inclines in a manner to go toward an opposite side of the cam chain chamber304, as going from the manifold collecting section541of the intake manifold54toward the intake port302.

Further, a length of each intake tube542is set to be a length capable of maintaining synchronism of each cylinder301. As described above, the manifold collecting section541of the intake manifold54has the cylindrical constitution, and each intake tube542extends from the outer periphery surface of the manifold collecting section541. Therefore, as a result that a position in the front and rear direction of a starting point of each intake tube542is made different from each other, the length of each intake tube542can be set. According to such a constitution, the length of each intake tube542can be set individually without enlarging a dimension in the front and rear direction of the intake manifold541. Thus, it is possible to set the length of the intake tube542to be a length necessary to obtain an inertia effect at a target rotation number and a length maintaining synchronism of each cylinder301, without enlarging a dimension in the front and rear direction of the motorcycle1. Therefore, it becomes possible to obtain a sufficient inertia effect in a whole area of opening degrees of the throttle valve534. As a result, improvement of an output, improvement of a fuel consumption, reduction of a noise, and improvement of a cleaning efficiency can be done.

Further, each intake tube542is provided with an injector502being a fuel injection device for mixing fuel into combustion air. Further, a fuel pipe501supplying fuel to each injector502is provided in the front diagonally upward of the manifold collecting section541of the intake manifold54. More detailedly, each injector502is provided on an upper surface of each intake tube542to protrude upward. The fuel pipe501is provided to be almost parallel to a center line CAof the manifold collecting section541of the intake manifold54.

Here, an assembling constitution of the intake pipe52, the throttle body53, and the intake manifold54will be described.

As shown inFIG. 3toFIG. 5andFIG. 7toFIG. 9, the throttle body53is provided on the side where the cam chain chamber304of the engine unit3is provided, the side being the end part on the upstream side of the manifold collecting section541of the intake manifold54. In particular, the throttle body53is fixed in a manner to be integral with the manifold collecting section541of the intake manifold54. Besides, the end part on the downstream side of the intake pipe52is connected to the end part on the upstream side of the throttle body53. Further, the throttle body53is disposed in a manner that a center line CT(center axis of a throttle bore) of the path533of combustion air thereof is parallel to the right and left direction. Further, as described above, the center line CAof the manifold collecting section541of the intake manifold54and the axis line of the end part on the downstream side of the intake pipe52are also parallel to the right and left direction. As just described, the end part on the downstream side of the intake pipe52, the throttle body53, and the manifold collecting section541of the intake manifold54are aligned in series in the right and left direction. The throttle body53and the manifold collecting section541of the intake manifold54are disposed coaxially.

According to such a constitution, joining of the intake pipe52and the throttle body53can be done smoothly. Further, because of the constitution in which the throttle body53and the intake manifold54are disposed to be aligned in the right and direction, increase in dimension in the front and rear direction of the intake system5can be prevented or suppressed.

Further, as shown inFIG. 4andFIG. 9, in side view, each intake tube542and the intake pipe52overlap each other. In other words, the intake pipe52, in side view, whose axis line is almost parallel to the top and bottom direction, extends from the rear part of the air cleaner51downward and reaches a center of the intake manifold54from above. Thus, in side view, the intake pipe52overlaps an upper part of the manifold collecting section541of the intake manifold54. On the other hand, each intake tube542of the intake manifold54reaches each intake port302of the engine unit3, while being bent in an arc shape to project upward with the upper part of the manifold collecting section541being a starting point. Therefore, in side view, the intake pipe52overlaps at least a part of each of the respective intake tubes542.

Further, as shown inFIG. 4,FIG. 9and so on, the throttle body53is disposed in a manner that the drive motor531is placed below the path533of combustion air. In particular, as shown inFIG. 7, in top view, the throttle body53is disposed in a manner that the drive motor531of the throttle valve534overlaps the path533of combustion air. In other words, as shown inFIG. 4,FIG. 9and so on, disposition positions of the drive motor531of the throttle body53and of the intake pipe52overlap each other in terms the forward and rear direction. Therefore, at least a part of the drive motor531of the throttle body53is positioned on a lower side (in particular, directly below) of a lower surface of the intake pipe52in side view. According to such a constitution, it is possible to prevent the drive motor531from protruding toward a rear side. Therefore, a dimension in the front and rear direction of the intake system5can be made smaller. Note that in the present embodiment there is shown a constitution in which a driving force transmission portion532of the throttle body53is positioned on the rear side of the intake pipe52in side view, but a constitution can be one where the driving force transmission portion532is positioned on a front side.

Next, positional relationships between the intake system5and the twin spar frame11and the fuel tank203will be described with reference toFIG. 10toFIG. 13, and so on.FIG. 10toFIG. 12are views schematically showing the positional relationships between the engine unit3and its intake system5, and the twin spar frame11.FIG. 10is a top view,FIG. 11is a right side view, andFIG. 12is a right side view showing the side frame112of the right side and so on being cut, respectively.FIG. 13is a cross-sectional view of the engine unit3, the intake system5, and the fuel tank203which are cut along a vehicle width center CB, and a view showing a relationship between the intake system5and the fuel tank203.

As shown inFIG. 10, the cylinder assembly31of the engine unit3, the intake pipe52, the throttle body53, and the intake manifold54are disposed between the right and left pair of side frames112of the twin spar frame11in top view.

The intake pipe52and the throttle body53are disposed in positions biased from the vehicle width center CBto a side on which the cam chain chamber304is formed in terms of the right and left direction. On the other hand, the intake manifold54is disposed in a position biased from the vehicle width center CBto the opposite side of the cam chain chamber304in terms of the right and left direction. Then, as shown inFIG. 10and so on, the intake pipe52, the throttle body53, and the intake manifold54are positioned, in top view, behind the cylinder head312, and in the inner side than a full width of the cylinder head312in terms of the right and left direction. More specifically, the disposition is as follows.

The engine unit3is of a side cam chain type, and the cam chain and the cam chain chamber304are provided on either one of sides (here, right side) in the right and left direction of the cylinder assembly31. Thus, the cylinder301and the center CIof the intake ports302of respective cylinders301are biased from the vehicle width center CBto an opposite side of the side on which the cam chain chamber304is provided. Then, the intake manifold54is disposed in a position biased from the vehicle width center CBto the opposite side of the cam chain chamber304in terms of the right and left direction, in correspondence with bias of the intake port302. Therefore, a space is formed behind the cam chain chamber304of the engine unit3. The intake pipe52and one throttle body53are disposed in this space behind the cam chain chamber304. In particular, as shown inFIG. 10andFIG. 12, the throttle body53is disposed in the rear diagonally downward of the intake port302of the cylinder301nearest to the cam chain chamber304.

According to such an assembling constitution, it is possible to dispose the intake pipe52, the throttle body53, and the intake manifold54in the inner side than the full width of the cylinder head312in terms of the right and left direction. Thus, it is possible to dispose the intake pipe52and the throttle body53without enlarging a distance between the right and left pair of side frames112of the twin spar frame11. Therefore, an increase in dimension in the right and left direction of the twin spar frame11can be prevented.

On the other hand, a constitution of the present embodiment is not a constitution in which a throttle body is provided in each intake port302, but is a constitution in which one throttle body53controls supply amounts of combustion air of all the intake ports302. According to such a constitution, it suffices if one throttle body53is disposed in the position biased from the vehicle width center CBto the side of the cam chain chamber304of the cylinder head312. Thus, a disposition space of the intake manifold54can be made larger. Therefore, compared with the constitution having individual throttle bodies, a passing path of combustion air in the intake manifold54can be made longer. Conventionally, an end part on an upstream side of an intake pipe52is extended to the inside of an air cleaner51in order to make a passing path of combustion air longer. In contrast, in the present embodiment, the passing path of combustion air can be made longer without extending the intake pipe52to the inside of the air cleaner51. Therefore, decrease in capacity of the air cleaner51can be prevented.

As shown inFIG. 13, the fuel tank203is disposed above the engine unit3. Further, the air cleaner51is disposed above the engine unit3, and the intake pipe52, the throttle body53, and the intake manifold54are disposed behind the cylinder assembly31of the engine unit31. Thus, in order to avoid interference with such an intake system5, a recess213recessed toward an upper side is formed in a lower part of the fuel tank203. At least a part of the intake system5gets into this recess213.

According to the present embodiment, since the dimension in the front and rear direction of the intake system5can be made smaller, interference between the intake system5and the fuel tank203can be made smaller, so that the recess213formed in the fuel tank203can be made smaller. Therefore, decrease of a capacity of the fuel tank203can be prevented or suppressed. In other words, a constitution in which a recess213is formed on a lower side of the fuel tank203decreases a capacity of a fuel tank203by a capacity of the recess213. Thus, when the recess213becomes larger, the capacity of the fuel tank203is decreased. In order to make the capacity of the fuel tank203larger with the recess213being as it is, it is necessary to enlarge an outside dimension of the fuel tank203. This leads to increase in size of the motorcycle1. In contrast, according to the constitution of the present embodiment, since the dimension in the front and rear direction of the intake system5can be made small, a dimension of the recess213formed in the fuel tank203can be made small. Therefore, it is possible to prevent interruption with the intake system5and to make the capacity of the fuel tank203larger without increasing the outside dimension.

Actions and effects of the intake system5according to the present embodiment are summarized as follows.

In the multi-cylinder parallel engine of the side cam chain type, the center CIof the intake ports302is biased from the vehicle width center CBto the opposite side of the cam chain chamber304in terms of the right and left direction. Thus, a space is formed behind the cam chain chamber304, in a range from the cylinder301nearest to the cam chain chamber304to the outer side end of the cylinder head312. The intake pipe52which connects the air cleaner51disposed on the upper side of the engine unit3and the throttle body53is disposed in this space. According to such a constitution, it is possible, without broadening a width of the right and left pair of side frames112disposed on both outer sides of the engine unit3, to dispose the intake pipe52therebetween.

Besides, one throttle body53is disposed closer to the vehicle width center CBthan the intake pipe52, in the rear diagonally downward of the intake port302of the cylinder301nearest to the cam chain chamber304. According to such a constitution, the intake pipe52can be disposed as described above, in consideration of assembling of the intake pipe52and the throttle body53.

When the constitution is such that the throttle body53is disposed being biased to the side on which the cam chain chamber304is provided in terms of the right and left direction, the space for disposing the intake manifold54can be secured behind the cylinder head312. All the intake tubes542of the intake manifold54gather in one manifold collecting section541, and the amount of combustion air is controlled by one throttle body53. Therefore, according to such a constitution, it is possible to make the passing path of combustion air in the intake system5have a length necessary for obtaining an inertia effect.

The throttle body53, the center line CT(center axis of a throttle bore) of the path533of combustion air being disposed almost in parallel to the right and left direction (vehicle body direction), is integrally fixed to the manifold collecting section541of the intake manifold54. Further, the intake pipe52is connected to the throttle body53. Besides, the throttle body53is disposed in the position biased to the side on which the cam chain chamber304is provided in a manner that the center line CTof the path533of combustion air is parallel to the right and left direction. Thus, it is possible to connect the throttle body53and the intake pipe52smoothly. Further, the intake manifold54is disposed on the opposite side of the cam chain chamber304, when viewed from the throttle body53, coaxially with the center line CTof the path533of combustion air of the throttle body53. Thus, it is possible to make the dimension in the front and rear direction of the intake system5as a whole smaller. Therefore, the motorcycle1can be made compact. Further, since the dimension in the front and rear direction of the intake system5can be made smaller, interference between the intake system5and the fuel tank203can be prevented or reduced. Therefore, decrease of the capacity of the fuel tank203can be prevented or suppressed.

The intake manifold54has one manifold collecting section541and the plural intake tubes542. The plural intake tubes542extend from the manifold collecting section541independently of each other and are linked to respective cylinders301. In other words, the plural intake tubes542integrally gather in the manifold collecting section541. The manifold collecting section541of the intake manifold54is disposed behind the cylinder assembly31, and above the crankcase assembly32. Further, the manifold collecting section541of the intake manifold54is disposed coaxially to the center line CTof the path533of combustion air of the throttle body53. Further, the manifold collecting section541of the manifold54is disposed on the downstream side of the throttle body53, and nearer to the vehicle width center CBthan a full width of the cylinder head312. According to such a constitution, it is possible to have an effect similar to the above.

The intake tube542of the intake manifold54is formed to be bent in an arc shape in side view. Further, the plural of two or more intake tubes542are formed in a manner that their center lines CPincline in the front and rear direction in top view. More specifically, the intake tubes542incline in a manner to move to the side of the cam chain chamber304, as going from the upstream side (side of the manifold collecting section541) to the downstream side (side of the intake port302).

According to such a constitution, it is possible to secure a length of the intake tube542necessary for obtaining an inertia effect at a target rotation number. Further, the length of the intake tube542can be changed to maintain synchronism of each cylinder301without enlarging the dimension in the front and rear direction of the intake system5. Thus, a sufficient inertia effect can be obtained in the whole area of opening degrees of the throttle valve524. Therefore, improvement of the output, improvement of the fuel consumption, reduction of the noise, and improvement of the cleaning efficiency can be done.

The injector502is disposed on an upper surface of each intake tube542of the intake manifold54in a manner to stand almost vertically (in a manner to protrude upward). The intake tube542of the intake manifold54and the intake pipe52overlap each other in side view. According to such a constitution, the dimension in the front and rear direction of the intake system5as a whole can be made smaller while the length of the intake tube542is secured to be a length sufficient for obtaining an inertia effect.

An electronic control type throttle body is applied to the throttle body53. The drive motor531for driving the throttle valve534is disposed to be positioned below the intake pipe52in side view. In other words, in top view, the drive motor531overlaps the path533of combustion air of the throttle body53. According to such a constitution, the dimension in the front and rear direction of the intake system5as a whole can be made smaller. Therefore, it becomes possible to dispose the intake system5in a small space.

In the present embodiment, as described above, the intake tube542can be formed to have a length sufficient for obtaining an inertia effect. Thus, the intake pipe52connecting the air cleaner51and the throttle body53is not required to be extended to the inside of the air cleaner51. With the constitution in which the intake pipe52is not extended to the inside of the air cleaner51, the capacity of the air cleaner51is not reduced. Therefore, an input can be improved.

FIG. 14is a left side view showing a state where an engine unit is mounted on a vehicle body frame in a motorcycle according to the present embodiment,FIG. 15is a right side view thereof, andFIG. 16is a top view thereof. A neighborhood of the engine unit will be described by usingFIG. 14toFIG. 16. Note that in the drawings used in the following description, a front of a vehicle is indicated by an arrow Fr, a rear of the vehicle is indicated by an arrow Rr, respectively, and a lateral right side of the vehicle is indicated by an arrow R, and a lateral left side of the vehicle is indicated by an arrow L, respectively, as appropriate.

In a motorcycle600, a seat rail601made of steel or an aluminum alloy material is connected to a rear part of a steering head pipe602, a right and left pair branching rearward, once widening from the steering head pipe602, and extending diagonally downward. A down tube603also extends downward by a steep angle from the steering head pipe603, and is bent rearward at a lower end part thereof. Further, a body tube604extends diagonally downward from around a rear part of the seat rail601, and a rear end part on the seat rail601is connected to a side tube605which inclines properly to rise rearward, and a vehicle body frame is constituted by the above frames. The vehicle body frame has a shape of almost a basket inside which a housing space for an engine unit60is formed, and the engine unit60is mounted as shown in the drawing.

Although illustration is omitted, an entire constitution of the motorcycle600will be described in summary. The motorcycle600is provided with a right and left pair of two front forks supported by the steering head pipe602in a pivotal manner from side to side. A handle bar is fixed to an upper end of the front fork, and a front wheel is rotatably supported at a lower end part thereof. Further, a swing arm606(seeFIG. 14and so on) is swingably coupled on a rear side of the engine unit60, and a rear wheel is rotatably supported by an end part thereof. The rear wheel is configured to be rotation-driven via a driven sprocket around which a chain transmitting a motive power of the engine unit60is wound.

A fuel tank is mounted above the engine unit60mounted on the vehicle body frame, and fuel is supplied from the fuel tank by a fuel supply system and air is supplied from an air cleaner constituting a later-described intake system. Air-fuel mixture made of such fuel and air is supplied to the engine unit60and exhaust gas after combustion in an engine is exhausted through an exhaust pipe.

Next, a constitution example of the engine unit (internal combustion engine) in the present embodiment will be described. The engine unit60of the present embodiment, as shown inFIG. 17, is made as a result that a cylinder block62, a cylinder head63, and a cylinder head cover64are integrally united in sequence on an upper part of a crankcase assembly61, and the above forms a cylinder assembly. The cylinder assembly is disposed in a posture inclining forward properly. Further, the engine unit60is integrally unite-supported by the vehicle body frame by being suspended by the down tube603and so on via a plurality of engine mounts. In this example, four cylinder blocks62are disposed in parallel in a right and left (vehicle width) direction, and a piston is fit into each cylinder bore of the cylinder block62in a freely slidable manner in a cylinder axis direction. Note that the cylinder blocks are No. 1 (#1), No. 2 (#2), No. 3 (#3), and No. 4 (#4) cylinders (combustion chambers) in sequence from the left side. A center line (FIG. 16, vehicle center line Y) in a vehicle front and rear direction is set to pass almost between the #2 cylinder and the #3 cylinder.

The crankcase assembly61, upper and lower halved upper crankcase and lower crankcase being united with each other, supports a shaft of a crankshaft65or the like by a uniting surface thereof. In the crankcase assembly61, the crankshaft65(outlined by a dashed line inFIG. 17) is disposed in the vehicle width direction. In this case, a plurality of journal portions of the crankshaft65is axially supported by a plurality of journal bearing portions set in the joining surface of the crankcase assembly61. A generator which includes a generator coil and a generator rotor is disposed and constituted in a left shaft end part of the crankshaft65. In this case, the generator coil is attached and supported by an inner side of a magneto cover66. Note that a sprocket for driving a cam being a valve moving device is attached to a right shaft end part of the crankshaft65.

A valve moving device open/close controlling an intake valve and an exhaust valve is housed in the cylinder head63. These valves are driven by an intake cam and an exhaust cam provided in an intake cam shaft and an exhaust cam shaft, respectively. Each cylinder has an intake port68which opens toward the rear of the cylinder head63and flowing into the cylinder of air-fuel mixture supplied from the intake system to the intake port68is controlled by the intake valve.

A rear part of the crankcase assembly61doubles as a transmission case69, and a transmission gear is housed and disposed inside thereof. In the transmission case69, a counter shaft70(outlined by a dashed line inFIG. 17) is disposed at a predetermined interval rearward in parallel to the crankshaft65, and further a drive shaft71(outlined by a dashed line inFIG. 17) is disposed at a predetermined interval rear downward. Among the above, the counter shaft70is supported by the joining surface of the crankcase assembly61. Further, in the transmission case69between the counter shaft70and the drive shaft71, a transmission system (speed change gear) with a predetermined stages is disposed and constituted, and a rotation of the counter shaft70is speed-changed and transmitted to the drive shaft71via the transmission system. A drive sprocket is attached to a left shaft end part of the drive shaft71, and via a chain wound between the drive sprocket and a sprocket bridged to an axle of the rear wheel, a motive power is transmitted from the engine unit60to the rear wheel.

A clutch device is axially provided in a clutch chamber72in a right shaft end part of the counter shaft70. The clutch device is covered by a clutch cover67. The clutch device is provided, in its clutch housing, with a plurality of drive plates and driven plates having radiuses in an orthogonal direction to a shaft direction of the counter shaft70, in a manner to overlap alternately. The clutch device itself is bulky, requiring a substantial disposing space. Further, in this example, it is constituted so that a breather case73is adjoined to a left side of the clutch device.

Here, as is known also fromFIG. 17, a space is secured in an upper part of the crankcase assembly61on a rear side of the cylinder assembly, combined with a fact that the cylinder assembly has the posture inclining forward properly. Further, the transmission case69is provided protrudingly rearward from a crankcase assembly61main body in a range from a middle of the #2 cylinder or from the #3 cylinder to the #4 cylinder in the vehicle width direction, and thus a space is secured also in a left side region of the transmission case69corresponding to almost the #1 cylinder. These spaces are indicated as a space S inFIG. 17.

In the engine unit60of the present embodiment, first, the engine itself is surrounded by the vehicle body frame, i.e., by the seat rail601with regard to an upper part thereof, the down tube603with regard to a front part, and the body tube604with regard to a rear part, respectively, as shown inFIG. 14and so on. An intake system74is disposed and constituted in an upper part of the crankcase assembly61and on a rear side of the cylinder assembly, as inFIG. 18. The intake system74includes, as shown inFIG. 14,FIG. 15and so on, an intake manifold76, an injector77being a fuel injection device, a throttle body78, and an intake pipe70between the cylinder assembly and an air cleaner75.

In the present embodiment, in particular the throttle body78is disposed as shown inFIG. 18on an opposite side of the clutch chamber72in the vehicle width direction, that is, on a leftward side of the crankcase assembly61. In other words, the single throttle body78is disposed by using the space S corresponding to almost the #1 cylinder.

In a concrete constitution of the intake system74, the air cleaner75is disposed in front of the body tube604constituting the vehicle body frame, as shown inFIG. 14and so on. Note that by using the vehicle body frame such as a body frame604, the air cleaner75can be supported at a position of almost the same height as that of the cylinder assembly. The air cleaner75is made of a casing extendedly provided horizontally in the vehicle width direction from almost the #1 cylinder to the #4 cylinder as also shown inFIG. 19, and an intake portion80for taking in air is attachedly provided to a rear part thereof. The intake portion80is equipped with a suction pipe81opening rearward, and air A is taken in from the suction pipe81as shown inFIG. 20. The suction pipe81can be one inclining toward the rear diagonally downward properly as inFIG. 20. A filter element82is installed between the air cleaner75and the intake portion80as inFIG. 20, and air cleaned by the air cleaner75led in.

As inFIG. 18, the intake manifold76is horizontally placed adjacent to a front side of the air cleaner75. The intake manifold76is, as shown inFIG. 19, extendedly provided horizontally in the vehicle width direction from almost the #2 cylinder to the #4 cylinder. In other words, disposition of the throttle body78in the space S corresponding to the #1 cylinder as described above is made possible.

The intake manifold76includes a plurality of separated and independent intake tubes83(83A,83B,83C,83D) linked to the cylinder heads63of the respective #1 to #4 cylinders, and a manifold collecting section84which collects those intake tubes integrally and is formed in a shape of a cylinder with a bottom. Note that the manifold collecting section84is constituted by a hollow barrel unit having a circular transverse cross section, and has a structure with a bottom where a right end part side is closed in this example. Note that the manifold collecting section84can be formed of a synthetic resin or an aluminum die-cast.

Each of intake tubes83A,83B,83C,83D, the front end part thereof being connected to the intake port68(seeFIG. 17) as shown inFIG. 20and so on, is formed in a curved shape to round in an upper part of the throttle body78from the front end part thereof toward the rear, as in side view ofFIG. 20. The curved shape can typically be a circular arc, and each is smoothly connected to an outer peripheral surface rear side of the manifold collecting section84smoothly. A base end side on which the respective intake tubes83A,83B,83C,83D are connected to the manifold collecting section84can be disposed at a practically same pitch interval as an arrangement pitch of the cylinders, that is, an interval in the vehicle width direction of the intake ports68.

Further, the intake tube83is disposed in a manner that an incline in relation to a longitudinal direction of the manifold collecting section84is increased gradually or decreased gradually in rear view. As shown inFIG. 21, with an axis line in the longitudinal direction of the manifold collecting section84being an axis line X, an inclination angles θ1, θ2, θ3, θ4in relation to the axis line X of the respective intake tubes83A,83B,83C,83D become gradually larger in sequence as going from the #1 cylinder to the #4 cylinder in this example. Note that the inclination angles θ of the intake tubes83A,83B,83C,83D are not limited to a case of the shown example inFIG. 21, but can be changed accordingly in correspondence with a disposition interval of the intake ports68or a linear dimension of the manifold collecting section84.

Here, the intake tube83A disposed farthest from the clutch chamber72among the respective intake tubes83A,83B,83C,83D is disposed, as inFIG. 19, in a manner to overlap the throttle body78in plan view. In other words, the intake tube83A corresponding to the #1 cylinder where the throttle body78is disposed is disposed above the throttle body78(seeFIG. 20and so on).

Next, in the present embodiment, what is called an electronic throttle is used.FIG. 22AandFIG. 22Bshow schematic constitution examples of an electronic throttle85according to the present embodiment. The throttle body78has a throttle bore inside which intake air flows, and the throttle valve87supported pivotally by a predetermined angle via a supporting shaft86is installed. The supporting shaft86is coupled to be rotation-driven by a drive motor88, and is constituted so that the drive motor88makes the supporting shaft86rotate based on a signal from an ECU mounted on the vehicle, thereby to control an opening degree of the throttle valve87. Note that the opening degree of the throttle valve87is detected by a throttle position sensor89, and its detection signal is feedbacked to an ECU. It is preferable that those components are integrally assembled to a base plate90and unitized.

In the electronic throttle, a throttle bore center axis X′ (FIG. 22AandFIG. 22B) of the throttle body78is set to be almost parallel to the vehicle width direction, and the throttle body78is integrally fixed to the intake manifold76. More specifically, as shown inFIG. 18and so on, the throttle body78is fixed to a left end part of the manifold collecting section84of the intake manifold76. The throttle bore of the throttle body78is communicated with the inside of the manifold collecting section84, and in such a case the throttle bore center axis X′ can be preferably disposed to be the same as the axis line X of the manifold collecting section84(seeFIG. 23).

Further, as described above, the throttle body78is disposed below the intake tube83A corresponding to the #1 cylinder. Thus, the throttle body78is disposed in an inner side of the cylinder assembly in the vehicle width direction. As shown inFIG. 19, the throttle body78is disposed in a range of a width W in the vehicle width direction of the cylinder head cover64which is basically the widest among the cylinder assembly.

Here, further, as shown inFIG. 19, in an upper part of each of intake tubes83A,83B,83C,83D, an injector77injecting fuel directing a back of an intake valve disposed in the interior of the intake port68is installed directly near the intake port68to which each of the intake tubes83A,83B,83C,83D is connected. Each injector77is tube-connected to a main gallery91, and fuel supplied from a fuel pump to the main gallery91is distributed to each injector77.

With reference toFIG. 14, the injector77, the intake tube83(83A), the throttle valve87and its drive motor88, from above in side view, are disposed tandem almost in parallel to a cylinder axis line Z, below the seat rail601in a space surrounded by the cylinder assembly, the air cleaner75, and the crankcase assembly61.

Further in a case of above, the air cleaner75is disposed in front of the body tube604, but the intake pipe79connecting the throttle body78and the air cleaner75is not extended to the inside of the air cleaner75as shown inFIG. 20. Note that the intake pipe79extends from a vicinity of a left end part near a front surface part of the air cleaner75toward the front, and is connected to the throttle body78while being bent rightward, as shown inFIG. 19,FIG. 20and so on.

In the intake system74, in particular, of the engine unit60constituted as above, as shown inFIG. 19andFIG. 20air A is led in from the suction pipe81and thereafter passes through the filter element82, thereby cleaned air being led in the air cleaner75. Air in the air cleaner75passes through the intake pipe79and is supplied to the throttle body (an arrow A0,FIG. 19). Air whose flow amount is controlled by the throttle valve87in the manifold collecting section84further passes through the intake tubes83A,83B,83c,83D and is supplied respectively to the intake ports68of the #1 to #4 cylinders as indicated by arrows A1, A2, A3, A4ofFIG. 19and so on.

Next, characteristic operation and effect of the present embodiment will be described. First, the throttle body78is disposed on an opposite side in the vehicle width direction of the clutch chamber72.

In a multi-cylinder engine as in the present embodiment, on a rear surface side of the cylinder head63a distance between the intake port68and an upper surface of the crankcase assembly61is largest on the opposite side of the clutch chamber72. In other words, the clutch chamber72has a bulky constitution as described above, and few margin is left in terms of a space above the clutch chamber72between the clutch chamber72and the intake port68. As a result that the single throttle body78is intensively disposed on a left side in the space S (seeFIG. 17) made to offset from such a clutch chamber72, the space is effectively secured between the cylinder block62and the air cleaner75. As a result of securing of the space, collectivizing of the air cleaner75and the intake manifold76becomes possible without reducing a capacity. Besides, since the sufficient inertia effect can be obtained in the whole area of the opening degrees of the throttle valve87while the sufficient intake tube length being secured, improvement of an output, improvement of a fuel consumption, reduction of a noise, improvement of a cleaning efficiency, and so on can be done.

Further, the throttle bore center axis X′ of the throttle body78is set to be almost parallel to the vehicle width direction, and the throttle body78is integrally fixed to the manifold collecting section84of the intake manifold74.

The throttle body78is disposed in a position offset in the vehicle width direction from a vehicle center line Y, on the opposite side of the clutch chamber72, in the left in this example. Thereby, the throttle bore center axis X′ is disposed almost in parallel to the vehicle width direction, and thus a width in a front and rear direction (riding direction) of the intake system as a whole can be effectively curtailed while the sufficient intake tube length and the capacity of the air cleaner75are secured. Therefore, it becomes possible to make also a vehicle as a whole compact.

Further, the throttle body78is disposed in the inner side of the cylinder assembly in the vehicle width direction, as shown inFIG. 19.

By disposing the throttle body78in such a way, a right and left direction width of the intake system as a whole can be made compact while the sufficient intake tube length is secured. The fact that the intake system is made slim and compact prevents a leg of a driver (rider) from touching the intake system in a state of riding, and can improve ridability, riding comfort, or the like. Incidentally, a range in which the driver's leg moves is outer than a dotted line M ofFIG. 16, and the intake system is housed compactly in the inner side thereof.

Further, the intake manifold76is constituted by including a plurality of intake tubes83and the manifold collecting section84which integrally assembles those intake tubes.

In other words, the manifold collecting section84is disposed in the upper part of the crankcase assembly61between the cylinder block62and the cylinder head63of the engine and the air cleaner75, and disposed in the same axis direction as the throttle bore center axis X′ in a downstream of the throttle body78, and is disposed in the inner side than a width of the cylinder head63in the vehicle width direction. Thereby, the sufficient intake tube length and the sufficient capacity of the air cleaner75can be secured.

Further, the respective intake tubes83A,83B,83C,83D are formed in the curved shape to round in the upper part of the throttle body78in side view and is disposed in a manner that inclines in relation to a longitudinal direction of the manifold collecting section84increase gradually or decrease gradually in rear view.

As a result that the intake tubes83A,83B,83C,83D are disposed in such a piping method, the intake tube lengths necessary for obtaining the inertia effect of intake at a target rotation number can be secured in a necessary and sufficient degree, and in addition, the vehicle front and rear width is curtailed to the minimum, so that adjustment of the lengths for maintaining synchronism of the respective cylinders becomes possible, enabling the sufficient inertia effect in the whole area of the throttle valve opening degrees.

In this example, as shown inFIG. 21, the inclination angles θ1, θ2, θ3, θ4of the intake tubes83A,83B,83C,83D are made smaller as going near to the throttle body78, and thereby the length of the intake tubes83a,83B,83C,83D themselves become longer as going toward the #1 cylinder. By inclinedly disposing the intake pipes83A,83B,83C,83D, the intake tube lengths are secured while the throttle body78is placed in a vehicle width direction center of the engine, so that an entire width of the intake pipes83A,83B,83C,83D can be made compact. Then, in the intake manifold76as a whole, flow path lengths of the intake of the intake pipes83A,83B,83C,83D can be practically the same length, and it becomes possible to balance the #1 to ″4 cylinders with one another.

Further, the intake tube83A disposed farthest from the clutch chamber72overlaps the throttle body78in plan view.

In other words, by disposing the intake tube83A above the throttle body78, it is possible to make the width of the intake system as a whole compact while making the intake tube length sufficient.

Further, in the space surrounded by the vehicle body frame, the crankcase assembly61, and so on, the injector77, the intake pipes83, the throttle valve87, and its drive motor88are disposed tandem almost in parallel to the cylinder axis line Z from above in side view, as shown inFIG. 14.

As a result that the respective functional components are disposed tandem in stages as above, the intake system as a whole can be laid out compactly. In other words, as a method for securing a sufficient intake tube length and necessary air cleaner volume to curtail a front and rear width of an intake system as a whole, quite an effective disposition method in which functional components are disposed tandem almost in parallel to a cylinder axis line Z is realized.

Further, the intake pipe79connecting the air cleaner75and the throttle body78is connected in a manner not to be extended to the inside of the air cleaner75.

By connecting the intake pipe79to the air cleaner75as above, the air cleaner capacity is not reduced, and consequently the engine output is improved.

Hereinabove, the embodiments and examples of the present invention are described in detail with reference to the drawings, but the above embodiments and examples merely illustrate concrete examples of implementing the present invention. The technical scope of the present invention is not to be construed in a restrictive manner by these embodiments. The present invention may be amended in various forms without departing from the technical spirit thereof, and such amendments are included in the technical scope of the present invention.

For example, in the above embodiment, the constitution in which the intake system of the internal combustion engine according to the present invention is applied to an on-road-type motorcycle is shown, but types of motorcycles to which the present invention is applied is not limited. The above motorcycle is merely an example of the motorcycle to which the present invention can be applied. The present invention can be applied to any type of motorcycle as long as the motorcycle has a side cam chain type parallel multi-cylinder engine. Further, the number of the cylinders shown in the present embodiment is an example, and the present invention can be applied regardless of the number of cylinders.

Further, though the example in which the manifold collecting section84has the circular transverse cross-sectional shape is described, it is possible to make a transverse cross-sectional shape be an oval shape or the like, in correspondence with a special relationship with neighboring components or members.

Further, if a clutch is disposed on a left side of the engine, contrary to the above embodiment, a throttle body is disposed on the opposite side, i.e., on the right side.

The present invention is also effectively applicable to a case where the number of cylinders in an engine unit is equal to or less than three or equal to or more than four.

According to the present invention, a collective single electronic control type throttle body is disposed in an extra space on an opposite side of a clutch chamber behind a cylinder assembly. By such a disposition constitution, intake manifolds are gathered, which has been impossible in a conventional motorcycle, and an intake amount is controlled by one throttle body. Thereby, securing of a sufficient length of an intake tube and an intake system capable of securing an air cleaner capacity as much as or more than conventionally can be realized.

Further, according to the present invention, one throttle body is disposed in a position biased to an arrangement direction of cylinders. Thus, the space is formed behind a cylinder head. The intake manifolds are gathered in this space and the intake amount is controlled by one collective throttle body. Thereby, the length of the intake tube can be made longer without enlarging the space necessary for disposition of the intake system.

The present invention is a technology effective to an engine unit of a motorcycle. According to the present invention, it is possible to realize an intake system enabling securing a sufficient intake tube length and securing an air cleaner capacity as much as or more than conventionally. Further, according to the present invention, the length of the intake tube can be made longer without making a space necessary for disposition of the intake system larger.