Overhead camshaft engine

The overhead camshaft engine (10) includes a cylinder block (11), a crankcase (12) attached to a lower part of the cylinder block to define a crankcase chamber (32), a bearing retaining member (60) attached to a part of the cylinder block, a crankshaft (20) rotatably supported by a pair of bearings (21, 22) supported by the cylinder block and the bearing retaining member, respectively, and a crankshaft pulley (53) attached to a part of the end of the crankshaft projecting outward from the bearing supported by the bearing retaining member.

TECHNICAL FIELD

The present invention relates to an improved overhead camshaft engine.

BACKGROUND ART

There is a desire to form an engine main body defining a cylinder and a crankcase chamber by using as small a number of component parts as possible, and support the crankshaft of the engine in a favorable manner in terms of structural integrity and ease of assembly. In a known overhead camshaft engine disclosed in JP2002-349340A, the engine main body includes a cylinder block that is integrally formed with a cylinder head, and a first bearing for supporting a first end of the crankshaft is fitted into a hole formed in the cylinder block. The lower end of the cylinder block is defined by a plane extending obliquely across the crankshaft, and a crankcase having a corresponding upper end is attached to the lower end of the cylinder block to define the crankcase chamber in cooperation with the cylinder block. The crankcase is formed with a hole, and a second bearing for supporting a second end of the crankshaft is fitted into the hole formed in the crankcase.

This engine main body essentially consists of two pieces. However, the crankcase is required to be made of relatively stiff member, and the assembling of the engine may not be as easy as wished. Also, because the crankshaft pulley for transmitting the power of the crankshaft to the cam mechanism via a timing belt is positioned on the part of the crankshaft located inward of the second bearing, the distance between the two bearings is increased by the width of this crankshaft pulley, and this is detrimental in maximizing the stiffness of the crankshaft. Also, the positioning of the timing belt complicates the cooling system of the engine.

BRIEF SUMMARY OF THE INVENTION

In view of such problems of the prior art, a primary object of the present invention is to provide an overhead camshaft engine that can be maximize the stiffness of the crankshaft.

A second object of the present invention is to simplify the cooling system of the engine.

A third object of the present invention is to simplify the structure of the engine without complicating the assembly process.

To achieve at least part of such objects, the present invention provides an overhead camshaft engine (10), comprising: a cylinder block (11) defining a cylinder (15) in cooperation with a cylinder head (14); a crankcase (12) attached to a lower part of the cylinder block to define a crankcase chamber (32) in cooperation with a part of the cylinder block; a first bearing (21) provided on the cylinder block to rotatably support a first end (23) of a crankshaft (20) of the engine; a second bearing (22) provided on a bearing retaining member (60) attached to a part of the cylinder block to rotatably support a second end (24) of the crankshaft; a camshaft (51) rotatably supported in an upper part of the cylinder head and provided with a camshaft pulley (52); a crankshaft pulley (53) attached to a part of the second end of the crankshaft projecting outward from the second bearing; and a timing belt (54) passed around the camshaft pulley and the crankshaft pulley.

In this arrangement, as the distance between the two bearings can be minimized owing to the presence of only the crankshaft webs and the crankpin between the two bearings, the stiffness of the crankshaft can be maximized.

As the second bearing is supported by the cylinder block via the bearing retaining member, the crankcase is required only to define the crankcase chamber, and is not required to support the second bearing. Therefore, the crankcase can be made of inexpensive material such as stamp formed sheet metal which may not have a high stiffness. Also, as the second end of the crankshaft is contained within the crankcase, the second bearing is not required to be fitted with an oil seal.

Preferably, the cylinder head is integrally formed with the cylinder block, for instance by casting the cylinder block including the cylinder head as a single piece component part.

According to a preferred embodiment of the present invention, the first bearing is fitted in a hole (34) formed in the cylinder block. Thereby, the structure can be simplified, and the assembly process can be simplified.

The bearing retaining member may consist of an integrally formed member. Typically, a central bore (61) is formed in the bearing retaining member, and the second bearing is fitted into the central bore.

According to a particularly preferred embodiment of the present invention, the cylinder block is integrally formed with a belt cover (17) that covers a part of the timing belt. Owing to the advantageous positioning of the crankshaft pulley, the timing belt cover can be integrally formed with the cylinder block.

The engine of the present invention can be most advantageously applied to a single cylinder engine for general purpose.

Preferably, the crankcase is attached to a lower end of the cylinder block at a parting plane (31) that extends obliquely across the crankshaft. Thereby, the parting plane between the cylinder block and the crankcase may consist of a single plane.

In a preferred embodiment of the present invention, the bearing retaining member is secured to a mating surface (35) of the cylinder block by using at least a pair of threaded bolts (73) and at least one locating pin (72) all extending through a parting plane (31) defined between the mating surface of the cylinder block and a corresponding mating surface (62) of the bearing retaining member; and the second bearing is secured in a central bore (61) formed in the bearing retaining member by using a circlip (71), the circlip being positioned to prevent dislodging of the locating pin.

Thereby, the locating pin can be prevented from being dislodged without requiring any additional component part even when the mating surfaces face a substantially vertical direction.

According to a preferred embodiment of the present invention, the bearing retaining member rotatably supports an oil slinger member (82) which is connected to the second end of the crankshaft via a power transmission mechanism (85,86).

The oil slinger member can be conveniently positioned in the bearing retaining member, and the power for rotating the oil slinger member can be readily obtained from the crankshaft. For instance, the power transmission mechanism may include a first gear (85) formed in the crankshaft pulley and a second gear (86) formed in the oil slinger member and meshing with the first gear.

Typically, the first end of the crankshaft is an output end of the crankshaft.

According to a particularly preferred embodiment of the present invention, the engine consists of an air cooled engine, and a plurality of fins (16) are formed on an outer peripheral surface of the cylinder block, and wherein the engine includes a belt cover (17) that defines a belt chamber (55) for receiving the timing belt therein, the belt cover including a wall (17a) separating the fins from the timing belt so that an air space (Sp1) communicating with outside is created between the fins and the wall of the belt cover.

In this arrangement, owing to the presence of an air space between the cooling fins and the belt cover, the engine can be cooled in a particularly favorable manner.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

A preferred embodiment of the present invention is described in the following with reference toFIGS. 1 to 6.

Referring toFIG. 1, the engine10of the illustrated embodiment includes a cylinder block11which integrally combines a cylinder head part14and a cylinder block part13which are formed as separate component parts in a more conventional arrangement, and internally defines a cylinder15therein. This engine10consists of a single cylinder, air cooled engine, and is provided with air cooling fins16on an outer periphery of the cylinder block part13.

The cylinder block11further includes a belt cover part17extending sideways (rightward inFIG. 1) and upward in the shape of letter L when seen from sideways, and internally defines a belt chamber55. The upper end of the belt chamber55is closed by a head cover57attached to an upper end of the belt cover part17to define a cam actuating mechanism chamber50. A camshaft pulley52is rotatably supported in the cam actuating mechanism chamber50by a camshaft51which is provided with cams (not shown in the drawings) for actuating an intake valve and an exhaust valve (not shown in the drawings) of the engine10in a per se known manner.

As shown inFIG. 1, the belt cover part17includes a wall17athat separates the belt chamber55from the finned outer peripheral part of the cylinder block11so that an air gap Sp1communicating with outside is created between the cylinder block11and the belt cover part17. The cylinder block11, including the cylinder block part13, the cylinder head part14and the belt cover part17, is typically made by casting iron, aluminum alloy or any per se known material as a one-piece cast product.

The lower end of the cylinder block11is defined by an oblique plane31, and a crankcase12is attached to the lower end of the cylinder block11at this oblique plane31by using a plurality of threaded bolts33. Thus, a crankcase chamber32is defined jointly by the crankcase12and the cylinder block11. The crankcase12may be made of stamp formed sheet metal or made of a cast member of suitable material. In the illustrated embodiment, the crankcase12is only required to define the crankcase chamber32jointly with the cylinder block11, and is not required to support a loading from any of the moving parts of the engine10. Therefore, the crankcase12may be made of highly light and/or economical material. The bottom part of the crankcase chamber32form an oil reservoir12afor storing a prescribed amount of lubricating oil Ju, and an oil level sensor81provided in a low point of the oil reservoir12afor measuring the level of the lubricating oil Ju therein. The side of the cylinder block11that is vertically longer is provided with a bearing hole34, and a first bearing21consisting of a ball bearing is fitted into this bearing hole34.

The side of the cylinder block11that is vertically shorter is provided with a substantially horizontal mounting surface35facing downward is a part of the cylinder block11adjoining the junction between the cylinder block part13and the belt cover part17. A bearing retaining member60defining a central bore61for receiving a second bearing22also consisting of a ball bearing is attached to this mounting surface35by using a pair of threaded bolts73and a pair of locating pins72or dowel pins which are passed upwards through holes64and65formed in the bearing retaining member60into corresponding holes formed in the cylinder block part13past the mating surface defined by the mounting surface35.

As shown inFIGS. 5 and 6, the bearing retaining member60has a rectangular configuration, and has a relatively small fore and aft dimension (the axial dimension of the crankshaft20). The bearing retaining member60has an upper end surface62which is planar, and abuts the mounting surface35. The threaded bolts73are passed through the holes64extending over the entire vertical length of the bearing retaining member60on either side of the central bore61, and threaded into the corresponding threaded holes formed in the mounting surface35. The upper part of the bearing retaining member60is provided with a pair of flanges66having a relatively small (vertical) thickness and projecting outward in the axial direction, and the holes65for the locating pins72are passed through these flanges66.

The central bore61of the bearing retaining member60fitted with the second bearing22is coaxial with the bearing hole34of the cylinder block11fitted with the first bearing21. A crankshaft20is received in the crankcase chamber32, and has a first end23rotatably supported by the first bearing21and a second end24rotatably supported by the second bearing22. The crankshaft20is provided with a crankpin26, and a piston25slidably received in the cylinder15is connected to the crankpin26via a connecting rod27in a per se known manner. In the illustrated embodiment, the inner race of each of the bearings21and22abuts a corresponding annular shoulder surface defined in the crankshaft20.

The first bearing21is provided with an oil seal28for preventing leakage of engine lubricating oil from the crankcase chamber32. The outer race of the first bearing21abuts an inwardly facing annular shoulder surface defined in the bearing hole34of the cylinder block11, and the inner race of the first bearing21abuts an outwardly facing annular shoulder surface defined in the crankshaft20so that the first bearing21is prevented from moving axially.

The first end23of the crankshaft20projects outward from the first bearing21, and is fitted with a cup shaped outer rotor42including a coaxial central disk45and an axial flange46extending axially inward from the peripheral edge of the central disk45. A plurality of permanent magnets47are fixedly attached to the inner circumferential surface of the axial flange46of the outer rotor42at a regular angular interval. An inner stator41is fixedly secured to the outer face of the cylinder block11, and includes a core43fixedly attached to the outer side of the outer wall of the cylinder block11in a coaxial relationship to the crankshaft20, and a plurality of windings44formed on the stator core43in such a manner that electric current is generated in the windings44as the outer rotor42is rotated relative to the stator core43. The produced current is conducted from the windings44, and used for powering an ignition plug48of the engine10via a per se known ignition circuit not shown in the drawings. The first end23of the crankshaft20serves as the output end of this engine10.

The outer side of the rotor42is covered by an engine cover (not shown in the drawings) which is attached to the outer side of the engine10to define a cooling air passage, and a plurality of cooling fans (not shown in the drawings) is provided on the axial end surface of the central disk45to create a cooling air flow that is conducted through the cooling air passage.

As best shown inFIG. 2, the second bearing22is fixed in position by a circlip71which is fitted into an annular radial groove63formed in an outer end part of the central bore61of the bearing retaining member60. Thus, the second bearing22is held in axial position owing to the annular shoulder surface of the crankshaft20, and the circlip71. The second end24of the crankshaft20includes a journal portion24asupported by the second bearing22and a reduced diameter portion24bprovided on the outer most part of the second end24. A crankshaft pulley53is fitted onto this reduced diameter portion24b, and fixedly secured in position by a threaded bolt56threaded into a threaded hole formed in the axial end of the second end24of the crankshaft20. As can be appreciated by a person skilled in the art, the circlip71is only an example, and any other circular or part circular retaining member may be used in place of the circlip71without departing from the spirit of the present invention,

As best illustrated inFIGS. 4 and 6, the radial groove63extends only over small angular ranges in side parts and an upper and lower part of the central bore61. Therefore, the circlip71received in the radial groove63is exposed in a pair of upper side parts and a pair of lower side parts where the material of the bearing retaining member50is removed. In particular, the part of the circlip71adjoining each locating pin72is exposed, and located such that an exposed part of the circlip71is located directly under the lower end72aof each locating pin72. Thereby, the downward movement of each locating pin72is prevented by the corresponding part of the circlip71abutting the end surface72bof the locating pin72. Therefore, even though the locating pins72are pushed into the corresponding holes in an upward direction, the dislodgement of the locating pins72can be prevented without requiring any particular measure or any additional component parts.

Because the lower ends72aof the locating pins72are exposed and visible in the crankcase chamber32as shown inFIG. 3, inadvertent omission of the locating pins72during the manufacturing or servicing process can be avoided. Also, the part of the bearing retaining member60adjoining each locating pin72is cut away, the locating pin72is positioned in a relatively accessible area so that the insertion and removal of the locating pin72can be performed without any difficulty.

As best shown inFIG. 2, while the journal portion24a(having an outer diameter D2) is fitted into the central bore of the inner race of the second bearing22, the reduced diameter portion24b(having an outer diameter D1which is smaller than the outer diameter D1of the journal portion24a) is fitted into the central bore of the crankshaft pulley53. Therefore, the outer diameter Dc of the crankshaft pulley53can be reduced as compared to the case where no reduced diameter portion is provided, and the part having the same outer diameter as the journal portion24ais fitted into the central bore of the crankshaft pulley53, for the given thickness Th of the hub53aof the crankshaft pulley53. The diameter of the camshaft pulley52is required to have a prescribed relationship to the diameter of the crankshaft pulley53in order to synchronize the operation of the engine valves in relation to the angular position of the crankshaft20. Therefore, when the diameter of the crankshaft pulley53is reduced, the diameter of the camshaft pulley52can be reduced by the same factor. Therefore, the reduction in the diameter of the crankshaft pulley53is beneficial in reducing the overall height of the engine10, and hence reducing the weight of the engine10.

A timing belt54is passed around the crankshaft pulley53and the camshaft pulley52so that the rotational movement of the crankshaft20may be transmitted to the camshaft51at half the speed of the crankshaft20in a synchronized relationship required for the four-stroke engine in a per se known manner. In the illustrated embodiment, the timing belt54consists of a cogged belt made of plastic material, but may also consist of other types of belts, or may consist of a chain. In the latter case, the camshaft pulley52and the crankshaft pulley53would consist of sprocket wheels.

In the illustrated embodiment, the crankshaft pulley53is positioned on the outer side of the second bearing22or on the other side of the cylinder15with respect to the second bearing22so that the distance between the first bearing21and the second bearing22can be minimized. In the illustrated embodiment, there is nothing between the first bearing21and the second bearing22except for the crank webs and the crankpin of the crankshaft20.

Also, owing to the advantageous positioning of the crankshaft pulley53of the illustrated embodiment, the timing belt54can be received in the belt cover part17which extends from the cylinder block part13so that the combustion heat can be transferred from the cylinder15to the cooling fins16in a favorable manner without being hindered by the presence of the timing belt54. The cooling air may be guided to the air gap Sp1between the cylinder block part13and the belt cover part17of the cylinder block11so that the cooling efficiency may be enhanced. Furthermore, according to the illustrated embodiment, the timing belt54is favorably protected from the heat of the engine10so that the service life of the timing belt54can be extended.

As shown inFIGS. 1 to 3, an oil slinger member82is rotatably supported by a lower extension83of the bearing retaining member60via a pivot shaft84extending in parallel with the axial line of the crankshaft20, and is provided with a first gear86formed along an outer periphery of the oil slinger member82. The crankshaft pulley53is provided with a second gear85along an outer periphery thereof, and the first gear86and the second gear85mesh with each other so that the oil slinger member82is rotated when the engine10is in operation.

In the illustrated embodiment, the second gear85is formed on the outer periphery of the crankshaft pulley53so that the axial length of the engine10may be minimized, but may also be formed as a separate gear member coaxially fitted on the second end24of the crankshaft20owing to the ample space Sp2available on the outer side of the second bearing22.

FIG. 7shows a second embodiment of the present invention. This embodiment differs from the first embodiment in the way the crankshaft pulley53is fitted on the second end24of the crankshaft20. In this embodiment, the crankshaft pulley53is press fitted onto the reduced diameter portion24bof the second end24of the crankshaft20. This fit is a tight fit so that the crankshaft pulley53is held rotationally fast to the crankshaft20.

This engine is particularly suitable for use as general purpose engines for powering various types of equipment, not exclusively, such as lawn mowers, power generators and snow blowers.

Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention.