Internal combustion engine having air cleaner

The present invention relates to an internal combustion engine having an air cleaner, and in particular to an internal combustion engine having an air cleaner combining a filter air dust collection unit and a plurality of cyclone dust collection units.

A known air cleaner for an internal combustion engine is provided with a cyclone dust collection unit and a filter dust collection unit that are connected in series in such a manner that dust with relatively large particle diameters are removed from the intake air in the cyclone dust collection unit and the intake air expelled from the cyclone dust collection unit is then forwarded to the filter dust collection unit to have dust of relatively small particle diameters removed from the forwarded intake air. See JPS61-108861A, for instance. According to this arrangement, the clogging of the filter element in the filter dust collection unit is reduced, and the interval of replacing the filter element can be thereby extended.

However, in the arrangement disclosed in JPS61-108861A, no consideration is made as to the positioning of the dust ejection hole so that the dust ejected from the dust ejection hole may be thrown to the operator who may be operating the recoil starter or the choke valve, or the apparatus on which the engine is mounted. Also, the dust ejected from the dust ejection hole may be deposited on the engine.

In view of such a problem of the prior art, a primary object of the present invention is to provide an internal combustion engine having an air cleaner including cyclone dust collection units which are configured to eject collected dust in a favorable manner.

According to the present invention, such an object can be accomplished by providing an internal combustion engine ( 1 ), comprising: an engine main body ( 2 ) including a crankcase ( 4 ), a cylinder block ( 6 ) connected to an upper end of the crankcase and defining a cylinder ( 5 ) having a rearwardly slanted axial line and a cylinder head ( 7 ) attached to an upper end of the cylinder block; and an air cleaner ( 32 ) positioned above an upper rear part of the engine main body, the air cleaner including a filter dust collection unit ( 35 ) defining a filter chamber ( 70 ) for receiving a filter element ( 43 ) therein, and a plurality of cylindrical cyclone dust collection units ( 34 ) commonly connected to an upstream end of the filter dust collection unit in a mutually parallel relationship; wherein the cyclone dust collection units are positioned in front of the filter dust collection unit and provided with axial lines that are in parallel to an axial line of the filter dust collection unit; and wherein each cyclone dust collection unit includes a main body ( 44 , 45 ) internally defining a separation chamber ( 45 C), and having an air inlet ( 31 ) communicating with outside and an air outlet ( 73 ) communicating with the filter chamber, the separation chamber being provided with a dust ejection hole ( 45 D) directed tangentially in an obliquely forward direction.

Thereby, the dust collected by the cyclone dust collection units can be expelled away from the operator of the engine and prevented from depositing on the engine.

According to a preferred embodiment of the present invention, the dust ejection holes are positioned on a front side of the main bodies of the cyclone dust collection units and are directed in a same direction.

Thereby, the flow of air that carries the dust away of the dust ejection holes are prevented from interfering one another so that the dust is expelled in the forward and/or oblique direction in a favorable manner.

Preferably, a lower part ( 45 ) of the main body of each cyclone dust collection unit is detachably attached to an upper part ( 44 ) of the main body of the cyclone dust collection unit, and is made of at least partly transparent material ( 45 B).

Thereby, the assembly of the cyclone dust collection units can be simplified, and the state of the separation chambers can be inspected from outside without removing any of the component parts. If any dust deposition in the separation chamber is required to be cleaned, the lower part of the main body may be simply detached, and the separation chamber can be cleaned without any difficulty.

According to a particularly preferred embodiment of the present invention, the lower parts ( 45 ) of the respective main bodies of the cyclone dust collection units are formed as cup-shaped members ( 45 B) integrally formed as a one-piece member.

Thereby, the number of component parts can be reduced, and the assembling and disassembling of the cyclone dust collection units can be simplified.

According to a particularly preferred embodiment of the present invention, each cyclone dust collection unit includes an air ejection pipe ( 44 C) having an upper end communicating with the filter chamber ( 70 ) and a lower end positioned in the corresponding separation chamber ( 45 C), an annular air inlet passage ( 44 E) defined between an outer circumferential surface of the air ejection pipe and a surrounding part of the main body of the cyclone dust collection unit for communicating the air inlet with the separation chamber, and guide vanes ( 44 D) provided in the annular air inlet passage for causing a swirl flow in an air flow that passes through the annular air inlet passage.

Thereby, the overall structure of the cyclone dust collection units can be simplified, and the cyclone dust collection units can be built as a compact unit.

Typically, a recoil starter ( 25 ) is provided on one side of the crankcase, and is provided with a starter rope ( 25 B) configured to be pulled in a rearward direction. In such a case, as the dust is expelled away from the operator, the operator is prevented from experiencing any discomfort.

When the air cleaner is located forward of a vertical profile defined by a rear end of the engine main body, the overall outer profile of the engine may be confined to a rectangular configuration so that the engine can be favorably installed in a limited space of an apparatus that utilizes the power of the engine. According to a particularly preferred embodiment of the present invention, a muffler ( 28 A) of an exhaust system of the engine is provided above the cylinder head and on inboard side of the air cleaner. This also contributes to a compact design of the engine.

In a particular preferred embodiment of the present invention, the engine further comprises a fuel tank provided above the main body and in front of the air cleaner, an engine cover surrounding at least a side part and an upper part of the main body to define a cooling air passage ( 23 ) in cooperation with an external surface of the engine main body, and a fan ( 21 ) connected to a crankshaft of the engine main body and positioned in the cooling air passage, the cooling air passage including a cooling air inlet ( 23 B) formed in a part of the engine cover facing the fan, and a cooling air outlet ( 23 A) defined in an upper part of the engine cover so as to open out into a gap defined between the fuel tank and the air cleaner.

According to this arrangement, the cooling air that has cooled the engine main body flows past the upper part of the engine. Therefore, the dust expelled from the dust ejection holes are entrained in the flow of the cooling air, and/or the flow of the cooling air promotes the flow of air from the dust ejection holes so that the dust is prevented from depositing on the upper surface of the engine.

Thus, in a most preferred embodiment of the present invention, the dust collected in the cyclone dust collection unit is expelled in a forward or oblique forward direction so that the operator of the engine is prevented from experiencing any discomfort, and the dust is prevented from depositing on the engine main body.

FIG. 1 is a side view of a general-purpose engine provided with an air cleaner embodying the present invention.

FIG. 2 is a plan view of the engine.

FIG. 3 is a sectional view taken along line III-III of FIG. 1 .

FIG. 4 is an exploded perspective view of the air cleaner.

FIG. 5 a is a perspective view of a passage member of the air cleaner with a flange part thereof omitted from the illustration.

FIG. 5 b is a plan view of the passage member with the flange part thereof omitted from the illustration.

FIG. 6 is a vertical sectional view of the air cleaner.

FIG. 7 is a sectional view taken along line VII-VII of FIG. 6 .

FIG. 8 is a sectional view taken along line VIII-VIII of FIG. 6 .

FIG. 9 is a graph showing the relationships of the intake flow rate and dust removal ratio to the mounting angle of guide vanes.

FIG. 10 is a simplified side view showing a concrete cutter fitted with the engine of the illustrated embodiment.

A preferred embodiment of the present invention is described in the following with reference to the appended drawings.

As shown in FIGS. 1 to 3 , the engine 1 consists of a single cylinder engine including an engine main body 2 having a rearwardly slanted cylinder axial line. The engine main body 2 includes a crankcase 4 internally defining a crankcase chamber 3 , a cylinder block 6 extending from an upper rear part of the crankcase 4 in a rearwardly and upwardly direction and internally defining a cylinder 5 communicating with the crankcase chamber 3 , a cylinder head 7 connected to the upper end of the cylinder block 6 and closing the upper end of the cylinder 5 , and a head cover 9 connected to the cylinder head 7 to define a valve actuating chamber jointly with the cylinder head 7 . A crankshaft 10 extending laterally is rotatably supported by the crankcase 4 , and a piston 13 connected to the crankshaft 10 via a connecting rod 12 is received in the cylinder 5 for a reciprocating movement along the cylinder axial line in a per se known manner. A plurality of legs 14 are provided on the bottom part of the crankcase 4 for supporting the engine main body 2 on an engine mount of a device that utilizes the power of the engine 1 .

The cylinder head 7 is formed with an intake port 15 and an exhaust port 16 communicating with the cylinder 5 . The intake port 15 extends rightward from the cylinder 5 and opens out on the right side face of the cylinder head 7 , and the exhaust port 16 extends leftward from the cylinder 5 and opens out on the left side face of the cylinder head 7 . The intake port 15 and the exhaust port 16 are provided with an intake valve 18 and an exhaust valve 19 , respectively, which are actuated by the crankshaft 10 via a valve actuating mechanism 17 mainly placed in the valve actuating chamber. The valve actuating mechanism 17 may consist of a per se known OHV (overhead valve) mechanism.

The left end and the right end of the crankshaft 10 are passed out of the side wall of the crankcase 4 , and project outward. The left end of the crankshaft 10 corresponds to the output end of the engine 1 , and the right end of the crankshaft 10 is fitted with a flywheel 21 consisting of a disk member. The outer end surface or the right end surface of the flywheel 21 is provided with a plurality of vanes 21 A, and thereby forms a centrifugal fan for cooling the engine 1 .

The right side end of the crankcase 4 and the cylinder block 6 is covered by an engine cover 23 which defines a cooling air passage 22 jointly with the crankcase 4 and the cylinder block 6 , and the flywheel 21 is received in this cooling air passage 22 . The front and lower edges of the engine cover 23 are positioned adjacent to the outer surface of the crankcase 4 , and the upper and rear edges of the engine cover 23 are positioned adjacent to the cylinder head 7 . The engine cover 23 extends along the flywheel 21 and the associated components with a relatively small gap, but defines an air flow passage of a relatively large cross sectional area extending in the fore and aft direction and around the cylinder block 6 which is provided with cooling fins. A cooling air outlet 23 A is defined by the gap between the upper rear edge of the engine cover 23 and the cylinder block 6 . An annular region concentrically opposing the flywheel 21 is formed with a plurality of through holes serving as a cooling air inlet 23 B. Thus, when the engine 1 is in operation, and the flywheel 21 is rotating, cooling air is introduced into the cooling air passage 22 via the cooling air inlet 23 B, and flows in a rearward direction before being expelled from the cooling air outlet 23 A in an upward and rearward direction as indicated by white arrows in FIG. 3 .

The engine cover 23 is fitted with a recoil starter 25 that includes a pulley 25 A coaxially and rotatably supported on the back side of the engine cover 23 , a rope 25 B having one end attached to the pulley 25 A and wound around the pulley 25 A, a grip 25 C attached to the outer end of the rope 25 B which is passed through a small hole formed in the engine cover 23 , a spiral spring 25 D interposed between the engine cover 23 and the pulley 25 A to urge the pulley 25 A in the direction to wind the rope 25 B around the pulley 25 A, a one-way clutch 25 E which transmits torque from the pulley 25 A to the flywheel 21 but not in the opposite direction. The grip 25 C is positioned such that the rope 25 B may be pulled in a tangential direction of the pulley 25 A which is directed in a rearwardly and upwardly direction.

A substantially rectangular fuel tank 27 is supported by the crankcase 4 via support arms (not numbered) so as to be positioned above the crankcase 4 and the engine cover 23 . The lateral width of the fuel tank 27 is substantially equal to the combined lateral width of the crankcase 4 and the engine cover 23 so that a substantially rectangular outer profile may be jointly defined.

An exhaust system 28 communicating with the exhaust port 16 is connected to the left side of the cylinder head 7 . The exhaust system 28 internally defines an exhaust passage, and a muffler 28 A is connected to the downstream end thereof. The muffler 28 A is surrounded by a substantially rectangular protective cover 28 B, and positioned above the cylinder head 7 and to the rear of the left half of the fuel tank 27 so that the exhaust gas is expelled in a rearward direction.

An intake system 30 communicating with the intake port 15 is connected to the right side of the cylinder head 7 . The intake system 30 internally defines an intake passage. An air cleaner 32 having an air inlet 31 is connected to the upstream end of the intake passage, and a carburetor 33 is connected between the air cleaner 32 and the intake port 15 . The carburetor 33 is positioned to the right of the cylinder head 7 and to the rear of the engine cover 23 . The air cleaner 32 is positioned above and to the right of the cylinder head 7 , and above the carburetor 33 . Also, the air cleaner 32 is positioned to the rear of the right half of the fuel tank 27 and to the right of the muffler 28 A. The air cleaner 32 , the muffler 28 A (protective cover 28 B) and the fuel tank 27 jointly define a substantially horizontal upper profile of the engine 1 .

As shown in FIGS. 4 to 8 , the air cleaner 32 includes three cyclone dust collection units 34 connected to the air inlet 31 in parallel to one another, and a filter dust collection unit 35 connected to the downstream ends of the cyclone dust collection units 34 . Each cyclone dust collection unit 34 guides the supplied air into a spiral flow, and separates dust from the air by centrifugal force. The filter dust collection unit 35 separates dust further from the air that has passed through the cyclone dust collection units 34 by filtration.

As shown in FIG. 4 , the air cleaner 32 includes a base member 41 , a cover member 42 , a filter element 43 , a passage member 44 and a case member 45 . The base member 41 , the cover member 42 and the filter element 43 jointly form the filter dust collection unit 35 . The cover member 42 , the passage member 44 and the case member 45 jointly form the cyclone dust collection units 34 . The passage member 44 and the case member 45 jointly form the main body of the individual cyclone dust collection units 34 .

As shown in FIGS. 4 and 6 , the base member 41 extends vertically, and defines a planar base wall 41 A. The base member 41 internally defines a vertically extending outlet passage 41 B. The upper end of the outlet passage 41 B is formed by a circular opening provided centrally in the base wall 41 A, and the lower end of the outlet passage 41 B opens out in a lower front part of the base member 41 . The base wall 41 A is surrounded by a vertically extending annular peripheral wall 41 C having a prescribed height, and the peripheral wall 41 C includes a rear section extending linearly in the lateral direction along the rear edge of the base wall 41 A, a pair of side sections extending linearly in the fore and aft direction along either side of the base wall 41 A and a front section extending along an arc concentric to the upper end of the outlet passage 41 B. The upper end of the peripheral wall 41 C is provided with an engagement groove 41 D extending along the entire length thereof. A cylindrical wall 41 E projects from the part of the base wall 41 A surrounding the outlet passage 41 B proud of the peripheral wall 41 C. A lower end of a connecting shaft 41 F is secured to an inner wall extending horizontally in a vertically middle part of the base member 41 , and extends vertically through the center of the upper end of the outlet passage 41 B. The connecting shaft 41 F extends out of the base wall 41 A by a prescribed length, and is provided with a male thread in an upper end part thereof.

As shown in FIG. 1 , a lower right part of the base member 41 is provided with openings 59 for passing a choke lever 57 for operating a choke valve (not shown in the drawings) of the carburetor 33 and a fuel shutoff valve lever 58 for operating a fuel shutoff valve (not shown in the drawings) for selectively shutting off the communication between the fuel tank 27 and the carburetor 33 . The choke lever 57 and the fuel shutoff valve lever 58 project from the right side of the engine 1 .

The grip 25 C, the choke lever 57 and the fuel shutoff valve lever 58 are thus positioned so as to be optimally accessible for the operator of the engine 1 standing to the right and rear of the engine 1 .

The filter element 43 includes a cylindrical and annular first filter element 43 A and a cylindrical and annular second filter element 43 B positioned concentrically outside of the first filter element 43 A. The first filter element 43 A and the second filter element 43 B are connected to each other by a first end plate 47 at the first axial ends thereof and by a second end plate 48 at the second axial ends thereof. Each of the end plates 47 and 48 is formed by a circular disk, and closes the interior of the first filter element 43 A and the second filter element 43 B from the corresponding ends thereof. The first end plate 47 is centrally formed with a first opening 47 A, and the second end plate 48 is also centrally formed with a substantially smaller second opening 48 A. The filter element 43 is positioned on top of the base wall 41 A such that the cylindrical wall 41 E fits into the first opening 47 A, and the connecting shaft 41 F extends centrally through the second opening 48 A. An annular first seal member 51 is fitted on the cylindrical wall 41 E so that the lower surface of the first end plate 47 and the base wall 41 A abut each other so as to seal off the first opening 47 A from the exterior of the filter element 43 . An annular second seal member 52 is placed on the part of the first end plate 47 surrounding the second opening 48 A, and the connecting shaft 41 F is passed through the central opening of the second seal member 52 to close the second opening 48 A in an air tight manner. A first wing nut 54 is threaded onto the male thread of the connecting shaft 41 F, and abuts the upper surface of the second seal member 52 via a washer 53 so that the first end plate 47 , the second end plate 48 and the filter element 43 are fixedly secured to the base member 41 .

As shown in FIG. 4 and FIGS. 6 to 8 , the cover member 42 includes a rectangular upper wall 61 and a tubular side wall depending from the rectangular outer edge of the upper wall 61 , the side wall including a front wall 62 , a rear wall 63 , a left side wall 64 and a right side wall 65 . The cover member 42 further includes a partition wall 66 extending between the left side wall 64 and the right side wall 65 . The upper edge of the partition wall 66 is spaced from the upper wall 61 . The lower edge of the partition wall 66 is substantially at the same elevation as the lower edge of the part of the side wall located behind the partition wall 66 (the rear wall 63 , a rear part of the left side wall 64 and a rear part of the right side wall 65 ). The lower edge of the remaining part of the side wall (the front wall 62 , a front part of the left side wall 64 and a front part of the right side wall 65 ) is located at a higher elevation than the lower edge of the part of the side wall located behind the partition wall 66 . In other words, the front lower part of the side wall is cut away as opposed to the rear lower part of the side wall.

The rear part of the side wall, the partition wall 66 and the upper wall 61 jointly define a recess or a filter chamber 70 opening out in the downward direction, and the base wall 41 A of the base member 41 serves as the bottom wall of the filter chamber 70 . The lower edge of the part of the side wall located behind the partition wall 66 is conformal to the upper edge of the peripheral wall 41 C of the base member 41 , and is fitted into the engagement groove 41 D formed in the upper edge of the peripheral wall 41 C of the base member 41 . The upper end of the connecting shaft 41 F extends through an opening 61 A formed in the upper wall 61 , and projects upward from the cover member 42 . A second wing nut 68 threaded onto the male thread of the connecting shaft 41 F fixedly secures the cover member 42 to the base member 41 .

The filter chamber 70 defined jointly by the rear part of the side wall, the partition wall 66 and the upper wall 61 receives the filter element 43 therein. The partition wall 66 is given with a curve formed as an arc centered around the central axial line of the filter element 43 or the central axial line of the connecting shaft 41 F, and is spaced from the outer circumferential surface of the filter element 43 .

The upper edge of the partition wall 66 is provided with a flange 66 A projecting in the forward direction, and extending along the length of the partition wall 66 . The lower surface of the flange 66 A defines a planar shoulder surface. A passage wall 71 curved in plan view so as to have three lobes extending substantially in a forward direction depends from the front part of the lower surface of the upper wall 61 or a part of the lower surface of the upper wall 61 located between the partition wall 66 and the front wall 62 . The two lateral ends of the passage wall 71 extend in a rearward direction, and are connected to the partition wall 66 . The flange 66 A and the passage wall 71 jointly define a vertically extending communication passage 72 , and the lower surface of the flange 66 A and the lower edge of the passage wall 71 jointly define a continuous annular shoulder surface 73 facing downward.

The upper wall 61 and the passage wall 71 jointly define a connecting chamber 75 opening out both in a rearward direction and a downward direction. As the connecting chamber 75 extends from the upper front end of the filter chamber 70 in a forward direction, the connecting chamber 75 may be considered as an extension of the filter chamber 70 . A passage chamber 76 is defined by the front wall 62 , the partition wall 66 , the front part of the left side wall 64 and the front part of the right side wall 65 .

The air inlet 31 is formed in the front wall 62 , and communicates with the passage chamber 76 . In the illustrated embodiment, the air inlet 31 is passed through the front wall 62 in the fore and aft direction, and is formed by a large number of small holes forming a mesh.

As shown in FIGS. 4 to 8 , the passage member 44 is provided with a plate-like base portion 44 A extending horizontally so as to close the lower end of the passage chamber 76 . The base portion 44 A is provided with a linear front edge and an arcuate rear edge in conformity with the cross sectional shape of the passage chamber 76 . In particular, the lateral end parts of the rear edge protrude rearwardly in comparison with the middle part of the rear edge. Three circular holes 44 B arranged along the lateral direction are passed through the base portion 44 A. The middle circular hole 44 B has a relatively smaller diameter than the remaining two circular holes 44 B having a relatively large diameter and arranged symmetric about the line passing through the central axial line of the middle circular hole 44 B and the central axial line of the filter elements 43 A and 43 B.

An air ejection pipe 44 C is positioned centrally in each circular hole 44 B, and is supported by the base portion 44 A via a plurality of guide vanes 44 D extending radially between the outer circumferential surface of the air ejection pipe 44 C and the opposing inner circumferential surface of the corresponding circular hole 44 B. Therefore, an annular air inlet passage 44 E is defined between the air ejection pipe 44 C and the circular hole 44 B all in a concentric arrangement. Each guide vane 44 D is slanted with respect to the axial line of the air inlet passage 44 E in such a manner that the air flowing downward in the air inlet passage 44 E is rotated in the clockwise direction around the central axial line of the air inlet passage 44 E when viewed from above.

The upper ends of the air ejection pipe 44 C are commonly connected to a flange 44 F defining a horizontal upper surface. The peripheral part of the upper surface of the flange 44 F abuts the shoulder surface 73 of the cover member 42 and the partition wall 66 so that the interior of each air ejection pipe 44 C is communicated with the connecting chamber 75 , and the passage chamber 76 is separated from the connecting chamber 75 by the air ejection pipes 44 C. In the illustrated embodiment, the base portion 44 A, the guide vanes 44 D and the air ejection pipes 44 C are formed by an integrally molded plastic member, and the flange 44 F is formed by a separately molded plastic member. The upper ends of the air ejection pipes 44 C are press fitted into, welded to or otherwise connected to the corresponding holes of the flange 44 F in an air tight manner. Alternatively, the base portion 44 A, the guide vanes 44 D, the air ejection pipes 44 C and the flange 44 F are formed by an integrally molded plastic member.

As shown in FIGS. 4 and 6 , the case member 45 includes an upper plate portion 45 A having an upper surface abutting the lower surface of the base portion 44 A and three cup portions 45 B recessed downward in positions corresponding to the lower ends of the respective air inlet passages 44 E and opening out upward. Each cup portion 45 B may be provided with a conical shape with a downward taper. The peripheral part of the upper plate portion 45 A extends outward slightly beyond the peripheral edge of the base portion 44 A. A downwardly facing annular shoulder surface 78 is formed in the rear surface of the front wall 62 , the right surface of the left side wall 64 , the left surface of the right side wall 65 and the front surface of the partition wall 66 . With the peripheral part of the upper surface of the upper plate portion 45 A brought into close contact with the shoulder surface 78 , a plurality of screws 82 are passed through holes formed in the peripheral parts of the upper plate portion 45 A and the base portion 44 A, and threaded into corresponding plate nuts 81 fixedly secured to the front wall 62 and the partition wall 66 . Thereby, the case member 45 and the passage member 44 are fixedly secured to the cover member 42 . The passage member 44 is thus interposed between the shoulder surface 73 jointly defined by the lower surface of the passage wall 71 and the lower surface of the flange 66 A, and the upper surface of the upper plate portion 45 A.

The interior of each cup portion 45 B defines a separation chamber 45 C, and a peripheral left front part of the bottom part of each cup portion 45 B is formed with a tangentially extending dust ejection hole 45 D. The case member 45 is made of transparent plastic material so that the interior of the separation chamber 45 C is visible from outside.

As shown in FIGS. 4 to 8 , the three cyclone dust collection units 34 are formed by the respective air inlet passages 44 E, guide vanes 44 D and air ejection pipes 44 C formed by the passage member 44 and the case member 45 . The three cyclone dust collection units 34 are arranged laterally, and the cyclone dust collection unit 34 in the middle is located centrally of the base portion 44 A with respect to the lateral direction while the remaining two cyclone dust collection units 34 are located symmetric to each other with respect to the central cyclone dust collection unit 34 . The central axial line of the central cyclone dust collection unit 34 and the central axial line of the filter element 43 are located on a central line of the air cleaner extending in the fore and aft direction. In each cyclone dust collection unit 34 , the circular hole 44 B, the air inlet passage 44 E, the separation chamber 45 C and the air ejection pipe 44 C are disposed coaxially to one another. The three cyclone dust collection units 34 are located on the front end of the air cleaner 32 , and the central axial lines of the three cyclone dust collection units 34 extend in parallel with the axial line of the filter element 43 and are arranged along the circumferential direction of the filter element 43 .

The central cyclone dust collection unit 34 has a smaller diameter than the remaining two cyclone dust collection units 34 in such a manner that the space defined between the rectangular outer profile of the air cleaner 32 and the circular outer profile of the filter element 43 may be fully utilized. The diameters of the air inlet passage 44 E, the air ejection pipe 44 C and the cup portion 45 B of each cyclone dust collection unit 34 are dimensioned in proportion to the overall diameter of the cyclone dust collection unit 34 .

As shown in FIGS. 7 and 8 , the central axial line of the central cyclone dust collection unit 34 and the central axial line of the filter element 43 extend vertically in parallel to each other, and are located on the central line of the air cleaner 32 extending in the fore and aft direction. The front ends of the three cyclone dust collection units 34 are aligned with each other with respect to the fore and aft direction while the central axial line of the central cyclone dust collection unit 34 is offset in the forward direction relative to the central axial lines of the remaining cyclone dust collection units 34 . Thus, the rear end of the central cyclone dust collection unit 34 is significantly offset in the forward direction relative to the rear ends of the remaining cyclone dust collection units 34 .

The combined lateral width of the three cyclone dust collection units 34 is substantially equal to the internal lateral width of the cover member 42 (distance between the left side wall 64 and the right side wall 65 ). In a preferred embodiment, the combined lateral width of the three cyclone dust collection units 34 is substantially equal to the diameter of the filter element 43 .

As shown in FIGS. 1 and 2 , the rear end of the air cleaner 32 is located at a substantially same position as the rear end of the engine main body 2 with respect to the fore and aft direction. The rear end of the protective cover 28 B of the muffler 28 A is also located at a substantially same position as the rear end of the engine main body 2 with respect to the fore and aft direction. The right end of the air cleaner 32 is located slightly to the left of the right end of the engine cover 23 . The upper end of the air cleaner 32 is located at a substantially same elevation as the upper end of the fuel tank 27 . In other words, the air cleaner 32 is positioned so as not to project beyond the generally rectangular outer profile of the engine 1 .

The mode of operation of the air cleaner 32 described above is discussed in the following. During the operation of the engine 1 , the negative pressure created in the cylinder 5 is transmitted to the outlet passage 41 B so that intake air is drawn into the passage chamber 76 of the air cleaner 32 via the air inlet 31 . The intake air introduced into the passage chamber 76 passes through one of the cyclone dust collection units 34 . In each cyclone dust collection unit 34 , as the air flows downward from the air inlet passage 44 E to the separation chamber 45 C, the guide vanes 44 D produce a swirl flow turning in clockwise direction when viewed from above and centered around the axial center of the cyclone dust collection unit 34 . In the separation chamber 45 C, dust in the air is forced radially outward by the centrifugal force, and drops down to be expelled from the dust ejection hole 45 D to the outside while the air freed from the dust flows upward from the separation chamber 45 C to the connecting chamber 75 via the air ejection pipe 44 C. The air that has passed through the cyclone dust collection units 34 merges in the connecting chamber 75 , and flows into the filter chamber 70 of the filter dust collection unit 35 . In the filter chamber 70 , the air passes through the filter element 43 in the radially inward direction, and foreign matters in the air are trapped by the filter element 43 . The air that has passed through the filter element 43 is forwarded to the carburetor 33 via the outlet passage 41 B.

In the illustrated embodiment, there are eight guide vanes 44 D in each air inlet passage 44 E which are arranged at a regular interval along the circumferential direction. The guide vanes 44 D are arranged in a non-overlapping relationship when viewed in the axial direction for the convenience of manufacture. The angle (mounting angle) of each guide vane 44 D with respect to a horizontal plane perpendicular to the axial line is in the range of 30 degrees to 37 degrees. FIG. 9 shows the changes in the air flow rate and the dust removal ratio in relation with this mounting angle. This graph was obtained by computational analysis based on the assumption that the outer diameter, the inner diameter and the length of the vanes 44 D are 40 mm, 27 mm and 10 mm, respectively, the inner diameter and the length of the air ejection pipe 44 C are 24 mm and 34 mm, respectively, the diameter and the length of the separation chamber 45 C are 40 mm and 40 mm, respectively, and the pressure on the downstream side of the air ejection pipe is −400 Pa. As can be seen from FIG. 9 , the air flow rate increases with an increase in the mounting angle of the guide vanes 44 D presumably because the flow resistance of the guide vanes 44 D decreases with an increase in the mounting angle. Meanwhile, the dust removal ratio is substantially constant in relation with the mounting angle when the mounting angle is smaller than 37 degrees, but starts declining sharply as the mounting angle increases beyond 37 degrees. It can be surmised that increasing the mounting angle diminishes the circumferential speed of the air that passes through the air inlet passage 44 E, and hence diminishes the centrifugal force that is effective in separating dust from the air. Based on these factors, in view of achieving a high dust removal ratio while maintaining a relatively high air flow rate, it is preferable to increase the mounting angle of the guide vanes 44 D without exceeding 37 degrees, and to select the mounting angle from a range of 30 degrees to 37 degrees, for instance.

Because the air cleaner 32 of the illustrated embodiment is provided with the three cyclone dust collection units 34 for the single filter dust collection unit 35 provided with the annular filter element 43 , and the middle cyclone dust collection unit 34 is smaller in diameter than the remaining cyclone dust collection units 34 on either side, the filter dust collection unit 35 and the cyclone dust collection units 34 can be accommodated in a rectangular space in an efficient manner, and this contributes to a compact design of the air cleaner having a prescribed performance level. As the outer profile of the air cleaner 32 is rectangular, the layout of the various associated components of the engine 1 such as the fuel tank 27 and the muffler 28 A may be optimized, and the engine 1 can be designed as a highly compact unit.

Because the dust ejection holes 45 D of the cyclone dust collection units 34 are all directed in the front right direction, the dust expelled from the respective dust ejection holes 45 D can be smoothly guided away from the engine 1 sideways without the individual flows of dust interfering with one another. The front right direction in which the dust is expelled from the cyclone dust collection units 34 faces away from the rear right direction in which the operator of the engine 1 is situated so that the dust expelled from the cyclone dust collection units 34 does not cause discomfort to the operator.

Because the cooling air outlet 23 A is positioned under the cyclone dust collection units 34 , and an air flow is thereby created along the upper surface of the engine main body 2 , the dust expelled from the cyclone dust collection units 34 is prevented from being deposited on the upper surface of the engine main body 2 .

According to the illustrated embodiment, because the cyclone dust collection units 34 are formed by combining three pieces, the cover member 42 , the passage member 44 and the case member 45 , the assembling and disassembling of the cyclone dust collection units 34 can be simplified. When the case member 45 is made of transparent material, the amount of dust collected in each separation chamber 45 C can be visually inspected from outside.

The engine 1 constructed as discussed above may be advantageously applied to a concrete cutter 100 , for instance. As shown in FIG. 10 , the concrete cutter 100 includes a frame 101 extending in the fore and aft direction, a pair of wheels 102 provided in the rear end of the frame 101 , a cutting disk 103 supported by the front end of the frame 101 so as to be rotatable around a rotational center line extending laterally, a cutting disk cover 104 attached to the frame 101 to cover an upper part of the cutting disk 103 , and a water tank supported on top of a rear part of the frame 101 . The engine 1 is supported on top of a front part of the frame 101 in such a manner that the crankshaft 10 extends laterally, and the crankcase 4 is located ahead of the cylinder head 7 . The output end of the crankshaft 10 of the engine 1 is connected to the center shaft of the cutting disk 103 via a power transmitting device 106 such as a belt-pulley system. In the engine 1 of the illustrated embodiment, the air cleaner 32 is positioned so as not to project from the rear end of the engine main body 2 . Thereby, the fore and aft length of the engine 1 may be minimized. Therefore, even when the water tank 105 is positioned in the rear part of the frame 101 , and the space available for the engine 1 may be limited, it is possible to install the engine on the frame in a spatially efficient manner.

The air cleaner of the present is particularly advantageous for applications where the engine 1 can be subjected to a dusty environment as is the case with concrete cutters. The use of the cyclone dust collection units 34 in the air cleaner 32 is highly effective in collecting dust before the fresh intake air is introduced into the filter element so that the service life of the filter element can be extended, and the performance of the engine 1 can be maintained over an extended period time without requiring frequent replacement of the filter element and the associated expenses.

Although the present invention has been described in terms of a preferred embodiment 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 which is set forth in the appended claims. The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application as well as the contents of the prior art references mentioned in this application are incorporated in this application by reference.