DESC:TECHNICAL FIELD
[0001] The present invention generally relates to an internal combustion engine of an automobile. More particularly, the present invention relates to an engine balancer assembly for the internal combustion engine of the automobile.

BACKGROUND
[0002] An internal combustion engine converts thermal energy obtained from burning of a fuel with an oxidizer (air) into mechanical energy, which can be used to do some kind of mechanical work. It is used in a wide range of applications including providing motive force for movement of an automobile. The main parts of the engine include a cylinder head, a reciprocating piston on a cylinder block and a connecting rod which connects the piston to the reciprocating crankshaft. The connecting rod in the engine transfers power generated by the reciprocating piston in the cylinder block of the engine and converts it into rotary motion of the crankshaft together forming a slider crank mechanism. When the piston reciprocates inside the cylinder block, there are unbalanced rotational inertia forces generated due to the weight of the piston and connecting rod assembly. These rotational inertial forces cause undesired engine vibrations and noise. These engine vibrations can be cancelled out, in part, by the counterweights integrally attached to a crankshaft. But, additional mechanisms maybe required to balance the internal combustion engine and reduce engine vibrations, noise and also to achieve good smoothness of the automobile. Such additional mechanisms include an engine balancer device to address this problem. In the engine balancer assembly, a counter-balancing weight corresponding to about just an equivalent mass to counter the rotational inertial forces, is mounted on the internal combustion engine and meshed with the crankshaft. The assembling of the engine balancer assembly is crucial as if there is a misalignment then, the rotational inertial forces may not be properly balanced.

BRIEF DESCRIPTION OF THE DRAWINGS
[0001] The detailed description is described with reference to the accompanying figures. The same numbers are used throughout the drawings to reference like features and components.
[0002] Fig. 1 illustrates the side view of a two wheeled vehicle employing an embodiment of the present invention.
[0003] Fig. 2a illustrates the side view of an internal combustion engine according to the embodiment of the present invention.
[0004] Fig. 2b illustrates the front perspective view of the internal combustion engine according to the embodiment of the present invention.
[0005] Fig. 3a illustrates the cross-sectional view (X-X) of the internal combustion engine showing an engine balancer assembly according to the embodiment of the present invention.
[0006] Fig. 3b illustrates another cross-sectional view (Y-Y) of the internal combustion engine showing the engine balancer assembly according to the embodiment of the present invention.
[0007] Fig. 4 illustrates the exploded view of the engine balancer assembly according to the embodiment of the present invention.
[0008] Fig. 5a illustrates the side view of the balancer gear assembly according to the embodiment of the present invention.
[0009] Fig. 5b illustrates the front view of the balancer gear assembly according to the embodiment of the present invention.
[00010] Fig. 5c illustrates the exploded view of the balancer gear assembly according to the embodiment of the present invention.
[00011] Fig. 6a illustrates the front view of a balancer gear according to the embodiment of the present invention.
[00012] Fig. 6b illustrates the front view of a bush member according to the embodiment of the present invention.

DETAILED DESCRIPTION
[00013] Various features and embodiments of the present invention here will be discernible from the following further description thereof, set out hereunder. In the ensuing exemplary embodiments, the vehicle is a two wheeled vehicle. However it is contemplated that the disclosure in the present invention may be applied to any automobile capable of accommodating the present subject matter without defeating the spirit of the present invention. The detailed explanation of the constitution of parts other than the present invention which constitutes an essential part has been omitted at suitable places.
[00014] Some internal combustion engines have the problem of vibrations and noise. A reciprocating piston inside the internal combustion engines gives rise to unbalanced rotational inertial force in accordance with the reciprocating movement of the piston. This rotational inertial force is exerted upon the crankshaft and can cause vibrations and noise. Counterweights are employed on the crankshaft so as to balance this rotational inertial force, but it is difficult to provide sufficient counterweights directly on the crankshaft so as to balance all the inertial forces. This problem is especially acute, when the internal combustion engine is a single cylinder engine and designed to be extremely compact and has layout constraints. Hence, in addition to providing counterweights on the crankshaft, there is an engine balancer assembly used to balance the rotational inertial forces completely.
[00015] The conventional engine balancer assembly has a counterbalancing weight corresponding to about a part of the reciprocating mass, while the rest of the counterbalancing weight is part of the crankshaft. The counterbalancing weight is integrally attached to a balancer shaft which is coupled to the crankshaft. Generally, the balancer shaft rotates at the same speed and in a direction opposite to that of the crankshaft to reduce the imbalance in rotational inertia forces produced by the unbalanced mass.
[00016] The important factors to be taken into consideration while designing an engine balancer assembly are size and compactness, hence generally two types of drives are adopted to connect the crankshaft to the balancer shaft. They are gear drive transmission and chain drive transmission. In gear drive transmission there is a gear on the crankshaft meshing with a corresponding gear on the balancer shaft. In chain drive transmission, a chain drive connects the crankshaft to the balancer shaft with aid of sprockets attached to both the shafts. Chain drive transmission is usually employed when due to layout constraints the balancer shaft cannot be disposed close to the crankshaft to connect it by gear drive transmission. Chain drive transmission is also used when multiple balancer shafts are used to balancer the rotational inertia forces completely.
[00017] In the gear drive transmission, the engine balancer device comprises the balancer shaft to which is integrally attached the counterbalancing weight. The balancer shaft has a balancer gear assembly which is rotated by the crankshaft, which in turn rotates the balancer shaft. The balancer gear assembly is made up of a balancer gear relatively coupled to a rotation member such a bush member through a damper system. The balancer gear is rotatably meshed with the crankshaft. The bush member is integrally mounted with the balancer shaft. The balancer gear and bush member are configured to be coupled such that the rotary motion is transferred from the balancer gear to the bush member. Hence, this arrangement ensures, the rotary motion of the crankshaft inputted to the balancer gear is relatively transmitted and supported by the bush member. A damper system is employed for damping the fluctuations during rotary motion transmission from the crankshaft to the balancer shaft. The damper system usually comprises of an elastic member disposed between the balancer gear and the bush member. The elastic member is usually a helical spring or an elastic damping material which can absorb and dampen the rotary motion fluctuations.
[00018] In the assembly of the balancer gear assembly, it is very crucial to assemble it in the right direction and orientation. In order to balance the rotational inertial forces generated by the reciprocating masses in the internal combustion engine, it is very crucial that the forces generated by the counterbalancing weight of the balancer shaft is always in a direction opposite of the forces generated by the reciprocating masses. Hence, these forces are equal and acting in a direction opposite to each other at all positions of the reciprocating piston, the rotational inertial forces are balanced, and vibrations and noise are reduced. The assembly of the engine balancer assembly is to be done with utmost care as slight misalignment with the assembly may cause the two opposite forces not to act in exactly the opposite direction, and may not eliminate the noise and vibrations completely. In this regard, one of the factors in ensuring perfect assembly is when the balancer gear is meshed with the gear in the crankshaft. Another crucial deciding factor relates to the assembly of the bush member in the balancer gear. This is crucial because, the bush member is relatively attached to the balancer shaft to transfer torque, and the bush member is attached to the balance gear through the transmitting means, hence any misalignment between the bush member and the balancer gear will cause the entire engine balancer assembly to be misaligned with respect to the reciprocating mass and the rotational inertial forces will not be balanced perfectly. Error free alignment between the bush member and the balancer gear is crucial and it is necessary to design the balancer gear assembly to avoid any chance of wrong fitment. Consequences of wrong assembly include vibrations and noise. Also, due to design of different mechanisms known, there maybe striking noises between the elements used to relatively couple the rotary member and balancer gear.
[00019] In a conventional state of art, a variety of mechanisms have been proposed for preventing vibration of the engine. Such mechanisms have mating portions that relatively connect the balancer gear to the bush member. But, the problem of occurrence of errors is still known in these mechanisms. Some solutions rely on skill and knowledge while assembling the components, and providing indentations, punch-marks and physical indications to indicate how to assemble it. In state of art, there are steps taken to prevent occurrence of error like, increase the height on one side and decrease the height in the other to cause a linear change in height of two mating portions engaged to transfer relative motion between the rotary member and balancer gear. These solutions have drawbacks, which include mechanism constraints, error when bush member is inserted in reverse assembly direction, and wearing out over a period of time. This can occur due to constant surface contact and high relative motion transfer which may cause the different lengths to get worn out over a period of time, and may cause misfit during serviceability of the automobile or during part replacement. The constraints posed by the mechanism include, rattle noise of two mating portions designed to transfer relative motion, and ineffective use of damping system.
[00020] Hence, it is the object of the present invention to prevent occurrence of error during assembly of the engine balancer assembly for the internal combustion engine. Such occurrence of error more specifically relates to assembly of the bush member which is to be inserted coaxially inside the balancer gear and avoidance of reverse assembly or flipped assembly or assembly in any other manner other than one intended.
[00021] Another object of the present invention is the ability to identify reference positions quickly during assembly, and use those references to assembly other components of the balancer gear assembly such as insertion of pins, and insertion of elastic member.
Another object of the present invention is to provide a foolproof solution to prevent errors in assembly from regular serviceability.
[00022] With the above design changes, the following advantages can be obtained such as no reverse assembly or flipped assembly possible or any other assembly other than normal assembly is possible, and the presence of reference points to do quick assembly in production line.
[00023] According to the present invention to attain the above mentioned objectives, there is provided an internal combustion engine which comprises a cylinder head a reciprocating piston inside a cylinder block, a rotatable crankshaft, a connecting rod connecting the reciprocating piston to the rotatable crankshaft and an engine balancer assembly configured to receive rotary motion from the rotatable crankshaft. The engine balancer assembly comprises of a balancer shaft, a counterbalancing weight integrally attached to the balancer shaft, a bush member, a balancer gear and a transmitting means configured to transfer rotary motion from the balancer gear to the bush member. The bush member has a boss portion attached to the balancer shaft and a plurality of outward slots arranged radially on the outer periphery of the bush member. The balancer gear is disposed coaxially with the bush member, said balancer gear having an annular portion and a plurality of inward slots arranged radially on the inner periphery of the balancer gear. The bush member and balancer gear are assembled such that, a recess is formed by one outward slot of said plurality of outward slots and one inward slot of said plurality of inward slots, said recess capable of accommodating a transmitting means configured to transfer rotary motion from the balancer gear to the bush member. Accordingly, in such a assembly at least one outward slot of said plurality of outward slots and one inward slot of said plurality of inward slots has a profile angularly dissimilar with respect to other outward slots of said plurality of outward slots and other inward slots of said plurality of inward slots.
[00024] The effect of the assembly of the present invention is that, at least one of the slots in the bush member and the corresponding mating slot on the inner periphery of the balancer gear forming the recess are angularly dissimilar with respect to other formed recesses. Hence, during assembly of this engine balancer assembly, any other way of assembly like reverse assembly, flipped assembly or any other way other than the normal assembly is attempted, then, the angular dissimilar profile of the slots in the bush member and the balancer gear mates with its corresponding symmetric slot. This arrangement prevents the assembly of further components such as pins and elastic members, as they are designed to be accommodated only if the angularly dissimilar slots on the corresponding bush member and balancer gear mate together. Thus, it is always possible to carry out assembly only in the normal way, and any assembly in other way is prevented. Thus, this provides a foolproof method of preventing any occurrence of error, and the engine balancer assembly would be able to function effectively.
[00025] Additionally, the engine balancer assembly according to the present invention has an indentation mark on the bush member matches with corresponding indentation in the balancer gear during assembly to further prevent occurrence of error of improper assembly.
[00026] The present invention along with all the accompanying embodiments and their other advantages would be described in greater detail in conjunction with the figures in the following paragraphs.
[00027] Fig. 1 illustrates a two wheeled vehicle (100), having an internal combustion engine (101) in accordance with one embodiment of the present invention. The vehicle further includes, a front wheel (110), a rear wheel (103), a body frame assembly, a fuel tank (107) and seat (106). The Body frame assembly includes a head pipe (111), a main tube (112), a down tube (113), and seat rails (not shown). The head pipe (111) supports a steering shaft (not shown) with two brackets – upper bracket (not shown) and lower bracket (not shown) at each end. Two telescopic front suspension (114) is attached to the lower bracket (not shown) on which is supported the front wheel (110). The upper portion of the front wheel (110) is covered by a front fender (115) mounted to the lower portion of the lower bracket at the end of the steering shaft (not shown). A handlebar (108) is fixed to upper bracket (not shown) and can rotate to both sides. A head light (109) is arranged on an upper portion of the head pipe (111). Down tube (113) is located in front of the internal combustion engine (101) and stretches slantingly downward from head pipe (111). A bracket (116) is provided at the lower end of down tube (113) for supporting the internal combustion engine (101). Main tube (112) is located above the internal combustion engine and stretches rearward from head pipe (111) and connects the rear of the internal combustion engine (101). A vertical pipe (117) is joined to the rear end of main tube (112) and stretches downward from the point where the main tube (112) joins the seat rails (not shown). Seat rails (not shown) are joined to main tube (112) and stretch rearward to support a seat (106) disposed above seat rails (not shown). Left and right rear swing arm bracket portions (not shown) support a rear swing arm (not shown) to swing vertically, and a rear wheel (103) is connected to rear end of the rear swing arm (not shown). Generally, two rear wheel suspensions (102) are arranged between rear swing arms. A tail light unit (104) is disposed at the end of the two-wheeled vehicle at the rear of the seat rails (not shown). A grab rail (105) is also provided on the rear of the seat rails (not shown). Rear wheel (103) is arranged below seat (106) and rotates by the driving force of the internal combustion engine (101) transmitted through a chain drive (118) from the engine (101). There is front brake (119) and back brake (not shown) arranged on the front wheel (110) and back wheel (103) respectively.
[00028] Fig. 2 illustrates the side view of the internal combustion engine (101) in accordance with the embodiment of the present invention. The engine is made up of a cylinder head (201), cylinder block (202), crankcase (203) and a sprocket (204) which transfers the rotary output to the rear wheel by means of the chain drive (118).
[00029] Fig. 3 illustrates a cross sectional view (X-X) of the internal combustion engine (101) showing the main parts which include a reciprocating piston (302), a cylinder block (202), a rotatable crankshaft (301), and an engine balancer assembly (300) according to the embodiment of the present invention. The cylinder head (201) consists of at least one inlet valve (not shown) and at least one outlet valve (not shown) which are operated by means of rocker arms (not shown) and a camshaft (304) which consists of at least one inlet cam lobe (not shown) and at least one outlet cam lobe (not shown) which actuates the rocker arms (not shown) when required. A cam-chain drive (303) is meshed between the rotatable crankshaft (301) and camshaft (304) in order to drive the camshaft (304) in the cylinder head (201). The inlet air fuel mixture from the carburettor (not shown) is connected to an inlet portion of the internal combustion engine in the cylinder head (201) and an exhaust system including a muffler (not shown) is connected to the outer portion of the cylinder head (201). The crankcase (203) all houses other systems such as starter assembly (not shown) and transmission system (not shown), lubrication system (not shown), cooling system (not shown) and exhaust system (not shown). The engine balancer assembly (300) is located inside the crankcase (203) and relatively in the front of the internal combustion engine (101) as viewed from the front of the two wheeled vehicle. The engine balancer assembly (300) comprising a balancer shaft (402), is assembled inside the internal combustion engine (101) in such as way that the balancer shaft (402) is parallel and located towards the side of the rotatable crankshaft (301) as illustrated in the Fig. 3a. Fig. 3b illustrates the cross-sectional view (Y-Y) of the internal combustion engine showing the engine balancer assembly (300) coupled to the rotatable crankshaft (301). There is a driving gear (308) integrally mounted with the rotatable crankshaft (301) and rotates along with it. The driving gear is coupled to the engine balance assembly (300) through the balancer gear assembly (401). Care must be taken to ensure that, the meshing between the driving gear (308) and balancer gear assembly is set right when the internal combustion engine is assembled. There are indentations present on both gears (307) and (504) and they must align to ensure the engine balancer assembly (300) is coupled perfectly to the rotatable crankshaft (301) to ensure the inertial forces are balanced perfectly. The gearing ratio between the driving gear (308) and the balancer gear assembly (401) is 1:1. Hence, the engine balancer assembly (300) rotates at the same rotational speed as that of the rotatable crankshaft (301) but in opposite direction.
[00030] Fig. 4 illustrates the exploded view of the engine balancer assembly (300) of the internal combustion engine (101) employing the embodiment of the present invention. The engine balancer assembly (300) comprises a balancer shaft (402) on which a balancer gear assembly (401) is disposed. The balancer shaft (402) has a counterbalance weight (403) integrally mounted on the balancer shaft (402). The balancer gear assembly is held by two spring retainers (404 and 405) which hold the torque transmitting means (510) disposed within the balancer gear assembly (401) in position. The spring retainers (404 and 405) are disposed on either side of the balancer gear assembly (401) during its assembly inside the balancer shaft (402). The balancer gear assembly and the spring retainer is held in position on the balancer shaft by means of an flange like structure such as a gear stopper (410) on one end. On the other end, a nut and a washer arrangement holds the balancer gear assembly (401) in position and also locks the spring retainers (404 and 405) in position to prevent the helical spring from getting displaced outside the gear balancer assembly. There is a flange like bearing stopper (409) located on the other end of the bearing shaft (402), which holds the shaft firmly in position when the balancer shaft surface at one end (412) it is inserted inside a bearing on the crankcase (203). Therefore, both the surface at both the ends of the balancer shaft (412 and 413) are supported on bearings (309 and 310) located on the crankcase (203).
[00031] Fig. 5a illustrates the front view of the balancer gear assembly (401) and Fig. 5b illustrates the sectional view (Y-Y) employing the embodiment of the present invention. The balancer gear assembly (401) comprises of a bush member (502), a balancer gear (501), the bush member (502) is held within the balancer gear (501) annularly by a torque transmitting means (510). The torque transmitting means (510) in present embodiment comprises a plurality of elastic members such as a helical spring (503) and a plurality of pins (507) to hold the helical spring (503) in position. The bush member (502) has a plurality of outward slots (602, 602a, 602b and 602c) arranged radially on its outer periphery. The balancer gear (501) has an annular portion (501a) and a plurality of inward slots (601, 601a, 601b and 601c) of similar dimension as that of the outward slots (602, 602a, 602b and 602c) on the bush member, and the inward slots (601, 601a, 601b and 601c) are arranged radially on the inner periphery around the annular portion (501a). The balancer gear (501) is assembled such that, it is disposed coaxially with the bush member (502) in such a manner that the outward slots (602, 602a, 602b and 602c) of the bush member match with the inward slots (601, 601a, 601b and 601c) of the balancer gear (501) to create a recess (509). The recess is made up of two halves, the upper half being the inward slots (601) of the balancer gear (501) and the bottom half being the outward slots (602) of the bush member (502). The bush member (502) has a boss portion (508) which is attached to the balancer shaft (402) by means of a woodruff key joint. The bush member (502) is placed such that, the bush keyway (509) perfectly mates with the corresponding shaft keyway (408). Subsequently, a semicircular key (407) is secured between the bush keyway (509) and the shaft keyway (408) in order to fix the bush member on the balancer shaft (402) and transfer relative motion to it.
[00032] Fig. 5c illustrates the exploded view of the gear balancer assembly (401). Here, we can see the assembly of the bush member (502) coaxially within the balancer gear (501) and connected by the torque transmitting means (510) is disposed in the recess (509). The function of the torque transmitting means (510) is to transfer the torque between the bush member (502) and the balancer gear (501) and provide damping effect along with minimizing surface contact. The helical spring (503) is disposed in the recess (509) and held there by means of a pin (507) disposed coaxially inside the helical spring (503) and acts like a base surface on which the helical spring (503) is disposed. The resulting balancer gear assembly (401) ensures that, the relative motion between the bush member (502) and balancer gear (501) is only through the combination of pin (507) and helical spring (503) arrangement. This minimizes surface contact between the bush member (502) the balancer gear (501).
[00033] There is a bush indentation mark (505) on the bush member (502) and a corresponding gear indentation mark (504) in the balancer gear (501). During assembly of the bush member (502) coaxially in the balancer gear (501), the bush indentation mark (505) is matched with the gear indentation mark (504) to further prevent occurrence of error of improper assembly.
[00034] Fig. 6a illustrates the balancer gear (501) and Fig. 6b illustrates the bush member (502) according to the embodiment of the present invention. One of the inward slot profile (601) of the balancer gear (501) is angularly dissimilar as compared to other plurality of slots (601a, 601b and 601c). In the present embodiment, the profile of the inward slot is deviated from the perpendicular axis drawn through the centre of the slot by a small angle (605) in the range 5 to 10 degrees. Similarly, one of the outward slot profile (602) is angularly dissimilar as compared to other plurality of slots (602a, 602b and 602c). In the present embodiment, the profile of the outward slot is deviated from the perpendicular axis drawn through the centre of the slot by a small angle (606) in the range 5 to 10 degrees. During assembly of the balancer gear assembly, the balancer gear (501) is coaxially disposed over the bush member (502) such that the surface of the inner periphery of the balance gear (603) and surface of the outer periphery of the bush member (604) come in contact and mate such that the inward slots (601, 601a, 601b and 601c) mate with its corresponding outward slots (602, 602a, 602b and 602c). This assembly ensures that, there is one recess formed which is angularly dissimilar as compared to other three similarly shaped recesses. Pins are assembled inside the recesses, over which helical springs (503) are disposed. The pins (507) and helical spring (503) are designed to perfectly match with the profile of the recess. In case wrong fitment, any other assembly other than normal assembly is attempted, the pin (507) and the helical spring (503) will not fit the resulting profile of recess so formed. Additionally, it is desirable that the dissimilar recess is fixed in relation to the bush indentation mark (505). This will ensure a relative positional relationship between the gear indentation mark (504) and driving gear indentation mark (307). This ensures, that keeping the dissimilar recess as reference, relative positions of all elements of the engine balancer assembly (300) can be fixed, ensuring completely foolproof error prevention during assembly.
[00035] Many modifications and variations of the present subject matter are possible in the light of above disclosure. Therefore, within the scope of claims of the present subject matter, the present disclosure may be practiced other than as specifically described.
,CLAIMS:We Claim:
1. An internal combustion engine (101) comprising:
a crankcase (203);
a cylinder block (202) disposed on the crankcase (203);
a reciprocating piston (302) inside the cylinder block (202) transferring reciprocating motion to a rotatable crankshaft (301) disposed within the crankcase (203); and
an engine balancer assembly (300) configured to receive rotary motion from the rotatable crankshaft (301), said engine balancer assembly (300) comprising:
a balancer shaft (402), including a counterbalancing weight (403);
a bush member (502), having a boss portion (508) attached to the balancer shaft (402) and a plurality of outward slots (602, 602a, 602b and 602c) arranged radially on the outer periphery of the bush member (502);
a balancer gear (501) disposed coaxially with the bush member (502), said balancer gear (501) having an annular portion (501a) and a plurality of inward slots (601, 601a, 601b and 601c) arranged radially on the inner periphery of the balancer gear (501); and
a recess (509) formed by one outward slot of said plurality of outward slots (602, 602a, 602b and 602c) and one inward slot of said plurality of inward slots (601, 601a, 6021 and 601c), said recess (509) capable of accommodating a torque transmitting means (510) configured to transfer rotary motion from the balancer gear (501) to the bush member (502);
wherein,
at least one outward slot (602) of said plurality of outward slots (602, 602a, 602b and 602c) and at least one inward slot (601) of said plurality of inward slots (601, 601a, 6021 and 601c) forming part of one recess (509a) of said plurality of recesses has a profile angularly dissimilar with respect to other outward slots (602a, 602b and 602c) of said plurality of outward slots (602, 602a, 602b and 602c) and other inward slots (601a, 601b and 601c) of said plurality of inward slots (601, 601a, 6021 and 601c).
2. The internal combustion engine (101) as claimed in claim 1, wherein the torque transmitting means (510) comprises plurality of pins (507) disposed in the recess (509), and plurality of elastic members (503) disposed in the recess (509), said elastic members disposed coaxially with the pins.
3. The internal combustion engine (101) as claimed in claim 1, wherein the bush member (502) has a bush indentation mark (505) which matches with a corresponding gear indentation mark (504) in the balancer gear (501) during assembly to prevent occurrence of error of improper assembly.
4. The internal combustion engine (101) as claimed in claim 1, wherein the rotatry motion is transferred from the rotatable crankshaft (203) to the balancer gear (501) by a driving gear (308) fixed to the rotatable crankshaft (203).
5. The internal combustion engine (101) as claimed in claim 1, wherein the bush member (502) includes a boss portion (508) having a keyway (509) and the attachment of the bush member to the balancer shaft is effected by a key (407).
6. The internal combustion engine (101) as claimed in claim 1, wherein the angularly dissimilar profile of at least one outward slot (602) of said plurality of outward slots (602, 602a, 602b and 602c) and the angularly dissimilar profile of at least one inward slot (601) of said plurality of inward slots (601, 601a, 6021 and 601c) is five degrees from an axis that is perpendicular to a centre axis of the balancer shaft (501).
7. A two-wheeled vehicle (100) including said internal combustion engine as claimed in claims 1 to 6.