DESC:TECHNICAL FIELD
[0001] The present subject matter relates generally to an internal combustion engine for a two wheeled vehicle. More particularly, the present subject matter relates to a chain tensioner in the internal combustion engine.
BACKGROUND
[0002] An internal combustion (IC) engine comprises a cylinder head, abutting a cylinder block to form a combustion chamber where the burning of air fuel mixture occurs. The forces generated due to combustion of air fuel mixture is transferred to a piston which is capable of reciprocating inside the cylinder block, and this reciprocating motion is transferred to rotary motion of the crankshaft through a connecting rod by the slider crank mechanism. The cylinder head comprises an intake port and an outlet port which allow the entry of air-fuel mixture and exit of burnt gases from the combustion chamber. The entry of air-fuel mixture to the combustion chamber and the exit of burnt gases are controlled by intake and exhaust valves which are configured to open and close based on the running cycle of the internal combustion (IC) engine. In this operation the precise movement and timing of the opening and closing of intake port and outlet port to the combustion chamber is essential for accurate performance of the internal combustion (IC) engine. The opening and closing operation must function effectively at a large range of speeds and operations. Generally, this opening and closing is controlled by a valve train mechanism present on the cylinder head and cylinder block and actuated by a camshaft by the transmission of drive from the crankshaft using a cam chain. The crankshaft drives the camshaft and this is achieved by a cam chain which operably meshes between the camshaft and crankshaft. The tension in the cam chain is a very essential parameter that determines internal combustion (IC) engine performance. As the chain in any cam chain drive mechanism is typically subject to elongation of its length owing to wear and tear of the chain links which result in slackness of the chain causes loss in the efficiency of the transmission and thus engine performance. Hence, a chain tensioner is used which exerts a constant pressure on the cam chain throughout its operation to avoid slackness of the chain. But, due to various factors affecting the chain tensioner, there is a constraint in the movement of a pushrod which exerts constant pressure on the cam chain. This results in rough performance of the internal combustion (IC) engine accompanied by noise and vibration resembling a rattle which increases with speed of the internal combustion (IC) engine. Hence, a chain tensioner system is proposed in the present subject matter in order to alleviate one or more drawbacks highlighted above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] 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.
[0004] Fig. 1a. illustrates the side view of a two wheeled vehicle employing an embodiment of the present subject matter.
[0005] Fig. 2a. illustrates the isometric view of an internal combustion engine employing the embodiment of the present subject matter.
[0006] Fig. 2b. illustrates the cut sectional side view (X-X) of the internal combustion engine according to the embodiment of the present subject matter.
[0007] Fig. 3a. illustrates a chain tensioner system according to the embodiment of the present subject matter.
[0008] Fig. 3b. illustrates the sectional view of the chain tensioner according to the embodiment of the present subject matter.
DETAILED DESCRIPTION
[0009] Various features and embodiments of the present subject matter here will be discernible from the following further description thereof, set out hereunder. According to an embodiment, an internal combustion (IC) engine described here operates in four cycles. Such an internal combustion (IC) engine is installed in a step through type two wheeled vehicle. It is contemplated that the concepts of the present invention may be applied to other types of vehicles employing the mechanical rack type chain tensioner within the spirit and scope of this invention. Further "front" and "rear", and "left" and "right" referred to in the ensuing description of the illustrated embodiment refer to front and rear, and left and right directions as seen from a rear portion of the two wheeled vehicle and looking forward. Furthermore, a longitudinal axis unless otherwise mentioned, refers to a front to rear axis relative to the engine, while a lateral axis unless otherwise mentioned, refers generally to a side to side, or left to right axis relative to the engine. The detailed explanation of the constitution of parts other than the present subject matter which constitutes an essential part has been omitted at suitable places.
[00010] An internal combustion (IC) engine comprises a cylinder block includes a cylinder bore, a piston reciprocating in the cylinder bore, a cylinder head located above the cylinder block and a combustion chamber interposed between the cylinder head and the cylinder block. During operation of the internal combustion (IC) engine, the burning of air fuel mixture occurs in the cylinder block. The forces generated due to combustion of air fuel mixture is transferred to a piston which is capable of reciprocating inside the cylinder block, and this reciprocating motion is transferred to rotary motion of the crankshaft though a connecting rod by the slider crank mechanism. The cylinder head comprises intake valve and outlet valve which control the intake of air fuel mixture inside the combustion chamber, and controls the exit of exhaust gases after combustion respectively. The intake valve and exhaust valve are controlled by a valve train mechanism and are synchronized using a transmission system such as a cam chain to transfer rotary movements of the crankshaft to drive a camshaft. The camshaft is disposed on the cylinder head which in turn actuates the valve train mechanism. The valve train mechanism comprises two rocker arms supported on two rocker shafts respectively. The rocker arms operate the intake valve to open and close the intake port and exhaust valve to open and close the exhaust port. The intake valve and exhaust valve is capable of reciprocating and the stroke movement of the valves is actuated by each of the rocker arm. While a forward stroke is actuated by the rocker arm, the return stroke is actuated by the bias force of an elastic member connected to the valve such as a return spring.
[00011] The main function of the cam chain is to transmit and synchronize the rotation of the crankshaft and the camshaft for optimum control of opening and closing of intake valve and exhaust valve during the particular IC engine cycle. Generally, the cam chain operably meshes between the camshaft and crankshaft and is enveloped inside a cam chain chamber formed together with the cylinder head and cylinder block. A driving gear is mounted on the crankshaft and a driven gear is mounted on the camshaft. The cam chain is mounted in the cam chain chamber and meshed between the driving gear and driven gear. Typically, the driven gear has a number of teeth twice as that of the number of teeth of the driving gear. This ensures that for every two rotation of the crankshaft, the camshaft rotates once. However, in some cases the driven gear has a number of teeth same as the number of teeth of the driving gear. This ensures that for every one rotation of the crankshaft, the camshaft rotates once. The cam chain is usually supported by plurality of chain guides to support and guide the cam chain to maintain tension and prevent chain slippage.
[00012] The cam chain transmission is a very important aspect in the effective design of the internal combustion (IC) engine as it results in reduced noise, and low vibration operation. During normal use of the internal combustion (IC) engine, there are zones of tension and slackness around different regions of the cam chain. Further, due to prolonged usage the cam chain stretches with normal use. Due to stretching and zones of tension and slackness whipping of cam chain occurs. Hence, to maintain optimum tension and support throughout the operating life of the cam chain, a chain tensioner system is used. Commonly in smaller capacity single cylinder internal combustion (IC) engines (below 200cc) a mechanical rake type tensioner is used, wherein a pushrod is operated by an elastic member such as a spring exerts a constant pressure of the cam chain. Generally, the cam chain is supported by two chain guides namely, a fixed chain guide, and a pivot chain guide. The chain tensioner is designed to facilitate the movement of a pushrod with sufficient forward stroke to maintain optimum tension in the cam chain by exerting pressure on the pivot chain guide. When the cam chain shows signs of decreasing tension, the tensioner pushes the cam chain to bring back the tension to the optimum value. The pushrod is designed to only move in one direction, and backward stroke is prevented by a ratchet mechanism in which a ratchet interlocks on a serrated teeth provided on an exposed part of the pushrod. The positive bias force on a pushrod by the elastic member pushes on the cam chain guide to maintain optimum tension. There is normally a housing which encloses the pushrod and the elastic member. The pushrod is keyed to the housing so that it cannot rotate along its length. The elastic member is secured to the housing and is connected to one end of the pushrod such that, the space occupied by the part of the pushrod and elastic member completely forms an enclosed chamber.
[00013] A typical internal combustion (IC) engines includes mechanical rake type tensioner, the pushrod is constantly lubricated on the surface on which it moves to have a smooth operation. This is achieved by utilizing the lubricating oil which splashes around the cam chain chamber in the internal combustion (IC) engine. But, this lubrication results in isolating the enclosed chamber in the housing with that of the cam chain chamber. This occurs because the lubricating oil penetrates in the chain tensioner and gets accumulated at the junction wherein the pushrod slides on the chain tensioner housing. This results in formation of partial vacuum inside the housing. This partial vacuum prevents the forward stroke movement of the pushrod which affects the exertion of optimum pressure on the cam chain. This problem is especially pronounced in internal combustion (IC) engines of horizontal type that is, a cylinder axis (the axis on which the piston of the internal combustion (IC) engine reciprocates) is almost parallel to the central longitudinal axis of the two wheeled vehicle. In such horizontal internal combustion (IC) engine, wherein due to accessibility the chain tensioner is oriented towards the bottom of the cylinder head, the cam chain chamber is so positioned to have a horizontal disposition, resulting in lubricating oil to form a layer deposited in the bottom around the location where the pushrod is exerting pressure on the cam chain guide. Since the entire pushrod is constantly submerged under a layer of lubricating oil, the probability of the surfaces of contact of pushrod with the housing getting blocked increases resulting in formation of partial vacuum. This partial vacuum prevents complete forward stroke movement of the push rod & thus hampers the normal performance of the IC engine.
[00014] The inability of complete forward stroke movement of the pushrod increases the noise generated from the movement of cam chain, and consequently results in cam chain whipping. Such issues restrict the internal combustion (IC) engine to reach its maximum performance. Hence, it is very essential to maintain optimum tension within the cam chain throughout as it increases cam chain life and provides for smooth operation of the internal combustion (IC) engine. It is known in prior art that to overcome the force of partial vacuum an elastic member with higher stiffness can be adopted, but such an arrangement is undesirable as it increases the frictional force cam chain is subjected to. Since, a large area of the cam chain is in contact with the chain guide, increased tension at large duration of operation causes the cam chain to run hot and result in excessive wear and hence age prematurely. Additionally, the cam chain is subjected to varying cyclic loads and any aggravating circumstances such as increase in tension results in failing due to fatigue load prematurely as compared to its rated life. Increased tension also result in cam chain clinging to the driving gear and decreases the efficiency.
[00015] Hence, to obviate the limitations, the proposed subject matter discloses, a chain tensioner for an internal combustion (IC) engine wherein one or more openings are provided on the housing to expose the enclosed chamber enclosing the elastic member to the cam chain chamber. This exposes the enclosed chamber to the internal combustion (IC) engine atmosphere for freely exchange of air and lubricating oil. This results in eliminating the pressure difference between the enclosed chamber and cam chain chamber.
[00016] With the above design changes, the following advantages is obtained such as reduced noise and vibrations originating from the internal combustion (IC) engine, substantial reduction in cam chain whipping, smooth performance of the internal combustion (IC) engine, increased mechanical efficiency and longer life of the cam chain.
[00017] According to the present subject matter to attain the above objectives, the present subject matter proposes an internal combustion (IC) engine of a two wheeled vehicle, said internal combustion (IC) engine comprising: a crankcase body, said crankcase body enclosing a crankshaft; a cylinder block mounted on the crankcase body; a cylinder head disposed on the cylinder block, said cylinder head enclosing a camshaft; a cam chain operably connected between the crankshaft and the camshaft, said cam chain; a chain tensioner integrally attached to the cylinder block, said chain tensioner comprising: a pushrod comprising a serrated teeth to exert a positive bias force on the cam chain; a housing includes an enclosed chamber, said enclosed chamber enclosing an elastic member and at least a part of the pushrod, and the elastic member exerting a positive bias force on the pushrod; wherein, said enclosed chamber comprising one or more openings configured to equalise pressure inside and outside said enclosed chamber allowing the elastic member to exert a positive bias force on the pushrod which being directly proportional to the number pitch length of the serrated teeth the pushrod has covered.
[00018] The present subject matter 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.
[00019] Fig. 1. illustrates a two wheeled vehicle, includes the chain tensioner (301) according to the embodiment of the present subject matter. In a preferred embodiment, the internal combustion (IC) engine (101) is horizontally disposed, that is, the cylinder block and crankcase lie horizontally with their axis being in a single plane. The internal combustion (IC) engine (101) is installed in a moped type motorcycle, and the present invention can be implemented in other two wheeled vehicles which includes a mechanical rack type chain tensioner (301) without deviating from the scope of the present subject matter. The two wheeled vehicle (100) has a single frame structure, which acts as the skeleton for bearing the loads. The single frame structure is a mono-tube type frame structure that includes a head tube (102), a first portion extending rearwardly downward called the main-tube (not shown), a second portion defining a step-through space (104) (forming part of the main-tube) and a third portion extending rearwardly upward called the rear-tube (105). The main-tube extends rearwardly downward from the head-tube (102). The step-through space (104) is a part of the main-tube that extends rearward, along a longitudinal axis of the two wheeled vehicle (100), from a rear portion of the main-tube. The rear-tube (105) extends inclinedly rearward from a rear portion of the step-through space (104) towards a rear portion R of the two wheeled vehicle (100). A handle bar assembly (113) is pivotally disposed through the head-tube (102). The handle bar assembly (113) is connected to a front wheel (106) by one or more front suspension(s) (110). A front fender (108) is disposed above the front wheel (106) for covering at least a portion of the front wheel (106). A fuel tank (112) is mounted to the main-tube of the frame assembly and it is disposed in the front portion F of a step-through space (104) of the single frame structure. The internal combustion (IC) engine (101) is mounted to the step-through space (104) of the main-tube (not-shown). The fuel tank (112) is functionally connected to the internal combustion (IC) engine (101). The internal combustion (IC) engine (101) is of horizontal type, i.e. parallel to a longitudinal axis of the two wheeled vehicle (100). A swing arm (121) is swingably connected to the frame assembly. A rear wheel (107) is rotatably supported by the swing arm (121). One or more rear suspension(s) (111) are connecting the swing arm (121) at an angle, to sustain both the radial and axial forces occurring due to wheel reaction, to the frame assembly. A rear fender (109) is disposed above the rear wheel (107). A seat assembly (119a, 119b) is disposed at a rear portion R of the step-through space (104). In an embodiment, the seat assembly (119) includes a rider seat (119a), and a pillion seat (119b). Further, the seat assembly (119a, 119b) is positioned above the rear wheel (107).The two wheeled vehicle (100) is supported by a centre stand (117) mounted to the single frame structure.
[00020] Fig. 2a. illustrates a perspective view of the internal combustion (IC) engine (101) employing the embodiment of the present subject matter. The internal combustion (IC) engine (101) is made up of a cylinder head assembly (203), cylinder block (204), a cylinder head cover (202) and crankcase (205). The crankcase (205) is made up of left-hand crankcase (205b) (hereafter LH crankcase) and right-hand crankcase (205c) (hereafter RH crankcase). The internal combustion (IC) engine (101) also comprises of a magneto assembly (205a) which performs the function of a magneto coil assembly during operation to recharge a battery (not shown). The internal combustion (IC) engine (101) also has a sprocket (not shown) disposed outside (on the other side) of the LH crankcase (205b) and is linked to the rear wheel (107) with an end transmission means such as a chain drive (not shown). The internal combustion (IC) engine (101) also comprises a kick-starter system which when actuated by the kick-lever (206a) helps in cranking the internal combustion (IC) engine (101) during start through spark plug housing (203a).
[00021] Fig. 2b. illustrates a cut section of the internal combustion (IC) engine (101) along line X-X showing the main components of the cam chain transmission system according to the embodiment of the present subject matter. The internal combustion (IC) engine (101) comprises a piston (not shown) reciprocating in the cylinder block (not shown) due to force imparted to it by the combustion of air fuel mixture. This reciprocating motion is converted and transferred to a rotary motion of a crankshaft (304) through a connecting rod (not shown). A driving gear (209) is connected to the crankshaft (304) and rotates integrally with it. The cylinder head (203) comprises of a valve train arrangement to control the intake of air fuel mixture and outlet of exhaust gases. The camshaft (208a) controls the movement of the valve train arrangement and comprises of at least one inlet cam lobe (not shown) and at least one outlet cam lobe (not shown) which actuates the valve train when required. Also, two chain guides are present within a cam chain chamber (210), namely a pivot chain guide (302) and a fixed chain guide (303) is disposed to support and guide a cam chain (201) during its operation. The cam chain (201) operably connects the crankshaft (304) and camshaft (208a) to drive the camshaft (208a) in the cylinder head (203). A driven gear (208) is integrally attached to the camshaft (208a) and the cam-chain (201) is configured to be meshed between the driving gear (209) and driven gear (208) to transfer rotary motion of the crankshaft (304) to the camshaft (208a). In the present embodiment, the ratio of the driven gear (209) is twice that of driving gear (208) such that for every two revolutions of crankshaft (304) the camshaft (208a) should rotate one revolution.
[00022] Fig. 3a. illustrates the camshaft transmission system according to the embodiment of the present subject matter. The cam chain (201) is loaded around the driving gear (209), and driven gear (208). Since transmission by the cam chain (201) is a positive drive, the transmission losses are minimal, provided parameters such as tension on the chain is maintained to the best optimum value. Further, two chain guides, namely the pivot chain guide (302) and the fixed chain guide (303) is disposed to support and guide the cam chain (201) during its operation. The chain guides serve two important functions, to support and guide the cam chain (201) within the cam chain chamber (210) to prevent wobbling and whipping action, and to maintain a constant pressure on the cam chain (201) by transferring the pressure exerted by the chain tensioner (301) to the cam chain (201). The latter functionality described is a very important function of the pivot chain guide (302). The pivot chain guide (302) is mounted on one side of the cam chain chamber (210), said pivot chain guide (302) pivoted with a tensioner blade (302b) and the tensioner blade (302b) comprising a projected portion (302a) which is configured to come in contact with the flat surface of the pushrod (301a) of the chain tensioner (301). Referring to Fig. 2b, in the present embodiment, the chain tensioner (301) is mounted on the cylinder block on the bottom facing surface of the cylinder block (204) such that the chain tensioner (301) is oriented downwards towards the ground level of the two wheeled vehicle. Further, a longitudinal axis of the chain tensioner (Y-Y) is approximately perpendicular to the imaginary line (X-X) joining the centre of the driving gear (209) and driven gear (208). Hence, the pushrod (301a) is maintained almost perpendicular to the to the pivot chain guide (302) when it comes in contact and exerts pressure on the contact portion of the pivot chain guide (302). As the tension within the cam chain (201) decreases below a predefined value which varies as per the intended design of the IC engine (101), the pushrod (301a) performs a forward stroke corresponding to the loss of tension which results in the pivot chain guide moving about the pivot to exert a corresponding pressure to the cam chain (201) to bring the cam chain tension to its correct value. Due to circulation of lubricating oil within the IC engine (101) especially by splash lubrication within the cam chain chamber (210), lubricating oil may penetrate the chain tensioner (301). In the present embodiment in addition to the splash of lubricating oil, due to the location of the chain tensioner (301) and the pushrod (301a) on the bottom portion of the IC engine (101), and due to the particular horizontal disposition of the IC engine (101), there may arise a possibility of a continuous layer of oil present in the cam chain chamber (210) and getting stagnated with being capable of flowing back inside to the crankcase. In such situations, there is a possibility of the pushrod (301a) of the chain tensioner (301) being submerged in lubricating oil.
[00023] Fig. 3b. illustrates the cut sectional view of the chain tensioner (301) according to the embodiment of the present invention. The chain tensioner (301) comprises a housing (301c) comprising an inner surface (301i), the pushrod (301a), said pushrod (301a) comprising a row of serrated teeth (301g) includes a predefined pitch (p) based on the desired forward stroke movement desired, a ratchet (301f) engageable with the serrated teeth (301g) to prevent backward stroke of the pushrod (301a), and an elastic member (301b) applying positive bias force on the pushrod (301a) to cause a forward stroke movement. One end of the pushrod (301a) has an operating part (301d) which has a piston like surface is in contact with the inner surface (301i) of the housing (301c) and configured to slide smoothly within the inner surface (301i) of the housing (301c). The piston like surface is loaded by the elastic member (301b) such that the positive bias force exerted by the elastic member (301b) is translated into forward stroke movement of the pushrod (301a) by the sliding of the piston surface over the inner surface (301i) of the housing (301c). To prevent backward stroke of the pushrod (301a) due to the constant forces the pushrod (301a) is subjected to by the cam chain (201) the ratchet (301f) and the serrated teeth (301g) arrangement ensures that, the backward return stroke is locked. Due to this arrangement, the forward stroke movement of the pushrod (301a) is defined in terms the number pitch length (p) of the serrated teeth (301g) of the pushrod (301a) has covered.
[00024] The elastic member (301b) is enclosed within the housing (301c) between one end of the housing (301c) and piston like surface forming an enclosed chamber (301k). The pushrod (301a) is constantly being subjected to oil splash during IC engine operation and sometimes even submerged within it. These results in oil entering inside the chain tensioner (301) and depositing within the contact surface of the piston like surface and the inner surface (301i) of the housing (301c). This oil layer on the contact surface may sometimes result in hermetically sealing the enclosed chamber (301k). This effect decreases the pressure inside the enclosed chamber (301k), while the pressure outside the piston like surface is higher. This pressure difference prevents the free forward movement of the pushrod (301a) towards the pivot chain guide (302). Hence, one or more openings (301e) are introduced connecting the enclosed chamber (301k) to the cam chain chamber (210). The one or more openings (301e) exposes the enclosed chamber (301k) to the cam chain chamber (210) to enable exchange of air and oil (if any) to and from the cam chain chamber (210). This ensures that even if the enclosed chamber (301k) is hermitically sealed due to oil deposit, the pressure inside the enclosed chamber (301k) is equalized as compared to the pressure outside the chain tensioner (301) within the cam chain chamber (210). Moreover, one or more openings (301e) should be disposed within an operating zone (L) of the pushrod (301a) where the pushrod (301a) moves inside the enclosed chamber (301k) for equalizing the pressure inside the enclosed chamber (301k) as compared to the pressure outside the chain tensioner (301) within the cam chain chamber (210). In present embodiment, one or more openings (301e) is of circular in shape for providing ease in attaining pressure equalization between the enclosed chamber (301k) and the pressure of area outside the chain tensioner (301) within the cam chain chamber (210). However, one or more openings (301e) are of various other shapes also such as rectangular slot, polygonal slot and any other shape which provide pressure equalization effectively within the cam chain chamber (210). Also, one or more openings (301e) in the enclosed chamber (301k) of the cam chain chamber (210) have an area which varies in the range of 0.02 to 0.99 times of the pushrod operating part (301d) surface area for reducing vacuum inside the enclosed chamber (301k). In an embodiment of the present subject matter, one opening (301e) is provided in the enclosed chamber (301k). In another embodiment, the number of openings (301e) varies from 1 to 15 in accordance to the area of openings (301e) in the enclosed chamber (301k) thereby ensuring that the pressure equalization within the cam chain chamber (210) is maintained.
[00025] The forward stroke movement of the pushrod (301a) is defined in terms the number pitch length (p) of the serrated teeth (301g) the pushrod (301a) has covered. As an illustration, the following condition is envisaged. However, the below example is by no means a limitation of the present subject matter in this application. A situation is illustrated wherein, the arrangement of one or more openings (301e) which enables communication of the enclosed chamber (301k) with the cam chain chamber (210), the partial vacuum in the enclosed chamber may prevent the complete movement of pushrod (301a), and if the required movement of the pushrod (301a) is one pitch length (p), the conditions inside the enclosed chamber (301k) prevent the movement of pushrod (301a) by one pitch length (p), and if due to excessive operation, if the movement required by the pushrod (301a) is two pitch lengths (2p), the conditions inside the enclosed chamber (301k) may allow the movement of pushrod (301a) for two pitch length (p). Hence, as a consequence a loss of one pitch length (p) results in loss of tension equivalent to one pitch length (p). Additionally, the loss of tension due to one pitch length (p) results in noise generation. Hence, the drawbacks associated with presence of vacuum inside the enclosed chamber (301k) such as excessive noise and cam chain (201) whipping are absent and chain tensioner (301) is effectively operated. One or more openings (301e) present in the enclosed chamber (301k) to the cam chain chamber (210) is one of the essential features of the present subject matter. Through one or more openings (301e) present in the enclosed chamber (301k) to the cam chain chamber (210) the positive bias force exerted by the elastic member (301b) on the pushrod (301a) is directly proportional to the number pitch length (p) of the serrated teeth (301g) the pushrod (301a) has covered. This minimizes the loss of tension due to pitch length (p) and results in reduction of noise generation and cam chain (201) whipping.

[00026] It will be appreciated that the present subject matter and its equivalent thereof offers many advantages, including those which have been described forthwith. 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 (IC) engine (101) of a two wheeled vehicle (100), said internal combustion engine IC engine (101) comprising:
a crankcase body (205), said crankcase body (205) enclosing a crankshaft (304);
a cylinder block (204) mounted on the crankcase body (205);
a cylinder head (203) disposed on the cylinder block (204), said cylinder head (203) enclosing a camshaft (208a);
a cam chain (201) operably connected between the crankshaft (304) and the camshaft (208a), said cam chain (201); and
a chain tensioner (301) integrally attached to the cylinder block (204), said chain tensioner (301) comprising:
a pushrod (301a) exerting a positive bias force on the cam chain (201);
a housing (301c) includes an enclosed chamber (301k), said enclosed chamber (301k) enclosing an elastic member (301b) and at least a part of the pushrod (301a), and the elastic member (301b) exerting a positive bias force on the pushrod (301a);
wherein,
said enclosed chamber (301k) comprising one or more openings (301e) configured to equalize pressure inside and outside said enclosed chamber (301k).

2. The internal combustion (IC) engine (101) as claimed in claim 1, wherein the internal combustion (IC) engine (101) has a cylinder axis substantially horizontal in orientation with respect to the longitudinal axis, said internal combustion (IC) engine (101) disposed below a step-through space (104) on the two wheeled vehicle (100).
3. The internal combustion (IC) engine (101) as claimed in claim 1, wherein the pushrod (301a) is prevented from a return stroke by a ratchet (301f) and a serrated teeth (301g) surface on the pushrod (301a) mechanism.
4. The internal combustion (IC) engine (101) as claimed in claim 1, wherein said one or more openings (301e) area varies in the range of 0.02 to 0.99 times of the pushrod operating part (301d) surface area.
5. The internal combustion (IC) engine (101) as claimed in claim 1, wherein the chain tensioner (301) is mounted to have a longitudinal axis of the chain tensioner (Y-Y) being approximately perpendicular to an imaginary line (X-X) joining the center of the driving gear (209) and driven gear (208) in the cylinder block (204).
6. The internal combustion (IC) engine (101) as claimed in claim 1, wherein said chain tensioner (301) is disposed on the cylinder head (203) facing downwards.
7. The internal combustion (IC) engine (101) as claimed in claim 1, wherein said one or more openings (301e) shape is selected from a group consisting of circular shape, rectangular shape and polygonal shape.
8. The internal combustion (IC) engine (101) as claimed in claim 1, wherein said one or more openings (301e) varies from 1 to 15.
9. The internal combustion (IC) engine (101) as claimed in claim 1, wherein the pushrod (301a) of the chain tensioner (301) in contact with the tensioner blade (302b) is completely submerged in lubricating oil.
10. A two wheeled vehicle (100) including said internal combustion (IC) engine (101) as claimed in claims 1 to 9.