MECHANICAL CAM PHASER

FIELD OF THE INVENTION

[0001] The present subject matter relates generally to the field of internal combustion engines, and more particularly, to a mechanical cam phaser system for varying the phase relationship between a crankshaft and a camshaft in a reciprocating internal combustion engine.

BACKGROUND OF THE INVENTION

[0002] A conventional internal combustion engine in a saddle type vehicle converts chemical energy into mechanical energy by combustion of air-fuel . mixture within a combustion chamber of the engine. The said engine, among other components, has a cylinder comprising a cylinder head atop the cylinder and receiving a reciprocating piston from the bottom. On combustion of the air-fuel mixture, the piston transfers the energy generated during combustion to a crankshaft through a connecting rod thereby driving the crankshaft. In this way, the reciprocatory motion of the piston is converted to rotatory motion of the crankshaft. The crankshaft rotation then powers the vehicle. The crankshaft is connected to a camshaft (comprising of a plurality of cams) by a cam chain mounted on a cam sprocket at the camshaft end. The timing of camlobes of the respective cams determines the opening and closing of associated intake and exhaust valve(s).

[0003] Despite being far more powerful and fuel efficient than their predecessors, present generation engines have several inherent compromises in their operation. One of the biggest drawbacks with small engines is the fixed valve timing regardless of engine speed. Hence, though these engines have optimal valve timing for a specific engine speed, they run sub optimally at all -other speeds. Valve timing significantly affects volumetric efficiency which has great influence on engine torque, emissions and drivability. Therefore, the timing of the intake valve has to be continuously changed with engine speed for obtaining the highest volumetric efficiency. The intake valve should be closed late at high engine speeds to utilize the inertia of the incoming air effectively. However, late intake valve closing reduces the volumetric efficiency at low speeds. With variable valve timing, optimal intake valve opening and closing times can be achieved at every engine speed. Further, a proper choice of valve timings can provide considerable reduction in pumping losses and can also be used to trap exhaust residuals for lowering NOx emissions. Thus, variable valve timing offers the great opportunities for improving the performance of an engine.

[0004] To this end, cam phasing systems are generally used in the valve train to allow a predetermined change in phase difference of one cam lobe with other cam lobe while the engine is running consequently resulting in change in valve timing. The phasing of the cam lobes is important because when the intake and exhaust valves open and close has a serious effect on engine performance. The prior art suggests various types of cam phasers using electrical, electromechanical or hydraulic means to achieve this end. However, each of those have their own limitations. Hydraulic systems have difficulty operating at extremes of temperatures particularly during engine start up when the oil is cold due to temperature related viscosity changes of the oil, whereas electrical systems are complex and unreliable.

[0005] The present invention is directed to overcome one or more problems as set forth above. It is an therefore an object of the present invention to disclose a cam phaser system using mechanical means and coupled to a crankshaft of an internal combustion engine to vary the phase relationship between the crankshaft and camshaft of such an engine. It is another object of the present invention to disclose a mechanical cam phaser system which can automatically control the opening and closing timings of an intake valve based on engine speed without any external assistance thereby improving volumetric efficiency and enabling good engine performace over the entire engine operating speed range. Yet another object of the present invention is to provide a mechanical cam phaser system with a decompression system for optimum engine performace. It is a further object of the present invention to provide a simple, compact, easy to manufacture and cost

effective mechanical cam phaser system with minimum modification to the cylinder head and cylinder head cover.
SUMMARY OF THE INVENTION
[0006] The present subject matter discloses a mechanical cam phaser system to enable variable valve timing in a valve train of an internal combustion engine comprising: at least two rotatable flanges supported on a camshaft, an exhaust flange provided with at least one arc shaped aperture on each side of the exhaust flange horizontal plane, an intake flange provided with at least one vertical aperture and at least two guiding grooves on each side of the intake flange horizontal plane; a cam sprocket provided with at least one arc shaped aperture on each side of the cam sprocket horizontal plane, the cam sprocket locked and simultaneously rotatable with the exhaust flange; a portioned flyweight disposed between the intake flange and the exhaust flange, and including an upper flyweight member and a lower flyweight member configured to slide centrifugally along the guiding grooves of the intake flange and held by at least two return springs, the upper flyweight member and the lower flyweight member also having a pin hole each; wherein at least one drive pin is provided on each side of the cam phaser system horizontal plane, the drive pin parallel to the camshaft axis and traversing across the arc shaped aperture of the cam sprocket, the vertical aperture of the intake flange, the pin hole of the upper and the lower flyweight members, and the arc shaped aperture of the exhaust flange, and wherein further, on rotation of the camshaft, the drive pins slide along the length of the respective apertures due to sliding movement of the upper flyweight member and the lower flyweight member in radially opposite directions resulting in a phase difference between a lobe of an exhaust flange operated exhaust cam and a lobe of an intake flange operated intake cam of the valve train, the phase difference being dependent on the engine speed.
[0007] According to an aspect of the present subject matter, the intake flange is disposed between the cam sprocket and the portioned flyweight. The intake flange, the exhaust flange and the cam sprocket are coaxially supported on the

split camshaft and, the intake flange is in relative motion with an exhaust cam assembly. Additionally, the cam phaser system also comprises a centrifugal force operated decompression arm having a pivoted end and a movable end, the decompression arm supported on the exhaust flange by a preloaded spring, and operating a decompression cam accommodated around the base circle of the exhaust cam causing the exhaust valve to have a small additional lift during compression stroke at engine startup and no additional lift once the engine speed crosses a threshold speed.
[0008] The foregoing objectives and summary provide only a brief introduction to the present subject matter. To fully appreciate these and other objects of the present subject matter as well as the subject matter itself, all of which will become apparent to those skilled in the art, the ensuing detailed description of the subject matter and the claims should be read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The above and other features, aspects, and advantages of the present invention will be better understood with regard to the following description, appended claims and accompanying drawings where:
FIG. 1 shows an exploded view of assembly of a mechanical cam phaser system from one side of camshaft rotational axis.
FIG. 2 depicts an exploded view of assembly of the mechanical cam phaser system from other side of camshaft rotational axis.
FIG. 3 shows a perspective view of a cam sprocket according to the present invention.
FIGs. 4(a) and 4(b) respectively show a front perspective view and a rear view of an intake flange according to the present invention, when seen from the cam sprocket end.
FIG. 5 shows a perspective view of an exhaust flange according to the present invention.

FIG. 6 shows a sectional view of an exhaust cam assembly according to the
present invention.
FIGs. 7(a) and 7(b) respectively show a front and rear view of an upper flyweight
member according to the present invention.
FIGs. 8(a) and 8(b) respectively show the front and rear view of assembled intake
flange with the flyweight.
FIG. 9 shows a sectional view of the mechanical cam phaser system.
FIG. 10 shows a top view of the mechanical cam phaser system.
FIGs. 11(a) and 11(b) respectively show the working of the decompression system of the mechanical cam phaser system.
FIGs. 12(a) and 12(b) respectively show the modified cylinder head and cylinder head cover.
FIG. 13 shows the functioning of the mechanical cam phaser system under low and high engine speed conditions.
FIG. 14 shows the change in valve lift of an intake valve under low and high engine speed condition.
DETAILED DESCRIPTION OF THE INVENTION
[00010] The subject matter described herein relates to a mechanical cam phaser system for valve train of an internal combustion engine. Various other features of the present subject matter here will be discernible from the following further description thereof, set out hereunder. The detailed explanation of the constitution of parts other than the subject matter which constitutes an essential part has been omitted at suitable places. Furthermore, a 'horizontal plane' refers to the horizontal plane of the cam phaser system or, wherever specified, any of its constituents thereof, and a 'vertical plane' refers to the vertical plane of the cam phaser system or, wherever specified, any of its constituents thereof. The ensuing

description is exemplified for intake cam phasing, however exhaust cam phasing is possible in similar way if the disclosure is applied to the exhaust cam.
[00011] The mechanical cam phaser system is now explained with the help of FIGs. 1- 14. As shown in FIG. 1, the mechanical cam phaser system comprises of a cam sprocket 104, a portioned flyweight 105, and at least two co-axial and rotatable flanges, namely an exhaust flange 101 and an intake flange 102. The intake flange 102 is disposed between the cam sprocket 104 and the portioned flyweight 105; and the intake flange 102, the exhaust flange 101 and the cam sprocket 104 are coaxial and supported on a camshaft 150.
[00012] FIG. 3 shows a perspective view of the cam sprocket 104 according to the present invention. The cam sprocket 104, preferably circular in shape, is operatively engaged with a crankshaft (not shown) of the internal combustion engine. Specifically, the said engagement is through an endless chain or belt means (not shown) meshed with a plurality of spaced teeth 107 over the radial surface of the cam sprocket 104 and being driven by a crankshaft sprocket. The cam sprocket 104, which then drives the valve train, is provided with at least one arc shaped aperture Ilia on each side of the cam sprocket horizontal plane and with at least one through hole 162 on each side of the its vertical plane. The said aperture Ilia functions to develop cam phasing as described later whereas the through hole 162 enables connection of the cam sprocket 104 to the exhaust flange 101. In a preferred embodiment, the cam sprocket 104 includes at least two arc shaped apertures Ilia, 111b and at least two through holes referred by the numeral 162, all in the aforementioned arrangement. The two arc shaped apertures Ilia, 111b are fabricated so as to curve in opposite directions.
[00013] FIG. 5 shows a perspective view of the exhaust flange 101 according to the present invention. The exhaust flange 101, preferably circular in shape, is operatively engaged with an exhaust cam 201 and has an opening 119 at its centre through which it is operatively connected to the exhaust cam 201. The exhaust flange 101 includes at least one arc shaped aperture 112a on each side of the exhaust flange horizontal plane for enabling cam phasing. In a preferred

embodiment, the exhaust flange 101 includes at least two arc shaped apertures referred by the numeral 112a, 112b. The two arc shaped apertures 112a, 112b are fabricated so as to curve in opposite directions, the said directions being identical to that of the arc shaped apertures Ilia, 111b of the cam sprocket 104. The exhaust flange 101 further comprises at least one boss 120 on each side of its vertical plane to enable connection of the cam sprocket 104 with itself 101. The exhaust flange 101 in combination with the exhaust cam 201, with a first ball bearing 109 between them, forms an exhaust cam assembly 203. FIG. 6 shows a sectional view of the exhaust cam assembly 203 according to the present invention. The exhaust cam assembly 203 is supported on an exhaust shaft 153, which is the shaft of the exhaust cam 201 and being received by the opening 119 of the exhaust flange 101. The exhaust shaft 153 defines a common pass-through groove 154 which receives the camshaft 150. Thus, as a standalone subsystem, the exhaust cam assembly 203 is supported on the exhaust shaft 153. When assembled, the exhaust shaft 153 receives the camshaft 150 through the common pass-through groove 154. A needle cage bearing 155 is provided in the common pass-through groove 154 to reduce friction between the camshaft 150 and the exhaust shaft 153 during phasing event. Preferably, the exhaust flange 101 is press fitted to the exhaust shaft 153 during assembly.
[00014] FIG. 4(a) and 4(b) respectively show a front view and a rear view of the intake flange 102 according to the present invention. The intake flange 102, preferably approximately circular in shape, is provided with at least one vertical aperture 114a on each side of the intake flange horizontal plane, and defines at least two guiding grooves (both referred by numeral 103a) on each side of the intake flange horizontal plane. The two guiding grooves 103a, 103a are adjacent to the vertical aperture 114a and disposed one each on each side of the vertical aperture 114a. In a preferred embodiment, the intake flange 102 includes at least two vertical apertures referred by the numeral 114a, 114b and at least four guiding grooves referred by the numeral 103a, 103a, 103b, 103b. The intake flange 102 is operatively engaged with an intake cam 202 which is supported on the camshaft 150. The camshaft 150 also supports the exhaust cam assembly. The

intake flange 102 also comprises of at least one recessed portion 115 on each side of the intake flange vertical plane to aid in the connection of the cam sprocket 104 with the exhaust flange 101. Each guiding groove 103a, 103b is located between the rotational axis of the intake flange 102 and the recessed portion 115. In an embodiment, the recessed portion 115 can be an opening in the fully circular intake flange for receiving a spacer (described later). However, in a preferred embodiment, a portion of the circular intake flange is removed to carve the recessed portion 115. This also changes the outer circular profile of the intake flange.
[00015] Further, the portioned flyweight 105 is disposed between the intake flange 102 and the exhaust flange 101. The portioned flyweight 105 operatively comprises two members, an upper flyweight member 105a above its horizontal plane and a lower flyweight member 105b below its horizontal plane. FIG. 7(a) and 7(b) show a front and rear view of the upper flyweight member 105a according to the present invention. The lower flyweight member 105b has a similar construction. The upper flyweight member 105a and the lower flyweight member 105b are slidably held together by at least two return springs 117a, 117b, each resting in at least one spring hole 127 provided therein, and are configured to slide centrifugally along the guiding grooves 103a, 103b of the intake flange in radially opposite directions by means of at least one sliding pin 126a for each guiding groove 103a. Furthermore, the upper flyweight member 105a and the lower flyweight member 105b have a pin hole 125 each to receive at least one drive pin 113a in each pin hole 125. The pin hole 125 lies at the centre of the respective flyweight member, 105a or 105b. The exhaust flange 101 comprises a small vertical groove 121 on each side of its vertical plane for providing relief to the return springs so that they do not interfere with the flange. Similar vertical groove may also be provided in the intake flange.
[00016] According to an aspect of the present invention, the intake flange 102 is operatively supported on the camshaft 150 by means of an integral hub 156. The integral hub 156 has a circular flyweight facing projection 157 supporting the

upper flyweight member 105a on its outer radial surface and a circular sprocket projection 158 supporting the cam sprocket 104 on its outer radial surface. The flyweight facing projection 157 and the sprocket projection 158 are contoured to develop into the integral hub 156 with a continuous opening 159. The flyweight facing projection 157 has a bigger diameter than the sprocket projection 158 and a D-shaped aperture 160 at its inner radial surface which helps it to lock with the camshaft 150.
[00017] During assembly, the intake cam 202 which is supported on the camshaft 150 is secured to the intake flange 102. A one end 151 of the camshaft 150 after passing through the common pass-through groove 154 is received by the D-shaped aperture 160 of the flyweight facing projection 157 of the intake flange hub 156. The D-shaped aperture 160 restricts the rotation of the one end 151 of the camshaft 150 with respect to the intake flange 102. The one end 151 of the camshaft 150 is further secured against a threaded flange bolt 106 traversing through the cam sprocket centre opening 161 and the continuous opening 159 of the integral hub 156. Additionally, the flange bolt 106 also ensures axial locking of the intake flange and acts as a stopper to the cam sprocket 104 as well as the mechanical cam phaser system. It maintains a clearance with the cam sprocket centre opening 161 and is secured by means of a washer 110. However, axial movement of the intake flange 102 is possible because an axial gap 130 is left between the flyweight facing projection 157 of the intake flange hub 156 and the exhaust shaft 153. The axial gap 130 helps the intake flange 102 to adjust itself during phasing. In a separate embodiment, the intake flange can be connected to the camshaft 150 through spliried connection. In that scenario, the hub would be replaced by a splined hub.
[00018] The connection of the cam sprocket with the exhaust flange is now explained. The cam sprocket 104 is secured to the exhaust flange 101 through at least one fastener 108a passing through the at least one through hole 162 and secured to the at least one boss 120 of the exhaust flange 101 in such a way that the exhaust cam assembly 203 and cam sprocket 104 are locked to each other and

simultaneously rotatable when the drive is received. Each said fastener 108a, 108b is provided with a spacer 118a, 118b on its outer periphery. The spacer 118a is disposed between the cam sprocket 104 and boss 120 of the exhaust flange 101 in such a way that the recessed portion 115 of the intake flange 102 partially engulfs the spacer 118a while allowing the phasing of the intake flange 102 with respect to the exhaust cam assembly 203. However, on rotation of the camshaft 150 when the intake flange 102 adjusts with respect to the exhaust cam assembly 203, the recessed portion 115 might touch the spacer 118a or 118b. The recessed portion 115 thus provides clearance between the spacer 118a or 118b, and the intake flange body on both sides of the spacer. In a separate embodiment, the length of the boss 120 can be increased in order to overcome the need for a separate spacer. In such a case, the boss would also increase the securing area with the threaded fastener. In an embodiment,, the recessed portion 115 can be supplanted by an opening in the fully circular intake flange, the opening maintaining clearance to the spacer.
[00019] According to another aspect of the present invention, the intake flange 102 is in relative motion with the fastened combination of cam sprocket 104 and exhaust cam assembly 203 due to the splitting of the cam shaft. Generally intake cam and exhaust cam are supported on a single camshaft but in the present invention, the camshaft is split, wherein the intake cam 202 is supported on a main and longer camshaft (referred to only as camshaft 150, though it acts as intake camshaft), and the exhaust cam assembly 203 with the exhaust cam 201 supported on the separate exhaust shaft 153. The exhaust shaft 153 and the intake shaft 150 are in limited relative motion with each other. During rotation of the camshaft 150, the exhaust cam rotates with it (i.e. exhaust cam rotation is fixed) whereas intake cam is capable to rotate relatively with the cam sprocket and exhaust cam assembly. However, this relative rotation is limited.
[00020] The working of the cam phaser system is now explained. The cam sprocket is flexibly connected to the intake cam through the intake flange and rigidly connected (no relative motion) with the exhaust cam. According to an

aspect of the present invention, at least one drive pin 113a is provided on each side of the cam phaser system horizontal plane. In a preferred embodiment, the cam phaser system has two drive pins 113a, 113b. Each drive pin 113a runs parallel to the camshaft axis and traverses across the arc shaped aperture Ilia of the cam sprocket 104, the vertical aperture 114a of the intake flange 102, the pin hole 125 of the upper or the lower flyweight member (as the case may be), and the arc shaped aperture 112a of the exhaust flange 101. Moreover, the upper and the lower flyweight members are supported on the respective drive pin and not on the integral hub. When the drive is received from the crankshaft of the internal combustion engine, the cam chain or belt drives the cam sprocket. The drive pins 113a, 113b supported in the cam sprocket arc shaped apertures drive the intake flange which in turn drives the camshaft 150. On rotation of the camshaft 150, the drive pins 113a, 113b slide along the length of the respective apertures due to sliding movement of the upper flyweight member 105a and the lower flyweight member 105b in radially opposite directions. The position of the drive pins decide the phase difference between the lobe of the exhaust flange operated exhaust cam and the lobe of the intake flange operated intake cam. The position of the drive pins itself is dependent upon the radially inward and outward sliding movement of the upper and the lower flyweight member whose positions themselves are dependent upon the cam shaft speed. The drive pin may have DLC coating to reduce friction during sliding motion. There can be axial movement in the drive pins during the rotation of the camshaft. Therefore, to arrest it, the each drive pin may be provided with a circlip 116 which would arrest the axial movement of the drive pin and prevent it from popping out of the cam phaser system.
[00021] The exhaust cam assembly 203 and cam sprocket 104 are locked with flange bolts 108a, 108b and have a fixed timing with crankshaft. The exhaust cam assembly 203 with cam sprocket 104 floats on camshaft 150 due to the axial gap 130 left. Flyweight members 105a, 105b along with the drive pins 113a, 113b mounted on the intake flange 102 have freedom to slide radially out in vertical apertures 114a, 114b against the load of the return spring 117a. This intake flange 102 is locked to the camshaft 150, as already described above, and rotates along

with it. The drive pins hold the two flyweight members, connect the cam sprocket and exhaust cam assembly as well as drive the intake cam by driving the intake flange. Since the exhaust flange and cam sprocket have similar arc shaped apertures, when the engine speed is increased, the flyweight members move radially out due to centrifugal force. Hence, the lobe of the intake cam with vertical aperture(s) rotates relatively and aligns with the cam sprocket's arc shaped aperture(s). Since the forces of the return spring is more than the centrifugal force at low engine speeds, the intake cam lobe rests at its installed position i.e. intake valve closure timing is advanced. At high engine speeds, the centrifugal force developed by flyweight members is more than the return spring force and the intake cam stays at its final position i.e. delays the closure of the intake valve. This is graphically demonstrated in FIG. 13 and FIG. 14. In the cam phaser system, all constituent elements are connected by mechanical links, constituting a forced drive. Therefore, the cam phaser system does not require a return force normally accomplished by implementing a return extension spring. The return spring constant, the mass of the portioned flyweight and the apertures are all carefully designed.
[00022] The system improves engine torque by 5-10% along entire operating speed range. This is attributed to the fact that at lower speeds, the cam phaser system retains intake cam in an advanced condition. Hence, early intake valve closure minimizes the backflow during the compression stroke so it results in reducing pumping losses and improves low speed range volumetric efficiency and torque. Similarly, at higher speeds increase in torque observed due to the late closure of the intake valve. This improves volumetric efficiency by utilizing the momentum of the air in the intake manifold at high speeds (utilization of inertia of the incoming charge) till the closure of the intake valve. The aforementioned description is exemplified for intake cam phasing but it can also be used for exhaust cam phasing is possible in similar way if the disclosure is applied to the exhaust cam. Therefore, the use of the term "intake" is interchangeable with "exhaust" in the description. In such case, wherever intake cam/flange is mentioned, it would mean exhaust cam/flange for exhaust cam phasing.

[00023] A lubrication channel 135 is defined for lubrication of the cam phaser system. The oil hole(s) in the camshaft 150 helps in reducing friction during phasing of the intake flange with the exhaust cam assembly. Further, the cam phaser system can be incorporated in the existing vehicle with minimum modifications in the cylinder head 170 and cylinder head cover 171.
[00024] The said mechanical cam phaser system is also provided with a decompression system. The cam phaser system further comprises a centrifugal force operated decompression arm 165 having a pivoted end and a movable end, the decompression arm 165 supported on the exhaust flange 101 by a preloaded spring, and operating a decompression cam 166 accommodated around the base circle of the exhaust cam 201 causing the exhaust valve to have a small additional lift during compression stroke at engine startup and no additional lift once the engine speed crosses a threshold speed (or during high engine speeds). The system works due to the effect of the centrifugal force. However, the maximum opening of the decompression arm in the direction of the centrifugal force is arrested by an arrester pin 121. FIGs. 11(a) and 11(b) respectively show the working of the decompression system of the mechanical cam phaser system.
[00025] Preferably, the proposed cam phaser system is usable in valve trains having single overhead camshaft systems. Though the above disclosure describes the cam phasing for an intake cam, it can also be used for phasing an exhaust cam in similar way for changing the exhaust valve timing. Further, for use in dual overhead camshaft systems, individually intake cam and exhaust cam can be phased using the aforementioned disclosure but on separate camshafts.
[00026] From the foregoing description, it will be appreciated that the present invention offers many advantages including those described above. The present cam phaser system offers a fully mechanical, simple, compact and cost effective phasing system. The proposed system fits into the available cylinder head space and intake valve opening and closing timings can be controlled automatically by engine speed without any external assistance. Further, the system improves volumetric efficiency over a wide engine speed range, preferably for small engine

applications. It is easy to manufacture, has a highly flexible design and has a good cost to benefit ratio. It also offers phasing accuracy and repeatability in the entire engine speed range.

[00027] The present subject matter is thus described. The description is not intended to be exhaustive nor is it intended to limit the invention to the precise form disclosed. It will be apparent to those skilled in the art that the disclosed embodiments may be modified in light of the above description. The embodiments described are chosen to provide an illustration of principles of the invention and its practical application to enable thereby one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore the . forgoing description is to be considered exemplary, rather than limiting, and the • true scope of the invention is that described in the appended claims.

We claim:
1. A mechanical cam phaser system to enable variable valve timing in a valve train of an internal combustion engine comprising:

at least two rotatable flanges supported on a camshaft, an exhaust flange provided with at least one arc shaped aperture on each side of the exhaust flange horizontal plane, an intake flange provided with at least one vertical aperture on each side of the intake flange horizontal plane, the intake flange also
provided with at least two guiding grooves on each side of the intake flange horizontal plane;

a cam sprocket provided with at least one arc shaped aperture on each side of the cam sprocket horizontal plane; the cam sprocket locked and simultaneously rotatable with the exhaust flange;

a portioned flyweight disposed between the intake flange and the exhaust flange, and including an upper flyweight member and a lower flyweight member configured to slide centrifugally along the guiding grooves of the intake flange and held by at least two return springs, the upper flyweight member and the lower flyweight member also having a pin hole each;

wherein at least one drive pin is provided on each side of the cam phaser system horizontal plane, the drive pin parallel to the camshaft axis and traversing across the arc shaped aperture of the cam sprocket, the vertical aperture of the intake flange, the pin hole of the upper and the lower flyweight members, and the arc shaped aperture of the exhaust flange,

and wherein further, on rotation of the camshaft, the drive pins slide along the length of the respective apertures due to sliding movement of the upper flyweight member and the lower flyweight member in radially opposite directions resulting in a phase difference between a lobe of an exhaust flange operated exhaust cam and a lobe of an intake flange operated intake cam of the valve train, the phase difference dependent on the engine speed.

2. The mechanical cam phaser system as claimed in claim 1, wherein the cam sprocket further comprises at least one through hole on each side of the its vertical plane, the exhaust flange further comprises at least one boss on each side of its vertical plane, and wherein further the cam sprocket is secured to the. exhaust flange through at least one fastener passing the said through hole and secured to the boss in such a way that the exhaust flange and cam sprocket are locked and simultaneously rotatable during rotation of the camshaft.

3. The mechanical cam phaser system as claimed in claim 2, wherein each said fastener is provided with a spacer on its outer periphery, the spacer held between the cam sprocket and exhaust flange boss in such a way that a recessed portion of the intake flange partially engulfs the spacer while allowing the phasing of the intake flange with respect to the exhaust cam assembly.

4. The mechanical cam phaser system as claimed in claim 1, wherein the exhaust flange and the exhaust cam are mechanically connected to each other with a first ball bearing in between to form an exhaust cam assembly, the exhaust cam assembly supported on an exhaust shaft and including a common pass-through groove with needle cage bearing to receive one end of the camshaft.

5. The mechanical cam phaser system as claimed in claim 4, wherein the intake flange is in relative motion with the exhaust cam assembly.

6. The mechanical cam phaser system as claimed in claim 1, wherein each vertical aperture of the intake flange is flanked by at least two guiding grooves, one each on either side of the vertical aperture.

7. The mechanical cam phaser system as claimed in claim 1, wherein the intake flange has an integral hub, the hub having a flyweight facing projection supporting the portioned flyweight on its outer radial surface and a sprocket projection supporting the cam sprocket on its outer radial surface, the flyweight facing projection and the sprocket projection developing into the integral hub with a continuous opening, the flyweight facing projection having a bigger radius than the sprocket projection and a D-shaped aperture at its inner radial surface.

8. The mechanical cam phaser system as claimed in claim 4 or claim 7 , wherein the one end of the camshaft after passing through the common pass-through groove is rotatably restricted in the D-shaped aperture of the flyweight facing projection of the intake flange integral hub.

9. The mechanical cam phaser system as claimed in claim 8, wherein one end of the camshaft is secured against a flange bolt, the flange bolt traversing through the cam sprocket centre opening and continuous opening of the intake flange integral hub.

10. The mechanical cam phaser system as claimed in claim 1, wherein the intake flange is disposed between the cam sprocket and the portioned flyweight and wherein further the intake flange, the exhaust flange and the cam sprocket are coaxially supported on the camshaft.

11. The mechanical cam phaser system as claimed in claim 1, wherein the cam phaser system further comprises a centrifugal force operated decompression arm having a pivoted end and a movable end, the decompression arm supported on the exhaust flange by a preloaded spring, and operating a decompression cam accommodated around the base circle of the exhaust cam causing the exhaust valve to have a small additional lift during engine compression stroke at engine startup and no additional lift once the engine speed crosses a threshold speed.