A ROCKER ARM ASSEMBLY FOR VARIABLE ROCKER RATIO

FIELD OF THE INVENTION

[0001] The present subject matter is related, in general to internal combustion engines, and in particular, but not exclusively to a rocker arm arrangement for providing variable valve lift by changing rocker ratio in a four stroke internal combustion engine.

BACKGROUND OF THE INVENTION

[0002] Usually, an internal combustion engine is provided with a cylinder head assembly containing valves to control the in and out movements of charge and exhaust gases in the cylinder relative to the piston. As shown in Figure 1, the engine body includes a crankcase 2, a cylinder block 1 coupled to the crankcase 2, a cylinder head 3 coupled to the upper part of the cylinder block 1 and a head cover 4 coupled to the upper part of the cylinder head 3. A piston slidably 5 fitted in the cylinder block 1 is connected via a connecting rod 6 with a crankshaft 7. The crankshaft 7 is rotatably supported by the crankcase 2. The crankcase 2 is constructed by connecting a left crankcase and a right crankcase together.

[0003] The induction and exhaust processes in such engine are controlled by a mechanical system known as a valve train responsible for operation of the valves. Typically such valve train has at least two valves, an intake valve 16 and an exhaust valve 17 generally inclined relative to the cylinder axis. An intake port 13 and an exhaust port 14 formed in the cylinder head 3 communicate with the combustion chamber 12 formed by being surrounded by the cylinder bore, the cylinder head 3 and the piston 5. The intake valve 16 is provided at the combustion chamber side opening of the intake port 13 and the exhaust valve 17 is provided at the combustion chamber side opening of the exhaust port 14. Rotational power is transmitted from the crankshaft 7 to the camshaft 22 by the valve train. The intake 16 and exhaust valves 17 are operated via rocker arms (25 & 26) connected to the camshaft 22 so as to open and close at required times. The camshaft 22 is rotatably supported by the cylinder head 3.

[0004] The performance and efficiency of such engine substantially depends on the amount of air that can be drawn by the intake valve into the combustion chamber as well as the amount of gas that can be evacuated out of the combustion chamber through the exhaust valve. The required air flow can be achieved in all operating conditions by varying the valve lift which in turn can be achieved by changing rocker ratio. A rocker arm multiplies the lift provided by a lobe of a camshaft. Change in ratio of the rocker arm changes the multiplication factor which in turn changes the valve lift. In the known disclosures, the variable valve lift has been obtained by using a valve train with more number of parts which has the effect of increasing the valve train mass considerably as well as increase in height of the cylinder head in some cases. Considerable increase in mass of the valve train also translates into limiting the speed of the engine for optimum performance.

[0005] The present invention is directed to overcoming all or any of the problems as set forth above and obviate the lacunae in the prior art. Hence it is an object of the present subject matter to improve the performance and efficiency of a motorcycle engine by employing a novel rocker arm assembly with variable rocker ratio.

[0006] It is another object of the present subject matter to disclose a rocker arm assembly that provides optimal air flow into and out of a combustion chamber.

[0007] It is yet another aspect of the present subject matter to provide a compact valve train including an additional rocker arm and a link arm with minimal increase in mass so that there is no significant impact on the speed of the engine.

SUMMARY OF THE INVENTION

[0008] To this end, the present subject matter discloses a variable ratio rocker arm assembly for increasing the amount of charge entering a combustion cylinder by changing the rocker ratio and thereby increasing the valve lift. The rocker arm assembly according to the present subject matter includes a first rocker arm pivotable about a first rocker shaft, a second rocker arm capable of rotation about a second rocker shaft, and a connecting means connecting the first rocker arm and the second rocker arm, wherein a shifting mechanism is provided for shifting the connecting means.

[0009] According to an aspect of the present subject matter, the first rocker arm includes a valve engagement arm and a link arm engagement arm on opposite ends, the second rocker arm includes a cam roller and a second rocker shaft on opposite ends, and a slot extending lengthwise in the second rocker arm, the connecting means is configured to slide in the slot of the second rocker arm between specified positions and the shifting mechanism shifts the said connecting means within the slot. The shifting mechanism further comprises of a motor actuating a shifting arm between specified positions which in turn shifts the connecting means within the slot. According to an embodiment, the connecting means is a link arm.

[00010] According to another aspect of the present subject matter, a change in the position of the link arm in the slot of the second rocker arm changes the effective rocker ratio.

[00011] According to another another aspect of the present subject matter, the rocker arm assembly also comprises a torsion spring at the first rocker shaft to . enable positive sliding of the link arm in the slot of the second rocker arm.

[00012] According to yet another aspect of the present subject matter, the rocker arm assembly is configured to facilitate the opening and closing of at least one valve based on the rotation of a cam. The cam operates the rocker arm assembly through a lobe. The valve can be an intake or an exhaust valve.

[00013] 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 invention and the claims should be read in conjunction with the accompanying drawings. The summary is not intended to identify essential features of the claimed subject matter nor is it intended for use in limiting the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[00014] The above and other features, aspects, and advantages of the present subject matter will be better understood with regard to the following description, appended claims and accompanying drawings where:

Figure 1 shows a sectional view of a typical four stroke internal combustion engine.

Figure 2 shows a perspective view of an embodiment of a rocker arm assembly according to the present subject matter when the cam is in base circle position and the valve is closed. Here the shift arm is in SI position and the link arm is in Rl position (SI, Rl described later).

Figure 3 shows a perspective view of the rocker arm assembly where the cam is in lift position, the shift arm is in SI position, the link arm is in Rl position and the valve is open.

Figure 4 shows a plan view of the rocker arm assembly and illustrate the forces operating on the link arm.

Figure 5 depicts a detailed view of a slot in the second rocker arm and illustrates the various positions of link arm with respect to the second rocker shaft.

Figure 6 shows a top view of the rocker arm assembly according to the present subject matter.

Figure 7 shows a perspective view of the rocker arm assembly where the cam is in base circle position, the shift arm is in S2 position, the link arm is in R2 position and the valve is closed (S2, R2 described later).

Figure 8 shows a perspective view of the present invention where the cam is in lift position, the shift arm is in S2 position, the link arm is in R2 position and the valve is open.

Figure 9 shows a valve lift graph illustrating valve lifts with different positions of the connecting means (link arm).

DETAILED DESCRIPTION OF THE INVENTION

[00015] The subject matter described herein relates to a variable ratio rocker arm assembly for a four stroke internal combustion engine wherein the position of connecting means between two rocker arms provided therein, decides the effective rocker ratio and valve lift. It is to be noted that generally either of the two types of valve actuating mechanisms, side valve mechanism and overhead valve mechanism, are used in internal combustion engines but the scope of the present subject matter is limited to an overhead valve mechanism where the intake and exhaust valves are mounted at overhead of the engine.

[00016] In the present subject matter, valve lift is enhanced by employing a higher ratio rocker arm assembly. Simply put, valve lift is the distance a valve moves from a completely closed to a completely open position and the rocker ratio is defined as the ratio between the valve lift and the cam lift. Various other features and embodiments of the rocker arm assembly according to the present subject matter will be discernible from the following further description thereof, set out hereunder. It is to be noted that the detailed explanation of the constitution of parts other than the subject matter which constitutes an essential part has been omitted at suitable places.

[00017] The rocker arm assembly according to the present subject matter includes at least two rocker arms, a first rocker arm and a second rocker arm and at least one connecting means connecting the first rocker arm and the second rocker arm. The rocker arm assembly is held in the cylinder head of the engine. Figure 2 shows the rocker arm assembly disclosed herein in detail. The first rocker arm 100 is pivotally mounted upon a first rocker shaft 103 for oscillatory motion around the first rocker shaft 103 which is located centrally to the first rocker arm 100. The first rocker arm 100 is configured to assist the opening and closing of at least one valve. This valve may be an exhaust valve or an intake valve. The first rocker arm 100 has a valve engagement arm 102 consisting of a tappet screw adapted to make contact with a valve stem 501 and a link arm engagement arm 101 in contact with a connecting means. The first rocker shaft 103 acts as a fulcrum about which the link arm engagement arm 101 of the first rocker arm 100 is lifted so that the valve engagement arm 102 at its other end goes down correspondingly.

[00018] The second rocker arm 200 is supported on a second rocker shaft 202 present at one of its end. The other end of the second rocker arm 200 has a cam arm 201 with a cam roller 203 adjustably secured to the said cam arm 201.

According to an aspect, the second rocker arm 200 has a slot 205 at its raised portion extending lengthwise of the second rocker arm 200.

[00019] Further, the engine cylinder head has at least one camshaft 401 rotationally supported within the engine receiving the drive from the crankshaft (not shown) and rotating at generally half the speed of the crankshaft owing to the four stroke nature of the engine. Generally for a single cylinder engine, there are two cams integrated to the camshaft 401, one for each of intake and exhaust valves. In the present subject matter, at least one cam 400, the shape of which resembles that of a base circle with an eccentric lobe, is mounted on the camshaft 401. On rotation of the camshaft 401, the cam lobe 402 transfers the drive to the rocker arm assembly by engaging with the cam roller 203 secured to the cam arm 201 of the second rocker arm 200 which ultimately leads to opening and closing of the valve.

[00020] According to an aspect, the connecting means connects the first rocker arm 100 and the second rocker arm 200. In an embodiment of the present subject matter, the connecting means is a link arm 300. The link arm 300 is positioned between the first rocker arm 100 and the second rocker arm 200 in such a way that the long axis of the link arm 300 is approximately perpendicular to the long axis of the first rocker arm 100 and the second rocker arm 200. The link arm 300 has two ends, a top end 301 towards the first rocker arm 100 and a bottom end 302 towards the second rocker arm 200. The bottom end 302 of the link arm 300 is configured to slide in the slot 205 of the second rocker arm 200 between specified positions. The position of link arm 300 decides effective rocker ratio and the valve lift.

[00021] According to another aspect, the top end 301 of the link arm 300 is secured to the link arm engagement arm 101 of the first rocker arm 100 by means of a revolute joint through a first connector pin 104. The bottom end 302 of the link arm 300 is secured to the slot 205 in the second rocker arm 200 by means of a revolute joint through a second connector pin 204. The first connector pin 104 and the second connector pin 204 form a revolute joint at respective interfaces. The distance between the longitudinal axis of the first connector pin 104 and the longitudinal axis of the first rocker shaft 103 is represented by LI whereas the distance between the first rocker shaft 103 to the centre of the tappet screw is represented by L2.

[00022] Further, the slot 205 is adjustably adapted to hold the bottom end 302 of the link arm 300 through the second connector pin 204 wherein the lower extremity and upper extremity is concave shaped. As shown in Figure 4, the said slot 205 is configured like an arc falling on the circumference of a perceived circle 600, the centre of the said circle being the first connector pin 104. The radius of the said circle 600 matches the length of the link arm 300. In other words, the slot 205 provided in the second rocker arm 200 is positioned as an arc of the circle 600 and one end of the link arm 300 can slide in the said slot 205 in a circular arc fashion. The length of the said slot 205 falls on the circumference of the said circle 600.

[00023] Figure 5 depicts the various positions of the link arm with respect to the second rocker shaft. The bottom end 302 of the link arm 300 is configured to slide in the slot 205 of the second rocker arm 200 between specified positions to provide different rocker ratios. The specified positions include a R2 position in which the said link arm is nearest to the second rocker shaft, and a Rl position in which the said link arm is farthest from the second rocker shaft. Thus, the farthest and nearest position of the link arm 300 with respect to the second rocker shaft 202 is determined by the radius Rl and R2 respectively considering the second rocker shaft 202 as the centre. Thus, when the link arm 300 is in R2 position, it is at the minimum distance from the second rocker shaft 202 whereas when the link arm 300 is in Rl position, it is at the maximum distance from the second rocker shaft 202. The radius R represents the distance between the cam roller 203 of the second rocker arm 200 and the second rocker shaft 202. Hence the minimum rocker ratio in the rocker arm assembly herein is defined as (R2/R)*(L2/L1) and the maximum rocker ratio is defined as (R1/R)*(L2/L1).

[00024] Further, the link arm 300 does not move from Rl to R2 position or vice versa on its own. Hence a shifting mechanism 700 is provided as shown in Figure 3. The shifting mechanism includes a shifting arm 702 having a linear motion when actuated by a motor 701 based on pre-defined inputs. The shifting arm 702 has a groove 703 which engages the link arm 300 through a projected portion of the second connector pin 204. The groove 703 is adapted not to interfere with the movement of the second rocker arm 200 in any given position of the link arm 300. The shifting arm 702 linearly moves between a SI and a S2 position, as defined, when actuated. SI is the position when the shifting arm 702 is farthest from the second rocker shaft 202. S2 is the position when the shifting arm 702 is nearest to the second rocker shaft 202 as shown in Figure 7. The linear movement of the shifting arm 702 from SI to S2 changes the position of the bottom end 302 of the link arm 300 from Rl to R2. Consequently, the bottom end 302 of the link arm 300 traverses from Rl to R2 along the circumference of the circle 600 with the first connector pin 104 as the centre. The locus of the slot 205 in the second rocker arm 200 and the arc with first connector pin 104 as its centre matches only when the cam 400 is in base circle position (not in profile). Thus the shifting of link arm 300 happens only during the valve non-actuating time due to which the actuating load is lower.

[00025] In one aspect of the present invention, the input for activating the shifting arm 702 can be engine speed. For example at lower engine speeds (< 5000 r.p.m) when the shifting arm 702 is in S2 position as shown in Figure 7, the link arm 300 is in R2 position resulting in lower rocker ratio due to which a lower valvelift is achieved. Similarly at higher engine speeds (> 5000 r.p.m) shifting mechanism will be actuated by the motor 701 through linear movement of the shifting arm 702 from S2 to SI, hence link arm 300 changes position from R2 to Rl resulting in higher rocker ratio leading to a higher valvelift. Other inputs can also be used to activate the shifting mechanism including a throttle.

[00026] Thus, the change in position of the bottom end 302 of the link arm 300 between the two rocker arms decides the effective rocker ratio and hence the valvelift. The shifting of link arm 300 from Rl to R2 keeps the valve lash same as there is no angular displacement in the second rocker arm 200 due to shifting. Briefly, valve lash is the gap or clearance between the valve engagement arm 102 of the first rocker arm 100 and butt end 502 of the valve stem 501.

[00027] Furthermore, any accidental change in the position of link arm 300, when the rocker arm assembly is in operation, is undesirable. Hence a torsional spring 105 is provided at the first rocker shaft 103 which creates a torque on the first rocker arm 100. Figure 4 illustrates the forces operating on the link arm 300. The force acting on the link arm 300 in direction A i.e. along the long axis of the link arm 300 is higher than the direction B i.e. along the short axis of the link arm 300 meaning that a position change in the link arm 300 is avoided without an external force. Moreover, the force along the short axis of the link arm 300 (direction B) depends on the instantaneous pressure angle and friction between the second rocker arm 200 and the link arm 300. Hence a preload between the first rocker arm 100 and the link arm 300 by the torsional spring 105 enables positive shifting of the link arm 300 with lower operating forces. The torsional spring 105 keeps the link arm 300 in the immediate previous position and does not allow the link arm 300 to shift from Rl to R2 position accidentally. Besides that, the present valve train also includes valve spring, spring retainer, and keeper groove like other conventional valve train systems (not shown)

[00028] The functioning of the rocker assembly provided in the present invention in brief is described now. When the cam is in base circle position and the valve is closed, the shifting arm moves to S2 position by the motor 701, based on pre-defined inputs, as depicted in Figure 7. Consequently, the link arm is shifted by the shifting mechanism 700 to R2 position. Now when the drive is transferred from crankshaft to camshaft 401, the cam 400 operates the cam roller 203 of the second rocker arm 200 directly through the cam lobe 402. The second rocker arm 200 pivots about the second rocker shaft 202 and the driving force on the second rocker arm 200 is transferred to the bottom end 302 of the link arm 300 in R2 position. This driving force is then passed to the top end 301 of the said link arm 300 and further onto the link arm engagement arm 101 of the first rocker arm 100. The first rocker arm 100 further pivots about the first rocker shaft 103 so that its valve engagement arm 102 moves downwards thereby actuating the valve. The valve is now open as shown in Figure 8 and gas exchange process is initiated. A similar process is followed when the lift arm is in Rl position and shifting arm is in SI position. Figures 2 and 3 represent the position of the rocker arm assembly before and after the valve lift event.

[00029] Figure 9 shows a valve lift graph illustrating valve lifts with Rl and R2 position of the link arm. The rocker ratio changes according to the change in position of the link arm from Rl to R2 and hence the valve lift changes enabling better engine efficiency and performance.

[00030] The present subject matter and its equivalent thereof offer many advantages, including those which have been described henceforth. The present subject matter has the advantage of presenting a novel rocker arm assembly to achieve variable valve lift. It keeps the layout of the engine head compact with respect to conventional systems. The increased height of cylinder head required due to inclusion of a connecting means (link arm) is offset by positioning the second rocker shaft lower than the horizontal axis. Thus the overall height increase of the cylinder head is kept low. Since the use of additional masses in the present rocker assembly is lower, there is no significant limitation to the engine speed.

[00031] 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 rocker arm assembly for an internal combustion engine facilitating the opening and closing of at least one valve based on the rotation of a cam, the said cam operating the said rocker arm assembly through a lobe, the said rocker arm assembly comprising:

a first rocker arm pivotable about a first rocker shaft, the said first rocker arm including a valve engagement arm and a link arm engagement arm on opposite ends thereof;

a second rocker arm capable of rotation about a second rocker shaft, the said second rocker arm including a cam roller and a second rocker shaft on opposite ends, and a slot extending lengthwise in the second rocker arm;

a connecting means connecting the said first rocker arm and the said second rocker arm, the connecting means configured to slide in the said slot of the second rocker arm between a plurality of specified positions; and,

a shifting mechanism for shifting the said connecting means within the said slot,

wherein a change in the position of the said connecting means in the said slot decides the effective rocker ratio.

2. The rocker arm assembly as claimed in claim 1, wherein the said connecting means is secured at one end, to the said link arm engagement arm of the said first rocker arm through a first connector pin and, at the other end to the said second rocker arm through a second connector pin, the end of the said connecting means towards the second rocker arm being capable of sliding in the said slot of the second rocker arm.

3. The rocker arm assembly as claimed in claim 1 or claim 2, wherein the said connecting means is a link arm.

4. The rocker arm assembly as claimed in claim 1, wherein the specified positions include a R2 position in which the said connecting means is nearest to the second rocker shaft, and a Rl position in which the said connecting means is farthest from the second rocker shaft.

5. The rocker arm assembly as claimed in claim 1 or claim 2, wherein the said rocker arm assembly also comprising a torsion spring at the first rocker shaft to enable positive sliding of the connecting means in the said slot of the second rocker arm.

6. The rocker arm assembly as claimed in claim 1, wherein the said valve may be an intake valve or an exhaust valve.

7. The rocker arm assembly as claimed in claim 1, wherein the said shifting mechanism further comprises of a motor actuating a shifting arm based on specified inputs from SI to S2 position as defined.

8. The rocker arm assembly as claimed in claim 1, 2, 3 or 4, wherein the shifting arm engages with the connecting means through the second connector pin.

9. The rocker arm assembly, substantially as herein described and illustrated with reference to the accompanying drawings.