TECHNICAL FIELD

[0001] The present subject matter, in general, relates to a power unit, and, in particular relates to variable valve timing system for the power unit.

BACKGROUND

[0002] Generally, a power unit like an internal combustion (IC) engine converts chemical energy into mechanical energy by combustion of air-fuel mixture within a combustion chamber of the IC engine. The IC engine, among other components, has a cylinder head assembly atop a cylinder block. The cylinder block defines the combustion chamber that accommodates a reciprocating piston. Combustion of air- fuel mixture causes the piston to undergo reciprocating motion transferring the energy generated during combustion to a crankshaft through a connecting rod thereby driving the crankshaft in a rotational manner.

[0003] Combustion of air-fuel mixture generates exhaust gases that needs to be scavenged from the combustion chamber through an exhaust system. Thus, the cylinder head assembly disposed atop the cylinder block is provided with plurality of valves that open and close at intervals for intake of air-fuel mixture into a combustion chamber and for scavenging of exhaust gases from the combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The detailed description is described with reference to the accompanying figures. In the figures, similar numbers are used throughout to reference like features and components.

[0005] Fig. 1 illustrates a front perspective view of a power unit, in accordance with an embodiment of the present subject matter.

[0006] Fig. 2 depicts atop view of a cylinder head assembly, in accordance with an embodiment of the present subject matter.

[0007] Fig. 3 depicts a side view of a cylinder head assembly, in accordance with an embodiment of the present subject matter.

[0008] Fig. 4 depicts a detailed schematic view of a variable valve timing system, in accordance with an embodiment of the present subject matter. [0009] Fig. 5 depicts an exploded view of selected components of a variable valve timing system, in accordance with an embodiment of the present subject matter. [00010] Fig. 6 depicts a schematic detailed view of a portion of variable valve timing system, in accordance with an embodiment of the present subject matter. [00011] Fig. 7 depicts a schematic view of a variable valve timing system, in accordance with an embodiment of the present subject matter.

[00012] Fig. 8 depicts a sectional view of a portion of a variable valve timing system taken along axis X-X’ as shown in Fig. 6, in accordance with an embodiment of the present subject matter.

[00013] Fig. 9 (a) depicts an exemplary graph of the working of a decompression system, in accordance with an embodiment of the present subject matter.

[00014] Fig. 9 (b) depicts an exemplary graph of Valve lift in an engages and a disengaged condition, in accordance with an embodiment of the present subject matter.

[00015] Fig. 10 depicts a schematic view of a variable valve timing system in an actuated condition, in accordance with an embodiment of the present subject matter.

DETAILED DESCRIPTION

[00016] Generally, the power unit is provided with plurality of valves. The plurality of valves corresponds to an intake and an exhaust of the power unit. For example, a basic configuration of valves may have a single intake valve and a single exhaust valve for intake of air-fuel mixture and for scavenging of exhaust gases, respectively. Depending upon the intake and the exhaust requirements, typically more than one valve is provided for intake and more than one valve is provided for exhaust. Generally, the plurality of valves is in normally closed condition and a valve timing mechanism is used to open each valve for a defined time interval. The valve timing mechanism comprises a camshaft for opening and closing the valves, wherein the camshaft can be driven by a chain driven.

[00017] Generally, a fixed valve timing is provided in smaller commuter vehicles like two-wheeler, three-wheelers or in small cars that incorporate power unit with lOOOcc or less. The term ‘valve timing’, typically refers to an opening time and a closing time of a valve. The fixed valve timing has fixed opening time and closing time of the valves. Since, the valve timing is dependent on the rotation of the crankshaft and the camshaft, a duration for which a valve is kept open keeps on reducing with increasing engine speed (due to increase in rotation per minute). The fixed valve timing of intake and exhaust system, significantly affects volumetric efficiency of the power unit. For example, during higher speeds of IC engine operation, breathability of the power unit may be affected due to a valve timing that is tuned for lower engine speeds. Similar problems may arise due to inadequate combustion time or inadequate scavenging time. As a consequence, the conventional IC engines perform efficiently only in a certain range of engine speed and the performance gets affected either at low speed or high speed.

[00018] Various attempts have been made in the past to address the above-stated problems relating to the fixed valve timing of the power unit by providing variable valve timing systems. However, such variable timing system are implemented in racing applications, or it is offered as a premium feature in vehicles owing to its cost and complexity. One such attempt was to achieve variable valve timing through cam profile switching. Such known systems suffer from the drawback of making the overall engine assembly bulkier. Thus, there has been a challenge to design the variable valve timing in compact power units like IC engines with single cylinder as the available space in cylinder head assembly region of such engines is small. The cylinder head typically has an additional requirement of adequate working space around its vicinity to enable operating space to service the system as well as its peripheral parts. Additionally, there is often need to install powertrain peripheral systems around the vicinity of cylinder head e.g. cooling system, sensors etc. which make the design more complex.

[00019] Further, the hydraulic based/ pressure-based systems that require provision of axial path inside the parts, makes the parts bulkier and the system requires maintenance at regular intervals due to use of hydraulics or pressure systems. Other setups such as a mechanical engaging unit between shafts for cam switching are also known in the art. However, the mechanical engaging units have higher weight to inertia thereby have the limitation of operating at higher engine speeds. Such engines with higher inertia require larger actuation system, which is a challenge in order to accommodate the same in compact power units. Further, such known systems are bulkier as camshaft is either axially extended to incorporate the switching system(s) or is radially extended to incorporate the hydraulic or mechanical actuation system making the camshaft and the valve timing assembly bulkier.

[00020] Thus, there is a need for providing a compact power unit with variable valve timing system comprising a compact camshaft and a compact valve timing assembly in order to be packaged even in a small and compact power unit layout with an otherwise crowded cylinder head assembly portion. Further, components used for variable valve timing system should have lower inertia in order to enable use of smaller actuation system that can be accommodated in even small sized engine and the lower inertia enables easy & reliable operation at higher speeds. [00021] The present subject matter provides a power unit with a cylinder head assembly. The cylinder head assembly involves variable valve timing system with a camshaft comprising a first cam and a secondary cam for actuating one or more first valves. A first rocker arm corresponding to the first cam actuates the one or more first valves. A secondary rocker arm corresponding to the secondary cam enables actuation of the one or more first valve selectively. During a first predetermined range of speed of the power unit, the one or more first valves are actuated by the first rocker arm and during a second pre-determined range of speed of the power unit the one or more first valves are actuated by the secondary rocker arm. Thus, the term ‘selectively’ defines that the secondary rocker arm and the corresponding secondary camshaft are operational at pre-determined range of speed of the power unit. The first rocker arm and the secondary rocker arm are capable of oscillating a first axis due to rotation of a camshaft. An engaging unit is also disposed about the first axis and is configured to selectively (‘selectively’ according to one embodiment refers to a pre-determined range of speed of the power unit) engage the secondary rocker arm with the first rocker arm.

[00022] It is an aspect that the engaging unit is configured to be in proximity to the first axis, which is an axis of oscillation of the first rocker arm and the secondary rocker arm. In one embodiment, the engaging unit operates about an operational axis and the operational axis overlaps with the first axis. Thus, the inertia of the system is lower, especially the inertia of the rocker arms is lower as the weight is concentrated about primarily the first axis. The effort or force required for actuation of the rocker arms is lower. Further, the rocker arms can be operated with ease even at higher speeds of engine operation.

[00023] It is a feature that the engaging unit capable of selectively engaging the first rocker arm to the secondary rocker arm & is disposed therebetween and the first rocker arm, the secondary rocker arm, and the engaging unit are comprising a common axis viz. the first axis. The engaging unit has one axial side engaged with one arm of the first rocker arm and the secondary rocker arm, and other axial side of the engaging unit is selectively engaged with other arm of the first rocker arm and the secondary rocker arm, wherein the engaging unit is configured to move about said first axis. Thus, the engaging unit is compactly accommodated between the first rocker arm and the secondary rocker arm thereby offering improved weight distribution. If the design is such that the engaging unit is disposed towards an axial end on the first rocker shaft, it will exert undesirable stress on rocker shaft, which would result in early failure of the shaft due to stress concentration at a single region or may cause bending of the rocker shaft that would affect clearance between the rocker arms and the camshaft.

[00024] The first rocker arm and the secondary rocker arm are selectively engaged when the engaging unit is in an actuated condition whereby the engaging unit couples the first rocker arm and the secondary rocker arm to oscillate both at the same time about the first axis. The coupling/ engagement is not restricted to mechanical means and includes other coupling means say magnetic coupling etc. Further, the rocker arm (say, secondary rocker arm) with a corresponding cam having a higher lift gets actuated first and due to the engagement or coupling between the rocker arms, other rocker arm (first rocker arm) is also actuated due to the engagement or coupling whereby a longer lift is achieved. The engagement is done for a pre-determined range of speed of the power unit/ engine speed. Thus, the rocker arms are selectively engaged.

[00025] The aforementioned one or more first valves are corresponding to at least one of intake or exhaust system. For example, the first valve can be an intake valve, which requires variable valve timing system according to engine operating conditions. The power unit comprises a third cam and a corresponding third rocker arm for driving at least one second valve, which can be an exhaust valve. In one embodiment, the third cam is disposed between the first cam and the secondary cam and correspondingly the third rocker arm disposed in opposite direction has at least a portion accommodated between the first rocker arm and the secondary rocker arm, wherein the secondary rocker arm is ancillary arm.

[00026] It is an aspect that third cam and third rocker arm (corresponding to the exhaust system) are disposed correspondingly between first cam and secondary cam, and first rocker arm and the secondary rocker arm respectively. This makes the assembly compact and enables it to be accommodated in the cylinder head assembly cylinder head assembly.

[00027] It is a feature that the engaging unit is substantially accommodated between the first rocker arm and the secondary rocker arm, and at least a portion of the engaging unit overlaps with at least a portion of a third cam of camshaft when viewed in radial direction. The current feature exemplifies the compact nature of the variable valve timing system as the space (considering radial direction) allocated for the third cam and the third rocker arm is also utilized for packaging the engaging unit thereby offering compact packaging.

[00028] It is a feature that the engaging unit is mounted to a first rocker shaft, which is rotatably supported on the cylinder head assembly through one or more rotation support members. In an embodiment, the rotation support member enables rotation of the first rocker shaft avoiding stress concentration on a certain portion of the first rocker shaft making it a durable & reliable construction.

[00029] Further, the first rocker shaft supports the first rocker arm and the secondary rocker arm enabling oscillation about the first axis. The present invention uses same axis i.e. the first axis for oscillation of rocker arms and operation of the engaging unit thereby maintaining weight concentration in proximity to the first axis. Thus, the amount of mass that is to be moved is lesser whereby the inertia is kept lower.

[00030] It is a feature that the engaging unit, in accordance with one embodiment, comprises a spline shaft, an interlocking member, and a locking portion. The first rocker shaft comprises at least a length comprising a smaller diameter to accommodate the spline shaft and the secondary rocker arm is mounted to spline shaft. Whereas, the first rocker arm is directly mounted to the first rocker shaft at a portion comprising larger width. It is an aspect of the present subject matter that the spline shaft, which is part of the engaging unit is compactly accommodated on the first rocker shaft without affecting the radial thickness of the first rocker shaft and the rocker arms. In one embodiment, the spline shaft is independently rotatable about the first rocker shaft.

[00031] It is a feature that the interlocking member is engaged with the spline shaft and the interlocking member is slidable in axial direction about the spline shaft (along the first axis). The interlocking member is capable of selectively engaging with the first rocker arm upon actuation. Till actuation, the secondary rocker arm and the first rocker arm are oscillating independent of each other.

[00032] It is a feature that the first rocker arm comprises a locking portion configured to engage with the interlocking member; Preferably, the locking portion is integrally formed with the first rocker arm for rigidity and to reduce number of parts for assembly. It is an aspect that the locking portion being part of the engaging unit is integrated with the first rocker arm to reduce the number of parts. It is an aspect that the engagement is done in base circle condition. The base circle is smallest circle drawn about an outer periphery of the cam and the radius of circle is the smallest, which is taken from center of the cam. Thus, the aforementioned engaging unit is operational during a condition when the rocker arms are in contact with the corresponding cam about the base circle portion of the cams, thence the engagement is done in the base circle condition of the cam.

[00033] It is a feature that the first cam provides a first lift with a first timing for actuating the one or more first valves and the secondary cam provides a second lift with a second timing for actuating the one or more first valves. The first timing is for shorter duration than the second timing. The first cam is active during lower engine speeds and the secondary cam is operational during higher speeds where breathing of the power unit is critical for delivering requisite power/ torque. [00034] It is a feature that the present invention with first cam comprising smaller lift enables provision of a decompression system coupled with the first cam and it is disposed adjacent to the first cam. Thus, the decompression system is not sandwiched between the cams, which would affect the compact layout of the rocker arms as additional space between the cams is to be provided for packaging the decompression system. Packaging the decompression system between the cams will also result in rocker arms being moved away from each other (as rocker arms work in conjunction with cams) thereby requiring a longer rocker shaft, which would add weight, occupies more space and adding to the cost.

[00035] It is a feature that the power unit comprises an actuator and the actuator is functionally connected to the interlocking member through a pivot arm. The pivot arm is pivoted about a pivot point and the actuator is configured to actuate the pivot arm, which cause the pivot arm to pivot thereby causing movement of the engaging unit, especially the interlocking member.

[00036] It is a feature that the actuator includes a solenoid or an electric motor or the like. It is an aspect that a solenoid that requires power only to change condition from actuated to -non-actuated condition & vice-versa may be used, which consumes less power. As the effort required is less due to lower inertia, a small size actuator can be used. Further, the actuator can be mounted on the cylinder head assembly or cylinder head-cover.

[00037] According to another aspect, the present subject matter is capable of being incorporated in a three valve or a four-valve engine. Power unit with variable valve timing system as explained above can be installed on the intake side and the exhaust side, both in single overhead cam systems (SOHC) and double overhead cam systems (DOHC).

[00038] Thus, the present invention offers improved drive characteristics as the power unit is capable of being implemented on a two-wheeler, a three-wheeler or a multi-wheeler, which is aimed at providing best low speed drivability with one low speed cam - for city condition; best high speed drivability with high speed cam lobe - for racing condition; and best low speed & high speed condition - combination of city & highway conditions.

[00039] It is a feature that the system is operated or the switching is performed at higher gears, where the power unit is operated at higher engine speed thereby providing improved acceleration.

[00040] In an embodiment, as the variable valve timing system is actuated using an actuator, the cutoff speed can be varied for different applications, and the actuation can be done using controller/ ECU or a manually operated switch. [00041] In one embodiment, when using an electronic actuator for the variable valve timing system, an electronic throttle can be synchronized to close in order to reduce speed for synchronization and engagement between cams.

[00042] These and other advantages of the present subject matter would be described in greater detail in conjunction with an embodiment of a power unit with the figures in the following description. Various other features and embodiments of the present subject matter here will be discernible from the following further description thereof, set out hereunder.

[00043] The detailed explanation of the present invention is explained with the help of a single cylinder type IC engine but the concepts introduced herein are applicable to multi-cylinder engines with single overhead cam (SOHC) or dual overhead cams (DOHC).

[00044] Fig. 1 illustrates a front perspective view of a power unit, in accordance with an embodiment of the present subject matter. The power unit 100 is an internal combustion engine with or without an electrically assisting motor. Hereinafter, the terms ‘power unit’ and ‘internal combustion (IC) engine’ are interchangeably used. The power unit 100 includes a crankcase assembly 104, 105, a cylinder block 103 coupled to the crankcase assembly 104, 105 and a cylinder head assembly 102 coupled to the upper part of the cylinder block 103, as per the depicted embodiment. A cylinder head-cover 101 is mounted to the cylinder head assembly 102 for covering a valve train system and other components mounted thereon. In the present embodiment, the cylinder block 103 defines a cylinder portion (not shown), which is a forwardly inclined type cylinder axis to enable minimizing the overall size of the power unit. A reciprocating piston (not shown) slidably fitted in the cylinder block 103 and the reciprocating piston is connected via a connecting rod (not shown) to a crankshaft (not shown). The crankshaft (not shown) is rotatably supported by the crankcase assembly 104, 105. The crankcase assembly 104, 105 is mounted with one or more covers 125 for covering components supported by the crankcase assembly 104, 105 from lateral direction(s) RH-LH.

[00045] The cylinder head assembly 102 comprises an intake port 114 (shown in Fig. 3) and an exhaust port (not shown) formed thereon. The intake port 114 allows air-fuel mixture to enter the combustion chamber, whereas after combustion of the air-fuel mixture, the exhaust gases are scavenged out of the combustion chamber through the exhaust port 115. A plurality of valves is provided in the cylinder head assembly 102, and the plurality of valves are closed and opened at per-determined timing to facilitate intake and exhaust process. In the depicted embodiment, the power unit 100 comprises two valves viz. a first valve 132 and a second valve 134 (shown in Fig. 2).

[00046] Fig. 2 depicts a top view of a cylinder head assembly in accordance with an embodiment of the present subject matter. Fig. 3 depicts a side view of a cylinder head assembly, in accordance with an embodiment of the present subject matter. The first valve 132 is supported in the intake side whereas the second valve 134 is supported in the exhaust side. The valves 132, 134 are driven by a camshaft 120 rotatably supported in the cylinder head assembly 102 so as to open and close them . Rotational power is transmitted from the crankshaft (not shown) to the camshaft 120 by a timing transmission mechanism (not shown). In an embodiment, the timing transmission mechanism includes a drive sprocket supported on the crankshaft, a driven sprocket supported on the camshaft and an endless cam chain connecting the drive sprocket with the driven sprocket.

[00047] The cylinder head assembly 102 defines a peripheral wall portion 110 with plurality of fins 111 defined on the periphery. A camshaft 120 is rotatably supported on the cylinder head assembly 102. The camshaft 120 comprises one or more bearings 121A, 12 IB for rotatably supporting it on the cylinder head assembly 102. The camshaft 120 comprises a sprocket 123 for being driven by the crankshaft or any actuator. The camshaft 120 comprises plurality of cams with cam lobes to drive the first valve 132, and the second valve 134 through rocker arms. One or more spark plugs 135 are provided on cylinder head assembly to enable combustion of air-fuel mixture. Further, Fig. 3 depicts a mounting-schema 205 for supporting an actuator 112. The mounting-schema 205 is an elevated portion with a profile provided to complement the profile of the actuator 112.

[00048] In the present embodiment, the camshaft 120 comprises a third cam 124 with a third cam lobe for driving the second valve 134. The third cam 124 enables oscillation of a third rocker arm (not shown) thereby causing opening/ closing of the second valve 134. Further, the present invention provides a power unit with variable valve timing system for altering of opening/ closing time of the first valve 132. The variable valve timing system 200 (hereinafter ‘variable valve timing system’ is briefly referred to as ‘system’) comprises an actuator 112 for enabling change of valve timing.

[00049] Fig. 4 depicts a detailed schematic perspective view of a variable valve timing system, in accordance with an embodiment of the present subject matter. The camshaft 120 comprises a first cam 142 that drives the first valve 132 (also shown in Fig. 3) through a first rocker arm 144. The first rocker arm 144 oscillates about a first rocker shaft 146 placed substantially parallel to a cam axis C-C’ of the camshaft 120. The first rocker shaft 146 has a first axis S-S’. A valve end of the first rocker arm 144 is disposed to be in contact with the first valve 132. Thus, upon rotation of the first cam lobe of the first cam 142, a cam follower of the first rocker arm 144 is lifted thereby causing the first rocker arm 144 to pivot about the axis S- S’ of the shaft 146 causing the first valve 132 to move down thereby creating opening of the port.

[00050] The camshaft 120 includes a secondary cam 152 with a secondary cam lobe. The system 200 comprises a secondary rocker arm 154 corresponding to the secondary cam 152, wherein the secondary cam 152 is capable of driving/operating the secondary rocker arm 154. In one embodiment, the secondary rocker arm 154 is also supported about first rocker shaft 146 and it oscillates about the first axis S- S’. The secondary cam 152 is axially spaced from the first cam 142 on the camshaft 120 and the secondary cam 152 drives a cam follower, which is mounted to a cam end of the secondary rocker arm 154.

[00051] Further, an engaging unit is disposed between the first rocker arm 144 and the secondary rocker arm 154 and is preferably supported on the first rocker shaft 146. The engaging unit is disposed about the first axis S-S’ and is configured to selectively engage the secondary rocker arm 154 with the first rocker arm 144. The engaging unit is connected to a fork member 165 that is functionally connected to a pivot arm 172. The pivot arm 172 is pivoted about a pivot point 174 with one end of the pivot arm 172 connected to the fork member 165 and other end connected to an actuator 112. In one embodiment, the third cam 124, corresponding to the second valve 134 (say exhaust valve) is disposed between the first cam 142 and the secondary cam 152 whereby a compact packaging of the valve timing assembly is achieved. In one embodiment, the secondary rocker arm 154 is shorter in length when compared to the first rocker arm 144 thereby making the system 200 further compact.

[00052] The actuator 112 can be a small sized solenoid (actuator 112 in Fig. 4 is shown in enlarged view and is not to be considered in relative size terms with other components). The actuator 112 can be mounted and configured within the cylinder head assembly 102 or the cylinder head-cover. Further, the actuator can be a solenoid or the like that requires power only to change from one state/ condition to another state/ condition thereby requiring low power.

[00053] Fig. 5 depicts an exploded view of selected components of the system, in accordance with an embodiment of the present subject matter. The first rocker shaft 146 supports the first rocker arm 144 and the secondary rocker arm 154. The first rocker shaft 146, according to the present embodiment, is provided with at least a portion of its length configured with a first diameter D1 and a remaining length configured with a second diameter D2, wherein as per an embodiment??? the second diameter D2 is additionally smaller than the first diameter D 1.

[00054] The system 200 comprises of a spline shaft 162 mounted to the first rocker shaft 146 about a portion comprising the second diameter D2. In one embodiment, the spline shaft 162 is freely rotatable about the first rocker arm. The secondary rocker arm 154 is mounted to the spline shaft 162. The spline shaft 162 comprises plurality of splines 162S and an interlocking member 160 comprising plurality of grooves 160G & it is slidably mounted to the spline shaft 162. The interlocking member 160 is provided with one or more engaging pins 160P that extend in an axial direction along the axis of the rocker shaft. Further, the interlocking member 160 comprises one or more grooves 160S at least partially provided on annular periphery of the interlocking member 160. The first rocker arm 144 comprises a locking portion 145 provided on one axial side, wherein the locking portion 145 comprises one or more slots corresponding to the one or more engaging pins 160P of the interlocking member 160. Upon engagement, the spline shaft 162, the first rocker arm 144, and the secondary rocker arm 152 act as a single component and oscillate together. [00055] In one embodiment, the first rocker arm 144, which is longer and/ or larger than secondary rocker arm 154, is additionally provided with grooves 1441 on outer periphery (either sides) to reduce weight and to attain lower inertia. The grooves 1441 provided on at least one of the either sides would result in an I-shaped section of the rocker arm 144, which is light weight and yet has structural high rigidity. [00056] Fig. 6 depicts a schematic detailed view of a portion of system, in accordance with an embodiment of the present subject matter. In an assembled condition as shown in Fig. 6, the interlocking member 160 is connected to a fork member 165, as per the current embodiment. Further, the fork member 165 is connected to a connector shaft 176. The system 200 further comprises a pivot arm 172 pivoted about a pivot point 174. One end of the pivot arm 172 is connected to the connector shaft 176 and other end of the pivot arm 172 is connected to the actuator 112. The actuator 112 in the present embodiment is schematically illustrated with larger size. However, a compact actuator like a solenoid or a compact electric switch/ motor may be used, which would be compact to be mounted to the cylinder head assembly 102. Actuation of the actuator 112 causes the pivot arm 172 to pivot about the pivot point 174 (direction of movement depicted using arrows), thereby causing the connector shaft 176 to move. The connector shaft 176 causes the interlocking member 160 to move thereby causing the engaging unit to engage with the first rocker arm 144 thereby establishing a connection between the secondary rocker arm 154 and the first rocker arm 144. For example, the engaging pins 160P of the interlocking member 160 would engage with the locking portion 145 of the first rocker arm 144 establishing a connection. Fig. 6 shows the engaging unit being in a non-actuated condition whereby the first rocker arm 144 and the secondary rocker arm 154 oscillate independently.

[00057] Fig. 7 depicts a schematic view of a system, in accordance with an embodiment of the present subject matter. Fig. 8 depicts a sectional view of a portion of the system taken along axis X-X’ as shown in Fig. 6, in accordance with an embodiment of the present subject matter. Referring Figs. 7 & 8, during a non- actuated condition, the first rocker arm 144 is driven by the first cam 142, which is comprising a lower lift in accordance with an embodiment. The first valve 132 is opened/ closed based on the oscillations of the first rocker arm 142. When the power unit 100 is operating at lower engine speeds, the actuator 112 is in a non-actuated condition. As shown in Fig. 7, the engaging pins 160P of the interlocking member 160 are in a non-engaging condition with the locking portion 145 of the first rocker arm 144. Thus, the secondary rocker arm 154 oscillates about the first rocker arm 146 without effecting opening/ closing of the first valve 132. The secondary rocker arm 154, is driven by the secondary cam 152. The secondary cam 152 is configured to provide a higher lift when compared to lift offered by the first cam 142. Translating to valve open time, the secondary cam 152 offers a higher open time when compared to a valve open time offered by the first cam 142. However, in the non-actuated condition, the timing or lift offered by the secondary cam 152 is not translated to the first valve 132.

[00058] The first rocker shaft 146 as per another embodiment is additionally supported by one or more rotation support member(s) 147 that enable the first rocker shaft 146 to rotate about the first axis S-S’, whereby the entire circumference of the shaft receives the forces instead of a single point being subjected to forces, which would cause bending of first rocker shaft thereby affecting the lift/ timing. [00059] Further, as shown in Fig. 7, the present subject matter offers a compact layout of variable valve timing system 200. The third cam 124 corresponding to the second valve 134 is disposed between the first cam 142 and the secondary cam 152, and the third rocker arm (not shown) corresponding to the second valve 134 is compactly configured between the first rocker arm 144 and the secondary rocker arm 154. Further, the engaging unit, which is constituted by the spline shaft 162, the interlocking member 160, and the locking portion 145 is substantially accommodated between the first rocker arm 144 and the secondary rocker arm 154. Further, when viewed in radial direction of the first rocker shaft 146, the engaging unit 160 overlaps OL (shown in dotted line) with at least a portion of the third cam 124 corresponding to the at least one second valve 134 (when viewed in radial direction).

[00060] Further, the engaging unit is disposed between the first rocker arm 144 and the secondary rocker arm 154 and the engaging unit is operational about the first axis S-S’ by performing engaging operation by movement about the first axis S-S’ . The engaging unit is movable about the first axis for selectively engaging the secondary rocker arm 154 with the first rocker arm 144 for altering lift / timing. The engaging unit is constituted by at least one of the spline shaft 162, the interlocking member 160, and the locking portion 145 is disposed about the first axis S-S’ thereby reducing the inertia. As the weight is accumulated in proximity to the first axis S-S’, the effort or force required to perform the shifting is less. Moreover, the system 200 can be operated at higher engine speeds due to the lower inertia. [00061] Further, in one embodiment, a decompression system 180 is provided on the intake side i.e. adjacent to the first cam 142 with smaller lift. Fig. 9 (a) depicts an exemplary graph of the working of the decompression system, in accordance with an embodiment of the present subject matter. The decompression system 180 is provided to facilitate reversing, braking, starting, changing compression ratio, or other specific operations. Preferably during starting of the power unit, the decompression system 180 enables reduction of compression pressure thereby reducing the effort required to manually or electrically start the power unit 100. The decompression system 180 is provided adjacent to the first cam 142 and the decompression system 180 comprises of a pin that is comprising a flat side and a bump side acting on the first cam 142. Thus, during lower speeds, the bump side is engaging with the rocker arm to perform decompression and at higher speeds, at speeds greater than idling speed, the flat side is operational not causing any valve lift. Thus, as show in graph, the curve E represents a lift of exhaust valve/ second valve, and curve I represents the lift of the intake valve / first valve and the curve D represents the decompression occurring during intake due to the decompression system 180.

[00062] Fig. 9 (b) depicts an exemplary graph of Valve lift in an engages and a disengaged condition, in accordance with an embodiment of the present subject matter. In the present embodiment, the variable valve timing system is configured on the intake valves. The curve E represents a lift of exhaust valve/ second valve. The curves EC and DC represent a valve lift of the intake valve(s)/ first valve(s) according to an engaged or disengaged (actuated or unactuated) condition of the variable valve timing system 200. In an unactuated condition, the first valve has smaller valve lift, represented by curve DC, due to a first cam and corresponding first rocker arm actuate the valves. In an actuated condition, the first valves have a larger lift, represented by curve EC, due to a secondary cam and corresponding secondary rocker arm actuate the first valves. Further, in the actuated condition, the secondary rocker arm and the first rocker arm are in an engaged condition and the secondary cam drives the secondary rocker arm. The present invention enables variable valve timing even at higher speeds without settling to a fixed valve lift that would be between curve DC and curve EC.

[00063] Fig. 10 depicts a schematic view of the system in an actuated condition, in accordance with an embodiment of the present subject matter. The system may include an electronic control unit (ECU) (not shown) or an integrated controller or a manual switch to enable the actuation of the system 200. In an actuated condition of the actuator 112, the connector shaft 176 is pushed by the pivot arm 172. The fork member 165 connected to the connector shaft 176 is in turn pushed. The interlocking member 160 connected to the fork member 165 is slid about the spline shaft 160. The engaging pins 160P of the interlocking member 160 with the slots provided in the locking portion 145 of the first rocker arm 144 thereby forming a rigid connection therebetween.

[00064] Thus, when the system 200 is actuated, the actuator 112 causes the engaging unit to slide axially towards the locking portion 145 as the at least one engaging pin is received by the at least one slot of the locking portion 145 of the first rocker arm 144. The actuation is performed during a base circle condition. The secondary rocker arm 154 gets operatively engaged with the first rocker arm 144 through the engaging unit and no free rotation between the two shafts exists during the actuated condition. In the actuated condition, the secondary cam 152 drives the first valve 132 as the secondary cam 152 with cam lobe comprising larger lift gets engaged first and the lift is transferred to the first rocker arm 144 thereby causing lift of the first valve. Since the secondary cam 152 has already caused the lift, the effect of the first cam 142 on the first rocker arm 144 is null or negligible.

[00065] It is to be understood that the aspects of the embodiments are not necessarily limited to the features described herein. 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. List of reference signs:

100 power unit 146 first rocker shaft

101 cylinder head-cover 25 147 support member

102 cylinder head assembly 152 secondary cam 103 cylinder block 154 secondary rocker arm

104, 105 crankcase assembly 160 interlocking member

110 peripheral wall portion 160G grooves

111 fins 30 160P engaging pins

112 actuator 160S fork slots 114 intake port 162 spline shaft

115 exhaust port 162S splines

120 camshaft 165 fork member

121 A, 12 IB bearings 35 172 pivot arm

123 sprocket 174 pivot point 124 third cam 176 connector shaft

125 cover 180 decompression system

132 first valve 200 variable valve timing system

134 second valve 40 205 mounting-schema

135 spark plug C-C’ cam axis 142 first cam D1 first diameter

144 first rocker arm D2 second diameter

1441 grooves S-S’ first axis

145 locking portion 45