TECHNICAL FIELD

[0001] The present subject matter, in general, relates to a power unit, and, in particular relates to a reciprocating member like a piston for the power unit.

BACKGROUND [0002] Generally, a power unit like an internal combustion engine is provided with air-fuel mixture for combustion that creates thermal energy. The thermal energy generated from the combustion process is converted into mechanical energy, which can be used to do some kind of mechanical work. Generally, the combustion process causes a reciprocating member like a piston to undergo a reciprocating motion and the reciprocating motion of the reciprocating member is converted into a rotational motion that gets transferred to a translational member like a crankshaft. The aforementioned power unit is used in a wide range of applications including providing of motive force for movement of a motor vehicle like two-wheeler, three-wheeler, or multi-wheelers. Thus, the reciprocating member plays a critical role in the power unit as it causes compression of the air- fuel mixture and also causing scavenging of the exhaust gases from the cylinder portion of the power unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The detailed description is described with reference to the accompanying figures. In the figures, similar numbers are used throughout the drawings to reference like features and components.

[0004] Fig. 1 depicts a right-side view of an exemplary motor vehicle, in accordance with an embodiment of the present subject matter.

[0005] Fig. 2 illustrates a left side view of a power unit, in accordance with an embodiment of the present subject matter.

[0006] Fig. 3 illustrates a schematic bottom-view of the cylinder head, in accordance with an embodiment of the present subject matter.

[0007] Fig. 4 depicts a perspective view of a reciprocating member, in accordance with an embodiment of the present subject matter. [0008] Fig. 5 depicts a top view of a reciprocating member, in accordance with an embodiment of the present subject matter.

[0009] Fig. 6 depicts another top view of a reciprocating member, in accordance with an embodiment of the present subject matter.

[00010] Fig. 7 depicts a partial sectional view of a power unit taken along an axis parallel to a cylinder axis of the power unit, in accordance with an embodiment of the present subject matter.

DETAILED DESCRIPTION

[00011] Generally, in a power unit provided in some smaller vehicles like two- wheelers or three-wheelers, a power unit is provided either in a swingable or fixed manner. The reciprocating member is provided to slidably move within a combustion chamber of the power unit. Constant effort is being put to improve the combustion efficiency of existing power units. Generally, one way of achieving the combustion efficiency is by operating the power unit at higher compression ratio, as the same combustion temperature can be achieved with less fuel. However, when operating the power unit with higher compression ratio, there is a possibility that the power unit is subjected to knocking and combustion noise. The knocking may occur due to detonation of an air-fuel mixture at a pocket outside a flame propagation path, which is generated by a spark from the spark plug. Knocking may cause excessive pressure in the combustion chamber affecting the cylinder portion and also the reciprocating member leading to rapid movement causing scuffing. This would increase wear and tear thereby drastically reducing life of the reciprocating member & durability of the engine. Thus, the challenge is to improve the combustion efficiency without resorting to a high compression ratio, which may cause the aforementioned and other problems known in the art. The aforementioned and other problems in the known art are prominent in power units that have smaller combustion chambers and that are provided with valves in asymmetrical manner or in odd number.

[00012] Generally, the position of the spark also plays a vital role in timely propagation of the flame and, also, in proper combustion of air-fuel mixture. The combustion process directly creates an effect on power and torque of the power unit, which define the performance of the power unit. However, in case of power unit having more than two-valves, say a three-valve type power unit, the cylinder head is crowded with the multiple valves thereby making it difficult for accommodating the spark plug with better & efficient flame propagation. Moreover, with multiple valves, the complete combustion of the air fuel mixture is difficult, which may result in HC emissions. Conventionally, more than one spark plug may be used for propagation of the flame; Nevertheless, use of more than one spark plug further creates space constraint in the cylinder head affecting the size of the cylinder head. This would require increasing size of the cylinder head, which would affect the clearance between the parts in the vehicle & packaging cum layout challenges in achieving a compact design of the vehicle as a whole. For example, the power unit may be mounted in a forwardly inclined cylinder orientation and such a layout has very small clearance between a storage unit (like utility box). Any increase in size of the cylinder head affects the clearance and increases heat propagation. Similarly, the clearances may get affected in power unit with vertical cylinder orientation. Moreover, typically engine is preferred to be standardized across platform of products with minor calibration changes based on markets segments & customer requirements. Additionally, engine being critical system is typically designed ahead of vehicle aggregate system design & layout such that engine or powertrain needs to be at the center & the vehicle layout is designed around it. Therefore, redesign of cylinder head of the power unit and redesign of the vehicle layout to address any adverse effects like knocking etc. is an expensive process, which is undesirable. For example, either the size of the cylinder head is to be increased, which would result in modification of the design or the bore diameter is to be increased which would affect the entire power unit configuration.

[00013] Thus, there is a need for a power unit having a reciprocating member that is capable of addressing the aforementioned and other challenges in the art for providing desired performance. Hence, the present subject matter provides a power unit with multiple valve (more than two) and a reciprocating member thereof, which is capable of offering desired performance without affecting the size of the power unit.

[00014] The present subject matter provides a power unit for a motor vehicle. The power unit comprises a cylinder head supporting two or more valves. The multiple valves help in improved breathing of the engine. A spark plug is also supported on the cylinder head. A reciprocating member of the power unit is slidably disposed within the cylinder portion. The reciprocating member is provided with an asymmetric bowl portion to enable improved squish effect. Thus, the squish effect, which creates a sudden turbulence of the air-fuel mixture as the reciprocating member approaches top dead center causes ‘squishing’ of the air-fuel mixture helping in better & efficient flame propagation with use of single spark plug enabling effective combustion.

[00015] The reciprocating member comprises a first region and a second region, which is part of periphery of the bowl portion. The first region and the second region are disposed substantially opposite to each other and along a pin-bore axis of the reciprocating member. Thus, the first region and second region that are disposed along the pin-bore axis due to which no additional side thrust is exerted on the reciprocating member. The first region and second region are asymmetrically provided (with respect to a pin-bore axis) and a first region is configured to be smaller than the second region.

[00016] Thus, the first region and the second region that are forming lands with different areas help in propagation of air-fuel mixture or combusted gases in a desired direction in the combustion thereby enabling use of such reciprocating members in power units with asymmetrically disposed valves. The reciprocating member with the larger second regions creates a sudden turbulence of the air-fuel mixture causing ‘squishing’ of the air-fuel mixture helping in improved combustion.

[00017] The first region and second region are preferably having non-uniform area whereby with respect to the regions, the bowl portion is asymmetric. However, in another embodiment, at least of the first region and the second region can also be asymmetrical with respect to the pin-bore axis. [00018] In one embodiment, the spark plug is disposed farther from the second region when compared to the distance between the spark plug and the first region. Thus, the present subject matter improves flow of air-fuel mixture and improved turbulence for combustion and for flame propagation whereby the overall combustion process is improved & problems like knocking are addressed.

[00019] In one embodiment, the second region is disposed away from the spark plug and the second region is larger than the first region. In other words, the second region extends farther into the bowl portion when compared to the first region.

[00020] The first region and the second region are disposed substantially opposite to each other and along a pin-bore axis of the reciprocating member whereby the first region and the second region are provided about the pin-bore axis, the thereby not affecting tilting of the reciprocating member about the pin-bore axis. [00021] Further, in one embodiment, a second diameter of the bowl portion taken about axis orthogonal to pin-bore axis is about 1.1 to 1.4 times of a first diameter of the bowl portion taken about pin-bore axis. The first region and the second region that occupy larger area are accommodated along long axis/ pin-bore axis in case of a reciprocating member having an oval profile.

[00022] In one embodiment, the power unit comprises a first intake valve and a second valve disposed towards one side of an imaginary partition line passing parallel to the pin-bore axis. The first intake valve is disposed in proximity to the spark plug. The second intake valve is disposed away from the spark plug. The reciprocating member with the larger second region causes the air and fuel from the second intake valve to propagate faster towards the spark plug causing complete combustion within the same time.

[00023] In one embodiment, the first region and the second region are having a convex profile as viewed from the center of the bowl portion, wherein the first region has a first radius of curvature, which is smaller than a second radius of curvature of the second region. The convex profile of the first region and the second region provides a desired turbulent flow without major disturbance. [00024] In one embodiment, the bowl portion comprises a first diameter taken along the pin-bore axis, which is being substantially smaller than a second diameter of the bowl portion taken in a direction orthogonal to the pin-bore axis. Major bowl portion is disposed towards the spark plug whereby during compression stroke, the air and fuel is maintained in a region in proximity to the spark plug. Also, deposition of the fuel in the crevice volume (a small volume at the interface between the reciprocating member and the cylinder) is reduced thereby reducing hydrocarbon emissions due to better propagation of flame thereat. [00025] In one embodiment, the reciprocating member comprises a top land having a non-circular profile. The non-circular profile has a long axis, which is parallel to the pin-bore axis of the reciprocating member. Thus, the reciprocating member is provided with sufficient clearance on a side orthogonal to the pin-bore axis enabling slapping of the reciprocating member. [00026] In one embodiment, a top land of the reciprocating member comprises a tapered edge and a corresponding portion on the cylinder head is provided with another tapered edge, wherein the tapered edge of the reciprocating member conforms and complements the tapered edge of the cylinder head. The tapered edge provides the desired crevice volume on the top land thereby reducing fuel deposition thereat.

[00027] In one embodiment, the reciprocating member comprises an asymmetric skirt portion. For example, a first skirt area is provided on thrust side and a second skirt area is provided on anti -thrust side. The second skirt area has a smaller area compared to the first skirt area thereby reducing overall friction experienced by the reciprocating member. Thus, the performance of the reciprocating member is improved.

[00028] In one embodiment, the reciprocating member is provided with a polymer coating or a diamond like carbon (DLC) coating to act as a lubricant for reducing scuffing of the reciprocating member. [00029] In one embodiment, the reciprocating member comprises a pin support portion having at least one of a convex, concave, or flat profile whereby a length of the pin-support portion is reduced thereby reducing reciprocating mass thereof. [00030] The reciprocating member, which plays a vital role, offers timely propagation of the flame and enables proper & effective combustion of air-fuel mixture.

[00031] The reciprocating member of the present subject matter offers improved combustion process with reduced friction thereby creating improved effect on power and torque of the power unit.

[00032] The power unit comprises two or more intake valves, say two intake valves, thereby offering improved breathing and improved swirl and turbulence creation.

[00033] The present subject matter enables retention of the existing size of the cylinder head with improved performance. For example, a single spark plug is used to achieve the desired combustion.

[00034] These and other advantages of the present subject matter would be described in greater detail in conjunction with one or more embodiments with the corresponding figures in the following description. In the figures F represents forward direction, R represents rearward direction, RH represents right side and LH represents left side with respect to a motor vehicle as reference.

[00035] Fig. 1 depicts a right-side view of an exemplary motor vehicle 100, in accordance with an embodiment of the present subject matter. The motor vehicle 100 includes a frame assembly 105 supporting a front wheel 130 and a rear wheel 132. The front wheel 130 and the rear wheel 132 are rotatably supported by front suspension system and the rear suspension system (not shown), respectively. In one embodiment, the rear wheel 132 may be additionally supported by a swingarm (not shown). In the depicted embodiment, the frame assembly 105 includes a main tube 107 extending rearwardly downward from a head tube 106 and one or more rear frames 108 extending inclinedly rearward, from the main tube 107, towards a rear portion of the vehicle 100. The frame assembly 105 defines a step-through portion 140 thereof, which can be used for load carrying or for the user to rest feet.

[00036] In the present embodiment, a power unit 200 is swingably mounted to one of the main tube 107 or to the rear tubes 108 of the frame assembly 105 and is disposed substantially rearward of the step-through portion 140. A utility box is supported by the rear tubes 108 and is disposed above the power unit 200 and below a seat assembly 150. In another embodiment, the power unit may be fixedly mounted to the main tube 107 of the frame assembly 105. The power unit 200 is connected to a muffler 155, which is part of an exhaust system, and is capable of attenuating noise and treating harmful exhaust gases before emitting the exhaust gases to the atmosphere. The power unit 200 is coupled to a transmission system 240 (shown in Fig. 3) for transferring power to the rear wheel 132, in accordance with the present embodiment. Further, the front wheel 130 is steerable by a handle bar assembly 150, which is functionally connected to the front wheel 130 for maneuvering the vehicle 100. The handle bar assembly 150 supports an instrument cluster, vehicle controls including throttle, clutch, or electrical switches.

[00037] Further, the seat assembly 150 is mounted to the frame assembly 105 and is disposed rearward of the step-through portion 140. The rider can operate the vehicle 100 in a seated position on the seat assembly 155. Further, the vehicle 100 is provided with plurality of panels 170A, 170B mounted to the frame assembly 105 for covering the frame assembly 105 and/or parts of the vehicle 100. Also, the vehicle 100 may be provided with plurality of mechanical, electronic, and electromechanical system including an anti-lock braking system, a synchronous braking system, a vehicle safety system, or an electronic control system.

[00038] Fig. 2 illustrates a detailed left side view of a power unit, in accordance with an embodiment of the present subject matter. The power unit 200 comprises a cylinder head cover 201, a cylinder head 202, a cylinder block 203, and a crankcase 204. The power unit 200 includes an intake port 208 to which an air- control device 209 like a carburetor or a throttle body is connected for regulating air flow. In one embodiment, the crankcase 204 is made of a first side casing (not shown) and a second side casing 206, which is capable of rotatably supporting various parts of the power unit 200 including a rotational member 214 like a crankshaft. One or more covers cover various parts of the power unit 200. For example, a first cover 225 is disposed to cover a leftward outer lateral side of the crankcase 204. In the depicted embodiment, the second side casing 206 extends substantially rearward with respect to a first side casing in order to support a transmission system and a final drive system (not shown).

[00039] The cylinder block 203 is supported by the crankcase 204. The cylinder block 203 includes a cylinder portion 220 about which the reciprocating member 300 is movable/ slidable. The reciprocating member 300 is disposed to traverse along a cylinder axis C-C\ The cylinder head 202 is mounted to the cylinder block 203 and the cylinder head 202 is supporting a camshaft and plurality of valves 233 (shown in Fig. 3). The power unit 200, in accordance with an embodiment of the present subject matter, includes more than two valves. The crankcase 204 rotatably supports the rotational member 214. The rotational member 214 is functionally connected to the reciprocating member 300, wherein the reciprocating motion of the reciprocating member 300 is converted into a rotational motion of the rotational member 214.

[00040] Fig. 3 illustrates a schematic bottom view of a cylinder head, in accordance with an embodiment of the present subject matter. The cylinder head 202 includes a body portion 231 made of light weight metal or a metal alloy. The body portion 231 comprises a lower surface that is adapted to be held in a sealing relationship with the cylinder block 203 (shown in Fig. 2). The cylinder head 202 supports a camshaft (not shown) and valve operating mechanism (not shown) mounted on an upper portion thereof. The cylinder head 202 supports plurality of valves 233 that are in functional connection with the camshaft through the valve operating mechanism. The term ‘camshaft’ is not limiting to a single camshaft as the cylinder head can have a single overhead cam (SOHC) or may even include a double overhead cam (DOHC). Similarly, the invention is not restricted to a single cylinder engine. [00041] In the depicted embodiment, the cylinder head 202 support three valves, wherein the three valves are formed by a first intake valve 234, a second intake valve 235 and an exhaust valve 237, wherein each of the valve 234, 235, 236 is movable along a predefined axis thereof during actuation by the valve operating mechanism. The valves (collectively referred using the reference sign ‘233’) are timed to open and close depending primarily on engine cycle (e.g. Otto cycle) through a timing chain connecting the crankshaft assembly (rotational member) 214 to the camshaft. Further, the cylinder head 202 supports a spark plug 237 that is capable of generating a spark for causing combustion of the air-fuel mixture in the combustion chamber 220. Further, the cylinder head 202 includes plurality of fins 238 that extend outward from the body portion 231 and the fins 238 are either selectively disposed or continuously disposed annularly around the cylinder head 202

[00042] The cylinder head 202 includes a cylindrical ring region 232 that is aligning substantially with the cylinder/ outer periphery of the combustion chamber 220 of the cylinder block 203. The valves 233 and spark plug 237 are disposed within the cylinder ring region 232. Specifically, each of the valves 233 comprises of a head having substantially a disc shape (when viewed in its axial direction or length direction) and a stem connected to head. The valves 233 are configured to be seated in a seat portion of the cylinder head 202. Similarly, the spark plug 237 includes a gap (not shown) formed between electrodes thereof and the gap of the spark plug 237 is extending from the cylinder head 202 into the combustion chamber 220.

[00043] In the present embodiment, the spark plug 237 is disposed to be substantially in line with a center of at least one of an intake valve or an exhaust valve (considering the pin-bore axis direction or a line orthogonal to the pin-bore axis direction). For example, the spark plug 237 is in line with the center of the first intake valve 234. The first intake valve 234 and the spark plug 237 are accommodated in a first half of the ring region 232 and the second intake valve 235 and the exhaust valve 236 are accommodated substantially in a second half of the ring region. The first half and the second half are imaginary portions divided by an imaginary partition line PL. The exhaust valve 236 disposed closer to the second intake valve 235 creates a space between the first intake valve 234 and the exhaust valve 236, wherein the spark plug 237 is disposed thereat with the sparking tip being as close as possible to an imaginary line joining the centers of the intake valve 234 & exhaust valve 236. This brings the spark plug 237 closer to both the intake valves 234, 235. The two-intake valves 234, 235 enable entry of higher volume of air enabling breathing of the engine 200. Accordingly, the present subject matter provides a piston that enables improved flow and combustion of the air-fuel mixture and the valves 233 and the spark plug 237 are compactly accommodated in the cylinder head 202.

[00044] In the depicted embodiment, the cylinder head 202 includes an intake manifold connection on one side and an exhaust manifold connection, wherein the intake valves 234, 235 are disposed towards the intake manifold connection side and the exhaust valve 236 is disposed towards the exhaust manifold connection side. The imaginary partition line PL is drawn between the intake manifold connection and the exhaust manifold connection whereby the partition line PL partitions the ring region 232 into half. The power unit 200 is provided with more than two-valves, which is three-valves in the present embodiment, wherein the cylinder head 202 accommodates the spark plug 237 with better flame propagation towards it. As the reciprocating member 300 complements the structure of the cylinder head 202 thereby enabling flow of the air-fuel mixture towards the spark plug 237 and also enabling flame propagation.

[00045] Fig. 4 depicts a perspective view of a reciprocating member, in accordance with an embodiment of the present subject matter. The reciprocating member 300 includes a crown portion 305, which defines a bowl portion 310. The reciprocating member 300 comprises atop land 315 defining the bowl portion 310 and one or more ring lands 320. Plurality of rings 325 are disposed between the ring lands 320 that offer minimal contact of the reciprocating member 300 with the combustion chamber 220 (as shown in Fig. 2). The plurality of rings 325 includes one or more compression rings and one or more oil control rings. Further, a skirt portion 330 is provided about an annular periphery thereof and the skirt portion 330 rubs against a cylinder liner of the cylinder block 203. A pin- bore portion 333 is provided to enable connection of the reciprocating member 300 to a connecting rod using a pin (not shown). The pin-bore portion 333 extends along a pin-bore axis PB-PB’. The pin-bore portion is also referred to as pin- support portion, hereinafter. The reciprocating member 300 acts as a bottom portion of the combustion chamber 220 and the reciprocating member 300 causes compression of air-fuel mixture, plays vital role in flame propagation and in flow of air-fuel mixture during combustion. The reciprocating member 300 is provided with a region forming a periphery of the bowl portion 310 that provides improved combustion and reduced hydrocarbon emissions.

[00046] Fig. 5 depicts a top view of the reciprocating member, in accordance with an embodiment of the present subject matter. The bowl portion 310 forms a bottom portion for the combustion chamber 220. The reciprocating member 300 moves towards top dead center (TDC) during compression stroke. The reciprocating member 300 is provided with the bowl portion 310 that provides improved control for movement of air and fuel mixture as the reciprocating member moves up during compression stroke. The reciprocating member 300 is provided with the improved region that forms the periphery of the bowl portion 310 whereby the asymmetric portion of the periphery of the bowl causes improved flow movement of the air-fuel mixture (say, a turbulent flow of air-fuel mixture by squish effect) towards the flame propagation resulting in effective combustion.

[00047] In one embodiment, the reciprocating member 300 is provided with asymmetric regions when viewed from a top of the reciprocating member 300. In the depicted embodiment, the reciprocating member 300 comprises a first region 335 and a second region 340 provided on a top portion thereof and the first region 335 and the second region 340 are disposed substantially opposite to each other and along a pin-bore axis PB-PB’ of the reciprocating member 300, 400. In other words, the pin-bore axis PB-PB’ passes through the first region 335 and the second region 340. The first region 335 has a larger land area extending in the bowl portion when compared to the second region 340. Further, the first region 335 is disposed away from the spark plug 237 when compared to the position of the spark plug 237 from the second region 240 wherein the air and fuel mixture is caused to flow towards the spark plug 237 for improved reach of flame and for combustion.

[00048] In the depicted implementation, as illustrated in Fig. 5, the crown portion 305 is provided with the first region 335 and the second region 340 provided on opposite sides along the pin-bore axis PB-PB’. The first region 335 is an elevated portion, when compared to the bowl portion, that extends inward into the bowl portion 310. The first region 335 and the second region 340 are forming at least a portion of an inner periphery of the bowl portion 310. The first region 335 and the second region 340 extend inward of the bowl portion 310, when compared to rest of the inner periphery of the bowl portion 310.

[00049] In the depicted embodiment, the regions 335, 340 have a convex profile as viewed from center of the bowl portion 310. The first region 335 is having a first radius curvature RC1. Similarly, the second region 340 is also an elevated portion that extends inward into the bowl portion 310 and the second region 340 is also having a convex profile. The second region 340 is having a second radius of curvature RC2, which is greater than the first radius of curvature RC 1 of the first region 335. The shape of the regions is not limited to convex profile and may include any other geometrical shape. The second radius of curvature RC2, which is larger, forms a larger land acting as a squish land for creating desired turbulence and mixture of air-fuel.

[00050] Further, as depicted in Fig. 5, the bowl portion 310 comprises a first diameter D1 (taken between inward surfaces of the bowl portion 310 taken along a pin-bore axis PB-PB’ being substantially smaller than a second diameter D2 of the bowl portion 310 taken in a direction orthogonal to the direction of the pin- bore axis PB-PB’. In one embodiment, the second diameter D2 is about 1.1 to 1.4 times of said first diameter Dl. Fig. 6 depicts another top view of a reciprocating member, in accordance with an embodiment of the present subject matter. The dotted circles represent the projection of valves (as shown in Fig. 3) and the spark plug on the reciprocating member 300. The first region 335 is disposed in proximity to the spark plug 237. The second region 340, with larger radius of curvature RC2, is disposed away from the spark plug 237. The regions 335, 340 enables flow of the air-fuel mixture towards the spark plug 237 but with varying velocity and turbulence to enable uniform combustion. For example, the first region 335 and the second region 340 causes a squish effect causing a turbulent flow of air-fuel mixture towards the spark plug 237. The spark plug 237 disposed substantially equidistant from at least one intake valve 234 and the exhaust valve

236 provides sufficient time for combustion process to occur before scavenging of burnt gases from the combustion chamber 220. The second intake valve 235, which is away from the spark plug 237 is disposed closer to the second region 340 whereby the air and fuel; flowing in proximity to the second region 340 is caused to flow at higher velocity towards the spark plug 237 so that the flame generated at the spark plug 237 can propagate with sufficient time for effective combustion. [00051] The second region 340 disposed opposite to the position of the spark plug

237 enables the air-fixture mixture to travel for a shorter time to reach the spark plug, when compared to a conventional reciprocating member. The flow reaching the spark from the first region 335, which has the smaller radius of curvature RC1, anyway takes smaller distance as the first region 335 is closer to the spark plug. Thus, the flow towards spark plug 237 is improved. This enables uniform propagation of flame resulting in improved combustion of air-fuel mixture. The complete combustion of air-fuel mixture is achieved without the need for higher compression ratios.

[00052] Fig. 7 (a) and Fig. 7 (b) depicts a first side view and a second side view of a reciprocating member, in accordance with an embodiment of the present subject matter. Fig. 7 (c) depicts a schematic sectional view of a portion of the power unit, in accordance with an embodiment of the present subject matter. In addition to the asymmetric regions provided for a bowl portion 410, a top land 415 of the reciprocating member 400 is having an oval profile. The oval profile has a long axis, which is parallel to the pin-bore axis PB-PB’. The reciprocating member 400 has a relatively smaller diameter along an axis orthogonal to a long axis (of oval profile)/ pin-bore axis PB-PB’. This leads to clearance reduction between the top land 415 and a cylinder bore enabling necessary tilt of the reciprocating member 400. A diameter of the top land 415 taken along the skirt surfaces 430 is lesser when compared with the diameter taken in a direction along the pin-bore axis PB-PB’.

[00053] Further, as depicted in Fig. 5, the second diameter D2 is about 1.1 to 1.4 times of the first diameter D1 and whereby, the first region 335 and the second region 340 that occupy larger area are accommodated along long axis/ pin-bore axis PB-PB’ about the oval profile. The oval profile of the top land 415 offers improved crevice volume thereby reducing deposition of any fuel mixture in the crevice volume. Thus, the oval profile improves the combustion process. Further, the top land height is optimized in the range of 2 mm to 4 mm. With a reduction in the height of the top land 415, the crevice volume is further reduced. In one embodiment, a ratio of the top land height to a diameter of the reciprocating member is kept in the range of 15 to 35 to maintain the desired crevice volume. Thus, the deposition of fuel mixture is reduced thereby reducing hydrocarbon emissions.

[00054] In one embodiment, as shown in Fig. 7 (c), the top land 415 is provided with a tapered edge 416. The top edge of the reciprocating member 400 is made to be tapered and corresponding edge 240 provided on the cylinder head 202 is made to complement and match the tapered edge 416 of the cylindrical profile. This tapered edge 416 on the reciprocating member 400 assists in flame travel towards the crevice volume around the top land 415 and thereby promoting improved combustion of the deposited fuel thereat.

[00055] As shown in Fig. 7 (a) (thrust side) and Fig. 7 (b) (anti-thrust side), the reciprocating member is provided with asymmetric skirts 431, 432. The thrust side and the anti-thrust side are the sides of the reciprocating member 400 that come in contact with a liner of the combustion chamber/ cylinder bore due to tilting of reciprocating member 400 about the pin-bore axis PB-PB’. For example, when moving towards a top dead center (TDC), the thrust side of the reciprocating member 400 scuffs against the liner. Similarly, once the reciprocating member 400 starts moving downward towards bottom dead center (BDC), an anti-thrust side of the reciprocating member 400 scuffs against the liner. Thus, the reciprocating member 400 has higher wear on thrust side. Thus, a first skirt portion, which is towards a thrust side of the reciprocating member 400 is having a first skirt area Al, which is greater than a second skirt area A2 of a second skirt portion 432, which is on the anti -thrust side. Analogous to the aforementioned statement, the first skirt area 431 has a first peripheral length Bl, which is larger than a second peripheral length B3 of the second skirt portion 432. This reduces the overall contact area of the reciprocating member 400 with the cylinder bore thereby reducing friction as the skirt area on the anti -thrust side (as shown in Fig. 7 (b)) has smaller area as it does not substantially come in contact with the liner. In addition to the improved combustion, the reciprocating member further improves performance due to reduced friction. Thus, larger surface area (Al) is provided on a side of the reciprocating member 400 where there contact related aspect.

[00056] In one embodiment, a pin bore support section is made to be at least one of a convex, concave, or flat structure based on a length of the pin. Preferably, a distance of the pin bore boss can be made shorter such that that the length of the pins is reduced. This reduces overall reciprocating mass of the reciprocating member thereby improving performance.

[00057] The power unit 200 according to the present subject matter may be implemented in a forwardly inclined cylinder orientation. As the present subject matter enables retention of substantially the same cylinder head 202 size with improved combustion process. The size of the cylinder head 202 is maintained whereby the clearance between a storage unit (like utility box) and the power unit 200 is maintained. Especially, maintaining the clearance between the cylinder head 202 and the utility box. Excessive heating of the utility box and the components therein is avoided. Also, a standardized & compact engine system is achieved to suit different platform of product.

[00058] 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: 237 spark plug

100 motor vehicle 238 fins (Cooling)

105 frame assembly 240 tapered edge (cylinder

106 head pipe 35 head) 107 main frame 300/ 400 reciprocating

108 rear frames member

130 front wheel 305 crown portion

132 rear wheel 310 bowl portion 140 step-through portion 40 315 top land 150 seat assembly 320 ring land(s)

151 handlebar assembly 325 ring(s) 155 muffler 330 skirt portion

170A/170B 335 first region Panel 45 340 second region 175 utility box 416 tapered edge (top land)

200 power unit D 1 first diameter

201 cylinder head-cover D2 second diameter

202 cylinder head PB-PB’ pin-bore axis

203 cylinder block 50 RC1 first radius of curvature 204 crankcase RC2 second radius of curvature

205 first side casing C-C’ piston axis

206 second side casing PL imaginary line

208 intake port F 1 bottom facing side

209 air control device 214 rotational member

220 cylinder portion

231 body portion

232 cylinder ring region

233 valves 234/235 intake valve 236 exhaust valve