TECHNICAL FIELD
[0001] The present invention generally relates to a transmission system for a two wheeled vehicle. More particularly, the present invention relates to a two speed automatic transmission for the internal combustion engine of the two wheeled vehicle.
BACKGROUND
[0002] Two-wheeled vehicles are powered by an internal combustion (IC) engine. Many two wheeled vehicles such as mopeds, small scooters and other small automobiles operate on single transmission system, wherein a crankshaft of the IC engine is directly connected to a wheel of the two wheeled vehicle through one stage reduction geartrain. Such two wheeled vehicles have common requirements of low cost, high efficiency, good control throughout the entire speed range. However, a trade off between torque requirement and fuel economy is difficult in such single speed transmission. At higher torque requirements the fuel economy is less as the transmission system is operating at a single speed transmission ratio and vice versa. The critical issues involved in the design of the transmission system are to consider improving efficiency, better operability and reduce transmission losses and at the same time retain its attractive features of low cost and easy drivability. Automatic transmission system and manual transmission system implemented in such two wheeled vehicles such as moped is known in art. Introducing automatic transmission systems in existing layout of the IC engine in such vehicles is difficult in view of the changes to be made to accommodate the additional transmission components such as clutch, geartrains and one way clutches. Hence, in order to alleviate the above drawbacks, the present subject matter proposes a two speed automatic transmission system using planetary gear system.

BRIEF DESCRIPTION OF THE DRAWINGS
[0003] The detailed description is described with reference to the accompanying figures. The same numbers are used throughout the drawings to reference like features and components.
i [0004] Fig. 1. illustrates the side view of a two-wheeled vehicle employing an embodiment of the present subject matter.
[0005] Fig. 2. illustrates side view of the internal combustion engine according to the embodiment of the present subject matter.
[0006] Fig. 3. illustrates the cross sectional view (X-X) of the internal i combustion engine according to the embodiment of the present subject matter.
[0007] Fig. 4. and Fig. 5. illustrates the exploded view of the transmission unit according to the embodiment of the present subject matter.
[0008] Fig. 7a. illustrates the side view of the clutch mounting member securing a first one way clutch according to the embodiment of the present subject matter.
[0009] Fig. 7b. illustrates the isometric view of the clutch mounting member according to the embodiment of the present subject matter. •
[00010] Fig. 8a, Fig. 8b, Fig. 8c. and Fig. 8d. illustrates the various views of the transmission unit according to the embodiment of the present subject matter.
i DETAILED DESCRIPTION
[00011] Various features and embodiments of the present invention here will be discernible from the following further description thereof, set out hereunder. In the ensuing exemplary embodiments, the vehicle is a two wheeled vehicle with a step-through frame colloquially called moped. However it is contemplated that the disclosure in the present invention may be applied to any two wheeled vehicle having single speed transmission system without defeating the spirit of the present . subject matter. The detailed explanation of the constitution of parts other than the

present invention which constitutes an essential part has been omitted at suitable places.
[00012] Generally, in a two-wheeled vehicle with a step-through type single tube frame structure called moped, a step-through space is provided. The step-though space has a floorboard extending either side in the lateral direction and may be used for carrying loads or for the rider to rest feet. In such vehicles, the frame structure starts from the head pipe, and extends downwards to the step-through space and again rises to form a driver seat, crosses a pillion seat and finishes at the tail lamp. A fuel tank can be mounted on the front side of the vehicle at a height above the step-through space, An IC engine is located below the step-through space on the forward side of the vehicle to form a low-slung engine type. The IC engine is of horizontal type that is, a cylinder axis (the axis on which the piston of the IC engine reciprocates) is almost parallel to the central longitudinal axis of the two wheeled vehicle. The IC engine is functionally connected to a rear wheel of the vehicle such as by a sprocket and chain drive, which provides the forward motion to the vehicle. Typically, the frame assembly acts as a skeleton for the vehicle that supports the vehicle loads.
[00013] A transmission system for such a two wheeled vehicle such as moped type vehicle, comprises a single speed transmission system. Such two wheeled vehicles have common requirements of low cost, high efficiency, good controllability throughout the entire speed range. Typically, the transmission system includes a single transmission stage, a centrifugal clutch, and driven shaft having a sprocket at its end, through which the final drive to a rear wheel is connected. The centrifugal clutch ensures that at low to idle speeds the power transmission from the IC engine is disengaged to the rear wheel. The final drive to the rear wheel of the two wheeled vehicle is usually a positive drive such as sprocket and chain arrangement. Typically, in the moped type two wheeled vehicle, a kick-start mechanism or the like is used to crank the IC engine. The kick-start mechanism includes various components such as kick-rod, various gear(s), a return spring, and a ratchet arrangement.

[00014] Conventionally, in the moped type two wheeled vehicle, there is a problem of low torque at low speeds. For example, when the two wheeled vehicle is climbing a gradient on the road or heavy load is to be' pulled there is requirement of a lot of torque at the rear wheel to pull the hybrid vehicle, and the transmission system may not be able to provide the same. Further, moving at low speeds with less torque results in loss of fuel economy. Furthermore, at higher speeds, a fixed transmission ratio gives limitation to speeds the vehicle can travel and results in loss of fuel economy. Hence, the transmission system may not be able to provide sufficient torque and the internal combustion engine may get switched off. Hence, there is a need for effective multi speed transmission systems.
[00015] The design of existing IC engine and its related components disposed within it is critical as it is designed in optimizing the engine layout to make in less bulky and easy to assemble. Implementing multi speed transmission system in the existing layout involves extensive design and layout changes, which is not only difficult but also cumbersome and difficult to access. Further, a multi speed transmission system should be implementable with minimal changes in existing layout and minimum modification of frame component supporting the IC engine. Furthermore, there should be commonisation of parts wherein single speed and two speed can be introduced based on customer needs and requirements. A multi speed automatic transmission is the need as it can affect the vehicle's mechanical efficiency, fuel consumption, and cost. It can accommodate the range of vehicle needs and can operate smoothly. In this regard, the there are many multi speed transmission mechanism known in art. Such systems include manually operated two speed reduction with fixed gear ratio, and automatic two speed transmission systems.
[00016] Conventional transmission systems such as the two speed automatic transmission systems have drawbacks wherein additional components are introduced causing layout constraints in the existing design. Such additional components include the introduction of a new transmission stage in the existing

single speed IC engine including geartrain mechanism and multiple centrifugal clutches. This requires complete overhaul of the IC engine layout and involves extensive research and development and considerable investment to design a new IC engine with two speed transmission system. The crankshaft can accommodate only selected components, and hence any addition of new components such as gear pump, electric start systems are difficult. This also increases the cost of the two wheeled vehicle extensively. Further, changes in IC engine layout affects its space occupied in the two wheeled vehicle and hence involves complete redesign of frame assembly to support the IC engine and change its location. Hence, there is a need to introduce two speed transmission systems with least and/or minimum changes in IC engine layout. However in two speed transmission systems in existing art, serviceability is difficult due to difficulty in removal and servicing.
[00017] Hence, it is the principal object of the present invention to provide an automatic transmission system for the two wheeled vehicle, which would automatically vary the torque based on the speed of the internal combustion engine with least modification to the existing layout of the IC engine. The said transmission system should be able to accommodate a range of vehicle needs with a basic design approach while retaining many key transmission components.
[00018] Another object of the present invention is to provide ease of serviceability of the transmission system.
[00019] With the above design changes, the following advantages can be obtained such as automatic transmission to provide different accelerations without the need to manually switch the gears, minimal changes to existing layout, reduced fuel consumption and improvement of efficiency, and better optimization and minimal changes required in accommodating starting systems such as kick starter system.. Additionally, the automatic transmission system can be optimized at conditions of low speed, heavy load or gradient surface to improve fuel efficiency.
[00020] The present subject matter is an internal combustion engine comprising wherein a transmission unit is operably connected to a crankshaft, and said transmission unit is configured to automatically vary the torque output to the

output shaft. The transmission unit comprises an outer hub operabty connected to the output shaft through a planetary gear system, a spring loaded first centrifugal shoe unit fixedly attached to the crankshaft and capable of expanding and engaging with the outer hub on rotation of the crankshaft beyond a first, predetermined speed, and a spring loaded second centrifugal shoe unit operably attached to the planetary gear system and enclosed within said outer hub, said second centrifugal clutch unit capable of expanding and engaging with the outer hub on rotation of the planetary gear system beyond a second predetermined speed. The planetary gear system includes a sun gear configured to be locked and unlocked by a first one way clutch when the rotational speed of the crankshaft decreases and increases beyond the second predetermined speed.
[00021] According to a second characteristic of the present subject matter, the planetary gear system comprises a ring gear portion, a planet carrier, plurality of planet gears, the sun gear and a driving gear member. The ring gear portion forms part of one end of the outer hub. The planet carrier is freely mounted on the crankshaft and comprises a carrier plate portion disposed rearward of the ring gear portion and having an externally splined portion at its end, and a second one way clutch. The plurality of planetary gears is disposed and freely mounted radially and equidistant around the carrier plate portion, and said plurality of planetary gears is disposed between and meshed with the ring gear portion of the outer hub and the sun gear. The sun gear is disposed coaxially to the ring gear portion and freely supported on the carrier driving portion of the planet carrier through a collar. The driving gear member has an internally splined portion being inserted annularly to the carrier driving portion of the planet carrier, said internally splined portion mating with the externally splined portion to transmit torque.
[00022] According to a third characteristic of the present subject matter, the first one way clutch is held securely to a crankcase of the internal combustion engine by a clutch mounting member. The clutch mounting member comprises a central ring member capable of accommodating said first one way clutch, and a plurality of mounting bosses spaced apart and formed on the outer periphery of said central

ring member, each of said plurality of mounting bosses comprising a hole at its outer edge configured to permit a securing member used to attach said clutch mounting member to the crankcase.
[00023] With the use of the transmission unit as described above, very less layout change is required. The use of the clutch mounting member enables the transmission unit to be mounted anywhere along the span of the crankshaft by varying the length of the plurality of mounting bosses. Hence, the flexibility of adjusting the transmission unit along the crankshaft in order to accommodate other components such as gear oil pump and starter system gives it flexibility in design. Further, the clutch mounting member is mounted on the wall of the RH crankcase which provides rigidity. Furthermore, since the axial load of the entire transmission unit is supported by the crankshaft detachably mounted along the clutch mounting member. During servicing, just on mere removal of the fasteners securing the clutch mounting member, the entire transmission unit can be disassembled on mere removal of clutch cover. Additionally, use of additional components or casing to secure the sun gear is avoided so that compactness can be achieved. The mere mounting of transmission unit on the LH crankshaft eliminates the need of any additional layout modifications as it can be interchangeably removed and mounted with an ordinary centrifugal clutch in order to obtain a single speed transmission. This aids in flexibility in approach and design.
[00024] The present subject matter along with all the accompanying embodiments and their other advantages would be described in greater detail in conjunction with the figures in the following paragraphs.
[00025] Fig. 1. illustrates a left side view of an exemplary two-wheeled type-through type vehicle, in accordance with an embodiment of present invention. The vehicle (100) has a mono-tube type frame assembly (105) extending from a front portion (F) to a rear portion (R) in a longitudinal axis (F-R) of the two wheeled vehicle (100), which acts as the skeleton for bearing the loads. The frame assembly (105) extends from a head tube (108) in the front portion (F) of the

vehicle till the vehicle rear portion (R). A steering shaft (not shown) is inserted through the head tube (108) and a handle bar assembly (115) is pivotally disposed on it. The steering shaft is connected to a front wheel (110) by one or more front suspension(s) (120). A front fender (125) is disposed above the front wheel (110) for covering at least a portion of the front wheel (110). A fuel tank (130) is mounted on the downward portion of the frame assembly (105) and it is disposed in the front portion (F). The frame assembly (105) forms a substantially horizontal step-through portion (106) with a floorboard to enable step-through mounting of a rider and to assist in carrying heavy loads. The IC engine (101) is mounted on the frame assembly (105) below the step-through portion (106) forming a low slung engine mounting. In an embodiment, a piston axis of the engine is horizontal i.e. parallel to a longitudinal axis of the vehicle (100). A swing arm (140) is swingably connected to the frame assembly (105). A rear wheel (145) is rotatably supported by the swing arm (140). One or more rear suspension(s) (150) are connecting the swing arm (140) at an angle, to sustain both the radial and axial forces occurring due to wheel reaction, to the frame assembly (105). A rear fender (155) is disposed above the rear wheel (145). A seat assembly (160A, 160B) is disposed at a rear portion (R) of the step-through portion for seating of the rider. In an embodiment, the seat assembly (160) includes a rider seat (160A) and a pillion seat (160B). Further, the seat assembly (160) is positioned above the rear wheel (145).The vehicle is supported by a centre stand (170) mounted to the frame assembly (105). The IC engine (101) is connected to the rear wheel (145) through a transmitting means, such as in the present embodiment sprockets linked to each other through a chain drive.
[00026] Fig. 2a illustrates the side view of the IC engine (101). The IC engine is made up of a cylinder head (203), cylinder block (204), crankcase (210) and a cylinder head cover (202). The crankcase (210) is made up RH crankcase (210a), LH crankcase (210b), and a clutch cover (210c). The clutch cover (210c) is disposed on the left hand side of the IC engine (101) adjacent to the LH crankcase (210b) and encloses a transmission unit (300). The internal combustion (IC) engine of the IC engine (101) includes an, an air intake system (205, 206), an

exhaust system (not shown), and a kickstarter system (not shown) using a kickshaft (207).
[00027] Fig. 3. illustrates the cross sectional view (X-X) of the internal combustion engine and a transmission unit according to the embodiment of the present subject matter. The IC engine comprises a reciprocating piston (401) reciprocating within the cylinder block (204), and a rotatable crankshaft (407a and 407b). Combustion occurs when air fuel mixture is burnt in the combustion chamber (not shown) which transfers the pressure created during combustion to the reciprocating piston (401). The reciprocating motion of the piston is converted to the rotary motion of the crankshaft (407) by a connecting rod (403) through a slider crank mechanism. The rotary motion of the crankshaft (407) is transferred to an engine sprocket (not shown) through the transmission unit (300) and a geartrain mechanism (302, 412). The crankcase (210) houses the kickstarter assembly (207, 409, 410, 423, 424) and comprises the kickshaft (207) which is connected to a ratchet mechanism (423). When the kicklever (409) is actuated by the foot of a rider, the ratchet moves and engages with another ratchet operably connected to the crankshaft (407). On withdrawal of pressure from the foot, the return spring (410) withdraws the kicklever (409) to starting position. The RH crankcase encloses a dry magneto assembly (406) disposed on the righthand side of the crankshaft (407a). The dry magneto assembly (406) is configured to rotate along with the crankshaft (407) to generate power which recharges a battery (not shown). Further, a centrifugal fan (405) is disposed in front of the magneto assembly (406) forming part of a cooling system to cool the IC engine (101). A shroud (402) (see Fig. 3) encloses the centrifugal fan (405) and covers the cylinder head (203) and cylinder block (204). The centrifugal fan (405) rotates along with the crankshaft and draws atmospheric air inside and circulates it throughout the interior portions of the shroud (402).
[00028] The IC engine (101) comprises the transmission unit (300) freely mounted on an extended portion (407c) of the LH crankshaft (407b). An output shaft (415) is disposed parallel to the crankshaft (407) towards the rear of the IC

engine (101) and is supported by two roller bearings (431 & 432). A driving gear member (302) is freely mounted on the LH crankshaft (407b) obtaining rotary motion from the transmission unit (300), and this rotary motion is transferred to the driving gear member (302). The driving gear member (302) is meshed with a first driven gear (412) mounted on said output shaft (415), and the transmission ratio between the driving gear member (302) and first driven gear (412) provides a gear ratio multiplication. The engine sprocket is disposed outside the LH clutch cover (210c) which receives rotary motion from the output shaft (415) of the geartrain mechanism. The engine sprocket is internally splined and mounted on external splines on the end of the output shaft (415) juxtaposing outside the rear portion of the LH crankcase (210b). A chain (not shown) connects the engine sprocket to a corresponding wheel sprocket (not shown) on the rear wheel (145). This way rotary motion is transferred to the rear wheel (145). One half of a crankshaft (407b) juxtaposes outside the LH crankcase (210b) over which the transmission unit (300) is operably secured. The transmission unit (300) is designed to be of a planetary-centrifugal clutch system with an external first one way clutch (301). The first one way clutch is held in position on the LH crankcase (210b) by a clutch mounting member (303). The LH crankcase (210c) is completely enclosed on its sides except for relevant opening for accommodating the crankshaft and the output shaft. The clutch mounting member (303) is secured to the lefthand wall of the LH crankcase (not shown). The IC engine (101) further comprises a clutch cover (210c) having an interior portion (210d) enclosing the transmission unit (300), and said interior portion (210d) comprising a support ball bearing (321) to provide axial support to the extended portion of the crankshaft (407c).
[00029] Fig. 4. and Fig. 5 illustrates the exploded view of the transmission unit (300) according to the embodiment of the present subject matter. The transmission unit (300) comprises of an outer hub (307) within which is enclosed two centrifugal shoe units (309 and 310) which are spring loaded. Each of the first centrifugal shoe unit (309) and the second centrifugal shoe unit (310) is secured to the extended portion of the crankshaft (407c) and enclosed by a single outer hub

(307) essentially functioning as two centrifugal clutches. The outer hub (307) is
connected to the output shaft (415) through a planetary gear system (350) and
geartrain (302, 412). On selective engagement of each of the first centrifugal shoe
unit (309) and the second centrifugal shoe unit (310) and the locking and
unlocking of the sun gear (313), automatic switching of two speed ratios having
different torque multiplication ratio can be obtained. The first and second
centrifugal shoe unit (309 & 310) each comprises a plurality of shoes each of
which is spring loaded with respect to a pivot. In the present embodiment, there
are three shoes equidistantly located. The spring stiffness is adjusted to actuate at
the right revolutions of the crankshaft.
[00030]. The planetary gear system (350) comprises the a ring gear portion (307a) forming part of the outer hub (307), a set of plurality of planetary gears (313), a sun gear (314) and a planet carrier (308) being connected to the driving gear member (302). The outer hub (307) has one end of its axial side, comprising an annular circular opening and comprises the ring gear portion (307a) having internal teethes around its inner periphery. The set of plurality of planetary gears (313) is in constant mesh with the ring gear portion (307a) of the outer hub (307) and are freely mounted by rivets on the planet carrier (308). The planet carrier
(308) is enclosed within the outer hub (307) and comprises a carrier plate portion
(308a), a carrier driving portion (308b), and a second one way clutch (308d). The
carrier plate portion (308a) is circular in shape and matching with the circular
dimensions of the outer hub (307) and said plurality of planet gears (313) is freely
secured on the axial surface of the carrier plate portion (308a). The carrier driving
portion (308b) projects axially outward of the annular circular opening from the
centre of the carrier plate portion (308a) which forms the output of the-
transmission unit (300). Further, the end portion (308c) of the carrier driving
portion (308b) comprises external splines through which rotary output can be
transmitted to the output shaft (415). The driving portion (308b) has a through
hole configured to be secured freely on the extended portion (407c) of the
crankshaft. The sun gear (314) is meshed between the plurality of planet gears
(313) and coaxial to the annular circular opening and the carrier driving portion

(308b). The internal portion of the sun gear (314) is splined to be engaged by a externally splined collar (315) which is supported freely on the outer radial surface of the carrier driving portion (308b). The internal portion of the first one way clutch (301) is press fitted on the collar (315) and the external portion (of the first one way clutch) is secured to the LH crankcase (210b) with the aid of clutch mounting member (303). This way, the rotation of the sun gear (314) can be locked and unlocked with the aid of the first one way clutch (301).
[00031] The two speed automatic transmission is brought about by engagement of selective engagement of the two centrifugal shoes unit (309, 310) and by locking and unlocking the sun gear (314). The locking and unlocking is achieved by employing the first one way clutch (301) being held by a rigid structural member called the clutch mounting member (303). The transmission unit (300) is designed to operate as a centrifugal clutch engagable at two different rotational speeds of the crankshaft by engaging either one or both the centrifugal shoes. The first centrifugal shoe (309) is designed to engage only at a particular rotational speed namely, the first predetermined speed of the IC engine. This rotational speed is usually just above idling rotational speed configured and designed for the IC engine during starting of the vehicle. In the present embodiment, the first predetermined speed is 2450 revolutions per minute. Hence, if the throttle is not operated by the rider, rotational speed of the IC engine decreases to idling speed and the transmission system is disengaged from the crankshaft (407).
[00032] The transmission is disengaged during starting of the two wheeled vehicle until the crankshaft begins to rotate at a particular first predetermined speed. Once, throttle is operated, the first centrifugal shoe unit (309) is swung outwardly and engages to the outer hub (307) to have frictional contact, and" the rotary motion is transmitted to the outer hub (307). The first centrifugal shoe unit (309) and the outer hub (307) rotate together, and the rotation of the outer hub (307) is transmitted to the planetary gears (313) through the mesh with the ring gear portion (307a). The planetary gears (313) rotate about their own axis and about the sun gear (314). The sun gear (314) is held at rest, since its rotation following

the rotation of the planetary gears (313) about their own axis is hindered by the first one way clutch (301). As the planetary gears (313) rotate about the sun gear (314), the planet carrier (308) supporting the planetary gears (313) is rotated, and the first driven gear (412) is rotated by the gear carrier driving portion (308b) which is integral with the planet carrier (308). This is a first stage of torque multiplication wherein the rear wheel (145) runs at a higher torque due the multiplication ratio between the ring gear portion (307a) of the outer hub (307) and the sun gear (314). .
[00033] Consider condition where the vehicle is operating at high speed. As highlighted above, the second centrifugal shoe unit (310) is attached on the one way clutch (308d) rearward of the carrier plate portion (308a) of the planet carrier (308). On achieving sufficient speed of the carrier plate portion (308a) namely a second predetermined speed, both the first centrifugal shoe unit (309) and the second centrifugal shoe unit (310) is swung outwardly and engages with the outer hub (307) to have frictional contact. This causes the planetary gears (313) to be integral with the outer hub (307a). Hence, they become an integral unit and no longer rotate about their own axes, but only rotate about the sun 'gear (314). The sun gear (314) is freely rotatable in the direction in which the planetary gears
(313) rotate due to the unlocking of the one way clutch (301) when the sun gear
(314) rotates in the opposite direction. Now, there is direct torque transmission from the crankshaft (407) to the output shaft (415) with no gear ratio multiplication as compared to the first stage. This is a second stage torque output, wherein, the rear wheel (145) runs at a lower torque and higher speed. In the present embodiment, the second- predetermined speed is 2000 revolutions per minute of outer hub (307).
[00034] As illustrated in Fig. 3, the cranks haft, has an extended portion (407c) on which the transmission unit (300) is supported. The extended crankshaft (407c) passes through the carrier driving portion (308b) of the planet carrier (308) and is splined to back-plate (306) disposed on the rearmost end of the outer hub (307). The first centrifugal shoe unit (309) is mounted on an axial projection of (306a)

from the centre of the back-plate (306). The projection (306a) comprises external splines and is used to locate the planet carrier (308). The external splines match with the internal splines on the planet carrier (308) and is used for support and location of the planet carrier (308), the plurality of planetary gears (313), the sun gear (314) and the outer hub (307). A second one way clutch (308d) is disposed on the planet carrier (308) wherein on its internal diameter, the projection of the back-plate (306a) is secured. The outer diameter of the second one way clutch (308d) is secured to the second "centrifugal shoe unit (310). This ensures that, the drive is not directly transferred from the crankshaft (407) to the second centrifugal shoe unit (310). Further, another function of the second one way clutch (308d) is to prevent the rotary motion drive from transferring back to the crankshaft (407) and the transmission unit (300) from the output shaft (415).
[00035] Fig. 6a. illustrates the clutch mounting member (303) secured to the LH crankcase (210b), and Fig. 6b. illus'trates the isometric view of the clutch mounting member (303) according to the embodiment of the present subject matter. In Fig. 7a, transmission unit (300) is removed retaining only the first one way clutch (301) for illustration. The clutch mounting member (303) comprises a central ring member (303a) capable of accommodating said first one way clutch (301) through a one way clutch cover (312), and a plurality of mounting bosses (303b) spaced apart and formed on the outer periphery of said central ring member (303a), each of said plurality of mounting bosses (303b) comprising a hole at its outer edge configured to permit a securing member (305a) used to attach said clutch mounting member (303) to the LH crankcase (210b). In one embodiment, the plurality of mounting bosses (303b) is three in number. The plurality of mounting bosses (303b) can be adjusted in length based on the position of the transmission unit (300). Hence, if any additional components is desired to be added such as gear oil pump, or electric start system, then can be accommodated in the crankshaft, and the transmission unit (300) can be axially moved and mounted by adjusting the height of the plurality of mounting bosses (303b).

[00036] 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.

We Claim:
1. An internal combustion engine (101) comprising:
a crankshaft (407) configured to receive rotary motion on burning" of air and fuel mixture within the internal combustion engine (101);
an output shaft (415) configured to receive rotary motion from the crankshaft (407); wherein a transmission unit (300) is operably connected to the crankshaft (407), said transmission unit (300) is configured to automatically vary the torque output to the output shaft (415), and said transmission unit (300) comprising:
an outer hub (307) operably connected to the output shaft (415) through a
planetary gear system (350);
a spring loaded first centrifugal shoe unit (309) fixedly attached to the
crankshaft (407) and capable of expanding and engaging with the outer
hub (307) on rotation of the crankshaft (407) beyond a first predetermined
speed;
a spring loaded second centrifugal shoe unit (310) operably attached to the
planetary gear system (350) and enclosed within said outer hub (307), said
second centrifugal clutch unit (310) capable of expanding and engaging
with the outer hub (307) on rotation of the planetary gear system (350)
beyond a second predetermined speed; and
said planetary gear system (350) includes a sun gear (314) configured to
be locked and unlocked by a first one way clutch (301) due to change in
direction of rotation of the sun gear (314) when the rotational speed of the
crankshaft (407) decreases and increases beyond the second predetermined
speed.
2. The internal combustion engine (101) as claimed in claim 1, wherein the
planetary gear system (350) comprising:

- a ring gear portion (307a) forming part of one end of the outer hub (307),
said ring gear portion (307a) having plurality of internal teethes disposed
around the internal periphery;
a planet carrier (308) freely mounted on the crankshaft (407), said planet
carrier (308) comprising a carrier plate portion (308a) disposed rearward
of the ring gear portion (307a) of the outer hub (307), a carrier driving
portion (308b) having an externally splined portion (308c) at its end, and a
second one way clutch (308d);
plurality of planetary gears (313) disposedand freely mounted radially and
equidistant around the carrier plate portion (308a) of the planet carrier
(308), and said plurality of planetary gears (313) disposed between and
meshed with the ring gear portion (307a) of the outer hub (307) and the
> sun gear (314);
said sun gear (314) disposed coaxially to the ring gear portion (307a) and
freely supported on the carrier driving portion (308b) of the planet carrier
(308) through a collar (315); and
a driving gear member (302) having an internally splined portion (302a)
being inserted annularly to the carrier driving portion (308b) of the planet
carrier (308), said internally splined portion (302a) mating with the
externally splined portion (308c) to transmit torque.
S. The internal combustion engine (101) as claimed in claim 1 or claim 2,
wherein said collar (315) securing the sun gear (314) is attached to the first
one way clutch (301) disposed coaxially over the collar (315).
[. The internal combustion engine (101) as claimed in claim 1 or claim 3, wherein said first one way clutch (301) is held securely to a crankcase (210b) of the internal combustion engine (101) by a clutch mounting member (303).-
>. The internal combustion engine (101) as claimed in claim 1, wherein said output shaft (415) comprises a driven gear (412) secured to it, and said driven gear (412) is meshed with said driving gear member (302) to transfer and multiply torque to the output shaft (415).

The internal combustion engine (101) as claimed in claim 1, wherein the planet carrier (308) comprises a second one way clutch (308d) through which the planet carrier (308) is supported on the crankshaft (407), and said second one way clutch (413) is configured to prevent the rotary motion drive from transferring to the second centrifugal shoe unit (310) from the crankshaft (407).
The internal combustion engine (101) as claimed in claim 1 or claim 4, wherein said clutch mounting member (303) comprises a central ring member (303a) capable of accommodating said first one way clutch (301), and a plurality of mounting bosses (303b) spaced apart and formed on the outer periphery of said central ring member (303a), each of said plurality of mounting bosses (303b) comprising a hole at its outer edge configured to permit a securing member (305a) used to attach said clutch mounting member (303) to the crankcase (210b).
The internal combustion engine (101) as claimed in claim 1, wherein the internal combustion engine (101) further comprises a clutch cover (210c) having an interior portion (210d) enclosing the transmission unit (300), and said interior portion (210d) comprising a support ball bearing (321) to provide axial support to the extended portion of the crankshaft (407c).
The internal combustion engine (101) as claimed in claim 1, wherein the first predetermined speed is 2450 revolutions per minute of the crankshaft (407), and second predetermined speed is 2000 revolutions per minute of the outer hub (307).
A step-through type two wheeled vehicle (100) including said internal combustion engine (101) as claimed in claims 1 to 9.