WO2018185643A1

WO2018185643A1 - A transmission system for a two-wheeled vehicle

Info

Publication number: WO2018185643A1
Application number: PCT/IB2018/052275
Authority: WO
Inventors: Kandregula SRINIVASA RAO; Thangaraj BOOPATHI RAJ
Original assignee: Tvs Motor Company Limited
Priority date: 2017-04-03
Filing date: 2018-04-03
Publication date: 2018-10-11
Also published as: CN110573418B; BR112019020759A2; CN110573418A

Abstract

The present subject matter discloses an internal combustion (IC) engine (101) of a vehicle (100) for efficient and automatic power transmission. The internal combustion (IC) engine (101) comprising a single-speed gear-train mechanism (411, 412) and a detachable additional gearbox (200). The additional gearbox (200) comprises a secondary gear-train mechanism (416a, 417) which selectively combines with the single-speed gear-train mechanism (411, 412) during a pre-determined operating range of the IC engine (101). The above-mentioned system through the additional gearbox (200) automatically varies the torque based on the speed of the internal combustion (IC) engine (101). Further, the present invention provides ease of serviceability and disassembly. Ultimately, the performance of the IC engine (101) improves through the present subject matter.

Description

A TRANSMISSION SYSTEM FOR A TWO-WHEELED

VEHICLE

FIELD OF INVENTION

[0001] The present invention generally relates to a transmission system for a saddle type vehicle. More particularly, the present invention relates to a housing to enclose additional transmission system components in the saddle type vehicle.

BACKGROUND

[0002] Saddle type vehicles e.g. two-wheeled vehicles are powered by an internal combustion (IC) engine or an electric motor or a hybrid system with gasoline as fuel for the IC engine & energy storage device like battery or fuel cell etc. Many two wheeled vehicles such as mopeds, small scooters and other small automobiles operate on single speed transmission system, wherein a crankshaft of the IC engine is directly connected to a wheel of the two wheeled vehicle through one stage reduction gear train. Such two wheeled vehicles have common requirements of low cost, high efficiency, good controllability 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 known 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 a clutch, gear trains and one way clutches. The size of the power train as well as its weight can have adverse impact & thus making it nearly impossible to package in the saddle type vehicle. Hence, in order to alleviate the above drawbacks, the present subject matter proposes an additional compact gearbox system which is disposed adjacent to the IC engine and houses the additional components to enable converting of the single speed transmission system to a multi speed transmission 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.

[0004] Fig. 1. illustrates the side view of a two-wheeled vehicle employing an embodiment of the present subject matter.

[0005] Fig. 2a. and Fig. 2b. illustrates side view and top view of the internal combustion engine with the additional gearbox according to the embodiment of the present subject matter.

[0006] Fig. 3. illustrates the exploded view of the additional gearbox operably mounted to the internal combustion engine according to the embodiment of the present subject matter.

[0007] Fig. 4. illustrates the cross sectional view (X-X) of the internal combustion engine and the additional gearbox according to the embodiment of the present subject matter.

[0008] Fig. 5a. illustrates the cross sectional view (X-X) of the IC engine according to another second embodiment of the present subject matter.

[0009] Fig. 5b. illustrates the cross sectional view (X-X) of the IC engine according to another third embodiment of the present subject matter.

[00010] Fig. 6a. illustrates the cross sectional view (X-X) of the IC engine according to another forth embodiment of the present subject matter. [00011] Fig. 6b. illustrates the cross sectional view (X-X) of the IC engine according another fifth embodiment of the present subject matter.

DETAILED DESCRIPTION

[00012] 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 typically 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.

[00013] 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 by suitable drive system 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. [00014] 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.

[00015] 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 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.

[00016] The design of 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 known layout involves extensive design and layout changes, which is not only difficult but also cumbersome and difficult to access. A multi speed automatic transmission is the need as it can affect the vehicleDs mechanical efficiency, fuel consumption, and cost. It can accommodate a range of vehicle needs and can operate smoothly. In this regard, there are many multi speed transmission mechanism known in art. Such systems include manually operated two speed reduction with fixed gear ratio, or automatic two speed transmission systems.

[00017] 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. The introduction of a new transmission stage in the existing single speed IC engine 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. 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 design a two speed transmission systems with least and/or minimum changes in IC engine layout solving all the above problems.

[00018] 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 with least modification to the existing layout of the IC engine based on a pre-determined operating range of the IC engine. In present embodiment, the pre-determined operating range of the IC engine is the speed of the internal combustion 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.

[00019] Another object of the present invention is to provide ease of serviceability of the transmission system.

[00020] Another object of the present invention is for easy disassembling of the transmission system from the main IC engine when two speed transmission system is not required. [00021] 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.

[00022] The present invention 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.

[00023] 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. A power unit (101) is mounted on the frame assembly (105) below the step-through portion (106) forming a low slung engine mounting. In present embodiment, the power unit (101) is an IC engine (101). However, the power unit (101) in other possible embodiments include a motor or a combination of motor and the IC engine. In present embodiment, a piston axis of the IC engine (101) 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 (301-see Fig. 3.).

[00024] Fig. 2a. illustrates the side view of the IC engine (101) and Fig. 2b. illustrates the top view of the IC engine (101) along with an additional gearbox (200). The IC engine (101) 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 first centrifugal clutch (421) and related transmission gear mechanisms. The additional gearbox (200) is disposed adjacent to the clutch cover (210c) and mounted on it. The clutch cover (210c) and the additional gearbox (200) comprises plurality of bosses (see 212) disposed at intervals around its outer periphery and said bosses comprises holes to receive fasteners. The additional gearbox (200) is mounted on the clutch cover (210c) and secured through fasteners mounted on said plurality of bosses (212). The bosses (see 212) are of sufficient length to accommodate an engine sprocket (414) in a space between the additional gearbox (200) and the clutch cover (210c). The internal combustion (IC) engine (101) includes an air intake system (205, 206), an exhaust system (not shown), and a kick-starter system (not shown) using a kick-shaft (409). The RH crankcase (210a) encloses a wet magneto assembly (406) disposed on the right hand side of the crankshaft (407a). The wet magneto assembly (406) is configured to rotate along with the crankshaft (407) and 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. 4) 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.

[00025] Fig. 3. illustrates the exploded view of the additional gearbox (200) operably mounted to the IC engine (101) according to the embodiment of the present subject matter. The engine sprocket (414) is disposed outside the LH clutch cover (210c) which receives rotary motion from an output shaft (415) of the transmission system. As highlighted, the chain (301) connects the engine sprocket (414) to a corresponding wheel sprocket (302) on the rear wheel (145). This way rotary motion is transferred to the rear wheel (145). The engine sprocket (414) comprises an outer gear teeth portion and an inner hollow portion comprising internal splines and which is configured to receive a secondary shaft (425) of the additional gearbox (200). A part of the secondary shaft (425) is projecting outwards and comprises external splines which correspond to the internal splines on the engine sprocket (414). Similarly, the output shaft (415) of the transmission system of the IC engine (101) comprises external splines. Hence, the engine sprocket (414) receives the output shaft (415) from the right side and the secondary shaft (425) from the left side. The clutch cover (210c) further comprises an opening (210d) which is configured to receive a primary shaft (416) of the additional gearbox (200). The primary shaft (416) is projecting outwards from the additional gearbox (200) and configured to be coaxially inserted inside the opening (210d) and operably engaged with the first centrifugal clutch (421) disposed within the IC engine (101).

[00026] Fig. 4. illustrates a cross sectional view (X-X) of the internal combustion engine (101) and an exploded view of the additional gearbox (200) 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 (407). Combustion occurs when air fuel mixture is burnt in the combustion chamber (422) 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 the engine sprocket (414) through the transmission system. The crankcase (210) houses the kickstarter assembly (409, 410, and 423) and comprises the kick-shaft (409) which is connected to a ratchet mechanism (423). When the kickshaft (409) is actuated by the foot of a rider, the ratchet moves and engages with another ratchet operably connected to the crankshaft (407) via a starter gear-train (404). On withdrawal of pressure from the foot, the return spring (410) withdraws the kickshaft (409) to starting position.

[00027] The transmission system comprises the first centrifugal clutch (421) disposed on the left hand side of the crankshaft (407b). A first driving gear (411) is freely mounted to the crankshaft (407b) and connected to the crankshaft by the first centrifugal clutch (421). The first centrifugal clutch (421) is designed to engage only at a particular rotational speed of the IC engine. This rotational speed is usually just above idling rotational speed configured and designed for the IC engine. Hence, if the throttle is not operated by the rider, rotational speed of the IC engine (101) decreases to idling speed and the transmission system is disengaged from the crankshaft (407). Once, throttle is operated, the first centrifugal clutch (421) engages, and the rotary motion is transmitted to the rest of the transmission system. Once, the vehicle speed is above idling speed and the first centrifugal clutch (421) is engaged the rotary motion is transferred to the first driving gear (411). The first driving gear (411) is meshed with a first driven gear (412), and the transmission ratio between the first driving gear (411) and first driven gear (412) provides the first speed reduction for the IC engine (101). The first driven gear (412) is disposed on an output shaft (415) and mounted on bearings. The engine sprocket (414) is disposed at one end of the output shaft (415) projecting out of the clutch cover (210c). The combination of the first driving gear (411) and first driven gear (412) is termed as a single-speed gear- train mechanism (411,412). The single-speed gear-train mechanism (411,412) setup provides the first transmission ratio operable at a pre-determined operating range of the IC engine (101) which includes low speeds and higher torque conditions. The first driven gear (412) is mounted on a one way clutch (413) to prevent the rotary motion drive from transferring back to the crankshaft (407b) from the output shaft (415).

[00028] The additional gearbox (200) is disposed beyond the clutch cover (210c). The additional gearbox (200) provides the second transmission ratio to be engaged during lower torque and higher speed conditions. The additional gearbox (200) comprises a primary shaft (416) configured to receive rotary motion from the LH crankshaft (407b), and a secondary shaft (425) connected to the engine sprocket (414) and configured to transfer the rotary motion to the engine sprocket (414) during conditions when the second speed transmission system is operable. The additional gearbox (200) further comprises a secondary gear-train mechanism (416a, 417) which includes a second driving gear (416a) and a second driven gear (417). Furthermore, the additional gearbox (200) comprises a second centrifugal clutch (420). The secondary gear-train mechanism (416a, 417) is externally mounted on the IC engine (101) and operably connected to the crankshaft (407) and the output shaft (415). Based on the IC engine (101) requirements and optimizing space and achieving good aesthetics, the second centrifugal clutch (420) can be disposed either on the primary shaft (416) or the secondary shaft (425). In case the second centrifugal clutch (420) is disposed on the primary shaft (416) then the second driving gear (416a) is mounted freely on the primary shaft

(416) and operably connected the primary shaft (416) through the second centrifugal clutch (420). Consequently, the secondary driven gear (417) is rigidly mounted on the secondary shaft (425). In case the second centrifugal clutch (420) is disposed on the secondary shaft (425) then the second driving gear (416a) is mounted freely on the secondary shaft (425) and operably connected the secondary shaft (425) through the second centrifugal clutch (420). Consequently, the second driven gear (417) is rigidly mounted on the primary shaft (416). The entire additional gearbox (200) components are enclosed in a housing (201) and a supportive housing (201a). The primary shaft (416) is supported by primary shaft bearings (418a and 418b) on either side. Similarly, the secondary shaft (425) is supported by secondary shaft bearings (419a and 419b) on either side of the housing (201 and 201a).

[00029] The present invention provides an automatic transmission system for the vehicle (100), which would automatically vary the torque with least modification to the existing layout of the IC engine (101) based on the pre-determined operating range of the IC engine (101). In present embodiment, the predetermined operating range of the IC engine (101) is the speed of the internal combustion engine (101). For speed of the internal combustion engine (101) there are various possible conditions, out of them few are considered here. Consider a condition where the vehicle (100) is operating at low speed. Here, the first centrifugal clutch (421) is engaged and the second centrifugal clutch (420) is disengaged. Here, the gear ratio between the first driving gear (411) and the first driven gear (412) is higher than the gear ratio in the high-speed transmission of the additional gearbox (200). The rotary motion is transmitted from the crankshaft (407b) to the first centrifugal clutch (421) wherein the clutch shoe expands due to centrifugal action and meshes with the outer hub. The outer hub is connected to the first driving gear (411) which is meshed with the first driven gear (412) and to the output shaft (415) through the one way clutch (413). The one way clutch (413) permits the transfer of rotary motion only from the first driven gear (412) to the output shaft (415) and slips when the relative motion of the output shaft exceeds the rotary motion of the first driven gear (412). Since the primary shaft (416) of the additional gearbox (200) is operably connected to the crankshaft (407b), the primary shaft (416) also rotates along with the crankshaft (407b) and transfers the rotary motion to the second driving gear (416a) integrally attached to the primary shaft (416). The second driving gear (416a) is meshed to the second driven gear (417). Since, the second driven gear (417) is freely mounted on the secondary shaft (425) and integrally attached to the second centrifugal clutch (420), further transfer is not permitted. Hence, the rear wheel (145) runs at a higher torque because the transmission occurs through the low-speed transmission. Consider another condition where the vehicle (100) is operating at high speed. Here, both the first centrifugal clutch (421) and the second centrifugal clutch (420) are engaged. Here, the motion from the second driven gear (417) is transferred to the second centrifugal clutch (420) which in turn transfers it to the engine sprocket (414) providing the high speed transmission. The one way clutch (413) prevents the relative motion between the crankshaft (407) and the output shaft (415). Since, the gear ratio between the second driving gear (416a) and second driven gear (417) is lower than the gear ratio in the low-speed transmission path. The rear wheel (145) runs at a lower torque because the transmission occurs through the high-speed transmission.

[00030] Fig. 5a. illustrates another second embodiment of the present subject matter. Here, the crankshaft (407) is projecting from the clutch cover (210c) and is inserted within a gearbox opening and operably attached to the primary shaft (416). Fig. 5b. illustrates another third embodiment wherein, the engine sprocket (414) is securely mounted on the output shaft (415) and the output shaft (415) extends further to be coaxially inserted into another opening in the additional gearbox (200) to be attached to the secondary shaft (425).

[00031] Fig. 6a. illustrates another fourth embodiment of the present subject matter. Here, the second centrifugal clutch (420) is disposed on the primary shaft (416) and the second driving gear (416a) is freely mounted on the primary shaft (416) and connected to the primary shaft (416) through the second centrifugal clutch (420). Here, the engine sprocket (414) comprises internal spines to receive the secondary shaft (425) and the output shaft (415) from either side. Fig. 6b. illustrates another fifth embodiment of the present subject matter, wherein the second centrifugal clutch (420) is freely mounted on the primary shaft (416) and the engine sprocket (414) is mounted on the output shaft (415) and the output shaft (415) extends further to be coaxially inserted into another opening in the additional gearbox (200) to be attached to the secondary shaft (425). [00032] 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.

Claims

We claim:

1. A vehicle (100) comprising: an internal combustion (IC) engine (101), said IC engine (101) comprising: a crankshaft (407) configured to receive rotary motion on burning of air and fuel mixture within said IC engine (101);

an output shaft (415) configured to receive rotary motion from said crankshaft (407);

an engine sprocket (414) disposed on one end of said output shaft (415) to provide rotary motion output to a rear wheel (145) of said vehicle (100); and a single-speed gear-train mechanism (411 , 412) interposed between said crankshaft (407) and said output shaft (415), said single-speed gear-train mechanism (411, 412) configured to provide single speed rotary motion output at said engine sprocket (414);

wherein, an additional gearbox (200) comprising a secondary gear-train mechanism (416a, 417) is externally and detachably mounted on said IC engine (101), said secondary gear-train mechanism (416a, 417) is operably connected to said crankshaft (407) and said output shaft (415).

The vehicle (100) as claimed in claim 1 , wherein the said secondary gear-train mechanism (416a, 417) selectively combines with the single-speed gear-train mechanism (411 , 412) during a pre-determined operating range of the IC engine (101).

3. The vehicle (100) as claimed in claim 1, wherein the additional gearbox (200) comprising:

a primary shaft (416) operably connected to the crankshaft (407);

a secondary shaft (425) operably connected to the engine sprocket (414); said secondary gear-train mechanism (416a, 417) interposed between the primary shaft (416) and the secondary shaft (425); and

1 a second centrifugal clutch (420) configured to automatically switch between low-speed gear-train and high-speed gear-train, said second centrifugal clutch (420) disposed at-least on one of the shaft (416,425).

The vehicle (100) as claimed in claim 1 or claim 2, wherein said secondary gear-train mechanism (416a, 417) comprises a second driving gear (416a) and a second driven gear (417).

The vehicle (100) as claimed in claim 1 or claim 2 or claim 3, wherein the second centrifugal clutch (420) disposed on the primary shaft (416), and the second driving gear (416a) mounted freely on the primary shaft (416) and operably connected the primary shaft (416) through the second centrifugal clutch (420).

The vehicle (100) as claimed in claim 1 or claim 4, wherein the secondary driven gear (417) is rigidly mounted on the secondary shaft (425).

7. The vehicle (100) as claimed in claim 1 or claim 2 or claim 3, wherein the second centrifugal clutch (420) disposed on the secondary shaft (425), and the second driving gear (416a) mounted freely on the secondary shaft

(425) and operably connected the secondary shaft (425) through the second centrifugal clutch (420).

8. The vehicle (100) as claimed in claim 1 or claim 6, wherein the second driven gear (417) is rigidly mounted on the primary shaft (416).

9. The vehicle (100) as claimed in claim 1, wherein said single-speed gear- train mechanism (411, 412) comprises a first driving gear (411) and first driven gear (412).

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10. The vehicle (100) as claimed in claim 1, wherein the additional gearbox (200) components are enclosed in a housing (201) and a supportive housing (201a).

11. The vehicle (100) as claimed in claim 1, wherein the engine sprocket (414) comprises internal spines to receive the secondary shaft (425) and the output shaft (415) from either side.

12. The vehicle (100) as claimed in claim 1, wherein the crankshaft (407) is projecting from the clutch cover (210c) and is inserted within a gearbox opening and operably attached to the primary shaft (416).

13. The vehicle (100) as claimed in claim 1, wherein the engine sprocket (414) is securely mounted on the output shaft (415) and the output shaft (415) extends further to be coaxially inserted into another opening in the additional gearbox (200) to be attached to the secondary shaft (425).

14. The vehicle (100) as claimed in claim 1, wherein the second centrifugal clutch (420) is disposed on the primary shaft (416) and the second driving gear (416a) is freely mounted on the primary shaft (416) and connected to the primary shaft (416) through the second centrifugal clutch (420).

15. The vehicle (100) as claimed in claim 1 or claim 2, wherein the second centrifugal clutch (420) is freely mounted on the primary shaft (416) and the engine sprocket (414) is mounted on the output shaft (415) and the output shaft (415) extends further to be coaxially inserted into another opening in the additional gearbox (200) to be attached to the secondary shaft (425).

16. A method of power transmission in a power unit (101) of a vehicle (100) comprising: transmitting power from a crankshaft (407) of said power unit (101) to an output shaft (415), by a single-speed gear-train mechanism (411, 412) at a predetermined operating range of the IC engine (101) including low speeds and higher torque conditions; and

transmitting power from said crankshaft (407) of said power unit (101) to said output shaft (415), by a secondary gear-train mechanism (416a, 417) at various conditions of the pre-determined operating range of the power unit (101). 17. The method of power transmission in the power unit (101) of the vehicle

(100) as claimed in claim 15, wherein various conditions of the predetermined operating range of the power unit (101) includes the power unit (101) operating at high speed or low speed for low torque or high torque transmission to a rear wheel (145) respectively.

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