CN110573418A

CN110573418A - Transmission system for two-wheeled vehicle

Info

Publication number: CN110573418A
Application number: CN201880027045.4A
Authority: CN
Inventors: K·斯里尼瓦萨罗; T·布帕提拉吉
Original assignee: TVS Motor Co Ltd
Current assignee: TVS Motor Co Ltd
Priority date: 2017-04-03
Filing date: 2018-04-03
Publication date: 2019-12-13
Anticipated expiration: 2038-04-03
Also published as: CN110573418B; BR112019020759A2; WO2018185643A1

Abstract

the present subject matter discloses an Internal Combustion (IC) engine (101) for an efficient and automatic power transmitting vehicle (100). An Internal Combustion (IC) engine (101) includes a single-speed gear train mechanism (411, 412) and a removable additional gearbox (200). The additional gearbox (200) includes a secondary gear train mechanism (416a, 417) that is selectively coupled to the single-speed gear train mechanism (411, 412) within a predetermined operating range of the IC engine (101). The system automatically changes torque through an additional gearbox (200) based on the speed of an Internal Combustion (IC) engine (101). Furthermore, the invention provides for easy maintenance and disassembly. Finally, the performance of the IC engine (101) is improved by the present subject matter.

Description

Transmission system for two-wheeled vehicle

Technical Field

The present invention generally relates to a transmission system for a saddle type vehicle. More particularly, the present invention relates to a housing for enclosing additional drive train components in a saddle type vehicle.

background

saddle-ride type vehicles, such as two-wheeled vehicles, are powered by Internal Combustion (IC) engines or electric motors or hybrid systems, using gasoline as a fuel for the IC engine and energy storage devices such as batteries or fuel cells. Many two-wheeled vehicles, such as mopeds, scooters and other small cars, operate on a single speed drive system in which the crankshaft of the IC engine is directly connected to the wheels of the two-wheeled vehicle through a primary reduction gear train. Such two-wheeled vehicles have the common requirement of low cost, high efficiency, good controllability over the entire speed range. However, in such single speed transmissions, a compromise between torque demand and fuel economy is difficult. At higher torque demands, fuel economy is lower because the transmission system operates in a single speed gear ratio, and vice versa. The key issues involved in the design of the transmission system are to consider improving efficiency, better operability and reducing transmission losses, while at the same time retaining their attractive properties of low cost and ease of driving. Automatic and manual transmission systems implemented in such two-wheeled vehicles (such as mopeds) are known in the art. It is difficult to incorporate an automatic transmission system in the known layout of the IC engine of such vehicles, taking into account the changes to be made to accommodate additional transmission components such as clutches, gear trains and one-way clutches. The size of the power train and its weight can be adversely affected, so that it is almost impossible to package it in a saddle type vehicle. Accordingly, to alleviate the above-mentioned disadvantages, the present subject matter proposes an additional compact gearbox system that is disposed adjacent to the IC engine and houses additional components to enable conversion of a single-speed drive system to a multi-speed drive system.

Drawings

The detailed description is made with reference to the accompanying drawings. Throughout the drawings, the same reference numerals are used to refer to like features and components.

FIG. 1 illustrates a side view of a two-wheeled vehicle employing an embodiment of the present subject matter.

Fig. 2a and 2b show side and top views of an internal combustion engine with an additional gearbox according to an embodiment of the present subject matter.

FIG. 3 illustrates an exploded view of an additional gearbox operably mounted to an internal combustion engine, according to an embodiment of the present subject matter.

FIG. 4 illustrates a cross-sectional view (X-X) of an internal combustion engine and additional gearbox according to an embodiment of the present subject matter.

Fig. 5a shows a cross-sectional view (X-X) of an IC engine according to a second embodiment of the present subject matter.

Fig. 5b shows a cross-sectional view (X-X) of an IC engine according to a third embodiment of the present subject matter.

Fig. 6a shows a cross-sectional view (X-X) of an IC engine according to a fourth embodiment of the present subject matter.

Fig. 6b shows a cross-sectional view (X-X) of an IC engine according to a fifth embodiment of the present subject matter.

Detailed Description

Various features and embodiments of the invention will be apparent from the further description that follows. In the exemplary embodiment that follows, the vehicle is a two-wheeled vehicle with a cross-frame, commonly referred to as a moped. However, it is contemplated that the present disclosure may be applied to any two-wheeled vehicle having a single-speed drive system without departing from the spirit of the present subject matter. A detailed description of the structure of the portions other than those constituting the main portion of the present invention is omitted where appropriate.

In general, in a two-wheeled vehicle having a cross-type single-tube frame structure called a scooter, a cross space is provided. The cross-space has a floor extending on either side in the transverse direction and can be used to carry loads or to place a rider on a foot. In such vehicles, the frame structure starts from the head tube and extends down to the cross-over space and then rises again to form the rider seat, passes through the rear seat and ends at the tail lights. The fuel tank may be mounted on the front side of the vehicle at a height higher than the step space. The IC engine is located below the cross-over space and on the front side of the vehicle to form a low swing angle engine type. The IC engine is of a horizontal type, i.e., the 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 the rear wheels of the vehicle by a suitable drive system, such as by a sprocket and chain drive, to provide forward motion to the vehicle. Typically, the frame assembly acts as a framework for a vehicle that supports the vehicle load.

the transmission system for such two-wheeled vehicles (such as mopeds) comprises a single-speed transmission system. Such two-wheeled vehicles have the common requirement of low cost, high efficiency, good controllability over the entire speed range. Typically, the transmission system includes a single gear stage, a centrifugal clutch, and a driven shaft having a sprocket at its end through which the final drive to the rear wheels is connected. The centrifugal clutch ensures that at low to idle speed, the power transmission from the IC engine is decoupled from the rear wheels. The final drive to the rear wheel of a two wheeled vehicle is typically a positive drive, such as a sprocket and chain arrangement. In general, in a portable two-wheeled vehicle, a kick start mechanism or the like is used to start an IC engine. The foot pedal activation mechanism includes various components such as a foot pedal lever, various gears, a return spring, and a ratchet gear.

Conventionally, in a lightweight two-wheeled vehicle, there is a problem of low torque at low speed. For example, when a two-wheeled vehicle climbs a slope on a road or pulls heavy objects, a large amount of torque needs to be applied at the rear wheels to pull the vehicle, and the transmission system may not be able to provide the large torque. Furthermore, moving at low speed with less torque results in wasted fuel. Furthermore, at higher speeds, a fixed gear ratio can limit the travel speed of the vehicle and result in wasted fuel. Thus, the driveline may not provide sufficient torque and the internal combustion engine may be shut down. Therefore, an efficient multi-speed transmission system is needed.

the design of an IC engine and associated components disposed therein is critical because it is designed to optimize engine layout to make it less bulky and easy to assemble. Implementing a multi-speed drive system in known arrangements involves a large number of design and layout changes, which are not only difficult, but also cumbersome and difficult to access. A multi-speed automatic transmission is required because it affects the mechanical efficiency, fuel consumption and cost of the vehicle. It can meet various vehicle requirements and can be operated smoothly. In this regard, many multi-speed drive mechanisms are known in the art. Such systems include manual two-speed reduction systems with fixed ratios, or automatic two-speed transmission systems.

Conventional transmission systems, such as two-speed automatic transmission systems, have drawbacks in that additional components are introduced, thereby causing layout limitations in existing designs. The introduction of new transmission stages in existing single speed IC engines requires extensive modification of the IC engine layout and extensive research and development and significant investment to design new IC engines with two speed transmission systems. This also greatly increases the cost of the two-wheeled vehicle. Furthermore, variations in the layout of the IC engine can affect its footprint in a two-wheeled vehicle, thus requiring a complete redesign of the frame assembly to support the IC engine and change its position. Accordingly, there is a need to design a two-speed transmission system with minimal and/or minimal variation of the IC engine layout to address all of the above-mentioned problems.

It is therefore a primary object of the present invention to provide an automatic transmission system for a two-wheeled vehicle that will automatically vary torque with minimal modification to the existing layout of the IC engine based on a predetermined operating range of the IC engine. In the present embodiment, the predetermined operating range of the IC engine is the speed of the internal combustion engine. The transmission system should be able to meet a wide variety of vehicle requirements through basic design methodology while retaining many of the critical transmission components.

It is another object of the present invention to provide ease of maintenance of the transmission system.

It is another object of the present invention to provide for easy removal of the transmission system from the main IC engine when a two-speed transmission system is not required.

By the above design modifications, advantages such as automatic transmissions providing different accelerations without manually switching gears, minimal changes to existing layout, reduced fuel consumption and efficiency improvements, and better optimization and minimal changes needed to accommodate a starting system (such as a foot-start system) can be obtained. In addition, the automatic transmission system can be optimized under conditions of low speed, heavy load, or inclined surfaces to improve fuel efficiency.

The invention, as well as all the appended embodiments and other advantages thereof, will be described in more detail in the following paragraphs with reference to the accompanying drawings.

FIG. 1 illustrates a left side view of an exemplary two-wheeled straddle-type vehicle in accordance with embodiments of the present invention. The vehicle (100) has a single tubular frame assembly (105) extending from the front (F) to the rear (R) on a longitudinal axis (F-R) of the two-wheeled vehicle (100), the single tubular frame assembly (105) serving as a load-bearing skeleton. The frame assembly (105) extends from a head pipe (108) in a front portion (F) of the vehicle to a rear portion (R) of the vehicle. A steering shaft (not shown) is inserted through the head pipe (108), and a handle assembly (115) is pivotably disposed on the steering shaft. The steering shaft is connected to the front wheels (110) by one or more front suspensions (120). The front fender (125) is disposed above the front wheel (110) to cover at least a portion of the front wheel (110). The fuel tank (130) is mounted on the lower portion of the frame assembly (105), and is disposed on the front portion (F). The frame assembly (105) forms a substantially horizontal cross-section (106) with a floor to enable a rider to cross over the boarding and to assist in carrying loads. The power unit (101) is mounted on the frame assembly (105), below the cross-over section (106), forming a low swing angle engine mount. In the present embodiment, the power unit (101) is an IC engine (101). However, in other possible embodiments, the power unit (101) includes an electric machine or a combination of an electric machine and an IC engine. In the present embodiment, the piston axis of the IC engine (101) is horizontal, i.e., parallel to the longitudinal axis of the vehicle (100). A swing arm (140) is swingably connected to the frame assembly (105). The rear wheel (145) is rotatably supported by the swing arm (140). One or more rear suspensions (150) connect the swing arms (140) at an angle to maintain radial and axial forces due to wheel reactions 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 cross portion for seating a rider. In one embodiment, the seat assembly (160) includes a rider seat (160A) and a rear seat (160B). Further, a seat assembly (160) is positioned above the rear wheel (145). The vehicle is supported by a central bracket (170) mounted to the frame assembly (105). The IC engine (101) is connected to the rear wheel (145) by a transmission, such as in this embodiment sprockets linked to each other by a chain drive (301-see fig. 3).

Fig. 2a shows a side view of the IC engine (101) and fig. 2b shows a top view of the IC engine (101) and the additional gearbox (200). An IC engine (101) is composed of a cylinder head (203), a cylinder block (204), a crankcase (210), and a cylinder head cover (202). The crankcase (210) is composed of a right crankcase (210a), a left crankcase (210b), and a clutch cover (210 c). A clutch cover (210c) is disposed on the left side of the IC engine (101), adjacent to the left crankcase (210b), and encloses a first centrifugal clutch (421) and associated gear mechanism. An additional gearbox (200) is disposed adjacent to and mounted on the clutch cover (210 c). The clutch cover (210c) and the additional gearbox (200) include a plurality of bosses (see 212) spaced around their periphery and the bosses include holes for receiving fasteners. The additional gear case (200) is mounted on the clutch cover (210c) and fixed by fasteners mounted on the plurality of bosses (212). The boss (see 212) has a sufficient length to accommodate the engine sprocket (414) in the space between the additional gearbox (200) and the clutch cover (210 c). An Internal Combustion (IC) engine (101) includes an intake system (205, 206), an exhaust system (not shown), and a kick starter system (not shown) using a kick shaft (409). The right crankcase (210a) encloses a wet magneto assembly (406) disposed on the right side of the crankshaft (407 a). The wet magneto assembly (406) is configured to rotate with the crankshaft (407) and generate electrical power that charges a battery (not shown). Further, a centrifugal fan (405) is arranged 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 the cylinder block (204). A centrifugal fan (405) rotates with the crankshaft and draws atmospheric air into the interior and circulates it inside the shroud.

Fig. 3 shows an exploded view of an additional gearbox (200) operably mounted to the IC engine (101) according to an embodiment of the present subject matter. An engine sprocket (414) is disposed outside the left clutch housing (210c), the engine sprocket (414) receiving rotational motion from an output shaft (415) of the drive train. As highlighted, the chain (301) connects the engine sprocket (414) to a corresponding sprocket (302) on the rear wheel (145). In this way, rotational motion is transmitted to the rear wheels (145). The engine sprocket (414) includes an outer gear tooth portion and an inner hollow portion including internal splines and configured to receive a layshaft (425) of the additional gearbox (200). A portion of the layshaft (425) protrudes outward and includes external splines corresponding to internal splines on the engine sprocket (414). Similarly, an output shaft (415) of a driveline of the IC engine (101) includes external splines. Thus, the engine sprocket (414) receives the output shaft (415) from the right side and the countershaft (425) from the left side. The clutch cover (210c) further includes an opening (210d), the opening (210d) configured to receive a main shaft (416) of the additional gearbox (200). A main shaft (416) projects outwardly from the extra gearbox (200) and is configured to be coaxially inserted into the opening (210d) and operatively engaged with a first centrifugal clutch (421) arranged within the IC engine (101).

Fig. 4 shows a cross-sectional view (X-X) of an internal combustion engine (101) and an exploded view of an additional gearbox (200) according to an embodiment of the present subject matter. The IC engine includes a reciprocating piston (401) reciprocating in a cylinder block (204) and a rotatable crankshaft (407). Combustion occurs as the air fuel mixture combusts in the combustion chamber (422), which transmits pressure generated during combustion to the reciprocating piston (401). The reciprocating motion of the piston is converted into the rotational motion of a crankshaft (407) by a connecting rod (403) through a slider crank mechanism. The rotational motion of the crankshaft (407) is transmitted to the engine sprocket (414) through a transmission system. The crank housing (210) houses a kick start assembly (409, 410, 423) and includes a kick shaft (409) connected to a ratchet mechanism (423). When the pedal shaft (409) is actuated by the rider's foot, the ratchet moves and engages another ratchet that is operably connected to the crankshaft (407) via the starter gear train (404). Upon withdrawal of pressure from the foot, the return spring (410) will withdraw the foot shaft (409) to the starting position.

The transmission system comprises a first centrifugal clutch (421) arranged on the left side of the crankshaft (407 b). The first drive gear (411) is freely mounted to the crankshaft (407) b, and is connected to the crankshaft through a first centrifugal clutch 421. The first centrifugal clutch (421) is designed to engage only at a specific rotational speed of the IC engine. This speed is typically just above the idle speed that is configured and designed for the IC engine. Therefore, if the throttle is not operated by the rider, the rotation speed of the IC engine (101) is reduced to the idling speed, and the transmission system is disengaged from the crankshaft (407). Once the throttle valve is operated, the first centrifugal clutch (421) is engaged and rotational motion is transmitted to the rest of the drive train. Once the vehicle speed is above the idle speed and the first centrifugal clutch (421) is engaged, the rotational motion is transmitted to the first drive gear (411). The first drive gear (411) meshes with the first driven gear (412), and the gear ratio between the first drive gear (411) and the first driven gear (412) provides a first reduction for the IC engine (101). The first driven gear (412) is disposed on the output shaft (415) and mounted on bearings. An engine sprocket (414) is arranged at one end of the output shaft (415) protruding from the clutch cover (210 c). The combination of the first driving gear (411) and the first driven gear (412) is referred to as a single-speed gear train mechanism (411, 412). The provision of the single-speed gear train mechanism (411, 412) provides a first gear ratio operable within a predetermined operating range of the IC engine (101), wherein the predetermined operating range includes low speed and high torque conditions. The first driven gear (412) is mounted on the one-way clutch (413) to prevent the rotational driving force from being transmitted from the output shaft (415) back to the crankshaft (407 b).

The additional gear case (200) is arranged beyond the clutch cover (210 c). The additional gearbox (200) provides a second gear ratio engaged at low torque and high speed conditions. The additional gearbox (200) includes a primary shaft (416) configured to receive rotational motion from the left-side crankshaft (407b) and a secondary shaft (425) connected to the engine sprocket (414) and configured to transmit rotational motion to the engine sprocket (414) under conditions in which the second speed drive system is operable. The additional gearbox (200) further comprises a secondary gear train mechanism (416a, 417), the secondary gear train mechanism (416a, 417) comprising a second drive 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 is operatively connected to the crankshaft (407) and the output shaft (415). The second centrifugal clutch (420) may be arranged on the primary shaft (416) or the secondary shaft (425) based on the demand for the IC engine (101), optimizing space and achieving good aesthetics. With the second centrifugal clutch (420) arranged on the main shaft (416), the second drive gear (416a) is freely mounted on the main shaft (416) and operatively connected to the main shaft (416) by the second centrifugal clutch (420). Thus, the secondary driven gear (417) is rigidly mounted on the countershaft (425). With the second centrifugal clutch (420) disposed on the countershaft (425), the second drive gear (416a) is freely mounted on the countershaft (425) and operatively connects the countershaft (425) through the second centrifugal clutch (420). Thus, the second driven gear (417) is rigidly mounted on the main shaft (416). The entire additional gearbox (200) components are enclosed in a housing (201) and a supporting housing (201 a). The main shaft (416) is supported on either side by main shaft bearings (418a and 418 b). Similarly, the secondary shaft (425) is supported by secondary shaft bearings (419a and 419b) on either side of the housing (201 and 201 a).

An automatic transmission system for a vehicle (100) is provided that will automatically change torque with minimal modification to the existing layout of an IC engine (101) based on a predetermined operating range of the IC engine (101). In the present embodiment, the predetermined operating range of the IC engine (101) is the speed of the internal combustion engine (101). There are various possible situations for the speed of the internal combustion engine (101), of which little consideration is given. Consider a case where a vehicle (100) is operating at a low speed. In this case, the first centrifugal clutch (421) is engaged and the second centrifugal clutch (420) is disengaged. In this case, the gear ratio between the first driving gear (411) and the first driven gear (412) is higher than that in the high-speed transmission of the additional gear box (200). Rotational motion is transmitted from the crankshaft (407b) to a first centrifugal clutch (421) in which the clutch shoes are centrifugally spread and engaged with the outer hub. The outer hub is connected to a first drive gear (411) that meshes with a first driven gear (412), and is connected to an output shaft (415) through a one-way clutch (413). The one-way clutch (413) allows transmission of rotational motion from only the first driven gear (412) to the output shaft (415) and slips when the relative motion of the output shaft exceeds the rotational motion of the first driven gear (412). Since the main shaft (416) of the additional gearbox (200) is operatively connected to the crankshaft (407b), the main shaft (416) also rotates with the crankshaft (407b) and transmits the rotational motion to a second drive gear (416a) integrally attached to the main shaft (416). The second drive gear (416a) meshes with the second driven gear (417). Since the second driven gear (417) is freely mounted on the layshaft (425) and integrally attached to the second centrifugal clutch (420), no further transmission is allowed. Thus, the rear wheels (145) operate at a higher torque because the transmission occurs through a low speed transmission. Consider another case where the vehicle (100) is operating at high speed. In this case, both the first centrifugal clutch (421) and the second centrifugal clutch (420) are engaged. In this case, motion from the second driven gear (417) is transmitted to the second centrifugal clutch (420), which in turn, the second centrifugal clutch (420) transmits it to the engine sprocket (414), providing a high speed drive. The one-way clutch (413) prevents relative movement between the crankshaft (407) and the output shaft (415). Therefore, the gear ratio between the second drive gear (416a) and the second driven gear (417) is lower than that in the underdrive path. Because the transmission occurs through a high speed transmission, the rear wheels (145) operate at a lower torque.

Fig. 5a shows a second embodiment of the present subject matter. In this embodiment, a crankshaft (407) protrudes from the clutch cover 210c and is inserted within the gearbox opening and operably attached to the main shaft (416). Fig. 5b shows a third embodiment, where the engine sprocket (414) is firmly mounted on the output shaft (415), and the output shaft (415) is further extended to be coaxially inserted into another opening of the additional gearbox (200) to be attached to the layshaft (425).

Fig. 6a shows a fourth embodiment of the present subject matter. In this embodiment, a second centrifugal clutch (420) is arranged on the main shaft (416), and a second drive gear (416a) is freely mounted on the main shaft (416) and connected with the main shaft (416) through the second centrifugal clutch (420). In this embodiment, the engine sprocket (414) includes internal ratchet teeth to receive the layshaft (425) and the output shaft (415) from either side. Fig. 6b shows a fifth embodiment of the present subject matter, where 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) is further extended to be coaxially inserted into another opening of the additional gearbox (200) to be attached to the secondary shaft (425).

many modifications and variations of the present subject matter are possible in light of the above disclosure. Therefore, within the scope of the claims of the present subject matter, the disclosure may be practiced other than as specifically described.

Claims

1. A vehicle (100) comprising:

An Internal Combustion (IC) engine (101), the IC engine (101) comprising:

A crankshaft (407) configured to receive rotational motion as an air and fuel mixture within the IC engine (101) combusts;

An output shaft (415) configured to receive rotational motion from the crankshaft (407);

an engine sprocket (414) disposed at one end of the output shaft (415) to provide a rotational motion output to a rear wheel (145) of the vehicle (100); and

A single speed gear train mechanism (411, 412) interposed between the crankshaft (407) and the output shaft (415), the single speed gear train mechanism (411, 412) configured to provide a single speed rotational motion output at the engine sprocket (414);

Wherein an additional gearbox (200) comprising a secondary gear train mechanism (416a, 417) is externally and removably mounted on the IC engine (101), the secondary gear train mechanism (416a, 417) being operatively connected to the crankshaft (407) and the output shaft (415).

2. the vehicle (100) of claim 1, wherein the secondary gear train mechanism (416a, 417) is selectively coupled with the single-speed gear train mechanism (411, 412) during a predetermined operating range of the IC engine (101).

3. The vehicle (100) according to claim 1, wherein the additional gearbox (200) comprises:

A main shaft (416) operatively connected to the crankshaft (407);

A countershaft (425) operatively connected to the engine sprocket (414);

The secondary gear train mechanism (416a, 417) is interposed between the primary shaft (416) and the secondary shaft (425); and

A second centrifugal clutch (420) configured to automatically switch between high speed gear trains in a low speed gear train, the second centrifugal clutch (420) being disposed on at least one of the shafts (416, 425).

4. The vehicle (100) according to claim 1 or 2, wherein the secondary gear train mechanism (416a, 417) comprises a second driving gear (416a) and a second driven gear (417).

5. A vehicle (100) according to claim 1 or 2 or 3, wherein the second centrifugal clutch (420) is arranged on the main shaft (416) and the second driving gear (416a) is freely mounted on the main shaft (416) and operatively connected to the main shaft (416) by the second centrifugal clutch (420).

6. a vehicle (100) according to claim 1 or 4, wherein the secondary driven gearwheel (417) is rigidly mounted on the countershaft (425).

7. A vehicle (100) according to claim 1 or 2 or 3, wherein the second centrifugal clutch (420) is arranged on the layshaft (425) and the second driving gear (416a) is freely mounted on the layshaft (425) and operatively connects the layshaft (425) through the second centrifugal clutch (420).

8. Vehicle (100) according to claim 1 or 6, wherein the second driven gear (417) is rigidly mounted on the main shaft (416).

9. The vehicle (100) of claim 1, wherein the single-speed gear train mechanism (411, 412) comprises a first drive gear (411) and a first driven gear (412).

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

11. the vehicle (100) of claim 1, wherein the engine sprocket (414) includes internal ratchet teeth to receive the layshaft (425) and the output shaft (415) from either side.

12. The vehicle (100) of claim 1, wherein the crankshaft (407) protrudes from the clutch cover (210c) and is inserted within a gearbox opening and operably attached to the main shaft (416).

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

14. The vehicle (100) of claim 1, wherein the second centrifugal clutch (420) is disposed on the main shaft (416), and the second drive gear (416a) is freely mounted on the main shaft (416) and connected to the main shaft (416) by the second centrifugal clutch (420).

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

16. a method of performing a power transmission in a power unit (101) of a vehicle (100), comprising:

Transmitting power from a crankshaft (407) of the power unit (101) to an output shaft (415) through a single-speed gear train mechanism (411, 412) within a predetermined operating range of the IC engine (101), the predetermined operating range including conditions of low speed and high torque; and

Transmitting power from the crankshaft (407) of the power unit (101) to the output shaft (415) through a secondary gear train mechanism (416a, 417) under various conditions of the predetermined operating range of the power unit (101).

17. A method of performing power transmission in a power unit (101) of a vehicle (100) according to claim 15, wherein the various conditions of the predetermined operating range of the power unit (101) include: the power unit (101) operates at high speed or low speed to transmit low torque or high torque, respectively, to the rear wheels (145).