Transmission system for a four cycle engine

Field of invention

The present invention relates to a "Transmission system for a four-cycle engine" and more particularly, to a use of dry centrifugal clutch layout for power transmission.

Background of the invention

There are a number of compact vehicles that need to have automatic or "single speed" transmission for ease of driving and for increased utility. These include mopeds, scooters and other small automobiles. These vehicles generally have common requirements for low cost, high efficiency, good controllability throughout the entire speed range. To meet these requirements these kinds of vehicles use centrifugal friction clutches to provide a releasable torsional connection between engine output and the associated transmission.

These centrifugal clutches engage at a predetermined rotational speed and are particularly useful in combination with internal combustion engines which tend to stall under starting load conditions, by delaying engagement until the engine is operating at a sufficiently high speed to produce enough torque to prevent stalling.

In recent years, regulations of exhaust emissions from internal combustion engine-mounted vehicles are becoming stringent in view of environment protection as a result of which the use of four-stroke engine is increasing. These four stroke engines generally have wet centrifugal clutch that utilizes the engine oil for lubrication and cooling.

However, these clutches are less efficient and also results in contamination of engine oil.

The above mentioned problems can be solved by providing an oil filter
provision but it makes system more complicated and expensive for such
vehicles.

Therefore, a principal object of this invention is to provide a dry centrifugal
clutch layout and further, in one embodiment the dry centrifugal clutch is
provided along with the cooling arrangement that accommodates the range of
vehicle needs with a basic design approach.

Brief description of the invention

Accordingly, the present invention provides a dry centrifugal clutch for a four¬cycle engine with single speed transmission. Said centrifugal friction clutches are particularly useful in connection with small displacement internal combustion engines such as are used in type of vehicles mentioned above, where any unnecessary size, weight and cost can be a disadvantage. In a preferred embodiment the present invention utilizes a crankcase that defines a wet chamber that houses the crankshaft along with the transmission system and a dry chamber for housing a centrifugal clutch that move into frictional engagement by application of centrifugal force further in an embodiment a cooling fan is provided such that it forces the ambient air through the said dry chamber housing the clutch, for cooling. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of

illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.

Brief description of drawings

Figure 1 illustrates a step through type two-wheeled vehicle using a single
stage reduction gear transmission and an embodiment of the present
invention.

Figure 2 illustrate a cross-sectional view of an engine used in vehicle shown %
in figure. 1.

Figure 3 illustrates in detail the layout of the dry clutch as per an embodiment
of the invention.

Detailed description of the preferred embodiment

Figure 1 shows a step-through type two-wheeled vehicle 01 having an engine 02 with dry centrifugal clutch layout of the present invention. As shown in Figure 2 the illustrated engine 02 is a four-stroke engine comprises a crankcase 03 and a cylinder 04 that is connected to the front end of the crankcase 03. In one configuration, the cylinder 04 preferably protrudes forward from a forward end of the crankcase 03 in a generally horizontal orientation such that it has an axis that extends in a longitudinal direction of the engine 02. Further, cylinder head 05 is secured to upper surface of the cylinder block. The crankshaft 06 housed in and supported by crankcase 03 is connected to a piston 07 positioned slide-ably within the cylinder bore of cylinder block 04 via connecting rod 08.

The engine has a kick-start provision

09 on the right side and a transmission system 10 on the left, connected to the crankshaft 06 through the said centrifugal clutch 11. In an embodiment of the invention the said crankcase 03 is formed by integrally joining right crankcase member 03A and a left crankcase member 03B. The left crankcase member 03B further comprises a partition wall 03C dividing the crankcase into a wet chamber 12 and a dry chamber 13. The said right crankcase member 03A and a left crankcase member 03B provides a right support bearing 14 and a left support bearing 15 respectively to support the crankshaft 06 rotatably at first and second support point spaced from each other in the wet chamber 12, in a manner to prevent radial motion of the crankshaft relative to the crankcase 03.

Towards the right crankcase member 03A in the wet chamber 12 a kick start system 09 and a magneto unit 16 is housed whereas, towards the left in the wet chamber a single gear reduction transmission 10 is housed, said transmission comprises a driven gear 17 for engaging with the output drive gear 18 of the clutch 11 and a sprocket 19 is secured to an output shaft 20 of the said transmission 10 to drive the rear wheel of the vehicle through a chain drive.

With reference to Figure 2 and 3, the centrifugal clutch 11 being a dry centrifugal plate type clutch is housed in the dry chamber 13. The said clutch comprises a generally cylindrical housing drum 21 of the clutch, adapted for transmission of torque from the crankshaft 06, when the clutch 11 is engaged. The cylindrical drum 21 is preferably coaxially aligned with and surrounds the clutch plates '22 and the pressure plates 23, with the drum interlocking with the clutch plates 22, the centrifugal clutch 11 includes a generally circular

ramp plate 24, having a central opening 24A allowing the crankshaft 06 to pass there through.

A drive hub 25 conforms to and fits over a crankshaft 06 that extends axially through the clutch 11. The drive hub 25 is mounted to the ramp plate 24 along with the spring ball type of centrifugal weights 26 surrounding the drive hub. The crankshaft 06 engages the drive hub 25, and input power is supplied to the clutch by the crankshaft 06. Said coaxially aligned clutch plates 22 and the pressure plates 23 follow the drive hub 24. The said pressure plates 23 have drive lug openings on the inside diameter for interconnection with the rotating drive hub 25 and the clutch plates 22 with drive lugs on the outside diameter for interconnection with the clutch drum 21 and finally a fixed plate 27 at the end bolted to the hub 25.

As the drive hub 25 rotates, the centrifugal weights 26, expands in radial direction and are pushed towards the said plates because of ramped profile of the ramp plate 24 that induces an axial force, that presses the pressure plates 23 and clutch plates 22 against the fixed plate 27. Once the drive hub 25 reaches a sufficient rotational speed, the force on the pressure plates 23 reaches a sufficient level to engage the frictional surfaces on the clutch plates 22. This causes the clutch plates 22 to rotate, which in turn causes the clutch drum 21 to rotate, transferring power to the transmission 10. In order to provide an output connector of the torque transmitted through the cylindrical housing drum 21 of the centrifugal clutch 11, a drive gear 18, ' having an extended stem 29 with a splined end' 30 is provided that passes through the partition wall 03C and engages with the corresponding groove 31 provided in an extended boss 31 at the back face of the cylindrical housing

drum 21 of the centrifugal clutch, and is thus conjoined with the back face 28 of the cylindrical housing drum 21. The extended stem 29 of the drive gear 18 is coaxially and rotatably supported by the crankshaft 06, that passes through the hollow cross-section of the extended stem 29. Thus, the drive gear 18 is disposed in the wet chamber 12 and engages with the driven gear 17 of the transmission system disposed inside the wet chamber 12 and the clutch 11 remains in the dry chamber 13.

The said partition wall 03C provides* an opening for the extended stem 29 to pass through. The peripheral boundary of the opening also provides radial clearance so that the extended stem 29 rotates freely. A first oil seal 32 is provided to prevent any leakage of oil from the wet chamber 12 to the dry chamber 13 through the radial clearance. Further, a second oil seal 33 is provided at the clearance between the crankshaft 06 and the extended stem 29 so as to prevent the oil does not leak into the dry chamber 13 and at the same time there is lubrication between the crankshaft 06 and the extended stem 29.

In another embodiment of the invention a cooling fan can also be provided in the dry chamber 13 in any appropriate orientation to force cooling air on the clutch assembly 11.
It should be understood that the foregoing description is that of a preferred embodiment of the invention and that various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.

Claims: We claim:

1. A motorcycle having a frame, a four-cycle engine mounted on the said
frame and at least two operational wheels operatively connected to
engine crankshaft through a transmission system comprising:
a centrifugal clutch coupling the said transmission system and the
crankshaft;
a crankcase defining a wet chamber for housing the said crankshaft
and the transmission system, a dry chamber for housing the said
centrifugal clutch assembly comprising a partition wall separating the
said wet chamber and dry chamber;
said centrifugal dutch assembly having an output member and an-input
member wherein, the said output member having an output shaft
extending into the wet chamber and having a drive gear fixed to the
shaft and disposed in the wet chamber;
said drive gear provides input to the transmission system;
said input member being connected to and driven by an end of the
crankshaft extending into the dry chamber;
said partition wall providing at least one pass through passage for said
input and output shaft with a radial clearance to allow free rotation of
the shafts;
a sealing means to prevent passage of oil from the wet chamber to dry
chamber through the said radial clearance; and
a cooling fan disposed in the dry chamber and adapted to force cooling
air through the said clutch assembly.

2. A motorcycle as claimed in claim 1, wherein the said transmission being a gear transmission, having a constant ratio and continuous power flow through the entire speed range of the vehicle.

3. A motorcycle as claimed in claim 1, wherein the said centrifugal clutch assembly is a dry plate type clutch assembly having frictional surfaces adapted to be engaged at a predetermined engagement speed of rotation of said crankshaft.

4. A motorcycle as claimed in claim 1, wherein the said centrifugal clutch assembly is a dry drum type clutch assembly having frictional surfaces adapted to be engaged at a predetermined engagement speed of rotation of said crankshaft.

5. A motorcycle as claimed in claim 1, wherein the crankshaft and the output member are coaxial.

6. A motorcycle having a frame, a four-cycle engine mounted on the said frame and at least two operational wheels operatively connected to engine crankshaft through a transmission system comprising:
a centrifugal clutch coupling the said transmission system and the
crankshaft;

a crankcase defining a wet chamber for housing the said crankshaft
and the transmission system, a dry chamber for housing the said
centrifugal clutch assembly comprising a partition wall separating the
said wet chamber and dry chamber;

said centrifugal clutch assembly having an output member and an input
member wherein, the said output member having an output shaft

extending into the wet chamber and having a drive gear fixed to the
shaft and disposed in the w et chamber;

said drive gear provides input to the transmission system;

said input member being connected to and driven by an end of the
crankshaft extending into the dry chamber;

said output shaft being provided with the hollow cross-section rotatably
and coaxially supported by said end of the crankshaft through a
bearing member wherein, the diameter of hollow cross section is
greater than the outer diameter of said end of the crankshaft to provide
radial clearance for accommodating the bearing member;

said output shaft being splined to the output member of the clutch
assembly;

a first oil sealing means provided, downstream of the said bearing
member towards the dry chamber to prevent passage of oil through the
said clearance to the dry chamber.

said partition wall providing a pass through passage for said output
shaft with a radial clearance to allow free rotation of the said output
shaft;
a second sealing means to prevent passage of oil from the wet
chamber to dry chamber through the said radial clearance;

a cooling fan disposed in the dry chamber and adapted to force cooling
air through the said clutch assembly.