Field of the invention:
The present invention relates to four stroke engine used for two wheeler, more particularly a timing chain arrangement using an idler gear sprocket assembly. The timing chain reduction from crankshaft to cam shaft is obtained in two stages. It makes the cam sprocket smaller to reduce the size of cylinder head in cam shaft chain pocket area.
Back ground of the invention:
In general four stroke engine used for two wheeler application, the cam shaft is driven by the crankshaft with the help of timing chain and two sprocket mounted on both the shaft. The cam shaft rotation speed is exactly half of crank shaft rotation speed as the valve operated by the cam shaft open or closed once in two rotation of crank shaft. So the size of cam shaft sprocket is two times the crankshaft sprocket size. The timing chain locus is symmetrical to the axis passing through camshaft and crankshaft center.

In this arrangement, there is restriction on the size of cylinder head cam shaft chain pocket area. Some times space becomes critical if vehicle is to be made compact as generally is the case with two wheeler scooter.
The common feature of geared four stroke engine is that the clutch is mounted on the main shaft and is driven by the engine gear. The engine gear also drives the oil pump gear and in some cases the clutch gear drives the oil pump gear. In this arrangement clutch is subjected to transmit higher torque then developed by engine at crankshaft by the magnitude of reduction ratio between crankshaft to main shaft which is in the range of 3 ~ 4 .
Prior Art
US patent 4,692,129 teaches, timing drive for driving an overhead mounted camshaft from the crankshaft of an internal combustion engine. The crankshaft has at least one main bearing surface formed on it for journaling the crankshaft for rotation. A sprocket is positioned adjacent the main bearing surface for driving the camshaft. The sprocket has a dedendum diameter smaller than the diameter of the main bearing surface.

Object of the invention:
The main object of the invention is to design a more compact cylinder head which is easier to accommodate in a limited space.
Another object of the invention is to provide a timing chain reduction from Crankshaft to cam shaft in two stages.
Summary of the invention:
According to the present invention , a four stroke engine comprising cam shaft (2) driven by a sprocket (4) , a sprocket (3), mounted on an idler gear (6) sprocket assembly (63) through timing change (5) mounted on the cam shaft (2), wherein the gear (6) of the sprocket assembly (63) driven by gear (9) is rigidly fixed to the crank shaft thus maintaining reduction ratio from crank shaft to cam shaft at 1:2.
According to another embodiment of the invention, the idler gear is adopted to derive the oil pump gear.

According to another embodiment of the invention, an idler gear pin is press fitted in bearing insert .
According to another embodiment of the invention, an idler gear pin and needle roller bearing are providing smooth movement of idler gear, sprocket and assembly.
According to another embodiment of the invention, a washer and circlip are provided to arrest the axial movement of the idler gear sprocket assembly.
Brief description of the drawings:
The invention will be described precisely with reference to the accompanying drawing , wherein
Fig 1 illustrates a detailed drawing of the four stroke engine system.
Fig 2 illustrates a systematic block diagram of the system.

Detailed description of the invention
In general four stroke engine used for two wheeler
application, the cam shaft is driven by the crankshaft with
the help of timing chain and two sprocket mounted on both
the shaft.
According to the present invention , as shown in FIG-1 the cam shaft (2) is driven by a sprocket (4) and sprocket (3) .
Sprocket (4) is mounted on cam shaft (2) and a sprocket (3) is mounted on idler sprocket assembly (63) through timing chain (5) .
Further, a gear (6) on idler sprocket assembly (63) is driven by a gear (9) rigidly fixed on crank shaft (1) along with the clutch assembly (7) . Thus the over all reduction ratios from crankshaft to cam shaft are maintained at 1:2.
According to the invention, the idler gear (6) is also used to drive the oil pump gear (8).

According to the invention as shown in fig 2, the crankshaft main bearing ( 25) is press fitted in a cast iron bearing insert (20 ) which is molded in aluminum crankcase (24). The cast iron bearing insert (20) has been modified to accommodate the idler gear pin (21) . The idler gear pin is press fitted in the bearing insert (20) . The idler gear (6) sprocket (3) assembly (63) is supported over idler gear pin (21) and needle roller bearing (22) is used for smooth movement. To idler gear sprocket assembly (63) axial movement is arrested with the use of washer (26) and circlip (23).
While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in art that various other changes and modifications can be made without departing from the sprit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are with in the scope of this invention.

We claim:
1. A four stroke engine comprising cam shaft driven by a sprocket (4) , a sprocket (3), mounted on an idler gear sprocket assembly (63) through timing change (5) mounted on the cam shaft (2), wherein the gear (6) of the sprocket assembly (63) driven by gear (9) is rigidly fixed to the crank shaft thus maintaining reduction ratio from crank shaft to cam shaft at 1:2.
2. A four stroke engine as claimed in claim 1, wherein the idler gear is adapted to derive the oil pump gear.
3. A four stroke engine as claimed in claim 1, wherein the idler gear pin is press fitted in cast iron bearing insert.
4. A four stroke engine as claimed in claim 3, wherein cast iron bearing insert is molded on aluminum crankcase.
5. A four stroke engine as claimed in claim 1, wherein the said idler gear pin and needle roller bearing are

providing smooth movement of ideal gear, sprocket and assembly.
6. A four stroke engine as claimed in claim 1, wherein a washer and circlip are provided to arrest the axial movement of the idler gear sprocket assembly.
7. A four stroke engine substantially as hereinbefore described with reference to the accompanying drawings.