DECOMPRESSION DEVICE FOR AN INTERNAL COMBUSTION

ENGINE

FIELD OF THE INVENTION

The present invention relates to decompression device for an internal combustion engine and more particularly to an auto decompression system for a four stroke internal combustion engine.

BACKGROUND OF THE INVENTION

An internal combustion engine is usually provided with a cylinder head assembly containing valves to control the in and out movements of charge and exhaust gases in the cylinder relative to the piston. As shown in Figure 1, the engine body includes a crankcase 2, a cylinder block i coupled to the crankcase 2, a cylinder head 3 coupled to the upper part of the cylinder block 1 and a head cover 4 coupled to the upper part of the cylinder head 3. A piston 5 slidably fitted in the cylinder block 1 is connected via a connecting rod 6 with a crankshaft 7. The crankshaft 7 is rotatably supported by the crankcase 2. The crankcase 2 is constructed by connecting a left crankcase and a right crankcase together.

The induction and exhaust processes in such engine are controlled by a mechanical system known as a valve train responsible for operation of the valves. Typically such valve train has at least two valves, an intake valve 16 and an exhaust valve 17 generally inclined relative to the cylinder axis.

An intake port 13 and an exhaust port 14 formed in the cylinder head 3 communicate with the combustion chamber 12 formed by being surrounded by the cylinder bore, the cylinder head 3 and the piston 5. The intake valve 16 is provided at the combustion chamber side opening of the intake port 13 and the exhaust valve 17 is provided at the combustion chamber side opening of the exhaust port 14. A camshaft CS is rotatably supported by the cylinder head 3 so as to open/close the intake valve 16 and the exhaust valve 17. Rotational power is transmitted from the crankshaft 7 to the camshaft CS by the valve train. The intake 16 and exhaust valves 17 are operated via rocker arms (25 & 26) connected to the camshaft CS.

Conventionally during cranking, the piston 5 in the cylinder 1 moves toward a combustion chamber 12 and has to overcome the gaseous pressure in the cylinder 1 as the intake and exhaust valves are closed. Manual cranking through a starter lever requires more effort by the operator of the motorcycle and is burdensome. Hence, to momentarily release the pressure during cranking, many auto decompression devices or mechanisms based on centrifugal effect have been proposed to relieve compression for starting an engine.

As shown in Figure 2 and 3, generally a decompression device disposed between a first cam lobe 101 and a bearing 103 comprises of an arm 105 held by a spring member 107 and rotatable about a pivot pin 106, an arm stopper and a cam pin 109 driven by the arm 105. The spring member 107 is further held by a spring groove 108. Arm opens due to centrifugal force and returns to initial position by spring force. The cam pin 109 lifts the cam follower which in turn lifts an intake valve or an exhaust valve momentarily by a fixed value to place the valve at an open position in an initial stage of the compression stroke of the engine consequently reducing the amount of charge in cylinder. The decompression mechanism keeps such valve at the open position and releases the valve from the open position when the rotational speed of the camshaft has reached a preset speed. Moreover for a given decompression system, the lift of the cam is fixed by the difference between base circle diameter and cam pin height in assembled condition.

Further, a decompression unit is generally mounted on a camshaft CS through a holder 104, the diameter of which is more than a first cam lobe 101 in base circle position. But for engines having small capacity, there is a considerable space constraint for mounting the holder of auto decompression unit as it consumes additional space and increases the length of the valve operating system.

Furthermore in the extremely open condition of the arm, the arm profile should be less than the cam lobe profile so as not to interfere with the functioning of other valve train components. Therefore, the main objective of the present invention is to provide a decompression device or unit that address all or any of the above problems and obviate the lacunae in the prior art.

SUMMARY OF THE INVENTION

It is therefore one of the objects of the present invention to improve the engine indicated above by incorporating an auto decompression device having a compact structure and can be disposed even in a small space.

Another object of the present invention is to provide a motorcycle engine equipped with an auto decompression device which is light in weight, compact in size and reliable in performance in starting the engine while requiring no more space than necessary for mounting it and further to reduce the length of the valve operating system so as to ensure a compact assembly.

To this end, the present invention discloses an improved decompression device operational during a compression stroke of an internal combustion engine comprising an arm operational by the effect of centrifugal force, an arm stopper to limit the spread of said arm, a spring member to bring the arm back to original position, a pivot pin about which the said arm rotates and a cam pin operated by the said arm through a connecting member, characterized in that the said decompression device is disposed between a cam lobe and a bearing on a camshaft in such a way that one end of the said pivot pin is ensconced in the said cam lobe, the other end of the said pivot pin is in contact with bearing circumference, the said arm is held by the said cam lobe through the said pivot pin and the arm rotates about the pivot pin, and wherein the profile of the said arm is such that the locus of periphery of arm during all operating conditions is within the envelope formed by the cam profile offset.

The foregoing objectives and summary provide only a brief introduction to the present subject matter. To fully appreciate these and other objects of the present subject matter as well as the subject matter itself, all of which will become apparent to those skilled in the art, the ensuing detailed description of the invention and the claims should be read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features, aspects, and advantages of the subject matter will be better understood with regard to the following description, appended claims and accompanying drawings where:

Figure 1 shows a typical sectional view of an internal combustion engine comprising of a valve train.

Figure 2 shows a perspective view of an auto decompression unit according to the prior art.

Figure 3 shows a front view of the auto decompression unit according to the prior art.

Figure 4 shows a perspective view of an auto decompression unit according to an embodiment of the present invention.

Figure 5 shows an exploded view of the auto decompression unit according to an embodiment of the present invention.

Figure 6 depicts a portion of the auto decompression unit according to an embodiment of present invention.

Figure 7 shows a sectional perspective view of a cylinder head incorporating an auto decompression unit mounted on a camshaft according to an embodiment of the present invention.

Figure 8 shows a perspective view of cylinder head with the decompression unit enclosed in the cylinder head according to an embodiment of the present invention.

Figure 9 illustrates a portion of the decompression unit mounted on the camshaft.

Figure 10 shows the interaction of the decompression unit with a rocker arm of a valve train.

Figure 11 shows the auto decompression unit in working condition wherein the arm is closed.

Figure 12 shows the auto decompression unit in non-working condition wherein the arm is open.

DETAILED DESCRIPTION OF THE INVENTION

The subject matter described herein relates to an improved auto decompression device for a valve train of an internal combustion engine used during compression stroke of the engine cycle to improve the device assembly on a camshaft and save space and to ease out the cranking effort during starting the engine.

Various other features and embodiments of the improved auto decompression unit system according to the present invention here will be discernible from the following further description thereof, set out hereunder. It is to be noted that the terms 'decompression device', 'decompression unit' and 'auto decompression device' are intended to mean the same. In the ensuing exemplary embodiments, the engine wherever cited is a four stroke overhead cam type single cylinder internal combustion engine. However it is contemplated that the disclosure in the present invention may be applied to any engine capable of accommodating the present subject matter without defeating the spirit of the invention. It is also to be noted that the present invention is not dependent on the type of a valve and therefore is equally workable with an intake or an exhaust valve.

The novelty of the present invention lies in the construction and assembly of an auto decompression device which is mounted on directly on the camshaft and support of the cam lobe thereby eliminating the need of a holder. Thus the decompression system present here do not have a holder. The functions of a holder are achieved jointly in arm and cam lobe. Additionally the construction of the arm profile is such that it works within an operational zone less than the diameter of the cam roller pin boss.

The improved auto decompression device is now described in detail in connection with the rendered figures 4-12. The detailed explanation of the constitution of parts other than the invention which constitutes an essential part has been omitted at suitable places.

Figure 4-6 show a decompression device according to the present invention. It is mounted on a camshaft CS containing at least two cams, the cams further comprising a first cam lobe 101 and a second cam lobe 102. The decompression device comprises an arm 205, an arm stopper 211 to limit the spread of said arm 205, a spring member 207 to bring the arm 205 back to original position, a pivot pin 206 about which the arm 205 rotates and a cam pin 109 operated by the arm 205 through a connecting member 110. The arm 205 thus has two ends, a cam pin end and a pivot pin end. The decompression device is disposed between the first cam lobe 101 and a bearing 103. The arm 205 is directly fitted into cam lobe side 101 (peak lift zone) of first cam through the pivot pin. The profile of the arm 205 is such that it does not interfere with the normal actuation of rocker arm and only provides the valve lift during cranking of the engine.

The pivot pin 206 has two ends, wherein one of the ends is held onto a groove provided in the first cam lobe 101 and the other end faces the bearing 103, with the body of the pivot pin 206 traversing through the arm 205 and the spring member 207. The pivot pin 206 is press fitted in the groove of the first cam lobe 101 and the arm 205 is free to rotate with respect to pivot pin 206 in the radial direction.

According to an embodiment of the present invention, a portion of the pivot pin end of the arm 205 develops into the arm stopper 211 partially enveloping the camshaft CS in the radial direction. The arm stopper 211 limits the spread of the arm 205 during radial rotation of the arm under the effect of the centrifugal force. Further in an embodiment of the present invention, the spring member 207 is preferably a loaded torsion spring. The torsion spring brings the arm back into its original position after the engine stops.

As shown in Figure 6, the cam pin 109 is located in a cam pin slot 111 on the cam lobe radial surface. Further, one half of the radial surface of the cam pin 109 is spherical whereas the other half is flat. The cam pin end of the arm 205 operates the cam pin 109 through the connecting member 110. The cam pin end of the arm 205 has a profile so as to lock the connecting member 110 in it as shown in Figure 6 and therefore operate the cam pin 109 on its rotation. Axial movement of cam pin 109 is prevented by arm 205 owing to the construction. The clearances of cam pin and cam pin slot are such that the load due to cam lift directly acts on the camshaft.

According to an embodiment of the present invention, the fitment of arm 205 in first cam lobe 101 is pivoted in peak lift zone of cam lobe and actuation of cam pin 109 is achieved in base circle zone. This means that the arm of the decompression device is in contact with the cam at two points. The arm stopper end of the arm is pivoted through the pivot pin in the peak lift zone of the cam lobe whereas the cam pin end of the arm is connected to the cam lobe through the cam pin at base circle position.

Figure 7 illustrates a sectional perspective view of the cylinder head 3 incorporating the decompression device mounted on the camshaft CS according to an embodiment of the present invention. The bearing is removed so that the decompression device is visible. Figure 8 shows the perspective view of cylinder head 3 with the decompression device enclosed in the cylinder head 3. The decompression device construction leads to compact packaging of the device inside the cylinder head.

The working of the decompression device is now explained with the help of Figure 9-12. It is to be noted that the decompression device is operable only before the engine rpm reaches a preset speed and before the arm spreads in radial direction due to the effect of centrifugal force of the system. The default position (initial position) of the arm 205 of the decompression device is closed as shown in Figure 11 i.e. the decompression device is functional as soon as the engine is cranked manually or electrically. The final position of the arm 205 is open i.e. the decompression device is not functional when the arm is open as shown in Figure 12. In the initial position, the radial surface of the cam pin visible is spherical and hence raised than the radial surface of the first cam lobe. Therefore when the camshaft CS starts rotating during cranking in the compression stroke, the raised profile of the cam pin 109 comes in contact with the cam roller rolling over the first cam lobe surface due to which the raised profile lifts the roller and hence the exhaust valve is open.

As the camshaft CS speed increases, the arm 205 opens under the effect of centrifugal force by rotating about the pivot pin 206 and the spread of the arm is limited by arm stopper 211 against camshaft. Mass of the arm stopper is relatively small compared to arm itself. The rotation of the arm rotates the cam pin due to which the flat half of the radial surface of the cam pin is now on the first cam lobe profile. Therefore the cam roller now does not make contact with the cam pin surface and the normal functioning of valve lift through the first cam lobe itself resumes. Thus the decompression device (or the arm) is functional only within an operating range of the engine. After the engine stops, the arm goes back to initial position owing to spring force and is activated when the engine is cranked again.

There is one more aspect to the present invention. In the conventional decompression systems working based on centrifugal force mechanisms, when the arm is open, the maximum diameter of arm with respect to the camshaft centre is higher than base circle diameter by atleast -1.25 times due to which it occupies additional axial space between the bearing and cam lobe. In the current invention, the arm 205 is placed adjacent to the first cam lobe 101. For a given base circle diameter A and rocker arm roller pin support boss dimensions, there is maximum diameter B with respect to centre of camshaft CS whose difference is the offset X. The profile of arm 205 has been designed such that locus of the periphery of the arm during all operating conditions is within the envelope formed by cam lobe profile X offset. Thus, in all working conditions, the maximum radial spread of the arm will always be less than the envelope formed by the profile of the first cam lobe as shown in Figure 10. Hence no additional axial space is required for arm.

The present subject matter and its equivalent thereof offer many advantages, including those which have been described henceforth. The decompression device in the present invention is firstly compact enough to be accommodated between a bearing and a cam lobe without additional axial space. Secondly the space reduction leads to reduction in the length of the valve train which can now be easily accommodated in the cylinder head of a small capacity engine. Further the reduced length of valve train frees more space for mounting of a spark plug just outside the cylinder head. Thirdly the need of a holder for mounting the decompression device is eliminated which results in additional advantage of weight and cost reduction. Fourthly construction of the present decompression device is such that the operational zone of the arm is always less than the envelope formed by the cam profile.

The present invention is thus described. The description is not intended to be exhaustive nor is it intended to limit the invention to the precise form disclosed. It will be apparent to those skilled in the art that the disclosed embodiments may be modified in light of the above description. The embodiments described are chosen to provide an illustration of principles of the invention and its practical application to enable thereby one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore the forgoing description is to be considered exemplary, rather than limiting, and the true scope of the invention is that described in the appended claims.

claim:

1. An improved auto decompression device operational during a compression stroke of an internal combustion engine comprising an arm operational by the effect of centrifugal force, an arm stopper to limit the spread of said arm, a spring member to bring the arm back to original position, a pivot pin about which the said arm rotates and a cam pin operated by the said arm through a connecting member,

characterized in that the said decompression device is disposed between a cam lobe and a bearing on a camshaft in such a way that one end of the said pivot pin is ensconced in the said cam lobe,

the other end of the said pivot pin is in contact with bearing circumference, the said arm is held by the said cam lobe through the said pivot pin and the arm rotates about the pivot pin,

wherein one end of the said arm is fitted to a cam at base circle position whereas the other end is connected to the cam at peak lift position, and

wherein further the profile of the said arm is such that the locus of periphery of arm during all operating conditions is within the envelope formed by the cam profile offset.

2. The improved auto decompression device as claimed in claim 1 An improved auto decompression device as claimed in claim 1 wherein said spring member is held between said arm and said cam lobe.

3. The improved auto decompression device as claimed in claim 1 wherein said spring member is preferably a torsion spring.

4. The improved auto decompression device as claimed in claim 1 wherein said pivot pin traverses through said arm as well as the said cam lobe.

5. The improved auto decompression device as claimed in claim 1 wherein the maximum opening of the said arm is limited by said arm stopper.

6. The improved auto decompression device as claimed in claim 1 wherein the said arm works in an operating speed range of the engine.

7. An internal combustion engine equipped with the said auto decompression device as claimed in any of the preceding claims.

8. The improved auto decompression device substantially as herein described and illustrated with reference to the accompanying drawings.