DECOMPRESSION DEVICE

Abstract

 A camshaft (2) comprising a decompression device (1) to reduce pressure in a cylinder when an engine starts, the camshaft (2) having first and second lobes (14, 15) for actuating cylinder valves (62). The decompression device (1) has a shaft (10) located axially within the camshaft (2), and rotatable relative to the camshaft (2) to extend or retract a first and a second lifter (12, 13) radially. The first and second lifters (12, 13) are respectively provided inwardly of lobe faces of the first and second lobes (14, 15) on the camshaft (2) in a radial direction of the camshaft (2) such that the first lifter and the second lifter (12, 13) can extend outwardly or retract inwardly in the radial direction from the lobe faces. The shaft (10) has first and second eccentric portions (10A, 10B) located in transverse slots (12B, 13B) in the first and second lifters (12, 13) to extend or retract the first and second lifters (12, 13) respectively. The second lifter (13) has a longitudinal cut-out (13C) extending to the transverse slot (13B) of the second lifter (13) to allow the second eccentric portion (10B) to enter the transverse slot (13B).

Description

Field of the Invention

[0001] This invention relates to a compression release system. Particularly, the invention relates to a decompressor system on an internal combustion engine.

Background to the Invention

[0002] Decompression devices are used on internal combustion engines to release in-cylinder pressure at low engine rpm such that starting the engine is made easier. This is usually achieved by a small opening of the exhaust valve when the engine is turning-over below its idle speed. This opening of the valve opens the cylinder to the atmosphere which reduces the pressure in the cylinder and thus reduces the starting torque of the engine. The compression release mechanism works to ease the starting of the engine by allowing it to accelerate to starting speed whilst reducing the work required to overcome the pumping action of the pistons in the cylinder. Once ignition occurs, the engine speed increases above a predetermined value and the decompressor no longer opens the exhaust valve.

[0003] The opening of the valve can be achieved by a local increase in the base circle height of the exhaust valve camshaft lobe, via a pin or similar lifter piece. The pin is provided in a bore in the camshaft lobe and when the speed of the engine is below a predetermined number of revolutions per minute the pin extends above the face of the camshaft lobe. The pin is commonly actuated by a bob-weight mounted to one end of the camshaft. The weight is pivoted such that it will open or close the lifter pin according to a predetermined engine speed. When the speed of the engine is above the predetermined engine speed, the centripetal force throws the bob-weight outwards such that the pin retracts below the face of the camshaft lobe.

[0004] The decompression device comprises a variety of parts and consequently increases the size of the cylinder head portion of the engine and also restricts the positions where components of the engine can be located.

[0005] The present invention has therefore been devised with the foregoing in mind. The invention seeks to overcome or ameliorate at least one of the disadvantages of the prior art, or provide a useful alternative.

Summary of the Invention

[0006] According to a first aspect of the present invention there is provided a camshaft comprising a decompression device for reducing pressure in a cylinder when an engine starts. The camshaft has at least one lobe for actuating a cylinder valve. The decompression device comprises a shaft located axially within the camshaft, and rotatable relative to the camshaft to extend or retract a lifter radially. The lifter is provided inwardly of the lobe face on the camshaft in a radial direction of the camshaft such that the lifter can extend outwardly or retract inwardly in the radial direction from the lobe face. The shaft has an eccentric portion, axially extending from an end of the shaft nearest the lifter. The eccentric portion locates in a transverse slot in the lifter such that the lifter is extended or retracted due to the eccentric rotation of the eccentric portion. This has the advantage of retracting and extending the lifter inside and outside of the cam face. Also, a reduced number of components of the shaft and lifter arrangement is required, there are reduced tolerances and the decompression device is easier to manufacture and assemble than other known devices.

[0007] There is a second lifter provided inwardly of a second lobe in a radial direction of the camshaft, wherein the shaft may have a second eccentric portion, axially extending from the opposing side of the shaft with respect to the other eccentric portion, wherein the second eccentric portion may locate in a transverse slot in the second lifter such that the second lifter is extended or retracted due to the eccentric rotation of the second eccentric portion. This gives the advantage of allowing pressure in another cylinder to be relived. Also, the activation of the lifters in both lobes rather than a more common method of only operating a single valve from a single lobe gives greater decompression in the cylinders of the engine and improved starting capability. This also means a smaller battery for starting the engine can be packaged saving further mass and cost.

[0008] The second lifter comprises a longitudinal slot for allowing the second eccentric portion of the shaft to enter the transverse slot. This has the advantage of allowing the second lifter to be fitted into the camshaft which holds the shaft and lifter arrangement together. The lifters are held within the cam face by the eccentric portions of the shaft and not allowed to extend too far outside of the cam face.

[0009] The camshaft may further comprise a flange and the decompression device may further comprise: a bias member for biasing a portion of a weight member toward the axis of the camshaft; the weight member pivotably connected to the camshaft for actuating a drive pin; the drive pin connecting the weight member and a shaft such that the weight member rotates the shaft; wherein the weight member and the drive pin are located on the camshaft between the flange and the lobe. Preferably the camshaft is supported for rotation at or near each end of the camshaft, the at least one lobe and the camshaft flange being disposed along the camshaft between the ends. In use, when the speed of the engine is below a predetermined rpm the lifters are radially extended, the valves are opened slightly and the cylinder pressure is relieved. This means it is easier for the pistons to be moved in the cylinders to reach the rpm of the engine where the engine has sufficient momentum to start through ignition. When the speed of the engine is above a predetermined rpm the lifters are radially retracted and the valves do not have the additional slight opening to relieve the cylinder pressure as it is not required.

[0010] The weight member and drive pin may be located between the first and second lobes. This has the advantage of simultaneously actuating both lifters in operation. This central mounting with subsequent dual activation gives a reduction in number of parts, mass of the engine and cost.

[0011] The flange may be located between the first and second lobes. This has the advantage of giving a reduction in engine width when compared to end mounted decompression devices and gives improved spark plug packaging/orientation.

[0012] There may be a camshaft sprocket, through which a rotational drive is provided to the camshaft, located between the flange and at least one lobe. This has the advantage of allowing adjustment of the cam timing without affecting the decompression function.

[0013] The weight member may be located between the camshaft sprocket and at least one lobe.

[0014] The sprocket may be located between the first and second lobes.

[0015] The weight member may be mounted at a pivot point to the flange. This has the advantage of allowing rotation of the weight member.

[0016] The camshaft sprocket may be mounted to the flange. This has the advantage of rotating the camshaft and the weight member with the sprocket.

[0017] The bias member may comprise a coil spring placed between the weight member and a connection pin mounted to the flange.

[0018] The pivot point of the weight member and an actuation end of the weight member may be on opposite sides of the camshaft axis.

[0019] According to a second aspect of the present invention there is provided a method of assembly of a shaft and lifters in a camshaft comprising: inserting the lifter into the lobe, inserting the shaft into the camshaft, rotating the shaft such that the eccentric portion of the shaft is inserted into the transverse slot of the lifter, inserting the second lifter into the second lobe, rotating the shaft such that the second eccentric portion of the shaft passes through the longitudinal slot and enters the transverse slot of the second lifter, rotating the shaft such that a drive pin can be inserted into the shaft, inserting the drive pin into the shaft. This has the advantage of assembling the shaft and lifter arrangement in such a way that the lifters are held in the camshaft by the shaft and the shaft, in turn is held in the camshaft by the lifters. This avoids the need for additional components which reduces cost of material and manufacture/assembly time.

[0020] According to a third aspect of the present invention there is provided a method of decompression for reducing pressure in a cylinder when an engine starts according to claim 13.

[0021] The method may be further comprising: biasing a portion of a weight member toward the centre axis of the camshaft, rotating the weight member located between a flange and the lobe on the camshaft by centripetal force, rotating a drive pin located between the flange and the lobe on the camshaft in contact with the weight member, and rotating the shaft with the drive pin.

Brief Description of the invention

[0022] An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a cross sectional view of a camshaft having a decompression device with a weight member in its outward position in accordance with an embodiment of the present invention;

Figure 2 shows a cross sectional view of a camshaft having a decompression device with a weight member in its inward position of the embodiment of Figure 1;

Figure 3 shows a perspective side view of the embodiment of Figure 1 with a weight member in its outward position;

Figure 4 shows a perspective side view of the embodiment of Figure 1 with a weight member in its inward position;

Figure 5 shows an axial cross sectional view of a pivot pin in a camshaft of the embodiment of Figure 1;

Figure 6 shows a perspective side view of a camshaft of the embodiment of Figure 1;

Figure 7 shows a radial cross sectional view of a weight member, in its outward position, of the embodiment of Figure 1;

Figure 8 shows a radial cross sectional view of a weight member, in its inward position, of the embodiment of Figure 1;

Figure 9 shows a perspective side view of the decompression device according to the embodiment of Figure 1 with rocker arm assemblies and exhaust valves;

Figure 10 shows a perspective view of a first lifter of the embodiment of Figure 1;

Figure 11 shows a perspective view of a second lifter of the embodiment of Figure 1;

Figure 12 shows a radial cross sectional view of a shaft and first lifter extending outside a cam of the embodiment of Figure 1;

Figure 13 shows a radial cross sectional view of a shaft and first lifter retracted within a cam of the embodiment of Figure 1;

Figure 14 shows a radial cross sectional view of a shaft and second lifter extending outside a cam of the embodiment of Figure 1;

Figure 15 shows a radial cross sectional view of a shaft and second lifter retracted within a cam of the embodiment of Figure 1;

Figure 16 shows a side view of a shaft and second lifter extending outside a cam of the embodiment of Figure 1;

Figure 17 shows a side view of a shaft and second lifter retracted within a cam of the embodiment of Figure 1;

Figure 18 shows a side view of a camshaft and a shaft of the embodiment of Figure 1;

Figure 19 shows a side view of a shaft and second lifter inserted within a cam of the embodiment of Figure 1;

Figure 20 shows a perspective side view of a shaft and second lifter inserted within a cam of the embodiment of Figure 1.

Description of the embodiments of the invention

[0023] With reference to Figure 1, there is provided a decompression device 1 incorporated in a camshaft 2 of an overhead cam type engine. The decompression device 1 comprises a weight member 4, a bias member 6 (i.e. a spring), a drive pin 8, a shaft 10 and lifters 12, 13.

[0024] The camshaft 2 extends axially between two camshaft ends 2A, 2B, and is supported for rotation about the camshaft axis. Two camshaft exhaust valve lobes (exhaust cams) 14, 15, two camshaft intake valve lobes (intake cams) 16, 17, and a camshaft flange 18 are located along the camshaft between the two ends 2A, 2B. A camshaft sprocket 20 abuts and affixes via bolts 19 to the camshaft flange 18. The sprocket 20 is a toothed wheel which is driven by a crankshaft (not shown) to rotate the camshaft 2. The flange 18 and the sprocket 20 are located substantially axially centrally in the camshaft 2 between the exhaust cams 14, 15. The shaft 10 is located concentrically with the camshaft 2 in a central bore 22 in the camshaft 2. The shaft 10 is substantially a cylindrical shaft extending between the exhaust cams 14, 15, where rotation of the shaft relative to the camshaft 2 causes lifters 12, 13, to be extended or retracted in a manner to be described below.

[0025] The shaft 10 has a hole 30 in an axially central portion that extends transversely through the shaft 10. The hole 30 is suitably sized for insertion of the drive pin 8 which is a cylindrical member longitudinally orientated perpendicularly to the axis of the shaft 10. The drive pin 8 is located in the shaft 10 and a first end of the drive pin extends radially outside the diameter of the shaft 10 in one direction through a recess 32 in the camshaft 2 to a distance greater than the diameter of the camshaft 2 main body. A second end of the drive pin 8 extends inside the shaft 10 to beyond the radial centre of the shaft 10 but does not extend through the shaft 10 fully. The drive pin 8 is a press fit into the shaft 10 at a controlled depth.

[0026] Figure 1 shows the lifter retracted and Figure 2 shows the lifter extended. The lifter 12 is positioned in the exhaust cam 14 in a radial bore 34 which extends from the face of the exhaust cam 14 through the central bore 22 in the camshaft 2 and into the exhaust cam 14 once more. There is a similar lifter 13 (not shown in Figure 2) in the exhaust cam 15. Although this embodiment shows the lifters 12, 13 are positioned in the exhaust cams 14, 15, in another embodiment a lifter 12 may be, instead, positioned in one or both of the intake cams 16, 17. In other embodiments, the camshaft 2 may have a single lobe, e.g. a single cylinder engine with a single valve. In other embodiments, the lifters may have a different structure. The structure of the shaft 10 and lifters 12, 13, will be described in more detail later.

[0027] The weight member 4 will now be described with reference to Figures 1 and 3. The weight member 4, or flyweight arm, is mounted adjacent, but not directly, to the sprocket 20 and has a pivot pin 42 that is mounted to the flange 18 and extends through a slot 43 in the sprocket 20. The slot 43 allows the camshaft 2 timing to be adjusted without affecting the position or timing of the decompression device 1. The weight member 4 is mounted in an axially central location on the camshaft 2, between the flange 18 and the exhaust cam 15. The sprocket 20 is located between the flange 18 and the weight member such that the weight member 4 is also located between the sprocket 20 and the exhaust cam 15. In another embodiment, the sprocket 20 may be on the opposite side of the flange 18. In another embodiment, gear teeth may be cut into the flange 18 and there would not be a separate sprocket 20. However, a separate sprocket 20 is preferred as it allows for angular adjustment of the sprocket 20 for adjustment of the camshaft 2 timing. With reference to Figure 5, the pivot pin 42 extends through a hole 44 in the flange 18 and is retained by an e-clip 45. The weight member 4 is partially enclosed in a sprocket recess 46 and has a radially extending portion 47 which contacts the inside lip 48 of the sprocket 20.

[0028] Referring to Figures 1 and 3 once more, the weight member 4 has a cut out section 49 where the bias member 6 connects to weight member 4 for clearance to the spring during operation and assembly. The weight member 4 has an outer periphery having a curvature radius which is less than a curvature radius of an outer periphery of the sprocket 20 and extends to a radius less than the outer periphery to an actuator end 50, i.e. the other end of the weight member 4 from the pivot pin 42 end, where it comes into contact with the drive pin 8. The actuator end 50, and also the first end of the drive pin 8, are located on the opposite side to the pivot pin 42 with respect to the camshaft 2 axis. The weight member 4 may be sized or orientated differently in other embodiments of the invention.

[0029] Referring to Figure 3, the bias member 6 is connected at one end to the weight member 4 relatively close to the pivot pin 42, i.e. in the cut out section 49, and extends to connect at its other end to a connection pin 51. The connection pin 51 is located at a substantially similar diameter around the camshaft 2 axis as the pivot pin 42 and at a different circumferential location. The connection pin 51 is mounted to the flange 18 through another hole in the sprocket 20 and extends from the flange 18 with a parallel axis to the camshaft 2. The connection pin 51 is retained by an e-clip (not shown), in a similar manner to the pivot pin 42 which is shown in Figure 5. The bias member 6 may be located in and connected to different locations in other embodiments of the invention.

[0030] Referring now to Figure 6, the camshaft 2 is shown without the sprocket 20 or the other components of the decompression device 1. The recess 32 is shown formed into the body of the camshaft 2 to provide space for the drive pin 8 to both insert through the body of the camshaft 2 into the shaft 10 and to partially rotate around the central axis of the camshaft 2 (the actuation of the drive pin 8 will be described later). The recess 32 is a cast-in pocket and is formed when the camshaft 2 is formed - e.g. by casting. The camshaft 2 and recess 32 may also be formed by other methods, such as forging or machining.

[0031] When the engine is off or during starting the engine, the speed of the engine is below a predetermined number of revolutions per minute (rpm), the weight member 4 is biased by the bias member 6 so that the weight member 4 is relatively close to the camshaft 2 and in its most radially inward position - see Figure 2 and Figure 4. When the engine is running, the camshaft 2 is rotated due to the sprocket 20 being driven by its connection to the crankshaft which is in turn rotated by the movement of the pistons in the cylinders of the engine (not shown). As the camshaft 2 rotates, it, and the other components that rotate with the camshaft, become subject to centripetal forces and as the weight member 4 is pivotably attached to the camshaft 2 it is free to move within its restrictions. When the speed of the engine exceeds the predetermined number of rpm, the centripetal force of the weight member 4 will overcome the bias force of the bias member 6 holding the weight member 4 in its inward position and the weight member 4 will swing around the pivot point (i.e. the pivot pin 42) to its most radially outward position - see Figure 1 and Figure 3.

[0032] Figure 7 shows the weight member 4 in its most outward position with the radially extending portion 47 in contact with the sprocket lip 48 restricting further outward movement. Figure 8 shows the weight member 4 in its most inward position with an inner portion of the weight member 4 resting on the camshaft surface 2 restricting further inward movement. With reference to both Figures 7 and 8, the drive pin 8 is in contact with the weight member 4 but not actively fixed to it. The weight member 4 has a recess 52 facing the sprocket 20 and suitably sized to allow the drive pin 8 to freely rotate. When the weight member 4 moves outwards from its most inward position, one radially extending wall 54 of the recess 52 impacts a first side of the drive pin 8 and rotates the drive pin 8 in an anti-clockwise direction (as viewed in Figures 7 and 8). When the weight member 4 moves inwards from its most outward position, a second radially extending wall 56 of the recess 52 impacts a second side of the drive pin 8, and rotates the drive pin 8 in a clockwise direction (as viewed in Figures 7 and 8).

[0033] As described above, the weight member 4 is in contact with the drive pin 8 and the drive pin 8 can rotate partially around the axis of the camshaft 2 due to the space created by the recess 32 and because the shaft 10 is freely rotatable inside the camshaft 2. When the drive pin 8 rotates, so too does the shaft 10 due to the drive pin 8 being inserted into the hole 30 in the shaft 10. Thus, when the weight member 4 swings due to the speed of the engine reaching the predetermined rpm, the shaft 10 will be rotated in the same direction along with the drive pin 8. When the speed of the engine returns below the predetermined rpm, the bias force of the bias member will overcome the centripetal force of the weight member 4 and the weight member 4 will return to its inward position. Although in this embodiment a drive pin 8 is used, any member that is suitable to connect the weight member 4 to the shaft 10 and actuate the shaft 10 may be used.

[0034] When the weight member 4 is in its inward position the lifter 12 radially extends outside the body of the exhaust cam 14. When the engine speed is above the predetermined rpm and the weight member 4 is therefore in its outward position the shaft 10 is rotated and the lifter 12 is fully retracted within the body of the exhaust cam 14. The way the shaft 10 moves the lifters 12, 13, to radially extend and retract them is described later.

[0035] With reference to Figure 9, the faces of the exhaust cams 14, 15 are in contact with cam followers 58 of exhaust valve rocker arm assemblies 60 and the irregular shape of the exhaust cams 14, 15 activates the rocker arms 60 to open exhaust valves 62 in the cylinder (not shown). While this embodiment shows the rocker arms 60 actuating two exhaust valves 62 each, the rocker arms 60 may actuate only one exhaust valve 62 or more than two exhaust valves 62. Also in different embodiments, the rocker arms 60 may actuate intake valves. When the lifters 12, 13 are radially extended the rocker arms 60 are also activated at this point in the rotation of the camshaft 2 to allow the exhaust valves 62 to be opened slightly and therefore the cylinder pressure is relieved. When the lifters 12, 13 are retracted the rocker arms 60 are not activated at this point in the rotation of the camshaft 2 and thus the cylinder pressure is not reduced. Due to nature of the decompression device 1, as described above, the lifters 12, 13 only extend when the speed of the engine is below the predetermined rpm. The opening of the exhaust valves 62 to the atmosphere at low rpm of the engine reduces the pressure in the cylinders and makes it easier for the pistons (not shown) to be moved in the cylinders to reach the rpm of the engine where the engine has sufficient momentum to start through ignition, i.e. the starting torque of the engine is reduced. At this point the lifters 12, 13 are retracted, the ignition is engaged and the normal engine cycle begins without the additional pressure release of the lifters 12, 13.

[0036] The arrangement of the shaft 10 and lifters 12, 13, will now be described in more detail. With reference to Figures 1 and 2, the shaft 10 is generally cylindrical with two eccentric end portions, a first eccentric portion 10A, and a second eccentric portion 10B, one at each end, protruding axially from the main body of the shaft 10. The eccentric portions 10A, 10B, are generally cylindrical but, in other embodiments, the eccentric portions 10A, 10B, may be any suitable shape. The first eccentric portion 10A is associated with the first lifter 12 and the second eccentric portion 10B is associated with the second lifter 13. The operation of the lifters 12, 13, will be described below.

[0037] Referring to Figure 10, the first lifter 12 is generally cylindrical with one end having a flat face, with a bevelled edge, and the other end having a partially spherical face. There is a cutaway portion 12A on the cylindrical surface of the first lifter 12. There is a transverse slot 12B partway up the body of the first lifter 12, extending fully across the body of the first lifter 12.

[0038] Referring now to Figure 11, the second lifter 13 is also generally cylindrical with one end having a flat face, with a bevelled edge, and the other end having a partially spherical face. There is a cutaway portion 13A on the cylindrical surface of the second lifter 13. There is also a transverse slot 13B partway up the body of the second lifter 13, extending fully across the body of the second lifter 13. The second lifter 13 also has a cut-out 13C which extends longitudinally from the slot 13B to the flat face making an opening to the slot 13B from the flat face of the second lifter 13. The lower (inward) side of the cut-out 13C does not extend transversely fully across the body of the second lifter 13 and thus a portion 13D of the second lifter 13 extends between the flat face and the slot 13B.

[0039] Referring once more to Figure 1, the first eccentric portion 10A of shaft 10 is shown inserted into the slot 12B of the first lifter 12 and the second eccentric portion 10B of shaft 10 is shown inserted into the slot 13B of the second lifter 13. Figure 1 shows the lifter 12 in the retracted position. Although not shown in the Figures 1 and 2, the second lifter 13 extends and retracts in a corresponding manner to the first lifter 12. The operation of the lifters 12, 13, and the fitting of the shaft 10 and lifters 12, 13, into the camshaft 2 will be described below.

[0040] Figure 12 shows the first lifter 12 in more detail in a radial cross section of exhaust cam 14 as indicated in Figure 2 (i.e. viewed from the direction of the axial centre of the shaft 10). The first lifter 12 is shown extended in the exhaust cam 14, within the radial bore 34 which extends from the face of the exhaust cam 14 to the central bore 22 in the camshaft 2. The flat face of the first lifter 12 is at an inner end, with respect to the radial centre of the camshaft 2, and the partially spherical face of the first lifter 12 is at an outer end. The first eccentric portion 10A of the shaft 10 is shown located in the slot 12B, between surfaces of the slot 12B. This means that the lifter 12 cannot move fully out of the radial bore 34 as it is held in by a radially facing wall of the slot 12B. Figure 13 shows the lifter 12 in its retracted position as indicated in Figure 1.

[0041] Figure 14 shows the second lifter 13 in more detail in a radial cross section of exhaust cam 15 (viewed from the direction of the axial centre of the shaft 10). The second lifter 13 is shown extended in the exhaust cam 15, within the radial bore 34A which extends from the face of the exhaust cam 15 to the central bore 22 in the camshaft 2. The flat face of the second lifter 13 is at an inner end, with respect to the radial centre of the camshaft 2, and the partially spherical face of the second lifter 13 is at an outer end. The second eccentric portion 10B of the shaft 10 is shown located in the slot 13B, between a surface of the slot 13B and the portion 13D of the second lifter 13. This means the second lifter 13 cannot move fully out of the radial bore 34A as it is held in by the portion 13D. Figure 15 shows the second lifter 13 in its retracted position.

[0042] The operation of the decompression device is now described with reference to Figures 1, 2 and 12-15. The weight member 4 functions in the same manner as described above and moves the drive pin 8 to rotate the shaft in order to extend or retract the lifters 12, 13. When the speed of the engine exceeds the predetermined number of rpm, the centripetal force of the weight member 4 will overcome the bias force of the bias member 6 holding the weight member 4 in its inward position and the weight member 4 will swing around the pivot point (i.e. the pivot pin 42) to its most radially outward position - see Figure 1. The drive pin 8 is rotated in an anti-clockwise direction (as viewed from the end of the camshaft 2 nearest the second lifter 13, i.e. as viewed from the same direction as Figures 12 and 13) when the weight member 4 moves to its outward position. This rotates the drive shaft 10 in the same anti-clockwise direction and rotates eccentric portion 10A from an outer position, at the centre of the slot 13B (see Figure 12) to a more inner position, at a side of the slot 13B (see Figure 13). The movement of the lifter 12 is due to the eccentric rotation of the eccentric portion 10A. As the eccentric portion 10A is located in the slot 13B, the rotation of the eccentric portion 10A to a more inner position means that the lifter 12 is moved to a more inner position and thus retracted from the face of the exhaust cam 14.

[0043] The second lifter 13 is moved in a similar manner with the rotation of the shaft being clockwise, as viewed in Figures 14 and 15, when the weight member 4 moves to its most outward position. Therefore, the second eccentric portion 10B rotates clockwise from an outer position, at the centre of the slot 13B (see Figure 14) to a more inner position, at a side of the slot 15 (see Figure 15). This rotation of the shaft 10 and subsequent movement of the second lifter 13 is also illustrated in Figures 16 and 17 (weight member 4 not shown). The movement of the lifter 12 is due to the eccentric rotation of the eccentric portion 10B. Figure 16 shows the second lifter 13 extended with the drive pin 8 in position after the weight member 4 has moved to its inward position while Figure 17 shows the second lifter 13 after being retracted by the rotation of the drive pin 8 and corresponding rotation of the second eccentric portion 10B.

[0044] When the speed of the engine drops below the predetermined number of rpm, the weight member 4 moves to its inward position and the drive pin 8 rotates the shaft 10 in the opposite direction, i.e clockwise as viewed in Figures 12 and 13, which is anticlockwise in Figures 14 and 15. The eccentric portions 10A, 10B, in turn rotate in the opposite direction from before to extend the lifters 12, 13, outside the face of the exhaust cams 14, 15, as shown in Figures 12, 14 and 16.

[0045] A particular way to assemble the shaft 10 and the lifters 12, 13, in the camshaft 2 is necessary in order for the shaft 10 to provide the function of holding the lifters 12, 13, in place within the radial bores 34, 34A, and extending and retracting the lifters 12, 13, as required. Firstly, the first lifter 12 is placed into the radial bore 34 and the shaft 10 is then inserted into the bore 22 of the camshaft 2 from the opposite axial end from the location of the lifter 12. The cutaway portion 12A of the first lifter 12 allows the main body of the shaft 10 to extend into the radial bore 34. Once the first eccentric portion 10A is inserted into slot 12B, which may require rotation of the shaft 10, the shaft 10 is situated at the second lifter 13 end of the camshaft 2 as shown in Figure 18. The second lifter 13 is then inserted into the radial bore 34A as shown in Figures 19 and 20. The second eccentric portion 10B must fit into the cut-out 13C and then enter slot 13B of the second lifter 13. In other words, the cut-out 13C of the second lifter 13 must slide past the second eccentric portion 10B and this is only possible by having the shaft 10 rotated such that the eccentric portion 10B is at a location to enable this to occur. The cutaway portion 13A allows the second lifter 13 to slide past the main body of the shaft 10 that extends into the radial bore 34A. Once the second lifter 13 is in position, this being known from the second lifter 13 being below the level of the face of the cam 15, the shaft 10 can be rotated (clockwise as viewed in Figures 14 and 15) such that it is in the position shown in Figure 16. This is the position of the shaft 10 when the second lifter 13 is extended and the weight member 4 would be in its inward position. Once the shaft 10 is in this position, the drive pin 8 can be inserted and thereafter the shaft 10 cannot be rotated back to the position to enable the second lifter 13 to be removed as the drive pin 8 would come into contact with the body of the camshaft defining the recess 32. Therefore, both the lifters 12, 13, are held in position within the camshaft 2, the shaft 10 can rotate using the drive pin 8, and the lifters 12, 13, can perform their function when the weight member 4 moves as described above.

[0046] It will be appreciated by persons skilled in the art that various modifications may be made to the above embodiment without departing from the scope of the present invention as defined by the claims. For example, whilst the above discussion has been concerned with a two cylinder engine, the invention is equally applicable to engines with one cylinder or more than two cylinders.

Claims

1. A camshaft (2) comprising a decompression device (1) for reducing pressure in a cylinder when an engine starts, the camshaft (2) having first and second lobes (14, 15) for actuating respective cylinder valves (62), the decompression device (1) comprising:

a shaft (10) located axially within the camshaft (2), and rotatable relative to the camshaft (2) to extend or retract a first lifter (12) and a second lifter (13) radially;

wherein the first lifter (12) and the second lifter (13) are provided inwardly of the lobe face of the first lobe (14) and the second lobe (15) respectively on the camshaft (2) in a radial direction of the camshaft (2) such that the first lifter (12) and the second lifter (13) can extend outwardly or retract inwardly in the radial direction from the respective lobe faces;

wherein the shaft (10) has a first eccentric portion (10A), axially extending from an end of the shaft (10) nearest the first lifter (12), and a second eccentric portion (10B), axially extending from the opposing end of the shaft (10) with respect to the first eccentric portion (10A);

wherein the first eccentric portion (10A) locates in a transverse slot (12B) in the first lifter (12) and the second eccentric portion (10B) locates in a transverse slot (13B) in the second lifter (13) such that the first lifter (12) and the second lifter (13) are extended or retracted due to the eccentric rotation of the first eccentric portion (10A) and the second eccentric portion (10B) respectively, and

wherein the second lifter (13) comprises a longitudinal cut-out (13C) extending from an inward end of the second lifter (13) to the transverse slot (13B) of the second lifter (13) for allowing the second eccentric portion (10B) of the shaft (10) to enter the transverse slot (13B) of the second lifter (13).

2. The camshaft (2) according to any preceding claim, the camshaft (2) further comprising a flange (18);
the decompression device (1) further comprising:

a bias member (6) for biasing a portion of a weight member (4) toward the axis of the camshaft (2);

the weight member (4) pivotably connected to the camshaft (2) for actuating a drive pin (8);

the drive pin (8) connecting the weight member (4) and the shaft (10) such that the weight member (4) rotates the shaft (10);

wherein the weight member (4) and the drive pin (8) are located on the camshaft (2) between the flange (18) and the lobe (14, 15).

3. The camshaft (2) according to claim 2, wherein the weight member (4) and drive pin (8) are located between the first and second lobes (14, 15).

4. The camshaft (2) according to claims 2 or 3, wherein the flange (18) is located between the first and second lobes (14, 15).

5. The camshaft (2) according to any one of claims 2-4, wherein a camshaft sprocket (20), through which a rotational drive is provided to the camshaft (2), is located between the flange (18) and at least one lobe (14, 15).

6. The camshaft (2) according to claim 5, wherein the weight member (4) is located between the camshaft sprocket (20) and at least one lobe (14, 15).

7. The camshaft (2) according to claim 5 or 6, wherein the sprocket (20) is located between the first and second lobes (14, 15).

8. The camshaft (2) according to any one of claims 2-7, wherein the weight member (4) is mounted at a pivot point to the flange (18).

9. The camshaft (2) according to any one of claims 5 to 8, wherein the camshaft sprocket (20) is mounted to the flange (18).

10. The camshaft (2) according to any one of claims 2-9, wherein the bias member (6) comprises a coil spring placed between the weight member (4) and a connection pin (51) mounted to the flange (10).

11. The camshaft (2) according to any one of claims 8 to 10, wherein the pivot point of the weight member (4) and an actuation end of the weight member (4) are on opposite sides of the camshaft (2) axis.

12. A method of assembly of a shaft (10) and lifters (12, 13) in a camshaft (2) according to any preceding claim, comprising:

inserting the first lifter (12) into the first lobe (14),

inserting the shaft (10) into the camshaft (2),

rotating the shaft (10) such that the eccentric portion (10A) of the shaft (10) is inserted into the transverse slot (12B) of the first lifter (12),

inserting the second lifter (13) into the second lobe (15),

rotating the shaft (10) such that the second eccentric portion (10B) of the shaft (10) passes through the longitudinal cut-out (13C) and enters the transverse slot (13B) of the second lifter (13),

rotating the shaft (10) such that a drive pin (8) can be inserted into the shaft (10), and

inserting the drive pin (8) into the shaft (10).

13. A method of decompression for reducing pressure in a cylinder when an engine starts, the engine comprising a shaft (10) located axially within a camshaft (2), the method comprising:

rotating the shaft (10) relative to the camshaft (2) to actuate a first lifter (12) and a second lifter (13) provided inwardly of respective lobe faces on the camshaft (2) in a radial direction of the camshaft (2) to extend and retract in the radial direction from the lobe faces due to the eccentric rotation of a first eccentric portion (10A) and a second eccentric portion (10B) of the shaft (10) located in respective transverse slots (12B, 13B) in the first lifter (12) and the second lifter (13), wherein the second lifter (13) comprises a longitudinal cut-out (13C) extending from an inward end of the second lifter (13) to the transverse slot (13B) of the second lifter (13) for allowing the second eccentric portion (10B) of the shaft (10) to enter the transverse slot (13B) of the second lifter (13).

14. A method of decompression for reducing pressure in a cylinder when the engine starts according to claim 13, further comprising:

biasing a portion of a weight member (4) toward the centre axis of the camshaft (2);

rotating the weight member (4) located between a flange (18) and the lobe (14, 15) on the camshaft (2) by centripetal force;

rotating a drive pin (8) located between the flange (18) and the lobe (14, 15) on the camshaft in contact with the weight member (4); and

rotating the shaft (10) with the drive pin (8).

Drawing