Power cutter with controller responsive to lubrication status

A power cutter including a housing; a two stroke internal combustion engine mounted within the housing; a throttle switch by which the engine can be accelerated when the engine is running; a support arm mounted on the housing and which projects forward of the housing. A blade mounting mechanism is rotatably mounted on the end of the support arm and is capable of being rotationally driven by the engine when the engine is running. A carburetor provides aerated fuel for the engine and an air intake provides air for the carburetor. An air filtration mechanism filters the air drawn in from the air intake for the carburetor. A fuel tank provides fuel to the carburetor. An exhaust is provided through which the exhaust gases generated by the operation of the engine are expelled from the engine. The power cutter also includes an engine controller that controls the operation of the engine; an oil tank that provides lubricating oil for the engine; an oil pump that pumps lubricating oil from the oil tank to mix it with fuel; and a sensing system that determines whether sufficient lubricating oil is being provided for mixing with the fuel in accordance with predetermined parameters. The sensing system provides an indication to the engine controller of when the lubricating oil is not being provided in accordance with the predetermined parameters. When the sensing system indicates that the lubricating oil is not being provided in accordance with the predetermined parameters, the engine controller places the engine either in an idle mode or switches it off.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national stage filing under 35 U.S.C. §371 of PCT/EP2008/058719 filed Jul. 4, 2008, which claims priority to GB 0712928.1 filed Jul. 4, 2007, both of which are incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a power cutter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

A typical power cutter comprises a housing in which is mounted a two stroke internal combustion engine. Attached to the side of the housing is a support arm which extends forward of the housing. Rotatably mounted on the end of the support arm is a cutting blade, usually in the form of a grinding disk. The motor is drivingly connected to the cutting blade via a drive belt. The rotary output of the engine rotatingly drives the cutting blade via the drive belt. The drive belt is driven via a centrifugal clutch which enables the out drive spindle of the engine to disengage from the belt when the engine is running at a slow speed, to allow the engine to continue running, whilst disengaging any drive to the cutting blade to allow the blade to be stationary.

Also mounted in the housing is a fuel tank which provides fuel for the engine via a carburetor. An oil tank can also be provided, which provides lubricating oil to mix with the fuel, to lubricate the engine.

Mounted on the rear of the housing is a rear handle for supporting the power cutter, which contains a trigger switch for accelerating the engine upon depressing. Depression of the trigger switch causes more of the aerated fuel/oil mixture to be injected into the engine which in turn causes the speed of the engine to accelerate.

GB2232913 and WO2005/056225 show examples of such power cutters.

2. Description of the Related Art

A known problem with all two stokes engines is the lubrication of the crank shaft114(using the reference numbers shown inFIG. 19) and piston1000. This is due to the fact that the fuel/air mixture first passes through a chamber18in the cylinder120below the piston1000before being forced into the chamber122above the piston16before being ignited. This prevents the use of lubricating oil being pumped around the crank shaft114on the underside of the piston1000. Therefore, in existing designs of two stoke engines, a mixture of aerated fuel and lubricating oil is burnt within the engine, the oil providing lubrication for the crank shaft114and piston1000prior to being burnt with the fuel during the combustion cycle. A common problem with the use of two stoke engines is that the operator forgets to add the oil to the fuel or adds it in an inconsistent manner, thus resulting in damage to the engine. This particularly so for power cutters as often power cutters are often hired out to operators. As such, operators are not familiar with using them; they are not in the operator's possession for long; the operator does not know who had it previously or how the previous operator used the power cutter; and, as the operator does not own it, has less reason to care for the long term maintenance of the power cutter. Therefore, it is desirable that a mechanism is provided to prevent or limit damage to the two stroke engine of the power cutter to ensure that sufficient lubrication oil is included in the aerated fuel which prior to entering the engine.

It is possible to add lubricating oil directly to the fuel in the fuel tank. However, this relies on the operator ensuring that the correct ratio of lubricating oil to fuel is achieved. This can be difficult as it may be hard to determine how much fuel is already in the fuel tank and how much lubricating oil has already been added. Therefore, it is preferable to have a separate oil tank which is filled with lubricating oil and which is then pumped from the tank and mixed with the fuel. This enables the amount of oil be added to the fuel to be controlled more accurately.

The two stroke engines of power cutters use a carburetor to provide aerated fuel for powering the engine. A typical design of such a carburetor is shown inFIG. 31. Referring toFIG. 31, the carburetor includes a housing1002through which is formed an air passageway1004through which air can pass in the direction of Arrow Q. The air passageway1004at its entrance and exit has the same cross-sectional area. However, formed part way along the length of the air passageway1004, is a restriction1006which reduces the size of the cross sectional area of the air passageway and which acts as a venturi; the air passageway1004narrowing and then expanding as the air passes through it. This causes the rate of flow of air through the narrow section1008of the air passageway1004to increase.

Fuel enters the carburetor via an inlet1010and fills a first chamber1012. The first chamber1012is connected to a second chamber1014via a fuel passageway1016. Fuel fills the second chamber1014via the fuel passageway1016.

An adjustable needle valve1020, which has a pointed tip and an elongate body, is mounted in the fuel passageway1016and which can axially slide within the passage way1016. The fuel passageway1016includes a narrow section1018. The tip of the adjustable needle valve1020projects towards the narrow section1018and can block the narrow section1018when the adjustable needle valve1020is moved towards it or open the narrow section1018when it is moved away from it.

The rear end, remote from the tip, of the adjustable needle valve1020projects into the second chamber1014. Attached to the rear end of the adjustable needle valve1020is a first lever1022which connects to a second lever1024via a pivot point1026. One of the walls of the second chamber1014is a flexible diaphragm1032which can move to adjust the volume of the second chamber1014. A hollow chamber1034is formed on the other side of the diaphragm1032. The end of the second lever1024connects to the diaphragm1032. The second lever1024also connects to a solid wall1028of the second chamber1014via a spring1030. The spring1030biases the second lever to a predetermine position, which in turn biases the first lever1022to a predetermined angular position. Flexing of the diaphragm1032causes pivotal movement of the first and second levers against the biasing force of the spring1030. Movement of the first lever1022causes an axial sliding movement of the adjustable needle valve1020moving its tip towards or away from the narrow section1018.

A first passageway1036connects to the second chamber1014via a high speed needle valve1038. The other end of the first passageway1036connects with the narrow section1008of the air passageway1004. A second passageway1040connects to the second chamber1014via an idle needle valve1042. The other end of the second passageway1040connects via three small vents1044with the air passageway1004down stream of the narrow section1008. The high speed needle valve1038is preset and limits the rate of flow of fuel through the first passageway1036. The idle needle valve1042is preset and limits the rate of flow of fuel through the second passageway1040.

Located in the air passageway1004, ahead of the narrow section1008, is a first pivotal plate1046which acts as the choke for the carburetor. The plate1046can be pivoted between an open position (as shown) where it extends in the direction of the air passageway1004, allowing the maximum amount of air to enter the passage1004, to a closed position where it extends across the air passageway1004, substantially reducing the amount of air able enter the air passageway1004.

Located in the air passageway1004, downstream of the narrow section1008, is a second pivotal plate1048which acts as the throttle for the carburetor. The plate1048can be pivoted between an open position where it extends in the direction of the air passage1004, allowing the maximum amount of air to leave the air passageway1004, to a closed position where it extends across the air passageway1004, substantially reducing the amount of air able to leave the air passageway1004. The plate1048is shown half way between its open and closed positions.

When the carburetor is in normal use, the first pivotal plate1046is in its open position. Air is drawn through the air passageway passing through the narrow section1008which causes it to speed up. The movement of the air through the narrow section1008causes fuel to be drawn out of the first passageway1036into the air flow and then pass through the air passageway1004. The amount of air, and hence the amount of fuel drawn out of the first passageway1036, is dependent on the angular position of the second pivotal plate1048. When it is in its open position, the maximum amount of air is able to pass through the air passageway, drawing out the maximum amount of fuel from the first passageway1036. When it is in its closed position, the minimum amount of air is able to pass through the air passageway, drawing out the minimum amount of fuel from the first passageway1036. In order to ensure that sufficient fuel enters the air flow in the air passageway1004when the second pivotal plate1048is in its closed position, the second passageway1040also provides fuel to the air flow. However, the exit of the second passageway1040connects to the air passageway down stream of the second pivotal plate1048to ensure that there is always sufficient fuel entering the air flow.

As fuel is drawn out of the two passageways1036;1040, the amount of fuel in the second chamber1014reduces. When the amount of fuel reduces, the diaphragm1032flexes, to reduce the volume of the second chamber1014to accommodate the loss of fuel. As the diaphragm1032flexes, it moves the first and second pivotal levers1024;1022against the biasing force of the spring1030, which in turn axially slides the adjustable needle valve1020, moving its tip away from the narrow section1018, opening it up and allowing fuel to flow from the first chamber1012into the second chamber1014. As the second chamber1014fills up, the diaphragm1032flexes to accommodate the additional fuel, pivoting the levers and moving the tip of the adjustable needle valve1020towards the narrow section1018and reducing the amount of fuel flowing through the fuel passageway. Movement of the diaphragm1032ensures movement of the tip of the adjustable needle valve1020relative to the narrow section1018is controlled to limit the amount of fuel in the second chamber1014.

When the engine is cold, the first pivotal plate1046is placed in its closed position. This reduces the amount of air entering the air passageway1004and therefore provides a higher ratio of fuel to air in the air passageway to enable the cold engine to run.

The angular position of the first pivotal plate1046is set using a Bowden cable connected to a separate lever which is adjusted manually by the operator of the power cutter. The angular position of the second pivotal plate1048is set using a Bowden cable connected to a trigger switch1070mounted on the handle which is manually adjusted by the operator.

A problem with this design of carburetor on a power cutter is that the operator has to be constantly adjusting the angular position of the first pivotal plate when the engine is cold to ensure the smooth operation of the engine. This is particularly difficult if the operator is also trying to use the power cutter.

SUMMARY OF THE INVENTION

Accordingly, there is provided a power cutter including a housing; a two stroke internal combustion engine mounted within the housing; a throttle switch by which the engine can be accelerated when the engine is running; a support arm mounted on the housing and which projects forward of the housing; a blade mounting mechanism rotatably mounted on the end of the support arm and which is capable of being rotationally driven by the engine when the engine is running; a carburetor for providing aerated fuel for the engine; an air intake for providing air for the carburetor; an air filtration mechanism to filter the air drawn in from the air intake for the carburetor; a fuel tank for providing fuel to the carburetor; and an exhaust through which the exhaust gases generated by the operation of the engine are expelled from the engine; an engine controller which controls the operation of the engine; an oil tank for providing lubricating oil for the engine; an oil pump to pump lubricating oil from the oil tank to mix it with fuel; and a sensing system which determines whether sufficient lubricating oil is being provided for mixing with the fuel in accordance with predetermined parameters. The sensing system provides an indication to the engine controller of when the lubricating oil is not being provided in accordance with the predetermined parameters, the engine controller switches the engine into either an idle mode or switching the engine off when the sensing system indicates that the lubricating oil is not being provided in accordance with the predetermined parameters.

It has been found that a two stoke engine can run for a long period of time without any lubricating oil being added to the fuel without damage to the engine occurring so long as the engine is run at a slow speed and without being under substantial stress. This is achieved by placing the engine in idle mode i.e. with the engine running at its minimal speed. As most power cutters drive the cutting blade via a centrifugal clutch, the drive of the engine on these types of power cutters is also disengaged from the cutting blade when the engine is running at a slow speed, reducing the stress on the engine. As such, when insufficient oil is detected, the engine can be placed in an idle mode, instead of stopping it, without the risk of incurring damage to the engine. The operator of the power cutter will realize that there is insufficient oil by the fact the engine has been placed into idle mode. Alternatively, the engine can be stopped to ensure that no damage occurs to it.

It will be appreciated by the reader that the oil may be pumped by the oil pump into the fuel tank to mix with the fuel within the tank or into the passage way carrying the fuel from the fuel tank to the carburetor. The fuel and oil mixture would then pass through carburetor where the oil/fuel mixture becomes aerated. Alternatively, the oil may be pumped directly into the carburetor to mix it with the fuel as it is being aerated. However, ideally, there is provided a passageway between the carburetor and the engine through which the aerated fuel generated by the carburetor passes from the carburetor to the engine, the oil pump pumping the oil into the passageway to mix with the aerated fuel within the passageway. If so, preferably the lubricating oil is pumped into the passageway in a liquid form, a spray form or in an atomized form.

The oil can be pumped into the passageway at a ratio relating to volume of around 1:50 in relation to the amount fuel entering the passageway.

When the engine controller has placed the engine in idle mode because the sensing system has indicated that the lubricating oil is not being provided in accordance with the predetermined parameters, the engine controller can prevent an operator from accelerating the engine by the operation of the throttle switch. When the engine has been placed into idle mode, it will override the action of the throttle switch which by the operator will try to use to accelerate the engine. This will prevent the operator from damaging the engine whilst indicating to the operator that the oil supply for the engine needs checking.

When the engine controller has switched the engine off because the sensing system indicates that the lubricating oil is not being provided in accordance with the predetermined parameters, the engine controller can prevent an operator from starting the engine again until sufficient oil is detected. When the engine is switched off by the engine controller. Again, the operator of the power cutter will realize that there is insufficient oil by the fact the engine has been switch off and is prevented from being started again.

Ideally, the engine comprises at least one spark plug, a piston slidably mounted within a cylinder and which is connected to a rotatable crank shaft, the ignition of the spark plug being controlled by the engine controller, wherein there is provided a sensor connected to the engine which monitors the angular position of crank shaft and provides the engine controller with a position signal dependent of the crank shaft angular position, the engine controller using the position to control the ignition of the spark plug to switch the engine to an idle mode or to switch the engine off.

If so, the engine controller can alter the timing of the ignition of the spark plug relative to the angular position of the crank shaft to place the engine in idle mode. In addition, the engine controller can alter the number of ignitions of the spark plug relative to the number of rotations of the crank shaft to place the engine in idle mode. The engine controller can ignite the spark ever two or three rotations of the crank shaft, for example providing only a half or a third of the power than if the spark plug was ignited every rotation, as would be the case during the normal operation of the engine.

Alternatively the engine controller stops the engine by stopping the ignition of the spark plug.

The sensor system can include a sensor located in a passageway through which the oil flows, the sensor measuring a parameter of the flow of the lubricating oil in the passageway and generating a signal which is a function of that parameter; and a signal processor which processes the signal to determine whether the lubricating oil is being pumped into or through the passageway in accordance with the predetermined parameters and provides an indication to the engine controller when the lubricating oil is not being pumped into the passageway in accordance with the predetermined parameters.

The passageway can be the passageway between the carburetor and the engine through which the aerated fuel generated by the carburetor passes from the carburetor to the engine. The passageway can also be an oil passageway.

The signal processor is integral with the sensor. Alternatively, the signal processor may form part of the engine controller.

The sensor may comprise at least two electrically conductive plates located in the passageway in close proximity to each other wherein the signal is dependent on the change in capacitance of the plates.

The oil pump may be driven by a crank shaft of the engine.

In an alternative design, the oil pump may be powered by an electrical power supply having a sensing system comprising monitoring means which monitors the voltage and/or current of the electrical power supply of the oil pump during the operation of the oil pump. The sensing system determines whether lubricating oil is being pumped into the passageway by analyzing the values of the voltage and/or current of the electrical power supply to determine whether sufficient lubricating oil is being provided compared with predetermined parameters and provides an indication to the engine controller when the lubricating oil is not being pumped in accordance with the predetermined parameters. For an electrically powered oil pump, it has been found that the voltage and/or current drawn by the oil pump during its operation is altered depending on how much oil is being pumped by the oil pump. In particular, there is a difference when the oil pump is pumping oil during the normal curse of operation and when it is operating with no oil to be pumped. In such a case, the sensing system can be incorporated into the engine controller.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIGS. 1 and 2, the power cutter comprises a body2in which is mounted a two stroke internal combustion engine24, a front handle4, a rear handle6, a support arm7, a cutting blade10, rotatably mounted on the support arm7and driven by the engine24via a rubber belt26and a blade guard22. A starter12is provided to activate the engine24. A plastic cover19covers the out side of the support arm7as shown inFIG. 2.

Referring toFIG. 3, the support arm7comprises two sections, a forward section8and a rearward section20. The rearward section20is made from cast iron and is rigidly attached to the side of the gear housing32using bolts (not shown). The forward section8is made of cast iron and is slidably mounted on the rearward section20. The forward section can slide in the direction of Arrow A. The method by which the forward section8is slidably mounted on the rearward section20is described in more detail below.

The engine24has an output shaft28on which is mounted a drive wheel30for the rubber belt26. The output shaft drives the drive wheel30via a centrifugal clutch (not shown) in known manner. A driven wheel32is rotatably mounted on the end of the forward section8of the support arm. The driven wheel32is connected to the cutting blade10which is also mounted on the forward section8as described below. The belt26passes around the rearward section20, between the two wheels30,32to transmit the rotation of the engine to the cutting blade10. The purpose of enabling the forward section8to slide in relation to the rearward section20is to enable the belt26to be tensioned as described below. A hub16covers the drive wheel30.

The interconnection between rearward and forward sections of the support arm7will now be described with reference toFIGS. 3,4,5and6.

Referring toFIGS. 3 and 4, the rearward section20comprises two elongate slots40formed through the width of the rearward section20. The elongate slots40are aligned with each other, are of equal length, and run lengthways along rearward section20of the support arm. Formed through the forward section8are two corresponding holes46. The part of the forward section8with the holes46is located alongside of the part of the rear section20with the elongate slots40so that the holes46align with a corresponding slot40. A bolt42passes through each hole46and corresponding slot40. A nut44is screwed onto the bolt42to sandwich the rearward and forward sections together and hold them in place using friction. The slots40enable the bolts42, and hence the forward section8, to slide relative to the rearward section20.

In order to slide the rearward section20relative to the forward section8, the nuts44are slackened so that the two can move relative to each other. The forward section8is then slid (using the belt tensioner described below) to the desired position, and then the nuts44are tightened to sandwich the rearward section20to the forward section8to prevent movement between the two.

The forward and rearward interconnection mechanism is designed so that the forward section8can be located on either side of the rearward section20as shown inFIG. 2. In normal operation, the forward section8is located on the same side of the rearward section20as the engine24. This is indicated as position B. In this position, the cutting blade10is located closer to the central axis of the power cutter. However, it is sometimes desirable to have the cutting blade located towards the edge of the body2to enable it to cut near to a wall. The forward section8can then be moved to the other side of the rearward section20and be rotated through 180 degrees about the longitudinal axis48of the forward section8as indicated by Arrow D to the position C. In both orientations, the driven wheel32locates in the same position so that it can be driven by the belt26.

The belt tensioner will now be described with reference toFIGS. 4,5and6

Referring toFIGS. 5 and 6, the belt tensioner comprises a metal plate50having two holes46through which the bolts42pass as seen inFIG. 4. The position of the plate50is fixed by the position of the bolts42. Formed on the metal plate50are two hoops52which form apertures which are aligned. The shaft of an elongate bolt54passes through the hoops52. The elongate bolt54can freely rotate and axially slide within the hoops52. Threadedly mounted onto the bolt54is a nut56. A spring58is sandwiched between the nut56and one of the hoops52. The spring58prevents the nut from rotating. Therefore, as the bolt54is rotated, the nut56travels along the length of the bolt54in a direction dependent on the direction of rotation of the bolt54. The position of the nut56is fixed relative to the hoop52by the spring58. A stop60is integrally formed on the rearward section20of the supporting arm.

In order to tension the belt26, the elongate bolt is rotated so that the nut moves towards the head62of the bolt54. As its position is fixed by the spring58, the nut remains stationary relative to the hoop52causing the bolt54to axially move within the hoops52so that the end64of the bolt54approaches the stop60. Upon engagement of the stop60, the end64and hence the bolt54, cannot move further and therefore the nut begins to move. The movement of the nut causes the spring58and hence the hoop52to move towards the head62of the elongate bolt54. This in turn results in the plate50, the two bolts42and the forward section8to move with the nut56, the bolts42sliding within the two slots40. However, when the belt26becomes tight, the forward section8, and hence the bolts42and plate50are prevented from moving further. However, if the elongate nut54is further rotated, the nut56will continue to travel along its length. Therefore, the spring58becomes compressed, applying a force onto the hoop, and hence plate50, which in turn transfers it to the forward section8. This tensions the belt26. The more compression of the spring58, the more force is applied to the belt26. An indicator66is added to the plate50to show when the amount of the compression of the spring58is sufficient to apply the correct amount of force to the belt26.

The blade mount on the forward section8will now be described.

Referring toFIG. 7, the driven wheel32is rotatably mounted on the forward section8. The driven wheel32is drivingly connected to the cutting blade10via a spindle70as will be described in more detail below. The blade guard22is pivotally mounted around the spindle70as will be described in more detail below. A grasp72is rigidly attached to the blade guard22which can be held by an operator in order to pivot the blade guard22.

FIGS. 12 to 18show the mechanism by which the forward support8rotatably supports the cutting blade10.

Referring toFIGS. 17 and 18, the driven wheel32is rigidly attached to the spindle70via a flanged nut74. Rotation of the driven wheel32results in rotation of the spindle70. The spindle70is mounted in the forward section8using two ball bearing races76, each comprising an inner track78rigidly connected to the spindle70, an outer track80rigidly connected to the forward support8, and a set of ball bearings82sandwiched between the two tracks78,80which allow the outer track80to rotate relative to the inner track78.

Formed along a part of the length of the spindle70are two flat surfaces84(seeFIG. 16). A second support disk86comprises a central hole which is predominantly circular with the same diameter of the spindle70, but with two flat sides which correspond in dimensions to the flat surfaces84of the spindle70. The second support disk86is mounted on the end of the spindle70and abuts against shoulders88on the spindle70formed by the two flat surfaces84. The flat surfaces84ensure that the second support disk is rotationally fixed to the spindle70so that rotation of the spindle70results in rotation of the second support disk86.

An adaptor90(described in more detail below) is mounted on the spindle70. The adaptor can freely rotate about the spindle70. The cutting blade10is mounted on the adaptor90.

A first support disk92comprises a central hole which is predominantly circular with the same diameter of the spindle70, but with two flat sides which correspond in dimensions to the flat surfaces84of the spindle70. The first support disk92is mounted on the end of the spindle70and abuts against the cutting blade10. The flat surfaces84ensure that the first support disk92is rotationally fixed to the spindle70so that rotation of the spindle70results in rotation of the first support disk86.

A threaded hole94is formed in the end of the spindle70(seeFIGS. 10 to 12). A second flanged nut96is screwed into the hole94. The flange of the nut96pushes the first support disk92against the blade10which in turn pushes the blade10against the second support disk86. The blade10becomes sandwiched between the two support disks86,92. Rotation of the support disks86,92by the spindle70results in rotation of the blade due to the frictional contact of the blade10being sandwiched between the two disks86,92. By frictionally driving the blade10, it allows rotational movement of the blade10relative to the spindle70if the blade becomes snagged during the operation of the power cutter.

The automatic blade support adjustment mechanism will now be described.

Cutting blades of different sizes can be used. Different sized cutting blades10have different sized holes in their centers through which the spindle70passes. It is intended that the present power cutter will be able to fit cutting blades10having two different sizes of hole through their centers. This is achieved by the use of the adaptor90.

Referring toFIGS. 17 and 18, the adaptor is mounted on the spindle70between the two support disks86,92. As well as being freely rotatable about the spindle70, the adaptor90can axially slide along the spindle70between the disks86,92.

The adaptor comprises a front section98and a rear section100. The front section98has a first outer diameter, the rear section100has a second larger outer diameter. The two sections allow blades10with holes of different diameters to be mounted onto the spindle70. InFIGS. 14 to 18, it can be seen that a blade10with a central hole of a first diameter is mounted on the rear section100of the adaptor90. InFIG. 13, it can be seen that a blade10with a central hole of a second diameter is mounted on the front section98of the adaptor90.

A spring102is sandwiched between the second support disk86and an inner shoulder104of the adaptor90. The spring102biases the adaptor towards the first support disk92. A circlip106is located around the spindle70which limits the maximum extent of axial travel of the adaptor90. When the adaptor90is allowed to slide to its maximum extent and abut against the circlip106, the rear section100is located centrally between the support disks86,92.

When a blade10, having a center hole with the same diameter of the rear section100of the adaptor is mounted onto the adapter, it fits onto the rear section100of the adaptor as shown inFIGS. 14 to 18. As such, the blade10is centrally located between the two support disks86.92. However, when a blade10, having a center hole with the same diameter of the front section98of the adaptor is mounted onto the adapter, it fits onto the front section98of the adaptor as shown inFIG. 13. It is prevented from sliding onto the rear section. In order for the blade10to be secured onto the spindle70by the support disks86,92, it must be located centrally between the two. When the first support disk92is mounted onto the spindle70after the blade, it pushes the blade10and adaptor90against the biasing force of the spring102, moving the adaptor90towards the second support disk86as shown inFIG. 13. When the blade is securely mounted on the spindle70, it is centrally located between the support disks. The front section is similarly mounted centrally. The adaptor enables two types of blade10to be used, it moving automatically in accordance with blade size.

The pivotal blade guard22will now be described.

Referring toFIG. 15, the blade guard22is held by being sandwiched between two pieces of rubber108,110. The blade guard22can pivot about the spindle70. However, it is frictionally held by the two pieces of rubber108,110. In order to pivot the guard22, the operator must overcome the friction between the guard22and the rubber108,110.

A metal bracket112is attached to the forward section8via four bolts114. The bolts pass freely through the forward section8and threadedly engage with threaded holes formed in the bracket112. A helical spring116is sandwiched between the head118of each bolt114and the forward section8, biasing the bolts114out of the holes, pulling the bracket112towards the forward section. Sandwiched between the bracket and the forward section8is a first piece of rubber,108, the guard22, a second piece of rubber110to form a rubber—guard—rubber sandwich. The strength of the spring116determines the amount of frictional force there is between the rubber108,110and the guard.

In order to pivot the guard the operator holds the grasp72and pivots the guard22by overcoming the frictional force between the guard and the rubber108,110.

A first design of oil and fuel management system will now be described with reference toFIG. 19.

The two stroke internal combustion engine comprises a cylinder120in which is slidably mounted a piston1000which is connected to a rotatable crank114. The reciprocating motion of the piston100in the cylinder120causes a rotational movement of the crank shaft114in well known manner. Movement of the piston is caused by the burning of aerated fuel/oil mixture in the cylinder, the ignition of which is caused by the ignition of a spark plug730. The engine burns the fuel in well known manner to generate rotary motion of its crank shaft114, which connects to the output shaft28. The exhaust gases are then expelled from the engine24through an exhaust146to the surrounding atmosphere. The speed of the engine is determined by the amount of aerated fuel/oil mixture the carburetor126provides to the engine which in turn is dependent on the amount the operator depresses the trigger switch1070.

The power cutter will comprise a fuel tank124in which is located fuel for driving the two stroke internal combustion engine24. Fuel will pass from the tank124via passageway generally indicated by dashed lines144through the carburetor126which will mix it with air prior to being forwarded to the cylinder120where it will be burnt. Details of the supply of air, including its filtration will be described in more detail below. A second tank128will also be mounted in the body2as shown, in which lubricating oil will be contained. The oil will be pumped out of the tank128via a standard design of oil pump130, which is mounted on the crank shaft housing which will be driven via a gear arrangement (not shown) from the crank shaft114. The oil pump130, will pump the oil through the oil passageways indicated by dashed lines142from the oil tank128via the pump130into the passageway132between the carburetor126and the cylinder120, in a suitable form, for example, as a liquid or as a spray or atomized, and then mixing the oil with the air/fuel mixture generated by the carburetor126. It will inject oil at the ratio 1:50 in relation to the fuel.

A sensor unit140will comprise a sensor which will be mounted within the passageway132between the carburetor126and cylinder120, or in the oil passageways142adjacent the passageway132. The sensor will measure a parameter of the flow of the oil within the passageway and determine, using a signal processor incorporated into the sensor unit, whether oil is being pumped correctly in accordance with predetermined parameters into the passageway132either by checking the pressure of the oil as it enters the passageway132or by detecting the presence of oil in the passageway132. Such a parameter could be the rate at which the oil is pumped into the passageway132.

The construction of one type of sensor unit will now be described with reference toFIG. 32. The sensor unit comprises a sensor consisting of two metal plates1050;1052which are connected via wires1056to a signal generator/signal processor1054. The metal plates are located within the passageway132between the carburetor126and cylinder120, down stream of the oil passageways142. The signal generator/signal processor1054sends a signal to the plates1050;1052to determine the capacitance of the two plates. The capacitance is dependent on the material between the plates1050;1052which will consist of aerated fuel and lubrication oil. As the amount of lubrication oil changes, the capacitance of the plates changes. This capacitance will be analyzed by the signal generator/signal processor1054which, if the capacitance extends outside of a predetermined limit, will provide a signal to an electronic ignition system.

The engine is controlled by the electronic ignition system. The sensor unit140will provide signals to the electronic ignition system about the amount of oil being pumped into the passageway132. In the event that insufficient, excessive or no oil is pumped into the passageway due to the fact that the oil tank is empty or there is a blockage in an oil pipe142or there is a fault with the pump, the sensor unit140will send the signal to the ignition system. The ignition system will then either switch the engine into an idle mode or switch the engine off entirely, depending on the settings of the ignition system. This will ensure that lubricating oil is always added to the fuel in the correct amount prior to combustion within the two stroke engine.

It will be appreciated by the reader that though the sensor unit140has been described as having the signal generator/signal processor1054incorporated in the sensor unit140, the signal generator/signal processor1054could be incorporated into the electronic ignition system with the sensor unit only comprising the sensor.

A second design of oil and fuel management system will now be described.

Referring toFIG. 33, the two stroke internal combustion engine comprises a cylinder120in which is slidably mounted a piston1000which is connected to a rotatable crank114. The reciprocating motion of the piston100in the cylinder120causes a rotational movement of the crank shaft114in well known manner. Movement of the piston is caused by the burning of aerated fuel/oil mixture in the cylinder, the ignition of which is caused by the ignition of a spark plug730. The engine burns the mixture in well known manner to generate rotary motion of its crank shaft114, which connects to an output shaft. The exhaust gases are then expelled from the engine through an exhaust146to the surrounding atmosphere. The engine is started using a pull cord in well know manner. The speed of the engine is determined by the amount of aerated fuel/oil mixture the carburetor126provides to the engine which in turn is dependent on the amount the operator depresses the trigger switch1070.

The power cutter will comprise a fuel tank124in which is located fuel for driving the two stroke internal combustion engine24. Fuel will pass from the tank124via passageway144through the carburetor126which will mix it with air from an air filter890, prior to being forwarded to the cylinder120where it will be burnt. A second tank128will also be mounted in the body as shown in which lubricating oil will be contained. The oil will be pumped out of the tank128via an oil pump700. The oil pump700will pump the oil through the oil passageways indicated by lines142from the oil tank128via the pump130into the passageway132between the carburetor126and the cylinder120, in a suitable form, for example, as a spray or atomized, which is then mixed with the air/fuel mixture generated by the carburetor126.

A sensor140is mounted within the passageway132between the carburetor126and cylinder120. The sensor monitors the amount of oil being added to the fuel/air mixture and sends a signal, via an electric cable701, indicative of the amount of oil in the passageway132back to an electronic controller716(seeFIG. 34). The electronic controller comprises a signal processor which processes the signal and determines whether it is in accordance with a predetermined parameter or not. If the electronic controller determines that the oil is not being supplied in sufficient amounts, it places the engine in an idle mode or stops it altogether.

Such a sensor can be of a capacitance type whereby the sensor monitors the change in capacitance between two plates, the capacitance being a function of the amount of oil there is in the fuel/air mixture. Such a sensor has been described previously with reference toFIG. 32.

The carburetor126will now be described with reference toFIG. 47. The design of the carburetor is similar to that previously described with reference toFIG. 31. Where the same features are present the same reference numbers have been used.

The main difference between that disclosed inFIG. 31and that inFIG. 47is that the first pivotal plate1046which acts as the choke for the carburetor has been removed. Instead, a third passageway1100has been added. The third passageway1100connects to the second chamber1014via a solenoid valve1102. The other end of the third passageway1100connects with the air passageway1004down stream of the narrow section1008. The solenoid valve1102comprises a solenoid714and a pin1106. The pin can axially slide (Arrow M) between a first position where the tip of the pin1106engages with a restriction1108in the third passageway1100to block the third passageway1100and a second position where the tip is located away from the restriction1108to open the third passageway1100. The pin1106is biased to its first position. Activation of the solenoid by the provision of an electric current, causes the pin1106to move against the biasing force from its first position to its second position opening the third passageway1100. Deactivation of the solenoid714by the removal of an electric current, causes the pin1106to move under the influence of the biasing force from its second position to its first position, closing the third passageway1100.

Under normal operation, the carburetor functions in the same manner as that described with reference toFIG. 31. No current is supplied to the solenoid714, and the pin1106engages with the restriction1108and blocks the third passage1100. However, when the engine is cold, a current can be supplied to the solenoid714, which causes the pin1106to slide away from the restriction, unblocking the third passageway1100and allowing fuel to pass through the third passageway1100and enter the air passageway1004. This increases the amount of fuel relative to the amount of air in the aerated fuel exiting the air passageway1004to enable the engine to run smoothly whilst it is cold. Once the engine is warm, the solenoid714can be switched off by removing the electric current to it, allowing the pin1106to slide towards the restriction1108to block the third passageway and prevent any further fuel from passing through it.

In addition to the third passageway1100, there is a fourth passageway1110which connects between the hollow chamber1034and the air passageway. This passageway provides an air passage between the two. When there are increases or decreases in the air pressure in the air passageway1004, these are transmitted to the hollow chamber1034. These in turn are transmitted to the second chamber1014via the diaphragm1032which influences the flow of fuel into the second chamber and into the air passageway1004. The fourth passageway1110acts as a feed back mechanism to improve performance of the carburetor.

The solenoid is used when the engine is cold to provide an air/fuel mixture which is richer in fuel to help start the engine. When the engine is warm, the solenoid is switched off. The temperature of the engine is measure using a sensor710located on the engine block. The solenoid714is used to replace the choke on exiting designs of carburetor whereby which an operator would manually adjust the valve to start the engine when it is cold. The electronic controller716operates the solenoid dependent on the temperature of the engine.

An alternative design of carburetor which uses a solenoid in a similar manner can be found in U.S. Pat. No. 7,264,230. This could be used to replace that described previously with reference toFIG. 47.

A second alternative design will now be described with reference toFIG. 48. The carburetor126will now be described with reference toFIG. 48. The design of the carburetor is similar to that previously described with reference toFIG. 31. Where the same features are present the same reference numbers have been used.

The main difference between that disclosed inFIG. 31and that inFIG. 47is that the first pivotal plate1046which acts as the choke for the carburetor has been removed and has been replaced by a heating element1200. When the engine is cold, the heating element1200is switched on and heats the air flow as it passes through the carburetor. This provides heated aerated fuel which enables the engine to run smoothly until it has achieved an acceptable running temperature. The heating element1200would be controlled by the electronic controller716. This could be used to replace that described previously.

The engine ignition system is controlled by an electronic controller716, the function of which is described in more detail below with reference toFIG. 34.

Mounted on the end of the end of the crank shaft114is a fly wheel702which contains a number of metal fins704which form an impeller. As the fly wheel702rotates, the impeller blows air around the out side of the engine. Adjacent the impeller702are two generators706;708. The two generators generate electricity using magnets and the change of inductance caused by the rotating flywheel702. As the fly wheel702rotates, it causes the two generators706;708to produce electricity. The first generator706is used to provide electricity for the ignition system of the engine and the electronic controller716. The second generator708is used to provide electricity for the oil pump700and the solenoid714in the carburetor. Both are connected to the electronic controller716via cables717.

Also mounted adjacent the flywheel are two sensors710;712. The first sensor710monitors the temperature of the engine block and sends a signal via an electric cable711indicative of the temperature to the electronic controller716. The second sensor712monitors the angular position of the flywheel702and sends a signal via an electric cable713indicative of the angular position of the flywheel702back to the electronic controller716. This signal can also be used by the electronic controller716to determine the rate of rotation of the fly wheel702, as well as its angular position.

The oil pump700is an electrically powered oil pump700, the power for which is supplied by the electronic controller716via electric cable715. The oil pump is shown inFIG. 35.

This type of oil pump is described in EP1236894. Referring toFIG. 35, the oil pump comprises a piston850which can axially slide within a housing1202over a limit range of axial movement which is determine by a stop mechanism1204. A spring854biases it to a predetermined position. A chamber852is located below the piston850. A solenoid1206surrounds the piston and moves it axially when an electric current is applied. The piston850is biased in direction of Arrow R by a spring854. Activation of the solenoid moves the piston against the biasing force of the spring854in the opposite direction to Arrow R. A passageway1208is formed through the piston850. Oil is fed into the passageway via an inlet1210. A first valve comprising a ball bearing1212and spring1214is located between the end of the passageway1208and the chamber852. A second valve comprising a ball bearing1216and a spring1218is located between the base the chamber852. When the piston is moving in the direction of Arrow R due to the biasing force of the spring854, enlarging the chamber852, the first valve opens and the second valve closes, filling the chamber with oil. When the piston is in the opposite direction of Arrow R due to activation of the solenoid1206, reducing the size the chamber852, the first valve closes and the second valve opens, expelling the oil from the chamber852and through an outlet1220.

The oil pump700is driven by the electronic controller716which sends a square shaped voltage signal892to the oil pump (seeFIG. 45A) When the voltage is at V1, it causes the piston850of the pump to move, reducing the size of the oil chamber852. This causes a preset amount of oil to be pump out of the chamber852. When voltage is “0”, the piston returns to its starting position due to the spring854, enlarging the chamber852and allowing the chamber852to fill with oil. The higher the frequency of the square shaped voltage signal892, the more oil the oil pump700pumps per unit of time. The oil pump is capable of running at two speeds (the first speed shown inFIG. 45A, the second speed being shown inFIG. 45Bwhere the frequency of the square shaped voltage signal892, and hence the movement of the piston850, is double) and its general operation is described in more detail below.

The spark plug730is connected to the electronic controller716via a cable732. Ignition of the spark plug is controlled by the electronic controller716.

A primer734is mounted on the rear wall736of the housing800of the power cutter. The primer is a manual pump. A pipe738connects from the fuel tank124to the primer734. A second pipe740connects from the primer to the carburetor126. A brief description of the principle of how the primer works will now be described with reference toFIG. 36. The primer consists of two valves742;744located in series which allow the fuel to flow one way through them only (indicated by Arrows A and B). Located between the two valves742;744is a chamber750having a rubber dome746forming a wall which is accessible to the user of the power cutter. One valve742only allowing fuel to enter the chamber750, the other only allowing fuel to leave the chamber750. In order to use the primer, the operator, compresses the rubber dome746(shown as dashed lines748). This reduces the amount of volume in the chamber750formed between the valves and hence the amount of space which can contain fuel. As such, fuel is ejected from the primer through the one of the valves744, as the second valve742remains closed, preventing fuel from leaving the chamber750via that valve742. When the operator releases the dome746, the volume of the chamber750increases, causing fuel to be sucked into the chamber750through the second valve742as the first valve remains closed744preventing fuel from entering the chamber750through that valve744. Repetitive compressing and releasing of the dome746results in the fuel being pumped through the primer734. The primer is arranged so that the operator can manually pump the fuel from the tank124to the carburetor126through the pipes738;740.

The purpose of the primer is to enable the operator to place fuel into the carburetor. Otherwise the operator has to spin the engine a number of times using the pull cord before a sufficient amount of fuel is sucked through into the carburetor126.

A DECO valve752is mounted on the side of the cylinder120. The valve752is opened manually by the operator prior to starting the engine. When opened, the DECO valve reduces the pressure within the cylinder120prior to ignition. This enables the starting of the engine using the pull cord to be made easier as the amount compression of the fuel/air mixture required is reduced. When the engine is started, the DECO valve automatically closes.

The electronic controller716has an on/off switch754in the form of a rotatable knob758. The switch is connected to the electronic controller via an electric cable756.

The knob758as a pointer764integrally formed on its periphery. The rotatable knob758has two angular positions between which it can rotate. In the first position, the switch is ON. In this position, the pointer764points to an ON label762(seeFIG. 1). In the second position, the switch is OFF. In this position, the pointer764points to an OFF label760. When the rotatable knob is in the ON position, the operator can start the engine and use the power cutter. When the rotatable knob758is in the OFF position, the engine is prevented from being started. If the rotatable knob758is moved from the ON to the OFF position when the engine is running, the engine is automatically switched off.

An operating button766is located in the center of the knob758. If the engine is running (i.e. the knob is in the ON position), depression of the operating button766will result in the engine being switched off. The knob758then automatically returns to the OFF position. If the knob758is prevented from returning to the OFF position after the operating button has been depressed, the engine will not be able to be started until the knob758has been allowed to return to the OFF position.

The construction of the assembly for the ON/OFF switch754, which includes the knob758and operating button766, will now be described.

The ON/OFF switch assembly consists of the rotatable knob758, a crank768, a switch cam770and the operating button766.

The crank768is rigidly fixed into the rear wall736of the housing800and prevented from rotation. The crank768comprises a socket772into which is rigidly mounted a micro switch774(seeFIG. 49C).

Rotatably mounted on the outside of the crank768is the knob758. Rotatably mounted on the inside of the crank768is the switch cam770. A bolt778, which passes through the base of a tubular recess776formed in the knob758, screws into the switch cam770and is rigidly attached to it. Sandwiched between the head of the bolt778and the base of the recess776is a spring780. The bolt778and spring780hold the knob758and switch cam770onto the crank768, biasing them towards each other as the spring biases the head of the bolt778away from the base of the recess776. The knob can rotate through a limited range of movement (between the ON and OFF positions) relative to the crank768. The range of positions is limited by pegs786formed on the underside of the knob engaging with recesses788formed in the edge of the rear wall736of the housing. The switch cam770can also rotate through a limited range of movement relative to the crank768. In addition, the switch cam770can axially slide relative to the crank768in a direction parallel to the longitudinal axis of the bolt778over a limited range of movement, the range being limited by the length of the bolt778within the recess776. The bolt778rotates and slides with the switch cam770.

The operating button766is mounted within the tubular recess776formed in the knob758and encloses the end of the bolt778located in the recess776and the spring780(seeFIG. 40). The operating button766can axially slide within the recess776towards or away from the switch cam770. The range of outward axial movement of the operating button is limited by stops782each engaging with an inner step of the knob758. The head of the bolt778directly abuts the underside of the operating button766. Depression of the operating button, causes the bolt778to be pushed through the base, compressing the spring780, moving the switch cam770away from the crank768and knob758.

Connected between the knob758and the crank768is a long helical spring784. The helical spring784locates in a circular channel790formed on the underside of the knob758as best seen inFIG. 41. One end abuts against a wall792at the end of the channel790. The other end abuts against a stop (not shown) formed on the crank772. The spring784rotationally biases the knob758relative to the crank to its OFF position.

Connected between the switch cam770and the crank768is a leaf spring794as best seen inFIGS. 43 and 44. One end of the leaf spring794is connected using a small bolt796to the switch cam770. The other end abuts a stop798on the crank768. The leaf spring794rotationally biases the switch cam770relative to the crank to an OFF position.

Formed on the underside of the knob758are two ramps820, each ramp having a ramp end822as best seen inFIG. 41. Formed on the side of the switch cam770which faces the knob758are ramp recesses824which have ramp recess ends826as best seen inFIG. 9. When the switch assembly is in the OFF position i.e. when both the knob758and the switch cam770in their OFF positions under the biasing force of their respective springs784;794, each of the two ramps820is located in a corresponding ramp recess824with the ramp ends822of each ramp820abutting directly against the ramp recess ends826of the corresponding ramp recess824.

Formed on the underside of the crank768are two crank ramps828, each ramp828having a crank ramp end830as best seen inFIG. 39C. Formed on the side of the switch cam770which faces the knob758are switch cam crank ramps832which have switch cam crank ramp ends834as best seen inFIG. 40. When the switch assembly is in the OFF position i.e. with both the knob and the switch cam770in their OFF positions under the biasing force of their respective springs784;794, each of the two switch cam crank ramps832are located against the low end (the end of the crank ramp828away from the crank ramp end830) of the corresponding crank ramp828as shown inFIG. 39C.

Formed around the edge of the switch cam770is a peripheral cam836as best seen inFIGS. 39A and 39B. The micro switch774comprises a pin838which projects from the body of the micro switch774. The pin838is capable of sliding axially in or out of the body of the micro switch774and biased to its outer most position by a spring (not shown) inside the micro switch774. The pin838engages the peripheral cam836. Rotation of the switch cam770causes the pin838to slide along the peripheral cam836, which causes it to be pushed into the body of the micro switch774against the biasing force of the spring, or allows it to slide out of the body of the micro switch774under influence of the spring. When the switch cam770is in its OFF position, the pin is pushed into the body of the micro switch774as shown inFIG. 8A. When switch cam is rotated to its ON position, the pin838extends to its outer most position as shown inFIG. 39B.

The way the assembly for the ON/OFF switch works will now be described.

Initially, the knob758and the switch cam770are both located in their OFF positions. The operator of the power cutter desires to turn the unit on using the ON/OFF switch. The operator uses their hand to rotate the knob758from its OFF position to its ON position. When the knob758is rotated, it causes the cam switch770to rotate in unison as the rotary movement is transferred from the knob758to switch cam770by the ramp ends822of each ramp820pushing the ramp recess ends826of each corresponding ramp recess824, against which it abuts, in the direction of Arrow M inFIG. 40, to cause the switch cam770to rotate with the knob758. As the switch cam770rotates, the two switch cam crank ramps832, which are initially located against the low end of the crank ramps828(shown inFIG. 39C), ride up the crank ramps828(shown inFIG. 39D), which are stationary. As the switch cam crank ramps832ride up the crank ramps828due to the rotation of the switch cam770, the switch cam770is forced to axially slide away from the knob758(direction of Arrow N inFIG. 40), causing the spring780to be compressed and the head of the bolt778to move towards the base of the recess776. When the switch cam has rotated sufficiently that the crank ramp ends830and the switch cam crank ramp ends834become aligned, the switch cam770axially slides under the biasing force of the spring780towards the knob758, ensuring that the crank ramp end830and the switch cam crank ramp ends834abut against each other as shown inFIG. 39E. When the crank ramp ends830and the switch cam crank ramp ends834abut each other as shown inFIG. 39E, the switch cam770is in its ON position and is prevented from returning to its OFF position, under the influence of the leaf spring794, as the crank ramp ends830and the switch cam crank ramp ends834prevent relative movement as they are jammed against each other. The knob758is prevented from returning to its OFF position under the influence of the spring784by the ramps820being held within the ramp recesses824by the action of the spring780which overrides the spring784. When the switch cam770rotates from the OFF position (seeFIG. 39A) to the ON position (FIG. 39B), the peripheral cam836rotates, which in turn allows the pin838to extend from the body of the micro switch774. This in turn makes a connection which allows the electric controller716to activate the power cutter and allow it to start when the pull cord is pulled.

As such, the assembly of the ON/OFF switch is now ON with the knob758and the switch cam770both in their ON positions, allowing the pin838to extend from the body of the micro switch774. There are two way of switching the ON/OFF switch assembly to its OFF position.

The first method comprises the depression of the operating button766. Depression of the operating button766causes the head of the bolt778to slide towards the base of the recess776of the knob758, compressing the spring780, which in turn causes the switch cam770to axially slide away from the knob758. As the switch cam770axially slides, the switch cam770moves away from the crank768, which in turn causes the crank ramps828and the switch cam crank ramps832to move away from each other, and thus causes the crank ramp ends830and the switch cam crank ramp ends834to disengage. As such, the switch cam770can now rotate back to its OFF position under the influence of the leaf spring794. As the knob is held in its ON position by the ramps820being held within the ramp recesses824, the knob858will also return to its OFF position as the ramp recesses824rotate with the switch cam770. Should the ramps820become disengaged from the ramp recesses824due to the sliding movement of the switch cam770relative to the knob758, the knob758will return to its OFF position under the influence of the spring784between the knob758and the crank768.

The second method of switching the ON/OFF switch assembly OFF comprises the rotation of the knob758. The operator rotates the knob758to its OFF position. As the ramps820are held within the ramp recesses824, rotation of the knob758urges rotation of the switch cam770. However, the switch cam770is prevented from rotating as the crank ramp ends830and the switch cam crank ramp ends834abut each other. Therefore, the ramps820slide out of the ramp recesses824, the ramp ends822moving away from ramp recess ends826. As the ramps820slide out of the ramp recesses824, the switch cam770, which is prevented from rotating, axially slides away from the knob858by the caming action of the ramps820and ramp recesses824. When the switch cam770has slid sufficiently far enough away from the knob758, the crank ramp ends830and the switch cam crank ramp ends834, which are sliding away from each other, become disengaged. Thus the switch cam770can rotate under the influence of the leaf spring794to its OFF position. The knob758will move under the influence of the operator and/or the spring784. As such, both the knob758and the switch cam770return to their OFF position where they are held by the springs784;794.

When both the knob and switch cam770moved to their OFF positions, the ramps820engage with the ramp recesses824so that the switch can be used again to switch on the power cutter.

The operation of the power cutter will now be described.

The operator first activates the DECO valve752and then pumps some fuel into the carburetor126using the primer734. The operator then switches the ON/OFF switch to ON by rotation of the knob758to its ON position. The operator then pulls the pull cord to rotate the crank114of the engine. As the crank114rotates, the fly wheel702also rotates causing the two generators706;708to produce sufficient electricity to operate the power cutter.

The electronic controller checks the temperature of the engine using sensor710. If the engine is cold, it uses the electricity from the second generator708to power the solenoid714in the carburetor to set the “automatic choke”. The second generator708is not powerful enough to power both the oil pump700and solenoid714at the same time. Therefore, when the electronic controller716is operating the solenoid714, it switches off the oil pump700. It has been found that the period during which lubricating oil is not required before the engine is damaged is greater than that required to heat up the engine.

The electronic controller supplies the power to the spark plug to cause combustion in the engine, the power being provided by the first generator706, the timing being determine by the electronic controller716based on the signal provided by the sensor712in relation to the angular position of the fly wheel702.

Once the engine commences firing, the DECO valve automatically closes. The electronic controller716continues to monitor the engine temperature and when it has reached a predetermine temperature, the electronic controller716switches the solenoid714in the carburetor126off. The electronic controller716then commences supplying a square shape voltage signal to the oil pump to commence pumping oil. The electronic controller monitors the speed of the engine using the signal provided by the sensor712monitoring the angular position of the fly wheel702to calculate the rotational speed. If the rotational speed is below a predetermined value, the electronic controller716sends a signal (FIG. 35A) to the oil pump700to cause it to pump at a slow speed. If the rotational speed is above a predetermined value, the electronic controller716sends a signal (FIG. 35B) to the oil pump700to cause it to pump at a higher speed. The speed of the engine is dependent on the operator squeezing a trigger switch which connects to the carburetor via a cable.

Whilst the engine is running the electronic controller716monitors the oil being added to the fuel/air mixture using the sensor140. If the sensor140sends a signal that indicates that the rate of flow of the oil being pumped by the oil pump700has dropped below a predetermine amount (e.g. there is a blockage in the oil pipe142or the tank128is empty), the electronic controller places the engine into an idle mode using the ignition system so that the engine runs, but at a minimal rate.

The engine controller can place the engine in idle mode by altering the timing of the ignition of the spark plug relative to the angular position of the crank shaft or the number of ignitions of the spark plug relative to the number of rotations of the crank shaft. The engine controller can ignite the spark ever two or three rotations of the crank shaft, for example, providing only a half or a third of the power than if the spark plug was ignited every rotation, as would be the case during the normal operation of the engine.

The operator cannot speed up the engine using the trigger switch1070until the sensor140detects the flow of oil. This protects the engine from damage due to a lack of lubrication. It has been found that the engine can run in idle mode for a considerable period of time before damage to the engine results.

It will be appreciated by the reader that, as an alternative to placing the engine in an idle mode, it could switch it off completely. In such a case, the electronic controller716would ensure it could not be started until oil was detected again by the sensor.

In order for the operator to stop the power cutter, the operator either depresses the operating button766or rotates the knob758to its OFF position.

An alternative system for sensing whether oil is being provided to the aerated fuel to that described previously will now be described. In the alternative design, it is intended to remove the sensor140and, instead, monitor the current being supplied to the oil pump700when it is running to determine if oil is being pumped by the oil pump.

Referring toFIGS. 46A,46B and46C,FIG. 46Ashows the voltage supplied to the oil pump700,FIG. 46Bshows the current supplied to the oil pump700when the oil pump is pumping but there is no oil being pumped, andFIG. 46Cshows the he current supplied to the oil pump when the oil pump is pumping oil. The oil pump700running at the slow speed.

A square wave voltage V1indicated by line900is supplied to the oil pump as shown inFIG. 46A. When oil is being pumped through the oil pump700(FIG. 46C), the current indicated by line902starts to rise from zero at t0, when the voltage applied rises to V1and increases towards I1. When it reaches I1the amount of current drops904before increasing906again to I1. The current drops after t2seconds before beginning to rise again at t3seconds. The drop in current occurs when the piston850in the oil pump700reaches its maximum amount of travel due to the solenoid1206and then bounces back slightly due to the piston's850impact at its maximum distance of travel. However, when no oil is being pumped by the oil pump700, but the pump is still operating, the speed of travel of the piston850in the oil pump increases as there is less resistance. As such it reaches its maximum amount of travel sooner. Therefore, as shown inFIG. 46B, it reaches a lower value of current I2before the value of current drops908and then rises910again towards I1. Furthermore, it only takes less time before the current drops908and rises910again than if the oil pump700was pumping oil. The signal processor in the electronic controller716can monitor when the current begins to drop versus the start of the voltage pulse and the value of the current at this point to determine whether the pump is pumping oil or not. If t is not, the electronic controller can place the engine in idle mode.

The construction of the fuel cap will now be described with reference toFIGS. 20 to 23.

The fuel tank124will be mounted within the body of the unit as generally indicated inFIG. 19. The tank124will be sealed by a fuel cap13as shown onFIG. 2.

The fuel cap will comprise an inner cap202, a clutch204and an outer cap206. The inner cap is of a tubular construction with one end210being sealed. Formed on the inside surface of a side wall212is a thread208. When the fuel cap is screwed onto the fuel tank, the thread208slidingly engages with a thread formed around the external surface of the neck of the fuel tank124.

Located inside the inner cap202adjacent the end210is a seal214. When the fuel cap is screwed onto the fuel tank, the seal214ensures that no fuel can escape from the tank. The inner cap2locates within the outer cap206. Sandwiched between the two is the clutch204. A clip216locates within a groove218of the inner cap and also engages with an inner groove220formed within the outer cap. The clip holds the inner cap inside the outer cap whilst allowing it to freely rotate within the outer cap206. The inner cap comprises a number of teeth222integrally formed with the inner cap. The teeth locate within corresponding slots224formed within the clutch, thus rotation of the inner cap causes rotation of the clutch204. Formed on the clutch204are a plurality of resilient arms226mounted on the ends of which are pegs228. The pegs228face towards the internal end wall230of the outer cap. Formed on the wall are a plurality of ridges232. The pegs on the clutch are arranged to co-operate with the ridges232in the outer cap.

Rotation of the outer cap206causes the ridges232to engage with the pegs228resulting in rotation of the clutch204, which in turn rotates the inner cap202via the teeth222. When the fuel cap is screwed onto the fuel tank, the inner cap202threadingly engages with the neck of the fuel tank, the rotation of the inner cap202being caused by rotation of the outer cap6via an operator rotating it using a finger grip234. When the seal214located within the inner cap engages with the end of the neck of the fuel tank, the inner cap202is prevented from further rotation. This in turn prevents further rotation of the clutch204. However as the operator continues to exert a rotational force on the outer cap206, the ridges232are caused to ride over the pegs228, the movement of the pegs228being allowed by the resilient arms226upon which they are mounted. In this way the operator can rotate the outer cap whilst the inner cap remains stationary thus preventing the operator from over-tightening the fuel cap onto the neck of the fuel tank.

The air filtration mechanism for any of the carburetors126previously described will now be described.

The two stroke engine comprises a carburetor126which mixes liquid fuel with air to generate a combustible mixture for powering the engine. However, due to the operation of the power cutter, a large amount of dust is generated which mixes with the surrounding air. This results in dust laden air. In order to ensure that the air entering the carburetor is free from dust it must pass through a filter system to remove the dust.

The filter system will now be described with reference toFIGS. 23 to 29.

Inside the body2is a filter unit316comprising a plastic base318and filter paper320folded to form pleats. The filter unit316is located within the body2so that the pleats320hang vertically downwards when the power cutter is in a storage position as shown inFIGS. 1 and 2.

Air will be sucked through the filter system by the carburetor126. Air enters slots314on the rear of the body2. Air passes (Arrow G) to a space322underneath the filter unit316and then passes through the filter paper320to a space324above the filter unit316. Any dust entrained within the air is trapped by the filter unit316and held within the pleats of the filter paper320.

The clean air then passes from the space324, through a hose326to the carburetor126located below the space322below the filter unit316.

In order to enable the operator to remove the dust trapped within the pleats of the filter paper320, a cleaning device is provided. The cleaning device comprises a rubber flap328, mounted on the top of a plastic base330, a brush332attached to the bottom of the plastic base330, a handle334attached to the plastic base330via to rigid arms338. The base330can slide within the space322below the filter unit316, widthways across the body2. Movement is caused by the operator pulling the handle334away from the side of the body2. Two springs336bias the handle334towards the side of the body2.

In order to clean the filter unit, the operator pulls the handle334, to move the base330across the width of the body2in the direction of Arrow H, and then releases it to allow it to return in the opposite direction under the biasing force of the springs336.

As the base330slides across the width, the rubber flap328engage with the pleats320, as best seen inFIG. 25, knocking the dust of the pleats320. The dust drops to the base340of the space322below the filter unit316.

The brush332slidingly engages with the base340of the space322. The brush332brushes the dust to one side or the other, depending on the direction of movement. An aperture344is formed on one side of the body2. As the brush approaches the side of the body, it pushes the dust being swept along the base through the apertures, expelling it from the body2.

ThoughFIG. 4shows the flap28moving perpendicularly to the direction of the pleats320, it will be appreciated by a person skilled in the art that is possible to rotate the filter paper320so that the pleats run in parallel to the sliding movement of the flap328as shown inFIG. 26. In such a scenario, the rubber flap28may be replaced by a plurality of brushes342.

The construction of the rear handle will now be described with reference toFIGS. 1 and 2.

The body of the power cutter is constructed in the form of a plastic casing constructed from a number of plastic clamshell rigidly connected together. The rear clam shell430connects to the rear handle6. In existing designs of power cutter, the rear handle6is integral with the rear clam shell430. However, if the handle6is broken, the whole clam shell430needs to be replaced. As handle breakage is common it is desirable to avoid this.

Therefore, the rear handle6in the present invention is constructed as a separate item to that of the rear clam shell430(or body2).

The rear handle6is constructed from a separate single clam shell431which is joined at its top432at two points434and at its bottom at a single point436. Each of the three points434,436is joined using a bolt which screws into the plastic clam shell430. Vibration dampening material may be used in conjunction with the bolts to reduce the amount of vibration transferred to the handle6from the body2. The use of such vibration dampening material allows limited movement of the handle6relative to the rear clam430at each of the three points. The movement could be either linear or rotational. One such construction is to surround the bolts with the dampening material in order to sandwich it between the bolts and parts of the clam shell of the rear handle6.

The top432of the handle6is in the form of a cross bar. The shape is such that the bolts fastening the top432of the handle to the rear of the clam shell430are aligned with each other and thus provides a pivot axis440for the rear handle6about which it can rotate by a limited amount.

A person skilled in the art will appreciate that the handle may be constructed from a number of clam shell connect rigidly together. Rubber soft grip over mold442may also be added to the handle for additional comfort.

A second embodiment of an air filtration system will now be described with reference toFIG. 30.

The filter device comprises a box400in which is mounted filter paper402which is pleated and which hangs down from the top section from inside the box. A space404is formed below the pleat. A large aperture406is formed in the side of the box below the filter paper and through which a drawer408can be slid. The drawer comprises a receptacle410which locates in the space404immediately below the filter paper402. The drawer408can be fastened into place via a screw412which threadedly engages a threaded hole414in the box. Air passes through slots314into the box and into the receptacle410in the space404below the filter paper402then through the filter paper402into a space416above the filter paper402and then exits the space416above the filter paper through a flexible tube418to the carburetor126. Any dust contained in the air entering the box400is blocked by the filter paper402.

A combination of two systems have been proposed to shake any dust within the filter paper402off the filter paper402into the drawer408so that the drawer408an be removed for emptying.

The first system is very similar to that disclosed in the first embodiment described above and comprises a rubber flap420which is attached to the front end of the drawer408. As the drawer408is inserted into the box400the rubber flap420engages with the pleated filter paper402. As the drawer408slides into the box400the rubber flap420successively hits the base of each pleat causing any dust on the pleats to be knocked off and into the drawer408. As such the action of inserting or removing the drawer408into the box400causes dust on the filter paper402to be loosened and allowed to be removed.

The second system relies on the starter cord422of the start12for the two stroke engine24of the power saw. When the engine is started, the power cord422needs to be pulled in order to cause it to rotate. As the cord422is pulled, it rotates a pulley wheel424which causes an eccentric pin426to rotate about the axis428of the pulley420. This causes one side of the box400to oscillate up and down as indicated by arrows Y. The other side of the box400is pivotally attached about an axis435to the body of the power cutter. The reciprocating motion of the box400causes dust in the filter402to be shaken off the filter paper402and into the drawer408.

Each system causes dust trapped in the filter paper402to fall into the drawer. When the operator first starts up the power cutter, the action of pulling the starter cord cleans the filter paper402. Then, the operator can subsequently clean the filter paper during the operation of the power cutter by inserting and removing the drawing408.

It will be appreciated by a person skilled in the art that the two systems could be used separately, as well as in combination, a power cutter having only one or the other system.