ENGINE SPEED LIMITATION CONTROL

TECHNICAL FIELD The present invention relates to a method and arrangement for controlling an engine speed of an internal combustion engine of a hand held power tool such as a chain saw or a cut off saw, preferably by controlling an ignition system of the engine, which ignition system has a primary firing pulse generator for charging a capacitor and also has an electronic switch for discharging the capacitor via an ignition coil to generate an ignition voltage. A microcomputer operates the switch to control the ignition timing of the generator and the microcomputer is communicatively connected with a speed detector, also referred to as detection means, that directly or indirectly detects the rotational speed of the engine. Under certain conditions and circumstances, a speed limitation control is utilized to limit the engine speed to a limitation speed.

BACKGROUND

Multi-purpose portable working machines such as chain saws, cutting tools and grass trimmers that have internal combustion engines are well known. Each of these types of machines has a working tool, such as a chain or cutting blade, which is brought to an operating rotational speed by the included engine. Since the operating tool is often close-by the operator, there is a risk of contact and an accidental injury occurring. Therefore, such machines are often equipped with a mechanical security brake for the tool, together with other security arrangements such as requiring two-hand-grip engagement by the operator in order to affect operation.

The machine is normally equipped with a centrifugal clutch that engages the tool when the engine exceeds a certain rotational speed. In normal operation, the clutch improves safety because the tool does not rotate when the engine speed is reduced below the clutch-in speed. The risk for bodily injury is therefore significantly reduced.

The machine is normally started with the throttle valve positioned in a starting position in order to ensure an efficient start-up. Because of the valve position, more air flows into the

motor causing the engine rotational speed to immediately increase above the clutch-in speed of the tool when the engine catches and starts. This can present a risk because the operator will not always be holding the machine in such a way that the security arrangements provide the intended protection. Still further, as the engine speed quickly rises upon starting, the clutch-in speed will be achieved before the operator is ready for the working portion (for example, a chain blade) to begin operation.

In U.S. Pat. No. 4,553,517, an arrangement is described that is intended to work in combination with the centrifugal clutch. The arrangement works in such a way that, simultaneously with the locking of the throttle valve in starting position, a circuit, as part of the ignition system, is activated. The circuit restricts the engine speed to a level below the clutch-in speed of the centrifugal clutch. The switch is deactivated when the throttle valve is no longer in the start position and thereby allows the engine to operate normally.

One problem with this solution is that it operates using a mechanical switch. This means that in case of switch failure, the arrangement will either continuously be in a speed limiting stage or never activate the speed limitation during start-up. Another problem is that the switch, to prevent failure, has to be very reliable and therefore is expensive. A further problem is that the switch cooperates mechanically with the start position knob on the machine and consequently relies on operator manipulation in order to be active during start-up. The switch is activated when activating the start position knob, and if the machine is started with half or wide open throttle valve without activating the knob, the start security system will fail to perform as intended. Still another problem is that the design of each mechanical switch is highly dependent upon the product into which it is being incorporated since the switch must cooperate and coexist with other physical components of the including machine. This means that a special technical design must be used for each product category such as power cutters, chain saws or grass cutters. In view of these drawbacks, an object of the present invention is to solve the above-outlined problems.

SUMMARY OF THE INVENTION

The present invention preferably relates to a method for controlling an ignition system of an internal combustion engine which has a primary firing pulse generator for charging a capacitor and an electronic switch for discharging the capacitor via an ignition coil to generate an ignition voltage. A microcomputer operates the switch to control the ignition

timing of the generator. The microcomputer has speed detection means for directly or indirectly detecting the rotational speed of the engine and a speed limitation control to limit the engine speed to a limitation speed below the clutch-in speed of a centrifugal clutch or alternatively shortly above the clutch-in speed of a centrifugal clutch or alternatively to a higher speed. The method can also be used by controlling a fuel system of the engine.

In one embodiment of the present invention, the speed limitation control is active or activated when starting the engine and is deactivated when the speed detection means detects a high speed state of the engine followed by a low speed state of the engine.

In an another embodiment of the invention, the speed limitation control initiates when certain operational problems are detected and limits the engine speed to a speed above the clutch-in speed. Further embodiments, characteristics and advantages are described below under detailed description with the support of the enclosed drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting an example on how the method operates at a starting period for the combustion engine.

FIG. 2 is a flow chart illustrating how the method operates at a starting period for the combustion engine.

Fig. 3 shows a chainsaw having a button to deactivate the engine speed limitation control.

Fig. 4 is a diagram depicting an example on how the method operates to limit the engine speed in case of an operational problem

DETAILED DESCRIPTION The invention relates to a method for providing a speed limitation control of an internal combustion engine. The speed limitation can be executed by having a microcomputer controlling the ignition of the engine and/or the fuel supply to the engine. The illustrative embodiments shall not be interpreted as a limitation of the invention. The purpose is instead to illustrate how the invention can be applied and to further illustrate the scope of the claimed invention.

Preferably, the combustion engine ignition system has a primary firing pulse generator for charging a capacitor and an electronic switch for discharging the capacitor via an ignition coil to generate an ignition voltage. The ignition system also includes a microcomputer that operates the switch to control ignition timing of the generator. The microcomputer, via the speed detection means, detects the rotational speed of the engine. Within the scope of the invention every speed detection means is considered, including both direct and indirect sensing of the speed of the internal combustion engine. Examples of direct detection or sensing would be the utilization of magnetics or hall-effect sensors for detecting the rotation of the shaft or an electric sensor for detecting the current generated by the primary firing pulse generator. An example of an indirect detection of the rotational speed of the engine would be the detection of the time of ignition in relation to a stroke of the piston. This would be applicable for cases where the engine speed is controlled by varying the time angle of the ignition in relation to the top dead end of the piston.

The engine speed can be reduced in different ways. One way is to switch off the ignition for some cycles. This is an easy way but may cause higher emissions at start-up. Further it will cause a somewhat oscillating engine speed, that could be annoying or could be used to catch operator attention. Another way is to control the timing of the ignition, e.g. reducing the engine torque by delaying the ignition timing. A third way is to control the fuel supply, e.g. in a direct injection system or other systems where a microcomputer can affect the fuel supply. Any of these ways will have the effect of an quick speed limitation, which is preferable. Of course two or possibly all three could be combined.

In the following we will describe an engine speed limitation control at startup of the engine as well as when an operational problem is detected.

Speed limitation at startup

FIG. 2 shows a flow chart illustrating how the method operates at a starting period for the combustion engine. The startup speed limitation control is active or activated when starting the engine and is deactivated when a deactivation condition is fulfilled. This means that when the operator starts the engine, the microcomputer system either immediately or after a short period activates the startup speed limitation control.

In the context of the present disclosure, starting the engine means any starting, either with choke (cold engine), normal start (warm engine), start with a wide-open throttle or start by pumping the choke. To avoid that some few lost ignitions or a tool that gets stuck will be misunderstood by the microcomputer as a turn-off of the engine, it could be preferable to have a short time period delay at start-up before activating the speed limitation control. This period should be so short that it does not cause any failure of the speed limitation control for a normal start.

The startup speed limitation control assures the engine speed is limited to a first limitation speed below the clutch-in speed of a centrifugal clutch. The intention is that this control will, when the operator starts the engine, strictly stop any attempt by the engine or the operator to bring up the speed so that the centrifugal clutch powers the cutting tool (working portion) into rotation. An uncontrolled rotation of the cutting tool could be dangerous for the operator, and the startup speed limitation control avoids the clutch being engaged at start-up. The activation of the startup speed limitation control is only dependent on the starting of the engine and cannot be stopped by the operator. This means that the activation is not related to any requirement except the fact that the engine is being started. The reason for this is to avoid a failure of the speed limitation system. For instance the varying speed at start could, if the activation was dependent on the speed control, result in that there is no activation.

The full drawn curve in Fig.l an illustrative example of the engine speed when starting the engine without any startup speed limitation control. As can be seen the engine speed reaches above the clutch-in speed shortly after start thereby engaging the clutch, which would be undesirable for safety reasons. This could happen in the common start position when the engine is started with the throttle in a start position, i.e. somewhat opened throttle valve.

The dashed curve in Fig. 1 shows the engine speed with the startup speed limitation control active. Here, the engine speed is limited to the first limitation Speed at start until the startup speed limitation control is deactivated by the microcomputer, in this example by having the engine speed at low speed state for a certain time period. To provide a margin, the first limitation speed is distanced from the clutch-in speed due to possible variations in

clutch-in speed. This provides a safer system that better assures keeping below the clutch- in speed even if variations exist.

A number of alternative deactivation conditions for the startup engine speed limitation will now be described below.

In the example shown in Fig.l the deactivation condition is that the speed detection means detects a sufficiently low speed state of the engine. The low speed state corresponds to an operation state of the engine in which the rotational speed of the engine is below the first limitation speed. This also means that the engine speed surely is below the clutch-in speed of the centrifugal clutch. For most engines this low speed state relates to a throttle valve being in its most closed position, i.e. the engines idle position.

Preferably, the speed limitation can only be deactivated after a predetermined time from starting the engine has lapsed. This is to avoid that the speed limitation is deactivated unintentionally at start, since for instance the engine speed varies substantially at the first cycles of ignition. After the predetermined time period has lapsed, the startup speed limitation control is deactivated when a sufficient low speed state is identified.

The pre-defined requirement of a low speed state for deactivation means that the operator, after having started the engine, in most cases has to grip the working machine with both hands, thereby being safely away from the cutting tool. Without doing so, he will not be able to deactivate the speed limitation control. This is because he has to press the handle throttle trigger to unlock the start position of the throttle valve and thereafter stop pressing the throttle trigger thereby bringing the speed down towards idle level. If the engine already is warmed up and started without the throttle valve in start position, the predetermined time period mentioned above prevents undesired early deactivation of the engine speed imitation control. In cases where the engine has a direct fuel injection system, there is no throttle valve to define the idle speed level. However, it should be realized by the person skilled in the art that a low speed state for any combustion engine is included within the scope of the invention.

Moreover, the low speed state has to last for a certain time period before the speed limitation control is deactivated. Preferably, the system will create an average of the speed for a period of 30-100 cycles before deactivating. The reason for this is to avoid

deactivation by mistake, for instance by speed variations at start due to properties of the air/fuel mixture or if the operator pumping the choke or the throttle trigger. The requirement of using an average speed to detect the low speed state means that the operator has to allow the engine to go down to the idle speed for a period of time, which in turn means he will have more control of the machine and probably be safely away from the cutting tool.

Another example of a deactivation condition(s) of the startup engine speed limitation control will now be described in relation to Fig. 1. To deactivate the startup speed limitation control after starting the engine, the speed detection means must first detect a high speed state of the engine and thereafter a low speed state of the engine. More specifically, after start the engine speed (preferably as an average of the speed for a period of e.g. 30-100 cycles) must reach an upper first engine speed threshold Nl. The first engine speed threshold Nl could e.g. be the first limitation speed or a value slightly below the first limitation speed. Secondly, the engine speed, preferably as an average of the speed for a period of e.g. 30-100 cycles, must pass a lower second engine speed threshold N2 lower than the first threshold Nl, for instance a value close to the idle speed. If these two conditions are fulfilled the startup speed limitation is deactivated. Alternatively for the second condition, instead of passing a lower threshold N2, the startup speed limitation is deactivated when the engine speed (preferably as an average of the speed for a period of e.g. 30-100 cycles) exhibits a retardation during a predetermined retardation time and/or if a retardation larger than a retardation derivate threshold.

This means that if the engine is started with the throttle valve in start position the operator has to grip the working machine with both hands, thereby being safely away from the cutting tool. Without doing so, he will not be able to deactivate the speed limitation control. This is because he has to press the handle throttle trigger to unlock the start position of the throttle valve and thereafter stop pressing trigger thereby bringing the speed down towards idle. If the engine already is warmed up and started without the throttle valve in start position he still needs to grip the working machine with both hands, thereby being safely away from the cutting tool. Without doing so, he will not be able to deactivate the speed limitation control. This is because he has to press the handle throttle trigger to bring up the speed to reach the first speed threshold Nl and thereafter release the actuation of the throttle trigger to bring the speed down.

According to another embodiment a distinguished manual input, (e.g. pressing a button) is required from the operator to close a circuit that deactivates the startup speed limitation control. For safety reasons it is preferred that the circuit is required to be open at start to allow deactivation by later closing it by actuating the manual input, i.e. so that if the operator tries to short circuit it to disable the startup engine speed limitation control at start, it will not be deactivated.

Preferably, the start up speed limitation can only be deactivated by the manual input after a predetermined time from starting the engine has lapsed. This is to avoid that the speed limitation is deactivated unintentionally at start.

Moreover, it is preferred that the startup speed limitation control cannot be deactivated by the user manual input when the engine speed is at the first limitation speed, rather it has to be lower than the first limitation speed. This could be implemented by having any threshold speed in the microcomputer between the first limitation speed and the engine idle speed. Of course the requirement could be that the average speed is lower than the threshold during a certain time period. This is to avoid that an accidental actuation of the manual input directly engages the clutch.

For instance starting with start the throttle in start position, the throttle trigger 4 (see Fig. 3) must first be pressed to unlock the start position (and of course the trigger lock 3 needs to be actuated to unlock the throttle trigger), secondly the throttle trigger 4 needs to be released to lower the engine speed below the first limitation speed, where lastly the user manual input can be actuated to deactivate the startup speed limitation control.

When the engine already is warm and started without having the throttle valve in start position, the user manual input can deactivate the startup speed limitation control as soon as the predetermined time period from start has lapsed, provided that the throttle trigger 4 is non-actuated.

The user manual input can be implemented by separate button located on the machine, preferably on the upper part of it to avoid that is unintentionally actuated if the machine is set down on the ground.

Another alternative for the user manual input could be closing a circuit when the trigger lock 3 (see Fig. 3) is actuated.

Another alternative for the user manual input could be closing a circuit when actuating the throttle trigger 4 while the trigger lock 3 is non-actuated.

Another alternative would be to integrate the user manual input with the choke actuator 2 of the machine 1 (see Fig. 3). For instance the choke actuator in WO2007/043930A1, herewith incorporated by reference, could easily be adapted to allow for an inward or upward push in the choke actuator position shown in Fig. 3 of WO2007/043930A1.

Moreover, the user manual input, could have visual or sounds means to inform the operator that the user manual input is ready to be actuated, for instance via an indicator lamp which is turned on when the engine speed is below the first limitation speed after the predetermined time period from start has lapsed.

Moreover, the user manual input could of course also be combined with the above described deactivation conditions described in relation to Fig. 1, where the user manual input could work as a final requirement to deactivate the startup speed limitation control.

Speed limitation when an operational problem occurs

In a related aspect, the speed limitation can be implemented when certain operational problems are encountered. A common problem arises when the working portion of the machine is overloaded to the extent that the engine slows under the load. An illustrative example is the chain blade of a chain saw that is being advanced through a tree log too quickly. As the operator presses down on the chain blade too hard, the chain slows, dragging the engine speed down with it. The corresponding occurs for the rotating saw blade of a cut off saw. Many times it is hard to notice that the tool has slowed or stopped.

A negative outcome usually develops. As described above, in general, the centrifugal clutch begins to engage and transfer torque when a sufficient initial engine speed is achieved, and which is referred to herein as the clutch-in speed. But as engine speed continues to increase, the clutch continues to engage more and more, permitting less and less relative slip until a fully engaged engine speed is reached. Once the fully engaged speed and clutch configuration is achieved, essentially no relative slip is permitted in the centrifugal clutch and a substantially direct drive connection is affected across the clutch. The speed range beginning with the speed at which initial clutch engagement occurs and

continuing until full clutch engagement occurs is referred to as the slip speed range.

As described above, the slip speed range is entered from the lower end at startup with the engine speed beginning at zero and increasing therefrom. After crossing engine idle speed and the first limitation speed, which is preferably slightly above the engine idle speed, the initial clutch-in speed is reached. During typical operation, the engine's speed continues to increase across the slip speed range until the clutch is fully engaged, and then beyond for high-speed, high-powered machine operation.

As intimated above, however, when the working tool is experiencing overload, the slowing engine enters the slip range speed from the upper end. Even though the engine is slowing under the overwhelming load, full or near full power is normally still being applied by the engine in an effort to urge the working tool back to the faster working speed. However, as the centrifugal clutch enters the slip speed range from the top end, slippage begins to be allowed, but heavy clutch engagement is still being affected. This is a detrimental situation because the permitted high-friction producing clutch slippage generates potentially harmful friction heat. Normally, the rotating tool, such as a binding chain blade, actually stops rotating and all of the engine torque is being dissipated in the centrifugal clutch - which can get damagingly hot. Still further, depending upon the power rating of the engine and the paired clutch, the situation can continue for prolonged periods until the operator becomes aware that clutch slippage is occurring. Because the actual rotation of the working tool is not always visible or otherwise obvious to the operator during operation due to such things as blade covers, flying saw dust and the fact that the working tool can be buried in the material being cut or otherwise worked on, detrimental and damaging operation can continue for long periods causing damage to the machine and potentially threatening the safety of the operator. The amount of generated heat-energy can be substantial; on the order of several kilowatts. At these levels, the generated heat can not only be damaging to the clutch itself, but also to surrounding covers and other nearby components of the machine such as the drive belt, bearings and clutch drum, among others.

Therefore, in another protective aspect of the present disclosure, there is an operational problem engine speed limitation control which is activated when it is detected that prolonged operation has occurred below a third engine speed threshold N3. The third engine speed threshold N3 can be in the slip speed range or alternatively shortly above the

slip speed range. Having the third engine speed threshold N3 shortly above the slip speed range activates the operational problem engine speed limitation control before the clutch starts to slip minimizing the wear of the clutch. Among other indicators, excessive clutch slip conditions can be sensed and analyzed using the microprocessor to determine whether excessive clutch slip is occurring by counting the number of consecutive engine revolutions that occur below the third engine speed threshold N3. When the number exceeds a predetermined limit, the operational problem engine speed limitation control is initiated.

Three alternative operational problem speed limitation controls are suggested. A first operational problem speed limitation control will limit the engine speed to the first limitation speed when an operational problem is detected, i.e. below the clutch-in speed. This first operational problem speed limitation control closely corresponds to the startup engine speed limitation. Since the speed is reduced below the clutch-in speed, there will be no substantial wear on the clutch.

It is desirable to have the acknowledgment of the operator that it is recognized that operational speed limitation control is active and that the throttle is no longer controlling engine speed. Since the microprocessor control takes engine speed down to a speed approaching idle speed, it is advantageous to require that the operator release the throttle trigger in a manner that would have otherwise also affected idle speed operation of the engine, much as it has been controlled to, independent of the operator. Therefore, in the preferred embodiment, once the first operational engine speed limitation control has been activated, the operator must release the throttle trigger for a resetting period of time after which the limitation control is deactivated and the engine speed can once again be increased for use. Further, it may also be advantageous to use this reset procedure when start-up engine speed limitation control is initiated so that the operator becomes accustomed to the routine for deactivating the control and readying the machine for active use.

In Fig. 4 a second operational problem speed limitation control will limit the speed to a second limitation speed below a forth engine speed threshold N4 being above the clutch-in speed. Preferably, the second limitation speed is within 2000 rpm from the clutch-in speed of the centrifugal clutch, preferably within 1000 rpm, more preferably within 500 rpm.

Limiting the engine speed to being close to the clutch-in speed reduces the engine torque and thereby the friction wear of the clutch is reduced to a sustainable level, but at the same time the operational function of the tool is not completely removed. In fact as the torque reduces, the fully engaged clutch speed will move towards the clutch-in speed, so that speed limitation control may here function as an anti-spin for the clutch, minimizing the slip speed range. Preferably the engine speed is reduced by controlling the fuel supply and/or by controlling the timing of the ignition. The operator may find this speed limitation control less abrupt while operating the tool. Here, the deactivation can be set to monitor a speed increase from the second limitations speed, i.e. if the operator removes the tool from the work piece or lessens the pressure on it the engine may speed up to full working speed.

In a third method an operating problem of the hand held power tool is detected by measuring a rotational speed of the working tool or its shaft and comparing it with the speed of the engine, and the degree of slip thus determined will result in a special limitation speed, the lowest being a speed below a clutch in speed of a centrifugal clutch. In a special alternative the hand held power tool is a cut off saw and the operating problem occurs above the end of clutch-in speed of the centrifugal clutch and is a slip problem of its drive belt detected by measuring the rotational speed of the working tool or its shaft and comparing it with the speed of the engine.

For all the above described speed limitations controls, to provide operator feedback that the engine speed limitation is active, operator perceivable signals, such as visible or audible signals, can be provided. An example would be indicator lamps; one lamp can be provided to generically indicate that an operating problem exists, or unique indicators can be provided that correlate to the particular cause of the problem or the component to which the problem relates such as the clutch, engine or the like.