Method for operating a handheld work apparatus having a combustion engine

Abstract

A handheld work apparatus has a combustion engine which drives a tool of the work apparatus via a centrifugal clutch. The centrifugal clutch couples in an engagement rotational speed range (n.sub.K) which extends between a lower engagement rotational speed (n.sub.u) and an upper engagement rotational speed (n.sub.o). The engine has a fuel supply device, an ignition device, a control device and a device for detecting the rotational speed (n) of the engine. A method for operating the handheld work apparatus makes provision for the rotational speed profile of the combustion engine to be monitored in the engagement rotational speed range (n.sub.K) and for the power (P) output for driving the tool to be increased from an operating power (P.sub.1) to an increased power (P.sub.2, P.sub.3) when the rotational speed profile corresponds with a predetermined rotational speed profile over a predetermined period of time (?t).

Claims

1. A method for operating a handheld work apparatus which includes a centrifugal clutch, a work tool and a combustion engine for driving said work tool via said centrifugal clutch, said centrifugal clutch defining a coupling engaging rotational speed range (n.sub.K) wherein said centrifugal clutch engages and said coupling engaging range extends between a lower coupling engaging rotational speed (n.sub.u) and an upper coupling engaging rotational speed (n.sub.o); said combustion engine including a fuel metering device, an ignition device, a control unit, a throttle lever, and a device for detecting the rotational speed (n) of said combustion engine, the method comprising the steps of: monitoring the rotational speed profile of said combustion engine in said coupling engaging rotational speed range (n.sub.K); and, when the throttle lever is fully actuated, increasing the power (P) delivered to drive said work tool from an operating power (P.sub.1) to an increased power (P.sub.2, P.sub.3) when said rotational speed profile corresponds to a pregiven rotational speed profile over a pregiven time span (?t) and when the rotational speed (n) is below the upper coupling engaging rotational speed (n.sub.o).

2. The method of claim 1, wherein said pregiven time span (?t) is at least 0.1 s.

3. The method of claim 1, wherein said increased power (P.sub.2, P.sub.3) is at least 103% of said operating power (P.sub.1).

4. The method of claim 1, wherein said power (P) is increased in a discontinuous jump.

5. The method of claim 1, wherein after the power increase, said power (P) is reduced at least once and again increased.

6. The method of claim 1, wherein said power (P) is set back to said operating power (P.sub.1) when said rotational speed (n) leaves said coupling engaging rotational speed range (n.sub.K).

7. The method of claim 1, wherein said power (P) is set back to said operating power (P.sub.1) after the elapse of a pregiven time (?t.sub.2).

8. The method of claim 1, wherein said power (P) is set back to said operating power (P.sub.1) after reaching a pregiven temperature of said combustion engine.

9. The method of claim 1, wherein said power (P) delivered by said combustion engine is increased by shifting the ignition time point (ZZP) of said combustion engine.

10. The method of claim 1, wherein said power (P) delivered by said combustion engine is increased by changing the metered fuel quantity (x).

11. The method of claim 1, wherein said power (P) delivered by said combustion engine is increased by changing the quantity of combustion air supplied to said combustion engine.

12. The method of claim 1, wherein said power (P) delivered by said combustion engine is increased by changing the quantity (y) of air/fuel mixture supplied to said combustion engine.

13. The method of claim 1, wherein, in addition to said work tool, said handheld work apparatus includes at least one additional consumer of energy which said combustion engine supplies with energy.

14. The method of claim 13, wherein said power (P) is increased by switching off said at least one additional consumer of energy.

15. The method of claim 13, wherein said power (P) is increased by reducing the energy supplied to said at least one additional consumer of energy.

16. The method of claim 1, wherein said work apparatus further includes an operator-controlled element and said power (P) of said combustion engine is increased when said operator-controlled element is actuated.

17. A method for operating a handheld work apparatus which includes a centrifugal clutch, a work tool and a combustion engine for driving said work tool via said centrifugal clutch, said centrifugal clutch defining a coupling engaging rotational speed range (n.sub.K) wherein said centrifugal clutch engages and said coupling engaging range extends between a lower coupling engaging rotational speed (n.sub.u) and an upper coupling engaging rotational speed (n.sub.o); said combustion engine including a fuel metering device, an ignition device, a control unit and a device for detecting the rotational speed (n) of said combustion engine, the method comprising the steps of: monitoring the rotational speed profile of said combustion engine in said coupling engaging rotational speed range (n.sub.K); and, increasing the power (P) delivered to drive said work tool from an operating power (P.sub.1) to an increased power (P.sub.2, P.sub.3) when said rotational speed profile corresponds to a pregiven rotational speed profile over a pregiven time span (?t), wherein said pregiven rotational speed profile is a constant rotational speed (n.sub.1).

18. A method for operating a handheld work apparatus which includes a centrifugal clutch, a work tool and a combustion engine for driving said work tool via said centrifugal clutch, said centrifugal clutch defining a coupling engaging rotational speed range (n.sub.K) wherein said centrifugal clutch engages and said coupling engaging range extends between a lower coupling engaging rotational speed (n.sub.u) and an upper coupling engaging rotational speed (n.sub.o); said combustion engine including a fuel metering device, an ignition device, a control unit and a device for detecting the rotational speed (n) of said combustion engine, the method comprising the steps of: monitoring the rotational speed profile of said combustion engine in said coupling engaging rotational speed range (n.sub.K); and, increasing the power (P) delivered to drive said work tool from an operating power (P.sub.1) to an increased power (P.sub.2, P.sub.3) when said rotational speed profile corresponds to a pregiven rotational speed profile over a pregiven time span (?t), wherein said handheld work apparatus has an additional drive motor and said power (P) is increased by switching in said additional drive motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described with reference to the drawings wherein:

(2) FIG. 1 is a schematic of a motor-driven saw;

(3) FIG. 2 is a schematic section through the motor-driven saw from FIG. 1;

(4) FIG. 3 is a schematic of the carburetor of the motor-driven saw from FIG. 1;

(5) FIG. 4 is a diagram which indicates, by way of example, the profile of the rotational speed of the combustion engine of the motor-driven saw from FIG. 1 over time;

(6) FIG. 5 is a diagram which schematically indicates a possible profile of the ignition time over time;

(7) FIG. 6 is a diagram which schematically indicates a possible profile of the quantity of fuel supplied over time;

(8) FIG. 7 is a diagram which schematically indicates a possible profile of the quantity of air/fuel mixture supplied over time;

(9) FIG. 8 is a diagram which schematically indicates possible profiles of the power of the combustion engine over time; and,

(10) FIG. 9 is a diagram which schematically indicates a further possible profile of the ignition time over time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

(11) FIG. 1 schematically shows a motor-driven saw 1 of an embodiment of a handheld work apparatus. However, the present invention can also be provided in the case of other handheld work apparatuses, such as, for example, cut-off machines, stone cutters, hedge shears or the like. The motor-driven saw 1 has a housing 2, a rear handle 3 and a bale handle 4. A guide bar 6, on which a saw chain 7 is arranged in a revolving manner, is fixed to the housing 2. A hand guard 5 which can serve at the same time for triggering a chain brake (not shown) is arranged on that side of the bale handle 4 which faces the guide bar 6.

(12) A combustion engine 8 arranged in the housing 2 serves for driving the saw chain 7. The combustion engine 8 is a single-cylinder engine, advantageously a mixture-lubricated engine, such as a two-stroke engine, or a mixture-lubricated four-stroke engine. The combustion engine 8 draws in combustion air via an air filter 28 and a carburetor 9. A fuel valve which supplies the fuel directly into the combustion engine 8 can also be provided instead of the carburetor 9. The combustion engine 8 has a spark plug 10 which is supplied with electrical power by an ignition module 11.

(13) A throttle lever 12, which is mounted pivotably on the rear handle 3, is provided for operating the combustion engine 8. In addition, a throttle lever lock 13, which prevents an unintentional actuation of the throttle lever 12, is mounted pivotably on the rear handle 3. An operating mode selector 14 is arranged on the housing 2 adjacent to the rear handle 3. The operating mode selector 14 advantageously serves for setting at least one starting position of the combustion engine 8 and for switching off the combustion engine 8. In addition, the motor-driven saw 1 has an operating element 15 which, in the embodiment, is arranged adjacent to the operating mode selector 14 and the function of which is also explained in more detail below.

(14) As FIG. 2 shows, the spark plug 10 protrudes into a combustion space 17 of the combustion engine 8. The combustion space 17 is bounded by a piston 16 which drives a crankshaft 19 in a rotating manner via a connecting rod 18. The crankshaft 19 is driven in a rotating manner about an axis of rotation 20. A flywheel 21 which can be, for example, a fan wheel is fixed to the crankshaft 19. The flywheel 21 has magnets (not shown) which, at the ignition module 11, induce a voltage which serves for generating the ignition spark at the spark plug 10. In the embodiment, a generator 22 is also fixed to the crankshaft 19, the generator serving to generate electrical energy. In the embodiment, the generator 22 is arranged in the region of the flywheel 21. However, a different arrangement may also be advantageous. A starter 23 for starting the combustion engine 8 is arranged on that side of the flywheel 21 which faces away from the combustion engine 8. In the embodiment, the starter 23 is an electric starter which comprises a drive motor 48. The starter 23 has a coupling device 49 via which the drive motor 48 acts on the crankshaft 19. However, the starter 23 may also be a starter which can be actuated manually, for example a pull-rope starter.

(15) A centrifugal clutch 24 is arranged on that side of the combustion engine 8 which is opposite the flywheel 21. The drive part 43 of the centrifugal clutch 24 is connected to the crankshaft 19 for rotation therewith. The drive part 43 advantageously comprises one or more centrifugal weights which are mounted so as to be pivotable radially outward with respect to the axis of rotation 20 and in a spring-loaded manner. The output part 44 is configured as a clutch drum which is connected to a drive pinion 26 for rotation therewith. The drive pinion 26 drives the saw chain 7 (not shown in FIG. 2). In addition, the output part 44 drives an oil pump 25 (shown schematically in FIG. 2) which serves for delivering lubricating oil for the saw chain 7.

(16) As FIG. 2 schematically shows, the combustion engine 8 has a temperature sensor 42. In the embodiment, the temperature sensor 42 is arranged adjacent to the combustion space 17. However, a different arrangement may also be advantageous. The temperature sensor 42 can be arranged, for example, on a crankcase of the combustion engine 8. The combustion engine 8 has a control device 41 which, in the embodiment, is integrated on the ignition module 11. However, a separate configuration of the control device 41 may also be advantageous. The temperature sensor 42 is connected to the control device 41. In order to detect the speed of the combustion engine 8, the control device 41 advantageously evaluates the signal of the voltage induced at the voltage module 11. However, a separate speed sensor may also be advantageous. A signal of the generator 22 can also be evaluated in order to determine the speed of the combustion engine 8.

(17) FIG. 3 schematically shows the carburetor 9 and the air filter 28. Combustion air is drawn in by the combustion engine 8 in a flow direction 30 by the air filter 28 and an intake channel 29 formed in the carburetor 9. A choke flap 34 with a choke shaft 36 is mounted pivotably in the intake channel 29. A throttle flap 35 with a throttle shaft 37 is mounted pivotably downstream of the choke flap 34 with respect to the flow direction 30. A different configuration of a throttle element and of a choke element may also be advantageous. The choke element may also be omitted. A venturi 31 is formed in the intake channel 29. A main fuel opening 32 leads into the intake channel 29 in the region of the venturi 31. A plurality of secondary fuel openings 33 lead into the intake channel 29 downstream of the main fuel opening 32. The fuel openings 32 and 33 can be fed by a fuel-filled control space of the carburetor 9. The carburetor 9 is advantageously a diaphragm-type carburetor which supplies the fuel depending on the negative pressure in the intake channel 29 and depending on a reference pressure. However, provision may also be made for the fuel openings 32 and 33 to be fed via a fuel valve, for example a solenoid valve. Provision may also be made for the fuel not to be supplied via a carburetor 9, but rather directly into the combustion engine 8, for example via one or more fuel valves. The fuel can be supplied here, for example, into the combustion space 17 or into a crank case of the combustion engine 8.

(18) In order to control the quantity of fuel supplied, the throttle flap 35 is mounted pivotably. The completely open position of the throttle flap 35 is defined by an end stop 38 which interacts with a lever 47 (shown schematically in FIG. 3) which is connected to the throttle shaft 37 for rotation therewith. The lever 47 and the end stop 38 are arranged outside the intake channel 29, advantageously on the outside of a housing of the carburetor 9.

(19) The throttle lever 12 advantageously acts on the throttle flap 35. When the throttle lever 12 is fully actuated, the lever 47 bears against the end stop 38. As indicated schematically by the dashed line in FIG. 3, the end stop 38 is adjustable via an actuator 39. If the actuator 39 is actuated, the throttle flap can be adjusted, for example, into the position 35 shown by a dashed line in FIG. 3. By actuation of the actuator 39, the quantity of combustion air supplied to the combustion engine 8 can be increased when the throttle lever 12 is fully actuated.

(20) FIG. 4 schematically shows, by means of a curve 40, a possible profile of the speed (n) of the combustion engine 8 over time (t). The speed (n) initially rises sharply. In the process, the speed (n) runs continuously through an engagement speed range n.sub.K which extends from a lower engagement speed n.sub.u to an upper engagement speed n.sub.o. When the lower engagement speed n.sub.u is reached, the at least one centrifugal weight of the drive part 43 is placed against the clutch drum of the output part 44. Until the upper engagement speed n.sub.o is reached, the centrifugal weight is pressed with increasing force against the clutch drum. As a result, when the speed of the drive part 43 increases, the power transmittable via the centrifugal clutch 24 rises. The curve 40 shows the speed profile (n) when the throttle lever 12 is fully actuated. The fluctuations in the speed (n) arise due to a different load, for example if the feed differs, that is, if the operator presses the motor-driven saw 1 more or less strongly into the workpiece to be cut. At the time t.sub.1, the speed (n) has dropped under the upper engagement speed n.sub.o to a speed n.sub.1. The speed remains constant after the speed n.sub.1 has been reached.

(21) The control device 41 monitors the profile of the speed (n) in the engagement speed range n.sub.K and recognizes that the speed n.sub.1 remains constant over a period of time ?t up to a predetermined second time t.sub.2. Speed fluctuations which, in the case of a combustion engine, occur within an engine cycle and over a plurality of engine cycles due to the design are not taken into consideration in the determination of the constant speed profile. The period of time here is advantageously at least 0.1 s, in particular at least 0.3 s, preferably at least 0.5 s. The period of time ?t is advantageously less than approximately 30 s, in particular less than approximately 10 s, preferably less than approximately 5 s. After the predetermined period of time ?t has elapsed, the control device 41 takes measures for briefly increasing the power output by the combustion engine 8 for driving the saw chain 7. As a result, the speed of the drive part 43 is increased, and the at least one centrifugal weight is pressed outward with greater force. By this means, the frictional force in effect is increased and therefore the power available for driving the saw chain 7 rises.

(22) In order to increase the power, it is possible, for example, for the ignition time ZZP to be adjusted. A possible profile of the ignition time is illustrated schematically in FIG. 5. Up to the ignition time ZZP.sub.2, the ignition time ZZP is subject to only small fluctuations. The ignition time ZZP may also be constant. At the time t.sub.2, that is, after the speed (n) has been constant for a period of time ?t, the ignition time is abruptly adjusted from an ignition time ZZP.sub.1 to an ignition time ZZP.sub.2. The ignition time ZZP.sub.2 is advantageously significantly earlier than the ignition time ZZP.sub.1. The ignition time ZZP.sub.2 can be located, for example, approximately 10? crankshaft angle before the first ignition time ZZP.sub.1. The ignition time ZZP.sub.2 is located here closer to the power-optimum ignition time than the ignition time ZZP.sub.1. By adjusting the ignition time from the first ignition time ZZP.sub.1 to the second ignition time ZZP.sub.2, the power output by the combustion engine 8 is increased. This is shown schematically in FIG. 8.

(23) At the second time t.sub.2, the power P output by the combustion engine corresponded to an operating power P.sub.1. Upon adjustment of the ignition time from the ignition time ZZP.sub.1 to the ignition time ZZP.sub.2 to advanced, the power has abruptly increased to an increased power P.sub.2. As indicated by the line 45 in FIG. 8, the increased power P.sub.2 remains approximately constant as far as a third time t.sub.3. As FIG. 4 shows, the speed increases again above the upper engagement speed n.sub.o before the third time t.sub.3 is reached. The power P can advantageously be reset to the operating power P.sub.1 when the speed (n) leaves the engagement speed range n.sub.K. However, it may also be advantageous to reset the power P to the operating power P.sub.1 after a predetermined time ?t.sub.2 has elapsed. The predetermined time ?t.sub.2 can advantageously be from approximately 0.1 s to approximately 60 s, in particularly from approximately 0.5 s to approximately 30 s, preferably from approximately 1 s to approximately 10 s. The power P can alternatively be reset to the operating power P.sub.1 if the temperature of the combustion engine 8, which temperature is determined by the temperature sensor 42, reaches a predetermined value. The power is reset from the increased power P.sub.2 to the operating power P.sub.1 at the third time t.sub.3. In an advantageous manner, a plurality of criteria for the resetting of the power to the operating power P.sub.1 are monitored and the power is reset to the operating power P.sub.1 as soon as one of the criteria is satisfied.

(24) Instead of adjusting the ignition time ZZP, provision may also be made to change the quantity of fuel (x) supplied to the combustion engine 8. This is shown schematically in FIG. 6. At the time t.sub.2, the quantity of fuel (x) supplied to the combustion engine 8 is changed from a first quantity of fuel x.sub.1 to a second quantity of fuel x.sub.2. In the embodiment shown, the supplied quantity of fuel (x) is reduced. The power P of the combustion engine 8 is also increased by reducing the quantity of fuel (x) supplied. The resulting profile of the power P corresponds to the profile shown in FIG. 8 by a dashed line 45 from the time t.sub.2 to the time t.sub.3. However, provision can also be made to increase the quantity of fuel (x) supplied in order to increase the power P if the combustion engine 8 is operated with a lean mix. In order to increase the power P, provision may also be made to change, preferably to increase, the quantity (y) of fuel/air mixture supplied to the combustion engine 8. This is shown schematically in FIG. 7. At the time t.sub.2, the quantity (y) of fuel/air mixture supplied to the combustion engine 8 is changed, increased in the embodiment shown, from a first quantity y.sub.1 to a second quantity y.sub.2. The profile of the power P that is shown by a dashed line 45 in FIG. 8 from the time t.sub.2 to the time t.sub.3 is then also produced.

(25) In order to increase the power provided by the combustion engine 8 for driving the saw chain 7, provision can also be made to switch off at least one additional consumer of the motor-driven chain saw 1 or to reduce the energy supplied to the consumer. For example, provision can be made to switch off the oil pump 25. This is indicated schematically in FIG. 2 by the arrow 27. For example, the oil pump 25 can be disengaged from the drive pinion 26 or the output part 44. Alternatively, the stroke of the oil pump 25 can be set to zero. In order to reduce the energy supplied to the oil pump 25, the stroke of the oil pump 25 can be reduced so as to reduce the delivery. Alternatively or in addition, the energy provided to the generator 22 can be reduced.

(26) In order to increase the power available for driving the saw chain 7, provision may also be made additionally to use the drive motor 48 of the starter 23 for driving the saw chain 7. The drive motor 48 can exert an additional driving torque on the crankshaft 19 via the coupling device 49. As a result, a significant increase in power can be achieved. The increased power can be, for example, approximately 150% to approximately 250% of the operating power. The drive motor 48 is advantageously battery-operated. For example, the drive motor can be supplied with energy by approximately five lithium-ion storage batteries. The storage batteries can be charged by the combustion engine 8 during operation. With a drive motor 48 of this type, it is possible, for example, to generate an additional torque of the order of magnitude of approximately 1 Nm. In order to avoid excessive heating of the drive motor 48, it is provided that the predetermined time, after which the power is reset again to the operating power, is up to approximately 10 s. The predetermined time is advantageously at least approximately 5 s.

(27) In order to increase the power of the combustion engine 8, provision may also be made to change the quantity of combustion air supplied to the combustion engine 8, for example by adjusting the end stop 38 into the position shown by dashed lines in FIG. 3. As a result, the quantity of combustion air supplied to the combustion engine 8 is increased, that is, the mixture supplied to the combustion engine 8 is made lean. This produces an increase in the power.

(28) It can also be advantageous to combine a plurality of measures for increasing the power P output by the combustion engine 8 for driving the saw chain 7. The measures mentioned for increasing the power P can be used in each case by themselves or in any combination for increasing the power P of the combustion engine 8.

(29) In order to assist breaking a tool loose from a cut, provision may be made, after the increase in the power, to reduce the power P of the combustion engine 8 again and to increase it again. In particular, provision may be made to increase the power P in an alternating manner. This is shown schematically in FIG. 9 for the ignition time ZZP. The ignition time is first of all adjusted from the ignition time ZZP.sub.1 to an earlier ignition time ZZP.sub.2 and subsequently to a somewhat later ignition time ZZP.sub.3 in order then to be adjusted as far as the third time t.sub.3 between the ignition times ZZP.sub.2 and ZZP.sub.3. This results in a zigzag-shaped profile of the ignition time point as a function of time. A corresponding zigzag-shaped profile is produced for the power P of the combustion engine 8, as shown schematically by the line 46 in FIG. 8. Provision may also be made to adjust the power between the first power P.sub.1 and the second power P.sub.2. This arises by adjusting the ignition time point between the first ignition time point ZZP.sub.1 and the second ignition point ZZP.sub.2. If the ignition time point is set to the third ignition time point ZZP.sub.3 which lies between the first ignition time point ZZP.sub.1 and the second ignition time point ZZP.sub.2, a third power P.sub.3 lying between the operating power P.sub.1 and the increased power P.sub.2 is produced. A wave-shaped profile of the power P and of the ignition time point ZZP or an abrupt change between two ignition time points and therefore between two power levels may also be advantageous.

(30) In order to increase the power P of the combustion engine 8, the operator can also actuate the operating element 15. The operation of the operating element 15 brings about an increase in the operating power P.sub.1 to an increased power P.sub.2 or P.sub.3. The increased power can be maintained for a predetermined period of time or until a predetermined, increased temperature of the combustion engine 8 is reached, in order then to be reset to the operating power P.sub.1. If the increased power (P.sub.2, P.sub.3) is of such a low level that it is suitable for continuous operation, provision may also be made to reset the power to the operating power P.sub.1 again only when the operator releases the operating element 15.

(31) It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.