METHOD FOR OPERATING A WORKING DEVICE WITH AN INTERNAL COMBUSTION ENGINE

Description

[0001] The invention relates to a method for operating a working device with an internal combustion engine according to the preamble of claim 1.

[0002] Common tools such as chainsaws, brush cutters, power cutters, blowers, and the like can be powered by an internal combustion engine, often a two-stroke or four-stroke engine. To operate the internal combustion engine, an ignition device is provided for triggering an ignition spark at a spark plug, which ignites a fuel / air mixture in the combustion chamber of the internal combustion engine. The piston defining the combustion chamber is accelerated downward by the combustion pressure and rotates a crankshaft, with the piston traveling through a stroke between top dead center and bottom dead center.

[0003] Depending on the rotational position of the crankshaft, the ignition device triggers the ignition spark at the spark plug at an ignition point in order to ensure the engine runs depending on the operating situation.

[0004] The rotating crankshaft of the internal combustion engine drives a tool via a clutch, which is preferably designed as a centrifugal clutch.

[0005] If a tool jams while operating, such as a chainsaw, a power cutter, or similar, the machine continues to draw in fuel at the same rate as at full load because the operator continues to apply full throttle. This leads to a rich mixture as the engine speed decreases. If the operator suddenly releases the throttle, this can cause the engine to stall.

[0006] A similar situation occurs when the operator starts the implement's combustion engine while the safety brake is engaged. If the operator notices this and suddenly releases the throttle, the machine often stalls on a rich fuel level.

[0007] The fuel / air mixture supplied to the combustion engine under full load is ignited at a predetermined ignition point before TDC, taking the full-load speed into account, and burns in such a way that sufficient pressure buildup is achieved to drive the piston. However, with a retarded ignition point, a rich fuel / air mixture ignited under full load does not have sufficient time for complete combustion or for the formation of the flame front and the associated pressure buildup necessary to drive the piston. This can lead to the combustion engine stalling "rich."

[0008] EP 2 270 325 A2 discloses a method for setting an operating point for an internal combustion engine. First, the current position of the operating point on the power curve is determined by adjusting the ignition timing and evaluating the engine's speed response. A desired operating point can then be set by adjusting the fuel supply.

[0009] EP 2 623 752 A2 describes a hand-held implement with an internal combustion engine whose operating speed is adjustable. This allows the implement to operate at reduced speed despite the full throttle setting.

[0010] US 2014 / 165 964 A1 describes another method for operating an internal combustion engine in a hand-held tool in which the ignition timing is adjusted depending on the speed.

[0011] The invention is based on the object of specifying a method for operating a working device with an internal combustion engine, in which a rich running out of the internal combustion engine is avoided even under unfavorable operating conditions.

[0012] The object is achieved according to the invention according to the features of claim 1.

[0013] The engine speed is monitored, preferably continuously. This allows it to be determined whether the engine speed remains within a specified speed range within a specified period of time.

[0014] If it is determined - for example via a speed monitoring circuit - that the speed is within a specified speed range for a specified period of time and then the speed of the combustion engine falls below a speed limit, the ignition timing set by the ignition device is advanced.

[0015] By significantly advancing the ignition timing, it is avoided that the rich fuel / air mixture is already highly compressed and difficult to ignite at the time of ignition. Advancing the ignition timing ensures that the ignition spark occurs at a time when the rich fuel / air mixture is still ignitable, thus initiating combustion and allowing the flame front to spread. This ensures that, for example, if a tool is blocked and the engine speed drops to the specified speed range, an abrupt release of the throttle trigger—and the associated sudden drop in the engine speed below a speed limit—does not lead to the engine stalling rich.

[0016] The conditions of the position of the speed in a predetermined speed range and a subsequent significant drop in speed below a speed limit can be easily linked by setting a status indicator when the first condition is met and then advancing the ignition timing when the drop in speed below the speed limit is detected when the status indicator is set.

[0017] Once set, a status indicator is held until the engine speed drops below the engine speed limit and the ignition is advanced. A set status indicator can also be held until the engine speed rises again and is above the specified engine speed range or exceeds the upper limit of the engine speed range.

[0018] It may be useful to hold the status indicator for a specified period of time. A period of 2 to 3 seconds may be advantageous.

[0019] The specified period of time within which the speed should remain within a specified speed range and / or the period of time for which a set status indicator is maintained can advantageously be determined by a specified number of crankshaft revolutions. In a simple manner, the period of time is determined by five to fifty consecutive crankshaft revolutions; in particular, the period of time is given by ten crankshaft revolutions. Determining the period of time by crankshaft revolutions has the advantage that the time is adapted to the current speed of the internal combustion engine without requiring any special measures.

[0020] It is advisable for the crankshaft revolutions that determine the duration to follow one another immediately.

[0021] The speed limit below which the engine must be undercut after the first condition is met is below the specified speed range. The speed limit is lower than the lower speed limit of the speed range.

[0022] The clutch between the crankshaft and the tool is preferably a centrifugal clutch. The engagement speed of this centrifugal clutch is below the specified speed range.

[0023] Further features of the invention will become apparent from the further claims, the description, and the drawing, which illustrates an exemplary embodiment of the invention described in detail below. They show: Fig. 1 shows a perspective view of a working device with an internal combustion engine using the example of a motor chainsaw, Fig. 2 shows a perspective view of a working device with an internal combustion engine using the example of a cut-off grinder, Fig. 3 shows a schematic view of a device for controlling the ignition point for the ignition of the internal combustion engine, Fig. 4 shows a flow chart for the operation of the working device, Fig. 5 shows an ignition point curve in a forked curve design.

[0024] In Fig. 1 As an example of a working device 1 with an internal combustion engine, a motor chainsaw is shown, the saw chain 3 of which rotates on a guide bar 2 and is driven by the internal combustion engine 5. The working device has a rear handle 11 that extends essentially in the longitudinal direction of the working device 1. A throttle lever 28 for controlling the internal combustion engine 5 is provided in the rear handle 11.

[0025] A front handle 12 is designed as a loop handle and spans the housing 13 of the implement 1 from one long side to the other. A hand guard 14 is provided in front of the front handle 12, which serves as a trigger for a safety brake 16 located beneath a sprocket cover 15.

[0026] In Fig. 2 As a further example of a working device 1 with an internal combustion engine, a cut-off grinder is shown, the cutting disc 3a of which is driven by the internal combustion engine 5. A hood 2a is assigned to the cutting disc 3a, which engages over the cutting disc 3a for protection and extends over a circumferential angle of 180°. The working device 1 designed as a cut-off grinder has a rear handle 11 that extends essentially in the longitudinal direction of the working device 1. A throttle lever 28 for controlling the internal combustion engine 5 is provided in the rear handle 11. A front handle 12 is designed as a bow handle and engages over the housing 13 of the working device 1 from one long side to the other long side.

[0027] The combustion engine 5 arranged in the housing of the working device 1 is shown in a schematic representation in Fig. 3 and comprises a cylinder 4 with a combustion chamber 6, which is delimited by a piston 7. The piston 7 drives a crankshaft 9 via a connecting rod 8, which is mounted in a crankcase 10.

[0028] The internal combustion engine 5 draws an ignitable fuel / air mixture into the crankcase 10 via an air filter 17 and a mixture formation device shown as an example, in the exemplary embodiment a carburetor 18. The mixture formation device can also be an electronically controlled carburetor or comprise a fuel injection pump. The ignitable fuel / air mixture formed in the mixture formation device is conveyed from the crankcase 10 into the combustion chamber 6 via at least one overflow channel 19 during a downward movement of the piston 7. During a subsequent upward movement of the piston 7, the mixture is compressed in the combustion chamber 6 and ignited by a spark plug 20. The combustion pressure building up during combustion drives the piston 7 downward again, with the combustion gases being discharged via an outlet 21.

[0029] The mixture formation device, in the embodiment the carburettor 18, has a throttle valve 22, the rotational position of which is controlled by the throttle lever 28 ( Fig. 1, 2 ) in the rear handle 11 of the working device 1.

[0030] The spark plug 20 is controlled by an ignition device 30 which, depending on the rotational position of the crankshaft 9, triggers an ignition spark 27 at the spark plug 20 at an ignition point ZZP.

[0031] The rotating crankshaft 9 drives the tool via a clutch 23, in the embodiment shown the saw chain 3 of the motor chain saw to Fig. 1 . For this purpose, the clutch 23 drives a chain wheel 24, via which the saw chain 3 is driven.

[0032] When the combustion engine is running at full load, for which the throttle valve 22 is fully open (dashed position), a rich fuel / air mixture is supplied to the combustion chamber 6. If, for example, the hand guard 14 of the safety brake 16, which acts as a trigger, is triggered, the output side of the clutch 23, which is designed as a centrifugal clutch, is braked. The engine speed drops; the combustion engine works against the applied safety brake 16.

[0033] A similar situation arises when the tool driven by the combustion engine via the centrifugal clutch becomes blocked. For example, the saw chain 3 or the cutting disc 3a may jam during the cut.

[0034] According to the invention, the rotational speed n of the internal combustion engine 5 is monitored, in particular continuously monitored. For this purpose, a rotational speed sensor 25 is arranged on the crankshaft, the output signal of which is fed to a rotational speed monitoring circuit 26. The rotational speed monitoring circuit 26 is preferably integrated into the ignition device 30 for adjusting the ignition point ZZP; the rotational speed monitoring circuit 26 is electrically connected to the ignition device 30.

[0035] During operation of the combustion engine 5, the speed sensor 25 reports the current speed n to the speed monitoring circuit 26. This processes the signal according to the flow chart in Fig. 4 . Firstly, a first element 50 monitors whether the rotational speed n lies within a rotational speed range which is determined by the minimum rotational speed n min and the maximum rotational speed n max. The engagement speed n clutch of the clutch 23, which is designed as a centrifugal clutch, lies outside this speed range n min < n < n max. The engagement speed n clutch can, for example, be approximately 3,500 revolutions per minute. Advantageously, the minimum rotational speed n min can be approximately 3,700 revolutions and the maximum rotational speed n max can be approximately 5,000 revolutions. The rotational speed range determined by the minimum rotational speed n min and the maximum rotational speed n max can also be referred to as the rotational speed window.

[0036] If the speed monitoring circuit 26 determines that the current speed n lies within the speed range n min < n < n max for a period of time ΔT, a first condition is met. If the first condition is met, a status indicator I is advantageously set in the speed monitoring circuit 26. Such a status indicator is also referred to as a "flag" in computer science.

[0037] A timer 51 is expediently used to monitor whether the first condition is present over a period of time ΔT. If this is the case and the speed subsequently drops below a speed limit n G , the speed monitoring circuit 26 acts on the ignition device 30 such that the current ignition point ZZP is advanced. This ensures that the internal combustion engine 5 does not run out rich even when the throttle lever 28 is released. The speed limit n G lies below the minimum speed n min of the specified speed range n min ; n max .

[0038] If, despite the current speed n remaining in the speed range n min < n < n max , the speed does not fall below the speed limit value n G even after the time ΔT in the timer 51 has elapsed, the system branches back to node 53 via branch 54 and continues to monitor the speed n. The ignition timing ZZP is not advanced.

[0039] If, after the first condition has been met, the speed drops below the speed limit value n G according to query 55, the ignition timing ZZP is advanced.

[0040] To store the state that the first condition was met, a status indicator I can be set, as indicated by field 60. If the status indicator I is set and the speed n falls below the speed limit value n G, the ignition timing ZZP is advanced.

[0041] If a status indicator I is set, it is cleared when the speed n is above the specified speed range. Deletion occurs when the speed n exceeds the maximum speed n max of the speed range. This is queried in field 61, and the status indicator I is cleared when the event occurs.

[0042] The status indicator can also be cleared if—as requested in field 55—the current speed n falls below the speed limit n G. This is done via branch 62.

[0043] It can be provided that the set status indicator I is maintained for a predetermined time period t. This time period t and the time period ΔT in field 51 can be a predetermined time span of, for example, 2 to 3 seconds. Advantageously, the time period t and / or the time period ΔT are determined by a predetermined number of consecutive crankshaft revolutions, with five to fifty crankshaft revolutions being advantageously provided. In particular, ten consecutive crankshaft revolutions are set to determine the time period, with the crankshaft revolutions advantageously following one another directly.

[0044] In Fig. 5 An ignition timing curve 70 is shown plotted against the speed n, which can also be referred to as a fork curve. One curve branch 71, shown as a solid line, represents the adjustment curve of the ignition timing ZZP that is effective for normal operation of the internal combustion engine 5 and is indicated on the Y-axis of the diagram. The dashed curve branch 72 represents the course of the adjustment curve of the ignition timing ZZP when the conditions for switching the ignition timing ZZP to "advance" are met. For this to happen, the speed n must be within the speed range n min ; n max for a specified period of time ΔT, i.e., greater than the minimum speed n min and less than the maximum speed n max. After this first condition is met, the speed must drop below a speed limit value n G that lies below the specified speed range n min < n < n max.If this condition is also met, the speed monitoring circuit 26 acts on the ignition device 30 in such a way that instead of curve branch 71, curve branch 72 is traversed, thus adjusting the ignition timing in the "advanced" direction. The advanced ignition timing (ZZP) lies in a range between 20° CA and 27° CA and ensures that the combustion engine 5 does not stall rich. As . Fig. 5 As also shows, the timing advance of the timing adjustment is earlier in the speed range below 3,000 rpm than the timing advance at speeds above 5,000 rpm. While on curve branch 71, in normal operation in a speed range above 5,000 rpm, the timing adjustment is up to 25° before TDC, after the advance according to the invention, at speeds below 3,000 rpm, the timing adjustment is set up to 27° CA before TDC.

[0045] If the combustion engine 5 is switched off, the speed monitoring circuit 26 and the ignition device 30 are also permanently de-energized. The states of the queries according to Fig. 4 are lost. When the combustion engine 5 is restarted, the system is reset, ie the system assumes a predetermined initial state. From this initial state, the system and the queries are processed according to Fig. 4 restarted.

Claims

1. Method for operating a work apparatus having an internal combustion engine, - wherein an ignition device (30) is provided for triggering an ignition spark (27) at a spark plug (20), - and the ignition spark is provided for igniting a fuel / air mixture in a combustion chamber (6) of the internal combustion engine (5), - wherein the combustion chamber (6) is bounded by a piston (7), - and the ignition device (30) triggers the ignition spark (27) at the spark plug (20) at an ignition point (ZZP) depending on the rotational position of the crankshaft (9), - and the crankshaft (9) of the internal combustion engine (1) drives a tool, - wherein a clutch (23) is arranged between the crankshaft (9) and the tool, - and the speed (n) of the internal combustion engine (5) is monitored via a speed monitoring circuit (26), characterized in - that the ignition point (ZZP) of the ignition device (30) is adjusted towards early if the following conditions are successively fulfilled: i. the speed (n) lies within a predefined period of time (ΔT) in a speed range (nmin; nmax) of 3000 rpm to 5000 rpm, wherein the predefined period of time (ΔT) is 2 to 3 seconds or is determined by a predefined number of successive crankshaft revolutions, with five to fifty crankshaft revolutions being provided, ii. the speed (n) of the internal combustion engine (5) drops below a speed limit value (nG) lying below the speed (nmin) of 3000 rpm after the first condition has been fulfilled.

2. Method according to Claim 1, characterized in that a status indicator (I) is set when the first condition is fulfilled, and, in the event of a drop in the speed (n) below a speed limit value (nG) when the status indicator is set, the ignition point (ZZP) is adjusted towards early.

3. Method according to Claim 2, characterized in that a set status indicator (I) is kept until the speed (n) drops below the speed limit value (nG).

4. Method according to Claim 2, characterized in that a set status indicator (I) is deleted if the speed (n) increases above the predefined speed range (nmin; nmax).

5. Method according to Claim 2, characterized in that a set status indicator (I) is kept for a predefined period of time (t).

6. Method according to one of Claims 1 to 5, characterized in that the period of time (t, ΔT) is determined by a predefined number of successive crankshaft revolutions.

7. Method according to Claim 6, characterized in that the period of time (t, ΔT) is determined by five to fifty successive crankshaft revolutions.

8. Method according to Claim 6 or 7, characterized in that the crankshaft revolutions directly follow one another.

9. Method according to one of Claims 1 to 8, characterized in that the clutch (23) is a centrifugal clutch and the engagement speed lies within the predefined speed range (nmin; nmax).