The present invention relates to an engine-driven working machine.
Engine-driven working machines have been known, such as a chain saw, a brush cutter, and a hedge trimmer.
The working machine includes: an internal-combustion engine having a carburetor; an operating unit (e.g., a chain with cutters in the case of the chain saw); and a centrifugal clutch disposed between the internal-combustion engine and the operating unit. The centrifugal clutch becomes engaged when the internal-combustion engine RPM is higher than a predetermined clutch-in RPM, transmitting rotations of the internal-combustion engine to the operating unit. On the contrary, when the engine RPM is lower than the clutch-in RPM, the centrifugal clutch becomes disengaged, interrupting coupling between the internal-combustion engine and the operating unit.
The internal-combustion engine of the working machine has a throttle valve controlling the engine output, the throttle valve being disposed in a mixture passage of the carburetor. The engine is designed so as to stably rotate at a lower RPM than the clutch-in RPM when the throttle valve is in its fully-closed position. The fully-closed state of the throttle valve is called “idle state”.
As to the engine startup, in the case of starting the engine when the engine is cool (so called “cold start”), it is general that the throttle valve is set half open. That is, by setting the throttle valve half open, the engine can be started with the increased amount of air (air-fuel mixture) fed to the engine. This can prevent the engine from stopping immediately after the engine starts. In other words, the reliability in the engine startup can be enhanced. The half-open state of the throttle vale is called “first idle state”. The engine can promptly be started by performing the engine startup operation in the first idle state.
In the case of starting the engine in the first idle state, however, the engine RPM exceeds the clutch-in RPM, with the result that the centrifugal clutch becomes engaged. When the centrifugal clutch becomes engaged, the operating unit abruptly acts. This operating unit action is unfavorable in terms of ensuring the worker's safety.
The working machine includes a brake system so that the operating unit can be braked by the brake system. For the purpose of ensuring the worker's safety in starting the engine, it is recommended to perform the engine startup operation with the brake system being activated. At the time of startup in the first idle state, in particular, the startup operation using the activated brake is strongly recommended to prevent the engine RPM from being higher than the clutch-in RPM.
It is left up to the worker's decision whether to turn on the brake at the engine startup. In case, for example, the worker performs the startup operation without using the brake system in the first idle state, the operating unit may act simultaneously with the engine startup. Since this operating unit action is an action unintended by the worker, it is desirable to provide the working machine with a means preventing the operating unit from acting at the engine startup.
In order to prevent the operating unit from acting at the engine startup, the working machine provided with a blade(s) or a cutter(s) in particular includes a control means having an RPM suppression mode. The RPM suppression mode has a function of inhibiting the engine RPM from exceeding the clutch-in RPM after the engine startup.
The instant that the internal-combustion engine starts, the RPM suppression mode begins action. In the RPM suppression mode, the engine RPM continues to be detected. When the started engine RPM is higher than the clutch-in RPM or when expected to become higher than the clutch-in RPM (i.e. when the engine RPM exceeds a predetermined RPM lower than the clutch-in RPM for example), control suppressing the engine RPM is executed. Examples of the engine RPM suppressing control can include misfire control thinning out the firing of the ignition device, ignition timing control considerably retarding the ignition timing, and air-fuel ratio control increasing the amount of the fuel component in the air-fuel mixture supplied to the engine.
The RPM suppression mode needs to be cancelled before a worker starts the work. If certain conditions are not satisfied, however, the RPM suppression mode cannot be cancelled. Accordingly, the engine does not respond even though the worker operates the throttle lever to fully open the throttle valve prior to cancelling the RPM suppression mode. That is, regardless of the worker's operation of the throttle lever, the engine RPM is inhibited from rising under the control of the RPM suppression mode. Thus, even if the worker operates the throttle lever to perform the work when the RPM suppression mode is not yet cancelled, the worker is faced with a situation where the worker cannot perform the work since the engine does not respond.
In order to ensure the worker's safety, the RPM suppression mode is desirably executed continuously until the engine RPM becomes stable at a low RPM (an idle RPM) in the state where the throttle valve is positioned at the idle position (closed position) with the first idle state cancelled. Thus, from the viewpoint of the safety, it is preferable to impose strict conditions as the RPM suppression mode cancelling conditions.
On the other hand, the RPM suppression mode needs to be cancelled before a worker operates the throttle lever to start the work. In other words, it is desirable to cancel the RPM suppression mode as early as possible. Thus, from the viewpoint of the workability, it is preferred that loose conditions be imposed as the RPM suppression mode cancelling conditions.
Patent Document 1 discloses cancelling the RPM suppression mode when a worker fully opens a throttle valve after the startup of the engine.
Patent Document 2 proposes cancelling the RPM suppression mode by detecting that the engine operation state has become idle after a worker fully closes the throttle valve to end the first idle state. That is, in Patent Document 2, the RPM suppression mode is cancelled by detecting that a time has elapsed enough for the engine to become stable at the idle RPM as a result of reduction in the engine RPM with the return of the throttle valve to the fully closed state by the worker.
[Patent Document 1] U.S. Pat. Application Publication No. 2012/0193112
[Patent Document 2] U.S. Pat. No. 7,699,039
As disclosed in Patent Document 1, it may be a preferred technique from the viewpoint of the operability that the RPM suppression mode is cancelled when a worker performs the operation opening the throttle valve. To securely detect the fully-opened state of the throttle valve, however, there is a need for e.g. a mechanical switch acting in response to the worker's operation to open the throttle valve or a sensor for detecting the fully opened state of the throttle valve. For example, employment of the mechanical switch leads to an increased cost of the working machine.
Patent Document 2 discloses a technique causing software to accurately execute a cancelation of the RPM suppression mode without using hardware like the mechanical switch. The technique disclosed in Patent Document 2, however, employs a condition that the engine RPM becomes stable at the idle RPM, as the RPM suppression mode cancelling conditions. As a result, the RPM suppression mode is executed until the internal-combustion engine RPM reaches the stable idle RPM. At the engine startup, however, a relatively long time may elapse before the engine RPM becomes stable at the idle RPM.
For example, if the engine RPM does not rise even though the worker operates the throttle lever to fully open the throttle valve for the purpose of starting the work, the worker will fall into an inexplicable feeling on why the engine RPM does not rise. The worker may think that some sort of hindrance occurs in cooperation between the throttle lever and the throttle valve, and may operate the throttle lever again and again. With the worker's opening operation of the throttle lever, the throttle valve opens and an excessive air-fuel mixture is supplied to the carburetor mixture passage.
The air-fuel mixture excessively supplied to the mixture passage acts so as to raise the engine RPM. The rise of the engine RPM not only activates the RPM suppression mode so that the engine RPM suppressing control is executed, but also delays more and more the timing to cancel the RPM suppression mode. In other words, the more the worker operates the throttle lever, the longer the RPM suppression mode continues, with the result that the cancelation of the RPM suppression mode is delayed increasingly. In consequence, the worker may not be able to work no matter how much time passes.
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