Patent ID: 12247539

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings.FIG.1is a diagram for explaining an overview of an engine system. The engine system designated at reference numeral1includes an engine body2, an intake system4supplying air-fuel mixture to the engine body2, and an exhaust system6discharging burned gas of the engine body2to the outside. In the diagram, reference numeral8denotes a muffler.

The engine body2has a combustion chamber12defined by a piston10, with an ignition plug14arranged facing the combustion chamber12. The piston10is coupled by a connecting rod16to a crankshaft18. The crankshaft18outputs engine driving force.

The engine body2shown is an illustrative four-cycle engine. The engine body2has an intake port20and an exhaust port22. The intake port20is opened or closed by an intake valve24. The exhaust port22is opened or closed by an exhaust valve26. The intake system4is coupled to the intake port20. The exhaust system6is coupled to the exhaust port22. The engine body2may be a two-cycle engine. As is well known, the two-cycle engine does not have the intake valve24and the exhaust valve26. In the two-cycle engine, the intake port20and the exhaust port22are opened or closed by the piston10. The two-cycle engine is currently used in work machines such as a brush cutter and a chain saw.

The intake system4has an air cleaner30at its upstream end and an intake passage32through which air filtered by the air cleaner3passes. A throttle valve34is disposed on the intake passage32and operated to regulate the amount of air introduced into the engine body2to thereby control the engine output. The intake system4has a carburetor36so that fuel F within a fuel tank38is supplied through the carburetor36to the intake passage32to generate air-fuel mixture. As a variant, a fuel injection valve may be employed instead of the carburetor36. The fuel injection valve stops fuel injection the instant it receives an engine stop signal that will be described later.

The engine system1includes a control unit40that receives a temperature signal from an engine temperature sensor42disposed on a mounting seat of the ignition plug14to detect the temperature of the engine body2and a rotation number signal from a rotation number sensor44detecting the engine rotation number. The control unit40receives an engine stop signal from an engine stop switch46operated by an operator.

FIG.2is a graph for explaining a run-on phenomenon. In the diagram, reference sign Pt denotes a point of time when engine stop control is executed based on receipt of the engine stop signal. When receiving the engine stop signal, power supply to the ignition plug14is interrupted. As a result, the ignition plug14becomes practically inactive and the engine body2goes to stopped state while continuing its rotational motion only by inertia. In consequence, the engine rotation number decreases. As can be seen fromFIG.2, after the engine rotation number drops to approx. 1000 rpm, there appear phenomena P1to P4where the engine rotation number rises intermittently several times. This is a phenomenon where, despite the fact that ignition control by the control unit40has been stopped, air-fuel mixture remaining within the combustion chamber12self-ignites interdependently on the temperature gradient in the combustion chamber12. The occurrence of this unintended combustion is the run-on phenomenon. Since the run-on phenomenon is an accidental combustion under the condition where the ignition control has already been stopped in this manner, it is difficult to control the phenomenon by quantitative and fixed control.

The carburetor36as a fuel supply device disclosed inFIG.1does not have an electronically controlled fuel supply cutoff valve. In the case of the carburetor36not having the fuel supply cutoff valve in this manner, when the pressure within the combustion chamber12goes negative, fuel is sucked out of the carburetor36, with the result that air-fuel mixture may be introduced into the combustion chamber12in spite of the state where the ignition has been stopped based on the engine stop signal. As a result, the run-on phenomenon after engine stop becomes easy to occur.

In this embodiment, duration time of the run-on phenomenon is reduced by engine control executed after engine stop. This engine control includes, in addition to control to interrupt power supply to the ignition plug14, ignition resumption control executed after a given condition is met after engine stop. In the ignition resumption control, firstly, when the control unit40receives the engine stop signal, the above engine stop control is executed. However, the current engine rotation number continues to be monitored by the rotation number sensor44. Secondly, when it is detected that the descending engine rotation number has reached or fallen below a threshold value Th (FIG.2), the ignition plug14is controlled based on resumption ignition timing that is timing deviating from an entire ignition timing range in an entire working area (normal working area) from idle operation to full throttle operation set in the engine body2. This is the ignition resumption control. This ignition resumption control is referred to as “run-on suppression control”. The run-on suppression control may be executed after the lapse of a first predetermined time (e.g. 2 to 4 sec. from engine stop) from a point of time when the engine stop signal was received. Thirdly, the run-on suppression control comes to an end when a second predetermined time (e.g. 5 to 6 sec. after engine stop) elapses.

The run-on suppression control detailed below is control intended for shortening the duration time of the run-on phenomenon. As described above, the resumption ignition timings, that is, run-on suppression ignition timings are set at timing deviating from the entire ignition timing range set in the normal working area in the engine body2. If, in the engine body2, for example, the entire ignition timing range set in the normal working area is the range of BTDC 5 to 40 degrees in crank angle, timing deviating from this range is set as the run-on suppression ignition timing. In case that e.g. the ignition timings upon idling included in the normal working area of the engine body2is set as the run-on suppression ignition timing, the engine may be activated by ignition of residual fuel by the ignition plug14, and return to idle operation. Hence, in the run-on suppression control, the ignition control is executed under the run-on suppression ignition timing within the ignition timing range not overlapping with the ignition timing range set in the normal working area.

In particular, it is desirable that the ignition timing range for use in the normal working area and the ignition timing range used for the run-on suppression control do not adjoin each other as the rotation number area and be apart from each other by a predetermined timing range.

Referring toFIG.3, all ignition timings Tg(Nor) set in the normal working area of the engine body2are shown by a broken line. The ignition timings Tg(Nor) set in the normal working area of this engine body2lie within the range of BTDC 30 to 8 degrees. Reference sign Tg(R-on) ofFIG.3denotes the run-on suppression ignition timings. The run-on suppression ignition timings Tg(R-on) are set within the range of ATDC 10 to 20 degrees. ATDC 10 to 20 degrees are the ignition timings that deviate to the retarded side from the range of the ignition timings Tg(Nor) set in the normal working area of that engine body2, i.e., BTDC 30 to 8 degrees.

More specifically, the example ofFIG.3relates to the setting of the run-on suppression timing, and shows the run-on suppression ignition timings are set to be retarded from the range of the ignition timings Tg (Nor) set in the normal operating region of the engine body2. This allows the ignition plug14to fire immediately before the timing when the run-on combustion occurs. Then, the flying spark can induce combustion of residual fuel. That is, combustion of residual fuel can be induced after ignition of the ignition plug14without directly igniting the residual fuel by the ignition plug14. As a variant, ignition timing deviating to the advanced side from the range of the ignition timing Tg(Nor) set in the normal working area of the engine body2may be set as the run-on suppression ignition timing.

In controlling the engine body2, it is preferable to prepare two programs and store these programs in the memory M (FIG.1). One program is for a normal operating control mode, wherein the engine body2is controlled under the ignition timings set for the entire working range from idle to full-throttle operation. The second program is for a ignition resumption control mode that is independent from the normal operating control mode and restarts ignition of the engine body2under the resumption ignition timings. When receiving the engine stop signal from the engine stop switch46, the control unit40switches the control mode from the normal operation control mode to the ignition resumption control mode, before resuming the ignition control. Then, when the engine rotation number has reached or fallen below the threshold value Th or when the first predetermined time has elapsed, the control unit40may execute the engine control, based on the ignition resumption control mode. To secure improved responsiveness, it is preferred that the resumption ignition timing be set based on a map of the run-on suppression ignition timing Tg(R-on) prepared in advance.

FIG.4is a flowchart related to an example of engine control succeeding receipt of the engine stop signal. Referring toFIG.4, when the engine stop signal is received from the engine stop switch46at step S1, the engine stop control is executed (S2). Here, the engine stop control includes control of interrupting power supply to the ignition plug14to render the ignition plug14inactive.

At next step S3, the current rotation number monitoring by the engine rotation number sensor44being executed from before the engine stop is continued even after the engine is stopped, and this rotation number monitoring is executed till the end of the run-on suppression control. The run-on suppression ignition timings Tg(R-on) are then set at step S4. For example, the engine control is switched from the normal operation control mode to the ignition resumption control mode.

If at nest step S5the first predetermined time has elapsed after receipt of the engine stop signal, the procedure goes to step S6to resume ignition control. The ignition resumption control is executed based on the above run-on suppression ignition timings. This resumed ignition control is executed till the lapse of the second predetermined time (S7). The second predetermined time is set to, for example, 5 to 6 sec. starting from the receipt of the engine stop signal. As a variant, the second predetermined time may be set starting from the point of time of resumption of the ignition control. If the second predetermined time has elapsed, the procedure goes to step S8to cut off power supply to the ignition plug14. The ignition resumption control thus terminates.

FIG.5is a flowchart related to another example of the run-on suppression control. In the flowchart ofFIG.5, the same reference signs are imparted to the same steps as those making up the flowchart ofFIG.4, which will not again be described. In the engine control example i.e. run-on suppression control shown inFIG.5, if at step S10the current engine rotation number has fallen below the threshold value Th (FIG.2), the ignition control is resumed at step S6.

FIG.6shows a variant of the run-on suppression ignition timings Tg(R-on) described with reference toFIG.3. As regards the run-on suppression ignition timing Tg(R-on) shown inFIG.6, BTDC 70 degrees is set as first timing Tg(R-on-1) in a first area including the engine rotation number of 1100 rpm. In a second area including the engine rotation number of 1160 rpm, ATDC 8 degrees is set as second timing Tg(R-on-2). In a third area including the engine rotation number of 1250 rpm, ATDC 12 degrees is set as third timing Tg(R-on-3).

BTDC 70 degrees as the first timing Tg(R-on-1) is ignition timing that deviates to the advanced side from the range of the ignition timings Tg(Nor) set in the entire operation range i.e. the normal working area of the engine body2, that is, the range of BTDC 30 degrees to 8 degrees. ATDC 8 degrees as the second timing Tg(R-on-2) and ATDC 12 degrees as the third timing Tg(R-on-3) are ignition timings deviating to the retarded side from the range of the ignition timings Tg(Nor) set in the normal working area of the engine body2.

As a preferred embodiment, the set values of the run-on suppression ignition timing Tg(R-on) are changed depending on the engine temperature. Describing with the run-on suppression ignition timings Tg(R-on) ofFIG.6, the set values Tg(R-on-1), Tg(R-on-2), and Tg(R-on-3) to be standards are shifted to the high rotation number side when the engine temperature is high (e.g. the temperature of the ignition plug seat is 250 degrees centigrade or more). On the other hand, when the engine temperature is low, the set values Tg(R-on-1), Tg(R-on-2), and Tg(R-on-3) to be standards are shifted to the low rotation number side.

FIG.7is a flowchart related to a further example of the run-on suppression control, the flowchart including a temperature-based setting change process of the run-on suppression ignition timings Tg(R-on). In this setting change process, at low temperature the standard set values i.e. the set values Tg(R-on-1), Tg(R-on-2), and Tg(R-on-3) shown inFIG.6of the run-on suppression ignition timing Tg(R-on) are shifted to the low rotation number side, whereas at high temperature they are shifted from the standard set values to the high rotation number side.

InFIG.7, the same reference signs are imparted to the same steps as those described in the flowchart ofFIG.4, which will not again be described. When the engine stop signal is received at step S1, the engine temperature is monitored by the engine temperature sensor42(S20), and this temperature monitoring continues until the ignition resumption control ends at step S8. When the ignition control is resumed at step S6, the procedure goes to step S21to determine whether first run-on combustion has occurred. This run-on combustion can be detected by detecting the acceleration of the engine rotation number. At the point of time when the first run-on combustion has occurred, ignition is correspondingly performed based on the first timing Tg(R-on-1) that is the standard set value. In relation to this first run-on combustion, ignition may be performed at timing shifted from the first timing Tg(R-on-1) as the standard set value in advance depending on the engine temperature.

At next step S22, corresponding to the second and subsequent run-on combustions, setting change is made to shift the first to third timings Tg(R-on-1), Tg(R-on-2), and Tg(R-on-3) as the standard set values depending on the engine temperature. This shift may be done gradually stepwise, or the setting change in timing may be completed by only one shift. In the case of normal engine temperature (e.g. 200 to 250 degrees centigrade in ignition plug seat temperature), the ignition control corresponding to the second and subsequent run-on combustions is executed based on the timings Tg(R-on-1), Tg(R-on-2), and Tg(R-on-3) as the standard set values. If the engine temperature is lower than that, the ignition control is executed at timings shifted from the standard set values to the low rotation number side. If the engine temperature is higher than that, the ignition control is executed at timings shifted from the standard set values to the high rotation number side.

Since residual fuel upon engine stop can be forced to burn by virtue of the above run-on suppression control, that is, the ignition resumption control, it becomes possible to sweep out residual fuel from the intake system4, the combustion chamber12, etc. and to shorten the duration time of the run-on phenomenon. The ignition control is resumed after a time interval following the engine stop based on the engine stop signal. In this resumed ignition control, the ignition control is executed based on the run-on suppression ignition timing i.e. the resumption ignition timing Tg(R-on) that is timing deviating from the entire ignition timing range set in the normal working area of the engine body2. This means that ignition of the ignition plug14is not intended to ignite residual fuel but is intended to encourage burning of residual fuel within the combustion chamber12after ignition of the ignition plug14. This can prevent the engine body2from being activated by the resumed ignition control to return to e.g. idle operation.