JP4490792B2 - Air-fuel ratio control system - Google Patents

Description

  The present invention relates to an air-fuel ratio control system for controlling an air-fuel ratio of an air-fuel mixture composed of air and fuel based on a detection result of an air-fuel ratio sensor.

2. Description of the Related Art Conventionally, for example, a technique for calculating the air-fuel ratio of an air-fuel mixture supplied to an engine serving as power, such as GHP (Gas Heat Pump), based on the oxygen concentration in exhaust gas is generally known. This oxygen concentration is measured by a so-called air-fuel ratio sensor or oxygen sensor (hereinafter simply referred to as “air-fuel ratio sensor”), and is connected to an ECU (Electronic Control Unit). Due to such a configuration, the ECU controls the opening degree of the throttle valve, the fuel control valve, etc. via the actuator based on the detection result of the air-fuel ratio sensor and the data and program stored in advance. Thus, the air-fuel ratio of the air-fuel mixture is controlled to an appropriate value. As an example of such an air-fuel ratio control system, there are those shown in Patent Documents 1 and 2.
JP 2002-295299 A JP-A-6-341335

  By the way, the exhaust gas exhausted from the engine contains oil or moisture such as oil, fuel, and water in the form of mist. Therefore, the mist-like oil or moisture adheres to the air-fuel ratio sensor provided in the exhaust pipe for detecting the oxygen concentration in the exhaust gas. Such a phenomenon will be described below simply as “oiling”. For example, in a situation where the engine is stable in a steady operating state, the air-fuel ratio sensor is heated to a high temperature by the heat of exhaust gas, a heater provided, and the like, and the exhaust gas has a flow rate. Even if oil is generated, oil and moisture are immediately evaporated or burned. Therefore, when the engine is in a steady operation state, the accuracy of the air-fuel ratio sensor is less likely to occur due to oiling. However, when the air-fuel ratio sensor itself is not at a high temperature immediately after the engine is started, or when a large amount of oil is generated in the air-fuel ratio sensor for some reason, the attached oil or moisture may not evaporate immediately. Conceivable. In this case, the air-fuel ratio sensor cannot measure the oxygen in the actual exhaust gas, measures the oil and moisture components that are mistakenly attached, and outputs the detection result as a state in which oxygen is very small. In this way, the state in which the exhaust gas has a small amount of oxygen corresponds to the case where the air-fuel ratio of the air-fuel mixture supplied to the engine is rich (fuel is rich), so the ECU has a rich air-fuel ratio of the air-fuel mixture. Control for avoiding the state is started. That is, the ECU controls the air-fuel ratio of the air-fuel mixture so that it becomes lean (fuel is thin). If such control is performed, when the air-fuel ratio of the actual air-fuel mixture is within a normal range, the air-fuel ratio of the air-fuel mixture is excessively controlled to the lean (fuel is thinner) side than necessary. Therefore, there is a problem that engine stall occurs. In other words, the occurrence of engine stall due to the occurrence of oil in the air-fuel ratio sensor significantly reduces the operating efficiency of the engine, and when an alarm is triggered by engine stall due to oil, Although it was not an original failure, there was a possibility of performing maintenance, etc., so it was not easy to use. Accordingly, the present invention has been made in view of the above circumstances, and the object of the present invention is to eliminate the influence of oil on its own when an engine stall occurs due to oil on the air-fuel ratio sensor. An air-fuel ratio control system capable of returning to normal operation is provided.

  The problem to be solved by the present invention is as described above. Next, means for solving the problem will be described.

In claim 1, in the air-fuel ratio control system having a control means for controlling the engine based on the output of the air-fuel ratio sensor, the air-fuel ratio sensor outputs a state where the air-fuel ratio is higher than usual because of oiling or the like. When the air-fuel ratio is controlled to be excessively lean and the engine is stopped, the control means restarts the engine and, after a predetermined time has elapsed, controls the air-fuel ratio based on the air-fuel ratio sensor. Is.

  According to a second aspect of the present invention, during the predetermined time, the control means operates the engine with a predetermined opening degree of the fuel control valve.

According to a third aspect of the present invention, when the engine is stopped again by starting control of the air-fuel ratio of the air-fuel mixture based on the air-fuel ratio sensor after the predetermined time has elapsed, the control means includes the air-fuel ratio sensor. Correct the sensitivity or measurement bandwidth.

  According to a fourth aspect of the present invention, the control means restarts the engine after correcting the sensitivity or measurement band of the air-fuel ratio sensor.

  According to claim 5, when the engine can be normally operated after starting the air-fuel ratio sensor based on the air-fuel ratio sensor after the predetermined time has elapsed, the control means temporarily stops and restarts the engine. To do.

  According to a sixth aspect of the present invention, when the engine is stopped again during the predetermined time, the control means determines that there is an abnormality in the engine.

  As effects of the present invention, the following effects can be obtained.

  According to the configuration of the first aspect, when the performance of the air-fuel ratio sensor is temporarily lowered due to oil, the engine is operated for a predetermined time to evaporate the oil adhering to the air-fuel ratio sensor and eliminate the influence of the oil. It becomes possible.

  According to the configuration of the second aspect, the engine can be easily restarted, and the minimum engine operation for evaporating oil or the like adhering to the air-fuel ratio sensor becomes possible.

  By correcting the sensitivity and the measurement band of the air-fuel ratio sensor according to the configuration of the third aspect, it is possible to eliminate the influence of the air-fuel ratio sensor over time and appropriately control the operation of the engine.

  According to the configuration of the fourth aspect, after correcting the air-fuel ratio sensor, it is possible to automatically start the engine and resume normal operation.

  According to the fifth aspect of the present invention, it is possible to surely wipe off the influence of oil on the air-fuel ratio sensor, and to appropriately perform correction due to the change with time of the air-fuel ratio sensor.

  According to the structure of claim 6, when the minimum operation for evaporating the moisture and oil adhering to the air-fuel ratio sensor is impossible, the control means recognizes that there is an abnormality in the engine, so that It is possible to notify the engine abnormality to the outside.

  Hereinafter, the best mode for carrying out the present invention will be described with reference to the accompanying drawings for understanding of the present invention. The following best mode for carrying out the present invention is an example embodying the present invention, and is not intended to limit the technical scope of the present invention. FIG. 1 is a schematic diagram showing a schematic configuration of an air-fuel ratio control system according to the best mode for carrying out the present invention, and FIG. 2 is a flowchart showing an example of a series of processes performed by the air-fuel ratio control system.

<Outline configuration>
First, the schematic configuration and function of an air-fuel ratio control system according to the best mode for carrying out the present invention will be described with reference to FIG. Further, the engine 60 to which the air-fuel ratio control system described below is applied may be a gas engine such as GHP (Gas Heat Pump) or a gasoline engine. First, the mixer 20 that generates air-fuel mixture by mixing air (outside air) taken from the air cleaner 30 and fuel such as gas will be described.

<Mixer>
As shown in FIG. 1, the mixer 20 mainly includes a venturi 23, a fuel control valve 22, a fixed valve 21, a fuel increase valve 27, and a throttle valve 25. First, the venturi 23 is arranged in an intake passage that takes in air through the air cleaner 30 and has a structure in which the intake passage is partially narrow. Further, in the venturi 23, since the fuel supply path communicates with the narrow portion, the negative pressure in the narrow portion where the flow of the air flow becomes fast is used to suck the fuel from the fuel supply path and It is possible to generate a mixture of fuel and fuel. The amount of fuel supplied to the venturi 23 is adjusted by the fuel control valve 22. Specifically, the amount of the fuel is adjusted by controlling a solenoid as an actuator that drives the valve body of the fuel control valve 22 by an ECU 10 (electronic control unit) which is an example of a control unit. The fuel control valve 22 may be abbreviated as “GVM”. As shown in FIG. 1, as a system for supplying fuel to the venturi 23, there is a system that passes through the fixed valve 21 in addition to the system that passes through the fuel control valve 22. This fixed valve 21 is for supplying only a predetermined amount of fuel to the venturi 23. That is, the fixed valve 21 is not controlled by the ECU 10 or the like, but is only adjusted in advance at the time of maintenance such as manufacture or installation of the mixer 20, and is fixed at a constant opening during operation. . That is, the ECU 10 controls the fuel control valve 22 so that the air-fuel ratio in the air-fuel mixture generated by the venturi 23 can be changed. The air-fuel mixture generated in this way reaches the throttle valve 25, and the flow rate of the air-fuel mixture passing through the throttle valve 25 varies depending on the opening degree of the throttle valve 25. Specifically, the opening degree of the valve body of the throttle valve 25 is changed by driving a stepping motor as an actuator, and the stepping motor is controlled by the ECU 10. Since it is configured in this way, it is possible to increase the flow rate of the air-fuel mixture passing by opening the throttle valve 25 and to decrease the flow rate of the air-fuel mixture passing by closing the opening. Become.

<Fuel increase valve>
Further, the fuel increase valve 27 may be provided downstream of the throttle valve 25 (engine 60 side). The function of the fuel increase valve 27 is opened when only the fuel supplied from the fuel control valve 22 is insufficient for the fuel contained in the air-fuel mixture. Specifically, when the opening degree of the throttle valve 25 (throttle opening degree) is close to full open, etc., it operates particularly when it is desired to make the air-fuel ratio of the air-fuel mixture rich. The control valve 22 operates complementarily in a control state in which the fuel is not completely supplemented. That is, the fuel increase valve 27 shown in FIG. 1 is not necessarily a necessary component for the air-fuel ratio control system of the present invention, but FIG. 1 shows an example in which the fuel increase valve 27 is provided. Yes.

<Engine, ECU>
Next, the engine 60 will be described. Further, the air-fuel mixture that has passed through the throttle valve 25 is sucked into the cylinder 40 of the engine 60 through the suction valve 41, compressed by the piston 45 with the suction valve 41 and the exhaust valve 42 closed, and then by the spark plug 43. Explosion occurs due to ignition, and the exhaust gas after the explosion is discharged through the exhaust valve 42. The crankshaft is rotated by the raising and lowering of the piston 45, and the rotation angle and the rotation speed are detected by the rotation speed sensor 44 and input to the ECU 10. The air-fuel ratio sensor 50 measures oxygen in the exhaust gas and is provided in an exhaust passage such as an exhaust manifold. The ECU 10 is sucked into the cylinder 40 based on the detection result of the air-fuel ratio sensor 50. It is possible to calculate the air-fuel ratio of the air-fuel mixture. Specifically, a measured value (output) that is a result of measurement by the air-fuel ratio sensor 50 is output as a current magnitude (hereinafter simply referred to as “IP value”), and this IP value is the ECU 10. Will be entered. Further, the air-fuel ratio sensor 50 described here outputs the IP value as a larger value as the amount of oxygen contained in the exhaust gas is larger, while the IP value is smaller as the amount of oxygen contained in the exhaust gas is smaller. As the output characteristic. That is, the IP value is proportional to the air-fuel ratio of the air-fuel mixture actually supplied to the engine 60. Therefore, the ECU 10 controls the fuel so that the IP value measured by the air-fuel ratio sensor 50 becomes the target IP value stored in advance in the ECU 10 in order to bring the actual air-fuel ratio of the air-fuel mixture closer to the target air-fuel ratio. The opening degree of the valve 22 and the throttle valve 25 is controlled. That is, the ECU 10 is an example of a control unit that controls the engine 60 based on the output of the air-fuel ratio sensor 50. In the following, “air-fuel ratio control” means that the ECU 10 controls the air-fuel ratio of the air-fuel mixture by controlling the opening degree of the fuel control valve 22 and the throttle valve 25 based on the detection result of the air-fuel ratio sensor 50. It means that. Further, this target IP value may be stored in advance as a map in the storage means (for example, ROM) of the ECU 10 in association with the engine speed and load detected by the engine speed sensor 44. The air-fuel ratio sensor 50 is heated by a heater or the like because it is necessary to activate the element itself at a certain temperature when starting measurement, and the temperature is also controlled by the ECU 10 or the like.

<Processing example>
Next, in the air-fuel ratio control system configured as described above, an example of a series of processes performed by the ECU 10, which is an example of a control unit, will be described with reference to FIG. Based on the rotational speed sensor 44 or the like, the ECU 10 determines whether the engine has stopped (that is, whether a so-called “engine stall” has occurred) in a state where the engine 60 is in a normal normal operation or start-up state (S10). (S20). In the following description, stopping the engine 60 unintentionally for some reason is referred to as “engine stall”. If it is determined in step S20 that an engine stall has occurred, the process proceeds to step S25. On the other hand, if it is determined that an engine stall has not occurred, the process in step S20 is repeated to generate an engine stall. Monitor.

<Judgment of air-fuel ratio>
When it is determined in step S20 that an engine stall has occurred, the ECU 10 determines whether the air-fuel ratio of the air-fuel mixture is richer (normal) based on the detection result of the air-fuel ratio sensor 50. (S25). This determination may be made as follows. As described above, the ECU 10 determines whether or not the IP value of the air-fuel ratio sensor 50 is smaller than a rich reference value that is a reference for determining that the air-fuel ratio of the air-fuel mixture is richer than usual. The rich reference value may be stored in the ECU 10 in advance. If it is determined in step S25 that the air-fuel mixture is rich, the process proceeds to step S30. On the other hand, if it is determined that the air-fuel mixture is not rich, the process proceeds to step S60. Here, a case where the process proceeds to step S30 will be described first.

<When proceeding to step S30>
When the engine stall occurs (S20) and it is determined that the air-fuel ratio of the air-fuel mixture is rich when the engine stall occurs (S25), the ECU 10 automatically restarts the engine 60 (S30). Then, during a predetermined time after the restart in step S30, the ECU 10 operates the engine 60 with the opening degree of the fuel control valve 22 or the like set to a predetermined opening degree. That is, during the predetermined time, the ECU 10 does not perform air-fuel ratio control of the air-fuel mixture, but only operates with the fuel control valve 22 and the like fixed at a predetermined opening. The specific length of the predetermined time may be, for example, about several minutes to several tens of minutes. As described above, the engine 60 can be operated without using the air-fuel ratio sensor 50 by operating the fuel control valve 22 and the like at a predetermined opening for a predetermined time. Therefore, for example, when the cause of the engine stall in a state where the air-fuel ratio of the air-fuel mixture is rich is due to a temporary deterioration in the performance of the air-fuel ratio sensor 50 due to oil, By operating 60 for the predetermined time, it is possible to evaporate the oil component adhering to the air-fuel ratio sensor 50 and eliminate the influence of oil. That is, by operating with the fuel control valve 22 and the like fixed at a predetermined opening degree, the minimum operation of the engine 60 for evaporating the oil and the like adhering to the air-fuel ratio sensor 50 becomes possible. Next, the ECU 10 determines whether an engine stall has occurred during the predetermined time (S35). If it is determined in step S35 that an engine stall has occurred, the process proceeds to step S36. If it is determined that an engine stall has not occurred, the process proceeds to step S40. Here, when the process proceeds to step S36, the engine stall occurs regardless of the operating state that is not based on the air-fuel ratio sensor 50 that operates with the fuel control valve 22 fixed at a predetermined opening degree. This is presumably due to the structure of the engine 60 other than the air-fuel ratio sensor 50. Therefore, in the process of step S36, the ECU 10 executes a process performed when it is determined that an abnormality has occurred in the engine 60 (S36). In the process of step S36, the ECU 10 may attempt to restart the engine 60 a predetermined number of times, for example, or issue a warning to notify the user that an abnormality has occurred in the engine 60. You may display on the operation panel etc. which are not shown in figure, and you may perform the process performed when abnormality in the engine 60 generate | occur | produces. As described above, the ECU 10 as the control means recognizes that there is an abnormality in the engine 60, so that the abnormality of the engine 60 can be notified to the outside at an early stage.

<When proceeding to step S40>
If it is determined in step S35 that engine stall has not occurred, it can be said that there is no abnormality in the structure of the engine 60 and the like because the operating state not based on the air-fuel ratio sensor 50 can be performed normally. Therefore, the ECU 10 executes air-fuel ratio control of the air-fuel mixture using the air-fuel ratio sensor 50 after elapse of a predetermined time in which the operation for fixing the fuel control valve 22 and the like described above is fixed to a predetermined opening degree (S40). . That is, during the predetermined time after the engine 60 is started in step S30, the fuel control valve 22 and the like are operated at a predetermined opening, so that the influence of oiling should be eliminated. In step S40, the air-fuel ratio control is started. Then, the ECU 10 determines whether or not an engine stall has occurred while performing air-fuel ratio control (S45). If it is determined in step S45 that an engine stall has occurred, the process proceeds to step S50. On the other hand, if it is determined that an engine stall has not occurred, the process proceeds to step S46.

<When moving to step S50>
In the situation where the process shifts to step S50, the air fuel ratio control is started in step S40 and the engine stall occurs (that is, the engine engine restarts in spite of the fact that engine stall has not occurred in the predetermined time as determined in step S35). Stop). In other words, there is no abnormality in the structure of the engine 60 and the influence of the oil coverage of the air-fuel ratio sensor 50 should have been eliminated. However, when air-fuel ratio control is started using the air-fuel ratio sensor 50, an engine stall occurs. Yes. This is because an engine stall may occur due to a change in sensitivity or measurement band due to a change over time due to the use of the air-fuel ratio sensor 50 or the like. That is, generally, the air-fuel ratio sensor 50 has a characteristic that the IP value decreases with time, so that the air-fuel ratio sensor 50 has a lower oxygen concentration in the exhaust gas (that is, the air-fuel mixture is rich). It is considered that the engine stall occurred as a result of the ECU 10 controlling the air-fuel ratio of the air-fuel mixture to make the air-fuel mixture leaner than necessary. Therefore, in the determination of step S20 and step S25, the engine stall occurs when the air-fuel ratio is rich because the sensitivity of the air-fuel ratio sensor 50 and the change in the measurement band are caused by the change of the air-fuel ratio sensor 50 over time. This is probably because the control to make the fuel ratio lean was performed. Since the sensitivity and the change in the measurement band due to the change with time of the air-fuel ratio sensor 50 are different from the failure, the ECU 10 determines the output IP value of the air-fuel ratio sensor 50 when performing the air-fuel ratio control according to the change with time. By correcting the correspondence relationship with the control parameters such as the fuel control valve 22 that is the control target (S50), it becomes possible to eliminate the influence of the temporal change. As a method of correcting the sensitivity and measurement band of the air-fuel ratio sensor 50 performed in step S50, for example, the following may be performed. In general, the performance and characteristics of the air-fuel ratio sensor 50 are known in advance at the manufacturing stage. Therefore, the time-varying characteristics of the air-fuel ratio sensor 50 when the air-fuel ratio sensor 50 is used in accordance with the environment in which the engine 60 is used are also: It is possible to make a prediction in the development stage of the air-fuel ratio control system of the present invention. Therefore, by storing the time-varying characteristics of the air-fuel ratio sensor 50 determined in the development stage of the air-fuel ratio control system in the ECU 10 or the like, the ECU 10 can cause the time-varying characteristics of the air-fuel ratio sensor 50 and the total start / stop of the engine 60. The correction in step S50 may be executed based on the correspondence with the parameters such as the number of times and the total operation time. By correcting the sensitivity and the measurement band of the air-fuel ratio sensor 50 in this way, it is possible to eliminate the influence of the change over time of the air-fuel ratio sensor 50 and appropriately control the operation of the engine 60. Then, the ECU 10 automatically restarts the engine 60 after the correction process of the air-fuel ratio sensor 50 (S50 → S10). As a result, after the air-fuel ratio sensor 50 is corrected, the engine 60 can be automatically started to resume normal operation.

<When proceeding to step S46>
The situation in which the process proceeds to step S46 in the determination of step S45 is that there is no abnormality in the structure of the engine 60, and the engine 60 operates normally even if the air-fuel ratio control is started using the air-fuel ratio sensor 50. This is a situation where engine stalls do not occur. In such a situation, it can be said that both the engine 60 and the air-fuel ratio sensor 50 are normal. However, at the determination of steps S20 and S25, at least the air-fuel mixture is rich and engine stall occurs. The influence of oil is not completely eliminated, and it is conceivable that some influence due to the change of the air-fuel ratio sensor 50 with time remains. Therefore, in order to completely wipe out the influence of oiling, in order to perform a process of operating with the fuel control valve 22 or the like fixed at a predetermined opening, the engine 60 is once stopped again (S46). The engine 60 is restarted (S30), and the processing loop of step S35, step S40, and step S45 described above is executed. If it is determined in step S45 in this processing loop that no engine stall has occurred, there is a disadvantage that the processing loop is repeated endlessly. Therefore, in order to eliminate such inconvenience, when the processing loop is repeated a predetermined number of times, the process automatically proceeds to the process of step S50, or the duration of the processing loop is set to a predetermined time. The ECU 10 may be programmed to automatically shift to the process of step S50 when the time has elapsed. By configuring in this manner, it is possible to surely wipe off the influence of oil on the air-fuel ratio sensor 50 and to appropriately perform correction due to changes with time of the air-fuel ratio sensor 50.

<When proceeding to step S60>
The case where it is determined in step S25 that the air-fuel mixture is not rich and the process proceeds to step S60 will be described. When the process proceeds to step S60, it is considered that the engine stall occurred in a situation where the air-fuel ratio of the air-fuel mixture was not relatively rich but was in a relatively appropriate state, and the air-fuel ratio sensor 50 was oiled. It is difficult to believe that an engine stall has occurred. In such a situation, there is a possibility that the engine 60 or the mixer 20 has failed for some reason. First, the ECU 10 counts the number of occurrences of engine stall (S60), corrects the air-fuel ratio sensor 50, and determines whether the correction is appropriate (S65). The determination as to whether the correction of the air-fuel ratio sensor 50 in the process of step S65 is appropriate may be made as follows. As already described above, when the processing shifts to step S60, it is considered that the air-fuel ratio control is relatively appropriately performed. Therefore, the sensitivity and measurement band of the air-fuel ratio sensor 50 are also in a relatively appropriate region. Conceivable. Therefore, the correction amount in the correction process in step S65 should be small in nature. Therefore, the ECU 10 determines that the correction of the air-fuel ratio sensor 50 is appropriate when the correction amount in the correction process in step S65 is within a predetermined range, and on the other hand, deviates from the range. It is judged that it is not appropriate if it is given.

  In the process of step S65, when it is determined that the correction of the air-fuel ratio sensor 50 is appropriate, the process proceeds to step S70. On the other hand, when it is determined that the correction of the air-fuel ratio sensor 50 is not appropriate, the process is step S75. Migrate to When the process proceeds to step S75, the correction of the air-fuel ratio sensor 50 has not been performed properly. Therefore, the ECU 10 determines that there is some failure in the air-fuel ratio sensor 50 itself, and the ECU 10 uses the air-fuel ratio sensor 50. Without controlling, the engine 60 is operated (S75). Specifically, this control may be a method of controlling the opening degree of the throttle valve 25 or the fuel control valve 22 in accordance with the rotational speed (output) of the engine 60, etc. Although it is impossible, the engine 60 can be operated as an emergency. Of course, a warning for notifying the user that an abnormality has occurred in the air-fuel ratio sensor 50 in this case may be shown on an operation panel (not shown). On the other hand, when the process proceeds to step S70, engine stall has occurred despite the fact that the correction of the air-fuel ratio sensor 50 has been properly performed, so it is considered that an abnormality has occurred in the mixer 20 or the engine 60 itself. Therefore, an ECU 10 warning or the like may be issued as described above (S75).

The schematic diagram which showed schematic structure of the air fuel ratio control system which concerns on the best form for implementing this invention. The flowchart which showed an example of the series of processes which an air fuel ratio control system performs.

10 ECU
20 Mixer 22 Fuel control valve 60 Engine

Claims (6)

In an air-fuel ratio control system having a control means for controlling the engine based on the output of the air-fuel ratio sensor, the air-fuel ratio sensor outputs a state where the air-fuel ratio is higher than usual due to oiling or the like, so the air-fuel ratio is excessively lean. The control means restarts the engine and controls the air-fuel ratio based on the air-fuel ratio sensor after a predetermined time elapses when the engine is stopped. Fuel ratio control system.   2. The air-fuel ratio control system according to claim 1, wherein during the predetermined time, the control means operates the engine with a predetermined opening degree of the fuel control valve. When the engine is stopped again by starting air-fuel ratio control based on the air-fuel ratio sensor after the predetermined time has elapsed, the control means corrects the sensitivity or measurement band of the air-fuel ratio sensor. The air-fuel ratio control system according to any one of claims 1 and 2.   The air-fuel ratio control system according to claim 3, wherein the control means restarts the engine after correcting the sensitivity or measurement band of the air-fuel ratio sensor.   The control means is configured to stop and restart the engine once when the engine can be normally operated after starting the air-fuel ratio sensor based on the air-fuel ratio sensor after the predetermined time has elapsed. The air-fuel ratio control system according to any one of claims 1 to 4.   The air-fuel ratio control system according to any one of claims 1 to 5, wherein when the engine is stopped again during the predetermined time, the control means determines that the engine is abnormal.

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