Control device for internal combustion engine

Abstract

Provided is a control device for an internal combustion engine that is used in a vehicle including a single-cylinder internal combustion engine and a centrifugal clutch engaged when the speed of the internal combustion engine has increased, and that can prevent unintended engagement of the centrifugal clutch during execution of idle operation control. When the engine speed of the internal combustion engine (2) in one cycle is plotted and a straight line connecting the maximum value of the engine speed in one cycle and the engine speed at the end of the exhaust stroke is defined as a reference line, if there is an engine speed plotted below the reference line in the exhaust stroke during execution of idle operation control, the control device (1) for the internal combustion engine controls the internal combustion engine (2) so that the engine speed decreases and prevent engagement in the centrifugal clutch (6).

Description

Control device for internal combustion engine

[0001] The present invention relates to a control device for an internal combustion engine.

[0002] A control device for an internal combustion engine that is applied to an internal combustion engine mounted on a vehicle and executes idle operation control when in an idle state is known. For example, Patent Document 1 discloses a control device for an internal combustion engine that performs early activation of an exhaust gas purification catalyst during idle operation control. According to this control device, when the temperature of the exhaust gas purification catalyst is relatively low (that is, when catalyst warm-up is required), the intake air amount is increased compared to normal idle operation. The control of increasing the rotational speed of the internal combustion engine (engine rotational speed) during idle operation to increase the intake air amount for catalyst warm-up is also referred to as "catalyst early activation control".

[0003] Patent Document 1: Japanese Patent Application Laid-Open No. 2000-179439

[0004] By the way, a centrifugal clutch may be interposed in the torque transmission path of a vehicle on which an internal combustion engine is mounted. A centrifugal clutch generally has an input shaft and an output shaft, and when the rotational speed of the input shaft increases, engagement occurs in the centrifugal clutch and torque is transmitted from the input shaft to the output shaft. If the input shaft is connected to the internal combustion engine so as to be torque-transmittable and the output shaft is connected to the drive wheels of the vehicle so as to be torque-transmittable, there is a possibility that unintended torque transmission from the internal combustion engine to the drive wheels may occur due to the engagement of the centrifugal clutch during the execution of idle operation control.

[0005] Therefore, a control device for an internal combustion engine applied to a vehicle equipped with a centrifugal clutch needs to avoid unintended engagement in the centrifugal clutch when executing control (including catalyst early activation control, also referred to as "idle speed increase control") that increases the engine rotational speed when in an idle state. However, in Patent Document 1, no consideration has been given to this point.

[0006] An object of the present invention is to provide a control device for an internal combustion engine that can execute idle operation control including idle speed increase control and suppress unintended engagement in a centrifugal clutch.

[0007] The inventors of this application investigated the implementation of catalyst early activation control (i.e., idle speed increase control) as disclosed in Patent Document 1 in a vehicle equipped with an exhaust gas purification catalyst and a centrifugal clutch.

[0008] In idle speed increase control, it is generally necessary to increase the engine speed. On the other hand, when the engine speed exceeds the clutch-engagement speed, rotation is transmitted from the internal combustion engine to the drive wheels due to the engagement of the centrifugal clutch. Therefore, in order to perform idle speed increase control in a vehicle equipped with a centrifugal clutch, it is necessary to increase the engine speed and keep the engine speed below the clutch-engagement speed.

[0009] Based on the above, the inventors of the present invention considered controlling the internal combustion engine such that, when idle speed increase control is performed, the engine speed is reduced if the engine speed exceeds a threshold rotational speed set to be lower than the clutch-engagement rotational speed.

[0010] In a centrifugal clutch, torque is transmitted from the input shaft to the output shaft by the frictional force generated between a coupling member (e.g., a clutch shoe) located on the input shaft and a coupling member (e.g., a clutch drum) located on the output shaft. Therefore, the clutch-in rotational speed can be said to be the engine rotational speed at which the frictional force in the centrifugal clutch becomes relatively large. As the engine rotational speed increases and the frictional force increases further, the output shaft rotates. That is, rotation is transmitted from the internal combustion engine to the power transmission means (specifically, the transmission and reduction gears included in the torque transmission path from the centrifugal clutch to the drive wheels).

[0011] The clutch-in rotational speed can vary depending on the environment in which the centrifugal clutch operates and / or individual differences in the centrifugal clutch itself. In other words, the actual clutch-in rotational speed has a range. That is, the actual clutch-in rotational speed is likely to differ from the theoretical clutch-in rotational speed, which will be discussed later. More specifically, the torque transmitted from the input shaft to the output shaft in a centrifugal clutch (transmitted torque) may vary depending on the temperature of the centrifugal clutch and the humidity around the centrifugal clutch. In addition, the transmitted torque may change depending on individual differences in the centrifugal clutch and changes over time.

[0012] Therefore, the inventors of this invention realized that in order to avoid an increase in transmitted torque when idle speed increase control is performed, the threshold rotational speed needs to be set to a value that is somewhat smaller than the theoretical clutch-in rotational speed. On the other hand, in order to terminate idle speed increase control (e.g., catalyst early activation control) early, it is desirable to maintain a relatively high engine rotational speed.

[0013] Therefore, the inventors of the present invention have come to realize that when performing idle speed increase control in a vehicle equipped with a centrifugal clutch, it is necessary to achieve two conflicting requirements: to maintain a relatively high engine rotation speed and to suppress an increase in the torque transmitted in the centrifugal clutch.

[0014] To fulfill these two requirements, the inventors of the present invention considered the following configuration. Specifically, when plotting the rotational speed of an internal combustion engine during one cycle of a single-cylinder internal combustion engine, a straight line connecting the maximum value of the rotational speed of the internal combustion engine during one cycle and the rotational speed of the internal combustion engine at the end of the exhaust stroke is defined as the reference line. If, during idle operation control, there is a rotational speed of the internal combustion engine plotted below the reference line during the exhaust stroke, the internal combustion engine is controlled to decrease the rotational speed of the internal combustion engine.

[0015] With this configuration, when idle operation control (especially idle speed increase control) is performed, the engine rotation speed is maintained at a relatively high level, while the internal combustion engine is controlled to decrease the engine rotation speed if there is an engine rotation speed plotted below the baseline.

[0016] The curve (graph) obtained by plotting the engine rotational speed as it changes over time will hereafter be referred to as the "plotted curve." In this configuration, if there is a section of the plotted curve that appears below the baseline during the exhaust stroke, the internal combustion engine is controlled to decrease the engine rotational speed. The reason for this is explained below.

[0017] When the centrifugal clutch is disengaged, no torque is transmitted from the input shaft to the output shaft. When the centrifugal clutch is fully engaged, the rotational speed of the output shaft is approximately equal to the rotational speed of the input shaft. That is, when fully engaged, the difference between the rotational speed of the input shaft and the rotational speed of the output shaft (also referred to as the "rotational speed difference") is approximately "0".

[0018] On the other hand, when the centrifugal clutch is partially engaged, torque is transmitted from the input shaft to the output shaft, while the rotational speed difference is greater than "0". Here, the state in which the rotational speed difference is greater than "0" includes the state in which the output shaft is not rotating (i.e., the rotational speed of the output shaft is approximately "0") and the state in which the output shaft is rotating (i.e., the rotational speed of the output shaft is greater than "0"). The state in which torque is transmitted from the input shaft to the output shaft while the output shaft is not rotating can also be described as the state in which the coupling members of the centrifugal clutch (e.g., the clutch shoe and clutch drum) slide against each other, generating frictional force, while rotation is not transmitted in the centrifugal clutch.

[0019] In other words, torque transmission in the centrifugal clutch occurs due to the contact between coupling members (and consequently, the frictional force generated by the contact between coupling members) caused by an increase in engine rotational speed. If the engine rotational speed increases further, the frictional force in the coupling members increases (i.e., the transmitted torque increases), and rotational transmission occurs in the centrifugal clutch. As the transmitted torque in the centrifugal clutch increases, the load acting on the internal combustion engine (engine load) increases. In other words, the frictional force in the centrifugal clutch hinders the rotation of the crankshaft of the internal combustion engine.

[0020] Incidentally, engine rotational speed fluctuates in each cycle of an internal combustion engine. More specifically, the engine rotational speed increases during the combustion stroke, and then decreases during the subsequent exhaust stroke. Therefore, in one cycle, it is possible that contact between coupling members occurs when the engine rotational speed increases, and then the contact between coupling members is released when the engine rotational speed decreases. In other words, when the engine rotational speed increases while the centrifugal clutch is open, an increase or decrease in the frictional force of the coupling members (and consequently, an increase or decrease in engine load) may occur in one cycle before rotational transmission occurs in the centrifugal clutch.

[0021] When the engine load decreases during the combustion stroke, following an increase in engine speed, the rate of decrease in engine speed decreases. The rate of decrease in engine speed is represented as the slope of the plotted curve. Therefore, when the rate of decrease in engine speed decreases, the magnitude of the slope at the plotted engine speed is likely to change from being greater than the baseline to being smaller. Consequently, when a decrease in engine load occurs during the exhaust stroke due to the release of contact in the coupling members, it is likely that the engine speed will be plotted below the baseline.

[0022] Therefore, the inventors of the present invention came up with the above configuration, which reduces the engine rotation speed when the engine rotation speed plotted below the reference line is present during the exhaust stroke when idle operation control is being performed. This configuration suppresses the transmission of rotation in the centrifugal clutch (i.e., unintended engagement in the centrifugal clutch) that occurs when the engine rotation speed further increases during idle operation control.

[0023] Based on the above findings, the control device for an internal combustion engine according to each aspect of the present invention comprises the following configuration.

[0024] (1) A control device for an internal combustion engine used in a vehicle equipped with a single-cylinder internal combustion engine and a centrifugal clutch that engages when the rotational speed of the internal combustion engine increases, wherein the control device performs idle operation control, and the straight line connecting the maximum value of the rotational speed of the internal combustion engine during one cycle and the rotational speed of the internal combustion engine at the end of the exhaust stroke is defined as the reference line, and when the rotational speed of the internal combustion engine is plotted below the reference line during the exhaust stroke while the idle operation control is being performed, the control device performs the internal combustion engine to reduce the rotational speed of the internal combustion engine.

[0025] If the engine rotational speed is plotted below the baseline during the exhaust stroke, it is highly likely that disengagement of the centrifugal clutch (specifically, contact of the coupling members in the centrifugal clutch) has occurred. In such cases, if the engine rotational speed increases further, it is highly likely that the transmitted torque in the centrifugal clutch will increase. With the above configuration, the engine rotational speed is reduced in such cases, thereby suppressing unintended engagement in the centrifugal clutch. In other words, with the above configuration, it is possible to increase the target rotational speed during idle operation control compared to conventional methods while avoiding an increase in transmitted torque.

[0026] According to one aspect of the present invention, a control device for an internal combustion engine can employ the following configuration: (2) The control device for an internal combustion engine according to (1), wherein the centrifugal clutch is configured to transmit the rotation of the internal combustion engine to a power transmission means when engaged, and the control device for the internal combustion engine controls the internal combustion engine to reduce the rotation speed of the internal combustion engine if, during the execution of the idle operation control, the rotation speed of the internal combustion engine is plotted below the reference line in the exhaust stroke and the rotation of the internal combustion engine is not transmitted to the power transmission means.

[0027] In the above configuration, the torque generated by the internal combustion engine is transmitted to the power transmission means via a centrifugal clutch. With this configuration, the engine rotation speed is reduced when the engine rotation speed is plotted below the baseline during the exhaust stroke and the centrifugal clutch is not transmitting rotation to the power transmission means. In other words, with the above configuration, the possibility of avoiding a further increase in engine rotation speed that would cause rotation to be transmitted to the power transmission means becomes even higher.

[0028] According to one aspect of the present invention, the control device for an internal combustion engine can adopt the following configuration: (3) The control device for an internal combustion engine according to (1), wherein the exhaust stroke is divided into two parts, and the region close to the combustion stroke is defined as the first half of the exhaust stroke, and the other region as the second half of the exhaust stroke, and during the execution of the idle operation control, if the load acting on the internal combustion engine in the first half of the exhaust stroke is greater than the load acting on the internal combustion engine in the second half of the exhaust stroke, the internal combustion engine is controlled to decrease the rotational speed of the internal combustion engine.

[0029] The load acting on an internal combustion engine (i.e., engine load) can be increased, for example, by applying a frictional force (from an external source) to the input rotating member in a centrifugal clutch. The input rotating member in a centrifugal clutch is, for example, an inner disc from which rotation is transmitted from the crankshaft of the internal combustion engine. During idle operation control, if the engine load in the first half of the exhaust stroke is greater than in the second half of the exhaust stroke, it is likely that the engine speed will be plotted below the baseline. In this case, the control device for the internal combustion engine will reduce the engine speed.

[0030] In other words, according to the above configuration, when an engine rotational speed plotted below the baseline appears, the control device for the internal combustion engine reduces the engine rotational speed, even if the cause is different from the elimination of contact in the coupling member of the centrifugal clutch. To put it another way, if the control device reduces the engine rotational speed when a frictional force is applied to the input-side rotating member such that the engine load in the first half of the exhaust stroke is greater than in the second half of the exhaust stroke, then it can be said that the control device is configured to control the internal combustion engine so that the engine rotational speed is reduced when an engine rotational speed plotted below the baseline exists during the exhaust stroke.

[0031] According to one aspect of the present invention, the control device for an internal combustion engine can adopt the following configuration: (4) The control device for an internal combustion engine according to (1) or (2), which, when it is detected that there is a rotational speed of the internal combustion engine plotted below the reference line during the exhaust stroke while the idle operation control is being performed, controls the internal combustion engine such that the area of ​​the region formed by the rotational speed of the internal combustion engine plotted below the reference line and the reference line is reduced.

[0032] In the above configuration, the control unit of the internal combustion engine determines whether or not there is an engine rotational speed plotted below the baseline when idle speed increase control is performed. Control to reduce the area of ​​the region (also referred to as the "lower region") formed by the rotational speed during the exhaust stroke of the internal combustion engine plotted below the baseline (i.e., a portion of the plotted curve) and the baseline is performed, for example, by reducing the engine rotational speed.

[0033] As engine speed decreases, the timing at which the contact between the centrifugal clutch coupling members is released during the exhaust stroke is advanced. In other words, the period from when the engine speed reaches its maximum during the combustion stroke until the contact between the coupling members is released becomes shorter. Therefore, the difference between the engine speed corresponding to the baseline and the engine speed appearing below the baseline is likely to decrease. Consequently, as engine speed decreases, the area of ​​the lower region is likely to decrease. Alternatively, as engine speed decreases, the lower region may disappear altogether. As a result, unintended engagement of the centrifugal clutch can be suppressed when idle control is performed.

[0034] A centrifugal clutch can switch the power of an internal combustion engine on and off by utilizing the centrifugal force acting on the clutch (more specifically, the coupling member of the centrifugal clutch) in accordance with the rotation of the internal combustion engine. The centrifugal clutch according to this disclosure is, for example, a wet multi-plate clutch. Alternatively, a wet shoe clutch may be used. For ease of understanding, this disclosure will use commonly used terminology for wet shoe clutches. In other words, the centrifugal clutch applied in each of the above configurations is not limited to a wet shoe clutch. The centrifugal clutch may be, for example, a dry shoe clutch.

[0035] Idle operation is an operating state in which the internal combustion engine is rotating but the rotation of the internal combustion engine is not being transmitted to the power transmission mechanism connected to the internal combustion engine. Idle operation control is a control performed when the internal combustion engine is in idle operation. Idle speed increase control is performed when it is necessary to increase the engine speed when the engine is idle. Idle speed increase control includes catalyst early activation control. Idle speed increase control may also include warm-up operation control performed when the internal combustion engine is cold-started.

[0036] The plotted curve described above can be identified by plotting the engine speed in each cycle of an internal combustion engine (particularly the engine speed during the combustion and exhaust strokes). The plotted curve is, for example, a curve drawn on a graph where the horizontal axis is the crank angle and the vertical axis is the engine speed, with the crank angle at top dead center (i.e., the end of the compression stroke) being represented as 0° and the crank angle at the next top dead center being represented as 720°. The plotted curve can be obtained based on measured values ​​of crank angle and engine speed acquired repeatedly by well-known methods.

[0037] When the crank angle is defined in this way, the exhaust stroke can be said to be the period during which the crank angle changes from 180° to 360°. One end of the baseline is determined by the combination of the crank angle at which the engine speed is maximum during the period during which the crank angle changes from 0° to 720°, and the maximum value of the engine speed. The other end of the baseline is determined by the combination of the crank angle at the end of the exhaust stroke (i.e., 360°) and the engine speed at the end of the exhaust stroke. The engine speed is generally maximum during the combustion stroke.

[0038] Once the plotted curve and baseline are identified, it is possible to determine whether there are any engine speeds plotted below the baseline during the exhaust stroke. In addition, based on the identified plotted curve and baseline, the region formed by the engine speed plotted below the baseline and the baseline (i.e., the lower region) can be identified. The plotted curve and baseline are hypothetical concepts introduced to illustrate this technology. Therefore, it is not necessary for the plotted curve and baseline to actually be drawn during the execution of idle operation control.

[0039] The power transmission means includes, for example, reduction gears and a transmission, and is connected to the output shaft of the centrifugal clutch. The state in which the rotation of the internal combustion engine is not transmitted to the power transmission means is the state in which the input shaft of the centrifugal clutch is rotating while the output shaft of the centrifugal clutch connected to the power transmission means is not rotating. The state in which there is an engine rotational speed plotted below the baseline during the exhaust stroke and the rotation of the internal combustion engine is not transmitted to the power transmission means is the state in which torque is transmitted from the input shaft to the output shaft of the centrifugal clutch while the rotation of the internal combustion engine is not transmitted to the power transmission means.

[0040] The first half of the exhaust stroke is the range of crank angles obtained by dividing the exhaust stroke into two, and is the region close to the combustion stroke. The second half of the exhaust stroke is the range of crank angles obtained by dividing the exhaust stroke into two, and is the region that appears after the first half of the exhaust stroke. The first half and second half of the exhaust stroke are concepts introduced hypothetically to explain this technology. Therefore, it is not necessary for the control by the control device of the internal combustion engine to switch between the first half and the second half of the exhaust stroke. If the crank angle is defined as ranging from 0° to 720° as described above, the first half of the exhaust stroke is the period when the crank angle is from 180° to 270°. The second half of the exhaust stroke is the period when the crank angle is from 270° to 360°.

[0041] If the frictional force in the centrifugal clutch during the first half of the exhaust stroke is greater than that during the second half of the exhaust stroke (i.e., if the engine load during the first half of the exhaust stroke is greater than that during the second half of the exhaust stroke), control is performed to reduce the engine speed. This control is performed, for example, by reducing the amount of intake air flowing into the combustion chamber. Alternatively, the control to reduce the engine speed may be a control that temporarily interrupts the combustion of the air-fuel mixture in the combustion chamber, a control that retards the combustion timing, or a control that increases the electrical load. When the engine speed decreases while the lower region is present, as described above, it is highly likely that the area of ​​the lower region will decrease or the lower region will disappear.

[0042] The technical terms used herein are intended to define only specific embodiments and are not intended to limit the invention. The term "and / or" as used herein includes any or all combination of one or more related enumerated components. Where used herein, the use of the terms "including," "comprising," or "having" and their variations identifies the presence of described features, processes, operations, elements, components and / or equivalents thereof, but may include one or more of the steps, operations, elements, components and / or groups thereof. Where used herein, the terms "attached," "connected," "joined" and / or equivalents are used broadly and include both direct and indirect attachment, connection and joining. Furthermore, "connected" and "joined" are not limited to physical or mechanical connection or joining, but may include direct or indirect electrical connection or joining. Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as generally understood by those skilled in the art to whom the invention belongs. Terms such as those defined in commonly used dictionaries should be interpreted as having meanings consistent with their meanings in the context of the relevant technology and this disclosure, and not as ideal or overly formal unless expressly defined herein. In the description of the present invention, it is understood that a number of technologies and steps are disclosed. Each of these has its own individual benefit, and each may be used in conjunction with one or more, or possibly all, of the other disclosed technologies. Therefore, for clarity, this description refrains from unnecessarily repeating all possible combinations of individual steps. Nevertheless, the specification and claims should be read with the understanding that all such combinations are within the scope of the invention and claims. This specification describes a control device for a new internal combustion engine. In the following description, for explanatory purposes, a number of specific details are given to provide a complete understanding of the invention.However, it will be apparent to those skilled in the art that the present invention can be practiced without these specific details. The present disclosure should be considered as an exemplification of the present invention and is not intended to limit the present invention to the specific embodiments shown in the following drawings or description.

[0043] According to the present invention, it is possible to suppress the occurrence of an unintended engagement in the centrifugal clutch during the execution of the idling operation control.

[0044] It is a time chart showing an example of the change in each cycle of the engine rotational speed controlled by the control device for an internal combustion engine according to the first embodiment, for each engine rotational speed. It is a diagram schematically showing an internal combustion engine and a centrifugal clutch. It is a time chart showing an example of the change in the engine rotational speed and the rotational speed of the output shaft (transmission rotational speed) of the centrifugal clutch whose input shaft is connected to the internal combustion engine. It is a time chart showing an example of the change in the engine rotational speed controlled by the control device for an internal combustion engine according to the third embodiment and the frictional force generated by the load generating device so as to act on the internal combustion engine. It is a partially enlarged view related to two changes in the engine rotational speed shown in FIG. 1.

[0045] Hereinafter, the present invention will be described based on embodiments with reference to the drawings.

[0046] [First Embodiment] FIG. 1 is a time chart showing an example of the change in the engine rotational speed NE, which is the rotational speed of the internal combustion engine 2 according to the first embodiment, for each engine rotational speed NE. FIG. 2 is a diagram schematically showing the internal combustion engine 2 and the centrifugal clutch 6. The internal combustion engine 2 is mounted as a power source, for example, on a saddle-type vehicle (not shown, also referred to as a "mounted vehicle").

[0047] The internal combustion engine 2 is controlled by an ECU 1 (control device, control unit, control means). The rotation of the internal combustion engine 2 is transmitted to the drive wheels (not shown) of the mounted vehicle via the centrifugal clutch 6. FIG. 3 is a time chart showing an example of the change in the engine rotational speed NE and the transmission rotational speed ND. The engine rotational speed NE is also the rotational speed of the input shaft in the centrifugal clutch 6. The transmission rotational speed ND is the rotational speed of the output shaft in the centrifugal clutch 6.

[0048] The internal combustion engine 2 is a four-stroke single-cylinder engine that repeats an intake stroke, a compression stroke, a combustion stroke (expansion stroke), and an exhaust stroke. As shown in FIG. 2, in the internal combustion engine 2, the reciprocating motion of the piston 21 is transmitted to the crankshaft 23 via the connecting rod 22, and as a result, the crankshaft 23 rotates.

[0049] The internal combustion engine 2 will be described more specifically. The intake pipe 31 and the intake port 32 form an intake passage for introducing intake air (fresh air) into the combustion chamber 33 of the internal combustion engine 2. The intake valve 34 opens and closes the communication portion between the intake port 32 and the combustion chamber 33 according to the rotational position of a camshaft (not shown) of the internal combustion engine 2. A throttle valve 35 is disposed in the intake pipe 31. The throttle valve 35 adjusts the opening degree of the intake pipe 31 (i.e., the throttle opening degree) according to the instruction of the ECU 1. A fuel injection valve 36 is disposed in the intake port 32. The fuel injection valve 36 injects fuel into the intake port 32 according to the instruction of the ECU 1.

[0050] Furthermore, a bypass flow path 37 that bypasses the throttle valve 35 is disposed in the intake pipe 31. A bypass valve 38 is disposed in the bypass flow path 37. The bypass valve 38 adjusts the bypass valve opening degree Vb according to the instruction of the ECU 1. The bypass valve opening degree Vb is the opening degree of the bypass valve 38.

[0051] An ignition plug 39 is disposed in the combustion chamber 33. The ignition plug 39 ignites the air-fuel mixture in the combustion chamber 33 (i.e., the intake air including the fuel injected from the fuel injection valve 36) according to the instruction of the ECU 1.

[0052] The exhaust port 41 and the exhaust pipe 42 form an exhaust passage for discharging the exhaust gas (combustion gas) generated in the combustion chamber 33. The exhaust valve 43 opens and closes the communication portion between the combustion chamber 33 and the exhaust port 41 according to the rotational position of the camshaft. A catalyst 44 for purifying exhaust gas is interposed in the exhaust pipe 42. The catalyst 44 is an exhaust gas purification device that contains a catalyst substance.

[0053] The rotation of the crankshaft 23 is transmitted to the drive transmission device 54 (power transmission means) via the centrifugal clutch 6, outer shaft 51, gears 52a to 52b that mesh with each other, and drive shaft 53. The crankshaft 23 is the input shaft of the centrifugal clutch 6, and the drive shaft 53 is the output shaft of the centrifugal clutch 6. The drive transmission device 54 includes a transmission and reduction gears, etc., and forms a torque transmission path from the drive shaft 53 to the drive wheels (not shown) of the mounted vehicle. In the mounted vehicle according to this embodiment, rotation is transmitted from the crankshaft 23 to the drive shaft 53 using gears 52a to 52b, but rotation may be transmitted by a belt and / or chain instead of gears 52a to 52b.

[0054] The centrifugal clutch 6 includes an inner disc 61, an outer drum 62, and a plurality of clutch shoes 63. Each of the outer drum 62 and clutch shoes 63 is a coupling member in the centrifugal clutch 6.

[0055] The inner disc 61 is mounted on the crankshaft 23. Therefore, the crankshaft 23 and the inner disc 61 rotate as a single unit. The outer drum 62 and the gear 52a are mounted on the outer shaft 51. Therefore, the outer drum 62, the outer shaft 51, and the gear 52a rotate as a single unit. The engine rotational speed NE mentioned above is the rotational speed of the crankshaft 23. The rotational speed of the outer shaft 51 is also referred to as the transmission rotational speed ND.

[0056] Each of the clutch shoes 63 is disposed on the inner disc 61 and rotates integrally with the inner disc 61. The inner disc 61 is provided with a plurality of elastic bodies (not shown; in this embodiment, coil springs) that bias the clutch shoes 63 toward the axis of the crankshaft 23.

[0057] As the rotational speed of the inner disc 61 (i.e., the engine rotational speed NE) increases, the clutch shoe 63 moves away from the axis of the crankshaft 23 against the biasing force of the elastic body due to centrifugal force. As a result, the clutch shoe 63 approaches the inner circumferential surface of the outer drum 62. When the clutch shoe 63 comes into contact with the inner circumferential surface of the outer drum 62 due to centrifugal force (i.e., when the clutch shoe 63 and the outer drum 62 engage), a frictional force is generated between the clutch shoe 63 and the outer drum 62. As a result, torque is transmitted from the crankshaft 23 to the driveshaft 53 via the clutch shoe 63 and the outer drum 62.

[0058] When the torque transmitted from the crankshaft 23 to the driveshaft 53 (transmitted torque) increases, the driveshaft 53 rotates. That is, rotation is transmitted from the crankshaft 23 to the driveshaft 53. When rotation is transmitted from the crankshaft 23 to the driveshaft 53, the drive wheels of the vehicle rotate. Weights may be provided on each of the clutch shoes 63 so that the clutch shoes 63 and the outer drum 62 engage even when the engine rotation speed NE is relatively low. By providing weights, the centrifugal force acting on the clutch shoes 63 increases. In the vehicle equipped with this embodiment, the centrifugal clutch 6 is provided on the crankshaft 23, but the centrifugal clutch 6 may also be provided on the driveshaft 53.

[0059] The ECU 1 is an electronic control unit that includes a CPU, RAM, and non-volatile memory (none of which are shown). The ECU 1 is connected to a crank angle sensor 71, a cam position sensor 72, an accelerator pedal operation amount sensor 73, an intake air volume sensor 74, a vehicle speed sensor 75, and temperature sensors 76a to 76b. The crank angle sensor 71 outputs a pulse signal to the ECU 1 each time the crankshaft 23 rotates by a predetermined angle. The cam position sensor 72 outputs a signal to the ECU 1 corresponding to the rotational position of the camshaft of the internal combustion engine 2.

[0060] The ECU 1 acquires the engine rotational speed NE based on the signal input from the crank angle sensor 71. In addition, the ECU 1 acquires the crank angle CA based on the signals input from the crank angle sensor 71 and the cam position sensor 72. The crank angle CA is a value that falls within the range of 0° to 720°. When the piston 21 is at top dead center of compression, the crank angle CA is 0°. When the piston 21 is at top dead center of exhaust, the crank angle CA is 360°.

[0061] The accelerator pedal operation amount sensor 73 acquires the accelerator pedal operation amount Ap. The accelerator pedal operation amount Ap is a value that represents the amount of operation (rotation angle) relative to the accelerator grip (not shown) located on the handlebars of the vehicle on which the vehicle is installed. When the accelerator grip is not operated by the driver of the vehicle on which the vehicle is installed, the accelerator pedal operation amount Ap is "0". When the accelerator grip is operated, the accelerator pedal operation amount Ap becomes greater than "0".

[0062] The intake air volume sensor 74 is positioned upstream of the bypass passage 37 in the intake manifold 31. The intake air volume sensor 74 acquires the intake air volume Ga. The intake air volume Ga is the amount of intake air flowing into the combustion chamber 33 per unit time. The vehicle speed sensor 75 acquires the vehicle speed Vs. The vehicle speed Vs is the driving speed of the vehicle on which the system is installed.

[0063] The temperature sensor 76a is positioned upstream of the catalyst 44 in the exhaust pipe 42. The temperature sensor 76a acquires the exhaust temperature Te1. The exhaust temperature Te1 is the temperature of the exhaust gas flowing into the catalyst 44. The temperature sensor 76b is positioned downstream of the catalyst 44 in the exhaust pipe 42. The temperature sensor 76b acquires the exhaust temperature Te2. The exhaust temperature Te2 is the temperature of the exhaust gas flowing out of the catalyst 44. The ECU 1 acquires the catalyst temperature Tc based on the exhaust temperatures Te1 to Te2. The catalyst temperature Tc is the temperature of the catalytic material contained in the catalyst 44. The catalyst temperature Tc may also be acquired directly by a temperature sensor installed in the catalyst 44.

[0064] (Control of the internal combustion engine by the ECU) The ECU 1 controls the throttle opening via the throttle valve 35 while the internal combustion engine 2 is operating. Generally, the larger the accelerator pedal input Ap, the larger the throttle opening, and as a result, the intake air volume Ga increases. In addition, the ECU 1 determines the fuel injection amount Qinj based on the intake air volume Ga. Generally, the larger the intake air volume Ga, the larger the fuel injection amount Qinj. When the crank angle CA reaches the fuel injection timing Cinj, the ECU 1 injects an amount of fuel equal to the fuel injection amount Qinj into the fuel injector 36.

[0065] (Idle Operation Control) Furthermore, when the ECU 1 determines that the internal combustion engine 2 should be in an idle state, it executes "idle operation control". Specifically, the ECU 1 executes idle operation control when the accelerator pedal input amount Ap is "0". When idle operation control is executed, the ECU 1 maintains the throttle valve 35 in a fully closed state. In addition, the ECU 1 controls the intake air volume Ga by adjusting the bypass valve opening Vb.

[0066] Idle operation control includes "first idle operation control" and "catalyst early activation control". First idle operation control is performed, for example, when the vehicle is in a stationary state. When first idle operation control is performed, ECU 1 adjusts the bypass valve opening Vb by feedback control so that the engine rotational speed NE approaches the reference idle rotational speed Nib. More specifically, ECU 1 sets the target rotational speed Ntgt to a value equal to the reference idle rotational speed Nib. In addition, ECU 1 adjusts the bypass valve opening Vb so that the difference between the target rotational speed Ntgt and the engine rotational speed NE becomes small.

[0067] On the other hand, if the ECU 1 determines that it is necessary to raise the temperature of the catalyst 44 early in order to activate it, it executes catalyst early activation control. For example, if the catalyst temperature Tc is lower than the threshold temperature Tth, the ECU 1 determines that it is necessary to raise the temperature of the catalyst 44 early. When executing catalyst early activation control, the ECU 1 sets the target rotational speed Ntgt to a value equal to the first temperature-raising rotational speed Nr1. The first temperature-raising rotational speed Nr1 is greater than the reference idle rotational speed Nib (i.e., Nr1 > Nib). Catalyst early activation control in which the target rotational speed Ntgt is set to a value greater than the reference idle rotational speed Nib is also called "idle speed increase control".

[0068] (Specific control in idle operation control) Furthermore, when the ECU 1 determines that a "specific condition" is met during idle operation control, it executes a "specific control" that reduces the engine rotation speed NE by a rotation speed reduction value Nf. The specific condition is a condition that is met when there is a high probability that transmission torque is being generated in the centrifugal clutch 6, as will be described later.

[0069] The specific control will be explained with reference to the solid line L5a and dashed line L6a shown in Figure 3. The solid line L5a shows an example of the change in engine rotational speed NE. The dashed line L6a shows the change in transmission rotational speed ND in this example. Note that the actual vehicle on which this system is installed does not have a sensor that directly detects the transmission rotational speed ND. During the period shown in Figure 3, the accelerator pedal input amount Ap is maintained at "0". In addition, during the period shown in Figure 3, the braking system (not shown) installed in the vehicle on which this system is installed is not operating.

[0070] In this example, cranking begins at time tb0. More specifically, from time tb0 onward, the starter motor (not shown) rotates the crankshaft 23. As a result, the internal combustion engine 2 begins operation. Since the accelerator pedal input amount Ap is "0", the ECU 1 performs idle operation control as soon as the internal combustion engine 2 starts operation. More specifically, since the catalyst temperature Tc at time tb0 is less than the threshold temperature Tth, the ECU 1 performs catalyst early activation control as idle operation control. That is, the ECU 1 sets the target rotational speed Ntgt to a value equal to the first temperature rise rotational speed Nr1.

[0071] Therefore, at time tb1, the engine rotation speed NE reaches the first heating rotation speed Nr1, and thereafter, the engine rotation speed NE is maintained in the vicinity of the first heating rotation speed Nr1. More specifically, the engine rotation speed NE fluctuates in the vicinity of the first heating rotation speed Nr1. In other words, "fluctuations" occur in the engine rotation speed NE.

[0072] As time tb2 approaches, fluctuations in the engine rotational speed NE increase, and as a result, the engine rotational speed NE becomes relatively larger than the first temperature-raising rotational speed Nr1. More specifically, the engine rotational speed NE at time tb2 is the rotational speed nb2. The rotational speed nb2 is greater than the first temperature-raising rotational speed Nr1. Due to the increase in engine rotational speed NE, the ECU1 determines that a specific condition is met at time tb2.

[0073] ECU1 determines that a specific condition is met and executes a specific control at time tb2. More specifically, ECU1 sets the target rotational speed Ntgt to a value that is smaller than the rotational speed nb2 by the rotational speed reduction value Nf. In this example, the target rotational speed Ntgt becomes the rotational speed nb1. The rotational speed nb1 is the difference between the rotational speed nb2 and the rotational speed reduction value Nf (i.e., nb1 = nb2 - Nf). As a result, from time tb2 onward, the engine rotational speed NE is maintained in the vicinity of the rotational speed nb1.

[0074] Subsequently, at time tb6, ECU1 determines that it is no longer necessary to prematurely heat up the catalyst 44. For example, ECU1 determines that it is no longer necessary to prematurely heat up the catalyst 44 when the catalyst temperature Tc becomes greater than the threshold temperature Tth. Therefore, from time tb6 onward, ECU1 executes the first idle operation control as idle operation control instead of the catalyst early activation control. That is, ECU1 sets the target rotational speed Ntgt to a value equal to the reference idle rotational speed Nib. As a result, from time tb6 onward, the engine rotational speed NE is maintained near the reference idle rotational speed Nib.

[0075] (Specific conditions in idle operation control) The specific conditions will be explained with reference to the solid line L1, dashed line L2, dashed line L3, and double dashed line L4 shown in Figure 1. The solid line L1, dashed line L2, dashed line L3, and double dashed line L4 are examples of changes in engine rotational speed NE when idle operation control is performed. More specifically, the solid line L1, dashed line L2, dashed line L3, and double dashed line L4 represent the changes in engine rotational speed NE in one cycle for the internal combustion engine 2, and the engine rotational speed NE increases in this order.

[0076] The solid line L1, dashed line L2, dashed line L3, and dashed line L4 are curves (i.e., plotted curves) obtained by plotting the engine rotation speed NE, which changes over time, on a graph (time chart). In Figure 1, the passage of time is represented by the crank angle CA.

[0077] The lines L1a to L4a shown in Figure 1 are "reference lines" corresponding to each of the plotted curves. The reference line is a straight line (line segment) obtained by connecting the maximum value of the engine rotational speed NE in one cycle of the internal combustion engine 2 in the plotted curve with the engine rotational speed NE at the end of the exhaust stroke in that cycle (i.e., exhaust top dead center). In other words, the reference line is the line segment in the plotted curve that connects the engine rotational speed NE that reached its maximum value (local maximum) during the combustion stroke with the engine rotational speed NE at the end of the exhaust stroke that follows the combustion stroke.

[0078] For example, along the dashed line L2, the maximum value of the engine rotational speed NE in one cycle is rotational speed na4. At the end of the exhaust stroke, the engine rotational speed NE is rotational speed na2. Along the dashed line L3, the maximum value of the engine rotational speed NE in one cycle is rotational speed na5. At the end of the exhaust stroke, the engine rotational speed NE is rotational speed na3.

[0079] The section of the plotted curve that appears below the baseline during the exhaust stroke (i.e., a portion of the plotted curve) is also referred to as the "lower section." The area between the plotted curve and the baseline within the lower section is also referred to as the "lower region."

[0080] In the example in Figure 1, the dashed line L2 shows a lower section (i.e., the section that appears below the dashed line L2 in the straight line L2a). Similarly, the dashed line L3 also shows a lower section. Regions R2 and R3 are the lower regions corresponding to the dashed line L2 and the dashed line L3, respectively. The solid line L1 and the double-dotted line L4 do not show a lower section. Therefore, there are no lower regions corresponding to the solid line L1 and the double-dotted line L4.

[0081] The reason why a downward section (and thus a downward region) may temporarily appear during the process of increasing engine rotational speed NE will be explained. As described above, as engine rotational speed NE increases, the centrifugal clutch 6 transitions from an open state to a semi-engaged state. More specifically, as engine rotational speed NE increases, the clutch shoe 63 comes into contact with the inner circumferential surface of the outer drum 62 (i.e., the clutch shoe 63 and the outer drum 62 engage), generating frictional force (clutch friction force) in the centrifugal clutch 6. Torque is transmitted to the drive shaft 53 by the clutch friction force, while the load acting on the internal combustion engine 2 (engine load) increases.

[0082] In other words, when the centrifugal clutch 6 is in a semi-engaged state and the engine speed NE decreases, the engagement between the clutch shoe 63 and the outer drum 62 is released (i.e., the centrifugal clutch 6 transitions to an open state), and the engine load decreases. The engine speed NE at which the engagement between the clutch shoe 63 and the outer drum 62 is released is also called the "release speed." During the period from when the engine speed NE reaches its maximum value until the end of the exhaust stroke, if the engine speed NE becomes less than the release speed, the engine load decreases. As a result, the rate of decrease in engine speed NE becomes smaller, and the lower section appears in the plotted curve.

[0083] In the example in Figure 1, the release rotational speed falls within the range of rotational speeds na3 to na4. Therefore, the lower section appears in the changes in engine rotational speed NE shown by the dashed line L2 and the dashed line L3, respectively. That is, in the examples of the dashed line L2 and the dashed line L3, the engine load decreases during the period from when the engine rotational speed NE reaches its maximum value until the end of the exhaust stroke. On the other hand, in the example of the solid line L1, since the maximum value is rotational speed na1 (which is smaller than rotational speed na3), there is no decrease in engine load during the period from when the engine rotational speed NE reaches its maximum value until the end of the exhaust stroke. That is, in the cycle of engine rotational speed NE shown by the solid line L1, the state in which the engine rotational speed NE is smaller than the release rotational speed is maintained, and therefore, no clutch friction force is generated. Therefore, the lower section does not appear in the plotted curve.

[0084] In other words, if a downward section appears in the plotted curve as a result of an increase in engine rotational speed NE, it is highly likely that clutch friction force is being generated. That is, it is highly likely that transmission torque is being generated in the centrifugal clutch 6. Therefore, when the ECU 1 detects that a downward section appears in the plotted curve during idle operation control (i.e., when there is an engine rotational speed NE plotted below the baseline), it determines that a specific condition has been met and executes a specific control.

[0085] (Significance of Specific Control) The reason why ECU1 performs specific control will be explained with reference to the dashed-dotted lines L5b and L6b shown in Figure 3. The dashed-dotted line L5b shows an example of the change in engine rotational speed NE assuming that the specific control was not started at time tb2. The dashed-dotted line L6b shows the change in transmission rotational speed ND in this example. The illustration of the dashed-dotted lines L5b and L6b for periods prior to time tb2 is omitted.

[0086] In this assumption, no specific control is performed, so from time tb2 onward, the engine speed NE temporarily rises above the rotational speed nb2. On the other hand, in this assumption, ECU1 sets the target rotational speed Ntgt to a value equal to the reference idle rotational speed Nib around time tb6. That is, as idle operation control, the first idle operation control is performed instead of the catalyst early activation control. Therefore, after the engine speed NE temporarily rises, the engine speed NE decreases to the reference idle rotational speed Nib.

[0087] In this case, as shown by the dashed line L6b, the transmission speed ND increases from "0" from time tb3, and at time tb4, the transmission speed ND is equal to the engine speed NE. Subsequently, from time tb5, the transmission speed ND becomes less than the engine speed NE, and at time tb7, the transmission speed ND is approximately "0". That is, the vehicle speed Vs is greater than "0" during the period from time tb3 to time tb7.

[0088] In other words, during the period from time tb3 to time tb4, and from time tb5 to time tb7, the transmitted rotational speed ND is greater than "0" and the rotational speed difference ΔN is greater than "0". The rotational speed difference ΔN is the difference between the engine rotational speed NE and the transmitted rotational speed ND (i.e., ΔN = NE - ND). Figure 3 shows an example of the rotational speed difference ΔN at time tb3a.

[0089] Specifically, during the period from time tb3 to time tb4, and from time tb5 to time tb7, the centrifugal clutch 6 is in a semi-engaged state (more specifically, a state in which rotation is transmitted). During the period from time tb4 to time tb5, the centrifugal clutch 6 is in a fully engaged state. During the period up to time tb3, and from time tb7 onward, rotation is not transmitted through the centrifugal clutch 6. At time tb3, the engine rotation speed NE is the clutch-in rotation speed Nci. At time tb7, the engine rotation speed NE is the clutch-out rotation speed Nco. Specifically, when the engine rotation speed NE decreases from a value greater than the clutch-out rotation speed Nco to the clutch-out rotation speed Nco, the transmission of rotation through the centrifugal clutch 6 stops.

[0090] As can be seen from Figure 3, the first heating rotation speed Nr1 is greater than the clutch-out rotation speed Nco. In addition, the clutch-in rotation speed Nci is greater than the first heating rotation speed Nr1 (i.e., Nco < Nr1 < Nci). The reason why the clutch-in rotation speed Nci is greater than the clutch-out rotation speed Nco is that when the transmitted rotation speed ND is greater than "0", the torque generated by the internal combustion engine 2 and acting on the inner disc 61 must be greater than "the torque required to start rotating each component included in the torque transmission path from the outer drum 62 to the drive wheels of the mounted vehicle against the maximum static friction force".

[0091] As can be understood from this assumption, if specific control is not performed when the engine rotational speed NE increases, the likelihood of unintended engagement in the centrifugal clutch 6 increases. That is, the transmitted torque in the centrifugal clutch 6 increases, and as a result, the transmitted rotational speed ND is likely to become greater than "0". On the other hand, as shown by the solid line L5a, if specific control is performed when the engine rotational speed NE increases, the likelihood of unintended engagement in the centrifugal clutch 6 being suppressed increases. In other words, before the transmitted torque in the centrifugal clutch 6 increases and the transmitted rotational speed ND becomes greater than "0", the engine rotational speed NE decreases, and as a result, the state in which the transmitted rotational speed ND is "0" is maintained.

[0092] Incidentally, the clutch-in rotational speed Nci and the clutch-out rotational speed Nco can both change. Similarly, the release rotational speed can also change. More specifically, as mentioned above, the transmission of torque (and consequently, the transmission of rotation) in the centrifugal clutch 6 is due to the frictional force generated when the clutch shoe 63 contacts the inner circumferential surface of the outer drum 62. The frictional force between the clutch shoe 63 and the outer drum 62 (i.e., the clutch friction force) can change depending on the temperature of the clutch shoe 63 and the outer drum 62, as well as the ambient humidity. In addition, the clutch friction force can change due to wear and deformation of the clutch shoe 63.

[0093] Furthermore, clutch friction may increase due to a decrease in the biasing force caused by the aging of the elastic body (i.e., coil spring) that biases the clutch shoe 63 toward the axis of the crankshaft 23. In other words, a decrease in biasing force can cause clutch friction to occur even at relatively low engine rotational speeds NE. Similarly, a decrease in the aging of the member (not shown) that holds the clutch shoe 63 can also cause clutch friction to occur even at relatively low engine rotational speeds NE.

[0094] When the release rotational speed changes, the engine rotational speed NE at which the lower section begins to appear during the exhaust stroke changes. When the lower section appears during the exhaust stroke, the ECU 1 determines that a specific condition has been met and executes specific control. In other words, according to the ECU 1, even if the engine rotational speed NE at which clutch friction force is generated in the centrifugal clutch 6 (i.e., the release rotational speed) decreases, the likelihood of unintended engagement in the centrifugal clutch 6 being suppressed increases. More specifically, when the release rotational speed decreases, the ECU 1 starts specific control earlier.

[0095] Therefore, the ECU 1 can maintain the engine rotational speed NE at the largest possible value when idle operation control (especially idle speed increase control) is performed, and can also suppress unintended engagement in the centrifugal clutch 6 (for example, transmission of rotation in the centrifugal clutch 6).

[0096] In the vehicle equipped with this actual configuration, the rotational speed of the inner disc 61 of the centrifugal clutch 6 (input shaft rotational speed) was equal to the engine rotational speed NE, but the input shaft rotational speed may be different from the engine rotational speed NE. For example, a gear mechanism may be interposed in the torque transmission path from the crankshaft 23 to the inner disc 61. Even in this case, the input shaft rotational speed increases as the engine rotational speed NE increases. Therefore, when the lower section appears in the exhaust stroke as the engine rotational speed NE increases, it is determined that a specific condition has been met and a specific control is executed.

[0097] The first temperature-raising rotational speed Nr1 was greater than the clutch-out rotational speed Nco, but it may also be less than the clutch-out rotational speed Nco and greater than the reference idle rotational speed Nib. In any case, the first temperature-raising rotational speed Nr1 may be set to a value such that a specific condition is not met when the engine rotational speed NE is near the first temperature-raising rotational speed Nr1. That is, the first temperature-raising rotational speed Nr1 may be set to be less than the engine rotational speed NE at which the specific condition is met.

[0098] The ECU 1 may perform idle speed increase control that is different from catalyst early activation control. For example, the ECU 1 may perform "warm-up operation control" as idle speed increase control when the internal combustion engine 2 is cold-started. When performing warm-up operation control, the ECU 1 sets, for example, the target rotational speed Ntgt to a value equal to the second temperature-raising rotational speed Nr2 (not shown). The second temperature-raising rotational speed Nr2 is greater than the reference idle rotational speed Nib (i.e., Nr2 > Nib).

[0099] The ECU 1 adjusted the bypass valve opening Vb according to the difference between the target rotational speed Ntgt and the engine rotational speed NE. In other words, when the ECU 1 decreases the engine rotational speed NE, it decreases the bypass valve opening Vb, thereby decreasing the amount of air flowing into the combustion chamber 33. However, the ECU 1 may also decrease the engine rotational speed NE by decreasing the fuel injection amount Qinj. Alternatively, the ECU 1 may decrease the engine rotational speed NE by retarding the ignition timing. The ECU 1 may also decrease the engine rotational speed NE by applying an electrical load. The ECU 1 may also decrease the engine rotational speed NE by temporarily causing a misfire (i.e., temporarily stopping fuel injection by the fuel injector 36 and ignition by the spark plug 39).

[0100] [Second Embodiment] In the second embodiment, the internal combustion engine 2 is controlled by the ECU 1a (see Figure 2). In the first embodiment, the ECU 1 determined that a specific condition was met when a lower section appeared on the plotted curve during idle operation control. In contrast, the ECU 1a determines that a specific condition is met when a lower section appears on the plotted curve during idle operation control AND the transmission rotational speed ND is approximately "0".

[0101] When ECU 1a determines that certain conditions are met, it executes specific control. In other words, when the transmitted rotational speed ND is approximately "0" (i.e., when rotation has not yet been transmitted from the internal combustion engine 2 to the drive transmission device 54), ECU 1a executes specific control when there is a high probability that transmission torque is being generated in the centrifugal clutch 6. That is, ECU 1a can more reliably suppress the transmission rotational speed ND from becoming greater than "0" as a result of unintended engagement in the centrifugal clutch 6 during idle operation control.

[0102] [Third Embodiment] In the third embodiment, the internal combustion engine 2 is controlled by the ECU 1, while the vehicle on which it is mounted may include a load generating device 81 (see Figure 2). Figure 4 is a time chart showing an example of a load test using the load generating device 81. Figure 4 uses the solid line L1 shown in Figure 1 (i.e., an example of change at engine rotational speed NE). The load generating device 81 can contact the crankshaft 23 (or inner disc 61) and generate a frictional force Fr that hinders the rotation of the crankshaft 23. When the frictional force Fr becomes greater than "0", the load acting on the internal combustion engine 2 (i.e., engine load) increases.

[0103] In this example, the load generating device 81 generates a frictional force Fr when the engine rotational speed NE becomes greater than the threshold rotational speed Nth. More specifically, as shown by the solid line L7 in Figure 4, if the engine rotational speed NE is greater than the threshold rotational speed Nth, the frictional force Fr increases as the engine rotational speed NE increases, and the engine load of the internal combustion engine 2 increases accordingly.

[0104] The dashed line L1m shown in Figure 4 represents the change in engine rotational speed NE in this case. In other words, the change in engine rotational speed NE when the frictional force Fr is maintained at "0" is shown by the solid line L1, while the change in engine rotational speed NE when the load generating device 81 generates a frictional force Fr is shown by the dashed line L1m.

[0105] The straight line L1ma is a reference line corresponding to the dashed line L1m. The dashed line L1m includes a section that appears below the straight line L1ma during the exhaust stroke (i.e., the lower section), and therefore the lower region R1m appears. In this case, the ECU1 determines that a specific condition is met and executes a specific control.

[0106] In other words, the increase in engine load that occurs with the transition from the open state to the semi-engaged state of the centrifugal clutch 6 due to an increase in engine rotational speed NE is simulated by the change in frictional force Fr by the load generating device 81. In this example, the threshold rotational speed Nth is a simulated release rotational speed. In other words, by simulating engine load during idle operation control, it is possible to determine whether the ECU 1 is configured to execute a specific control when a specific condition is met.

[0107] This means that even if the release rotational speed decreases due to environmental changes or aging of the centrifugal clutch 6, the ECU 1 can execute specific control when the engine rotational speed NE is relatively low, thereby suppressing unintended engagement of the centrifugal clutch 6. In other words, even if the release rotational speed changes, the ECU 1 can determine whether the specific condition is met (i.e., whether specific control is necessary) according to the actual release rotational speed.

[0108] The generation of frictional force Fr by the load generating device 81 will be explained in more detail. In the example shown in Figure 4, the load generating device 81 generates frictional force Fr when the crank angle CA is within the range from angle ca1 to angle ca2. As can be seen from Figure 4, angle ca1 is less than 180°. Angle ca2 is within the range from 180° to 270°. Therefore, the engine load in the "first half of the exhaust stroke" (i.e., the load acting on the internal combustion engine 2) is greater than the engine load in the "second half of the exhaust stroke". The first half of the exhaust stroke is the region closest to the combustion stroke, obtained by dividing the exhaust stroke (i.e., the region in which the crank angle CA is within the range from 180° to 360°) into two parts. The second half of the exhaust stroke is the other region (following the first half of the exhaust stroke).

[0109] The engine load in the first half of the exhaust stroke can be said to be the integrated value of the friction force Fr in the first half of the exhaust stroke. Similarly, the engine load in the second half of the exhaust stroke can be said to be the integrated value of the friction force Fr in the second half of the exhaust stroke. In the example in Figure 4, the engine load in the first half of the exhaust stroke is greater than the engine load in the second half of the exhaust stroke, resulting in a downward pressure zone, and consequently, specific control is executed.

[0110] [Fourth Embodiment] In the fourth embodiment, the internal combustion engine 2 is controlled by the ECU 1b (see Figure 2). The ECU 1b performs a "determination process" to determine whether a specific condition is met. Figure 5 is a time chart referenced to explain the determination process. Figure 5 incorporates the dashed line L2 and the dashed line L3 shown in Figure 1 (i.e., examples of changes at engine rotational speed NE). More specifically, in Figure 5, parts of the dashed line L2 and the dashed line L3 are shown in a magnified view compared to Figure 1, along with the straight lines L2a to L3a.

[0111] The determination process determines whether or not there is an engine rotation speed NE plotted below the baseline during the exhaust stroke when idle operation control is being executed. The determination process may be performed by either ECU1 or ECU1a as described above.

[0112] To describe the determination process in more detail, during idle operation control, ECU 1b stores the engine speed NE from the start of the combustion stroke to the end of the exhaust stroke in its RAM along with the crank angle CA as a unit of time elapses. When the exhaust stroke ends, ECU 1b obtains the maximum value of the engine speed NE during the combustion stroke, the crank angle CA at which the engine speed NE was at its maximum, and the engine speed NE at the end of the exhaust stroke. In addition, ECU 1b identifies a baseline based on these obtained values. Furthermore, ECU 1b determines whether or not there are any engine speed NE values ​​obtained during the exhaust stroke that are plotted below the baseline.

[0113] For example, in the case of the dashed line L2, when the crank angle CA is angle cd, the engine rotational speed NE is rotational speed nd1. Angle cd is an example of the crank angle CA included in the exhaust stroke. On the other hand, the engine rotational speed NE corresponding to angle cd on the straight line L2a is rotational speed nd2. Rotational speed nd1 is smaller than rotational speed nd2 (i.e., nd1 < nd2). Therefore, when the crank angle CA is angle cd, the engine rotational speed NE is plotted below the baseline (straight line L2a in the case of the dashed line L2).

[0114] Therefore, in the example of the dashed line L2, ECU1b determines that a specific condition is met as a result of executing the judgment process. Therefore, ECU1b executes a specific control. That is, ECU1b reduces the engine rotational speed NE by the rotational speed reduction value Nf. When the engine rotational speed NE decreases, it is suppressed that the engine rotational speed NE becomes greater than the release rotational speed mentioned above during the combustion stroke. As a result, the area of ​​the lower region in the exhaust stroke decreases, or the lower region disappears (i.e., the area of ​​the lower region becomes "0"). Therefore, it can be said that the specific control is a control that reduces the area of ​​the lower region in the exhaust stroke.

[0115] 1, 1a, 1b ECU 2 Internal Combustion Engine 21 Piston 22 Connecting Rod 23 Crankshaft 31 Intake Pipe 32 Intake Port 33 Combustion Chamber 34 Intake Valve 35 Throttle Valve 36 Fuel Injector 37 Bypass Passage 38 Bypass Valve 39 Spark Plug 41 Exhaust Port 42 Exhaust Pipe 43 Exhaust Valve 44 Catalytic Converter 51 Outer Shaft 52a, 52b Gears 53 Drive Shaft 54 ​​Drive Transmission 6 Centrifugal Clutch 61 Inner Disc 62 Outer Drum 63 Clutch Shoe 71 Crank Angle Sensor 72 Cam Position Sensor 73 Accelerator Pitch Sensor 74 Intake Air Volume Sensor 75 Vehicle Speed ​​Sensor 76a, 76b Temperature Sensor 81 Load Generator

Claims

1. A control device for an internal combustion engine used in a vehicle equipped with a single-cylinder internal combustion engine and a centrifugal clutch that engages when the rotational speed of the internal combustion engine increases, and which performs idle operation control, wherein, when the rotational speed of the internal combustion engine in one cycle is plotted, a straight line connecting the maximum value of the rotational speed of the internal combustion engine in one cycle and the rotational speed of the internal combustion engine at the end of the exhaust stroke is defined as a reference line, and when the rotational speed of the internal combustion engine is plotted below the reference line during the exhaust stroke while idle operation control is being performed, the control device for an internal combustion engine controls the internal combustion engine so as to decrease the rotational speed of the internal combustion engine.

2. A control device for an internal combustion engine according to claim 1, wherein the centrifugal clutch is configured to transmit the rotation of the internal combustion engine to a power transmission means when engaged, and the control device for the internal combustion engine controls the internal combustion engine to reduce the rotation speed of the internal combustion engine when, during the execution of the idle operation control, the rotation speed of the internal combustion engine is plotted below the reference line in the exhaust stroke and the rotation of the internal combustion engine is not transmitted to the power transmission means.

3. A control device for an internal combustion engine according to claim 1, wherein the exhaust stroke is divided into two parts, with the region closer to the combustion stroke defined as the first half of the exhaust stroke and the other region as the second half of the exhaust stroke, and during the execution of the idle operation control, if the load acting on the internal combustion engine in the first half of the exhaust stroke is greater than the load acting on the internal combustion engine in the second half of the exhaust stroke, the control device for an internal combustion engine controls the internal combustion engine to reduce the rotational speed of the internal combustion engine.

4. A control device for an internal combustion engine according to claim 1 or 2, wherein, when it is detected that there is a rotational speed of the internal combustion engine plotted below the reference line during the exhaust stroke while the idle operation control is being performed, the control device controls the internal combustion engine such that the area of ​​the region formed by the rotational speed of the internal combustion engine plotted below the reference line and the reference line decreases.

Images