Internal combustion engine boost pressure diagnostic apparatus

An internal combustion engine boost pressure diagnostic apparatus includes a boost pressure sensor, a throttle valve opening degree sensor, a bypass valve opening degree sensor and a control unit. The boost pressure sensor detects a pressure between a forced induction device and a throttle valve. The throttle valve opening degree sensor detects a throttle valve opening degree of the throttle valve. The bypass valve opening degree sensor detects a valve opening degree of a bypass valve. The control unit determines that the pressure between the forced induction device and the throttle valve is abnormal upon the boost pressure sensor detecting the pressure being equal to or larger than a prescribed value, the throttle valve opening degree sensor detecting the throttle valve opening degree being in a prescribed opening degree region, and the bypass valve opening degree sensor detecting the bypass valve opening degree being in a prescribed opening degree region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2011-011549, filed on Jan. 24, 2011. The entire disclosure of Japanese Patent Application No. 2011-011549 is hereby incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention generally relates to a boost pressure diagnostic apparatus for an internal combustion engine equipped with a forced induction device. More specifically, the present invention relates a boost pressure diagnostic apparatus that determines if a boost pressure is abnormal in an internal combustion engine equipped with a forced induction device.

2. Background Information

Japanese Laid-Open Patent Application Publication No. 2007-77897, for example, discloses an apparatus comprising a throttle valve, a forced induction device provided on upstream the throttle valve, a bypass valve provided in a passage arranged to circumvent the forced induction device, and a boost pressure sensor configured to detect a pressure upstream of the throttle valve. The apparatus is configured to determine that an abnormality exists in the bypass valve when the pressure upstream of the throttle valve is larger than a preset pressure determination value and has remained larger than the preset pressure determination value continuously for a prescribed amount of time after the bypass valve was opened.

SUMMARY

It has been discovered that with the apparatus disclosed in the above mentioned Japanese patent publication, a determination regarding the pressure upstream of the throttle valve is made based solely on a detection value of the boost pressure sensor even if the detection value of the boost pressure sensor includes noise. Consequently, there is a possibility that an incorrect pressure determination value will occur due to the detection value of the boost pressure sensor containing noise such that the pressure upstream of the throttle valve will exceed the pressure determination value.

In view of the state of the known technology, one aspect presented in the present disclosure is to provide a boost pressure diagnostic apparatus for an internal combustion engine according that determines a pressure between a forced induction device and a throttle valve is abnormal when a pressure detected by a boost pressure sensor detects a pressure between the forced induction device and the throttle valve that is equal to or larger than a prescribed value and each of an opening degree of the throttle valve and an opening degree of a bypass valve provided in a bypass passage allows intake air to circumvent the forced induction device is in a prescribed opening degree region.

Another aspect presented in the present disclosure is to provide an internal combustion engine boost pressure diagnostic apparatus that basically comprises a boost pressure sensor, a throttle valve opening degree sensor, a bypass valve opening degree sensor and a control unit. The boost pressure sensor detects a pressure between a forced induction device and a throttle valve. The throttle valve opening degree sensor detects a throttle valve opening degree of the throttle valve. The bypass valve opening degree sensor detects a valve opening degree of a bypass valve. The control unit determines that the pressure between the forced induction device and the throttle valve is abnormal upon the boost pressure sensor detecting the pressure being equal to or larger than a prescribed value, the throttle valve opening degree sensor detecting the throttle valve opening degree being in a prescribed throttle valve opening degree region, and the bypass valve opening degree sensor detecting the bypass valve opening degree being in a prescribed bypass valve opening degree region.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring initially toFIG. 1, an internal combustion engine1is schematically is illustrated with a boost pressure diagnostic apparatus in accordance with one illustrative embodiment. In the illustrated embodiment, the internal combustion engine1is installed in a passenger vehicle. The internal combustion engine1includes a plurality of combustion chambers2. In most case, the internal combustion engine1for a passenger vehicle has four or more cylinders that define four or more combustion chambers. For the sake of brevity, only one of the combustion chambers2and the associated structures will be discussed and illustrated herein.

As shown inFIG. 1, the internal combustion engine1includes at least one intake valve3for regulating intake air flow into the combustion chamber2from an intake passage4. The intake passage4is connected to the internal combustion engine1for supplying intake air into the combustion chamber2through an intake port that is selectively opened and closed by the intake valve3. The internal combustion engine1further includes at least one exhaust valve5for regulating exhaust air flowing out from the combustion chamber2. The internal combustion engine1further includes an exhaust passage6that is connected to the combustion chamber2through an exhaust port that is selectively opened and closed by the exhaust valve5. Typically, a spark plug7is arranged in a central upper section of the combustion chamber2. A fuel injection valve8is often arranged on a side portion of the combustion chamber2that is located closer to the intake valve3. In the illustrated embodiment, the fuel injection valve8injects fuel directly into the combustion chamber2. Fuel supplied to the fuel injection valve8has been regulated to a prescribe pressure by a high-pressure fuel pump9or the like. The fuel is delivered through a high-pressure fuel passage10.

Also in the illustrated embodiment, as shown inFIG. 1, the exhaust system of the internal combustion engine1includes a first or primary catalytic converter11, a second or secondary catalytic converter12and a muffler13. The catalytic converters11and12and the muffler13are provided in the exhaust passage6in order as mentioned from upstream to downstream.

Also in the illustrated embodiment, the intake passage4is provided with an air flow meter14and a throttle valve15. The air flow meter14is configured to detect an intake air flow rate. The throttle valve15is arranged in a position downstream of the air flow meter14for controlling the intake air flow rate of the intake air flowing in the intake passage4to the combustion chamber2.

Also as shown inFIG. 1, an intake air collector16is positioned downstream of the throttle valve15, and an air cleaner17is positioned upstream of the air flow meter14. The throttle valve15is provided with an actuator15athat includes an electric motor. In the illustrated embodiment, a valve opening degree (throttle valve opening degree) of the throttle valve15is detected by two throttle valve opening degree sensors15band15cinstalled inside the throttle valve15. The actuator15acontrols the throttle valve15to a target throttle valve opening degree in accordance with a control signal issued from a control unit30. That is, in the illustrated embodiment, the boost pressure diagnostic apparatus can determine if a detected throttle valve opening degree is a correct detection of an actual throttle valve opening degree by detecting the throttle valve opening degree with the throttle valve opening degree sensors15band15c. For example, if there is a large difference between a throttle valve opening degree value detected by the first throttle valve opening degree sensor15band a throttle valve opening degree value detected by the second throttle valve opening degree sensor15c, then the apparatus can determine that one of the two throttle valve opening degree sensors15bor15cis malfunctioning.

The intake air system of the internal combustion engine1is also provided with an atmospheric pressure sensor18, a forced induction (boost pressure) device19, an intercooler20and a boost pressure sensor21in the intake passage4. The atmospheric pressure sensor18, the forced induction device19, the intercooler20and the boost pressure sensor21are arranged between the air flow meter14and the throttle valve15in order as listed from upstream to downstream. The boost pressure diagnostic apparatus is configured to accurately determine if a pressure between the forced induction device19and the throttle valve15is abnormal as discussed below.

The atmospheric pressure sensor18is positioned upstream of the forced induction device19and detects an atmospheric pressure, and the boost pressure sensor21is positioned downstream of the forced induction device19and detects a pressure (boost pressure) on an upstream side of the throttle valve15. In the illustrated embodiment, the forced induction device19is a mechanical forced induction device (e.g., a so-called “supercharger”) driven by a crankshaft22of the engine1. The forced induction device19is equipped with an electromagnetic clutch23. The electromagnetic clutch23is configured to connect and disconnect a drive force from the crankshaft22to the forced induction device19in accordance with a control signal issued from the control unit30. The electromagnetic clutch23is configured to be engaged (connected) and released (disconnected) in accordance with one or more operating conditions that are inputted to the control unit30. When the electromagnetic clutch23is engaged (connected), a drive force from the crankshaft22rotates a rotor19aof the forced induction device19and pumps intake air downstream. When the electromagnetic clutch23is released (disconnected), a drive force is not transmitted from the crankshaft22and the rotor19aof the forced induction device19does not rotate.

The intake air system of the internal combustion engine1is also provided with a bypass passage24. The bypass passage24is connected to the intake passage4and arranged to allow intake air to circumvent (bypass) the forced induction device19and the intercooler20. The bypass passage24is arranged parallel to the forced induction device19and the intercooler20such that one end is connected to the intake passage4downstream of the intercooler20and the other end is connected to the intake passage4upstream of the forced induction device19. A bypass valve25is arranged in the bypass passage24.

The bypass valve25has an actuator25acomprising an electric motor. The bypass valve25is configured to be controlled to a target bypass valve opening degree by driving the actuator25ain accordance with a control signal issued from the control unit30. The bypass valve25is configured to employ an elastic force of a spring such that the valve opening degree (bypass valve opening degree) of the bypass valve25is fully open when electric power is not supplied to the actuator25afrom an onboard battery (not shown).

In this embodiment, the bypass valve opening degree is detected by the two bypass valve opening degree sensors25band25cinstalled inside the bypass valve25. That is, in this embodiment, the boost pressure diagnostic apparatus can determine if a detected bypass valve opening degree is a correct detection of an actual bypass valve opening degree by detecting the bypass valve opening degree with two bypass valve opening degree sensors25band25c. For example, if there is a large difference between the first bypass valve opening degree value detected by the first bypass valve opening degree sensor25band the second bypass valve opening degree value detected by the second bypass valve opening degree sensor25c, then the boost pressure diagnostic apparatus can determine that one of the bypass valve opening degree sensors25bor25cis malfunctioning.

In addition to receiving detection signals from the air flow meter14, the atmospheric pressure sensor18and the boost pressure sensor21, the control unit30also receives detection signals from a variety of other sensors, including a crank angle sensor26, an accelerator position sensor27and a coolant temperature sensor28. The crank angle sensor26is configured to detect a crank angle position and output a crank angle position signal that is indicative of an engine rotational speed (engine speed). The accelerator position sensor27configured to detect a depression amount (accelerator position) of an accelerator pedal operated by a driver and output an accelerator pedal depression amount signal. The coolant temperature sensor28configured to detect a coolant temperature of the engine1and output a coolant temperature signal. The control unit30also receives information related to a voltage of the onboard battery (not shown) and output a voltage signal. Based on the detection signals received by the control unit30, the control unit30determines and controls a fuel injection quantity, a fuel injecting timing and an ignition timing of the engine1. Also based on the detection signals received by the control unit30, the control unit30determines and controls a throttle valve opening degree, a bypass valve opening degree and a drive/stop status of the forced induction device19(engage/release status of the electromagnetic clutch23).

The control unit30preferably includes a microcomputer with a various control program that controls the engine1as well as a boost pressure diagnostic program as discussed below. The control unit30can also include other conventional components such as an input interface circuit, an output interface circuit, and storage devices such as a ROM (Read Only Memory) device and a RAM (Random Access Memory) device. The microcomputer of the control unit30is programmed to control the operation of the engine1. The memory circuit stores processing results and control programs such as ones for boost pressure diagnostic program that are run by the processor circuit.

In addition to using a detection value of the boost pressure sensor21, the boost pressure diagnostic apparatus uses output values from one or both of the throttle valve opening degree sensor15band15cand from one or both the bypass valve opening degree sensors25band25cto determine if an abnormal pressure exists between the forced induction device19and a throttle valve15. As a result, even if the detection value of the boost pressure sensor21contains electrical noise, a determination regarding whether the pressure between the forced induction device19and the throttle valve15is abnormal and has exceeded an appropriate range can be accomplished accurately and as quickly as possible.

As used herein, the term “boost region” refers to a situation in which the electromagnetic clutch23is engaged and the forced induction device19is driven. On the other hand, the term “non-boost region” refers to a situation in which the electromagnetic clutch23is released and the forced induction device19is not driven. The boost region and the non-boost region are determined in advance as preset operating regions that are based on engine speed and target torque for a particular vehicle. More specifically, the non-boost region is an operating region in which a target torque is smaller than a maximum torque obtainable from the engine1for a particular vehicle when the electromagnetic clutch23is released. Meanwhile, the boost region is an operating region in which the target torque is larger than a maximum torque obtainable from the engine1for a particular vehicle when the electromagnetic clutch23is released. In the boost region with the throttle valve15fully open, the torque of the engine1is controlled by the control unit30controlling the bypass valve opening degree. Meanwhile, in the non-boost region with the bypass valve25fully open, the torque of the engine1is controlled by the control unit30controlling the throttle valve opening degree. The target torque is calculated based on, for example, a detection vehicle speed and a detected accelerator depression amount or value from the accelerator position sensor27. Alternatively, the detected accelerator depression amount can be detected by a throttle valve opening degree.

When excessive boost occurs and the pressure between the forced induction device19and the throttle valve15becomes excessively high, the control unit30increases the bypass valve opening degree. Also, if the forced induction device19is being driven (i.e., the electromagnetic clutch23is engaged) during excessive boost and the excessively high pressure between the forced induction device19and the throttle valve15, the control unit30further stops driving the forced induction device19(i.e., the electromagnetic clutch23is released) such that the control unit30controls the pressure between the forced induction device19and the throttle valve15to a pressure level that is not excessive. Although it is possible for the control unit30to determine if the pressure between the forced induction device19and the throttle valve15is excessively high (i.e., if the pressure between the forced induction device19and the throttle valve15is abnormal) based solely on the detection value of the boost pressure sensor21, the control unit30of the boost pressure diagnostic apparatus of the illustrated embodiment takes into account the possibility that there could be electrical noise in the detection value of the boost pressure sensor21. In particular, the control unit30takes into account the possibility of electrical noise in the detection value of the boost pressure sensor21by taking into account the throttle valve opening degree and the bypass valve opening degree when determining if the pressure between the forced induction device19and the throttle valve15is excessively high.

More specifically, the boost pressure diagnostic apparatus uses an excessive boost limit valve opening degree map like that shown inFIG. 2to calculate an excessive boost limit valve opening degree of the bypass valve25based on the rotational speed (engine speed) of the engine1and the throttle valve opening degree detected by the first throttle valve opening degree sensor15b. The excessive boost limit valve opening degree map can be preset in advance for a particular vehicle as shown inFIG. 2. If the bypass valve opening degree detected by the first bypass valve opening degree sensor25bis smaller than the excessive boost limit valve opening degree and the detection value, i.e., boost pressure, detected by the boost pressure sensor21is larger than a prescribed pressure A (prescribed value), then the pressure between the forced induction device19and the throttle valve15is determined to be excessively high. The excessive boost limit valve opening degree of the bypass valve25is a limit value of the bypass valve opening degree for ensuring that the pressure between the forced induction device19and the throttle valve15does not exceed a preset prescribed value (the prescribed pressure A) for a given throttle valve opening degree and a given engine speed that exist. If the bypass valve opening degree is smaller than the excessive boost limit opening degree, then the pressure between the forced induction device19and the throttle valve15is excessively high (higher than the prescribed pressure A).

The excessive boost limit valve opening degree map is configured to take into account whether the forced induction device19is driven or stopped. Also the excessive boost limit valve opening degree map sets the excessive boost limit valve opening degree such that the excessive boost limit valve opening degree increases as the throttle valve opening degree decreases and the rotational speed (engine speed) of the engine1increases. Since the determination of whether to drive or stop the forced induction device19is made based on the rotational speed of the engine and the throttle valve opening degree, the pressure between the forced induction device19and the throttle valve15is determined based on the throttle valve opening degree, the bypass valve opening degree, and the rotational speed of the engine1. Thus, as shown inFIG. 3, a correlation diagram is calculated experimentally in advance for different engine speeds. This correlation diagram ofFIG. 3expresses a relationship among the throttle valve opening degree, the bypass valve opening degree, and the pressure between the forced induction device19and the throttle valve15(i.e., the boost pressure). The correlation diagrams for each of the engine speeds can then be combined to obtain the aforementioned excessive boost limit opening degree map. As shown inFIG. 3, the boost pressure increases with decreasing throttle valve opening degree, decreasing bypass valve opening degree, and increasing engine speed. Also, the lines P1, P2and P3shown inFIG. 3are lines of constant boost pressure satisfying the relationship P1<P2<P3, where P3is comparatively the highest boost pressure.

More specifically, the boost pressure between the forced induction device19and the throttle valve15at a given engine speed is determined by the throttle valve opening degree and the bypass valve opening degree. Therefore, when the boost pressure between the forced induction device19and the throttle valve15is at a given value, the bypass valve opening degree can be increased if the throttle valve opening degree is decreased, and the throttle valve opening degree can be increased if the bypass valve opening degree is decreased. Also, as the rotational speed of the engine1increases, the throttle valve opening degree and the bypass valve opening degree required for setting the boost pressure between the forced induction device19and the throttle valve15to a given value also become comparatively larger. In other words, when the pressure between the forced induction device19and the throttle valve15is excessively high (i.e., higher than the prescribed pressure A), the throttle valve opening degree can assume a value within a prescribed range (i.e., within a prescribed throttle valve opening degree region). Similarly, when the pressure between the forced induction device19and the throttle valve15is excessively high (higher than the prescribed pressure A), the bypass valve opening degree can assume a value within a prescribed range (i.e., within a prescribed bypass valve opening degree region). The prescribed bypass valve opening degree region is a prescribed range in which the bypass valve opening degree can lie while keeping the pressure between the forced induction device19and the throttle valve15excessively high (higher than the prescribed opening degree). Thus, the prescribed range for the throttle valve prescribed opening degree region becomes larger as the bypass valve opening degree decreases. Also the prescribed range (prescribed opening degree region) is larger at higher engine speeds than at lower engine speeds. Meanwhile, the prescribed range (prescribed bypass valve opening degree region) in which the bypass valve opening degree can lie while keeping the pressure between the forced induction device19and the throttle valve15excessively high (higher than the prescribed opening degree) becomes larger as the throttle valve opening degree decreases and is larger at higher engine speeds than at lower engine speeds.

In this embodiment, if the first and second throttle valve opening degrees detected by the first and second throttle valve opening degree sensors15band15cdiffer by an amount larger than a throttle valve opening degree sensor warning determination value, then the pressure between the forced induction device19and the throttle valve15will be determined to be excessively high when the detection value, i.e., the boost pressure, detected by the boost pressure sensor21is larger than the prescribed pressure A (prescribed value). Also if the first and second bypass valve opening degrees detected by the first and second bypass valve opening degree sensors25band25cdiffer by an amount larger than a bypass valve opening degree sensor warning determination value, then the pressure between the forced induction device19and the throttle valve15will be determined to be excessively high when the detection value, i.e., the boost pressure, detected by the boost pressure sensor21is larger than the prescribed pressure A (prescribed value).

FIG. 4is a flowchart showing control steps executed by the control unit30in relation to determining a status of the boost pressure and controlling the engagement and release of the electromagnetic clutch23and the opening degree of the bypass valve25in response to the status of the boost pressure. These control steps are repeatedly executed by the control unit30once per prescribed time period (e.g., 10 ms).

In step S1, the control unit30receives various information including, but not limited to, the engine speed, the first and second throttle valve opening degrees, and the first and second bypass valve opening degrees from the various sensors.

In step S2, the control unit30uses an excessive boost limit opening degree map (FIG. 2) to calculate an excessive boost limit opening degree based on the throttle valve opening degree and the rotational speed of the engine1. Based on this calculation, the control unit30determines if the bypass valve opening degree is smaller than an excessive boost limit valve opening degree. If the bypass valve opening degree is smaller than the excessive boost limit valve opening degree, then the control unit30proceeds to step S3. If the bypass valve opening degree is equal to or larger than the excessive boost limit valve opening degree, then the control unit30proceeds to step S4.

Although in this embodiment the throttle valve opening degree used to calculate the excessive boost limit valve opening degree is the throttle valve opening degree detected by the first throttle valve opening degree sensor15b, it is also possible to use the throttle valve opening degree detected by the second throttle valve opening degree sensor15cor an average value (arithmetic mean) of the throttle valve opening degrees detected by the first and second throttle valve opening degree sensors15band15c. Similarly, although in this embodiment the bypass valve opening degree used in the calculation of the excessive boost limit valve opening degree is the bypass valve opening degree detected by the first bypass valve opening degree sensor25b, it is also possible to use the bypass valve opening degree detected by the second bypass valve opening degree sensor25cor an average value (arithmetic mean) of the first and second bypass valve opening degrees detected by the first and second bypass valve opening degree sensors25band25c.

In step S3the control unit30determines if the electromagnetic clutch23is engaged (i.e., the electromagnetic clutch23is determined to be engaged if the engine1is determined to be operating in a boost region based on a target torque and an engine speed). Also the control unit30determines if a voltage of the onboard battery is equal to or larger than a preset prescribed voltage (e.g., 11 V). If the battery voltage is equal to or larger than the prescribed voltage, then the control unit30proceeds to step S6. If the battery voltage is smaller than the prescribed voltage, then the control unit30proceeds to step S4.

In step S4, the control unit30determines if the absolute value of a value obtained by subtracting the throttle valve opening degree detected by the second throttle valve opening degree sensor15cfrom the throttle valve opening degree detected by the first throttle valve opening degree sensor15bis larger than a preset throttle valve opening degree sensor warning determination value. If so, then the control unit30proceeds to step S6. If not, then the control unit30proceeds to step S5.

When operating conditions are such that the control unit30proceeds from S4to S6, at least one of the two throttle valve opening degree sensors15band15cis not detecting an actual throttle opening correctly and the detected throttle valve opening degree is less reliable than when both of the throttle valve opening degree sensors15band15care detecting the actual throttle valve opening degree correctly. That is, in step S4, the control unit30can determine if a malfunction of the throttle valve opening degree sensors15band15cis occurring.

In step S5, the control unit30determines if the absolute value of a value obtained by subtracting the second bypass valve opening degree detected by the second bypass valve opening degree sensor25cfrom the first bypass valve opening degree detected by the first bypass valve opening degree sensor25bis larger than a preset bypass valve opening degree sensor warning determination value. If so, then the control unit30proceeds to step S6. If not, then the control unit30clears (sets to zero) an excessive boost failsafe counter T (ms) (explained later) and ends the current cycle of the routine. When operating conditions are such that the control unit30proceeds from S5to S6, at least one of the bypass valve opening degree sensors25band25cis not detecting an actual bypass valve opening degree correctly and the detected bypass valve opening degree is less reliable than when both of the bypass valve opening degree sensors25band25care detecting the actual bypass valve opening degree correctly. That is, in step S5, the control unit30can determine if a malfunction of the bypass valve opening degree sensors25band25cis occurring.

In step S6, the control unit30determines if a relative pressure value obtained by subtracting an atmospheric pressure detected by the atmospheric pressure sensor18from the boost pressure detected by the boost pressure sensor21is larger than a preset boost pressure determination pressure. If the relative pressure is larger, then the control unit30increments the excessive boost failsafe counter T (ms) and proceeds to step S7. If not, then the control unit30clears (zeroes) the excessive boost failsafe counter T (ms) and ends the current cycle of the routine.

In step S7, the control unit30determines if the excessive boost failsafe counter T is larger than a preset excessive boost failsafe determination delay t (ms). If so, then the control unit30proceeds to step S8. If not, then the control unit30ends the current cycle of the routine. In other words, in step S7, the control unit30determines if the pressure between the forced induction device19and the throttle15is excessively high and proceeds to step S8if it determines that the pressure between the forced induction device19and the throttle15is excessively high.

In step S8, the control unit30turns off an electric power supply to the actuator25aof the bypass valve25such that the bypass valve25is fully open. The control unit30also releases the electromagnetic clutch23such that the pressure between the forced induction device19and the throttle valve15, i.e., the boost pressure, is lowered.

In step S8, the control unit30also determines if the number of times the excessive boost failsafe counter T has been determined to be larger than the preset excessive boost failsafe determination delay t (ms) during a single session of operation (i.e., a single period from when the ignition is turned on until the ignition is turned off) is equal to or larger than a preset excessive boost abnormality determination count C. If the number of times is larger than the determination count C, then the control unit30illuminates a warning lamp installed in an instrument panel of the vehicle in which the engine1is installed. The warning lamp is installed in a position where a driver can see it and serves to inform the driver that the boost pressure is abnormal (i.e., the pressure between the forced induction device19and the throttle valve15is abnormal). Once the warning lamp is illuminated, it continues to shine during operation of the vehicle until the vehicle is inspected at a repair shop or the like.

When, for example, the throttle valve15(or the bypass valve25) malfunctions and enters a prescribed opening degree region and the boost pressure becomes excessively high, the control unit30can accurately determine that a boost pressure abnormality exists by executing the steps S2→S3→S6→S7of the flowchart explained above.

Meanwhile, if there is electrical noise in the detection value of the boost pressure sensor21, then in step S2the control unit30will determine that the bypass valve opening degree is equal to or larger than the excessive boost limit valve opening degree and will not determine that a boost pressure abnormality exists. Also, although the control unit30can proceed through steps S4and S5to step S6after determining in step S2that the bypass valve opening degree is equal to or larger than the excessive boost limit valve opening degree, this will only occur if any of the throttle valve opening degree sensors15band15cand the bypass valve opening degree sensors25band25cis malfunctioning.

There are situations in which although the throttle valve15has failed and the throttle valve opening degree has entered a prescribed opening degree region, the fact that the throttle valve opening degree entered the prescribed opening degree region because at least one of the throttle valve opening degree sensors15band15cfailed cannot be detected. In such a situation, a failure of the throttle valve opening degree sensor15bor15ccan be determined in step S4even if the throttle valve opening degree is not in the prescribed opening degree region. Additionally, if the boost pressure is abnormal, then the abnormal boost pressure can be accurately recognized by following the steps S2→S4→S6→S7of the flowchart.

There are situations in which the bypass valve25fails and the bypass valve opening degree enters a prescribed opening degree region, but the fact that the bypass valve opening degree entered the prescribed opening degree region because at least one of the bypass valve opening degree sensors25band25cfailed cannot be detected. In such a situation, a failure of the bypass valve opening degree sensor25bor25ccan be determined in step S5even if the bypass valve opening degree is not in the prescribed opening degree region. Additionally, if the boost pressure is abnormal, then the abnormal boost pressure can be accurately recognized by following the steps S2→S4→S5→S6→S7of the flowchart.

As explained heretofore, in addition to using a detection value of the boost pressure sensor21, the present invention uses output values from the throttle valve opening degree sensors15band15cand the bypass valve opening degree sensors25band25cto determine if an excessively high pressure exists between the forced induction device19and the throttle valve15. As a result, even if the detection value of the boost pressure sensor21contains electrical noise, a determination regarding whether the pressure between the forced induction device19and the throttle valve15is abnormal and has exceeded an appropriate range can be accomplished accurately and as quickly as possible.

When the force induction pressure is determined to be excessively high, the bypass valve25is opened fully and the electromagnetic clutch23is released if the forced induction device19is being driven. As a result, a situation in which the engine1continues to operate with the boost pressure excessively high can be avoided and the parts of the air induction system can be reliably protected.