Exhaust passage switching unit and method for internal combustion engine

A hydrocarbon adsorption apparatus is provided in an exhaust passage of an internal combustion engine. The hydrocarbon adsorption apparatus is equipped with a main passage, a switching valve for opening and closing the main passage, and a bypass passage for bypassing the main passage. The switching valve is coupled to a diaphragm of a diaphragm mechanism. A variable chamber in the diaphragm mechanism is connected to an intake manifold through a negative pressure feed line and a vacuum switching valve. When the vacuum switching valve is turned on, a negative pressure is supplied to the variable pressure chamber so that the diaphragm is deflected. In response to the deflection of the diaphragm, the switching valve is closed. The state of the switching valve is judged based on a tendency of changes in pressure of the negative pressure feed line when the vacuum switching valve is turned on.

INCORPORATION BY REFERENCE
 The disclosures of Japanese Patent Application Nos. 11-143210 filed on May
 24, 1999 and 2000-85369 filed on Mar. 24, 2000 including the
 specifications, drawings and abstracts are incorporated herein by
 reference in their entire ICS .
 BACKGROUND OF THE INVENTION
 1. Field of Invention
 The present invention relates to an exhaust passage switching unit and
 method for an internal combustion engine and, more particularly, to an
 exhaust passage switching unit and method capable of precisely judging the
 state of a switching mechanism for switching an exhaust passage.
 2. Description of Related Art
 Exhaust passage switching units as disclosed, for example, in Japanese
 Patent Application No. 8-334014 have been known. A switching unit of this
 type is equipped with a switching valve which switches exhaust gases to a
 main exhaust passage in which a catalyst is disposed or to a bypass
 passage. In an operating state with a high rotational speed and a high
 load, deterioration caused by overheating of the catalyst is prevented by
 switching the exhaust passage to the bypass passage.
 When the main exhaust passage serves as the exhaust passage, the flow
 resistance of the exhaust passage increases because of the resistance of
 the catalyst. If the flow resistance of the exhaust passage increases, the
 negative pressure in an intake pipe decreases accordingly. Hence,
 according to the switching unit of the related art, it is determined,
 based on a change in negative pressure in the intake pipe when a switching
 signal is supplied to the switching valve, whether or or the exhaust
 passage has been switched, that is, whether or not the switching valve is
 in normal operation.
 However, the negative pressure in the intake pipe changes depending not
 only on a flow resistance of the exhaust passage but also on an operating
 state of the internal combustion engine. Thus, it is difficult to
 distinguish between a change in negative pressure in the intake pipe
 caused by the switching of the exhaust passage and a change in negative
 pressure in the intake pipe caused by an operating state of the internal
 combustion engine. The method in the aforementioned switching unit of the
 related art does not necessarily permit precise judgment of an operational
 state of the switching valve.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide an exhaust passage switching
 unit for an internal combustion engine which permits precise judgment of
 the state of a switching mechanism for switching an exhaust passage.
 In a first aspect of the invention, an exhaust passage switching unit for
 an internal combustion engine comprises a switching mechanism which has a
 moving member driven by a fluid pressure and a pressure transmitting
 portion for transmitting a fluid pressure of a fluid pressure source to
 the moving member, wherein the switching member switches an exhaust
 passage in accordance with a movement of the moving member, a transmitting
 portion pressure detector that detects a pressure of the pressure
 transmitting portion, a controller that determines a state of the
 switching mechanism based on a pressure detected by the transmitting
 portion pressure detector.
 In the first aspect of the present invention, the pressure transmitting
 portion transmits a fluid pressure to the moving member. The moving member
 is driven by the fluid pressure transmitted by the pressure transmitting
 portion, whereby the exhaust passage is switched. If a fluid pressure in
 the fluid pressure source is supplied to the pressure transmitting
 portion, the pressure in the pressure transmitting portion changes towards
 the pressure in the fluid pressure source. In this case, the volume in the
 pressure transmitting portion changes in accordance with an amount of
 movement of the moving member, whereby the pressure in the pressure
 transmitting portion also changes. That is, the change in pressure of the
 pressure transmitting portion reflects an operating state of the moving
 member. Thus, the controller can determine a state of the switching
 mechanism based on a pressure in the pressure transmitting portion
 detected by the transmitting portion pressure detector.
 In the case where the switching mechanism is in normal operation, while the
 moving member moves in the course of a change in pressure of the pressure
 transmitting portion towards the pressure in the fluid pressure source,
 the volume of the pressure transmitting portion changes in such a
 direction as to attenuate changes in pressure therein. As a result, the
 absolute value of a gradient of changes in pressure decreases. On the
 contrary, if the switching mechanism is locked, the moving member does not
 move. Therefore, the aforementioned change in gradient of changes in
 pressure does not occur.
 In the first aspect of the present invention, the controller may judge
 presence or absence of locking of the switching mechanism based on whether
 or not there is a period when an absolute value of a gradient of changes
 in pressure detected by the transmitting portion pressure detector becomes
 equal to or smaller than a predetermined value within a predetermined
 period after start of a switching operation of the exhaust passage by the
 switching mechanism.
 The controller may judge presence or absence of locking of the switching
 mechanism based on whether or not there is a period when an absolute value
 of a change amount of a gradient of changes in pressure detected by the
 transmitting portion pressure detector becomes equal to or smaller than a
 predetermined value within a predetermined period after start of a
 switching operation of the exhaust passage by the switching mechanism.
 If the switching mechanism is locked, the volume of the pressure
 transmitting portion changes depending on the locking position of the
 switching mechanism. In this case, the gradient of changes in pressure at
 the time of supply of a fluid pressure of the fluid pressure source to the
 pressure transmitting portion changes in accordance with a volume of the
 pressure transmitting portion.
 Thus, in the aforementioned aspect, if it is determined that the switching
 mechanism is locked, the controller may determine a locking position of
 the switching mechanism based on a gradient of changes in pressure
 detected by the transmitting portion pressure detector.
 If there arises a malfunction such as pressure leakage in the pressure
 transmitting portion, the pressure in the pressure transmitting portion
 does not reach the pressure in the fluid pressure source.
 Thus, in the first aspect of the present invention, the controller may be
 constructed to judge presence or absence of a malfunction in the pressure
 transmitting portion based on a pressure detected by the transmitting
 portion pressure detector after lapse of a predetermined length of time
 since start of a switching operation of the exhaust passage by the
 switching mechanism.
 While the moving member moves in the course of a change in pressure of the
 pressure transmitting portion towards the pressure in the fluid pressure
 source, the volume of the pressure transmitting portion changes in such a
 direction as to attenuate changes in pressure therein. As a result, the
 absolute value of the gradient of changes in pressure decreases. That is,
 the absolute value of a change amount of the gradient of changes in
 pressure increases. If the moving member has moved to its maximum after a
 decrease in absolute value of the gradient of changes in pressure, the
 volume of the pressure transmitting portion does not change afterwards. As
 a result, the absolute value of the gradient of changes in pressure
 increases. That is, the absolute value of the change amount of the
 gradient of changes in pressure increases.
 Thus, in the aforementioned aspect, the may be constructed controller to
 determine an amount of movement of the moving member based on a period
 from a moment when an absolute value of a change amount of a gradient of
 changes in pressure detected by the transmitting portion pressure detector
 becomes equal to or greater than a first predetermined value after start
 of a switching operation of the exhaust passage by the switching mechanism
 to a moment when the absolute value becomes equal to or greater than a
 second predetermined value.
 In the course of a change in pressure of the pressure transmitting portion
 towards the pressure in the fluid pressure source, the period required for
 the absolute value of the gradient of changes in pressure to increase
 after its decrease changes in accordance with the pressure in the fluid
 pressure source.
 Thus, in the aforementioned aspect, the exhaust passage switching unit may
 comprise a fluid pressure source pressure detector or that detects a
 pressure of the fluid pressure source. Also, the controller may be
 constructed to determine an amount of movement of the moving member based
 on a pressure detected by the fluid pressure source pressure detector.
 In the course of a change in pressure of the pressure transmitting portion
 towards the pressure in the fluid pressure source, the absolute value of
 the gradient of changes in pressure changes, after its decrease, in
 accordance with a magnitude of friction of the switching mechanism.
 Thus, in the aforementioned aspect, the controller may determine a
 magnitude of friction of the switching mechanism based on a gradient of
 changes in pressure detected by the transmitting portion pressure detector
 after an absolute value of a change amount of a gradient of changes in the
 pressure becomes equal to or greater than a predetermined value since
 start of a switching operation of the exhaust passage by the switching
 mechanism.
 In the course of a change in pressure of the pressure transmitting portion
 towards the pressure in the fluid pressure source, the period required for
 the absolute value of the gradient of changes in pressure to increase
 after its decrease changes in accordance with a magnitude of friction
 generated in the switching mechanism.
 Thus, in the aforementioned aspect, the exhaust passage switching unit may
 comprise a friction detector that detects a friction of the switching
 mechanism. Also, the controller may determine an amount of movement of the
 moving member based on a magnitude of the friction detected by the
 friction detector.
 In the course of a change in pressure of the pressure transmitting portion
 towards the pressure in the fluid pressure source, the period required for
 the absolute value of the gradient of changes in pressure to increase
 after its decrease changes in accordance with a magnitude of friction
 caused by thermal expansion of the switching mechanism.
 Thus, in the aforementioned aspect, the exhaust passage switching unit may
 comprise a temperature detector that detects a temperature of the
 switching mechanism. Also, the controller may determine an amount of
 movement of the moving member based on a temperature detected by the
 temperature detector.
 In the case where the switching mechanism is in normal operation, the
 amount of movement of the moving member is confined to a predetermined
 range. On the contrary, if there arises a malfunction such as unlinking
 between the moving member and the member for switching the exhaust
 passage, the amount of movement of the moving member may exceed the
 predetermined range.
 Thus, in the aforementioned aspect, the controller may judge presence or
 absence of a malfunction in the switching mechanism based on whether or
 not an amount of movement of the moving member is equal to or greater than
 a predetermined threshold value.
 In the course of a change in pressure of the pressure transmitting portion
 towards the pressure in the fluid pressure source, there arises a
 phenomenon wherein the absolute value of the gradient of changes in
 pressure increase after its decrease. If a long period of time elapses
 until emergence of the phenomenon wherein the absolute value of the
 gradient of changes in pressure increases after its decrease, it can be
 determined that the amount of movement of the moving member is great and
 that there is a malfunction such as unlinking of the switching mechanism
 between the moving member and the member for switching the exhaust
 passage.
 Thus, in the first aspect of the present invention, the controller judge
 presence or absence of a malfunction in the switching mechanism based on
 an elapsed time after an absolute value of a change amount of a gradient
 of changes in pressure detected by the transmitting portion pressure
 detector becomes equal to or greater than a predetermined value since
 start of a switching operation of the exhaust passage by the switching
 mechanism.
 In the course of a change in pressure of the pressure transmitting portion
 towards the pressure in the fluid pressure source, the pressure at the
 beginning of a decrease in absolute value of the gradient of changes in
 pressure changes in accordance with a magnitude of friction generated in
 the switching mechanism. If the absolute value of the pressure is great,
 there may arise a circumstance where the pressure is unable to change
 towards the pressure in the fluid pressure source. In this case, it is
 possible to determine that there is a malfunction resulting from friction
 of a great magnitude in the switching mechanism.
 Thus, in the first aspect of the present invention, the controller may
 judge presence or absence of a malfunction in the switching mechanism
 based on a pressure detected by the transmitting portion pressure detector
 when an absolute value of a change amount of a gradient of changes in
 pressure detected by the transmitting portion pressure detector becomes
 equal to or greater than a predetermined value after start of a switching
 operation of the exhaust passage by the switching mechanism.
 In the case where the intake passage of the internal combustion engine
 serves as the fluid pressure source for transmitting a fluid pressure to
 the moving member, the internal pressure of the intake passage
 (hereinafter referred to as an intake pressure) changes in accordance with
 an operating state of the internal combustion engine. The pressure in the
 pressure transmitting portion changes in accordance with an intake
 pressure. Hence, if the intake pressure fluctuates, the pressure in the
 pressure transmitting portion fluctuates accordingly. For this reason, the
 state of the switching mechanism cannot precisely be judged based on the
 pressure in the pressure transmitting portion.
 Thus, in the first aspect of the present invention, the fluid pressure
 source may be designed as an intake passage of the internal combustion
 engine. Also, the controller may judge the instability of an internal
 pressure of the intake passage based on an operating state of the internal
 combustion engine. Further, if the controller determines that an internal
 pressure of the intake passage is stabilized, the controller may judge a
 state of the switching mechanism based on a pressure detected by the
 transmitting portion pressure detector.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 FIG. 1 is a structural view of a system in accordance with the invention.
 The system of this embodiment is controlled by an electronic control unit
 (hereinafter referred to as an ECU) 10. As shown in FIG. 1, the system of
 this embodiment is equipped with an internal combustion engine 12. The
 internal combustion engine 12 is equipped with an exhaust manifold 14 and
 an intake manifold 16. The exhaust manifold 14 is connected to a first
 exhaust pipe 18. A start catalyst 20 is provided in the neighborhood of an
 upstream end of the first exhaust pipe 18. A second exhaust pipe 22 is
 connected to a downstream side of the first exhaust pipe 18. A main
 catalyst 24 is provided in the second exhaust pipe 22. An intake pressure
 sensor 25 is disposed in the intake manifold 16. The intake pressure
 sensor 25 outputs an electric signal corresponding to an internal pressure
 of the intake manifold 16, that is, an intake manifold negative pressure
 (hereinafter referred to as an intake pressure PM). An output signal from
 the intake pressure sensor 25 is supplied to the ECU 10. The ECU 10
 detects the intake pressure PM based on the output signal from the intake
 pressure sensor 25. It is also possible to estimate an intake pressure PM
 based on a load (e.g. an amount of intake air) of the internal combustion
 engine 12 without providing the intake manifold 16 with the intake
 pressure sensor 25.
 An HC adsorption equipment 26 is provided at a joint between the first
 exhaust pipe 18 and the second exhaust pipe 22. The HC adsorption
 equipment 26 is equipped with a main passage 27, a switching valve 28 and
 a bypass passage 30. The main passage 27 establishes communication between
 the first exhaust pipe 18 and the second exhaust pipe 22 at their
 respective central portions with a large opening area. The switching valve
 28 opens and closes the main passage 27. The bypass passage 30 bypasses
 the periphery of the main passage 27 and establishes communication between
 the first exhaust pipe 18 and the second exhaust pipe 22. An HC adsorbent
 31 is disposed in the bypass passage 30. The HC adsorbent 31 has a
 characteristic of adsorbing hydrocarbon (HC) contained in exhaust gas.
 The system of this embodiment is equipped with a diaphragm mechanism 32. A
 diaphragm 34 is provided inside the diaphragm mechanism 32. The diaphragm
 34 divides the internal space of the diaphragm mechanism 32 into a
 variable pressure chamber 36 on the left and an atmospheric pressure
 chamber 38 on the right as shown in FIG. 1. The switching valve 28 is
 coupled to the diaphragm 34 through an actuating rod 40. When the
 diaphragm 34 is not deflected, the switching valve 28 assumes its
 full-opening state and makes the main passage 27 passable. When the
 diaphragm 34 is deflected towards the variable pressure chamber 36 to its
 maximum, the switching valve 28 assumes its full-closure state and shuts
 the main passage 27 off.
 The pressure in the atmospheric pressure chamber 38 is always maintained at
 an atmospheric pressure. A negative pressure feed line 42 is connected at
 one end to the variable pressure chamber 36 and at the other end to a
 vacuum switching valve (hereinafter referred to as a VSV) 44. A pressure
 sensor 46 communicates with the negative pressure feed line 42 and outputs
 a signal corresponding to a pressure therein, that is, a pressure supplied
 to the variable pressure chamber 36 (hereinafter referred to as a feed
 pressure P). The output signal from the pressure sensor 46 is supplied to
 the ECU 10. The ECU 10 detects the feed pressure P based on the output
 signal from the pressure sensor 46. It is also possible to provide the
 variable pressure chamber 36 with the pressure sensor 46.
 A negative pressure passage 48 communicating with the intake manifold 16 is
 connected to the VSV 44. The VSV 44 is equipped with an atmospheric
 opening 44a which is open to the atmosphere. When the VSV 44 is off, it
 establishes communication between the negative pressure feed line 42 and
 the atmospheric opening 44a. Upon receipt of an ON-signal from the ECU 10,
 the VSV 44 is turned on and shuts the negative pressure feed line 42 off
 from the atmospheric opening 44a so that the negative pressure feed line
 42 is connected to the negative pressure passage 48.
 An NE sensor 52 and a throttle position sensor 54 are connected to the ECU
 10. The NE sensor 52 outputs a signal corresponding to a rotational speed
 of the internal combustion engine 12 (hereinafter referred to as an engine
 speed NE). The throttle position sensor 54 outputs a signal corresponding
 to an opening degree of a throttle valve for controlling an amount of
 intake air (hereinafter referred to as a throttle opening degree TA). The
 ECU 10 detects the engine speed NE based on the output signal from the NE
 sensor 52 and the throttle opening degree TA based on the output signal
 from the throttle position sensor 54.
 A coolant temperature sensor 56 and an intake air temperature sensor 58 are
 connected to the ECU 10. The coolant temperature sensor 56 outputs a
 signal corresponding to a temperature of the coolant flowing inside a
 water jacket of the internal combustion engine 12 (hereinafter referred to
 as a coolant temperature THW). The intake air temperature sensor 58
 outputs a signal corresponding to a temperature of the air flowing inside
 the intake manifold 16 (hereinafter referred to as an intake air
 temperature THA). The ECU 10 detects the coolant temperature THW based on
 the output signal from the coolant temperature sensor 56 and the intake
 air temperature THA based on the output signal from the intake air
 temperature sensor 58. Based on these temperatures, the ECU 10 estimates a
 temperature of the exhaust gas discharged from the internal combustion
 engine 12 and a temperature of a later-described switching mechanism 50.
 According to the aforementioned construction, when the VSV 44 is off, an
 atmospheric pressure is introduced into the variable pressure chamber 36
 of the diaphragm mechanism 32 through the negative pressure feed line 42.
 In this case, since the variable pressure chamber 36 and the atmospheric
 pressure chamber 38 are equal in pressure, the diaphragm 34 is not
 deflected.
 As described above, when the diaphragm 34 is not deflected, the switching
 valve 28 assumes its full-opening state. Thus, when the VSV 44 is kept
 off, the switching valve 28 assumes its full-opening state, whereby most
 exhaust gas flows from the first exhaust pipe 18 into the second exhaust
 pipe 22 through the main passage 27 without passing through the bypass
 passage 30, that is, without passing through the HC adsorbent 31.
 On the other hand, when the VSV 44 is on, an intake pressure PM is supplied
 to the variable pressure chamber 36 of the diaphragm mechanism 32 through
 the negative pressure feed line 42. In this case, the variable pressure
 chamber 36 becomes lower in pressure than the atmospheric pressure chamber
 38, whereby the diaphragm 34 is deflected towards the variable pressure
 chamber 36. As described above, when the diaphragm 34 is deflected towards
 the variable pressure chamber 36 to its maximum, the switching valve 28
 assumes its full-opening state. Thus, when the VSV 44 is turned on, the
 switching valve 28 assumes its full-closure state, whereby the exhaust gas
 discharged into the first exhaust pipe 18 flows from the first exhaust
 pipe 18 into the second exhaust pipe 22 through the bypass passage 30,
 that is, through the HC adsorbent 31.
 In this manner, the switching valve 28, the actuating rod 40, the diaphragm
 mechanism 32, the negative pressure feed line 42 and the VSV 44 constitute
 a mechanism which, with the aid of the intake pressure PM, switches a path
 of exhaust gas in the HC adsorption equipment 26 between the main passage
 27 and the bypass passage 30. The switching valve 28, the actuating rod
 40, the diaphragm mechanism 32, the negative pressure feed line 42 and the
 VSV 44 will hereinafter be referred to generically as the switching
 mechanism 50.
 In the system of this embodiment, when the internal combustion engine 12 is
 cold, the ECU 10 supplies an ON-signal to the VSV 44. In this case, as
 described above, exhaust gas passes through the HC adsorbent 31 and is
 thereby removed of the HC contained therein. Thus, during the cold
 operation wherein the start catalyst 20 and the catalytic converter 24 do
 not purify exhaust gas, the exhaust gas containing HC is prevented from
 being discharged to the outside.
 While the internal combustion engine 12 is in operation, the switching
 valve 28 is always exposed to exhaust gas. Hence, the unburnt gas, carbon
 and the like contained in exhaust gas adhere to the switching valve 28 and
 may thereby cause an abnormality in the switching valve 28 (hereinafter
 referred to as a stroke abnormality in the switching valve 28). That is,
 the switching valve 28 may be locked or unable to be opened or closed
 beyond a certain position.
 In the case where there is a malfunction in the VSV 44, where the negative
 pressure feed line 42 cracks or collapses, where the joint disengages, or
 where the variable pressure chamber 36 undergoes an abnormality such as
 insufficient sealability (hereinafter referred to as the case where there
 is a malfunction in a negative pressure transmission system), even if an
 ON-signal is supplied to the VSV 44, the variable pressure chamber 36 of
 the diaphragm mechanism 32 is not supplied with a sufficient negative
 pressure and hence the switching valve 28 cannot be closed. The system of
 this embodiment is advantageous for its capability to precisely judge the
 occurrence of the abnormalities as mentioned above in the switching
 mechanism 50.
 FIG. 2 shows an example of time-dependent changes in feed pressure P when
 the VSV 44 is switched from off to on, in the case A where the switching
 valve 28 functions normally, in the case B where the switching valve 28 is
 locked in its full-opening state, and in the case C where the switching
 valve 28 has an abnormal stroke and can only be closed from its
 full-opening state to its half-opening state. The cases A, B and C are
 indicated by a solid line, a broken line and an alternate long and short
 dash line respectively. FIG. 2 also shows whether the VSV 44 is on or off
 at each moment.
 It will first be described how the feed pressure P changes in the case
 where the switching valve 28 functions normally. In the case where the
 switching valve 28 functions normally as indicated by a solid line A in
 FIG. 2, the VSV 44 is kept off prior to a moment t0 and thus the feed
 pressure P is maintained at an atmospheric pressure Pa. If the VSV 44 is
 turned on at the moment t0, the negative pressure feed line 42 is supplied
 with an intake pressure PM and thus the feed pressure P starts to
 decrease. If the feed pressure P reaches a predetermined pressure P1 at a
 moment t1, the diaphragm 34 starts to be deflected towards the variable
 pressure chamber 36. If the diaphragm 34 is deflected towards the variable
 pressure chamber 36, the variable pressure chamber 36 decreases in volume
 accordingly. In this case, since the volume of a space to be aspirated by
 means of the intake pressure PM (i.e. an internal space formed by the
 negative pressure feed line 42 and the variable pressure chamber 36;
 hereinafter referred to as an aspiration space) decreases, the descending
 gradient of the feed pressure P decreases discontinuously in comparison
 with the moment when the diaphragm 34 starts to be deflected. A point
 where the descending gradient of the feed pressure P decreases
 discontinuously upon deflection of the diaphragm 34 will hereinafter be
 referred to as a first change point Q1 of the feed pressure P.
 If the feed pressure P passes through the first change point Q1 and
 decreases to a predetermined pressure P2 at a moment t2, the diaphragm 34
 is deflected to its maximum possible extent and the variable pressure
 chamber 36 does not change in volume afterwards. Hence, the descending
 gradient of the feed pressure P increases discontinuously at the moment
 t2. A point where the descending gradient of the feed pressure P increases
 discontinuously due to the maximum deflection of the diaphragm 34 will
 hereinafter be referred to as a second change point Q2 of the feed
 pressure P. If the feed pressure P reaches the intake pressure PM at a
 moment t3, the feed pressure P is kept substantially equal to the intake
 pressure PM.
 On the other hand, in the case where the switching valve 28 is locked in
 its full-opening state, even if the feed pressure P becomes equal to or
 lower than the predetermined pressure P1, the diaphragm 34 is not
 deflected and thus the variable pressure chamber 36 does not decrease in
 volume. Thus, unlike the case where the switching valve 28 functions
 normally, neither the first change point Q1 nor the second change point Q2
 appears on the curve of the feed pressure P as indicated by the broken
 line B in FIG. 2.
 In this manner, based on whether or not the first change point Q1 or the
 second change point Q2 appears on the curve of the feed pressure P after
 the VSV 44 has been turned on (in other words, based on whether or not the
 descending gradient of the feed pressure P becomes equal to or smaller
 than a predetermined value within a predetermined period after the VSV 44
 has been turned on), it is possible to determine whether or not the
 switching valve 28 is locked in its full-opening state.
 In the case where the switching valve 28 has an abnormal stroke and can
 only be closed from its full-opening state to its half-opening state, the
 maximum deflection amount of the diaphragm 34 decreases in comparison with
 the case where the switching valve 28 functions normally. Thus, as
 indicated by an alternate long and two short dashes line C in FIG. 2, the
 second change point Q2 appears on the curve of the feed pressure P earlier
 in comparison with the case where the switching valve 28 functions
 normally. The timing at which the second change point Q2 is generated
 becomes earlier as the maximum deflection amount of the diaphragm 34
 decreases. Therefore, the maximum deflection amount of the diaphragm 34,
 namely, the on-off stroke of the switching valve 28 can be determined
 based on the timing at which the second change point Q2 is generated.
 FIG. 3 shows an example of time-dependent changes in feed pressure P when
 the VSV 44 is switched from off to on, in the case A where the switching
 valve 28 is locked in its full-opening state, in the case B where the
 switching valve 28 is locked in its half-opening state, and in the case C
 where the switching valve 28 is locked in its full-closure state. The
 cases A, B and C are indicated by a solid line, a broken line and an
 alternate long and short dash line respectively. As is the case with FIG.
 2, FIG. 3 also shows whether the VSV 44 is on or off at each moment.
 In the case where the switching valve 28 is locked, as the locking position
 of the switching valve 28 becomes closer to the full-closure position, the
 diaphragm mechanism 32 is maintained in a state where the diaphragm 34 has
 been deflected more greatly, that is, in a state where the variable
 pressure chamber 36 has more drastically decreased in volume. In the case
 where the VSV 44 is turned on, as the variable pressure chamber 36 becomes
 smaller in volume, the aspiration space becomes smaller in volume.
 Therefore, the feed pressure P decreases rapidly. As indicated by A
 through C in FIG. 3, as the locking position of the switching valve 28
 becomes closer to the full-closure position, the feed pressure P decreases
 more rapidly after the turning-on of the VSV 44.
 Thus, based on one of the descending gradient of the feed pressure P
 immediately after the turning-on of the VSV 44, the value of the feed
 pressure P after the lapse of a predetermined length of time since the
 turning-on of the VSV 44, and the time required for the feed pressure P to
 reach a predetermined pressure, it is possible to determine a locking
 position of the switching valve 28.
 In this manner, the system shown in FIG. 1 makes it possible to determine,
 based on the tendency of changes in feed pressure P after the turning-on
 of the VSV 44, whether or not the switching valve 28 is locked, whether or
 not the switching valve 28 has a stroke abnormality, and where the locking
 position is if the switching valve 28 is locked.
 In the case where there arises a malfunction in the negative pressure
 transmission system, even if the VSV 44 is turned on, the feed pressure P
 does not reach the intake pressure PM. Thus, if the feed pressure P does
 not become equal to or lower than a predetermined value even after the
 lapse of a sufficient length of time since the turning-on of the VSV 44,
 it can be determined that there has arisen a malfunction in the negative
 pressure transmission system. In particular, in the case where there is a
 malfunction in the VSV 44, even if an ON-signal is supplied to the VSV 44,
 the intake pressure PM is not introduced into the negative pressure feed
 line 42. Thus, in the case where the feed pressure P does not change even
 if an ON-signal has been supplied to the VSV 44, it can be determined that
 there is a malfunction in the VSV 44.
 If the intake pressure PM changes greatly while the occurrence of an
 abnormality in the switching mechanism 50 is judged based on changes in
 feed pressure P, the feed pressure P may change accordingly and cause a
 deterioration in the precision of judging the occurrence of an abnormality
 in the switching valve 28. In order to avoid such an inconvenience, the
 negative pressure passage 48 is provided with, for example, a regulator
 valve so that the negative pressure supplied to the negative pressure feed
 line 42 is kept constant.
 As the negative pressure feed pipe 42 and the variable pressure chamber 36
 increase in volume, the decreasing rate of the volume of the aspiration
 space at the time of deflection of the diaphragm 34 decreases. Hence, the
 change in gradient of the feed pressure P at the first change point Q1
 also decreases. For this reason, in the switching mechanism 50, the
 volumes of the negative pressure feed line 42 and the negative pressure
 chamber 36 are made small enough to elucidate a change in gradient at the
 first change point Q1.
 As the maximum deflection amount of the diaphragm 34 decreases, the period
 of time between generation of the first change point Q1 to generation of
 the second change point Q2 shortens. Hence, it becomes difficult to
 determine a moment of generation of the second change point Q2 with high
 precision. For this reason, in the switching mechanism 50, the maximum
 deflection amount of the diaphragm 34 is made large enough to allow a
 moment of generation of the second change point Q2 to be determined with
 high precision.
 As the deflection rigidity of the diaphragm 34 increases, the speed of
 deflection of the diaphragm 34 decreases. Hence, the decreasing rate of
 the descending gradient of the feed pressure P in the range from the first
 change point Q1 to the second change point Q2 decreases. For this reason,
 in the switching mechanism 50, the deflection rigidity of the diaphragm 34
 is made small enough to elucidate the first change point Q1 and the second
 change point Q2.
 In addition, the tendency of change in feed pressure P also depends on a
 pressure-detecting position of the pressure sensor 46. For this reason, in
 the switching mechanism 50, the pressure sensor 46 is disposed at such a
 position as to allow the occurrence of an abnormality in the switching
 valve 28 to be judged precisely.
 The processings performed by the ECU 10 to judge the occurrence of an
 abnormality in the switching mechanism 50 in this embodiment will be
 described hereinafter with reference to FIGS. 4 and 5. FIG. 4 is a
 flowchart of an exemplary routine executed by the ECU 10 in this
 embodiment. FIG. 5 explains a method of judging the occurrence of an
 abnormality in the switching valve 28 in the routine shown in FIG. 4, and
 shows time-dependent changes in feed pressure P in the case A where the
 switching valve 28 functions normally, in the case B where the switching
 valve 28 has an abnormal stroke and can only be closed to its half-opening
 state, in the case C where the switching valve 28 is locked in its
 full-opening state, and in the case D where the switching valve 28 is
 locked in its full-closure state. The cases A, B, C and D are indicated by
 a solid line, a broken line, an alternate long and short dash line, and an
 alternate long and two short dashes line respectively.
 In the routine shown in FIG. 4, the state of the switching valve 28 is
 judged based on an elapsed time Tth required for the feed pressure P to
 reach a predetermined reference pressure Ps after the supply of an
 ON-signal to the VSV 44. As shown in FIG. 5, the reference pressure Ps is
 set, for example, to a value smaller than the feed pressure P at the
 second change point Q2 and sufficiently greater than the intake pressure
 PM.
 As shown in FIG. 5, the elapsed time Tth is equal to T1 in the case A where
 the switching valve 28 functions normally, equal to T2 in the case B where
 the switching valve 28 has an abnormal stroke and can only be closed to
 its half-opening state, equal to T3 in the case C where the switching
 valve 28 is locked in its full-opening state, and equal to T4 in the case
 D where the switching valve 28 is locked in its full-closure state. The
 elapsed time Tth becomes shorter in the sequence of A, B, C and D. That
 is, the elapsed time Tth reflects changes in the tendency of changes in
 feed pressure P corresponding to the state of the switching valve 28 as
 has been described above with reference to FIGS. 2 and 3. Hence, the
 routine shown in FIG. 4 determines a judgment time Ts in consideration of
 error factors for the value T1 of the elapsed time Tth in the case where
 the switching valve 28 functions normally. If the elapsed time Tth that
 has been measured is shorter than the judgment Ts, it is determined that
 there arises an abnormality in the switching valve 28.
 As described above, if there arises a malfunction in the negative pressure
 transmission system, for example, in the case where there arises a
 malfunction in the VSV 44, where the negative pressure feed line 42
 cracks, collapses or disengages, or where the variable pressure chamber 36
 undergoes an abnormality such as insufficient sealability, the feed
 pressure P is unable to reach the reference pressure Ps. Hence, in this
 case, the intake pressure PM detected based on an output signal from the
 intake pressure sensor 25 is compared with the feed pressure P after the
 lapse of a sufficient length of time since the turning-on of the VSV 44.
 If the difference therebetween exceeds a predetermined value, it is
 determined that there is a malfunction in the negative pressure
 transmission system.
 A concrete description will hereinafter be made as to the contents of the
 routine shown in FIG. 4. The routine shown in FIG. 4 is repeatedly
 activated every time its processings are completed. Upon activation of the
 routine shown in FIG. 4, the processing in STEP 100 is first of all
 performed.
 It is determined in STEP 100 whether or not the pressure sensor 46 is in
 normal operation. This determination is made, for example, based on the
 result of initial check in the internal combustion engine 12. If the
 result confirms that the pressure sensor 46 is not in normal operation, it
 is determined that the judgment of the occurrence of an abnormality in the
 switching valve 28 based on the feed pressure P cannot be made, and then
 the present routine is terminated. On the other hand, if it is determined
 in STEP 100 that the pressure sensor 46 is in normal operation, the
 processing in STEP 102 is performed next.
 It is determined in STEP 102 whether or not an ON-signal has been supplied
 to the VSV 44. If the result confirms that an ON-signal has not been
 supplied to the VSV 44, atmosphere is introduced into the negative
 pressure feed line 42. In this case, an atmospheric pressure Pa is
 detected based on an output signal from the pressure sensor 46 in STEP
 104, and then the present routine is terminated. In this routine, the
 atmospheric pressure Pa detected in STEP 104 is used to correct the output
 signal from the pressure sensor 46, whereby the feed pressure P on the
 basis of the atmospheric pressure Pa is detected. On the other hand, if an
 ON-signal has been supplied to the VSV 44 in STEP 102, the processing in
 STEP 106 is performed next.
 It is determined in STEP 106 whether or not the condition for judging the
 occurrence of an abnormality in the switching mechanism 50 based on
 changes in feed pressure P (the abnormality judgment condition) is
 established. To be more precise, for example, in the case where the intake
 pressure PM has settled to a value equal to or lower than a predetermined
 pressure and where the outside air temperature is equal to or higher than
 a predetermined temperature, the abnormality judgment condition is
 established. If the abnormality judgment condition is not established in
 STEP 106, the present routine is terminated. On the other hand, if the
 abnormality judgment condition is established, the processing in STEP 108
 is performed next.
 In STEP 108, the elapsed time Tth required for the feed pressure P to reach
 the reference pressure Ps after the start of supply of the ON-signal to
 the VSV 44 is measured. It is determined in STEP 110 whether or not a
 predetermined length of time T0 has elapsed after the start of supply of
 the ON-signal to the VSV 44. The predetermined length of time T0 is
 preliminarily determined as a length of time required for the feed
 pressure P to reach the intake pressure PM. If the predetermined length of
 time T0 has not elapsed in STEP 110, the present routine is terminated. On
 the other hand, if the predetermined length of time T0 has elapsed in STEP
 110, the processing in STEP 112 is performed next.
 In STEP 112, the current feed pressure P is stored as a base pressure
 Pbase. Thus, the base pressure Pbase is equal to the intake pressure PM
 unless there is a malfunction in the negative pressure transmission system
 It is determined in STEP 114 whether or not the base pressure Pbase is
 lower than the reference pressure Ps. If the result confirms that the
 relation Pbase&lt;Ps is established, the processings for judging a state
 of the switching valve 28 based on the elapsed time Tth are performed in
 STEP 116 and STEP 118. On the other hand, if the relation Pbase&lt;Ps is
 not established in STEP 114, the processings for judging the occurrence of
 a malfunction in the negative pressure transmission system are performed
 in STEP 120 and STEP 122.
 In STEP 116, the judgment time Ts is determined from the base pressure
 Pbase. In the case where the switching valve 28 is in normal operation, as
 the base pressure Pbase becomes lower (i.e. as the intake pressure PM
 becomes lower), the feed pressure P decreases more rapidly and the elapsed
 time Tth becomes shorter. Hence, the values of the elapsed time Tth
 corresponding to various intake pressures PM in the case where the
 switching valve 28 functions normally are preliminarily stored as a map.
 In STEP 116, the judgment time Ts is determined from the base pressure
 Pbase by referring to the map.
 It is determined in STEP 118 whether or not the elapsed time Tth is equal
 to or longer than the judgment time Ts. If the result confirms that the
 relation Tth.gtoreq.Ts is established, it is determined that the switching
 valve 28 is in its normal operation. Next in STEP 124, a provisional
 abnormality flag F1 is reset to be turned off. After a normal flag F2 has
 been turned on in STEP 126, the present routine is terminated. Meanwhile,
 if the relation Tth&gt;Ts is not established in STEP 118, it is
 provisionally determined that there is an abnormality in the switching
 valve 28. The processing in STEP 130 is performed next.
 It is determined in STEP 130 whether or not the provisional abnormality
 flag F1 is on. If the result confirms that the provisional abnormality
 flag F1 is on, the occurrence of an abnormality has successively been
 judged twice. In this case, it is determined that there is an abnormality
 in the switching valve 28. Next in STEP 132, an abnormality flag F3 is
 turned on. In STEP 132, the abnormality flag F3 is turned on. After a
 warning indicative of the occurrence of an abnormality in the switching
 mechanism 50 has been issued in STEP 134, the present routine is
 terminated. On the other hand, if the provisional abnormality flag F1 is
 not on in STEP 130, the provisional abnormality flag F1 is turned on in
 STEP 136, and then the present routine is terminated.
 In STEP 120, the intake pressure PM is detected based on an output signal
 from the intake pressure sensor 25.
 It is determined in STEP 122 whether or not the difference .DELTA.P between
 the base pressure Pbase and the intake pressure PM calculated in STEP 120
 (=Pbase-PM) is equal to or lower than a predetermined value .DELTA.P0. If
 the result confirms that the relation .DELTA.P.ltoreq..DELTA.P0 is not
 established, it is provisionally determined that there arises a
 malfunction in the transmission system. Then, the processings in STEP 130
 and the following STEPS are performed, and the present routine is
 terminated. On the other hand, if the relation .DELTA.P.ltoreq..DELTA.P0
 is established in STEP 122, it is determined that there is no malfunction
 in the transmission system. Then, the processing in STEP 126 is performed,
 and the present routine is terminated.
 As described above, in this embodiment, the occurrence of an abnormality in
 the switching mechanism is judged based on the tendency of changes in feed
 pressure P after the turning-on of the VSV 44. The tendency of changes in
 feed pressure P after the turning-on of the VSV 44 almost exclusively
 depends on the state of the switching mechanism 50. Thus, this embodiment
 makes it possible to precisely judge the occurrence of an abnormality in
 the switching mechanism 50.
 In the routine shown in FIG. 4, if the elapsed time Tth is equal to or
 shorter than the reference time Ts, it is determined that there is an
 abnormality in operation of the switching valve 28. However, as has been
 described with reference to FIG. 5, the elapsed time Tth is shorter in the
 case where the switching valve 28 is locked than in the case where the
 switching valve 28 has a stroke abnormality. Moreover, in the case where
 the switching valve 28 is abnormally locked, as the locking position
 becomes closer to the full-closure position, the elapsed time Tth becomes
 shorter. Hence, the locking of the switching valve 28 and the stroke
 abnormality may separately be judged based on the elapsed time Tth. In
 addition, if the switching valve 28 is locked, the locking position of the
 switching valve 28 may be determined.
 Next, a second embodiment of the present invention will be described. In
 this embodiment, as has been described with reference to FIGS. 2 and 3,
 the locking of the switching valve 28, the stroke abnormality of the
 switching valve 28 and the locking position of the switching valve 28 are
 judged based on the presence or absence of the first change point Q1, the
 moment of generation of the second change point Q2 and the descending
 gradient of the feed pressure P at a predetermined moment respectively.
 FIG. 6 is a flowchart of an exemplary routine executed by the ECU 10 in
 this embodiment. In FIG. 6, the steps for performing processings similar
 to those of the routine shown in FIG. 4 are denoted by the same reference
 numerals as in FIG. 4 and will not be described again In the routine shown
 in FIG. 6, after the processing in STEP 104 has been completed, the
 processing in STEP 200 is performed next.
 In STEP 200, it is determined whether or not the elapsed time T since the
 supply of an ON-signal to the VSV 44 is equal to or shorter than a
 predetermined length of time Ta, or it is determined whether or not a
 locking normal flag Fa is off. The predetermined length of time Ta is made
 longer than an elapsed time required for generation of the second change
 point Q2 since the turning-on of the VSV 44 in the case where the
 switching valve 28 functions normally. Also, as will be described later,
 the locking normal flag Fa is turned on upon detection of the first change
 point Q1. If the result in STEP 200 turns out to be affirmative, the
 processing in STEP 202 is performed next. On the other hand, if the result
 in STEP 200 turns out to be negative, the processing in STEP 204 is
 performed next.
 It is determined in STEP 202 whether or not the descending gradient dP
 (=-dP/dt) of the feed pressure P is equal to or smaller than a
 predetermined value C0. If the result confirms that the relation
 dP.ltoreq.C0 is established, it is determined that the descending gradient
 at the first change point Q1 has decreased. In this case, the locking
 normal flag Fa is turned on in STEP 206, and then the present routine is
 terminated. On the other hand, if the relation dP.ltoreq.C0 is not
 established in STEP 202, the present routine is terminated without
 performing the processing in STEP 206.
 It is determined in STEP 204 whether or not the locking normal flag Fa is
 on. If the result confirms that the locking normal flag Fa is on, the
 processing in STEP 208 is performed next. On the other hand, if the
 locking normal flag Fa is not on in STEP 204, the processing in STEP 210
 is performed next.
 It is determined in STEP 208 whether or not the descending gradient dP is
 equal to or greater than a predetermined value C1. The predetermined value
 C1 is set to a lower limit value of the descending gradient dP at the
 moment past the second change point Q2 in the case where the switching
 valve 28 functions normally. Thus, if the relation dP.gtoreq.C1 is
 established in STEP 208, it is determined that the second change point Q2
 has been generated. The processing in STEP 212 is then performed. On the
 other hand, if the result in STEP 208 turns out to be negative, the
 present routine is terminated.
 It is determined in STEP 212 whether or not the elapsed time T after the
 start of supply of an ON-signal to the VSV 44 is equal to or longer than a
 predetermined value Tb. The value Tb is set to a lower limit value of the
 elapsed time required for generation of the second change point Q2 since
 the turning-on of the VSV 44 in the case where the switching valve 28
 functions normally. Thus, if the relation T.gtoreq.Tb is established in
 STEP 212, it is determined that there is no stroke abnormality in the
 switching valve 28. In this case, a stroke normal flag Fb is turned on in
 STEP 214, and the present routine is terminated. On the other hand, if the
 relation T.gtoreq.Tb is not established in STEP 212, it is determined that
 there occurs a stroke abnormality in the switching valve 28. A warning
 indicative of the occurrence of the stroke abnormality is issued in STEP
 216, and the present routine is terminated.
 In STEP 210, a warning indicative of the locking of the switching valve 28
 is issued. It is determined in STEP 218 whether or not the elapsed time T
 after the start of supply of an ON-signal to the VSV 44 has reached a
 predetermined length of time Tc. The predetermined length of time Tc is
 preferably set to such a timing that the difference in feed pressure P
 based on the locking position of the switching valve 28 assumes its
 maximum value. If the elapsed time T has not reached the predetermined
 length of time Tc in STEP 218, the present routine is terminated. On the
 other hand, if the elapsed time T has reached the predetermined length of
 time Tc in STEP 218, the locking position of the switching valve 28 is
 judged based on the current feed pressure P in STEP 220, and then the
 present routine is terminated. In STEP 218 and STEP 220, as has been
 described with reference to FIG. 3, the locking position of the switching
 valve 28 may be judged based on the descending gradient of the feed
 pressure P at a predetermined moment or based on the elapsed time required
 for the feed pressure P to reach a predetermined pressure.
 A third embodiment of the invention will be described with reference to
 FIGS. 7 and 8.
 In the aforementioned first and second embodiments, while the presence or
 absence of a locking abnormality in the switching valve 28 is judged based
 on the presence or absence of the first change point Q1, the presence or
 absence of the first change point Q1 is judged based on whether or not the
 descending gradient of the feed pressure P has become equal to or smaller
 than a predetermined value within a predetermined period after the
 turning-on of the VSV 44.
 FIGS. 7A and 7B explain a method of judging the occurrence of a locking
 abnormality in the switching valve 28 in this embodiment. FIG. 7A shows,
 with a solid line, time-dependent changes in feed pressure P after the
 tuning-on of the VSV 44 in the case A where the switching valve 28
 functions normally. FIG. 7B shows, with a broken line, time-dependent
 changes in gradient of changes in feed pressure P in the case B where the
 switching valve 28 is locked.
 In the case where the switching valve 28 functions normally, the first
 change point Q1 and the second change point Q2 appear on the curve of the
 feed pressure P within a predetermined period after the turning-on of the
 VSV 44 at a moment t0. That is, the descending gradient of the feed
 pressure P decreases discontinuously at a moment t1, and then the
 descending gradient of the feed pressure P increases discontinuously at a
 moment t2. On the other hand, in the case where the switching valve 28 is
 locked at all events, neither the first change point Q1 nor the second
 change point Q2 appears on the curve of the feed pressure P. In other
 words, the aforementioned state of the descending gradient is not
 realized.
 In this manner, it is possible to determine whether or not the switching
 valve 28 is locked, based on whether or not the first change point Q1
 appears on the curve of the feed pressure P within a predetermined period
 after the turning-on of the VSV 44, that is, whether or not there is a
 period when the amount of change in descending gradient of the feed
 pressure P is greater than a predetermined value. Hence, this embodiment
 employs the aforementioned method to judge the locking of the switching
 valve 28.
 FIG. 8 is a flowchart of an exemplary control routine executed by the ECU
 10 in this embodiment. In FIG. 8, the steps for performing processings
 similar to those of the routine shown in FIG. 6 are denoted by the same
 reference numerals as in FIG. 6 and will not be described again. In the
 routine shown in FIG. 8, if the result in STEP 200 turns out to be
 affirmative, the processing in STEP 240 is performed next.
 It is determined in STEP 240 whether or not the absolute value of a
 difference .DELTA.dP (=dP2-dP1) between a descending gradient dP1 of the
 feed pressure P during the last processing and a descending gradient dP2
 of the feed pressure P during the present processing is equal to or
 greater than a predetermined value D0. The predetermined value D0 is set
 to a lower limit value which makes it possible to determine that the
 descending gradient of the feed pressure P decreases discontinuously after
 the VSV 44 has been turned on under the circumstance where the switching
 valve 28 functions normally.
 If the relation .vertline..DELTA.dP.vertline..gtoreq.D0 is established, it
 can be determined that the descending gradient of the feed pressure P has
 decreased discontinuously, namely, that the tendency of a decrease in feed
 pressure P has weakened. In this case, it is possible to determine that
 the first change point Q1 has appeared on the curve of the feed pressure P
 and that the switching valve 28 is in normal operation. Thus, if it is
 determined in STEP 240 that the relation
 .vertline..DELTA.dP.vertline..gtoreq.D0 is established, the locking normal
 flag Fa is turned on in STEP 206 and then the present routine is
 terminated. On the other hand, if it is determined that the relation
 .vertline..DELTA.dP.vertline..gtoreq.D0 is not established, the present
 routine is terminated without performing the processing in STEP 206.
 If the result in STEP 200 turns out to be negative, the processing in STEP
 204 is performed next. If the result in STEP 204 turns out to be
 affirmative, the processing in STEP 242 is performed next.
 It is determined in STEP 242 whether or not the absolute value of a
 difference .DELTA.dP (=dP2-dP1) between a descending gradient dP1 of the
 feed pressure P during the last processing and a descending gradient dP2
 of the feed pressure P during the present processing is equal to or
 greater than a predetermined value D1. The predetermined value D1 is set
 to a lower limit value which makes it possible to determine that the
 descending gradient of the feed pressure P increases discontinuously after
 passage of the second change point Q2 under the circumstance where the
 switching valve 28 functions normally.
 If the relation .vertline..DELTA.dP.vertline..gtoreq.D1 is established, it
 can be determined that the descending gradient of the feed pressure P has
 increased discontinuously, namely, that the tendency of a decrease in feed
 pressure P has strengthened. In this case, it can be determined that the
 second change point Q2 has appeared on the curve of the feed pressure P.
 Thus, if it is determined in STEP 242 that the relation
 .vertline..DELTA.dP.vertline..gtoreq.D1 is established, the processings in
 STEP 212 and the following STEPS are performed. On the other hand, if the
 relation .vertline..DELTA.dP.vertline..gtoreq.D1 is not established, the
 present routine is terminated without performing the processings in STEP
 212 and the following STEPS.
 The aforementioned processings make it possible to judge the presence or
 absence of the locking of the switching valve 28 based on whether or not
 there is a period when the amount of change in the descending gradient of
 the feed pressure P is greater than a predetermined value within a
 predetermined period after the turning-on of the VSV 44.
 Next, a fourth embodiment of the invention will be described with reference
 to FIG. 9.
 In the first through third embodiments of the present invention, in order
 to prevent the feed pressure P from changing due to fluctuations of the
 intake pressure PM when judging the occurrence of an abnormality in the
 switching mechanism 50, a regulator valve is provided in the negative
 pressure passage 48 so that the negative pressure supplied to the negative
 pressure feed line 42 is kept constant.
 On the other hand, in this embodiment, the occurrence of an abnormality in
 the switching mechanism 50 is judged only when the intake pressure PM is
 kept substantially constant. This method makes it possible to change the
 feed pressure P depending only on the state of the switching mechanism 50
 without providing a regulator valve. Thus, this embodiment makes it
 possible to precisely judge the occurrence of an abnormality in the
 switching mechanism 50 based on the tendency of changes in feed pressure
 P.
 FIG. 9 is a flowchart of an exemplary routine of a control routine executed
 by the ECU 10 to realize the aforementioned function in this embodiment.
 The routine shown in FIG. 9 is repeatedly activated every time its
 processings are completed. Upon activation of the routine shown in FIG. 9,
 the processing in STEP 300 is first of all performed.
 It is determined in STEP 300 whether or not a predetermined length of time
 T10 has elapsed since the beginning of the starting operation of the
 internal combustion engine 12. The predetermined length of time T10 is a
 preliminarily determined period which is required for the internal
 combustion engine 12 to operate stably after the beginning of the starting
 operation of the internal combustion engine 12. The processing in STEP 300
 is repeatedly performed until it is determined that the aforementioned
 condition is established. If the result turns out to be affirmative, the
 processing in STEP 302 is performed.
 It is determined in STEP 302 whether or not the operating state of the
 internal combustion engine 12 is stabilized. To be more specific, it is
 determined in STEP 302 whether or not the intake pressure PM detected by
 means of the intake pressure sensor 25, the engine speed NE detected by
 means of the NE sensor 52, and the throttle opening degree TA detected by
 means of the throttle position sensor 54 are maintained within
 predetermined ranges respectively. The processing in STEP 302 is
 repeatedly performed until it is determined that the aforementioned
 condition is established. If those parameters are maintained within the
 predetermined ranges respectively, it is possible to determine that the
 operating state of the internal combustion engine 12 is stabilized and
 that the intake pressure PM is kept substantially constant. Thus, if the
 result in STEP 302 turns out to be affirmative, the processing in STEP 304
 is performed next.
 In STEP 304, the processing of supplying an ON-signal to the VSV 44 is
 performed. If the processing in STEP 304 is performed, the intake pressure
 PM is thereafter introduced into the negative pressure feed line 42 so
 that the switching valve 28 is closed.
 It is determined in STEP 306 whether or not the operating state of the
 internal combustion engine 12 is still stabilized. If the result in STEP
 306 turns out to be affirmative, the processing in STEP 308 is performed
 next On the other hand, if the result in STEP 306 turns out to be
 negative, the processing in STEP 312 is performed next.
 It is determined in STEP 308 whether or not a predetermined length of time
 T11 has elapsed since the start of supply of an ON-signal to the VSV 44.
 The predetermined length of time T11 is set to a period which is longer
 than an estimated period required for generation of the first change point
 Q1 after the turning-on of the VSV 44 in the case where the switching
 valve 28 functions normally. If the result in STEP 308 turns out to be
 negative, the processing in STEP 306 is performed again. On the other
 hand, if the result in STEP 308 turns out to be affirmative, the
 processing in STEP 310 is performed next.
 In STEP 310, the processing of judging the presence or absence of an
 abnormal state in the switching mechanism 50 based on the tendency of
 changes in feed pressure P is performed. To be more specific, the
 processings shown in the first through third embodiments are performed.
 Upon completion of the processing in STEP 310, the present routine is
 terminated.
 In STEP 312, the processing of prohibiting the judgment of the presence or
 absence of an abnormal state in the switching mechanism 50 based on the
 feed pressure P is performed. Upon completion of the processing in STEP
 312, the present routine is terminated.
 The aforementioned processing makes it possible to prohibit the judgment of
 an abnormality in the switching mechanism 50 based on the feed pressure P
 under the circumstance where the operating state of the internal
 combustion engine 12 is not stabilized. Thus, this embodiment makes it
 possible to change the feed pressure P in the negative pressure feed line
 42 and the variable pressure chamber 36 depending only on the state of the
 switching mechanism 50. Therefore, this embodiment makes it possible to
 precisely judge the occurrence of an abnormality in the switching
 mechanism 50 based on the tendency of changes in feed pressure P and to
 enhance the precision in judging the occurrence of the abnormality.
 In the fourth embodiment, it is determined whether or not the operating
 state of the internal combustion engine 12 is stabilized, using the intake
 pressure PM, engine speed NE and throttle opening degree TA of the
 internal combustion engine 12. In addition, however, the stability of the
 operating state of the internal combustion engine 12 may be judged based
 on the result of a determination whether or not the amount of change in
 fuel injection quantity in the internal combustion engine 12 and the
 amount of change in vehicle speed are maintained within predetermined
 ranges respectively. In the case where the internal combustion engine 12
 is fitted with intake and exhaust valves with variable opening and closing
 timings, the stability of the operating state of the internal combustion
 engine 12 may be judged based on the result of a determination whether or
 not the opening and closing timings are maintained within predetermined
 ranges respectively. The intake pressure PM tends to be destabilized
 immediately after the start of the internal combustion engine 12. Thus,
 the stability of the operating state of the internal combustion engine 12
 may be judged in consideration of the result of a determination whether or
 not the engine speed NE has exceeded a predetermined value, that is,
 whether or not the internal combustion engine 12 has at least reached its
 idle state.
 A fifth embodiment of the invention will be described with reference to
 FIGS. 10 through 12.
 In the case where there arises a stroke abnormality in the switching valve
 28, the deflection amount of the diaphragm 34 from the start of its
 deflection to the end of its deflection (hereinafter referred to as a
 movable amount L) is smaller in comparison with the case where the
 switching valve 28 functions normally. That is, the timing at which the
 second change point Q2 is generated after the emergence of the first
 change point Q1 on the curve of the feed pressure P becomes earlier as the
 movable amount L of the diaphragm 34 becomes smaller. Thus, it is possible
 to judge the movable amount L of the diaphragm 34 based on a time interval
 between a moment of generation of the first change point Q1 on the curve
 of the feed pressure P and a moment of generation of the second change
 point Q2 on the curve of the feed pressure P (hereinafter referred to as a
 change-point-interval time TIME). Thus, it is possible to judge the
 occurrence of a stroke abnormality in the switching valve 28 based on
 whether or not the movable amount L of the diaphragm 34 is small.
 In the aforementioned switching mechanism 50, the switching valve 28, which
 is coupled to the diaphragm 34 through the actuating rod 40, is opened or
 closed in accordance with a state of deflection of the diaphragm 34. In
 this construction, there may be caused an abnormality of disengagement in
 a joint between the diaphragm 34 and the switching valve 28 (hereinafter
 referred to as an unlinking abnormality). In this circumstance, even if an
 ON-signal has been supplied to the VSV 44, deflection of the diaphragm 34
 is not transmitted to the switching valve 28 so that the switching valve
 28 cannot be closed.
 In general, the diaphragm 34 is designed to be deflectable beyond a
 deflection amount at which the switching valve 28 assumes its full-opening
 state. However, the movable amount L of the diaphragm 34 is normally
 confined to a predetermined range. If there arises an unlinking
 abnormality in the switching mechanism 50, the movable amount L of the
 diaphragm 34 is greater in comparison with the case where the switching
 mechanism 50 functions normally. That is, in this case, the timing at
 which the second change point Q2 is generated after the emergence of the
 first change point Q1 on the curve of the feed pressure P becomes later as
 the movable amount L of the diaphragm 34 becomes greater. Thus, the
 occurrence of an unlinking abnormality in the switching mechanism 50 can
 be judged based on whether or not the movable amount L of the diaphragm 34
 is great, or based on an elapsed time from generation of the first change
 point Q1 on the curve of the feed pressure P to emergence of the second
 change point Q2.
 In this manner, the system of this embodiment makes it possible to judge
 the occurrence of a stroke abnormality in the switching valve 28 and the
 occurrence of an unlinking abnormality in the switching mechanism 50.
 In the case where the negative pressure supplied to the negative pressure
 feed pipe 42 is kept constant by a regulator valve or the like, the feed
 pressure P changes depending only on a state of the switching mechanism
 50. In this case, the movable amount L of the diaphragm 34 can be
 determined based only on a change-point-interval time TIME. However, in
 the case where the negative pressure supplied to the negative pressure
 feed line 42 is not kept constant, that is, in the case where the negative
 pressure fluctuates in accordance with an intake pressure PM, even if the
 switching mechanism 50 is in normal operation, the change-point-interval
 time TIME may fluctuate.
 FIG. 10 shows an example of time-dependent changes in feed pressure P when
 the VSV is switched from off to on, in the case A where the intake
 pressure PM assumes a great value on the negative pressure side and in the
 case B where the intake pressure PM assumes a small value on the negative
 pressure side. The cases A and B are indicated by a solid line and a
 broken line respectively.
 As shown in FIG. 10, as the intake pressure PM becomes greater on the
 negative pressure side, the feed pressure P decreases more smoothly. That
 is, the decreasing speed of the feed pressure P fluctuates in accordance
 with the intake pressure PM. The time from the start of deflection of the
 diaphragm 34 to the maximum possible deflection of the diaphragm 34,
 namely, the change-point-interval time TIME becomes shorter as the intake
 pressure PM becomes greater on the negative pressure side (=TIME 1), and
 becomes longer as the intake pressure PM becomes smaller on the negative
 pressure side (=TIME 2&gt;TIME 1). Thus, in the case where the negative
 pressure supplied to the negative pressure feed line 42 may fluctuate in
 accordance with the intake pressure PM, it is possible to precisely
 determine a movable amount L of the diaphragm 34 based on a
 change-point-interval time TIME and an intake pressure PM.
 FIG. 11 is a flowchart of an exemplary control routine executed by the ECU
 10 to judge the occurrence of a stroke abnormality in the switching valve
 28 and an unlinking abnormality in the switching mechanism 50. The routine
 shown in FIG. 11 is repeatedly activated every time its processings are
 completed. In FIG. 11, the steps for performing processings similar to
 those of the routine shown in FIG. 4 are denoted by the same reference
 numerals as in FIG. 4 and will not be described again. If the result in
 STEP 106 turns out to be affirmative, the processing in STEP 340 is
 performed next.
 It is determined in STEP 340 whether or not the first change point Q1 has
 appeared on the curve of the feed pressure P. To be more specific, it is
 determined in STEP 340 whether or not the descending gradient of the feed
 pressure P decreases discontinuously, that is, whether or not the absolute
 value of an amount .DELTA.dP of change in the descending gradient is equal
 to or greater than a predetermined value. If the result confirms that the
 first change point Q1 has not been generated, the present routine is
 terminated. On the other hand, if it is determined that the first change
 point Q1 has been generated, the processing in STEP 342 is performed next.
 In STEP 342, the processing of counting up from "0" using a counter COUNT
 for counting the change-point-interval TIME is performed.
 It is determined in STEP 344 whether or not the second change point Q2 has
 appeared on the curve of the feed pressure P. To be more specific, it is
 determined in STEP 344 whether or not the descending gradient of the feed
 pressure P increases discontinuously, that is, whether or not the absolute
 value of an amount .DELTA.dP of change in the descending gradient is equal
 to or greater than a predetermined value. The processing in STEP 344 is
 repeatedly performed until it is determined that the second change point
 Q2 has been generated. If the result confirms that the second change point
 Q2 has been generated, the processing in STEP 346 is performed next
 In STEP 346, the processing of terminating the counting operation of the
 counter COUNT is performed. If the processing in STEP 346 is performed,
 the change-point-interval time TIME is calculated.
 It is determined in STEP 348 whether or not the intake pressure PM has
 continuously been settled to a value equal to or lower than a
 predetermined pressure from generation of the first change point Q1 to the
 present moment. If the result in STEP 348 turns out to be negative, the
 intake pressure PM fluctuates. Therefore, the movable amount L of the
 diaphragm 34 cannot precisely be detected. Thus, in the case of such a
 negative determination, the present routine is terminated. On the other
 hand, if the result turns out to be affirmative, the processing in STEP
 350 is performed next.
 In STEP 350, the movable amount L of the diaphragm 34 is calculated based
 on the change-point-interval time TIME and the intake pressure PM.
 FIG. 12 shows a two-dimensional map of change-point-interval time TIME and
 intake pressure PM, which is designed to calculate a movable amount L of
 the diaphragm 34. In STEP 350, the movable amount L of the diaphragm 34 is
 calculated by referring to the map shown in FIG. 12.
 It is determined in STEP 352 whether or not the movable amount L of the
 diaphragm 34 calculated in STEP 350 is smaller than a predetermined value
 L1. The predetermined value L1 is a lower limit value of the movable
 amount L of the diaphragm 34 which makes it possible to determine that the
 switching valve 28 functions normally. If the result confirms that the
 relation L&lt;L1 is established, a warning indicative of the occurrence of
 a stroke abnormality in the switching valve 28 is issued in STEP 354, and
 then the present routine is terminated. On the other hand, if it is
 determined in STEP 352 that the relation L&lt;L1 is not established, the
 processing in STEP 356 is performed next.
 It is determined in STEP 356 whether or not the movable amount L of the
 diaphragm 34 has exceeded a predetermined value L2. The predetermined
 value L2 is an upper limit value of the movable amount L of the diaphragm
 34 which makes it possible to determine that the switching valve 28
 functions normally. If the result confirms that the relation L&gt;L2 is
 established, a warning indicative of the occurrence of an unlinking
 abnormality in the switching mechanism 50 is issued, and then the present
 routine is terminated.
 If it is determined that the relation L&gt;L2 is not established, it is
 possible to determine that the movable amount of the diaphragm 34 is
 maintained within a predetermined range. Thus, in the case of such a
 determination, the processing in STEP 360 is performed next.
 In STEP 360, the processing of turning on a normal flag Fc indicative of a
 normality in the switching mechanism 50 is performed. Upon completion of
 the processing in STEP 360, the present routine is terminated.
 The aforementioned processing makes it possible to precisely determine a
 movable amount of the diaphragm 34 based on the change-point-interval TIME
 and the intake pressure PM, even in the case where the negative pressure
 supplied to the negative pressure feed line 42 may fluctuate in accordance
 with the intake pressure PM. Hence, this embodiment makes it possible to
 judge the occurrence of a stroke abnormality in the switching valve 28 and
 the occurrence of an unlinking abnormality in the switching mechanism 50.
 In the fifth embodiment, the occurrence of an unlinking abnormality in the
 switching mechanism 50 is judged based on whether or not the movable
 amount L of the diaphragm 34 is great. However, the occurrence of the
 unlinking abnormality may also be judged based on an elapsed time from
 generation of the first change point Q1 to emergence of the second change
 point Q2.
 Next, a sixth embodiment of the invention will be described with reference
 to FIGS. 13 and 14.
 FIG. 13 shows an example of time-dependent changes in feed pressure P when
 the VSV is turned on, in the case A where the switching mechanism
 functions normally and in the case B where the switching mechanism
 undergoes a considerable friction in the course of operation. The cases A
 and B are indicated by a solid line and a broken line respectively.
 During the period from the start of deflection of the diaphragm 34 to the
 end of deflection of the diaphragm 34 after the turning-on of the VSV 44,
 even if the feed pressure P remains unchanged, the deflection amount of
 the diaphragm 34 changes in accordance with the friction acting on the
 switching mechanism 50. As the friction acting on the switching mechanism
 50 increases, deflection of the diaphragm 34 becomes more difficult.
 Hence, as the friction acting on the switching mechanism 50 increases, the
 decreasing rate of the volume of the aspiration space becomes smaller and
 the feed pressure P decreases more smoothly. The magnitude of the friction
 which acts on the switching mechanism 50 while the switching valve 28 is
 being opened or closed can be judged based on a descending gradient after
 the start of deflection of the diaphragm 34, that is, after generation of
 the first change point Q1 on the curve of the feed pressure P.
 The change-point-interval time TIME changes in accordance with the friction
 which acts on the switching mechanism 50 while the switching valve 28 is
 being opened or closed. To be more specific, as the magnitude of friction
 increases, the change-point-interval time TIME is lengthened. For this
 reason, the movable amount L of the diaphragm 34 needs to be determined in
 consideration of the magnitude of the friction which acts on the switching
 mechanism 50 while the switching valve 28 is being opened or closed. Thus,
 in this embodiment, the movable amount L of the diaphragm 34 is determined
 by referring to a predetermined three-dimensional map based not only on
 the change-point-interval time TIME and the intake pressure PM as in the
 aforementioned fifth embodiment but also on the magnitude of the friction
 which acts on the switching mechanism 50 while the switching valve 28 is
 being opened or closed.
 FIG. 14 is a flowchart of an exemplary control routine executed by the ECU
 10 to judge the occurrence of a stroke abnormality in the switching valve
 28 and the occurrence of an unlinking abnormality in the switching
 mechanism 50 in this embodiment. In FIG. 14, the steps for performing
 processings similar to those of the routines shown in FIGS. 4 and 11 are
 denoted by the same reference numerals as in FIGS. 4 and 11 and will not
 be described again. That is, if the result in STEP 348 turns out to be
 affirmative, the processing in STEP 400 is performed.
 In STEP 400, the processing of calculating a descending gradient of the
 feed pressure P from generation of the first change point Q1 to generation
 of the second change point Q2 is performed.
 In STEP 402, the movable amount L of the diaphragm 34 is calculated based
 on the descending gradient of the feed pressure P calculated in STEP 400
 as well as the change-point-interval time TIME and the intake pressure PM.
 Even if the movable amount L of the diaphragm 34 remains unchanged, the
 change-point-interval time TIME is lengthened as the descending gradient
 of the feed pressure P increases, that is, as the magnitude of the
 friction which acts on the switching mechanism 50 while the switching
 valve 28 is being opened or closed increases. In other words, even if the
 change-point-interval time TIME and the intake pressure PM remain
 unchanged, the movable amount L of the diaphragm 34 decreases as the
 descending gradient of the feed pressure P increases.
 Upon completion of the processing in STEP 402, the processings in STEP 352
 and the following STEPS are performed. These processings make it possible
 to precisely determine a movable amount L of the diaphragm 34 even if the
 magnitude of the friction acting on the switching mechanism 50 is great.
 Therefore, this embodiment makes it possible to precisely judge the
 occurrence of a stroke abnormality in the switching valve 28 and the
 occurrence of an unlinking abnormality in the switching mechanism 50 and
 enhance the precision in judging such abnormalities.
 In the sixth embodiment, the magnitude of the friction acting on the
 switching mechanism 50 is determined based on the descending gradient of
 the feed pressure P from generation of the first change point Q1 to
 generation of the second change point Q2. However, the magnitude of the
 friction fluctuates due to the thermal expansion of the switching
 mechanism 50. Thus, the magnitude of the friction acting on the switching
 mechanism 50 may be determined using a temperature of the switching
 mechanism 50 in place of or in addition to the descending gradient of the
 feed pressure P. In this case, the system of this embodiment may be
 designed to detect a temperature of the switching mechanism 50 using the
 coolant temperature sensor 56 and the intake air temperature sensor 58. In
 this case, the ECU 10 detects the temperature of the switching mechanism
 based on output signals from the coolant temperature sensor 56 and the
 intake air temperature sensor 58, whereby "temperature detector" of the
 present invention is realized. In the case where sensors for detecting
 temperatures of exhaust gas and the main catalyst 24 are installed, these
 temperature may be used to detect a temperature of the switching mechanism
 50.
 Next, a seventh embodiment of the invention will be described with
 reference to FIGS. 15 and 16.
 FIG. 15 shows an example of time-dependent changes in feed pressure P when
 the VSV is switched from off to on, in the case A where the switching
 mechanism functions normally and in the case B where the switching
 mechanism undergoes a considerable friction prior to the start of
 operation. The cases A and B are indicated by a solid line and a broken
 line respectively.
 Before the diaphragm 34 starts to be deflected after the turning-on of the
 VSV 44, the switching valve 28, the actuating rod 40 or the diaphragm 34
 may undergo an abnormality of entrapment in foreign matters (hereinafter
 referred to as an entrapment abnormality). If there arises an entrapment
 abnormality in the switching mechanism 50, deflection of the diaphragm 34
 requires a high feed pressure P. Hence, even if the first change point Q1
 has been generated on the curve of the feed pressure P, there is a
 possibility that the negative pressure required for generation of the
 second change point Q2 may not be supplied to the variable pressure
 chamber 36 and that the switching valve 28 may not be opened or closed
 appropriately.
 As described above, if there arises an entrapment abnormality in the
 switching mechanism 50, deflection of the diaphragm 34 requires a high
 feed pressure P. Thus, the magnitude of the friction acting on the
 switching mechanism 50 prior to the start of operation can be determined
 based on the feed pressure P at the time of generation of the first change
 point Q1 after the turning-on of the VSV 44. Also, the presence or absence
 of an entrapment abnormality in the switching mechanism 50 can be judged.
 FIG. 16 is a flowchart of an exemplary control routine executed by the ECU
 10 to judge the occurrence of an entrapment abnormality in the switching
 mechanism 50 in this embodiment. In FIG. 16, the steps for performing
 processings similar to those of the routine shown in FIGS. 4 and 11 are
 denoted by the same reference numerals as in FIGS. 4 and 11 and will not
 be described again. That is, if it is determined in STEP 340 that the
 first change point Q1 has been generated, the processing in STEP 440 is
 performed next.
 It is determined in STEP 440 whether or not the feed pressure P at the time
 of generation of the first change point Q1 is equal to or lower than a
 predetermined threshold value Psh. The predetermined threshold Psh is a
 lower limit value of the feed pressure P required for generation of the
 second change point Q2 after generation of the first change point Q1. If
 the result confirms that the relation P.ltoreq.Psh is not established, it
 can be determined that the feed pressure P required to open and close the
 switching valve 28 appropriately is ensured. Thus, in the case of such a
 determination, the processing in STEP 442 is performed next. On the other
 hand, if the relation P.ltoreq.Psh is established, it can be determined
 that the feed pressure P has decreased to such an extent that the
 switching valve 28 cannot be opened or closed appropriately. Thus, in the
 case of such a determination, the processing in STEP 444 is performed next
 In STEP 442, the processing of turning on an entrapment normal flag Fd is
 performed. Upon completion of the processing in STEP 442, the present
 routine is terminated.
 In STEP 444, a warning indicative of the occurrence of an entrapment
 abnormality in the switching mechanism 50 resulting from an increase in
 friction prior to the start of operation is issued. Upon completion of the
 processing in STEP 444, the present routine is terminated. The
 aforementioned processing makes it possible to judge the occurrence of an
 entrapment abnormality in the switching mechanism 50 based on the feed
 pressure P at the time of generation of the first change point Q1 on the
 curve of the feed pressure P.
 In the first through seventh embodiments, the description of a stroke
 abnormality in the switching valve 28 has been made as to the case where
 the switching valve 28 can only be closed from its full-opening state to
 its half-opening state. However, there may arise a stroke abnormality
 wherein the switching valve 28 cannot be opened to its full-opening state.
 In the case of the occurrence of such a stroke abnormality, the diaphragm
 32 is deflected towards the variable pressure chamber 36 even under the
 circumstance where the VSV 44 has been turned off. Therefore, the
 aspiration space becomes smaller in volume in comparison with the case
 where the switching valve 28 assumes its full-opening state. Hence, the
 feed pressure P decreases with a great gradient when the VSV 44 is turned
 on, and the first change point Q1 is generated earlier. Thus, the moment
 of generation of the first change point Q1 is detected, whereby the
 judgment can be made with various stroke abnormalities being distinguished
 from one another.
 In the first through seventh embodiments, the occurrence of an operational
 abnormality in the switching valve 28 is judged based on the tendency of
 changes in feed pressure P after the turning-on of the VSV 44. However,
 even after the VSV 44 has been switched from on to off, if the switching
 valve 28 assumes a certain state, the feed pressure P demonstrates a
 tendency of changes which is obtained by vertically inverting what is
 shown in FIGS. 2 and 3. Thus, it is possible to judge the occurrence of an
 abnormality in the switching mechanism 50 even when the VSV 44 is switched
 from on to off. In other words, the judgment of the occurrence of an
 abnormality can be made twice by turning the VSV 44 on or off once.
 In the case where the occurrence of an abnormality in the switching
 mechanism 50 is judged based on the feed pressure P after the turning-off
 of the VSV 44, as the flow resistance at an atmospheric opening of the VSV
 44 increases, the gradient of changes in feed pressure P after the
 turning-off of the VSV 44 decreases. Therefore, the degree of changes in
 gradient at the first change point Q1 and the second change point Q2 also
 decreases. On the other hand, as the flow resistance at the atmospheric
 opening decreases, the feed pressure P increases more smoothly. In this
 case, the period from generation of the first change point Q1 to
 generation of the second change point Q2 is shortened, which causes a
 deterioration in precision of judgment at the time of generation of the
 second change point Q2. Thus, it is preferable to set the flow resistance
 (e.g. the opening area) at the atmospheric opening of the VSV 44 to a
 suitable value so that the first change point Q1 and the second change
 point Q2 emerge distinctly and that the moment of generation of the second
 change point Q2 can be determined with high precision.
 In the first through seventh embodiments of the invention, the VSV 44, the
 negative pressure feed line 42 and the variable pressure chamber 36
 correspond to "a pressure transmitting portion", the diaphragm 34 to "a
 moving member", and the intake manifold 16 to "a fluid pressure source"
 and "an intake passage". In the first through seventh embodiments, the ECU
 10 performs the processings in STEPS 118, 122, 124, 126, and 130 through
 136 in the routine shown in FIG. 4, the processings in STEPS 200 through
 220 in the routine shown in FIG. 6, the processings in STEPS 240 and 242
 in the routine shown in FIG. 8, the processings in STEPS 340 through 360
 in the routine shown in FIG. 11, the processings in STEPS 400 and 402 in
 the routine shown in FIG. 14, or the processings in STEPS 440 through 444
 in the routine shown in FIG. 16, whereby "controller" of the invention is
 realized.
 In addition, in the first through seventh embodiments, the ECU 10 detects a
 feed pressure P based on an output signal from the pressure sensor 46 to
 thereby realize "transmitting portion detector" of the present invention,
 detects an intake pressure PM based on an output signal from the intake
 pressure sensor 25 to thereby realize "fluid pressure source pressure
 detector" of the present invention, determines a magnitude of the friction
 acting on the switching mechanism 50 during the opening or closing of the
 switching valve 28 based on a descending gradient after generation of the
 first change point Q1 on the curve of the feed pressure P to thereby
 realize "friction detector" of the present invention, and performs the
 processings in STEPS 302 and 306 to thereby realize "controller" of the
 present invention.
 While the invention has been described with reference to preferred
 embodiments thereof, it is to be understood that the invention is not
 limited to the disclosed embodiments or constructions. On the contrary,
 the invention is intended to cover various modifications and equivalent
 arrangements. In addition, while the various elements of the disclosed
 invention are shown in various combinations and configurations which are
 exemplary, other combinations and configurations, including more, less or
 only a single embodiment, are also within the spirit and scope of the
 invention.