JPH04292528A - Air suction controller of internal combustion engine - Google Patents

Abstract

PURPOSE:To improve combustion and fuel consumption while reducing pumping loss. CONSTITUTION:Although an air suction control valve is closed in the halfway of suction process, average opening degree of the air suction control valve is reduced to a smaller opening degree (for examle, half-open) than full open. Therefore, the time when opening of the air suction contorl valve is completed for sucking the pridetermined amount of air is delayed comparing to the time when the valve is fully opened conventionally. As a result, the period of adiabatic expansion is shortened, and the degree of temperature drop of air in a cylinder is reduce. At the same time, the degree of reduction of swirl is reduced. Consequently, it becomes possible to improve combustion and fuel efficiency by far while maintaining the reduction of pumping loss which the air suction control valve has.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an intake control system for an internal combustion engine using an intake control valve.

0002

2. Description of the Related Art Conventionally, there has been an intake control device that controls the amount of intake air by providing an intake control valve in the intake passage of each cylinder of an internal combustion engine to improve fuel efficiency at partial load (for example, in Japanese Patent Laid-Open No. 6
3-65138). In this conventional intake control device, the intake period can be substantially shortened regardless of the cam profile by independently controlling the opening and closing of the intake control valves provided in the intake passages of each cylinder using actuators. FIG. 18 illustrates the relationship between the opening/closing timing of the intake control valve and the pressure at the upstream portion of the intake passage in a conventional device. As shown in the figure, in the conventional device, the intake control valve can be closed early in the intake stroke, so the throttle valve can be fully opened (WOT) and a predetermined amount of air at approximately atmospheric pressure can be taken in. Therefore, the loss, so-called pumping loss, caused by suction of negative pressure air is significantly reduced, and fuel efficiency can be improved.

0003

[Problems to be Solved by the Invention] Although the above-mentioned conventional device has achieved a reduction in pumping loss, there is a problem in that combustion may deteriorate, making it difficult to further improve fuel efficiency. This is because the intake control valve closes the intake passage early in the intake stroke, as shown in FIG. In other words,
The long period from the closure of the intake control valve to the end of the intake stroke results in adiabatic expansion, which lowers the temperature of the air in the cylinder, and at the same time, air intake is stopped early in the intake stroke, reducing the swirl and combustion due to swirl. This is because the improving effect of These factors lead to deterioration of combustion, making it difficult to further improve fuel efficiency.

The purpose of the intake air control device for an internal combustion engine of the present invention is to solve the above problems, improve combustion, and make it possible to further improve fuel efficiency.

[0005]

[Means and operations for solving the problems] As illustrated in FIG. 1, an intake control device for an internal combustion engine according to the present invention includes an intake control valve control means that controls an intake control valve provided in an intake passage of each cylinder of an internal combustion engine. In the intake control device for an internal combustion engine, which opens and closes at a predetermined timing to control the amount of intake air, the average opening degree of the intake control valve during the opening period of the intake control valve by the intake control valve control means is set to an opening degree smaller than fully open. It is characterized by providing means for reducing the average opening degree.

[0006] In the intake control device for an internal combustion engine having the above structure, the average opening degree reduction means reduces the average opening degree during the opening period of the intake control valve by the intake control valve control means to a predetermined degree of opening less than full opening. In order to obtain the amount of intake air of Therefore, the adiabatic expansion period from the closing of the intake control valve to the end of the intake stroke is shortened, and the degree of temperature drop of the air in the cylinder is reduced. Also,
The timing at which the intake control valve stops sucking air in the middle of the intake stroke is delayed, and the degree of reduction in swirl becomes smaller. These factors result in better combustion and further improvement in fuel efficiency.

[0007]

[Embodiment] An embodiment of the present invention will be described below. FIG. 2 shows the system configuration of an engine equipped with the intake air control device of this embodiment.

As shown in FIG. 2, the engine system configuration includes a four-cylinder engine 1 and an intake system 1 of this engine 1.
It is composed of an intake control unit 3 provided in a, an electronic control unit (hereinafter simply referred to as ECU) 5 that controls these, and the like.

[0009] The engine 1 includes four cylinders 7. Each cylinder 7 is provided with an intake valve 9 and an exhaust valve 11 that are opened and closed by a high-speed compatible cam. A throttle valve 13 as a pressure regulating valve is disposed in the intake system 1a of the engine 1. The throttle valve 13 changes the throttle opening degree by drive control of the throttle actuator 15. An exhaust pipe 1b is connected to each exhaust valve 11.

Each intake port 17 branched from the intake system 1a communicates with each cylinder 7. An intake control valve 19 is provided inside each intake port 17 . Each intake control valve 1
Reference numeral 9 changes the opening degree of the intake port 17 independently for each cylinder by drive control of the actuator 21. Intake port 17
An injector 23 for injecting fuel is provided at the upstream portion 17a of the
is provided.

The engine 1 is equipped with various sensors as detectors. For example, there is a crank angle sensor 25 that outputs a pulse signal when the piston of each cylinder 7 is located at top dead center (TDC). Additionally, a rotational speed sensor 27 outputs a pulse signal at each predetermined crank angle, a sensor 29 detects torque or combustion for each cylinder (for example, a cylinder pressure sensor, a torque sensor, a knock sensor), and an air amount sensor 29 for each cylinder. A sensor 31 (for example, an intake pipe pressure sensor), a load detection means 33 (for example, a throttle sensor, an accelerator sensor) that detects a load state, a noise/vibration detection means 35 that detects noise or vibration, and an emission detection means that detects an emission state. There are 37 etc.

Furthermore, the engine 1 is equipped with various control means. For example, an injection control means 39 that controls the injection amount and injection timing by the injector 23, an ignition timing control means 41 that controls the ignition timing, a supercharging means 43 that supercharges intake air, and a learning system that performs learning control according to the operating state. There are a control means 45, an intake air heating means 47 for heating intake air, and a cooling water temperature adjusting means 49 for adjusting the temperature of cooling water.

[0013]ECU5 includes CPU5a, ROM5b, RA
It is configured as an arithmetic logic circuit centered around M5c, and is connected to an input/output section 5e via a common bus 5d to perform input/output with the outside. Detection signals from the sensors 25 to 37 and signals from the control means 41 to 49 are input to the CPU 5a from the input/output section 5e. The CPU 5a outputs control signals to the throttle actuator 15, the actuator 21 of the intake control valve 19, the supercharging means 43, and the intake air heating means 47 via the input/output section 5e. Further, the ECU 5 detects the opening/closing period of the intake valve 9 of each cylinder 7 based on the signals from the crank angle sensor 25 and the rotational speed sensor 27.

Next, the intake control valve 19 and actuator 21 will be explained with reference to FIGS. 3 to 5. 3 is a sectional view showing the structure of the intake control valve 19 and the actuator 21, FIG. 4 is a sectional view taken along line 4--4 in FIG. 3, and FIG. 5 is an explanatory diagram showing the operation of the intake control valve 19. As shown in FIG. 3, the intake control valve 19 is a butterfly-type circular valve plate 50 provided inside the intake port 17. The circular valve plate 50 is rotatably supported by a support shaft 51, and is connected to the intake port 17.
It swings around the support shaft 51 in a non-contact manner while having an extremely narrow clearance with respect to the wall surface (see FIG. 5). Note that one end of the support shaft 51 is supported by the intake port 17 by a bearing 53, and the other end of the support shaft 51 is connected to the actuator 21.

The actuator 21 includes a support shaft 57, a magnet member 59, electromagnetic coils 61, 63, permanent magnets 65, 67, and the like. The support shaft 57 is rotatably supported within the casing 55 and is connected to the support shaft 51 of the intake control valve 19 . A magnet member 59 is fitted around the outer periphery of the support shaft 57 . Magnetic poles that are symmetrically different in the circumferential direction are formed in the magnet member 59 . A pair of electromagnetic coils 61 and 63 are provided on the inner wall of the casing 55 with the magnet member 59 interposed therebetween. In addition, a pair of permanent magnets 6
5 and 67 are provided on the inner wall of the casing 55 so as to face each other with the magnet member 59 in between and to be orthogonal to the pair of electromagnetic coils 61 and 63.

Next, a control circuit for the electromagnetic coils 61 and 63 will be explained. As shown in Figure 6, the control circuit consists of four FETs.
This is a known chopper driving circuit including 70A, 70B, 70C, 70D (transistors may also be used), a DC power supply 71, and the like. This control circuit driver (
(not shown) is provided. One ends of the electromagnetic coils 61 and 63 are connected in series. The other end of the electromagnetic coil 61 is FE
It is connected between T70A and FET70B, and the other end of electromagnetic coil 63 is connected between FET70C and FET70D.

The intake control valve 19 controlled by the above control circuit basically operates as follows. The timing chart of FIG. 7 shows the relationship between the input signal of the driver, the source voltages of FETs 70A to 70D, and the drive voltages. When an “H” signal with a high voltage level is input to the driver, the driver
Change the source voltage of ET70B and 70C to a low voltage level.
At the same time, the source voltage of the FET 70A is set to the high level "H", and the source voltage of the FET 70D is switched between the "H" and "L" levels by PWM control. Therefore, a positive drive voltage is applied, and a current with a magnitude determined by the duty ratio (ratio between period T2 and period T3) flows from point a to point b in FIG. 61 and 63 generate magnetic force according to the magnitude of the current. And the electromagnetic coil 61
, 63 and the magnetic poles formed by the permanent magnets 65, 67. As a result, the support shaft 57 integral with the magnet member 59 swings, and the cylindrical valve body 50 swings toward the fully open position shown by the two-dot chain line in FIG. The amount of swing (stable open position) is dut
It is changed by changing the y ratio.

[0018] Also, if the driver has a low voltage level "L"
When a signal is input, the driver switches FETs 70A and 70D.
The source voltage of the FET 70C is set to the "L" level, the source voltage of the FET 70C is set to the "H" level, and the source voltage of the FET 70B is switched between the "H" and "L" levels by PWM control. Therefore, a negative drive voltage is applied, and a current having a magnitude determined by the duty ratio (ratio between period T5 and period T6) flows from point b to point a in FIG. swings in the opposite direction. As a result, the cylindrical valve body 50 swings toward the fully closed position shown by the two-dot chain line in FIG. The amount of rocking (closed stable position) is changed by changing the duty ratio.

Further, when the electromagnetic coils 61 and 63 are de-energized, the magnet member 59 is held only by the magnetic poles of the permanent magnets 65 and 67. Therefore, the cylindrical valve body 50 swings to the neutral position (half-open position) and becomes stable, as shown by the solid line in FIG. The neutral position is a position where the rotor magnet and control are stable by cogging torque (detent torque) when the actuator 21 is not energized.

Therefore, in the above configuration, the stable open position is determined by the ratio of the period T2 and the period T3.
If T2 is long, it will be in a fully open state, and as the period T2 is shortened, it will become stable at a position approaching a half-open state. Then, if no current is applied, that is, period T2 = 0, the neutral position,
That is, it becomes a half-open state. When setting the opening position to an open position where the degree of opening is between the neutral position and the fully closed position, the ratio of periods T5 and T6 is changed. As the period T6 is lengthened, it becomes stable at a position closer to the fully closed state. In this way the duty
By changing the ratio, the intake control valve 19 can be opened to an arbitrary degree.

It should be noted that the periods T1 and T4 are used to compensate for the rise characteristics of the control valve. By increasing the widths of the periods T1 and T4, the moving speed of the intake control valve 19 increases and responsiveness improves, as shown in diagram A in FIG. 8. Period T
1. If the width of T4 is short, the response is not good as shown in diagram B. Further, the driver input signal in FIG. 7 is a signal input from the ECU 5 to the driver, and is a signal for selecting the swinging direction of the cylindrical valve body 50 from the open side and the closed side.

The operation of the intake control device having the configuration described above will be explained below. The intake air amount Q for each cylinder is based on the input signal (intake pressure P) from the sensor 31 that detects the air amount for each cylinder, and the control signal (intake time T) of the intake control valve 19 output to the actuator 21. Of these, the intake pressure P is determined by the opening degree of the throttle valve 13, and the intake time T is determined by the intake control valve 19 and the intake valve 9. FIG. 9 shows the relationship between the opening degree of the throttle valve 13 and the amount of accelerator depression. As shown in the figure, in the embodiment, the opening degree of the throttle valve 13 is suppressed to a constant value when the amount of accelerator depression exceeds a predetermined value. The basic closing timing of the intake control valve 19 is set as shown in Table 1. As shown in Table 1,
The closing timing of the intake control valve is basically adjusted depending on the load. In Table 1, the valve closing timing indicates the amount of advance relative to the bottom dead center at the end of the intake stroke. Furthermore, although the above control substantially controls the closing timing of the intake valve 9 using the intake control valve 19,
The actual intake time may be controlled by opening and closing the intake control valve within the opening time of the intake valve 9.

[0023]

[Table 1]

The valve opening timing is given as shown in Table 2. That is, the valve opening timing is set according to the engine speed, and the valve opening timing in Table 2 indicates the amount of advance relative to the top dead center at the end of the compression stroke. Opening/closing control of the intake control valve 19 is executed as follows with respect to the above-mentioned opening/closing valve timing.

[0025]

[Table 2]

FIG. 10 shows a timing chart of the driving voltage of the actuator 21, and FIG. 11 shows a timing chart of the operation of the intake valve 9 and the intake control valve 19. For comparison, FIG. 11 shows the operation of the intake control valve according to the conventional device using a chain double-dashed line.

As explained above in relation to the basic operation of the control circuit (FIG. 6), the intake control valve 1 is controlled depending on the duty ratio.
The opening degree of 9 is set arbitrarily. Therefore, Fig. 10 (B
), when the period T3 during which a positive drive voltage is applied is set to the value 0, the operation of the intake control valve 19 becomes as shown in diagram A in FIG. 11, and the intake control valve 19 is in the neutral position. oscillate. Therefore, the opening degree becomes half open, and in order to obtain a predetermined amount of intake air, the closing completion timing of the intake control valve 19 is shifted closer to the bottom dead center BDC than in the past.

Furthermore, as shown in FIG. 10(A), when the period T3 during which a positive drive voltage is applied is made shorter than the period T6 during which a negative drive voltage is applied for full closing, the intake control valve The operation of No. 19 is as shown in diagram B of FIG. The intake control valve 19 is stabilized between the half open position and the fully open position. As a result, when the intake control valve 19 is completely closed, it deviates toward the bottom dead center BDC side compared to the conventional case in order to obtain a predetermined amount of intake air, but the amount of deviation is smaller than when it is half-open.

Furthermore, as shown in FIG. 10(C), a positive drive voltage is applied for a period T1 for rising, and thereafter a negative drive voltage is applied, and a negative drive voltage for fully closing is applied. When the period T8 during which a negative drive voltage is applied is made shorter than the period T6 during which a drive voltage of 1 is applied, the operation of the intake control valve becomes as shown in line C in FIG. The intake control valve 19 is stabilized between a half-open position and a fully closed position. As a result, in order to obtain a predetermined amount of intake air, when the intake control valve 19 is completely closed, it is further shifted toward the bottom dead center BDC side than when it is half-open.

In addition to the above, by changing the duty ratio, the valve can be controlled to close from a half-open state at a low speed, as shown in line D in FIG. 11, or to be fully open, as shown in line E. After moving from the position to the half-open state and once holding it, control is realized to change the state from the half-open state to the fully closed state.

Among the controls shown in each of the diagrams A to E above, the timing chart of FIG. 12 shows the timing chart of FIG. The relationship between the operation and the pressure at the upstream portion 17a of the intake port is shown.

[0032] In the control for one cycle of period, the following phenomenon occurs. Since each intake control valve 19 closes the intake port 17 during the intake stroke of each cylinder 7, the pressure in the upstream section 17a greatly drops from atmospheric pressure to negative pressure as shown in the figure.
The minimum value is reached at the bottom dead center of the piston. Thereafter, as the piston rises, the pressure slightly returns to the atmosphere side, but when the intake valve 9 is closed, the pressure in the upstream portion 17a is maintained at negative pressure. Next, after the intake valve 9 is closed,
When the intake control valve 19 is opened again, air suddenly flows into the upstream section 17a from upstream of the intake port 17, and the air suddenly flows into the upstream section 17a.
The pressure at 7a rises rapidly and causes pressure fluctuations near atmospheric pressure.

In the above cycle, the intake control valve 19
Since the stable open position of is half open, each intake control valve 1 is opened during the intake stroke of each cylinder 7 in order to obtain a predetermined amount of intake air.
The timing to fully close 9 is delayed on the BDC side. As a result, the period of adiabatic expansion is shortened, and the degree of decrease in the temperature of the air within the cylinder is reduced. Furthermore, since the timing at which the intake control valve 19 stops sucking air in the middle of the intake stroke is delayed, the degree of reduction in swirl is reduced. As a result, combustion becomes better. Note that although the pumping loss increases, the amount of increase is very small. Figure 13 shows the PV diagram of this engine. The pumping loss of the engine is the area surrounded by the symbol AHBXG in the figure. The pumping loss of the conventional intake control device is the area surrounded by the symbol GHBX in the figure. The amount of increase in pumping loss is slightly with the sign AHG
It is only a small area surrounded by This is sufficiently small compared to the pumping loss (the area surrounded by the symbol AJBX in the figure) when the intake control valve 19 is not used. In other words, by delaying the closing timing of the intake control valve 19 through half-open control of the intake control valve 19, it is possible to sufficiently reduce the pumping loss while minimizing the reduction in combustion temperature and swirl.

The operation of the ECU 5 during the above-mentioned intake valve control will be explained below based on the flowchart of FIG. 14. This routine is an interrupt process executed in the ECU 5 at every bottom dead center of the intake stroke, and the case shown in diagram A in FIG. 11 is illustrated.

When this routine is started, first, the engine speed and load are read (step S10), and then, based on the engine speed and load, maps corresponding to Tables 1 and 2 stored in advance in the ROM 5b are referred to. The opening timing and closing timing are read (step S20). In addition, in Tables 1 and 2, the opening timing and closing timing are shown by the amount of advance from intake bottom dead center or intake top dead center, but in the above map, the opening timing and closing timing are shown by the advance angle from intake bottom dead center corresponding to the above-mentioned opening timing and closing timing. A timer count is used.

Next, the open side condition and the close side condition are read (step S30). Here, the open and close conditions refer to T1 to T6 shown in FIG. This routine exemplifies the case of diagram A in FIG. 11, where the open condition is half open and the close condition is fully closed.

By the way, as described above, the opening degree of the intake control valve is determined by the ratio of T2 and T3 and the ratio of T5 and T6. The relationship between this ratio and the control valve opening degree is shown in FIG. That is,
When T3/(T2+T3) is about 40%, it is fully open, and when -T6/(T5+T6) is about -40%, it is fully closed. Therefore, when the valve is half open, T3 has a value of 0, and when it is fully closed, -T6/(T5+T6) is approximately 40%. Further, T1 and T4 have values corresponding to the absolute value of the above ratio.

When the open condition (half-open) and close condition (fully closed) are read in step S30, the timer starts counting (step S40), and it is determined whether the count value corresponds to the opening timing (step S50). If the count value corresponds to the opening timing and the determination is "YES", an open signal (half-open signal) is output to the actuator 21 (S70).

Further, if the count value does not correspond to the open state and the determination is "NO" in step S50, a close signal (fully closed signal) is output (step S60), and the count value corresponds to the open state. Output of the closed signal (step S60
)repeat.

Next, it is determined whether the count value corresponds to the closing time (step S80). If the count value corresponds to the closing timing and it is determined as "YES", a closing signal (fully closed signal) is output to the actuator 21 (step S90).
. Since the count value does not correspond to the closing timing, "N" is returned in step S80.
If it is determined as "O", the process returns to step S70 and an open signal is output, and the open signal output in step S70 is repeated until the count value corresponds to the closing timing.

According to the intake control device for an internal combustion engine described above, since the timing of completion of closing of the intake control valve 19 can be delayed, the effect of reducing the pumping loss can be substantially maintained while reducing the combustion temperature and reducing the swirl. This has an excellent effect in that it is possible to suppress the decrease in fuel consumption, improve combustion, and significantly improve fuel efficiency. Further, since combustion is stabilized, there is an advantage that drivability is improved. Note that the control that delays the closing timing of the intake control valve 19, which achieves these effects, reduces the average opening degree of the intake control valve 19 to an opening degree smaller than the fully closed degree, while ensuring the same amount of intake air as during the fully open control. Regarding the control, various patterns of control can be considered as shown in the diagrams A to E in FIG. 11.

Furthermore, in a configuration in which control is performed to set the opening degree of the intake control valve 19 to half-open (diagram A in FIG. 11), the half-open state can be maintained without supplying electric power. This has the effect of reducing power consumption required for drive control. In addition, a configuration in which the opening degree is set between half-open and fully closed has a smaller amount of swing to reach fully closed, so power consumption is slightly lower than a configuration in which the opening is set between half-open and fully open. However, both are larger than the half-open control configuration described above.

Although the embodiments have been described above, the present invention is not limited to the embodiments in any way, and it goes without saying that it can be implemented in various forms without departing from the spirit of the invention. For example, as an intake control valve, JP-A-63-6
The configuration described in Japanese Patent No. 5138, in which the intake passage is opened and closed by moving a plurality of plate members, may also be used. Although the throttle valve 40 has been described as being fully open, it may be at a constant position close to fully open. Alternatively, the opening/closing control of the intake control valve (
The intake air amount may be controlled by combining both the opening/closing time (determining the opening/closing time) and the opening degree of the throttle valve 40 (determining the air density). For example, a graph defining the relationship (stepwise relationship or continuous relationship) between the intake control valve opening and the load (throttle opening) illustrated in FIG. 16 is stored in the memory as a map. By referring to this map and setting the opening degree of the intake control valve 19 according to the load, the timing at which the intake control valve 19 completes closing may be delayed. The valve closing completion timing may be 80 to 120 degrees before bottom dead center (BDC). Further, it is sufficient to find through testing the optimum valve closing completion timing that reduces pumping loss while improving combustion. The opening degree of the intake control valve 19 is influenced more by the above-mentioned load than the rotation speed, but if it is configured to be controlled in relation to the rotation speed, and considering the condition that the amount of intake air is constant, the opening timing of the intake valve 9 will be Since it decreases in proportion to the engine speed, the closing start time of the intake timing control valve 19 can be set in a linear relationship as shown in FIG.

[0044]

As explained above, according to the intake control device for an internal combustion engine of the present invention, the opening degree of the intake control valve is maintained at an opening degree smaller than full open, compared to the conventional device which controls the intake control valve to be fully open. This shortens the adiabatic expansion period by delaying the time when the intake passage is completely closed by the intake control valve, so it is possible to suppress the reduction in combustion temperature and swirl while maintaining most of the pumping loss reduction effect. This has the excellent effect of making it possible to significantly improve fuel efficiency.

[Brief explanation of the drawing]

FIG. 1 is a block diagram illustrating the basic configuration of the present invention.

FIG. 2 is a block diagram showing an entire engine system to which an air intake control device according to an embodiment of the present invention is applied.

FIG. 3 is a sectional view showing the structure of an intake control valve.

FIG. 4 is a cross-sectional view of FIG. 3 taken along line 4-4.

FIG. 5 is an explanatory diagram showing the structure of an intake control valve.

FIG. 6 is a circuit diagram showing a control circuit of an actuator of an intake control valve.

FIG. 7 is a timing chart showing the relationship between the input signal of the control circuit of the actuator of the intake control valve and the drive voltage.

FIG. 8 is a timing chart showing the operation of the intake control valve obtained by the embodiment device.

FIG. 9 is a graph showing the relationship between throttle opening and accelerator depression amount.

FIG. 10 is an explanatory diagram showing an aspect of the driving voltage of the intake control valve.

FIG. 11 is a timing chart showing the operation of the intake control valve.

FIG. 12 is a timing chart showing the relationship between the operation of the intake control valve and the pressure upstream of the intake port.

FIG. 13 is a PV diagram obtained by the example device.

FIG. 14 is a flowchart showing interrupt processing performed in the ECU.

FIG. 15 is a characteristic diagram showing the relationship between duty ratio and control valve opening degree.

FIG. 16 is a graph showing the relationship between intake control valve opening and load.

FIG. 17 is a graph showing the relationship between the intake control valve opening degree and the intake valve closing timing.

FIG. 18 is a timing chart showing the relationship between the operation of the intake control valve and the pressure upstream of the intake port in a conventional device.

[Explanation of symbols]

Claims (1)

[Claims] 1. An intake control device for an internal combustion engine, wherein an intake control valve provided in an intake passage of each cylinder of the internal combustion engine is opened and closed at a predetermined timing by an intake control valve control means to control an intake air amount. An intake control device for an internal combustion engine, comprising an average opening reducing means for reducing the average opening of the intake control valve during a valve opening period by the valve control means to an opening smaller than a full opening.