Supercharging pressure control system for an automotive engine
BACKGROUND OF THE INVENTION 
     The present invention relates to a system for controlling supercharging pressure in an automotive engine having a turbocharger, and more particularly to a system for electronically controlling the operation of a wastegate valve. 
     In an automotive engine having a turbocharger, the wastegate valve is provided in a bypass around a turbine of the turbocharger so as to divert exhaust gas away from the turbine so that the supercharging pressure is kept substantially constant to prevent knocking of the engine. The wastegate valve is generally controlled by a feedback control system in accordance with a desired valve. 
     Japanese Patent Application Laid-Open 61-55316 discloses a feedback control system wherein the difference between an actual supercharging pressure detected by a supercharging pressure sensor and a desired intake pressure is calculated and the supercharging pressure is controlled by a PI control dependent on the calculated difference. 
     However, when the vehicle is rapidly accelerated, the control operation of the wastegate valve delayed with respect to the opening speed of the throttle valve of the vehicle, which causes extremely high supercharging pressure because of the overshooting of the control. 
     If a feedforward control is employed, such an overshooting can be reduced. However, the feedforward control can not satisfactorily control the supercharging pressure to a desired value. Therefore, it is preferable to correct the deviation of the controlled supercharging pressure from the desired value with the feedback control. However, if both the controls are effected at the same time, overshoot values in a transient state are added which excessively increases the supercharging pressure. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a control system where excessive supercharging pressure may be decreased. 
     The other objects and features of this invention will be apparently understood from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic diagram showing a system of the present invention; 
     FIG. 2 is a block diagram showing a control unit of the present invention; 
     FIGS. 3a and 3b show tables storing basic duty ratios and increments and decrements of feedback duty ratio, respectively; 
     FIGS. 4a and 4b are flowcharts showing the operation of the present invention; and 
     FIG. 5 is a graph showing characteristics of the duty ratio and supercharging pressure at rapid acceleration. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a turbocharger 4 includes a compressor 4a having an impeller and a turbine 4b consisting of a turbine wheel. An inlet port of the compressor 4a is communicated with an air cleaner 2 through an intake pipe 3, and an outlet port thereof is communicated with cylinders of an engine 1 through a throttle valve 5, a chamber 6 and an intake manifold 7. The turbine 4b is located between an exhaust manifold 8 and an exhaust pipe 9. The impeller of the compressor 4a is mounted on the same shaft as the turbine wheel. When the turbine 4b is operated by exhaust gas from the engine 1 through the exhaust manifold 8, the impeller is rotated by the turbine for compressing the intake air. Thus, the compressed intake air is supplied to the engine cylinders. 
     An exhaust bypass 12 is provided around the turbine 4b, and a wastegate valve 11 which is operated by a diaphragm operated actuator 13 is provided in the bypass 12. A chamber of the diaphragm operated actuator 13 is communicated with the intake pipe 3 through a passage 17, a three-way solenoid operated valve 16 and a passage 15 downstream of the compressor 4a. 
     The solenoid operated valve 16 has a solenoid 16a and a valve body 16b. Upon energization of the solenoid 16a in response to a control signal in the form of pulses applied from a control unit 30, the valve body 16b is moved to open a pressure supply port 16c communicated with the passage 15 and to close leak port 16d communicated with an upstream portion of the compressor 4a through a passage 18 having an orifice 19. Thus the intake pressure is applied to the actuator 13 through the passage 15, valve 16 and passage 17. When the pressure of the intake air becomes higher than a predetermined value, the diaphragm of the actuator 13 is deflected to open the wastegate valve 11. To the contrary, when the leak port 16d is opened, the intake air leaks therefrom so that the actuator 13 is applied with a low pressure to close the wastegate valve 11. The opening quantity of the wastegate valve 11 is thus dependent on the duty ratio of the control pulse signal fed from the control unit 30 to the solenoid 16a. 
     The turbocharger 4 further has a vane 4c at the inlet of the turbine 4b. The vane 4c is operated by a diaphragm operated actuator 14, a chamber of which is also communicated with the valve 16 through the passage 17. When a high pressure is applied to the chamber of the actuator 14, the diaphragm of the actuator 14 is deflected to rotate the vane 4c so as to decrease the opening area of the exhaust pipe 9 at the vane. Thus, the exhaust gas is discharged passing through the wastegate valve 11 without driving the turbine 4b. 
     The system is further provided with an intake pressure sensor 21 downstream of the throttle valve 5, an engine speed sensor 22 and a throttle position sensor 23, outputs of which are supplied to the control unit 30. 
     Referring to FIG. 2, the control unit 30 has, as a feedforward control system, a basic duty ratio determining section 31 to which engine speed Ne from the engine speed sensor 22 and intake pressure Pb from the intake pressure sensor 21 are applied. At the duty ratio determining section 31, a basic duty ratio D B  for the pulse signal is derived from a basic duty ratio table 32 which is schematically shown in FIG. 3a. The values of the basic duty ratios D B  are determined so as to control the pressure Pb to a limit intake pressure Pt in steady driving states under various conditions dependent on the intake pressure Pb and the engine speed Ne. The limit pressure Pt is set to, for example, a maximum pressure for the full throttle state of the engine. Namely, in a low engine speed and low intake pressure range, the value of the basic duty ratio D B  is large so that the solenoid valve 16 is operated to open the leak port 11d. The actuating pressure applied to the actuators 13 and 14 is decreased, thereby closing the wastegate valve 11 and rotating the vane 4c to increase the opening area. As the engine speed Ne and the intake pressure Pb increase, the actuating pressure is increased, opening the wastegate valve 11 and decreasing the opening area at the vane 4c. 
     The control unit 30 further has, as a feedback control system, a pressure difference calculator 34 to which the intake pressure Pb is fed for calculating the difference (error) ΔP between the limit pressure Pt and the actual intake pressure Pb in accordance with ΔP=Pb-Pt. The difference ΔP is applied to a feedback duty ratio calculator 35 where a feedback duty ratio D FB  is calculated in dependency on the error. The feedback duty ratio D FB  includes a proportional plus integral (PI) component. The feedback duty ratio D FB  is represented as follows. 
     
         D.sub.FB =D.sub.In +Dp 
    
     where D In  is an integral component and Dp is a proportion component. D In  is represented as 
     
         D.sub.In =Do+ΔI 
    
     where Do is an integral component calculated at the last routine of a program, and ΔI is an increment of the integral component. The proportion component Dp and the increment ΔI are stored in a table 36 in dependency on the difference ΔP as shown in FIG. 3b. In the table, Dpu and ΔIu are provided for increasing the duty ratio D FB  and Dpd and ΔId are provided for decreasing the ratio D FB . Each of the proportion components and increments is an increasing function of the difference ΔP. 
     The feedback duty ratio D FB  thus calculated is fed to a duty ratio calculator 33 to which the basic duty ratio D B  is also fed to calculate a duty ratio D in accordance with 
     
         D=D.sub.B +D.sub.FB 
    
     A control pulse signal having the duty ratio D is applied to the solenoid operated valve 16 through a driver 37. 
     In accordance with the present invention, the control unit 30 further has a system for correcting the duty ratio when the vehicle is rapidly accelerated. A rapid acceleration determining section 38 determines the rapid acceleration of the vehicle when a throttle opening degree θ detected at the throttle position sensor 23 is larger than a predetermined reference degree θ1 (θ≧θ1). 
     In addition a pressure peak detector 40 is provided to detect that the intake pressure passed a peak. The detector 40 detects the pressure peak when pressure changing rate dPd/dt is smaller than zero (dPb/dt&lt;0), and feeds a pressure peak signal to a control mode determining section 39. The determining section 39 is provided with a timer to calculate a time T from the detection of the rapid acceleration to the detection of the pressure peak. 
     The control operation is described hereinafter with reference to FIGS. 4a, 4b and 5. The turbine 4b of the turbocharger 4 is driven by the exhaust gas, thereby driving the compressor 4a. The intake air is compressed by the compressor and supplied to the cylinders of the engine. At the partial load state of the engine represented as θ&lt;θ 1  (step 101), in order to maintain the intake pressure below the limit pressure Pt, the control mode determining section 39 decides to execute the feedforward control and the feedback control only for decreasing the actual intake pressure (step 115). Then, as the feedforward control, one of the basic duty ratios D B  is derived from the table 32 in dependency on the intake pressure Pb and the engine speed Ne. On the other hand, since the optimum intake pressure at the partial load is below the limit pressure Pt, the feedback duty ratio D FB  calculated at the feedback duty ratio calculator 35 is zero, D FB  =0. Accordingly, the duty ratio D depending on the basic duty ratio D B  is outputted from the duty ratio calculator 33 to operate the solenoid operated valve 16. The actuating pressure controlled by the solenoid operated valve is applied to the actuators 13 and 14 through the passage 17 to actuate the wastegate valve 11 and the vane 4c. When the intake pressure Pb and the engine speed Ne are low, the basic duty ratio D B  is large, so that opening period of the leak port 16d is much longer than closing period. As a result, a low pressure is applied to the actuator 13 so that the diaphragm is urged by a spring of the actuator to close the wastegate valve 11. On the other hand, the vane 4c is operated by the actuator 14 to increase the opening area of the exhaust pipe 9 at the vane. Thus a large quantity of air is supplied to the turbine 4b to increase the supercharging pressure. 
     As the intake pressure Pb and the engine speed Ne increase, the ratio D B  derived from the table 32 decreases so that the pressure applied to the actuators 13 and 14 rises. Thus the wastegate valve 11 opens a predetermined amount and the vane 4c is rotated to decrease the amount of air supplied to the turbine 4b. Therefore, the intake pressure Pb is kept below the limit pressure Pt, thereby preventing damaging of the engine 1. 
     If the intake pressure Pb exceeds the limit pressure Pt at the partial load, the feedback duty ratio calculator 35 calculates a feedback duty ratio D FB  based on the proportion decreasing component Dpd and integral decrement ΔId derived from the table 36 in dependency on the difference ΔP which is calculated at the calculator 34. The feedback ratio D FB  for decreasing the intake pressure Pb is added to the basic duty ratio D B  at the calculator 33 to obtain a corrected duty ratio D. Thus the duty ratio D is decreased stepwisely, thereby discharging the exhaust gas through the wastegate valve 11 to rapidly decrease the intake pressure. 
     The feedback control operation at the rapid acceleration is described hereinafter with reference to FIGS. 4a, 4b and 5. 
     Referring to FIG. 4a, when the throttle opening degree θ is smaller than the reference degree θ1 at a step 101, a decreasing feedback duty ratio is added to the basic duty ratio and the above-described operation is performed at a step 115. 
     When the throttle opening degree θ exceeds the reference degree θ1, the timer in the section 39 is set at a step 104. 
     When it is determined that the actual pressure Pb is between reference pressures P2 and P1, (steps 105, 106), the program proceeds to a step 108 through a step 107. 
     Time T from the detection of the rapid acceleration (θ≧θ 1 ) to the detecting of the peak is compared with a reference time To at a step 108. If the time T is shorter than the time To, (T≦To), the program goes to a step 109 where it is determined whether the peak of the intake pressure is detected by the detector 40. When the peak is detected, the program goes to a step 110 where a predetermined short time Δt after the peak of the pressure is counted to delay for improving the reliability of the control. After the time Δt, the program proceeds to a step 112 where an ordinary PI feedback control is carried out, so that the intake pressure Pb is maintained approximately at the limit pressure Pt in accordance with increasing and decreasing the intake pressure by feedback control until the throttle opening degree lowers under the reference degree θ1. 
     When the time T is larger than the reference time To, the program goes to the step 112 where the ordinary feedback control is performed, so that the intake pressure Pb immediately converges to the limit pressure Pt. 
     In the period T+Δt, as shown in FIG. 5, i.e. when Pd≦P 2 , Pd≧P 1 , or when dPd/dt≧0, the program goes to the step 115 to perform the pressure decreasing feedback control dependent on the proportion decreasing component Dpd and the decrement ΔId. 
     The above-described system may be arranged such that the basic duty ratio D B  and the feedback duty ratio D FB  are derived further in accordance with coolant temperature and atmospheric pressure. 
     According to the present invention, since the intake pressure is controlled in accordance with the basic duty ratio for producing the optimum intake pressure at the partial load state, stable control is maintained. Furthermore, since the feedback control for increasing and decreasing the intake pressure is executed after the detection of the peak of the intake pressure in the acceleration state, the overshooting of the intake pressure from the limit pressure is immediately reduced and the intake pressure converges to the limit pressure. 
     While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.