Abstract:
A control system for operating vanes of a turbocharger turbine ( 16 T) and for operating a turbine-shunting bypass valve ( 22 ) according to a strategy wherein a processor executes an algorithm for selectively unenabling the control system to operate the bypass valve when the control system is operating the vanes to adjust exhaust back-pressure on the engine within a range of effectiveness for the vanes to control the exhaust back-pressure and enabling the control system to operate the bypass valve when the control system has operated the mechanism to a limit of the range of effectiveness.

Description:
FIELD OF THE INVENTION 
     This invention relates to turbocharged internal combustion engines, particularly a motor vehicle diesel engine that has a two-stage turbocharger where one turbine stage has variable geometry. 
     BACKGROUND OF THE INVENTION 
     Turbocharged diesel engines are common powerplants of trucks that are presently being built. A known turbocharged engine comprises a two-stage turbocharger that comprises high- and low-pressure turbines in series flow relationship in the exhaust system that operate high- and low-pressure compressors in series flow relationship in the intake system to develop boost. The high-pressure turbine of a particular type of two-stage turbocharger has vanes that can be controlled by an actuator to control both torque that operates the high-pressure compressor and exhaust back-pressure. Such a turbocharger is sometimes called a variable geometry turbocharger, or VGT for short. 
     The high-pressure VGT stage is typically designed to have a relatively smaller size that is optimized for low-end engine performance while the low-pressure stage is typically designed with a relatively larger size for high-end performance. The high-pressure stage has the ability to respond well to transient demands at lower engine speeds and is the main contributor to boost over that speed range. At higher speeds, and at larger loads, the low-pressure stage becomes the main contributor to boost because it can provide the necessary greater air-handling capacity. Over a portion of an engine operating range, the high-pressure stage may however interact with the low-pressure stage in ways that affect turbocharger performance. 
     Compensation for such interaction can be achieved by the inclusion of two bypass valves, one shunting the high-pressure compressor stage and another shunting the high-pressure turbine stage. By opening in the higher speed and load range to shunt flows around the high-pressure stages, the bypass valves prevent the high-pressure stages from choking the flows. 
     The operation of each bypass valve is controlled in concert with operation of the other, and their operation is coordinated with control of the VGT vanes. The engine control system processes various data according to algorithms to provide control functions for the VGT vanes and the bypass valves such that exhaust back-pressure and engine boost are regulated in a way deemed appropriate for the manner in which the engine is being operated. 
     For various reasons that bear on engine performance and/or emission control, the ability to accurately control exhaust back-pressure is important to an engine control strategy. A typical strategy processes various data to develop a data value for a desired set-point for exhaust back-pressure. Changes in engine operation that affect that set-point typically call for the control system to respond promptly and accurately to force the actual exhaust back-pressure to follow the changes in the desired set-point. 
     In the lowest speed range, exhaust back-pressure can be controlled entirely by control of the VGT vanes. When the engine operating conditions change such that exhaust back-pressure can no longer be controlled solely by the VGT vanes, the bypass valves should open. It is desirable that the transition from VGT control to bypass valve control, and vice versa, occur in ways that avoid interactions between the VGT vanes and the bypass valves that would result in undesired effects on control accuracy, such as delayed response, pressure spikes, etc. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method for a coordinated control strategy for a VGT and associated bypass valves that strives to avoid such undesired effects in control of the exhaust back-pressure set-point. 
     The invention is effective to control the opening of the bypass valves as engine speed and load increase beyond the range where exhaust back-pressure can be controlled by the VGT vanes alone, and similarly to control their closing as speed and load return to the range where the VGT vanes alone can be effective to control back-pressure. 
     While the disclosed preferred embodiment of the invention relates to a two-stage turbocharger having a variable geometry high-pressure turbine, the most general principles of the invention are believed applicable to certain other turbocharger configurations. 
     A generic aspect of the present invention relates to an internal combustion engine comprising an intake system for developing charge air for the engine, combustion chambers in which admitted charge air and injected fuel combust to operate the engine, an exhaust system for conveyance of exhaust gas resulting from combustion from the combustion chambers, a two-stage turbocharger comprising an upstream compressor upstream of a downstream compressor in the intake system operated respectively by a downstream turbine downstream of an upstream turbine in the exhaust system, a turbine-shunting bypass valve shunting one of the turbines with which a mechanism for adjusting exhaust back-pressure that the turbocharger is creating on the engine is associated, and a control system. 
     The control system comprises a processor that processes data to provide control data for operating the mechanism and for operating the turbine-shunting bypass valve. The processor executes an algorithm for selectively unenabling the control system to operate the bypass valve when the control system is operating the mechanism to adjust back-pressure within a range of effectiveness for the mechanism to control exhaust back-pressure and enabling the control system to operate the bypass valve when the control system has operated the mechanism to a limit of the range of effectiveness. 
     A further generic aspect of the present invention relates to the method that is inherent in the one generic aspect described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general schematic diagram of an engine comprising a two-stage turbocharger and associated bypass valves controlled by a strategy in accordance with principles of the present invention. 
         FIGS. 2A and 2B  collectively show a first portion of a software strategy diagram representing algorithms programmed in an engine control system in accordance with principles of the present invention. 
         FIG. 3  shows a second portion of the software strategy diagram. 
         FIG. 4  shows a third portion of the software strategy diagram. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows an exemplary internal combustion engine  6  having an intake system  8  through which air for combustion enters the engine and an exhaust system  10  through which exhaust gasses resulting from combustion of air-fuel mixtures in engine cylinders  11  exit the engine. Engine  6  is by way of example, a turbocharged diesel engine comprising a two-stage turbocharger  12  that has a low-pressure stage  14  and a high-pressure stage  16 . 
     Air drawn into intake system  8  follows a path indicated by arrows  18 , leading first to a compressor  14 C of low pressure stage  14 . A compressor  16 C of high-pressure stage  16  is in downstream series flow relationship to compressor  14 C and is shunted by a normally-closed valve  20  that may at times be referred to as a bypass valve or a bypass control valve. From compressor  14 C there are two possible paths for airflow, one through compressor  16 C, the other through bypass valve  20  when open. The charge air then enters an intake manifold  21  to which cylinders  11  are open when associated cylinder intakes valves are open. Fuel is injected into cylinders to combust with the charge air and release energy for powering the engine. Exhaust gasses from combustion exit through exhaust system  10 . 
     The exhaust gasses leaving cylinders  11  entrain in an exhaust manifold  23  from whence they pass through exhaust system  10 , as marked by arrows  24 . From manifold  23 , there are two possible flow paths for the exhaust gasses. One is through a high-pressure stage turbine  16 T of stage  16 . The other is through a bypass valve  22  that shunts stage  16 T and that although normally closed, passes flow when operated open. Exhaust gasses then pass through a turbine  14 T of stage  14  before exiting exhaust system  10 . 
     Bypass valves  20  and  22  are proportional valves controlled by the engine control system (ECS). The engine control system processes various data to control valves  20  and  22  such that exhaust back-pressure, and consequently engine boost, are regulated in a appropriate manner according to the manner in which the engine is being operated. 
     By keeping bypass valves  20  and  22  closed during lower-speed engine operation the entire exhaust gas flow passes through both turbines  16 T,  14 T, and the entire charge air flow passes through both compressors  14 C,  16 C. In that speed range, exhaust back-pressure can be adequately controlled by an actuator that controls vanes of turbine stage  16 T. 
     At speeds beyond the lower-speed range where the VGT vanes have reached their control limit, valves  20  and  22  open to an extent controlled by the engine control system to achieve desired boost and exhaust back-pressure. 
     The inventive strategy is embodied in one or more processors of the engine control system as algorithms for processing data. Through coordinated control of the VGT vanes and bypass valves  20  and  22 , the strategy controls the set-point for exhaust back-pressure. 
     A parameter TCBC_PWM, shown in  FIG. 4 , controls valves  20  and  22 . The data value for TCBC_PWM represents the duty cycle of a pulse width modulated signal that is applied to electric actuators of the two valves to control the extent to which they are open. The data value is developed by the engine control system&#39;s processing of various data in accordance with the strategy shown in  FIGS. 2A ,  2 B,  3 , and  4 . The strategy employs various maps, or look-up tables, and various processing functions. Input data, which includes indicated engine torque TQI_BPA and engine speed N, is obtained from sensors, calculated from other data, and/or obtained from some other appropriate source. 
     When engine operation allows exhaust back-pressure to be controlled solely by the VGT vanes, only VGT control is used. It is when the VGT vanes reach their control limit that TCBC control is needed. TCBC control is enabled by the setting of a TCBC flag LV_TCBC_EN seen in  FIG. 2B  where a switch function  52  serves to selects one of two sub-strategies for enabling TCBC using a selection parameter LC_TCBC_EN_SEL. 
     When the data value for LC_TCBC_EN_SEL is a logic “0”, a sub-strategy SS 1  ( FIG. 2A ) that is based on the exhaust back-pressure set-point EGBP_SP, engine speed N, and engine torque TQI BPA is selected. When the data value for LC_TCBC_EN_SEL is a logic “1”, a sub-strategy SS 2  ( FIG. 2A ) that is based on engine speed N, engine torque TQI_BPA, and several additional parameters is selected. Which of the two sub-strategies is selected typically depends on the particular engine and how it has been calibrated for a particular engine model in a particular motor vehicle. 
     Sub-strategy SS 1  comprises a map, or look-up table,  34  and a comparison function  36 . Map  34  is populated with data values for EGBP_SP_MAX_TCBC_EN each correlated with a respective pair of data values representing a particular engine speed range and a particular engine torque range. Current data values for engine speed N and indicated torque TQI BPA cause the corresponding data value for EGBP_SP_MAX_TCBC_EN to be selected as the input to comparison function  36 . The parameter EGBP_SP_MAX_TCBC_EN represents an exhaust back-pressure, based on current engine speed and torque, that can be satisfied by VGT control alone without invoking turbocharger bypass control (TCBC) using valves  20  and  22 . Comparison function  36  compares the selected data value for EGBP_SP_MAX_TCBC_EN with a data value for the exhaust back pressure set point (parameter EGBP_SP) for the purpose of enabling TCBC control by setting LV_TCBC EN (the TCBC enable flag) to a logic “1” whenever VGT control becomes incapable of satisfying the exhaust back-pressure set-point by itself. As long as VGT control is capable of controlling exhaust back-pressure by itself, TCBC control remains unenabled. 
     Stated another way, comparison function  36  compares a data value representing a set-point for desired exhaust back-pressure (EGBP_SP) and a data value (EGBP_SP_MAX_TCBC EN) representing a set-point defining a maximum that is based on at least one parameter indicative of current engine operation (the embodiment shown here uses two—speed and torque) and that needs to be exceeded by the set-point for desired exhaust back-pressure in order to enable the control system to operate the bypass valves. The strategy will enable the control system to operate the bypass valves when the comparison function discloses that the set-point for desired exhaust back-pressure exceeds the defined maximum. 
     Sub-strategy SS 2  comprises two maps, or look-up tables,  30 ,  32 , four comparison functions  38 ,  40 ,  44 , and  46 , two AND logic functions  42 ,  48 , and a latch function  50 . Functions  38 ,  40 , and  42  coact to set latch function  50  when certain conditions are satisfied, and functions  44 ,  46 , and  48  coact to reset function  50  when certain other conditions are satisfied. 
     One necessary condition for setting latch function  50  is that the exhaust back-pressure error (meaning the difference between actual exhaust back-pressure and the exhaust back-pressure set-point) be less than a defined value of a parameter C_EGBP_DIF_TCBC_EN calibrated for the particular engine. The existence or non-existence of that condition is determined by comparison function  38 . 
     A second necessary condition is that the duty cycle output to the VGT control, represented by a parameter BPAPWM, be less than a duty cycle, based on current engine speed and load, represented by a parameter BPAPWM_MIN_TCBC_EN. This second condition assures that the VGT is being controlled in a way that, for current engine speed and torque, the use of TCBC control will not be counterproductive to attaining the desired exhaust back-pressure set-point. Map  30  is populated with data values for BPAPWM_MIN_TCBC_EN each correlated with a respective pair of data values representing a particular engine speed range and a particular engine torque range. Current data values for engine speed and indicated torque cause the corresponding data value for BPAPWM_MIN_TCBC_EN to be selected as the input to comparison function  40 . 
     AND logic function  42  will set latch function  50  when the two necessary conditions are simultaneously satisfied. Other conditions are necessary to reset latch function  50 . 
     The coaction of functions  38 ,  40 , and  42  causes latch function  50  to be operated to the set state when comparison function  38  discloses that the difference between the actual exhaust back-pressure and the set-point for desired exhaust back-pressure is disclosing a need to enable the control system to operate the bypass valves and when comparison function  40  at the same time is disclosing that the setting to which the adjustable vanes are being currently commanded is not within the range of settings within which the adjustable vanes can be effective by themselves in controlling exhaust back-pressure based on one or more current engine operating parameters, those parameters being speed and torque in this embodiment. 
     One necessary condition for resetting latch function  50  is that the exhaust back-pressure error EGBP_DIF_BPA be greater than the value of a parameter C_EGBP_DIF_TCBC_EXIT. The existence or non-existence of that condition is determined by comparison function  44 . 
     A second necessary condition is that the data value for TCBC_PWM, be greater than that of a parameter TCBC_PWM_MAX_EXIT obtained from map  32 . Map  32  is populated with data values for TCBC_PWM_MAX_EXIT each correlated with a respective pair of data values representing a particular engine speed range and particular engine torque range. Current data values for engine speed and indicated torque cause the corresponding data value for TCBC_PWM_MAX_EXIT to be selected as the input to comparison function  46 . 
     AND logic function  48  discloses when both conditions are simultaneously satisfied by resetting latch function  50 . 
     The coaction of functions  44 ,  46 , and  48  causes latch function  50  to be operated to the reset state when comparison function  44  discloses that the difference between the actual exhaust back-pressure and the set-point for desired exhaust back-pressure is disclosing a need to unenable the control system to operate the bypass valves and when comparison function  46  at the same time is disclosing that the setting to which the adjustable vanes are being currently commanded is within the range of settings within which the adjustable vanes can be effective by themselves in controlling exhaust back-pressure based on one or more current engine operating parameters. 
     When the selected sub-strategy, either SS 1  or SS 2 , has enabled TCBC control via switch function  52 , LV_TCBC_EN causes a switch function  54  to select a parameter EGBP_DIF_BPA for further processing. Two further switch functions  55 ,  56  assure that conditions are appropriate for actual use of EGBP_DIF_BPA. Switch function  55  is under the control of a parameter STAT_ENG_MODE_GES to assure that the engine has been started and is running. Switch function  56  is under the control of a parameter LV_LIH_TCBC_OL whose purpose is to indicate an actual or potential fault whose occurrence places the engine control system in what is referred to as a “limp-home” mode for the purpose of minimizing risk of potential damage because of the fault. This allows the engine to continue to operate so that the vehicle can be driven to a service facility for service to investigate the fault signal and make corrections as needed. 
     Hence, with the engine running and in the absence of any indicated fault, the parameter EGBP_DIF_BPA is subjected to processing using a function  58 . The data value of EGBP_DIF_BPA represents the data value of exhaust back-pressure error, meaning the difference between actual exhaust back-pressure and the exhaust back-pressure pressure set-point. Function  58  defines upper and lower limits for a data value EGBP_DIF_TCBC used in subsequent processing. If the data valve for EGBP_DIF_BPA is greater than a data value representing the upper limit (C_TCBC_EGBP_DIF_MAX), the upper limit value is used in further processing as a parameter EGBP_DIF_TCBC, and if the data value for EGBP_DIF_BPA is less than that representing the lower limit (C_TCBC_EGBP_DIF_MIN), the lower limit value is used in further processing as the data value for EGBP DIF_TCBC. Otherwise, the data value for EGBP_DIF_BPA becomes the data value for EGBP_DIF_TCBC. 
     The further processing of EGBP_DIF_TCBC is performed by a PID controller  60  that performs one or more of proportional, integral, and derivative functions on EGP_DIF_TCBC to provide data values  60 P,  601 , and  60 D that are summed by a summing function  62 . The processing may include the use of other data not specifically shown here. Subsequent processing of the sum provided by function  62  is allowed by a switch function  64  when TCBC control is enabled. Otherwise PID controller  60  provides a zero output. 
     The data value output of PID controller  60  is summed with the data value of a parameter TCBC_PWM_PCTL by a summing function  66 . The sum is processed by the portion of the strategy shown in  FIG. 4  to yield a data value for TCBC_PWM, as will be more fully explained hereinafter. With TCBC control enabled, the output of PID controller  60  represents a closed-loop control component for valves  20  and  22  because it has been developed by processing of exhaust back-pressure error. TCBC_PWM_PCTL represents a feed-forward, open-loop component for TCBC control. As will be more fully explained in subsequent description, this open-loop component is available for use in controlling valves  20  and  22  regardless of whether PID controller  60  is providing any control component for TCBC control, but whether any open-loop component is actually summed at summing function  66  depends on certain conditions affecting the portion of the strategy shown in  FIG. 3 . 
     That Figure shows how parameter TCBC_PWM_PCTL is developed. Four maps  70 ,  80 ,  90 , and  100  are used in conjunction with engine speed N and indicated torque TQI BPA to develop data values for respective parameters TCBC_PWM_PCTL_PROT, TCBC_PWM_PCTL_EGR_OFF, TCBC_PWM_PCTL_EGR_ON, and TCBC_PWM_PCTL_TCBC_OFF. The respective maps are populated with data values for those four respective parameters, each data value being correlated with a respective pair of data values representing a particular engine speed range and a particular engine torque range. Current data values for engine speed and indicated torque cause the corresponding data value that populates the respective map to be made available for further processing under appropriate conditions. 
     The portion of the strategy shown in  FIG. 3  further includes summing functions  82 ,  88 , multiplication functions  84 ,  86 , and switch functions  92 ,  94 , and  114 . 
     When the engine control system has been placed in limp-home mode, LV_LIH_TCBC_OL operates switch function  114  to cause map  70  alone to provide the data value for TCBC_PWM_PCTL. When the engine control system is not in limp-home mode, the data value for a parameter TCBC_PWM_PCTL_BAS serves as the data value for TCBC_PWM_PCTL. 
     How the data value for TCBC_PWM_PCTL_BAS is calculated depends on whether TCBC control is enabled. If TCBC control is not enabled, switch function  94  causes map  100  alone to provide the data value for TCBC_PWM_PCTL_BAS, and hence TCBC_PWM_PCTL, provided that map  100  has been populated with data, such populating being done to allow a feed-forward component to be applied to open valve  20  and  22  in certain situations when TCBC control has not enabled by LV_TCBC_EN. 
     If TCBC control is enabled, switch function  94  causes maps  80  and  90  to be used in various ways depending on the value of a parameter FAC_EGR_SP_BAS to provide the data value for TCBC_PWM_PCTL_BAS, and hence TCBC_PWM_PCTL. 
     If a parameter LC_PCTL_TCBC is operating switch function  92  to a first switch state, the data value for TCBC_PWM_PCTL_BAS is obtained solely from map  90 . If LC_PCTL_TCBC is operating switch function  92  to a second switch state, the data value for TCBC_PWM_PCTL_BAS is obtained either 1) solely from map  80 , 2) solely from map  90 , or 3) by interpolating data from both maps  80  and  90 . Which of those three possibilities is actually used depends on the data value for FAC EGR_SP BAS, which can be any value in the range extending from 0 to 1 inclusive, and which represents a dynamic correction multiplier for performing the interpolation using functions  82 ,  84 ,  86 , and  88  in the manner shown. 
     TCBC_PWM is the result of processing performed by the portion of the strategy shown by  FIG. 4  that comprises functions  116 ,  118 ,  120 ,  122 , and  124 . The data value from summing function  66  in  FIG. 2B  is represented by the parameter TCBC_PWM_BAS and is shown as a control input to function  116  in  FIG. 4 . 
     Function  116  is a switch function that can be used to substitute a selectable parameter C_TCBC_CONF for TCBC_PWM_BAS when manual control of TCBC_PWM is needed, such as for development or diagnostic purposes. 
     Function  118  is a limiting function that limits the input data value to a maximum C_TCBC_PWM_MAX and to a minimum C_TCBC_PWM_MIN. 
     Function  120  is a switch function that when limp-home mode is indicated by LV_LIH_TCBC_DFT applies a default input value (parameter C_TCBC_PWM_DFT) to switch function  122  instead of the control value passed by function  118 . The same default parameter can also be passed by switch function  122  based on engine state as determined by comparison function  124 . 
     While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention apply to all embodiments falling within the scope of the invention that is generally described as follows.