Abstract:
A method for coordinating control of exhaust gas recirculation ( 18 ) in a turbocharged internal combustion engine ( 10 ) with control of engine boost. When actual boost deviates from a desired boost set-point developed by a boost control strategy ( 32 ), such as during a sudden acceleration or deceleration, the EGR control strategy ( 34 ) provides a prompt adjustment of exhaust gas recirculation (EGR) seeking to null out the boost disparity.

Description:
FIELD OF THE INVENTION  
       [0001]    This invention relates to turbocharged internal combustion engines, particularly a motor vehicle diesel engine that in addition to having a turbocharger for developing boost has exhaust gas recirculation control. 
       BACKGROUND OF THE INVENTION  
       [0002]    Turbocharged diesel engines are powerplants of many trucks that are presently being manufactured in North America, with single- and two-stage turbochargers being representative of those used. A two-stage turbocharger 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 is one example of a turbocharger. A single-stage stage turbocharger has only a single turbine and a single compressor. 
         [0003]    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. A single-stage turbocharger can also have a variable geometry turbine for boost and exhaust back-pressure control. Such turbochargers are sometimes called variable geometry turbochargers, or VGT&#39;s for short. 
         [0004]    Sometimes, bypass valves are associated with the high-pressure compressor and turbine stages of a two-stage turbocharger and controlled in conjunction with VGT control. 
         [0005]    For various reasons that bear on engine performance and/or emission control, the ability to accurately control boost is important to an engine control strategy. A typical strategy processes various data to develop a data value for a desired set-point for boost. Changes in engine operation that affect that set-point typically call for the control system to respond promptly and accurately to force the actual boost to follow the changes in the desired set-point. 
         [0006]    Engine accelerations and decelerations create transient conditions where actual boost may temporarily lower or higher than appropriate. While a processor-based engine control system can rapidly process data, mechanical devices controlled by the control system may have slower response characteristics, and one example of this is turbo lag. 
         [0007]    Such limitations can have unfavorable implications for engine/vehicle performance and also for tailpipe emissions. Consequently, a control strategy that can minimize undesirable consequences of such limitations on engine performance and tailpipe emissions in certain situations would be a meaningful improvement in engine/vehicle technology. 
       SUMMARY OF THE INVENTION  
       [0008]    The present invention is directed to such a control strategy. 
         [0009]    Principles of the invention can be embodied in an engine control strategy without the inclusion of additional mechanical devices, making implementation of the inventive strategy cost-effective. Moreover, the favorable effect on tailpipe emissions can make a meaningful contribution toward compliance with applicable laws and regulations. 
         [0010]    Briefly, when actual boost deviates from a desired boost set-point developed by a boost control strategy, such as during a sudden acceleration or deceleration, the inventive strategy provides a prompt adjustment of exhaust gas recirculation (EGR) seeking to null out the boost disparity. To accomplish this several calculations are made. Before discussing them, some discussion of the EGR control system and the turbocharger control system is appropriate. 
         [0011]    The strategy for control of the EGR valve establishes a desired EGR set-point based on several parameters, including engine speed, indicated engine torque, and mass flow rate of fresh air entering the intake system. A typical EGR valve is controlled by a duty-cycle signal that is based on the EGR set-point. Changes in the EGR set-point change the duty cycle of the duty signal through a controller, typically a PID (proportional-integral-derivative) controller embodied as a virtual controller in the processing strategy. The response characteristics of any particular PID controller are typically determined during engine development to accommodate acceptable EGR valve response over relevant engine operating conditions that include steady-state conditions, i.e. non-transient conditions, and changing conditions, i.e. transient conditions. 
         [0012]    The strategy for control of turbocharger boost establishes a desired boost set-point based on several parameters, including engine speed and indicated engine torque. The boost set-point is processed by a control strategy for controlling the turbocharger, specifically controlling the position of the vanes of a VGT turbocharger. Vane position is typically controlled by an actuator to which a duty-cycle signal based on boost set-point is applied. The duty-cycle signal may also be developed by a PID controller in the boost control strategy. 
         [0013]    Because the response characteristic of a PID controller is often the result of a compromise between various operating conditions to enable the controller to perform reasonably satisfactorily for essentially all engine operating conditions, a PID controller may not provide quick enough response for certain more extreme transients that are more severe than slowly changing ones. Sudden accelerations and decelerations are examples of more extreme transients, and they may affect tailpipe emissions in undesirable ways. Principles of the present invention can ameliorate the adverse effect of such transients on tailpipe emissions. 
         [0014]    In accordance with those principles, various calculations are made. One calculation performed by a suitably appropriate algorithm uses actual boost to provide the mass flow rate through the engine cylinders. Another calculation, performed in any suitably appropriate way, provides the actual mass flow rate of fresh air entering the engine intake system. The mass flow rate of recirculated exhaust gas that entrains with the fresh air entering the intake system is then calculated as the difference between the calculated mass flow rate through the engine cylinders and the actual mass flow rate of fresh air entering the intake system. 
         [0015]    The EGR valve is modeled in such a way that for certain prevailing conditions that bear on mass flow rate through the EGR valve, such as exhaust gas temperature and pressure differential between the valve inlet and outlet, a correlation between mass flow rate through the valve and the extent to which the valve is open is defined. 
         [0016]    To null out the boost disparity during a sudden acceleration or deceleration, the control system uses the correlation between flow rate through the EGR valve and the extent to which the EGR valve is open to define an adjustment for the valve opening that will adjust the mass flow through the EGR valve in a way that seeks to null out the boost discrepancy. 
         [0017]    For example, when more boost is needed for engine acceleration, the EGR valve will be promptly operated in its closing direction to quickly reduce the mass flow rate of exhaust gas through the EGR valve so that less exhaust gas is introduced into the engine cylinders. Because engine fueling is being quickly increased to accelerate the engine, the quickly reduced amount of EGR facilitates the ensuing in-cylinder combustion processes and turbocharger operation in accordance with the strategy seeking to null the boost discrepancy as the engine accelerates. Quick response of the EGR is accomplished by using a feed-forward strategy by-passing the EGR PID controller. A significant reduction in tailpipe smoke can be noticed. 
         [0018]    When less boost is needed, the EGR valve will be promptly operated in its opening direction to quickly increase the mass flow rate of exhaust gas through the EGR valve so that more exhaust gas is introduced into the engine cylinders. The quickly increased amount of EGR can limit NOx formation. Quick response of the EGR is accomplished by using the feed-forward strategy by-passing the EGR PID controller. 
         [0019]    One generic aspect of the present invention relates to a method for coordinating control of exhaust gas recirculation from a exhaust system of a turbocharged internal combustion engine to an intake system of the engine with control of engine boost. 
         [0020]    The method comprises: developing data representing the mass flow rate of fresh air that is entering the intake system; calculating data representing the mass flow rate of recirculated exhaust gas that is entraining with the fresh air entering the intake system by calculating data representing mass flow rate through the engine cylinders and calculating the difference between the data representing the calculated mass flow rate through the engine cylinders and the data representing the mass flow rate of fresh air entering the intake system; calculating data representing expected mass flow rate through the engine cylinders that would occur if boost were equal to a desired set-point; calculating data representing actual mass flow rate through the engine cylinders using actual boost; calculating data representing the difference between the data representing actual mass flow rate through the engine cylinders and the data representing the expected mass flow rate through the engine cylinders; and using the data representing the difference between the data representing actual mass flow rate through the engine cylinders and the data representing the expected mass flow rate through the engine cylinders as a feed-forward adjustment of the mass flow rate of recirculated exhaust gas in a direction of adjustment that seeks to null out the difference between desired boost set point and actual boost. 
         [0021]    A further generic aspect of the present invention relates to an engine system comprising an engine having cylinders, a turbocharger, an intake system, including a compressor of the turbocharger, for delivering charge air to the engine cylinders, an exhaust system, including a turbine of the turbocharger, for conveying exhaust gas from the engine cylinders, an exhaust gas recirculation system, including an EGR valve, for recirculating exhaust gas from the exhaust system to the intake system, and a control system. 
         [0022]    The control system coordinates control of exhaust gas recirculation and comprises a processor for: a) developing data representing the mass flow rate of fresh air that is entering the intake system, b) calculating data representing the mass flow rate of recirculated exhaust gas that is entraining with the fresh air entering the intake system by calculating data representing mass flow rate through the engine cylinders and calculating the difference between the data representing the calculated mass flow rate through the engine cylinders and the data representing the mass flow rate of fresh air entering the intake system, c) calculating data representing expected mass flow rate through the engine cylinders that would occur if boost were equal to a desired set-point, d) calculating data representing actual mass flow rate through the engine cylinders using actual boost, and e) calculating data representing the difference between the data representing actual mass flow rate through the engine cylinders and the data representing the expected mass flow rate through the engine cylinders. 
         [0023]    The control system performs feed-forward adjustment of the mass flow rate of recirculated exhaust gas in a direction of adjustment that seeks to null out the difference between desired boost set point and actual boost by processing the data representing the difference between the data representing actual mass flow rate through the engine cylinders and the data representing the expected mass flow rate through the engine cylinders to develop a feed-forward adjustment signal that is applied to the EGR valve to cause the adjustment. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0024]      FIG. 1  is a general schematic diagram of a motor vehicle engine system. 
           [0025]      FIG. 2  is a graph plot useful in explaining an aspect of the inventive strategy. 
           [0026]      FIG. 3  is a schematic diagram illustrating principles of the inventive strategy. 
           [0027]      FIG. 4  is another graph plot useful in explaining the inventive strategy. 
           [0028]      FIG. 5  is another graph plot useful in explaining the inventive strategy. 
           [0029]      FIG. 6  shows a series of data traces representing various parameters affected by the inventive strategy. 
           [0030]      FIG. 7  shows another series of data traces representing various parameters affected by the inventive strategy. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]      FIG. 1  shows an exemplary internal combustion engine system  10  comprising an engine  12  containing cylinders in which combustion occurs, an intake system  14  through which charge air can enter engine  12  and an exhaust system  16  through which exhaust gasses resulting from combustion of air-fuel mixtures in the cylinders exit. An EGR system  18  provides for exhaust gas to be recirculated from exhaust system  16  to intake system  14 . 
         [0032]    Engine system  10  is representative of a turbocharged diesel engine comprising a turbocharger  20  that has turbine  20 T in exhaust system  16  operating a compressor  20 C in intake system  14 . A charge air cooler  22  is downstream of compressor  20 C. 
         [0033]    EGR system  18  comprises an EGR cooler  26  through which exhaust gas passes before reaching an EGR valve  26  that is controlled by a duty-cycle signal applied to an electric actuator of the valve to set the extent to which the EGR valve is open. 
         [0034]    The inventive strategy is embodied in one or more processors of an engine control system as algorithms for processing data. Through control of EGR valve  26  in coordination with control of boost, sudden transients have less adverse effect on tailpipe emissions. 
         [0035]    The strategy includes modeling EGR valve  26  such that for certain prevailing conditions, such as exhaust gas temperature and pressure differential across the valve, that bear on mass flow rate through the valve, a correlation between mass flow rate through the valve and the extent to which the valve is open is defined.  FIG. 2  shows an example of valve modeling where the vertical axis represents mass flow rate through the valve M EGR  and the horizontal axis represents an amount of valve opening. 
         [0036]    A first plot DP 1  defines a relationship between mass flow rate and valve opening at a certain differential pressure DP 1 . A second plot DP 2  defines a relationship between mass flow rate and valve opening at another differential pressure DP 2 . A third plot DP 3  defines a relationship between mass flow rate and valve opening at still another differential pressure DP 3 . 
         [0037]    Thus data storage in the processors of the control system may be populated with data defining data values for X EGR  each correlated with a respective pair of data values for differential pressure and mass flow rate. 
         [0038]    Knowing how EGR valve  26  has been modeled, attention is directed to  FIG. 3  for more explanation of the strategy  30 . 
         [0039]    A general turbocharger control strategy is designated by the reference numeral  32 . Vanes of turbine  20 T are positioned by a duty cycle signal VGT_DTY applied to an actuator that sets vane position. Strategy  32  seeks to position the vanes so that compressor  20 C develops boost corresponding to a desired boost set-point represented by a parameter MAP_SP(N,TQ). The control system uses engine speed N and indicated engine torque TQ to select an appropriate data value for MAP_SP(N,TQ) from a map for processing by strategy  32 . Strategy  32  contains a closed-loop controller that compares a data value for actual boost, parameter MAP, with the desired set-point to develop an error signal that is processed to create a value for VGT_DTY that will secure correspondence of actual boost to the desired set-point. 
         [0040]    The EGR control strategy is designated by the reference numeral  34 . A desired set-point for EGR is represented by a parameter EGR_SP which like the boost set-point depends on engine speed N and indicated engine torque TQ, with the control system selecting an appropriate data value for EGR_SP from a map for processing by strategy  34 . A portion of the processing designated by the reference numeral  36  processes not only EGR_SP but also data representing engine fueling, parameter M fuel , and the mass flow rate of fresh air entering intake system  14 , parameter MAF. A data value for MAF is calculated in any suitably appropriate way, such as by converting a MAF sensor output into a corresponding data value. 
         [0041]    The result of processing  36  is used as one input to an algebraic summing function  38  that provides output data X EGR  to an EGR PID controller  40  that in turn provides an input to another algebraic summing function  42 . It is the output of summing function  42  that sets the duty cycle signal EGR_DTY applied to the actuator of EGR valve  26 . 
         [0042]    Strategy  34  comprises a suitably appropriate algorithm  44  that develops a data value for actual mass flow rate through engine  12 , represented by a parameter M eng . The data value for M eng  is an input to an algebraic summing function  46 . Actual mass flow is a function of several variables shown here as boost (MAP), air temperature (MAT), volumetric efficiency (Vol eff), and engine displacement (Displ). It is data values for those parameters that are processed by algorithm  44  to develop the data value for M eng . 
         [0043]    Strategy  34  further comprises a suitably appropriate algorithm  47  that develops a data value for mass flow rate through engine  12  that is based on the same variables processed by algorithm  44  except for MAP. Instead of using MAP, algorithm  47  uses desired boost set-point MAP_SP(N,TQ). The result provided by algorithm  47  is represented by a parameter M eng* . The data value for M eng*  is an input to an algebraic summing function  48 . 
         [0044]    Summing function  48  calculates the difference between M eng  and M eng* . The difference is represented by a parameter ΔM ENG  that is one of several inputs for a boost coupling algorithm  50 . This algorithm performs calculations that yield a data value for a parameter ΔX EGR  that is subtracted by summing function  42  from the data value for X EGR  provided by EGR PID controller  40 . 
         [0045]    Summing function  46  calculates the mass flow rate through EGR valve  26 , represented by a parameter M EGR , by subtracting from the data value for M eng  the data values for MAF and M fuel . The data value for M EGR  is another input to algorithm  50 . It is also subtracted by summing function  38  from the data value calculated by processing  36 . 
         [0046]    Additional inputs for algorithm  50  are parameters ΔP the pressure across the EGR valve and ρ density (Willy, I think I know what these two symbols represent but I&#39;m not sure and don&#39;t want to guess as to how their data values are developed, so please clarify and explain briefly.) 
         [0047]    During steady-state and near steady-state operation of the engine, there is little or no disparity between the data values for ΔM ENG  and M EGR . As a result, boost coupling strategy  50  provides little or no adjustment of EGR via ΔX EGR  because the data value for ΔX EGR  is small or zero. The EGR mass flow rate error input to EGR PID controller  42  provides closed-loop control of EGR that continually forces the EGR rate toward the set-point EGR_SP. 
         [0048]    During non-steady-state operation that is significantly more non-steady-state that merely near steady-state (sudden accelerations and decelerations for example), the disparity between the data values for ΔM ENG  and M EGR  becomes significant. As a result, boost coupling strategy  50  provides adjustment of EGR via ΔX EGR  because the data value for ΔX EGR  has now become significant. EGR PID controller  42  still provides a closed-loop component to control of EGR by virtue of ΔX EGR , but the additional component provided by ΔX EGR  is quickly reflected in EGR_DTY because it is not delayed by the slower response that is inherent in the compromised design of the PID controller. 
         [0049]    The strategy is graphically portrayed by  FIGS. 4 and 5 . When the desired boost set-point suddenly changes, as shown by the step in MAP_SP in  FIG. 4 , actual MAP changes as portrayed by the trace labeled MAP. The change in flow rate ΔM ENG  creates a data value for ΔM EGR  that requires a corresponding change in valve opening ΔX EGR . M EGR  is processed by algorithm  50  to define the location on the appropriate ΔP plot where the EGR valve is presently operating. ΔM EGR  defines the amount of change in EGR mass flow rate that is needed, and use of the valve model embodied as stored data in the processing system converts the change to a change in valve opening. The disparity in boost (difference between actual boost and desired boost set-point) may be considered as a boost deficit that can be either positive or negative. The invention provides immediate feed-forward adjustment of the EGR valve because the strategy bypasses PID controller  40  when applying ΔX EGR  to the EGR valve. The signal EGR_DTY may be considered a composite signal composed of a closed-loop component from the PID controller  40  and an open-loop, feed-forward component from algorithm  50 . 
         [0050]    In a motor vehicle powered by engine system  10 , a sudden depression of the acceleration pedal by the driver will cause EGR valve  26 , if open, to be promptly operated in the direction of closing quickly reducing the mass flow rate of exhaust gas through the EGR valve. The immediate effect is a corresponding reduction in exhaust gas being introduced into the engine cylinders. Because engine fueling is being quickly increased to accelerate the engine, the quickly reduced amount of EGR facilitates the ensuing in-cylinder combustion processes and turbocharger operation toward more quickly nulling out the boost discrepancy as the engine accelerates. 
         [0051]    A sudden deceleration, like that resulting from release of the accelerator, will quickly drop the desired boost set-point. The inventive strategy causes EGR valve  26  to be promptly operated in its opening direction to quickly increase the mass flow rate of exhaust gas through the EGR valve so that more exhaust gas is introduced into the engine cylinders. The quickly increased amount of EGR can limit NO x  formation during the deceleration. 
         [0052]    A comparison of the traces shown in  FIG. 6  with those shown in  FIG. 7  are representative of the effectiveness of the inventive strategy during an acceleration. The traces marked “set-point” and “boost pressure” in both Figures show that a sudden increase in the desired set-point will cause boost to increase to the higher desired set-point in about two seconds, and to slightly overshoot before settling at the new set point. When the desired set-point suddenly drops to the original set-point, boost drops off to the original in about one second. 
         [0053]    In both  FIGS. 6 and 7 , the traces marked EGRP and VGT represent the amount of ERG valve opening and turbocharger vane position respectively, and the traces marked EGR and MAF represent the ERG mass flow rate and fresh air mass flow rate respectively. The traces EGRP, VGT, EGR and MAF in  FIG. 6  show how the sudden changes in desired boost set-point affect the respective parameters in a typical engine system that does not have the inventive strategy. The traces EGRP, VGT, EGR and MAF in  FIG. 7  show how the sudden changes in desired boost set-point affect the respective parameters in a typical engine system that does have the inventive strategy. (Willy, your explanation of the significance of the differences—I guess VGT especially—would be helpful. I suppose we should also add Figures showing the smoke and NOx traces—let me know please.) 
         [0054]    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.