Abstract:
A variable nozzle turbocharger ( 12 ) creates engine boost. Boost is controlled by controlling the position of vanes within turbocharger. A processor develops a control signal ( 29 ) for controlling vane position. The processor develops a value for desired boost and processes that value with a value corresponding to the amount of boost being created by the turbocharger to generate error data ( 48 A) defining error between the amount of boost being created by the turbocharger and the desired boost, and the processor develops a component of the control signal by P-LI-D processing ( 62 ) of the error data. Other components of the control signal are a feed-forward value from a look-up table ( 34 ) and a value from an overspeed protection function ( 60 ).

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to control of turbocharged diesel engines that propel motor vehicles, and in particular to control of a variable nozzle turbocharger of such an engine.  
         BACKGROUND AND SUMMARY OF THE INVENTION  
         [0002]    A turbocharger is one type of device that is used to supercharge an internal combustion engine. A diesel engine that is supercharged by a turbocharger is sometimes referred to as a turbocharged diesel. A turbocharger comprises a turbine that is powered by engine exhaust gas and coupled by a shaft to operate a compressor that boosts pressure in the engine air intake system downstream of the compressor. One way to control boost pressure is to control turbine operation.  
           [0003]    There are several different forms of turbine control. One form of control involves the construction of the turbocharger itself. A turbocharger that has a variable geometry, or variable nozzle, is capable of changing the manner in which exhaust gas that flows through the turbocharger interacts with the turbine, and hence controlling the pressure, i.e. boost, that the compressor creates in the engine intake manifold. One type of variable geometry, or variable nozzle, turbocharger comprises movable vanes whose positions are selectively controlled to in turn selectively control the nature of exhaust gas interaction with the turbine, and hence the boost pressure developed by the turbocharger. The turbocharger includes a device for interfacing an electric control with the movable vanes. That device comprises an electromechanical actuator having a solenoid for setting vane position according to the extent to which the solenoid is electrically energized. With the solenoid placed under the control of the engine electronic control system, the extent to which the solenoid is energized, and hence vane position, are determined by the degree of modulation of a pulse width modulated (PWM) signal created by the electronic control system. The device may utilize a medium like fluid power, hydraulics for example, that is controlled by the solenoid actuator to impart movement to the vanes.  
           [0004]    U.S. Pat. Nos. 4,428,199; 4,660,382; 4,671,068; 4,685,302; 4,691,521; 4,702,080; 4,732,003; 4,756,161; 4,763,476; 4,765,141; 4,779,423; 5,123,246; 5,867,986; 6,000,221; and International Application WO 99/23377 relate to control of turbocharged internal combustion engines. Certain of those documents relate to control of variable geometry turbochargers. Both documents WO 99/23377 and 6,000,221 disclose systems for control of the variable geometry of a turbocharger utilizing a signal from a turbocharger vane position sensor as feedback in closed-loop control of the vanes.  
           [0005]    The present invention is distinguished by a closed-loop control system for controlling boost without a vane position sensor by utilizing certain data already available in an engine control system.  
           [0006]    Certain of the documents disclose systems that employ PID functions for control purposes.  
           [0007]    The present invention is distinguished from those systems by a P-LI-D function in which the integration function is selectively, or conditionally, employed depending on prevailing conditions.  
           [0008]    One aspect of the present invention relates to a novel strategy for control of a variable geometry, or variable nozzle, turbocharger of an internal combustion engine. The disclosed strategy is implemented in a microprocessor-based engine control system, and utilizes certain data that is already available to the control system and/or developed by the processor. Certain data may be programmed into the control system.  
           [0009]    Individual data may be categorized as: an input variable; a local variable; or an output variable. Input variables include barometric pressure; manifold pressure; engine load; and engine speed. Programmable parameters include an enable feature; high engine idle speed; and low engine idle speed. Each variable is calibrated in any suitable unit of measurement.  
           [0010]    The input variables and the programmed parameters are applied to the general control strategy. The control operates on those variables and parameters in accordance with the general strategy to develop a PWM signal applied by a driver circuit to the solenoid that controls the turbocharger vane position.  
           [0011]    One general aspect of the invention relates to control of a variable nozzle turbocharger of an internal combustion engine for changing boost according to changes in both engine speed and engine load to achieve desired boost appropriate to various combinations of engine speed and engine load so that boost appropriate to each particular combination is consistently achieved as the engine operates.  
           [0012]    Another aspect relates to control of a variable nozzle turbocharger of an internal combustion engine for avoidance of turbine shaft speeds that exceed a predefined maximum.  
           [0013]    Still another aspect relates to control of a variable nozzle turbocharger of an internal combustion engine for adjusting desired boost according to changing barometric conditions, like those that may be experienced when a vehicle being powered by such an engine is driven at different altitudes.  
           [0014]    Still other aspects of the invention relate to details of the disclosed control strategy and its various sub-strategies. While the conditional integration provided by the P-LI-D control sub-strategy is useful in turbocharger boost control, it may provide advantages in other closed-loop control systems.  
           [0015]    One general aspect of the claimed invention relates to an internal combustion engine comprising a turbocharger that creates engine boost and has a selectively positionable mechanism for controlling the amount of boost created by passage of exhaust gas through the turbocharger. A control selectively positions the mechanism to control the amount of boost in accordance with data inputs. The control comprises a processor for processing data, including the data inputs, to develop a control signal for selectively positioning the mechanism. A first data input to the processor comprises data corresponding to engine load, and a second data input to the processor comprising data corresponding to engine speed. A look-up table is programmed with values representing desired boost corresponding to sets of values representing various combinations of engine speed and engine load. A third data input to the processor comprises data corresponding to the amount of boost being created by the turbocharger.  
           [0016]    The processor selects from the look-up table a value for desired boost corresponding to values of the first data input and the second data input. The processor processes the value of the third data input and the selected value for desired boost from the look-up table to generate error data defining error between the amount of boost being created by the turbocharger and the desired boost. The processor further processes the error data according to the value of the error data to cause the control signal to position the mechanism to reduce the error such that when the error data is less than a predetermined value, further processing comprises processing the error data with proportional, integral, and derivative control, and when the error data is not less than the predetermined value, the further processing comprises processing the error data with proportional and derivative control but without integral control.  
           [0017]    Another general aspect of the claimed invention relates to an internal combustion engine comprising a turbocharger that creates engine boost and has a selectively positionable mechanism for controlling the amount of boost created by passage of exhaust gas through the turbocharger. A control selectively positions the mechanism to control the amount of boost in accordance with data inputs. The control comprises a processor for processing data, including the data inputs, to develop a control signal for selectively positioning the mechanism. A first data input to the processor comprises data corresponding to engine load, a second data input to the processor comprises data corresponding to engine speed, and a third data input to the processor comprises data corresponding to the amount of boost being created by the turbocharger. A first look-up table is programmed with values representing desired boost corresponding to sets of values representing various combinations of engine speed and engine load, and a second look-up table is programmed with values representing feed-forward values for use in developing the control signal correlated with sets of values representing various combinations of engine speed and engine load. A function generator is programmed with values for turbocharger speed corresponding to values of boost for a given barometric pressure.  
           [0018]    The processor selects from the first look-up table a value for desired boost corresponding to values of the first data input and the second data input, from the second look-up table, a feed-forward value corresponding to values of the first data input and the second data input, and from the function generator, a value for turbocharger speed corresponding to the value of the third data input. The processor processes the value of the third data input and the value of desired boost selected from the first look-up table to generate a value for error data defining error between the amount of boost being created by the turbocharger and the desired boost. The processor further processes the error data according to the value of the error data to create a first component of the control signal for causing the mechanism to reduce the error such that when the value of error data is less than a predetermined value, further processing comprises processing the error data with proportional, integral, and derivative control, and when the value of error data is not less than the predetermined value, the further processing comprises processing the error data with proportional and derivative control signal but without integral control. The processor processes the selected feed-forward value from the second look-up table to create a second component of the control signal, and the processor processes the selected turbocharger speed from the function generator to create a third component of the control signal for limiting turbocharger speed to a predetermined maximum speed during a condition when the control signal would otherwise be calling for a turbocharger speed greater than the predetermined maximum.  
           [0019]    Another general aspect of the claimed invention relates to an internal combustion engine comprising a variable nozzle turbocharger powered by passage of exhaust gas through the turbocharger for creating and controlling engine boost and a control for controlling vane position of the variable nozzle turbocharger to control the amount of boost in accordance with data inputs. The control comprises a processor for processing data, including the data inputs, to develop a control signal for controlling the vane position.  
           [0020]    The processor processes certain data to develop a value for desired boost and processes that value with a value corresponding to the amount of boost being created by the turbocharger to generate error data defining error between the amount of boost being created by the turbocharger and the desired boost, and the processor develops the control signal by further processing of the error data.  
           [0021]    Still another general aspect of the claimed invention relates to an internal combustion engine comprising a device that comprises a selectively positionable mechanism in a flow path through the engine for controlling a pressure in the flow path. A control selectively positions the mechanism in accordance with data inputs. The control comprises a processor for processing data, including the data inputs, to develop a control signal for selectively positioning the mechanism. The processor generates error data for positioning the mechanism, and processes the error data according to the value of the error data to cause the control signal to position the mechanism to reduce the error such that when the value of error data is less than a predetermined value, the error data is processed using proportional, integral, and derivative control, and when the value of error data is not less than the predetermined value, the error data is processed using proportional and derivative control but without integral control.  
           [0022]    Other general aspects of the claimed invention relate to the methods for controlling boost and pressure in engines as described above.  
           [0023]    The foregoing, along with further aspects, features, and advantages of the invention, will be seen in this disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, briefly described below, and contains a detailed description that will make reference to those drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a general schematic diagram showing input variables and programmed parameters applied to a general strategy to develop a PWM signal delivered through a driver circuit to a solenoid that controls turbocharger vane position, in accordance with principles of the present invention.  
         [0025]    [0025]FIGS. 2A and 2B collectively form a more detailed schematic diagram of the general strategy.  
         [0026]    [0026]FIG. 3 is a detailed schematic diagram of a first sub-strategy within the general strategy.  
         [0027]    [0027]FIG. 4 is a detailed schematic diagram of a second sub-strategy within the general strategy.  
         [0028]    [0028]FIG. 5 is a detailed schematic diagram of a third sub-strategy within the general strategy.  
         [0029]    [0029]FIGS. 6A and 6B collectively form a detailed schematic diagram of a modified general strategy.  
         [0030]    [0030]FIG. 7 is a detailed schematic diagram of one sub-strategy within the modified general strategy of FIGS. 6A and 6B.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    [0031]FIG. 1 illustrates a general strategy  10 , in accordance with principles of the present invention, for control of a variable nozzle turbocharger  12  of an internal combustion engine  14  that powers a motor vehicle. An example of such a vehicle is a medium or heavy truck having a chassis containing a powertrain in which engine  14  is a fuel-injected diesel engine operatively coupled through a drivetrain to driven wheels for propelling the vehicle. Engine  14  comprises individual fuel injectors that inject diesel fuel into individual engine cylinders in properly timed relation to engine operation.  
         [0032]    An electronic engine control  18  that possesses digital processing capability is associated with engine  14 . Control  18  may comprise one or more microprocessors that process data from various input data signal sources in accordance with programmed algorithms to develop certain signals used in the performance of various functions associated with operation of engine  14 . The signals processed by control  18  may be ones that originate at external sources (input variables) and/or ones that are generated internally of control  18  (local variables).  
         [0033]    One of the primary functions of control  18  is to operate engine  14  in a way that produces output torque appropriate to certain variable input signals, including a driver input from a sensor sometimes referred to as an accelerator position sensor (not shown in the Figures). An accelerator position sensor is linked with an accelerator pedal of the vehicle (also not shown) and delivers to control  18  a signal indicating the extent to which the driver is depressing the accelerator pedal. Control  18  operates engine  14  in manner that strives to faithfully respond to the accelerator position signal, while also taking into account certain variables that are relevant to achieving proper engine operation, but without operating the engine in ways that are considered inappropriate.  
         [0034]    In operating engine  14 , control  18  performs the function of opening and closing the fuel injectors at proper times during the engine operating cycle so that appropriate amounts of fuel are injected at the proper times. Control  18  therefore develops fuel injector control signals by processing various input data signals in accordance with pre-programmed fuel control algorithms. Another of the functions performed by control  18  is control of turbocharger boost, such as control of turbine vane position in a variable nozzle turbocharger. FIG. 1 represents that portion of engine control  18  for accomplishing that function in accordance with principles of the present invention.  
         [0035]    Input variables to control  18  for accomplishing turbocharger vane control include: Engine Speed  20 A; Barometric Pressure  22 A; Engine Load  24 A; and Manifold Pressure  26 A. The blocks  20 ,  22 ,  24 , and  26 , labeled Crankshaft Position Signal Processing, Barometric Pressure Signal Processing, Fuel or Load Signal Processing, and Manifold Pressure Signal Processing respectively, represent certain processing that may be used to develop, from raw data sources, the respective data values of these four input variables that are digitally processed by control  18 . Any suitable raw data sources may be used. For example, differentiating a crankshaft position signal may provide engine speed data. Engine load may be indicated by how heavily the engine is being fueled, in which case a fueling command derived from the accelerator position sensor may be used to develop engine load data.  
         [0036]    Programmed parameters for turbocharger vane position control include: Enable Feature  28 A, which enables the variable nozzle control function to be performed; High Idle Speed  28 B; and Low Idle Speed  28 C. The block  28 , labeled Programmable Parameters, denotes the fact that these three parameters may be programmed for the particular engine and/or vehicle.  
         [0037]    The processing of data representing these input variables and programmable parameters by control  18 , in accordance with the inventive control strategy, yields a Control Signal  29  that is an input to a block  30 , labeled PWM Driver, denoting a circuit that creates a corresponding pulse width modulated signal suitable for energizing the solenoid that controls turbocharger vane position.  
         [0038]    Certain additional data used in processing are calibration constants that will be mentioned from time to time in the ensuing description. A calibration constant is programmed in control  18  during engine or vehicle manufacture for the particular engine and vehicle.  
         [0039]    [0039]FIGS. 2A and 2B show more detail of the control strategy. Control  18  comprises two look-up tables, or two-dimensional maps  34 ,  36 : namely, a feed-forward look-up table  34  and a desired boost look-up table  36 . It also comprises six limiting functions  23 ,  38 ,  40 ,  42 ,  44 ,  46 : namely, a barometric pressure limiting function  23  that establishes a maximum limit 23MX and a minimum limit 23MN for barometric pressure; an engine load limiting function  38  that establishes a maximum limit 38MX and a minimum limit 38MN for engine load; an engine idle speed limiting function  40  that uses High Idle  28 B as a maximum limit 40HI for engine speed and Low Idle  28 C as a minimum limit 40LO for engine speed; a desired boost limiting function  42  that establishes a maximum limit 42MX and a minimum limit 42MN for desired boost; a manifold pressure limiting function  44  that establishes a maximum limit 44MX and a minimum limit 44MN for manifold pressure; and a duty cycle limiting function  46  that establishes a maximum duty cycle limit 46MX and a minimum duty cycle limit 46MN. There are also: two summing functions  48 ,  50 ; four low-pass filtering functions  25 ,  54 ,  56 ,  58 ; an overspeed protection sub-strategy  60 ; a P-LI-D (proportional, conditional integral, derivative) function  62 ; and a switch function  66 .  
         [0040]    In order for the data values of Barometric Pressure  22 A, Engine Load  24 A, Engine Speed  20 A, and Manifold Pressure  26 A to be considered valid for processing by control  18 , the value of each must lie within a respective predetermined range defined between upper and lower limit values of the respective limiting function  23 ,  38 ,  40 ,  44 . Whenever the value of any of these three inputs falls outside the respective range, a respective default value defined by the appropriate limit is substituted and processed by control  18 .  
         [0041]    A primary purpose for processing each input through a respective limiting function is to guard against subsequent processing of data that is likely to be false, and therefore apt to produce an incorrect, and potentially undesirable, result. For example, false information could be given by a sensor error or malfunction or by a processing error.  
         [0042]    The low-pass filtering functions serve to prevent too large a change in data value from one processing iteration to the next from having an immediate effect on the result. For example, such a large change could be due to a random processing glitch that should not be allowed to have an effect of the control process, and therefore should be ignored. A succession of large changes however will be allowed to have an effect because they are indicative of actual change as distinguished from a random glitch.  
         [0043]    In the ensuing description, reference to processed Barometric Pressure, processed Engine Speed, processed Engine Load, and processed Manifold Pressure should be understood in context to refer to data that has been subjected to processing by the respective limiting function and the respective low-pass filtering function so as to be both appropriate for subsequent processing and presumptively valid and indicative of the value of the actual variable, except possibly when being limited by a limiting function.  
         [0044]    Look-up tables  34 ,  36  make use of both processed Engine Load data and processed Engine Speed data. The processed Engine Speed data is also used by overspeed protection sub-strategy  60 .  
         [0045]    Look-up table  34  correlates various feed-forward values of Control Signal  29  with various combinations of engine load and engine speed over relevant load and speed operating ranges. Look-up table  36  correlates various desired boost values with various combinations of engine load and engine speed over relevant load and speed operating ranges. Boost is understood to be the same as intake manifold pressure.  
         [0046]    [0046]FIG. 3 discloses detail of overspeed protection sub-strategy  60 . The substrategy comprises: two comparison functions  70 ,  72 ; five function generators, or one-dimensional maps,  74 ,  76 ,  78 ,  80 ,  82 ; three switch functions  84 ,  86 ,  88 ; two multiplication functions  90 ,  92 ; a relay function  94 ; and a summing function  96 . The strategy of overspeed protection  60  is to predict rotational speed of the turbine shaft of turbocharger  12  and act upon the duty cycle signal applied to the turbocharger solenoid in such a way that the turbine shaft speed value  90 A may not exceed a predetermined maximum value corresponding to a maximum allowable speed for the turbine shaft.  
         [0047]    Comparison function  70  compares the data value of processed Engine Speed to a defined value  70 A. Comparison function  72  compares the data value of processed Engine Speed to a defined value  72 A. The defined value  70 A is greater than the defined value  72 A. If the data value of processed Engine Speed is greater than the defined value  70 A, comparison functions  70 ,  72  act via switch functions  84 ,  86  so as to cause function generator  76  to provide the first of the two inputs to multiplication function  90 . If the data value of processed Engine Speed is between the defined values  70 A and  72 A, inclusive, comparison functions  70 ,  72  act via switch functions  84 ,  86  so as to cause function generator  74  to provide the first input to multiplication function  90 . If the data value of processed Engine Speed is less than the defined value  72 A, comparison functions  70 ,  72  act via switch functions  84 ,  86  so as to cause function generator  78  to provide the first input to multiplication function  90 .  
         [0048]    The particular function generation characteristic of each function generator  74 ,  76 ,  78  is empirically determined. The processed Manifold Pressure is a common input to all three function generators  74 ,  76 ,  78 . The defined values  70 A and  72 A are chosen to divide the entire range of turbine shaft speeds into three sub-ranges, or portions, within a first of which engine speed is less than the speed defined by data value  72 A, within a second of which engine speed lies within a range between speeds defined by data values  72 A and  70 A, inclusive, and within a third of which engine speed is greater than the speed defined by data value  70 A. Over the respective portion of the speed range defined by the respective function generator  74 ,  76 ,  78 , the function embedded in the function generator correlates predicted shaft speed with manifold pressure for a given barometric pressure and engine speed. Each function may be empirically derived by statistical correlation techniques, and the extent to which an entire speed range is sub-divided may depend on statistical results. Hence, the use of three sub-ranges in the disclosed embodiment may be considered merely illustrative.  
         [0049]    Function generator  80  correlates values of perceived altitude to values of barometric pressure. The value of the perceived altitude provided by function generator  80  for subsequent processing is determined by the value of processed Barometric Pressure. A value of perceived altitude from function generator  80  is used in an altitude compensation calculation. Multiplication function  92  multiplies that value by an altitude compensation factor  93 , and the value of unity ( 1 ) is added to that product by summing function  96 . The result of summing function  96  forms the second input to multiplication function  90 , and that result serves to adjust the predicted turbine shaft speed for altitude. As altitude increases above sea level, the shaft will tend to run at increasing speed because of the decrease in density of atmospheric air. Function  90  multiplies the first and second inputs to it to develop an altitude-compensated data value for Predicted Shaft Speed  90 A. The data values contained in function generator  80  may be considered as baseline data obtained by operating a turbocharger in a given setting. The altitude compensation factor is specified by the turbocharger manufacturer for a particular model of turbocharger used in a particular model of engine, and hence altitude compensation factor  93  is a calibration constant that is programmed in control  18  for that engine model.  
         [0050]    The data value for Predicted Shaft Speed  90 A is processed by function generator  82  and by relay function  94 . Function generator  82  correlates values of a duty cycle with predicted shaft speed. In general, the relationship is one where the duty cycle increases as predicted shaft speed increases. Relay function  94  and switch function  88  cooperate to impart a certain hysteresis to the through-switching of a data value for a duty cycle  82 A obtained from function generator  82  to summing function  50 . The switch function is effective to allow the data value of duty cycle  82 A to pass as an input to summing function  50  when the data value for Predicted Shaft Speed  90 A exceeds a maximum allowable actual speed for the turbocharger shaft. One data value defines a maximum allowable actual speed  94 A, and another data value defines a minimum allowable actual speed  94 B for relay function  94 . Relay function  94  is effective upon the data value for Predicted Shaft Speed  90 A exceeding that maximum allowable actual speed to cause switch function  88  to pass the data value obtained from function generator  82  for duty cycle  82 A. A data value for duty cycle  82 A will continue to be passed until the data value for Predicted Shaft Speed  90 A falls below the data value for minimum allowable actual speed  94 B that is itself somewhat below the data value for maximum allowable actual speed  94 A. The effect on the turbocharger is a reduction in shaft speed. Only when predicted shaft speed, as sensed by relay function  94 , falls below the value for minimum allowable actual speed  94 B will the relay function be effective via switch function  88  to once again pass a zero value to summing function  50  instead of a duty cycle value  82 A from function generator  82 . The interaction of relay function  94  and switch function  88  with function generator  82  assures that having once approached, or perhaps even reached, maximum allowable shaft speed, actual shaft speed will have to decrease some predetermined amount before it is again allowed to increase toward maximum allowable speed. The interaction amounts to what may be considered a buffering of shaft speed that avoids the occurrence of repeating decelerations and accelerations near maximum speed. The output of the overspeed protection sub-strategy which is supplied as an input to summing function  50  in FIG. 2B is subtracted from the sum of a value from look-up table  34  and a value from P-LI-D function  62 .  
         [0051]    In FIG. 2A, a value from look-up table  36  represents the desired boost, meaning desired manifold pressure, that turbocharger  12  should be producing in the engine intake manifold, subject to any limiting imposed by limiting function  42 . A data value for processed Manifold Pressure  26 A, representing actual boost being produced by the turbocharger unless limited by limiting function  44 , is subtracted from a data value for desired boost that is called for by the processed Engine Speed and the processed Engine Load to create a data value for Manifold Pressure Error  48 A that is subsequently processed by P-LI-D function  62 . One may therefore perceive that the value of processed Manifold Pressure, or processed boost, represents negative feedback and that the value of Manifold Pressure Error  48 A represents an error signal input for the closed loop control of turbocharger  12  via P-LI-D function  62 .  
         [0052]    P-LI-D function  62  represents the primary control for developing the pulse width modulated signal applied to the control solenoid of turbocharger  12  that sets vane position. Overspeed protection sub-strategy  60  and look-up table  34  are arranged to interact with the primary control provided by P-LI-D function  62 . The extent of the interaction however depends on certain circumstances. The interaction is performed by summing function  50 , which algebraically sums a data value from P-LI-D function  62  (considered positive), a data value from look-up table  34  (considered positive), and the data value output of overspeed protection sub-strategy  60  (considered negative) to create a value for Control Signal  29 . The latter signal is subject to limiting by limiting function  46 . Moreover, only when the turbocharger feature is enabled by engine control  18  is Control Signal  29  actually applied to turbocharger  12 . Hence, when the turbocharger feature is enabled, the Enable Feature parameter acts via switch function  66  to cause Control Signal  29  to be applied to PWM driver  30 .  
         [0053]    P-LI-D function  62  is capable of processing Manifold Pressure Error data  48 A through proportional, integral, and derivative functions. However, these three functions are not necessarily always simultaneously applied. In particular, the integral function is selectively, or conditionally, employed depending on prevailing conditions. Hence, for some conditions, P-LI-D function  62  may actually perform proportional, integral, and derivative functions, but for other conditions, it may actually perform only proportional and derivative functions.  
         [0054]    Detail of P-LI-D function  62  appears in FIG. 4. The proportional function is performed by a multiplication function  100  that multiplies the data value for Manifold Pressure Error data  48 A by a data value for a Proportional Gain factor  101 . A data value for a Proportional Contribution  103  is the resulting product of the multiplication and forms a first input to a summing function  102 .  
         [0055]    The derivative function of P-LI-D  62  is performed in the following manner. A data value for Manifold Pressure Error  48 A is filtered by a low-pass filtering function  106  and then differentiated by a derivative function  104 . A multiplication function  108  multiplies the data value resulting from the differentiation by a Derivative Gain factor  107 . The product of that multiplication is a second data input to summing function  102 .  
         [0056]    A third input to summing function  102  is a data value for an Integral Contribution  115 . A multiplication function  110  multiplies the data value for Manifold Pressure Error  48 A by an Integral Gain factor  117 , and the multiplication product  119  is integrated by an integration function  112 . The integration result is processed by a limiting function  105  and if necessary, limited maximally or minimally before being allowed to pass to a conditional integral function  114  for further processing. The data value passed to the conditional integral function is designated Integral Value  105 A.  
         [0057]    Proportional Gain factor  101 , Derivative Gain factor  107 , and Integral Gain factor  117  are calibration constants.  
         [0058]    Conditional integral function  114  determines if the data value for Integral Value  105 A will be allowed to pass to summing function  102 . If the data value is allowed to pass, then P-LI-D function  62  acts as a proportional, integral, derivative controller (PID controller). But if the data value is not allowed to pass, then P-LI-D function  62  acts as a proportional and derivative controller. When acting as a PID controller, P-LI-D function utilizes Control Signal  29  as feedback for limiting the contribution that the integration function can make under certain conditions.  
         [0059]    Detail of the sub-strategy embodied in conditional integral function  114  is shown in FIG. 5. Function  114  comprises an absolute value function  116 , two comparison functions  118 ,  120 , a timer function  122 , an AND logic function  124 , and a switch function  126 . A data value for an Error Band  121  defines a range of data values for manifold pressure error for which it has been determined that the integral function of P-LI-D function  62  will be active in contributing to summing function  102 , provided that the data value for Manifold Pressure Error  48 A has remained within that range for a defined time. Absolute value function  116  and comparison function  118  coact to determine whether the manifold pressure error leaves the range. A data value for an Integral Activation Time  123  sets the defined time. So long as the data value for Manifold Pressure Error  48 A is within the range, timer function  122  runs, and when the running time finally exceeds the defined time set by the Integral Activation Time  123 , AND function  124  acts via switch function  126  to pass the data value for Integral Value  105 A to be the data value for Integral Contribution  115 . Should the data value for Manifold Pressure Error  48 A leave the defined range, AND function  124  acts via switch function  126  to cause the data value for Integral Contribution  115  to be set to zero value, and timer function  122  is reset to zero.  
         [0060]    The conditional-I sub-strategy is believed to be a useful technique for optimizing response and accuracy in turbocharger boost control. By utilizing only P-D control when error is relatively larger, the control strategy is endowed with faster response and less overshoot than would be the case if P-I-D control were employed. Exclusive reliance on P-D control however never allows the error to be reduced to zero, or at least substantially zero. By allowing integral control to become effective once error has been reduced to less than the relatively larger error during which only P-D control was effective, then it becomes possible to reduce the error to zero, or substantially zero.  
         [0061]    [0061]FIGS. 6A and 6B show a control strategy that in a number of respects is like the one described with reference to FIGS. 2 and 3. Hence, like reference numerals and designations in the two pairs of Figures represent like functions and data. The strategy of FIGS. 6A and 6B differs from that of FIGS. 2A and 2B in that the Feed Forward Contribution to Control Signal  29  and the Desired Manifold Pressure are developed in different ways. There are also differences in the overspeed protection sub-strategy that will be explained with reference to FIG. 7 later.  
         [0062]    In FIGS. 6A and 6B, processed Barometric Pressure performs a slewing function in the development of the Feed Forward Contribution to Control Signal  29  and the Desired Manifold Pressure, i.e. the desired boost. There are two look-up tables  34 A,  34 B that correlate Feed Forward data values with sets of processed Engine Speed and processed Engine Load data values. Table  34 A applies for a high altitude range, while table  34 B applies for a low altitude range. A function generator  34 C correlates slewing factor values with values of barometric pressure. For a processed Barometric Pressure, the processing uses function generator  34 C to determine a corresponding slewing factor value. That value is used in slewing (reference numeral  35 ) of respective feed-forward values that are obtained from respective tables  34 A and  34 B in correlation with present processed Engine Speed and processed engine load to yield the Feed Forward Contribution to Control Signal  29 . In this way, the Feed Forward Contribution is compensated for changes in altitude.  
         [0063]    Collectively, the two look-up tables  34 A,  34 B and the stewing function  34 C would be equivalent to a single three-dimensional look-up table of feed-forward values that would cover the entire altitude range for all sets of values of processed Engine Speed and processed engine load.  
         [0064]    Processed Barometric Pressure is likewise used in the performance of a slewing function to develop Desired Manifold Pressure. Two look-up tables  36 A,  36 B correlate Desired Manifold Pressure data values with sets of processed Engine Speed and processed Engine Load data values. Table  36 A applies for a high altitude range, while table  36 B applies for a low altitude range. A function generator  36 C correlates slewing factor values with values of barometric pressure. For a processed Barometric Pressure, the processing uses function generator  36 C to determine a corresponding slewing factor value. That value is used in slewing (reference numeral  37 ) of respective values that are obtained from respective tables  36 A and  36 B in correlation with present processed Engine Speed and processed engine load to yield a value for Desired Manifold Pressure. The value yielded is still subject to limiting by limiting function  42 . In this way, the processed Desired Manifold Pressure also is compensated for changes in altitude.  
         [0065]    Collectively, the two look-up tables  36 A,  36 B and the slewing function  36 C would be equivalent to a single three-dimensional look-up table that would cover the entire altitude range for all combinations of engine speed and fueling.  
         [0066]    [0066]FIG. 7 shows an enhanced form of overspeed protection sub-strategy that provides a different form of barometric pressure compensation for the predicted turbine shaft speed. Processed Barometric Pressure is used in the performance of a slewing function to develop a predicted value for turbine shaft speed. Two look-up tables  74 A,  76 A correlate shaft speed data values with sets of processed Engine Speed and processed Manifold Pressure. Table  74 A applies for a high altitude range, while table  74 B applies for a low altitude range. A function generator  75 A correlates slewing factor values with values of barometric pressure. For a processed Barometric Pressure, the processing uses function generator  75 A to determine a corresponding slewing factor value. That value is used in slewing (reference numeral  77 A) of respective values that are obtained from the respective tables  74 A and  76 A in correlation with present processed Engine Speed and processed Manifold Pressure to yield a value for shaft speed.  
         [0067]    The overspeed protection sub-strategy further comprises four function generators  140 ,  142 ,  144 ,  146 , two comparison functions  148 ,  150 , an AND logic function  152 , a latch function  154 , a multiplication function  156 , and a switch function  158 .  
         [0068]    The value of processed Barometric Pressure is used to select from function generator  144  a value for barometric pressure compensation. Shaft speed is used to select from function generator  146  a value for Overspeed Protection Command Signal Correction. The two selected values are processed by multiplication function  156  to yield a data value for Overspeed Protection Contribution to Control Signal  29 .  
         [0069]    Switch function  158 , when set, allows the overspeed protection contribution to Control Signal  29  to pass to summing function  50 . When the switch function is not set, it does not allow the contribution to pass, and instead makes a zero value contribution.  
         [0070]    The purpose of latch function  154  is to control the setting and resetting of switch function  158  as did relay function  94  in FIG. 3. Hysteresis continues to be imparted to the switch function characteristic, but now with the hysteresis band being adjusted for changes in barometric pressure.  
         [0071]    Hence, turbine shaft speed data is compared with values corresponding to the hysteresis band limits by the respective comparison functions  148 ,  150 . Function generator  140  sets an upper limit for the band according prevailing barometric pressure. Function generator  142  sets a lower limit for the band according prevailing barometric pressure.  
         [0072]    Whenever shaft speed equals or exceeds the upper band limit, comparison function  148  sets latch function  154 . The setting of the latch function in turn sets switch function  158 , causing the calculated overspeed protection contribution to the control signal to pass to summing function  50  where it is subtracted from the sum of the other two inputs to the summing function.  
         [0073]    Whenever shaft speed equals or falls below the lower limit of the hysteresis band, comparison function  150  resets latch function  154 . The resetting of the latch function in turn resets switch function  158 , terminating passage of the calculated overspeed protection contribution to the control signal to summing function  50 , and instead passing a zero value contribution.  
         [0074]    While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention are applicable to all embodiments and uses that fall within the scope of the following claims.