Abstract:
A valve ( 50 ) selectively shunts exhaust gas around a stage ( 20 T) of a turbocharger turbine ( 20 ) under control of a control system that selectively renders an EGR system ( 38 ) active and inactive and that develops a value for a set-point of operation for the valve. The control system comprises a first map set ( 60, 142 ) containing data that the control system uses to the exclusion of data in a second map set ( 80, 148 ) to develop the set-point value when the EGR system is active. When the EGR system is inactive, the control system uses the data in the second map set to the exclusion of the data in the first map set to develop the set-point value.

Description:
FIELD OF THE INVENTION 
   This invention relates to internal combustion engines, such as diesel engines, that have turbochargers, such as two-stage turbochargers. 
   BACKGROUND OF THE INVENTION 
   A known turbocharger system for an engine comprises a two-stage turbocharger that comprises high- and low-pressure turbines in series flow relationship and a bypass valve that is in parallel flow relationship to the high-pressure turbine and under the control of the engine control system. The engine control system processes various data to control the bypass valve such that exhaust back-pressure and engine boost are regulated in an appropriate way according to the manner in which the engine is being operated. The high-pressure stage can be designed to have a relatively smaller size that is optimized for low-end engine performance while the low-pressure stage can be designed with a relatively larger size for high-end performance. 
   Exhaust gas recirculation (EGR) is typically used over a wide range of engine operating conditions under various ambient conditions to aid in controlling tailpipe emissions for achieving compliance with applicable laws and regulations. Engine engineers may however deem it appropriate, without violating applicable laws and/or regulations, to temporarily suspend the use of EGR during certain conditions that affect engine operation in certain ways. Reasons for such temporary non-use of EGR may include the following: EGR is simply unnecessary; EGR has no significant effect; or EGR is actually detrimental. Another possible reason is that sufficiently precise control of EGR cannot be realized while such conditions prevail. 
   A turbocharger control strategy may take into account many engine operating parameters, including EGR, over a wide range of engine operation. When an engine operates while prevailing conditions require EGR to be inactive, a turbocharger control strategy may require adaptation for inactive EGR. 
   Failure to adapt the strategy for inactive EGR can cause the engine to operate in ways that are detrimental to engine performance and/or durability. One consequence is an undesired increase in exhaust back-pressure (EBP) caused by a turbocharger control strategy striving to increase boost when it is inappropriate to do so because certain prevailing conditions, such as an engine being cold or being extremely hot to the point of overheating stipulate that EGR be rendered inactive during those conditions. 
   SUMMARY OF THE INVENTION 
   The present invention in one respect relates to a system and method for adapting a turbocharger control strategy to avoid such undesired increases in engine EBP. 
   The disclosed preferred embodiment of the invention employs multiple map sets containing data values for turbocharger set-point. One map set is used to the exclusion of another depending on whether the engine control system is rendering the EGR system active or inactive. When engine temperature is cold or too hot, the EGR system is rendered inactive and one map set is used to the exclusion of another to control turbocharger set-point and consequently both EBP at an engine exhaust manifold and manifold absolute pressure (MAP) at an engine intake manifold. When engine temperature is neither cold nor too hot, the EGR system is rendered active and the other map set is used to the exclusion of the one map set. 
   The set-point map may contain data values based on EBP, MAP, or MGP (manifold gauge pressure). The system preferably uses a closed-loop control strategy using a data value for a set-point from a selected map as a command input and a data value from a suitable data source as a feedback input that is subtracted from the command input to provide an error signal that the strategy continually seeks to null out for causing the turbocharger to operate as closely as possible to the set-point. The particular source of feedback is appropriate to the particular parameter represented by data values in the maps, i.e. EBP, MAP, or MGP. 
   Accordingly, one generic aspect of the present invention relates to an internal combustion engine that has an intake system including a turbocharger compressor for developing combustion charge air for the engine, combustion chambers where charge air and fuel are combusted to operate the engine, and an exhaust system through which exhaust gas resulting from combustion pass from the combustion chambers. The exhaust system also includes a turbocharger turbine that uses exhaust gas to operate the turbocharger compressor. 
   An EGR system recirculates some exhaust gas from the exhaust system to the intake system when active, but at times may be rendered inactive by the control system, particularly when the engine is either cold or too hot. 
   A bypass valve shunts a stage of the turbine and when open, shunts exhaust gas around the stage. 
   The control system renders the EGR system selectively active and inactive. It also develops a value for a set-point of operation for the valve defining the extent to which the valve shunts exhaust gas around the turbine stage, thereby setting a set-point for turbocharger operation. 
   The control system comprises a first map set containing data that the control system uses to the exclusion of data in a second map set in developing the set-point value when the EGR system is active. The control system uses the data in the second map set to the exclusion of the data in the first map set when the EGR system is inactive. 
   Still other generic aspects relate to the control system and to a method for turbocharger set-point control. 
   The foregoing, along with further features and advantages of the invention, will be seen in the following 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, now briefly described as follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a general schematic diagram of an exemplary internal combustion engine having an engine control system in accordance with principles of the present invention. 
       FIG. 2  is first portion of a software strategy diagram representing algorithms programmed in the engine control system in accordance with principles of the present invention. 
       FIG. 3  is a second portion of the software strategy diagram. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows an exemplary internal combustion engine  10  having an intake system  12  through which air for combustion enters the engine and an exhaust system  14  through which exhaust gas resulting from combustion exits the engine. Engine  10  is, by way of example, a turbocharged diesel engine comprising a two-stage turbocharger  16  that has a low-pressure stage  18  and a high-pressure stage  20 . By way of example, engine  10  is a multi-cylinder V-type engine having intake manifolds  22  and exhaust manifolds  24 , and when used in a motor vehicle, such as a truck, is coupled through a drivetrain (not shown) to propel the vehicle. 
   Air drawn into intake system  12  follows an entrance path indicated by arrows  26  leading to a compressor  18 C of low-pressure stage  18 . A compressor  20 C of high-pressure stage  20  is in downstream series flow relationship to compressor  18 C via a path marked by arrows  28 . A path marked by arrows  30  continues from compressor  20 C through a charge air cooler  32  and an intake throttle valve  34  to intake manifolds  22 . 
   From intake manifolds  22 , charge air enters engine cylinders  36  into which fuel is injected to form a mixture that is combusted to power the engine. Gas resulting from combustion is exhausted through exhaust system  14 , but some portion may be recirculated through an exhaust gas recirculation (EGR) system  38 . Recirculated exhaust gas from exhaust manifolds  24  follows a path marked by arrows  40  through an EGR cooler  42  and an EGR valve  44  back to intake manifolds  22 . 
   Upon leaving exhaust manifolds  24 , exhaust gas that is not recirculated is constrained to take one or both of two parallel paths marked by respective arrows  46 ,  48 . Path  46  comprises a turbine  20 T of high-pressure stage  20 , and path  48 , a bypass valve  50 . After turbine  20 T and valve  50 , the paths  46 ,  48  merge into a common path  52  leading to a turbine  18 T of low-pressure stage  18 . Beyond turbine  18 T, exhaust gas may pass through one or more exhaust gas treatment devices, such as a catalyst  54 , before being exhausted to atmosphere. 
   Exhaust bypass valve  50  is under the control of the engine control system. The engine control system processes various data to control valve  50  such that exhaust back-pressure and engine boost are regulated in an appropriate manner according to the manner in which the engine is being operated. An advantage of having two turbines  20 T,  18 T in series flow relationship, with valve  50  providing for control of the amount of exhaust gas allowed to bypass turbine  20 T, is that high-pressure stage  20  can be designed to be smaller in size and optimized for low-end engine performance, while low-pressure stage  18  can be designed to be larger in size for better high-end performance. 
   By closing exhaust bypass valve  50  during low-end engine operation the entire exhaust gas flow passes through both turbines  20 T,  18 T, and high-pressure compressor  20 C will develop higher outlet pressure that so that the charge air is developed by both compressor stages. This can provide desirable increased low-end boost. 
   Over a mid-speed range and high end of engine operation, valve  50  may be operated to partially open or fully open condition as appropriate to achieve desired boost and back-pressure. 
   The inventive turbocharger bypass control (TCBC) strategy is embodied in the engine control system which comprises one or more processors containing algorithms for processing data. Through control of valve  50 , the strategy may be considered to control the set-point for turbocharger operation. 
   A data value for desired turbocharger bypass control of valve  50  in a particular engine is a parameter TCBC_DES shown in  FIG. 2 . The data value is developed by the engine control system to set the extent to which valve  50  is open. The data value for TCBC_DES is a function primarily of engine speed and engine load. Data values for TCBC_DES are contained in a generic map  60  shown in  FIG. 2  where speed data represented by the parameter N and engine load represented by engine fueling data MFDES are inputs to map  60 . 
   A data value for TCBC_DES is selected from map  60  based on the input data values for speed and load. Compensation and filtering for certain transient conditions and certain parameters such as barometric pressure and engine temperature may be present in some systems, and limiting of various data may be performed as appropriate. Map  60  may actually comprise a single map or multiple individual maps each providing a corresponding range of set-point data values correlated appropriately with speed and load data values. The stored data values may represent any of the parameters EPB, MAP, or MGP. 
   The basic strategy for controlling valve  50  is premised on repeatedly calculating a set-point that defines the extent to which the valve should be open by repeatedly processing data for the appropriate parameters such as engine speed and engine fueling. The set-point changes as values of relevant parameters change. The basic strategy may also take in account the occurrences of certain transients by including their effects in the set-point calculation. The strategy then uses the calculated set-point as a control input to a control portion of the strategy that strives to operate valve  50  so that the amount of valve opening corresponds as closely as possible to the set-point in real time. Such processing may use a combination of feed-forward and feedback control, with the latter using proportional and integral (PI) control, or it may use only feedback control. 
   Certain operating conditions may affect engine operation in ways that make it appropriate to temporarily discontinue use of EGR while those conditions continue. It has been observed that when EGR is shut off, i.e. rendered inactive, because an engine is either cold or very hot, the turbocharger increases boost and hence mass airflow into the engine, and as a result can create unacceptable increase in engine EBP due in significant part to the increased mass flow. 
   The present invention is directed to a solution for avoiding such EBP increases. As will be explained herein, the solution is embodied by providing several additional maps that, when the strategy renders EGR temporarily inactive, are used in substitution of other maps that are used when EGR is active. In this way the basic control strategy is maintained, but is adapted in an especially convenient way to avoid unacceptable EBP increases resulting from conditions like those just described. 
   The data value for TCBC_DES results from evaluation of a data value for a parameter TCBC_DES_SP (also in  FIG. 2 ) performed by an evaluation function  70  for compliance with minimum and maximum limits represented by respective parameters TCBC_DES_LMN and TCBC_DES_LMX. If the data value for TCBC_DES_SP is within the limits, it is passed by function  70 . If it is above the maximum, the value passed is the value of TCBC_DES_LMX. If it is below the minimum, the value passed is the value of TCBC_DES_LMN. The switch functions  72 ,  74  shown inside the broken line rectangles designated KOER Diagnostics and P-I Cal Development are provided for diagnostic and development purposes. When the turbocharged engine is operating in a production motor vehicle, like a truck, those switch functions pass the data value of TCBC_DES_SP to function  70  for evaluation. 
   The data value for TCBC_DES_SP may be either the data value for a parameter TCBC_SP or the data value for a parameter TCBC_SPF, as selected by the coaction of a switch function  76  and a switch function  78  based on various modes numbered “1” through “5”. Mode “0” represents the normal operating mode in a production motor vehicle. Other modes are used for diagnostic or development purposes. 
   In mode “0”, the data value for TCBC_DES_SP is the data value for TCBC_SPF. Depending on the state of switch function  78 , the data value for TCBC_SPF may be either the data value for TCBC_SP or a data value from a map  80 . In accordance with certain principles of the present invention, switch function  78  is controlled by a parameter EGR_DISABLE. When use of EGR is discontinued, EGR_DISABLE causes switch function  78  to provide a data value from map  80  as the data value for TCBC_SPF. When use of EGR is resumed, EGR_DISABLE causes switch function  78  to provide the data value for TCBC_SP as the data value for TCBC_SPF. 
   As will be more apparent from further description, it is this selective use of: A) a data value from map  80  that changes the set-point in a way that avoids undesired increase in EBP when the engine control system renders EGR inactive because the engine is either cold or too hot, and B) the data value for TCBC_SP when the engine control system renders EGR active, that are significant to principles of the present invention. 
   The data value for a parameter MFDES corresponds to the rate at which the engine is being fueled, and may be derived from any suitably appropriate source. The data value for a parameter N corresponds to engine speed, and may be derived from any suitably appropriate source. MFDES and N are inputs to the generic map  60 , which as noted earlier may comprise multiple individual maps. 
   Map  60 , whether a single map or multiple maps, contains a number of data values, each of which correlates with a respective pair of data values, one for engine fueling MFDES and one for engine speed N. Each data value for engine fueling MFDES represents a corresponding fractional span of a range of engine fueling while each data value for engine speed represents a corresponding fractional span of a range of engine speeds. For any given combination of engine fueling and engine speed, engine fueling will fall within one of its fractional spans in the map, and engine speed will fall within one of its fractional spans, causing the particular data value stored in the map in correlation with the two respective fractional spans to be supplied for further processing by function  70 , and it is that basic set-point value that is further processed through the strategy when EGR is active. As will be explained later, it is the data value from a different map  80  that is processed further through the strategy when EGR is not active. 
   The data value passed through evaluation function  70  forms an input for closed-loop control of valve  50 . A summing function  120  (see  FIG. 2 ) is where the loop is closed. In mode “0”, function  120  subtracts a data value TCBC_EBP_EST from the data value for TCBC_DES. The data value for TCBC_EBP_EST may be obtained in any suitably appropriate way, such as by estimation, or by actual measurement using a device like a sensor at an appropriate location in the engine system. The difference is a data value representing the error between the two. 
   When the data stored in map  60  represents EBP, the data value for TCBC_EBP_EST represents actual or estimated EBP. When the data stored in map  60  represents MAP, the data value for TCBC_EBP_EST represents actual or estimated MAP. When the data stored in map  60  represents MGP, the data value for TCBC_EBP_EST represents actual or estimated MGP. 
   The error difference is next evaluated by an evaluation function  122  against minimum and maximum preset limits, as shown by  FIG. 2 . If the data value for the error difference is more positive than the data value for the maximum preset limit (parameter TCBC_ERR_LMX), then the data value for TCBC_ERR_LMX is passed. If the data value for the error difference is more negative than the data value for the minimum preset limit (parameter TCBC_ERR_LMN), then the data value for TCBC_ERR_LMN is passed. If the data value for the error difference is between the limits, the data value for the actual error difference itself is passed. 
   The error data value that is passed is designated by a parameter TCBC_ERR, which is then processed by TCBC P-I &amp; Feed-Forward Control that is shown in  FIG. 3  to comprise both a proportional function  130  and an integral function  132  that process TCBC_ERR. A respective gain is associated with each function  130 ,  132 , the gain KP being associated with proportional function  130  and the gain KI being associated with integral function  132 . Each gain is itself a function of engine fueling and engine speed. 
     FIG. 3  further shows TCBC P-I &amp; Feed-Forward Control to comprise a map  134  for setting the gain for proportional function  130  and a map  136  for setting the gain for integral function  134 . 
   Map  134  contains a number of data values of proportional gain KP, each of which correlates with a respective pair of data values, one for engine fueling MFDES and one for engine speed N. Each data value for engine fueling MFDES represents a corresponding fractional span of a range of engine fueling while each data value for engine speed represents a corresponding fractional span of a range of engine speeds. For any given combination of engine fueling and engine speed, engine fueling will fall within one of its fractional spans in map  134 , and engine speed within one of its fractional spans, causing the particular data value for proportional gain KP corresponding to the two respective fractional spans to be supplied to a multiplication function  138 . 
   Map  136  contains a number of data values of integral gain KI, each of which correlates with a respective pair of data values, one for engine fueling MFDES and one for engine speed N. Each data value for engine fueling MFDES represents a corresponding fractional span of a range of engine fueling while each data value for engine speed represents a corresponding fractional span of a range of engine speeds. For any given combination of engine fueling and engine speed, engine fueling will fall within one of its fractional spans in map  136 , and engine speed within one of its fractional spans, causing the particular data value for integral gain corresponding to the two respective fractional spans to be supplied to an integrator  140  of integral function  132 . Integrator  140  includes clamp-logic for constraining the integration rate to maximum and minimum limits. 
   The data value for a parameter TCBC_FFD represents an approximate target value a feed-forward component for the TCBC set-point. The data value for TCBC_FFD may be either the data value for a parameter TCBC_EGR_FFD or a data value selected from a map  142 , as selected by the coaction of a switch function  144  and a switch function  146  based on the various modes “0” through “5”. 
   In mode “0”, the states of switch functions  144 ,  146  cause the data value for TCBC_FFD to be determined by the state of switch function  146  that is under the control of EGR_DISABLE. When use of EGR is discontinued, EGR_DISABLE causes switch function  146  to provide a data value from a map  148  as the data value for TCBC_EGR_FFD. When use of EGR is resumed, EGR_DISABLE causes switch function  146  to provide the data value for TCBC_EGR_FFD as a data value selected from map  142 . 
   Map  142  that contains a number of data values representing set-point target values that are to be used when EGR is active. Each set-point target data value correlates with a respective pair of data values, one for engine speed N and one for engine fueling MFDES. Each data value for engine speed represents a corresponding fractional span of the total engine speed range while each data value for fueling represents a corresponding fractional span of the total range of engine fueling. For any given combination of engine speed and engine fueling, engine speed will fall within one of the fractional speed spans in the map, and engine fueling within one of the fractional fueling spans, causing the particular set-point target value corresponding to the two respective fractional spans to be selected for further processing when EGR is not disabled. 
   Map  148  that contains a number of data values representing set-point target values that are to be used when EGR is inactive, i.e. disabled. Each set-point target data value correlates with a respective pair of data values, one for engine speed N and one for engine fueling MFDES. Each data value for engine speed represents a corresponding fractional span of the total engine speed range while each data value for fueling represents a corresponding fractional span of the total range of engine fueling. For any given combination of engine speed and engine fueling, engine speed will fall within one of the fractional speed spans in the map, and engine fueling within one of the fractional fueling spans, causing the particular set-point target value corresponding to the two respective fractional spans to be selected for further processing when EGR is disabled. 
   One aspect of the present invention relates to this selective use of: A) a data value from map  148  that changes the set-point in a way that avoids undesired increase in EBP when the engine control system temporarily discontinues, i.e. disables, EGR because the engine is either cold or becomes too hot, and B) a data value from map  142  when EGR is active, i.e. not disabled. 
   It should be noticed that the feed-forward target set-point selection using speed and engine fueling is an open-loop function, whereas the proportional and integral control provided by functions  130 ,  132  are closed-loop functions. The strategy therefore relies on an open-loop, feed-forward function to approximate the desired TCBC set-point and a closed-loop function acting in concert with the open-loop function to actually attain the desired set-point. 
   Rather than relying on speed and desired fueling exclusively for the open-loop approximation of TCBC set-point, the disclosed strategy also includes barometric pressure BARO_KPA and an offset as additional factors. A function generator  149  and an offset (parameter TCBC_DTY_OFSET) provide two additional data values that are summed by a summing function  150  with the data value obtained from either map  142  or map  148 , depending on whether EGR is active, to create a data value for a parameter TCBC_DTY_FF representing a target data value that at least approximates desired TCBC. 
   The data value for TCBC_DTY_FF, the data value for TCBC_DTY_P provided by proportional function  130 , and the data value for TCBC_DTY_I provided by integral function  132  are algebraically summed by a summing function  152 . The data value resulting from the summation is the data value for a parameter TCBC_DTY_PIF that controls a conventional actuator (not shown) for valve  50 . 
   In summary then, the disclosed strategy has been shown to develop desired TCBC as an input to a control system for forcing actual TCBC to correspond as closely as possible to that input. Various forms of compensation can be applied to the desired TCBC input ahead of the point where the feedback loop is closed. Generic principles of the invention are broad enough to encompass any, all, or none of those various forms of compensation. The strategy may be executed at any appropriate execution rate, such as 125 hz for example. 
   The combination of proportional control and integral control, i.e. P-I control, is considered a preferred form of feedback control that is most appropriate for control of TCBC. The combination of feed-forward, open-loop control with P-I closed-loop control may also be desirable in certain applications. Principles of the invention relating to the selective disabling of EGR may however be practiced with specific control strategies other than the disclosed feed-forward, open-loop control and P-I closed-loop control. In certain control strategies the use of feed-forward control may be unnecessary, in which case, only feedback control is employed.  FIG. 3  has however illustrated the selective use of maps  142 ,  148  as an aspect of the invention that by itself is within the generic principles of the invention, and in conjunction with the selective use of map  80  is a further enhancement to generic principles represented by the selective use of map  80 . 
   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 following claims.