Patent Publication Number: US-11378026-B2

Title: Self-learning torque over boost combustion control

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of International Patent Application No. PCT/US19/60887 filed on Nov. 12, 2019, which claims the benefit of the filing date of U.S. Provisional Application Ser. No. 62/769,302 filed on Nov. 19, 2018, which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to combustion control for an internal combustion engine, and more particularly is concerned with combustion control of the engine using a self-learned torque over boost (TOB) reference. 
     BACKGROUND 
     A spark ignited engine can employ NOx feedback in a control algorithm, such as in a flame speed compensator algorithm, to determine combustion parameters such as spark timing and/or air-fuel ratio (AFR) in the engine cylinders. Typically a physical NOx sensor that measures engine-out NOx is used on most applications. However, for certain applications and/or operating conditions, a NOx sensor has a very short useful life and is not recommended or desirable for use, or has failed or is not reliable or active and cannot be used for combustion control. 
     One alternative method to employing a physical NOx sensor involves determining NOx with a “virtual” NOx sensor. One virtual NOx sensor technique involves a torque over boost (TOB) determination for NOx estimation. One example of TOB NOx estimation is provided in U.S. Pat. No. 5,949,146, which is incorporated herein by reference. 
     TOB is determined by the brake mean effective pressure (BMEP) (or torque output or braking power of the engine) times the ratio of the intake manifold temperature (IMT) to the intake manifold pressure (IMP). However, TOB NOx estimation may not provide the desired accuracy or robustness for the control system to provide the desired system performance. For example, TOB can vary based on varying operating conditions and particular individual engines, which creates challenges for calibration development and engine commission. Thus, there remains a need for additional improvements in systems and methods for NOx estimation and in the control of spark ignited engine operations. 
     SUMMARY 
     Unique systems, methods and apparatus are disclosed for controlling operation of a spark ignited internal combustion in response to a self-learned TOB reference. In one embodiment, a spark ignited internal combustion engine is controlled in response to a self-learned TOB reference. The self-learned TOB reference is based on a difference between a learned TOB offset and a desired TOB from a sensed or target TOB. The learned TOB offset at a given operating condition, such as charge pressure, can be found by interpolating between the learned charge pressure breakpoints in the TOB learning algorithm. 
     In a further embodiment, the TOB learning algorithm can include using a filtered charge pressure value to indicate the engine load at which the TOB offset (the difference between the desired TOB and sensed TOB) is learned. An index determination is made using a look up table with charge pressure as an input and an array index of learned charge pressure and associated learned TOB offset as outputs to the combustion control algorithm. 
     This summary is provided to introduce a selection of concepts that are further described below in the illustrative embodiments. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a portion of an internal combustion engine system with a charge pressure sensor. 
         FIG. 2  is a schematic illustration of a cylinder of the internal combustion engine system of  FIG. 1 . 
         FIG. 3  is a diagram of an example control logic for learning a TOB offset for controlling operation of the internal combustion engine. 
         FIG. 4  is a diagram of an example control logic for integrating the learned TOB offset in a combustion control algorithm. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, any alterations and further modifications in the illustrated embodiments, and any further applications of the principles of the invention as illustrated therein as would normally occur to one skilled in the art to which the invention relates are contemplated herein. 
     With reference to  FIG. 1 , an internal combustion engine system  20  is illustrated in schematic form. A fueling system  21  is also shown in schematic form that is operable with internal combustion engine system  20  to provide fueling for engine  30  from a first fuel source  102 . In one embodiment, only one fuel source  102  is provided and fuel source  102  is located so that the fuel is pre-mixed with the charge flow upstream of the combustion chambers of engine cylinders  34 . In another embodiment, the fuel from first fuel source  102  is injected directly into the cylinder(s) via direct injection or via port injection. In yet another embodiment, fueling system  21  includes an optional second fuel source  104  for also providing fueling, and internal combustion engine system  20  is a dual fuel system. 
     Internal combustion engine system  20  includes engine  30  connected with an intake system  22  for providing a charge flow to engine  30  and an exhaust system  24  for output of exhaust gases in an exhaust flow. In certain embodiments, the engine  30  includes a spark ignited internal combustion engine in which a gaseous fuel flow is pre-mixed with the charge flow from first fuel source  102 . The gaseous fuel can be, for example, natural gas, bio-gas, methane, propane, ethanol, producer gas, field gas, liquefied natural gas, compressed natural gas, or landfill gas. 
     In another embodiment, engine  30  includes a lean combustion engine such as a diesel cycle engine that uses a liquid fuel in second fuel source  104  such as diesel fuel as the sole fuel source, or in combination with a gaseous fuel in first fuel source  102  such as natural gas. However, other types of liquid and gaseous fuels are not precluded, such as any suitable liquid fuel and gaseous fuel. In the illustrated embodiment, the engine  30  includes six cylinders  34   a - 34   f  in a two cylinder bank  36   a ,  36   b  arrangement. However, the number of cylinders (collectively referred to as cylinders  34 ) may be any number, and the arrangement of cylinders  34  unless noted otherwise may be any arrangement including an in-line arrangement, and is not limited to the number and arrangement shown in  FIG. 1 . 
     Engine  30  includes an engine block  32  that at least partially defines the cylinders  34 . A plurality of pistons, such as piston  70  shown in  FIG. 2 , may be slidably disposed within respective cylinders  34  to reciprocate between a top-dead-center position and a bottom-dead-center position while rotating a crankshaft  78 . Each of the cylinders  34 , its respective piston  70 , and the cylinder head  72  form a combustion chamber  74 . One or more intake valves, such an intake valve  92 , and one or more exhaust valves, such as exhaust valve  94 , are moved between open and closed positions by a conventional valve control system, cam phaser, or a variable valve timing system, to control the flow of intake air or air/fuel mixture into, and exhaust gases out of, the cylinder  34 , respectively. 
       FIG. 2  shows a single engine cylinder  34  of the multi-cylinder reciprocating piston type engine shown in  FIG. 1 . The control system of the present invention could be used to control fuel delivery and combustion in an engine having only a single cylinder or any number of cylinders, for example, a four, six, eight or twelve cylinder or more internal combustion engine. In addition, control system may be adapted for use on any internal combustion engine having compression, combustion and expansion events, including a rotary engine, two stroke cycle engines, four stroke cycle engines, N stroke cycle engines, HCCI engine, PCCI engines, and a free piston engine. In other embodiments system  20  includes a motor/generator and an energy storage system configured to provide hybrid operations in which power is selectively provided by the engine, the energy storage system and motor/generator, and combinations of these. The control system of the present invention may also be employed with any suitable ignition system, including spark plug  80 , diesel pilot ignition, plasma, laser, passive or fuel fed pre-chamber, and integrated pre-chamber spark plug ignition systems, for example. 
     The control system may further include a cylinder sensor  96  for sensing or detecting an engine operating condition indicative of the combustion in combustion chamber  74  and generating a corresponding output signal to controller  100 . Cylinder sensor  96  permits effective combustion control capability by detecting an engine operating condition or parameter directly related to, or indicative of, the combustion event in cylinder  34  during the compression and/or expansion strokes. For example, cylinder sensor  96  can measure cylinder pressure (average or peak), charge pressure, knock intensity, start of combustion, combustion rate, combustion duration, crank angle at which peak cylinder pressure occurs, combustion event or heat release placement, effective expansion ratio, a parameter indicative of a centroid of heat release placement, location and start/end of combustion processes, lambda, and/or an oxygen amount. 
     In one embodiment, engine  30  is a four stroke engine. That is, for each complete engine combustion cycle (i.e., for every two full crankshaft  78  rotations), each piston  74  of each cylinder  34  moves through an intake stroke, a compression stroke, a combustion or power stroke, and an exhaust stroke. Thus, during each complete combustion cycle for the depicted six cylinder engine, there are six strokes during which air is drawn into individual combustion chambers  74  from intake supply conduit  26  and six strokes during which exhaust gas is supplied to exhaust manifold  38 . As discussed further below, the present invention measures an exhaust manifold pressure with at least one exhaust manifold pressure sensor  98  at one or more locations in exhaust manifold  38  and determines an estimate of the NOx output from the one or more cylinders  34  based at least in part on the exhaust manifold pressure. 
     The engine  30  includes cylinders  34  connected to the intake system  22  to receive a charge flow and connected to exhaust system  24  to release exhaust gases produced by combustion of the fuel(s). Exhaust system  24  may provide exhaust gases to a turbocharger  40  (or multiple turbochargers in a single stage), although a turbocharger is not required. In still other embodiments, multiple turbochargers are included to provide high pressure and low pressure turbocharging stages that compress the intake flow. 
     Furthermore, exhaust system  24  can be connected to intake system  22  with one or both of a high pressure exhaust gas recirculation (EGR) system  50  and a low pressure EGR system  60 . EGR systems  50 ,  60  may include a cooler  52 ,  62  and bypass  54 ,  64 , respectively. In other embodiments, one or both of EGR systems  50 ,  60  are not provided. When provided, EGR system(s)  50 ,  60  provide exhaust gas recirculation to engine  30  in certain operating conditions. In any EGR arrangement during at least certain operating conditions, at least a portion the exhaust output of cylinder(s)  34  is recirculated to the engine intake system  22 . 
     In the high pressure EGR system  50 , the exhaust gas from the cylinder(s)  34  takes off from exhaust system  24  upstream of turbine  42  of turbocharger  40  and combines with intake flow at a position downstream of compressor  44  of turbocharger  40  and upstream of an intake manifold  28  of engine  30 . In the low pressure EGR system  60 , the exhaust gas from the cylinder(s)  34   a - 34   f  takes off from exhaust system  24  downstream of turbine  42  of turbocharger  40  and combines with intake flow at a position upstream of compressor  44  of turbocharger  40 . The recirculated exhaust gas may combine with the intake gases in a mixer (not shown) of intake system  22  or by any other arrangement. In certain embodiments, the recirculated exhaust gas returns to the intake manifold  28  directly. In yet another embodiment, the system  20  includes a dedicated EGR loop in which exhaust gas from one or more, but less than all, of cylinders  34  is dedicated solely to EGR flow during at least some operating conditions. 
     Intake system  22  includes one or more inlet supply conduits  26  connected to an engine intake manifold  28 , which distributes the charge flow to cylinders  34  of engine  30 . Exhaust system  24  is also coupled to engine  30  with engine exhaust manifold  38 . Exhaust system  24  includes at least one exhaust conduit  46  extending from exhaust manifold  32  to an exhaust valve. In the illustrated embodiment, exhaust conduit  46  extends to turbine  42  of turbocharger  40 . Turbine  42  may include a valve such as controllable waste gate  48  or other suitable bypass that is operable to selectively bypass at least a portion of the exhaust flow from turbine  42  to reduce boost pressure and engine torque under certain operating conditions. In another embodiment, turbine  42  is a variable geometry turbine with a size-controllable inlet opening. In another embodiment, the exhaust valve is an exhaust throttle that can be closed or opened. Turbocharger  40  may also include multiple turbochargers. Turbine  42  is connected via a shaft  43  to compressor  44  that is flow coupled to inlet supply conduit  26 . 
     In yet another embodiment, the exhaust system  24  includes exhaust conduit  46  connected with one of the banks  36   a  of cylinders  34  (e.g. cylinders  34   a - 34   c ) and another, second exhaust conduit  46 ′ connected to the other of the banks  36   b  of cylinders  34  (e.g. cylinders  34   d - 34   f .) The exhaust conduits  46 ,  46 ′ may each include an exhaust sensor  47 ,  47 ′ that measures engine-out NOx. Engine out NOx or an average knock index may be used as feedback control of the engine  30  in a closed loop combustion control algorithm, such as for flame speed compensation. 
     An aftertreatment system (not shown) can be connected with an outlet conduit  66 . The aftertreatment system may include, for example, oxidation devices (DOC), particulate removing devices (PF, DPF, CDPF), constituent absorbers or reducers (SCR, AMOX, LNT), reductant systems, and other components if desired. In one embodiment, exhaust conduit  46  is flow coupled to exhaust manifold  32 , and may also include one or more intermediate flow passages, conduits or other structures. Exhaust conduit  46  extends to turbine  42  of turbocharger  40 . A second turbocharger may be provided if a second exhaust conduit  46 ′ is included with system  20 . 
     Compressor  44  receives fresh air flow from intake air supply conduit  23 . Fuel source  102  may also be flow coupled at or upstream of the inlet to compressor  44  which provides a pre-mixed charge flow to cylinders  34 . Intake system  22  may further include a compressor bypass (not shown) that connects a downstream or outlet side of compressor  44  to an upstream or inlet side of compressor  44 . Inlet supply conduit  26  may include a charge air cooler  56  downstream from compressor  44  and intake throttle  58 . In another embodiment, a charge air cooler  56  is located in the intake system  22  upstream of intake throttle  58 . Charge air cooler  56  may be disposed within inlet air supply conduit  26  between engine  30  and compressor  44 , and embody, for example, an air-to-air heat exchanger, an air-to-liquid heat exchanger, or a combination of both to facilitate the transfer of thermal energy to or from the flow directed to engine  30 . 
     In operation of internal combustion engine system  20 , fresh air is supplied through inlet air supply conduit  23 . The fresh air flow or combined flows can be filtered, unfiltered, and/or conditioned in any known manner, either before or after mixing with the EGR flow from EGR systems  50 ,  60  when provided. The intake system  22  may include components configured to facilitate or control introduction of the charge flow to engine  30 , and may include intake throttle  58 , one or more compressors  44 , and charge air cooler  56 . The intake throttle  58  may be connected upstream or downstream of compressor  44  via a fluid passage and configured to regulate a flow of atmospheric air and/or combined air/EGR flow to engine  30 . Compressor  44  may be a fixed or variable geometry compressor configured to receive air or air and fuel mixture from fuel source  102  and compress the air or combined flow to a predetermined pressure level before engine  30 . The charge flow is pressurized with compressor  44  and sent through charge air cooler  56  and supplied to engine  30  through intake supply conduit  26  to engine intake manifold  28 . 
     Fuel system  21  is configured to provide either fueling from a single fuel source, such as first fuel source  102  or second fuel source  104 . In another embodiment, dual fueling of engine  30  from both of fuel sources  102 ,  104  is provided. In one dual fuel embodiment, fuel system  21  includes first fuel source  102  and second fuel source  104 . First fuel source  102  is connected to intake system  22  with a mixer or connection at or adjacent an inlet of compressor  44 . Second fuel source  104  is configured to provide a flow of liquid fuel to cylinders  34  with one or more injectors at or near each cylinder. In certain embodiments, the cylinders  34  each include at least one direct injector  76  for delivering fuel to the combustion chamber  74  thereof from a liquid fuel source, such as second fuel source  104 . In addition, at least one or a port injector at each cylinder or a mixer at an inlet of compressor  44  can be provided for delivery or induction of fuel from the first fuel source  102  with the charge flow delivered to cylinders  34 . 
     A direct injector, as utilized herein, includes any fuel injection device that injects fuel directly into the cylinder volume (combustion chamber), and is capable of delivering fuel into the cylinder volume when the intake valve(s) and exhaust valve(s) are closed. The direct injector may be structured to inject fuel at the top of the cylinder or laterally of the cylinder. In certain embodiments, the direct injector may be structured to inject fuel into a combustion pre-chamber. Each cylinder  34 , such as the illustrated cylinders  34  in  FIG. 2 , may include one or more direct injectors  76  in the duel fuel engine embodiment. The direct injectors  76  may be the primary fueling device for liquid fuel source  104  for the cylinders  34 . 
     A port injector, as utilized herein, includes any fuel injection device that injects fuel outside the engine cylinder in the intake manifold to form the air-fuel mixture. The port injector injects the fuel towards the intake valve. During the intake stroke, the downwards moving piston draws in the air/fuel mixture past the open intake valve and into the combustion chamber. Each cylinder  34  may include one or more port injectors (not shown). In one embodiment, the port injectors may be the primary fueling device for first fuel source  102  to the cylinders  34 . In another embodiment, the first fuel source  102  can be connected to intake system  22  with a mixer upstream of intake manifold  28 , such as at the inlet or upstream of compressor  44 . 
     In certain dual fuel embodiments, each cylinder  34  includes at least one direct injector that is capable of providing all of the designed primary fueling amount from liquid fuel source  104  for the cylinders  34  at any operating condition. First fuel source  102  provides a flow of a gaseous fuel to each cylinder  34  through a port injector or a natural gas connection upstream of intake manifold  28  to provide a second fuel flow (in the dual fuel embodiment) or the sole fuel flow (in a single fuel source embodiment) to the cylinders  34  to achieve desired operational outcomes. 
     In the dual fuel embodiment, the fueling from the second, liquid fuel source  104  is controlled to provide the sole fueling at certain operating conditions of engine  30 , and fueling from the first fuel source  102  is provided to substitute for fueling from the second fuel source  104  at other operating conditions to provide a dual flow of fuel to engine  30 . In the dual fuel embodiments where the first fuel source  102  is a gaseous fuel and the second fuel source  104  is a liquid fuel, a control system including controller  100  is configured to control the flow of liquid fuel from second fuel source  104  and the flow of gaseous fuel from first fuel source  102  in accordance with engine speed, engine loads, intake manifold pressures, and fuel pressures, for example. In single fuel embodiments where the sole fuel source  102  is a gaseous fuel, a control system including controller  100  is configured to control the flow of gaseous fuel from first fuel source  102  in accordance with engine speed, engine loads, intake manifold pressures, and fuel pressures, for example. In single fuel embodiments where the sole fuel source  104  is a liquid fuel, a control system including controller  100  is configured to control the flow of liquid fuel from second fuel source  104  in accordance with engine speed, engine loads, intake manifold pressures, and fuel pressures, for example. 
     One embodiment of system  20  shown in  FIG. 2  includes each of the cylinders  34  with a direct injector  76  (in dual fuel embodiment) and/or a spark plug  80 , associated with each of the illustrated cylinders  34   a - 34   f  of  FIG. 1 . Direct injectors  76  are electrically connected with controller  100  to receive fueling commands that provide a fuel flow to the respective cylinder  34  in accordance with a fuel command determined according to engine operating conditions and operator demand by reference to fueling maps, control algorithms, or other fueling rate/amount determination source stored in controller  100 . Spark plugs  80  are electrically connected with controller  100  to receive spark or firing commands that provide a spark in the respective cylinder  34  in accordance with a spark timing command determined according to engine operating conditions and operator demand by reference to fueling maps, control algorithms, or other fueling rate/amount determination source stored in controller  100 . 
     Each of the direct injectors  76  can be connected to a fuel pump (not shown) that is controllable and operable to provide a flow or fuel from second fuel source  104  to each of the cylinders  34  in a rate, amount and timing determined by controller  100  that achieves a desired torque and exhaust output from cylinders  34 . The fuel flow from first fuel source  102  can be provided to an inlet of compressor  44  or to port injector(s) upstream of cylinders  34 . A shutoff valve  82  can be provided in fuel line  108  and/or at one or more other locations in fuel system  21  that is connected to controller  100 . The gaseous fuel flow is provided from first fuel source  102  in an amount determined by controller  100  that achieves a desired torque and exhaust output from cylinders  34 . 
     Controller  100  can be connected to actuators, switches, or other devices associated with fuel pump(s), shutoff valve  82 , intake throttle  58 , waste gate  48  or an inlet to a VGT or an exhaust throttle, spark plugs  80 , and/or injectors  76  and configured to provide control commands thereto that regulate the amount, timing and duration of the flows of the gaseous and/or liquid fuels to cylinders  34 , the charge flow, and the exhaust flow to provide the desired torque and exhaust output in response to an estimated NOx amount based at least in part on the measured exhaust manifold pressure and a predetermined engine out NOx limit. 
     In addition, controller  100  can be connected to physical and/or virtual engine sensor(s)  90  to detect, measure and/or estimate one or more engine operating conditions outside of cylinders  34  such as charge pressure, IMT, IMP, mass charge flow (MCF), EGR flow, an oxygen amount or lambda in the exhaust, engine speed, engine torque, spark timing, waste gate or turbine inlet position, and other operating conditions. An EMP sensor  98  can measure exhaust manifold pressure during engine operation. Controller  100  can be connected to a charge pressure sensor  97  to detect or measure a pressure in the charge flow during engine operation. 
     As discussed above, the positioning of each of the actuators, switches, or other devices associated with fuel pump(s), shutoff valve  82 , intake throttle  58 , waste gate  48  or an inlet to a VGT or an exhaust throttle, spark plug(s)  80 , injector(s)  76 , intake and/or intake valve opening mechanisms, cam phasers, etc. can be controlled via control commands from controller  100 . In certain embodiments of the systems disclosed herein, controller  100  is structured to perform certain operations to control engine operations and fueling of cylinders  34  with fueling system  21  to provide the desired engine speed, torque outputs, spark timing, lambda, and other outputs or adjustments in response to the exhaust manifold pressure measurement from EMP sensor  98 . 
     In certain embodiments, the controller  100  forms a portion of a processing subsystem including one or more computing devices having memory, processing, and communication hardware. The controller  100  may be a single device or a distributed device, and the functions of the controller  100  may be performed by hardware or software. The controller  100  may be included within, partially included within, or completely separated from an engine controller (not shown). The controller  100  is in communication with any sensor or actuator throughout the systems disclosed herein, including through direct communication, communication over a datalink, and/or through communication with other controllers or portions of the processing subsystem that provide sensor and/or actuator information to the controller  100 . 
     The controller  100  includes stored data values, constants, and functions, as well as operating instructions stored on computer readable medium. Any of the operations of exemplary procedures described herein may be performed at least partially by the controller. Other groupings that execute similar overall operations are understood within the scope of the present application. Modules may be implemented in hardware and/or on one or more computer readable media, and modules may be distributed across various hardware or computer implemented. More specific descriptions of certain embodiments of controller operations are discussed herein in connection with  FIGS. 3 and 4 . Operations illustrated are understood to be exemplary only, and operations may be combined or divided, and added or removed, as well as re-ordered in whole or in part. 
     Certain operations described herein include operations to interpret or determine one or more parameters. Interpreting or determining, as utilized herein, includes receiving values by any method, including at least receiving values from a datalink or network communication, receiving an electronic signal (e.g., a voltage, frequency, current, or pulse-width modulation (PWM) signal) indicative of the value, receiving a software parameter indicative of the value, reading the value from a memory location on a computer readable medium, receiving the value as a run-time parameter by any means known in the art, and/or by receiving a value by which the interpreted or determined parameter can be calculated, and/or by referencing a default value that is interpreted or determined to be the parameter value. 
     In one embodiment, controller  100  is configured to perform operations such as shown in  FIGS. 3 and 4  for real-time learning and updating of a TOB reference used in the control and operation of engine  30  based on the virtual NOx sensor measurements provided by TOB. In one embodiment, the updated TOB reference is an updated TOB error that is used as a virtual sensor for NOx error for combustion control of engine  30  when NOx sensor(s)  47 ,  47 ′ have failed or are not active. Learning of the TOB reference reduces effort in tuning and calibrating TOB to the specific engine attributes and operating conditions, and facilitates integration of TOB into the combustion control algorithm for engine  30 . 
     Engine out NOx concentration is directly correlated to adiabatic flame temperature (AFT), which is the temperature of complete combustion products in the constant volume combustion process without doing work, no heat transfer, or changes in kinetic or potential energy. One type of combustion control algorithm is a flame speed compensator, which is a closed loop combustion control algorithm that uses engine out NOx or an average knock index as feedback to control operation of the spark ignition engine  30 . The flame speed compensator control algorithm actively switches closed loop control feedback between knock and NOx based on the knock and NOx error. When the NOx sensor(s)  47 ,  47 ′ fail or are not active, the NOx error in the control algorithm is replaced by the updated TOB error determined according to the logic and procedures disclosed herein. 
     Referring to  FIG. 3 , a control logic diagram  300  for TOB self-learning is illustrated. TOB has a strong correlation with engine out NOx, but does not have a one-to-one relationship at different operating conditions, and TOB is sensitive to engine part-to-part variation. Diagram  300  includes a first input  302  for a charge pressure of a charge flow to one of more of the cylinders  34  of engine  30 . The charge pressure inputs  302  are processed in a low pass filter  304 . In the illustrated embodiment, charge pressure is used to represent engine load, and a filtered charge pressure value is used to indicate the engine load at which the TOB is learned. However, the use of other operating parameters to indicate the condition at which the TOB is learned is not precluded. 
     Diagram  300  also includes a desired TOB input  306  and a sensed TOB input  308 , and the difference between these inputs is determined as a TOB error and passed through low pass filter  310 . When the engine is at steady state, the sensed TOB identifies an appropriate combustion condition. The desired TOB is tuned in a test cell environment for nominal operating conditions. The error between the desired TOB and sensed TOB is the TOB error that is filtered and learned as the learned TOB offset  312  at a measured engine load condition indicated by the learned charge pressure  316 . 
     An index determination block  314  receives the filtered charge pressure from low pass filter  304  as an input, and outputs an array index to determine the learned TOB offset  312  and the learned charge pressure  316 . In one embodiment, the index determination is a two-dimensional look-up table. Based on the index determined by the input charge pressure at block  314 , the learned TOB offset  312  and learned charge pressure  316  are stored in an appropriate array index. The learned TOB offsets  312  are thus identified at varying load conditions and other associated operating conditions (e.g. fuel quality, humidity, altitude, exhaust back pressure, spark timing, air/fuel ratio and/or any other captured conditions) at that load condition, and the learned TOB offsets  312  and the learned charge pressure  316  are stored in a memory of the controller  100  as a power down save. 
     Referring to  FIG. 4 , there is shown control logic diagram  400  that captures the integration of the learned TOB offset  312  in the combustion control algorithm, such as a flame speed compensator (FSC). The learned TOB offset  312  and learned charge pressure  316  are provided to a calculator  402  that determines a learned final desired TOB at a given operating condition. The learned final desired TOB at calculator  402  can be found by interpolating between the learned charge pressure breakpoints in the learning algorithm. 
     The updated learned TOB error provided to block  408  is determined by subtracting the learned final desired TOB determined by calculator  402  from the desired TOB input  404 , and then subtracting this difference from the sensed TOB input  406 . Since the units of TOB are different than NOx, a loop gain multiplier is used to convert the updated learned TOB error to a NOx error at block  408 . 
     The output from block  408 , along with the NOx sensor status from block  410  and NOx error from block  412 , are provided to an evaluation block  420 . Under conditions in which the NOx sensors are inoperable or inactive, the NOx error conversion based on the learned TOB error is provided as an input to the combustion control algorithm  422 . The combustion control algorithm  422  determines a combustion control error  418  based on the NOx error from either the NOx sensor(s) or updated TOB error if the NOx sensor(s) are inactive or disabled, the control state  414  of the algorithm, and the knock error  416 . The final error  418  can be used by an engine control module of controller  100  to output an operating lever adjustment command to meet or maintain an engine operating performance target and/or emissions target. 
     The adjustment in the one or more operating conditions and/or operating lever adjustment includes, for example, adjusting at least one operating lever of system  20  associated with one or more of the lambda and spark timing in order to deliver one or more of a target engine out NOx amount, a target knock margin, a target brake thermal energy (BTE), and/or a target coefficient of variance for the GIMEP. Levers of system  20  that effect the engine out NOx amount and that can be controlled in response to the estimated engine out NOx amount to meet a NOx target include one or more of IMT, humidity, spark timing, coolant temperature, compression ratio, intake/exhaust valve timing (opening and closing), swirl, lambda, air-fuel ratio, water injection, steam injection and membranes, for example. 
     Possible levers of system  20  that can be adjusted to meet emissions or other performance targets may include, for example, valves, pumps and/or other actuators that control a fuel flow to cylinders  34  and/or an air flow to cylinders  34 . Further example levers include an intake air throttle position, a waste gate position, a turbine inlet opening size, a compressor bypass, variable valve actuator, a cam phaser, a variable valve timing, switching between multiple lift profiles/cams, compression braking, Miller cycling (early and/or late intake valve closing), cylinder bank cutout, cylinder cutout, intermittent cylinder deactivation, exhaust throttle, spark timing, IMT regulation, changing displacement of engine, changing number of strokes in cycle (e.g. 2 stroke vs. 4 stroke), pressure relief valve venting in the intake and/or exhaust, bypassing one or more of the compressors or turbines in a single stage turbocharger system or two stage turbocharger system or in a multiple turbine system, switching turbines in and out, and activating electrically activated turbocharging/supercharging, power-turbine (coupled to crank or alternator), turbo-compounding, exhaust throttle control downstream of one or more of the turbines, and EGR flow from one or more of a dedicated EGR, high pressure EGR loop, low pressure EGR loop, and internal EGR. 
     Various aspects of the systems and methods disclosed herein are contemplated, including those in the claims appended hereto and in the discussion above. For example, one aspect is directed to a method including: determining a pressure in a charge flow to at least one of a plurality of cylinders of an internal combustion engine system; determining a TOB error associated with the pressure in the charge flow; learning a TOB offset and a charge pressure at the associated pressure in the charge flow; determining an updated TOB error in response to the learned TOB offset, a desired TOB, and a sensed TOB; and adjusting an operating condition of the at least one engine in response to the updated TOB error. 
     In an embodiment, the internal combustion engine system includes an intake system connected to the plurality of cylinders and at least one fuel source operably connected to the internal combustion engine system to provide a flow of fuel to each of the plurality of cylinders. The intake system is coupled to each of the plurality of cylinders to provide the charge flow from the intake system to a combustion chamber of the respective cylinder. The internal combustion engine system further includes an exhaust manifold connected to an exhaust system. In one refinement of this embodiment, the exhaust system includes first and second exhaust conduits connected to respective ones of first and second exhaust conduits of the exhaust system. In a further refinement, the first and second exhaust conduits include respective ones of first and second exhaust sensors. In yet a further refinement, the first and second NOx sensors are failed or not active. 
     In another embodiment of the method, learning the TOB offset and the charge pressure includes applying an index value to the TOB error that is based on the pressure in the charge flow. In one refinement, the method includes storing the learned TOB offset and the learned charge pressure in an array index of a look-up table. In a further refinement, the method includes associating one or more engine operating conditions with the learned TOB offset at the learned charge pressure. In yet a further refinement, the one or more operating conditions include one or more of fuel quality, humidity, altitude, exhaust back pressure, spark timing, and air/fuel ratio. 
     In another embodiment, the pressure in the charge flow is indicative of an engine load. In yet another embodiment, the TOB error is determined in response to a difference between a desired TOB and a second TOB. In still another embodiment, the method includes converting the updated TOB error to a NOx error. 
     According to another aspect, a system includes an internal combustion engine including a plurality of cylinders and at least one engine sensor, an exhaust system configured to receive exhaust from the plurality of cylinders, and an intake system configured to direct a charge flow to the plurality of cylinders. The system also includes a fuel system including at least one fuel source operable to provide a flow of fuel to the plurality of cylinders and a controller connected to the internal combustion engine and the at least one engine sensor. The controller is configured to receive a pressure signal indicative of the charge flow pressure and determine a TOB error associated with the charge flow pressure, learn a TOB offset and learn a charge pressure at the associated charge flow pressure, determine an updated TOB error in response to the learned TOB offset, a desired TOB, and a sensed TOB, and adjust an operating condition of the internal combustion engine in response to the updated TOB error. 
     In one embodiment, the fuel is selected from the group consisting of natural gas, bio-gas, methane, propane, ethanol, producer gas, field gas, liquefied natural gas, compressed natural gas, or landfill gas. In another embodiment, the controller is configured to adjust at least one of the following in response to the engine out NOx amount: a spark timing in the at least one cylinder in response to the engine out NOx amount; and a lambda in the at least one cylinder in response to the engine out NOx amount. 
     According to yet another aspect, an apparatus includes an electronic controller. The controller is operable to: determine a pressure in a charge flow to at least one of a plurality of cylinders of an internal combustion engine system; determine a TOB error associated with the pressure in the charge flow; learn a TOB offset and a charge pressure at the associated pressure in the charge flow; determine an updated TOB error in response to the learned TOB offset, a desired TOB, and a sensed TOB; and adjust an operating condition of the at least one engine in response to the updated TOB error. 
     In one embodiment, the controller is configured to: learn the TOB offset and the charge pressure at the associated pressure by applying an index value to the TOB error that is based on the pressure in the charge flow; store the learned TOB offset and the learned charge pressure in an array index of a look-up table; and associate one or more engine operating conditions with the learned TOB offset at the learned charge pressure. 
     In another embodiment, the pressure in the charge flow is indicative of an engine load. In still another embodiment, the TOB error is determined in response to a difference between a desired TOB and a second TOB. In yet another embodiment, the controller is configured to convert the updated TOB error to a NOx error. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain exemplary embodiments have been shown and described. Those skilled in the art will appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. 
     In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary.