Patent Publication Number: US-11384700-B2

Title: Spark ignition engine control with exhaust manifold pressure sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 15/351,721 filed on Nov. 15, 2016, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to spark ignited engines, and more particularly is concerned with NOx estimation and control of spark ignited engine operations in response to exhaust manifold pressure outputs from an exhaust manifold pressure sensor. 
     BACKGROUND 
     A spark ignited engine can employ NOx feedback to control spark timing, lambda, 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, such as those that employ an aggressive fuel, such as type D fuel, the NOx sensor has a very short useful life and is not recommended or desirable for use. 
     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, particularly as NOx limits decrease. For example, as engine-out limits decrease, the uncertainty in the estimation in the engine out NOx amount may result in operating condition adjustments that cause an engine out NOx amount occurring that is lower than a threshold amount, resulting in a lambda below a lean limit and high coefficient of variation (COV) of cylinder gross indicated mean effective pressure (GIMEP) and misfire. 
     Other methods for controlling emissions targets and engine operations, such as those that employ targeting an air-fuel ratio or lambda, also suffer from deficiencies due to various conditions such as variation in fuel composition, fuel quality, and humidity, and also due to sensor drift error and sensor condition deterioration over time. In addition, pumping mean effective pressure (PMEP) estimates employed in engine controls that are made without an exhaust manifold pressure sensor suffer from uncertainties due to waste gate position, variable geometry turbine inlet position, and variable valve timing and cam phaser position. Thus, there remains a need for additional improvements in systems and methods for NOx estimation and in the control spark ignited engine operations. 
     SUMMARY 
     Unique systems, methods and apparatus are disclosed for controlling operation of a spark ignited internal combustion in response to an exhaust manifold pressure measurement of the engine. In one embodiment, an engine out NOx amount for at least one cylinder is determined at least in part in response to the exhaust manifold pressure measurement and a BMEP of the at least one cylinder. An operating condition of the engine is adjusted in response to the engine out NOx amount. 
     In another embodiment, the exhaust manifold pressure determination is employed to determine a mass charge flow which approximates trapped air mass in the at least one cylinder, and a traditional TOB determination of the engine out NOx amount is modified by replacing the IMP and IMT inputs in the TOB determination with the mass charge flow to estimate the engine out NOx amount in response to a ratio of the BMEP to the mass charge flow, and also in response to the exhaust manifold pressure and the spark timing. 
     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 an exhaust manifold pressure sensor. 
         FIG. 2  is a schematic illustration of a cylinder of the internal combustion engine system of  FIG. 1 . 
         FIG. 3  is a flow diagram of an example procedure for controlling operation of the internal combustion engine in response to exhaust manifold pressure. 
         FIG. 4  is a schematic illustration of a controller apparatus. 
         FIG. 5  is a flow diagram of another embodiment procedure for controlling operation of the internal combustion engine in response to exhaust manifold pressure. 
         FIG. 6  is a flow diagram of other embodiment procedures for controlling operation of the internal combustion engine in response to exhaust manifold pressure. 
     
    
    
     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. 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), 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 , 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 an 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 . 
     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 . 
     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 fuelling from a single fuel source, such as first fuel source  102  or second fuel source  104 . In another embodiment, dual fuelling 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 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. Controller  100  can also be connected to exhaust manifold pressure sensor  98  to detect or measure an exhaust manifold pressure so that an engine out NOx amount can be estimated that is determined at least in part in response to the exhaust manifold pressure measured by EMP sensor  98  during operation of engine  30 . 
     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 5-6 . 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 a procedure  300  such as shown in  FIG. 3  for operating a virtual engine out NOx sensor. 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. The mean in-cylinder AFT can be predicted based on gross power, which can be represented by GIMEP, and in-cylinder trapped air mass, which can be represented by the mass charge flow (MCF) and cylinder trapped air mass. In addition, NOx is affected by peak AFT, which is based on the centroid of heat release location. Therefore engine out NOx estimates are determined as a function of peak AFT, which is a function of the ratio of GIMEP to MCF, and also the centroid of heat release (which is related to combustion or spark timing), as follows:
 
NOx= f (GIMEP/MCF,centroid)  Equation 1
 
     Procedure  300  uses an EMP measurement from EMP sensor  98  to estimate GIMEP based on a measured or estimated BMEP and PMEP, where PMEP is the difference between the EMP and IMP. With GIMEP instead of BMEP in the NOx estimation, engine out NOx estimation is more accurate and more robust to uncertainties caused by waste gate positioning, cam phaser positioning, exhaust back pressure, turbo degradation, and other component uncertainties and wear since the pressure in the exhaust manifold is directly measured by EMP sensor  98 . 
     In one embodiment, procedure  300  includes an operation  302  to determine BMEP in response to an indicated torque or horsepower and displacement volume of engine  30  and to determine net indicated mean effective pressure (NIMEP). The NIMEP is determined from the sum of BMEP (brake power) and friction mean effective pressure (FMEP). Since FMEP is a function of engine power in general, it can be estimated by BMEP, IMP and EMP. Procedure  300  continues at operation  304  to receive an exhaust manifold pressure measurement input from EMP sensor  98  and estimate PMEP as the difference between the EMP and the IMP. Procedure  300  continues at operation  306  to determine the GIMEP from the sum of the NIMEP and the PMEP. 
     With the determination of GIMEP, procedure  300  continues at operation  308  to determine the engine speed and mass charge flow (MCF) and residual mass from engine sensor(s)  90 . Procedure  300  further continues at operation  310  to estimate an engine out NOx amount for the measured engine speed based on a ratio of the GIMEP to mass charge flow since NOx is directly correlated to this ratio. Procedure  300  continues at operation  312  to adjust one or more operating conditions of engine system  20  in response to the estimated engine out NOx amount to meet a targeted performance, such as an emissions target. 
     In addition to providing a more robust virtual sensor estimation of engine out NOx amount due to basing the determination at least in part on GIMEP, the exhaust manifold pressure measurement from EMP sensor  98  also improves the volumetric efficiency calculation that can be used to estimate the mass charge flow. In addition, the EMP changes with the residual mass charge flow changes, which allows the engine out NOx estimate to account for charge flow temperature effects, diluent effects, volumetric-efficiency effects, and charge uniformity effects. 
     Referring to  FIG. 4 , there is shown one embodiment of a controller apparatus such as controller  100  which includes a NOx estimation module  402  and an operations control module  404 . NOx estimation module  402  receives an EMP sensor input  406  from EMP sensor  98  and other inputs (virtual and/or physical)  408  from engine sensor(s)  90  and/or cylinder sensor(s)  96 . NOx estimation module  402  is configured to determine an estimated engine out NOx amount  410  for one or more cylinders  34  in response to inputs  406 ,  408 . 
     In one embodiment, NOx estimation module  402  is configured to determine BMEP in response to an indicated torque or horsepower and displacement volume, and also to determine NIMEP based on BMEP. NOx estimation module  402  also determines PMEP from the EMP input  406  and an IMP input. NOx estimation module  402  also determines the GIMEP from the sum of the NIMEP and the PMEP, and also to determine MCF. The engine out NOx amount is estimated for a determined engine speed based on a ratio of the GIMEP to mass charge flow since NOx is directly correlated to this ratio. Operations control module  404  is configured to receive the engine out NOx amount and output an operating lever adjustment command  412  to meet or maintain an engine operating performance target and/or emissions target. 
     In operation  312  and/or operating lever adjustment command  412 , 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 in response to the exhaust manifold pressure sensor measurement to control operations of engine system  20  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 includes 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. 
     Other inputs  408  in addition to the exhaust manifold pressure sensor measurement from EMP sensor  98  are also contemplated at operations  302 ,  304  of procedure  300 . Examples of other inputs include positions or indicators associated with a target throttle margin, WG, VGT, compressor bypass, VVA, cam phaser, spark timing, lambda, intake throttle, exhaust throttle, cylinder deactivation of one or more cylinders or cylinder banks, engine load, engine derate, addition of additional engines, stored power, TOB or NOx target, fuel flow, air flow, charge flow, EGR flow, compression ratio, valve events, fuel composition, water injection, IMT, IMP, jacket water temperature, engine speed, variable RPM of a genset, knock, low temperature coolant temperature (including inlet and outlet temperatures), high temperature coolant temperature (including inlet and outlet temperatures), RPM and RPM error, any error relative to target, kW load, combustion feedback, heat release, centroid of heat release, state of charge, EER, combustion rate (peak, etc.), cylinder pressure, peak cylinder pressure (PCP), PCP normalized by IMP, pressure ratio in cylinder, rate of pressure rise, cylinder pressure sensing, RPM based combustion feedback, knock sensor based combustion feedback, crankshaft twist based feedback, magnetostrictive torque sensors, strain gauge based cylinder pressure measurements, instrumented head bolts, instrumented gaskets, instrumented injector/plug hold-down bolts, ionization sensing, optical sensing, grid parallel, breaker status, island mode, transient compensation, load pickup, load shedding, compressor surge, engine protection, engine faults, misfire detection, charge pressure (IMP), water flow (for water injection), and any other engine sensors that quantify the operating condition. 
     In one embodiment of the procedures disclosed herein and/or of controller  100 , the exhaust manifold pressure measurement from EMP sensor  98  is used in conjunction with the virtual sensors disclosed herein to determine an engine out NOx amount, and/or used to adjust the above described levers to meet a targeted engine performance, including emissions, power, combustion, knock and efficiency of engine operations. 
     Another embodiment procedure  500  shown in  FIG. 5  in which an engine out NOx amount based on a modified TOB (indicated as TOB′) is determined. Procedure  500  includes an operation  502  where an exhaust pressure manifold measurement from EMP sensor  98  is determined. Procedure  500  further continues at operation  504  to determine a MCF and spark timing. 
     Traditional TOB includes determining BMEP, IMT, and IMP as inputs to estimate TOB, and engine out NOx is a function of TOB, spark timing (ST) and waste gate position (WG), as follows:
 
TOB=Brake Power/(IMP/IMT)  Equation 2
 
NOx= f (TOB,ST,WG)  Equation 3
 
     Brake power can be an output torque from engine  30 , such as from operating tables, from a kW-load signal from an alternator, or any other suitable technique. Since EMP is a function of WG, and since MCF is a function of IMP/IMT, engine speed and volumetric efficiency, the determination of engine out NOx with a modified TOB can be improved as follows:
 
NOx= f (TOB′,EMP,ST)  Equation 4,
 
     In Equation 4, the IMP/IMT term in Equation 2 is replaced by MCF to determine TOB′. 
     Procedure  500  includes an operation  506  to determine an engine out NOx amount from TOB′, EMP and ST. One or more operating levers of engine system  20  can then be adjusted as discussed above in response to the engine out NOx amount to adjust an operating condition of engine  30  in response to the engine out NOx amount. 
     In another embodiment of procedures  300 ,  500 , the virtual estimate of the engine out NOx amount and the EMP measurement from EMP sensor  98  can be employed to balance NOx and knock. A NOx target can be achieved by adjusting engine operating levers to obtain different lambdas and spark timings. The performance of engine system  20  can be optimized by adjusting spark timing as needed to avoid knock conditions while staying within the NOx target. 
     Another approach of estimating NOx employs a “black box” model. The black box model can include, for example, regression equations, maps, neural network model, and other types of information. The black box model takes measurements of GIMEP, IMP, IMT, engine speed, spark timing, and EMP to estimate engine out NOx. 
     In another embodiment, the throttle margin is targeted with the waste gate position WG. The throttle margin is measured and compared to a target throttle margin. For a given engine speed and load and engine out NOx amount, there is a target WG that will deliver the desired throttle margin. This allows BTE to be maximized within the throttle margin requirements for stability. While this could be employed to meet targeted NOx without an EMP sensor, an EMP measurement from the EMP sensor  98  improves the accuracy of the virtual engine out NOx sensor. 
     In another embodiment, the systems and methods in which the virtual NOx sensor is included with EMP sensor  98  also includes a physical engine out NOx sensor  99  ( FIG. 1 ). The physical engine out NOx sensor  99  is employed to determine engine out NOx unless one or more fault conditions are detected that indicate that the physical engine out NOx measurement is not accurate, such as, for example, the physical engine out NOx sensor  99  being too cold, defective, and/or providing unreasonable or unsteady measurements. In these fault conditions, the virtual NOx sensor with EMP sensor  98  is used to determine the engine out NOx amount as discussed above. 
     In a further embodiment of the systems and methods disclosed herein, such as shown in  FIG. 6 , the virtual NOx sensor disclosed herein is employed for diagnostics/prognostics of physical NOx sensor  99 , of another virtual NOx sensor, or of engine health. For example, the virtual NOx sensor is used to diagnose engine health by comparing expected values to estimated values. In one procedure  600 , an engine out NOx amount is determined according to a traditional TOB analysis or from a measured NOx amount at operation  602 . At operation  604 , the engine out NOx amount is estimated based on exhaust manifold pressure. At operation  606  the measured and/or estimated NOx amounts are compared to one another and if the engine out NOx amounts differ by more than a threshold amount, a fault can be issued. 
     In another embodiment of procedure  600 , the engine out NOx amount from a physical NOx sensor  99  and/or from the virtual NOx sensors discussed herein are used to estimate engine torque output at operation  608 . The torque output of the engine is measured at operation  610 . At operation  612  the measured and estimated torque outputs are compared to diagnose an engine condition. For example, if the estimated torque output differs from the measured torque output by more than a threshold amount, a fault can be issued. 
     Other diagnostic procedure embodiments include diagnostics of the waste gate  48  or of a cam phaser using the EMP measurement from EMP sensor  98 . For example, a change in position of these components can be expected to produce a certain change in EMP, and a fault is issued is the actual change in EMP differs from the expected change in EMP by more than a threshold amount. In another procedure, a change in spark timing is expected to produce a certain response in EMP at a fixed throttle margin. If the actual EMP pressure change and expected pressure change differ by more than a threshold amount, a fault can be issued. 
     In another embodiment, the estimated engine out NOx amount is determined and an estimated lambda is compared to a measured lambda (with a lambda sensor). A determination about fuel quality can be made from the difference between the measured and estimated lambdas, and used to improve knock control. This determination could also be used to diagnose one or more fuel mass flow sensors, or improve a fuel mass flow estimate. 
     In another embodiment procedure, the measured EMP from EMP sensor  98  is used to determine PMEP at a given operating condition and compare it to an expected PMEP value at the operating condition. If the expected and estimated PMEP values differ by more than a threshold amount, a turbo degradation fault may be indicated. The accuracy of this approach could be improved with turbo speed sensing. This procedure can also be used to automatically recalibrate the engine if the turbocharger was changed (for example a different nozzle ring for improved altitude capability). 
     This measured PMEP determined from EMP sensor  98  and the IMP at the given operating condition also provides a way to estimate GIMEP. With measured PMEP from the EMP and IMP, NIMEP is estimated. Since BMEP is measured, friction mean effective pressure (FMEP) can be determined. An increase in the FMEP over time can provide an indication of operating issues with engine  30  and an opportunity to determine or predict engine failures. 
     Various aspects of the systems and methods disclosed herein are contemplated. For example, one aspect relates to a method that includes operating an internal combustion engine system. The engine system includes an intake system connected to an engine with a 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 a charge flow from the intake system to a combustion chamber of the respective cylinder. The internal combustion engine system further includes an exhaust system including an exhaust manifold. The method includes determining a pressure in the exhaust manifold during a combustion cycle associated with at least one of the plurality of cylinders; determining a BMEP in the exhaust manifold during the combustion cycle of the at least one of the plurality of cylinders; determining an engine out NOx amount for the at least one cylinder in response to the pressure in the exhaust manifold and the BMEP; and adjusting an operating condition of the at least one engine in response to the engine out NOx amount. 
     In one embodiment, the method includes determining a GIMEP of the at least one of the plurality of cylinders in response to the pressure in the exhaust manifold and the BMEP of the at least one cylinder and determining the engine out NOx amount for the at least one cylinder in response to the GIMEP. In a refinement of this embodiment, the GIMEP is determined in response to a sum of a PMEP and a NIMEP. In a further refinement, the PMEP is determined in response to a differential between the pressure in the exhaust manifold and an intake manifold pressure, and the NIMEP is based on the BMEP. In another refinement of the embodiment, the method includes determining a mass charge flow and a spark timing associated with the least one cylinder, and wherein determining the engine out NOx amount is based on a ratio of the GIMEP to the mass charge flow and the spark timing. 
     In another embodiment, the method includes comparing the engine out NOx amount from the at least one cylinder in response to an engine out NOx amount threshold and adjusting the operating condition of the engine to reduce the engine out NOx amount in response to the engine out NOx amount exceeding the engine out NOx amount threshold. In a refinement of this embodiment, adjusting the operating condition includes at least one of the following: opening or closing at least one of an intake throttle in the intake system, a compressor bypass valve in a compressor bypass, a waste gate of a turbine in the exhaust system, and a variable geometry turbine inlet in the exhaust system; and varying a lift profile of at least one of an intake valve and an exhaust valve of the cylinder. 
     In another embodiment, adjusting the operating condition includes adjusting a spark timing to avoid a knock condition in the at least one cylinder. In yet another embodiment, the method includes comparing the engine out NOx amount to a measured engine out NOx amount from the physical NOx sensor and diagnosing a condition of the physical NOx sensor in response to the comparison. In a refinement of this embodiment, the method includes estimating a torque output of the internal combustion engine in response to the measured engine out NOx amount; measuring a torque output of the internal combustion engine with at least one engine sensor; comparing the estimated torque output with the measured torque output; and diagnosing a condition of the internal combustion engine in response to the comparison. 
     According to another aspect, a method includes operating an internal combustion engine system including an intake system connected to an engine with a 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 a charge flow from the intake system to a combustion chamber of each of the respective cylinders. The internal combustion engine system further includes an exhaust system with an exhaust manifold. The method further includes determining a pressure in the exhaust manifold during a combustion cycle associated with at least one of the plurality of cylinders; determining a BMEP in the exhaust manifold during the combustion cycle of the at least one of the plurality of cylinders; determining a mass charge flow and a spark timing associated with the at least one cylinder during the combustion cycle; determining a modified TOB from a ratio of the BMEP to the mass charge flow; determining an engine out NOx amount for the at least one cylinder in response to the modified TOB, the pressure in the exhaust manifold and the spark timing; and adjusting an operating condition of the at least one engine in response to the engine out NOx amount. 
     According to another aspect, a system includes an internal combustion engine including a plurality of cylinders and at least one engine sensor and an exhaust system configured to receive exhaust from the plurality of cylinders. The exhaust system includes an exhaust manifold pressure sensor in communication with the exhaust from at least one of the plurality of cylinders. The system also includes an intake system configured to direct a charge flow to the plurality of cylinders and a fuel system including at least one fuel source operable to provide a flow of fuel to the plurality of cylinders. A controller is connected to the internal combustion engine, the at least one engine sensor, and the exhaust manifold pressure sensor. The controller is configured to receive a pressure signal indicative of the exhaust manifold pressure and an engine signal indicative of a BMEP of the at least one cylinder during a combustion cycle associated with at least one cylinder. The controller is further configured to determine an engine out NOx amount for the at least one cylinder in response to the pressure in the exhaust manifold and the BMEP, and adjust an operating condition of the at least one engine in response to the engine out NOx amount. 
     In one embodiment, the controller is configured to determine a GIMEP of the at least one of the plurality of cylinders in response to the pressure in the exhaust manifold and the BMEP of the at least one cylinder and determine the engine out NOx amount for the at least one cylinder in response to the GIMEP. In a refinement of this embodiment, the controller is configured to determine a mass charge flow and a spark timing associated with the least one cylinder, and the engine out NOx amount is based on a ratio of the GIMEP to the mass charge flow and the spark timing. 
     In another 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 another aspect, an apparatus includes an electronic controller operatively connected with an internal combustion engine. The engine includes a plurality of cylinders and is connected to an exhaust manifold 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 electronic controller is also operatively connected with an exhaust manifold pressure sensor of the exhaust manifold and a fuel system including at least one fuel source operable to provide a flow of fuel into the charge flow to the plurality of cylinders. The electronic controller includes a plurality of modules including: a NOx estimation module configured to determine an engine out NOx amount in response to an exhaust manifold pressure from the exhaust manifold pressure sensor and a BMEP of at least one of the plurality of cylinders and an operation control module configured to adjust operations of the internal combustion engine in response to the engine out NOx amount. 
     In one embodiment, the NOx estimation module is configured to determine a GIMEP in response to the exhaust manifold pressure and the BMEP. In a refinement of this embodiment, the NOx estimation module is configured to determine a mass charge flow and a spark timing associated with the least one cylinder, and the NOx estimation module is further configured to determine the engine out NOx amount based on a ratio of the GIMEP to the mass charge flow and the spark timing. 
     In another embodiment, the operation control module is configured to adjust at least one of adjust a spark timing and a lambda in the at least one cylinder of the internal combustion engine in response to the engine out NOx amount. 
     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.