METHOD AND SYSTEM FOR DETERMINING AN AIR-FUEL IMBALITATION

Abstract

Methods for an engine, including: during a cylinder deactivation process, sequentially deactivating each cylinder of a cylinder group with two or more cylinders; estimating a lambda deviation for each cylinder after the sequential deactivation of each cylinder of the cylinder group; Learning an air error for each cylinder based on the estimated lambda deviation; and Display of an air-fuel imbalance for each cylinder based on the learned air error for that cylinder.

Description

Area

[0001] The present description generally relates to methods and systems for determining an air-fuel imbalance of a cylinder in an internal combustion engine of a vehicle. Background / Brief description

[0002] Compliance with engine emissions regulations requires accurate detection of air-fuel imbalance between engine cylinders. Air-fuel imbalance between engine cylinders can occur due to various factors. For example, cylinder-to-cylinder imbalance can occur due to air leaks from some cylinders, exhaust gas recirculation failures, clogged intake valves, misfiring fuel injectors, and faulty exhaust gas sensors. In addition to deteriorating emissions, air-fuel imbalances can reduce fuel efficiency and engine performance.

[0003] Cylinder-to-cylinder air-fuel imbalance may be monitored using an exhaust gas sensor to estimate an air-fuel error magnitude by associating a sensor signal with a measured air-fuel deviation. An exemplary approach for monitoring air-fuel variation in a multi-cylinder engine is described by Behr et al. in US 7,802,563 B2. In this document, exhaust gas from a first group of cylinders is directed to an exhaust gas sensor, and during selected operating conditions, an air-fuel imbalance in at least one of the cylinders is indicated based on a response of the exhaust gas sensor operating at or above the firing frequency of cylinders in the first group.By indicating an air-fuel imbalance in response to a reading from the exhaust gas sensor at or above the cylinder firing frequency, a feedback control interaction can be isolated to achieve a consistent indication of an air-fuel fault.

[0004] Further prior art is known from the documents US 2012 / 0 253 642 A1 and US 2012 / 0 053 820 A1.

[0005] However, the inventors of this application have recognized potential problems with such an air-fuel imbalance detection system. For example, poor or insufficient exhaust gas mixing at the exhaust sensor can create deviations in the sensor readings. Thus, estimates of the air-fuel error under such exhaust mixture ratios may not represent the actual cylinder imbalance. Furthermore, the geometry of the exhaust system can create additional problems in learning the air-fuel imbalance. In addition to the uneven mixing of flow from the cylinders, the flow from some cylinders may be hidden from the exhaust sensor by the flow from other cylinders. Thus, there may be some cylinders whose flow never passes through the exhaust sensor. Other disadvantages can include reduced sensitivity of the exhaust sensor during certain engine operating conditions.For example, during cold start conditions, the exhaust gas sensor may not be sufficiently warmed up and may register sensor readings with deviations, thereby impairing the learning of the cylinder's air-fuel imbalance.

[0006] In alternative concepts, the air-fuel imbalance can be learned using in-cylinder pressure or torque errors. However, such sensors can be expensive. Yet another concept is based on exhaust pressure sensors. However, such sensors can be unreliable, especially if the pressure in the exhaust manifold is measured further downstream of the cylinder outlet. Still other concepts can aggressively drive engine cylinders very lean or very rich to identify the imbalance. However, such aggressive concepts can lead to excessive emissions.

[0007] The object of the present invention is therefore to provide an improved method and system for determining an air-fuel imbalance of a cylinder in an internal combustion engine of a vehicle.

[0008] The problem is solved by the features of the independent patent claims. Advantageous developments of the invention are described in the subclaims.

[0009] In one example, the disadvantages described above may be at least partially addressed by a method for an engine comprising: during a cylinder deactivation process, sequentially deactivating each cylinder of a cylinder group having two or more cylinders; estimating a lambda deviation for each cylinder after sequentially deactivating each cylinder of the cylinder group; and learning an air error for each cylinder based on the estimated lambda deviation. In this way, the air error in cylinders of a multi-cylinder engine may be reliably and opportunistically identified while accounting for deviations generated by exhaust geometry, sensor sensitivity, and exhaust mixture.

[0010] As one example, an engine may include a plurality of cylinders located in a first and a second cylinder bank. Under conditions when engine load is low, one or more cylinders, such as all cylinders in a cylinder bank, may be selectively deactivated (e.g., fuel and spark may be deactivated) while the remaining active cylinders operate at a higher average load to reduce engine pumping losses and improve fuel economy. Before cylinder deactivation, an air-fuel ratio when all cylinders are firing may be noted. During the cylinder deactivation process, the cylinders to be deactivated may be sequentially deactivated, and a lambda deviation (from the air-fuel ratio when all cylinders are firing) for each cylinder after sequential deactivation may be determined.Since the deactivated cylinders are not receiving fuel, any lambda deviation can be attributed to air flowing through the cylinder. In this way, the air error can be learned for each cylinder. Additionally, the lambda deviation can be compared to an expected lambda deviation to learn an air error for each cylinder. A cylinder deactivation sequence can be adjusted so that the air error can be learned for each engine cylinder during the deactivation process. The learned air errors can then be used to determine air-fuel imbalance between cylinders. By learning the air error in each cylinder of the first and second cylinder banks based on the estimated lambda deviation, issues related to exhaust geometry, sensor sensitivity, and exhaust mixture can be addressed.

[0011] The approach described herein may have several advantages. For example, the method provides improved learning of air-fuel imbalance between cylinders of a multi-cylinder engine. By opportunistically deactivating each cylinder of a cylinder group during a cylinder deactivation mode of engine operation while the remaining engine cylinders are active, individual cylinder air errors can be learned regardless of the exhaust manifold geometry and even in the presence of non-uniform cylinder flow. Furthermore, cylinder imbalance can be reliably determined using an existing exhaust gas sensor. By learning the air-fuel imbalance between cylinders, engine operation can be adjusted to account for and / or compensate for such an imbalance.Thus, exhaust emissions can be reduced by reducing air-fuel ratio variations from cylinder to cylinder in an engine and fuel efficiency can be improved.

[0012] It should be understood that the foregoing summary is provided to introduce, in a simplified manner, a selection of concepts that are further described in the detailed description. It is not intended to identify central or essential features of the claimed subject matter, the scope of which is defined solely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that resolve any disadvantages noted above or in any part of this disclosure. Brief description of the drawings Fig. 1 shows an exemplary exhaust system arrangement of a variable displacement engine 10 (VDE). Fig. 2 shows a partial view of an internal combustion engine. Fig. 3 shows a flowchart for an exemplary method for opportunistically estimating cylinder-by-cylinder air-fuel imbalance parameters during a VDE mode of engine operation. 15 Fig. 4 shows a flowchart of an example method for identifying a cylinder-to-cylinder air-fuel imbalance. Fig. 5 shows a flowchart of an example method for identifying cylinder-to-cylinder air-fuel imbalance based on a composition index estimated using air-fuel ratio, exhaust pressure, and cylinder torque. Fig. 6 shows an example graphical representation for identifying a cylinder imbalance based on air error, fuel error, and air-fuel error. Detailed description

[0013] The following description relates to systems and methods for identifying cylinder-to-cylinder imbalance in a vehicle engine operating with variable displacement. As such, the variable displacement engine (VDE), such as the one in Fig. 1-2, can switch between operation in which all cylinders fire and operation in which some of the cylinders fire by selectively deactivating fuel and spark and changing the actuation of the intake and exhaust valves of the selected cylinders. Fig. Figure 2 shows a partial view of a single cylinder in a multi-cylinder engine system. An engine controller may be configured to execute a routine, such as the example routine shown in Fig. 3-4 for opportunistic estimation of cylinder-by-cylinder air-fuel imbalance parameters during a VDE operating mode of a variable displacement engine. Fig. 5 shows an example routine that may be used by the controller to identify a cylinder-by-cylinder air-fuel imbalance based on a composition index determined by a weighted air-fuel ratio based imbalance with a first confidence level, an exhaust pressure based imbalance with a second confidence level, and a cylinder torque based imbalance with a third confidence level. Fig. 6 shows an example graphical representation for identifying a cylinder imbalance based on an air fault, a fuel fault, and an air-fuel fault in cylinders of a multi-cylinder engine.

[0014] Fig. 1 shows an exemplary variable displacement (VDE) engine 10 in which cylinders (e.g., cylinders A1-A4 in cylinder bank 15A and cylinders B1-B4 in cylinder bank 15B) may have cylinder valves that are held closed during one or more engine cycles. The cylinder valves may be deactivated via hydraulically actuated lifters or via a cam profile switching (CPS) mechanism that uses a zero-lift cam for deactivated valves. Other valve deactivation mechanisms may also be used. As shown herein, the engine 10 is a V8 engine having two cylinder banks 15A and 15B (each bank containing four cylinders) with an intake manifold 44 (with throttle 62) and an exhaust manifold 48 coupled to an emissions control device 70 including one or more catalyst(s) and exhaust gas sensor(s).

[0015] During selected conditions, for example, when the full torque capability of the engine is not required, one or more cylinders in a first cylinder group and a second cylinder group may be selected for deactivation (also referred to herein as a VDE operating mode). In particular, one or more cylinders of the selected group may be deactivated by turning off the respective fuel injectors while maintaining operation of the intake and exhaust valves so that air can continue to be pumped through the cylinders. While the fuel injectors of the deactivated cylinders are turned off, the remaining activated cylinders continue to perform combustion with the fuel injectors active and functional.To meet torque requirements, the engine produces the same amount of torque in those cylinders for which the injectors remain activated. In other words, the remaining active cylinders operate at higher average cylinder loads. This requires higher manifold pressures, resulting in reduced pumping losses and increased fuel efficiency. The smaller actual surface area (only of the activated cylinders) exposed to combustion also reduces engine heat losses, improving engine thermal efficiency.

[0016] Based on a drop in torque demand, one or more cylinders may be selectively deactivated. Furthermore, the cylinders may be grouped for deactivation based on their position along the engine block, within an engine bank, and their deactivation history. As one example, the cylinders from different cylinder banks (e.g., cylinder banks 15A and 15B) may be grouped for deactivation. For example, during a first VDE condition, cylinders A1, B1, A4, and B4 may be deactivated, while cylinders A2, B2, A3, and B3 may be deactivated during a second VDE condition. In an alternative example, the first VDE pattern may include a different identity and number of cylinders than the second VDE pattern.

[0017] The engine 10 may be operated using a variety of substances that may be delivered via a fuel system 172. The engine 10 may be controlled, at least in part, by a control system including a controller 12. The controller 12 may receive various signals from sensors 4 connected to the engine 10 and send control signals to various actuators 22 connected to the engine and / or the vehicle. Additionally, the controller 12 may receive an indication of cylinder knock or pre-ignition from one or more knock sensors distributed along the engine block. If included, the plurality of knock sensors may be distributed symmetrically or asymmetrically along the engine block. Further, the one or more knock sensors may include accelerometers, ionization sensors, or cylinder pressure transducers.

[0018] Fig. 2 is a schematic representation of a cylinder of engine 10, which may be included in a propulsion system of a motor vehicle. Engine 10 may be controlled at least in part by a control system, including controller 12, and by input from a vehicle operator 132 via an input device 130. In this example, input device 130 includes an accelerator pedal and a pedal position sensor 134 for generating a pedal position proportional signal PP. Combustion chamber (i.e., cylinder) 30 of engine 10 may have combustion chamber walls 32 with a piston 36 positioned therein. Piston 36 may be connected to a crankshaft 40 such that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft 40 may be connected to at least one drive wheel of a vehicle via an intermediate gearing system.The crankshaft 40 may also be connected to a starter motor via a flywheel to initiate operation of the engine 10. Furthermore, a crankshaft torque sensor may be connected to the crankshaft 40 for monitoring cylinder torque. In one example, the torque sensor is a laser torque sensor or a magnetic torque sensor. Other torque sensors may be used. The cylinder torque may be estimated using the measured position signals from the torque sensor. Other methods may also be used to estimate the cylinder torque. As shown in FIG. Fig. As highlighted in Figures 4-5, an engine control system can infer an air-fuel imbalance based on the torque sensor output.

[0019] Combustion chamber 30 may receive intake air via an intake passage 42 from an intake manifold 44 and may expel exhaust gases via an exhaust passage 48. Intake manifold 44 and exhaust passage 48 may selectively communicate with combustion chamber 30 via a respective intake valve 52 and exhaust valve 54. In some embodiments, combustion chamber 30 may include two or more intake valves and / or two or more exhaust valves. In this example, intake valve 52 and exhaust valve 54 may be controlled by cam actuation via one or more cams and may utilize one or more of a cam profile shift (CPS) system, a variable cam timing (VCT) system, a variable valve timing (VVT) system, and / or a variable valve lift (VVL) system, which may be actuated by controller 12 to vary valve function.The position of intake valve 52 and exhaust valve 54 may be determined by position sensors 55 and 57, respectively. In alternative embodiments, intake valve 52 and / or exhaust valve 54 may be controlled by electric valve actuation. For example, cylinder 30 may alternatively include an intake valve controlled by electric valve actuation and an exhaust valve controlled by cam actuation with CPS and / or VCT systems.

[0020] In some embodiments, each cylinder of engine 10 may be configured with one or more fuel injectors for providing fuel thereto. As a non-limiting example, cylinder 30 is illustrated as including a fuel injector 66 supplied with fuel from fuel system 172. Fuel injector 66 is illustrated as being directly coupled to cylinder 30 for injecting fuel directly therein in proportion to the pulse duration of signal FPW received from controller 12 via electronic driver 68. In this manner, fuel injector 66 provides what is known as direct injection (hereinafter referred to as "DI") of fuel into combustion cylinder 30.

[0021] It should be appreciated that in an alternative embodiment, injector 66 may be an intake injector that delivers fuel into the intake port upstream of the cylinder. It is also understood that cylinder 30 may receive fuel from a plurality of injectors, such as a plurality of intake injectors, a plurality of direct injectors, or a combination thereof.

[0022] Further with reference to Fig. 2, the intake passage 42 may include a throttle 62 having a throttle plate 64. In this particular example, the position of the throttle plate 64 may be varied by the controller 12 via a signal provided to an electric motor or actuator integrated with the throttle 62, a configuration generally referred to as electronic throttle control (ETC).

[0023] In this manner, throttle 62 may be actuated to vary the intake air provided to combustion chamber 30, among other engine cylinders. The position of throttle valve 64 may be provided to controller 12 by throttle position signal TP. Intake passage 42 may include a mass air flow (MAF) sensor 120 and a manifold air pressure (MAP) sensor 122 for providing the respective MAF and MAP signals to controller 12.

[0024] An ignition system 88 may provide an ignition spark to the combustion chamber 30 via the spark plug 92 in response to the spark advance signal SA from the controller 12 during selected operating modes. Although spark ignition components are shown, the combustion chamber 30 or one or more other combustion chambers of the engine 10 may, in some embodiments, be operated in a compression ignition mode, with or without an ignition spark.

[0025] A pressure sensor 124 may be connected to the exhaust passage 48 downstream of the exhaust valve 54 and upstream of the emissions control device 70. The pressure sensor 124 is preferably located near the exhaust valve 54 to measure the exhaust manifold pressure (EMP). In one embodiment, the pressure sensor may be a pressure transducer. As shown in Fig. As highlighted in Figures 4-5, an engine control system can infer an air-fuel imbalance based on the pressure sensor output.

[0026] An upstream exhaust gas sensor 126 is shown connected to the exhaust passage 48 upstream of the emissions control device 70. The upstream sensor 126 may be any suitable sensor for providing an indication of the exhaust air-fuel ratio, such as a linear wideband oxygen sensor or universal exhaust gas oxygen sensor (UEGO), a narrowband dual-state oxygen sensor or EGO, or a heated exhaust gas oxygen sensor (HEGO). In one embodiment, the upstream exhaust gas sensor 126 is a UEGO sensor configured to provide an output, such as a voltage signal, that is proportional to the amount of oxygen present in the exhaust gas. The controller 12 uses the output to determine the exhaust air-fuel ratio. As shown in Fig. As highlighted in Figures 4-5, an engine control system can infer an air-fuel imbalance based on the exhaust gas sensor output.

[0027] The emissions control device 70 is shown disposed along the exhaust passage 48 downstream of the exhaust gas sensor 126. The device 70 may be a three-way catalyst (TWC) configured to reduce NOx and oxidize CO and unburned hydrocarbons. In some embodiments, the device 70 may be a NOx trap, various other emissions control devices, or combinations thereof.

[0028] A second, downstream exhaust gas sensor 128 is shown connected to the exhaust passage 48 downstream of the emissions control device 70. The downstream sensor 128 may be any suitable sensor for providing an indication of an exhaust air-fuel ratio, such as a UEGO, EGO, HEGO, etc. In one embodiment, the downstream sensor 128 is a HEGO sensor configured to indicate a relative richness or leanness of the exhaust gas after passing through the catalyst. As such, the HEGO sensor may provide an output in the form of a switch point, or the voltage signal at the point at which the exhaust gas switches from lean to rich.

[0029] Further, in the disclosed embodiments, an exhaust gas recirculation (EGR) system may direct a desired portion of exhaust gas from the exhaust passage 48 into the intake passage 42 via an EGR passage 140. The amount of EGR provided to the intake passage 42 may be varied by the controller 12 via an EGR valve 142. Further, an EGR sensor 144 may be disposed within the EGR passage and may provide an indication of one or more of pressure, temperature, and concentration of exhaust gas. Under some conditions, the EGR system may also be used to regulate the temperature of the air-fuel mixture within the combustion chamber.

[0030] The control 12 is in Fig. 2 as a microcomputer having a microprocessor unit 102, input / output ports 104, an electronic storage medium for executable programs and calibration values, shown as a read-only memory chip 106 in this particular example, a random access memory 108, a life support memory 110, and a data bus.The controller 12 may receive, in addition to the previously discussed signals, various signals from sensors connected to the engine 10, including an inducted mass air flow (MAF) measurement from the mass air flow sensor 120; an exhaust manifold pressure (EMP) from the pressure sensor 124; an engine coolant temperature (ECT) from the temperature sensor 112 connected to a coolant jacket 114; an ignition profile recording (PIP) signal from a Hall effect sensor 118 (or other type) connected to the crankshaft 40; a cylinder torque from the crankshaft torque sensor connected to the crankshaft 40; a throttle position (TP) from a throttle position sensor; and a manifold absolute pressure (MAP) signal from sensor 122. An engine speed signal, RPM, may be generated by the controller 12 from the PIP signal. The controller 12 may also control the various actuators of the . Fig. 2 to adjust motor operation based on the received signals and instructions stored in a controller memory.

[0031] The read-only memory storage medium 106 may be programmed with computer-readable data representing instructions executable by the processor 102 to perform the methods described above, as well as other variations that are conceivable but not expressly listed.

[0032] During operation, each cylinder within the engine 10 typically undergoes a four-stroke cycle: The cycle includes the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. During the intake stroke, the exhaust valve 54 closes and the intake valve 52 generally opens. Air is directed through the intake manifold 44 into the combustion chamber 30, and the piston 36 moves toward the bottom of the cylinder to increase the volume within the combustion chamber 30. The position where the piston 36 is near the bottom of the cylinder and at the end of its stroke (e.g., when the combustion chamber 30 has reached its largest volume) is commonly referred to by those skilled in the art as bottom dead center (BDC).

[0033] During the compression stroke, the intake valve 52 and the exhaust valve 54 are closed. The piston 36 moves toward the cylinder head to compress the air within the combustion chamber 30. The point at which the piston 36 is at the end of its stroke and closest to the cylinder head (e.g., when the combustion chamber 30 has reached its smallest volume) is commonly referred to by those skilled in the art as top dead center (TDC). In a process referred to herein as injection, fuel is introduced into the combustion chamber. In a process referred to herein as ignition, the injected fuel is ignited by known ignition devices, such as the spark plug 92, resulting in combustion.

[0034] During the power stroke, the expanding gas pushes the piston 36 back to the BDC. The crankshaft 40 converts the piston motion into torque on the rotating shaft. Finally, during the exhaust stroke, the exhaust valve 54 opens to release the combusted air-fuel mixture to the exhaust manifold 48, and the piston returns to the TDC. It should be noted that the above is shown only as an example, and that intake and exhaust valve opening and / or closing timing may vary to provide positive or negative valve overlap, late intake valve closing, and various other examples.

[0035] As described above, Fig. 2 only one cylinder of a multi-cylinder engine and each cylinder can equally contain its own set of intake / exhaust valves, fuel injectors, spark plugs, etc.

[0036] Referring to Fig. 3, an exemplary method 300 for identifying a cylinder air-fuel imbalance in a variable displacement engine is shown. Instructions for performing method 300 and the remaining methods included herein may be executed by controller 12 based on instructions stored in a memory of the controller and in conjunction with signals received from sensors of engine system 30, such as those previously described with respect to Fig. 1-2. The controller can use the engine system's engine actuators to adjust engine operation according to the procedures described below.

[0037] At 302, method 300 includes determining, estimating, and / or measuring current engine operating conditions. The operating conditions may include, among other things, an engine speed load, a torque demand, a boost pressure, a manifold air pressure, an engine temperature, a combustion air-fuel ratio, an exhaust pressure, and an engine temperature. Method 300 proceeds to 304 after the engine operating conditions are determined.

[0038] At 304, method 300 determines whether one or more air-fuel imbalance detection (AFIM) conditions are met. The AFIM detection conditions may include a threshold duration or a threshold travel distance of the vehicle since the last AFIM detection. As another example, the AFIM detection may be performed once every drive cycle. If the AFIM conditions are not met, method 300 proceeds to 306 to operate the engine with the variable displacement engine (VDE) mechanism enabled based on at least one driver demand. Specifically, the engine may operate in a VDE mode where one or more cylinders are deactivated when the driver demand is lower, and in a non-VDE mode where all cylinders are active when the driver demand is higher. The method may then end.If one or more AFIM conditions are met, the answer is YES and method 300 proceeds to 308.

[0039] At 308, method 300 determines whether VDE conditions are met. VDE conditions may be met if the driver demand is less than a threshold. If VDE conditions are met, the method continues to 310. At 310, method 300 may include determining a number of cylinders to deactivate based on the decrease in driver demand, where the number is higher as the driver-requested torque decreases. Additionally, an identity of the cylinders to deactivate may be determined. In one example, control may select an initial VDE pattern for cylinder deactivation and the duration of cylinder deactivation based on current engine operating conditions.As elaborated herein, the initial VDE pattern may be adjusted 30 in response to the AFIM sensing conditions being met so as to learn air errors for each cylinder and thus opportunistically learn cylinder-by-cylinder air-fuel imbalance during VDE mode. Method 300 may then proceed to 312. Returning to 308, if the VDE conditions are not met, the answer is NO, and method 300 proceeds to 312 to intrusively learn cylinder air errors and cylinder-by-cylinder air-fuel imbalance.

[0040] At 312, method 300 may include estimating an exhaust air-fuel ratio (or lambda value), an exhaust pressure, and individual cylinder torque values ​​when all cylinders are active. For example, the air-fuel ratio may be measured at an exhaust gas sensor (e.g., exhaust gas sensors 126 and / or 128 in Fig. 2). The control system can determine an average lambda (LAM_ALL) over an engine cycle (2 revolutions) when all cylinders are active. The exhaust pressure can be measured at an exhaust pressure sensor (e.g., pressure sensor 124 in Fig. 2) and individual cylinder torque can be measured at a crankshaft torque sensor (such as the torque sensor connected to a crankshaft 40 of each cylinder as shown in Fig. 2 shown).

[0041] After determining the air-fuel ratio, exhaust pressure, and cylinder torque when all cylinders are active, method 300 proceeds to 314. At 314, one or more cylinders may be deactivated according to the VDE pattern. In one example, a first cylinder is deactivated. For example, a cylinder of the selected VDE pattern may be deactivated while the remaining engine cylinders are kept active. Deactivation may include turning off the fuel injector and spark of the selected cylinder while the cylinder's intake and exhaust valves continue to open and close to pump air through the selected cylinder. While the fuel injector of the deactivated cylinder is deactivated, the remaining activated cylinders continue to perform combustion with the fuel injectors active and functional.For example, an engine may have two banks of cylinders, with each bank containing four cylinders (e.g., cylinders A1-A4 in bank 15A and cylinders B1-B4 in bank 15B in . Fig. 1). In one example, the selected VDE pattern may include cylinders listed according to a firing order (e.g., cylinders A1, B1, A4, B4, B3, A2, B2, and A3), where each cylinder may be selectively deactivated individually while the remaining engine cylinders remain active. By deactivating a cylinder individually, any air fault may be attributed to the deactivated cylinder. It should be appreciated that while the above examples suggest sequentially deactivating a single cylinder to learn the cylinder's air-fuel imbalance, in alternative examples, a plurality of cylinders (e.g., two or more) of the selected VDE pattern may be deactivated simultaneously. In such a case, a more complicated calculation may be required to determine the air-fuel imbalance of each cylinder and to distinguish the air faults associated with each deactivated cylinder.

[0042] After selecting one or more cylinders of the selected VDE pattern for deactivation, method 300 proceeds to 316. At 316, an air-fuel ratio / lambda, exhaust pressure, and cylinder torque may be determined, estimated, and / or measured while the individual (or one or more) cylinders of the VDE pattern are deactivated and the remaining cylinders are active. For example, the controller may deactivate one cylinder at a time using the VDE mechanisms and collect the lambda over an engine cycle for each cylinder deactivation (e.g., LAM_1 for cylinder 1, LAM_2 for cylinder 2, LAM_8 for cylinder 8 in an 8-cylinder engine). After determining lambda, exhaust pressure, and cylinder torque while one or more cylinders of the selected VDE pattern are deactivated, method 300 proceeds to 318.At 318, method 300 assesses whether lambda, exhaust pressure, and cylinder torque values ​​have been determined for all cylinders of the selected VDE pattern. If the answer is NO, the routine continues to 320. At 320, the routine reactivates the previously selectively deactivated cylinder(s) and deactivates the next cylinder (or bank of cylinders) of the selected VDE pattern and returns to 316 to determine lambda, exhaust pressure, and cylinder torque values ​​for the deactivated cylinders while keeping the remaining cylinders active. For example, if the previous cylinder selected for deactivation was A1, the next cylinder selected for deactivation may be B1. In another example, if the previous cylinders selected for deactivation were A1 and A3, the next cylinders selected for deactivation may be B1 and B3.The lambda, exhaust pressure and cylinder torque values ​​are determined while cylinder B1 (or B1 and B3) is deactivated and the remaining cylinders are active.

[0043] In one example, the cylinders to be deactivated may each be sequentially deactivated according to the selected cylinder pattern. Then, the cylinders may be reactivated, and the remaining cylinders may be sequentially deactivated, allowing all engine cylinders to be deactivated at least once during the AFIM detection of the VDE mode. In one example, the engine is a four-cylinder engine (with cylinders 1-4), and in response to the decrease in driver demand, a cylinder is to be deactivated during the VDE mode. Cylinder 1 may initially have been selected to be deactivated for the entirety of the VDE mode. However, cylinder 1 may be deactivated during the AFIM detection of the VDE mode, and a cylinder 1 air fault may be learned.Then, while VDE conditions still exist, cylinder 1 can be reactivated, and cylinder 2 can be deactivated, and an air fault of cylinder 2 can be learned. Then cylinder 2 can be reactivated, and cylinder 3 can be deactivated, and an air fault of cylinder 3 can be learned. Finally, cylinder 3 can be reactivated, and cylinder 4 can be deactivated, and an air fault of cylinder 4 can be learned. In this way, the cylinders can be deactivated sequentially during VDE mode until an air fault of every cylinder of the engine has been opportunistically learned during VDE mode.

[0044] Returning to 318, if lambda, exhaust pressure, and cylinder torque values ​​of all cylinders have been evaluated, the routine continues to 324. At 324, the engine resumes VDE operation based on current engine operating conditions. This includes keeping one or more cylinders deactivated if VDE conditions are still present. Otherwise, if the cylinder reactivation conditions are met, the deactivated cylinders are reactivated. Method 400 then continues to 402 of method 400 to determine an air-fuel imbalance between cylinders. As in Fig. As highlighted in Figure 4, the controller can calculate the difference in lambda for each cylinder from the all-cylinder value and use this difference relative to a threshold to determine whether a cylinder imbalance exists. The controller can evaluate cylinder-specific torque and exhaust pressure estimates in a similar manner. If an imbalance is detected, a diagnostic trouble code (DTC) can be set.

[0045] It should be acknowledged that, while the procedure of Fig. 3, an air-fuel imbalance between cylinders is estimated during a VDE mode by sequentially deactivating engine cylinders and learning a corresponding lambda deviation (from a value when all cylinders are firing). In further examples, the learning may also be performed during engine idle and medium load conditions with an engaged transmission and a torque converter securely coupled between the engine 30 and the transmission. This may further increase the likelihood of cylinder flow being detected at the downstream exhaust sensor, as the flow pattern is likely to change at higher flows (during medium load conditions) compared to lower flows (during idle conditions).By comparing the cylinder-specific lambda deviation to a value learned by sequentially deactivating cylinders during VDE conditions, relative to idle and medium load conditions, air-fuel imbalances caused by cylinder-specific air errors can be more reliably learned. Additionally, the stability of imbalance detection is improved. For example, false detections and missed detections of imbalance are reduced.

[0046] Fig. 4 illustrates an exemplary method 400 for learning an air-fuel imbalance between cylinders in a multi-cylinder engine. The method 400 is described in FIG. 10 herein with reference to components and systems described in Fig. 1-2, particularly with respect to engine 10, cylinder banks 15A and 15B, and controller 12. Method 400 may be performed by the controller executing instructions stored on computer-readable media. It will be appreciated that method 400 may be applied to other engine systems having different configurations without departing from the scope of this disclosure.

[0047] The approach described herein detects changes in the output of each of an exhaust gas sensor, a pressure sensor, and a torque sensor related to combustion events in cylinders that are sequentially deactivated during air-fuel imbalance learning. The exhaust gas sensor outputs a signal proportional to the oxygen concentration in the exhaust gas. The pressure sensor outputs a signal proportional to the exhaust gas pressure, while the torque sensor outputs a signal corresponding to the torque applied to the cylinders during combustion.

[0048] By deactivating a single cylinder of the selected VDE pattern while the remaining engine cylinders are allowed to combust air and fuel, the outputs of the exhaust gas sensor, pressure sensor, and torque sensor can be used to indicate a cylinder air-fuel imbalance for the deactivated cylinder. Thus, the present approach can increase a signal-to-noise ratio to determine a cylinder air-fuel imbalance. In one example, a UEGO or HEGO sensor output voltage (converted to air-fuel ratio or lambda (e.g., difference between air-fuel and air-fuel stoichiometry)) is sampled for cylinder firings after exhaust valves of the cylinders receiving fuel are open while the single cylinder of the selected VDE pattern is deactivated.The sampled oxygen sensor signal is then evaluated to determine a lambda value or an air-fuel ratio. In another example, the pressure sensor output is sampled to determine exhaust pressure, and the torque sensor output is sampled to determine cylinder torque for cylinder firings after exhaust valves of the cylinders receiving fuel are opened while the individual cylinder of the selected VDE pattern is deactivated.

[0049] Method 400 begins at 402, where air-fuel ratio / lambda, exhaust pressure (P), and cylinder torque (TQ) values ​​for each deactivated cylinder (n) of the selected VDE pattern are compared to average values ​​of lambda (LAMavg), exhaust pressure (Pavg), and cylinder torque (TQavg) when all cylinders are active. In particular, the comparison may include calculating a lambda difference (LAM_diff n ), a pressure difference (P_diff n) and a cylinder torque difference (TQ_diff n ) for each deactivated cylinder of the selected VDE pattern, as shown in the following equations. LAM_diffn=LAMavg−LAMn, P_diffn=Pavg−Pn TQ_diffn=TQavg−TQn,

[0050] After comparing the lambda, exhaust pressure, and cylinder torque values ​​for each deactivated cylinder with the average values ​​of lambda, exhaust pressure, and cylinder torque when all cylinders are active, method 400 proceeds to 404.

[0051] At 404, the differences in lambda, exhaust pressure, and cylinder torque are used to learn a torque error for each cylinder. For example, a first air error (resulting in a corresponding first torque error) for a given cylinder may be determined based on the lambda deviation after deactivation of that cylinder relative to lambda when all cylinders are firing. As another example, a second torque error for that cylinder may be determined based on the exhaust pressure deviation after deactivation of that cylinder relative to exhaust pressure when all cylinders are firing. As yet another example, a third torque error for that cylinder may be determined based on the crankshaft speed after deactivation of that cylinder relative to crankshaft speed when all cylinders are firing.The first, second, and third errors can then be compared to determine an average error for that cylinder. The same steps can then be repeated to learn the error for each engine cylinder.

[0052] In another example, the differences in lambda, exhaust pressure, and cylinder torque may be compared to thresholds to identify the presence of a cylinder air / torque fault and a corresponding air-fuel / torque imbalance between cylinders. Specifically, the lambda difference for each deactivated cylinder of the selected VDE pattern is compared to a threshold lambda difference, where the threshold lambda difference is based on an imbalance that produces either excessive emissions (e.g., higher than a threshold for emissions) or unacceptable vibration (e.g., higher than a threshold for vibration). For example, if the lambda difference is greater than the threshold lambda difference, an air-fuel imbalance may be indicated for the currently deactivated cylinder.Otherwise, if the lambda difference is less than the threshold lambda difference, no cylinder air-fuel imbalance is detected for each selected cylinder of the selected VDE pattern. Similarly, the exhaust pressure difference for the deactivated cylinder of the selected VDE pattern is compared to a threshold pressure difference, where the threshold pressure is based on a pressure imbalance that produces either excessive emissions or unacceptable vibration. If the exhaust pressure difference is greater than the threshold pressure difference, an air-fuel imbalance may be indicated for the deactivated cylinder. Otherwise, if the exhaust pressure difference is less than the threshold pressure difference, no air-fuel imbalance is detected for the deactivated cylinder.In yet another example, the 30-cylinder torque difference for the deactivated cylinder of the selected VDE pattern is compared to a threshold torque difference, where the threshold torque difference is based on a torque imbalance that is producing either excessive emissions or unacceptable vibration. If the cylinder torque difference is greater than the threshold torque difference, an air-fuel imbalance may be indicated for the currently deactivated cylinder. Otherwise, if the cylinder torque difference is less than the threshold torque difference, no air-fuel imbalance is detected for the deactivated cylinder.

[0053] The selection of a parameter (i.e., one or more of lambda, exhaust pressure, and cylinder torque differences) used for learning the cylinder's air-fuel imbalance may be selected based on a reliability of an estimate of the difference parameter, as determined based on operating conditions. For example, if the exhaust gas is insufficiently mixed and / or the exhaust sensor is insufficiently warmed up, the lambda difference may be used to learn the air-fuel imbalance of the deactivated cylinder with increased reliability. In another example, the exhaust temperature may be less than a threshold temperature during a cold-start condition, and the exhaust sensor is not sufficiently warmed up. Then, the lambda difference estimated under such conditions may be unreliable or inaccurate.Therefore, a parameter other than the lambda difference, such as the exhaust pressure difference or the cylinder torque difference, can be given greater weight during cold-start conditions to learn the air-fuel imbalance for the deactivated cylinder of the selected VDE pattern. This can improve the learning of the air-fuel imbalance of the deactivated cylinder.

[0054] After determining the air-fuel imbalance based on difference parameters for each deactivated cylinder of the selected VDE, method 400 proceeds to 404. At 404, an air-fuel imbalance (or torque deviation) is learned for each engine cylinder based on the differences. For example, the imbalance for each cylinder may be determined based on the first, second, and third errors learned from lambda deviation, exhaust pressure deviation, and crankshaft acceleration, respectively.

[0055] At 406, an airflow error is learned for each cylinder of the engine based on the corresponding lambda deviation. Specifically, since the lambda deviation is learned when a single cylinder is selectively deactivated, the error is attributed to an air error since no fueling is occurring at that time. In this way, an air error component of the air-fuel imbalance for a cylinder can be distinguished from a fuel error component of the air-fuel imbalance.

[0056] At 408, the routine assesses whether decelerated fuel shutoff (DFSO) conditions exist. DFSO conditions may include one or more of the following: an accelerator pedal not depressed, a constant or decreasing vehicle speed, and a brake pedal depressed.

[0057] Returning to 408, if DFSO conditions are met, the routine continues to 4 to learn a fuel fault for fuel injectors of each engine cylinder. Otherwise, if the DFSO conditions are not met, the routine continues to 420.

[0058] Next, at 410, a predetermined amount of fuel is sequentially injected into each cylinder, and the air-fuel mixture is combusted to learn the fuel injection error for each cylinder. In one example, injecting an amount of fuel includes injecting a set amount of fuel into a selected cylinder while keeping the remaining cylinders deactivated (e.g., no fuel is injected) while the engine continues to rotate. After injecting fuel into the selected cylinder, the cylinder may be fired one or more times to create a perturbation of the exhaust air-fuel ratio, or lambda, after combustion products are expelled after each combustion event in the firing cylinder.The air-fuel ratio, or lambda, value can then be related to the amount of fuel injected into the cylinder, and the amount of fuel injected into the cylinder can be provided by adjusting a fuel pulse width applied to a fuel injector of the cylinder receiving fuel. After the lambda values ​​are determined, it is assessed whether or not a lambda change is present. In particular, a deviation in lambda after injection from a maximum lean air-fuel ratio during DFSO can be estimated and compared to an expected lambda (due to the amount of fuel injected). The actual lambda value of a cylinder may differ from the expected lambda value due to a fuel injection fault of the cylinder, which is then learned.

[0059] Next, at 412, the routine may learn a fueling fault associated with each cylinder's fuel injector based on the lambda change estimated during DFSO. The cylinder air-fuel imbalance may result from an air-fuel ratio of one or more cylinders deviating from a desired or expected engine air-fuel ratio. A difference between the actual cylinder lambda and the expected lambda may be determined for one or an average of lambda values, and an injection fueling fault may be learned based on the average lambda values. Learning the fueling fault includes determining whether the cylinder's air-fuel ratio is leaner (e.g., excess oxygen) or richer (e.g.,excess fuel) than expected and storing the learned error for future operation of the cylinder after DFSO is complete. For example, if a lambda value of a selected cylinder is 2.1 and the expected lambda value is 1.9, then a rich air-fuel ratio change with a magnitude of 0.2 may be present. The magnitude may be learned and applied to future combustion in the cylinder after DFSO so that a fuel injection can compensate for the lambda change of 0.2 in the cylinder experiencing the change (e.g., inject an amount of fuel more than the determined amount, with the additional fuel proportional to the magnitude of 0.2). After learning the fuel error for each cylinder, the routine continues to 414.

[0060] At 414, the routine may include learning the air-fuel imbalance for each cylinder based on one or more of the learned air error, the learned fuel injection error, and a comparison of air error to fuel injection error. The air error may occur when a cylinder receives either less air or more air than expected, such as due to the specific geometry of the cylinders. The magnitude of the air error may depend on a position of the cylinder relative to the air intake system. For example, engine cylinders close to the air intake system may receive more air than cylinders farther away. The cylinder's fuel error may occur due to a fuel injector injecting more or less fuel into a cylinder than intended.Depending on the magnitude of the air and fuel imbalances in a given cylinder, a combination of air and fuel imbalances can result in an overall air-fuel imbalance for that cylinder relative to other cylinders. The cylinder imbalance can be a lean air-fuel imbalance if the air imbalance is greater than the fuel imbalance. Alternatively, the fuel imbalance for that cylinder can be greater than the air imbalance, resulting in a rich air-fuel imbalance. In other cases, the air and fuel imbalances in that cylinder can cancel each other out, resulting in no air-fuel imbalance.

[0061] Next, at 416, the routine judges whether the air-fuel imbalance for a given cylinder is greater than a threshold imbalance estimate (e.g., greater than 0.2). If the answer is yes, the routine continues to 418. Otherwise, if the air-fuel imbalance is less than the threshold imbalance estimate, the routine ends.

[0062] At 418, the routine sets a diagnostic code by noting the identity of the unbalanced cylinders and the corresponding degree of imbalance. In one example, the diagnostic code may only be removed after the cylinder has been serviced by a technician. Further, operation of the unbalanced cylinder may be limited while the code is set. For example, engine load may be limited. In another example, when the diagnostic code is set, the engine may enter a fault mitigation mode, where the fault mitigation mode is an FMEM mode that reduces misdiagnosis of affected systems and damage to engine components. In the fault mitigation mode, an engine load (including air flow and total fuel mass) may be limited.The limit may be based on the degree of imbalance identified, with the engine load being limited to a lower level if the degree of imbalance is greater from cylinder to cylinder and / or if a large number of cylinders are imbalanced. After the diagnostic code sets, the routine continues to 420.

[0063] At 420, the routine adjusts cylinder operation of all cylinders experiencing an air-fuel imbalance, as determined at 414. Adjusting may include adjusting amounts of fuel injected into the engine cylinders by varying the fuel injection amount. The fuel injection adjustments may be proportional to the air-fuel error, as described at 412. Adjusting may further include injecting a greater amount of fuel or a lesser amount of fuel based on the type of cylinder air-fuel imbalance. For example, a given cylinder may exhibit a rich air-fuel imbalance at 414. The fuel adjustments may include injecting less fuel into the given cylinder.Alternatively, if the given cylinder exhibits a lean air-fuel imbalance, the fuel adjustments may include injecting more fuel into the given cylinder. By adjusting the amount of fuel injected into the imbalanced cylinders based on the air-fuel imbalance, engine efficiency and operation may be improved while reducing emissions. Method 400 may end after the adjustments corresponding to the learned air-fuel imbalance are applied for each cylinder.

[0064] Fig. 5 shows an alternative method 500 for identifying cylinder air-fuel imbalance. In the example method 500, the air-fuel imbalance is determined based on three different imbalance estimates, each estimate weighted with a confidence level based on engine operating conditions. Engine torque, exhaust oxygen sensor signals, and exhaust pressure signals are processed and stored after each combustion event. A separate register memory is used to store the average torque, lambda, and pressure for each individual cylinder by utilizing cylinder combustion spark timing information. In this way, method 500 can reliably determine air-fuel imbalance over a wide range of operating conditions without interrupting engine operation.The method 500 is described herein with reference to components and systems described in . Fig. 1-2, particularly with respect to engine 10, cylinder banks 15A and 15B, and controller 12. Method 500 may be performed by the controller executing instructions stored on computer-readable media. It will be appreciated that method 500 may be applied to other engine systems having different configurations without departing from the scope of this disclosure.

[0065] Method 500 proceeds to 502 based on a first operating condition. The first operating condition may include one or more of a moderate engine load, an idle condition, uniform exhaust gas mixture conditions, and sufficient exhaust gas sensor warm-up. The routine may select an air-fuel ratio (AFR) corresponding to a deactivated cylinder of the selected VDE pattern (measured or estimated in Method 0). The selected air-fuel ratio is normalized to one percent of an averaged air-fuel ratio (LAMavg) estimated when all engine cylinders are active.

[0066] Next, at 504, a first confidence level (c1) for the air-fuel ratio estimate for the deactivated cylinder of the selected VDE pattern is determined based on the first operating condition. The first confidence level may reflect the reliability or accuracy of the air-fuel ratio estimate based on current engine conditions. The confidence level may be set to a highest value of one (indicating the greatest confidence) and may be set to a lowest value of zero if the cylinder imbalance estimate is unavailable or unreliable. A higher confidence level indicates that the imbalance estimate is more reliable, while a lower confidence level indicates that the imbalance estimate is less reliable.For example, the first confidence level may be increased when the exhaust gas mixture at the exhaust sensor is sufficient or above a threshold mixture level. In another example, the first confidence level may be decreased during cold-start conditions when the exhaust sensor is not sufficiently warmed up, which may make the air-fuel ratio estimate unreliable. The first confidence level may be different for each cylinder of the selected VDE pattern. As one example, the exhaust sensor readings may be affected by the location of a cylinder relative to the position of the exhaust sensor such that flow from some cylinders may be detected at the exhaust sensor while flow from other cylinders may not be detected.Thus, cylinders whose flow is detected by the exhaust gas sensor can be assigned higher probabilities of information compared to cylinders whose flow is not detected.

[0067] If the vehicle is in the second operating condition, method 500 proceeds to 506. The second condition may be a mid-load steady-state condition or an idle steady-state condition. Further, the second operating condition may be a change in exhaust valve timing that exceeds a threshold timing. Additionally, the second operating condition may be that an average distance between a pressure sensor and the exhaust valve of the combusting cylinders is less than a threshold distance. Thus, the second operating condition may include any one or any combination of the above operating conditions. The exhaust pressure (P n) for each deactivated cylinder of the selected VDE pattern (previously estimated in Procedure 300) is normalized to one percent of the average exhaust pressure (Pavg) when all cylinders are active.

[0068] At 508, a second confidence level for an air-fuel imbalance estimate is determined for each deactivated cylinder of each selected VDE pattern based on the second operating condition. The second confidence level may be increased for a smaller change in valve timing and decreased for a larger change in valve timing. The second confidence level may also be set to a value below a threshold when the average distance between the pressure sensor and the exhaust valve of the combusting cylinders is greater than a threshold distance. Furthermore, the second confidence level may be set to a value above a threshold when the distance is less than the threshold distance.

[0069] If the vehicle is in the third operating condition, method 500 proceeds to 510. The third condition may be a cold start condition. For example, the cold start condition may be determined when the exhaust temperature is less than a threshold temperature and the exhaust gas is not sufficiently mixed at the exhaust gas sensor. Additionally, the third condition may be lean engine operation. Thus, the third operating condition may include any one or any combination of the above operating conditions. When the third operating condition is met, a cylinder torque (TQ n ), measured or estimated in method 300, normalized to one percent of an average cylinder torque (TQavg) for each deactivated cylinder of each selected VDE pattern.

[0070] At 512, a third confidence level (c3) for a cylinder torque imbalance estimate is determined for each deactivated cylinder of the selected VDE pattern based on the third operating condition. The third confidence level may be decreased with better exhaust gas mixing at the exhaust sensor and increased with insufficient exhaust gas mixing at the exhaust sensor. The third confidence level may be increased with a leaner air-fuel ratio and decreased with a richer air-fuel ratio. After estimating the normalized imbalance estimates and all confidence levels for each deactivated cylinder of each selected VDE pattern, method 500 may proceed to 514.

[0071] At 514, a comprehensive normalized parameter (CNP) is estimated for each deactivated cylinder of the selected VDE pattern based on the probabilities and the normalized imbalance estimates. For example, the comprehensive normalized parameter for a deactivated cylinder (n) of the selected VDE pattern can be calculated as shown below: CNP=c1LAMnLAMavg+c2PnPavg+c3TQnTQavg

[0072] At 516, method 500 determines whether the comprehensive normalized parameter for each deactivated cylinder of the selected VDE pattern is greater than a threshold parameter. The threshold parameter may be a value of a threshold or an averaged comprehensive normalized parameter determined when all cylinders are active. If the answer is NO (i.e., the CNP is less than the threshold parameter), method 500 proceeds to 518, where no air-fuel imbalance is detected in a deactivated cylinder and the routine ends.

[0073] Returning to 516, if the answer is YES (i.e., the CNP is greater than the threshold parameter), method 500 proceeds to 520. At 520, each deactivated cylinder of the selected VDE pattern is identified as having an air-fuel imbalance. The imbalanced cylinder may be identified based on a deviation of the deactivated cylinder's comprehensive normalized parameter from the threshold parameter. The magnitude of the deviation may correspond to the magnitude of the air-fuel imbalance. For example, if the VDE pattern includes four cylinders (e.g., cylinders A1-A4 and B1-B4 in Fig. 1) is selected for air-fuel imbalance learning. In one example, under cold-start conditions, cylinder A1 of the selected VDE pattern may be deactivated and the air-fuel error determined. If the first confidence level is 0.2, the second confidence level is 0.4, and the third confidence level is 0.4. Additionally, if the normalized exhaust air-fuel ratio is 1.33 (0.8 / 0.6), the normalized exhaust pressure is 0.86 (1.2 / 1.4), and the normalized cylinder torque is 0.92 (2.4 / 2.6). The CNP is calculated to be 0.98, but the threshold parameter is 0.8, then an air-fuel error of 0.18 is determined for cylinder A1.

[0074] Method 500 then proceeds to 522 to update a diagnostic code containing information about the unbalanced cylinders. For example, the diagnostic code for all unbalanced cylinders may be modified based on the CNP deviation from the threshold parameter determined at 516. In another example, the diagnostic code may be updated based on the difference between the current CNP deviation and a previous CNP deviation in the diagnostic code from a previous period in which the engine was operated. Further, the imbalance history of all cylinders may be updated. After updating the diagnostic code, method 500 may end.

[0075] In one example, engine torque, exhaust oxygen sensor signals, and exhaust pressure signals are processed and stored after each combustion event. A combined average of all cylinders is then calculated for each signal type. For each signal type, the individual cylinder values ​​are normalized to one percent of the combined average to obtain up to three complete sets of normalized results for each cylinder. For each cylinder, the normalized results are weighted by a confidence level (1.0 nominal) and then summed together to produce a comprehensive normalized result. The method with the highest value receives the highest confidence level (e.g., 1.0). The comprehensive normalized result for each cylinder is compared to the other cylinders. If the spread between cylinder torques exceeds a threshold, an imbalance is detected and determined.The cylinder(s) that are furthest from the combined mean of the comprehensive normalized cylinder results (or exceed a threshold) are identified by setting a corresponding diagnostic code.

[0076] Returning to Fig. Figure 6 shows an example graphical representation of an air-fuel imbalance in individual cylinders of a four-cylinder engine (e.g., an in-line engine with cylinders 1-4). The sequence of Fig. 6 can be executed by executing instructions in the system of Fig. 1-2 according to the procedures of the Fig. 3-4. The individual cylinders of the engine are plotted on the X-axis, while air error, fuel error, and air-fuel imbalance are plotted on the Y-axis. The air error, fuel injection error, and air-fuel imbalance values ​​are determined for each cylinder when the DFSO conditions are met, as in Fig.4. Air error values ​​for cylinders 1-4 are plotted at curve 602, zero air error is plotted by line 604. The fuel error for each cylinder is plotted at curve 606, and the air-fuel imbalance for each cylinder is plotted at curve 610. Lines 608 and 612 represent zero fuel error and zero air-fuel imbalance, respectively. While the illustrated example shows zero errors at 604, 608, and 612, in alternate examples they may represent a combined average of this parameter based on the estimation of this parameter for all cylinders, and the solid circles show deviations from the combined average.

[0077] Referring to curve 602, air error values ​​for each cylinder are plotted. As shown, cylinder 1 has a relatively higher air error value, while cylinders 2 and 3 have relatively lower air error values. In particular, cylinder 1 deviates the most from the mean. Cylinders 1 and 3 are receiving more air than expected, but cylinder 1 is receiving more air compared to cylinder 3. Cylinder 2 is receiving less air than expected, while cylinder 4 does not indicate an air error (604) because the cylinder is receiving the expected amount of air. For example, cylinders 1-3 may display air error values ​​of 0.5, 0.2, and 0.1, respectively, while cylinder 4 displays no air error. The air error of 0.5 in cylinder 1 indicates that the cylinder is receiving a larger amount of air than expected. Cylinder 2 displays an air error of 0.2, indicating that the cylinder is receiving a smaller amount of air than expected.Cylinder 3 shows an air error of 0.1, which indicates that the cylinder is receiving a larger amount of air than expected, but the amount of air received by cylinder 3 is less compared to the amount received by cylinder 1.

[0078] Next, curve 606 shows the fuel error values ​​for cylinders 1-4. As shown, cylinders 1-3 are showing fuel errors, while cylinder 4 is not showing a fuel error (608). Cylinders 1 and 3 are receiving a smaller amount of fuel than expected, while cylinder 2 is receiving a larger amount of fuel than expected. Since cylinder 4 is not showing a fuel error, the cylinder is receiving the expected amount of fuel. For example, cylinders 1-3 may show fuel error values ​​of 0.2, 0.15, and 0.1, respectively, while cylinder 4 is not showing a fuel error.

[0079] Next, curve 610 shows the air-fuel imbalance values ​​for cylinders 1-4. As shown, cylinder 1 has a relatively higher air-fuel imbalance, while cylinder 2 has a relatively lower air-fuel imbalance. Cylinders 3-4 show no air-fuel imbalance. Cylinder 1 shows a lean air-fuel deviation, while cylinder 2 shows a rich air-fuel deviation. Since cylinder 1 receives a larger amount of air than expected but less fuel, a large lean air-fuel deviation can be detected. Cylinder 2 receives a smaller amount of air than expected but a larger amount of fuel, thus a rich air-fuel deviation can be observed. The air and fuel errors in cylinder 3 cancel out, resulting in no air-fuel imbalance in that cylinder (612).Since cylinder 4 has no air and fuel faults, no air-fuel imbalance is detected (612).

[0080] The air fault, fuel fault, and air-fuel imbalance of each imbalanced cylinder are noted and stored in the diagnostic code. A controller may adjust cylinder operation of all cylinders experiencing air-fuel imbalance as determined above. Adjusting may include adjusting amounts of fuel injected into the imbalanced engine cylinders via altering fuel injection timing, such as by advancing or retarding fuel injection timing. The fuel injection timing adjustments may be proportional to the determined air-fuel fault. Adjusting may further include injecting a greater amount of fuel or a lesser amount of fuel based on the type of cylinder air-fuel imbalance.For example, cylinder 1 may exhibit a lean air-fuel imbalance, while cylinder 2 exhibits a rich air-fuel imbalance. Fuel adjustments may include injecting more fuel into cylinder 1 but less fuel into cylinder 2 to bring the air-fuel ratio of both cylinders closer to stoichiometric. By adjusting the amount of fuel injected into the imbalanced cylinders based on the air and fuel errors and the air-fuel imbalance estimate, engine efficiency and operation can be improved while reducing emissions.

[0081] In this way, the air-fuel imbalance of the cylinders can be determined reliably and stably, without having to resort to the air-fuel ratio and without relying on expensive sensors. Furthermore, the imbalance can be determined for cylinders regardless of their geometry. By improving the stability of imbalance detection and reducing false and missed detections, warranty costs are reduced.

[0082] In one example, a method for an engine includes: during a cylinder deactivation process, sequentially deactivating each cylinder of a cylinder group having two or more cylinders; estimating a lambda deviation for each cylinder after sequentially deactivating each cylinder of the cylinder group; and learning an air error for each cylinder based on the estimated lambda deviation. The above example may additionally or optionally further include: Differentiating the air fault for each cylinder from a fuel fault for fuel injectors of each cylinder of the cylinder group. Any or all of the above examples additionally or optionally further includes: during a deceleration fuel shutoff (DFSO) event, sequentially firing each cylinder of the cylinder group with a fuel pulse duration selected to provide an expected lambda deviation, and learning the fuel injection fault for each cylinder of the cylinder based on an actual lambda deviation relative to the expected air-fuel deviation. Any or all of the above examples additionally or optionally further includes: indicating an air-fuel imbalance for each cylinder based on the learned air fault for the cylinder.In any or all of the preceding examples, estimating the lambda deviation additionally or optionally includes estimating a deviation from an average lambda when all cylinders are firing prior to the cylinder deactivation event. In any or all of the preceding examples, additionally or optionally, the cylinder deactivation event is in response to a drop in driver demand, and a number and identity of cylinders in the cylinder group selected for sequential deactivation is based on the drop in driver demand. In any or all of the preceding examples, an order of sequential deactivation is additionally or optionally based on, respectively, a firing order of each cylinder of the cylinder group and a duration elapsed since a last air fault diagnostic for each cylinder of the cylinder group.In any or all of the above examples, indicating includes, additionally or optionally, indicating an air-fuel imbalance for a given cylinder in response to the learned air error for the given cylinder being greater than a threshold error.

[0083] Furthermore, in any or all of the preceding claims, additionally or optionally, the learned fault is a first fault, the method further comprising: during engine idle conditions, sequentially deactivating each cylinder of the cylinder group and learning a second air fault for each cylinder based on the estimated lambda deviation; during an engine load greater than a threshold load and with the torque converter sensed, sequentially deactivating each cylinder of the cylinder group and learning a third air fault for each cylinder based on the estimated lambda deviation; and indicating the air-fuel imbalance for each cylinder based on the first, second, and third air faults.

[0084] Any or all of the above examples additionally or optionally further includes: in response to indicating an air-fuel imbalance in a first cylinder of the cylinder group, after reactivating the cylinder group, adjusting fueling based on the learned air error for the first cylinder, and further adjusting fueling to the remaining cylinders of the cylinder group based on the learned air error to maintain the air-fuel ratio at or near stoichiometry. Any or all of the above examples additionally or optionally further includes: learning a torque error for each cylinder of the cylinder group based on one or more of crankshaft accelerations and exhaust pressure pulsations during sequential deactivation, and indicating the air-fuel imbalance based on the learned air error in relation to the learned torque error.In any or all of the above examples, the cylinder group is, additionally or optionally, a first cylinder group and the lambda deviation is estimated based on an output of a first common exhaust gas sensor that selectively receives exhaust gas from each cylinder of the first cylinder group, the engine including a second, different cylinder group and a second common exhaust gas sensor that selectively receives exhaust gas from each cylinder of the second cylinder group, the method further comprising: distinguishing a fault of the first common exhaust gas sensor from a fault of the second common exhaust gas sensor based on an air-fuel ratio imbalance of the first cylinder group with respect to an air-fuel ratio imbalance of the second cylinder group.

[0085] In another example, a method for an engine may include: estimating a first lambda when all cylinders are firing; selectively deactivating a first cylinder and estimating a second lambda; then reactivating the first cylinder while selectively deactivating a second cylinder and estimating a third lambda; learning a first air error for the first cylinder based on the second lambda with respect to the first lambda; learning a second air error for the second cylinder based on the third lambda with respect to the first lambda; and upon reactivating the first and second cylinders, adjusting fueling of the first and second cylinders based on each of the first and second air errors, and operating the engine at or near stoichiometry.The above example may additionally or optionally further include: estimating a maximum lean lambda when all cylinders are deactivated; selectively fueling the first cylinder and learning a fuel error for the first cylinder based on an actual change in lambda with respect to an expected change in lambda; then deactivating the first cylinder while selectively fueling the second cylinder and learning a fuel error for the second cylinder based on the actual change in lambda with respect to the expected change in lambda; and upon reactivating the first and second cylinders, adjusting the fueling of the first cylinder based on the first fuel error and the fueling of the second cylinder based on the second fuel error to operate the engine at or near stoichiometry.In any or all of the above examples, additionally or optionally, the selective deactivation is in response to a decrease in torque demand from the driver, and all cylinders are deactivated in response to deceleration fuel cutoff conditions. Any or all of the above examples additionally or optionally further include: distinguishing air-fuel sensor faults for a common air-fuel sensor associated with each of the first and second cylinders.

[0086] Another exemplary vehicle system includes: an engine cylinder group having two or more cylinders; selectively deactivatable fuel injectors connected to each cylinder of the cylinder group; an exhaust air-fuel ratio sensor receiving exhaust gas from each cylinder of the cylinder group; a controller having computer-readable instructions stored in non-transitory memory for: sequentially deactivating each cylinder of the cylinder group in response to cylinder deactivation conditions and learning an air fault for each cylinder of the cylinder group based on a first lambda deviation,estimated at the exhaust air-fuel ratio sensor after the sequential deactivation; sequentially fueling each cylinder of the cylinder group in response to deceleration fuel cut-off conditions and learning a fuel injection fault for each cylinder of the cylinder group based on a second lambda deviation estimated at the exhaust air-fuel ratio sensor after the sequential fueling; and indicating the cylinder's air-fuel imbalance based on the learned air fault in relation to the learned fuel injection fault. In any or all of the preceding claims, the controller further comprises, additionally or optionally, instructions for: adjusting the fueling of each cylinder of the cylinder group during subsequent engine operation when all cylinders are firing based on each of the learned air faults,the learned fuel injection fault and the cylinder air-fuel imbalance. In any or all of the preceding claims, the controller further comprises, additionally or optionally, instructions for: in response to the cylinder air-fuel imbalance for a cylinder being greater than a threshold, setting a diagnostic code and entering a fault mitigation mode, wherein the fault mitigation mode is an FMEM mode that prevents misdiagnosis of affected systems and damage to engine components. In any or all of the preceding claims, the engine load (including air quantity and total fuel mass) is, additionally or optionally, limited in the fault mitigation mode. In any or all of the preceding claims, the controller further comprises, additionally or optionally,Instructions for: Learning an exhaust air-fuel ratio sensor shift based on the learned air error in relation to the learned fuel injection error.

[0087] It should be noted that the example control and estimation routines contained herein may be used with various engine and / or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memory and may be executed by the control system, including the controller in combination with various sensors, actuators, and other engine components. The particular routines described herein may represent one or more of any number of processing strategies, such as event-driven, interrupt-driven, multitasking, multithreading, and the like. As such, various illustrated acts, operations, and / or functions may be performed in the illustrated order, in parallel, or in some cases omitted.Likewise, the processing order is not required to achieve the features and advantages of the embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts, operations, and / or functions may be performed repeatedly, depending on the particular strategy employed. Further, the described acts, operations, and / or functions graphically represent code to be programmed into a non-transitory memory of the computer-readable storage medium in the engine control system, in which the described acts are carried out by executing the instructions in a system including the various engine hardware components in combination with the electronic controller.

[0088] It should be appreciated that the configurations and routines disclosed herein are exemplary in nature and that these particular embodiments are not intended to be limiting, as many variations are possible. For example, the above technology may be applied to a V6, an I4, an I6, a V12, a horizontally opposed four-cylinder engine, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations and other features, functions, and / or characteristics disclosed herein.

[0089] The following claims particularly point out certain combinations and subcombinations that are considered novel and non-obvious. These claims may refer to "a" element or "a first" element, or the equivalent thereof. These claims should be understood to include the inclusion of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations or subcombinations of the disclosed features, functions, elements, and / or properties may be claimed by amending the present claims or by setting forth new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope with respect to the original claims, are also considered to be included within the subject matter of the present disclosure.

Claims

[1] A method for an engine, comprising: During a cylinder deactivation process, sequentially deactivating each cylinder of a cylinder group comprising two or more cylinders; estimating a lambda deviation for each cylinder after the sequential deactivation of each cylinder of the cylinder group; Learning an air error for each cylinder based on the estimated lambda deviation; and Display an air-fuel imbalance for each cylinder based on the learned air error for that cylinder. [2] The method of claim 1, further comprising distinguishing the air fault for each cylinder from a fuel injection fault for fuel injectors of each cylinder of the cylinder group. [3] The method of claim 2, further comprising, during a deceleration fuel cut-off (DFSO) event, sequentially firing each cylinder of the cylinder group with a fuel pulse duration selected to provide an expected lambda deviation, and learning the fuel injection error for each cylinder of the cylinder group based on an actual lambda deviation relative to the expected lambda deviation. [4] The method of claim 1, wherein estimating the lambda deviation comprises estimating a deviation from an average lambda when all cylinders are firing prior to the cylinder deactivation event. [5] The method of claim 1, wherein the cylinder deactivation event is in response to a decrease in driver demand, and wherein a number and identity of cylinders in the cylinder group selected for sequential deactivation is based on the decrease in driver demand. [6] The method of claim 5, wherein an order of sequential deactivation is based on a firing order of each cylinder of the cylinder group and a duration elapsed since a last air fault diagnosis for each cylinder of the cylinder group. [7] The method of claim 1, wherein the indicating comprises indicating an air-fuel imbalance for a given cylinder in response to the learned air error for the given cylinder being greater than a threshold error. [8] The method of claim 5, further comprising, in response to indicating an air-fuel imbalance in a first cylinder of the cylinder group, after reactivating the cylinder group, adjusting the fueling of the first cylinder based on the learned air error for the first cylinder, and further adjusting the fueling of the remaining cylinders of the cylinder group based on the learned air error to maintain the air-fuel ratio at or near stoichiometry. [9] The method of claim 5, wherein the learned error is a first error, the method further comprising: during engine idle conditions, sequentially deactivating each cylinder of the cylinder group and learning a second air fault for each cylinder based on the estimated lambda deviation; during an engine load higher than a threshold load and with the torque converter detected, sequential deactivation of each cylinder of the cylinder group and Learning a third air error for each cylinder based on the estimated lambda deviation; and Display the air-fuel imbalance for each cylinder based on the first, second, and third air faults. [10] The method of claim 5, further comprising learning a torque error for each cylinder of the cylinder group based on one or more of crankshaft accelerations and exhaust pressure pulsations during the sequential deactivation and indicating the air-fuel imbalance based on the learned air error in relation to the learned torque error. [11] The method of claim 1, wherein the cylinder group is a first cylinder group and the lambda deviation is estimated based on an output of a first common exhaust gas sensor that selectively receives exhaust gas from each cylinder of the first cylinder group, the engine including a second, different cylinder group and a second common exhaust gas sensor that selectively receives exhaust gas from each cylinder of the second cylinder group, the method further comprising distinguishing a fault of the first common exhaust gas sensor from a fault of the second common exhaust gas sensor based on an air-fuel ratio imbalance of the first cylinder group with respect to an air-fuel ratio imbalance of the second cylinder group, and distinguishing the air fault from the exhaust gas sensor fault. [12] Engine system comprising: an engine cylinder group with two or more cylinders; sequentially deactivatable fuel injectors associated with each cylinder in the cylinder group; an exhaust air-fuel ratio sensor that receives exhaust gas from each cylinder of the cylinder group; a controller with computer-readable instructions stored in non-volatile memory for: sequentially deactivating each cylinder of the cylinder group in response to cylinder deactivation conditions and learning an air error for each cylinder of the cylinder group based on a first lambda deviation estimated at the exhaust air-fuel ratio sensor after the sequential deactivation; sequentially fueling each cylinder of the cylinder group in response to deceleration fuel cut-off conditions and learning a fuel injection fault for each cylinder of the cylinder group based on a second lambda deviation estimated at the exhaust air-fuel ratio sensor after the sequential fueling; and Display cylinder air-fuel imbalance based on learned air error relative to learned fuel injection error. [13] The system of claim 12, wherein the controller further comprises instructions to: Adjusting the fueling of each cylinder in the cylinder group during subsequent engine operation when all cylinders are firing based on each of the learned air error, the learned fuel injection error, and the cylinder air-fuel imbalance. [14] The system of claim 12, wherein the controller further comprises instructions for: in response to the cylinder air-fuel imbalance for a cylinder being greater than a threshold, setting a diagnostic code and entering a fault mitigation mode. [15] The system of claim 12, wherein the controller further comprises instructions to: Learning an exhaust air-fuel ratio sensor shift based on the learned air error relative to the learned fuel injection error.

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