System and method for a variable compression ratio engine

Abstract

A method and system for improving the calibration of a variable compression ratio engine is provided. Cylinder-to-cylinder compression ratio variations are detected and accounted for by comparing cylinder fuel flow and IMEP at each compression ratio setting. Dilution parameters including EGR and VCT schedules are also calibrated to account for the cylinder-to-cylinder compression ratio variations.

Description

Technical Field

[0001] The present description generally relates to methods and systems for controlling the compression ratio of a variable compression ratio engine.

[0002] Background technology / invention content

[0003] The compression ratio (CR) of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom dead center (BDC) to the cylinder volume when the piston is at top dead center (TDC). In general, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. Variable compression ratio (VCR) engines have been developed in which the compression ratio of each cylinder can be varied between a higher setting and a lower setting to improve engine performance. For example, a higher compression ratio setting can be used during a knock-free condition to take advantage of high thermal efficiency, while a lower compression ratio setting can be used during a knock-prone condition. In a VCR engine, a connecting rod or other mechanism (e.g., an eccentric) can be coupled to the piston of each cylinder to mechanically vary the compression ratio between a higher setting and a lower setting.

[0004] An example of a VCR engine is shown in Caswell in US 4,469,055. Therein, during engine operation, the CR of the engine is adjusted based on engine operating conditions. For example, the CR can be optimized for engine fuel efficiency or engine performance or both. The CR calibration, that is, the CR command based on engine speed and load, can be calibrated based on a prototype engine.

[0005] However, the inventors have discovered potential problems with such systems. As an example, CR adjustment during engine operation requires accurate knowledge of the actual CR. However, due to manufacturing tolerances, each engine may have a slightly different compression ratio (CR) in each cylinder. In a VCR engine, in addition to the normal variations of non-VCR engines, each component of the VCR mechanism may also have manufacturing tolerances that cause significant part-to-part variations. Therefore, calibration of parameters that affect engine dilution (such as exhaust gas recirculation (EGR) calibration or variable cam timing (VCT) calibration based on CR data from a prototype engine) may not be optimal for a production engine. The resulting higher cylinder-to-cylinder CR variation may produce unstable combustion on those cylinders with a CR below the average, thereby causing higher noise, vibration and harshness (NVH), higher risk of misfire, and lower efficiency on those cylinders. The use of high-quality manufacturing methods and / or "pick-and-match" parts can be used to control or address CR differences between cylinders, but such methods add significant costs.

[0006] In one example, the above problem can be at least partially solved by a method comprising: calibrating a compression ratio schedule of a variable compression ratio engine based on each of a fuel flow rate and a peak torque per cylinder at each compression ratio setting of the engine; and adjusting exhaust gas recirculation (EGR) flow to the engine according to an updated EGR calibration schedule based on the compression ratio schedule calibration. In this way, dilution optimization of a VCR engine is improved.

[0007] As an example, the actual CR of each cylinder of a VCR engine can be quantified based on each VCR mechanism setting. For example, the CR of each engine cylinder at each of a plurality of compression ratio settings can be quantified. In one example, the CR of each cylinder can be quantified based on the learned fuel flow and IMEP of each cylinder at each CR setting of the VCR engine. Then, the lowest of the quantified compression ratios can be identified. Then, the nominal EGR or VCT plan can be updated based on the identified lowest CR of all cylinders. A nominal EGR / VCT plan can be created based on mapping data from a prototype engine. During conditions when the engine load is above a threshold load, engine dilution can be provided according to the nominal EGR / VCT plan when the engine dilution requirement is low. However, during conditions when the engine load is below a threshold load, engine dilution can be provided according to an updated EGR / VCT plan when the engine dilution requirement is high. Specifically, as the minimum CR decreases, the provided engine dilution can be reduced, such as by reducing the opening of the EGR valve or by advancing one or more of the intake valve closing timing and the exhaust valve closing timing (thereby reducing positive valve overlap). In one example, the actual CR may be significantly lower than the commanded CR due to degradation of the VCR mechanism, or due to failure to meet entry conditions for the optimal CR (e.g., failure to meet temperature, oil pressure, and current limit conditions). By adjusting the EGR / VCT schedule during such conditions, combustion stability and engine performance can be improved while compensating for the lack of optimal CR.

[0008] In this way, the efficiency of the VCR engine can be improved by better adjusting the engine dilution in view of the actual cylinder-to-cylinder compression ratio variation. By knowing the fuel flow and IMEP of all cylinders according to each CR setting of the VCR engine, the CR variation of the actual engine can be known, rather than relying on a prototype engine that may be significantly different from a given engine. In addition, the EGR and VCT schedules of the VCR engine can be calibrated without relying on expensive manufacturing methods and / or components. The technical effect of modifying the nominal EGR / VCT schedule of the VCR engine based on the lowest mapped CR of all engine cylinders is that the efficiency of the VCR engine can be improved by allowing the basic EGR / VCT calibration to operate closer to the stability limit, while still protecting the engine with a relatively low CR (due to manufacturing variations) on one or more cylinders. By modifying the EGR and VCT schedule of the engine based on the actual CR map, the dilution control at low engine load is improved, thereby allowing improved combustion stability and reduced NVH. In general, the engine performance and fuel efficiency of the VCR engine can be improved.

[0009] It should be understood that the above summary of the invention is provided to introduce in a simplified form a selection of some concepts further described in the detailed description. It is not intended to identify the key or essential features of the claimed subject matter, the scope of which is limited only by the claims following the detailed description. In addition, the claimed subject matter is not limited to embodiments that address any disadvantages noted above or in any part of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 An exemplary variable compression ratio (VCR) engine system is shown.

[0011] Figure 2 An exemplary high level flow chart for optimizing CR calibration and dilution calibration of a VCR engine is depicted.

[0012] Figure 3 Another exemplary high level flow chart for optimizing CR calibration and dilution calibration of a VCR engine is depicted.

[0013] Figure 4 An exemplary table showing the difference between the actual CR and the expected CR of a VCR engine is depicted.

[0014] Figure 5 An exemplary difference between a nominal EGR and VCT schedule and a modified EGR and VCT schedule for a VCR engine is depicted.

[0015] Figure 6 A prophetic example of VCR engine control is depicted.

[0016] Figure 7 An exemplary difference between a nominal CR schedule and a modified CR schedule for a VCR engine is depicted. DETAILED DESCRIPTION

[0017] The following description relates to systems and methods for an engine system configured with a variable compression ratio (VCR) mechanism, as described with reference to Figure 1 The controller may be configured to execute a control routine (such as Figures 2 to 3 ) to calibrate the commanded CR at a given engine speed-load by learning the actual cylinder-to-cylinder CR variation based on the difference in fuel economy and output torque for each cylinder at each CR setting for the VCR engine. During conditions when dilution control is needed, the controller may also modify the nominal EGR (exhaust gas recirculation) or VCT (variable camshaft timing) schedule based on the mapped cylinder-to-cylinder CR variation. Exemplary modifications of CR calibration and EGR calibration are described in Figures 4 to 5 and Figure 7 Reference Figure 6 , shows an example of adjusting engine operation based on CR calibration and EGR calibration. In this way, the performance and fuel economy of the VCR engine can be improved.

[0018] Figure 1 An exemplary embodiment of a combustion chamber or cylinder of an internal combustion engine 10 is depicted. Engine 10 may be included in a vehicle system 5, such as a vehicle configured for on-road propulsion. Engine 10 may receive control parameters from a control system including controller 12, and input from a vehicle operator 130 via an input device 132. In this example, input device 132 includes an accelerator pedal, and a pedal position sensor 134 for generating a proportional pedal position signal PP. Cylinder 14 of engine 10 (also referred to herein as a "combustion chamber") may include combustion chamber walls 136 with a piston 138 positioned therein. Piston 138 may be coupled to a crankshaft 140 such that reciprocating motion of the piston is transformed into rotational motion of the crankshaft. Crankshaft 140 may be coupled to at least one drive wheel of a passenger vehicle via a transmission system. Additionally, a starter motor may be coupled to crankshaft 140 via a flywheel to enable a starting operation of engine 10.

[0019] The engine 10 may be configured as a variable compression ratio (VCR) engine in which the compression ratio (CR) of each cylinder (that is, the ratio of the cylinder volume when the piston is at bottom dead center (BDC) to the cylinder volume when the piston is at top dead center (TDC)) may be mechanically varied. The CR of the engine may be varied via a VCR actuator 192 actuating a VCR mechanism 194. In some exemplary embodiments, the CR may vary between a first, lower CR (where the ratio of the cylinder volume when the piston is at BDC to the cylinder volume when the piston is at TDC is smaller) and a second, higher CR (where the ratio is higher). In other exemplary embodiments, there may be a predefined number of stepped compression ratios. Additionally, the CR may vary continuously between a first, lower CR and a second, higher CR (to any CR therebetween).

[0020] In the depicted example, VCR mechanism 194 is coupled to piston 138 so that the VCR mechanism can change the piston TDC position. For example, piston 138 may be coupled to crankshaft 140 via piston position changing VCR mechanism 194, which moves the piston closer to or further away from the cylinder head, thereby changing the size of combustion chamber 14. Position sensor 196 may be coupled to VCR mechanism 192 and may be configured to provide feedback to controller 12 regarding the position of VCR mechanism 194 (and thus the compression ratio) applied to the cylinder.

[0021] In one example, changing the position of the piston in the combustion chamber also changes the relative displacement of the piston in the cylinder. The piston position changing VCR mechanism can be connected to a conventional crank system (cranktrain) or an unconventional crank system. Non-limiting examples of unconventional crank systems to which the VCR mechanism can be connected include variable head distance (distance head) crankshafts and variable motion length crankshafts. In one example, the crankshaft 140 can be configured as an eccentric shaft. In another example, an eccentric wheel can be connected to the area of ​​the piston pin, or in the area of ​​the piston pin, the eccentric wheel changes the position of the piston in the combustion chamber. The movement of the eccentric wheel can be controlled by the oil passage in the piston rod.

[0022] It will be appreciated that other VCR mechanisms that mechanically change the compression ratio may be used. For example, the CR of the engine may be changed via a VCR mechanism that changes the cylinder head volume (that is, the clearance volume in the cylinder head). In yet another example, the VCR mechanism may include hydraulic pressure, air pressure, or a mechanical spring reaction piston. In addition, the VCR mechanism may include a multi-link mechanism or a bent rod mechanism. Still other VCR mechanizations may be possible. It will be appreciated that, as used herein, a VCR engine may be configured to adjust the CR of the engine via mechanical adjustments that change the piston position or the cylinder head position or the cylinder head volume. Therefore, the VCR mechanism does not include an effective CR adjustment that is achieved via adjustments to the valve timing or cam timing.

[0023] By adjusting the position of the piston in the cylinder, the actual (static) compression ratio of the engine (that is, the difference between the cylinder volume at TDC and the cylinder volume at BDC) can be changed. In one example, reducing the compression ratio includes reducing the displacement of the piston in the combustion chamber by increasing the distance between the top of the piston and the cylinder head. For example, the engine can send a signal to the controller to actuate the VCR mechanism to a first position to operate at a first lower compression ratio, in which the piston has a smaller effective displacement in the combustion chamber. As another example, the engine can send a signal to the controller to actuate the VCR mechanism to a second position to operate at a second higher compression ratio, in which the piston has a larger effective displacement in the combustion chamber. Changes in the engine compression ratio can be used to improve fuel economy. For example, a higher compression ratio can be used to improve fuel economy under light to medium engine loads until the spark delay from the onset of early knock erodes the fuel economy benefit. Then, the engine can be switched to a lower compression ratio, thereby compromising the efficiency benefits of a higher compression ratio with the efficiency benefits of optimized combustion phasing. The continuous VCR system can continuously optimize the tradeoff between combustion phasing and the efficiency benefits of a higher compression ratio to provide an optimal compression ratio between a higher compression ratio limit and a lower compression ratio limit under given operating conditions. In one example, the engine controller can reference a lookup table to select a compression ratio to apply based on engine speed-load conditions. As described in detail below, the selection can include selecting a lower compression ratio at higher engine loads and selecting a higher compression ratio at lower engine loads.

[0024] Cylinder 14 may receive intake air via a series of intake passages 142, 144, and 146. Intake passage 146 may communicate with other cylinders of engine 10 in addition to cylinder 14. In some embodiments, one or more of the intake passages may include a boost device, such as a turbocharger or a supercharger. For example, Figure 1Engine 10 is shown configured with a turbocharger including a compressor 174 arranged between intake passages 142 and 144, and an exhaust turbine 176 arranged along exhaust passage 148. Compressor 174 may be powered at least in part by exhaust turbine 176 via shaft 180, where the boost device is configured as a turbocharger. However, in other examples, such as examples where engine 10 is provided with a supercharger, exhaust turbine 176 may optionally be omitted, where compressor 174 may be powered by mechanical input from a motor of the engine. Throttle 20 including throttle plate 164 may be disposed along an intake passage of the engine to vary the flow rate and / or pressure of intake air provided to the engine cylinders. For example, throttle 20 may be positioned downstream of compressor 174, such as in an embodiment of the present invention. Figure 1 As shown, or alternatively, may be provided upstream of compressor 174 .

[0025] Exhaust passage 148 may receive exhaust gas from other cylinders of engine 10 in addition to cylinder 14. Exhaust gas sensor 128 is shown coupled to exhaust passage 148 upstream of emission control device 178. Sensor 128 may be selected from a variety of suitable sensors for providing an indication of the exhaust air / fuel ratio, such as a linear oxygen sensor or UEGO (universal or wide range exhaust gas oxygen), a two-state oxygen sensor or EGO (as depicted), HEGO (heated EGO), NOx, HC, or CO sensors. Emission control device 178 may be a three way catalyst (TWC), a NOx trap, various other emission control devices, or combinations thereof.

[0026] Exhaust temperature may be estimated by one or more temperature sensors (not shown) located in exhaust passage 148. Alternatively, exhaust temperature may be inferred based on engine operating conditions such as speed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhaust temperature may be calculated by one or more exhaust gas sensors 128. It will be appreciated that exhaust temperature may alternatively be estimated by any combination of the temperature estimation methods listed herein.

[0027] Each cylinder of engine 10 may include one or more intake valves and one or more exhaust valves. For example, cylinder 14 is shown including at least one intake poppet valve 150 and at least one exhaust poppet valve 156 located at an upper region of cylinder 14. In some embodiments, each cylinder of engine 10 (including cylinder 14) may include at least two intake poppet valves and at least two exhaust poppet valves located at an upper region of the cylinder.

[0028] Intake valve 150 may be controlled by controller 12 via cam actuation via cam actuation system 151. Similarly, exhaust valve 156 may be controlled by controller 12 via cam actuation system 153. Cam actuation systems 151 and 153 may each include one or more cams, and may vary valve operation using one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT), and / or variable valve lift (VVL) systems operable by controller 12. The positions of intake valve 150 and exhaust valve 156 may be determined by valve position sensors 155 and 157, respectively. In alternative embodiments, the intake valve and / or exhaust valve may be controlled by electric valve actuation. For example, cylinder 14 may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and / or VCT systems. In other embodiments, the intake and exhaust valves may be controlled by a common valve actuator or actuation system, or a variable valve timing actuator or actuation system.

[0029] Cylinder 14 can have a compression ratio, which is the ratio of the volume of piston 138 when it is at bottom dead center to when it is at top dead center. Conventionally, the compression ratio is in the range of 9:1 to 10:1. However, in some examples using different fuels, the compression ratio can be increased. For example, this may occur when using a fuel with a higher octane rating or a fuel with a higher potential enthalpy of vaporization. If direct injection is used, the compression ratio may also be increased due to its effect on engine knock. The compression ratio can also be mechanically changed based on driver demand via adjustment of the VCR actuator 192 that actuates the VCR mechanism 194, thereby changing the effective position of the piston 138 within the combustion chamber 14. The compression ratio can be inferred based on feedback from sensor 196 about the position of the VCR mechanism 194.

[0030] In some embodiments, each cylinder of engine 10 may include a spark plug 192 for initiating combustion. Under selected operating modes, ignition system 190 may provide an ignition spark to combustion chamber 14 via spark plug 192 in response to spark advance signal SA from controller 12. However, in some embodiments, spark plug 192 may be omitted, such as where engine 10 may initiate combustion by auto-ignition or by injection of fuel, as is the case with some diesel engines.

[0031] 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 14 is shown including one fuel injector 166. Fuel injector 166 is shown as being directly coupled to cylinder 14 for injecting fuel directly therein in proportion to the pulse width of signal FPW received from controller 12 via electronic driver 168. In this manner, fuel injector 166 provides so-called direct injection (hereinafter also referred to as "DI") of fuel into combustion cylinder 14. Although Figure 1 Injector 166 is shown as a side injector, but it can also be located on the top of the piston, such as near the spark plug 192. When operating with alcohol-based fuels, this location can improve mixing and combustion due to the lower volatility of some alcohol-based fuels. Alternatively, the injector can be located on the top of the intake valve and close to it to improve mixing. The fuel can be delivered to the fuel injector 166 from a high-pressure fuel system 8 including a fuel tank, a fuel pump and a fuel rail. Alternatively, the fuel can be delivered by a single-stage fuel pump at a lower pressure, in which case the timing of the direct fuel injection during the compression stroke can be more restrictive than when using a high-pressure fuel system. In addition, although not shown, the fuel tank can have a pressure sensor that provides a signal to the controller 12. It will be understood that in an alternative embodiment, the injector 166 can be an intake port injector that provides fuel to the intake port upstream of the cylinder 14.

[0032] It will also be appreciated that while the depicted embodiment shows the engine operating by injecting fuel via a single direct injector; in alternative embodiments, the engine may be operated by using two or more injectors (e.g., one direct injector and one port injector per cylinder, or two direct injectors / two port injectors per cylinder, etc.) and varying the relative injection amounts from each injector into the cylinder.

[0033] During a single cycle of the cylinder, fuel may be delivered to the cylinder via an injector. In addition, the distribution and / or relative amount of fuel delivered from the injector may vary with operating conditions. In addition, for a single combustion event, multiple injections of the delivered fuel may be performed per cycle. Multiple injections may be performed during the compression stroke, the intake stroke, or any suitable combination thereof. Moreover, fuel may be injected during the cycle to adjust the air-to-fuel ratio (AFR) of the combustion. For example, fuel may be injected to provide a stoichiometric AFR. An AFR sensor may be included to provide an estimate of the in-cylinder AFR. In one example, the AFR sensor may be an exhaust gas sensor, such as EGO sensor 128. The sensor may determine the AFR by measuring the amount of residual oxygen (for a lean mixture) or unburned hydrocarbons (for a rich mixture) in the exhaust gas. Therefore, the AFR may be provided as a lambda (λ) value, that is, as a ratio of the actual AFR of a given mixture to the stoichiometric ratio. Thus, a lambda of 1.0 indicates a stoichiometric mixture, richer than stoichiometric mixtures may have lambda values ​​less than 1.0, and leaner than stoichiometric mixtures may have lambda values ​​greater than 1.0.

[0034] As mentioned above, Figure 1 Only one cylinder of a multi-cylinder engine is shown. Therefore, each cylinder may similarly include its own set of intake / exhaust valves, fuel injectors, spark plugs, etc.

[0035] Fuel tanks in the fuel system 8 may hold fuels having different fuel qualities, such as different fuel compositions. These differences may include different alcohol content, different octane ratings, different heats of vaporization, different fuel blends, and / or combinations thereof, among others.

[0036] The engine 10 may also include a knock sensor 90 coupled to each cylinder 14 for identifying abnormal cylinder combustion events. In an alternative embodiment, one or more knock sensors 90 may be coupled to a selected position of the engine block. The knock sensor may be an accelerometer on the cylinder block, or an ion sensor configured in the spark plug of each cylinder. The output of the knock sensor may be combined with the output of the crankshaft acceleration sensor to indicate an abnormal combustion event in the cylinder. In one example, based on the output of the knock sensor 90 in one or more defined windows (e.g., a crank angle timing window), abnormal combustion caused by one or more of knock and pre-ignition may be identified and distinguished. For example, knock may be identified in response to the knock sensor output estimated in the knock window being higher than a knock threshold, while pre-ignition may be identified in response to the knock sensor output estimated in the pre-ignition window being higher than a knock threshold, the pre-ignition being higher than the knock threshold, the pre-ignition window being earlier than the knock window. In addition, abnormal combustion may be addressed accordingly. For example, knock can be solved by reducing the compression ratio and / or retarding the spark timing, while pre-ignition can be solved by enriching the engine and / or limiting the engine load. In addition, reducing the compression ratio also reduces the change of further pre-ignition.

[0037] Controller 12 is shown as a microcomputer including a microprocessor unit 106, input / output ports 108, an electronic storage medium for executable programs and calibration values ​​(shown in this particular example as a read-only memory chip 110), a random access memory 112, a keep alive memory 114, and a data bus. Controller 12 may also receive various signals from sensors coupled to engine 10, including, in addition to those previously discussed, a measurement of intake mass air flow (MAF) from mass air flow sensor 122; engine coolant temperature (ECT) from temperature sensor 116 coupled to cooling sleeve 118; a surface ignition sensing signal (PIP) from a Hall effect sensor 120 (or other type) coupled to crankshaft 140; throttle position (TP) from a throttle position sensor; an absolute manifold pressure signal (MAP) from sensor 124; cylinder AFR from EGO sensor 128; abnormal combustion from knock sensor 90 and crankshaft acceleration sensor; and VCR mechanism position from position sensor 196. Engine speed signal RPM may be generated by controller 12 based on signal PIP. Manifold pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum or pressure in the intake manifold. Figure 1 The signals of various sensors are used Figure 1The controller may use various actuators to adjust engine operation based on received signals and instructions stored in the controller's memory. For example, based on engine speed and load, the controller may adjust the engine's compression ratio by sending a signal to a VCR actuator, which actuates a VCR mechanism to mechanically move the piston closer to or further away from the cylinder head, thereby changing the volume of the combustion chamber.

[0038] Non-transitory storage medium read-only memory 110 can be programmed with computer readable data representing instructions executable by processor 106 for performing the methods described below as well as other variants that are anticipated but not specifically listed.

[0039] In some examples, the vehicle 5 may be a hybrid vehicle having multiple torque sources available for one or more wheels 55. In other examples, the vehicle 5 is a conventional vehicle having only an engine, or an electric vehicle having only a motor. In the example shown, the vehicle 5 includes an engine 10 and a motor 52. The motor 52 may be a motor or a motor / generator. When one or more clutches 56 are engaged, the crankshaft 140 of the engine 10 and the motor 52 are coupled to the wheels 55 via a transmission 54. In the depicted example, a first clutch 56 is provided between the crankshaft 140 and the motor 52, and a second clutch 56 is provided between the motor 52 and the transmission 54. The controller 12 may send a signal to the actuator of each clutch 56 to engage or disengage the clutch so as to connect or disconnect the crankshaft 140 with the motor 52 and with the components connected thereto, and / or connect or disconnect the motor 52 with the transmission 54 and with the components connected thereto. The transmission 54 may be a gearbox, a planetary gear system, or other types of transmissions. The powertrain may be configured in various ways, including parallel, series, or series-parallel hybrid vehicles.

[0040] The electric machine 52 receives power from the traction battery 58 to provide torque to the wheels 55. The electric machine 52 may also operate as a generator to provide power to charge the battery 58, for example, during a braking operation.

[0041] The actual CR of each cylinder affects the knock limit of the cylinder, especially under high load, and also affects the dilution limit of the cylinder, especially under light load. Due to the manufacturing tolerance of the VCR mechanism of each cylinder 30 connected to the engine 10, there may be significant part-to-part variation between the actual CR of each cylinder and the expected CR of the cylinder. In addition, for a given expected CR, there may be significant actual CR variation between cylinders. Due to these differences, the CR calibration may not be optimal. Since the CR of the engine also affects the dilution tolerance of the engine, errors in CR estimation may also cause non-optimal EGR or VCT (or VVL, etc.) calibration. As an example, a lower CR setting can be commanded in response to a high load condition. However, since the actual CR of the cylinder is higher than expected, the resulting non-optimal CR may be higher than expected, causing the cylinder to become over-knock limited. As another example, a higher CR setting can be commanded in response to a low load condition. However, since the actual CR of the cylinder is lower than expected, the resulting non-optimal CR may be lower than expected, causing the cylinder to become combustion stable and NVH limited.

[0042] As reference Figures 2 to 3 As described in detail, the engine controller can update the CR calibration (that is, the calibration of the CR commanded at a given engine speed and load) based on the calculated differences in fuel usage and torque output of each cylinder at each CR setting of the VCR engine. The engine controller can also update the EGR and / or VCT calibration (that is, the calibration of the dilution commanded at a given engine speed and load) based on the calculated differences in fuel usage and torque output of each cylinder at each CR setting of the VCR engine. As a result, VCR engine performance can be improved. Turning now to Figure 2 , describes an exemplary routine 200 for calibrating a VCR engine. This method reduces performance losses due to cylinder-to-cylinder CR variations due to manufacturing tolerances. Instructions for executing method 200, as well as other methods included herein, may be generated by a controller based on instructions stored on a memory of the controller and in conjunction with sensors from the engine system (such as those referenced above). Figure 1 According to the method described below, the controller can use the engine actuator of the engine system to adjust the engine operation.

[0043] At 202, method 200 includes estimating and / or measuring engine operating conditions. Engine operating conditions may include, for example: driver power demand (e.g., based on output of a pedal position sensor coupled to an operator pedal); ambient temperature, pressure, and humidity; engine speed, engine temperature; manifold pressure (MAP); manifold air flow (MAF); catalyst temperature; intake air temperature; boost level; fuel octane rating of fuel available in a fuel tank; etc.

[0044] It will be appreciated that in alternative examples, method 200 may be triggered during the first engine start after the engine is manufactured to allow calibration of the engine. In other examples, method 200 may be triggered in response to engine repair or maintenance (as indicated by a disconnected battery, or input from a diagnostic tool, or input from a graphical user interface (GUI)).

[0045] At 204, method 200 includes selecting a desired compression ratio for operating the engine based on the estimated engine operating conditions. The engine may be configured with a VCR mechanism (e.g., Figure 1 VCR mechanism 194) that mechanically changes the engine compression ratio between a first lower compression ratio setting and a second higher compression ratio setting. The VCR mechanism can achieve this by mechanically changing the piston position in the cylinder. Alternatively, multiple compression ratios between the first compression ratio and the second compression ratio are possible. The controller can calculate the fuel efficiency at each possible compression ratio of the engine under a given driver power demand, and select the compression ratio that provides the highest fuel efficiency. The controller can compare the fuel efficiency at each compression ratio by comparing the brake fuel consumption (BSFC) of the engine at each compression ratio, for example, according to a lookup table stored in the memory of the controller, which is filled during the initial engine calibration based on a prototype engine with substantially the same CR on each cylinder. The fuel efficiency of the engine at each compression ratio can be determined via a table, a map, an algorithm and / or an equation, each stored according to operating conditions (e.g., engine speed, torque, temperature, humidity, inferred fuel octane number, etc.), and the settings filled during the initial engine calibration are based on the prototype engine. In general, as engine load or BMEP increases, the selected compression ratio may decrease due to a tradeoff between the efficiency benefits of higher CR (which dominates at lower loads) and the efficiency losses of knock-limited combustion phasing (which dominates at higher loads). Thus, a lower compression ratio is selected at higher engine loads, and a higher compression ratio is selected at lower engine loads.

[0046] At 206, the method includes retrieving the actual compression ratio setting for each cylinder at the desired nominal compression ratio setting. For example, reference may be made to a value such as Figure 4 A lookup table of a table is used to determine whether the actual CR of a given cylinder exceeds or falls below the desired nominal CR setting.

[0047] At 208, the method includes calculating the fuel economy (or fuel use) associated with each cylinder at the retrieved actual compression ratio. For example, if the actual CR of the selected cylinder is higher than the desired nominal CR setting, it can be determined that the fuel economy of the cylinder is reduced at high load due to the additional spark retard (followed by knock-limited combustion phasing). At 210, the method includes determining the overall fuel economy of the engine by summing the fuel economy of each cylinder.

[0048] At 212, the method includes calculating the fuel loss associated with each cylinder at the retrieved actual compression ratio. For example, if the actual CR of the selected cylinder is higher than the desired nominal CR setting, and the engine is currently operating at a high load (which is knock limited in this case), then it can be determined that the cylinder has a fuel loss that is a function of the difference between the actual CR of the selected cylinder and the desired nominal CR setting. At 214, the method includes determining the total fuel loss of the engine by summing the fuel losses of each cylinder.

[0049] As an example, the number of cells which can be obtained from a lookup table stored in a memory of the controller, such as Figure 4 The actual CR data can be retrieved from Table 400 of the controller. The lookup table can be populated with data immediately after the engine is manufactured, replaced, or overhauled. For example, the CR of each cylinder can be quantified by measuring the dimensions of key engine parts during manufacturing. Alternatively, the CR of each cylinder can be quantified during end-of-line spin testing by measuring the cylinder pressure in each cylinder, or by using a radio frequency transceiver in each cylinder, or by measuring the crankshaft acceleration curve determined by the crank angle. The known CR of each cylinder can be stored in the memory of the controller immediately after the engine is manufactured and, if necessary, updated by a service technician after the engine is replaced or overhauled. In doing so, for each cylinder, it can be determined whether the actual CR of a given cylinder is above or below a given CR setting. For example, referring to Figure 4 In table 400, the actual CR of cylinder 1 is significantly higher than the expected setting, while the actual CR of cylinder 4 is significantly lower than the expected setting, and the difference between the expected CR and the actual CR varies with the nominal CR. When operating the engine at higher loads, the higher than expected actual CR of cylinder 1 may cause cylinder 1 to be more knock limited than the other cylinders, thereby requiring additional spark retard. This causes a fuel efficiency loss for cylinder 1 at high loads.

[0050] At 216, the total fuel loss due to the actual CR and the nominal CR may be compared to a threshold. If the loss is below the threshold, that is, if there is no significant fuel loss associated with the actual CR and the nominal CR, then at 218, the method includes continuing engine operation at the desired nominal CR setting selected at 204. For example, an engine with less variation in actual CR between cylinders will operate most efficiently at the desired nominal CR selected at 204 because this desired nominal CR was determined based on testing a prototype engine with less variation in actual CR between cylinders (see, e.g., Figure 7 702 of the mapping diagram 700).

[0051] Otherwise, if there is a significant fuel economy penalty associated with the actual CR versus the nominal CR, then at 220, the method includes actuating the VCR mechanism to a lower CR setting. For example, before repeating the sequence of method 200, the controller may send a signal to the VCR actuator to move the VCR mechanism to reduce the CR by a ratio of 0.2. For an engine operating at high loads (where the engine is knock limited) where one or more cylinders have a CR higher than the nominal CR, the optimal CR will be lower than the desired nominal CR (e.g., Figure 7 ), the desired nominal CR is determined based on a prototype engine having a small variation in actual CR between test cylinders.

[0052] From each of 218 and 220, the method moves to 222 to determine whether engine dilution control is required. In one example, engine dilution control is required when the engine load is less than a threshold load, where combustion stability is a constraint on the EGR and / or VCT (or VVL, etc.) schedule. In another example, engine dilution control is required below a threshold load, which varies with engine speed, temperature, or other factors. If dilution control is required, then at 224, the nominal VCT and / or EGR schedule can be maintained. Otherwise, if dilution control is required, then at 228, the nominal VCT and / or EGR schedule can be updated. Specifically, the controller can modify the nominal EGR / VCT schedule based on the lowest CR of all cylinders, as determined at 206. Lower CR causes reduced combustion stability at light loads, which reduces dilution tolerance and therefore reduces the optimal EGR rate. It also moves the optimal VCT / VVL schedule toward lower "internal EGR" (lower overlap and / or earlier exhaust valve closing time) and / or toward higher effective CR (earlier intake valve closing time). Mapping data from a prototype engine with less actual CR variation between cylinders can be used to quantify the optimal (combustion stability limited) EGR and / or VCT schedule based on CR. The combustion stability limit is determined by the "worst case" cylinder, in this case the cylinder with the lowest CR. Therefore, the combustion stability limited EGR and / or VCT settings at light loads are simply calculated by using the CR of the lowest CR cylinder rather than using the nominal CR. For example, for an engine operating below a load threshold (where combustion stability is a constraint) in which one or more cylinders have a CR below the nominal CR, a lower EGR amount and / or lower valve overlap and / or earlier EVC settings (such as Figure 5 506 and 508).

[0053] like Figure 3 As detailed, the EGR and / or VCT schedule may be modified at any time that the VCR mechanism degrades, such as may occur due to component degradation or due to unsatisfied entry conditions. Specifically, it may be determined whether the actual VCR mechanism position is different from the desired VCR. Unsatisfied VCR actuator entry conditions may include conditions related to temperature, oil pressure, current limits, etc. If VCR degradation is detected, then a lower EGR amount and / or lower valve overlap and / or earlier EVC settings may be applied, such as Figure 5 shown.

[0054] Now turn to Figure 3 , another exemplary method 300 for calibrating a VCR engine is shown.

[0055] At 302, as at 202, method 300 includes estimating and / or measuring engine operating conditions. Engine operating conditions may include, for example: driver power demand (e.g., based on output of a pedal position sensor coupled to an operator pedal); ambient temperature, pressure, and humidity; engine speed, engine temperature; manifold pressure (MAP); manifold air flow (MAF); catalyst temperature; intake air temperature; boost level; fuel octane rating of fuel available in a fuel tank; etc.

[0056] It will be appreciated that in some cases, method 300 may be triggered during the first engine start after the engine is manufactured to allow the engine to be calibrated. In other cases, method 300 may be triggered in response to engine repair or maintenance (as indicated by, for example, a disconnected battery, or input from a diagnostic tool, or input from a graphical user interface (GUI)).

[0057] At 304, method 300 includes quantifying the actual CR of each cylinder of the engine at each CR setting of the engine. As an example, a lookup table (such as Figure 4 The actual CR data may be retrieved from table 400 of the controller. The lookup table may be populated with data immediately after an engine is manufactured, replaced, or overhauled. For example, the CR of each cylinder may be quantified during an off-line overspeed test, based on measured cylinder pressure, or using a radio frequency transceiver in each cylinder, or based on a crankshaft acceleration curve resolved by a measured crank angle, or based on measured dimensions of key engine parts. The known CR for each cylinder may be stored in the controller's memory immediately after the engine is manufactured and, if necessary, updated by a service technician after an engine replacement or overhaul. In doing so, for each cylinder, it may be determined whether the actual CR of a given cylinder is above or below a given CR setting. For example, referring to Figure 4 In table 400, the actual CR of cylinder 1 is significantly higher than the expected setting, while the actual CR of cylinder 4 is significantly lower than the expected setting, and the difference between the expected CR and the actual CR varies with the nominal CR. When operating the engine at higher loads, the higher than expected actual CR of cylinder 1 may cause cylinder 1 to be more knock limited than the other cylinders, requiring additional spark retard. This results in a loss of fuel efficiency.

[0058] At 306, method 300 includes quantifying the fuel flow and maximum in-cylinder mean effective pressure (IMEP) for each cylinder based on the VCR mechanism setting (nominal CR) at the current operating conditions. For each possible setting of the nominal CR, the controller calculates the fuel flow and IMEP for each individual cylinder at its actual compression ratio. The controller can calculate the fuel flow and IMEP at each compression ratio by using a lookup table stored in the controller's memory, which is populated during an initial engine calibration based on a prototype engine with substantially the same CR on each cylinder. The fuel flow for each cylinder on the prototype engine is simply the total fuel flow divided by the number of cylinders. The IMEP is based on cylinder pressure data on the prototype engine. At 306, the IMEP and fuel flow for each cylinder are calculated for each nominal CR (VCR mechanism setting), but the actual CR of each cylinder is used. If the engine has a small cylinder-to-cylinder CR variation, the calculated IMEP and fuel flow in each cylinder are almost the same, and the minimum total engine IMEP and fuel flow are achieved at the same CR as the minimum fuel flow and IMEP of the prototype engine. However, for engines with higher cylinder-to-cylinder CR variations, such as Figure 4 ), when operating at high loads with engine knock limited, the calculated fuel flow and IMEP will be different in each cylinder. Figure 4 For cylinder 1 of the engine shown in , the CR used in the fuel flow calculation will be higher due to knock-limited combustion phasing, and IMEP will be lower at high CR. The effects of knock-limited combustion phasing are non-linear, so the optimum nominal CR cannot be determined by simply averaging the CRs of all cylinders; the cylinder with the highest CR has a disproportionate effect under knock-limited conditions. At high loads, the optimum nominal CR for this engine will be lower than that of the prototype engine, as shown in Figure 1. Figure 7 As shown by the dotted line in .

[0059] It will be appreciated that while the method suggests quantifying fuel flow and IMEP per cylinder, this is not meant to be limiting, and in alternative examples, other cylinder parameters indicative of fuel economy and performance may be used. For example, in alternative examples, the metric quantified by the controller (under part load conditions, such as when below a threshold load) may include efficiency or brake specific fuel consumption (BSFC), while the metric quantified under higher load conditions (such as near peak load or above a threshold load) may include torque, power, or brake specific air consumption (BSAC).

[0060] At 310, it may be determined whether the driver demand is above a threshold. The threshold may be based on the accelerator pedal position. Thus, above the threshold, it may be inferred that the driver prioritizes performance over efficiency, while below the threshold, maximum performance is not required and VCR control may be optimized for efficiency.

[0061] If the driver demand is below the threshold, then at 312, the method includes selecting a VCR setting corresponding to a minimum total engine fuel flow. By selecting a VCR setting corresponding to a minimum total engine fuel flow during part load conditions, fuel consumption and CO2 emissions can be minimized. Otherwise, at 314, if the driver demand is above the threshold load, then the method includes selecting a VCR setting corresponding to a maximum total IMEP. By selecting a VCR setting corresponding to a maximum total IMEP during peak load conditions, vehicle acceleration performance can be maximized.

[0062] From each of 312 and 314, the method moves to 316 to determine whether engine dilution control is required. In one example, engine dilution control is required when the engine load is less than a threshold load. If dilution control is required, then at 320, the nominal VCT and / or EGR schedule can be updated. Specifically, the controller can modify the nominal EGR / VCT schedule based on the lowest CR of all cylinders. Lower CR causes reduced combustion stability at light loads, which reduces the dilution tolerance and therefore reduces the optimal EGR rate. It also moves the optimal VCT schedule toward lower "internal EGR" (lower overlap and / or earlier exhaust valve closing time) and / or toward higher effective CR (earlier intake valve closing time). Mapping data from a prototype engine with less actual CR variation between cylinders can be used to quantify the optimal (combustion stability limited) EGR and / or VCT schedule based on CR. The combustion stability limit is determined by the "worst case" cylinder, in this case the cylinder with the lowest CR. Thus, the combustion stability-limited EGR and / or VCT settings at light loads are calculated based on the lowest CR cylinder rather than using the nominal CR. For example, for an engine operating below a load threshold where combustion stability is a constraint where one or more cylinders have a CR below the nominal, a lower EGR amount and / or lower valve overlap and / or earlier EVC settings may be applied, as further described below. Figure 5 shown.

[0063] If engine dilution control is not required, then at 318, it may be determined whether there is any degradation or failure of the VCR mechanism. Specifically, it may be determined whether the actual VCR mechanism position is different from the desired VCR. VCR degradation may be determined in response to VCR component degradation or because VCR actuator entry conditions are not met. Unmet VCR actuator entry conditions may include conditions related to temperature, oil pressure, current limit, etc. If VCR degradation is detected, then a lower EGR amount and / or lower valve overlap and / or earlier EVC settings may be applied, such as Figure 5 shown.

[0064] If VCR degradation is not confirmed, then at 322, the nominal VCT and / or EGR schedule may be maintained. The nominal EGR and / or VCT schedule may be based on engine operating conditions. Otherwise, if VCR degradation is determined, the method returns to 320 to operate with a modified EGR and / or VCT schedule.

[0065] Figure 5 An effective EGR schedule for load (BMEP) sweeps at various CRs is shown based on data from a prototype engine with substantially the same CR on each cylinder. Curve 504 shows an EGR schedule with a 12:1 CR, curve 506 shows an EGR schedule with a 10:1 CR, and curve 508 shows an EGR schedule with an 8:1 CR. It will be appreciated that the EGR schedule may also be a function of engine speed, engine temperature, air temperature and humidity, etc. The EGR schedule has a maximum EGR rate at an intermediate BMEP. At higher BMEPs, the EGR rate may be reduced due to the negative effect of EGR on volumetric efficiency. At lower BMEPs, the EGR rate may be reduced due to the negative effect of EGR on combustion stability. However, at lower CRs, combustion stability is also poorer. For example, a load threshold 502 is shown, below which combustion stability is reduced. Therefore, when the CR of one or more cylinders is lower, a lower EGR rate is used. This table was created using mapping data from a prototype engine with substantially the same CR on each cylinder, but the cylinder with the lowest actual CR was used with the table to determine the current desired EGR rate. A similar approach would be used to limit "internal EGR" by changing the schedule for VCT, VVL, etc. based on the cylinder with the lowest CR. Figure 6The trends of fuel consumption versus load (BMEP) for various CRs are shown. These trends are well known to those skilled in the art because they result from a basic tradeoff between the efficiency benefits of higher CRs and the efficiency penalties of knock limited combustion phasing. The results of these tradeoffs determine the optimum CR for each BMEP (the optimum CR also varies with engine speed, fuel octane, intake air temperature, humidity, etc.). The efficiency benefits of higher CR dominate at lower BMEPs, while the efficiency penalties of knock limited combustion phasing dominate at higher BMEPs, as shown by the load limited curve 604. Thus, at low BMEPs where the engine is not knock limited, the optimum CR is high, and Figure 6 It is shown that the lowest fuel consumption is achieved at a high CR of 13:1. Curves 602a to 602h show that fuel consumption decreases with increasing CR. At high BMEP where the engine is most knock limited, the optimum CR is low, and Figure 6 It is shown that the lowest fuel consumption occurs at the lowest CR of 8: 1. At intermediate BMEPs, these two factors compromise to varying degrees, and the lowest fuel flow occurs at various CRs between low and high. Figure 6 The trends shown in are quantified based on a test prototype engine and are used to determine the optimum or desired nominal CR based on BMEP, engine speed, fuel octane, intake air temperature, humidity, etc., which is, for example, Figure 2 Step 204 and Figure 3 The example of step 306 is used.

[0066] Using the above method, compression ratio variations between cylinders can be better detected and responded to. By knowing the fuel flow and IMEP of all cylinders based on each nominal desired CR setting of the VCR engine, the actual CR variation of the engine can be known and distinguished from the CR data obtained on the prototype engine. In addition, the VCR engine can be reliably calibrated in a more cost-effective manner while relying on existing sensors and actuators. By adjusting the CR setting to provide the highest fuel economy under low load conditions and the highest engine output under high load conditions, engine performance can be improved despite the CR variation between cylinders. By also adjusting the EGR and VCT schedule of the VCR engine based on the mapped CR of all engine cylinders, dilution control is improved, allowing the engine to operate closer to the combustion stability limit with fewer NVH issues. In general, engine performance and fuel efficiency are improved by improving the calibration of the VCR engine.

[0067] An exemplary method for an engine includes: calibrating a compression ratio schedule of a variable compression ratio engine based on each of a fuel flow and a peak torque for each cylinder at each compression ratio setting of the engine; and adjusting an exhaust gas recirculation (EGR) flow to the engine according to an updated exhaust gas recirculation (EGR) calibration schedule based on the compression ratio schedule calibration. In the foregoing examples, additionally or optionally, the calibration includes learning, for each cylinder, a difference between an actual compression ratio and a commanded compression ratio at each of a plurality of compression ratio settings. In any or all of the foregoing examples, additionally or optionally, the adjusting includes: identifying an engine cylinder having a highest difference between an actual compression ratio and a commanded compression ratio, wherein the actual compression ratio is lower than the commanded compression ratio; and adjusting the EGR flow to the engine based on the actual compression ratio of the identified engine cylinder. In any or all of the foregoing examples, additionally or optionally, the adjusting includes: identifying an engine cylinder having a lowest actual compression ratio; and adjusting the EGR flow to the engine based on the actual compression ratio of the identified engine cylinder. In any or all of the foregoing examples, additionally or optionally, the adjustment according to the updated EGR calibration plan includes adjustment based on the lowest of a plurality of compression ratio settings for individual cylinders of the engine. In any or all of the foregoing examples, additionally or optionally, the adjustment also includes: when the engine load is below a threshold load, reducing the EGR flow as the lowest of the plurality of compression ratio settings decreases, reducing the EGR flow includes one or more of: reducing the opening of the EGR valve, adjusting the cylinder valve timing to advance exhaust valve closing, and adjusting the cylinder valve timing to reduce intake valve and exhaust valve positive overlap. In any or all of the foregoing examples, additionally or optionally, the engine is coupled in a vehicle, and wherein the updated EGR calibration plan is updated according to a nominal EGR calibration plan based on engine testing prior to vehicle manufacture, the vehicle comprising a hybrid electric vehicle. In any or all of the foregoing examples, additionally or optionally, the adjusting further comprises adjusting the EGR flow to the engine according to a default EGR calibration schedule when engine load is above the threshold load.In any or all of the foregoing examples, additionally or optionally, the compression ratio schedule calibration includes: estimating each of the fuel flow and peak torque of each cylinder at each of a plurality of compression ratio settings of the engine; learning a first parameter indicating the fuel flow of the engine at each of a plurality of compression ratio settings; learning a second different parameter indicating the torque of the engine at each of a plurality of compression ratio settings; and actuating a variable compression ratio mechanism of each engine cylinder based on a selection of one of the first parameter and the second parameter, the selection being based on a driver torque demand. In any or all of the foregoing examples, additionally or optionally, the selection includes: selecting the first parameter instead of the second parameter when the driver torque demand is below a threshold, and selecting the second parameter instead of the first parameter when the driver torque demand is above the threshold, the first parameter including one of a total engine fuel flow and a total brake fuel consumption rate, and the second parameter including one of a total engine torque and a total cylinder mean effective pressure. In any or all of the foregoing examples, additionally or optionally, the actuation based on the selection includes: actuating the variable compression ratio mechanism based on the engine cylinder having the lowest value of the first parameter when the driver torque demand is below a threshold, and actuating the variable compression ratio mechanism based on the engine cylinder having the highest value of the second parameter when the driver torque demand is above the threshold.

[0068] Another example method includes: comparing a commanded compression ratio to an actual compression ratio for each cylinder of a variable compression ratio engine having multiple compression ratio settings; and adjusting exhaust gas recirculation (EGR) flow to the engine based on the actual compression ratio of the engine cylinder having the lowest actual compression ratio. In the foregoing examples, additionally or optionally, the EGR flow to the engine is reduced from a nominal EGR flow as the value of the lowest actual compression ratio decreases. In any or all of the foregoing examples, additionally or optionally, adjusting the EGR flow is in response to an engine load below a threshold load, and the method further includes maintaining the nominal EGR flow in response to an engine load above the threshold load. In any or all of the foregoing examples, additionally or optionally, reducing the EGR flow includes one or more of: reducing an opening of an EGR valve, changing valve timing to reduce positive valve overlap, and advancing exhaust valve timing to an earlier exhaust valve closing timing. In any or all of the foregoing examples, additionally or optionally, the method further comprises, when the engine load is above a threshold load, transitioning to a lower compression ratio via mechanical actuation of a variable compression ratio mechanism in response to a difference between the commanded compression ratio and the actual compression ratio of the engine cylinder being above a threshold difference.

[0069] Another exemplary engine system includes: an engine including a plurality of cylinders; a VCR mechanism coupled to a piston of each of the plurality of cylinders for applying one of a plurality of compression ratio settings in a given cylinder via a mechanical change in a piston position within the given cylinder; an EGR passage including an EGR valve for recirculating exhaust gas from an engine exhaust port to an engine intake port; and a controller having computer-readable instructions stored on a non-transitory memory for: updating a nominal compression ratio calibration of the engine based on each of a fuel flow and a peak torque of each cylinder at each of the plurality of compression ratio settings; adjusting an EGR flow to the engine based on the nominal compression ratio calibration at a load above a threshold engine load; and adjusting the EGR flow to the engine based on a lowest compression ratio of the updated compression ratio calibration at a load below a threshold engine load. In the foregoing examples, additionally or optionally, adjusting the EGR flow based on the minimum compression ratio includes: identifying one of the plurality of cylinders having a lowest actual compression ratio; estimating engine dilution for the identified one of the plurality of cylinders; and adjusting the opening of the EGR valve based on the estimated engine dilution. In any or all of the foregoing examples, additionally or optionally, adjusting the EGR flow based on the minimum compression ratio includes reducing the opening of the EGR valve when the minimum compression ratio decreases. In any or all of the foregoing examples, additionally or optionally, the engine is coupled in a hybrid electric vehicle, and wherein each of the nominal compression ratio calibration and the nominal EGR calibration is based on engine test data collected prior to vehicle manufacture.

[0070] Another exemplary method for an engine includes: actuating a variable compression ratio mechanism of the engine to mechanically adjust a target compression ratio of the engine according to an updated calibration, the updated calibration being based on each of a fuel flow rate and a peak torque per cylinder at each compression ratio setting of the mechanism. In the foregoing example, additionally or optionally, the updated calibration includes: estimating the fuel flow rate and the peak torque per cylinder at a plurality of compression ratio settings; quantifying the total engine fuel flow rate of the engine at each of the plurality of compression ratio settings as the sum of the fuel flow rates per cylinder at the corresponding compression ratio setting; and learning the total torque of the engine at each of the plurality of compression ratio settings as the sum of the torque per cylinder at the corresponding compression ratio setting. In any or all of the foregoing examples, additionally or optionally, the actuation includes: actuating the mechanism to one of the plurality of compression ratio settings having the lowest total engine fuel flow rate when a driver torque demand is below a threshold; and actuating the mechanism to another of the plurality of compression ratio settings having the highest total engine torque when a driver torque demand is above the threshold. In any or all of the foregoing examples, additionally or optionally, the threshold is based on one or more of accelerator pedal position, engine speed, fuel octane, ambient temperature, and ambient humidity. In any or all of the foregoing examples, additionally or optionally, the method further comprises adjusting the engine dilution calibration based on the lowest of a plurality of compression ratios of individual cylinders. In any or all of the foregoing examples, additionally or optionally, the adjusting the engine dilution comprises using less dilution as the lowest of the plurality of compression ratios decreases when the engine load is below a threshold. In any or all of the foregoing examples, additionally or optionally, the method further comprises selecting the target compression ratio of the engine based on a nominal calibration. In any or all of the foregoing examples, additionally or optionally, the engine is coupled to a vehicle, the method further comprising updating the nominal calibration of the engine in response to engine operation after vehicle manufacture, the nominal calibration being based on engine testing prior to vehicle manufacture, the vehicle comprising a hybrid electric vehicle.

[0071] Yet another example method includes: comparing a target compression ratio of an engine with an actual compression ratio of each cylinder of the engine; calculating a fuel penalty associated with the target compression ratio based on a total difference between the actual compression ratio of each cylinder of the engine and the target compression ratio; and if the fuel penalty exceeds a threshold, transitioning to a lower compression ratio. In the foregoing examples, additionally or optionally, each of the target compression ratio and the actual compression ratio is a defined compression ratio setting for a variable compression ratio mechanism, the defined compression ratio setting being one of a plurality of compression ratio settings for the engine. In any or all of the foregoing examples, additionally or optionally, the transitioning includes actuating a variable compression ratio mechanism to mechanically change the actual compression ratio of each cylinder of the engine. In any or all of the foregoing examples, additionally or optionally, the method further includes adjusting the engine dilution in response to the transition, the adjustment including: comparing the actual compression ratio of each cylinder of the engine when the engine load is below a threshold; and applying the engine dilution setting corresponding to the lowest of the actual compression ratios of each cylinder. In any or all of the foregoing examples, additionally or optionally, adjusting engine dilution includes adjusting one of an exhaust gas recirculation (EGR) amount and a variable cam timing schedule. In any or all of the foregoing examples, additionally or optionally, applying one of a lower EGR amount, a lower valve overlap amount, and an earlier exhaust valve closing timing as the lowest of the actual compression ratios for each cylinder.

[0072] Another exemplary engine system includes; an engine comprising a plurality of cylinders; a VCR mechanism coupled to a piston of each of the plurality of cylinders for applying one of a plurality of compression ratio settings in a given cylinder via a mechanical change in a piston position within the given cylinder; an EGR passage comprising an EGR valve for recirculating exhaust gas from an engine exhaust port to an engine intake port; and a controller having computer-readable instructions stored on a non-transitory memory for performing the following operations: updating a compression ratio calibration of the engine based on each of a fuel flow rate and a peak torque of each cylinder at each of the plurality of compression ratio settings; and actuating a variable compression ratio mechanism of the engine to mechanically adjust a target compression ratio of the engine according to the updated calibration, the updated calibration being based on each of a fuel flow rate and a peak torque of each cylinder at each compression ratio setting of the mechanism. In the foregoing examples, additionally or optionally, the updating includes estimating the fuel flow and the peak torque for each of the plurality of cylinders at each of a plurality of compression ratio settings; quantifying, for each cylinder, the total engine fuel flow as the sum of the fuel flow at each of the plurality of compression ratio settings; and learning the total torque of the engine at each of the plurality of compression ratio settings as the sum of the torques for each of the plurality of cylinders at a corresponding one of the plurality of compression ratio settings. In any or all of the foregoing examples, additionally or optionally, the controller includes additional instructions for: actuating the mechanism to one of the plurality of compression ratio settings with the lowest total engine fuel flow when the driver torque demand is below a threshold; and actuating the mechanism to another of the plurality of compression ratio settings with the highest total engine torque when the driver torque demand is above the threshold. In any or all of the foregoing examples, additionally or optionally, the updating is calibrated according to a nominal compression ratio based on engine testing prior to vehicle manufacturing. In any or all of the foregoing examples, additionally or optionally, the vehicle is a hybrid electric vehicle, and wherein the updating is responsive to engine operation after vehicle manufacturing. In any or all of the foregoing examples, additionally or alternatively, the controller includes further instructions for: updating an EGR calibration for the engine based on a lowest compression ratio setting for one of the plurality of engine cylinders in the updated compression ratio calibration; and actuating the EGR valve based on engine load and also based on the updated EGR calibration.

[0073] It should be noted that the exemplary control and estimation routines included herein can be used with various engine and / or vehicle system configurations. The control methods and routines disclosed herein can be stored as executable instructions in a non-transitory memory and can be executed by a control system including a controller combined with various sensors, actuators and other engine hardware. The specific routines described herein can represent one or more of any number of processing strategies (such as event-driven, interrupt-driven, multi-tasking, multi-threaded, etc.). Therefore, the various actions, operations and / or functions described can be executed in the described sequence, executed in parallel, or omitted in some cases. Similarly, the processing order is not necessarily required to achieve the features and advantages of the exemplary embodiments described herein, but is provided for ease of description and description. One or more of the actions, operations and / or functions described can be repeatedly performed according to the specific strategy used. In addition, the described actions, operations and / or functions can graphically represent the code in the non-transitory memory of the computer-readable storage medium to be programmed into the engine control system, wherein the described actions are executed by executing instructions in a system including various engine hardware components combined with an electronic controller.

[0074] It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments should not be viewed in a limiting sense, as many variations are possible. For example, the above techniques may be applied to V-6, I-4, I-6, V-12, opposed 4, 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 properties disclosed herein.

[0075] The following claims particularly point out certain combinations and subcombinations deemed novel and non-obvious. The claims may refer to "an" element or a "first" element or the equivalent thereof. The claims should be understood to include combinations of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and / or properties may be claimed by amendment of the present claims or by presentation of new claims in this or a related application. Such claims, whether broader, narrower, the same, or different in scope to the original claims, are deemed to be included in the subject matter of the present disclosure.

[0076] According to the present invention, a method for an engine is provided, comprising: calibrating a compression ratio plan of a variable compression ratio engine based on each of the fuel flow and peak torque of each cylinder at each compression ratio setting of the engine; and adjusting the exhaust gas recirculation (EGR) flow to the engine according to an updated EGR calibration plan based on the compression ratio plan calibration.

[0077] According to one embodiment, the calibration comprises learning, for each cylinder, a difference between an actual compression ratio and a commanded compression ratio at each of a plurality of compression ratio settings.

[0078] According to one embodiment, the adjusting includes identifying an engine cylinder having a highest difference between an actual compression ratio and a commanded compression ratio, wherein the actual compression ratio is lower than the commanded compression ratio; and adjusting EGR flow to the engine based on the actual compression ratio of the identified engine cylinder.

[0079] According to one embodiment, the adjusting comprises identifying the engine cylinder having the lowest actual compression ratio; and adjusting EGR flow to the engine based on the actual compression ratio of the identified engine cylinder.

[0080] According to one embodiment, said adjusting according to an updated EGR calibration schedule comprises adjusting based on a lowest of a plurality of compression ratio settings for individual cylinders of said engine.

[0081] According to one embodiment, the adjusting further comprises: when the engine load is lower than a threshold load, reducing the EGR flow as the lowest of the plurality of compression ratio settings decreases, reducing the EGR flow comprises one or more of: reducing an opening of an EGR valve, adjusting cylinder valve timing to advance exhaust valve closing, and adjusting the cylinder valve timing to reduce positive overlap of an intake valve and an exhaust valve.

[0082] According to one embodiment, the engine is coupled in a vehicle, and wherein the updated EGR calibration schedule is updated according to a nominal EGR calibration schedule based on engine testing prior to vehicle manufacture, the vehicle comprising a hybrid electric vehicle.

[0083] According to one embodiment, said adjusting further comprises, when engine load is above said threshold load, adjusting said EGR flow to said engine according to a default EGR calibration schedule.

[0084] According to one embodiment, the compression ratio schedule calibration includes: estimating each of the fuel flow and peak torque of each cylinder at each of a plurality of compression ratio settings of the engine; learning a first parameter indicative of the fuel flow of the engine at each of a plurality of compression ratio settings; learning a second different parameter indicative of the torque of the engine at each of a plurality of compression ratio settings; and actuating a variable compression ratio mechanism of each engine cylinder based on a selection of one of the first parameter and the second parameter, the selection being based on a driver torque demand.

[0085] According to one embodiment, the selection includes: selecting the first parameter instead of the second parameter when the driver torque demand is lower than a threshold, and selecting the second parameter instead of the first parameter when the driver torque demand is higher than the threshold, the first parameter including one of the total engine fuel flow and the total brake fuel consumption rate, and the second parameter including one of the total engine torque and the total cylinder mean effective pressure.

[0086] According to one embodiment, the actuation based on the selection includes: actuating the variable compression ratio mechanism based on the engine cylinder having the lowest value of the first parameter when the driver torque demand is below a threshold, and actuating the variable compression ratio mechanism based on the engine cylinder having the highest value of the second parameter when the driver torque demand is above the threshold.

[0087] According to the present invention, a method includes: comparing a commanded compression ratio with an actual compression ratio for each cylinder of a variable compression ratio engine having multiple compression ratio settings; and adjusting exhaust gas recirculation (EGR) flow to the engine based on the actual compression ratio of the engine cylinder having the lowest actual compression ratio.

[0088] According to one embodiment, the EGR flow to the engine is reduced from a nominal EGR flow as the value of the lowest practical compression ratio decreases.

[0089] According to one embodiment, adjusting the EGR flow is responsive to engine load being below a threshold load, the method further comprising maintaining the nominal EGR flow responsive to engine load being above the threshold load.

[0090] According to one embodiment, reducing the EGR flow includes one or more of: reducing an opening of an EGR valve, changing valve timing to reduce positive valve overlap, and advancing exhaust valve timing to an earlier exhaust valve closing timing.

[0091] According to one embodiment, the invention is further characterized by transitioning to a lower compression ratio via mechanical actuation of a variable compression ratio mechanism in response to a difference between the commanded compression ratio and the actual compression ratio of an engine cylinder being above a threshold difference when engine load is above a threshold load.

[0092] According to the present invention, an engine system includes: an engine, the engine including a plurality of cylinders; a VCR mechanism, the VCR mechanism being coupled to a piston of each of the plurality of cylinders for applying one of a plurality of compression ratio settings in the given cylinder via a mechanical change in a piston position within the given cylinder; an EGR passage, the EGR passage including an EGR valve for recirculating exhaust gas from an engine exhaust port to an engine intake port; and a controller having computer-readable instructions stored on a non-transitory memory for performing the following operations: updating a nominal compression ratio calibration of the engine based on each of a fuel flow rate and a peak torque of each cylinder at each of the plurality of compression ratio settings; adjusting an EGR flow to the engine based on the nominal compression ratio calibration at an engine load above a threshold; and adjusting the EGR flow to the engine based on a lowest compression ratio of the updated compression ratio calibration at an engine load below a threshold.

[0093] According to one embodiment, adjusting the EGR flow based on the lowest compression ratio includes: identifying one of the plurality of cylinders having a lowest actual compression ratio; estimating engine dilution for the identified one of the plurality of cylinders; and adjusting an opening of the EGR valve based on the estimated engine dilution.

[0094] According to one embodiment, adjusting the EGR flow rate based on the minimum compression ratio includes reducing the opening of the EGR valve when the minimum compression ratio decreases.

[0095] According to one embodiment, the engine is coupled in a hybrid electric vehicle, and wherein each of the nominal compression ratio calibration and the nominal EGR calibration is based on engine test data collected prior to vehicle manufacture.

Claims

1. A method for an engine, the method comprising: calibrating a compression ratio schedule for a variable compression ratio engine based on each of fuel flow and peak torque per cylinder at each compression ratio setting of the engine; as well as An EGR calibration schedule is updated based on the calibrated compression ratio schedule of the variable compression ratio engine, and EGR flow to the engine is adjusted according to the updated EGR calibration schedule. 2 . The method of claim 1 , wherein calibration of the compression ratio schedule includes learning, for each cylinder, a difference between an actual compression ratio and a commanded compression ratio at each of a plurality of compression ratio settings.

3. The method of claim 1, wherein the adjusting comprises: identifying an engine cylinder having a highest difference between an actual compression ratio and a commanded compression ratio, wherein the actual compression ratio is lower than the commanded compression ratio; as well as EGR flow to the engine is adjusted based on the identified actual compression ratio of the engine cylinders.

4. The method of claim 1, wherein the adjusting comprises: Identify the engine cylinder with the lowest actual compression ratio; as well as EGR flow to the engine is adjusted based on the identified actual compression ratio of the engine cylinders. 5 . The method of claim 1 , wherein the adjusting according to an updated EGR calibration schedule comprises adjusting based on a lowest of a plurality of compression ratio settings for individual cylinders of the engine.

6. The method of claim 5, wherein the adjusting further comprises: When the engine load is below a threshold load, the EGR flow is reduced as the lowest of the plurality of compression ratio settings decreases, and reducing the EGR flow includes one or more of: reducing an opening of an EGR valve, adjusting cylinder valve timing to advance exhaust valve closing, and adjusting the cylinder valve timing to reduce intake valve and exhaust valve positive overlap.

7. The method of claim 6, wherein the engine is coupled in a vehicle, and wherein the updated EGR calibration schedule is updated according to a nominal EGR calibration schedule based on engine testing prior to vehicle manufacture, the vehicle comprising a hybrid electric vehicle.

8. The method of claim 7, wherein the adjusting further comprises: When engine load is above the threshold load, the EGR flow to the engine is adjusted according to a default EGR calibration schedule.

9. The method of claim 1, wherein the calibration of the compression ratio schedule comprises: estimating each of a fuel flow rate and a peak torque per cylinder at each of a plurality of compression ratio settings for the engine; learning a first parameter indicative of fuel flow of the engine at each of the plurality of compression ratio settings; learning a different second parameter indicative of torque of the engine at each of the plurality of compression ratio settings; as well as A variable compression ratio mechanism for each engine cylinder is actuated based on selection of one of the first parameter and the second parameter, the selection being based on a driver torque demand.

10. The method of claim 9, wherein the selecting comprises: The first parameter is selected instead of the second parameter when the driver torque demand is below a threshold, and the second parameter is selected instead of the first parameter when the driver torque demand is above the threshold, the first parameter comprising total engine fuel flow, and the second parameter comprising total engine torque.

11. The method of claim 1 , wherein the calibration of the compression ratio schedule comprises: A variable compression ratio mechanism is actuated for each engine cylinder based on selection of one of a first parameter and a second parameter, the selection being based on the driver torque demand, the first parameter comprising a total brake specific fuel consumption and the second parameter comprising a total cylinder mean effective pressure.

12. The method of claim 9, wherein the actuating based on the selection comprises: The variable compression ratio mechanism is actuated based on the engine cylinder having the lowest value of the first parameter when the driver torque demand is below a threshold, and the variable compression ratio mechanism is actuated based on the engine cylinder having the highest value of the second parameter when the driver torque demand is above the threshold.

13. An engine system, comprising: an engine, the engine comprising a plurality of cylinders; a VCR mechanism coupled to a piston of each of the plurality of cylinders for applying one of a plurality of compression ratio settings in a given cylinder via a mechanical change in piston position within the given cylinder; an EGR passage including an EGR valve for recirculating exhaust gas from an engine exhaust port to an engine intake port; as well as A controller having computer readable instructions stored on a non-transitory memory for: updating a nominal compression ratio calibration of the engine based on each of the fuel flow and the peak torque per cylinder at each of the plurality of compression ratio settings; adjusting EGR flow to the engine based on the nominal compression ratio calibration at above a threshold engine load; as well as The EGR flow to the engine is adjusted based on a lowest compression ratio calibrated to the updated nominal compression ratio below a threshold engine load.

14. The system of claim 13, wherein adjusting the EGR flow based on the minimum compression ratio comprises: identifying one of the plurality of cylinders having a lowest actual compression ratio; estimating engine dilution for the identified one of the plurality of cylinders; as well as An opening of the EGR valve is adjusted based on the estimated engine dilution.

15. The system of claim 13, wherein adjusting the EGR flow based on the minimum compression ratio includes decreasing an opening of the EGR valve as the minimum compression ratio decreases.

16. The system of claim 13, wherein the engine is coupled in a hybrid electric vehicle, and wherein each of the nominal compression ratio calibration and the nominal EGR calibration are based on engine test data collected prior to vehicle manufacture.

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