Systems and methods for a variable compression ratio engine

Abstract

Methods and systems are provided for improving the calibration of a variable compression ratio engine. 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] This specification generally relates to methods and systems for controlling the compression ratio of a variable compression ratio engine.

[0002] BACKGROUND ART / SUMMARY OF THE INVENTION

[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). Generally, 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 non-knock conditions to take advantage of the high thermal efficiency, while a lower compression ratio setting can be used during knock-prone conditions. In a VCR engine, a connecting rod or other mechanism (e.g., an eccentric wheel) can be coupled to the piston of each cylinder to mechanically vary the compression ratio between the higher and lower settings.

[0004] Caswell shows an example of a VCR engine in US 4,469,055. Therein, during engine operation, the CR of the engine is adjusted based on the 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 according to the engine speed and load, can be calibrated based on a prototype engine.

[0005] However, the inventors have found 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 slightly different compression ratios (CRs) in each cylinder. In a VCR engine, in addition to the normal variations of non-VCR engines, each component of the VCR mechanism may have manufacturing tolerances to cause significant variations between components. VCR calibration based on the average CR (that is, the average of the CRs of all engine cylinders) may result in additional spark retard use in those cylinders having a higher-than-average CR, causing much lower efficiency in these cylinders. Using high-quality manufacturing methods and / or "select-fit" components can be used to control the CR difference between cylinders, but such methods increase significant costs. Since VCR engines maximize the compression ratio as much as possible, they tend to be knock-limited over most of the engine operating map and most of the driving cycle. Without knowing the actual CR of each cylinder and the variations between cylinders, it may be difficult to optimize the CR calibration, resulting in a loss of engine performance.

[0006] In another example, the above problem can be solved at least in part by a method that includes: actuating a variable compression ratio mechanism of an engine to mechanically adjust a target compression ratio of the engine according to an updated calibration, the updated calibration being based on each of fuel flow and peak torque of each cylinder at each compression ratio setting of the mechanism. In this way, CR optimization of the VCR engine is improved.

[0007] As an example, the actual CR of each cylinder of a VCR engine can be quantified according to each VCR mechanism setting. For example, when operating the VCR engine at a lower CR setting, the CR of each engine cylinder can be quantified first. Then, when operating the VCR engine at a higher CR setting, the CR of each engine cylinder can be quantified. Then, the fuel flow and the maximum IMEP of each cylinder can be quantified according to each VCR mechanism setting. Additionally, parameters can be quantified according to existing engine operating conditions (such as engine speed, engine torque, fuel octane number, intake air temperature, humidity, etc.). Then, the sum of the fuel flow and IMEP of all cylinders can be obtained to quantify the total engine fuel flow and the total IMEP of the engine according to each VCR mechanism setting under the current operating conditions. Thereafter, under each engine operating condition where the driver requires less than a threshold, the engine controller can select the VCR mechanism setting that gives the minimum total engine fuel flow. Under each operating condition where the driver requires more than the threshold, the controller can select the VCR mechanism setting that gives the maximum total engine IMEP. The threshold can be a predetermined value, or it can be adjusted according to the current engine speed, fuel octane number, ambient temperature, humidity, etc.

[0008] In this way, the efficiency of the VCR engine can be improved by detecting and compensating for the inter-cylinder variation of the compression ratio. The technical effect of obtaining the fuel flow and IMEP of all cylinders according to each CR setting of the VCR engine is that the CR variation of the actual engine can be learned, rather than relying on a prototype engine that may be significantly different from a given engine. In addition, the VCR engine can be calibrated without relying on expensive manufacturing methods and / or components. By selecting the CR setting of the VCR engine corresponding to the minimum total engine fuel flow when the operator torque requirement is low, fuel consumption and carbon dioxide (CO2) emissions can be minimized. By selecting the CR setting of the VCR engine corresponding to the maximum total torque when the operator torque requirement is high, engine performance can be maximized. Generally speaking, the engine performance and fuel efficiency of the VCR engine can be improved.

[0009] It should be understood that the above invention content provides a selection of some concepts further described in the detailed description in a simplified form. It is not intended to identify the key or essential features of the claimed subject matter, and the scope of the claimed subject matter is defined only by the claims following the detailed description. Additionally, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0011] Figure 2 An example high-level flowchart for optimizing the CR calibration and dilution calibration of a VCR engine is shown.

[0012] Figure 3 Another example high-level flowchart for optimizing the CR calibration and dilution calibration of a VCR engine is shown.

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

[0014] Figure 5 An example difference between the nominal EGR and VCT schedules and the modified EGR and VCT schedules of a VCR engine is shown.

[0015] Figure 6 A predictive example of VCR engine control is shown.

[0016] Figure 7 An example difference between the nominal CR schedule and the modified CR schedule of a VCR engine is shown. 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 for the engine system of reference Figure 1 The controller may be configured to execute control routines (such as the example routine of Figures 2 to 3 CR settings to calibrate the commanded CR at a given engine speed-load by learning the actual cylinder-to-cylinder variation of CR based on the difference in fuel economy and output torque of each cylinder at each CR setting of the VCR engine. During conditions when dilution control is required, 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. Example modifications to the CR calibration and EGR calibration are shown in the tables of Figures 4 to 5 and Figure 7 Reference Figure 6, an example of adjusting engine operation based on CR calibration and EGR calibration is shown. In this way, the performance and fuel economy of the VCR engine can be improved.

[0018] Figure 1 An example embodiment of a combustion chamber or cylinder of an internal combustion engine 10 is shown. Engine 10 may be included in a vehicle system 5 (such as a vehicle configured for propulsion on a road). Engine 10 may receive control parameters from a control system including a controller 12 and receive 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. A cylinder (also referred to herein as a "combustion chamber") 14 of engine 10 may include a combustion chamber wall 136 within which a piston 138 is positioned. Piston 138 may be connected to a crankshaft 140 such that the reciprocating motion of the piston is transformed into a rotational motion of the crankshaft. Crankshaft 140 may be coupled via a transmission system to at least one drive wheel of a passenger vehicle. Additionally, a starter motor may be coupled to crankshaft 140 via a flywheel to effect a starting operation of engine 10.

[0019] Engine 10 may be configured as a variable compression ratio (VCR) engine, where the compression ratio (CR) of each cylinder (i.e., 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 changed. The CR of the engine may be changed by actuating a VCR actuator 192 of a VCR mechanism 194. In some example 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 example embodiments, there may be a pre-defined number of stepped compression ratios. Additionally, the CR may vary continuously between the first lower CR and the second higher CR (to any CR therebetween).

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

[0021] In one example, changing the position of the piston within the combustion chamber also changes the relative displacement of the piston within the cylinder. The piston position changing VCR mechanism can be coupled to a conventional cranktrain or an unconventional cranktrain. Non-limiting examples of unconventional cranktrains to which the VCR mechanism can be coupled include variable distance head crankshafts and variable stroke length crankshafts. In one example, the crankshaft 140 can be configured as an eccentric shaft. In another example, an eccentric can be coupled to or within the area of the piston pin, and the eccentric changes the position of the piston within the combustion chamber. Movement of the eccentric can be controlled through an oil passage within the piston rod.

[0022] It will be appreciated that other VCR mechanisms for mechanically changing the compression ratio can be used. For example, the CR of an engine can be changed via a VCR mechanism that changes the cylinder head volume (i.e., the clearance volume within the cylinder head). In yet another example, the VCR mechanism can include a hydraulic, air pressure, or mechanical spring reaction piston. Additionally, the VCR mechanism can include a multi-link mechanism or a bent lever mechanism. Other VCR mechanisms can be possible. It will be appreciated that, as used herein, a VCR engine can be configured to adjust the CR of the engine via mechanical adjustment of the piston position or the cylinder head position or the cylinder head volume. Thus, a VCR mechanism does not include an effective CR adjustment achieved via adjustment of valve timing or cam timing.

[0023] By adjusting the position of the piston within the cylinder, the actual (static) compression ratio of the engine can be changed (i.e., the difference in cylinder volume at TDC relative to the cylinder volume at BDC). In one example, reducing the compression ratio includes reducing the displacement of the piston within the combustion chamber by increasing the distance between the top of the piston and the cylinder head. For example, the engine can be operated at a first, lower compression ratio by a controller sending a signal to actuate the VCR mechanism to a first position where the piston has a smaller effective displacement within the combustion chamber. As another example, the engine can be operated at a second, higher compression ratio by a controller sending a signal to actuate the VCR mechanism to a second position where the piston has a larger effective displacement within the combustion chamber. The variation in the engine compression ratio can be advantageously used to improve fuel economy. For example, a higher compression ratio can be used to improve fuel economy at light to moderate engine loads until spark retard from early knock begins to erode the fuel economy benefit. Subsequently, the engine can be switched to a lower compression ratio, thereby trading off the efficiency benefit of the higher compression ratio with the efficiency benefit of optimized combustion phasing. A continuously variable VCR system can continuously optimize the tradeoff between combustion phasing and the efficiency benefit of the higher compression ratio to provide an optimal compression ratio between a higher compression ratio limit and a lower compression ratio limit for a given operating condition. In one example, the engine controller can reference a lookup table to select the compression ratio to apply based on the engine speed-load condition. As detailed below, the selection can include selecting a lower compression ratio at higher engine loads and a higher compression ratio at lower engine loads.

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

[0025] In addition to cylinder 14, the exhaust passage 148 can also receive exhaust from other cylinders of the engine 10. The exhaust sensor 128 is shown as being coupled upstream of the emission control device 178 to the exhaust passage 148. The sensor 128 can 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 shown), a HEGO (heated EGO), a NOx, HC, or CO sensor. The emission control device 178 can be a three-way catalyst (TWC), a NOx trap, various other emission control devices, or a combination thereof.

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

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

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

[0029] The cylinder 14 may have a compression ratio that is the ratio of the volume when the piston 138 is at bottom dead center to the volume when 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 may be increased. For example, this may occur when using a fuel with a higher octane rating or a fuel with a higher latent heat of vaporization. If direct injection is used due to its effect on engine knock, the compression ratio may also be increased. The compression ratio may also be mechanically changed based on driver needs by adjusting the VCR actuator 192 that actuates the VCR mechanism 194 to change the effective position of the piston 138 within the combustion chamber 14. The compression ratio may be inferred based on feedback from the sensor 196 regarding the position of the VCR mechanism 194.

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

[0031] In some embodiments, each cylinder of the engine 10 may be configured with one or more fuel injectors for supplying fuel thereto. As a non-limiting example, the cylinder 14 is shown, and the cylinder 14 includes a fuel injector 166. The fuel injector 166 is shown as being directly coupled to the cylinder 14 for directly injecting fuel therein in proportion to the pulse width of a signal FPW received from the controller 12 via an electronic driver 168. In this manner, the fuel injector 166 provides so-called direct injection (hereinafter also referred to as "DI") of fuel into the combustion cylinder 14. Although Figure 1 the injector 166 is shown as a side injector, it may also be located at the top of the piston, such as in a position close to the spark plug 192. This position may improve mixing and combustion when operating the engine with an alcohol-based fuel due to the lower volatility of some alcohol-based fuels. Alternatively, the injector may be located at the top of the intake valve and close thereto to improve mixing. Fuel may be delivered from a high-pressure fuel system 8 including a fuel tank, a fuel pump, and a fuel rail to the fuel injector 166. Alternatively, the fuel may be delivered by a single-stage fuel pump at a lower pressure, in which case the timing of direct fuel injection during the compression stroke may be more restrictive than in the case of using a high-pressure fuel system. Additionally, although not shown, the fuel tank may have a pressure sensor that provides a signal to the controller 12. It will be appreciated that in alternative embodiments, the injector 166 may be an intake port injector that supplies fuel into the intake port upstream of the cylinder 14.

[0032] It will also be appreciated that although the illustrated embodiments show the engine being operated by injecting fuel via a single direct injector; in alternative embodiments, the engine can be operated and the relative amount of fuel injected into the cylinders from each injector varied 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.).

[0033] During a single cycle of a cylinder, fuel can be delivered to the cylinder via an injector. Additionally, the distribution and / or relative amount of fuel delivered from the injector can vary with the operating conditions. Further, for a single combustion event, multiple injections of the fuel delivered can be performed per cycle. The multiple injections can be performed during the compression stroke, intake stroke, or any suitable combination thereof. Also, fuel can be injected during the cycle to adjust the air - fuel ratio (AFR) of the combustion. For example, fuel can be injected to provide a stoichiometric AFR. An AFR sensor can be included to provide an estimate of the in - cylinder AFR. In one example, the AFR sensor can be an exhaust sensor, such as the EGO sensor 128. By measuring the amount of residual oxygen (for lean mixtures) or unburned hydrocarbons (for rich mixtures) in the exhaust, the sensor can determine the AFR. Thus, the AFR can be provided as a lambda (λ) value, that is, as the ratio of the actual AFR of a given mixture to the stoichiometric ratio. Thus, λ of 1.0 indicates a stoichiometric mixture, and can have a λ value less than 1.0 for mixtures richer than the stoichiometric mixture and greater than 1 for mixtures leaner than the stoichiometric mixture.

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

[0035] The fuel tank in the fuel system 8 can hold fuels having different fuel qualities (such as different fuel compositions). These differences can include different alcohol contents, different octane ratings, different heats of vaporization, different fuel mixtures, and / or combinations thereof, etc.

[0036] The engine 10 may also include knock sensors 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 selected locations of the engine block. The knock sensors may be accelerometers on the cylinder block or ionization sensors configured in the spark plugs of each cylinder. The output of the knock sensors may be combined with the output of the crankshaft acceleration sensor to indicate abnormal combustion events in the cylinders. In one example, based on the output of the knock sensors 90 in one or more defined windows (e.g., crank angle timing windows), 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 estimated knock sensor output in the knock window being higher than the knock threshold, while pre-ignition may be identified in response to the estimated knock sensor output in the pre-ignition window being higher than the knock threshold, the pre-ignition window being earlier than the knock window. Additionally, the abnormal combustion may be addressed accordingly. For example, knock may be addressed by reducing the compression ratio and / or retarding the spark timing, while pre-ignition may be addressed by increasing the engine speed and / or limiting the engine load. Additionally, reducing the compression ratio also reduces the likelihood of further pre-ignition.

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

[0038] The non - transitory storage medium read - only memory 110 can be programmed with computer - readable data representing instructions executable by the processor 106 for performing the methods described below and other variations that are contemplated but not specifically listed.

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

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

[0041] The actual CR of each cylinder affects the knock limit of that cylinder, particularly at high loads, and also affects the dilution limit of that cylinder, particularly at light loads. Due to manufacturing tolerances of the VCR mechanism coupled to each cylinder 30 of the engine 10, there may be significant inter-component variations between the actual CR of each cylinder and the expected CR of that cylinder. Additionally, for a given expected CR, there may be significant intra-cylinder variations in the actual CR. Due to these differences, 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 result in non-optimal EGR or VCT (or VVL, etc.) calibration. As an example, a lower CR setting may 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 desired, causing the cylinder to become overly knock-limited. As another example, a higher CR setting may 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 desired, causing the cylinder to become combustion-stable and NVH-limited.

[0042] As referenced Figures 2 to 3 in detail, the engine controller may update the CR calibration (i.e., 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 may also update the EGR and / or VCT calibration (i.e., 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. Accordingly, VCR engine performance may be improved.

[0043] Now turning to Figure 2 , an example routine 200 for calibrating a VCR engine is described. Due to manufacturing tolerances, the method reduces performance losses due to intra-cylinder variations in CR. The instructions for performing method 200 and other methods included herein may be executed by a controller based on instructions stored on the controller's memory and in conjunction with signals received from sensors of the engine system, such as the sensors referenced Figure 1 above. In accordance with the methods described below, the controller may employ engine actuators of the engine system to adjust engine operation.

[0044] At 202, method 200 includes estimating and / or measuring engine operating conditions. Engine operating conditions can include, for example: driver power demand (e.g., based on the 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 number of the fuel available in the fuel tank; and so on.

[0045] It will be appreciated that in alternative examples, method 200 can be triggered during the first engine start after the engine is manufactured to allow for calibration of the engine. In other examples, method 200 can be triggered in response to an engine repair or service (as indicated by a disconnected battery, input from a diagnostic tool, or input from a graphical user interface (GUI)).

[0046] At 204, method 200 includes selecting a desired compression ratio for operating the engine based on the estimated engine operating conditions. The engine can 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 within the cylinder. Alternatively, multiple compression ratios between the first and second compression ratios are possible. The controller can calculate the fuel efficiency at each possible compression ratio of the engine for 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 specific fuel consumption (BSFC) of the engine at each compression ratio, e.g., via a look-up table stored in the controller's memory, the look-up table being populated during the initial engine calibration of a prototype engine having substantially the same CR on each cylinder. The fuel efficiency of the engine at each compression ratio can be determined by tables, graphs, algorithms, and / or equations, each stored according to operating conditions (e.g., engine speed, torque, temperature, humidity, inferred fuel octane number, etc.), the settings populated during the initial engine calibration being based on the prototype engine. Generally, as the engine load or BMEP increases, the selected compression ratio may decrease due to a trade-off between the efficiency benefits of a higher CR (which dominates at lower loads) and the efficiency losses due to 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.

[0047] At 206, the method includes retrieving the actual compression ratio setting of each cylinder at the desired nominal compression ratio setting. For example, reference can be made to such as Figure 4a lookup table of a table to determine whether the actual CR of a given cylinder exceeds or is lower than the desired nominal CR setting.

[0048] At 208, the method includes calculating the fuel economy (or fuel usage) associated with each cylinder at the retrieved actual compression ratio. For example, if the actual CR of a selected cylinder is higher than the desired nominal CR setting, then it can be determined that the fuel economy of that cylinder is reduced at high load due to additional spark retard (followed by knock-limited combustion phasing). At 210, the method includes determining the total fuel economy of the engine by summing the fuel economy of each cylinder.

[0049] 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 a selected cylinder is higher than the desired nominal CR setting and the engine is currently operating at high load (which is knock-limited in this case), then it can be determined that there is fuel loss in that cylinder, which is based on 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 loss of each cylinder.

[0050] As an example, the actual CR data can be retrieved from a lookup table (such as Figure 4 Table 400) stored in the memory of the controller. The lookup table can be populated with data immediately after engine manufacture, replacement, or overhaul. For example, the CR of each cylinder can be quantified by measuring the dimensions of key engine components during manufacture. Alternatively, the CR of each cylinder can be quantified during an off-line overspeed test by measuring the cylinder pressure in each cylinder, or by using a radio frequency transceiver in each cylinder, or by measuring the crank angle-resolved crankshaft acceleration curve. The known CR of each cylinder can be stored in the memory of the controller immediately after engine manufacture and, if necessary, updated by a service technician after engine replacement or overhaul. In doing so, for each cylinder, it can be determined whether the actual CR of a given cylinder is higher or lower than a given CR setting. For example, referring to Figure 4 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 a higher load, the higher-than-expected actual CR of cylinder 1 may cause cylinder 1 to be more knock-limited than other cylinders, thus requiring additional spark retard. This results in a loss of fuel efficiency in cylinder 1 at high load.

[0051] At 216, the total fuel loss due to the actual CR and the nominal CR can 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 a small variation in the actual CR between cylinders will operate most efficiently at the desired nominal CR selected at 204, since this desired nominal CR was determined by testing a prototype engine with a small variation in the actual CR between cylinders (see, for example Figure 7 for curve 702 of graph 700).

[0052] Otherwise, if there is a significant fuel economy loss associated with the actual CR and 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 can send a signal to the VCR actuator to move the VCR mechanism to decrease the CR by 0.2 ratio. For an engine operating at high load (where the engine is knock-limited) with one or more cylinders having a CR higher than the nominal CR, the optimal CR will be lower than the desired nominal CR (as Figure 7 shown by curve 704 of graph 700), the desired nominal CR being determined by testing a prototype engine with a small variation in the actual CR between cylinders.

[0053] From each of 218 and 220, the method moves to 222 to determine if engine dilution control is needed. In one example, engine dilution control is needed 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 needed at loads below a threshold load that varies with engine speed, temperature, or other factors. If dilution control is needed, then at 224, the nominal VCT and / or EGR schedule can be maintained. Otherwise, if dilution control is needed, 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. A lower CR causes reduced combustion stability at light loads, which reduces the dilution tolerance and thus the optimal EGR rate. It also shifts the optimal VCT / VVL schedule towards lower "internal EGR" (lower overlap and / or earlier exhaust valve closing times) and / or towards a higher effective CR (earlier intake valve closing times). Mapping data from a prototype engine with little variation in CR 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, which in this case is the cylinder with the lowest CR. Thus, the combustion stability-limited EGR and / or VCT settings at light loads are 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 one or more cylinders have a CR lower than nominal (where combustion stability is limited), a lower EGR amount and / or lower valve overlap and / or an earlier EVC setting can be applied, as shown by curves 506 and 508 of Figure 5 shown.

[0054] As Figure 3 shown, the EGR and / or VCT schedule can be modified at any time the VCR mechanism deteriorates, such as may occur due to component degradation or due to non-satisfaction of the actuator entry conditions. Specifically, it can be determined whether the actual VCR mechanism position is different from the desired VCR. Unsatisfied VCR actuator entry conditions can 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 an earlier EVC setting can be applied, as shown by Figure 5 shown.

[0055] Now turning to Figure 3, shows another example method 300 for calibrating a VCR engine. At 302, as at 202, method 300 includes estimating and / or measuring engine operating conditions. Engine operating conditions can include, for example: driver power demand (e.g., based on the output of a pedal position sensor coupled to the operator's 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 number of the fuel available in the fuel tank; and so on.

[0056] It will be appreciated that, in some cases, method 300 can be triggered during the first engine start after engine manufacture to allow the engine to be calibrated. In other cases, method 300 can be triggered in response to an engine repair or maintenance (as indicated, for example, by a disconnected battery, 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, the actual CR data can be retrieved from a look-up table stored in the memory of the controller (such as Figure 4 table 400). The look-up table can be populated with data immediately after engine manufacture, replacement, or overhaul. For example, the CR of each cylinder can be quantified during a downline overspeed test, based on the measured cylinder pressure, or using a radio frequency transceiver in each cylinder, or based on the measured crank angle resolved crankshaft acceleration curve, or based on the measured dimensions of critical engine components. The known CR of each cylinder can be stored in the memory of the controller immediately after engine manufacture and, if necessary, updated by a service technician after engine replacement or overhaul. In doing so, for each cylinder, it can be determined whether the actual CR of a given cylinder is higher or lower than a given CR setting. For example, referring to Figure 4 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 the engine is operated at a higher load, the higher-than-expected actual CR of cylinder 1 may cause cylinder 1 to be more knock-limited than the other cylinders, thus requiring additional spark retard. This results in a loss of fuel efficiency.

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

[0059] It will be appreciated that while the method proposes to quantify the fuel flow rate and IMEP for each cylinder, this does not mean a limitation, and in alternative examples, other cylinder parameters indicating fuel economy and performance can be used. For example, in an alternative example, the metric quantified by the controller (under part-load conditions, such as when below a threshold load) can include efficiency or brake specific fuel consumption (BSFC), while the metric quantified under higher load conditions (such as near peak load or when above a threshold load) can include torque, power, or brake specific air consumption (BSAC).

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

[0061] If the driver's demand is below the threshold, then at 312, the method includes selecting a VCR setting corresponding to the minimum total engine fuel flow. By selecting a VCR setting corresponding to the minimum total engine fuel flow during part - load conditions, fuel consumption and CO2 emissions can be minimized. Otherwise, at 314, if the driver's demand is above the threshold load, then the method includes selecting a VCR setting corresponding to the maximum total IMEP. By selecting a VCR setting corresponding to the 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 needed. In one example, engine dilution control is needed when the engine load is less than a threshold load. If dilution control is needed, 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. A lower CR causes a reduction in combustion stability at light loads, which reduces the dilution tolerance and thus the optimum EGR rate. It also moves the optimum VCT schedule towards lower "internal EGR" (lower overlap and / or earlier exhaust valve closing times) and / or towards a higher effective CR (earlier intake valve closing times). Mapping data from a prototype engine with little cylinder - to - cylinder variation in actual CR can be used to quantify the optimum (combustion - stability - limited) EGR and / or VCT schedules. The combustion stability limit is determined by the "worst - case" cylinder, which in this case is 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 the load threshold (where combustion - stability - limited operation) where one or more cylinders have a CR lower than nominal, a lower EGR amount and / or lower valve overlap and / or earlier EVC settings can be applied, as further described below Figure 5 as shown.

[0063] If engine dilution control is not required, then at 318, it can be determined whether there is any deterioration or malfunction in the VCR mechanism. Specifically, it can be determined whether the actual VCR mechanism position is different from the desired VCR. VCR deterioration can be determined in response to VCR component deterioration or due to non - satisfaction of the VCR actuator entry conditions. The non - satisfied VCR actuator entry conditions can include conditions related to temperature, oil pressure, current limit, etc. If VCR deterioration is detected, then a lower EGR amount and / or a lower valve overlap and / or an earlier EVC setting can be applied, as Figure 5 shown.

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

[0065] Figure 5 The effective EGR schedules for load (BMEP) sweeps at various CRs based on data from a prototype engine having substantially the same CR on each cylinder are shown. Curve 504 shows the EGR schedule for a 12:1 CR, curve 506 shows the EGR schedule for a 10:1 CR, and curve 508 shows the EGR schedule for an 8:1 CR. It will be appreciated that the EGR schedule can also be according to engine speed, engine temperature, air temperature and humidity, etc. The EGR schedule has a maximum EGR rate at the intermediate BMEP. At higher BMEPs, the EGR rate may be reduced due to the negative impact of EGR on volumetric efficiency. At lower BMEPs, the EGR rate may be reduced due to the negative impact of EGR on combustion stability. However, the lower the CR, the worse the combustion stability. For example, a load threshold 502 is shown below which the combustion stability is reduced. Thus, when the CR of one or more cylinders is lower, a lower EGR rate is used. Mapping data from a prototype engine having substantially the same CR on each cylinder is used to create this table, but the cylinder with the lowest actual CR is used with this table to determine the currently desired EGR rate. A similar method will be used to limit the "internal EGR" by changing the schedules for VCT, VVL, etc. according to the cylinder with the lowest CR. Figure 6Shows the trends of fuel consumption versus load (BMEP) for various CRs. These trends are well-known to experts in the field as they result from the fundamental trade-off between the efficiency benefits of higher CRs and the efficiency losses due to knock-limited combustion phasing. The outcome of these trade-offs determines the optimal CR for each BMEP (the optimal CR also varies with engine speed, fuel octane number, intake air temperature, humidity, etc.). The efficiency benefits of higher CRs dominate at lower BMEPs, while the efficiency losses due to 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 optimal CR is high, and Figure 6 shows that the lowest fuel consumption occurs at a high CR of 13:1. Curves 602a to 602h show that fuel consumption decreases with increasing CR. At high BMEPs where the engine is most knock-limited, the optimal CR is low, and Figure 6 shows that the lowest fuel consumption occurs at the lowest CR of 8:1. At intermediate BMEPs, these two factors trade off to varying degrees, and the lowest fuel flow occurs at various CRs between low and high. Figure 6 The trends shown in Figure 2 are quantified by testing a prototype engine and are used to determine the optimal or desired nominal CR based on BMEP, engine speed, fuel octane number, intake air temperature, humidity, etc., which is used, for example, in the Figure 3 example in step 204 and

[0066] step 306.

[0067] Using the above method, the cylinder-to-cylinder variation in compression ratio can be better detected and accounted for. By knowing the fuel flow and IMEP for all cylinders based on the set of each nominal desired CR of the VCR engine, the actual CR variation of the engine can be learned and distinguished from the CR data obtained on the prototype engine. Additionally, 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 at low load conditions and the highest engine output at high load conditions, the engine performance can be improved despite the cylinder-to-cylinder variation in CR. By also adjusting the EGR and VCT schedules of the VCR engine based on the mapped CR of all engine cylinders, dilution control is improved so that the engine can operate closer to the combustion stability limit with fewer NVH problems. Overall, the engine performance and fuel efficiency are improved by improving the calibration of the VCR engine.An example 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 fuel flow and peak torque of each cylinder at each compression ratio setting of the mechanism. In the foregoing example, additionally or optionally, the updated calibration includes: estimating the fuel flow and the peak torque of each cylinder at a plurality of compression ratio settings; quantifying a total engine fuel flow of the engine at each compression ratio setting of the plurality of compression ratio settings as a sum of the fuel flow of each cylinder at the corresponding compression ratio setting; and ascertaining a total torque of the engine at each compression ratio setting of the plurality of compression ratio settings as a sum of the torque of each cylinder at the corresponding compression ratio setting. In any or all of the foregoing examples, additionally or optionally, the actuating includes: when a driver torque demand is below a threshold, actuating the mechanism to a compression ratio setting having a lowest total engine fuel flow among a plurality of compression ratio settings; and when the driver torque demand is above the threshold, actuating the mechanism to another compression ratio setting having a highest total engine torque among the plurality of compression ratio settings. 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 number, ambient temperature, and ambient humidity. In any or all of the foregoing examples, additionally or optionally, the method further includes adjusting an engine dilution calibration based on a lowest one 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 includes using less dilution as the lowest one 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 includes selecting the target compression ratio of the engine according to a nominal calibration. In any or all of the foregoing examples, additionally or optionally, the engine is coupled in a vehicle, and the method further includes updating the nominal calibration of the engine in response to engine operation after vehicle manufacture, the nominal calibration being based on engine tests before vehicle manufacture, the vehicle including a hybrid electric vehicle.

[0068] 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 loss 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 transitioning to a lower compression ratio if the fuel loss exceeds a threshold. In the foregoing example, additionally or optionally, each compression ratio setting of the target compression ratio and the actual compression ratio is a defined compression ratio setting for a variable compression ratio mechanism, and the defined compression ratio setting is one of a plurality of compression ratio settings of the engine. In any or all of the foregoing examples, additionally or optionally, the transition 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 engine dilution in response to the transition, the adjustment including: when the engine load is below a threshold, comparing the actual compression ratio of each cylinder of the engine; and applying an engine dilution setting corresponding to the lowest one 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, when the lowest one of the actual compression ratios of each cylinder decreases, applying one of a lower EGR amount, a lower valve overlap amount, and an earlier exhaust valve closing timing.

[0069] An example 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 the piston position within the given cylinder; an EGR passage including an EGR valve for recirculating exhaust 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 the engine compression ratio calibration based on each of the fuel flow and 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 the fuel flow and peak torque of each cylinder at each of the compression ratio settings of the mechanism. In the foregoing example, additionally or optionally, the updating includes estimating the fuel flow and the peak torque of each of the plurality of cylinders at each of the plurality of compression ratio settings; quantifying a total engine fuel flow for each cylinder as a sum of the fuel flows at each of the plurality of compression ratio settings; and learning a total torque of the engine at each of the plurality of compression ratio settings as a sum of the torques of each of the plurality of cylinders at the corresponding compression ratio setting 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 having the lowest total engine fuel flow 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 the driver torque demand is above the threshold. In any or all of the foregoing examples, additionally or optionally, the updating is from a nominal compression ratio calibration based on engine testing prior to vehicle manufacture. 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 manufacture. In any or all of the foregoing examples, additionally or optionally, the controller includes additional instructions for: updating the engine EGR calibration based on a lowest compression ratio setting of one of the plurality of engine cylinders in the updated compression ratio calibration; and actuating the EGR valve based on engine load and further based on the updated EGR calibration.

[0070] Another example method for an engine includes: calibrating a compression ratio schedule of a variable compression ratio engine based on each of fuel flow and peak torque of each cylinder at each compression ratio setting of the engine; and adjusting an EGR flow to the engine according to an updated exhaust gas recirculation (EGR) calibration schedule based on the calibrated compression ratio schedule. In the foregoing example, additionally or optionally, the calibration includes learning, for each cylinder, a difference between an actual compression ratio and a commanded compression ratio at each compression ratio setting among a plurality of compression ratio settings. In any one or all of the foregoing examples, additionally or optionally, the adjustment includes: identifying an engine cylinder having a highest difference between the actual compression ratio and the 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 one or all of the foregoing examples, additionally or optionally, the adjustment 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 one or all of the foregoing examples, additionally or optionally, the adjustment according to the updated EGR calibration schedule includes adjusting based on a lowest one among a plurality of compression ratio settings of individual cylinders of the engine. In any one or all of the foregoing examples, additionally or optionally, the adjustment further includes: when an engine load is below a threshold load, reducing the EGR flow as the lowest one among the plurality of compression ratio settings decreases, and reducing the EGR flow includes one or more of the following: reducing an opening degree of an EGR valve, adjusting cylinder valve timing to advance closing of an exhaust valve, and adjusting the cylinder valve timing to reduce positive overlap between an intake valve and an exhaust valve. In any one or all of the foregoing examples, additionally or optionally, the engine is coupled in a vehicle, and wherein the updated EGR calibration schedule is updated from a nominal EGR calibration schedule based on engine tests before vehicle manufacture, and the vehicle includes a hybrid electric vehicle. In any one or all of the foregoing examples, additionally or optionally, the adjustment further includes: when the engine load is above the threshold load, adjusting the EGR flow to the engine according to a default EGR calibration schedule.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; obtaining a first parameter indicative of the fuel flow of the engine at each of the plurality of compression ratio settings; obtaining a second different parameter indicative of the torque of the engine at each of the 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 driver torque demand. In any or all of the foregoing examples, additionally or optionally, the selection includes: selecting the first parameter rather than the second parameter when the driver torque demand is below a threshold, and selecting the second parameter rather than the first parameter when the driver torque demand is above the threshold, the first parameter including one of total engine fuel flow and total brake specific fuel consumption, and the second parameter including one of total engine torque and total in-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 the 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.

[0071] Yet another example method includes; comparing a commanded compression ratio to an actual compression ratio for each cylinder of a variable compression ratio engine having a plurality of compression ratio settings; and adjusting an 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 example, additionally or optionally, reducing the EGR flow to the engine 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 performed in response to an engine load below a threshold load, the method further including 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 the 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 includes transitioning to a lower compression ratio via mechanical actuation of a variable compression ratio mechanism when the engine load is above the threshold load in response to a difference between the commanded compression ratio and the actual compression ratio of an engine cylinder being above a threshold difference.

[0072] 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 the piston position within the given cylinder; an EGR passage including an EGR valve for recirculating exhaust 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 rate and a peak torque of each cylinder at each of the plurality of compression ratio settings; adjusting an EGR flow rate to the engine based on the nominal compression ratio calibration at an engine load above a threshold; and adjusting the EGR flow rate to the engine based on a lowest compression ratio of the updated compression ratio calibration at an engine load below the threshold. In the foregoing example, additionally or optionally, adjusting the EGR flow rate based on the lowest compression ratio includes: identifying one of the plurality of cylinders having the 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. In any one or all of the foregoing examples, additionally or optionally, adjusting the EGR flow rate based on the lowest compression ratio includes decreasing an opening of the EGR valve when the lowest compression ratio decreases. In any one 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.

[0073] Note that the example control and estimation routines included herein can be used with a variety of 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 executed by a control system including a controller in combination 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.). Accordingly, the various acts, operations, and / or functions illustrated can be executed in the illustrated sequence, executed in parallel, or in some cases omitted. Similarly, the order of processing is not necessarily required to implement the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the acts, operations, and / or functions illustrated can be repeatedly executed in accordance with the particular strategy used. Additionally, the acts, operations, and / or functions described can be graphically represented as code to be programmed into the non-transitory memory of a computer-readable storage medium for an engine control system, where the described acts are executed by performing instructions in a system including various engine hardware components in combination with an electronic controller.

[0074] It will be appreciated that the configurations and routines disclosed herein are exemplary in nature and these specific embodiments should not be considered to have a limiting meaning as many variations are possible. For example, the above techniques can 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 sub-combinations regarded as novel and non-obvious. These claims may refer to "an" element or "a first" element or the equivalent thereof. These claims are to be understood to include the combination of one or more such elements, neither requiring nor precluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and / or properties can 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 from the original claims, are regarded as included within the subject matter of the present disclosure.

[0076] According to the present invention, there is provided an example method for an engine, the method having: 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 fuel flow and peak torque of each cylinder at each compression ratio setting of the mechanism.

[0077] According to one embodiment, the updated calibration includes: estimating the fuel flow and the peak torque of each cylinder at a plurality of compression ratio settings; quantifying the total engine fuel flow of the engine at each compression ratio setting among the plurality of compression ratio settings as the sum of the fuel flows of each cylinder at the corresponding compression ratio setting; and learning the total torque of the engine at each compression ratio setting among the plurality of compression ratio settings as the sum of the torques of each cylinder at the corresponding compression ratio setting.

[0078] According to one embodiment, the above invention is further characterized in that when the driver torque requirement is lower than a threshold, actuating the mechanism to a compression ratio setting having the lowest total engine fuel flow among the plurality of compression ratio settings; and when the driver torque requirement is higher than the threshold, actuating the mechanism to another compression ratio setting having the highest total engine torque among the plurality of compression ratio settings.

[0079] According to one embodiment, the threshold is based on one or more of accelerator pedal position, engine speed, fuel octane number, ambient temperature, and ambient humidity.

[0080] According to one embodiment, the above invention is further characterized in that the engine dilution calibration is adjusted based on the lowest of the plurality of compression ratios of individual cylinders.

[0081] According to one embodiment, the adjusting of the engine dilution includes using less dilution as the lowest of the plurality of compression ratios decreases when the engine load is lower than a threshold.

[0082] According to one embodiment, the above invention is further characterized in that the target compression ratio of the engine is selected according to a nominal calibration.

[0083] According to one embodiment, the engine is coupled in a vehicle, and the method further includes updating the nominal calibration of the engine in response to engine operation after vehicle manufacture, the nominal calibration being based on engine tests before vehicle manufacture, the vehicle including a hybrid electric vehicle.

[0084] According to the present invention, a method is provided, the method having: comparing a target compression ratio of an engine with an actual compression ratio of each cylinder of the engine; calculating a fuel loss 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 loss exceeds a threshold, then transitioning to a lower compression ratio.

[0085] According to one embodiment, the features of the above invention further lie in that each of the target compression ratio and the actual compression ratio is set for a defined compression ratio of a variable compression ratio mechanism, and the defined compression ratio setting is one of a plurality of compression ratio settings of the engine.

[0086] According to one embodiment, the transition includes actuating a variable compression ratio mechanism to mechanically change the actual compression ratio of each cylinder of the engine.

[0087] According to one embodiment, the features of the above invention further lie in that in response to the transition, the engine dilution is adjusted, and the adjustment includes: when the engine load is lower than a threshold, comparing the actual compression ratio of each cylinder of the engine; and applying an engine dilution setting corresponding to the lowest one of the actual compression ratios of each cylinder.

[0088] According to one embodiment, adjusting the engine dilution includes adjusting one of an exhaust gas recirculation (EGR) amount and a variable cam timing schedule.

[0089] According to one embodiment, the features of the above invention further lie in that as the lowest one of the actual compression ratios of each cylinder decreases, one of a lower EGR amount, a lower valve overlap amount, and an earlier exhaust valve closing timing is applied.

[0090] According to the present invention, an engine system is provided, the engine system having: 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 the 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 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 of the compression ratio settings of the mechanism.

[0091] According to one embodiment, the invention is further characterized in that the fuel flow rate and the peak torque of each of the plurality of cylinders are estimated at each of a plurality of compression ratio settings; for each cylinder, the total engine fuel flow rate is quantified as the sum of the fuel flow rates at each of the plurality of compression ratio settings; and the total torque of the engine at each of the plurality of compression ratio settings is ascertained as the sum of the torques of each of the plurality of cylinders at the corresponding compression ratio setting among the plurality of compression ratio settings.

[0092] According to one embodiment, the controller includes additional instructions for performing the following operations: when the driver torque demand is below a threshold, actuating the mechanism to one of the plurality of compression ratio settings having the lowest total engine fuel flow rate; and when the driver torque demand is above the threshold, actuating the mechanism to another of the plurality of compression ratio settings having the highest total engine torque.

[0093] According to one embodiment, the update is from a nominal compression ratio calibration based on engine testing prior to vehicle manufacture.

[0094] According to one embodiment, the vehicle is a hybrid electric vehicle, and wherein the update is responsive to engine operation after vehicle manufacture.

[0095] According to one embodiment, the controller includes additional instructions for performing the following operations: updating the EGR calibration of the engine based on the lowest compression ratio setting of one of the plurality of engine cylinders in the updated compression ratio calibration; and actuating the EGR valve based on engine load and further based on the updated EGR calibration.

Claims

1. A method for an engine, the method comprising: Actuating a variable compression ratio mechanism of the engine to mechanically adjust a target compression ratio of the engine according to an updated compression ratio calibration of the engine, the updated compression ratio calibration of the engine being based on each of fuel flow and peak torque of each cylinder at each compression ratio setting of the mechanism.

2. The method according to claim 1, wherein the updated compression ratio calibration of the engine comprises: Estimating the fuel flow and the peak torque of each cylinder at a plurality of compression ratio settings; Quantifying a total engine fuel flow of the engine at each compression ratio setting among the plurality of compression ratio settings as a sum of the fuel flows of each cylinder at the corresponding compression ratio setting; And Learning a total torque of the engine at each compression ratio setting among the plurality of compression ratio settings as a sum of the torques of each cylinder at the corresponding compression ratio setting.

3. The method according to claim 1, wherein the actuating comprises: When a driver torque demand is below a threshold, actuating the mechanism to a compression ratio setting having a lowest total engine fuel flow among a plurality of compression ratio settings; And When the driver torque demand is above the threshold, actuating the mechanism to another compression ratio setting having a highest total engine torque among the plurality of compression ratio settings.

4. The method according to claim 3, wherein the threshold is based on one or more of an accelerator pedal position, an engine speed, a fuel octane number, an ambient temperature, and an ambient humidity.

5. The method according to claim 1, the method further comprising: Adjusting an engine dilution calibration based on a lowest one of a plurality of compression ratios of individual cylinders.

6. The method according to claim 5, wherein adjusting the engine dilution includes: Using less dilution as the lowest one of the plurality of compression ratios decreases when the engine load is below a threshold.

7. The method according to claim 1, wherein the method further comprises: Selecting the target compression ratio of the engine according to a nominal calibration.

8. The method according to claim 1, wherein the engine is coupled in a vehicle, the method further comprising updating a nominal calibration of the engine in response to engine operation after vehicle manufacture, the nominal calibration being based on engine tests before vehicle manufacture, the vehicle including a hybrid electric vehicle.

9. An engine system, the engine system comprising: An engine including a plurality of cylinders; A variable compression ratio mechanism, i.e., 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 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 fuel flow and peak torque of each cylinder at each compression ratio setting among the plurality of compression ratio settings; And Actuate the variable compression ratio mechanism of the engine to mechanically adjust the target compression ratio of the engine according to an updated calibration, the updated calibration being based on each of the fuel flow and peak torque of each cylinder at each compression ratio setting of the mechanism.

10. The system of claim 9, wherein the update comprises: estimating the fuel flow and the peak torque of each of the plurality of cylinders at each of the plurality of compression ratio settings; quantifying, for each cylinder, the total engine fuel flow as the sum of the fuel flows at each of the plurality of compression ratio settings; and ascertaining the total torque of the engine at each of the plurality of compression ratio settings as the sum of the torques of each of the plurality of cylinders at a corresponding one of the plurality of compression ratio settings.

11. The system of claim 10, wherein the controller comprises additional instructions for: when the driver torque demand is below a threshold, actuating the mechanism to a compression ratio setting of the plurality of compression ratio settings having the lowest total engine fuel flow; and when the driver torque demand is above the threshold, actuating the mechanism to another compression ratio setting of the plurality of compression ratio settings having the highest total engine torque.

12. The system of claim 9, wherein the update is made from a nominal compression ratio calibration based on engine testing prior to vehicle manufacture.

13. The system of claim 12, wherein the vehicle is a hybrid electric vehicle, and wherein the update is in response to engine operation after vehicle manufacture.

14. The system of claim 10, wherein the controller comprises additional instructions for: updating the EGR calibration of the engine based on the lowest compression ratio setting of one of the plurality of engine cylinders in the updated compression ratio calibration; and actuating the EGR valve based on engine load and further based on the updated EGR calibration.

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