Valve timing detection in fuel system using aborted cycle sweep

Abstract

Operating a fuel system for an engine includes moving a plunger between an advanced position and a retracted position during a pumping cycle, and pulsing a solenoid actuator in a circuit with a plurality of current pulses of different durations during at least one abortion cycle of the plunger, in which the plunger is driven by the plunger to move between an advanced position and a retracted position during the at least one abortion cycle. The plunger is neither advanced nor retracted. An electrical characteristic, such as a freewheeling response, in the circuit caused by moving the valve in response to each respective current pulse is monitored, and a valve timing of the valve is determined based on the monitored electrical characteristic. A fuel system including a fuel injector may be electronically trimmed based on the determined valve timing. Related devices and control logic are also disclosed.

Description

Technical Field

[0001] The present disclosure relates generally to operating a fuel system for an engine, and more particularly to determining valve timing of a valve in the fuel system based on an electrical characteristic induced in a solenoid actuator circuit. Background Art

[0002] Internal combustion engine systems employ a series of operational and logical strategies to control the associated fuel system. In a typical fuel system, multiple fuel injectors are each associated with a different combustion cylinder. The fuel injectors are typically electronically controlled and receive a control current generated by the engine or fuel control system. The control current energizes the solenoid or other electrical actuator of the fuel injector to adjust the valve therein to determine the timing and manner of fuel injection.

[0003] A widely adopted fuel system configuration used in compression ignition diesel engines employs a directly operated nozzle check that opens and closes with timing to start and end fuel injection based on hydraulic pressure applied to the surface of the nozzle check. The relief valve of some fuel injectors in this type of fuel injector changes the connection between the plunger cavity and the low-pressure space or outlet. When the relief valve is open, the plunger in the fuel injector passively reciprocates to exchange fuel between the plunger cavity and the low-pressure space. With the relief valve closed, the plunger can pressurize the fuel in the fuel injector.

[0004] For decades, engineers have experimented with various strategies to energize and control the electric actuators of the valves in fuel injectors. Precise control of fuel injection timing and the so-called fuel injection rate shape can result in various desired characteristics related to fuel injection timing, injection rate, pressure, etc. The performance of individual fuel injectors may change during their service life, sometimes causing valve operation to change in response to control currents that may affect fuel injection quantity, timing, rate shape, or other factors. Electronic fine-tuning techniques are often used in an effort to shift time-varying fuel injection parameters toward a desired state. Engineers are constantly seeking new and improved strategies to monitor and control specific aspects of fuel injector operation (including strategies for electronic fine-tuning). U.S. Patent Application Publication No. US20210140386A1 shows a typical overflow valve fuel injector arrangement. Summary of the invention

[0005] In one aspect, a method of operating a fuel system for an engine includes: moving a plunger in the fuel system between an advanced position and a retracted position during a pumping cycle of the plunger. The method also includes: pulsing a solenoid actuator in a circuit with a plurality of current pulses of varying durations during at least one rest cycle of the plunger; and monitoring an electrical characteristic in the circuit caused by moving a valve between a first position and a second position in response to each respective current pulse of the plurality of current pulses. The method also includes: determining valve timing of the valve based on the monitored electrical characteristic.

[0006] On the other hand, a fuel system includes a fuel pump and a relief valve, the fuel pump having a plunger, the plunger being movable between an advanced position and a retracted position in a plunger cavity formed in a housing, the relief valve being movable between an open position and a closed position to respectively open and close the fluid communication between the plunger cavity and a fuel port formed in the housing. The fuel system also includes a control valve assembly having a solenoid actuator and a control valve, the control valve being movable in the housing in response to energizing the solenoid actuator. The fuel system also includes a control system having an electrical circuit and an electronic control unit, the electrical circuit including the solenoid actuator. The electronic control unit is configured to pulse the solenoid actuator with a plurality of current pulses of different durations when the relief valve is in an open position, and monitor electrical characteristics in the circuit caused by moving the control valve between a first position and a second position in response to each corresponding current pulse in the plurality of current pulses. The electronic control unit is further configured to determine valve timing of the control valve based on the monitored electrical characteristics.

[0007] In yet another aspect, a fuel control system includes an electronic control unit configured to pulse a solenoid actuator in the fuel system with a plurality of current pulses during at least one abort cycle of a plunger in the fuel system, and monitor an electrical characteristic in a circuit including the solenoid actuator caused by moving a valve in the fuel system in response to each respective one of the plurality of current pulses. The electronic control unit is further configured to determine a valve timing of the valve based on the monitored electrical characteristic, and electronically fine-tune the valve based on the determined valve timing. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a cross-sectional schematic diagram of an internal combustion engine system according to one embodiment;

[0009] Figure 2 is a diagram of a current pulse according to one embodiment;

[0010] Figure 3is another diagram illustrating abort loop sweep according to one embodiment; and

[0011] Figure 4 is a flowchart illustrating an exemplary method and logic flow according to one embodiment. DETAILED DESCRIPTION

[0012] refer to Figure 1 , an internal combustion engine system 10 according to one embodiment is shown. The engine system 10 includes an internal combustion engine 12 in which a combustion cylinder 14 is formed. The combustion cylinder 14 can be one of any number of combustion cylinders in the engine 12, and the combustion cylinders are arranged in any suitable arrangement, such as in-line, V-type or other arrangements. The engine 12 will generally be equipped with an intake system, an exhaust system, engine valves, and various other devices not explicitly shown. The piston will generally be movable between the top dead center position and the bottom dead center position in the combustion cylinder 14 in a conventional four-stroke mode. The engine 12 can be compression-ignited and operated with a suitable compression-ignition fuel (such as diesel distillate fuel), but the present disclosure is not limited thereto. The engine 12 may also include a rotatable crankshaft (not shown) coupled to a rotatable camshaft 16 through a gear train, the camshaft having a cam lobe 18 and defining a base circle 17. As further discussed, the camshaft 16 generally includes a plurality of cam lobes arranged to operate a device including a fuel injector in the engine system 10.

[0013] The engine system 10 also includes a fuel system 20. The fuel system 20 generally includes a plurality of fuel injectors, each of which is positioned to extend into one of a plurality of combustion cylinders in the engine 12. Figure 1 1, one fuel injector 22 is shown associated with the combustion cylinder 14, and it should be understood that the description and discussion of the fuel injector 22 should be understood to refer by analogy to any other such fuel injector in the engine system 10. The fuel injector 22 includes an injector housing 24 having a nozzle 26 that extends into the combustion cylinder 14 and sprays fuel into the combustion cylinder 14 by means of a plurality of nozzle outlets 30 formed in the nozzle 26.

[0014] The fuel injector 22 also includes a direct operating check or DOC 28 that is movable within the injector housing 24 to open and close the nozzle outlet 30 to directly inject liquid fuel. The DOC 28 is directly hydraulically operated based on fluid pressure (typically the fluid pressure of the fuel) in a pressure control chamber 38. In the illustrated embodiment, the DOC 28 includes a needle valve or needle check.

[0015] The fuel injector 22 also includes an injection control valve assembly 32. The injection control valve assembly 32 is operable to control the closing hydraulic pressure in the pressure control chamber 38 to achieve the opening and closing of the DOC 28. The injection control valve assembly 32 includes an injection control valve 34, which is movable in the fuel injector 22 to open and close the valve seat 36. In some embodiments, the injection control valve 34 may include one or more valves, which include separate but contacting valve members. When the valve seat 36 is open, the pressure control chamber 38 can be fluidly connected to a low pressure space 54, such as a fuel port defined by the injector housing 24, so that the DOC 28 can open in response to the fuel pressure and allow fuel to be sprayed from the nozzle outlet 30. When the valve seat 36 is closed, an increased hydraulic pressure is seen in the pressure control chamber 38, and the increased hydraulic pressure causes the DOC 28 to close. The armature 40 is coupled to the injection control valve 34. The armature 40 is associated with a solenoid actuator 42 that can be energized to magnetically attract the armature 40 and open the valve seat 36. When the solenoid actuator 42 is de-energized, the biasing spring 52 pushes the injection control valve 34 to close against the valve seat 36. Thus, the injection control valve 34 can move between a first position and a second position in the fuel injector 22. In an embodiment, the first position includes a closed stop position, and the second position includes an open stop position. When the solenoid actuator 42 is de-energized, the biasing spring 52 pushes the injection control valve 34 to close against the valve seat 36. Thus, the injection control valve 34 can move in the fuel injector 32 in response to energizing the solenoid actuator 42 to change the closing hydraulic pressure on the DOC 28.

[0016] The fuel injector 22 also includes a relief valve assembly 44. The relief valve assembly 44 includes a relief valve 46 coupled to an armature 48, and a solenoid actuator 50. When the solenoid actuator 50 is energized, the armature 48 is magnetically attracted toward the solenoid actuator 50. When the solenoid actuator 50 is de-energized, the biasing spring 52 pushes the armature 48 and the relief valve 46 away from the solenoid actuator 50. Therefore, the relief valve 46 is understood to be movable in the fuel injector 22 in response to energizing the solenoid actuator 50 and is biased toward an open position. In other embodiments, separate multiple springs may be used instead of using a single spring to bias both the armature 40 and the armature 48. In addition, embodiments are envisioned in which the relief valve 46 is positioned outside the fuel injector housing 24.

[0017] The fuel injector 22 also includes a plunger 56 that is movable in a plunger cavity 58 formed in the injector housing 24 and fluidly connected to the relief valve 46 via a relief passage 64. In an embodiment, the plunger 56 is mechanically cam-driven by the rotation of the camshaft 16. During the pumping cycle of the plunger 56, the camshaft 16 rotates to engage the cam lobe 18 against the tappet 19 coupled to the plunger 56. The rotation of the camshaft 16 pushes the tappet 19 away from the camshaft 16 to move the plunger 56 between the advanced position and the retracted position during the pumping cycle of the plunger 56. The camshaft 16 also defines a base circle 17. When the base circle 17 rotates into contact with the tappet 19, the plunger 56 is understood to be in an idle cycle, neither advancing nor retracting. When the relief valve 46 is open, the upward movement of the plunger 56 causes fuel to be drawn into the plunger cavity 58 from the low pressure space or fuel port 54, for example, through the relief passage 64. The downward movement (advancement) of the plunger 56 causes the fuel to be discharged from the plunger chamber 58 through the relief passage 64. When the relief valve 46 is closed, the fluid communication between the plunger chamber 58 and the fuel port 54 is blocked, and the advancement of the plunger 56 causes the fuel pressure in the plunger chamber 58 to increase. The increased fuel pressure is transmitted to the vicinity of the nozzle outlet 30 through the nozzle supply passage 60. The increased fuel pressure is also transmitted to the pressure control chamber 38, thereby keeping the DOC 28 closed. When the DOC 28 is lifted, at the desired time, by opening the injection control valve assembly 32, the fuel is sprayed out of the nozzle outlet 30 from the nozzle supply passage 60. Another fluid passage 62 is fluidly connected between the nozzle supply passage 60 and the injection control valve 34.

[0018] As shown in the drawings and noted above, the relief valve assembly 44 is located in the fuel injector 22, but in other embodiments, it may be located externally. In addition, the plunger 56 may be understood to form part of a fuel pump for pressurizing fuel for a single fuel injector. In other embodiments, a plunger in a fuel pump may be used to pressurize fuel for multiple fuel injectors. It should also be understood that although the tappet 19 may form a direct mechanical connection between the camshaft 16 and the plunger 56, in other embodiments, an intervening structure, such as a rocker arm assembly, may also be used.

[0019] The fuel system 20 also includes a fuel control system 70. The fuel control system 70 includes an electronic control module or ECM 72, which includes an electronic control unit or ECU 74. The ECU 74 can be or include a programmable logic controller (e.g., a microprocessor or microcontroller), and a suitable computer readable memory storing program control instructions that, when executed, cause the fuel injector 22 and the fuel system 20 to operate in accordance with the present disclosure. Any suitable computer readable memory can be used, such as RAM, ROM, flash memory, hard drive, etc. The fuel control system 70 also includes a lower voltage power supply such as a battery 78 and a boost voltage power supply 80. The battery 78 is shown as part of the ECM 72, but can be a separate device. The boost voltage power supply 80 can be part of the ECM, or physically separated in other embodiments. The circuit 53 is electrically connected to the ECM 72 and the ECU 74, and includes the solenoid actuator 42.

[0020] The concept of electronic trimming will be familiar to those skilled in the art. In a fuel system, control field electronic trimming can be used to change the timing, duration, amplitude, and possibly other characteristics of the electrical control current sent to the electrical actuator in the fuel injector to improve or optimize fuel injector performance parameters, including, for example, valve opening timing, valve closing timing, and injection rate shape. The present disclosure provides a unique valve timing detection and determination strategy that enables electronic trimming to improve fuel injector performance. An exemplary control target of electronic trimming may include valve opening timing, such as the timing for the valve to reach a fully open position. As further discussed herein, electronic trimming according to the present disclosure may include changing the start of the timing of the current used to energize the solenoid actuator 42 to produce a valve timing that is as close to the nominal valve timing as possible.

[0021] To this end, the electronic control unit 74 may be configured to determine the valve timing of the valves in the fuel system 20 through an abort cycle sweep strategy. As described above, when the base circle 17 rotates into contact with the tappet 19, the plunger 56 is idle. The relief valve 46 will also typically be in an open position. The fuel injector 22 is not operated to inject fuel. It has been found that in such circumstances, an assessment of the valve timing, particularly the valve timing of the control valve 34, can be performed to accurately detect the valve opening timing, which can be used as the basis for electronic trimming of the fuel injector 22 (including electronic trimming of the control valve 34). In the abort cycle, the DOC 28 does not move, and therefore the control valve 34 can be opened and closed to observe its performance without interacting with the hydraulic pressure or other phenomena in the fuel injector 22, or with minimal interaction therewith. The idle cycle sweep may include generating a plurality of currents of different durations in the circuit 53 during at least one idle cycle (in which the plunger 56 is neither advanced nor retracted and the relief valve 46 is open) to pulse the solenoid actuator 42. The electronic control unit 72 may be further configured to monitor an electrical characteristic in the circuit 53 caused by moving the control valve 34 between its first position and its second position in response to each respective current pulse of the plurality of current pulses. As further discussed herein, the valve timing of the control valve 34 is determined based on the monitored electrical characteristic.

[0022] The monitored electrical characteristic may include a freewheeling response in the circuit 53, and in an embodiment, a duration of the freewheeling response. The duration of the freewheeling response generated by the current pulse may indicate a plurality of different durations for moving the control valve 34 between its first position and its second position, such as a duration for returning the control valve 34 to the first position. As described above, the first position may include a closed stop position, and the second position may include an open stop position. The plurality of different durations may include a plurality of valve closing time (VCT) durations. Thus, determining the valve timing may include determining a valve opening time (VOT) duration for moving the control valve 34 from the closed stop position to the open stop position. In addition, and as also discussed below, the plurality of VCT durations may include a VCT max duration, and the determined VOT duration may be based on VCT max Duration. Pulsing the solenoid actuator 42 may include pulsing the solenoid actuator 42 with a plurality of current pulses of successively increasing or successively decreasing durations.

[0023] Reference now Figure 2, a diagram 100 is shown, which shows an exemplary current pulse waveform that can be generated in the circuit 53 to energize the solenoid actuator 42. The electrical control current is shown at 110. The valve arrival is shown at 140. As discussed herein, the valve arrival 140 can reflect the timing of the control valve 34 reaching its second position. Number 120 identifies the pulse duration. Number 130 shows the timing window for the control valve 34 to return to the first position. During operation of the fuel system 20 to inject fuel, each time the control valve 34 is opened during the pumping cycle of the plunger 56, a pulse similar to Figure 2 As discussed herein, during operation of the fuel system 20 to perform an abort cycle sweep, during one or more abort cycles of the plunger 56, a current pulse similar to Figure 2 , but with different durations. In order to electronically fine-tune the control valve 34 after the valve timing is established, the start of the current timing of the control current 110 can be advanced or delayed to advance or delay the valve arrival timing 140, thereby reducing the error in the fuel injection amount or rate shape by shifting the fuel injection amount or rate shape to closer to the nominal condition. The present disclosure contemplates pulsing the circuit 53 and the solenoid actuator 42 multiple times, monitoring the electrical characteristics induced in the circuit 53, and determining which of the monitored electrical characteristic events represent the duration of the current pulse corresponding to the VOT duration. In considering Figure 3 This phenomenon will become more apparent later.

[0024] Reference now Figure 3 , shows a trace 220, which represents a plurality of current pulses 222 of continuously increasing durations for energizing the solenoid actuator 42 in the circuit 53. Trace 210 includes a plurality of measurements 212 of the freewheeling response duration (the monitored electrical characteristic) which continuously increase in duration until VCT max 214. With VCT max 214 corresponds to VOT duration 224, which means that VOT duration 224 is the time that causes VCT. maxThe duration of the current pulse. It will be appreciated that the freewheeling response in the circuit 53 is generated as the control valve 34 and the armature 40 move relative to the solenoid actuator 42. A relatively short current pulse in the circuit 53 may cause the control valve 34 to move away from its closed stop position, but return to the closed stop position before reaching its open stop position (i.e., fully open). A longer current pulse duration may cause the control valve 34 and the armature 40 to reach the open stop position and rebound toward the closed stop position. A current pulse duration that is sufficient to cause the valve 34 and the armature 40 to only reach the closed stop position and then return to the open stop position will tend to be the current pulse duration of the longest duration that causes the control valve 34 and the armature 40 to complete its movement cycle from the closed stop position to the open stop position and then return to the closed stop position. In other words, VCT max is the longest duration valve closing timing because the armature 40 traverses the full travel distance between its stop positions but does not bounce or bounces minimally. Therefore, the pulse duration that causes the longest duration valve closing timing is substantially the same as the duration it takes for the valve 32 to be fully open. According to the present disclosure, this discovery is utilized to electronically fine-tune the fuel injector 22. The different durations of the pulses may be continuously increasing, continuously decreasing, or potentially varying according to some other pattern. With the VOT determined, when the fuel injector 22 is again operated to inject fuel, the start of the current timing or possibly other characteristics (e.g., current amplitude) or the end of the current timing may be adjusted as discussed herein.

[0025] Industrial Applicability

[0026] Still referring generally to the drawings, but now focusing on Figure 4 , a flowchart 300 is shown, which illustrates an exemplary method and logic flow according to one embodiment. At box 310, in a pumping cycle, the camshaft 16 is rotated so that the cam lobe 18 contacts the tappet 19 to move the plunger 56. Figure 1 In FIG. 3 , the camshaft 16 and the tappet 19 are generally shown as they may appear during a pumping cycle when approaching the end of the moving plunger 56 so that the plunger 56 is retracted. The flowchart 300 advances from box 310 to box 315 to rotate the camshaft 16 so that the base circle 17 contacts the tappet 19 and the plunger 56 is in an idle cycle. Figure 1 Camshaft 16 and lifter 17 are shown as they might appear shortly before starting the method of block 315 .

[0027] Flowchart 300 proceeds from box 315 to box 320 to pulse the solenoid actuator 42 during the stop cycle. As discussed herein, it should be understood that circuit 53 is pulsed with multiple current pulses to move valve 32 between its first position and second position in response to each corresponding current pulse in the multiple current pulses. It is contemplated that a stop cycle sweep with multiple pulses may occur during one revolution of camshaft 16. However, the present disclosure is not limited to this, and in other cases, multiple pulses of different durations may be performed using multiple consecutive stop cycles. At box 325, a freewheeling response in circuit 53 is monitored. Monitoring the freewheeling response may include monitoring the duration of the back EMF generated in circuit 53 in response to the relative motion between armature 40 and solenoid actuator 42. Monitoring the freewheeling response does not require measuring the back EMF itself, but rather requires the duration of the back EMF being generated. Flowchart 300 proceeds from box 325 to box 330 to determine valve timing based on the monitored freewheeling response. As discussed herein, the electronic control unit 72 monitors the afterflow response to determine the VCTmax corresponding to the longest duration of valve closing timing. From box 330, the flowchart 300 proceeds to box 335 to electronically trim the fuel injector 22 and the valve 34 based on the determined valve timing. Box 335 may be performed by actually performing the electronic trimming, or, for example, by updating a trim table or other data structure for electronically trimming the fuel injector 22.

[0028] This specification is for illustrative purposes only and should not be interpreted as narrowing the scope of the present disclosure in any way. Therefore, it will be understood by those skilled in the art that various modifications may be made to the currently disclosed embodiments without departing from the complete and reasonable scope and spirit of the present disclosure. By viewing the accompanying drawings and the appended claims, other aspects, features and advantages will become apparent. As used herein, the articles "a" and "an" are intended to include one or more items and can be used interchangeably with "one or more". When only one item is intended to be represented, the term "one" or similar language is used. In addition, as used herein, the term "has, have, having" and the like are intended to be open terms. In addition, the phrase "based on" is intended to mean "based, at least in part, on", unless otherwise expressly stated.

Claims

1. A method of operating a fuel system for an engine, comprising: moving a plunger in a fuel system between an advanced position and a retracted position during a pumping cycle of the plunger; pulsing a solenoid actuator in the circuit with a plurality of current pulses of varying durations during at least one rest cycle of the plunger; monitoring an electrical characteristic in the circuit caused by moving the valve between a first position and a second position in response to each respective current pulse of the plurality of current pulses; as well as Valve timing of the valve is determined based on the monitored electrical characteristic. 2 . The method of claim 1 , wherein monitoring the electrical characteristic comprises monitoring a freewheeling response duration, and wherein the monitored freewheeling response duration indicates a plurality of different durations for moving the valve between the first position and the second position. 3 . The method of claim 2 , wherein the first position comprises a closed stop position and the second position comprises an open stop position, and the plurality of different durations comprises a plurality of valve closing time (VCT) durations. 4 . The method of claim 3 , wherein determining valve timing comprises determining a valve open time (VOT) duration for the valve to move from the closed stop position to the open stop position.

5. The method of claim 4, wherein the plurality of VCT durations comprises VCT max duration, and the determined VOT duration is based on the VCT max Duration.

6. A method according to any preceding claim, wherein pulsing a solenoid actuator comprises pulsing the solenoid actuator with a plurality of current pulses of successively increasing or successively decreasing duration.

7. A method according to any preceding claim, wherein: The valve comprises an injection control valve in a fuel injector, and moving the plunger comprises moving the plunger in response to rotation of a cam; and The fuel injector includes an outlet check in a closed position during at least one aborted cycle of the plunger, and a relief valve in an open position during at least one aborted cycle of the plunger.

8. A fuel system comprising: a fuel pump including a plunger movable between an advanced position and a retracted position in a plunger cavity formed in the housing; a relief valve movable between an open position and a closed position to respectively open and close fluid communication between the plunger cavity and a fuel port formed in the housing; a control valve assembly including a solenoid actuator and a control valve movable within the housing in response to energizing the solenoid actuator; A control system having an electric circuit and an electronic control unit, wherein the electric circuit includes the solenoid actuator, and the electronic control unit is configured to: pulsing the solenoid actuator with a plurality of current pulses of varying durations when the relief valve is in the open position; monitoring an electrical characteristic in the circuit caused by moving the control valve between a first position and a second position in response to each respective current pulse of the plurality of current pulses; and Valve timing of the control valve is determined based on the monitored electrical characteristic.

9. The fuel system of claim 8, wherein the valve timing comprises a valve open time (VOT) duration, and wherein the monitored electrical characteristic indicates a plurality of different valve close time (VCT) durations; and The electrical characteristics monitored include the follow-on response.

10. The fuel system of claim 9, wherein the plurality of VCT durations comprises VCT max duration, and the determined VOT duration is based on the VCT max duration; and Wherein the first position comprises a closed stop position, and the second position comprises an open stop position.

11. The fuel system of any one of claims 8-10, further comprising a fuel injector comprising the control valve assembly, an outlet check operably coupled to the control valve, and a cam rotatable to move the plunger between the advanced position and the retracted position; and The electronic control unit is further configured to perform pulsing the solenoid actuator and monitoring the electrical characteristic during at least one rest cycle of the plunger.

12. A fuel control system comprising: An electronic control unit, the electronic control unit being configured to: pulsing a solenoid actuator in a fuel system with a plurality of current pulses during at least one idle cycle of a plunger in the fuel system; monitoring an electrical characteristic in a circuit including the solenoid actuator caused by moving a valve in the fuel system in response to each respective one of the plurality of current pulses; determining valve timing of the valve based on the monitored electrical characteristic; and The valve is electronically trimmed based on the determined valve timing.

13. The fuel control system of claim 12, wherein: The valve timing includes valve opening timing; and The monitored electrical characteristic is indicative of a plurality of different valve closing time (VCT) durations.

14. The fuel control system of claim 13, wherein the plurality of VCT durations include VCT max duration, and the valve opening timing includes a time constant based on the VCT max Duration of the valve open time (VOT) duration.

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