Method for controlling camshaft orientation to improve engine restarts of an engine with start-stop capability

Abstract

A method includes determining a value of a run motor current limit and setting a value of a start current limit equal to a predetermined value that exceeds the value of the run motor current limit if a set of predetermined conditions is satisfied. The method includes determining that operation of an engine has been interrupted, operating an electric motor of a variable valve timing mechanism with a current having a magnitude less than or equal to the start current limit after determining that operation of the engine has been interrupted, determining that operation of the engine has been restored, and operating the electric motor of the variable valve timing mechanism with a current having a magnitude less than or equal to the run motor current limit after determining that operation of the engine has been restored.

Description

Technical Field

[0001] This invention relates to a method for controlling the camshaft direction to improve engine restart of an engine with start-stop capability. Background Technology

[0002] This section provides background information related to the present invention, which is not necessarily prior art.

[0003] Modern car engines typically employ variable valve timing (VVT) mechanisms to change the phase or timing (relative to the crankshaft's rotational position) of the intake and / or exhaust valves. VVT mechanisms generally adjust the phase of the intake and / or exhaust valve opening by rotating a camshaft that controls the opening and closing of the valves. This type of VVT mechanism typically uses an electric motor to rotate the camshaft.

[0004] It is known in the art that a VVT mechanism is employed in engines with start-stop capability to place the engine in a state more suitable for restarting. In this case, the VVT ​​mechanism is used to preposition the camshaft in a direction more suitable for restarting the engine. The VVT ​​mechanism can also be used to adjust the camshaft (relative to the crankshaft) phasing during engine starting sequence to improve engine starting capability, for example, by reducing the amount of torque required to start the crankshaft by delaying engine timing. When using techniques that reduce the amount of torque required to start the crankshaft and delay engine timing together, the camshaft phasing needs to be advanced gradually but rapidly. While known techniques for operating VVT mechanisms are suitable for their intended purpose, such techniques are still susceptible to modification. Summary of the Invention

[0005] This section provides a general overview of the invention and is not a complete disclosure of its entire scope or all its features.

[0006] In one form, the invention provides a method for operating a variable valve timing mechanism that controls the phasing of a camshaft in an engine, wherein the variable valve timing mechanism has an electric motor. The method includes determining a value for an operating motor current limit, and, if a set of predetermined conditions are met, setting the value of the starting current limit to be equal to a predetermined value exceeding the value of the operating motor current limit, or conversely, using the value of the operating motor current limit as the value of the starting current limit. The method includes: determining that engine operation has been interrupted; after determining that engine operation has been interrupted, operating the electric motor of the variable valve timing mechanism with a current having a value less than or equal to the starting current limit; determining that engine operation has resumed; and after determining that engine operation has resumed, operating the electric motor of the variable valve timing mechanism with a current having a value less than or equal to the value of the operating motor current limit.

[0007] In some forms, the set of predetermined conditions includes the motor temperature being below a predetermined temperature threshold. In some forms, the magnitude of the predetermined value is based on the motor's lifespan. In some forms, the magnitude of the predetermined value is based on the motor's temperature. In some forms, the value of the operating motor current limit is based on the motor's temperature. In some forms, the value of the operating motor current limit is determined by a step function based on the motor's temperature. In some forms, determining that engine operation has been restored is at least partially based on control signals provided by the engine controller. In some forms, determining that engine operation has been restored is based on an algorithm utilizing one or more parameters of the engine. In some forms, determining that engine operation has been restored is based on one or more parameters of the variable valve timing mechanism. In some forms, determining that engine operation has been restored is based on the value of a timer.

[0008] The present invention also provides a method for operating a variable valve timing mechanism that controls the phasing of a camshaft in an engine, wherein the variable valve timing mechanism has an electric motor. The method includes determining that engine operation has been interrupted, and, upon determining that engine operation has been interrupted, operating the electric motor in a restart mode to control the phasing of the camshaft. The method also includes determining, independently of a dedicated restart signal, that engine operation has resumed, and, upon determining that engine operation has resumed, operating the electric motor in an engine operation mode to control the phasing of the camshaft.

[0009] In some forms, determining that engine operation has been interrupted includes determining whether the electric motor's rotational speed is less than or equal to a predetermined motor speed threshold. In some forms, determining that engine operation has been interrupted includes determining that the duration for which the electric motor's rotational speed has been less than or equal to the predetermined motor speed threshold is greater than or equal to a predetermined time threshold. In some forms, determining that engine operation has been interrupted includes determining that the value of the duty cycle supplying power to the electric motor is within a predetermined range. In some forms, determining that engine operation has been interrupted includes determining that the value of the duty cycle supplying power to the electric motor is less than a predetermined duty cycle threshold. In some forms, determining that engine operation is resuming includes determining whether the electric motor's rotational speed is greater than a predetermined motor speed threshold. In some forms, determining that engine operation is resuming includes determining that the duration for which the electric motor's rotational speed has been greater than or equal to the predetermined motor speed threshold is greater than or equal to a predetermined time threshold. In some forms, determining that engine operation is resuming includes determining that the value of the duty cycle supplying power to the electric motor is greater than or equal to a predetermined duty cycle threshold. In some forms, determining that engine operation is resuming includes determining that the magnitude of the duty cycle supplying power to the electric motor is greater than or equal to a predetermined duty cycle threshold.

[0010] Other applications will become apparent from the description provided herein. The descriptions and specific examples in this overview are for illustrative purposes only and are not intended to limit the scope of the invention. Attached Figure Description

[0011] The accompanying drawings described herein are for illustrative purposes only, representing selected embodiments and not all possible embodiments, and are not intended to limit the scope of the invention.

[0012] Figure 1 This is a schematic diagram of an exemplary engine having a variable valve timing mechanism configured to perform a camshaft pre-positioning function according to the teachings of the present invention.

[0013] Figure 2 yes Figure 1 The diagram shows a portion of the engine, with the variable valve timing mechanism shown in more detail.

[0014] Figure 3 This is a flowchart illustrating an exemplary method for performing a camshaft pre-positioning function according to the teachings of the present invention; and

[0015] Figure 4 This is a flowchart-style schematic diagram of an exemplary method for adjusting current limits according to the teachings of the present invention.

[0016] In the several views of the accompanying drawings, the corresponding reference numerals indicate the corresponding parts. Detailed Implementation

[0017] Exemplary embodiments will now be described more fully with reference to the accompanying drawings.

[0018] Refer to the attached diagram. Figure 1 An exemplary internal combustion engine 10 is shown. Engine 10 is described as having a V-shaped configuration comprising a first cylinder bank 12 and a second cylinder bank 14, each cylinder bank having one or more cylinders 16. However, it should be understood that the teachings of the present invention can be applied to other engine configurations. Intake air passes through an air filter 18 and flows to a throttle valve 20, which regulates the flow of fresh air for combustion in engine 10. The air flowing through the throttle valve 20 enters an intake passage 22 and is received into the cylinders 16 of engine 10 during the intake stroke. Fuel can be introduced into the intake air in a portion of the intake passage 22 near the cylinders 16 before air and fuel enter the cylinders 16 (i.e., port injection) and / or after air has been introduced into the cylinders 16 (i.e., direct injection). The air-fuel mixture in the cylinders is ignited and burned by a spark plug 28, producing gases that push the piston down in the cylinders 16 to rotate the crankshaft 30. The gases produced by the combustion of the air-fuel mixture in cylinder 16 are then discharged from cylinder 16.

[0019] Each cylinder 16 has one or more intake valves 34 and one or more exhaust valves 36, which can be opened to allow air to flow into or out of the cylinder 16. The opening and closing of the intake valves 34 and exhaust valves 36 are controlled by one or more camshafts. In the provided example, each cylinder bank 12, 14 has an intake valve camshaft 40 and an exhaust valve camshaft 42. A ring drive is typically used to rotatably connect the camshafts 40, 42 to the crankshaft 30. The ring drive typically includes sprockets or toothed pulleys connected to the camshafts 40, 42 and the crankshaft 30, and a chain or toothed belt fitted around the sprockets or toothed pulleys. Depending on the engine configuration, the ring drive can be configured to provide a desired reduction speed between the crankshaft 30 and the camshafts. In the provided example, the engine 10 is a four-stroke engine; therefore, the ring drive is configured such that the intake valve camshaft 40 and the exhaust valve camshaft 42 rotate at half the speed of the crankshaft 30.

[0020] Engine 10 also includes one or more variable valve timing (VVT) mechanisms that are selectively used to change the phasing of the camshaft (i.e., the timing of the rotational position of the crankshaft 30 relative to the opening and closing of an associated set of valves). In the provided example, each cylinder bank 12, 14 has an intake VVT ​​mechanism 50 and an exhaust VVT mechanism 52. The intake VVT ​​mechanism 50 and the exhaust VVT mechanism 52 are substantially similar and are well known in the art. Therefore, a discussion of the intake VVT ​​mechanism 50 will suffice.

[0021] See also Figure 2 The intake VVT ​​mechanism 50 is operated by an actuator 60, which is controlled by an engine control module (ECU) 62. In short, the ECU 62 controls the actuator 60 as needed to advance or delay the timing of the camshaft 40 during engine 10 operation according to a predetermined control method, typically to reduce the fuel consumption rate of the engine 10. The actuator 60 includes an electric motor 64, which can operate to cause relative rotation between the camshaft 40 and an annular drive element (i.e., sprocket 68 in the specific example provided) that provides rotational power to the camshaft 40.

[0022] Engine 10 has start-stop capability, wherein the operation of engine 10 will cease (via ECU 62) when a vehicle (not shown) driven by engine 10 stops for a predetermined time interval. Various VVT mechanisms on engine 10 (i.e., intake VVT ​​mechanism 50 and exhaust VVT mechanism 52) can be used to reposition the intake valve camshaft 40 and exhaust valve camshaft 42 respectively, such that engine 10 is in a state deemed more suitable for restarting at the end of a start-stop cycle, for example, to allow engine 10 to start faster and / or emit less. In the provided example, each of intake VVT ​​mechanism 50 and exhaust VVT mechanism 52 includes a mechanism controller 70 configured to monitor and operate actuator 60; however, it should be understood that some or all of the monitoring and control functions performed by mechanism controller 70 may be performed by ECU 62. Each mechanism controller 70 is configured to perform… Figure 3 and 4 The routine is illustrated schematically. In short, the mechanism controller 70 is configured to operate the electric motor 64 of the actuator 60 at an elevated current level under certain conditions until the engine 10 restarts. This configuration allows the electric motor 64 to be “overdriven”, allowing for a more rapid change in the camshaft phase during the restart of the engine 10, thereby not only improving starting capability but also reducing emissions and improving fuel economy.

[0023] exist Figure 3 In the process, control begins at bubble 100 and proceeds to decision box 102, where the control determines whether the mechanism controller 70 has detected that the engine is stopping. The mechanism controller 70 can use various criteria to determine whether the engine is stopping, such as signals generated by the ECU 62. In the example described in more detail herein, the mechanism controller 70 uses two example criteria to identify whether the engine is stopping; however, it should be understood that any type of criterion can be used to determine whether the engine is stopping.

[0024] A first example criterion that the mechanism controller 70 can use to identify a situation where engine operation is stopping involves the rotational speed of engine 10, and more specifically, whether the rotational speed of engine 10 has dropped below a predetermined speed threshold for a predetermined time. It should be understood that the rotational speeds of other components (e.g., camshaft 40, crankshaft 30, electric motor 64, etc.) can be used to determine whether engine operation is stopping. Because the rotational speed of electric motor 64 is proportional to the rotational speed of engine 10, the mechanism controller 70 can use the rotational speed of electric motor 64 to determine the rotational speed of engine 10. Electric motor 64 includes a position sensor that senses the rotational position of the rotor of electric motor 64 and responsively generates a rotor position signal transmitted to mechanism controller 70. Mechanism controller 70 can use the position sensor to control the commutation of electric motor 64 and to determine the rotational speed of electric motor 64. Alternatively, the rotational speed of the rotor of electric motor 64 can be determined by mechanism controller 70 based on the magnitude of the reverse electromagnetic force (emf) generated by electric motor 64. Once mechanism controller 70 determines that the rotor speed is below a speed associated with a predetermined speed threshold of engine 10, mechanism controller 70 can start a timer. Therefore, at a certain point in time, the first criterion is met, at which point the rotor speed is kept below the speed associated with the predetermined speed threshold of engine 10 for a predetermined time period equal to or exceeding the predetermined time.

[0025] A second example criterion that the mechanism controller 70 can use to identify a situation where engine operation is stopping involves the magnitude of the requested duty cycle for supplying power to the electric motor 64. If the pre-positioned camshaft is not needed or desired, the electric motor 64 of the actuator 60 will not require power if the engine 62 has stopped operating. Therefore, a requested duty cycle having a magnitude outside a predetermined duty cycle range or region indicates that engine 10 has stopped operating. Thus, the second criterion is met when the requested duty cycle has a magnitude outside the predetermined duty cycle range.

[0026] In the provided example, the control determines that the engine is stopping when both the first and second criteria are met. However, it should be understood that the control can determine that the engine 10 is stopping if only the first criterion is met, only the second criterion is met, or either the first or the second criterion is met.

[0027] In decision block 102, if the control does not determine that the engine 10 is stopping, the control loops back to decision block 102. However, if the control determines that the engine 10 is stopping, the control proceeds to block 104.

[0028] In block 104, the desired camshaft closing position is obtained. As described above, the desired camshaft closing position is the position to which the camshaft is prepositioned during the "stop" portion of a "start-stop" cycle when the engine 10 is not running, such that the engine 10 is in a state considered more suitable for restarting at the end of the "start-stop" cycle. As an example, the mechanism controller 70 may obtain the desired camshaft closing position from the ECU 62, which may store the desired camshaft closing position or dynamically calculate the desired camshaft closing position based on various electrical and mechanical characteristics, such as those of the electric motor 64.

[0029] In block 106, the current limit for controlling and adjusting the current supplied to the motor 64 of the actuator 60 is specified. The current limit can be adjusted based on at least one of the following: the current limit, a predetermined current limit, the current motor temperature, a predetermined temperature limit, and a motor temperature profile. See below for further details. Figure 4 A more detailed description of the example current regulation control performed by mechanism controller 70.

[0030] In block 108, control maintains the camshaft at a desired camshaft pre-start position. As an example, mechanism controller 70 may supply current to motor 64 having a magnitude less than or equal to the current limit determined at block 106 in order to maintain the camshaft at the desired camshaft pre-start position.

[0031] In decision block 110, the control determines whether engine 10 is restarting. The determination that engine 10 is restarting can be in response to power supply to the starter motor, the use of a flag indicating that engine 10 will restart or other control signals, or the occurrence of any or all of the conditions used in decision block 110 to determine that engine operation is stopped. If the control does not determine that engine 10 is restarting, the control loops back to block 110. If the control determines that engine 10 is restarting, the control proceeds to block 112.

[0032] In block 112, control repositions or repositions the camshaft from its current phase toward or to a desired camshaft phase. As an example, before / during engine restart, control delays the phase of camshafts 40, 42 relative to the phase of crankshaft 30. As another example, before / during engine restart, control advances the phase of camshafts 40, 42 relative to the phase of crankshaft 30. In one form, mechanism controller 70 may supply current to motor 64 having a magnitude less than or equal to the updated current limit.

[0033] In decision block 114, the control determines whether the engine restart meets the engine restart conditions. In one example, meeting the engine restart conditions may indicate that the engine 10 has resumed operation and meets various engine parameters, while not meeting the engine restart conditions may indicate that the engine 10 has not resumed operation or that various engine parameters were not met during / at the time of engine restart. Determining whether various engine parameters are met may be based on a comparison of the crankshaft 30 speed with a threshold, a comparison of the engine 10 speed with a threshold, whether the duty cycle used to supply power to the electric motor 64 is within a predetermined duty cycle range, and other engine parameters.

[0034] If the control determines at block 114 that the engine restart condition is not met, the control proceeds to block 116, where it determines whether predetermined operating parameters of engine 10 are met. In one example, meeting the predetermined operating parameters when the engine restart condition is not met may instruct engine 10 to continue operating normally (or to operate according to a set of predetermined criteria), and failing to meet the predetermined operating parameters when the engine restart condition is not met may indicate that engine 10 is not operating normally (e.g., ECU 62 and / or mechanism controller 70 detects an engine / mechanism fault). The control may determine whether the predetermined operating parameters of engine 10 are met based on various engine parameters, such as crankshaft speed, engine speed, the value of the duty cycle used to supply power to electric motor 64, the value of a timer, and other engine parameters used to determine normal operation of engine 10.

[0035] Additionally or alternatively, the determination of predetermined operating parameters may be based, in whole or in part, on threshold time values ​​and timer values ​​of mechanism controller 70, which increase proportionally to the elapsed time of one or more predetermined events, such as determining that engine 10 is restarting or crankshaft 30 speed exceeds a predetermined threshold.

[0036] As an example, predetermined operating parameters are met when the timer value indicates that the engine has completed restarting within a predetermined time period. As another example, predetermined operating parameters may not be met when the timer value indicates that the engine has not completed restarting within a predetermined time period. It can be understood from the above that determining that engine 10 has restarted includes controlling the determination that all necessary conditions or parameters (including the elapsed timer) related to the resumption of engine 10 operation have been sufficiently met, and does not involve the ability of engine 10 to operate without a rotational input supply from any substance other than combustion within engine 10.

[0037] As another example, when the value of the duty cycle used to supply power to motor 64 is within a predetermined duty cycle range and the speed of motor 64 is greater than or equal to a minimum predetermined threshold speed (e.g., 0), the control can determine that predetermined operating parameters are met. As yet another example, when the value of the duty cycle used to supply power to motor 64 is outside the predetermined duty cycle range and the speed of motor 64 is less than or equal to a minimum predetermined threshold speed (e.g., 0), the control can determine that predetermined operating parameters are not met.

[0038] If the control determines at box 116 that the predetermined operating parameters are met, the control proceeds to box 112. However, if the control determines at box 116 that the predetermined operating parameters of engine 10 are not met, the control proceeds to box 118.

[0039] Returning to box 114, if the control determines that the engine restart meets the restart conditions, the control proceeds to box 118, where it adjusts the current limit. In one form, the current limit can be adjusted based on at least one of a predetermined current limit, the current motor temperature, a predetermined temperature limit, a motor temperature profile, and an engine temperature profile. As an example, the control can reset the current limit used to supply power to the electric motor 64 to a lower level associated with normal operation of the engine 10.

[0040] exist Figure 4 In, it is shown that in Figure 3 At block 106, current regulation control is performed by mechanism controller 70. Current regulation control begins at decision block 202, where it determines whether the current current limit is greater than a predetermined current limit. Mechanism controller 70 uses the current current limit to limit the amount of current supplied to motor 64 of actuator 60. If, in decision block 202, the current regulation control determines that the current current limit is not greater than the predetermined current limit, control proceeds to block 208, where the current regulation control uses the temperature of motor 64 and a motor temperature profile to determine an updated current limit (e.g., a new value for the current current limit). The motor temperature profile may include algorithms, lookup tables, or mappings of various parameters, including the temperature of motor 64, which correlate various parameters with the current limit of motor 64.

[0041] If the current regulation control determines that the current current limit is greater than a predetermined current limit, the current regulation control proceeds to decision block 204. In decision block 204, the current regulation control determines whether the temperature of the motor 64 of the actuator 60 is greater than or equal to a predetermined motor temperature limit. If, in decision block 204, the temperature of the motor 64 is greater than or equal to the predetermined motor temperature limit, the current regulation control proceeds to block 208, where an updated current limit is determined in the manner described above.

[0042] Returning to decision block 204, if the control determines that the motor temperature is not greater than or equal to the predetermined motor temperature limit, the current regulation control proceeds to block 206. In block 206, the current regulation control updates the current limit to the predetermined high current limit. In decision block 202, the predetermined high current limit may be equal to or different from the predetermined current limit. Then the control proceeds to decision block 202.

[0043] It should be understood that the set of predetermined conditions used to determine whether to use the predetermined high current limit may include other conditions, such as parameters related to the usage of the motor 64. For example, if the motor 64 has been used to the point where a predetermined percentage (e.g., 80%) of its service life has been consumed or used, the current regulation control may be configured to prohibit the use of the predetermined high current limit. Additionally or alternatively, the predetermined high current limit may vary throughout the service life of the motor 64.

[0044] For illustrative and descriptive purposes, the foregoing description of embodiments has been provided. It is not intended to be exhaustive or limiting of the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but are instead interchangeable where applicable and can be used in selected embodiments, even if not explicitly shown or described. Similarly, variations are possible in many ways. Such variations should not be considered a departure from the invention, and all such modifications are intended to be included within the scope of the invention.

[0045] As used herein, the phrases A, B, and C at least one should be interpreted as meaning the logic of using the non-exclusive logic "OR" (A or B or C), and should not be interpreted as meaning "at least one of A, at least one of B, and at least one of C".

[0046] The description of this invention is merely exemplary in nature, and therefore, variations that do not depart from the essence of the invention are intended to be within its scope. Such variations should not be considered as departing from the spirit and scope of the invention.

[0047] In the accompanying drawings, the direction of the arrows, as indicated by the arrows, generally represents the flow of information (e.g., data or instructions) of interest. For example, when components A and B exchange various types of information, but the information transmitted from component A to component B is relevant to the illustration, the arrow can point from component A to component B. This unidirectional arrow does not imply that no other information is transmitted from component B to component A. Furthermore, for information sent from component A to component B, component B can send a request for the information to component A or receive an acknowledgment of the information.

[0048] In this application, the term "controller" may refer to, belong to, or include: application-specific integrated circuits (ASICs); digital, analog, or mixed-signal discrete circuits; digital, analog, or mixed-signal integrated circuits; combinational logic circuits; field-programmable gate arrays (FPGAs); processor circuits (shared, dedicated, or grouped) that execute code; memory circuits (shared, dedicated, or grouped) that store code executed by the processor circuits; other suitable hardware components that provide the functions described; or some or all of the above, such as in a system-on-a-chip.

[0049] The term memory is a sub-device of the term computer-readable medium. As used herein, the term computer-readable medium does not include transient electrical or electromagnetic signals propagating through a medium (e.g., on a carrier wave); therefore, the term computer-readable medium can be considered tangible and non-transient. Non-limiting examples of non-transient, tangible computer-readable media are non-volatile memory circuits (e.g., flash memory circuits, erasable programmable read-only memory circuits, or mask read-only circuits), volatile memory circuits (e.g., static random access memory circuits or dynamic random access memory circuits), magnetic storage media (e.g., analog or digital magnetic tape or hard disk drives), and optical storage media (e.g., CDs, DVDs, or Blu-ray discs).

[0050] The apparatus and methods described in this application can be implemented, in part or in whole, by a special-purpose computer created by configuring a general-purpose computer to perform one or more specific functions contained in a computer program. The aforementioned function blocks, flowchart components, and other elements serve as software specifications that can be translated into a computer program through the routine work of a skilled technician or programmer.

Claims

1. A method for operating a variable valve timing mechanism, the variable valve timing mechanism controlling the phase angle of a camshaft in an engine, the variable valve timing mechanism having an electric motor, the method comprising: Power with a first magnitude of current is supplied to the electric motor to operate the electric motor in an engine operating mode that controls the phase angle of the camshaft; It has been determined that the engine operation has been interrupted; After determining that the engine operation has been interrupted, power with a second magnitude of current is supplied to the electric motor to operate the electric motor in restart mode to control the phase angle of the camshaft, the second magnitude being greater than the first magnitude and greater than the operating motor current limit to overdrive the electric motor. It has been determined that the engine operation has been restored; as well as In response to determining that the engine operation has been restored, power with a current having the first magnitude is supplied to the electric motor to operate the electric motor in the engine operating mode to control the phase angle of the camshaft.

2. The method of claim 1, wherein determining that the operation of the engine has been interrupted includes determining that the rotational speed of the electric motor is less than or equal to a predetermined motor speed threshold.

3. The method of claim 2, wherein determining that the operation of the engine has been interrupted includes determining that the time length during which the rotational speed of the electric motor has been less than or equal to the predetermined motor speed threshold is greater than or equal to the predetermined time threshold.

4. The method of claim 1, wherein determining that the operation of the engine has been interrupted includes determining that the duty cycle for supplying power to the electric motor is within a predetermined range.

5. The method of claim 1, wherein determining that the operation of the engine has been interrupted includes determining that the duty cycle for supplying power to the electric motor is less than a predetermined duty cycle threshold.

6. The method of claim 1, wherein determining that the engine operation has been restored includes determining that the rotational speed of the electric motor is greater than a predetermined motor speed threshold.

7. The method of claim 6, wherein determining that the engine operation has been restored includes determining that the time length during which the rotational speed of the electric motor has been greater than or equal to the predetermined motor speed threshold is greater than or equal to the predetermined time threshold.

8. The method of claim 6, wherein determining that the engine operation has been restored includes determining that the magnitude of the duty cycle for supplying power to the electric motor is greater than or equal to a predetermined duty cycle threshold.

9. The method of claim 1, wherein determining that the engine operation has resumed includes determining that the magnitude of the duty cycle for supplying power to the electric motor is greater than or equal to a predetermined duty cycle threshold.

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