Coordinated engine start in hybrid vehicle

A method for controlling a hybrid vehicle powertrain, where the powertrain includes a motor and an engine, includes providing a first motor torque in response to an engine start request. The first motor torque cranks the engine through a first compression event. The method additionally includes providing a second motor torque in response to the engine being cranked through the first compression event. The second motor torque is less than the first motor torque.

TECHNICAL FIELD

The present disclosure relates to hybrid vehicles and to methods of controlling engine starts in such vehicles.

BACKGROUND

Hybrid electric vehicles (HEVs) utilize a combination of an internal combustion engine with at least one electric motor to provide power to vehicle traction wheels. HEVs may be configured to shut down the engine under certain operating conditions and operate in an electric only mode. In such situations, the electric motor provides all of the power to propel the vehicle. The engine may subsequently be started in response to, for example, an increase in drive power demand or a decrease in battery state of charge.

SUMMARY

A method for controlling a hybrid vehicle powertrain, where the powertrain includes a motor and an engine, includes providing a first motor torque in response to an engine start request. The first motor torque cranks the engine through a first compression event. The method additionally includes providing a second motor torque in response to the engine being cranked through the first compression event. The second motor torque is less than the first motor torque and cranks the engine through subsequent compression events.

In one embodiment, the engine start request is a first start request subsequent to a key-on event. In such an embodiment, the first motor torque is based on a calibratable baseline torque and at least one engine operating condition including temperature, pressure, an engine friction estimate, and an elapsed time with the engine off. In another embodiment, the engine start request is at least a second start request subsequent to a key-on event. In such an embodiment, the first motor torque is based on a calibratable baseline torque and at least one engine operating condition including an engine stop position, temperature, pressure, an engine friction estimate, and an elapsed time with the engine off. In additional embodiments, the second motor torque is based on a calibratable baseline torque at least one engine operating condition including temperature, pressure, an engine friction estimate, and an elapsed time with the engine off. In some embodiments, the first motor torque is recalculated after a calibratable interval elapses.

In some embodiments, the method further includes providing a motor drive torque to vehicle traction wheels in response to a driver torque request. In such an embodiment, providing a first motor torque comprises commanding the motor to provide the first motor torque and the motor drive torque. Similarly, providing a second motor torque comprises commanding the motor to provide the second motor torque and the second drive torque.

A hybrid vehicle according to the present disclosure includes an internal combustion engine, a motor configured to provide drive torque to vehicle wheels and cranking torque to the engine, and at least one controller. The controller is configured to command the motor to provide a first torque to crank the engine through a first compression event in response to an engine start request. The controller is additionally configured to command the motor to provide a second torque in response to the engine being cranked through the first compression event, where the second torque is less than the first torque.

In one embodiment, the engine start request is a first start request subsequent to a key-on event. In such an embodiment, the first torque is based on a calibratable baseline torque and at least one engine operating condition including temperature, pressure, an engine friction estimate, and an elapsed time with the engine off. In another embodiment, the engine start request is at least a second start request subsequent to a key-on event. In such an embodiment, the first torque is based on a calibratable baseline torque and at least one engine operating condition including an engine stop position, temperature, pressure, an engine friction estimate, and an elapsed time with the engine off. In additional embodiments, the second torque is based on a calibratable baseline torque at least one engine operating condition including temperature, pressure, an engine friction estimate, and an elapsed time with the engine off. In some embodiments, the controller is further configured to intermittently recalculate the first torque and second torque at calibratable intervals.

A method for controlling a motor in a hybrid vehicle according to the present disclosure includes providing drive torque to vehicle traction wheels when an engine is stopped. The method additionally includes providing a first additional torque to crank the engine through a first compression event in response to an engine start request. The method further includes providing a second additional torque in response to the engine being cranked through the first compression event. The second additional torque is less than the first additional torque.

In one embodiment, the first additional torque is calculated based on at least one of: a calibratable baseline torque, an engine stop position, temperature, pressure, an engine friction estimate, and an elapsed time with the engine off. In such an embodiment, the method may further include recalculating the first additional torque at calibratable intervals. In another embodiment, the second additional torque is calculated based on at least one of: a calibratable baseline torque, temperature, pressure, an engine friction estimate, and an elapsed time with the engine off. In such an embodiment, the method may further include recalculating the second additional torque at calibratable intervals.

Embodiments according to the present disclosure provide a number of advantages. For example, the present disclosure provides a system and method for starting an engine in a hybrid vehicle using lower motor torque than known methods, resulting in increased efficiency. This may result in an increased efficiency engine start. In addition, the present disclosure provides a system and method for using a motor/generator to efficiently start an engine without disturbing torque provided to vehicle traction wheels.

The above and other advantages and features of the present disclosure will be apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.

DETAILED DESCRIPTION

Referring toFIG. 1, a schematic diagram of a hybrid electric vehicle (HEV)10is illustrated according to an embodiment of the present disclosure.FIG. 1illustrates representative relationships among the components. Physical placement and orientation of the components within the vehicle may vary. The HEV10includes a powertrain12. The powertrain12includes an engine14that drives a transmission16, which may be referred to as a modular hybrid transmission (MHT). As will be described in further detail below, transmission16includes an electric machine such as an electric motor/generator (M/G)18, an associated traction battery20, a torque converter22, and a multiple step-ratio automatic transmission, or gearbox24.

The engine14and the M/G18are both drive sources for the HEV10. The engine14generally represents a power source that may include an internal combustion engine such as a gasoline, diesel, or natural gas powered engine, or a fuel cell. The engine14generates an engine power and corresponding engine torque that is supplied to the M/G18when a disconnect clutch26between the engine14and the M/G18is at least partially engaged. The M/G18may be implemented by any one of a plurality of types of electric machines. For example, M/G18may be a permanent magnet synchronous motor. Power electronics56condition direct current (DC) power provided by the battery20to the requirements of the M/G18, as will be described below. For example, power electronics may provide three phase alternating current (AC) to the M/G18.

When the disconnect clutch26is at least partially engaged, power flow from the engine14to the M/G18or from the M/G18to the engine14is possible. For example, the disconnect clutch26may be engaged and M/G18may operate as a generator to convert rotational energy provided by a crankshaft28and M/G shaft30into electrical energy to be stored in the battery20. The disconnect clutch26can also be disengaged to isolate the engine14from the remainder of the powertrain12such that the M/G18can act as the sole drive source for the HEV10. Shaft30extends through the M/G18. The M/G18is continuously drivably connected to the shaft30, whereas the engine14is drivably connected to the shaft30only when the disconnect clutch26is at least partially engaged.

The M/G18is connected to the torque converter22via shaft30. The torque converter22is therefore connected to the engine14when the disconnect clutch26is at least partially engaged. The torque converter22includes an impeller fixed to M/G shaft30and a turbine fixed to a transmission input shaft32. The torque converter22thus provides a hydraulic coupling between shaft30and transmission input shaft32. The torque converter22transmits power from the impeller to the turbine when the impeller rotates faster than the turbine. The magnitude of the turbine torque and impeller torque generally depend upon the relative speeds. When the ratio of impeller speed to turbine speed is sufficiently high, the turbine torque is a multiple of the impeller torque. A torque converter bypass clutch34may also be provided that, when engaged, frictionally or mechanically couples the impeller and the turbine of the torque converter22, permitting more efficient power transfer. The torque converter bypass clutch34may be operated as a launch clutch to provide smooth vehicle launch. Alternatively, or in combination, a launch clutch similar to disconnect clutch26may be provided between the M/G18and gearbox24for applications that do not include a torque converter22or a torque converter bypass clutch34. In some applications, disconnect clutch26is generally referred to as an upstream clutch and launch clutch34(which may be a torque converter bypass clutch) is generally referred to as a downstream clutch.

The gearbox24may include gear sets (not shown) that are selectively placed in different gear ratios by selective engagement of friction elements such as clutches and brakes (not shown) to establish the desired multiple discrete or step drive ratios. The friction elements are controllable through a shift schedule that connects and disconnects certain elements of the gear sets to control the ratio between a transmission output shaft36and the transmission input shaft32. The gearbox24is automatically shifted from one ratio to another based on various vehicle and ambient operating conditions by an associated controller, such as a powertrain control unit (PCU)50. The gearbox24then provides powertrain output torque to output shaft36.

It should be understood that the hydraulically controlled gearbox24used with a torque converter22is but one example of a gearbox or transmission arrangement; any multiple ratio gearbox that accepts input torque(s) from an engine and/or a motor and then provides torque to an output shaft at the different ratios is acceptable for use with embodiments of the present disclosure. For example, gearbox24may be implemented by an automated mechanical (or manual) transmission (AMT) that includes one or more servo motors to translate/rotate shift forks along a shift rail to select a desired gear ratio. As generally understood by those of ordinary skill in the art, an AMT may be used in applications with higher torque requirements, for example.

As shown in the representative embodiment ofFIG. 1, the output shaft36is connected to a differential40. The differential40drives a pair of wheels42via respective axles44connected to the differential40. The differential transmits approximately equal torque to each wheel42while permitting slight speed differences such as when the vehicle turns a corner. Different types of differentials or similar devices may be used to distribute torque from the powertrain to one or more wheels. In some applications, torque distribution may vary depending on the particular operating mode or condition, for example.

The powertrain12further includes an associated powertrain control unit (PCU)50. While illustrated as one controller, the PCU50may be part of a larger control system and may be controlled by various other controllers throughout the vehicle10, such as a vehicle system controller (VSC). It should therefore be understood that the powertrain control unit50and one or more other controllers can collectively be referred to as a “controller” that controls various actuators in response to signals from various sensors to control functions such as starting/stopping engine14, operating M/G18to provide wheel torque or charge battery20, select or schedule transmission shifts, etc. Controller50may include a microprocessor or central processing unit (CPU) in communication with various types of computer readable storage devices or media. Computer readable storage devices or media may include volatile and nonvolatile storage in read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM), for example. KAM is a persistent or non-volatile memory that may be used to store various operating variables while the CPU is powered down. Computer-readable storage devices or media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by the controller in controlling the engine or vehicle.

The controller communicates with various engine/vehicle sensors and actuators via an input/output (I/O) interface that may be implemented as a single integrated interface that provides various raw data or signal conditioning, processing, and/or conversion, short-circuit protection, and the like. Alternatively, one or more dedicated hardware or fin mare chips may be used to condition and process particular signals before being supplied to the CPU. As generally illustrated in the representative embodiment ofFIG. 1, PCU50may communicate signals to and/or from engine14, disconnect clutch26, M/G18, launch clutch34, transmission gearbox24, and power electronics56. Although not explicitly illustrated, those of ordinary skill in the art will recognize various functions or components that may be controlled by PCU50within each of the subsystems identified above. Representative examples of parameters, systems, and/or components that may be directly or indirectly actuated using control logic executed by the controller include fuel injection timing, rate, and duration, throttle valve position, spark plug ignition timing (for spark-ignition engines), intake/exhaust valve timing and duration, front-end accessory drive (FEAD) components such as an alternator, air conditioning compressor, battery charging, regenerative braking, M/G operation, clutch pressures for disconnect clutch26, launch clutch34, and transmission gearbox24, and the like. Sensors communicating input through the I/O interface may be used to indicate turbocharger boost pressure, crankshaft position (PIP), engine rotational speed (RPM), wheel speeds (WS1, WS2), vehicle speed (VSS), coolant temperature (ECT), intake manifold pressure (MAP), accelerator pedal position (PPS), ignition switch position (IGN), throttle valve position (TP), air temperature (TMP), exhaust gas oxygen (EGO) or other exhaust gas component concentration or presence, intake air flow (MAF), transmission gear, ratio, or mode, transmission oil temperature (TOT), transmission turbine speed (TS), torque converter bypass clutch34status (TCC), deceleration or shift mode (MDE), for example.

An accelerator pedal52is used by the driver of the vehicle to provide a demanded torque, power, or drive command to propel the vehicle. In general, depressing and releasing the pedal52generates an accelerator pedal position signal that may be interpreted by the controller50as a demand for increased power or decreased power, respectively. Based at least upon input from the pedal, the controller50commands torque from the engine14and/or the M/G18. The controller50also controls the timing of gear shifts within the gearbox24, as well as engagement or disengagement of the disconnect clutch26and the torque converter bypass clutch34. Like the disconnect clutch26, the torque converter bypass clutch34can be modulated across a range between the engaged and disengaged positions. This produces a variable slip in the torque converter22in addition to the variable slip produced by the hydrodynamic coupling between the impeller and the turbine. Alternatively, the torque converter bypass clutch34may be operated as locked or open without using a modulated operating mode depending on the particular application.

To drive the vehicle with the engine14, the disconnect clutch26is at least partially engaged to transfer at least a portion of the engine torque through the disconnect clutch26to the M/G18, and then from the M/G18through the torque converter22and gearbox24. The M/G18may assist the engine14by providing additional power to turn the shaft30. This operation mode may be referred to as a “hybrid mode” or an “electric assist mode.”

To drive the vehicle with the M/G18as the sole power source, the power flow remains the same except the disconnect clutch26isolates the engine14from the remainder of the powertrain12. Combustion in the engine14may be disabled or otherwise OFF during this time to conserve fuel. The traction battery20transmits stored electrical energy through wiring54to power electronics56that may include an inverter, for example. The power electronics56convert DC voltage from the battery20into AC voltage to be used by the M/G18. The PCU50commands the power electronics56to convert voltage from the battery20to an AC voltage provided to the M/G18to provide positive or negative torque to the shaft30. This operation mode may be referred to as an “electric only” operation mode.

In any mode of operation, the M/G18may act as a motor and provide a driving force for the powertrain12. Alternatively, the M/G18may act as a generator and convert kinetic energy from the powertrain12into electric energy to be stored in the battery20. The M/G18may act as a generator while the engine14is providing propulsion power for the vehicle10, for example. The M/G18may additionally act as a generator during times of regenerative braking in which rotational energy from spinning wheels42is transferred back through the gearbox24and is converted into electrical energy for storage in the battery20.

It should be understood that the schematic illustrated inFIG. 1is merely exemplary and is not intended to be limited. Other configurations are contemplated that utilize selective engagement of both an engine and a motor to transmit through the transmission. For example, the M/G18may be offset from the crankshaft28, an additional motor may be provided to start the engine14, and/or the M/G18may be provided between the torque converter22and the gearbox24. Other configurations are contemplated without deviating from the scope of the present disclosure.

One advantage of hybrid configurations, such as the exemplary vehicle illustrated inFIG. 1, is that torque provided by a motor/generator may be applied to a crankshaft to crank an engine during an engine start event. In known vehicles, a motor/generator may provide a generally constant cranking torque to an engine until the engine achieves a threshold speed.

However, during an engine start event the amount of cranking torque required to turn the engine varies. When rotation begins from a dead stop, the torque required to turn the crankshaft through the first vacuum-producing stroke (i.e. a power stroke of the cycle) combined with the torque for the subsequent compression producing stroke may substantially exceed the torque required to turn the engine through the remaining portion of the engine start event. The torque required for an engine start may thus be divided into two separate torque requirements, a “first compression torque” required to turn the engine through a first compression event and a “cranking torque” required to continue cranking the engine thereafter.

Referring toFIG. 2, an engine start event is illustrated. At time t0the engine speed is turned off, as illustrated at numeral60, and the motor is providing a motor torque τm,0. In this exemplary start event τm,0is non-zero, i.e. the motor is providing torque to vehicle traction wheels at time t0to operate the vehicle in an electric-only mode. It should be noted, of course, that this method may be implemented when τm,0is zero as well.

At time t1an engine start request is issued. In response to the engine start request, the motor torque at time t1is increased to τm,1, where τm,1is at least sufficient to satisfy τm,0and a first compression torque τm,FC, where τm,FCis the required motor torque to crank the engine through a first compression event. Additionally, the pressure in the disconnect clutch26is controlled to transmit τm,FCto the engine. It should be noted that in some circumstances the disconnect clutch may not be able to transmit a torque magnitude equal to τm,FCbased on the current clutch pressure. In such a scenario, the motor torque may be reduced according to the torque the clutch is capable of transmitting. The motor torque may subsequently be increased to the target τm,1as pressure in the disconnect clutch26increases.

In an exemplary embodiment, τm,FCis calculated using a calibratable baseline torque and a set of multipliers or scale factors based on engine operating conditions. The multipliers may be based on crankshaft position, engine temperature, barometric pressure, an engine friction estimate, and soak time. The baseline torque may be, for example, 100 Nm. The engine friction estimate may vary according to engine temperature, engine speed, and barometric pressure for a given engine class, and may be adapted over time. Other appropriate variables may, of course, be used to calculate τm,FC. The value of τm,FCis preferably intermittently recalculated, for example at one second intervals, during periods when the engine is off. In addition, the value of τm,FCmay be recalculated as the engine position changes during a start event, for example at calibratable crank intervals.

It should be noted that the calculation of τm,FCmay in some embodiments be modified for an engine start event that is the first engine start following a key-on event. For example, a crankshaft position reading may be unavailable when first starting the engine, and thus the calculation of τm,FCmay be based in part on a “worst case” crankshaft stop position. In a six cylinder engine, the worst case position may be 120 degrees before top dead center (TDC), as at this position one cylinder is at TDC on a power stroke, necessitating increased torque as it draws a vacuum. As another example, the baseline torque may be increased for a first start, for example to 120 Nm.

Returning toFIG. 2, at time t1the engine begins to rotate due to motor torque transmitted through the disconnect clutch26, as illustrated at numeral62. At time t2the engine completes a first compression event. In response to the engine completing the first compression event, the motor torque is reduced to τm,2, where τm,2is at least sufficient to satisfy τm,0and a cranking torque τm,C, where τm,Cis the required motor torque to crank the engine through the remainder of the engine start.

In an exemplary embodiment, τm,Cis calculated using a calibratable baseline torque and a set of multipliers or scale factors based on engine operating conditions. The multipliers may be based on engine temperature, barometric pressure, an engine friction estimate, and soak time. The baseline torque may be, for example, 20 Nm. Other appropriate variables may, of course, be used to calculate τm,C. The value of τm,Cis preferably intermittently recalculated, for example at one second intervals, during periods when the engine is off. The engine continues to crank under the influence of τm,Cand increase in speed until the engine start is complete, as illustrated at numeral64. Once the engine has started, at least one controller may control the engine, disconnect clutch, and motor according to any appropriate hybrid operation logic.

It should be noted that when the vehicle operates in an electric only mode, the motor continues to provide torque to traction wheels to satisfy driver torque demands until the engine starts. Thus, although τm,0is shown as being constant inFIG. 2for illustrative purposes, τm,0may vary during the engine start according to driver torque demands.

In a preferred embodiment, at least one software “flag” is implemented to signal that various steps of the algorithm are complete. For example, a “first compression complete” flag frst_comp_flag may be activated in response to the engine completing the first compression event. The motor torque may be reduced from τm,1to τm,2in response to the frst_comp_flag being activated. The frst_comp_flag may subsequently be reset when the engine stops, for example in response to the engine speed being zero. In addition, the flag may be set to activate upon completion of a second compression event if the engine stop position is too close to top dead center. As an additional example, a separate flag may be used to signal that the first engine start following a key-on event has occurred. Such a flag may be reset in response to a key-off event.

Referring toFIG. 3, a first compression complete flag is illustrated. In response to the engine speed reaching zero at time t0′ frst_comp_flag is reset. In this exemplary embodiment, calculated values for τm,FCand τm,Care recalculated in response to frst_comp_flag being reset. These calculated values may also be intermittently recalculated while the engine is stopped. At time t1′ an engine start request is received, and the engine is started generally as described above with respect toFIG. 2. At time t2′ the engine completes the first compression event, and frst_comp_flag is activated. The motor torque (not illustrated) will be reduced in response to this flag being activated, as illustrated inFIG. 2.

Referring toFIG. 4, a method for controlling a vehicle according to the present disclosure is illustrated in flowchart form. At the outset, the engine is off, as illustrated at block70. While the engine is off, a required motor torque for a first compression event τm,FCand a required motor cranking torque τm,Care recalculated at calibratable time intervals, as illustrated at block72. τm,FCand τm,Cmay be calculated based upon various engine conditions, as illustrated at block74. These conditions may include a calibratable baseline torque, a crankshaft position, engine temperature, barometric pressure, an engine friction estimate, soak time, or other appropriate conditions. An engine start request is received, as illustrated at block76. The motor and clutch are controlled to provide τm,FCto the engine, as illustrated at block78. This may be an additional torque above a torque supplied to vehicle traction wheels. At block80, a determination is made of whether a first compression event is complete. If no, the motor continues to provide τm,FC.If yes, then the motor and clutch are controlled to provide τm,C, as illustrated at block82. At block84, a determination is made of whether the engine has started. If no, the motor continues to provide τm,C. If yes, then the algorithm ends, as illustrated at block86.

Variations of the above method are of course possible. For example, a calculation for a complete torque profile for an engine start may be provided. In such an embodiment, motor torque may be varied through an engine start according to a profile calculated based on similar engine parameters as discussed above. Such an embodiment may provide a more efficient engine start, but would also be more calculation-intensive.

As can be seen from the above embodiments, the present invention provides a system and method for using a motor/generator to efficiently start an engine at lower torque levels without disturbing torque provided to vehicle traction wheels.

While the best mode has been described in detail, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. While various embodiments may have been described as providing advantages or being preferred over other embodiments with respect to one or more desired characteristics, as one skilled in the art is aware, one or more characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments discussed herein that are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.