Method for increasing electric operation in hybrid electric vehicles

A hybrid electric vehicle includes an internal combustion engine configured to provide power to traction wheels, and a controller. The controller is configured to, in response to the engine being off and a power request exceeding an engine-start threshold, delay an engine start when vehicle speed is below a predetermined value. Delaying an engine start may include providing an engine-start-threshold offset. The offset decreases as vehicle speed increases. The engine is started when the power request exceeds a sum of the engine-start threshold and the engine-start-threshold offset.

TECHNICAL FIELD

This disclosure relates to systems and methods for controlling the operation of an engine in an internal combustion vehicle.

BACKGROUND

Hybrid electric vehicles generally include both an engine and at least one traction motor. One method of improving the fuel economy in an HEV is to shut down the engine during times that the engine operates inefficiently, or is not otherwise needed to propel the vehicle. In these situations, the traction motor is used in an electric-only drive mode to provide all of the power needed to propel the vehicle.

SUMMARY

A hybrid electric vehicle according to the present disclosure includes an internal combustion engine configured to provide power to traction wheels, and a controller. The controller is configured to, in response to the engine being off and a power request exceeding an engine-start threshold, delay an engine start when vehicle speed is below a predetermined value. Delaying an engine start may include providing an engine-start-threshold offset. The offset decreases as the vehicle speed increases. In such embodiments, the engine is started when the power request exceeds a sum of the engine-start threshold and the engine-start-threshold offset.

In one embodiment, the engine-start-threshold offset has a maximum value based on a battery-discharge limit. The engine-start-threshold offset may be based on vehicle speed and the engine-start threshold, and may be stored in and obtained from a lookup table.

In some embodiments, the controller is configured to delay the engine start in response to the engine being off, the power request exceeding the engine-start threshold, and a battery state of charge (SOC) exceeding an SOC threshold. In such embodiments, when the battery SOC is below the SOC threshold, the engine is started when the power request exceeds the engine-start threshold.

A method of controlling a hybrid electric vehicle according to the present disclosure, wherein the vehicle has an internal combustion engine, includes starting the engine in response to the engine being off, a first driver power request exceeding an engine-start threshold and current vehicle speed being above a predetermined value. The method further includes delaying an engine start event in response to the engine being off, a second driver power request exceeding the engine-start threshold and current vehicle speed being below the predetermined value.

A hybrid electric vehicle according to the present disclosure includes traction wheels, an electric machine configured to provide power to the traction wheels, an internal combustion engine configured to provide power to the traction wheels, and a controller. The controller is configured to coordinate the electric machine and internal combustion engine to satisfy a driver power request, wherein in response to a driver power request exceeding an engine-start threshold when the engine is off and the vehicle speed is below a predetermined value, the controller delays an engine start event.

Embodiments according to the present disclosure provide a number of advantages. For example, systems and methods according to the present disclosure may avoid unnecessary engine starts during launch events, thus improving customer perception of overall fuel economy. Furthermore, systems and methods according to the present disclosure may avoid engine restarts during brief temporary increases in driver power request (“pedal noise”).

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 firmware 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.

Hybrid-electric vehicles may provide significant fuel economy advantages relative to conventional engine-powered vehicles due to reduced engine usage. Hybrid-electric vehicles are generally configured to operate in a plurality of modes, including at least one operating mode with the engine on and an electric-only mode (i.e. with the engine off). Hybrid-electric vehicles are usually configured to operate in the various operating modes according to an algorithm calibrated for maximum fuel efficiency.

However, some customers may perceive electric-only mode to be necessarily more efficient. Customer satisfaction may thus be generally increased by prolonged operation in electric-only mode. Consequently, it may be desirable to increase the duration and quantity of intervals of electric-only operation relative to the default (e.g. optimized for fuel efficiency) algorithm.

When decelerating to a full stop, known hybrid-control logic will frequently result in the engine being stopped. Subsequently, when accelerating from the full stop, known hybrid-control logic will start the engine when the driver power demand exceeds an engine-start threshold. Generally speaking, the engine-start threshold is based on current battery state of charge (SOC) and charging/discharging limits. Similarly, when driving at speed in electric-only mode, when the driver power demand exceeds an engine-start threshold the engine will be started.

In embodiments according to the present invention, a cushion or buffer is provided to enable continued electric-only operation when the driver power request exceeds the threshold. In a preferred embodiment, the buffer takes the form of an offset added to the engine-start threshold.

FIG. 2illustrates an engine-start-threshold offset according to one embodiment of the present invention. A driver power request60varies over time, for example as the driver adjusts an accelerator pedal position. At time t1, the driver power request60exceeds a base engine-start threshold62. The base engine-start threshold may be based on battery state of charge, discharge limits, and/or other factors. Thus, according to the base hybrid logic, the engine would ordinarily be started at time t1.

An engine-start-threshold offset64is provided and added to the base engine-start threshold62to generate a modified engine-start threshold66. The engine-start-threshold offset64may be a function of vehicle speed. In a preferred embodiment, the engine-start-threshold offset64has a maximum value when the vehicle is stopped and decreases as vehicle speed increases. In a further preferred embodiment, the engine-start-threshold offset is configured to decay to zero over a predefined interval after the driver power request60exceeds the base engine-start threshold62, as discussed in further detail below.

Because the driver power request60does not exceed the modified engine-start threshold66, the engine is not started. The power request60is thus satisfied while maintaining vehicle operation in electric-only mode, enhancing the perception of efficient operation.

In a preferred embodiment, the maximum value of the engine-start-threshold offset64is limited based on a battery discharge limit68. The battery discharge limit68corresponds to a maximum power deliverable by the high voltage battery at current operating conditions. The current operating conditions may include current battery state of charge, accessory power draw, and inherent system discharge limits.

In a further preferred embodiment, the engine-start-threshold offset is further limited to an adjusted discharge limit70based on a discharge limit buffer72. While possible to limit the offset only on the discharge limit68, it is desirable to provide a buffer72to ensure that sufficient battery discharge capacity is reserved to start the engine if necessary. The magnitude of the buffer72is thus preferably based on a power quantity required to start the engine.

Referring now toFIG. 3, a method of controlling a vehicle according to the present disclosure is illustrated in flowchart form. The method begins at block80. The hybrid vehicle is travelling in electric-only mode, i.e. with the engine off, as illustrated at block82. An engine-start threshold Pstartis determined and stored, as illustrated at block84. The engine-start threshold Pstartis determined, for example, based on battery state of charge, discharge limits, and/or other factors.

An engine-start-threshold offset Poffsetis then determined and stored, as illustrated at block86. The engine-start-threshold offset Poffsetmay be based on factors including a battery discharge limit, a buffer to ensure power to start the engine, current vehicle speed, and/or a decay scale factor, as illustrated at block88. The decay scale factor will be discussed in further detail below with respect to block100.

A driver power request Prequestis received and monitored, as illustrated at block90. The driver power request Prequestmay be received, for example, by way of a driver actuation of an accelerator pedal. The power request Prequestmay also be received based on outputs from a cruise control algorithm, self-driving vehicle algorithm, or other partially- or fully-automated driving systems.

A determination is then made of whether the driver power request Prequestexceeds the engine-start threshold Pstart, as illustrated at operation92. If no, then the vehicle is controlled in electric-only mode with the engine off, as illustrated at block94. Control then returns to block84. Thus, while the driver power request remains below the engine-start threshold, the vehicle continues in electric-only mode with the engine off. The engine-start threshold Pstartand the engine-start-threshold offset Poffsetare recalculated each cycle, as a change in current operating conditions may cause a change in at least one of the respective offsets.

If yes, then a determination is made of whether the driver power request Prequestexceeds a sum of the engine-start threshold Pstartand the engine-start-threshold offset Poffset, as illustrated at operation96. If no, then the engine start is delayed, as illustrated at block98. An offset-decay scale factor is determined, as illustrated at block100. The offset-decay scale factor is provided to decay Poffsetto zero over a defined interval after a driver power request Prequestexceeds the engine-start threshold Pstart. The offset-decay scale factor may be a function of time elapsed after the driver power request Prequestexceeds the engine-start threshold Pstart, current vehicle speed, battery state of charge, and/or other appropriate variables. Operation then continues to block94.

If yes, then the engine is started, as illustrated at block102. The driver power request may then be satisfied at least in part using power from the internal combustion engine.

Variations on the above are, of course, possible. As an example, in some embodiments, a hybrid vehicle is provided with an “ECO” button. Various vehicle systems are configured to operate in a first mode in response to the ECO button being inactive and a second mode in response to the ECO button being active. In some such embodiments, the engine-start-threshold offset is provided only when the ECO button is active. In other such embodiments, the engine-start-threshold offset is provided only when the ECO button is inactive.

As may be seen from the above description, the present invention provides a method of controlling a hybrid vehicle that delays engine starts when permissible, thus avoiding engine restarts when the vehicle takes off from a dead stop or during brief temporary increases in driver power request.