Preventing gear shift cycling of a hybrid electric vehicle

In a powertrain for a motor vehicle that includes a power source, a transmission driveably connected to the power source and wheels of the vehicle, an electric machine able to operate as an electric motor for transmitting power to at least some of the vehicle wheels, a method for controlling the powertrain includes operating the power source and the transmission in a desired gear to produce a first wheel torque in response to a demanded wheel torque, increasing the demanded wheel torque while turning the vehicle along a curved path, and using the electric motor to provide a second wheel torque, such that a combined magnitude of the first wheel torque produced in the desired gear and the second wheel torque is equal to or greater than the increased demanded wheel torque.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a powertrain for a hybrid electric vehicle (HEV), and in particular to avoiding cyclic shifting among gears of a transmission while the vehicle corners or turns on a winding road.

2. Description of the Prior Art

A motor vehicle powertrain having an internal combustion engine and an automatic transmission, in which discrete gear ratios are produced, typically has a shift schedule calibrated to schedule downshifts when the engine runs out of reserve power and to schedule upshifts as close as possible to tractive effort crossovers. The shift schedule is calibrated for use in a vehicle that is unloaded and on straight road since the vehicle generally operates in these conditions.

When the vehicle is negotiating a curve or turning a corner, the straight road calibration can cause a condition, wherein the transmission repetitively upshifts and downshifts between gears. Excessive gear shifting occurs when the vehicle begins to decelerate in the turn the driver tips-in, i.e., depresses the accelerator pedal either at a high rate or to a substantial portion of its travel, to maintain vehicle speed. As a result of the tip-in, the transmission downshifts. Because the vehicle begins to accelerate beyond the desired speed after the downshift is completed, the driver tips-out of the throttle causing an upshift to occur. This cyclic gear shifting continues until the vehicle exits the curve.

To avoid this frequent gear shifting, it is conventional to perform the downshift and to prevent a subsequent upshift. Although this procedure mitigates the shift frequency problem, it can lead to driver dissatisfaction due to the downshift that occurs upon entering the turn. Furthermore, the early downshift causes a fuel economy penalty attributable to remaining in the lower gear for longer period, whereas fuel economy is maximized in the higher gear.

SUMMARY OF THE INVENTION

In a powertrain for a motor vehicle that includes a power source, a transmission driveably connected to the power source and wheels of the vehicle, an electric machine able to operate as an electric motor for transmitting power to at least some of the vehicle wheels, a method for controlling the powertrain includes operating the power source and the transmission in a desired gear to produce a first wheel torque in response to a demanded wheel torque, increasing the demanded wheel torque while turning the vehicle along a curved path, and using the electric motor to provide a second wheel torque, such that a combined magnitude of the first wheel torque produced in the desired gear and the second wheel torque is equal to or greater than the increased demanded wheel torque.

The method uses an electric motor to provide torque to the wheels during cornering, in addition to the torque that transmitted from the transmission output, and to allow the vehicle to stay in the higher gear and therefore to reduce gear shift frequency. Preferably, the electric motor may be an electric rear axle drive (ERAD) or a crankshaft-integrated starter/generator (CISG), or a combination of these.

The electric machine torque can increase the available torque to supplement the current gear torque so that the desired wheel torque can be attained while remaining in the higher gear. Shift cycle frequency is reduced and a greater range of authority is attained with the accelerator pedal while remaining in the higher gear.

Vehicle acceleration is proportional to the degree of accelerator pedal displacement as the driver tips in to the accelerator pedal when beyond the midpoint of the corner without unnecessary gear changes. This reduces the effort of the driver while cornering by enabling the driver to adjust vehicle acceleration smoothly and precisely.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first toFIG. 1, the powertrain10for a hybrid electric vehicle includes a first power source such as an internal combustion engine12, a diesel engine or a gasoline engine; an automatic transmission14producing multiple forward and reverse gear ratios; an electric machine16driveably connected to the engine crankshaft and transmission input18, such as a crankshaft-integrated starter/generator (CISG) for providing starter/generator capability; and an additional electric machine20driveably connected to a rear axle differential mechanism36, such as an electric rear axle drive (ERAD), for providing additional propulsion capability in either an electric drive or hybrid drive mode. The transmission output24is connected through a final drive unit and differential mechanism26to the front axles28,30, which drive the front wheels32,33, respectively. ERAD20drives the rear wheels34,35through ERAD gearing36, a differential mechanism36, rear axles22,23and wheels34,35.

The powertrain10comprises a first power path driveably connected to the load that includes CISG16, transmission14, final drive unit26, axles28,30and the wheels32,33. A gear of the transmission must be engaged between input18and output24and the input clutch38or39that is associated with the engaged gear must be engaged to complete a drive path between CISG16and the vehicle wheels32,33. Powertrain10also comprises a second power path driveably connected to the load that includes ERAD20, ERAD gearing48, a differential mechanism36, rear axles22,23and wheels34,35.

An electronic engine control module (ECM)24controls operation of engine12. An electronic transmission control module (TCM)27controls operation of transmission14and the input clutches38,39. An integrated starter controller (ISC)40controls operation of CISG16, ERAD20and the system for charging an electric storage battery42, which is electrically coupled to the electric machines16,20.

The undesired gear shifting is illustrated inFIG. 2with the HEV42entering a curve44while in fourth gear and having its wheel brakes applied at45. The accelerator pedal46is depressed at48as the vehicle starts to exit the curve, causing a downshift to third gear. The driver eases off accelerator pedal46at50as the vehicle10accelerates to the desired speed52. In response to this movement of the accelerator pedal46, transmission14upshifts at54to fourth gear.

If the vehicle42were entering the curve44while accelerating in third gear, an upshift to fourth gear occurs at56when the curve is entered. Thereafter, the driver would tip-out of the accelerator pedal46, potentially causing a 3-4-3-4 gear sequence through the curve. The shift hunting condition can be even more extreme if the vehicle were operating in several switch back curves (s-curves), which are frequently encountered in mountainous driving.

Gear changes in a discrete ratio transmission14are produced according to a gear shift schedule60, such as that illustrated inFIG. 3. The gear shift schedule is calibrated to schedule downshifts when the engine runs out of reserve power and upshifts as close as possible to the tractive effort crossovers. Driver demanded wheel torque is represented either by engine throttle position, for a vehicle without electronic throttle control (ETC); or by accelerator pedal position, for a vehicle with ETC. One of the criteria for shift scheduling calibration is to have good shift spacing so that the transmission does not cycle between upshifts62-64and downshifts65-67too frequently.

FIGS. 4 and 5illustrate the occurrence of multiple upshifts and downshifts as the vehicle42turns through a series of curves44. The driving pattern is a deceleration of the vehicle and a downshift67to third gear from fourth gear as it enters the curve, the wheel brakes are applied and the operating state, defined by demanded wheel torque and vehicle speed, crosses the 3-4 gear shift curve. This is followed by acceleration of the vehicle and an upshift64to fourth gear as the vehicle exits the curve when the driver tips into the accelerator pedal46.

FIG. 6shows the steps of an algorithm for preventing gear shift cycling of a HEV42while the vehicle is turning through a curving roadway44or cornering. The algorithm is executed repetitively at intervals of about 8 ms.

After entering the cornering shift control at80, a test is made at step82to determine whether the vehicle is cornering or entering a curve. Various techniques for detecting whether the vehicle is entering a curve include referencing a global position system, a steering angle sensor that produces a signal representing angular displacement of the vehicle's steering wheel from a reference position, or speed sensors that produce a signal representing the difference in speed of wheels on opposite sides of the vehicle, preferably wheels34,35that are not being driven by a power source.

If the result of test82is logically true, at step84a test is made to determine whether an electric machine16that is driveably connected to the transmission input shaft18can provide sufficient wheel torque in the current gear in additional to the wheel torque provided by the engine12in the current gear to meet or exceed the wheel torque being demanded by the driver. Step84determines whether the CISG16is able currently to produce wheel torque in the current gear that is equal to or greater than the difference between the demanded wheel torque and the wheel torque produced by the engine in the current gear.

If the result of test82is logically false, at step86a flag called “Upshift Inhibit” is cleared to allow upshifts to occur, and at step88the algorithm is terminated and control returns to step80.

If the result of test84is true, control advances to step90to determine whether a downshift is scheduled to occur. If the result of test90is false, at step92execution of the algorithm is terminated and control returns to step80.

The preferred result of the algorithm is to remain in the current gear, the higher gear as long as possible while passing through the curve. The least preferable result is to downshift since it can accelerate the vehicle beyond the current driver demand and initiate shift cycling and reduce fuel economy.

If the result of test90is true, at step94the scheduled downshift is prevented.

At step96, a check is made to determine whether the demanded wheel torque is greater than the wheel torque that can produced in the current gear by the engine12; any other power source or sources, such as CISG16, driveably connected to the input shaft; or a combination of the engine and the other power source or sources. If the result of test96is false, at step98execution of the algorithm is terminated and control returns to step80.

If the result of test96is true, at step100an electric machine that is not driveably connected to transmission input shaft18, such as ERAD20, is used to provide sufficient wheel torque in addition to the torque transmitted to input shaft18, thereby avoiding a downshift. At step102, execution of the algorithm is terminated and control returns to step80.

If the result of test84is false, control advances to step104, to determine whether a downshift is scheduled. If the result of test104is false, execution of the algorithm is terminated at step106and control returns to step80.

If the result of test104is true, the transmission14performs the scheduled downshift at step108.

At step110, the torque transmitted to input shaft18is used to produce the demanded wheel torque in the lower gear.

At step112, the flag is set to prevent an upshift from the lower gear. At step114, the algorithm is terminated and control returns to step80.

FIG. 7illustrates use of torque120produced by electric machine20in response to depressing the accelerator pedal46at48. Torque120increases the wheel torque produced in the current gear, fourth gear, by torque transmitted to transmission input shaft18in order to meet the demanded wheel torque without need for a downshift.