1. Field of Invention
The present invention relates generally to a hybrid electric vehicle, and specifically to a method and system to optimize collecting regenerative braking energy in a parallel hybrid electric vehicle (HEV) while minimizing torque disturbance to the powertrain.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and pollutants from automobiles and other vehicles powered by an internal combustion engines (ICE""s) is well known. Vehicles powered by electric motors have attempted to address these needs. However, electric vehicles have limited range and limited power coupled with the substantial time needed to recharge their batteries. An alternative solution is to combine both an ICE and electric traction motor into one vehicle. Such vehicles are typically called hybrid electric vehicles (HEV""s). See generally, U.S. Pat. No. 5,343,970 (Severinsky).
The HEV has been described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. A series hybrid electric vehicle (SHEV) is a vehicle with an engine (most typically an ICE) which powers a generator. The generator, in turn, provides electricity for a battery and motor coupled to the drive wheels of the vehicle. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) is a vehicle with an engine (most typically an ICE), battery, and electric motor combined to provide torque to power the wheels of the vehicle.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both the PHEV and the SHEV. The PSHEV is also known as a torque (or power) splitting powertrain configuration. Here, the torque output of the engine is given in part to the drive wheels and in part to an electrical generator. The generator powers a battery and motor that also provide torque output. In this configuration, torque output can come from either source or both simultaneously. The vehicle braking system can even deliver torque to drive the generator to produce charge to the battery.
The desirability of combining the ICE with an electric motor is clear. The ICE""s fuel consumption and pollutants are reduced with no appreciable loss of performance or range of the vehicle. Nevertheless, there remains substantial room for development of ways to optimize the HEV""s operational parameters. Two such areas of development are engine start/stop and regenerative braking. Engine start/stop strategies turn off the engine during times of low power demand from the driver, thereby reducing fuel usage and emission production directly.
Regenerative braking (regen) captures the kinetic energy of the vehicle as it decelerates. In conventional vehicles, kinetic energy is usually dissipated as heat at the vehicle""s brakes or engine during deceleration. Regen converts the captured kinetic energy through a generator into electrical energy in the form of a stored charge in the vehicle""s battery. This stored energy is used later to power the electric motor. Consequently, regen also reduces fuel usage and emission production. In certain vehicle configurations, the engine can be disconnected from the rest of the powertrain thereby allowing more of the kinetic energy to be converted into stored electrical energy.
Successful implementation of an efficient regen strategy must consider, among other things, the effects of ICE braking on the vehicle. In conventional vehicles, engine braking is well known and is typically characterized by two types of negative powertrain torques including engine friction and pumping losses. Both types of losses result from the engine being driven by the wheels through the powertrain. Engine friction losses result from the piston rings sliding along the cylinder walls and rotation in the bearings of the engine. Engine pumping refers to the compression of the air in each of the engine""s cylinders as the engine moves through its stroke. Engine braking allows the driver to reduce vehicle speed without applying force to the brake pedal.
Regenerative braking (regen) is known for conventional ICE vehicles in the prior art. A primitive regen system is described in U.S. Pat. No. 5,086,865 to Tanaka, et. al. In Tanaka, a regen controller decouples the engine from the vehicle""s powertrain. Based on vehicle speed and gear selection, an electromagnetic clutch couples the powertrain to a hydraulic pump/motor whereby the vehicle""s kinetic energy is transferred to a high pressure oil accumulator. The pressure can be transferred back to the powertrain during, for example, the next acceleration of the vehicle.
Regen in an HEV is also known in the prior art. In U.S. Pat. No. 5,839,533 to Mikami, et. al., a rapid response drive source brake controller for engine braking and regen is described. The Mikami controller determines the gearshift lever position manually set by the driver (e.g., low gear). The engine""s brake force (negative torque) increases as the speed ratio of an automatic transmission increases. The controller can engage both engine braking and regenerative braking if the manually selected braking exceeds the maximum regen force that can be generated by the electric generator.
Taga, et. al., U.S. Pat. No. 5,915,801, discloses a regen controller to simulate ICE braking torque. This controller disengages the engine from the powertrain via a disconnect clutch and accumulates braking energy (negative torque) in an on-board accumulator such as a generator and battery. The Taga controller improves the speed and efficiency of the regen by, for example, determining the target braking torque according to the release speed of the accelerator pedal. Thus, when large braking torque is required, the controller makes it possible to produce a large amount of regen without delay even before the brake pedal is depressed. This decreases the need for the driver to operate the manual shift lever to a lower gear or further depress the brake pedal. The controller can additionally use input from brake pedal position, vehicle speed, vehicle weight, and gradient information to determine target braking torque.
Using the Taga controller during regen, the engine may or may not be connected to the powertrain. If the engine is disconnected during regen, there is no engine friction and pumping. This allows the recapture of even more kinetic energy without exceeding the deceleration limits for the vehicle. Obviously this is advantageous for an HEV from an energy management perspective.
The tradeoff for disconnecting the engine to capture more regen energy is that with the engine disconnected, the transition back to an engine driving state becomes significantly more complex. If the engine is left connected during regen and the driver depresses the accelerator pedal, it is a straightforward process to restart the engine, if desired, simply by reinitializing fueling of the engine. Any torque disturbance to the powertrain due to the engine restarting would be small, and not completely unexpected by the driver, given the change in demand. Alternatively, if the engine is disconnected from the powertrain during regen, starting the engine would involve maintaining the vehicle""s response to the driver""s demand using the motor while simultaneously closing the disconnect clutch and starting the engine.
Torque supply to the powertrain should be transferred from the motor to the engine smoothly in order to avoid any disturbance to the driver. Nevertheless, the Taga patent, while attempting to simulate engine braking and improve vehicle drivability, does not address the common situation where a driver suddenly changes from decelerating to accelerating. It is therefore necessary to develop a strategy to keep the engine connected to the powertrain during regen if a change in driver demand (from decelerating to accelerating) is anticipated. With two modes of regen possible, it will also be necessary to transition the compression braking torque from the engine to the motor as the engine is disconnected from the powertrain in going from one mode to the other.
The present invention provides a method and system for controlling regenerative braking energy in a parallel hybrid electric vehicle. The controller: (1) determines a target braking torque based on a basic quantity which is at least one of, a driver demand and a vehicle operating status, (2) determines whether to disconnect an engine connector to a vehicle powertrain, and (3) controls a resultant increasing regenerative braking torque during an engine disconnect to minimize powertrain disturbance. Driver demand can be determined using brake pedal position and accelerator position. Operating status can include engine on status, motor fault condition, battery state of charge, transmission gear, transmission shift status, battery current sink capability and vehicle speed.
The controller also determines whether to disconnect the connecting means of the engine to the vehicle powertrain, whereby increased regenerative braking energy can be collected, and similarly controls the resultant increasing regenerative braking torque during engine reconnect to minimize powertrain disturbance. Engine disconnect factors could include determining whether a predetermined vehicle speed has been reached, whether the driver demand (e.g., brake pedal position and accelerator pedal position) indicates lower expected power demand, and whether a fault condition exists in a vehicle motor.
An important feature of this invention is that the controller minimizes powertrain disturbance during the disconnecting of the engine from the powertrain. This is accomplished by continuously adjusting the amount of regenerative braking to correspond to the changing torque of the engine on the powertrain during disconnect.