Patent Application
20130218408
The present disclosure relates to improving fuel economy by optimizing electrical energy generation and consumption.
Presently, fuel economy benefits related to the control of electrical energy generation and consumption are limited to discretized methods that rely on particular operating conditions of a vehicle, such as load level, deceleration, cruising, and the like. Primarily, these discretized methods are based on the principles of minimizing unnecessary alternator operation and taking advantage of vehicle kinetic energy during deceleration. Moreover, batteries are normally kept in a charging state, regardless of efficiency and state of charge.
These approaches focus on the efficiency of individual subsystems, and do not consider the efficiency of the overall system; that is, the relationship between the engine, alternator and battery. A new approach that maximizes overall system efficiency and examines both electrical energy generation, storage and engine operation to improve fuel economy is therefore warranted.
In various example embodiments, the present disclosure provides methods and apparatus for improving vehicle fuel economy using an energy storage efficiency model. Broadly, the disclosure proposes a management strategy to control electrical energy generation and consumption in a vehicle so as to improve fuel economy. When the vehicle is operating under conditions that electrical energy can be produced and stored efficiently, the generator will be fully engaged, even if demand is not high. The excess electrical energy is stored for later consumption. An efficiency model takes into consideration engine efficiency, generator efficiency, and energy storage efficiency. To fully take advantage of high efficiency conditions and ensure smooth operation of the vehicle, the level of state of charge of electrical energy storage is constantly monitored and carefully controlled.
The efficiency model used herein is based on the operational efficiencies of the generator (typically an alternator), energy storage medium (typically a battery), and engine, within the constraint of maintaining a minimum required state of charge in the energy storage medium. Both the generator and energy storage medium have their own “most efficient” operating conditions. For instance, turning the generator on and off effectively changes the engine operating point, thus altering engine operation efficiency. Vehicle fuel economy ultimately depends on the overall system efficiency, not directly on its individual subsystem efficiencies.
One such embodiment includes a method for improving fuel economy in a vehicle having an engine, a generator driven by the engine, and an energy storage unit by using an energy storage efficiency model. The method includes evaluating a current efficiency state of the engine, evaluating a current efficiency state of the generator, and evaluating a current efficiency state of the energy storage unit. Based on the results of the evaluations and the energy storage efficiency model, the generator is controlled to provide an optimized voltage output. The method further includes determining whether the vehicle is decelerating, and if so, maximizing voltage output so as to accumulate maximum charge in the energy storage unit. A voltage regulator implementing this method is also described.
Evaluating the current efficiency state of the engine generally includes determining whether an increase in generator load will result in a positive or negative change in efficiency, given a current torque output of the engine. This can further include using an efficiency curve to determine whether an increase in generator load will result in a positive or negative change in efficiency. Evaluating the current efficiency state of the generator includes determining generator rotor speed and field current. Evaluating the current efficiency state of the energy storage unit includes determining a current regulator output voltage, a state of charge of the energy storage unit, and a temperature of the energy storage unit.
One exemplary energy storage efficiency model provides that voltage output will be set sufficiently high so as to accumulate charge in the energy storage unit if: a) a state of charge of the energy storage unit is below a minimum threshold, b) the efficiency state change of the engine due to the generator engagement is positive, the efficiency states of the generator and energy storage unit are high, and the state of charge of the energy storage unit is below an overcharge threshold; or c) the efficiency state change of the engine is positive, the efficiency state of the generator is high, and the state of charge of the energy storage unit is below a midpoint threshold. The efficiency model further provides that voltage output will be set sufficiently high so as to maintain a state of charge in the energy storage unit if a) the efficiency state change of the engine is positive, the efficiency states of the generator and energy storage unit are high, and the state of charge of the energy storage unit is above an overcharge threshold; or b) the efficiency state change of the engine is positive and the efficiency state of the generator is high and the efficiency state of the energy storage unit is low.
Further areas of applicability of the present disclosure will become apparent from the detailed description and claims provided hereinafter. It should be understood that the detailed description, including disclosed embodiments and drawings, are merely exemplary in nature intended for purposes of illustration only and are not intended to limit the scope of the invention, its application or use. Thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention.
As noted above, one overall objective of the methods disclosed herein is to improve the electrical energy generation, storage and consumption efficiency of the vehicle 10. This is accomplished using an energy storage efficiency model, as will be described in more detail below with reference to
For instance, an energy storage efficiency model can automatically maximize generator 13 output during vehicle 10 deceleration, so as to recoup kinetic energy generated by vehicle 10 as it slows. This allows energy storage unit 15 to be charged at a maximum possible level. Of course, the state of charge of the energy storage unit 15 needs to generally be maintained at less than 100% in order to store electrical energy generated during deceleration. Accordingly, during normal vehicle 10 operation, the energy storage unit 15 is maintained at a state of charge of less than 100%. Within vehicle 10, other electrical energy consumers can also be instructed to fully take advantage of this ‘free’ energy without compromising performance and functionality. For instance, a cooling fan of engine 12 might increase output RPMs during deceleration. Other examples of subsystems that can take advantage of deceleration include vehicle 10's climate control subsystem.
As to evaluating efficiency states of the various components of the vehicle 10, in order to evaluate the current engine 12 efficiency state, a torque efficiency curve, such as that depicted in
As noted above,
When the state of charge of the energy storage unit 15 is low and close enough to a critical state (Smin), constraining generator 13 engagement to be within the range of Tpeak and Tactual may not allow generator 13 to generate enough output to maintain the current state of charge of energy storage unit 15. Accordingly, rather than letting the state of charge of energy storage unit 15 decrease to the critical state and then charging energy storage unit 15 unconditionally, increasing the torque increment range to twice the difference between Tpeak and Tactual permits maintaining the state of charge without much compromise of engine 12 operation efficiency. Specifically, since the efficiency curve as compared with torque at a given engine speed is generally smooth, it is reasonable to assume some symmetry of the efficiency curve around Tpeak, especially within a relatively small range of Tpeak. Accordingly, engaging generator 13 can also be based, at least in part, on whether engine 12 efficiency will be within this established range, when generator 13 is active.
In such determinations, the engine 12 efficiency change due to alternator load can be determined as a function of the current engine 12 torque, the range of Tpeak and Tactual, and the increase in torque associated with activating the generator 13.
Another component to be evaluated for efficiency is generator 13. The efficiency state of generator 13 is primarily a function of rotor speed and field current. The rotor speed generally has a fixed ratio over the engine speed and the field current is proportional to the output duty cycle rate. The field current directly determines the additional torque applied to the engine and is a function of generator 13's target output voltage, energy storage unit 15's open circuit voltage, total electrical load, and state of charge. Lowering generator 13's target output voltage without changing energy storage unit 15's open circuit voltage, total electrical load, and state of charge does not result in a significant torque reduction.
Energy storage unit 15 should also be evaluated for its efficiency state, which is generally a function of generator 13's output voltage, energy storage unit 15's state of charge, and energy storage unit 15's temperature.
