Patent Application
20100076634
1. Field of the Invention
The invention relates to an operating strategy for a micro-hybrid electric vehicle using an engine start and stop sequence to obtain optimum fuel economy.
2. Background Art
Known hybrid electric vehicle powertrains typically include separate power flow paths from an electric machine power source and from a mechanical power source, such as an internal combustion engine. Transmission gearing distributes power from the separate power sources to vehicle traction wheels. The electric machine may act as a motor or as a generator. When it acts as a generator in a power regenerative mode, mechanical inertia energy may be distributed to the generator, which converts it to electric energy to charge the battery. The electric machine acts as a generator, for example, to charge a powertrain battery during engine braking.
Since the engine is mechanically coupled to the generator, the generator may act as a motor to start the engine. When the generator and starter are combined into one machine, the combination is often referred to as an integrated starter-generator system (ISG). In a conventional vehicle powertrain, the starting and generating functions are accomplished separately by two electrical machines. This is a separated starter-generator system (SSG). The term starter-generator will be used hereinafter to designate either.
Because of the dual function of a starter-generator in known hybrid electric vehicle powertrains, as it develops regenerative power under some operating conditions and electric motive power under other operating conditions, the size, cost and weight of a starter-generator in known hybrid electric vehicle powertrains may not be suitable in certain vehicle applications.
It is an objective of the present invention to use a so-called micro-hybrid electric vehicle architecture to reduce size, cost and weight of a hybrid electric vehicle powertrain without significantly affecting the operating characteristics of the powertrain. A micro-hybrid electric vehicle (HEV) powertrain can be defined as any HEV system with a kilowatt capacity less than approximately 3 kw at 12 volts that can stop and start the engine of the powertrain using a starter-generator. A micro-HEV using the method of the invention has less than all of the available functions of a conventional full-hybrid electric vehicle powertrain. For example, it does not provide electric vehicle launch torque nor full regenerative power. The functions that are used include an engine stop and start function while providing only a minimal engine power assist and a minimal regenerative energy recovery (e.g., <3 kw).
Because of the power limitations of a micro-HEV using the control method of the invention, the starter-generator results in a fuel economy benefit associated primarily with an engine stop and start function, which turns off the engine during engine idle when the vehicle is at rest. At that time the engine is not required to provide motive power. This function may be of more significance than a regenerative braking function for energy recovery.
If the power requirement of the hybrid electric vehicle is approximately 10 kw, for example, a fuel economy benefit (e.g., an EPA metro-highway fuel economy benefit) for the micro-hybrid powertrain of the invention due to the engine stop and start function may be as high as approximately 5%. Any braking energy recovery benefit, on the other hand, would be in addition to this fuel economy benefit, and would vary between 1% and 5%. If the regenerative braking capability of the micro-HEV would increase, the cost-benefit ratio would decrease relative to that of a conventional hybrid electric vehicle powertrain.
The micro-HEV control method of the present invention will shut off the engine and disconnect the engine for all braking events rather than using the starter-generator to collect braking energy. The engine will be disconnected from the traction wheels using the neutral gear of a power-shift, multiple-ratio automatic transmission, which is part of the vehicle powertrain. Using the design approach of the present invention, the starter-generator need only be sized for rapid and warm engine starts, rather than braking energy recovery. Due to the absence of a full regenerative mode during braking, the battery can be made smaller and less expensive than a battery for hybrid electric vehicle powertrains with more regenerative energy recovery ability.
During operation of a micro-HEV using the method of the invention, the driver may lift his or her foot off the brake pedal when the vehicle is warm and at a rest, and the starter-generator can be used to start the engine. The engine then will provide all of the driver requested power. When the vehicle is warm and moving, and the driver actuates the brake pedal, the transmission, under the control of a transmission control unit, will shift to a neutral gear. Simultaneously, the engine is stopped.
When the vehicle is moving and the driver lifts his or her foot off the brake pedal, the starter-generator will start the engine and the transmission will shift from neutral to the desired gear determined by a transmission control unit. The engine speed is synchronized to provide the driver with requested power.
When the vehicle again slows to rest and the driver presses the brake pedal, the transmission will be shifted into neutral gear and the engine will be stopped. At that time, if the vehicle is to be restarted and the engine is warm, the starter-generator will restart the engine as previously described. In the case of a cold start, a conventional starter motor can be used, and the engine may remain running until it is warm.
A starter motor, schematically shown at 22, under the control of a low voltage battery, not shown, can be used to start engine 10 under cold start conditions. An electronic throttle control for the engine 10 is shown at 24 in block diagram form.
The engine 10 is drivably connected to a crankshaft pulley, which drives a belt-driven starter-generator unit 26 in the exemplary embodiment of the invention disclosed herein. Although a belt-drive is disclosed, a driving connection between the engine and the starter-generator 26, other types of drives could be used. For example, a flexible chain drive or a geared drive could be used, depending on design choice. The starter-generator 26 is electrically coupled to a voltage source, such as a low voltage battery 28 or a high voltage battery 54. The high voltage battery 54 may be connected to the starter-generator 26 through a DC/AC inverter 30. Hybrid vehicle accessories, such as an air conditioning compressor 34, a fuel pump 36 and a power steering pump 38, which may be electrically powered by low voltage battery 28, also are illustrated in
A powertrain microprocessor controller 40 may include an input/output signal portion 42, a central microprocessor unit 44, a random access memory section 46 and a read-only memory section 48. Controller 40 may be of conventional design for controlling a transmission control unit 50 and a battery control module 52, which is electrically coupled to the high voltage battery 54.
In the case of a conventional hybrid electric vehicle powertrain, a large motor is provided to provide driver-requested torque when the electric motor is the sole driving power source. This electric torque is not available in the micro-hybrid electric vehicle of the present invention. Thus, the strategy of the present invention will quickly and smoothly restart the engine using only the starter-generator, while simultaneously re-engaging the engine by terminating the neutral state of the transmission.
A typical time required for the driver to move his or her foot from the brake pedal to the accelerator pedal may be about 0.2 seconds. If the engine is restarted in this time interval, the driver will not be able to feel any torque deficit. Any additional torque delay can be suitably calibrated so that the driver will not experience a driving “feel” that is significantly different from a comparable “feel” due to a turbo lag in a turbo powered engine.
In calibrating the time required for engine starts, the transmission controls should allow enough engagement time for transmission friction clutch and brake during a change from neutral to a targeted restart gear. The targeted engine restart time may be about 0.6 seconds from the engine start command to the instant when the engine delivers 80% of the requested torque. The neutral-engage portion of this target time may be about 0.4 seconds. A front, electrically-driven auxiliary pump in the transmission may be used if needed to keep the transmission clutches and brakes primed when the engine is off.
The engine control may also control a fueling strategy in order to meet emissions targets so that there will be no flow across the catalyst in an engine catalytic converter when the engine is off. Since there will be no gas flow across the catalyst, there will be no oxygen loading, and the catalyst temperature should remain high during short shut-off intervals.
The duration of the engine start delay can be calibrated. If the delay is perceptible to the driver, a more aggressive deceleration engine shut off could be selected by the driver using a driver-controlled high fuel economy switch, which could be located in the driver's compartment.
After the engine is on, as determined at step 78, the routine will determine whether the brake pedal is on or off, as indicated at decision block 80. If the brake pedal is off, the routine will not continue and the engine will remain on. If the brake pedal is on, it is determined whether driver intent to stop criteria meet precalibrated threshold values, as indicated at decision block 82. One threshold value may be a speed threshold, which is calibrated. In the alternative, a time threshold can be used, either with the speed threshold or independently of the speed threshold. The time threshold would be a calibrated time period within which the brake pedal would be depressed. If the brake pedal has been depressed by the driver for a time greater than the time threshold, that would indicate a driver's intent to stop or decelerate. The control routine then will confirm that the strategy should continue and cause the engine to stop. An incidental brake pressure increase that does not indicate the driver's intent to stop or decelerate will not cause the engine to stop. Other possible thresholds that can be used to confirm the driver's intent to stop are a calibrated brake pedal travel, a calibrated brake pedal force or a calibrated brake fluid brake line pressure. These thresholds can be used independently, or more than one can be used together.
If the threshold criteria at step 82 are satisfied the routine to continue, and the engine is stopped, as shown at 84. The transmission, under the control of the transmission control unit 50, simultaneously will shift the transmission 12 into neutral, thereby disconnecting the crankshaft of the engine from the torque input elements of the automatic transmission 12.
In executing the control functions, the vehicle system control, as shown at 40 in
Although an embodiment of the invention has been disclosed, it will be apparent to persons skilled in the art that modifications may be made without departing from the scope of the invention. All such modifications and equivalents thereof are intended to be covered by the following claims.
