Electrical System for a Motor Vehicle

1. Field of the Invention

The present invention relates to electrical systems for motor vehicles.

2. Description of the Related Art

In city driving, a substantial amount of the fuel consumed by a motor vehicle is consumed when the vehicle is at rest or decelerating. In fact, some studies indicate that fuel economy on the "Federal Urban Drive Schedule" can be increased by up to 20% by turning off the vehicle's engine while the vehicle is stopped or decelerating. When the engine is off, vehicle electrical loads would be supplied from the vehicle's storage battery.

In a vehicle whose engine is turned off during such fuel-saving opportunities, the engine will be cranked more frequently than in other vehicles. Providing the very high-power (though relatively low-energy) electrical pulse to crank and start the engine is, however, very stressful on storage batteries. In fact, the ability of a storage battery to provide the high-power pulse begins to degrade after only a relatively few repetitions, although the overall energy capacity of the battery is not degraded. Thus, a system which would reduce the exposure of the vehicle's storage battery to such high-power engine cranking pulses will prove advantageous.

The addition of a capacitor to the vehicle's electrical system to provide the high-power engine cranking pulse has been proposed, for example, in U.S. Pat. No. 5,146,095, issued to Tsuchiya et al. This system, however, relies on relays to manage the charging and discharging of the capacitor. Other designs may be more cost-efficient and reliable.

In vehicles whose engine stops and starts frequently, for example to take advantages of fuel savings which can be realized, quick vehicle restarts are preferable to minimize annoyance to the driver of the vehicle. Such quick vehicle restarts can be facilitated by providing a particularly large amount of power to the engine cranking motor. A cranking motor operating at a higher voltage than the conventional 12 volts of a motor vehicle electrical system can provide higher power without correspondingly higher electrical currents, an advantageous situation. But, if a cranking motor operating at a higher voltage is used in a system which otherwise uses 12-volt electrical loads, a decision must be made as to the voltage at which electrical energy will be generated and stored in the system. Generating and storing energy at 12 volts will require a voltage up-converter to provide the higher voltage for the engine cranking motor. Using an up-converter to convert a relatively lower voltage to a relatively higher voltage for supply to a higher-voltage motor is known; however, high-power up-converters are very expensive.

Thus, a system which facilitates the use of a higher voltage for vehicle cranking, if desired, while not requiring a voltage up-converter will prove cost-advantageous.

The present invention provides an electrical system for a motor vehicle. The system comprises a capacitor and an engine cranking motor coupled to receive motive power from the capacitor. The system further comprises a storage battery and an electrical generator having an electrical power output, the output coupled to provide electrical energy to the capacitor and to the storage battery. Also, the system includes current-limiting limiting means for limiting current flow from the battery to the engine cranking motor.

The present invention also provides a second electrical system for a motor vehicle. The system comprises a capacitor and an engine cranking motor coupled to receive motive power from the capacitor. The system additionally comprises a storage battery and an electrical generator having an electrical power output, the output coupled to provide electrical energy to the capacitor and to the storage battery. Further, the system includes blocking means for allowing current flow through the blocking means from the output of the electrical generator to the storage battery and for preventing current flow through the blocking means from the storage battery to the engine cranking motor.

The present invention provides an advantage over the prior art by using a capacitor for engine cranking without requiring relay control of the charging and discharging of the capacitor. A high-reliability and highly cost-effective system can result. Further, the present invention facilitates systems which employ higher-voltage cranking of the vehicle, if desired, without requiring an expensive high-power voltage up-converter to provide that higher voltage.

FIG. 1 is an electrical schematic diagram of an electrical system according to one embodiment of the present invention.

FIG. 2 is an electrical schematic diagram of an electrical system according to a second embodiment of the present invention.

Referring first to FIG. 1, an electrical system for a motor vehicle is illustrated. The system includes a storage battery 12. The system further includes a starter/alternator 14. Although starter/alternator 14 is preferably a single component, a separate starter motor and alternator can be provided. Some examples of combined starter/alternators known in the art are disclosed in U.S. Pat. Nos. 4,720,638, 4,916,345, 5,001,412, 5,097,140 and 5,469,820. The disclosures of those patents are hereby incorporated by reference.

The system of FIG. 1 further includes a capacitor 16. Capacitor 16 is of sufficient energy storage capacity to provide engine cranking power to motor/alternator 14 to quickly crank an internal combustion engine (not shown) to which starter/alternator 14 is coupled. Further, to minimize the cranking time required to start the engine, starter/alternator 14 is preferably designed to operate at higher than the 12 volts which is typical in a motor vehicle. Higher power can be delivered for cranking the internal combustion engine without requiring correspondingly higher currents.

Also provided in the electrical system is a DC-to-DC converter 22. DC-to-DC converter 22 converts the relatively higher voltage stored by storage battery 12 into a relatively lower voltage (such as, for example, 12 volts) to power other electrical loads 24 on the vehicle.

The electrical system also may include further relatively-higher voltage loads 25, should the electrical system designer decide to replace some of the vehicle's electrical loads with higher voltage loads. Some of the vehicle's motors, for example, might be run more efficiently and made smaller in size if designed to operate at higher than conventional 12-volt vehicle system voltage. Higher-voltage electrical loads 25 can also include one or more electric "traction" motors adapted to help propel a "hybrid" electric vehicle. In such a case, an inverter might be provided to convert the DC energy from storage battery 12 to AC for the motors.

Further included in the electrical system are a resistor 26 and a diode 28. Resistor 26 functions to limit current which can flow from storage battery 12 to starter/alternator 14. Thus, when the internal combustion engine is to be cranked, capacitor 16 will provide essentially all of the cranking energy and will therefore bear the high-power pulse provided for cranking the internal combustion engine. Storage battery 12 is therefore spared the requirement of providing the high-power cranking pulse.

When starter/alternator 14 instead operates as an alternator to generate electrical energy when the internal combustion engine is running, diode 28 allows starter/alternator 14 to charge storage battery 12 with reasonably high efficiency. That is, losses which would occur if the charging current were to pass through resistor 26 are substantially avoided.

After providing energy for cranking the internal combustion engine, capacitor 16 is recharged by either starter/alternator 14, or if such recharging is not complete when the internal combustion engine 10 is next turned off, by storage battery 12. Electrical losses incurred by recharging capacitor 16 through resistor 26 are insignificant because capacitor 16 stores relatively little energy and because the capacitor-recharging event occurs relatively infrequently.

Thus, the electrical system of FIG. 1 advantageously allows the use of a starter/alternator 14 optimized to perform both the cranking and electrical generating functions at high voltage. Equivalently, the system allows the use of a separate starter and alternator in place of starter/alternator 14 and each designed to take advantage of operating at high voltage.

In a variation of the system of FIG. 1, FIG. 2 shows the addition of a second current-limiting resistor 30 and a second diode 32. Current-limiting resistor 30 limits, if desired, the recharging current provided from starter/alternator 14 to capacitor 16. Diode 32 allows the engine cranking energy provided by capacitor 16 to starter/alternator 14 without current limitation by current-limiting resistor 30.

It should be noted that current-limiting resistor 26, diode 28, current-limiting resistor 30 and diode 32 are all passive devices. Thus, the systems according to the embodiments of FIGS. 1 and 2 can be very cost-effective. Alternatively, the functions of some or all of those components can be performed by "active" components, such as transistors, to better optimize performance of the various functions.

Various other modifications and variations will no doubt occur to those skilled in the arts to which this invention pertains. Such variations which generally rely on the teachings through which this disclosure has advanced the art are properly considered within the scope of this invention. This disclosure should thus be considered illustrative, not limiting; the scope of the invention is instead defined by the following claims.