Abstract:
This invention relates to plug-in hybrid propulsion systems where the energy storage element of the hybrid drive train may be charged with externally supplied electricity as well as energy from the engine or regenerative braking. The invention is a plug-in hybrid system with a fast energy storage and delivery system. In a preferred embodiment the invention comprises a fuel powered engine, a battery, a fast energy storage system, power converters, controllers, drive motors, an electrical distribution system, and a drive train. Additionally, the invention relates to plug-in hybrids that provide services to the electrical utility when the vehicle is connected to the utility grid.

Description:
FIELD OF INVENTION  
       [0001]     This invention relates to plug-in hybrid propulsion systems for cars, trucks, and buses where the energy storage element of the hybrid drive train may be charged with externally supplied electricity as well as energy from the engine or regenerative braking.  
         [0002]     In particular, the invention relates to plug-in hybrids that can provide services to the electrical utility when the vehicle is connected to the utility grid.  
       RELATED ART  
       [0003]     Existing products commonly found in the marketplace include conventional hybrid vehicles such as the Toyota Prius, Honda Insight, and Ford Escape Hybrid. More closely related to the present invention are prototype Daimler Chrysler Sprinter plug-in hybrid vans.  
         [0004]     The main problem with existing conventional hybrids is that the full potential of conventional hybrid propulsion is not realized due to limited all-electric travel range. This problem is addressed by increasing the energy storage capacity of the vehicle to allow for greater all-electric range. The energy storage system may be charged from an external supply to offset and reduce fuel consumption. This type of propulsion system is known as a plug-in hybrid.  
         [0005]     The main problems with existing plug-in hybrid vehicles are the cost of batteries and limited battery life. While conventional hybrid propulsion sized for a car uses energy storage with a capacity of about 1 kWh, plug-in hybrid cars require energy storage exceeding 5 kWh. The plug-in hybrid battery system must be deeply cycled in order to obtain useful all-electric range within a reasonable physical weight and volume. Deep cycling reduces battery life. The plug-in hybrid battery is much larger and proportionately more expensive than the battery in a conventional hybrid vehicle. Therefore, the cost penalty associated with a plug-in hybrid is more severe than for a conventional hybrid.  
         [0006]     An additional problem with batteries used for hybrid vehicles is that maximizing available energy storage requires constraining such environmental conditions of the batteries as temperature. Conditioning of the environment of the battery is required to attain tolerable durability.  
         [0007]     Some electric power grids in the U.S. are operated not by utilities but by separate regional entities called Independent System Operators. For example, the California Independent System Operator (CAISO) controls the grid for the entire State of California. An ISO purchases a variety of ancillary services that are used to regulate power flows, voltage stability and frequency on the grid. These include:  
         [0008]     peak power  
         [0009]     baseload power  
         [0010]     spinning reserves  
         [0011]     regulation services  
         [0012]     backup power  
         [0013]     renewable energy time shifting  
         [0014]     Regulation service is necessary because the electric load on the grid is constantly changing as people turn on or off appliances, motors and machinery. This constantly changing load leads to small voltage imbalances at various locations on the grid. Changing load can also cause small changes in generator speed leading to frequency variations above and below 60 Hz. Since motors, clocks, computers and most other devices require constant electric frequency to operate accurately, utilities maintain a 60 Hz frequency within specified tolerances. Deviations from the 60 Hz frequency are called Area Control Error (ACE).  
         [0015]     Regulation service is provided today by small, frequent adjustments to the output of each power plant. Computer models use frequency measurements throughout the grid to determine which power plants should generate more and which should generate less in order to minimize ACE at all points on the grid. Often one power plant will be providing Regulation Up service at the same time that another is providing Regulation Down service.  
         [0016]     In California, regulation service is provided partly by the electric utilities (self-provided regulation) and partly by merchant power plants that are paid for their services by the ISO. A power plant cannot operate at its maximum output level if it is committed to offering Regulation Up service. That is because it must be prepared to make further upward adjustments in generation to boost voltage if called upon. In California, regulation service providers must be capable of changing their output within 10 minutes of being asked to do so and must be able to stay online at the required output level for at least two hours.  
       BRIEF SUMMARY OF THE INVENTION  
       [0017]     The invention is a hybrid electric vehicle powertrain comprising an internal combustion engine; a first electric motor-generator connected to the engine and used to start the engine or supply electricity to the second motor-generator; a second electric motor generator that supplies traction power to the vehicle wheels, a first electrical energy storage device, a second electrical energy storage device, a power electronics system, a control system, and a charging system.  
         [0018]     The first electrical energy storage device is a battery that delivers or absorbs electrical energy when the vehicle is operated as a hybrid or when the vehicle is operated using stored electrical energy only.  
         [0019]     The second electrical energy storage device is a flywheel, a capacitor or ultracapacitor or supercapacitor, or a battery that absorbs or delivers current only as necessary to protect the first energy storage device from current above the damage threshold for the first energy storage device.  
         [0020]     A power electronics system, responsive to the control system, transfers electrical energy from each electrical energy storage device or electric motor-generator to each other electrical energy storage device or electric motor-generator.  
         [0021]     The control system has means to determine and control the energy flow path through the power electronics. The charging system uses externally supplied electricity to recharge either or both the first energy storage device and the second energy storage device.  
         [0022]     The system comprises (contains at least) either a fuel powered engine or a fuel cell, a battery, a fast energy storage system, power converters, controllers, drive motors, an electrical distribution system, and a drive train.  
         [0023]     One of three devices can be used for fast energy storage. (1) A flywheel apparatus comprises a rotor, a motor-generator, bearings, a housing, a power converter and controller, and ancillary subsystems. (2) A small battery optimized for high cycle life. (3) A super-capacitor bank comprises a number of electrostatic energy storage components.  
         [0024]     In broad terms, a preferred embodiment of the apparatus comprises a battery pack, a fast energy storage device, an engine, a transmission, power electronics, and controls. A preferred embodiment of the method comprises use of the fast energy storage device to perform short, frequent, high intensity charge and discharge functions to preserve the battery to provide average power for driving in electric-only mode.  
         [0025]     The power electronics package comprises a number of power conversion devices to manage the flow of power between the various subsystems. One conversion device is used for grid interface. Associated with the grid interface power electronics are a controller to manage the two-way flow of power between the vehicle and the grid which may include a device for communicating with a utility, independent system operator, aggregator of services, or other relevant entity.  
         [0026]     The grid interface system may comprise some or all of the following elements:  
         [0027]     GPS-based vehicle location sensing device  
         [0028]     Two way data communications  
         [0029]     Charge/discharge control unit  
         [0030]     “Plug” or hookup device capable of bi-directional power flows  
         [0031]     External (garage-installed) charger for bi-directional power flows  
         [0032]     Charger(s) at workplace parking lots and garages  
         [0033]     A purpose of the invention is to provide a plug-in hybrid drive-train system that will yield at least a 150,000 mile durability for a passenger car in ordinary use.  
         [0034]     A second purpose of the invention is to combine the battery and a separate fast energy storage element of a plug-in hybrid to provide services to the electric distribution grid when the vehicle is connected to the grid where Vehicle to Grid (V2G) systems allow the electric power grid to benefit from many small power sources connected to it at random and dispersed locations. Regulation services have the best potential to use the capabilities of a vehicle-based energy storage system to add value to the grid but other ancillary services may be comparably beneficial. Ancillary services provided by a dispersed fleet of vehicles may be cheaper and more effective than regulation services provided by power plants today. A large population of V2G vehicles would be able to perform frequency stabilization by sourcing or sinking energy pulses thereby mitigating the need for frequent adjustment of power plant output. With respect to V2G applications, the main problem with existing conventional hybrids is that providing ancillary services to the electrical distribution system is likely to entail a sufficient number and depth of charge-discharge cycles to degrade the performance of the battery and reduce battery life. This problem is exacerbated when cycling involves high current operation at a low state of charge.  
         [0035]     A third purpose of the invention is to provide distributed storage for non-firm sources of electricity such as wind. Wind patterns in some parts of the world (the American Midwest and West Texas for example) are such that there is more wind at night than in the daytime. A large population of V2G vehicles would be able to absorb non-dispatchable, off-peak wind generation to charge their batteries during off-peak hours when energy is inexpensive.  
         [0036]     One advantage of the invention is a reduced number of charge discharge cycles of the battery with correspondingly increased battery life. This is accomplished by using a fast energy storage system that has a cycle life exceeding battery cycle life by at least 10× and sizing the fast energy storage system to source or sink brief, frequent pulses and provide most or all of the V2G ancillary services.  
         [0037]     A second advantage of the invention is that fast energy storage reduces the life-cycle cost of a battery. Without fast energy storage, the battery will experience a large number of shallow cycles and occasional high current pulses when the battery is at a low state of charge. The invention reduces the number of shallow charge cycles that the battery would experience by as much as 90%-100% and protects the battery from high current pulses. In particular, the fast energy storage system protects the battery from the deleterious effects of high rate discharge while at a low state of charge. By doing so, the invention extends life of the battery so that replacement is not required for the life of the vehicle.  
         [0038]     A third advantage of the invention is reduced total weight of the energy storage system. The combined weight of the battery and fast energy storage device are less than the weight of a battery sized to handle frequent cycling and high current pulses.  
         [0039]     A fourth advantage of the invention is improved durability of the energy storage system.  
         [0040]     A fifth advantage of the invention is that fuel economy with respect to conventional vehicles is improved. The invention enables as much as 2× improvement in fuel efficiency and an improvement in fuel economy that, depending on the driving cycle, may be as much as 5× for the typical driver. Current hybrids provide a 30-50% improvement in fuel economy.  
         [0041]     The invention requires integration of electrochemical, electrostatic, and electro-kinetic storage technology. The apparatus and its function have application as a distributed energy system.  
         [0042]     Further aspects of the invention will become apparent from consideration of the drawings and the ensuing description of preferred embodiments of the invention. A person skilled in the art will realize that other embodiments of the invention are possible and that the details of the invention can be modified in a number of respects, all without departing from the inventive concept. Thus, the following drawings and description are to be regarded as illustrative in nature and not restrictive. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]     The features of the invention will be better understood by reference to the accompanying drawings that illustrate presently preferred embodiments of the invention. In the drawings:  
         [0044]      FIG. 1  is a drawing of the plug-in hybrid system.  
         [0045]      FIG. 2  is a drawing of a plug-in hybrid system with clutches.  
         [0046]      FIG. 3  is a drawing of a plug-in parallel hybrid system.  
         [0047]      FIG. 4  is a drawing of a plug-in series hybrid system.  
         [0048]      FIG. 5  is a drawing of a plug-in series hybrid system with four-wheel drive.  
         [0049]      FIG. 6  is a drawing of a plug-in series hybrid system with a fuel cell.  
         [0050]      FIG. 7  is a drawing of an electronic switch assembly.  
         [0051]      FIG. 8  is a schematic drawing of the power converter and controller and attached devices for a system using three motor/generators and a capacitor fast energy storage system.  
         [0052]      FIG. 9  is a schematic drawing of the power converter and controller and attached devices for a system using three motor/generators and a flywheel fast energy storage system. 
     
    
       [0053]     The following reference numerals are used to indicate the parts and environment of the invention on the drawings: 
     1  Engine      2  First motor/generator      3  Second motor/generator      4  Transmission      5  Clutch      6  Power converter and controller      7  Battery      8  Fast energy storage (flywheel, high cycle battery, or capacitor)      9  Differential      10  Axle      11  Wheel      12  Plug/disconnect      13  Stationary V2G interface      14  Grid      15  Driveshaft      16  Fuel Cell      17  switch module      18  DC bus      19  DC bus common      20  DC bus capacitor      21  inductor      22  transformer      23  AC link      24  charger input/output      25  DC supply      26  flywheel fast energy storage      27  gate drive      28  controller      29  diode      30  insulated gate bipolar transistor (IGBT)      31  rear motor/generator (MGR)    
 
       DETAILED DESCRIPTION OF THE INVENTION  
       [0085]     Referring to  FIG. 1 , a preferred embodiment of apparatus is disclosed. The invention comprises an engine  1  connected to a transmission  4 . The engine  1  may be connected through a clutch or may be connected through fixed gears or shafting. A first motor/generator  2  is connected to the shaft either on the engine  1  or the transmission  4  side of the engine/transmission interface. The transmission  4  transfers power from the engine  1  and first motor/generator  2  through the transmission  4  to the driveshaft  15 , differential  9 , and then the axle  10  and the wheels  11 . A second motor/generator  3  is connected to the transmission  4  at a point in the transmission  4  closer to the output. The drivetrain may use zero, one, two, or three clutches to selectively disengage the engine  1  or an individual motor/generator  2 ,  3 .  
         [0086]     The drivetrain comprises the driveshaft  15  and differential  9 , which both may be part of the transmission  4  or separate from it, and the axle  10  or split shaft.  
         [0087]     The motor/generators  2 ,  3  may be mounted in-line with the drive train or may be connected to the drivetrain through gears, belts or chains, or hydraulics.  
         [0088]     The battery  7  preferentially uses lithium chemistry but may also use NiMH, NiCAD, or Pb:acid. The fast energy storage device  8  may comprise a flywheel, a capacitor, or a high power battery. Preferentially, the flywheel uses a high-speed rotor housed in an evacuated chamber and integral electronics to drive the flywheel. Alternatively, the flywheel may be of any type that incorporates a rotor and built in motor/generator so that energy may be stored and retrieved electrically. The capacitor may be of any type including super capacitors, ultra capacitors and electrolytic capacitors. The fast energy storage device  8  may have an energy storage capacity that is considerably smaller than the capacity of the battery  7 .  
         [0089]     In the preferred embodiment, the engine  1  is a small piston engine  1  fueled with gasoline. Alternatively, the engine  1  may be an internal combustion engine  1  fueled with gasoline, ethanol, flex-fuel, diesel fuel, bio-diesel, natural gas, propane, or hydrogen.  
         [0090]     A fast energy storage device  8  and a battery  7  are connected to a power converter and controller  6 . The power converter and controller  6  directs the flow of energy between the flywheel or capacitor  8 , the battery  7 , the first motor/generator  2  and the second motor/generator  3 . All the elements that store or use electricity ( 2 ,  3 ,  7 , and  8 ) may either source or sink electricity. The power converter and controller  6  may comprise a single assembly or subassemblies. The subassemblies may be collocated within a single module or they may be housed as separate modules. They may be located together or dispersed throughout the vehicle.  
         [0091]     The interface between the vehicle and the grid may comprise a plug and receptacle  12  where AC power to the vehicle is converted to DC power on-board. The AC supply may by 110V, 220V, 480V single or three phase or other commercially supplied AC electricity. Alternatively, a stationary V2G interface  13  that has bi-directional power handling capability may provide V2G service. The stationary V2G interface  13  may communicate with a utility in order to dispatch the V2G resource or to allow isolation by the utility. The stationary V2G interface  13  may connect to the vehicle through a DC or AC link and a plug/receptacle  12 .  
         [0092]     Operation of the invention involves driving in a number of different modes of operation.  
         [0093]     In the first driving mode, the engine  1  supplies motive power to the wheels  11  and the first motor/generator  2  and the second motor/generator  3  are free to rotate but are not energized.  
         [0094]     In the second driving mode of operation, the engine  1  is turned off and all motive power is provided using either the first motor/generator  2  or the second motor/generator  3  or both. This mode is referred to as EV mode. In this mode, electrical energy is supplied by either the fast energy storage device  8  and the battery  7  in a combination determined by the power converter and controller  6 .  
         [0095]     In the third driving mode of operation, motive power is provided by both the engine  1  and either the first motor/generator  2 , the second motor/generator  3  or both motor/generators. In this mode, electrical energy is supplied by either the fast energy storage device  8  and/or the battery  7  in a combination determined by the power converter and controller  6 .  
         [0096]     In the fourth driving mode of operation, the vehicle is decelerating or descending on a grade and energy is recovered regeneratively. In this mode of operation, retarding torque is applied to the transmission  4  by either the first motor/generator  2 , the second motor/generator  3  or both motor/generators. In this mode of operation, one or both motor/generators functions as generators and convert recovered kinetic energy of the vehicle into electricity. The electricity is delivered to either the fast energy storage device  8  or the battery  7 . The flow of electricity to the energy storage devices is directed by the power converter and controller  6 . In this mode of operation, the engine  1  may be rotating or not rotating.  
         [0097]     In the fifth driving mode of operation, the engine  1  drives the first motor/generator  2  so that it produces electricity to charge the battery  7  or the fast energy storage device  8  or both in a combination determined by the power converter and controller  6 . In this mode, the vehicle may be either stopped or moving.  
         [0098]     Performing V2G services involves a number of different V2G modes of operation.  
         [0099]     In the first V2G mode of operation, the grid sources energy through the stationary V2G interface  13  to the plug/receptacle  12  and subsequently the power converter and controller  6 . The controller  6  uses this energy to charge either the fast energy storage device  8  or the battery  7  or both.  
         [0100]     In the second V2G mode of operation, energy from the fast energy storage device  8  is extracted by the power converter and controller  6  and supplied to the grid  14  via the plug/receptacle  12  and the stationary V2G interface  13 .  
         [0101]     In the third V2G mode of operation, energy from battery  7  is extracted by the power converter and controller  6  and supplied to the grid  14  via the plug/receptacle  12  and the stationary V2G interface  13 .  
         [0102]     In the fourth V2G mode of operation, energy from engine  1  is converted to electricity by either or both of the motor generators  2  and  3  and is then extracted by the power converter and controller  6  and supplied to the grid  14  via the plug/receptacle  12  and the stationary V2G interface  13 . Any of the modes of V2G operation may be commanded automatically by software residing in the vehicle or may be commanded by an outside entity such as a utility, an independent system operator, an aggregator of services, or any other end user.  
         [0103]     The fast energy storage device  8  is tolerant of frequent cycling and high power operation while the battery  7  is not. In all modes of operation, the power converter and controller  6  typically directs the flow of energy such that the number of charge and discharge events experienced by the battery  7  is minimized. Additionally, the fast energy storage device  8  is operated to minimize the magnitude and extent of high power operation of the battery  7 . By protecting the battery  7  from excessive cycling and excessive high power operation, several benefits accrue. The durability of the combined energy storage system is improved compared to using a battery  7  without a fast energy storage device  8 . The battery  7  may be operated over a deeper depth of discharge than would otherwise be possible without the protection of the fast energy storage device  8 . Thus a given all-electric range of travel can be attained with a much smaller battery  7  than would be possible without the protection of a fast energy storage device  8 .  
         [0104]     Many other modes are possible where the functions of the five defined modes are used in combination.  
         [0105]     Many variations of the invention will occur to those skilled in the art.  
         [0106]     Referring to  FIG. 2 , the first variation uses one or more clutches  5  to selectively disengage the engine  1 , the first motor/generator  2  or the second motor/generator  3 .  
         [0107]     Referring to  FIG. 3 , a second variation is the parallel configuration in which only one motor/generator (the first motor/generator  2 ) is used. The transmission  4  may be an automatic or manual transmission that may include zero, one, or two clutches  5 .  
         [0108]     In either the first variation or the second variation, the transmission  4  may support two-wheel drive as shown. Alternatively, the transmission  4  may be capable of full time or part time four-wheel drive.  
         [0109]     Referring to  FIG. 4 , the third variation eliminates the transmission  4  entirely. In this case, the first motor/generator  2  is connected directly to the engine  1 . The first motor/generator  2  functions primarily as a generator but may also function as a motor that could be used to start the engine  1 . The second motor/generator  3  powers the wheels  11  directly or indirectly. The second motor/generator  3  is connected to the differential  9 , the driveshaft, or the wheels  11  directly. The second motor/generator  3  may be connected through fixed gearing or other compact and limited drivetrain components or subassemblies. All motive power is transmitted from a point of point of generation or storage to the drive motor electrically. This configuration is a series plug-in hybrid or a series hybrid.  
         [0110]     Referring to  FIG. 5 , a fourth variation uses multiple drive motors instead of a single second motor/generator  3 .  
         [0111]     Referring to  FIG. 6 , a fifth variation uses the series hybrid configuration from above and uses a fuel cell to generate electricity. The fuel cell replaces the engine  1  and the first motor/generator  2 . In this variation, a fast energy storage device  8  and a battery  7  are connected to a power converter and controller  6 . The power converter and controller  6  directs the flow of power between the fast energy storage device  8 , the battery  7 , and the second motor/generator  3 . All the elements that store or use electricity ( 3 ,  7 , and  8 ) may either source or sink electricity. In this variation the fast energy storage device  8  protects the battery  7  from severe or frequent charge and discharge events. Additionally, in this configuration, the fast energy storage device  8  protects the fuel cell  16  by providing immediate power for acceleration where the fuel cell has poor throttle response and could be damaged by such an event.  
         [0112]      FIGS. 7, 8  and  9  disclose the details of the power converter and controller  6 .  FIG. 7  shows nomenclature for a switch  17  comprising a diode  29  and a solid-state switching device  30 . Preferentially, the solid-state switching device  30  is an insulated gate bipolar transistor (IGBT) although other switching devices may be used. The switch  29  is commanded to open or close through signals from the controller to the gate drive  27 .  
         [0113]      FIGS. 8 and 9  indicate the controller  28  that issues commands to each switch  29  in the system. For clarity, only a few representative connections are shown. In practice, all switches  29  receive input from the controller  28 . Additionally, the controller  28  may receive information from each switch  29  including temperature, state (open or closed), and fault condition (clear, warning, fault).  
         [0114]     Each switch  29  is switched open or closed in response to a command from the controller  28 . Switching is conducted to energize or disable components or subsystems, for commutation, chopping, or to synthesize an AC waveform. The power ratings of the attached devices vary. The corresponding power ratings of the associated switches  15  may also vary in order to allow minimization of the overall size, weight and cost of the power converter and controller  6 .  
         [0115]     The power converter and controller  6  has a DC bus with one bus bar  18  at elevated potential and a second bus  19  at a common potential. An H-bridge leg comprises two switches  29  connected in series where the pair of switches  29  connect the two bus bars of the DC bus  18 ,  19  and the point between the switches connects to one phase leg of the connected AC device.  
         [0116]     A DC bus capacitor  20  serves several purposes individually or simultaneously. Mainly, the bus capacitor  20  provides dynamic energy storage necessary for the motor drive and buck-boost functions conducted by the inverter legs. A single DC bus capacitor  20  serves all of the phase legs in the power converter and controller  6 .  
         [0117]     The power converter and controller  6  sources or sinks power from the motor/generators,  2 ,  3 ,  31  flywheel fast energy storage  26 , and the charging port  24  in AC format. The H-bridge legs of the power converter and controller can operate as a rectifier, an active rectifier, a motor drive, or an AC inverter in order to interface with these devices.  
         [0118]     The H-bridge legs may also function as a chopper, or perform any other power processing accomplished by switching, such as those used for DC-DC conversion. These configurations are used for the interface to the battery  7  and the fast energy storage capacitor  8 .  
         [0119]     Inductance is required for buck-boost functions and as part of the motor drive circuitry. Motors have non-negligible inductance that may be sufficient for this purpose. For devices with low inherent inductance such as batteries  7  or energy storage capacitors  8 , an inductor  21  may be incorporated in the circuit.  
         [0120]     For motors MG 1   2 , MG 2   3 , MGR  31 , and the motor/generator in the flywheel  26 , portions of the power converter and controller  6  function as a bi-directional motor drive. Three-phase drive is typical but other numbers of phases may be used as well.  FIGS. 3 and 4  show 3-phase drive configurations. To produce torque, a number of control strategies may be implemented including pulse width modulation (PWM), space vector control, and simple commutation.  
         [0121]     Buck/boost converters perform DC to DC voltage conversion by using high frequency switching to cause dynamic response in an inductance. A capacitor  20  smooths out transients associated with the switching frequency of the converter. An inductor  21  or inherent inductance, a capacitor  20 , and a switch  15  are required to perform either a buck or boost function.  FIGS. 8 and 9  show buck/boost circuits for the battery  7 .  FIG. 8  shows a buck/boost circuit for the fast energy capacitor  8 . In these examples, the use of two switches  17  for each buck/boost stage allows the inductor  21  and the capacitor  20  to be used for either buck or boost operation without reconfiguration.  
         [0122]     Buck/boost converters are used for DC-DC conversion for higher power attached devices. An AC link  23  and transformers  22  are used for AC voltage conversion to the charger port  24 . An internal AC link  23  is used to allow transformation to a lower voltage so that a separate inverter subassembly can provide lower voltage output (12V, 42V) at the DC supply ports  25 .  
         [0123]     The charger port  24  is shown as a single-phase system but a 3-phase system may be used as well. When the vehicle is at rest and connected to a utility grid, the charger circuitry may deliver energy to the DC bus  18 ,  19  and from there to any of the attached devices. During V2G operation, energy from the battery  7 , fast energy system  8 ,  26  or engine  1  via MG 1   2  may be delivered to the grid.  
         [0124]     The DC output ports  25  are energized by a small active rectifier that operates at a voltage that is different from the voltage of the principal DC bus  18 ,  19 . This active rectifier uses switches  17  of the type used throughout the power converter and controller  6  and communicate with the controller  28 . The configuration shown in  FIGS. 8 and 9  can source low power DC at two voltages, preferentially 12V and 42V.  
         [0125]     All such variations are intended to be within the scope and spirit of the invention.