Abstract:
A truck includes an auxiliary power unit having components specifically selected such that they form a stand-alone unit that can fit within an auxiliary compartment of a vehicle and deliver heating, cooling, and additional electric power to the vehicle. Included is an auxiliary engine, an auxiliary alternator, and an auxiliary condenser to provide coolant for a personnel compartment mounted evaporator. An auxiliary voltage regulator provides a ramp-up feature to minimize excessive start-up loads, limits maximum available current from the auxiliary alternator when the auxiliary compressor is engaged, and selectively disables power to electrical components in the event of low vehicle battery voltage. The auxiliary engine includes a radiator system to provide heated fluid for a personnel compartment mounted heat exchanger.

Description:
RELATED APPLICATIONS  
       [0001]     This application is related to and claims priority from provisional application Ser. No. 60/491,401, filed Jul. 31, 2003, titled “Voltage Regulator for Auxiliary Power Unit.” 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to auxiliary power systems for vehicles, more specifically to power regulation of an auxiliary power unit to operate the auxiliary air conditioning and heating system.  
         [0004]     2. Description of the Related Art  
         [0005]     Many vehicles in particular truck tractors typically referred to as semi-tractor-trailers are equipped with sleeping and resting compartments. These compartments are provided to save driver the time and expense of procuring local sleeping arrangements during extended down-time and to allow the driver to provide personal security to the vehicle, along with other incidental benefits.  
         [0006]     A problem faced by the drivers of these vehicles is that during very warm weather and very cold weather, the drivers have traditionally needed to run their main engines to keep the vehicle cabin area comfortable enough for the driver to obtain adequate rest, to allow the driver amenities such as the ability to watch television or listen to the radio, or to have refrigerated foods. Additionally, particularly in cold weather, when the vehicle is not being driven or stopped briefly, and where external electric power outlets are not readily available to the drivers, the drivers have traditionally needed to run their main engines to assure the engines would maintain sufficient compression and adequate lubrication to restart. Running the main vehicle engine during an extended stop, however, is not only very inefficient, is recognized that it is an environmentally unsound practice, and due to the recent increase in cost of fuel, it is becoming a less financially viable option.  
         [0007]     One solution to the problem of maintaining an ergonomic environment for the drivers, and maintaining the ability for the vehicle to restart during cold weather, has been to use an auxiliary power plant consisting of an auxiliary engine and a generator, typically co-positioned with the main vehicle engine to directly power the vehicles air-conditioning, heating, or electrical components normally powered by the main engine. There is, however, limited ability to position such power plant in the main engine compartment. Additionally, positioning such components in the main engine compartment has resulted in increased difficulty of performing maintenance.  
         [0008]     A recent innovation in the art has been to position an auxiliary power unit and an existing auxiliary compartment. These units, however, generally only consist of an engine or power plant and a generator and do not provide independent air-conditioning and heating systems and thus, have many of the same maintainability issues as the power plants or power sources positioned in the main engine compartment.  
         [0009]     Recognized, therefore, is the need for a compact auxiliary power unit which is generally self-contained and can be housed in an existing or modified auxiliary compartment of the vehicle, and that includes within the auxiliary compartment a generator to provide electric power to power various electronic components in the vehicle cabin, a heating system, an air-conditioning system. Recognized also is the need for an auxiliary power unit which includes sensors and circuitry to monitor and control the electrical components and power requirements.  
         [0010]     Another problem faced by drivers employing prior auxiliary power unit systems involve difficulty in starting the units and reduced service life due to the units being subjected to heavy loads during startup. Thus, also recognized is the need for a compact auxiliary power unit which includes an RPM sensor or is equipped with a ramp-up which will prevent excessive load on the engine during startup.  
         [0011]     A further problem faced by drivers using auxiliary power systems, similar to those faced by drivers that do not use an auxiliary power source, is that the drivers do not realize they are drawing power over and above that capable of the unit, and thus, deplete the vehicle battery resulting in an inability to start the main engine. Thus, also recognized is the need for a compact auxiliary power unit which includes a voltage or current regulator which will interrupt or limit current output of its power generator to various high-drain electrical components during periods of very high loading to prevent vehicle battery depletion, thus preventing an “engine no-start incident” from occurring.  
       SUMMARY OF THE INVENTION  
       [0012]     In view of the foregoing, embodiments of the present invention advantageously provide a vehicle, typically a semi-tractor trailer or other truck having a main engine, a main battery, a personnel compartment preferably including a compartment often referred to as “the sleeper,” and at least general provisions for an external auxiliary compartment often used to store tools or accessories and which can carry an auxiliary power unit assembly to provide auxiliary power to operate an auxiliary air conditioning and heating system. For example, in an embodiment of the present invention, the unit includes an auxiliary power unit assembly positioned in the external auxiliary compartment. The auxiliary power unit assembly includes various components specifically selected such that they form a stand-alone unit that can fit within the external auxiliary compartment when the compartment is in a closed position and require minimum retrofitting to the vehicle in order to deliver heating, cooling, and additional electric power to the vehicle. The primary component is an auxiliary engine, preferably a lightweight single cylinder diesel that can draw from the vehicle fuel supply. An auxiliary alternator is positioned adjacent to and powered by the auxiliary engine to provide DC electric power to a plurality of selected electrical components including an inverter which provides AC electric power. An auxiliary condenser is also positioned adjacent to and powered by the auxiliary engine to provide cooling fluid through a condenser and an expansion valve to an evaporator which can be retrofitted inside or adjacent to the sleeper. An auxiliary voltage regulator is electrically connected to the auxiliary alternator to control voltage and amperage generated by the auxiliary alternator.  
         [0013]     Advantageously, in the preferred embodiment of the present invention, the voltage regulator can selectively disable power to at least one of the electrical components in the event of low vehicle battery voltage, when so existing, to thereby prevent excessive battery depletion. This prevents the driver having to succumb to a “no-start” situation where the driver inadvertently depletes the vehicle batteries to a point where there is insufficient power to restart the vehicle. The vehicle engine includes a block heater electrically preferably connected to the vehicle battery, the auxiliary alternator, and the voltage regulator. In the preferred embodiment of the present invention, the voltage regulator is adapted to selectively disable power to the block heater and/or the inverter in the event of low vehicle battery voltage, when so existing, to thereby prevent excessive battery depletion. This allows the alternator to charge the battery while still providing either heating or cooling to the sleeper. The monitoring and disabling function can be accomplished according to a duty cycle to, among other things, prevent any erroneous effect due to momentary voltage fluctuations. The voltage regulator also can sense engine speed “RPM” during start-up to allow engine speed to obtain a minimum preselected speed prior to enabling the auxiliary alternator to supply power. Further, the voltage regulator can limit maximum available current from the auxiliary alternator when the auxiliary compressor is engaged to avoid overloading the auxiliary engine and can disengage the alternator from supplying power in the event the engine speed decreases below a preselected speed.  
         [0014]     The auxiliary engine is preferably cooled by a liquid coolant fluid based radiator system similar to that provided to a main engine of the vehicle. An auxiliary radiator is synergistically included in the external auxiliary compartment to cool an engine coolant fluid. An associated auxiliary fan can selectively provide cooling air to the auxiliary radiator when a temperature of the engine coolant fluid exceeds a preselected temperature. This radiator system provides an auxiliary heat exchanger positioned within the vehicle passenger compartment heated fluid to heat air in the vehicle passenger compartment. Further, the auxiliary fan can be set to not be powered when the temperature of the engine coolant fluid is below the preselected temperature to enhance heating of the vehicle passenger compartment during cold weather operation.  
         [0015]     Embodiments of the present invention also include a method of power regulation of an auxiliary power unit assembly to operate an auxiliary air conditioning and heating system. For example, in an embodiment of the present invention the method includes operating the auxiliary engine while the main engine is off, thereby generating DC voltage with the auxiliary alternator, applying a charging voltage from the auxiliary alternator to the main battery, converting some of the DC voltage to AC voltage and powering an AC electrical component, and ceasing to power the AC electrical component if the main battery voltage drops below a selected minimum. Its regulator senses the engine speed (RPM) of an auxiliary engine during start-up to allow engine speed to obtain a minimum selected speed prior to enabling an auxiliary alternator to drive an electrical load until after the auxiliary engine obtains the minimum selected speed, determining whether an auxiliary compressor is being driven by the auxiliary engine, and responsive to such determination, limiting maximum available current from the auxiliary alternator to power the various electrical components when the auxiliary compressor is engaged, to avoid overloading the auxiliary engine. The method can also include selectively disabling power to the block heater and optionally the inverter in the event of low vehicle battery voltage to prevent excessive vehicle battery depletion, and to provide a maximum voltage to the battery to recharge the battery when simultaneously providing heating or cooling, as desired, to a vehicle personnel compartment.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in more detail, a more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.  
         [0017]      FIG. 1  is a perspective view of the passenger side of a truck cab having a side compartment containing an auxiliary power unit mounted beneath the passenger door, each being constructed according to an embodiment of the present invention.  
         [0018]      FIG. 2  is a schematic representation of an air conditioning system powered by the engine of the truck shown in  FIG. 1 , and a schematic representation of the auxiliary power unit including an auxiliary air conditioning system and positioned in the side compartment shown in  FIG. 1  according to an embodiment of the present invention.  
         [0019]      FIG. 3  is a flow chart illustrating power regulation for the auxiliary power unit system of  FIG. 2  according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0020]     The present invention will now be described more fully hereinafter with reference to the accompanying drawings which illustrate embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the prime notation, if used, indicates similar elements in alternative embodiments.  
         [0021]      FIG. 1  illustrates a truck tractor  11  that has a forward passenger compartment or cab  13  and a rear sleeping and resting compartment  15 . The driver is situated in cab  13  while driving truck  11 , and uses the rear passenger compartment  15  for storage, a living area, entertainment center, and sleeping at other times. Truck tractor  11  has an engine compartment  17  in front of or below cab  13 . Typically, a side compartment  19  is mounted to a side frame of truck  11 , below cab  13 . Typically, the side compartment  19  is used for storage and as a step for persons to use while entering and exiting cab  13 . In the present invention, side compartment  19  preferably encloses an auxiliary power unit  21  ( FIG. 2 ). Compartment  19  could alternately be located elsewhere, such as behind fuel tank  20 , where it would not be used as a step to cab  13 . Compartment  19  is preferably adapted to be mounted on rails (not shown) to allow for easy removal. Further, compartment  19  preferably has the size and dimensions of a standard Peterbilt® toolbox and includes bottom vents and side air holes (not shown) to provide cooling and ventilation for the components of the auxiliary power unit  21  positioned inside the compartment  19 , described below.  
         [0022]     Referring to  FIG. 2 , truck tractor  11  has a primary air conditioning system  23  with standard components, which perform their standard functions known in the art of heating, ventilation, and air conditioning. Primary air conditioning system  23  is powered by the primary diesel engine  25  of truck tractor  11 . Primary engine  25  drives compressor  27  through a belt drive. A condenser  29  is mounted in front of an engine radiator  30 . An expansion valve  31  and evaporator  33  are mounted in forward passenger compartment  13 . A primary fan  35  is driven by primary engine  25  for discharging air through condenser  29  and radiator  30 . A primary blower  37  discharges air through evaporator  33  and a conventional heater coil  39 .  
         [0023]     Primary air conditioning system  23  provides cooler air when truck  11  is being driven or stopping briefly. Generally, when primary air conditioning system  23  is used to provide cooler air to passenger compartments  13  and  15  ( FIGS. 1 and 2 ), high-pressure gaseous refrigerant passes from compressor  27  to condenser  29  where it is liquefied. It then passes through expansion valve  31  where it expands into a gas, and into evaporator  33  where air from forward compartment  13  passes through evaporator  33  and is cooled by the cold refrigerant. The warm gaseous refrigerant then flows to compressor  27 .  
         [0024]     More specifically, compressor  27  is driven by engine  25  and compresses the refrigerant from a gaseous phase to a saturated gas. The refrigerant exits compressor  27  and flows into condenser  29 . Primary engine fan  35  blows air across condenser  29 . While in condenser  29 , heat transfers from the refrigerant to the air passing across condenser  29 , thereby making the refrigerant cooler. The cooler refrigerant condenses into liquid, and flows from condenser  29  to expansion valve  31 , where the refrigerant expands and flows into evaporator  33 . Blower  37 , which is electrically driven, supplies air to passenger compartments  13  and  15  ( FIGS. 1 and 2 ), blows air across evaporator  33 . While the refrigerant passes through evaporator  33 , heat transfers from the air flowing across evaporator  33  to the refrigerant inside of evaporator  33 . By transferring heat from the air passing over evaporator  33  to the refrigerant inside of evaporator  33 , the air entering passenger compartments  13  and  15  ( FIGS. 1 and 2 ) is cooled. The heat transferred from the air passing over evaporator  33  to the refrigerant inside causes the refrigerant to evaporate. The refrigerant then passes from evaporator  33  back to compressor  27 .  
         [0025]     Heater  39  provides warmer air when truck  11  is being driven or stopping briefly. Hot water from engine  25  is transferred to heater  39 . Primary blower  37  forces air from cab  13  through heater  39 , which warms the air with hot water from primary engine  25 . The water from engine  25  is cooled due to the loss of heat, and returns to radiator  30  for further cooling before returning to engine  25 .  
         [0026]     Primary engine  25  also drives a conventional primary alternator  41  that charges batteries  43 . Primary engine  25  may also have a block heater  45 , which has a conventional electrical resistance element for heating the block of engine  25  while engine  25  is not operating.  
         [0027]     Still referring to  FIG. 2 , truck  11  can be equipped with an auxiliary power unit  21  located and generally enclosed in compartment  19  and including an auxiliary air conditioning system  46 . The auxiliary power unit  21  has a small engine  47 , which is preferably a four-stroke diesel engine that runs on fuel from the same source as the engine of truck tractor  11 . Fuel line connections can be readily made with minor modifications to a fuel pickup area (not shown) of the vehicle fuel tank. The auxiliary air conditioning system  46  is powered by auxiliary engine  47 . The system  46  includes an auxiliary compressor  49  that can be driven by a belt  51 . The system  46  has a condenser  55  that is also mounted in compartment  19 . Condenser  55  has an intake (not shown) connected by a conduit or refrigeration line to compressor  49  and an outlet (not shown) connected by a conduit or refrigeration line (not shown) to an expansion valve  57  and an evaporator  59  mounted to sleeping compartment  15 . Evaporator  59  is shown mounted inside sleeping compartment  15 , however, it could also be mounted to the rear or below the compartment  15 . These components are used in a similar fashion as the components in primary air conditioning system  23  to cool the passenger compartments  13  and  15  ( FIGS. 1 and 2 ).  
         [0028]     The auxiliary air conditioning system  46  also has an auxiliary radiator  61  for cooling auxiliary engine  47 . A water line  63  selectively transfers hot water from radiator  61  to an auxiliary heater exchanger or coil  65  mounted adjacent evaporator  59 . Auxiliary engine  47  drives an auxiliary generator preferably in the form of a DC alternator  67 , which supplies preferably DC power to drive an auxiliary fan  69  for discharging air through radiator  61  and condenser  55 . Alternator  67  also supplies electrical power to a blower  71  for discharging air through heater  65  and evaporator  59  into sleeping compartment  15 . Note, a DC alternator is an alternator equipped with rectifiers to produce a DC output. Although there are significant benefits in producing and then rectifying alternating current voltage to produce a direct current voltage for a vehicle, use of a DC generator is also within the scope of the present invention. Thus, the term “alternator,” used throughout, refers to either a DC alternator or a DC generator.  
         [0029]     The auxiliary air conditioning system  46  supplies cooled air to passenger compartments  13  and  15  ( FIGS. 1 and 2 ) in substantially the same way as primary air conditioning system  23 , when primary air conditioning system  23  is not operating. Auxiliary compressor  49  is driven by auxiliary engine  47  and compresses cooling fluid or refrigerant from a gaseous phase to a saturated gas. The refrigerant exits auxiliary compressor  49  and flows into condenser  55 . Auxiliary fan  69 , which is preferably electrically driven, blows air across condenser  55 . While in condenser  55 , heat transfers from the refrigerant to the air passing across condenser  55 , which causes the refrigerant to condense into liquid. The refrigerant flows from the condenser  55  to expansion valve  57  along a conduit or refrigerant line, where the refrigerant expands and flows into evaporator  59 . Blower  71 , which is also electrically driven, blows air across evaporator  59  to cool the passenger compartments  13  and  15  ( FIGS. 1 and 2 ). The heat transferred from the air passing over evaporator  59  to the refrigerant inside causes the refrigerant to evaporate. The refrigerant then passes from evaporator  59  back to compressor  49  via a second conduit or refrigerant line.  
         [0030]     Similarly, during cold weather when primary engine  25  is not operating, heated water from engine  47  passes through water line  63  to heater coil  65 , to heat air blown over heater coil  65  by auxiliary blower  71 , to thereby heat passenger compartments  13  and  15  ( FIGS. 1 and 2 ). In the warm air mode, heater coil  65  can function as the primary cooling radiator for auxiliary engine  47  with the auxiliary fan  69  not powered unless the water temperature for the auxiliary engine  47  exceeds a preselected value, such as, for example, 190 degrees F. or the refrigerant pressure of the cooling system exceeds a preselected value. A control panel  72  can be mounted in compartment  15  to provide for automatic or manual temperature control of the temperature within compartment  15 . When selecting heat on the control panel  72 , a potentiometer (not shown) sets a flowrate of the water or radiator fluid of radiator  61  to heater coil  65 , thereby controlling the amount of heat delivered to the passenger compartments  13 ,  15 . When selecting cold on the control panel  72 , the selection controls a cycling of compressor  49 , thereby controlling the amount of refrigerant or cooling fluid delivered to the evaporator  59 .  
         [0031]     Auxiliary power unit  21  may optionally have an inverter  73 , also preferably positioned within auxiliary compartment  19 , to supply 110 VAC power for operating appliances in sleeping compartment  15 , such as a microwave oven, a coffee maker, and a television set. A voltage controller or regulator  75 , also preferably positioned within auxiliary compartment  19 , is incorporated with the circuitry of auxiliary power unit  21  for controlling voltage and amperage generated by alternator  67 . Voltage regulator  75  also can selectively control the duty cycle of the block heater  45  and can selectively turn on and off block heater  45  and inverter  73  in the event of low battery voltage.  
         [0032]     Embodiments of the present invention include a ramp-up feature whereby either the rotational speed of the auxiliary engine  47  is sensed or a specific time delay, such as, for example, 10-30 seconds, is instituted such that the engine  47  reaches a minimum speed prior to engaging the compressor  49 , alternator  67 , or both. For example, referring to  FIG. 3 , when auxiliary power unit  21  is first started, voltage regulator  75  will sense the rotational speed of auxiliary engine  47 . As per steps  77 ,  79  and  81 , voltage regulator  75  will electrically disengage the auxiliary alternator  67 . That is, the voltage regulator  75  will cause an open circuit condition in the auxiliary alternator  47 , which will prevent auxiliary alternator  67  from supplying any voltage until the rpm of engine  47  reaches a selected level, for example, 2500 rpm. A potentiometer (not shown), or other device having a similar function, can be used to vary the engine speed setting or time delay selection. Steps  77 ,  79  and  81  prevent overloading engine  47  at startup. Further, though not depicted, in the preferred embodiment of the present invention, if the speed of the auxiliary engine  47  were again to drop below the selected level, the auxiliary alternator  47  would be disengaged  
         [0033]     As indicated by step  83 , voltage regulator  75  will also sense whether auxiliary compressor  49  is on. If so, as indicated by step  85 , to avoid overloading auxiliary engine  47 , voltage regulator  75  will limit the output of alternator  67  to a less than maximum amount, preferably 65 amps. Compressor  49  cycles on and off due to its own control circuitry that is based on the operator controls  72  and/or the temperature monitored in sleeping compartment  15 . Normally, when the temperature is cool enough based on the operator selection, a clutch releases compressor  49  from rotating even though auxiliary engine  47  keeps running. The release and actuation of the clutch is monitored in step  83  as well as physically turning off the auxiliary air conditioning system, such as during cold weather. If compressor  49  is off for either reason, voltage regulator  75  will allow the amperage output of alternator  67  to be much higher, such as 110 amps, as indicated by step  87 .  
         [0034]     Referring to step  89 , the output of alternator  67 , whether the lower limit or the higher, may be used to power auxiliary fan  69 , inverter  73 , auxiliary blower  71 , block heater  45 , and truck lights and accessories, such as the radio. Alternator  67  also charges the main truck batteries  43 . Normally, auxiliary power unit  21  does not have a special purpose battery, but rather, relies on the main truck batteries  43 . Voltage regulator  75  also monitors the voltage of batteries  43  to make sure that enough voltage is always present to start primary engine  25 .  
         [0035]     Still referring to  FIG. 3 , during cold weather operations, the operator may selectively turn on or off block heater  45  ( FIG. 2 ). Step  89  indicates voltage regulator  75  providing power to the block heater  45 . Because of the power requirements of the block heater  45 , the voltage regulator  75  can be set to either continuously or cyclically check the voltage of the batteries  43  for a selected time, such as, for example, four minutes, as indicated by step  91 . As indicated in step  93 , if the voltage regulator  75  determines that the voltage of the batteries  43  has dropped below a selected value, such as, for example, 12.5 volts, power to the block heater  45  is then turned off according to step  95 . Power remains off to the block heater  45  until the voltage of batteries  43  exceed a second selected voltage, such as, for example, 13.5 volts, at which time power is available for the above listed electrical components including the block heater  45  as indicated in step  101 . On the other hand, as indicated in step  97 , if the load on alternator  67  is heavy enough so that the voltage level is below a third selected value, such as, for example, 10.5 volts, or if even after the block heater  45  was turned off, the voltage continues to drop, then power to the inverter  73  is also turned off, as indicated in step  99  and the inverter  73  will remain off until the desired voltage is reached, as indicated in step  101 . With the block heater  45  and/or the inverters  73  off, the voltage regulator  75  will continue to monitor voltage, preferably according to a duty cycle, such as, for example four minutes, as indicated in step  103 .  
         [0036]     Referring to  FIGS. 1 and 2 , in operation, the operator of truck tractor  11  would normally use primary air conditioning system  23  while driving from one place to another. The operator turns on auxiliary power unit  21  when the operator reaches a destination where the operator intends to stay at for a while, and the operator needs to turn off primary engine  25  of truck tractor  11 . Auxiliary power unit  21  supplies cooled air to forward and rear passenger compartments  13  and  15  after primary engine  25  of truck tractor  11  is turned off and no longer supplying power to primary air conditioning system  23 .  
         [0037]     The operator of truck  11  would normally use primary heater  39  to heat passenger compartments  13  and  15  while driving from one place to another. The operator turns on auxiliary air conditioning system  48  to heat passenger compartments  13  and  15  when the operator reaches a destination where the operator intends to stay for a while, and the operator needs to turn off the truck. Auxiliary air conditioning system  46  will supply heated air through auxiliary heater  65  or cold air through evaporator  59  to forward and rear passenger compartments  13 ,  15 , generally after primary engine  25  of truck  11  is turned off.  
         [0038]     The invention has several unique advantages. For example, embodiments of the present invention provide a compact auxiliary power unit including both cooling and heating components, entirely isolated from the vehicles engine, which are generally self-contained and can be housed in a slightly modified existing auxiliary compartment. Advantageously, such unit can provide electric power to power various electronic components, heating units, air-conditioning systems, in the vehicle personnel compartments, and/or vehicle engine heating units, and can include sensors and circuitry to monitor and control the electrical components and power requirements. Also for example, embodiments of the present invention provide a compact auxiliary power unit which includes an RPM sensor or provides a time delay which will prevent excessive load on the engine during startup by limiting application of a load to the engine from the alternator. Advantageously, embodiments of the present invention include a compact auxiliary power unit which has a voltage or current regulator which will limit current output of its alternator during periods of very high loading. Further, embodiments of the present invention provide an auxiliary power unit, which includes a voltage or current regulator that prevents depletion of the vehicle main battery by selectively disabling powering various electrical components, thus preventing a no-start condition from occurring.  
         [0039]     In the drawings and specification, there have been disclosed a typical preferred embodiment of the invention, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The invention has been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the invention as described in the foregoing specification. For example, the auxiliary power unit assembly was described as having a DC alternator. Use of a DC generator is within the scope of the present invention. Further, the voltage regulator was described as monitoring engine a particular engine speed providing any ramp-up feature. This ramp-up feature is not limited to the speeds provided in the illustration. Further, specific amperages were described with respect to a load reduction feature. These particular amperages were for a particular embodiment. Varying the size of the engine or capacity of the alternator or operator selection would result in different optimal amperages. Still further, specific voltages and time delays were described with respect to a battery saving feature. These voltages and the time delay are merely described with respect to the preferred embodiment and can be set differently.