Abstract:
A personal vehicle for transporting a user over a surface including an external combustion engine. The vehicle includes a generator for converting the mechanical energy produced by the external combustion engine to electrical energy and an energy storage device for storing power provided by the generator and for providing power to the external combustion engine and the assembly. The personal vehicle includes a controller for controlling a total power load placed on the external combustion engine providing short term regulation of external combustion engine parameters.

Description:
[0001]    This application is a divisional of co-pending U.S. application Ser. No. 10/395,028 filed on Mar. 21, 2003, herein incorporated by reference which claims priority from U.S. application Ser. No. 09/517,808, filed Mar. 2, 2000, now issued U.S. Pat. No. 6,536,207. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention pertains to hybrid electric vehicles utilizing an external combustion engine and in particular, a Stirling cycle engine. 
       BACKGROUND OF THE INVENTION 
       [0003]    In response to energy and environmental issues and concerns, hybrid electric vehicles, such as buses and cars, have been developed in an attempt to provide efficient, low emission vehicles. In general, a hybrid electric vehicle combines a combustion engine with a battery and an electric motor. Typically, the combustion engine is an internal combustion engine. Some hybrid electric vehicles have been developed using external combustion engines, such as a Stirling engine. 
         [0004]    As mentioned, one type of external combustion engine which may be used in a hybrid electric vehicle is a Stirling cycle engine. A Stirling cycle engine produces both mechanical energy and heat energy appropriate for space heating. The history of Stirling cycle engines is described in detail in Walker, Stirling Engines, Oxford University Press (1980), herein incorporated by reference. The principle of operation of a Stirling engine is well known in the art. Stirling cycle engines have not generally been used in practical applications, such as hybrid electric vehicles, due to several daunting engineering challenges in their development. These involve such practical considerations as efficiency, vibration, lifetime and cost. 
         [0005]    For example, Stirling cycle engines generally make poor traction motors due to poor throttle response and limited power in comparison to an internal combustion. The response time of a Stirling cycle engine is limited by the heat transfer rates between the external combustion gases and the internal working fluid of the engine and may be on the order of 30 seconds. The response time of an internal combustion engine, on the other hand, is very short because the combustion gas is the working fluid and can be directly controlled by the fuel flow rate. Prior attempts to increase the responsiveness of a Stirling cycle engine provided a variable dead space for the working fluid as described in U.S. Pat. No. 3,940,933 to Nystrom and U.S. Pat. No. 4,996,841 to Meijer or controlled the pressure of the working fluid as described in U.S. Pat. No. 5,755,100 to Lamos. The foregoing references are hereby incorporated by reference in their entirety. However, both these approaches tend to increase the complexity, size, and weight of the engine design. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with an embodiment of the invention, a personal vehicle for transporting a user over a surface includes a support for supporting the user, a ground contacting module having at least one ground contacting member and a drive arrangement for causing locomotion of the support, ground contacting module and user over a surface. The drive arrangement includes an external combustion engine for generating mechanical energy and thermal energy, a generator for converting the mechanical energy produced by the external combustion engine to electrical energy and an energy storage device for storing power provided by the generator and for providing power to the external combustion engine and the assembly. The external combustion engine and the generator may be housed in a hermetically sealed pressure vessel. In addition, the personal vehicle includes a controller for controlling a total power load placed on the external combustion engine so as to provide short term regulation of external combustion engine parameters. 
         [0007]    In one embodiment, the external combustion engine is a Stirling cycle engine. The thermal energy produced by the external combustion engine may be used to provide heat to an area surrounding the personal vehicle. In another embodiment, the personal vehicle includes a power output coupled to the energy storage device for providing power to an external load. In yet another embodiment, the personal vehicle has modes in which it is not statically stable. The personal vehicle may have balancing capability on lateral and foe-aft places defined by the support. 
         [0008]    In another embodiment of the invention a personal vehicle for transporting a user over a surface includes a support for supporting the user, a ground contacting module having at least one ground contacting member and a drive arrangement for causing locomotion of the support, ground contacting module and user over a surface. The drive arrangement includes an external combustion engine for generating mechanical energy and thermal energy, a generator for converting the mechanical energy produced by the external combustion engine to electrical energy and an energy storage device for storing power provided by the generator and for providing power to the external combustion engine and the assembly. The vehicle further includes a power output coupled to the energy storage device for providing power to an external load, while the vehicle is stationary. The vehicle may include an inverter coupled to the energy storage device so that alternating current power may be derive for an external load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention will be more readily understood by reference to the following description taken with the accompanying drawings, in which: 
           [0010]      FIG. 1  is a schematic diagram of a personal hybrid electric vehicle using a Stirling engine in accordance with an embodiment of the invention; 
           [0011]      FIG. 2  is a schematic diagram of a personal hybrid electric vehicle in accordance with an alternative embodiment of the invention; 
           [0012]      FIG. 3  is a schematic diagram of a personal hybrid electric vehicle in accordance with an alternative embodiment of the invention; 
           [0013]      FIG. 4  is a schematic block diagram of the power, drive and control components for the personal hybrid electric vehicle of  FIG. 1  in accordance with an embodiment of the invention; 
           [0014]      FIG. 5  is a cross section view of a Stirling cycle engine in accordance with a preferred embodiment of the invention; 
           [0015]      FIG. 6A  is a schematic block diagram of the power control system for the engine of the personal vehicle of  FIG. 1  in accordance with an embodiment of the invention 
           [0016]      FIG. 6B  is a schematic block diagram of a method of control for the power control system of  FIG. 6A  in accordance with an embodiment of the invention; 
           [0017]      FIG. 7  illustrates the circuitry for the power control system in  FIG. 6A  in accordance with an embodiment of the invention; 
           [0018]      FIG. 8  is a schematic block diagram of the power control system of the personal vehicle of  FIG. 1  including a burner controller in accordance with an embodiment of the invention; 
           [0019]      FIG. 9  is a schematic block diagram of the power control system of the personal vehicle of  FIG. 1  including a burner controller in accordance with an alternative embodiment of the invention; 
           [0020]      FIG. 10   a  is a side view in cross section of the burner and exhaust heat recovery assembly, in accordance with an embodiment of the invention; 
           [0021]      FIG. 10   b  shows a perspective top view of a heater head including heat transfer pin arrays in accordance with an embodiment of the invention; 
           [0022]      FIG. 10   c  shows a perspective view of an alternative heater head including heater transfer tubes in accordance with an embodiment of the invention; 
           [0023]      FIG. 11   a  shows a cross-sectional view from the side of a fuel intake manifold for a Stirling cycle engine in accordance with a preferred embodiment of the invention; 
           [0024]      FIG. 11   b  shows a cross-sectional view from the top of the fuel intake manifold of  FIG. 11   a  taken through cut BB; and 
           [0025]      FIG. 11   c  is a cross-sectional view from the top of the fuel intake manifold of  FIG. 1   a  taken through cut AA, showing the fuel jet nozzles. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    In accordance with an embodiment of the invention, a personal vehicle is provided that includes a hybrid Stirling engine/generator to provide power to the vehicle. The hybrid Stirling engine/generator as described herein has the benefits of a Stirling engine such as low emissions, long life and quiet operation. In addition, the hybrid Stirling engine/generator has good throttle response and instant power as required by a vehicle for operation. The hybrid Stirling engine/generator is advantageously of a small size (5 kW or less) with ultra low emissions and may be implemented in a variety of personal vehicles. 
         [0027]    The invention may be implemented in a wide range of embodiments.  FIG. 1  is a schematic diagram of a personal hybrid electric vehicle using a Stirling engine in accordance with an embodiment of the invention. A personal hybrid electric vehicle as used in the description and the following claims means a vehicle with a weight less than 1400 lbs, with an engine power output less than 5 kW and at least one ground contacting member, such as a wheel.  FIG. 1  shows a scooter type personal vehicle  100  with two ground contacting members  110 . The scooter  100  is powered by an external combustion engine  105 , preferably a Stirling cycle engine. A support  104  covers the components of the scooter and serves to support a user of the scooter  100 . The support  104  includes a seat  112  on which a user may sit while using the vehicle. 
         [0028]    Scooter  100  includes a drive arrangement to provide the power to cause the locomotion of the scooter. The drive arrangement includes a Stirling cycle engine and generator combination  105  and an energy storage device  107 . The outputs of the Stirling cycle engine  105  during operation typically include both mechanical energy and residual heat energy. The Stirling cycle engine  105  is coupled to the generator and the generator/engine combination may be housed in a sealed pressure vessel. Alternatively, the generator may be external to the pressure vessel containing the engine. The pressure vessel contains a high pressure working fluid, preferably helium, nitrogen or a mixture of these gases at 20 to 30 atmospheres pressure. The generator converts the mechanical energy produced by the Stirling cycle engine to electrical energy. The working gas of the Stirling engine is heated by heat from an external thermal source, such as burner  102 . Burner  102  burns a fuel provided by a fuel supply  103 . 
         [0029]    The Stirling cycle engine and generator combination  105  produces electrical energy that may be used to power the scooter. Accordingly, the generator is used to power a motor  106  coupled to the ground contacting members  110 . The electrical energy produced is also stored in the energy storage device  107 . In a preferred embodiment, the energy storage device is a rechargeable battery. The energy storage device may also be used to power the scooter  100 . Accordingly, energy storage device  107  is coupled to the motor  106 . 
         [0030]    Scooter  100  may also include a radiator  108  coupled to the Stirling cycle engine  105  to provide cooling during operation of the engine. A controller  109  is coupled to the Stirling cycle engine and generator  105 , a fuel regulator (not shown, a blower (not shown) and the energy storage device  107 . Controller  109  is used to control the power output produced by the Stirling engine and generator. An electrical output  114  is optionally connected to the energy storage device  107  to provide electricity to an external load when the scooter is not being used for transportation. 
         [0031]      FIG. 2  and  FIG. 3  show alternative forms of a personal hybrid electric vehicle.  FIG. 2  shows an exemplary electric wheelchair. A support arrangement  212  includes a chair, on which a user  213  may be seated. A pair of laterally disposed ground contacting members  211  is used to suspend the user  213  over a surface with respect to which the user is being transported. In a further embodiment, the hybrid Stirling engine/generator may be used to replace the battery pack used in may standard electric wheelchairs.  FIG. 3  shows a scooter on which a user stands. A support arrangement includes a platform  311  on which the user stands and holds grip  317  on handle  313  attached to the platform. The scooter also includes ground contacting members  314 . In a further embodiment, the hybrid Stirling engine/generator may be used in a personal vehicle that includes a set of pedals on the support arrangement that may be used by the user during operation. 
         [0032]    In alternative embodiments, the personal vehicle is configured such that the vehicle lacks inherent stability at least a portion of the time with respect to a vertical in a fore-aft plane but is relatively stable with respect to a vertical in the lateral plane. These dynamically stabilized personal vehicles include a control system that actively maintains the stability of the personal vehicle while the vehicle is operating. The control system maintains the stability of the personal vehicle by continuously sensing the orientation of the vehicle, determining the corrective action to maintain stability, and commanding the wheel motors to make the corrective action. Dynamically stabilized vehicles are discussed in more detail in U.S. Pat. Nos. 5,701,965 and 5,971,091, both of which are herein incorporated by reference. 
         [0033]      FIG. 4  is a schematic block diagram of the power, drive and control components of the personal vehicle of  FIG. 1 , in accordance with an embodiment of the invention. As discussed above with respect to  FIG. 1 , the personal vehicle includes a Stirling engine  401  coupled to a generator  402 . The outputs of the Stirling cycle engine  401  during operation include both mechanical energy and residual heat energy. Heat produced in the combustion of a fuel in a burner  404  is applied as an input to the Stirling cycle engine  401 , and partially converted to mechanical energy. The unconverted heat or thermal energy accounts for 65 to 85% of the energy released in the burner  404 . This heat is available to provide heating to the local environment around the scooter. The exhaust gases are relatively hot, typically 100 to 300.degree. C., and represent 10 to 20% of the thermal energy produced by the Stirling engine  401 . The cooler rejects 80 to 90% of the thermal energy at 10 to 40.degree. C. above the ambient temperature. The heat is rejected to either a flow of water, or, more typically, to the air via a radiator  407 . 
         [0034]    As mentioned above, burner  404  combusts a fuel to produce hot exhaust gases which are used to drive the Stirling engine  401 . A controller  408  is coupled to the burner  404  and a fuel supply  410 . Controller  408  delivers a fuel from the fuel supply  410  to the burner  404 . The controller  408  also delivers a measured amount of air to the burner  404  to advantageously ensure substantially complete combustion. The fuel combusted by burner  404  is preferably a clean burning and commercially available fuel such as propane. A clean burning fuel is a fuel that does not contain large amounts of contaminants, the most important being sulfur. Natural gas, ethane, propane, butane, ethanol, methanol and liquefied petroleum gas (“LPG”) are all clean burning fuels when the contaminants are limited to a few percent. One example of a commercially available propane fuel is HD-5, an industry grade defined by the Society of Automotive Engineers and available from Bernzomatic. Alternatively, the fuel may be any commercially available liquid fuel including diesel, is gasoline, kerosene, methanol and ethanol. In accordance with an embodiment of the invention, and as discussed in more detail below, the Stirling engine  401  and burner  404  provide substantially complete combustion in order to provide high thermal efficiency as well as low emissions. The characteristics of high efficiency and low emissions are highly desired characteristics of a hybrid electric vehicle. 
         [0035]    Controller  408  also controls the power output produced by the Stirling cycle engine  401  and generator  402 . Generator  402  is coupled to a crankshaft (not shown) of Stirling engine  401 . In an alternative embodiment, the external combustion engine  401  is a free piston Stirling engine and the generator is coupled mechanically to the pistons of the Stirling engine. The term “generator”, as used in the specification and in any appended claims, unless context requires otherwise, will encompass the class of electric machines such as generators wherein mechanical energy is converted to electrical energy or motors wherein electrical energy is converted to mechanical energy. The generator  402  is preferably a permanent magnet brushless motor. An energy storage device  413  is coupled to the controller  408  and is used to provide power at various points during operation. For example, energy storage device  413  may be used to provide starting power for the personal transport vehicle  100  as well as direct current (“DC”) or alternating current (“AC”) power to a wheel motor. In a preferred embodiment, the energy storage device  413  is a rechargeable battery. In an alternative embodiment, the personal transport vehicle may include an AC outlet to provide power to an external load. An inverter  416  is coupled to the battery  413  in order to convert the DC power produced by battery  413  to AC power. 
         [0036]    In the course of operation, Stirling engine  401  also produces heat from, for example, the exhaust gases of the burner  404  as well as the supply and extraction of heat from a working fluid. Accordingly, the excess heat produced by the Stirling engine  401  may be used to advantageously heat the atmosphere surrounding the personal vehicle or the user of the personal vehicle. In this manner, the Stirling engine may be used to provide both electrical power and heat for the vehicle. 
         [0037]    The operation of Stirling cycle engine  401  will now be described in more detail with respect to  FIG. 5  which is a cross-sectional view of a Stirling engine in accordance with an embodiment of the invention. The configuration of Stirling engine  401  shown in  FIG. 5  is referred to as an alpha configuration, characterized in that a compression piston  500  and an expansion piston  502  undergo linear motion within respective and distinct cylinders: compression piston  500  in a compression cylinder  504  and expansion piston  502  in an expansion cylinder  507 . The principle of operation of a Stirling engine configured in an “alpha” configuration and employing a first “compression” piston and a second “expansion” piston is described at length in U.S. Pat. No. 6,062,023 which is herein incorporated by reference. The alpha configuration is discussed by way of example only, and without limitation of the scope of any appended claims. 
         [0038]    In addition to compression piston  500  and expansion piston  502 , the main components of Stirling engine  401  include a burner (not shown), a heater heat exchanger  522 , a regenerator  524 , and a cooler heat exchanger  528 . Compression piston  500  and expansion piston  502 , referred to collectively as pistons, are constrained to move in reciprocating linear motion within respective volumes  508  and  510  defined laterally by compression cylinder  504  and expansion cylinder liner  512 . The volumes of the cylinder interior proximate to the burner heat exchanger  522  and cooler heat exchanger  528  will be referred to, herein, as hot and cold sections, respectively of engine  401 . The relative phase (the “phase angle”) of the reciprocating linear motion of compression piston  500  and expansion piston  502  is governed by their respective coupling to drive mechanism  514  housed in crankcase  516 . Drive mechanism  514  may be one of various mechanisms known in the art of engine design which may be employed to govern the relative timing of pistons and to interconvert linear and rotary motion. For additional information relating to a preferred drive mechanism  514 , see U.S. Pat. No. 6,253,550, “Folded Guide Link Stirling Engine,” which is incorporated herein by reference. 
         [0039]    Compression piston  500  and expansion piston  502  are coupled, respectively, to drive mechanism  514  via a first connecting rod  518  and a second connecting rod  520 . The volume of compression cylinder  508  is coupled to cooler heat exchanger  528  via duct  515  to allow cooling of compressed working fluid during the compression phase. Duct  515 , more particularly, couples compression volume  508  to the annular heat exchangers comprising cooler heat exchanger  528 , regenerator  524 , and heater heat exchanger  522 . The burner (not shown) combusts a fuel in order to provide heat to the heater heat exchanger  522  of a heater head  530  of the Stirling engine. The expansion cylinder and piston are disposed within a heater head  530  such that the working fluid in the expansion cylinder may be heated via the heater heat exchanger  522 . For additional information relating to a preferred configuration of a burner, regenerator  524  and heater head  530 , see U.S. Pat. No. 6,381,958, entitled “Stirling Engine Thermal System Improvements,” which is incorporated herein by reference in its entirety. 
         [0040]    Returning to  FIG. 4 , as mentioned, the Stirling cycle engine  401  and generator  402  may be disposed within a pressure vessel  418 . The pressure vessel  418  contains a high pressure working fluid, preferably helium or nitrogen at 20 to 30 atmospheres pressure. The expansion cylinder and piston (shown in  FIG. 5 ) of the Stirling engine  401  extend through the pressure vessel  418  and a cold collar (or cooler)  403 . In an alternative embodiment, the cold collar may be disposed within the pressure vessel  418 . The end of the expansion cylinder (including heater head  530 ) is contained within the burner  404 . The cold collar  403  circulates a cooling fluid through cooling lines  406  and through radiator  407 . The cooling fluid is pumped through the cold collar  403  by a cooling pump  405 . A fan  411  may be used to force air past the radiator  407  thereby heating the air and cooling the cooling fluid. The heated air may then be forced through openings in the body of the personal vehicle to heat the surrounding area including the person on the personal vehicle. In alternative embodiments, the excess heat created by the combustion within burner  404  may be directly provided to the surrounding ambient air. 
         [0041]    The pressure vessel  418  has a pass-through port for an electrical connection  419  between the generator  402  contained within the pressure vessel  418  and the controller  408 . The controller  408  supplies power to cooling pump  405  and fan  411  through power supply lines  415 . The controller  408  also controls the power output of the Stirling engine  401  and generator  402  as well as the charge level of the battery  413  by varying the speed and temperature of the Stirling engine. Controller  408  provides command signals to the burner  404  in order to control the temperature of the Stirling engine  401 . Controller  408  also provides command signals to generator  402  in order to control the speed of the Stirling engine  401 . In one embodiment, the controller  408  varies the temperature of the heater head of the Stirling engine to meet the power demands, while the engine is allowed to operate at speeds permitted by a simple rectifier. The temperature of the heater head may be controlled by varying the fuel flow. The temperature of the heater head, however, is subject to maximum temperature limits. Alternatively, the controller  408  varies the speed of the Stirling engine to meet the power demands, while the heater head temperature is held constant at the maximum allowed temperature. In other embodiments of the invention, the generator is disposed outside the pressure vessel and a sealed coupling between engine and generator is effected. 
         [0042]    Preferably, the power output of generator  402  and Stirling engine  401  are controlled using controller  408  so as to maintain the optimal charge and voltage levels in the battery  413 . Electrical loads (e.g., the motor used to propel the ground contacting members of the vehicle) will reduce the charge and voltage of the battery  413  causing the controller  408  to command additional power from the engine.  FIG. 6A  is a schematic block diagram of the power control system included in the controller  408  (shown in  FIG. 4 ) in accordance with an embodiment of the invention. The power control system controls the speed and temperature of the Stirling engine in order to provide the necessary power to meet the demand (or load) placed on the Stirling engine and generator by the wheel motor of the scooter and maintaining the charge level of the battery. The power control system as shown in  FIG. 6A  includes a motor/generator  602 , a motor-amplifier  605 , and a battery  613 . 
         [0043]    As discussed above with respect to  FIG. 4 , the generator  602  is coupled to the crankshaft of a Stirling engine (not shown). The Stirling engine provides mechanical power (P.sub.mech) to the generator  602  which in turn converts the mechanical power to three-phase electrical power. Generator  602  also, as discussed in more detail below, acts as an adjustable load on the engine in order to control the speed of the engine. Generator  602  delivers the three-phase electrical power to motor-amplifier  605 . Motor-amplifier  605  transfers electrical power produced by the motor generator  602  to a high voltage DC bus (P.sub.amp). The power provided to the high voltage DC bus (P.sub.amp) is delivered to a DC to DC converter  606  (P.sub.dcdch) which steps down the power to a low voltage DC bus for delivery to the battery  613  (P.sub.bat). The DC to DC converter  606  may also be used to step up the power to the high voltage DC bus used for power control and AC power conversion. Alternate embodiments may omit the DC to DC converter and connect the high voltage DC bus directly to the battery  613 . Battery  613  is used to start the Stirling engine and to provide power to auxiliary circuitry  608  of the APU such as fans, pumps, etc., as well as to provide output power when the load on the APU exceeds the power produced by the motor/generator  602 . In addition, the battery  613  provides power to the vehicle, while the engine is warming up, thereby allowing immediate operation of the vehicle. As described further below, battery  613  acts as an energy reservoir during the operation of the personal vehicle. 
         [0044]    An emergency shunt  607  may be used to remove excess power from the high voltage DC bus in the case of an overvoltage condition in either DC bus. In one embodiment, the emergency shunt resistors are located in the water of the radiator  108  (shown in  FIG. 1 ). In this manner, the excess heat produced by the shunt resistors when they are utilized to remove excess power, is advantageously absorbed by the same system used to dissipate the excess heat of the personal vehicle (i.e., radiator  108 ). Alternatively, the emergency shunt may be located on the frame of the vehicle or in the open air. An inverter  616  is used to deliver AC power (P.sub.out) to a load  610 . The inverter  616  draws power (P.sub.inv) from the DC bus. 
         [0045]    The charge level of the battery  613  reflects changes in the load  610  over time (e.g., the requirements of the motor used to propel the vehicle). In order to provide the required power output, the power control system of  FIG. 6A  attempts to keep the battery  613  at its optimum charge, without overcharging, in response to changes in the output load  610 . The optimum charge is not necessarily a full charge and may be 80-100% of the full charge. The optimum charge is a tradeoff between keeping the battery ready for extended periods of discharge and increasing the battery cycle life. Charging the battery to nearly 100% of full charge maximizes the availability of the battery for extended periods of discharge but also stresses the battery, resulting in a shorter battery cycle life. Charging the battery to less than full charge reduces the stresses placed on the battery and thereby extends the battery cycle life but also reduces the energy available in the battery for sudden load changes. The selection of the optimum charge will depend on the expected load variations of the personal vehicle and the battery capacity and is well within the scope of one of ordinary skill in the power management art. In a preferred embodiment, the optimum charge is set at 90% of full charge. In another embodiment, the battery may be brought to above 100% of its theoretical charge capacity to extend the life of certain types of batteries, such as lead-acid batteries. Another goal of the power control system is to reduce the fuel consumption of the engine by maximizing the efficiency from fuel input to power output. The power control system of  FIG. 6A  adjusts the engine temperature and the engine speed in order to produce the desired battery charge and thus, the required power output. 
         [0046]    The charge of the battery  613  may be roughly estimated by the battery voltage which is roughly related to the battery charge. Monitoring only the battery voltage provides a simpler and cheaper method to determine the battery charge. As described above, differences between the load power (P.sub.out) and the power generated by the Stirling engine (P.sub.mech or P.sub.amp) will result in power flow to or from the battery  613 . For example, if the engine does not produce enough power to meet the demand of the load  610 , the battery  613  will provide the remaining power necessary to support the load  610 . If the engine produces more power than required to meet the demand of the load  610 , the excess power may be used to charge the battery  613 . The power control system determines whether it is necessary to command the engine to produce more or less power in response to changes in the load. The engine speed and engine temperature are then adjusted accordingly to produce the required power. When the battery  613  is being discharged (i.e. the demand from the load  610  is greater than the power produced by the engine for extended periods of time), the engine temperature and speed are adjusted so that the engine produces more power. Typically, the engine temperature and speed are increased in order to produce more power. Preferably, when more power is needed, raising engine temperature is given preference over raising engine speed. Conversely, when the battery  613  is being charged for extended periods of time (i.e., the engine is producing more power than the load  610  demands), the engine temperature and speed are decreased to decrease the amount of power produced by the engine. Typically, the engine temperature and speed are adjusted to decrease the amount of power produced by the engine. Preferably, when less power is needed, reducing engine speed is given preference over reducing engine temperature. 
         [0047]    Once the power control system determines the desired engine temperature and speed based on the desired battery power, the power control system sends a temperature command to the controller  109  (shown in  FIG. 1 ) indicating the desired engine temperature and a speed command to the generator  602  indicating the desired engine speed. As mentioned above, the speed of the engine may be controlled by modulating the torque applied to the crankshaft of the engine by the motor/generator  602  using the motor amplifier  605 . As such, the generator  602  acts as an adjustable load on the engine. When the generator  602  increases demand on the engine, the load on the crankshaft increases and thereby slows down the speed of the engine. The motor amplifier  605  adjusts the motor current in order to obtain the necessary torque in the motor and accordingly the necessary engine speed. 
         [0048]    A Stirling cycle engine (or other external combustion engine) typically has a long response time to sudden changes in the load (i.e., there is a time lag between the engine&#39;s receipt of an increase or decrease temperature command and the engine reaching the desired temperature). The power control system, therefore, is designed to account for the lengthy response time of a Stirling cycle engine. For a sudden increase in the load  610 , the torque load applied by the generator  602  on the crankshaft of the engine is reduced, thereby allowing the crankshaft to speed up and temporarily maintain an increased power output of the generator  602  until an increased temperature command sent to the controller  109  (shown in  FIG. 1 ) takes effect. For a sudden load decrease, the torque applied by the generator  602  on the crankshaft of the engine may be increased in order to slow down the crankshaft and decrease the power output until a decreased temperature command sent to the burner control unit takes effect. The excess charge or power produced by the generator  602  may be used to charge the battery  613 . As discussed above, any further excess electrical energy may also be directed to the emergency shunt  607 . The process of controlling the temperature of the engine using the controller  109  is described in more detail below with respect to  FIGS. 8-11   c.    
         [0049]      FIG. 6B  is a schematic block diagram of a method for determining the desired engine temperature and speed in order to provide the required electrical power to maintain the optimal charge for the battery and meet the applied load. First, at block  620 , the power control system estimates the state of charge of the battery. The estimated battery state of charge (Q.sub.est) is determined using the measured battery current (I.sub.B) as well as, when necessary, an adjustment current (I.sub.adj) as shown in the following equation: 
         [0000]        Q.sub.est ( t )= Q.sub.est ( t−dt )+ I.sub.B ( t ) dt+I.sub.adj ( t ) dt,   (Eqn. 1)
 
         [0050]    in block  620 . When the engine is first started, the initial estimated state of charge (Q.sub.est) is set to a preselected value. In a preferred embodiment, the initial state of charge value is 10% of full charge. The adjustment current is then used to correct the battery current such that Q.sub.est approaches a value near the actual state of charge. By selecting a low initial value for Q.sub.est at startup, faster correction is achieved because a lower value for Q.sub.est allows for a higher charging current. 
         [0051]    The adjustment current may be selected based on the known V-I characteristics of the battery. In a preferred embodiment, the battery is a lead-acid battery. The determination of the V-I plane for a particular battery is well within the scope of one of ordinary skill in the art. The V-I plane for the battery  613  (shown in  FIG. 6A ) may be divided into operating regions where the state of the charge of the battery is reasonably known. The measured battery voltage, V.sub.B, and battery current, I.sub.B, are used to identify the to current state of the battery in the V-I plane. The estimated charge Q.sub.est is then compared to the identified state of charge corresponding to the region of the V-I plane in which the measured battery voltage and current fall. The adjustment current, I.sub.adj, is estimated by taking the product of a constant, which is a function of the measured voltage and current of the battery, and the difference between the estimated state of charge Q.sub.est and the state of charge estimated using the V-I plane and the measured battery voltage and current. 
         [0052]    At block  622 , a power error P.sub.err is determined by comparing the desired battery power P.sub.batdes and the actual battery power P.sub.B. The power error P.sub.err is indicative of whether the APU must produce more or less power output. The actual battery power P.sub.B is the measured battery power flowing into the battery (I.sub.BV.sub.B). The desired battery power may be estimated using two methods. The first method is based on the charging voltage of the battery V.sub.chg and the second method is based on the estimated state of charge Q.sub.est of the battery. In the following discussion, the desired battery power estimated using the first method will be referred to as P.sub.V and the desired battery power estimated using the second method will be referred to as P.sub.Q. 
         [0053]    The first method estimates a desired battery power, P.sub.V, using the charging voltage of the battery (V.sub.chg). In a preferred embodiment, P.sub.V is estimated using the following equation: 
         [0000]        P.sub.V=V.sub.chg *MAX[ I.sub. min, I.sub.B]−I.sub.OC,   (Eqn. 2).
 
         [0054]    The charging voltage V.sub.chg is the optimum battery voltage to keep the battery charged and is typically specified by the manufacturer of a particular battery. For example, in a preferred embodiment, the lead-acid battery has a charging voltage of 2.45V/cell. V.sub.chg is multiplied by the larger of either the measured battery current (I.sub.B) or a predetermined minimum current value (I.sub.min). I.sub.min may be selected based on the known characteristics of the V-I plane of the battery. For example, in one embodiment, when the measured battery voltage V.sub.B is much less than V.sub.chg, I.sub.min may be set to a high value in order to quickly increase the voltage of the battery, V.sub.B, up to V.sub.chg. If V.sub.B is near V.sub.chg, I.sub.min may be set to a low value as it will not require as much additional energy to bring the battery voltage V.sub.B up to V.sub.chg. If V.sub.B is greater than V.sub.chg, however, an overcharge current I.sub.OC may be subtracted from the greater of I.sub.B and I.sub.min in order to avoid an overvoltage condition. 
         [0055]    The second method estimates a desired battery power P.sub.Q based on the estimated state of charge (Q.sub.est) of the battery (as determined in block  620 ). In a preferred embodiment, P.sub.Q is estimated using the following equation: 
         [0000]        P.sub.Q=K.sub.Q ( Q.sub.G−Q.sub.est )−(. eta.I.sub.busV.sub.bus−I.sub.BV.sub.B −),  (Eqn. 3)
 
         [0056]    where: 
         [0057]    K.sub.Q is a gain constant that may be configured, either in design of the system or in real-time, on the basis of current operating mode and operating conditions as well as the preference of the user; 
         [0058]    Q.sub.G is the desired state of charge of the battery; 
         [0059]    I.sub.bus is the measured bus current exiting the motor amplifier; 
         [0060]    V.sub.bus is the measured bus voltage; and 
         [0061]    .eta. is an estimated efficiency factor for the DC/DC converter (shown in  FIG. 4A ) between the motor amplifier and the battery. 
         [0062]    The desired power P.sub.Q is based on the difference between the desired charged state Q.sub.G of the battery and the estimated charge state Q.sub.est of the battery. Q.sub.G is a predetermined value between 0 (fully discharged) and 1 (fully charged) and represents the state of charge the controller is trying to maintain in the battery. In a preferred embodiment, the desired state of charge of the battery is 90% of full charge. The farther away the estimated battery charge Q.sub.est is from the desired charge state Q.sub.G, the more power which can safely be requested to charge the battery. The closer Q.sub.est is to Q.sub.G, the less power that is needed to bring the battery voltage, V.sub.B, up to V.sub.chg. 
         [0063]    The estimation of the desired battery power P.sub.Q is also adjusted to account for possible load changes. If the load on the Stirling engine and generator were suddenly decreased, the excess power produced by the engine must be directed elsewhere until the amount of power generated by the engine may be reduced (i.e., the system has time to react to the sudden change in load). The excess power represents the worst case additional power that could flow into the battery if the load were suddenly removed from the system. Accordingly, it is desirable to select a desired battery power which leaves room in the battery to absorb the excess power produced by a change in the load. The excess power is subtracted from P.sub.Q in order to leave additional room in the battery to absorb the excess power. The excess power may be determined by comparing the power generated by the engine to the power entering the battery and is represented by the term .eta.I.sub.busV.sub.bus-I.sub.BV.s-ub.B in Eqn. 3 above. The power generated by the engine is estimated using the bus voltage V.sub.bus measured at the motor amplifier and the bus current I.sub.bus measured exiting the motor amplifier. The power entering the battery is the product of the measured battery voltage and current (I.sub.BV.sub.B). 
         [0064]    At block  622 , the minimum of the two estimated desired battery powers P.sub.V and P.sub.Q is used to determine the power error P.sub.err. The power error P.sub.err is the difference between the selected desired battery power and the measured power flowing into the battery as shown by the following equation: 
         [0000]        P.sub.err =MIN[ P.sub.V,P.sub.Q]−I.sub.BV.sub.B,   (Eqn. 4)
 
         [0065]    The measured power P.sub.B flowing into the battery is the product of the measured battery current I.sub.B and the measured battery voltage V.sub.B. As mentioned above, the power error P.sub.err is indicative of whether the APU must produce more or less power output. In other words, if the actual battery power is less than the desired battery power, the APU will need to produce more power (i.e., increase speed and temperature). If the actual battery voltage is greater than the desired battery voltage, the APU will need to produce less power (i.e., decrease speed and temperature). 
         [0066]    In response to the power error signal P.sub.err, the power control system produces an engine temperature command signal output (T) and an engine speed command signal output (.omega.) at block  624  which indicate the engine temperature and speed required to produce the desired power. In a preferred embodiment, the engine temperature T is proportional to the engine speed and the integral of a function of P.sub.err. In this embodiment, T is governed by the control law 
         [0000]      T=.intg.fdt,  (Eqn 5)
 
         [0067]    where: 
         [0068]    f=K.sub.itP.sub.err when .omega.sub.mot&lt;.omega.sub.motidle; 
         [0069]    f=K.sub.itP.sub.err+K.sub.drift when P.sub.err.gtoreq.0 and .omega.sub.mot.gtoreq.omega.sub.motidle; and 
         [0070]    f=K.sub.drift when P.sub.err&lt;0 and .omega.sub.mot&gt;.omega.su-b.motidle. 
         [0071]    In the above control law, .omega.sub.mot is the measured engine speed, .omega.sub.motidle is a predetermined nominal engine speed, and K.sub.it is a gain constant. When the speed of the engine is greater than a nominal motor speed, an additional drift term (K.sub.drift) is added which has the effect of slowly increasing the engine temperature as well as indirectly decreasing the engine speed to the nominal speed of the engine. Operation of the engine at the nominal engine speed maximizes the efficiency of the engine. 
         [0072]    In a preferred embodiment, the speed of the engine (.omega.) is proportional to the power error P.sub.err and the integral of P.sub.err and is governed by the following control law: 
         [0000]      .omega.=.omega. sub .min+ K.sub.pwP.sub.err+K.sub.iw.intg.P.sub.errdt   (Eqn. 6)
 
         [0073]    where: 
         [0074]    .omega.sub.min represents the minimum allowable engine speed; and 
         [0075]    K.sub.pw and K.sub.iw are gain constants. 
         [0076]    The motor speed, .omega., is limited to be at least some minimum speed .omega.sub.min. The engine speed is also limited to a maximum speed .omega.sub.max to reduce the engine cooling effect when the speed increases. 
         [0077]      FIG. 7  shows the structural details of the power electronics circuitry of  FIG. 6A . The generator  702  is coupled to a battery  713 , an inverter  716 , an amplifier  705  and an emergency shunt  707 . The behavior of these elements is similar to that described above with respect to  FIGS. 6A and 6B . 
         [0078]    As discussed above with respect to  FIGS. 6A and 6B , once the power control system determines the desired engine temperature and speed required to maintain the optimal charge level of the battery, a speed command (.omega.) is sent to the generator  602  (shown in  FIG. 6A ) indicating the desired engine speed and a temperature command (T) is sent to the controller  109  (shown in  FIG. 1 ) indicating the desired engine temperature. Returning to  FIG. 4 , the controller  408  controls the burner  404  to achieve the desired engine temperature. The controller  408  delivers a clean burning fuel, preferably propane, supplied from a fuel supply  410  to the burner  404 . The controller  408  also delivers a measured amount of air to the burner  404  to ensure substantially complete combustion of the fuel. The controller  408  sets the fuel and air flow rates to provide the required engine temperature and to minimize emissions. 
         [0079]    Preferred methods of improving thermal efficiency and providing low emissions of Stirling engine  401  will now be discussed in more detail in reference to  FIGS. 8-11 . Components of such thermal efficiency include efficient pumping of an oxidant (typically air, and, referred to herein as “air”) through the burner  404  to provide combustion, and the recovery of hot exhaust leaving the heater head  530  (shown in  FIG. 5 ) of the Stirling engine. In many applications, air (or other oxidant) is pre-heated, prior to combustion, nearly to the temperature of the heater head  530 , so as to achieve thermal efficiency. There is still a considerable amount of energy left in the combustion gases after the heater head of the Stirling engine has been heated, and, as known to persons skilled in the art, a heat exchanger may be used to transfer heat from the exhaust gases to the combustion air prior to introduction into burner  404 . A preheater assembly is discussed in more detail below with respect to  FIG. 10 . 
         [0080]    In addition, minimizing emissions of carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx) requires a lean fuel-air mixture which still achieves complete combustion. A lean fuel air mixture has more air than a stoichiometric mixture (i.e., 15.67 grams of air per gram of propane, for example). As more air is added to the fuel, the emissions of CO, HC and NOx decrease until the amount of air is large enough that the flame becomes unstable. At this point, pockets of the fuel-air mixture will pass through the burner without complete combustion. Incomplete combustion of the fuel-air mixture produces large amounts of CO and HC. The CO and HC emissions will continue to increase as more air is added to the fuel-air mixture until the flame extinguishes at a Lean Blow-Out limit (“LBO”). The LBO will increase as the temperature of the incoming air (i.e., the preheated air) increases. As a result, the optimal fuel-air ratio decreases as the temperature of the preheated air increases during the warmup phase of the engine. Once the engine is warmed up, the fuel-air ratio is adjusted to minimize the emissions produced and to maintain a stable flame. As used in this description and the following claims, a fuel-air ratio is the ratio of the mass of the fuel to the mass of the air flowing into the combustion chamber of the burner. 
         [0081]    Accordingly, the fuel-air ratio is first controlled by the controller (shown in  FIGS. 1 and 4 ) to provide the optimal fuel-air ratio for ignition. Once the flame is proved, the fuel-air ratio is controlled to minimize emissions based upon the temperature of the preheated air and the fuel type. The controller then controls the fuel flow rate to bring the heater head  530  temperature up to the commanded temperature. The air flow rate is adjusted in order to maintain a desired level of oxygen in the exhaust of the engine as the fuel flow rate changes and as the air preheat temperature changes. 
         [0082]      FIG. 8  is a schematic block diagram of the power control system including the burner controller  809  and an engine controller  811 . Engine controller  811  calculates the required engine temperature and engine speed at block  806  as discussed above with respect to  FIGS. 6A and 6B . The desired engine temperature (i.e. the desired temperature of the heater head) is provided as a temperature command input  807  to the burner controller  809 . A slew rate limiter  801  is advantageously used to limit the increase in engine temperature so that the temperature increase is gradual in order to minimize temperature over- and under-shoot. Upon receiving a temperature command  807  from the engine controller  811  for an engine temperature above a minimum operating temperature, the burner controller  809  initiates a lighting sequence for the burner  804 . A water pump (not shown) and a radiator fan (not shown) are controlled to maintain the temperature of the coolant. 
         [0083]    A given fuel will only ignite over a limited range of fuel-air ratios. At ignition, an ignition fuel-air ratio chosen which is equal to or less than the stoichiometric fuel-air ratio corresponding to the fuel being used. In a preferred embodiment, where the fuel is propane, the ignition fuel-air ratio is set to 0.1 IB grams propane per grams air. The ignition fuel air ratio is maintained until the flame stabilizes and the temperature of the interior of the combustion chamber of the burner  804  increases to a warmup temperature. In a preferred embodiment, the ignition fuel-air ratio is maintained until the heater head  530  temperature reaches 300.degree. C. 
         [0084]    Once the flame is stabilized, and the temperature of the combustion chamber of the burner reaches the desired warmup temperature, the fuel-air ratio is then controlled based on the air preheat temperature and the fuel type. As described above, the optimal fuel-air ratio of the fuel-air mixture decreases as the temperature of the preheated air increases. The optimal fuel-air ratio first decreases linearly from a “start” fuel-air ratio for room temperature air to a “run” fuel-air ratio, for warmed up preheated air temperature. The air is considered fully warmed up when it exceeds its known ignition temperature. For example, the ignition temperature for propane is 490.degree. C. In a preferred embodiment, where the fuel is propane, the “start” fuel-air ratio is 0.052 grams fuel to gram air, which results in approximately 4% oxygen in the exhaust of the engine. The “run” fuel-air ratio in the preferred embodiment is 0.026 grams fuel to gram air, which results in approximately 13% oxygen in the exhaust of the engine. Once the air reaches its warmed up preheated temperature, the air flow rate is adjusted to maintain the optimal fuel-air ratio for the warmed up preheated temperature. The air flow rate may be adjusted, for example, in response to a change in the fuel flow rate or a change in the air preheat temperature. 
         [0085]    In the embodiment of  FIG. 8 , the fuel-air ratio may be determined by measuring the air and fuel mass flow rates. The air flow rate may be measured with a pressure sensor and a venturi tube at the blower  805 . The fuel flow rate may be determined from the pressure upstream and downstream of a set of fuel control valves and monitoring which valves are currently commanded open. In an alternative embodiment, the fuel-air ratio may be based on the measurement of the oxygen content in the exhaust of the APU as shown in  FIG. 9 . An oxygen sensor may be placed in the engine to sample the exhaust gas and measure the percentage of oxygen in the exhaust. 
         [0086]    Returning to  FIG. 8 , the engine temperature (T.sub.head) is measured and compared to the desired engine temperature  807  using a feed back loop. The engine temperature will continue to be increased (by increasing the fuel and air flow rates) until the engine temperature reaches the desired engine temperature. As discussed previously, the slew rate limiter  801  provides a gradual increase in the temperature to minimize temperature over- and under-shoot. When the engine controller  811  commands a heater head temperature below a minimum heater head temperature, the burner controller  809  turns off the fuel and air and controls the water pump and radiator fan to avoid coolant boil-over. 
         [0087]    In addition to providing the optimal fuel-air ratio, the fuel and air combusted in burner  804  must be well-mixed with sufficient amounts of oxygen to limit the emission of carbon monoxide (CO) and hydrocarbon (HC) and, additionally, must be burned at low enough flame temperatures to limit the formation of oxides of nitrogen (NO.sub.x). The high temperature of pre-heated air, which as described above is desirable for achieving high thermal efficiency, complicates achieving low emission goals by making it difficult to premix the fuel and air and requiring large amounts of excess air in order to limit the flame temperature. As used herein, the term “auto-ignition temperature” is defined as the temperature at which a fuel will ignite without a temperature-decreasing catalyst under existing conditions of air and fuel pressure. The typical preheated air temperature exceeds the auto-ignition temperature of most fuels, potentially causing the fuel air mixture to ignite before entering the combustion chamber of the burner. One solution to this problem is to use a non-pre-mixed diffusion flame. However, since such diffusion flames are not well mixed, higher than desirable emissions of CO and NOx result. A detailed discussion of flame dynamics is provided by Turns, An Introduction to Combustion: Concepts and Applications, (McGraw-Hill, 1996), which is incorporated herein by reference. An increased air flow provided to limit flame temperature typically increases the power consumed by an air pump or blower, thereby degrading overall engine efficiency. 
         [0088]    In accordance with an embodiment of the present invention, low emissions and high efficiency may be provided by producing a pre-mixed flame even in the presence of air heated above the auto-ignition temperature of the fuel, and additionally, by minimizing the pressure drop between the air inlet and the flame region thereby minimizing blower power consumption. 
         [0089]    The term “flame speed” is defined as the speed at which a flame front will propagate through a particular fuel-air mixture. Within the specification and the following claims, the term “combustion axis” shall refer to the direction of predominant fluid flow upon combustion of the fluid. 
         [0090]    Typical components of the burner and preheater assemblies, in accordance with embodiments of the present invention, are described with reference to  FIG. 10   a . The target range for combustion gases is 1700-2300K, with a preferred range of 1900-1950K. Operating temperatures are limited by the strength of heater head  530  which must contain working fluid at an operating pressure of typically several atmospheres and by the oxidation resistance of the burner structure. Since the strength and oxidation resistance of metals typically decreases at high temperatures, it is important to shield metal components from the high combustion temperatures. To that end, burner  122  is surrounded by a ceramic combustion chamber  1004 , itself encased in a metal combustion chamber liner  1006 , thermally sunk to heater head  530  and cooled by incoming air from the preheater path or by exhaust gases  1010 . Additionally, heater head  530  is shielded from direct flame heating by head flame cap  1002 . The exhaust products of the combustion process follow path  1008  past heater head  530  through a channel providing for efficient transfer of heat to the heater head and to the working gas contained within the heater head. 
         [0091]    The overall efficiency of a thermal engine is dependent in part on the efficiency of heat transfer between the combustion gases and the working fluid of the engine. In order to increase the efficiency of heat transfer from exhaust products of the combustion process generated by burner  122 , to the working fluid contained within heater head  530  of the engine, a large wetted surface area, on either side of heater head  530  is required. Referring to  FIG. 5 , heater head  530  is substantially a cylinder having one closed end  532  (otherwise referred to as the cylinder head) and an open end  534 . Closed end  532  is disposed in burner  122  as shown in  FIG. 10   a.    
         [0092]    Referring to  FIG. 10   b , in accordance with a preferred embodiment of the invention, fins or pins may be used to increase the interfacial area between the hot fluid combustion products and the solid heater head  530  so as to transfer heat, in turn, to the working fluid of the engine. Heater head  530  may have heat transfer pins  152 , disposed on the exterior surface as shown in  FIG. 10   b , so as to provide a large surface area for the transfer of heat by conduction to heater head  530 , and thence to the working fluid, from combustion gases flowing from burner  122  (shown in  FIG. 10   a ) past the heat transfer pins. Heat transfer pins may also be disposed on the interior surface (not shown) of heater head  530 . Interior-facing heat transfer pins serve to provide a large surface area for the transfer of heat by conduction from heater head  530  to the working fluid. 
         [0093]    The use and method of manufacture of heat transfer pins is described in copending U.S. Pat. No. 6,381,958, titled “Stirling Engine Thermal System Improvements”, incorporated by reference above. 
         [0094]    Depending on the size of heater head  530 , hundreds or thousands of inner transfer pins and outer heat transfer pins may be desirable. In accordance with certain embodiments of the invention, individual arrays of pins  150 , comprise arcuate fractions of the circumferential distance around the heater head  530 . This is apparent in the top view of the heater head assembly shown in perspective in  FIG. 10   b . Between successive heat transfer pin array segments  150  are trapezoidal dividers  506  which are baffled to block the flow of exhaust gases in a downward direction through any path other than past the heat transfer pins. Since exhaust gases do not flow through dividers  506 , a temperature sensor, such as thermocouple  138  is advantageously disposed in divider  506  in order to monitor the temperature of heater head  530  with which the temperature sensor is in thermal contact. 
         [0095]    Temperature sensing device  138  is preferably disposed within divider  506  as depicted in  FIG. 10   b . More particularly, temperature sensing tip  139  of temperature sensor  138  is preferably located in the slot corresponding to divider  506  as nearly as possible to cylinder head  332  in that this area is typically the hottest part of the heater head. Alternatively, temperature sensor  138  might be mounted directly to cylinder head  332 ; however location of the sensor in the slot, as described, is preferred. Engine performance, in terms of both power and efficiency, is highest at the highest possible temperature, yet the maximum temperature is typically limited by metallurgical properties. Therefore, sensor  138  should be placed to measure the temperature of the hottest, and therefore the limiting, part of the heater head. Additionally, temperature sensor  138  should be insulated from combustion gases and walls of divider  506  by ceramic insulation (not shown). The ceramic can also form an adhesive bond with the walls of the divider to retain the temperature sensor in place. Electrical leads  144  of temperature sensor  138  should also be electrically insulated. 
         [0096]    Returning to  FIG. 10   a , exhaust gases follow path  1008  past heater head  530  and are then channeled up along path  1010 , between chamber liner  1006  and inner insulation  1012 , thereby absorbing additional heat from chamber liner  1006 , with the additional advantage of preventing overheating of the chamber liner. The exhaust gases are then returned downward through preheater  1014  and exhausted around the circumference of heater head  530  as shown by arrows designated  1016 . Preheater  1014  allows for exchange of heat from the exhaust gases to air taken in from the ambient environment, typically by an air pump or blower. Preheater  1014  may be fabricated from corrugated folder fins, typically, Inconel, however, any means for exchange of heat from the exhaust to the input air is within the scope of the present invention. 
         [0097]    In an alternative embodiment, heater tubes may be used to transfer heat from the hot fluid combustion products to the working fluid of the engine.  FIG. 10   c  shows an exemplary heater head including heater tubes. Additional information on a preferred heater tube design is discussed in U.S. Pat. No. 6,543,215, entitled, Thermal Improvements for an External Combustion Engine, which is herein incorporated by reference. 
         [0098]    Referring now to  FIGS. 11   a - 11   c , an intake manifold  1199  is shown for application to a Stirling cycle engine or other combustion application in accordance with an embodiment of the invention. In accordance with a preferred embodiment of the invention, fuel is pre-mixed with air that may be heated above the fuels auto-ignition temperature and a flame is prevented from forming until the fuel and air are well mixed.  FIG. 11  a shows a preferred embodiment of the apparatus including an intake manifold  1199  and a combustion chamber 
         [0099]    The intake manifold  1199  has an axisymmetrical conduit  1101  with an inlet  1103  for receiving air  1100 . Air  1100  is pre-heated to a temperature, typically above 900K, which may be above the auto-ignition temperature of the fuel. Conduit  1101  conveys air  1100  flowing inward radially with respect to combustion axis  1120  to a swirler  1102  disposed within the conduit  1101 . 
         [0100]      FIG. 11   b  shows a cross sectional view of the conduit  1101  including swirler  1102  in accordance with an embodiment of the invention. In the embodiment of  FIG. 11   b , swirler  1102  has several spiral-shaped vanes  1126  for directing the flow of air  1100  radially inward and imparting a rotational component on the air. The diameter of the swirler section of the conduit decreases from the inlet  1124  to the outlet  1122  of swirler  1102  as defined by the length of the swirler section conduit  1101 . The decrease in diameter of swirler vanes  1126  increases the flow rate of air  1100  in substantially inverse proportion to the diameter. The flow rate is increased so that it is above the flame speed of the fuel. At outlet  1122  of swirler  1102 , fuel  1106 , which in a preferred embodiment is propane, is injected into the inwardly flowing air. 
         [0101]    In a preferred embodiment, fuel  1106  is injected by fuel injector  1104  through a series of nozzles  1128  as shown in  FIG. 11   c . More particularly,  FIG. 11   c  shows a cross sectional view of conduit  1101  and includes the fuel jet nozzles  1128 . Each of the nozzles  1128  is positioned at the exit of the swirler vanes  1126  and is centralized between two adjacent vanes. Nozzles  1128  are positioned in this way for increasing the efficiency of mixing the air and fuel. Nozzles  1128  simultaneously inject the fuel  1106  across the air flow  1100 . Since the air flow is faster than the flame speed, a flame will not form at that point even though the temperature of the air and fuel mixture is above the fuel&#39;s auto-ignition temperature. In a preferred embodiment, where propane is used, the preheat temperature, as governed by the temperature of the heater head, is approximately 900 K. 
         [0102]    Referring again to  FIG. 11   a , the air and fuel, now mixed, referred to hereafter as “air-fuel mixture”  1109 , is transitioned in direction through a throat  1108  which has a contoured flairing and is attached to the outlet  1107  of the conduit  1101 . Fuel  1106  is supplied via fuel regulator  1132 . Throat  1108  has an inner radius  1114  and an outer dimension  1116 . The transition of the air-fuel mixture is from a direction which is substantially transverse and radially inward with respect to combustion axis  1120  to a direction which is substantially parallel to the combustion axis. The contour of the flairing of throat  1108  has the shape of an inverted bell such that the cross sectional area of throat  1108  with respect to the combustion axis remains constant from the inlet  1111  of the throat to outlet  1112  of the throat. The contour is smooth without steps and maintains the flow speed from the outlet of the swirler to the outlet of the throat  1108  to avoid separation and the resulting recirculation along any of the surfaces. The constant cross sectional area allows the air and fuel to continue to mix without decreasing the flow speed and causing a pressure drop. A smooth and constant cross section produces an efficient swirler, where swirler efficiency refers to the fraction of static pressure drop across the swirler that is converted to swirling flow dynamic pressure. Swirl efficiencies of better than 80% may typically be achieved by practice of the invention. Thus, the parasitic power drain of the combustion air fan may be minimized. 
         [0103]    Outlet  1112  of the throat flares outward allowing the air-fuel mixture  1109  to disperse into the chamber  1110  slowing the air-fuel mixture  1109  thereby localizing and containing the flame and causing a toroidal flame to form. The rotational momentum generated by the swirler  1102  produces a flame stabilizing ring vortex as well known in the art. The operation of the fuel intake valve as shown in  FIGS. 11   a - 11   c  is further described in U.S. Pat. No. 6,062,023, which is herein incorporated by reference. 
         [0104]    The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.