Abstract:
A method of combusting fuel and air in a burner of an external combustion engine having a heater head. The fuel and air are combined to form a fuel-air mixture which is characterized by a fuel-air ratio. An exhaust gas product is produced when the fuel-air mixture is combusted in the burner of the external combustion engine. A flame is formed by igniting the fuel-air mixture at a first fuel-air ratio produced by a first air flow rate and a fuel flow rate. The air flow rate is then increased to produce a second fuel-air ratio. The fuel flow rate is also controlled based upon a temperature of the heater head of the external combustion engine. The flame is maintained at the second fuel-air ratio by adjusting the air flow rate based on the fuel flow rate. The external combustion engine may be, for example, a Stirling cycle engine.

Description:
The present application is a continuation-in-part of U.S. application Ser. No. 09/115,383, filed Jul. 14, 1998, now U.S. Pat. No. 6,062,023, and claims priority from 06/052,535, filed Jul. 15, 1997, both of which applications are herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention pertains to improvements to a Stirling cycle heat engine or refrigerator and more particularly to improvements relating to mechanical and thermal components of a Stirling cycle heat engine or refrigerator which contribute to increased engine operating efficiency and lifetime, and to reduced size, complexity and cost. 
     BACKGROUND OF THE INVENTION 
     Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker,  Stirling Engines,  Oxford University Press (1980), incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression. The Stirling cycle refrigerator is also the mechanical realization of a thermodynamic cycle which approximates the ideal Stirling thermodynamic cycle. In an ideal Stirling thermodynamic cycle, the working fluid undergoes successive cycles of isovolumetric heating, isothermal expansion, isovolumetric cooling and isothermal compression. Practical realizations of the cycle, wherein the stages are neither isovolumetric nor isothermal, are within the scope of the present invention and may be referred to within the present description in the language of the ideal case without limitation of the scope of the invention as claimed. 
     Various aspects of the present invention apply to both Stirling cycle engines and Stirling cycle refrigerators, which are referred to collectively as Stirling cycle machines in the present description and in any appended claims. Additional aspects of Stirling cycle machines and improvements thereto are discussed in a co-pending U.S. patent application entitled “Stirling Cycle Machine Improvements,” filed Jul. 14, 1998, and incorporated herein by reference. 
     The principle of operation of a Stirling cycle engine is readily described with reference to FIGS. 1 a - 1   f , wherein identical numerals are used to identify the same or similar parts. Many mechanical layouts of Stirling cycle engines are known in the art, and the particular Stirling engine designated generally by numeral  10  is shown merely for illustrative purposes. In FIGS. 1 a  to  1   d , a piston  12  (otherwise referred to herein as a “compression piston”) and a second piston (also known as an “expansion piston”)  14  move in phased reciprocating motion within cylinder  16 . Compression piston  12  and expansion piston  14  may also move within separate, interconnected, cylinders. Piston seals  18  prevents the flow of a working fluid contained within cylinder  16  between piston  12  and piston  14  from escaping around either piston  12 . The working fluid is chosen for its thermodynamic properties, as discussed in the description below, and is typically helium at a pressure of several atmospheres. The volume of fluid governed by the position of expansion piston  14  is referred to as expansion space  22 . The volume of fluid governed by the position of compression piston  12  is referred to as compression space  24 . In order for fluid to flow between expansion space  22  and compression space  24 , whether in the configuration shown or in another configuration of Stirling engine  10 , the fluid passes through regenerator  26 . Regenerator  26  is a matrix of material having a large ratio of surface area to volume which serves to absorb heat from the working fluid when the fluid enters hot from expansion space  22  and to heat the fluid when it passes from compression space  24  returning to expansion space  22 . 
     During the first phase of the engine cycle, the starting condition of which is depicted in FIG. 1 a , piston  12  compresses the fluid in compression space  24 . The compression occurs at a constant temperature because heat is extracted from the fluid to the ambient environment. In practice, a cooler  68  (shown in FIG. 2) is provided, as will be discussed in the description below. The condition of engine  10  after compression is depicted in FIG. 1 b . During the second phase of the cycle, expansion piston  14  moves in synchrony with compression piston  12  to maintain a constant volume of fluid. As the fluid is transferred to expansion space  22 , it flows through regenerator  26  and acquires heat from regenerator  26  such that the pressure of the fluid increases. At the end of the transfer phase, the fluid is at a higher pressure and is contained within expansion space  22 , as depicted in FIG. 1 c.    
     During the third (expansion) phase of the engine cycle, the volume of expansion space  22  increases as heat is drawn in from outside engine  10 , thereby converting heat to work. In practice, heat is provided to the fluid in expansion space  22  by means of a heater  64  (shown in FIG. 2) which is discussed in greater detail in the description below. At the end of the expansion phase, the hot fluid fills the full expansion space  22  as depicted in FIG. 1 d . During the fourth phase of the engine cycle, the fluid is transferred from expansion space  22  to compression space  24 , heating regenerator  26  as the fluid passes through it. At the end of the second transfer phase, the fluid is in compression space  24 , as depicted in FIG. 1 a , and is ready for a repetition of the compression phase. The Stirling cycle is depicted in a P-V (pressure-volume) diagram as shown in FIG. 1 e  and in a T-S (temperature -entropy) diagram as shown in FIG. 1 f . The Stirling cycle is a closed cycle in that the working fluid is typically not replaced during the course of the cycle. 
     The principle of operation of a Stirling cycle refrigerator can also be described with reference to FIGS. 1 a - 1   e , wherein identical numerals are used to identify the same or similar parts. The differences between the engine described above and a Stirling machine employed as a refrigerator are that compression volume  22  is typically in thermal communication with ambient temperature and expansion volume  24  is connected to an external cooling load (not shown). Refrigerator operation requires net work input. 
     Stirling cycle engines have not generally been used in practical applications, and Stirling cycle refrigerators have been limited to the specialty field of cryogenics, due to several daunting engineering challenges to their development. These involve such practical considerations as efficiency, vibration, lifetime, and cost. The instant invention addresses these considerations. 
     SUMMARY OF THE INVENTION 
     A method of combusting a fuel and air in a burner of an external combustion engine, the fuel and air combined to form a fuel-air mixture having a variable fuel-air ratio, the fuel-air mixture when combusted producing an exhaust gas product includes igniting the fuel-air mixture to form a flame at a first fuel-air ratio produced by a first air flow rate and a fuel flow rate, increasing the air flow rate to produce a second fuel-air ratio, controlling the fuel flow rate based at least on a temperature of the heater head, and maintaining the flame at the second fuel-air ratio by adjusting the air flow rate based at least on a temperature of the air and an oxygen concentration in the exhaust gas product. Igniting the fuel and air where the fuel having an auto-ignition temperature and a flame speed includes propelling the air at a speed above the flame speed into an inlet of a throat, the throat also having an outlet and a constant cross sectional area from inlet to outlet and mixing fuel into the air forming the fuel-air mixture, the fuel-air mixture exiting the outlet, such that a flame is created in the air fuel mixture outside the outlet of the throat. 
     In accordance with another embodiment of the invention, the second fuel-air ratio is maintained by adjusting the air flow rate based on an oxygen concentration in the exhaust gas. In a further embodiment, the second fuel-air ratio may be maintained by adjusting the air flow rate based at least on a temperature of the air and the fuel flow rate. In an alternative embodiment, the second fuel-air ratio is maintained by adjusting the air flow rate based at least on a temperature of the air and an oxygen concentration in the exhaust gas. 
     A system for operating a combustion chamber of an external combustion engine having a heater head, the combustion chamber characterized by a combustion axis and for delivering heat to the heater head of the engine by combusting a fuel in air to produce heat and an exhaust gas product includes a swirler having axial symmetry about the combustion axis of the combustion chamber for conveying inwardly flowing air, a fuel injector for injecting fuel into the radially inwardly flowing air in such a manner that the air and fuel mix to form an air-fuel mixture having a specified air-fuel ratio and a fuel supply regulator for delivering fuel at a specified rate of fuel delivery. The system further includes a blower for delivering air to the burner at a specified air flow rate so as to produce the specified air-fuel ratio, a swirler air temperature sensor for measuring the temperature of the air delivered to the combustion chamber, and a controller for governing the rate of air delivery based at least on the temperature of the air delivered to the combustion chamber. 
     In a further embodiment, the system includes a heater head temperature sensor for measuring the temperature of the heater head and a controller for governing the rate of fuel delivery based at least upon the temperature of the heater head and the temperature of the air delivered to the combustion chamber. In yet another embodiment, the system further includes a gas composition sensor for monitoring a gas concentration in the exhaust product of the burner and a controller for governing the rate air delivery based at least upon the temperature of the air delivered to the combustion chamber and the gas composition in the exhaust gas product. The system may also include a flow sensor for measuring the rate of fuel delivery where the controller includes a controller based at least on he temperature of the air delivered to the combustion chamber and the measured rate of fuel delivery. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
     FIGS. 1 a - 1   e  depict the principle of operation of a prior art Stirling cycle machine; 
     FIG. 2 is a side view in cross section of a Stirling cycle engine in accordance with an embodiment of the present invention; 
     FIG. 3 is a cross-sectional view of a folded guide link drive mechanism for a two-piston machine such as a Stirling cycle machine in accordance with a preferred embodiment of the invention. 
     FIG. 4 is a perspective view of one embodiment of the folded guide link drive mechanism of FIG.  3 . 
     FIG. 5 a  is a cross-sectional view of a Stirling cycle engine employing a pin heat exchanger, in accordance with an embodiment of the present invention; 
     FIG. 5 b  is a magnified perspective detail view of the pin heat exchanger of FIG. 5 a ; 
     FIG. 5 c  shows a cross-sectional view of the heater head assembly of FIG. 5 a  with heat transfer pins shown schematically, not to scale, in accordance with a preferred embodiment of the invention. 
     FIG. 6 a  is a perspective view from the bottom of the Stirling cycle engine of FIG. 2, showing branching ducts for enhancing flow uniformity in accordance with an embodiment of the present invention; 
     FIG. 6 b  is a planar view of the system of branching ducts of FIG. 9 a ; 
     FIG. 7 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; 
     FIG.7 b  shows a cross sectional view from the top of the fuel intake manifold of FIG. 10 a  taken through cut BB; 
     FIG.7 c  is a cross sectional view from the top of the fuel intake manifold of FIG. 10 a  taken through cut AA, showing the fuel jet nozzles.; 
     FIG. 8 is a cross section of a burner and heater head assembly showing the placement of temperature sensors in accordance with an embodiment of the invention. 
     FIG. 9 shows the relationship of the optimal fuel-air ratio to the air preheat temperature for propane in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 2, one embodiment of a Stirling cycle engine is shown in cross-section and is designated generally by numeral  28 . While the invention will be described generally with reference to the Stirling engine shown in FIG. 2, it is to be understood that many engines as well as refrigerators may similarly benefit from various embodiments and improvements which are subjects of the present invention. The configuration of Stirling engine  28  shown in FIG. 2 is referred to as an alpha configuration, characterized in that compression piston  30  and expansion piston  32  undergo linear motion within respective and distinct cylinders: compression piston  30  in compression cylinder  34  and expansion piston  32  in expansion cylinder  36 . The alpha configuration is discussed by way of example only, and without limitation of the scope of any appended claims. 
     In addition to compression piston  30  and expansion piston  32 , the main components of Stirling engine  28  include heater  64 , regenerator  66 , and cooler  68 . Compression piston  30  and expansion piston  32 , referred to collectively as pistons, are constrained to move in reciprocating linear motion within respective volumes  38  and  40 . A cylinder liner  42  may line the respective cylinder surfaces. The volumes of the cylinder interior proximate to the heater  64  and cooler  68  will be referred to, herein, as hot and cold sections, respectively, of engine  28 . The relative phase (the “phase angle”) of the reciprocating linear motion of compression piston  30  and expansion piston  32  is governed by their respective coupling to drive mechanism  44  housed in crankcase  46 . Drive mechanism  44 , discussed in greater detail below, may be employed to govern the relative timing of pistons and to interconvert linear and rotary motion. Compression piston  30  and expansion piston  32  are coupled, respectively, to drive mechanism  44  via a first connecting rod  48  and a second connecting rod  50 . The volume  38  of compression cylinder  34  is coupled to cooler  68  via duct  45  to allow cyclic cooling of working fluid. Duct  45 , more particularly, couples compression volume  38  to the annular heat exchangers comprising cooler  68 , regenerator  66 , and heater  64 . Branching of flow between duct  45  and annular plenum  47  is discussed below with reference to FIG.  6 . 
     The operation of drive mechanism  44  is now discussed with reference to FIGS. 3 and 4. FIG. 3 is a cross-sectional view of a dual folded guide link drive mechanism designated generally by numeral  300 . For a more detailed discussion of a preferred folded guide link drive, see pending U.S. patent application Ser. No. 09/335,392, filed Jun. 17, 1999, entitled “Folded Guide Link Stirling Engine”, which is incorporated herein by reference. The drive mechanism  300  in FIG. 3 comprises two folded guide links  303  and  313 . Pistons  301  and  311  are the displacer and compression pistons, respectively, of a Stirling cycle engine such as described above with respect to FIG.  2 . As used in this description and the following claims, a displacer piston is either a piston without a seal or a piston with a seal (commonly known as an “expansion” piston). Displacer piston  301  is rigidly coupled to the piston end of guide link  303  at a piston connection point  302 . Guide link  303  is rotatably connected to a connecting rod  306  at a rod connection point  305 . The piston connection point  302  and the rod connection point  305  define the longitudinal axis  324  of guide link  303 . 
     Connecting rod  306  is rotatably connected to a crankshaft  308  at a crankshaft connection point  307  which is offset a fixed distance from the crankshaft axis of rotation  326 . The crankshaft axis of rotation  326  is orthogonal to the longitudinal axis  324  of the guide link  303  and the crankshaft axis of rotation  326  is disposed between the rod connection point  305  and the piston connection point  302 . In a preferred embodiment, the crankshaft axis of rotation  326  intersects the longitudinal axis  324 . 
     An end  328  of guide link  303  is constrained between a pair of rollers  304 . In a preferred embodiment, one of the rollers  304  is spring loaded to maintain rolling contact with the guide link  303 . Alignment of the longitudinal axis  324  of the guide link  303  with respect to piston cylinder  322  is maintained by the rollers  304  and by the piston  301 . As crankshaft  308  rotates about the crankshaft axis of rotation  326 , the rod connection point  305  traces a linear path along the longitudinal axis  324  of the guide link  303 . 
     Piston  301  and guide link  303  form a lever with the piston  301  at one end of the lever and the rod end  328  of the guide link  303  at the other end of the lever. The fulcrum of the lever is on the line defined by the centers of the rollers  304 . The lever is loaded by a force applied at the rod connection point  305 . As rod connection point  305  traces a path along the longitudinal axis of the guide link  303 , the distance between the rod connection point  305  and the fulcrum, the first lever arm, will vary from zero to one-half the stroke distance of the piston  301 . The second lever arm is the distance from the fulcrum to the piston  301 . The lever ratio of the second lever arm to the first lever arm will always be greater than one, preferably in the range from 5 to 15. The lateral force at the piston  301  will be the forced applied at the rod connection point  305  scaled by the lever ratio; the larger the lever ratio, the smaller the lateral force at the piston  301 . 
     The compression piston  311  is rigidly coupled to the piston end of guide link  313  at a piston connection point  312 . Guide link  313  is rotatably connected to a connecting rod  316  at a rod connection point  315 . The piston connection point  312  and the rod connection point  315  define the longitudinal axis of guide link  313 . Connecting rod  316  is rotatably connected to the crankshaft  308  at a crankshaft connection point  317  which is offset a fixed distance from the crankshaft axis of rotation  326 . An end  330  of guide link  313  is constrained between a pair of rollers  314 . As discussed above, in a preferred embodiment on of the rollers  314  is spring loaded to maintain rolling contact with the guide link  313 . The operation of guide link  313  is similar to that described above with respect to guide link  303 . Alignment of the longitudinal axis of guide link  313  with respect to piston cylinder 320  is maintained by the rollers  314  and by the piston  301 . As crankshaft  308  rotates about the crankshaft axis of rotation  326 , the rod connection point  305  traces a linear path along the longitudinal axis of the guide link  313 . 
     FIG. 4 is a perspective view of the dual folded guide link drive mechanism shown in FIG.  3 . Compression piston  311  and displacer piston  301  undergo linear motion within respective and distinct cylinders: compression piston  311  in compression cylinder  320  and displacer piston  301  in expansion cylinder  322 . Guide link  303  and guide link  313  are rigidly coupled to displacer piston  301  and compression piston  311  at piston connection points  302  and  312  respectively (shown in FIG.  3 ). Connecting rods  306  and  316  are rotationally coupled at connection points  305  and  315  of the distal ends of guide links  303  and  313  to crankshaft  308  at crankshaft connection points  307  and  317  (shown in FIG.  3 ). Lateral loads on guide links  303  and  313  are taken up by roller pairs  304  and  314 . 
     Referring now to FIGS. 5 a - 5   c , a novel structure is depicted, in accordance with an embodiment of the present invention, for transferring large amounts of heat from the combustion source to the interior of Stirling cycle engine  28 , shown in cross section. In order to increase the efficiency of heat transfer from hot gases  300 , generated by burner  150 , to the working fluid contained in the interior volume  306  of the engine, a large wetted surface area, on either side of heater head  64  is required. To achieve the high surface area, a large number of metal pins  310  are fabricated on either one of or both the interior surface  312  and exterior surface  314  of heater head  64 . Fabrication may be accomplished at low cost, such as by investment casting. Metal pins  310  not only increase the wetted surface area on either side of heater head  64  but also create turbulent wakes that increase fluid mixing and thereby further increase the flow of heat. This structure may also be employed for heat transfer at the cooler  68  (shown in FIG. 2) or in any application where efficient heat transfer is required between volumes of gases. FIG. 5 c  shows a cross-sectional view of the heater head assembly of FIG. 5 a  with het transfer pins  130  and  124  shown schematically in accordance with a preferred embodiment of the invention. In FIG. 5 c , inner heat transfer pins  124  and outer heat transfer pins  130  are located along the sides of the heater head  64 . The use and method of manufacture of heat transfer pins is described in copending U.S. patent application Ser. No. 09/115,381, filed Jul. 14, 1998 and copending U.S. patent application Ser. No. 09/517,245, filed Mar. 2, 2000, titled “Stirling Engine Thermal System Improvements,” which are herein incorporated by reference. 
     Referring to FIG. 6 a , a perspective view is shown of a system of header ducts  400  providing for the flow of working fluid between compression volume  38  and the annular region of fluid flow through the heat exchange network, namely past cooler head  68 , through regenerator  66  (shown in FIG.  2 ), and past heater head  64  (shown in FIG.  2 ). The annular flow of working fluid culminates at annular header  47  to which branching ducts  400  are coupled for creating equal-length flow passages between cylinder volume  38  and the entire annular region of header  47 . By substantially equalizing the flow impedance between every portion of the annular flow region and the cylinder volume, losses due to flow non-uniformities through the heat exchangers may be advantageously reduced, and, additionally, the flow of working fluid within a loop confined to the heat exchange region and thereby lost for purposes of mechanical work may be minimized. FIG. 6 b  shows a schematic of the system of branching ducts  400  of FIG. 6 a , “unwrapped” into a planar view, showing the fluid communication via branching ducts  400  between compression space  38  and annular header  47 . 
     While Stirling engines are capable of providing high thermal efficiency and low emission of pollutants, these objectives impose requirements of thermal efficiency, in particular, on a burner  806  employed to heat heater head  808  of the Stirling engine as shown in FIG.  8 . Components of such thermal efficiency include the efficient pumping of oxidant (typically, air, and, referred to herein and in any appended claims, without limitation, as “air”) through the burner  806  to provide combustion, and the recovery of hot exhaust leaving the heater head  808 . In many applications, air (or other oxidant) is pre-heated, prior to combustion, nearly to the temperature of the heater head  808 , so as to achieve the stated objectives of 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 the burner. In order to achieve high efficiency and low emissions, the burner must provide substantially complete combustion. In order to achieve substantially complete combustion, a measured amount of air as well as a clean burning fuel, preferably propane, are delivered to the burner. The fuel and air flow rates are controlled in order to allow for ignition of a flame in the burner as well as for clean emissions after ignition. The fuel and air must also be well-mixed with sufficient amounts of oxygen to limit the emission of carbon monoxide (CO) and hydrocarbons and, additionally, must be burned at low enough flame temperatures to limit the formation of oxides of nitrogen (NO x ). 
     The high temperature of preheated air, 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 and in any appended claims, 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. One solution to this problem is to use a non-premixed diffusion flame. However, since such diffusion flames are not well-mixed, higher than desirable emissions of CO, HC and NO x  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. Any increased air flow provided to limit flame temperatures typically increases the power consumed by an air pump or blower, thereby degrading overall engine efficiency. 
     In accordance with 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. 
     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. 
     Referring now to FIGS. 7 a - 7   c , an intake manifold  699  is shown for application to a Stirling cycle engine or other combustion application in accordance with an embodiment of the present invention. In accordance with a preferred embodiment of the invention, fuel is pre-mixed with air that may be heated above the fuel&#39;s auto-ignition temperature and a flame is prevented from forming until the fuel and air are well-mixed and in the combustion chamber  809  (shown in FIG.  8 . FIG. 7 a  shows a preferred embodiment of the apparatus including an intake manifold  699  and a combustion chamber  710 . The intake manifold  699  has an axisymmetrical conduit  701  with an inlet manifold  703  for receiving air  700  supplied via the blower  728 . Air  700  is pre-heated to a temperature, typically above 1000 K, which may be above the auto-ignition temperature of the fuel. Conduit  701  conveys air  700  flowing inward radially with respect to combustion axis  720  to a swirler  702  disposed within the conduit  701 . 
     FIG. 7 b  shows a cross sectional view of the conduit  701  including swirler  702  in accordance with an embodiment of the invention. In the embodiment of FIG. 7 b , swirler  702  has several spiral-shaped vanes  730  for directing the flow of air  700  radially inward and imparting a rotational component on the air. The diameter of the swirler section of the conduit decreases from the inlet  732  to the outlet  734  of swirler  702  as defined by the length of swirler vanes  730 . The decrease in diameter of swirler vanes  730  increases the flow rate of air  700  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  734  of swirler  702 , fuel  706 , which in a preferred embodiment is propane, is injected into the inwardly flowing air. 
     In a preferred embodiment, fuel  706  is injected by fuel injector  704  through a series of nozzles  736  as shown in FIG. 7 c . More particularly, FIG. 7 c  shows a cross sectional view of conduit  701  and includes the fuel jet nozzles  736 . Each of the nozzles  736  is positioned at the exit of the swirler vanes  730  and is centralized between two adjacent vanes. Nozzles  736  are positioned in this way for increasing the efficiency of mixing the air and fuel. The fuel jet nozzles  736  are sized to provide jets of fuel that extend at least half way across the conduit  701  (shown in FIGS. 7 a  and  7   b ). Calculations to size the fuel jet nozzles  736  are well known in the art and described in Boer and Chigier, “Combustion Aerodynamics,” John Wiley &amp; Sons, 1972. Nozzles  736  simultaneously inject the fuel  706  across the air flow  700 . 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. 
     Referring again to FIG. 7 a , the air and fuel, now mixed, referred to hereafter as “air-fuel mixture”  709 , is transitioned in direction through a throat  708  which has a contoured fairing  722  and is attached to the outlet  707  of the conduit  701 . Fuel  706  is supplied via fuel regulator  724 . Throat  708  has an inner radius  714  and an outer dimension  716 . The transition of the air-fuel mixture is from a direction which is substantially transverse and radially inward with respect to combustion axis  720  to a direction which is substantially parallel to the combustion axis. The contour of the fairing  722  of throat  708  has the shape of an inverted bell such that the cross sectional area of throat  708  with respect to the combustion axis remains constant from the inlet  711  of the throat to outlet  712  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  708  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. 
     Outlet  712  of the throat flares outward allowing the air-fuel mixture  709  to disperse into the chamber  710  slowing the air-fuel mixture  709  thereby localizing and containing the flame and causing a toroidal flame to form. The rotational momentum generated by the swirler  602  produces a flame stabilizing ring vortex as well known in the art. 
     As discussed above, the fuel and air flow rates are controlled in order to allow for ignition of a flame in the burner as well as for clean emissions after ignition. Referring to FIG. 7 a , burner controller  726  is used to control the fuel and air flow rates provided by fuel regulator  724  and blower  728  respectively. The fuel regulator  724  is set to an initial value for ignition. Once the flame is proved, the burner controller  726  varies the fuel flow rate to control the heater head temperature as measured by a head temperature sensor  804  (shown in FIG.  8 ). A flame is proved when a flame detector detects the presence of the flame. There are several types of flame detectors including thermocouples and ultraviolet sensors known in the art. Flame detection methods are discussed in copending U.S. patent application Ser. No. 09/517,808, filed Mar. 2, 2000, which is herein incorporated by reference. 
     The output (or air mass flow rate) of the combustion air blower  728  is set by the burner controller  726  to control the fuel-air ratio in the combustion chamber  809  (shown in FIG.  8 ). The fuel-air ratio is the ratio of the fuel mass flow rate over the air mass flow rate and is the primary factor affecting emissions. The blower  728  controls the fuel-air ratio by increasing or decreasing the air mass flow rate relative to the fuel mass flow rate. For example, in order to hold the fuel-air ratio constant, the burner controller  726  will increase the blower output as the fuel regulator  724  increases its output and vice versa. The desired fuel-air ratio and the fuel flow rate may be changing at the same time, so the burner controller  726  will change the output of the blower  728  to accommodate both the change in desired fuel-air ratio and the fuel flow rate. 
     Minimizing the emissions of carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (N ox ) 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 a fixed amount of fuel, the emissions of CO, HC and NO x  will decrease until the amount of air in the fuel-air mixture 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 quickly 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 of the fuel-air mixture decreases as the temperature of the preheated air increases during the warmup phase of the Stirling engine. Once the engine is warmed up, the fuel-air ratio is held constant. 
     Accordingly, the fuel-air ratio must first be controlled to provided 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. When the fuel flow rate is increased or decreased to adjust the temperature of the heater head, the air flow rate is also adjusted to maintain the desired fuel-air ratio. 
     A given fuel will only ignite over a limited range of fuel-air ratios. At ignition, an ignition fuel-air ratio is 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 grams propane per gram of air. The ignition fuel-air ratio is maintained until the flame stabilizes and temperature of the interior of the combustion chamber increases to a warmup temperature. Referring to FIG. 8, the temperature of the combustion chamber  809  is typically determined by measuring the temperature of the heater head  808  or by allowing a predetermined time interval for the combustion chamber to heat. A temperature sensor, such as thermocouple  804 , may be used to measure the temperature of heater head  808 . In a preferred embodiment, the ignition fuel-air ratio is held until the heater head temperature reaches 300° C. and the flame has been lit for 5 seconds. 
     Once the flame is stabilized, and the temperature of the combustion chamber  809  reaches the desired warmup temperature, the fuel-air ratio is then controlled based upon the air preheat temperature and the fuel type. As described above, the optimal fuel-air ratio  901  of the fuel-air mixture decreases as the temperature of the preheated air  903  increases as shown in FIG.  9 . The temperature of the preheated air is measured using a temperature sensor, such as a thermocouple  810 , in an air swirler  802  coupled to the combustion chamber  806  as shown in FIG.  8 . The air preheat temperature can also be inferred from the heater head  808  temperature by subtracting several hundred degrees Celsius from the heater head temperature. In a preferred embodiment, the air preheat temperature is taken as the heater head temperature minus 300° C. 
     The optimal fuel-air ratio will first decrease linearly with the preheated air temperature from a “start” fuel-air ratio for room temperature air to a “run” fuel air ratio, for a warmed up preheated air temperature. The air is considered fully warmed up when it exceeds the known auto-ignition temperature for the fuel. For example, the auto-ignition temperature for propane is 490° C. In a preferred embodiment, where the fuel is propane, the “start” fuel-air ratio is 0.052 grams of fuel to grams of air, which results in approximately 4% oxygen in the exhaust of the Stirling engine. The “run” fuel-air ratio in the preferred embodiment, is 0.026 grams of fuel to grams of air, which results in approximately 13% oxygen in the exhaust of the Stirling engine. 
     The fuel-air ratio may be determined by measuring the air and fuel flow rates. A pressure sensor may be used to measure the air-flow rate at the blower  728  (shown in FIG. 7 a ). The fuel flow rate may be determined by measuring the pressure upstream and downstream of a set of fuel control valves of fuel regulator  724  (shown in FIG. 7 a ) and by monitoring which of the valves is currently open. In an alternative embodiment, the fuel-air ratio may be based on a measurement of the oxygen content in the exhaust of the Stirling engine. An oxygen sensor may be placed in the engine to sample the exhaust gas and measure the percentage of oxygen in the exhaust of the engine. 
     The devices and methods described herein may be applied in other applications besides the Stirling engine in terms of which the invention has been described. 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.