Patent Publication Number: US-7895985-B2

Title: Compliant walled combustion devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation and claims priority under U.S.C. §120 from U.S. patent application Ser. No. 11/763,148, filed Jun. 14, 2007 and entitled “COMPLIANT WALLED COMBUSTION DEVICES FOR PRODUCING MECHANICAL AND ELECTRICAL ENERGY”, which is incorporated herein for all purposes; this Ser. No. 11/763,148 application claimed priority under U.S.C. §120 from U.S. patent application Ser. No. 11/134,077, filed May 19, 2005 and entitled “COMPLIANT WALLED COMBUSTION DEVICES”, issued as U.S. Pat. No. 7,237,524 on Jul. 3, 2007, which is incorporated herein for all purposes; the Ser. No. 11/134,077 application claimed priority under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 60/574,891 filed May 26, 2004, naming R. Pelrine et al. as inventors, and titled “Polymer Engines For Lightweight Portable Power”, which is incorporated by reference herein in its entirety for all purposes; the Ser. No. 11/134,077 application also claimed priority under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 60/608,741 filed Sep. 9, 2004, which is also incorporated by reference herein in its entirety for all purposes. 
    
    
     U.S. GOVERNMENT RIGHTS 
     This invention was funded in part with Government support under contract number DAAD19-03C-0067 awarded by the United States Army. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to combustion devices that convert chemical energy stored in a fuel to mechanical energy. More particularly, the present invention relates to combustion devices that include one or more compliant sections or walls that deform in response to combustion. 
     BACKGROUND OF THE INVENTION 
     Combustion devices that employ a metal piston and rigid combustion chamber to generate mechanical power are well developed and widely used. 
     Conventional combustion devices tend to be relatively heavy and non-portable. At smaller scales and lower weights, the efficiency of combustion systems rapidly decreases. Small-scale engines also suffer from leakage in the piston-cylinder gap, which is normally a negligible loss for larger engines. Since the piston-cylinder gap cannot be readily scaled down with engine size, leakage becomes more problematic as engine size decreases. Other problems associated with rigid combustion-based systems—at any size—include corrosion, temperature warping in small gaps, and wear. Rigid combustion systems of any size also need to be relatively heavy to achieve the rigidity needed to maintain tight tolerances in the piston-cylinder gap. 
     Many portable devices employ one or more batteries as a power source. Disposable or rechargeable batteries are used in most portable electronic devices for example. Intermittent bursts of power are important in the design and operation of many portable devices, where batteries often fall short. Batteries by themselves also offer no mechanical output; electrical output from them must be supplied to a motor to produce mechanical work. 
     In view of the foregoing, alternative power generation and combustion devices, particularly those suitable for mobile and portable use, would be desirable. 
     SUMMARY OF THE INVENTION 
     Combustion devices of the present invention employ a compliant wall or segment that borders at least a part of a combustion chamber and deforms in response to pressure generated during combustion of a fuel in the combustion chamber. 
     Some compliant walls or segments stretch during combustion. The compliant segment may decrease in thickness during the stretch. Compliant segment thickness decreases often lead to a dynamic increase in combustion chamber volume. This raises maximum volume for a combustion chamber, which increases combustion efficiency and volume displacement for a given linear displacement. 
     Compliant segments and walls may also dynamically vary surface area of the combustion chamber, which improves thermal management. During and after combustion, compliant walls may increase their surface area and provide a greater area for conductive heat transfer out of the chamber. When a compliant wall thins, the conductive heat transfer path through the wall also shortens, which further increases thermal dissipation. 
     Some combustion devices elastically stretch a compliant wall during combustion. Elastic return of the compliant wall may be used to facilitate exhaust of combustion products from a combustion chamber. 
     In one aspect, the present invention relates to a combustion device for producing mechanical energy from a fuel. The combustion device comprises a set of walls that border a combustion chamber. The set of walls include a compliant segment configured to deform to increase volume of the chamber during combustion of the fuel in the combustion chamber. The combustion device also comprises a coupling portion that translates the increase in the volume of the chamber into mechanical output. The combustion device further comprises one or more ports configured to inlet an oxygen source and fuel into the combustion chamber and to outlet exhaust gases from the combustion chamber. 
     In another aspect, the present invention relates to a combustion device for producing mechanical energy from a fuel. The combustion device comprises a constraint that reduces deformation of a portion of a compliant segment during combustion. 
     In yet another aspect, the present invention relates to a method for producing mechanical energy from a fuel. The method comprises providing a fuel and oxygen into a combustion chamber. The method also comprises combusting the fuel in the combustion chamber. The method further comprises decreasing thickness for a portion of a compliant segment included in a set of walls that border the combustion chamber such that volume for the combustion chamber increases with the thickness decrease. 
     In still another aspect, the present invention relates to a method for improving thermal management of a combustion device. The method comprises stretching a compliant segment included in a set of walls that border the combustion chamber. Stretching the compliant segment increases surface area for the set of walls that border the combustion chamber. The method also comprises dissipating heat produced in the combustion chamber through the stretched compliant segment. 
     In another aspect, the present invention relates to a combustion device for producing mechanical energy from a fuel. The combustion device comprises a set of walls that border a substantially cylindrical combustion chamber. The set of walls include a substantially cylindrical compliant segment configured to axially stretch during combustion of the fuel in the combustion chamber such that a diameter for the substantially cylindrical combustion chamber increases during combustion of the fuel. 
     In yet another aspect, the present invention relates to a combustion device for producing mechanical energy from a fuel. The combustion device comprises a set of walls that border a combustion chamber. The set of walls include a compliant segment configured to stretch during combustion of the fuel in the combustion chamber such that thickness for the compliant segment decreases during combustion of the fuel and such that volume for the combustion chamber increases as a result of the thickness decrease in the compliant segment. 
     In still another aspect, the present invention relates to a combustion cycle for producing mechanical energy from a fuel. The cycle comprises providing a fuel and oxygen into a combustion chamber. The cycle also comprises combusting the fuel in the combustion chamber. The cycle further comprises, using forces generated in the combustion, stretching a compliant segment included in a set of walls that border the combustion chamber. The cycle additionally comprises at least partially exhausting combustion products using elastic return of the stretched segment. 
     These and other features and advantages of the present invention will be described in the following description of the invention and associated figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a simplified combustion device in accordance with one embodiment of the present invention. 
         FIG. 1B  illustrates the combustion device of  FIG. 1A  after combustion. 
         FIG. 2A  illustrates a simplified cross-section of a cylindrical combustion device, before combustion, in accordance with one embodiment of the present invention. 
         FIG. 2B  illustrates the cylindrical combustion device of  FIG. 2A  after combustion. 
         FIG. 3A  illustrates a simplified cross-section of a cylindrical combustion device, before combustion, in accordance with one embodiment of the present invention. 
         FIG. 3B  illustrates the cylindrical combustion device of  FIG. 3A  during intake of fuel and an oxygen source. 
         FIG. 3C  illustrates the cylindrical combustion device of  FIG. 3A  during combustion. 
         FIG. 3D  illustrates the cylindrical combustion device of  FIG. 3A  after exhaust is complete. 
         FIG. 4A  illustrates a cross-section of a cylindrical combustion device, before combustion, in accordance with another embodiment of the present invention. 
         FIG. 4B  illustrates the cylindrical combustion device of  FIG. 4A  during combustion. 
         FIG. 5A  illustrates a simplified cross-section of a radial combustion device, before combustion, in accordance with one embodiment of the present invention. 
         FIG. 5B  illustrates the radial combustion device of  FIG. 5A  after fuel intake. 
         FIG. 5C  illustrates the radial combustion device of  FIG. 5A  after combustion. 
         FIG. 6A  illustrates a simplified cross-section of a sheathed combustion device in accordance with one embodiment of the present invention. 
         FIG. 6B  illustrates the sheathed combustion device of  FIG. 6A  after combustion. 
         FIG. 7A  illustrates a simplified cross-section of a bellows combustion device in accordance with another embodiment of the present invention. 
         FIG. 7B  illustrates bellows combustion device of  FIG. 7A  after combustion. 
         FIG. 8A  illustrates a simplified cross-section of a bellows combustion device in accordance with another embodiment of the present invention. 
         FIG. 8B  illustrates the bellows combustion device of  FIG. 8A  after combustion. 
         FIG. 9A  illustrates a simplified cross-section of a combustion device in accordance with another embodiment of the present invention. 
         FIG. 9B  illustrates the combustion device of  FIG. 9A  after combustion. 
         FIG. 10A  illustrates a shape changing combustion device in accordance with one embodiment of the present invention. 
         FIG. 10B  illustrates the combustion device of  FIG. 10A  after combustion. 
         FIG. 10C  illustrates the combustion device of  FIG. 10A  after exhaust. 
         FIG. 11A  illustrates a combustion device including a compliant wall that is configured to provide a compliant wall in one direction of the sealed combustion chamber in accordance with another embodiment of the present invention. 
         FIG. 11B  illustrates the combustion device of  FIG. 11A  after combustion. 
         FIG. 12A  illustrates a membrane fuel control combustion device in accordance with another embodiment of the present invention. 
         FIG. 12B  illustrates the combustion device of  FIG. 12A  after fuel intake. 
         FIG. 12C  illustrates the combustion device of  FIG. 12A  after combustion. 
         FIGS. 13A and 13B  illustrate dynamic dimensions for the combustion device of  FIG. 2A . 
         FIG. 14A  illustrates a process flow for producing mechanical energy from a fuel in accordance with one embodiment of the present invention. 
         FIG. 14B  illustrates a process flow for improving thermal management of a combustion device in accordance with one embodiment of the present invention. 
         FIG. 15A  illustrates a combustion cycle for producing mechanical energy from a fuel in accordance with one embodiment of the present invention. 
         FIG. 15B  illustrates a process flow for producing mechanical energy from a fuel in accordance with another embodiment of the present invention. 
         FIG. 16  illustrates a perspective view of a simplified motor in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described in detail with reference to a few preferred embodiments as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. 
     Overview 
     Combustion refers to a rapid chemical change that produces mechanical energy. The chemical change usually burns a fuel to produce heated gases and pressure resulting from expansion of the heated gases. Combustion thus allows a small amount of fuel, when ignited in a combustion chamber, to produce mechanical energy in the form of an expanding gas. 
     Combustion devices of the present invention include a compliant wall or compliant segment that stretches in response to mechanical energy communicated by an expanding gas. Coupling to a portion of the combustion device permits the mechanical energy to perform useful work. In some embodiments, a combustion device includes a single material (other than any mechanisms employed for inlet to and exhaust from the combustion chamber) where one portion of the material moves, another portion remains stationary, and a compliant segment that deforms to permit relative motion between the moving and stationary portions. 
       FIG. 1A  shows a simplified combustion device  10  in accordance with one embodiment of the present invention.  FIG. 1B  illustrates device  10  after combustion in combustion chamber  14 . Combustion device  10  relies on deformation of a segment  19  of a compliant wall  15  to harness combustion energy and provide mechanical output. While the present invention will now be discussed in terms combustion devices and components include therein, those skilled in the art will appreciate that the following discussion will also illuminate methods and discrete steps for using combustion devices and for producing mechanical energy from a fuel. 
     Combustion device  10  includes a set of walls  12  and  15  that border a combustion chamber  14 . Walls  12  are rigid, while wall  15  is compliant. In general, a combustion device of the present invention may include any number of walls of any geometry suitable for bounding and defining dimensions a combustion chamber  14 . At least one wall—or a portion thereof—in device  10  includes a compliant segment  19  or compliant wall  15  that deforms, e.g., stretches, in response to forces generated by combustion of a fuel in combustion chamber  14 . As will be described below, compliant wall  15  may constitute varying proportions of the wall surface surrounding combustion chamber  14  and may include numerous geometries based on a particular combustion device design. The compliant wall  15  may also include one or more rigid portions, e.g.  19  may be a metal or rigid plastic reinforcement of complaint wall  15 . In some cases, noncompliant walls  12  may be included such that mechanical energy in chamber  14  acts on a smaller area for compliant wall  15  or segment  19  and increases the force or displacement of compliant wall  15  and mechanical output  23 . Combustion chamber  14  geometries, compliant wall  15  and compliant segment  19  configurations, and chamber wall configurations may vary. For example, the combustion chamber and compliant wall may include a diaphragm, tubular (cylindrical), balloon, or other volume-enclosing arrangement. Several exemplary geometries and configurations are described below. 
     Unconstrained portions of compliant wall  15 , such as compliant segment  19 , deform in response to expanding gases and pressure generated by combustion of a fuel  25  in combustion chamber  14 . In general, deformation of a compliant segment or wall refers to any stretch, displacement, expansion, bending, contraction, torsion, linear or area strain, combinations thereof, or any other deformation of a portion of the compliant wall  15 . In one embodiment, compliant segment  19  stretches in response to expanding gases and pressure caused by combustion of fuel  25 . Elastic stretching of a compliant wall  15  or segment  19  also stores elastic mechanical energy. Several embodiments of the present invention make use of elastic energy storage in wall  15  or segment  19 . For example, after combustion, compliant wall  15  may elastically return to a pre-combustion state or position, which provides a mechanism for assisting exhaust of combustion gases from chamber  14 . While some designs elastically stretch to expand the combustion chamber, other designs employ more of a bending mode, or both bending and stretching. Various materials and configurations for compliant wall  15  are described in further detail below. 
     For the device  10  of  FIG. 1 , compliant wall  15  forms a top wall of the combustion chamber  14 . In some cases, compliant wall  15  includes portions that do not stretch, such as those used for fixing compliant wall  15  to one or more rigid walls included in the set of walls  12  or a mechanical output. For the device of  FIG. 1 , a central portion of compliant wall  15  attaches to a rigid mechanical output  23 . This leaves a compliant segment  19  that includes all portions of compliant wall  15  not attached to mechanical output  23  or portions of compliant wall  15  used to attach to rigid walls  12 . When combustion chamber  14  is substantially cylindrical and mechanical output  23  is round, compliant segment  19  resembles a donut shape on wall  15 . In another embodiment, central segment  19  is not compliant and includes a stiffer material than compliant wall  15 . In this case, the central segment  19  is relatively rigid and the compliant segment for device  10  includes an outer ring around the central rigid segment  19 ; this allows compliant wall/segment  15  to expand and drive the central rigid segment  19  and mechanical output  23  attached thereto. 
     The set of walls  12  (including compliant wall  15 ) cooperate to form and enclose combustion chamber  14 . As the term is used herein, a combustion chamber refers to an enclosed space in which combustion of a fuel occurs to produce mechanical energy. A wide variety of physical configurations may be used for the combustion chamber. By way of example, suitable physical configurations may include spherical geometries, square and rectangular geometries, cylindrical geometries, oval and elliptical geometries, and a variety of other geometries (several of which are described below). In general, the present invention is not limited to any particular combustion chamber design or shape. 
     The volume of combustion chamber  14  varies as compliant wall  15  deforms. Combustion chamber  14  typically has a maximum volume and a minimum volume. ‘Displacement’ refers to the difference between the maximum and minimum volume. Typically, increasing displacement permits greater mechanical output for a combustion device. For some combustion devices, the maximum volume additionally increases as a compliant wall  15  or segment  19  stretches and its thickness decreases. 
     In one embodiment, combustion device  10  includes no piston that translates within the combustion chamber. In many cases, combustion device includes no moving parts internal to combustion chamber  14  other than any inlet or outlet valve mechanisms (or parts thereof) disposed within the combustion chamber. These designs avoid friction between moving parts within the combustion chamber  14  and reduce energy losses that result from frictional heat generation. These designs also avoid the need for lubrication in combustion chamber  14  between moving parts. Some designs may include a piston as mechanical output coupled to the outside of compliant wall  15  and acting as a linear mechanical output  23  to use energy produced within chamber  14 , but even in these instances, the designs include no piston that translates within the combustion chamber. This is in contrast to conventional combustion chambers where the piston is internal to the combustion chamber (or it forms a wall that translates in the cylinder, requires sealing, and requires lubrication internal to the cylinder to reduce friction between moving parts). 
     Combustion device  10  includes one or more ports configured to inlet an oxygen source such as air and fuel into combustion chamber  14  and to outlet exhaust gases from combustion chamber  14 . Inlet and outlet of reactants and products into and out from a combustion chamber is well known to one of skill in the art and the present invention is not limited by how reactants are provided to a combustion chamber and how products are removed from the combustion chamber. Slightly pressurized fresh fuel-air can be injected through and inlet port to force out exhaust through an outlet port, for example. Other higher efficiency methods are known in the prior art and some are described later in this patent. Some combustion device designs may include a single and common inlet/outlet port. In other designs two ports may be provided. By way of example, in the embodiment shown in  FIG. 1A , device  10  includes two ports: an inlet port  20  and an outlet port  22 . In other designs three or more ports may be employed. 
     Intake port  20  permits an oxygen source and fuel passage into combustion chamber  14 . Intake port  20 , also commonly referred to as an intake valve, opens at specified times to let in air and/or fuel into combustion chamber  14 . Device  10  inlets a combined air/fuel mixture. In a specific embodiment, intake port  20  includes valve sealed by an electrostatic clamp or an electroactive polymer actuated valve, or a valve incorporating both. Other actuated valves such as solenoid valves are known in the prior art and can be used. In another embodiment, device  10  includes separate and dedicated air and fuel ports  20 . 
     An oxygen source is supplied to combustion device  10 . Air readily provides oxygen, but other oxygen sources and oxidizing agents may be used. For example, the oxygen source may include O 2 -enriched air, or pure oxygen. O 2  enrichment in the combustion air can reduce inert gas volume (i.e., N 2 ) and increase combustion capacity. The oxidizing agent may include a chemical oxidant beyond oxygen or air, as one of skill in the art will appreciate. While the present invention will now primarily be described with respect to air as the oxygen source in a combustion device, it is understood that other oxidants beyond oxygen or air may also be used. 
     Fuel  25  acts as a source of chemical energy for combustion device  10 . Fuel  25  may be stored in a separate storage device, such as a tank. In some embodiments, a pump of some type transfers fuel  25  from storage to fuel inlet  20 . In other embodiments, the fuel is stored under a pressure that is higher than atmospheric pressure, and its intake regulated by a valve. If the combustion device includes carburetion, the pump may also move external air or a stored oxidizer into combustion chamber  14 . Fuel  25  may be stored in a liquid, gaseous, solid or gel-state. Exemplary fuels  25  suitable for use with the present invention include hydrocarbon based fuels such as propane, butane, natural gas, kerosene, gasoline, diesel, coal-derived fuels, JP8, hydrogen and the like. As with most engines, butane or propane are relatively easier fuels to burn. 
     Exhaust port  22  permits the discharge of combustion products. Exhaust port  22 , which is also commonly referred to as an exhaust valve, opens at specified times in a combustion cycle to let out exhaust gases. The exhaust includes chemical products of the combustion process, along with any unprocessed reactants such as unconsumed fuel or extra air. Device  10  may include multiple exhaust ports  22  to improve exhaust of combustion products from combustion chamber  14 . Additional exhaust system components may receive exhaust gases from port  22  and direct them as desired. For example, mechanical devices may be included to decrease back pressure for removing gases from combustion chamber  14 . Outlet of exhaust from a combustion chamber is well known to one of skill in the art and the present invention is not limited by how products are exhausted from a combustion chamber. 
     Coupling portions  18  and  13  each generally refer to a portion of device  10  that permits external mechanical attachment to device  10 . Typically, one of coupling portions  18  and  13  remains stationary relative to device  10 , while the other is configured to move relative to the stationary portion during combustion of fuel  25  in combustion chamber  14  and deformation of compliant wall  15 . As shown in  FIG. 1A , coupling portion  18  includes stationary rigid wall  12   a . Attachment to coupling portion  18  prevents rigid portions of combustion device  10  from moving (e.g. rigid walls  12   a - c ). Coupling portion  13  includes a central portion of compliant wall  15  that translates with deformation of compliant segment  19 . An adhesive may be used to attach an external object to a wall or portion of device  10 , such as an adhesive that attaches mechanical output  23  to complaint wall  15 , or another adhesive that attaches wall  12   a  to a fixed object. Suitable adhesives will depend on the materials being joined, as one of skill in the art will appreciate. Screws may also be used to attach to a portion of device  10 , such as fixing wall  12   a  to a stationary object. 
     Deformation of compliant segment  19  allows mechanical output from combustion device  10  for mechanical energy produced by combustion within chamber  14 . This deformation may be used to do mechanical work. 
     Output  23  couples to portion  13  and provides mechanical work. Coupling portion  13  includes a central area on the outer surface of complaint wall  15  that is externally attached to. Coupling between mechanical output  23  and portion  13  may include a) direct attachment between an outer surface of compliant wall  15  and mechanical output  23  and/or b) indirect attachment via one or more objects interconnected between the two components. Motion of output  23  may be constrained to linear translation by bearings (not shown) that limit movement of a shaft  23  to a single linear direction. In another embodiment, mechanical output  23  attaches to a large portion of the outside surface of compliant wall  15 . This avoids instances where the compliant wall  15  may deform around coupling portion  13  and resistive mechanical output  23 , and better converts combustion pressure to mechanical output  23 . One or more joints or other flexibility may be left in the coupling to allow vertical deformation of a large surface on compliant wall  15 . 
     Coupling to a combustion device may vary. For a cylindrical and linearly actuating combustion device  10  having a compliant cylindrical body (see  FIG. 2A ), coupling portion  13  may be disposed at one end of the cylindrical body, while stationary coupling portion  18  is disposed at the other cylindrical end and may attach to a pin that permits the combustion device  10  to pivot about the pin. Mechanical output  23  in this case may include connecting rod that interfaces with bearings and a crankshaft (see  FIG. 16 ). In this case, combustion of a fuel in the combustion chamber forces the compliant body to expand and coupling portion  13  to rotate about the crankshaft. Other examples of coupling portions  18  and  13  and output mechanisms  23  that convert mechanical energy in the form of expanding gas in the combustion chamber to useful mechanical work are described below. In general, the present invention is not limited to any mechanical output or coupling used to harness mechanical energy from a combustion device. It is understood that additional mechanical output or coupling may be added to device  10  to facilitate external attachment and use of device  10  in a particular application. In general, any external attachment communicates forces with combustion device  10  and the point or locations at which the forces enter or exit combustion device  10  may be considered a coupling portion. 
     Ignition mechanism  17  (see  FIG. 1B ) ignites the air/fuel mixture and initiates combustion in combustion chamber  14 . Common ignition mechanisms  17  include spark plugs and glow plugs, although other suitable ignition mechanisms may be used as well. A spark plug generates a spark via electrical input, and is typically timed according to a cycle such as at peak compression of an air/fuel mixture or position of the combustion chamber stroke. Some combustion devices of the present invention do not include an ignition mechanism and may rely on compression of the fuel to initiate spontaneous combustion. 
     In operation, air and fuel  25  enters combustion chamber  14 . The air/fuel mixture ignites (via either compression or active ignition). The resulting combustion creates expanding gases, typically at an elevated temperature, that increase pressure within combustion chamber  14 . The expanding gases and pressure stretch unconstrained portions of compliant wall  15  such as compliant segment  17 . Compliant segment  19  continues to stretch until mechanical forces balance the combustive forces driving the stretch (or until a crankshaft coupled to mechanical output  23  that drives displacement determines otherwise). The mechanical forces include elastic restoring forces of the compliant wall  15  material and any external resistance provided by a device and/or load(s) coupled to mechanical output  23 . The amount of stretching for wall  15  as a result of a combustion may also depend on a number of other factors such as the geometry and size of combustion chamber  14 , the number and size of compliant walls  15  in device  10 , the thickness and elastic modulus of each wall, the amount and type of fuel combusted, the compression ratio, the shape and size of mechanical output  23 , the amount of air present, etc. Typically, both inlet port  20  and exhaust port  22  are closed during compression and combustion. After combustion, exhaust port  22  opens and releases exhaust gases from combustion chamber  14 . Compression may be achieved, for example, using a crankshaft that couples to mechanical output  23  and drives compliant wall downward to decrease volume in the combustion chamber. 
     Compliant Walls 
     Having discussed an overview of a simplified combustion device in accordance with a specific embodiment of the present invention, exemplary compliant walls and materials will now be discussed. 
     As the term is used herein, a compliant wall generally refers to a wall that deforms in response to pressures or forces generated within a combustion chamber. In many instances, an entire wall is not free to deform in response to combustion forces. A compliant segment refers to a portion of a combustion chamber wall that deforms in response to pressures or forces generated within a combustion chamber. For example, ends of a compliant wall may be fixed while a central segment of the compliant wall is free to deform. Similarly, coupling portions of a compliant wall may be constrained from movement while another segment (such as the donut shape described above) is free to deform. In many embodiments, a compliant wall or compliant wall segment is configured to stretch during combustion of the fuel in the combustion chamber. While the discussion will now focus on compliant walls, it is understood that the following materials discussion also applied to compliant segments. 
     Stiffness of a compliant wall may vary according to design. In one embodiment, a stretching compliant wall includes an elastic modulus less than about 1 GPa. Bending walls may include a higher elastic modulus, such as Kevlar or another rigid material used in a bending design. A stretching compliant wall comprising an elastic modulus less than about 100 MPa is suitable for some applications. In a specific embodiment, a stretching compliant wall includes an elastic modulus less than about 10 MPa. Stiffness may be tailored for a device to achieve a desired amount of deformation, toughness, or device longevity. Decreasing stiffness provides more volumetric displacement within the combustion chamber for a given combustion pressure. Some devices may include a compliant wall with an elastic modulus between about 5 MPa and about 100 MPa. 
     Thickness of a compliant wall may be widely varied and the appropriate thickness will generally be a function of many factors, including size of the device or engine that incorporates the combustion chamber, the nature of the compliant material used, a desired useful life of the combustion chamber, a desired expansion of the combustion chamber, etc. By way of example a compliant wall thicknesses in the range of about 0.25 mm to about 4 cm (before combustion and deformation) is appropriate for many applications. In many applications, a compliant wall thickness in the range of about 5 mm to about 2 cm is suitable. Other thicknesses may be used. For example, walls thicker than 4 cm may also be used, although as the base thickness of the wall increases, it typically becomes more desirable to provide a cooling mechanism for the combustion device. After combustion, thickness of a compliant wall may vary with a number of factors such as pressures generated within the combustion chamber, temperatures generated within the chamber and stiffness for the compliant wall (based on the material elastic properties and any mechanical attachments). 
     For thick walls, the combustion device may include cooling structures such as water-cooled tubes within the wall. If the tubes are themselves compliant, action of the combustion device resulting from combustion may squeeze the tubes. Connecting one-way valves to the tubes then permits the device to pump its own cooling liquid. With these and other techniques, it should be noted that the effective thermal thickness of the wall (the distance heat needs to travel before being removed) may be less than the actual physical wall thickness. 
     In general, materials suitable for use with compliant walls described herein may include any material having suitable elastic properties and able to withstand the thermal loading associated with combustion. Exemplary materials may include polymers, acrylics, plastics, silicones, rubbers, reinforced fabrics (such as Kevlar), high temperature ceramic fabrics and papers provided they have minimal leakage (can be coated on the outer surface with an elastomer such as silicone), and structures made from combinations of rigid materials with flexible and compliant materials, for example. Exemplary polymers include high-density polyethylene and polyimide. Polymers with good temperature tolerance, such as high temperature acrylics and high temperature silicones, may be used. Polymer compliant walls suitable for use may include any compliant polymer or rubber (or combination thereof) having suitable elastic and thermal properties. Preferably, the polymer deformation is reversible over a wide range of strains. In a specific embodiment, compliant walls used with device  70  of  FIG. 2A  include HS IV RTV High Strength Moldmaking Silicone Rubber as produced by Dow Corning, Midland, Mich. 
     Relative to metals, most polymers include lower thermal conductance and thermal capacitance. As a result, the polymers absorb less heat from combustion within the combustion chamber and thereby increase efficiency. 
     With regard to heat tolerance, internal combustion gas temperatures may be much higher than the temperature of a chamber wall—due to localized cooling of the combustion gases. This is the approach taken in conventional engine designs. Indeed, the wall temperature of many conventional engines is typically limited to 150-260° C. (300-500° F.) because of oil lubricant usage. Some combustion devices made in accordance with the present invention have been operated with wall temperatures about 260° C., while many silicone materials for example are thermally rated above 300° C. 
     Experimental tests have established the viability of using high-temperature-combustion gases in compliant walled combustion devices. Firing frequencies in the range of 0.1 to 15 Hz have been used. Higher and lower frequency operation is contemplated. The combustion devices provided compliant wall tolerance to transient heating and used internal combustion gases in excess of 1000° C.; some tests used gases estimated to be in excess of 1500° C. Butane and propane were demonstrated in a combustion chamber up to 10,000 cycles, corresponding to about 3 hours of continuous operation at 1 Hz. Hydrogen fuel was also demonstrated. Longer lifetimes were also feasible in this instance; when the polymer engine tests were stopped upon reaching a 10,000-cycle target the combustion devices were still intact and functioning. In summary, internal combustion devices with compliant polymer walls and gas temperatures sufficiently high to enable useful and high efficiency have been developed and verified. 
     Varying Wall Thicknesses and Chamber Volumes 
     In many embodiments, compliant wall  15  decreases in thickness as a result of the stretching and expansion in an orthogonal planar direction. Decreasing thickness for a compliant wall increases combustion chamber volume for many designs. 
     In some cases, a compliant wall of the present invention can be described as substantially incompressible in volume for modeling and description purposes. That is, the compliant wall has a substantially constant volume under stress. For an incompressible compliant wall, the compliant wall decreases in thickness as a result of the expansion in an orthogonal planar direction. Decreasing thickness for a compliant wall may have volumetric and efficiency benefits for a combustion device. It is noted that the present invention is not limited to incompressible compliant walls and deformation of a compliant wall may not conform to such a simple relationship. 
     In one embodiment, thickness for a compliant wall—or portion of a compliant wall—decreases in response to combustion in the combustion chamber. Referring to  FIGS. 13A-13B  for example, device  50  may be characterized before combustion ( FIG. 13A ) by the following dimensions: an initial outer diameter, D o , an initial inner diameter, d o , an initial height, H o , and an initial wall thickness, t o . After combustion ( FIG. 13B ), device  50  may be characterized by the following dimensions: outer diameter, D o , inner diameter, d e , height, H e , and wall thickness, t e . As compliant wall  54  expands and stretches in height, thickness of compliant wall  54  decreases in the radial direction from t o  to t e . Thickness changes may occur for any compliant wall or segment for a combustion device described herein and not just the illustrative example shown in  FIGS. 13A and 13B . 
     In one embodiment, thickness for a compliant wall—or portion of a compliant wall—decreases by more than about 1 millimeter as a result of stretching due to combustion. Some combustion devices may include a compliant wall or wall portion that decreases in thickness by more than about 2 millimeters. In a specific embodiment, thickness for a compliant wall—or portion thereof—decreases by more than about 5 millimeters as a result of combustion. The degree of thickness change may also be characterized relative to initial dimensions of the compliant wall. In one embodiment, thickness for a portion of a compliant wall decreases by more than about 20% of an original thickness for the portion before combustion. In a specific embodiment, the compliant wall decreases by more than about 40% of an original thickness for the portion before combustion. It is understood that some portions of a compliant wall may thin more than other portions. For example, combustion device  70  of  FIG. 3C  includes a cylindrical compliant wall  74  whose thickness varies along axial direction  85 . In this case, thickness is at a minimum in a central portion of the compliant wall  74  and increases towards end plates  72 . 
     Combustion chamber volumes may also be configured to increase as a result of a thickness decrease in a compliant wall—or compliant segment. Referring again to  FIGS. 13A-13B  for example, as thickness of compliant wall  54  decreases in the radial direction from t o  to t e , the inner diameter of compliant wall  54  increases from d o  to d e . Outer diameter, D o , remains relatively constant due to constraints  58 , which limit radial expansion of the outer surface of compliant wall  54 . Thus, the inner diameter—and volume—of the combustion chamber dynamically increases during combustion. 
     In an illustrative example, to starts at about 1 cm, d o  starts at about 2 cm (D o  will stay relatively constant at about 4 cm), and H o  starts at about 2 cm. After combustion, compliant wall  54  includes a combustion device  50  is configured such that to drops to about 0.4 cm, d e  peaks at about 2.8 cm and H e  peaks at about 5.5 cm. This results in a volume increase of about 5 times the initial volume. For a conventional cylinder where wall thickness or internal diameter does not change, the same change in height for the device only produces a volume increase of about 2.75 times the initial volume. 
     As one of skill in the art will appreciate, increasing maximum volume for a combustion chamber increases the engine displacement. The displacement provides an indication of how much energy per firing a combustion device can produce. As displacement increases, so does energy available to a combustion device for one firing. For example, larger displacement increases energy and efficiency since more fuel may be burned during each combustion or cycle and a larger combustion volume for a given surface area reduces thermal losses. This dynamic combustion chamber increase is not limited to the example of  FIG. 13  and may include any device describer herein or any compliant walled combustion device of the present invention. 
     Combustion chamber dimensions may be configured to take advantage of decreasing wall thicknesses and dynamic combustion chamber volume increases. In one embodiment, a combustion chamber is configured such that the diameter for a substantially cylindrical combustion chamber increases during combustion of the fuel. For the cylindrical embodiments, this occurs as a result of maintaining a substantially fixed outer diameter for the combustion chamber walls during expansion of the chamber. When expansion occurs, the thickness of the cylindrical chamber walls decrease, which causes a corresponding double increase in the inner diameter of the chamber. Since volume of a cylinder increases with the square of the radius change, increasing dynamic diameters may result in significant displacement improvement for a combustion device (e.g., for a radius increase from 1 cm to 1.5. cm, the planar area and thus the cylindrical volume for a chamber having a fixed height increases by a factor of 2.25 (i.e., 1.5 2 )). Changes in the height (or length) of the cylindrical combustion chamber amplify this dynamic diameter gain. If the height of the cylindrical combustion chamber doubles for the previous example, then the volume increases by a factor of 4.5 (2×2.25). This is a significantly larger increase in volume than just a linear expansion alone. A conventional rigid walled combustion device would only increase in volume by a factor of 2 for the same doubling in height and no change in inner diameter. 
     The amount of volumetric increase based on reduced wall thicknesses during combustion will depend upon the thickness of any compliant walls included in the combustion device and configuration for the combustion device. Some combustion devices include relatively thick combustion chamber walls that provide significant opportunity for wall thinning and volumetric increase. Configuration also affects the volumetric increase. In some embodiments, a combustion device of the present invention may include a greater initial outer diameter that an initial height (D o &gt;H o ) to capitalize the square of radius changes. In another embodiment, the combustion chamber is spherical (see  FIGS. 8 and 9 ) and the volume increases with the cube of a thickness decrease and corresponding radius increase. 
     There are many ways to characterize dynamic volumetric changes for a combustion device of the present invention. For a cylindrical or spherical combustion chamber, changes in inner diameter for the chamber provide a good indication of volumetric increase benefits based on a decreasing wall thickness. Inner diameter changes will vary with the size of the combustion device, the thickness and elastic properties of the walls, the amount of fuel consumed in a combustion, etc. In one embodiment, inner diameter for a combustion chamber increases by more than about 2 millimeters during combustion of the fuel in the combustion chamber. Some combustion chambers may include an inner diameter that increases by more than about 4 millimeters. In a specific embodiment, inner diameter for a combustion chamber increases by more than about 10 millimeters as a result of combustion. The degree of change may also be characterized relative to initial dimensions for the inner diameter. In one embodiment, inner diameter of the combustion chamber increases by more than about 10% relative to an inner diameter for the combustion chamber before combustion. In a specific embodiment, the inner diameter increases by more than about 20% relative to the original inner diameter. It is understood that some portions of a combustion chamber may increase in inner diameter more than other portions (see  FIG. 3C  for example). 
     Other combustion devices and designs described herein may be configured to include wall thicknesses that decrease with combustion. Many of these devices may also witness dynamic volumetric increases based on changing wall thicknesses. For example, combustion device  120  of  FIG. 5A  may be configured with compliant walls  122  that decrease in thickness and increase volume of combustion chamber  132  during combustion. Similarly, the spherical wall  182  of combustion chamber  180  of  FIG. 8A  may be configured with thick wall that diminishes in thickness during combustion and increase volume of chamber  184 . 
     In one aspect, the present invention relates to methods for using combustion devices. Since compliant walled combustion devices offer new designs that are quite different from conventional rigid-walled piston designs, the present invention opens up new regimes in combustion device operation. One method decreases thickness of a wall during deflection. Another method increases volume of a combustion chamber dynamically in multiple directions or as a wall changes in thickness. The present invention also enables new combustion cycles. One cycle uses elastic energy stored in a stretching wall to facilitate exhaust. The present invention also improves mechanical/electrical hybrid systems, which will be described in further detail below. 
       FIG. 14A  illustrates a process flow  300  for producing mechanical energy from a fuel in accordance with one embodiment of the present invention. Other combustion devices and figures described herein may also help illustrate combustion methods described herein. 
     Process flow  300  begins by providing a fuel and oxygen into a combustion chamber ( 302 ). Typically this employs an inlet port or valve that opens into the combustion chamber and pressure to move the fuel and oxygen. A fuel system may supply the fuel and mix it with air so that a desired air/fuel mixture travels through the inlet port. Three common fuel delivery techniques include: carburetion, port fuel injection, and direct fuel injection. In carburetion, a carburetor mixes fuel (typically in a gaseous state) into air before provision into the combustion chamber. In a fuel-injected engine, a desired amount of fuel is injected into the combustion chamber either above the intake valve (port fuel injection) or directly into the chamber (direct fuel injection). 
     The fuel is then combusted in the combustion chamber ( 304 ). Typically, this occurs after the intake valve has been closed and while an exhaust port is also closed. In one embodiment, the present invention employs ignition to initiate combustion. This may occur with or without compression of the fuel/air mixture before ignition. In another embodiment, the present invention does not rely on ignition from an external device. Instead, heat and pressure of a compression stroke cause the fuel to spontaneously ignite. Compression devices compress the air/fuel mixture more, which may lead to increased efficiency. Further discussion of combustion and different combustion cycles suitable for use with a device of the present invention is provided below. 
     Process flow  300  proceeds by decreasing thickness ( 306 ) for a portion of a compliant wall such that volume for the combustion chamber increases with the thickness decrease. Typically, thickness changes in a compliant wall employ pressure and forces generated during combustion. In one embodiment, a compliant wall stretches in a direction that is substantially orthogonal to a direction of the thickness decrease. The combustion device may be constrained and prevented from moving in all directions save an intended direction of stretch, which then influences where and how the thickness change will occur. 
     The sizes of the combustion chambers formed in accordance with the present invention may be widely varied. By way of example, maximum combustion chamber volumes, after combustion, ranging from about 2 cubic centimeters to about 40 cubic centimeters work well. Other maximum combustion chamber volumes may be used. Combustion chamber volume may be varied according to the needs of an application. Since the polymer components and described systems can be quite small and light weight, engines incorporating the described combustion chambers are very well suited for use in relatively lower power requirement applications, including applications that do not traditionally use internal combustion engines as the power sources. By way of example, maximum combustion chamber volumes ranging from about 2 cubic centimeters to about 25 cubic centimeters work well in many applications. However, again, it should be appreciated that both larger and smaller combustion chamber volumes may also be used. 
     Changing wall thickness may also have other benefits. In many cases, the inner surface area of the combustion chamber increases with decreasing wall thickness and as the compliant wall stretches. This increases the surface area for heat dissipation from the combustion chamber, which may increase efficiency for the combustion device over a large number of cycles where steady-state heat dissipation affects efficiency. For example, a cylindrical combustion chamber has a surface area proportional to the inner diameter and height. As the inner diameter increases with decreasing thickness, so does surface area for heat dissipation. A spherical combustion chamber will increase in inner surface area with the square of the inner radius and which depends on thickness changes.  FIG. 14B  illustrates a process flow  320  for improving thermal management of a combustion device in accordance with one embodiment of the present invention. 
     Process flow  320  provides fuel and oxygen into a combustion chamber, e.g., similar to that described above with respect to step  302  in process flow  300 . The fuel is then burned to produce heat in the combustion chamber to produce heat ( 322 ). 
     Process flow  320  then stretches a compliant segment or wall included in a set of walls that border the combustion chamber such that surface area for the set of walls increases ( 324 ). For cylindrical combustion devices and compliant walls described above, the surface area bounding the combustion chamber will increase with both diameter and height increases. The amount of surface area increase will vary with design of the combustion chamber and device, elasticity and thickness of the compliant wall, and any load coupled to the mechanical output. 
     A unique feature of the present invention is that compliant wall thicknesses and inner diameters for a combustion chamber dynamically change during combustion. In one embodiment, the compliant wall includes a first thickness when combustion begins and a reduced thickness when combustion ends. This may be doubly beneficial for combustion. First, the compliant wall includes a greater thickness at the beginning of combustion—when heat should be contained to maximize mechanical output of the combustion device (and increase efficiency of a single combustion). Second, and oppositely, surface area for the combustion chamber also maximizes at the end of a stroke. This produces a greater area for thermal transfer out through the walls—when is often desirable for heat to be released from the combustion chamber. A compliant segment or wall that stretches or otherwise thins also includes a reduced thickness at the end of combustion. This reduces the thermal outlet path or cooling distance for dissipating heat from the combustion chamber through the compliant walls, again, when it is desirable to dissipate heat out from the combustion chamber at the end of the stroke. This reduced thermal path will also facilitate and expedite cooling of internal walls for the combustion chamber. Thus, the compliant segment or wall is thick when heat should be contained and thin and larger in surface area when heat should be dissipated. It is understood that thickness changes may vary across different portions of a compliant wall, thus altering thermal performance of the compliant wall as a function of position and configuration for the device. 
     Heat produced in the combustion chamber is then dissipated through the stretched compliant segment ( 326 ). Typically, this will occur as long as the temperature within the combustion chamber is greater than the temperature outside the combustion chamber. The heat may come from a current combustion or heat generated by previous combustion in the chamber. 
     In some designs, such as those that use a bending mode (e.g. a bellows) to respond to compression pressures, then the surface area doesn&#39;t significantly increase. For a bellows, the inner surface area of the folds stays the same, but as they unfold from axial expansion, the inner volume increases. These designs will also not see a significant decrease or change in thickness as described in process flow  300 . 
     Combustion 
     The present invention contemplates a wide array of internal combustion engine designs and cycles it is not limited to any particular design or cycle. One well-known combustion cycle suitable for use with the present invention is the four-stroke combustion cycle, or Otto cycle. The Otto cycle includes four strokes: an intake stroke, a compression stroke, a combustion stroke, and an exhaust stroke. Such a four-stroke cycle is suitable for use with many combustion devices described above. Other suitable cycles include Miller, Diesel, Sterling, detonation (knock) cycles and various 2-stroke cycles. The Miller cycle is attractive in terms of its performance and natural fit to a compliant combustion device with electrical loading ability (such as using an electroactive polymer in conjunction with a combustion device) to effectively implement different compression and expansion strokes. In some cases, a crankshaft is used and piston-based cylinders are replaced with piston-less compliant combustion devices that expand uniaxially like conventional piston-based cylinders (see  FIG. 16 ). Combustion devices of the present invention are also well suited for use with cycles and at high speeds. 
     Unique features provided by the present invention may also create new combustion cycles and alter conventional combustion cycles.  FIG. 15A  illustrates a combustion cycle  340  for producing mechanical energy from a fuel in accordance with one embodiment of the present invention. 
     Process flow  340  provides fuel and oxygen into a combustion chamber ( 302 ). The fuel is then burned in the combustion chamber to produce heat ( 304 ). A compliant segment or wall is then stretched in response to the combustion ( 342 ). The compliant segment is included in a set of walls that border the combustion chamber. The compliant wall receives mechanical energy from the combustion and stores a portion of the mechanical energy as elastic energy. As will be described in further detail below, a constraint may influence deformation of the compliant wall and force it along a desired direction of output. Some constraints, such as a helical spring, may also store mechanical energy provided in the combustion as it deforms. 
     After combustion is complete, combustion products are exhausted from the combustion chamber using elastic return of the stretched portion ( 344 ). More specifically, elastic energy stored in the compliant wall returns the compliant wall to position that reduces volume in the combustion chamber. Typically, an exhaust port is opened just before elastic return begins. The amount of force available in the compliant wall for expelling exhaust from the combustion chamber will depend on the amount of force produced during the combustion, elastic properties of compliant wall, and the ratio of mechanical energy provided to the compliant wall relative to that provided to a mechanical output or load. A helical spring used as a constraint may also assist elastic return and exhaust of combustion products. In this manner, elastic return of the compliant wall provides a mechanism for automatically and passively exhausting combustion gases from a chamber after combustion. 
     Compression ratio is a basic efficiency parameter for many combustion devices. Compression ratios of 6-12 are typical for Otto cycles. Higher compression ratios can theoretically deliver higher efficiency, but detonation (knock), which adversely affects engine lifetime, typically limits the use of high compression ratios in conventional engines. Many compliant walled combustion devices may offer advantages for knock engines because of their shock resistance and compact configuration. Compression ratios of 6-12 are feasible using any one of a number of different compliant combustion device configurations. One may use dormant spacers  82  ( FIG. 3A ) if needed to reduce the top dead center volume (minimum chamber volume) and increase the compression ratio. Thus, compression ratios greater than 6-12 may be used with devices described herein. 
     Relative to conventional metal combustion devices, some compliant walled combustion devices described herein reduce surface-to-volume ratios at a given volume, operate at higher inner wall temperatures than oil-lubricated metal engines, eliminate piston-cylinder leakage and mechanical friction in from piston-cylinder sliding contact, reduce heat transfer to the inner wall by expanding the chamber with the combustion gases rather than having a relative velocity between the two, and (if desired, using an electroactive polymer or other electrical device or control) adjust timing and pressure variables at electronic speeds. Any of these may improve combustion and conversion of chemical energy in the fuel to useful mechanical energy. 
     Hybrid Electrical Energy Functionality 
     The present invention also permits electrical energy generation using combustive energy. In one embodiment, an electroactive polymer transducer is used to generate electrical energy based on mechanical energy provided by combustion. Electroactive polymers are a class of compliant polymers whose electrical state changes with deformation. Exemplary electroactive polymers may include electrostrictive polymers, dielectric elastomers (a.k.a. electroelastomers), conducting polymers, IPMC, gels, etc. In a specific embodiment, a compliant wall included in a combustion device includes a composite structure that includes a compliant wall as described herein for enclosing a combustion chamber and an electroactive polymer transducer disposed external to the compliant wall. 
     Some electroactive polymers are multifunctional, so the same electroactive polymer transducer can be used a) as a generator (convert mechanical to electrical energy, e.g., to power a spark plug), b) as an actuator (convert electrical energy to mechanical energy, e.g., in a “turbo” mode where mechanical output of the device is increased by using both electrical actuation and a combustion drive working together), and/or c) as a sensor (read electrical changes, e.g., to detect deformation). The sensing function may also be used to monitor and optimize combustion or other polymer engine parameters. Sensing could be used to monitor mechanical loading conditions of interest. For electrical energy generation, the combustion is used to deform or stretch the electroactive polymer in some manner. 
     The present invention also permits new hybrid mechanical and electrical output systems and methods.  FIG. 15B  illustrates a process flow  360  for producing mechanical energy from a fuel in accordance with one embodiment of the present invention. 
     Process flow  360  provides fuel and oxygen into a combustion chamber ( 302 ). The fuel is then combusted to produce heat in the combustion chamber ( 304 ). A compliant segment or compliant wall is then stretched ( 342 ). The compliant segment or wall is included in a set of walls that define the combustion chamber. 
     Mechanical energy produced in the combustion is then provided for mechanical output ( 362 ). For example, a mechanical output coupled to the combustion chamber may be used to do work on a load. In a robotics application, the mechanical output may be used for locomotion. 
     Process flow  360  also deforms an electroactive polymer as the compliant segment or wall stretches ( 364 ). The compliant segment of the wall may itself by made of an electroactive polymer. The electroactive polymer may be used to assist mechanical output, intake or compress fuel-air mixture, alter mechanical output via electrical loading, as a sensor, and/or to generate electrical energy. Actuating the polymer—or applying an electric field to the electroactive polymer during combustion—may increase the amount of mechanical output for the combustion device. Applying an electric field to the electroactive polymer before the electroactive polymer contracts from a stretched position may be used to generate electrical energy using the electroactive polymer as it contracts from the stretched position. Applying an electric field to the electroactive polymer before combustion is complete may alter the electroactive polymer stiffness, which alters mechanical load on the hybrid device and effects combustion efficiency. This allows combustion device controllers and designers to dynamically and electrically tailor combustion output. Alternatively, the electroactive polymer may be used as a sensor where an electrical state of the electroactive polymer is read as compliant segment or wall deforms. Electroactive polymers may also be used as an actuator to intake fuel-air mixtures into the combustion chamber, or to force exhaust gases out after combustion. 
     In a specific embodiment, the electroactive polymer attaches or couples to compliant segment or wall, such as the outer surface, and stretches with the compliant segment or wall. For example, an electroactive polymer may be wrapped once or rolled multiple times around compliant cylindrical wall  54  of combustion device  50  in  FIG. 2A . For electrical energy generation with some electroactive polymers, charge is placed on compliant electrodes attached to an electroactive polymer at some elevated planar expansion. When the electroactive polymer contracts, positive charges on one face of the polymer are pushed farther away from the negative charges on the opposite face of the polymer, thus raising their voltage and electrical energy. In addition, as the electroactive polymer contracts, charges on each face (positive charges on a electrode and face or negative charges on a second electrode) become closer and raise voltage and electrical energy of any charge on the electrodes. Gains in contracted energy of 3-5 times the energy initially placed on the polymer are common, with smaller and greater gains possible, depending on the area strain of the stretched electroactive polymer, loading conditions and electrical harvesting controls. 
     To generate electrical energy over an extended time period, the electroactive polymer may be stretched and relaxed over many cycles. For electrical energy harvesting from a combustion device, mechanical energy from combustion is applied to the electroactive polymer in a manner that allows electrical energy to be removed from the electroactive polymer. Generation and utilization of electrical energy may require conditioning electronics of some type. For instance, circuitry may be used to remove electrical energy from the transducer. Further, circuitry may be used to increase the efficiency or quantity of electrical generation or to convert an output voltage to a more suitable value. Further discussion of conditioning electronics suitable for use with the present invention is described in commonly owned U.S. Pat. No. 6,628,040 and entitled “Electroactive Polymer Thermal Electric Generator” naming R. Pelrine et al. as inventors. This application is incorporated herein by reference in its entirety for all purposes. 
     In another specific embodiment, a compliant wall for the combustion device includes an electroactive polymer that is actuated to intake combustion chamber reactants or exhaust combustion chamber products. For intake, the electroactive polymer compliant wall is actuated to increase combustion chamber volume (e.g., elongate the chamber), create a negative pressure, and draw in fuel and/or air. Electrical energy to the electroactive polymer may then be turned off to compress the fuel before ignition via elastic return of the polymer. The electroactive polymer offers a simple alternative to draw in fuel and air without requiring a pressurized source or a camshaft that actuates the valves in a piston-cylinder engine. For example, wall  244  of combustion device  240  ( FIG. 12A ) may include an electroactive polymer. In this case, the electroactive polymer is being used in actuator mode to perform fuel control functions. In other embodiments, charge can be reapplied to an electroactive polymer at top dead center to oppose contraction forces momentarily. Further, charge can be reapplied to the electroactive polymer at top dead center if running the electroactive polymer in generator mode. 
     Conventional engines basically execute a sinusoidal motion of the piston (sinusoidal displacement relative to time); a necessity imposed by the inertia of the device and crankshaft motion constraints in a conventional engine. Compliant walled combustion devices that include an electroactive polymer may execute more advanced motions and are not limited to sinusoidal output. Loading can be electronically varied in a conventional engine generator but only in a gross, average way by electronically loading the external generator. Motion or frequency (e.g., in a free piston engine) constraints in conventional engines may also cause suboptimal performance. For example, it is well known that the ideal Sterling cycle is a reversible cycle theoretically capable of Carnot efficiency. But practical implementations of Sterling engines usually only approximate the ideal Sterling cycle because they cannot execute discontinuous, independent motions of the hot and cold sides of the engine (the two are typically mechanically coupled by a crankshaft, for example, in a conventional design). 
     By contrast, an electroactive polymer and compliant walled combustion device could, for example, be controlled to expand rapidly, completely stop for a significant part of the cycle period, and then slowly contract (by varying an electrical state applied onto the electroactive polymer that increases stiffness of the electroactive polymer or mechanical force applied by the electroactive polymer). The pressure profile in the polymer engine could even be adjusted electronically on the fly, for example in response to startup conditions, changes in load, changes in environmental conditions, or changes in sensed combustion parameters. The ability to electronically change control parameters generally leads to improved combustion and systems. Further description of electroactive polymers suitable for use with the present invention is described in commonly owned U.S. Pat. No. 6,628,040, which is incorporated herein by reference in its entirety for all purposes. 
     Materials suitable for use as an electroactive polymer with the present invention may include any substantially insulating polymer or rubber (or combination thereof) that deforms in response to an electrostatic force or whose deformation results in a change in electric field. One suitable material is NuSil CF19-2186 as provided by NuSil Technology of Carpenteria, Calif. Other exemplary materials suitable for use as a pre-strained polymer include silicone elastomers, acrylic elastomers such as VHB 4910 acrylic elastomer as produced by 3M Corporation of St. Paul, Minn. 
     Applications 
     The present invention finds wide use. Compliant walled combustion devices described herein may be used in any application that traditionally employs conventional piston-based engines. For example, combustion devices of the present invention may be used in lawnmowers, leaf blowers, pumps, compressors, and other tools and equipment. Combustion devices described herein also find wide use as a fast acting actuator. Locomotion applications may include automotive applications where mechanical and/or electrical power is generated from a fuel. 
     Compliant walled combustion devices encompass a large design space, even larger than piston-cylinder engines because of their greater design flexibility. Further, existing limitations in piston-cylinder engine designs, particularly on small scales, are overcome using compliant walled combustion devices. 
     Although the present invention has primarily been described with respect to mechanical output of a single combustion device, many systems have more than one combustion device. Four, six and eight cylinder systems are common. Multiple cylinders may be arranged in a number of ways: in-line, V, or flat (also known as horizontally opposed or boxer). 
     The Department of Defense (DoD) has diverse needs for power sources ranging from micro air vehicles (MAVs) and small autonomous robots to portable power sources for foot soldiers to large power sources for vehicles and spacecraft. Most DoD power sources are designed for mobile applications, and many therefore have common requirements such as lightweight, high efficiency, and high power density. The present invention is well suited for use in these applications. Combustion devices described herein also find use for small, lightweight, efficient 20 W power sources for various generic missions. In particular, the MAV (micro air vehicle) and small robot missions where power output, longevity and weight are important may benefit from the present invention. 
     The present invention provides a portable energy alternative with a high power to weight ratio and the ability to generate power over a significant time period. Hydrocarbon based fuels have a relatively high energy density as compared to batteries. For instance, the energy density of a hydrocarbon based fuel may be 20 times higher than a density of a battery. 
     Compliant combustion engines described herein are also easily adapted to include electrical energy generation. Adding an electroactive polymer that produces electrical energy from combustion also increases applicability of the present invention. Many applications require both mechanical and electrical power. Robotics often requires mechanical output in addition to electrical energy generation. Some compliant engines may electronically control the ratio of the two—which is useful for robots and mobile applications. In contrast, robotics devices that employ fuel cells and batteries also an entire separate subsystem (e.g., a motor) to produce mechanical output. Relative to conventional piston-based combustion devices, an entire subsystem—the electromagnetic generator—has been eliminated. 
     In one embodiment, electrical input using an electroactive polymer is used to alter the combustion loading (i.e., the combustion pressure-volume profile) electronically in real time. The idea of using electrical loading on a generator to optimize the combustion efficiency of an engine has been applied to hybrid cars. However, with compliant walled combustion engines loading can be controlled more quickly—potentially within much less than the period of one cycle. 
     Using polymers for compliant walls may also lower costs of combustion devices described herein since polymers are generally less expensive than metals. Embodiments that do not include metal components may also avoid the need for precision machining of metals and associated costs thereof. In some cases, combustion devices of the present invention are inexpensive enough to be made as a disposable item if desired. 
     The present invention may include low cost polymers in construction. This permits the possibility of disposable engines. Custom molding of polymers also allows a designer to fabricate a variety of combustion volume shapes (e.g. oval, flatter, etc.) and customize a device in shape for a particular application. 
     The polymers also provide mass advantages as lightweight materials, e.g., higher power density per gram, or, for a given engine mass, the ability to make a larger combustion volume which typically increases efficiency. The invention also reduces extra mass needed to maintain rigidity in the tight tolerances of conventional metal piston-cylinders. 
     Also, the present invention opens the option of using dirty fuels because tight sliding seals have been eliminated from inside the combustion chamber. 
     The compliant wall approach offers numerous potential advantages such as light weight, quiet, simplicity, high efficiency, an ability to electronically vary between electrical and mechanical outputs to optimize the system, low cost, and tremendous design flexibility. Many compliant combustion devices and engines described herein simplify combustion device technology. Much of the conventional rigid engine hardware may be eliminated such as pistons, piston rings and lubricants in piston designs. 
     The piston rings in a conventional engine provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder. The rings serve two purposes: they provide the fuel/air mixture and exhaust in the combustion chamber from leaking during compression in combustion; and keep oil from leaking into the combustion chamber, where would be burned and lost. Since the present invention need not include a piston internal to the combustion chamber, piston rings internal to the combustion chamber may be avoided. Also eliminated in this case are combustion chamber leakage issues associated with piston rings. 
     The present invention also offers light weight, low noise signature (quiet operation), simplicity and improved efficiency designs. The low inertia of polymer components enables higher efficiency than that of metal components. Light weight not only reduces power plant weight but also increases efficiency. Each time the combustion device changes direction, it uses energy to stop travel in one direction and start travel in another. The lighter the combustion device, the less energy changing directions takes. The potential for higher wall temperatures than in oil-lubricated engines is also an opportunity for increased efficiency. 
     A noteworthy design feature of many combustion devices described herein, such as devices  180  and  200 , is that the combustion chamber  184  is isolated from any mechanical moving parts, such as between outer surfaces or seals included in piston  206  and the inner surface of housing  204 . As a result, any lubrication used for minimizing friction between piston  206  and housing  204  need not mix with any components in combustion chamber  184  and need not be subject to the high temperature conditions that are found inside combustion chambers. 
     Compliant combustion devices claimed herein may also achieve attractive power densities at sub-acoustic frequencies, eliminate other noise sources such as metal-to-metal contact in gears and bearings. 
     Combustion Devices 
     Having discussed compliant walled combustion devices independent of design, several benefits and various modes of operation, numerous exemplary designs will now be expanded upon. 
       FIG. 2A  illustrates a simplified cross-section of a cylindrical combustion device  50 , before combustion, in accordance with one embodiment of the present invention.  FIG. 2B  illustrates device  50  after combustion. Combustion device  50  includes rigid walls  52 , compliant wall  54 , combustion chamber  56  and constraint  58 . 
     Compliant wall  54  is substantially cylindrical and circumferentially borders combustion chamber  56  along an axial length of chamber  56 . Cylindrical wall  54  axially stretches in direction  55  in response to combustion of a fuel in combustion chamber  56 . In one embodiment, thickness for wall  56  is substantially constant, before combustion in chamber  56 , for the entire circumference taken through an axial cross-section of wall  54 . In some cases, the thickness may vary during and after combustion. Cylindrical wall  54  includes a material whose elastic strength is low enough to permit axial stretching based on combustion in chamber  56 . In a specific embodiment, compliant wall  54  includes a stretchable elastomer, such as silicone having a desired stiffness. Additional details on suitable compliant wall materials are elastic properties were provided above. 
     Rigid walls  52  resemble end caps on the substantially cylindrical compliant wall  54 . Rigid wall  52   a  is disposed at a first end  54   a  of compliant wall  54 , while rigid wall  52   b  is disposed at a second end  54   b  of compliant wall  54 . Rigid wall  52   a  is externally fixed and remains relatively stationary during combustion within combustion chamber  56 . Rigid wall  52   b  moves relative to rigid wall  52   a  in axial direction  55  as a result of combustion within combustion chamber  56  and stretching of compliant wall  54 . While not shown in  FIG. 2A , rigid walls  52  may include one or more coupling mechanisms to allow attachment to fixed or mechanical outputs. Compliant wall end portions  54   a  and  54   b  may attach to rigid walls  52   a  and  52   b , respectively, using a suitable adhesive, for example. 
     Combustion chamber  56  is defined in size by the inner surfaces of rigid walls  52  and compliant wall  54 . More specifically, a tubular inner surface of compliant wall  54  and substantially flat end portions of rigid walls  52  cooperate to form a substantially cylindrical volume for combustion chamber  56 . While not shown in  FIG. 2A , device  50  may also include one or more inlet and outlet ports to communicate reactant and product gases into and out of combustion chamber  56 . Ignition and combustion of a fuel within chamber  56  increases pressure within chamber  56  and causes compliant wall  54  to axially stretch in direction  55 . 
     Constraint  58  reduces radial expansion of the compliant wall  54  during combustion of the fuel in combustion chamber  56 . In the absence of constraint  58 , combustion and pressure generation within chamber  56  causes compliant wall  54  to deform and stretch a) radially away from a central cylindrical axis and b) linearly along direction  55 . By reducing radial expansion of compliant wall  54 , constraint  58  increases mechanical output efficiency in a desired output direction, such as direction  55  when rigid wall  52   a  is fixed. 
     In one embodiment, constraint  58  includes a high tensile element  58  that wraps circumferentially about the substantially cylindrical compliant wall  54 . For example, the high tensile element may include one or more high tensile fibrous strands  58 , such as Kevlar, a metal wire or a nylon fiber. The high tensile fibrous strands  58  prevent radial expansion of outer portions of compliant wall  54 . 
     In a specific embodiment, high tensile fibrous strands  58  wrap around an outside surface of compliant wall  54 . Alternatively, a high tensile element  58  may be integrated into the wall thickness of compliant wall  54 . In this case, the high tensile fibrous strands are embedded in, such as halfway, between the inner surface of compliant wall  54  and the outer surface. In a specific embodiment, constraint  58  includes a coil (such as a spring) with flat windings (flattened normal to the direction of expansion and contraction) embedded in the structure of compliant wall  54 . The flat windings resist vacuum formation within the compliant wall around each winding as the wall axially deforms. In another embodiment, the constraint  58  may be a set of disks or rings separated by spacers in a few locations. Using a silicone or other polymer for compliant wall  54  and a lightweight fiber such as Kevlar for constraint  58  provides a combustion device  50  that is significantly lighter than conventional metal piston-based combustion cylinders. 
     The amount and geometry of winding for the high tensile element between ends of compliant wall  54  in direction  55  may vary. In a specific embodiment, separate strands or high tensile elements  58  are included along the axial direction  55  of compliant wall  54 . In this case, constraint  58  may include anywhere from two to dozens to several hundred individual strands, counted along the axial direction, that circumferentially surround combustion chamber  56 . In another embodiment, a single high tensile element wraps helically about the substantially cylindrical compliant wall from one end  54   a  of compliant wall to the other end  54   b  (or to some lesser degree if the entire axial length of compliant wall  54  is not used for expansion, see  FIG. 3A ). In this case, the high tensile element may include a helical spring—such as a spring formed of a suitably stiff plastic or metal. 
     In many embodiments, such as high tensile fibrous strands wrapped around compliant wall  54 , constraint  58  does not substantially inhibit axial deformation of the substantially cylindrical compliant wall  54  along direction  55 . As mentioned above, by reducing radial expansion of compliant wall  54 , constraint  58  increases mechanical output efficiency in direction  55  since pressure generated within combustion chamber  56  results primarily in expansion of endplate  54   b  axially in direction  55 . 
     A helical spring used as constraint  58  restricts radial deformation of compliant wall  54 . However, the spring will store elastic mechanical energy as the spring  58  and compliant wall  54  stretch in direction  55 . This is useful in some designs. For example, elastic return of the spring and compliant wall  54  provides a mechanism for automatically and passively exhausting combustion gases from chamber  56  after combustion. 
     It is understood that the cylindrical shape of combustion chamber  56  may deviate from a perfect cylinder, particularly during combustion of a fuel within chamber  56 . As described above, compliant wall  154  may decrease in thickness as it stretches in axial direction  55 . As shown in  FIG. 2B  after combustion (or during combustion to a lesser degree), end portions  54   a  and  54   b  of compliant wall that attach to rigid walls  52   a  and  52   b  are restricted from axial stretching and radial thinning in this region. This rounds the cylindrical corners of combustion chamber  56 . In addition, in the absence of constraint  58 , compliant wall  56  may deform radially during combustion and initial rapid expansion of gases within chamber  56 , and thus deviate from a perfect cylinder for the volume of chamber  56 . Alternatively, chamber  56  may intentionally be made non-cylindrical even without combustion expansion; for example, chamber  56  may include a flat oval to better fit an application with flat constraints. 
     In a specific embodiment, constraint  58  includes a helical spring configured with a negative spring force when combustion device  50  is in a contracted state as shown in  FIG. 2A . This increases linear output in axial direction  55 . In some cases, this may also increase mechanical output and efficiency for combustion device  50  (where efficiency is defined as the ratio of mechanical output to chemical input). 
       FIG. 3A  illustrates a simplified cross-section of a cylindrical combustion device  70 , before combustion, in accordance with one embodiment of the present invention.  FIG. 3B  illustrates device  70  during intake of fuel and air.  FIG. 3C  illustrates combustion device  70  during combustion.  FIG. 3D  illustrates combustion device  70  after exhaust.  FIGS. 3A-3D  also illustrate a combustion cycle where elastic energy of a compliant wall is used to expel exhaust gases. Combustion device  70  includes rigid end plates  72 , compliant wall  74 , combustion chamber  76 , spacers  82 , output shaft  78 , and port  84 . 
     Compliant wall  74  includes a single piece of compliant material whose internal dimensions define combustion chamber  76 . For convenience, compliant wall is described with multiple segments: a substantially cylindrical segment  74   a  that radially borders combustion chamber  76  along an entire axial direction of chamber  76 , and end walls  74   b  and  74   c  that form substantially flat end portions to the cylindrical combustion chamber  76 . In a specific embodiment, compliant wall  74  includes a soft elastomer singly molded into a desired shape and dimensions for device  70 . 
     Constraint  80  includes a spring-like structure that reduces radial expansion of cylindrical portion  74   a  during combustion of the fuel in combustion chamber  76  and forces uniaxial expansion for the cylindrical portion  74   a  of compliant wall  74  in direction  85 . 
     Combustion device  70  includes two spacers  82  internal to combustion chamber  76 . Specifically, lower spacer  82   a  attaches to a flat inner surface of compliant wall  72   a  while upper spacer  82   b  attaches to a flat inner surface of compliant wall  72   b . Spacers  82  reduce dead space in combustion chamber  76  before combustion of a fuel in chamber  76 . Spacers  82  also increase the axial length of compliant wall  74  in direction  85 . In some cases, this may reduce strain on compliant wall  74 . Before fuel and air intake, as shown in  FIG. 2C , spacers  82  consume a large proportion of the volume within combustion chamber  76 . In this case, no space is provided between spacers  82 . In another embodiment, cylindrical compliant wall  74   a  extends axially beyond spacers  82  and combustion chamber  76  includes space in an axial direction beyond spacers  82 . As one of skill in the art will appreciate, reducing dead space in a combustion chamber before combustion increases efficiency. Although combustion device  70  is illustrated with two spacers  82 , combustion device of the present invention may include one or any other suitable number of spacers that reduce dead space in combustion chamber  76 . The spacers may also include any geometry that facilitates combustion in the combustion chamber. As shown, lower spacer  82  includes a channel that passes therethrough to allow communication of combustion gases and products. In one embodiment, spacers  82  are compliant. 
     Rigid end plates  72   a  and  72   b  couple to an outside surface of each compliant end wall  74   b  and  74   c , respectively. In a specific embodiment, an adhesive adheres each end plate  72  to an outside surface of each wall  74   b  and  74   c . Rigid plate  72   b  is fixed and does not substantially move for device  70 . An output shaft  78  is coupled to endplate  72   a . The output shaft may be coupled using any suitable mechanism, as for example a threaded engagement with a threaded hole in endplate  72   a . The output shaft  78  provides mechanical output for combustion device  70 . In this case, device  70  includes a first coupling portion  77   a  disposed on a first end wall  74   b  where it interfaces with end plate  72   a . The first coupling portion  77   a  is disposed proximate to a first end of the substantially cylindrical compliant wall  74   a . Device  70  includes a second coupling portion  77   b  disposed on the second end wall  74   c , which is disposed proximate to a second end of the substantially cylindrical compliant wall  74   a.    
     An intake and exhaust tube  84  passes through an aperture in rigid endplate  72   b  and an aperture in compliant wall  74   c . Although device  70  is illustrated with a single port  84 , it is understood that device  70  may employ separate tubes for intake and exhaust. In the embodiment shown, port  84  passes through a portion of device  70  that is fixed or relatively stationary during combustion. While not shown to prevent obscuring the present invention, device  70  may also include one or more valves to facilitate inlet of combustion reactants and exhaust of combustion products. 
     Although device  70  shows rigid plate  72   b  fixed, other designs are contemplated. For example, a central portion of compliant wall  74  may be fixed, while both ends of the cylinder are arranged to move upon combustion. In other words, a central portion of the substantially cylindrical compliant wall  74  is fixed while the ends are free to move and do mechanical work. This creates compliant segments on each side of the point of fixation and zero displacement. External attachment to each end plate  72  thus permits two mechanical outputs for a single combustion chamber. For example, a first mechanical output may be attached to rigid plate  72   b  while a second mechanical output is attached to rigid plate  72   a . Axially offsetting where combustion device  74  is fixed away from a mechanical center for device  70  provides a different force and output for the mechanical outputs on opposing end of the cylinder, e.g., rigid plate  72   b  receives greater mechanical output than rigid plate  72   a.    
       FIG. 3B  illustrates cylindrical combustion device  70  during intake of fuel and air. Numerous techniques and mechanisms may be used to inlet combustion reactants into chamber  76 . One technique employs external pressure to supply fuel and air into combustion chamber  76 . This may create a positive pressure in chamber  76  that stretches compliant segment  74   a  and creates a volume within combustion chamber  76 . In this case, the compliance of segment  74   a  and inlet pressure may be designed to achieve a desired compression ratio for the air and fuel mixture. In another technique, device  70  is actuated or externally moved to the position shown in  FIG. 3B . For example, an electroactive polymer may be used to stretch compliant wall  74   a  and increase volume in combustion chamber  76 . Alternatively, output shaft  78  may couple to a crankshaft whose rotational motion stretches compliant wall  74   a  and increases volume combustion chamber  76  (see  FIG. 16 ). Other mechanism for moving device  70  to the state shown in  FIG. 3B  may be used. 
       FIG. 3C  illustrates combustion device  70  during combustion  84 . Since rigid endplate  72   b  and compliant portion  74   c  are fixed, and constraint  80  restricts radial expansion of compliant segment  74   a , compliant segment  74   a  stretches axially in direction  85  as shown. Compliant segment  74   a  also thins in a radial direction substantially orthogonal to the direction of axial stretch. Typically, compliant segment  74   a  thinning occurs for the entire circular perimeter. Output shaft  78  (along with rigid plate  72   a  and compliant wall  74   b ) translates linearly in direction  85  away from rigid endplate  72   b  and compliant portion  74   c  as a result of combustion in chamber  76 . 
       FIG. 3C  also illustrates combustion device  70  at peak expansion. After combustion is complete and/or maximum deformation has been achieved, an outlet valve may be opened to permit the release of exhaust gases from combustion chamber  76 . In one embodiment, elastic return of compliant wall  74   a  assists and expedites exhaust of gases from combustion chamber  76 . More specifically, contraction forces stored as elastic energy in a material of compliant wall  74  act to return compliant wall  74  to a resting state, which in this case reduces the volume of combustion chamber  76  and pushes any gases included therein out an open exhaust port  84 . In addition, a helical spring used as constraint  80  may also store elastic energy at peak expansion that becomes a contractile force in the axial direction to exhaust gases from a contracting combustion chamber  76 . 
       FIG. 3D  illustrates combustion device  70  after exhaust is complete. In this case, spacers  82  also facilitate the removal of exhaust gases from the combustion chamber by reducing dead space in chamber  76  and forcing combustion gases out from the chamber. Combustion device  70  is then suitable to begin a new combustion cycle. For example, a two stroke cycle may include a first stroke that includes intake ( FIG. 3B ), and power ( FIG. 3C ) stroke segments and a second stroke that accomplishes exhaust ( FIG. 3D ). 
     In a specific embodiment, output shaft  78  connects to a crankshaft by a connecting rod (see  FIG. 16 ). As the crankshaft revolves, it sets timing for a combustion cycle in combustion chamber  76 . For example, combustion device  70  may work as follows. For an intake stroke, output shaft  76  starts at the bottom ( FIG. 3A ), an intake valve opens, and the crankshaft pulls the output shaft  76  up while air and a fuel are injected into combustion chamber  76 . When output shaft  76  reaches some desired position of its stroke, a spark plug (not shown) emits a spark to ignite the fuel. The fuel in combustion chamber  76  combusts, driving output shaft  76  upwards, which drives the crankshaft. Once output shaft  76  hits the top of its stroke, an exhaust valve opens and exhaust gases from the combustion leave combustion chamber  76 . 
       FIG. 4A  illustrates a cross-section of a cylindrical combustion device  90 , before combustion, in accordance with another embodiment of the present invention.  FIG. 4B  illustrates combustion device  90  during combustion. Combustion device  90  includes compliant wall  92 , inlet port  94 , output port  96 , ignition mechanism  98 , combustion chamber  100  and linear translation mechanism  102 . 
     Compliant wall  92  attaches to a stationary portion  93  of device  90  and to a moving head  95 . For example, stationary portion  93  may include a metal or other suitably rigid material that fixes one end  92   a  of compliant wall  92 . The other end  92   b  of compliant wall  92  is attached to moving head  95 . Moving head  95  includes a compliant wall coupling portion  95   a  and an external coupling portion  95   b.    
     An active segment  92   c  of compliant wall  92  refers to a portion of compliant wall  92  permitted to expand and stretch during combustion. In one embodiment, the active segment  92   c  includes any portions of compliant wall  92  not fixed or attached to a rigid structure or otherwise constrained in deformation during combustion within chamber  100 . In this case, distal ends of compliant wall  92  are routed and attached within stationary portion  93  and moving head  95 . Unattached material between these two distal ends forms the active segment  92   c  for compliant wall  92 . A length,  1 , axially characterizes the active segment  92   c . Compliant segment  92   c  is substantially cylindrical along length,  1 . 
     Linear translation mechanism  102  constrains deformation of device  90 . Linear translation mechanism  102  includes concentric cylindrical shells  97   a  and  97   b  and bearings  99 . Cylindrical shells  97   a  and  97   b  share a cylindrical axis and move relative to each other via bearings  99 . In a specific embodiment, cylindrical shells  97  each include a rigid material such as metal tubing, plastic or teflon. Cylindrical shell  97   a  indirectly couples to compliant wall  92  by attaching to stationary portion  93 , which attaches to one end of compliant wall  92 . Cylindrical shell  97   b  indirectly couples to the other end of compliant wall  92  by attaching to moving head  95 , which attaches to the other end of compliant wall  92 . 
     Linear translation mechanism performs several functions for device  90 . Firstly, linear slide  102  constrains deformation of moving head  95  to one direction: linearly to and from stationary portion  93  parallel to an axial center of the concentric cylindrical shells. Secondly, inner cylindrical shell  97   b  may be sized to fit outside of compliant wall  92  and prevent radial expansion of compliant wall  92  upon combustion within combustion chamber  100 . Grease or another suitable lubricant may be used between the outside of compliant wall  92  and the inner surface of cylindrical shell  97   b  to decrease friction between the two surfaces. In a specific embodiment, compliant wall  92  includes a low friction surface on its outside surface. Thirdly, slide  102  acts as a constraint that reduces bending of compliant wall  92  away from the axial direction of expansion. 
     In another embodiment, a combustion device includes electrostatic clamps that apply holding forces at select moments of combustion. For example, device  90  may include electrostatic clamping between two metal shells  97  at various times during a combustion cycle. The electric clamp may be arranged to hold moving head  95  at one or more particular positions in the stroke, such as at peak stroke. Holding a position may be useful in some instances. For example, a device may hold a position immediately after ignition to allow more complete fuel combustion before expansion begins for higher efficiency; or may hold a position at peak expansion to allow the gases time to cool. This second hold at peak stroke may create a partial vacuum in the chamber and allow the device to harvest return stroke energy that would otherwise be sent out as waste heat, thereby potentially increasing efficiency. Further, the two metal shells may be used for sensing and to monitor position of moving head  95 . Further description of electrostatic clamping materials suitable for use with the present invention are described in commonly owned and co-pending patent application Ser. No. 11/078,678, and titled “Mechanical Meta-Materials”. This application is incorporated by reference herein in it entirety for all purposes. In a specific embodiment, an electrostatic clamping material is disposed about a compliant wall and externally activated to lock the combustion device at a desired position, or otherwise alter force vs. displacement for the combustion device. 
     In operation, fuel and air enters combustion chamber  100  via inlet port  94 . Ignition mechanism  98  includes an electrode, which when electrically activated, creates a spark that ignites a fuel and initiates combustion within chamber  100 . As shown in  FIG. 4   b , combustion within chamber  100  drives moving head  95  linearly away from stationary portion  93 . 
     In a specific embodiment, device  90  is dimensioned as follows. Compliant wall  92  is about 1 inch in outer diameter, cylindrical shell  97   b  is about 1 inch in inner diameter, and cylindrical shell  97   a  is about 1 inch in inner diameter plus the thickness of cylindrical shell  97   b . Along an axial direction of cylindrical device  90 , stationary portion  93  is between about 4 and 7 inches in length, s; compliant wall  92  is about 3 inches in active length,  1 , before combustion; compliant wall coupling portion  95   a  is about ½ inch in length, M 1 ; and external coupling portion  95   b  is about ½ inch in length, M 2 . This creates a combustion device  90  with a total length between about 5 and 8 inches before combustion. After combustion, compliant wall  92  may be about 3½ to about 7 inches in active length,  1 . For example, moving head  95  may be controlled in dimensions (e.g., by attaching moving head  95  to a bearing on the crankshaft) such that active length,  1 , extends to a desired length, e.g., about 5 inches. 
       FIGS. 5A-5C  illustrate a radial—or tubular—combustion device  120  in accordance with a fourth embodiment of the invention.  FIG. 5A  is a simplified cross-section view of the tubular combustion device  120 , at the beginning of a new cycle before intake or combustion.  FIG. 5B  illustrates radial combustion device  120  after fuel intake.  FIG. 5C  illustrates tubular combustion device  120  during combustion at peak expansion. In the illustrated embodiment, combustion device  120  includes tubular compliant wall  122 , inlet valve  124  exhaust valve  128 , ignition mechanisms  130 , combustion chamber  132  and frame  134 . 
     Compliant wall  122  attaches at its opposing ends  122   a  and  122   b  to frame portions  134   a  and  134   b , respectively. Frame  134  includes rigid portions  134   a  and  134   b . Frame  134  attaches to opposite end portions of compliant wall  122  and prevents axial expansion of compliant wall  122 . Specifically, frame portion  134   a  fixes to—and prevents motion of—an end portion  122   a  of compliant wall  122 , while frame portion  134   b  fixes to—and prevents motion of—an opposite end portion  122   b . Since both opposite tubular ends of compliant wall  122  are fixed to prevent axial deformation, tubular compliant wall  122  radially stretches during combustion of a fuel in combustion chamber  132 . 
     Combustion chamber  132  is formed by inner surfaces of compliant wall  122  and surfaces of walls on frame  134  that neighbor chamber  132 . In this case, the shape of combustion chamber  132  changes with deformation and stretching of compliant wall  122 . As shown in  FIG. 5A , compliant wall  122  includes extra material, which forms bends  136  according to the pressure in combustion chamber  132 , e.g., when the pressure is low. 
     Inlet valve  124  regulates fuel and air provision through an inlet  125 , which proceeds through frame portion  134   a  and opens into combustion chamber  132 . Similarly, outlet valve  126  regulates exhaust passage via an exhaust outlet  127  that opens into combustion chamber  132 . Combustion device  120  includes multiple ignition mechanisms  130 , each of which includes spark electrodes for ignition of fuel within combustion chamber  132 . The multiple ignition mechanisms  130  create more consistent radial expansion along the tubular axis. 
       FIG. 5B  illustrates radial combustion device  120  after fuel intake. At this point, compliant wall  122  is substantially cylindrical or tubular between ends  122   a  and  122   b . Upon combustion, compliant wall  122  expands radially and directions  138  as shown in  FIG. 5C . Mechanical coupling  139  is attaches to an external surface of a central portion of compliant wall  122  and provides mechanical output for combustion device  120 . 
     Combustion device  120  provides mechanical output in 360° of radial expansion for compliant wall  122  about the tubular axis. A combustion device need not include such a large expansion area for a compliant segment or wall. Indeed, some combustion devices limit expansion of a compliant wall to smaller segments. This increases combustive forces on the smaller area. 
       FIG. 6A  illustrates a simplified cross-section of a sheathed combustion device  140 , before combustion, in accordance with another embodiment of the present invention.  FIG. 6B  illustrates sheathed combustion device  140  after combustion. 
     Combustion device  140  includes a rigid sheath  141  that is configured to restrict expansion of compliant wall  142  during combustion of a fuel in combustion chamber  144  to within an aperture  146  in rigid sheath  141 . Specifically, rigid sheath  141  surrounds compliant wall  142  with the exception of an opening provided by aperture  146 . Thus, a compliant segment  145  that is free to expand is formed by the lack of rigid sheath  141  in aperture  146 . Although not shown, corners of rigid sheath  141  may be rounded to prevent pinching portions of compliant wall  142  that bend around sheath  141 . 
     In one embodiment, compliant segment  145  is cylindrical as described above with respect to combustion device  120  and the cylindrical axis passes in direction  148  ( FIG. 6A ). In another embodiment, combustion device  140  is cylindrical and the cylindrical axis passes in direction  150  ( FIG. 6B ). In this case, deformation and stretching of compliant segment  145  through aperture  146  resembles a diaphragm based on the geometry and size of aperture  146 . Mechanical coupling  149  attaches to an external surface of compliant segment  145  in a region that passes through aperture  146 . Coupling  149  provides substantially linear mechanical output for combustion device  120 . To facilitate linear mechanical output, coupling mechanism  149  may also include one or more sets of bearings that constrain motion of an output shaft included in coupling mechanism  149  to a single degree of linear deformation. 
     So far, combustion devices have linearly linked mechanical output to compliant segment or wall displacement. The present invention also contemplates indirect relationships where a coupling mechanism transfers changes in the combustion device to provide mechanical output. 
       FIG. 7A  illustrates a simplified cross-section of a bellows combustion device  160  in accordance with another embodiment of the present invention.  FIG. 7B  illustrates bellows combustion device  160  after combustion. Combustion device  160  includes combustion device  120  of  FIG. 5A  and a coupling mechanism  162 . 
     Coupling mechanism  162  receives the mechanical energy produced within combustion chamber  132  and converts the mechanical energy into a linear direction of deformation  164 . More specifically, coupling mechanism  162  is configured to receive a volumetric increase in combustion chamber  132  when compliant wall  122  stretches during combustion. Coupling mechanism  162  converts the volumetric increase into linear extension of a movable element  170  in direction  164 . Coupling mechanism  162  includes a bellows device  166  having a limited volume  168 . In one embodiment, bellows device  166  includes and seals in an incompressible liquid  169  or gel that transfers volume displacement of combustion device  120  to linear translation of a moveable element  170  along direction  164 . Thus, an increase in volume for combustion chamber  132  causes expansion of side bellows  167  in direction  164  when compliant wall  122  stretches during combustion. Bellows device  166  is suitably sized to receive an increase in volume for combustion chamber  132  that causes extension of bellows  167  and element  170 . This implies that bellows  167  and the volume  168  within bellows device  166  can service volumetric changes for combustion within device  120 . More specifically, bellows  167  includes a position that accommodates a maximum volume for combustion chamber  132  and a position that accommodates a minimum volume for chamber  132 . 
       FIG. 8A  illustrates a simplified cross-section of a bellows combustion device  180  in accordance with another embodiment of the present invention.  FIG. 8B  illustrates bellows combustion device  180  after combustion. Bellows combustion device  180  includes compliant wall  182 , combustion chamber  184 , coupling mechanism  186  and fluid  188 . 
     In one embodiment, compliant wall  182  is substantially spherical and defines a substantially spherical combustion chamber  184 . Spherical combustion chambers allow the minimal surface-to-volume ratios of any geometry, and thus minimize parasitic heat losses for a given combustion volume through the walls of the combustion chamber. In this case, compliant wall  182  resembles a balloon that expands and contracts in response to the pressure status within combustion chamber  184 . For spherical compliant wall  182 , the set of walls that border combustion chamber  184  only includes a spherical single wall. In another embodiment, the profile shown in  FIGS. 8A and 8B  extends linearly in a direction normal to the cross-section shown. In this case, compliant wall  182  and combustion chamber  184  are both substantially cylindrical and extend for a length normal to the cross-section shown as determined by design. 
     Coupling mechanism  186  is configured to receive a volumetric increase in combustion chamber  184  and converts a combustion generated volumetric increase into linear output in direction  187 . A bottom surface  185  of mechanism  186  permits mechanical attachment and coupling to mechanism  186 . As shown, bottom surface  185  attaches to a rigid and non-moving wall  189 . An outlet port  192  passes through non-moving wall  189  and bottom surface  185 . Although not shown, device  180  may also include a separate inlet port. A top surface  183  of mechanism  186  is free to linearly move relative to bottom surface  185 . Top surface  183  is rigid and permits external attachment to mechanism  186 . 
     Coupling mechanism  186  includes one or more flexible bellows walls  191  that extend on opposite sides of mechanism  186  from top surface  183  to bottom surface  185 . Bellows walls  191  expand in direction  187  in response to volumetric increases in combustion chamber  184 . In a specific embodiment, bellows mechanism  186  includes a commercially available bellows device, such as one of the Silicone BL-SIT series as provided by Anver Corporation of Hudson, Mass. Bellows mechanism  186  may also be custom made for a combustion device. Other bellows devices may be used to transfer mechanical energy. In a specific embodiment, bellows  183  includes a sealed elastomer having a spring or wound high tensile fiber about its periphery that constricts deformation of the elastomer to linear displacement in direction  187 . Exemplary spring and high tensile fiber geometries were described above. 
     A liquid or gel  188  is disposed within bellows mechanism  186  and transfers volume displacement of combustion chamber  184  into expansion of bellows mechanism  186  which causes the top surface the top surface  183  to move linear in direction  187 . In other words, liquid  188  acts as a hydraulic drive responsive to pressure changes within combustion chamber  184 . During combustion, when pressure within chamber  184  rises rapidly, liquid  188  pushes a) directly upwards on top surface  183  and b) on bellows walls  191  that indirectly convert the pressure into upwards movement of top surface  183 . In other words, bellows mechanism  186  is constrained to linearly expand only in direction  187  and does so in response to spherical expansion of combustion chamber  184 . 
     Exhaust of combustion gases from chamber  184  may be achieved in a number of manners. In a specific embodiment, exhaust is driven mechanically by an output shaft and crankshaft coupled to top surface  183 , for example (see  FIG. 16 ). In this case, fluid  188  and transfers compressive forces from top surface  183  onto compliant wall  182  to force gases out through port  192 . 
     Fluid  188  also facilitates cooling of combustion device  180 . More specifically, heat transferred into compliant wall  182  generated by combustion within chamber  184  dissipates convectively into fluid  188 . Fluid  188  may then be cycled through a cooling system to actively cool device  180 . 
     A spherical compliant wall  182  and combustion chamber  184  reduces the surface to volume ratio for combustion chamber  184 . Often, the amount of heat lost to a wall in a combustion chamber is proportional to the surface area of the wall. Spherical compliant wall  182  minimizes heat loss into wall  182  initially when combustion begins. In addition to increasing efficiency for the device (less energy is lost his heat), this also reduces the amount of cooling needed. 
       FIG. 9A  illustrates a simplified cross-section of a combustion device  190  in accordance with another embodiment of the present invention.  FIG. 9B  illustrates bellows combustion device  190  after combustion. Combustion device  190  includes compliant wall  182 , combustion chamber  184 , a hydraulic coupling mechanism  192  and fluid  188 . 
     Compliant wall  182 , fluid  188  and combustion chamber  184  are similar to that described above with respect to  FIG. 8A . Coupling mechanism  192  in this case includes a hydraulic cylinder including a rigid cylinder housing  194  and a piston  196  that linearly translates within housing  194 . Housing  194  and piston  196  also cooperate to seal in fluid  188 . Combustion of a fuel within chamber  184  causes compliant wall  182  to push on fluid  188 , which in turn pushes up on piston  196  (housing  194  is rigid and thus receives no mechanical work from fluid  188 ). 
     Piston  196  may also be used to facilitate exhaust of combustion gases from chamber  184 . An output shaft and crankshaft coupled to piston  196 , for example, may be used to drive exhaust of combustion gases from chamber  184  (see  FIG. 16 ). Similar to that described above, fluid  188  transfers forces between piston  196  and combustion chamber  184 —including both forces generated within chamber  184  (e.g. combustion) and forces applied by piston  196  (e.g. crankshaft forces). 
       FIGS. 8 and 9  illustrate ways in which a compliant wall combustion device may couple its mechanical output to a liquid (which may itself couple to a linear output device such as a piston or bellows). The mechanical pressure exerted on the liquid by the compliant wall combustion device may itself be the desired output for a pump. In this case, the piston-cylinder or bellows is instead a rigid fixed volume chamber with liquid input and output valves, thereby allowing the compliant walled combustion device to act as a pump. 
     Liquid piston engines are known to those skilled in the art. However, compared to conventional liquid piston engines, compliant wall combustion devices of the present invention keep the combustion chamber separate from the liquid, thus eliminating liquid surface breakup, frothing, and other problems associated with conventional liquid piston pumps. 
     So far, combustion devices have been discussed where combustion energy stretches a wall. Other designs are possible with the present invention. In some cases, walls of a combustion device change shape during a combustion cycle. 
       FIG. 10A  illustrates a shape changing and compliant walled combustion device  200  in accordance with another embodiment of the present invention.  FIG. 10B  illustrates the shape changing combustion device  200  after combustion.  FIG. 10C  illustrates the shape changing combustion device  200  after exhaust. Combustion device  200  includes wall  202 , constraint  204 , combustion chamber  206 , rigid end plates  208 , at least one port  210 , and output shaft  312 . 
     Compliant wall  202  includes a compliant material whose internal dimensions define combustion chamber  206 . Compliant wall  202  will be described in terms of four wall portions: a substantially cylindrical wall segment  202   a , frustoconical wall segment  202   b , top flat wall segment  202   c  and bottom flat wall segment  202   d . Cylindrical segment  202   a  radially borders combustion chamber  206  along an axial direction from bottom flat segment  202   d  to a bending point  214  in wall  202 . Frustoconical segment  202   b  radially borders combustion chamber  206  along an axial direction from bending point  214  to top flat wall segment  202   c . Frustoconical wall portion  202   b  decreases in diameter from a maximum diameter at bending point  214  according to the diameter of cylindrical wall  202   a  to a minimum diameter at top flat wall segment  202   c  which matches the diameter of the top flat wall segment  202   c . At rest, frustoconical wall segment  202   b  resembles a reducing diameter tube whose wall thickness remains about constant. Top and bottom flat wall segments  202   c  and  202   d  form substantially flat end portions to combustion chamber  206 . Top flat wall segment  202   c  is sized with a diameter such that it may fit within the inner diameter of cylindrical segment  204   a . In a specific embodiment, compliant segment  202  includes a soft elastomer molded into a desired shape and dimensions for device  200 . 
     Rigid end plates  208   a  and  208   b  couple to an outside surface of each compliant end segment  202   c  and  202   d , respectively. Rigid plate  208   b  is externally fixed and does not substantially move. An output shaft  212  attaches to rigid endplate  208   a  and provides mechanical output. Device  200  also includes one more ports  210  for communicating gases into and out of combustion chamber  206 . Constraint  204  prevents radial expansion of compliant wall  202  and is dimensioned to the outer diameter of compliant wall  202  for both cylindrical segment  202   a  and frustoconical segment  202   b.    
     Operationally,  FIG. 10A  illustrates device  200  during fuel and air intake.  FIG. 10B  illustrates device  200  during combustion at peak expansion.  FIG. 10C  illustrates device  200  at the end of exhaust. Bending point  214  facilitates bending of wall  202  and allows frustoconical portion  202   b  to collapse into chamber  206  such that frustoconical portion  202   b  and top flat wall portion  202   c  fit within cylindrical portion  202   a . This facilitates exhaust of gases out from combustion chamber  206 . Folding in compliant wall  202  as shown also reduces dead space within chamber  206 . 
     While  10 C illustrates device  200  having minimal dead space within chamber  206 , the amount of dead space within chamber  206  after exhaust may vary. In a specific embodiment, output shaft  212  connects to a crankshaft that drives displacement of rigid end plate  208   a  and the amount dead space within chamber  206  after exhaust. Some designs may include complete collapse as shown (see  FIG. 16 ). Other embodiments including a frustoconical design may not collapse as completely as shown (top flat wall portion  202   c  may not reach bottom flat wall portion  202   d ). In one embodiment, combustion chamber  206  includes an exhaust volume that is less than about 50% of a peak expansion volume for combustion chamber  206 . In a specific embodiment, combustion chamber  206  includes an exhaust volume that is less than about 25% of a peak expansion volume for combustion chamber  206 . 
     In one embodiment, a crankshaft attached to output rod  212  controls displacement of top flat wall portion  202   c  and frustoconical portion  202   b  and drives collapse of top flat wall portion  202   c  into combustion chamber  206  (see  FIG. 16 ). In another embodiment, elastic energy stored in compliant wall  202  at peak expansion returns top flat wall portion  202   c  at least partially into combustion chamber  206 . 
     Combustion device  200  may include features described above with respect to combustion devices  50  and  70  described above. For example, rigid end plates that attach to the cylindrical and frustoconical sidewalls (and form inner walls for combustion chamber  206 ) may replace top and bottom flat wall portions  202   c  and  202   d . In addition, constraint  204  may include examples described above with respect to constraints  58  and  80 . 
     Combustion device  50  of  FIG. 2A  illustrates a substantially cylindrical geometry while combustion device  200  of  FIG. 10A  illustrates a combined cylindrical and frustum design. Alternatively, a combustion device of the present invention may include a frustum design from one end to another. 
     So far, the present invention has been described primarily with respect to compliant walls that stretch in response to combustion within a combustion chamber. Deflection of a compliant wall may also include other forms of the deflection, such as contraction in response to combustion within a combustion chamber, shape changes in response to combustion within a combustion chamber, etc. 
       FIG. 11A  illustrates a combustion device  220  including a compliant wall  228  including a complaint segments  228  that is configured to contract in response to combustion in accordance with another embodiment of the present invention.  FIG. 11B  illustrates combustion device  220  after combustion. Device  220  produces mechanical energy from a fuel and includes a housing  222 , piston  224 , bearings  226  and compliant wall  228 . 
     Housing  222  includes a rigid structure and a set of rigid walls. Rigid walls for housing  222  include a cylindrical wall  222   a  and a bottom wall  222   b . Inner walls of housing  222  cooperate with an inner surface of compliant wall  228  to define dimensions of combustion chamber  230 . Housing  222  may include a suitably stiff material such as a metal or plastics. Other materials are suitable provided they have a stiffness that does not react to combustion and can withstand heat generation within combustion chamber  230 . Housing  222  also includes two ports: an inlet port  234  that permits combustion reactants to enter chamber  230 , and port  236  that allows combustion products to exit chamber  230 . 
     Combustion device  220  is notably different from other combustion devices described herein in that device  220  includes a piston. However, device  220  separates itself and conventional piston cylinder engines in that compliant wall  228  separates piston  224  from the combustion chamber  230 . In this case, piston  224  acts as mechanical output for device  220 . Piston  224  translates into and out of combustion chamber  230  with the help of bearings  226 . As the term is used herein, a piston refers to a rigid member that translates relative to a combustion chamber in response to combustion within the combustion chamber. Bearings  226  neighbor piston  224  and guide linear translation of piston  224 . More specifically, bearings are disposed on an upper wall of housing  222  and permit low friction movement of piston  224  relative to bearings  226  and combustion chamber  230 . 
     Compliant wall  228  spans the top portion of rigid wall  222   a . Compliant wall  228  also couples to a bottom surface of piston  224 . The coupling may include direct attachment between an outer surface of compliant wall  228  and the bottom surface of piston  224  or indirect attachment via another object placed between the two components. Notably, piston  224  does not include a surface or portion that forms a wall of combustion chamber  230 . 
     Combustion devices described so far have been configured such that a compliant wall stretches in response to combustion within a combustion chamber. Combustion device  220 , however, is different since compliant wall  228  may be configured to either expand or contract in response to combustion within combustion chamber  230 . Compliant wall  228  includes multiple portions: a fixed portion  228   b  attached to the bottom of piston  224  and compliant segment  228   a  that deforms in response to combustion in chamber  230 . In a specific embodiment, device  230  is cylindrical and piston  224  and fixed portion  228   b  are round while compliant segment  228   a  takes a frustoconical shape. In one embodiment, compliant wall  228  is contiguous beyond its dimensions within chamber  230  and includes a portion that is secured between bearings  226  and a top portion  222   c  of housing  222 . 
     In operation, combustion within combustion chamber  230  causes an increase in volume for combustion chamber  230 , which forces piston  224  to move from a position as shown in  FIG. 11A  to a position shown in  FIG. 11B . Piston  226  may couple to a crankshaft ( FIG. 16 ) that drives motion of piston  224  back into combustion chamber  230  to facilitate exhaust of gases from chamber  230  ( FIG. 11A ). 
     Compliant wall  228  has several functions for combustion device  230 . Firstly, wall  228  seals chamber  230 . Thus, compliant wall  228  prevents combustion products and gases from escaping combustion chamber  230  through the piston  224 /cylinder  222  gap. In addition, compliant wall  228  prevents heat loss through the piston cylinder gap, which increases efficiency for combustion device  230 . Secondly, since piston  224  does not include movable parts and a potential gap (that loses combustion gases or heat) within the combustion chamber  230 , tolerances on piston  224  or bearings  226  may be relaxed. Thirdly, since there are no moving parts within combustion chamber  230 , lubrication oil is not required within combustion chamber  230 . In one embodiment, combustion chamber  230  does not include a lubricant other that any fuel used and chamber  230 . Fourthly, compliant wall  228  may include a heat insulating material that reduces heat loss from combustion chamber  230  and increases efficiency of combustion device  220 . 
       FIG. 12A  illustrates a simplified cross-sectional view of a membrane fuel control combustion device  240  in accordance with another embodiment of the present invention.  FIG. 12B  illustrates the membrane fuel control combustion device  240  after fuel intake.  FIG. 12C  illustrates the membrane fuel control combustion device  240  after combustion. Device  240  includes a first compliant wall  242 , second compliant wall  244 , coupling mechanism  246 , porous separator  248 , rigid support  250 , housing  258 , and intake valve  252 , and outlet valve  254 . 
     Combustion device  240  differs from combustion devices described so far in that compliant wall  242  is configured to change shape when deforming from a negative cup angle to a positive cup angle based on combustion within combustion chamber  256 . Compliant wall  242  may also stretch as a result of combustion. More specifically, compliant wall  242  starts out stretched in a direction and position that reduces the volume of combustion chamber  256 , deforms (as a result of combustion) through an inflection point where surface area for wall  242  may decrease, changes shape from cupped to bowed, and then may stretch in a direction and to a position that increases the volume of combustion chamber  256 . 
     In the cross-sectional views shown, a combustion chamber  256  is formed by a bottom side of compliant wall  242 , a top side of compliant wall  244  and sidewalls included in housing  258  on either side of combustion chamber  256  as shown. The volume and shape of combustion chamber  256  will vary with the position of each compliant wall  244  and  242 . In one embodiment, housing  258  is substantially cylindrical out its top opening and compliant wall  242  attaches to housing  258  about a perimeter of the circular hole and spans the circular hole, thereby forming a top compliant wall for combustion chamber  256 . 
     A porous separator  248  is disposed in combustion chamber  256  and laterally spans the combustion chamber from one wall of housing  258  to an opposite wall of housing  258 . Porous separator  248  permits gaseous and fluidic transport through its surfaces from one side to the other. For example, porous separator  248  may include a plastic disk having numerous holes or a metal screen comprising a mesh of thin wires. During exhaust of combustion products from chamber  256 , the solid compliant walls  244  and  242  are restricted from contacting each other via porous separator  248 , which also sets a minimum volume within chamber  256  according to its dimensions (thickness and surface area). 
     For fuel intake, fuel and air are pumped into combustion chamber  256  through inlet valve  252  and compliant wall  244  deflects from the position shown in  FIG. 12A  to the position shown in  FIG. 12B . In one embodiment, the fuel and air are pressurized and wall  244  has a reduced compliance that allows it to expand and open up chamber  256 . In a specific embodiment, wall  244  includes an electroactive layer so it can move down by applying voltage to open up the combustion chamber. In addition, the pressure used to supply the air and gas is insufficient to move coupling member  246 . Each valve  252  and  254  includes an aperture in housing  258  that opens into combustion chamber  256 . In another embodiment, the fuel control membrane is made of an actuated electroactive polymer material. When voltage is applied to the electroactive polymer, any small pressure difference between the fuel inlet side and the atmosphere will cause the fuel control membrane to bulge in that direction, thus allowing fuel intake. 
     For combustion, an ignition mechanism included in the device  248  ignites the fuel and chamber  256  initiates combustion. In a specific embodiment, electric leads are disposed on porous separator  248  and reaches central portion of the cross-sectional area for separator  248  and combustion chamber  256 . Combustion of the fuel forces compliant wall  242  to deflect upwards as shown in  FIG. 12C . Rigid support  250  limits motion and deflection of compliant wall  244 . In one embodiment, rigid support  250  prevents compliant wall  244  from moving past a desired position after fuel intake. In a specific embodiment, rigid support  250  includes a porous plastic or metal cup shaped to a desired profile for the static position of compliant wall  244  during combustion. Upon combustion, compliant wall  244  thus assumes the shape, profile and stiffness of rigid support  250  and mechanical energy generated from combustion of the fuel goes into moving compliant wall  242  and mechanical coupling  246  attached thereto. Coupling mechanism  246  attaches to an outside surface of compliant wall  242 . Combustion of the fuel within chamber  256  pushes coupling mechanism  246  upwards. 
     In another embodiment, the separator  248  is not present, and the shape of the fuel control  244  membrane is defined by the compliant wall  242  before air-fuel intake and rigid wall  250  after air-fuel intake. 
     An alternate embodiment involves replacing fuel control membrane  244 , porous separator  248  and rigid support  250  with a non-porous rigid structure that is of the same shape as rigid support  250 . In this case, fuel intake is achieved through external valves and not through a fuel control membrane  244 . The rigid wall in this case provides a fixed constraint, thus allowing the compliant wall  242  to undergo shape change similar as described in  FIGS. 12B and 12C . 
     Motor Designs 
     In general, a motor in accordance with the present invention includes one or more compliant walled combustion devices configured in a particular motor design. The design converts repeated deformation of a compliant walled combustion device into continuous rotation of a power shaft included in a motor. There are an abundant number of motor and engine designs suitable for use with the present invention—including conventional motor and engine designs retrofitted with one or more combustion devices described herein and custom motor designs specially designed for compliant walled combustion device usage. Several motor and engine designs suitable for use with the present invention will now be discussed. These exemplary designs convert deformation of one or more combustion devices into output rotary motion for a rotary motor or linear motion for a linear motor. 
       FIG. 16  illustrates a perspective view of a simplified rotary motor  500  in accordance with one embodiment of the present invention. Motor  500  converts linear mechanical output from one or more combustion devices to rotary mechanical power. As shown in  FIG. 16 , rotary crank motor  500  includes four elements: a compliant walled combustion device  502 , a crank pin  504 , a power shaft  506 , and a crank arm  508 . 
     As the term is used herein, a crank refers to the part of a rotary motor that provides power to the power shaft  506 . For motor  500 , the crank includes combustion device  502 , crank pin  504 , and crank arm  508 . Combustion device is capable of reciprocal translation in a direction  509 . Crank pin  504  provides coupling between combustion device  502  and crank arm  508 . Crank arm  508  transmits force between the crank pin  504  and the power shaft  506 . Power shaft  506  is configured to rotate about an axis  503 . In this case, rotational direction  514  is defined as clockwise rotation about axis  503 . 
     A bearing  511  facilitates coupling between combustion device  502  and crank pin  504 . Bearing  511  is attached on its inner surface to the crank pin  504  and attached on its outer surface to a mechanical output shaft or connecting rod  512  that is attached to a moving end of combustion device  502 . Bearing  511  allows substantially lossless relative motion between connecting rod  512  and crank pin  504 . 
     Connecting rod  512  is connected on one end to combustion device  502  and on the opposite end to crank pin  504  to connectivity between combustion device  502  and crank pin  504 . In this case, the top end of connecting rod  512  is connected to combustion device  502  and translates up-and-down in direction  509 . The opposite end of connecting rod  512  couples to crank pin  504  and rotates around power shaft  506  with crank pin  504 . A pin  511  allows combustion device  502  to pivot while connecting rod  512  traces an orbital path about axis  503 . Upon combustion within combustion device  502 , the upper end of the connecting rod moves downward with the combustion device  502  in direction  509 . The opposite end of the connecting rod moves down and in a circular motion as defined by crank arm  508 , which rotates about crankshaft  506 . 
     Combustion of a fuel within combustion device  502  moves crank pin  504  down and causes power shaft  506  to rotate. As bearing  511  translates downward in direction  509 , crank pin  504  rotates about power shaft  506  in clockwise direction  514 . Combustion of a fuel within combustion device  502  may be referred to as the ‘power stroke’ for the motor  500 . As linear deformation of combustion device  502  continues, crank pin  504  follows an orbital path around power shaft  506  as defined by the geometry of crank arm  508 . 
     In a specific embodiment, crank pin  504  reaches its furthest downward displacement in direction  509  (bottom dead center) as combustion for combustion device  502  finishes. Momentum of crank pin  504  and crank arm  508  continue to move crank pin  504  in direction  514  around power shaft  506  at bottom dead center. Elastic return of a compliant wall  510  may also cause output shaft  512  to deflect upwards. Elastic return of the compliant wall  510 , and momentum of crank pin  504  and crank arm  508 , continues to move crank pin  504  upwards in direction  514  around power shaft  506 . When the crank pin  504  passes its minimal downward displacement in the direction  509  (top dead center), combustion of a fuel in combustion device  502  begins again. Combustion and elastic return in this manner may be repeatedly performed to produce continuous rotation of the power shaft  506  about axis  503 . 
     For motor  500 , movement of combustion device  502  from top dead center to bottom dead center is called a downstroke, and movement of the combustion device  502  from bottom dead center to top dead center is called an upstroke. As illustrated, combustion device  502  combusts a fuel during the downstroke and uses elastic return of the compliant wall  510  during the upstroke to make one complete revolution of the power shaft  506 . Other embodiments are permissible. For example, the upstroke and downstroke can be switched be re-positioning the combustion device  502  below the power shaft  506 . In this case, combustion within combustion device  502  and elastic return of compliant wall  510  contribute to separate portions of the rotation of power shaft  506 . 
     The combustion device  502  may include any device described herein where connecting rod  512  is used as the mechanical output. One advantage of the rotary output provided by motor  500  is exhaust of gases from combustion device  502 , and control of combustion chamber dimensions during an exhaust stroke, can be achieved by tailoring the length of crank arm  508 . For example, the degree of collapse for combustion device  200  may be controlled using length of crank arm  508 . 
     As shown in  FIG. 16 , power shaft  506 , crank arm  508 , and crank pin  504  are a single continuous structure, also referred to as a crankshaft. A crankshaft is a shaft with an offset portion—a crank pin and a crank arm—that describes a circular path as the crankshaft rotates. In another embodiment, the power shaft  506 , the crank arm  508 , and the crank pin  504  are separate structures. For example, crank arm  508  may be a rigid member rotably coupled to the crank pin  504  at one end and attached to the power shaft  506  at another end, e.g., similar to a bicycle pedal crank. 
     The exemplary motor shown in  FIG. 16  has been simplified in order to not unnecessarily obscure the present invention. As one of skill in the art will appreciate, other structures and features may be present to facilitate or improve operation. For example, the end of combustion device  502  opposite to rod  512  may be grounded or coupled to a pin that permits pivoting. In addition, combustion device  502  may be significantly larger than as shown to reduce the amount of compliant wall  510  strain needed to rotate the crank pin  504 . As shown, combustion device  502  may rely on large linear strain to fully rotate the crank, which is suitable for some combustion devices of the present invention. However, a larger combustion device  502  may be used to reduce the amount of strain needed in compliant wall  510  to rotate the crank pin  504 . For example, the size of combustion device  502  may be selected to produce a strain of about 20 percent to about 100 percent linear strain in the compliant wall  510  to rotate crank pin  504 . 
     Using a single combustion device  502  as described with respect to the motor  500  may result in uneven power distribution during rotation of power shaft  506 . Full reliable rotation of the shaft may also require substantial rotational inertia and speed to prevent the shaft from merely rotating in an oscillatory fashion (i.e. less than 360 degrees rotation). In one embodiment, a rotary motor of the present invention includes multiple combustion devices that provide power to rotate a power shaft. The multiple combustion devices may also be configured to reduce dead spots in rotation of the power shaft, e.g., by offsetting the combustion devices at different angles, thus producing a more consistent and continuous flow of output power for the motor. 
     Although  FIG. 16  shows combustion device  502  coupled to a single crank pin, motors of the present invention may include multiple crank pins, or multiple throws, each coupled to a combustion device  502 . For example, a plurality of cranks may be arranged substantially equally about a crankshaft, where each crank is dedicated to a combustion device  502 . The present invention may encompass any suitable number of cranks arranged in a suitable manner around the power shaft. 2, 4, 6, and 8 crank arrangements are common. In one embodiment, combustion devices are arranged around the power shaft such that they counterbalance each other. 
     Motors of the present invention comprising multiple combustion devices may be described according to the arrangement of the combustion devices about a power shaft. In one embodiment, combustion devices are aligned about a power shaft in an opposed arrangement with all combustion devices cast in a common plane in two side rows about the power shaft, each opposite the power shaft. In another embodiment, combustion devices are aligned about power shaft in an in-line arrangement about the power shaft. In yet another embodiment, combustion devices are aligned about power shaft in a Vee about the power shaft, with two banks of combustion devices mounted in two inline portions about the power shaft with a Vee angle between them. Combustion devices in the Vee may have an angle between about 0 degrees and 180 degrees. Multi-input motor arrangements are well-known to one of skill in the art and not detailed herein for sake of brevity. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents that fall within the scope of this invention which have been omitted for brevity&#39;s sake. For example, although the present invention has been described with respect to a few output mechanisms for employing mechanical energy created in the combustion chamber, one of skill in the art is aware of additional mechanisms to harness mechanical energy produced by a combustion device. It is therefore intended that the scope of the invention should be determined with reference to the appended claims.