Abstract:
A recuperating four-stroke internal combustion engine obtains improves Carnot efficiency by use of a new and novel cylinder head which captures thermal energy normally thrown away in the exhaust and re-introduces it to the working cycle. This result, long sought by others, has been achieved by incorporating within the head a compact internal recuperative heat exchanger in series with a combustion chamber or pre-chamber. A recuperator-protecting valve segregates the recuperator from hot combustion gases until the gases reach maximum expansion in the cylinder. Recuperators of both common-duct and seperated-duct design are described, the separated duct recuperator permitting higher recuperator temperature and increased efficiency and a reduction in the number of valves necessary to control gas flow. A preferred embodiment employs four valves per cylinder unit, a separated duct recuperator, and an insulating liner that surrounds both the combustion chamber and the recuperator. A similar prototype recuperative engine has demonstrated that recuperation can reduce exhaust temperature as much as 600° Rankine below that attainable in an equivalent Otto Cycle engine.

Description:
REFERENCE TO OTHER APPLICATIONS 
     This utility application claims the benefit of U.S. Provisional Patent Application No. 60/051,714 filed Jul. 03, 1997 and entitled: 
     RECUPERATIVE INTERNAL COMBUSTION ENGINE CYCLE AND MECHANISM. 
     This utility application is related to my U.S. application Ser. No. 09/110,740 filed Jul. 03, 1998 and entilted: 
     RECUPERATIVE INTERNAL COMBUSTION ENGINE WITH SEPARATE COMPRESSOR. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to piston-type internal combustion engines having a single piston and an exhaust heat recuperator which preheats the compressed air charge and passes it into a pre-chamber for combustion. The invention relates further to an engine having a recuperator and a protective valve to protect the recuperator from the combustion process. 
     2. Description of the Related Art 
     Internal combustion engines today, with the exception of Diesels, operate on what is commonly known as an Otto cycle originally patented in France in 1862 by Alphonse Beau de Rochas. In 1876, the Rochas compression cycle was incorporated into a practical engine by Nicholas A. Otto. Otto engines were immediately more efficient than Lenoir non-compressing gas engines which had been in production since 1862. Then in 1892, Rudolf Diesel invented the compression ignition engine with higher efficiency than an Otto engine. At the time, their efficiencies were about 3 to 4% for the Lenoir, 12% for the Otto, and 24% for the Diesel, and compared with their expansion ratios of approximately 1.5: 1, 2.5: 1, and 16: 1. 
     The low efficiencies are related to the large amount of energy remaining in the engine exhaust at the moment of release by the exhaust valve. Exhaust temperatures of 1,450° Fahrenheit or more for example were reported for the Lenoir and Otto, and around 900° F. for the Diesel. Actual gas temperatures inside the cylinders when expansion was complete were surely much higher. This is because a great deal of heat transfers to the exhaust valve and then to the exhaust port walls. For example, gas reaches about 90% equilibrium with wall temperature after flowing only ten diameters along the length of a straight pipe. In early engines, exhaust valves and ports were labyrinthine in design and thus absorbed much of the heat from the exhaust before it exitted the engine. 
     The better efficiency of the Diesel came about due to its very high expansion ratio, a result of the high compression ratio needed to create high temperature sufficient to auto-ignite the injected fuel. The high compression ratio and attendant gas and bearing pressures required greatly increased strength and with it, increased weight and cost. In fact, the Diesel is two to three times the weight and cost of a comparable Otto engine. 
     In present day Otto engines such as those used in automobiles, compression/expansion ratios are typically around 8:1 and expansion of the combustion gases is far from complete. Thus, at full load, exhaust is released at 90 to 120 pounds per square inch and 2,500 to 3,200° F. and carries away to the coolant and the atmosphere nearly half of the input fuel energy. 
     A partial solution to energy wastage with the exhaust has been the turbo-expansive conversion of exhaust energy to rotative energy, the rotative energy then being used to drive a turbo-compressor for boosting input air pressure to an engine. Turbines however cannot tolerate direct exhaust heat from an Otto engine nor are they suited to the pulsating exhaust flow from a single cylinder. Exhaust from several cylinders is merged to smooth the flow and passed through exposed pipes to dump part of the heat to the atmosphere so as to cool the gases by 1,000 to 1,500° F. Exhaust turbines have thus been only minimally effective in raising the efficiency of Otto engines. 
     Recuperative gas turbines were developed for stationary use beginning around 1950 wherein residual exhaust heat is captured and returned to the compressed air before it enters the burners. Gas turbines operate on the Brayton gas cycle and differ from the Otto cycle in that combustion takes place at constant pressure and that expansion continues typically back to atmospheric pressure. Brayton and Otto cycles are similar however in that the exhaust exitting from a gas turbine is usually at a higher temperature than the compressed air entering the combustion chamber. The opportunity thus exists for a reduction in the fuel input required, since the heat being exhausted can be used (at the cost of a heat exchanger and connecting ducting) to provide part of the heat input, with consequent increase in efficiency. A brief review of the regenerative gas-turbine cycle may be found in  Engineering Thermodynamics  by M. C. Potter and C. W. Somerton, McGraw-Hill, 1993. A more complete analysis of recuperative gas turbines may be found in  Marine Gas Turbines  by John B. Woodward, John Wiley &amp; Sons, 1975. 
     Gas turbines with heat exchangers are under development today for military use as evidenced by a request for proposal published in the Department of Defense Fiscal Year 1998 Small Business Innovation Research (SBIR) Program #A98-013 Titled:  Advanced Ultra Compact Heat Exchangers . In this request, it is noted that present day, state-of-the-art recuperated gas turbine engines use metallic heat exchangers which are exceedingly heavy and larger in volume than all the engine turbomachinery components combined, thereby precluding their use in air vehicles. 
     In the prior art, recuperative engines have generally had adequate recovery of exhaust heat but their transfer of heat to the working charge has been inefficient. This is because the high temperature, high grade thermal energy available in and recovered from the exhaust has been allowed to degrade prior to its transfer to the working charge. Early inventors of heat engines sought effective use of recuperators, but often compromised thermal efficiency by reducing temperatures either to protect working materials and surfaces or to avoid problems with detonation or pre-ignition in the combustible mixture. 
     The first known recuperative internal combustion engine of the prior art is described in U.S. Pat. No. 155,087 granted Sep. 15, 1874 to Joseph Hirsch. It has two cylinders in a vee plus an air pump adjoined at their working ends by a duct containing a regenerator and is described as a hot-air engine. 
     In U.S. Pat. No. 328,970 granted Oct. 27, 1885, inventor James F. Place describes a recuperative engine that uses two stage, counter-flow transfer of exhaust heat to the compressed charge. Place used two cylinders approximately 70° apart in phase to provide separate compression and expansion in his engine. 
     Looking now at the art related to single piston recuperative engines, U.S. Pat. No. 1,190,830 granted to J. F. Wentworth describes a single cylinder engine having transfer of exhaust heat to the incoming air charge. Since the transfer is prior to compression, the engine cycle is not advantageously recuperative. Wentworth recognized at an early date, the utility of a thermally insulating liner in the combustion chamber and on the piston cap to reduce heat losses from combustion. The refactory metal liner was separated by a space from the cylinder head and the space filled with a refractory insulation such as asbestos. 
     U.S. Pat. No. 1,945,818 granted to H. L. McPherson and J. W. Weatherford describes an engine with no recuperator but having a separated combustion chamber connected by a duct to a single cylinder with a reciprocating piston therein, the piston being shaped to provide minimum clearance with the cylinder head. Fuel input and a spark plug provide for combustion within the chamber while a poppet valve controls gas flow through the duct between the chamber and the cylinder. The poppet valve is perforated by holes  23 , believed to be for cooling. 
     U.S. Pat. No. 2,671,311 granted to H. Rohrback is similar to the engines of some of the earlier patents in the use of a liquid coolant, in this instance, by injection of coolant into the cylinder after the work stroke to cool the cylinder and piston. A condenser for the volatized liquid coolant acts as a heat exchanger to heat air being drawn therethrough for injection into the cylinder by a supercharger. The Carnot efficiency of this engine would not be significantly improved since the cycle is not recuperative. 
     U.S. Pat. No. 3,591,958 granted to William H. Nebgen and assigned to Treadwell Corporation, teaches the use of an external turbo-compressor to add chilled compressed air to the single cylinder of an internal combustion engine for the expressed purpose of cooling the internal parts of the engine including the piston and cylinder. At the end of the compression stroke, the compressed air is expelled from the cylinder and enters a manifold to pass through a heat recuperator. After the compressed air has been returned to the cylinder, combined with fuel, ignited and expanded in the cylinder to perform work on the piston, the exhaust is expelled to an exhaust manifold, passed through two heat recuperators in series and then to the atmosphere. Nonetheless, this hot exhaust gas is then fed back into the cylinder for admixture with additional cold, compressed air. The recuperators do not appear to be intimately associated with the working cylinder and would probably have large radiation and convection heat losses. Additionally, energy must be expended to operate the refrigerator and the turbo-compressor, which energy is not fully recoverable. As a consequence, the thermal efficiency is less than optimum. 
     U.S. Pat. No. 4,280,468 granted to Millman and four patents including U.S. Pat. No. 5,632,255 granted to Ferrenberg describe a single cylinder engine having a moving plate recuperator with diameter equal to the piston and including a mechanism to mechanically reciprocate the recuperator in the cylinder between the piston and the cylinder head. While this engine offers improved thermodynamic efficiency, the recuperator is located within the combustion space and exposed to combusting gases. 
     U.S. Pat. No. 4,284,055 granted to Anthony C. Wakeman and assigned to Lucas Industries, Limited, of Great Britain proposes to provide a low compression ratio reciprocating piston internal combustion engine wherein exhaust heat is recovered in a recuperative heat exchanger located within the working volume of the cylinder above the top dead center of the piston and below the cylinder head. With this arrangement, it again appears that the recuperative heat exchanger is directly exposed to combusting gases. Wakeman uses an additional piston to displace gases through the recuperator. 
     There is of course another group of engines, generally called Stirling engines, which have recuperators for internal gas exchange. This art is considered unrelated since the engines have external combustion and the temperature at which the recuperator operates is considerably lower than in the cited art. 
     The problem with most of these cited internal combustion engines employing a heat exchanger or recuperator of one type or another is the large radiative and convective heat loss caused by the exposed location and/or the large size of the heat exchanging element. As pointed out in the discussion of individual patents, heat losses from the recuperator lower the Carnot efficiency. In the very few instances in the prior art where the recuperator is not subject to radiation and convection losses, the recuperator is located in the working cylinder or in a duct directly connected with the working cylinder and the recuperator is thus directly exposed to the flame front of the ignited charge. 
     BRIEF SUMMARY OF THE INVENTION 
     Intermediate between the Otto and the Diesel there has remained the possibility of an engine type which would extract more energy from the combustion gas but without the weight and cost penalties of the Diesel. A direct approach has been to capture heat from the exhaust and put it back into a subsequent engine cycle. This process of recovering heat has been referred to in the prior art as regeneration, heat exchanging, heat recycling and recuperation. The latter term will be used herein. Recuperation is believed to be effective only when the recovered heat is put into the charge after the charge is trapped in the working chambers of the engine. The term “recuperative engine” will also be reserved in this specification for those engines which have positive displacement, an actual throughput of combustion gas, and combustion taking place internally within a working chamber. 
     A recuperative or heat exchanging cycle, referred to hereafter as the Hx Cycle (pronounced “wix”), will be used herein to refer to an engine cycle wherein heat captured from the exhaust is used to heat a subsequent compressed charge. 
     The maximum efficiency that a heat engine such as the Otto Cycle engine can have is calculable by Carnot&#39;s Law. This law states:          Maximum  possible  efficiency     =     η   =     1   -       T   Low       T   High                                  
     T High  in the above equation is the combustion temperature, and T LOW  is the exhaust temperature. 
     The maximum possible efficiency of the Hx Cycle, where DT rec  is the temperature reduction achieved in the engine exhaust by the recuperator, is:          Max.  possible  recuperative  efficiency     =     η   =     1   -         T   Low     -     Δ                   T   rec             T   High     +     Δ                   T   rec                                      
     Putting in typical temperatures in degrees Rankine for an Otto engine at peak output, but with recuperation, we get:          Maximum efficiency     ,     η   =       1   -     (         3        ,        000     -   600         5        ,        500     +   600       )       =   .606                              
     For the Otto engine without recuperator, the efficiency maximum is:          Maximum efficiency     ,     η   =       1   -     (       3        ,        000       5        ,        500       )       =   .455                              
     Six hundred Rankine degrees of recuperation, the amount obtained in tests of a prototype, thus raises the ideal efficiency from 45.5% to 60.6%. It must be understood that these numbers are good only for comparative purposes and in actuality, one may obtain only around one-half of their value, the Otto engine today providing about 22 to 25% efficiency in automotive use. Many factors have not been accounted for such as heat losses to the combustion chamber and recuperator walls, gas flow losses due to resistance to compressed charge flow through the recuperator into the combustion chamber, and to exhaust flow from the expander cylinder out through the recuperator to atmosphere, and the variation of specific heat of air and combustion gases with temperature, which variation necessitates a correction factor for DT rec . making it larger when below and smaller when above its mean value. 
     The problems of Otto engines and known recuperative engines of the prior art are solved by my invention wherein I provide a new and novel cylinder head which provides a working four-stroke cycle unit over each piston of an engine. The cylinder head contains an internal recuperator for heating the compressed air charge which air is then fed into an internal combustion chamber (also known in the art as a pre-chamber), admixed with fuel, and combusted. Life shortening of the recuperator through exposure to the flaming fuel-air mixture has also been eliminated by the use of a recuperator protective valve. The recuperator protective valve closes to separate the recuperator from the combustion chamber just prior to combustion. It reopens at the end of the expansion stroke to release the exhaust through the recuperator to atmosphere. The protective valve is needed in all but the smallest, lowest output engines to protect the recuperator from damage from the combustion flame front. 
     This arrangement of combustion chamber and compact recuperator obtains improved Carnot efficiency in a simple, cost effective manner, the heat losses caused by radiation and convection being reduced due to the recuperator having a volume on the order of only 5% of the cylinder displacement. Thermal losses are then reduced further by the use of a novel insulating liner around the recuperator and combustion chamber. In a preferred embodiment, a separated duct recuperator built around the exhaust valve provides further enhanced efficiency whereby a recuperative exhaust valve may be built as an integral unit. 
     In larger engines, residual energy still in the exhaust after it leaves the recuperator (perhaps as much as 25% of fuel energy), may be partially recovered with an exhaust driven turbo-alternator to produce electric energy. Since most engine applications already use an alternator to provide electric power, capture of alternator drive energy energy from the recuperative engine exhaust can take the place of the present alternator system and further improve efficiency. 
     The recuperative cylinder head may be engineered to match existing conventional engine blocks such as are used in modern gasoline fueled automobiles, thereby reducing the cost of changeover to new production. In addition, the head may be engineered to retrofit to the block of engines which originally had drive means such as a notched timing belt for operating overhead valves. 
     OBJECTS OF THE INVENTION 
     In this invention, an exhaust heat recuperator extracts heat from the exhaust, thereby lowering the heat rejection temperature and subsequently adds this heat to the compressed working charge prior to combustion, thereby raising the pre-combustion temperature which produces an equivalent increase (after adjustment for the change in specific heat) in the peak combustion temperature. 
     A first object of this invention is therefore to maximize, through the method of recuperation, the potential Carnot efficiency and the related actual working efficiency of a single piston internal combustion engine. 
     Another object is to provide effective recuperation in an engine with the minimum number of additional valves, pistons, recuperators, and other ironmongery, which in the preferred embodiment is reduced to a single reciprocating piston and four valves per engine unit. 
     A third object of this invention is to provide a coaxial, symmetric method of construction for a combustion chamber, recuperator, valving, and insulative housing all within a common cylinder head. 
     Another object is to provide a means for insulating a combustion chamber and recuperator with a simple mechanical construction that due to symmetry is resistant to thermal stress fracture and temperature non-uniformities. 
     Another object of the invention is to provide for an engine, a recuperator unit made in thermally segregated sections with reduced axial heat flow loss. 
     Another object is to provide a combination recuperator and exhaust valve in an assembly having separated parallel coaxial ducts comprising an outer one which carries exhaust from the combustion chamber out of the engine in a first direction, and an inner duct which carries compressed air in an opposite direction into the engine combustion chamber. 
     Another object is to provide a common duct recuperator having a tapering cross section along it flow axis to compensate for the changing viscosity of gases with temperature. 
     Yet another object is to provide a separated duct recuperator having tapering cross sections along its flow axes, the hotter ends of each duct having larger area, to compensate for the increasing viscosity of the gas flows with temperature. 
     Another object is to provide a method for installing a recuperator such that it may be easily removed for servicing, much like a spark plug. 
     Yet another object is to provide a recuperative cylinder head for easy attachment to new engine blocks and also easy retrofit to existing conventional engine blocks to replace conventional Otto Cycle cylinder heads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be clearly understood and readily carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
     FIG. 1 is a sectional view of the preferred embodiment of the recuperative engine operable on a four-stroke cycle and having a single piston, a coaxial, separated duct recuperator, and four poppet valves. 
     FIG. 2 is a cross section of the separated duct recuperator taken along plane  2 — 2  of FIG.  1 . 
     FIGS. 3 a - 3   h  are sequential sectional views illustrating the gas cycle of the recuperative engine of FIG.  1 . 
     FIG. 4 is a sectional view of the cylinder head of a recuperative engine operable on a four-stroke cycle and having a single piston, a separated duct recuperator, and five poppet valves. 
     FIG. 5 is a sectional view of the cylinder head of a recuperative engine operable on a four-stroke cycle and having a single piston, a common duct recuperator, and four poppet valves. 
     FIG. 6 is a linear pressure vs. volume graph comparing Hx, Otto and Diesel cycles. 
     FIG. 7 is a log p vs. log (V/V max ) graph comparing Hx, Otto and Diesel cycles. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A clear and better understanding of my invention can be had by reference to this description when taken together with the appended drawings, wherein like reference indicia refer to like elements of the invention. 
     In the preferred embodiment of the engine shown in FIG. 1, a conventional liquid cooled cylinder block  10  similar to that used on automotive engines includes a cylinder  12  in which a piston  14  operates reciprocably between a top dead center and bottom dead center position to define a working volume  20 . The head of piston  14  and the opposing surface of a recuperative head  16  are shaped such that volume  20  is minimized when piston  14  is at top dead center. The improvement comprises recuperative cylinder head  16  attached at joint  18  to enclose the open end of cylinder block  10  above the top dead center position. Recuperative cylinder head  16  defines a combustion chamber  34  and contains a recuperator  26  which are separated by a recuperator inlet valve  58  and a recuperator exhaust valve  80 , all being concentric with a combustor valve  50  which separates chamber  34  from cylinder volume  20 . An insulating liner  40  is illustrated containing chamber  34  and recuperator  26 , but is not essential to operation of the recuperative engine. Combustor valve  50  is provided with an axial transfer duct  60  to pass compressed charge from cylinder  12  up through ports  25   a  and  25   b  defined by valves  50  and  58  to admit the charge to recuperator  26 . Cylinder head  16  also defines an intake port  36  containing an inlet valve  38  for the admission of ambient air charge  42  into cylinder  12 . 
     During operation, air is inducted into cylinder  12  on a downstroke of piston  14 , then compressed to a small fraction of its ambient volume at which compression it is released by the opening of recuperator inlet valve  58  allowing the charge to flow through the inner duct  83  of recuperator  26  and into combustion chamber  34  which is also a small fraction of the ambient volume of the charge. During this flow, fuel is injected into the charge such as by injector  80 . When piston  14  nears top dead center, recuperator inlet valve  58  closes, combustor valve  50  opens, and ignition occurs at spark plug  66 . Combustion products now flow past valve  50  into cylinder  12  and expand against piston  14  driving it towards bottom dead center. When piston  14  nears bottom dead center, recuperator exhaust valve  80  opens allowing exhaust to blow back through combustor valve  50  which has remained open and through the combustion chamber and through duct  81  of recuperator  26  and through port  72  to atmosphere. 
     In engines designed for higher efficiency, a exhaust driven turbo-expander may be attached to port  72  to receive cooled gases from the engine and extract further work from them. The rotative work obtained may be used to drive an electric alternator, or to drive a turbo-compressor provide boost for the engine inlet air. 
     Referring now to FIGS. 1 and 2, in this preferred embodiment of the recuperative engine, recuperator  26  is a separated, coaxial duct arrangement wherein the exhaust passes through an outer annular duct  81  towards port  72  and the compressed air charge passes through inner annular duct  83 , ducts  81  and  83  being concentric and in close thermal contact by means of thermally conductive recuperator fins  84  which pass radially through and conduct heat through tubular wall  99  of exhaust valve  58 . The recuperator flows are countercurrent whereby they flow in opposite directions for good thermal transfer efficiency. 
     Recuperator  26  is tapered, ducts  81  and  83  being larger in cross section at the hotter end of the recuperator adjacent combustion chamber  34  and smaller in cross section at the end adjacent exhaust port  72 . 
     The hot end of recuperator  26  for best cycle efficiency is maintained at close to the maximum temperature tolerance of the recuperator element comprising fins  84  and wall  99 , a temperature presently around 2,000° F. when exposure is limited to exhaust gases consisting mainly of nitrogen with negligible oxygen content. The portion of fins  98  projecting into exhaust duct  81  operates hotter than the portion of fins projecting inward into compressed air duct  83 . Since duct  83  contains air with oxygen, it must operate at a lower temperature. This is in fact how the temperature gradient will reside in normal operation. 
     Higher output engines may need a controller to limit air and fuel input to avoid exceeding the temperature tolerance of the recuperator. For this control, an electronic computer can be used which reads exhaust temperature in port  72  with sensor means  101  such as a thermocouple. The controller would receive these and other inputs such as RPM, ignition spark advance, throttle opening, engine coolant temperature, oil temperature, and possibly combustion noise to determine optimum fuel and air inputs with the goal being to attain high efficiency while preventing thermal or oxidation damage to the recuperator. 
     Temperature of the recuperator can also be controlled by limiting the surface area of fins  98  where they project into exhaust duct  81  to reduce their heat pickup and enlarging the surface area of fins  98  where they project into compressed air duct  83  to increase their heat transfer to the charge. 
     It should be understood that the recuperator temperature maximum is a mean temperature. The initial peak exhaust temperature during exhaust blowdown can run several hundred degrees above this temperature since the recuperator operates at an average between the mean exit temperature for the exhaust and the mean entrance temperature for the compressed charge. 
     The end of the recuperator adjacent to the exhaust port operates substantially cooler at about 1000° F. During operation of the engine, recuperator temperatures are higher when the engine is at full load and high RPM and lower when the engine is at minimum output and at low RPM. Recuperator temperatures also run higher when the fuel-air mixture is lean and above stoichiometric and the burning speed of the charge is lower. 
     While operating recuperatively, the fuel-to-air ratio can be leaned significantly more than when operated as an Otto engine, to an estimated 20:1 air-to-fuel ratio. The lean burn capability is believed to be due to the higher temperature of the fuel-air charge at the instant of ignition. 
     An additional benefit of the recuperator was that exhaust from the engine was appreciably quieter after it had passed through the recuperator. 
     Recuperator  26  is preferably segregated into at least two elements such as “cool element”  28  and a “hot element”  30 , illustrated, the elements being thermally segregated in the axial direction by splits  103  to reduce detrimental axial flow of heat from the hot end to the cool end. Cool gases such as compressed air, flowing through the recuperator from the cool end to the hot end, are considered as flowing in a charging direction and are heated by the recuperator. Hot gases such as exhaust, flowing from the hot end towards the cool end, flow in an exhausting direction and lose their heat to the recuperator. 
     Cool element  28  and particularly hot element  30  of recuperator  26  are fabricated from a material that combines high thermal conductivity with resistance to oxidizing gas conditions up to the maximum temperature expected at that point in the recuperator. For example, hot element  30  may operate at 2,000° F. at the hot end adjacent recuperator valve  58  and require fabrication from a refractory material such as tungsten, while cool element  28  may operate at 1,200° F. at its hot end which may be tolerated by pure nickel or chromium plated nickel. Tubular stem  99  of recuperator exhaust valve  80  must also be resistant to high temperature and also requires high strength, but does not require high thermal conductivity. 
     The common duct recuperator is designed to have minimum volume commensurate with providing minimal resistance to compressed air and exhaust gas flows. This recuperator volume can be kept below 8% and is preferably held to less than 5% of the cylinder volume  20 . Minimizing the gas volume within recuperator  26  and exhaust chamber  37  reduces the loss of air charge that remains trapped within the recuperator once the combustion chamber has been filled and the recuperator valve closes. This trapped air is swept out with the subsequent exhaust flow and constitutes a loss in efficiency. To reduce thermal losses from recuperator  26 , it is mounted within liner  40  which provides thermal insulation against heat flow into cylinder head  16 . 
     Referring to FIG. 4, cylinder head  16  defines a duct  60  which allows compressed gases to flow from cylinder volume  20  into recuperator  26  as per flow arrows  22 . A transfer valve  56  may be used in duct  60  to more closely control the flow of compressed air from volume  20  to recuperator  26 . Without this valve, exhaust gas may bypass, during the exhaust stroke, from volume  20  through duct  60  to the exhaust space  37  and out past exhaust valve  70  to atmosphere if the recuperator is somewhat restrictive to exhaust gas flow. 
     The volume of duct  60 , like the volume of recuperator  26 , is minimized for efficiency of the cycle, and duct  60  volume need only be large enough to provide charge transfer with a minimum of flow resistance and losses. This generally need only be on the order of 1 per cent of cylinder volume  20 . Communication between recuperator  26  and combustion chamber  34  is controlled by the movement of recuperator valve  58  which controls gas flow between recuperator  26  and combustion chamber  34 . The stem of recuperator valve  58  is sealed by high pressure seal  76  against pressures up to several hundred pounds per square inch created during charging of the combustion chamber  34 , the seal preventing leakage along the valve stem to atmosphere. 
     A fuel injector  64  provides means for admixing fuel  65  with air within chamber  34 . This fuel injection is preferably designed to admix efficiently with air flow as it passes around recuperator valve  58  into chamber  34 . Once the compressed air charge is within chamber  34 , recuperator valve  58  closes to protect the recuperator from combustion heat. 
     Ignition of air-fuel mixtures is provided by a spark plug  66  or its equivalent. Combustion gases from chamber  34  have access to the working volume  20  of cylinder  12 , above piston  14 , via port  68  defined by cylinder head  16 , the port being valved by combustor valve  50 . The stem of combustor valve  50  is sealed by high pressure seal  78  against combustion pressures up to one thousand pounds per square inch within combustion chamber  34  from leakage along the valve stem to atmosphere. 
     Combustor valve  50  operates hot since it passes hot combustion gases into volume  20  and then passes hot, spent gases back from volume  20  to chamber  34 . Conventional cooling of combustor valve  50  occurs through its valve seat  25   a  during periods when the valve is closed tightly by spring pressure, which seat  25   a  is cooled by normal cooling means such as coolant passages  72  for cylinder head  16 . Heat is also carried away by valve guide  53  defined by cylinder head  16 , and by coolant passages  74  or by the equivalent cooling fins in the case of a direct air cooled engine. Additional cooling of combustor valve  50  is provided by a small portion, perhaps five per cent, of the bleed air charge during the compression stroke which is allowed to leak through multiple tiny holes  48  in the head of valve  50  into chamber  34 . 
     After the combustion gases have expanded against piston  14  to produce work, and piston  14  is almost at bottom dead center, the gases are released by the opening of recuperator valve  58  and exhaust valve  70  whereby the gases reverse their flow, passing back through combustor valve  50 , combustion chamber  34 , recuperator valve  58 , recuperator  26 , exhaust chamber  37 , exhaust valve  70 , and exhaust port  72  defined by cylinder head  16 . 
     In review, FIG. 4 illustrates an improved four-stroke internal combustion engine  10  having a single cylinder  12  and a single piston  14  reciprocating therein between top dead center and bottom dead center positions, wherein my improvement comprises a recuperative cylinder head  16  in which is located a recuperator  26  connected to a combustion chamber  34 . An inlet duct  36  defined by cylinder head  16  and an inlet valve  38  located in duct  36  control the flow of atmospheric air  42  into cylinder  12  where the air is compressed on the next stroke of piston  14 . After partial compression, recuperator valve  58  opens allowing the compressed air  22  to flow through duct  60  and through recuperator  26  and past recuperator valve  58  into combustion chamber  34  in series. An injector  64  adds fuel  62  to the compressed air  22  as it enters chamber  34  and a spark plug  66  ignites the fuel-air mixture within chamber  34 . The burning gases are then released by combustor valve  50  to flow into cylinder  12  where they expand and force piston  14  towards bottom dead center. Combustor valve  50  remains open and after the gases expand in cylinder  12 , they are released by the opening of recuperator valve  58  and exhaust valve  70  to flow back through combustor valve  50 , combustion chamber  34 , recuperator valve  58 , recuperator  26  and an exhaust valve  70  which releases the gases through port  72  to atmosphere. 
     For better control over flow  22 , a transfer valve  56  may be installed in duct  60  to release compressed air from cylinder  12  into recuperator  26  at the same instant that recuperator valve  58  opens to release the air into combustion chamber  34 . 
     Turning now to FIG. 3 a  through  3   h , there is shown the recuperative gas cycle carried out in an engine similar to the preferred engine embodiment of FIG.  1 . The cycle proceeds as follows: 
     On a first stroke of the piston  14  from top dead center to bottom dead center, as illustrated in FIG. 3 a , inlet valve  38  opens at top dead center and allows a charge of ambient air to be inducted through inlet duct  36  and into cylinder  12 . 
     As piston  14  passes bottom dead center, inlet valve  38  closes and for a major portion of the return stroke to top dead center, piston  14  compresses and adiabatically heats the air charge within space  20 , as illustrated in FIG. 3 b.    
     During the remainder of the second stroke, shown in FIG. 3 c , recuperator valve  58  opens briefly to allow the charge of compressed hot air to be forcibly transferred from space  20  through duct  60 , recuperator  26 , past recuperator inlet valve  58 , and into combustion chamber  34 . During this transfer, the hot air charge is allowed to leak from cylinder space  20  through multiple small cooling holes in combustor valve  50  into combustion chamber  34 . This leakage flow provides cooling for valve  50 . 
     As piston  14  passes top passes top dead center in FIG. 3d, recuperator valve  58  closes and the doubly heated air charge, now within chamber  34  and containing fuel, is ignited and combusts to produce high temperature and pressure gaseous products within the chamber. 
     Combustor valve  50  opens immediately and as piston  14  leaves top dead center on its third stroke in FIG. 3 e , hot, high pressure combustion gases flow from combustion chamber  34  into cylinder space  20 , expand against piston  14 , and generate work. 
     In FIG. 3 f , the exhaust gases are partially expanded, but still hot and at high pressure, while piston  14  is approaching bottom dead center. Recuperative exhaust valve  70  now opens to allow these gases to blow down through combustor valve  50 , exhaust duct  82 , and exit to atmosphere. During this gas blowdown, most of the residual gas heat is transferred to the wall of the combustion chamber  34 , the head of recuperative exhaust valve  80 , and recuperator  26 . 
     During a fourth stroke of the piston towards top dead center as shown in FIG. 3 g , residual, hot exhaust gases remaining in the cylinder at close to ambient pressure are forcibly ejected to atmosphere during which ejection, further heat is captured by the chamber walls, recuperator valve  58 , recuperator  26 , and exhaust valve  70 . 
     All valves close briefly as piston  14  passes top dead center at the end of its fourth stroke down shown in FIG. 3 h.    
     FIGS. 3 a  through  3   h  illustrate the complete recuperative cycle performed in a single piston engine. FIG. 3 a  shows inlet valve  38  having just opened and piston  14  on a first stroke towards bottom dead center, thereby drawing in a charge of fresh air. 
     In FIG. 3 b , inlet valve  38  has closed and piston  14  is on its second and return stroke to top dead center, compressing the charge against minimal clearance volume  20  between the head of piston  14  and the cylinder head  16 . Some air flows through the hollow stem of valve  50 ′ thereby cooling it, and into the space within the recuperator air duct  83  during initial compression of inducted air in cylinder  12  and volume  20 . The actual combined volume of recuperator  26  and duct  60  is only a few per cent of the maximum cylinder volume  20 . Another few per cent of the compressed air flows through tiny holes  48  in combustor valve  50 ′ and into combustion chamber  34  thereby providing cooling for the head of combustor valve  50 ′. 
     In FIG. 3 c , recuperator inlet valve  58  opens as piston  14  compresses the air through between 65% and 90% of its full stroke, and the compressed air flows through recuperator air duct  83  past valve  58  into combustion chamber  34 , picking up heat during the transfer. 
     In FIG. 3 d , piston  14  has reached top dead center and forced all but a few per cent of the inducted air charge into combustion chamber  34 . Recuperator inlet valve  58  closes at the same instant piston  14  reaches top dead center and fuel  65  has been spraying from injector  64  into chamber  34  to create a combustible mixture. Ignition now occurs by spark plug  66 , combusting the mixture within combustion chamber  34 . 
     Combustor valve  50 ′ also opens as the mixture combusts in FIG. 3 e , releasing the high pressure gases to flow into cylinder space  20  and force piston  14  on its third and power stroke towards bottom dead center. 
     In FIG. 3 f , piston  14  is about to pass bottom dead center and recuperative exhaust valve  58  opens to release the exhaust gases to blowdown past combustor valve  50 ′, through combustion chamber  34 , past recuperative exhaust valve  80 , through recuperator duct  81  and out exhaust port  72 . 
     In FIG. 3 g , blowdown has finished and piston  14  makes its fourth stroke towards top dead center, driving residual exhaust gases out the same route described above. 
     In FIG. 3 h  piston  14  has completed its exhaust and fourth stroke to top dead center, all valves close momentarily, and the cycle is ready to repeat at FIG. 3 a.    
     The engine illustrated in FIG. 1 is a four-stroke engine with an operating cycle as follows: 
     1. on a first stroke of piston  14 , inlet valve  38  is opened and the piston makes a stroke to bottom dead center to draw in a charge of ambient air; 
     2. on a first portion of a second stroke towards top dead center, the air charge is compressed through between 65% and 90% of the full stroke, at which point recuperator valve  58  opens releasing the compressed air to travel through duct  60  and ports  25   a  and  25   b  and through duct  83  of recuperator  26  and into combustion chamber  34 ; 
     3. during the remainder of the second stroke, fuel is admixed with the air charge as the air is driven through the recuperator and into combustion chamber  34  by piston  14  as it approaches top dead center; 
     4. recuperator valve  58  now closes and ignition occurs at the same instant that combustor valve  50  is opened, all occurring as piston  14  passes top dead center; 
     5. the combustion products now expand through combustor valve  50  into cylinder  12 , forcing piston  14  on its third stroke towards bottom dead center and performing work against the piston; 
     6. at the end of the third stroke near bottom dead center, combustor valve  50  remains open and recuperator valve  58  and exhaust valve  70  open to allow the partially spent gases to blow down through combustion chamber  34 , duct  81  of recuperator  26 , and exhaust port  82  in series to atmosphere. 
     7. During the fourth stroke and return to top dead center, the residual products of combustion are forcibly expelled through the combustion chamber, duct  81  of the recuperator, and exhaust port in series to atmosphere at which point the combustor valve, recuperator valve, and exhaust valve close, and inlet valve  38  re-opens for the next cycle. 
     FIG. 5 illustrates an alternative embodiment of my invention which differs from FIG. 1 in that it has a common duct recuperator  26 . Recuperative cylinder head  16  is split into two parts comprising a cap  17   a  and a base  17   b , the cap being connected to the base at joint  21 . A pocket  39 , defined by cap  17   a  and base  17   b  at joint  21 , encloses and holds combustion chamber liner  40  in place. Liner  40  provides thermal insulation for combustion chamber  34  and recuperator  26  due to an insulating interface  43  which the liner forms with pocket  39 . Inlet port  36  is defined in this embodiment by base head  17   b    
     Liner  40  is preferably made of a refractory material similar to that used for exhaust valves and capable of operating at high temperature in excess of 2,000° F. The liner may be thin and mechanically supported by the pocket, while the interface still provides substantial insulation effect. Alternatively, the liner may be thick enough to mechanically support combustion pressures and provided with additional clearance at interface  43 , which interface may then be fitted with thermal insulation such as asbestos. Fuel injector  64  and ignition means such as spark plug  66  are preferably fitted gas tight into liner  40  to prevent the leakage of combustion gases into interface  43 . 
     This embodiment of the single piston recuperative engine in FIG. 5 also employs three valves and a recuperator arranged in a group coaxial with liner  40 . The inner valve in this group is combustor valve  50 ′ which has additional provisions for cooling. The first provision comprises bore  60  of the hollow stem of valve  50 ′ which serves to transfer compressed air to holes  25   a  and  25   b  defined by valves  50 ′ and  58 ′ and thence into exhaust chamber  37 , and to cool the stem of combustor valve  50 ′ while said transfer occurs. Multiple holes  48  defined by valve  50 ′ also provide for cooling by allowing a small percentage of compressed air to leak from cylinder  12  into combustion chamber  34  during the compression stroke. 
     The middle valve in the group is a recuperator valve  58 ′ which has a hollow stem to serve as a valve guide for combustor valve  50 ′. The outer valve in the group is exhaust valve  70  with a hollow stem that serves as a valve guide for recuperator valve  58 ′. 
     Recuperator  26 ′ in this embodiment has a tapered cross section, being larger at the hot end than is at the cool end. This provides for a more uniform velocity of the gases flowing through the recuperator as the gases change temperature and therefore volume during their passage from hot to cool end in the case of exhaust gases, and from cool to hot end for air flowing towards the combustion chamber. In addition to compensating for change in gas volume during passage of each gas charge, the taper also compensates for change in viscosity with the change in temperature, since the viscosity of gases increases considerably with temperature and the increasing sectional area in recuperator  26 ′ prevents the viscous drag from becoming excessive. Since both the volume and the viscosity of the gas flow change with temperature, the sectional change of the recuperator should change approximately 1.5 times the change in absolute temperature. Thus if the exhaust gas temperature drops by 600° Rankine and the initial temperature was 3,000° Rankine, the percentage change is 20% and the cross section should taper and reduce by 30% in that flow direction. 
     FIG. 2 is a sectional view of the preferred embodiment of my invention. The recuperative cylinder head  16  is attached to an engine block  10  with a cylinder  12  and a piston  14  therein. which reciprocates between top dead center and bottom dead center positions. The recuperative cylinder head  16  is joined to engine block  10  at joint  18  so as to enclose the end cylinder  12  above the top dead center. Cylinder head  16  is built in two sections, a base head  17   a  and a cap  17   b ′ which adjoins base head  17   a  at joint  21 , thereby to enclose combustion chamber liner  41  within pocket  39 . 
     This preferred embodiment in FIG. 1 of the single piston recuperative engine also employs three valves arranged in a coaxial group within combustion chamber liner  41 . The inner valve in this group is combustor valve  50  for control of gas flow between combustion chamber  34  and cylinder  12 . Combustor valve  50  slides in a hollow stem defined by the recuperator valve  58  located in the middle of the group of valves. 
     Referring to both FIGS. 1 and 5, the outer valve in the group is a multifunction recuperative exhaust valve  80 . This valve has a tubular portion  99  which is spaced from the inner surface of liner  41  to provide an annular exhaust duct  81  for the passage of exhaust gas to the atmosphere. Radial fins  84  made of a high thermal conductivity material capture exhaust heat and conduct it into an annular compressed air duct  83  formed on the inside of tubular portion  81  where the captured heat is transferred to the compressed charge. Recuperative exhaust valve  80  has a hollow stem  90  that serves as a valveguide for recuperator valve  58 . The construction shown in FIGS. 1 and 5 is representative of only one of many possible constructions for the recuperator. 
     To produce a combustible mixture, injector  64  may be used to spray fuel  62  directly into combustion chamber  34 . Alternatively, more rapid and more efficient mixing of fuel with the entering compressed air may be obtained by spraying fuel across the air as it enters chamber  34  through the opening of recuperator valve  58 . One such method is to pipe fuel in along the stem of the recuperative inlet valve  58  and released radially from the underside of the head of valve  58  as per flow arrows  88 . 
     Referring now to FIG. 6, a linear pressure versus volume (P-V) graph has been drawn to illustrate differences between the four-stroke Hx, Otto, and Diesel cycles in a reciprocating piston engine. In this graph, the Hx, Otto, and Diesel P-V curves have been normalized to essentially equal swept volume whereby they start from points, a Hx , a O , and a D , and induct an air charge along a common curve to maximum cylinder volume at a common point b, where the piston is at bottom dead center. 
     Compression of the air charges continues along common curves from point b to point c. From point c, the Diesel curve is dotted and continues with further compression to point c″ at which point fuel is injected under approximately constant pressure and increasing volume to point d″, and from d″, combustion gases expand along dotted line to e″, and are then exhausted along a substantially common curves to g″. 
     The Otto cycle continues from point c with a dashed line to c′ where combustion occurs which drives the pressure up to point d′, followed by expansion of combustion gases along dashed line to e′ and exhaust of gases along the substantially common curves to point g′. 
     The Hx cycle undergoes a perturbation at point c, when the compressed charge is released to pass through the recuperator into the combustion chamber, and travels an indeterminate path up to point d Hx . The release causes the pressure to momentarily fall as the gas expands into and through the recuperator and then heating of the compressed air causes a rapid increase in pressure up to point d Hx  with no input yet of fuel energy. At point d Hx , the compressed air has been sealed in the combustion chamber, admixed with fuel, and ignited causing an increase in pressure to point d c  comparable to that which occurs in the Otto cycle but reaching a higher pressure. From d c  the combustion gases are expanded along the solid curve to point e where they are released to flow through the recuperator (with some slight back pressure causing the curve to ride above the curves for Otto and Diesel), and then be expelled to atmosphere along solid curve to point g Hx . 
     In FIG. 6, one characteristic of a linear P-V graph is that the area inside a closed curve is proportional to the indicated mean effective pressure produced by an engine, and which also correlates with the output power of the engine. Within the equivalent accuracy of the three gas cycles, it may be seen that the Hx curve has substantially more area than the Otto for a similar input of fuel and is not much less than the area displayed by the Diesel. It is also shown in FIG. 6 that the Hx cycle achieves substantially greater power while working at just slightly higher pressures than the Otto cycle. The peak working pressure of an engine, such as at points d c , d′, and d″ is generally a direct indicator of the engine&#39;s weight. The Hx engine therefore appears to not be at any great disadvantage weight wise with the Otto engine. 
     FIG. 7 illustrates the same Hx, Otto, and Diesel cycles as FIG. 6 but on a logarithmic pressure versus volume curve. A log-log graph accentuates low pressure effects and shows much more clearly those low pressure changes which occur during induction and during exhaust expulsion. The expulsion of the Hx cycle from point f to g Hx  has more back pressure due to the presence of the recuperator than the f to g′ and f to g″ expulsion curves of the Otto and Diesel respectively. This back pressure illustrates the importance of designing the recuperator for low back pressure on the exhaust flow, and also illustrates why the separated flow recuperator is preferred as it allows freer flow for the exhaust while avoiding the waste of trapped air which occurs with the common duct recuperator. The log-log P-V illustrates clearly the parallality of the expansion curves d′ to e′, d″ to e″, and d c  to e to the compression curve b to c, common to all three gas cycles, and distinguishes clearly the point at which compression ends and heat addition to the gas begins. 
     MODE OF OPERATION 
     During operation of the engine of FIG. 1, inlet valve  38  is opened as piston  14  starts to descend on a first stroke from top dead center towards bottom dead center, inducting a charge of ambient air as shown by arrow  42  through inlet duct  36  and into working volume  20  of cylinder  12 . During the induction of ambient air, combustor valve  50  and recuperator valve  58  remain in a closed state. Inlet valve  38  also closes as piston  14  begins its second stroke and return to top dead center which compresses said air within cylinder  12 . A small percentage of the air undergoing compression bleeds into combustion chamber  34  through multiple coolant passages  48  in valve  50 . The remainder of the air undergoing compression is compressed in cylinder  12  against closed valves to a volume approximately equal to the combined volume of duct  60 , recuperator  26 , and combustion chamber  34  and then passed in a compressed state from working volume  20  through duct  60 , past now open transfer valve  56 , and thence through recuperator  26 , where the compressed air is heated by approximately 400 to 600° F. or more during its passage, and past now open recuperator valve  58  into combustion chamber  34 . Exhaust valve  70  remains closed during the third stroke of piston  14  back to bottom dead center, as well as during its fourth stroke and return to top dead center. Fuel injection, as indicated at  62  and provided by injector  64  is carried on during the flow of compressed air into chamber  34 , and recuperator valve  58  remains open until piston  14  reaches top dead center initiated prior to piston  14  reaching top dead center, and valve  58  remains open until piston  14  reaches top dead center to allow for completion of the flow of compressed air and the simultaneous mixing of the injected fuel with the compressed heated air flowing into chamber  34 . 
     Valve  58  is now closed simultaneously with initiation of ignition, as by spark plug  66 , and combustor valve  50  is opened before there has been any significant rise in pressure from combustion of the gas-fuel mixture in combustion chamber  34 . With valve  50  open, the gaseous combustion products are free to pass through duct  68  into working volume  20  to urge piston  14  towards bottom dead center and produce work. (After piston  14  has proceeded about two-thirds of the distance towards bottom dead center, the gas pressure in working volume  20  and combustion chamber  34  will drop below the gas pressure in recuperator  26 . At this point, it is advantageous to re-open valve  58  to allow residual hot compressed gas from recuperator  26  to reach working volume  20  via combustion chamber  34  and duct  68  and thereby perform additional useful work by urging piston  14  towards bottom dead center with renewed urgency.) When piston  14  has advanced roughly 95 per cent of the distance towards bottom dead center, exhaust valve  70  which has hitherto been closed is now opened to allow the gas in working volume  20  to exit in series via duct  68  valved by combustor valve  50 , combustion chamber  34 , recuperator  26 , exhaust valve  70 , and duct  72  to the atmosphere during the subsequent motion of the piston towards top dead center. 
     It should be understood that FIG. 1 does not show standard components of an internal combustion engine except for those which are essential for an understanding of my invention. Thus, there is no illustration of a crankshaft, or a connecting rod, or a cam shaft to actuate piston  14  and the four valves  38 ,  50 ,  58 , and  70 . Those of normal skill in the art of internal combustion engines will have no difficulty in applying their knowledge to mentally supply these standard components which were not included in the drawing. 
     Those same persons of normal skill in the art will appreciate the fact that inlet valve  38  obtrudes duct  36  leading to working volume  20 , which volume is defined by piston  14  moving in cylinder  12  between limits defining a top dead center and a bottom dead center. When valve  38  is open, ambient air is drawn into volume  20  as piston  14  moves towards bottom dead center, and the air is compressed in working volume  20  as piston  14  ascends towards top dead center, and when near top dead center, valve  58  opens to allow the compressed air to pass as per arrow  22  into and through duct  60 , past now open valve  56 , through recuperator  26  where it is heated during its passage, and past valve  58  into combustion chamber  34 , the compressed air to be heated during transit through recuperator  26 . A small amount of compressed air from working volume  20  passes through the coolant openings  48  in valve  50  and directly into combustion chamber  34  via port  68 . This system comprising valve  38 , working volume  20 , piston  14 , duct  60 , recuperator  26  and valve  58  constitute a means for selective control of compressed air passing through the recuperator  26  to the combustion chamber  34 . 
     It is also shown in FIG. 1 that fuel  62  is added to the heated compressed air in combustion chamber via injector  64  and the resulting mixture ignited by spark plug  66  causing burning and generation of gaseous ignition products which are released from combustion chamber  34  via port  68  into working volume  20 , with valve  50  open, to act upon piston  14 , urging it towards bottom dead center. These components operating in sequence constitute a means for adding fuel to the compressed air in combustion chamber  34  and for igniting same and for selectively controlling the release of the gaseous ignition products to the cylinder to urge the piston towards bottom dead center. As piston  14 , under control of the crank shaft and the connecting rod, not shown, ascends from bottom dead center, the expanded gaseous ignition products are expelled from the working volume  20  of cylinder  12  to exhaust to the atmosphere via port  68 , combustion chamber  34 , and recuperator  26 , while valve  70  is open to exhaust duct  72 . These elements, when working in the described sequence, constitute a means for selectively controlling the release and subsequent forcible expulsion of gaseous ignition products from the cylinder via the combustion chamber and recuperator, in series, to exhaust to the atmosphere. 
     During operation of the engine of FIG. 5, inlet valve  38  is opened as piston  14  starts to descend from top dead center towards bottom dead center, drawing in ambient air as shown by arrow  42  through inlet duct  36 . During the induction of ambient air, valve  50 ′,  58 ′ and  70  remain in a closed state. Inlet valve  38  is closed as piston  14  begin its ascent from bottom dead center. As previously mentioned, a small portion of the air  46  undergoing compression is bled into combustion chamber  34  through coolant passages  48  in valve  50 ′. The remainder of the gas undergoing compression is compressed to approximately the compression ratio of the engine and then released by the quick opening of valve  58 ′ to allow passage of air in a compressed state from working volume  20  through duct  60 ′ and holes  25   a  and  25   b  and thence through recuperator  26 , where it is heated during its passage, and past valve  58  into combustion chamber  34 . Exhaust valve  70  remained closed during the downstroke of piston  14  to bottom dead center, as well as during its return to top dead center. Fuel injection, as indicated at  62 , by injector  64  is initiated as soon as compressed air begins its transfer into chamber  34 , and continues until piston  14  has reached top dead center and the compressed air transfer is completed. Valve  50 ′ now opens. The gaseous products mixture urge piston  14  towards bottom dead center. Hot compressed air travel through the recuperator after valve  58 ′ open. Part of the hot gases also bleed through the holes  31   a  and  31   b . Exhaust valve  70  allows the burnt gases to exit to the ambient atmosphere. 
     OPERATING RESULTS 
     Development of the recuperative cylinder head was an iterative process during which a series of seven prototype cylinder heads were built and tested. The last two of these were a single piston and a dual piston version of a recuperative engine. Results from them demonstrated that the recuperative gas cycle is effective and able to deliver substantially lower exhaust temperature, the single piston delivering approximately 400° F. reduction and the dual piston over 600° F. reduction in exhaust temperature. 
     In the dual piston prototype recuperative engine operable for development purposes in either the recuperative mode or the approximate Otto mode by the insertion or removal of the recuperator, exhaust from recuperative mode operation exitted the engine cylinder head at about 650° F. versus approximately 1,250° F. (600° F. cooler) when the engine was operated in the Otto mode with the recuperator element removed. By Carnot&#39;s Law, this reduction in exhaust temperature translates to an improvement in gas cycle efficiency for the recuperative cycle of approximately 30% over the Otto. 
     The dual piston two-stroke embodiment was similar to that illustrated in FIG.  4 . The dual piston recuperative head was fabricated to fit on a Ford Pinto 4 cylinder 2.0 liter engine block which was used as a basic test block. Cylinders  1  and  4  were blanked off and the pistons  1  and  4  were left in for mechanical balance. Cylinders  2  and  3  were used for the compressor and combustor pistons which operated in phase. The compressor and the combustor cylinders were left at their basic displacement of 30.6 cubic inches each. 
     An inlet duct and valve was built into the cylinder head to admit an air charge into the compressor cylinder. A recuperator duct having an adjustable internal diameter of either 0.75″ or 1.00″ and a length of 2.7″ was built into the recuperative cylinder head between the cylinders. The initial recuperator element was 0.060″ thick copper sheet rolled into a spiral 1.25″ long×1.00″ diameter, fixed in place in the duct by transverse pins. A transfer valve comprising a poppet valve 0.5″ diameter was built over the compressor cylinder to release charge from the compressor cylinder into an 0.4″ diameter duct 3.4 long which connected with the “cool end” of the heat exchanger duct. At this same “cool end” of the recuperator duct, an 0.9″ in diameter exhaust valve was fitted to release spent exhaust gases to the atmosphere. 
     The opposite or “hot” end of the recuperator duct adjoined a combustion chamber 0.8″ deep×3.0″ diameter which was also built into the recuperative head, the hot end of the recuperator being fitted with a 1.4″ diameter recuperator valve to control the flow of gases between the recuperator and the combustion chamber and to shield the recuperator from combustion gases. The compression/expansion ratio of this cylinder was 6.40:1. 
     A variable fuel feed carburetor was used for test purposes to admix fuel with the inducted air prior to compression, although it is preferred and recommended in practicing the invention that fuel be admixed with the air after compression and after passage at least part way through the recuperator. Admixture of fuel is preferred to occur at or beyond a point in the recuperator where the temperature is high enough to avoid buildup of deposits on the recuperator element. 
     Upon startup of the engine, the recuperator  26 , which is initially at ambient temperature, quickly warms up and after a few dozen engine cycles, reaches operating temperature, as indicated by changing operation of the engine. Initial operation of the engine is thus much like that of an engine operating on an Otto cycle but as the recuperator warms up, the engine cycle becomes recuperative. One indication of recuperator warm-up is that the fuel feed can be leaned for about 60 seconds after startup, and beyond that time, the optimum fuel adjustment remains constant. Another indication of recuperator warm-up is exhaust gas temperature which rises quickly at first startup and then assymptotically approaches a constant value. 
     Having complied with the statute by clearly and fully describing the practice of my invention including the construction and operation of same in such clear and concise manner that those inventors and artisans of normal skill in the art and in allied arts will be able to understand and practice same without need for experimentation, I wish it to be known that this invention is not to be limited by the examples in the drawings and specifications. The scope of the invention is only limited by the appended claims.