High-efficiency linear generator

Abstract

To provide a highly efficient linear combustion engine. [Solution] Various embodiments of the present invention are directed to a linear combustion engine comprising: a cylinder (105) having a cylinder wall and a pair of ends, the cylinder (105 including a combustion section (130) disposed in a central portion of the cylinder (105); a pair of opposed piston assemblies (120) adapted to move linearly within the cylinder (105), each piston assembly (120) disposed on one side of the combustion section (130) opposite the other piston assembly (120), each piston assembly (120) including a spring rod and a piston (125) having a solid front section adjacent to the combustion section (130) and a gas section; and a pair of linear electromagnetic machines (200).

Description

Technical Field

[0001] (Reference to Related Applications) This application is a continuation-in-part of U.S. Patent Application No. 13 / 298,206, filed Nov. 16, 2011, which is a continuation-in-part of U.S. Patent Application No. 13 / 102,916, filed May 6, 2011, which is a continuation-in-part of U.S. Patent Applications Nos. 12 / 953,277 and 12 / 953,270, filed Nov. 23, 2010. These applications are hereby incorporated by reference in their entirety.

[0002] (Field of the Invention) The present invention relates to high-efficiency linear combustion engines. More specifically, some embodiments relate to high-efficiency linear combustion engines capable of reaching high compression / expansion ratios by using a free piston engine structure with a linear electromechanical device for work extraction and an innovative combustion control method.

Background Art

[0003] Over the past 30 years, improvements in engine output density and emissions have been made. However, the overall efficiency has remained relatively constant. It is well known in the engine industry that increasing the geometric compression ratio of an engine increases the theoretical efficiency limit of the engine. Additionally, increasing the geometric expansion ratio of the engine above that compression ratio further increases that theoretical efficiency limit. For convenience, "compression ratio" and "expansion ratio" are used to refer to "geometric compression ratio" and "geometric expansion ratio," respectively.

[0004] Figure 1 (prior art) shows the theoretical efficiency limits (Otto and Atkinson) of two cycles commonly used in internal combustion engines. In particular, Figure 1 is a comparison of the ideal efficiencies of the Otto and Atkinson cycles as a function of compression ratio. Model assumptions include (i) the pressure at bottom dead center ("BDC") is equal to 1 atmosphere, and (ii) the premixed stoichiometric ideal gases, methane and air, with variable properties, dissociation products, and equilibrium during expansion.

[0005] As shown in Figure 1, the theoretical efficiency limits for both cycles increase significantly with increasing compression ratio. The ideal Otto cycle is divided into three steps: 1) isentropic compression, 2) adiabatic constant-volume combustion, and 3) isentropic expansion to the original volume in the BDC. The expansion ratio for the Otto cycle is equal to its compression ratio. The ideal Atkinson cycle is also divided into three steps: 1) isentropic compression, 2) adiabatic constant-volume combustion, and 3) isentropic expansion to the original BDC pressure (equal to 1 atmosphere in this embodiment). The expansion ratio for the Atkinson cycle is always greater than its compression ratio, as shown in Figure 1. The Atkinson cycle has a higher theoretical efficiency limit for a given compression ratio than the Otto cycle, but has a significantly lower energy density (power per unit mass). In practical applications, there is a trade-off between efficiency and energy density.

[0006] Today, well-designed / engineered engines on the market typically achieve braking efficiencies of 70-80% of their theoretical efficiency limit. The efficiencies of several commercially available engines are shown in Figure 2 (Prior Art). Specifically, Figure 2 is a comparison between the ideal Otto cycle efficiency limit and several commercially available engines on the market today. Model assumptions include propane and air, premixed stoichiometric ideal gases with variable properties, dissociation products, and equilibrium during expansion. The effective compression ratio is defined as the ratio of the gas density at top dead center ("TDC") to the gas density at BDC. The effective compression ratio provides a means of comparing boosted and naturally aspirated engines under fair conditions. Similarly, for a well-designed engine to have a braking efficiency above 50% (i.e., at least 70% of its theoretical efficiency), an engine operating under the Otto cycle must have a compression ratio above 102, corresponding to an expansion ratio of 54, as shown in Figure 1, and an engine operating under the Atkinson cycle must have a compression ratio above 14.

[0007] Due to the inherent structure of such engines, it is difficult to achieve high compression / expansion ratios (above 30) in conventional slider-crank reciprocating engines ("conventional engines"). A schematic diagram illustrating the structure of conventional engines and the challenges that limit them from achieving high compression ratios is shown in Figure 3 (Prior Art). Typical internal combustion ("IC") engines have a bore / stroke ratio of 0.5–1.2 and a compression ratio of 8–24. (Heywood, J. (1988). Internal Combustion Engine Fundamentals. McGraw-Hill). As the compression ratio of an engine is increased while maintaining the same bore / stroke ratio, the surface / volume ratio at top dead center (TDC) increases, the temperature rises, and the pressure increases. This has three main consequences: 1) increased heat transfer from the combustion chamber, 2) increased difficulty in adjusting the combustion phase, and 3) increased friction and mechanical losses. Heat transfer increases because the thermal boundary layer occupies the majority of the overall volume (i.e., the aspect ratio at TDC becomes smaller). The aspect ratio is defined as the ratio of the bore diameter to the length of the combustion chamber. Achieving combustion phase adjustment and complete combustion is difficult due to the small volume at TDC. The increase in combustion chamber pressure is directly translated into an increase in force. These large forces can overload both the mechanical couplings and the piston rings.

[0008] Free-piston internal combustion engines are not new, but typically, with the exception of research at Sandia National Laboratory, they have not been utilized or developed to achieve compression / expansion ratios exceeding 30:1. See Patent Document 1. The number of documents and patents relating to free-piston engines is enormous. However, the documents focus on free-piston engines with short stroke lengths and therefore, when high compression / expansion ratios are achieved, they have similar challenges to reciprocating engines, namely combustion control challenges and large heat transfer losses. Free-piston engine configurations can be classified into three categories: 1) two opposing pistons, single combustion chamber, 2) single piston, double combustion chamber, and 3) single piston, single combustion chamber. Schematic diagrams of the three common free-piston engine configurations are shown in Figure 4 (Prior Art). The single-piston, double combustion chamber, and free-piston engine configurations have limited compression ratios because the strong forces experienced at high compression ratios are not balanced, which can lead to mechanical instability.

[0009] As mentioned above, several free-piston engines have been proposed in research and patent documents. Of the many proposed free-piston engines, only a few have been physically implemented (to our knowledge). Research by Mikalsen and Roskilly describes free-piston engines at West Virginia University, Sandia National Laboratory, and the Royal Institute of Technology (Sweden) (Mikalsen R., Roskilly APA review of free-piston engine history and applications. Applied Thermal Engineering. 2007;27:2339-2352). Other research efforts are also being reported at Czech Technical University (http: / / www.lceproject.org / en / ), INNAS BV (Netherlands) (http: / / www.innas.com / ), and Pempek Systems (Australia) (http: / / www.freepistonpower.com / ). All known physically implemented free-piston engines have short stroke lengths and therefore, when high compression / expansion ratios are achieved, they face similar challenges to reciprocating engines, namely combustion control challenges and large heat transfer losses.In addition, with the exception of the prototype at Sandia National Laboratory (Aichlmayr, HT, Van Blaigan, P. Modeling and Experimental Characterization of a Permanent Magnet Linear Alternator for Free-Piston Engine Applications, ASME Energy Sustainability Conference, San Francisco, CA, July 19-23, 2009) and the prototype developed by OPOC (Patent Document 2), all engines have a single-piston, double-combustion-chamber configuration. Therefore, the strong forces experienced at high compression ratios are not balanced, which can lead to mechanical instability and thus limit the compression ratio.

[0010] Given the inherent structural limitations of conventional engines as described above, several manufacturers have attempted, and continue to attempt, to increase engine efficiency by achieving a high effective compression ratio through the use of turbochargers or superchargers. Boosting an engine via a turbocharger or supercharger provides a means to achieve a high effective compression ratio while maintaining the same geometric compression ratio. Boosting an engine does not avoid the challenges caused by the above-normal pressure and force experienced at and near TDC. Thus, the force can overload both the mechanical connections within the engine (piston pins, piston rods, and crankshaft) which can lead to mechanical failure, and the pressure-excited rings which can lead to increased friction, wear, or failure. Boosting an engine also typically leads to significant heat transfer losses because the elapsed time at and near TDC (i.e., when the temperature is highest) is not shortened sufficiently to account for the above-normal temperatures experienced at and near TDC. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] U.S. Patent No. 6,199,519 [Patent Document 2] International Publication No. 03 / 078835 [Overview of the Initiative] [Means for solving the problem]

[0012] Various embodiments of the present invention provide a highly efficient linear combustion engine. Such embodiments address the challenges that prevent conventional engines from achieving a high compression / expansion ratio by using a free-piston engine structure in conjunction with a linear electromagnet for work extraction and an innovative combustion control scheme. The present invention disclosed herein provides means for increasing the thermal efficiency of an internal combustion engine to over 50% at a scale (5kW-5MW) suitable for distributed power generation and / or hybrid vehicles.

[0013] One embodiment of the present invention relates to a linear combustion engine comprising: a cylinder having a cylinder wall and a pair of ends, including a combustion chamber located in the central portion of the cylinder; a pair of opposing piston assemblies adapted to move linearly within the cylinder, each piston assembly located on one side of the combustion chamber opposite to the other piston assembly, and each piston assembly including a piston having a spring rod and a solid front compartment adjacent to the combustion chamber, and a hollow rear compartment, the piston having a gas spring that directly provides at least some compression work during the compression stroke of the engine; and a pair of linear electromagnets adapted to directly convert the kinetic energy of the piston assemblies into electrical energy, and adapted to directly convert the electrical energy into the kinetic energy of the piston assemblies to provide compression work during the compression stroke, wherein the engine includes a variable expansion ratio greater than 50:1.

[0014] Another embodiment of the present invention relates to a linear combustion engine comprising a cylinder having a cylinder wall and a combustion compartment located at one end of the cylinder, and a piston assembly, the piston assembly comprising a piston having a spring rod and a piston having a solid front compartment adjacent to the combustion compartment and a hollow rear compartment with a gas spring that directly provides at least some compression work during the compression stroke of the engine, and the piston assembly is adapted to move linearly within the cylinder, and a linear electromagnet adapted to directly convert the kinetic energy of the piston assembly into electrical energy and, during the compression stroke, to directly convert the electrical energy into the kinetic energy of the piston assembly to provide compression work, wherein the engine includes a variable expansion ratio greater than 50:1.

[0015] Other features and aspects of the present invention will become apparent from the following modes for carrying out the invention, which will be considered, for example, in conjunction with the accompanying drawings illustrating the features according to embodiments of the invention. The disclosure of the invention is not intended to limit the scope of the invention as defined solely by the claims appended herein. This specification provides, for example, the following items: (Item 1) A cylinder having a cylinder wall and a pair of ends, wherein the cylinder includes a combustion compartment located in the central part of the cylinder, A pair of opposing piston assemblies adapted to move linearly within the cylinder, wherein each piston assembly is positioned on one side of the combustion chamber opposite to the other piston assembly. A pair of drive compartments, each drive compartment comprising a compression mechanism that provides at least some compression work during the compression stroke of the engine, A pair of linear electromagnetic machines and Equipped with, The pair of linear electromagnetic machines described above are adapted to directly convert the kinetic energy of the piston assembly into electrical energy, and are adapted to provide compression work by directly converting the electrical energy into the kinetic energy of the piston assembly during the compression stroke, and each linear electromagnetic machine is located distal to the end of the cylinder. Linear combustion engine. (Item 2) The above-mentioned engine is a linear combustion engine as described in item 1, which includes a variable compression ratio less than or equal to a variable expansion ratio. (Item 3) A linear combustion engine as described in item 1, wherein the length of the combustion chamber at top dead center is 0.2 inches to 4 inches. (Item 4) The above-mentioned engine is a linear combustion engine as described in item 1, including a variable expansion ratio greater than 50:1. (Item 5) The above-mentioned engine is a linear combustion engine as described in item 1, including a variable expansion ratio greater than 75:1. (Item 6) Each piston assembly comprises a piston, a piston seal, and a piston rod, the piston rod moving linearly inside and outside the cylinder along a bearing and sealed by a gas seal fixed to the cylinder, the linear combustion engine as described in item 1. (Item 7) Each piston assembly comprises two pistons, two piston seals, and one piston rod. A linear combustion engine as described in item 1, wherein each piston assembly is enclosed by the cylinder and configured to move linearly within the cylinder. (Item 8) The linear combustion engine according to item 1, wherein each linear electromagnetic machine comprises a stator and a transducer attached to a piston assembly and moving linearly within the stator. (Item 9) Each linear electromagnetic machine comprises a permanent magnet machine, an induction machine, a switched reluctance machine, or a combination thereof, as described in item 1, for the linear combustion engine. (Item 10) The compression mechanism includes a linear alternator or a gas spring that operates as a motor, and the gas spring includes a certain volume of gas located within the drive section that communicates with the piston assembly. The linear combustion engine according to item 1. (Item 11) Fuel is directly injected into the combustion section through a fuel injector, or is mixed with air before or during air intake. The engine can operate by lean, stoichiometric, or rich combustion using liquid or gaseous fuel. The linear combustion engine according to item 1. (Item 12) One or more exhaust / injection ports that allow exhaust gas and fluid to flow into and out of the cylinder, One or more intake ports that allow intake of air, an air / fuel mixture, or an air / fuel / combustion product mixture, One or more drive gas removal ports that allow removal of drive gas, One or more drive gas supply ports that allow absorption of supply gas for the drive section The linear combustion engine according to item 1, further comprising. (Item 13) The engine operates using a two-stroke piston cycle that includes an output stroke and a compression stroke. The linear combustion engine according to item 1. (Item 14) Near bottom dead center between the output stroke and the compression stroke, the engine discharges combustion products and intakes air, an air / fuel mixture, or an air / fuel / combustion product mixture. The linear combustion engine according to item 11. (Item 15) During the output stroke, a portion of the kinetic energy of the piston assembly is converted into electrical energy by the linear electromagnetic machine, and another portion of the kinetic energy performs compression work on the gas within the drive section. The linear combustion engine according to item 11. (Item 16) The above engine is a linear combustion engine as described in item 1, which operates using a four-stroke piston cycle including intake stroke, compression stroke, output stroke, and exhaust stroke. (Item 17) A linear combustion engine as described in item 16, wherein during the output stroke, a portion of the kinetic energy of the piston assembly is converted into electrical energy by the linear electromagnetic machine, and another portion of the kinetic energy performs compression work on the gas in the drive compartment. (Item 18) A linear combustion engine as described in item 16, wherein the exhaust stroke continues until all exhaust ports are closed and the speed of the piston is zero, and at least some combustion products remain in the combustion chamber. (Item 19) A linear combustion engine as described in item 16, in which the intake stroke continues until the speed of the piston above becomes zero and all intake ports are closed. (Item 20) A linear combustion engine as described in item 1, wherein engine ignition is achieved by spark or compression ignition, and optimal combustion is achieved by adjusting the gas temperature in the combustion chamber to a suitable level so that it reaches its auto-ignition temperature at its optimal volume. (Item 21) The linear combustion engine described in item 1, wherein the cylinder comprises a main cylinder portion housing the combustion compartment and a pair of outer cylinder portions, the outer cylinder portions being located on either side of the main cylinder, and each outer cylinder portion housing a drive compartment. (Item 22) The above cylinder is a linear combustion engine according to item 1, housing the above combustion compartment and the above drive compartment. (Item 23) The above-mentioned drive section provides at least some compression work during the compression stroke, in the linear combustion engine described in item 1. (Item 24) A cylinder having a cylinder wall and a pair of ends, wherein the cylinder includes a combustion compartment located in the central part of the cylinder, A pair of opposing piston assemblies adapted to move linearly within the cylinder, each piston assembly positioned on one side of the combustion chamber opposite to the other piston assembly, and each piston assembly includes a spring rod and a piston having a solid front section and a drive section adjacent to the combustion chamber, A pair of linear electromagnetic machines and Equipped with, The pair of linear electromagnetic machines described above are adapted to directly convert the kinetic energy of the piston assembly into electrical energy, and are adapted to provide compression work by directly converting the electrical energy into the kinetic energy of the piston assembly during the compression stroke. Linear combustion engine. (Item 25) The linear combustion engine described in item 24, wherein the drive compartment comprises a hollow rear compartment containing a gas spring that directly provides at least some compression work during the compression stroke of the engine. (Item 26) The linear combustion engine according to item 25, wherein the piston assembly further comprises an external bearing located between the combustion chamber and the linear electromagnetic machine, and an internal bearing located within the hollow chamber of the piston. (Item 27) A linear combustion engine as described in item 25, wherein one end of the spring rod is provided with one face of the gas spring. (Item 28) The linear combustion engine according to item 24, wherein the piston assembly further comprises a magnet attached to the piston, the magnet moving linearly with the piston within the stator of the linear electromagnetic machine. (Item 29) The linear combustion engine according to item 24, wherein the piston assembly further comprises a front seal fixed to the piston at or near its front end to prevent gas from moving out of the combustion chamber, and a rear seal to prevent intake gas or blow-by gas from moving outwards. (Item 30) A linear combustion engine as described in item 25, wherein the spring rod includes a central lumen that allows mass to be transferred from the drive section to a reservoir section communicating with the surroundings. (Item 31) A linear combustion engine as described in item 24, in which the length of the combustion chamber at top dead center is 0.2 inches to 4 inches. (Item 32) The above-mentioned engine is a linear combustion engine as described in item 24, which includes a variable compression ratio less than or equal to a variable expansion ratio. (Item 33) The above-mentioned engine is a linear combustion engine as described in item 24, which includes a variable expansion ratio greater than 50:1. (Item 34) The above-mentioned engine is a linear combustion engine as described in item 24, which includes a variable expansion ratio greater than 75:1. (Item 35) Each linear electromagnetic machine is a linear combustion engine as described in item 24, comprising a permanent magnet machine, an induction machine, a switched reluctance machine, or a combination thereof. (Item 36) A linear combustion engine as described in item 24, wherein the fuel is either injected directly into the combustion chamber via a fuel injector or mixed with air before or during air intake, and the engine is capable of operating by lean, stoichiometric, or rich combustion using liquid or gaseous fuel. (Item 37) One or more discharge / injection ports that allow exhaust gas and fluid to flow into and out of the cylinder, One or more intake ports that allow intake of air, an air / fuel mixture, or an air / fuel / combustion product mixture, One or more drive gas removal ports that enable the removal of drive gas, One or more drive gas supply ports that enable the absorption of supply gas for the above drive compartment, A linear combustion engine, as described in item 24, further comprising the features described therein. (Item 38) The above engine is a linear combustion engine as described in item 24, which operates using a two-stroke piston cycle including an output stroke and a compression stroke. (Item 39) The linear combustion engine described in item 36, wherein the engine discharges combustion products and draws in air, an air / fuel mixture, or an air / fuel / combustion product mixture near the bottom dead center between the output stroke and the compression stroke. (Item 40) A linear combustion engine as described in item 36, wherein during the output stroke, a portion of the kinetic energy of the piston assembly is converted into electrical energy by the linear electromagnetic machine, and another portion of the kinetic energy performs compression work on the gas in the drive compartment. (Item 41) The above engine is a linear combustion engine as described in item 24, which operates using a four-stroke piston cycle including intake stroke, compression stroke, output stroke, and exhaust stroke. (Item 42) A linear combustion engine as described in item 41, wherein during the output stroke, a portion of the kinetic energy of the piston assembly is converted into electrical energy by the linear electromagnetic machine, and another portion of the kinetic energy performs compression work on the gas in the drive compartment. (Item 43) A linear combustion engine as described in item 41, wherein the exhaust stroke continues until all exhaust ports are closed and the speed of the piston is zero, and at least some combustion products remain in the combustion chamber. (Item 44) A linear combustion engine as described in item 41, in which the intake stroke continues until the speed of the piston above becomes zero and all intake ports are closed. (Item 45) A linear combustion engine as described in item 24, wherein engine ignition is achieved by spark or compression ignition, and optimal combustion is achieved by adjusting the gas temperature in the combustion chamber to a suitable level so that it reaches its auto-ignition temperature in its optimal volume. [Brief explanation of the drawing]

[0016] The present invention, in one or more different embodiments, will be described in detail with reference to the following drawings. The drawings are provided for illustrative purposes only and merely depict typical or exemplary embodiments of the invention. These drawings are provided to facilitate the reader's understanding of the invention and are not to be considered limitations on the breadth, scope, or availability of the invention. It should be noted that for clarity and ease of illustration, these drawings are not necessarily made to exact scale. [Figure 1] Figure 1 (Prior Art) is a chart illustrating the theoretical efficiency limits of two cycles commonly used in internal combustion engines. [Figure 2] Figure 2 (Prior Art) is a chart comparing the ideal Otto cycle efficiency limit with several commercially available engines currently on the market. [Figure 3] Figure 3 (Prior Art) is a schematic diagram illustrating the structure of a conventional engine and the challenges that limit its ability to achieve a high compression ratio. [Figure 4] Figure 4 (prior art) is a schematic diagram of three common free-piston engine configurations. [Figure 5] Figure 5 is a chart illustrating a comparison between experimental data from a prototype at Stanford University and the ideal Otto cycle efficiency limit. [Figure 6] Figure 6 is a cross-sectional view illustrating an embodiment of a two-piston, two-stroke internal combustion engine with an integrated gas spring, based on the principles of the present invention. [Figure 7] Figure 7 is a schematic diagram illustrating the two-stroke piston cycle of the two-piston integrated gas spring engine shown in Figure 6. [Figure 8] Figure 8 is a cross-sectional view illustrating a two-piston, four-stroke, integrated gas spring embodiment of an internal combustion engine based on the principles of the present invention. [Figure 9] Figure 9 is a schematic diagram illustrating the four-stroke piston cycle of the two-piston, integrated gas spring engine shown in Figure 8, based on the principle of the present invention. [Figure 10]Figure 10 is a cross-sectional view illustrating an alternative two-piston, two-stroke, single-combustion-chamber, fully integrated gas-spring and linear electromagnetism engine based on the principles of the present invention. [Figure 11] Figure 11 is a cross-sectional view illustrating an alternative two-piston, two-stroke, single-combustion-chamber, separated-gas-spring engine based on the principles of the present invention. [Figure 12] Figure 12 is a cross-sectional view illustrating a single-piston, two-stroke, integrated gas spring engine based on the principles of the present invention. [Figure 13] Figure 13 is a schematic diagram illustrating the two-stroke piston cycle of the single-piston, two-stroke, integrated gas spring engine shown in Figure 12, according to the principles of the present invention. [Figure 14] Figure 14 is a cross-sectional view illustrating a single-piston, four-stroke, integrated gas spring engine based on the principles of the present invention. [Figure 15] Figure 15 is a schematic diagram illustrating the four-stroke piston cycle of the single-piston, two-stroke, integrated gas-spring engine shown in Figure 14, according to the principles of the present invention. [Figure 16] Figure 16 is a cross-sectional view illustrating another single-piston, two-stroke, single-combustion-chamber, fully integrated gas-spring and linear electromagnetism engine according to the principles of the present invention. [Figure 17] Figure 17 is a cross-sectional view illustrating another single-piston, two-stroke, single-combustion-chamber, isolated gas spring engine according to the principles of the present invention. [Figure 18] Figure 18 is a cross-sectional view illustrating a single-piston, two-stroke version of the IIGS structure according to one embodiment of the present invention. [Figure 19] Figure 19 is a cross-sectional view illustrating an embodiment of a gas spring rod according to the principle of the present invention. [Figure 20] Figure 20 is a cross-sectional view illustrating a two-piston, two-stroke version of the IIGS engine according to one embodiment of the present invention. [Modes for carrying out the invention]

[0017] The figures are not intended to be comprehensive or to limit the invention to the precise forms disclosed. It should be understood that the invention can be practiced with modifications and alterations, and is limited only by the claims and their equivalents.

[0018] The present invention generally relates to a highly efficient linear combustion engine capable of achieving a high compression / expansion ratio by using a free-piston engine structure in conjunction with a linear electromagnet for work extraction and an innovative combustion control system.

[0019] A single-cylinder, single-piston prototype has been built and is operational at Stanford University. This prototype demonstrates conceptual feasibility, achieving 60% of the indicated work efficiency. A plot of some experimental results is shown in Figure 5. In particular, Figure 5 is a chart illustrating a comparison between experimental data from the prototype at Stanford University and the ideal Otto cycle efficiency limit. The model assumptions are as follows: diesel #2 and air, dissociation products, and equilibrium during expansion, including a 0.3 equivalence ratio and variable characteristics.

[0020] Various embodiments of the present invention relate to free-piston, linear combustion engines characterized by a thermal efficiency greater than 50%. In at least one embodiment, the engine comprises (i) at least one cylinder, (ii) at least one piston assembly per cylinder arranged for linear displacement within the cylinder, (iii) at least one linear electromagnet that directly converts the kinetic energy of the piston assembly into electrical energy, and (iv) at least one gas chamber that provides at least some compression work during the compression stroke. In addition, in some configurations, the internal combustion engine has the following physical characteristics: (i) a variable expansion ratio greater than 50:1, (ii) a variable compression ratio less than or equal to the expansion ratio, and (iii) a combustion chamber length at TDC from 0.2 to 4 inches. However, it should be noted that further embodiments may include various combinations of the identified features and physical characteristics described above.

[0021] Figure 6 is a cross-sectional view illustrating a two-piston, two-stroke, integrated gas-spring embodiment of an internal combustion engine 100. This free-piston internal combustion engine 100 directly converts chemical energy in the fuel into electrical energy via a pair of linear electromagnetic machines 200. As used herein, the term “fuel” refers to a substance that reacts with an oxidizer. Such fuels include, but are not limited to, (i) hydrocarbon fuels such as natural gas, biogas, gasoline, diesel, and biodiesel; (ii) alcohol fuels such as ethanol, methanol, and butanol; and (iii) mixtures of any of the foregoing. The engines described herein are suitable for both steady-state and portable power generation (e.g., for use in a vehicle).

[0022] Figure 6 illustrates one embodiment of a two-piston, two-stroke, integrated gas spring engine 100. In particular, the engine 100 comprises one cylinder 105 with two opposing piston assemblies 120 that intersect in a combustion compartment 130 (or combustion chamber) in the center of the cylinder 105. The placement of the combustion compartment 130 in the center of the engine 100 equalizes the combustion forces. Each piston assembly 120 comprises a piston 125, a piston seal 135, and a piston rod 145. The piston assembly 120 moves freely and linearly within the cylinder 105. The piston rod 145 moves along a bearing and is sealed by a gas seal 150 fixed to the cylinder 105. In the illustrated embodiment, the gas seal 150 is a piston rod seal. As used herein, the term “bearing” means any part on which another part moves, slides, or rotates, and includes, but is not limited to, slide bearings, flexible bearings, ball bearings, roller bearings, gas bearings, and / or magnetic bearings. In addition, the term “surroundings” refers to the area outside the cylinder 105 and includes, but is not limited to, the direct environment, auxiliary piping, and / or auxiliary equipment.

[0023] Referring further to Figure 6, the volume between the back of the piston 125, the piston rod 145, and the cylinder 105 is referred to herein as the drive compartment 160. The drive compartment 160 may also be referred herein as the “gas compartment,” “gas spring,” or “gas spring compartment.” Each drive compartment 160 is sealed from the surroundings and the combustion compartment 130 by the piston rod seal 150 and the piston seal 135. In the illustrated embodiments, the gas in the drive compartment 160 acts as a flywheel (i.e., a gas spring) during the cycle to provide at least some compression work during the compression stroke. Thus, some embodiments of the present invention feature a gas spring for providing work. Other embodiments include a highly efficient linear AC generator operating as a motor and do not require a gas spring for generating compression work.

[0024] In some embodiments, to obtain high thermal efficiency, the engine 100 has a variable expansion ratio greater than 50:1. In additional embodiments, the variable expansion ratio is greater than 75:1. In further embodiments, the variable expansion ratio is greater than 100:1. In addition, some embodiments feature a compression ratio less than or equal to the expansion ratio, and a combustion chamber length at TDC of 0.2–4 inches. As used herein, “combustion chamber length at TDC” is the distance between the fronts of the two pistons 125 at TDC.

[0025] The aforementioned specifications indicate that engine 100 has a significantly longer stroke length than conventional engines, where the term “stroke length” refers to the distance traveled by each piston 125 between TDC and BDC. Combustion ignition can be achieved via compression ignition and / or spark ignition. Fuel can be injected directly into the combustion chamber 130 via a fuel injector ("direct injection") and / or mixed with air prior to and / or during air intake ("premixed injection"). Engine 100 can operate with lean, stoichiometric, or rich combustion using liquid and / or gaseous fuels.

[0026] Continuing with reference to Figure 6, cylinder 105 includes an exhaust / injection port 170, an intake port 180, a drive gas removal port 185, and a drive gas replenishment port 190 for exchanging material (solid, liquid, gas, or plasma) with its surroundings. As used herein, the term “port” includes any opening or set of openings (e.g., porous material) that allow for the exchange of material between the inside of cylinder 105 and its surroundings. Some embodiments do not require all of the ports depicted in Figure 6. The number and type of ports depend on the engine configuration, injection method, and piston cycle (e.g., a two- or four-stroke piston cycle). In this two-piston two-stroke embodiment, the exhaust / injection port 170 allows exhaust gases and fluids to flow in and out of the cylinder, the intake port 180 is for the intake of air and / or an air / fuel mixture, the drive gas removal port 185 is for the removal of drive gases, and the drive gas replenishment port 190 is for the intake of replenishment gases for the drive compartment 160. The locations of the various ports are not necessarily fixed. For example, in the illustrated embodiment, the discharge / injection port 170 is substantially located at the midpoint of the cylinder. However, these ports may alternatively be located adjacent to the intake port 180 and away from the midpoint.

[0027] The aforementioned ports may be opened and closed via or without valves. The term “valve” refers to any actuated flow controller or other actuated mechanism for selectively passing a substance through an opening, and may include, but is not limited to, ball valves, plug valves, butterfly valves, choke valves, check valves, gate valves, leaf valves, piston valves, poppet valves, rotary valves, slide valves, solenoid valves, two-way valves, or three-way valves. Valves may be actuated by any means, which may include, but is not limited to, mechanical, electrical, magnetic, camshaft-driven, hydraulic, or pneumatic means. In most cases, ports are required for exhaust, drive gas removal, and drive gas replenishment. In embodiments where direct injection is the desired ignition method, injection ports and air intake ports are also required. In embodiments where premixed compression ignition or premixed spark ignition is the desired combustion method, air / fuel intake ports may also be required. In embodiments where a hybrid premixed / direct injection method with compression ignition and / or spark ignition is the desired combustion method, injection ports and air / fuel intake ports may also be required. In all engine configurations, exhaust gases from the previous cycle can be mixed with intake air or an air / fuel mixture for the subsequent cycle. This process is called exhaust gas recirculation (EGR) and can be used to optimize combustion timing and peak temperature.

[0028] Referring further to Figure 6, the engine 100 further comprises a pair of linear electromagnetic machines (LEMs) 200 for directly converting the kinetic energy of the piston assembly 120 into electrical energy. Each LEM 200 is also capable of directly converting electrical energy into the kinetic energy of the piston assembly 120 to provide compression work during the compression stroke. As shown, the LEM 200 comprises a stator 210 and a transducer 220. Specifically, the transducer 220 is attached to the piston rod 145 and moves linearly within the stationary stator 210. The volume between the transducer 220 and the stator 210 is called the air gap. The LEM 200 can include any number of configurations. Figure 6 shows one configuration in which the transducer 220 is shorter than the stator 210. However, the transducer 220 may be longer than the stator 210, or may be substantially the same length. In addition, the LEM 200 may be a permanent magnet machine, an induction machine, a switched reluctance machine, or several combinations of the three. The stator 210 and the transducer 220 can each include magnets, coils, iron, or several combinations thereof. Since the LEM200 converts the kinetic energy of the piston into electrical energy and directly from there (i.e., there is no mechanical coupling), mechanical and frictional losses are minimized compared to conventional engine-generator configurations.

[0029] The embodiment shown in Figure 6 operates using a two-stroke piston cycle. A schematic diagram illustrating the two-stroke piston cycle 250 of the two-piston integrated gas spring engine 100 of Figure 6 is shown in Figure 7. As used herein, the term “piston cycle” refers to any series of piston movements that can be started and ended by the piston 125 in substantially the same configuration. One common embodiment is a four-stroke piston cycle, which comprises an intake stroke, a compression stroke, an output (expansion) stroke, and an exhaust stroke. Additional alternative strokes may form part of the piston cycle, as described throughout this disclosure. A two-stroke piston cycle is characterized as having an output (expansion) stroke and a compression stroke.

[0030] As illustrated in Figure 7, near the BDC between the output and the compression stroke, the engine expels combustion products (through the exhaust port 170) and draws in air, an air / fuel mixture, or an air / fuel / combustion product mixture (through the intake port 180). This process may be referred to herein as “breathing” or “breathing in or near the BDC.” It will be understood by those skilled in the art that many other types of port and breathing configurations are possible without departing from the scope of the present invention. When the BDC is in or near the BDC, and the drive compartment is to be used to provide compression work, the pressure of the gas in the drive compartment 160 exceeds the pressure in the combustion compartment 130, biasing the piston 125 inward toward each other. The gas in the drive compartment 160 can be used to provide at least a portion of the energy required to perform the compression stroke. The LEM200 can also provide a portion of the energy required to perform the compression stroke.

[0031] The amount of energy required to perform the compression stroke depends on the desired compression ratio, the pressure in the combustion chamber 130 at the start of the compression stroke, and the mass of the piston assembly 120. The compression stroke continues until combustion occurs, when the velocity of the piston 125 is zero or nearly zero. The point where the velocity of the piston 125 is equal to zero indicates the piston's TDC position for that cycle. Combustion causes an increase in temperature and pressure in the combustion chamber 130, biasing the piston 125 outward toward the LEM 200. During the output stroke, some of the kinetic energy of the piston assembly 120 is converted into electrical energy by the LEM 200, and another portion of the kinetic energy performs compression work on the gas in the drive chamber 160. The output stroke continues until the velocity of the piston 125 becomes zero, indicating the piston's BDC position for that cycle.

[0032] Figure 7 illustrates a single-port configuration for breathing, where the intake port 180 is located in front of both pistons near the BDC, and the exhaust port 170 is located near the TDC. Various alternative port configurations exist, for example, but are not limited to, positioning the exhaust port 170 in front of one piston 125 near the BDC and the intake port 180 in front of the other piston 125 near the BDC to enable what is called uniflow scavenging, or uniflow breathing. The opening and closing of the exhaust port 170 and the intake port 180 are controlled independently. The locations of the exhaust port 170 and the intake port 180 can be selected to allow a range of compression and / or expansion ratios. The time during a cycle when the exhaust port 170 and the intake port 180 are activated (opened and closed) can be adjusted during and / or between cycles to vary the compression and / or expansion ratios and / or the amount of combustion products retained in the combustion chamber 130 at the start of the compression stroke. Retaining combustion gases within the combustion compartment 130 is called residual gas trapping (RGT) and can be used to optimize combustion timing and peak temperature.

[0033] During the piston cycle, gases may potentially move across the piston seal 135 between the combustion compartment 130 and the drive compartment 160. This movement of gases is referred to as "blow-by." Blow-by gases may contain air and / or fuel and / or combustion products. Engine 100 is designed to manage blow-by gases by having at least two ports within each drive compartment 160, one port 185 for removing drive gases and the other port 190 for supplying replenishment drive gases. The removal of drive gases and the intake of replenishment drive gases occur independently, controlled, to minimize losses and maximize efficiency.

[0034] Figure 7 shows one method for exchanging the drive gas, where the removal of the drive gas occurs at some point during the expansion stroke, and the intake of replenishment drive gas occurs at some point during the compression stroke. The removal and intake of drive gas can also occur in the reverse order of the stroke or within the same stroke. The removed drive gas can be used as part of the intake for the combustion chamber 130 during the subsequent combustion cycle. The amount of gas in the drive chamber 160 can be adjusted by varying the compression ratio and / or expansion ratio. The expansion ratio is defined as the ratio of the volume of the combustion chamber 130 when the piston 125 has zero velocity after the power stroke to the volume of the combustion chamber 130 when the piston 125 has zero velocity after the compression stroke. The compression ratio is defined as the ratio of the volume of the combustion chamber 130 when the pressure in the combustion chamber 130 begins to increase due to the inward motion of the piston 125 to the volume of the combustion chamber 130 when the piston 125 has zero velocity after the compression stroke.

[0035] Combustion is optimally controlled by mitigating (e.g., cooling) the temperature of the gas in the combustion compartment 130 prior to combustion. Temperature control can be achieved by pre-cooling the combustion compartment intake gas and / or cooling the gas in the combustion compartment 130 during the compression stroke. Optimal combustion occurs when the combustion compartment 130 reaches a volume at which the thermal efficiency of the engine 100 is maximized. This volume is referred to as the optimal volume and can occur around TDC. Depending on the combustion method (ignition and injection method), the combustion compartment intake gas may be air, an air / fuel mixture, or an air / fuel / combustion product mixture (combustion products originating from EGR and / or recycled drive gas), and the gas in the combustion compartment 130 may be air, an air / fuel mixture, or an air / fuel / combustion product mixture (combustion products originating from EGR and / or RGT and / or recycled drive gas).

[0036] If compression ignition is the desired ignition method, optimal combustion is achieved by moderately adjusting the temperature of the gas in the combustion chamber 130 so that it reaches its auto-ignition temperature at the optimal volume. If spark ignition is the desired ignition method, optimal combustion is achieved by moderately adjusting the temperature of the gas in the combustion chamber 130 so that it remains below its auto-ignition temperature at the optimal volume before the spark ignites. The spark is externally controlled to ignite at the optimal volume. The combustion chamber intake gas can be pre-cooled by a cooling cycle. The gas in the combustion chamber 130 can be cooled during the compression stroke by injecting a liquid into the combustion chamber 130 and then evaporating it. The liquid can be water and / or other liquids such as fuel or coolant, but is not limited to water. The liquid can be cooled prior to its injection into the combustion chamber 130.

[0037] Given the engine geometry and exhaust and intake port locations, the power output from the engine 100 can be varied from cycle to cycle by varying the pre-combustion air / fuel ratio and / or the amount of combustion products in the combustion compartment 130, and / or the compression ratio and / or expansion ratio. For a given air / fuel ratio in a cycle, the peak combustion temperature can be controlled by varying the amount of combustion products from previous cycles present in the combustion compartment gas prior to combustion. Combustion products in the pre-combustion combustion compartment gas may originate from EGR and / or RGT and / or recycled drive gas. Piston synchronization is achieved through a control scheme that modulates the operating characteristics of the LEM and drive compartment using information on piston position, piston speed, combustion compartment structure, and cylinder pressure.

[0038] The configurations in Figures 6 and 7, referred to as engine 100, include a single unit defined by a cylinder 105, a piston assembly 120, and an LEM 200. However, many units can be installed in parallel and collectively referred to as an "engine." Some embodiments of the present invention are modular so that they can be arranged to operate in parallel, allowing end users to scale the engine as needed. In addition, it is not necessary for all units to be the same size or to operate under the same conditions (e.g., frequency, stoichiometry, or breathing). When units operate in parallel, but are not limited, there is potential for integration between engines, such as gas exchange between units and / or feedback between LEM 200s of units.

[0039] The free-piston structure allows for large and variable compression and expansion ratios while maintaining a sufficiently large volume at TDC, minimizing heat transfer and achieving moderate combustion. In addition, the piston spends less time at and near TDC than when mechanically connected to the crankshaft. This is useful for minimizing heat transfer (and maximizing efficiency) because less time is spent at the highest temperature. Furthermore, because the free-piston structure has no mechanical connection, mechanical and friction losses are minimized compared to conventional engines. The large and variable compression and expansion ratios, sufficiently large volume at TDC, direct conversion of kinetic energy to electrical energy by LEM200, the inherently short time at and near TDC, and the ability to control combustion all combine to enable engine 100 to achieve a thermal efficiency of over 50%.

[0040] During operation, losses within the engine 100 include combustion losses, heat transfer losses, electrical conversion losses, friction losses, and blow-by losses. In some embodiments of the present invention, combustion losses are minimized by performing combustion in a high internal energy state, which is achieved by having the ability to reach a high compression ratio while mitigating the combustion compartment temperature. Heat transfer losses are minimized by having a sufficiently large volume when and near combustion occurs, such that the thermal boundary layer is only a small portion of the volume. Heat transfer losses are also minimized by using a free piston profile rather than a slider-crank profile, thereby reducing the time spent at high temperatures. Friction losses are minimized because there is no mechanical coupling. Blow-by losses are minimized by using a drive gas that contains unburned fuel as part of the intake for the next combustion cycle, with a well-designed piston seal.

[0041] As described, the embodiments described above with respect to Figures 6 and 7 comprise a two-piston, single-combustion-chamber, two-stroke internal combustion engine 100. Several alternative embodiments are described below and illustrated in the corresponding figures. These embodiments are not intended to be limiting. As will be understood by those skilled in the art, various modifications and alternative configurations may be used and other changes may be made without departing from the scope of the invention. Unless otherwise stated, the physical and operational characteristics of the embodiments described below are similar to those described in the embodiments of Figures 6 and 7, and identical elements are marked where appropriate. Furthermore, all embodiments may be configured in parallel (i.e., in multiple unit configurations for scaling purposes) as described above.

[0042] Figure 8 illustrates a four-stroke embodiment of the present invention, comprising a two-piston, four-stroke, integrated gas spring engine 300. The main physical difference between the four-stroke engine 300 in Figure 8 and the two-stroke engine 100 in Figure 6 is related to the location of the ports. In particular, in the four-stroke engine 300, the exhaust, injector, and intake ports 370 are located midpoint and / or near the midpoint of the cylinder 105 between the two pistons 125.

[0043] Figure 9 illustrates a four-stroke piston cycle 400 for the two-piston integrated gas spring engine 300 of Figure 8. The four-stroke piston cycle is characterized as having an output (expansion) stroke, an exhaust stroke, an intake stroke, and a compression stroke. The output stroke begins after combustion occurs at optimal volume and continues until the speed of the piston 125 becomes zero and the piston reaches the output stroke BDC position for that cycle.

[0044] During the output stroke, a portion of the kinetic energy of the piston assembly 120 is converted into electrical energy by the LEM 200, and another portion of the kinetic energy performs compression work on the gas in the drive compartment 160. When the output stroke BDC is at or near it, and the drive compartment is attempting to provide at least some compression work, the pressure of the gas in the drive compartment 160 exceeds the pressure of the gas in the combustion compartment 130, biasing the piston 125 inward toward the midpoint of the cylinder 105. In the illustrated embodiment, the gas in the drive compartment 160 can be used to provide at least some of the energy required to perform the exhaust stroke. In some cases, the LEM 200 may also provide some of the energy required to perform the exhaust stroke. The exhaust port 370 opens at or near the output stroke BDC, which may be before or after the start of the exhaust stroke. The exhaust stroke continues until the velocity of the piston 125 becomes zero, indicating the piston's exhaust stroke TDC position for that cycle. The exhaust port 370 closes at some point before the piston 125 reaches its exhaust stroke TDC position. Therefore, at least some combustion products remain within the combustion section 130. This process is called residual gas trapping.

[0045] Referring further to Figure 9, at or near the exhaust stroke TDC, the pressure in the combustion chamber 130 exceeds the pressure in the drive chamber 160, biasing the piston 125 outward. The trapped residual gas acts as a gas spring, providing at least some of the energy required to perform the intake stroke. LEM200 can also provide at least some of the energy required to perform the intake stroke. The intake port 370 opens at some point during the intake stroke after the pressure in the combustion chamber 130 has fallen below the pressure of the intake gas. The intake stroke continues until the velocity of the piston 125 becomes zero and the piston reaches the intake stroke BDC position for that cycle. The intake stroke BDC position for a given cycle does not necessarily have to be the same as the output stroke BDC position. The intake port 370 closes at or near the intake stroke BDC. The compression stroke continues until combustion occurs, which is when the velocity of the piston 125 is zero or near zero. The position of piston 125, where its velocity is equal to zero, indicates the TDC (Time to Destruction) position of the piston's compression stroke for that cycle. At and near the compression stroke TDC, the gas pressure in the drive chamber 160 exceeds the gas pressure in the combustion chamber 130, biasing piston 125 inward. The gas in the drive chamber 160 is used to provide at least a portion of the energy required to perform the compression stroke. LEM200 may also provide a portion of the energy required to perform the compression stroke.

[0046] Figure 9 illustrates one method for exchanging the drive gas, where the removal of the drive gas occurs at some point during the expansion stroke, and the intake of replenishment drive gas occurs at some point during the compression stroke. As in the two-stroke embodiment, the removal and intake of the drive gas can also occur in the reverse order of the strokes or within the same stroke. However, since the four-stroke embodiment has a separate exhaust stroke that requires less energy to perform the compression stroke, adjusting the amount of air in the drive compartment 160 may require different approaches depending on the extent to which the LEM200 is used to provide and extract energy during the four strokes.

[0047] Figure 10 illustrates a second two-piston, two-stroke, fully gas-springed, and integrated linear electromagnetisme (LEM) embodiment of the internal combustion engine 500. Similar to engine 100 in Figure 10, engine 500 comprises a cylinder 105, two opposing piston assemblies 520, and a combustion chamber 130 located in the center of cylinder 105. In the illustrated configuration, each piston assembly 520 comprises two pistons 525, a piston seal 535, and a piston rod 545. Unlike the previously described embodiment, the piston assemblies 520 and transducer 620 are entirely located within the cylinder, and the LEM 600 (including the stator 610) is positioned around the outer periphery of cylinder 105. The piston assemblies 520 move freely linearly within cylinder 105. Cylinder 105 further includes an exhaust / injection port 170, an intake port 180, a drive gas removal port 185, and a drive gas supply port 190. Referring further to Figure 10, this embodiment can operate using a two- or four-stroke piston cycle, using the same methodology as described above with reference to Figures 7 and 9.

[0048] Figure 11 illustrates a third two-piston, two-stroke, single-combustion-chamber, isolated-gas-spring embodiment of the internal combustion engine 700. Similar to engine 100 in Figure 6, engine 700 comprises a main cylinder 105, two opposing piston assemblies 120, and a combustion chamber 130 located in the center of cylinder 705. However, the illustrated engine 700 has some physical differences compared to engine 100. Specifically, engine 700 includes a pair of outer cylinders 705, each containing an additional piston 135, and LEM200 is located between the main cylinder 105 and the outer cylinders 705. Each outer cylinder 705 includes a drive chamber 710 located between piston 125 and the distal end of cylinder 705, and a drive rear chamber 720 located between piston 125 and the proximal end of cylinder 705. In addition, cylinder 105 includes a pair of combustion rear chambers 730 located between piston 125 and the distal end of cylinder 105. The drive rear section 720 and the combustion rear section 730 are maintained at or near atmospheric pressure. Thus, the drive rear section 720 is not sealed (i.e., the linear bearing 740 does not have a gas seal), while the combustion rear section 730 is sealed (i.e., via the seal 150) but has ports for blow-by gas removal (i.e., blow-by removal port 750) and replenishment gas (i.e., replenishment air port 760). In the illustrated configuration, each piston assembly 120 comprises two pistons 125, a piston seal 135, and a piston rod 145. The piston assembly 120 moves freely linearly between the main cylinder 105 and the outer cylinder 705, as depicted in Figure 11. The piston rod 145 moves along the bearing and is sealed by the gas seal 150 which is fixed to the main cylinder 105. The cylinder 105 further includes an exhaust / injection port 170 and an intake port 180. However, the drive gas removal port 185 and the drive gas supply port 190 are located on a pair of outer cylinders 705, each containing one of the two pistons 125 of the piston assembly 120. Referring further to Figure 11, this embodiment can operate using a two or four-stroke piston cycle, using the same methodology described above with reference to Figures 7 and 9.

[0049] Figure 12 illustrates one embodiment of a single-piston, two-stroke, integrated gas-spring engine 1000. In particular, the engine 1000 includes a vertically positioned cylinder 105 with a piston assembly 120 that is sized to move within the cylinder 105 in response to reactions within the combustion chamber 130 (or combustion chamber) near the bottom end of the cylinder 105. A collision plate 230 is provided at the bottom end of the vertically positioned cylinder to provide stability and collision resistance during combustion. The piston assembly 120 includes a piston 125, a piston seal 135, and a piston rod 145. The piston assembly 120 moves freely and linearly within the cylinder 105. The piston rod 145 moves along a bearing and is sealed by a gas seal 150 fixed to the cylinder 105. In the illustrated embodiment, the gas seal 150 is a piston rod seal.

[0050] Referring further to Figure 12, the volume between the back of the piston 125, the piston rod 145, and the cylinder 105 is referred to herein as the drive compartment 160. The drive compartment 160 may also be referred herein as the “gas spring” or “gas spring compartment.” In the illustrated embodiment, the drive compartment 160 is sealed from the surroundings and the combustion compartment 130 by the piston rod seal 150 and the piston seal 135. The gas in the drive compartment 160 acts as a flywheel (i.e., a gas spring) for the duration of the cycle to provide at least some compression work during the compression stroke. Thus, some embodiments of the present invention feature a gas spring to provide work. Other embodiments include a highly efficient linear AC generator, which operates as a motor and does not require a gas spring to generate compression work.

[0051] In some embodiments, to obtain high thermal efficiency, the engine 1000 has a variable expansion ratio greater than 50:1. In additional embodiments, the variable expansion ratio is greater than 75:1. In further embodiments, the variable expansion ratio is greater than 100:1. In addition, some embodiments feature a compression ratio less than or equal to the expansion ratio, and a combustion chamber length at TDC of 0.1–2 inches. As used herein, “combustion chamber length at TDC” is the distance between the head of the combustion chamber and the front of the piston 125.

[0052] The aforementioned specifications indicate that engine 1000 has a significantly longer stroke length than conventional engines, where the term “stroke length” refers to the distance traveled by the piston 125 between TDC and BDC. Stroke is the distance traveled by the piston between TDC and BDC. Combustion ignition can be achieved via compression ignition and / or spark ignition. Fuel can be injected directly into the combustion chamber 130 via a fuel injector ("direct injection") and / or mixed with air prior to and / or during air intake ("premixed injection"). Engine 1000 can operate using liquid and / or gaseous fuels by lean, stoichiometric, or rich combustion.

[0053] Continuing with reference to Figure 12, the cylinder 105 includes an exhaust / injection port 170, an intake port 180, a drive gas removal port 185, and a drive gas supply port 190 for exchanging material (solid, liquid, gas, or plasma) with its surroundings. As used herein, the term “port” includes any opening or set of openings (e.g., porous material) that allow for the exchange of material between the inside of the cylinder 105 and its surroundings. Some embodiments do not require all of the ports depicted in Figure 12. The number and type of ports depend on the engine configuration, injection method, and piston cycle (e.g., a two- or four-stroke piston cycle). In this single-piston, two-stroke embodiment, the exhaust / injection port 170 allows exhaust gases and fluids to flow in and out of the cylinder, the intake port 180 is for the intake of air and / or an air / fuel mixture, the drive gas removal port 185 is for the removal of drive gases, and the drive gas supply port 190 is for the intake of supply gases for the drive compartment 160. The locations of the various ports are not necessarily fixed. For example, in the illustrated embodiment, the discharge / injection port 170 is substantially located at the midpoint of the cylinder. However, these ports may alternatively be located adjacent to the intake port 180 and away from the midpoint.

[0054] Referring further to Figure 12, the engine 1000 further includes a linear electromagnetic machine (LEM) 200 for directly converting the kinetic energy of the piston assembly 120 into electrical energy. The LEM 200 is also capable of directly converting electrical energy into the kinetic energy of the piston assembly 120 to provide compression work during the compression stroke. As shown, the LEM 200 comprises a stator 210 and a transducer 220. Specifically, the transducer 220 is attached to the piston rod 145 and moves linearly within the stationary stator 210. The volume between the transducer 220 and the stator 210 is called the air gap. The LEM 200 can include any number of configurations. Figure 6 shows one configuration in which the transducer 220 is shorter than the stator 210. However, the transducer 220 may be longer than the stator 210, or may be substantially the same length. In addition, the LEM 200 may be a permanent magnet machine, an induction machine, a switched reluctance machine, or several combinations of the three. The stator 210 and the transducer 220 can each include magnets, coils, iron, or several combinations thereof. Since the LEM200 converts the kinetic energy of the piston into electrical energy and directly from there (i.e., there is no mechanical coupling), mechanical and frictional losses are minimized compared to conventional engine-generator configurations.

[0055] The embodiment shown in Figure 12 operates using a two-stroke piston cycle. A schematic diagram illustrating the two-stroke piston cycle 1250 of the single-piston integrated gas spring engine 1000 of Figure 12 is shown in Figure 13. The engine exhausts combustion products (through exhaust port 170) and draws in air, an air / fuel mixture, or an air / fuel / combustion product mixture (through intake port 180) near the BDC between the power and compression strokes. This process may be referred to herein as “breathing” or “breathing in or near the BDC.” It will be understood by those skilled in the art that many other types of port and breathing configurations are possible without departing from the scope of the invention. When the BDC is in or near the BDC, and the drive compartment is to be used to provide compression work, the pressure of the gas in the drive compartment 160 exceeds the pressure in the combustion compartment 130, biasing the piston 125 inward toward each other. The gas in the drive compartment 160 can be used to provide at least a portion of the energy required to perform the compression stroke. The LEM200 can also provide some of the energy required to perform the compression stroke.

[0056] The amount of energy required to perform the compression stroke depends on the desired compression ratio, the pressure in the combustion chamber 130 at the start of the compression stroke, and the mass of the piston assembly 120. The compression stroke continues until combustion occurs, when the velocity of the piston 125 is zero or near zero. When the velocity of the piston 125 is equal to zero, it indicates the piston's TDC position for that cycle. Combustion causes an increase in temperature and pressure in the combustion chamber 130, biasing the piston 125 outward toward the LEM 200. During the output stroke, some of the kinetic energy of the piston assembly 120 is converted into electrical energy by the LEM 200, and another portion of the kinetic energy performs compression work on the gas in the drive chamber 160. The output stroke continues until the velocity of the piston 125 becomes zero, and the piston reaches its BDC position for that cycle.

[0057] Figure 13 illustrates a single-port configuration 1300 for breathing, where the intake port 180 is located in front of the piston near the BDC and the exhaust port 170 is located near the TDC. The opening and closing of the exhaust port 170 and the intake port 180 are controlled independently. The locations of the exhaust port 170 and the intake port 180 can be selected to allow a range of compression and / or expansion ratios. The time during a cycle when the exhaust port 170 and the intake port 180 are activated (opened and closed) can be adjusted during and / or between cycles to vary the compression and / or expansion ratio and / or the amount of combustion products retained in the combustion chamber 130 at the start of the compression stroke. Retaining combustion gases in the combustion chamber 130 is called residual gas trapping (RGT) and can be used to optimize combustion timing and peak temperature.

[0058] During the piston cycle, gases may potentially move beyond the piston seal 135 between the combustion compartment 130 and the drive compartment 160. This movement of gases is referred to as "blow-by." Blow-by gases may contain air and / or fuel and / or combustion products. Engine 1000 is designed to manage blow-by gases by having at least two ports within the drive compartment 160, one port 185 for removing drive gases and the other port 190 for supplying replenishment drive gases. The removal of drive gases and the intake of replenishment drive gases occur independently, controlled, to minimize losses and maximize efficiency.

[0059] Figure 13 shows one method for exchanging the drive gas, where the removal of the drive gas occurs at some point during the expansion stroke, and the intake of replenishment drive gas occurs at some point during the compression stroke. The removal and intake of drive gas can also occur in the reverse order of the stroke or within the same stroke. The removed drive gas can be used as part of the intake for the combustion chamber 130 during the subsequent combustion cycle. The amount of gas in the drive chamber 160 can be adjusted by varying the compression ratio and / or expansion ratio. The expansion ratio is defined as the ratio of the volume of the combustion chamber 130 when the piston 125 has zero velocity after the output stroke to the volume of the combustion chamber 130 when the piston 125 has zero velocity after the compression stroke. The compression ratio is defined as the ratio of the volume of the combustion chamber 130 when the pressure in the combustion chamber 130 begins to increase due to the inward motion of the piston 125 to the volume of the combustion chamber 130 when the piston 125 has zero velocity after the compression stroke.

[0060] The configurations in Figures 12 and 13, referred to as engine 1000, include a single unit defined by cylinder 105, piston assembly 120, and LEM200. However, many units can be installed in parallel and collectively referred to as “engine”. Some embodiments of the present invention are modular so that they can be arranged to operate in parallel, allowing end users to scale up the engine as needed. In addition, it is not necessary for all units to be the same size or to operate under the same conditions (e.g., frequency, stoichiometry, or breathing). When units operate in parallel, but not limited to, there is potential for integration between engines, such as gas exchange between units and / or feedback between LEM200s of units.

[0061] As described, the embodiments described above with respect to Figures 12 and 13 comprise a single-piston, single-combustion-chamber, two-stroke internal combustion engine 1000. Several alternative embodiments are described below and illustrated in the corresponding figures. These embodiments are not intended to be limiting. As will be understood by those skilled in the art, various modifications and alternative configurations may be utilized without departing from the scope of the invention, and other changes may be made. Unless otherwise stated, the physical and operational characteristics of the embodiments described below are similar to those described in the embodiments of Figures 12 and 13, and identical elements are marked where appropriate. Furthermore, all embodiments may be configured in parallel (i.e., in multiple unit configurations for scaling purposes) as described above.

[0062] Figure 14 illustrates a four-stroke embodiment of the present invention, comprising a single-piston, four-stroke, integrated gas-spring engine 1400. The main physical difference between the four-stroke engine 1400 in Figure 14 and the two-stroke engine 1000 in Figure 12 is related to the location of the ports. In particular, in the four-stroke engine 1400, the exhaust, injection, and intake ports 370 are located at and / or near the bottom of the cylinder 105 adjacent to the impact plate 230.

[0063] Figure 15 illustrates a four-stroke piston cycle 1500 for the single-piston integrated gas spring engine 1400 of Figure 14. The four-stroke piston cycle is characterized as having an output (expansion) stroke, an exhaust stroke, an intake stroke, and a compression stroke. The output stroke begins after combustion occurs at optimal volume and continues until the speed of the piston 125 becomes zero and the piston reaches the output stroke BDC position for that cycle.

[0064] During the output stroke, a portion of the kinetic energy of the piston assembly 120 is converted into electrical energy by the LEM 200, and another portion of the kinetic energy performs compression work on the gas in the drive compartment 160. When the output stroke BDC is at or near it, and the drive compartment is attempting to provide at least some compression work, the pressure of the gas in the drive compartment 160 exceeds the pressure of the gas in the combustion compartment 130, biasing the piston 125 inward toward the midpoint of the cylinder 105. In the illustrated embodiment, the gas in the drive compartment 160 can be used to provide at least some of the energy required to perform the exhaust stroke. In some cases, the LEM 200 may also provide some of the energy required to perform the exhaust stroke. The exhaust port 370 opens at or near the output stroke BDC, which may be before or after the start of the exhaust stroke. The exhaust stroke continues until the velocity of the piston 125 becomes zero, indicating the piston's exhaust stroke TDC position for that cycle. The exhaust port 370 closes at some point before the piston 125 reaches its exhaust stroke TDC position. Therefore, at least some combustion products remain within the combustion section 130. This process is called residual gas trapping.

[0065] Referring further to Figure 15, at or near the exhaust stroke TDC, the pressure in the combustion chamber 130 exceeds the pressure in the drive chamber 160, biasing the piston 125 upward. The trapped residual gas acts as a gas spring, providing at least some of the energy required to perform the intake stroke. LEM200 may also provide some of the energy required to perform the intake stroke. The intake port 370 opens at some point during the intake stroke after the pressure in the combustion chamber 130 has fallen below the pressure of the intake gas. The intake stroke continues until the velocity of the piston 125 becomes zero and the piston reaches the intake stroke BDC position for that cycle. The intake stroke BDC position for a given cycle does not necessarily have to be the same as the output stroke BDC position. The intake port 370 closes at or near the intake stroke BDC. The compression stroke continues until combustion occurs, which is when the velocity of the piston 125 is zero or near zero. The position of piston 125, where its velocity is equal to zero, indicates the TDC (Time to Depression) position of the piston's compression stroke for that cycle. At and near the compression stroke TDC, the gas pressure in the drive chamber 160 exceeds the gas pressure in the combustion chamber 130, biasing piston 125 downward. The gas in the drive chamber 160 is used to provide at least a portion of the energy required to perform the compression stroke. LEM200 may also provide a portion of the energy required to perform the compression stroke.

[0066] Figure 15 illustrates one method for exchanging the drive gas, where the removal of the drive gas occurs at some point during the expansion stroke, and the intake of replenishment drive gas occurs at some point during the compression stroke. As in the two-stroke embodiment, the removal and intake of the drive gas can also occur in the reverse order of the strokes or within the same stroke. However, since the four-stroke embodiment has a separate exhaust stroke that requires less energy than the compression stroke, different approaches may be required depending on the extent to which the LEM200, which regulates the amount of air in the drive compartment 160, is used to supply and extract energy during the four strokes.

[0067] Figure 16 illustrates a second single-piston, two-stroke, fully gas-springed, and integrated linear electromagnetisme embodiment of the internal combustion engine 1600. The engine 1600 comprises a cylinder 105, a piston assembly 520, and a combustion chamber 130. In the illustrated configuration, the piston assembly 520 comprises two pistons 525, a piston seal 535, and a piston rod 545. Unlike the previously described embodiment, the piston assembly 120 and transducer 620 are entirely located within the cylinder, and the LEM 600 (including the stator 610) is positioned around the outer periphery of the cylinder 105. The piston assembly 520 moves freely linearly within the cylinder 105. The cylinder 105 further includes an exhaust / injection port 170, an intake port 180, a drive gas removal port 185, and a drive gas supply port 190. Referring further to Figure 16, this embodiment can operate using a two- or four-stroke piston cycle, using the same methodology as described above.

[0068] Figure 17 illustrates a third two-piston, two-stroke, single-combustion-chamber, isolated-gas-spring embodiment of the internal combustion engine 1700. Similar to engine 1000, engine 1700 comprises a main cylinder 105, a piston assembly 120, and a combustion chamber 130. However, the illustrated engine 1700 has some physical differences compared to engine 1000. Specifically, engine 1700 includes an outer cylinder 705 with an additional piston 125, and LEM200 is located between the main cylinder 105 and the outer cylinder 705. The outer cylinder 705 includes a drive chamber 710 located between piston 125 and the distal end of cylinder 705, and a drive rear chamber 720 located between piston 135 and the proximal end of cylinder 705. In addition, cylinder 105 includes a combustion rear chamber 730 located between piston 135 and the distal end of cylinder 105. The drive rear section 720 and the combustion rear section 730 are maintained at or near atmospheric pressure. Thus, the drive rear section 720 is not sealed (i.e., the linear bearing 740 does not have a gas seal), while the combustion rear section 730 is sealed (i.e., via the seal 150) but has ports for blow-by gas removal (i.e., blow-by removal port 750) and replenishment gas (i.e., replenishment air port 760). In the illustrated configuration, each piston assembly 120 comprises two pistons 125, a piston seal 135, and a piston rod 145. The piston assembly 120 moves freely linearly between the main cylinder 105 and the outer cylinder 705. The piston rod 145 moves along the bearing and is sealed by the gas seal 150 fixed to the main cylinder 105. The cylinder 105 further includes an exhaust / injection port 170 and an intake port 180. However, the drive gas removal port 185 and the drive gas supply port 190 are located on the outer cylinder 705, which includes one of the two pistons 125 of the piston assembly 120. This embodiment can operate using a two or four-stroke piston cycle, using the same methodology described above.

[0069] The embodiments disclosed herein include single-piston and two-piston configurations, including (i) an integrated gas spring with a separate linear electromagnetic machine (Figures 6-9 and 12-15), (ii) a fully integrated gas spring and linear electromagnetic machine (Figures 10 and 16), and (iii) a separated gas spring and linear electromagnetic machine (Figures 11 and 17). Figure 18-20 illustrates a further embodiment of the invention featuring an integrated internal gas spring, in which the gas spring is integrated inside the piston and the linear electromagnetic machine (LEM) is separated from the combustion cylinder. Table 1 summarizes the key differences between the four structures described herein.

[0070] [Table 1] (Integrated internal gas spring) As illustrated in Figure 18-20 and summarized in Table 1, the integrated internal gas spring (IIGS) structure is similar in length to the integrated gas spring with the separated LEM structure illustrated in Figures 6-9 and 12-15. However, the IIGS structure eliminates the issues related to blow-by gases from the combustion chamber flowing into the gas spring, which also occur in fully integrated gas springs and LEM structures.

[0071] Figure 18 is a cross-sectional view illustrating a single-piston, two-stroke version of the IIGS structure according to one embodiment of the present invention. Many components, such as the combustion chamber 130, are similar to those in the previously described embodiments (e.g., Figure 12) and are labeled as appropriate. The engine 1800 comprises a vertically positioned cylinder 105 with a piston assembly 1820 that is sized to move within the cylinder 105 in response to reactions in the combustion chamber 130 near the bottom end of the cylinder 105. A collision plate may be provided at the bottom end of the vertically positioned cylinder to provide stability and collision resistance during combustion. The piston assembly 1820 comprises a piston 1830, a piston seal 1835, and a spring rod 1845. The piston assembly 1820 moves freely linearly within the cylinder 105. The piston rod 1845 moves along a bearing and is sealed by a gas seal 150 fixed to the cylinder 105. In the illustrated embodiment, the gas seal 150 is a piston rod seal. Cylinder 105 includes intake ports 1870, 1880 for air, fuel, exhaust gas, air / fuel mixture, and / or air / exhaust gas / fuel mixture, and exhaust of combustion products, as well as / or injectors. Some embodiments do not require all of the ports depicted in Figure 18. The number and type of ports depend on the engine configuration, injection method, and piston cycle (e.g., a two- or four-stroke piston cycle).

[0072] In the illustrated embodiment, the engine 1800 further includes an LEM 1850 (including a stator 210 and a magnet 1825) to directly convert the kinetic energy of the piston assembly 1820 into electrical energy. The LEM 1850 is also capable of directly converting electrical energy into the kinetic energy of the piston assembly 1820 to provide compression work during the compression stroke. The LEM 1850 can be a permanent magnet machine, an induction machine, a switched reluctance machine, or some combination of the three. The stator 210 can include magnets, coils, heat, or some combination thereof. Since the LEM 1850 converts the kinetic energy of the piston into electrical energy and directly from there (i.e., there is no mechanical coupling), mechanical and frictional losses are minimized compared to conventional engine-generator configurations.

[0073] Referring further to Figure 18, the piston 1830 comprises a solid front section (combustion side) and a hollow rear section (gas spring side). The area inside the hollow section of the piston 1830 between the front of the piston and the spring rod 1845 contains gas, which acts as a gas spring 160 and provides at least some of the work required to perform the compression stroke. The piston 1830 moves linearly within the stator 210 of the combustion section 130 and the LEM 1850. The motion of the piston is guided by bearings 1860, 1865, which may be integral bearings, hydraulic bearings, and / or pneumatic bearings. In the illustrated embodiment, the engine 1800 includes both an external bearing 1860 and an internal bearing 1865. In particular, the external bearing 1860 is located between the combustion section 130 and the LEM 1850, and the internal bearing 1865 is located inside the hollow section of the piston 1830. The external bearing 1860 is fixed from the outside and does not move with the piston 1830. The internal bearing 1865 is fixed to the piston 1830 and moves with the piston 1830 relative to the spring rod 1845.

[0074] Continuing to refer to Figure 18, the spring rod 1845 serves as one face of the gas spring 160 and is fixed from the outside. The spring rod 1845 has at least one seal 1885 located at or near its end, which serves the purpose of retaining gas within the gas spring compartment 160. The magnet 1825 is mounted on the back of the piston 1830 and moves linearly with the piston 1830 within the stator 210 of the LEM 1850. The piston 1830 has a seal 1835, which retains gas within its respective compartments. The illustrated embodiment includes (i) a front seal fixed to the piston 1830 at or near its front end to prevent gas from moving out of the combustion compartment 130, and (ii) a rear seal fixed to the cylinder 105 to prevent intake gas and / or blow-by gas from moving outwards.

[0075] Figure 19 is a cross-sectional view illustrating an embodiment 1900 of a gas spring rod 1845 according to the principles of the present invention. Specifically, the spring rod 1845 includes a central lumen 1910 that allows mass transfer between a gas spring section 160 and a reservoir section 1920 that communicates with the surroundings. Communication with the surroundings is controlled through a valve 1930. The amount of mass in the gas spring 1845 is adjusted to control the pressure within the gas spring 1845 so that sufficient compression work is available for the next piston cycle.

[0076] Figure 20 is a cross-sectional view illustrating a two-piston, two-stroke version of the IIGS engine 2000 according to one embodiment of the present invention. Most of the elements of the two-piston embodiment are similar to those of the single-piston embodiment in Figure 18, and identical elements are labeled as appropriate. In addition, the operating characteristics of the single and two-piston embodiments are similar to those described in the above embodiments, including all aspects such as the linear AC generator, breathing, and combustion method.

[0077] While various embodiments of the present invention have been described herein, it should be understood that they are presented only as examples and not limiting. Similarly, various schematic diagrams depict exemplary structures or other configurations for the present invention, which may be done to aid in understanding the features and functionalities that may be included in the present invention. The present invention is not limited to the illustrated exemplary structures or configurations, and desired features can be implemented using various alternative structures and configurations. Indeed, it will be obvious to those skilled in the art how alternative functional, logical, or physical divisions and configurations can be implemented to implement desired features of the present invention. Furthermore, numerous different component module names other than those described herein can be applied to various divisions. In addition, with respect to flowcharts, operation descriptions, and method claims, the order in which the steps are presented herein does not obligate various embodiments to be implemented to perform the functionalities enumerated in the same order, unless otherwise indicated by the context.

[0078] While the present invention has been described in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects, and functionalities described in one or more of the individual embodiments are not limited to their availability in the specific embodiment described thereby. Rather, such embodiments may be applied, individually or in various combinations, to one or more other embodiments of the present invention, whether described or not, and whether such features are presented as part of the described embodiment. Therefore, the breadth and scope of the present invention should not be limited by any of the exemplary embodiments described above.

[0079] The terms and phrases used in this book, and their variations thereof, should be interpreted as non-restrictive, as opposed to restrictive, unless expressly stated otherwise. As such, the term “including” should be read as “including, but not limiting” or equivalent; the term “example” should be used to provide illustrative examples of an item in discussion, not an exhaustive or restrictive list thereof; the terms “a” or “an” should be read as “at least one,” “one or more” or equivalent; and adjectives such as “conventional,” “traditional,” “usual,” “standard,” “known,” and similar terms should not be interpreted as limiting the item described to items available in a given time period or at a given point in time, but rather as encompassing conventional, traditional, common, or standard techniques that may be available or known at present or future time. Similarly, where this book refers to a technique that would be obvious or known to those skilled in the art, such a technique encompasses those that are obvious or known to those skilled in the art at present or future time.

[0080] In some instances, the presence of broader words and phrases such as “one or more,” “at least,” “not limited to,” or other similar terms is not to be interpreted as meaning that the narrower definition is intended or required in cases where such broader terms may be absent. The use of the term “module” does not imply that all components or functionalities described or claimed as part of a module are contained within a common package. In fact, some or all of the various components of a module, whether control logic or other components, can be combined within a single package or maintained separately, and furthermore, can be distributed across multiple groups or packages or across multiple locations.

[0081] In addition, the various embodiments described herein are explained in terms of illustrative block diagrams, flowcharts, and other illustrations. As will be obvious to those skilled in the art after careful reading of this document, the illustrated embodiments and their various alternatives can be implemented without being limited to the illustrated examples. For example, the block diagrams and their accompanying descriptions should not be construed as obligating any particular structure or configuration.

Claims

1. 1. A method for operating a reciprocating device comprising a pair of opposed free piston assemblies, each free piston assembly of the pair of opposed free piston assemblies in contact with a respective drive compartment and a reaction compartment between the opposed free piston assemblies, the method comprising: translating the pair of opposed free piston assemblies along one or more respective gas bearings according to a cycle, the cycle comprising at least an expansion stroke and a stroke subsequent to the expansion stroke; converting kinetic energy of said pair of opposed free piston assemblies into electrical energy using a linear electromagnetic machine during said expansion stroke and during strokes subsequent to said expansion stroke; controlling an amount of energy stored in each drive section, whereby said amount of energy is sufficient to perform the stroke subsequent to said expansion stroke; using said pair of opposed free piston assemblies to perform said subsequent stroke using said amount of energy from said drive section; A method comprising:

2. The method of claim 1 , wherein the stroke following the expansion stroke comprises a compression stroke.

3. 2. The method of claim 1, wherein each drive section comprises a gas spring, the gas spring coupled to a first port for removing drive gas and a second port for providing make-up drive gas.

4. Controlling the amount of energy stored in each drive segment comprises: controlling the removal of the drive gas from the first port; controlling the intake of the make-up drive gas into the second port; and The method of claim 3 , comprising:

5. 2. The method of claim 1, wherein each drive section comprises a gas spring, the gas spring comprising a gas, and the gas spring configured to convert between kinetic energy of a respective free piston assembly and stored energy in the gas.

6. 6. The method of claim 5, further comprising adjusting the amount of gas in the gas spring to vary a compression ratio, an expansion ratio, or both.

7. The method of claim 1 , wherein performing the subsequent stroke includes using only the amount of energy from each drive segment.

8. 1. A system comprising: a pair of opposed free piston assemblies configured for translation along one or more respective gas bearings according to a cycle, each free piston assembly of the pair of opposed free piston assemblies contacting a respective drive section of a pair of drive sections; a reaction compartment between said pair of opposed free piston assemblies; The cycle includes at least a pair of opposed free piston assemblies having an expansion stroke and a stroke subsequent to the expansion stroke; a linear electromagnetic machine configured to convert kinetic energy of the pair of opposed free piston assemblies into electrical energy during the expansion stroke and during strokes subsequent to the expansion stroke; Equipped with a control system in which an amount of energy stored in each drive section of the pair of drive sections is controlled such that the amount of energy is sufficient to perform a stroke subsequent to the expansion stroke, and each free piston assembly of the pair of opposed free piston assemblies performs the subsequent stroke using the amount of energy from its respective drive section of the pair of drive sections.

9. The system of claim 8 , wherein the stroke following the expansion stroke comprises a compression stroke.

10. 9. The system of claim 8, wherein each drive section of the pair of drive sections includes a gas spring, the gas spring coupled to a first port for removing drive gas and a second port for providing make-up drive gas.

11. 9. The system of claim 8, wherein each drive section of the pair of drive sections comprises a gas spring, the gas spring comprising a gas, and the gas spring configured to convert between kinetic energy of a free piston assembly of the pair of opposed free piston assemblies and stored energy in the gas.

12. 12. The system of claim 11, further comprising adjusting the amount of gas in the gas spring to vary a compression ratio, an expansion ratio, or both.

13. 1. A method for operating a reciprocating device comprising a pair of opposed free piston assemblies, each free piston assembly of the pair of opposed free piston assemblies in contact with a respective drive compartment and a reaction compartment between the pair of opposed free piston assemblies, the method comprising: performing an expansion stroke as part of a cycle, the reaction section urging each free piston assembly of the pair of opposed free piston assemblies to translate along one or more respective gas bearings; performing a compression stroke as part of said cycle, each drive section urging a respective free piston assembly of said pair of opposed free piston assemblies in translation; storing an amount of energy in each drive section during the expansion stroke so as to provide at least a portion of the energy required to perform the compression stroke; Moderating reaction timing or peak temperatures in the cycle by retaining exhaust gases from the expansion stroke in the reaction section at the beginning of the compression stroke; A method comprising:

14. 14. The method of claim 13, further comprising providing air to the reaction zone at the start of the compression stroke.

15. The method of claim 13 further comprising providing air and fuel to the reaction zone at the beginning of the compression stroke.

16. 14. The method of claim 13, further comprising converting kinetic energy of each free piston assembly of said pair of opposed free piston assemblies into electrical energy during said expansion stroke and said compression stroke using a linear electromagnetic machine.

17. The method of claim 13 further comprising controlling the intake and exhaust ports to achieve uniflow scavenging.

18. The method of claim 17 , wherein retaining exhaust gases includes using the uniflow scavenging.

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