Patent Description:
Document <CIT> describes an engine includes a piston slidably disposed in a cylinder with a closed end and a converter operable with the piston to convert mechanical energy of the first piston from and to electrical energy within a piston cycle.

Document <CIT> describes a free-piston engine comprises a cylinder and a single double-ended piston which partitions the cylinder into two separate combustion chambers, each of which is supplied from one or more intake means, the piston being arranged to move over and past the intake means during each stroke such that air is replenished within one combustion chamber while the piston compresses air held in the other combustion chamber.

Document <CIT> describes a free-piston generator for generating electricity with the linear reciprocal motion of a piston is provided with a combustion chamber and a spring portion disposed on mutually opposite sides of the piston and comprises: an engine unit in which the piston reciprocally moves by the combustion pressure generated upon combustion of a fuel in the combustion chamber and the restoring force of the spring portion compressed by the piston; an electricity generation unit for generating electricity with the reciprocal motion of the piston; and a control means for controlling the drives of the engine unit and the electricity generation unit.

According to aspects of the present invention, a free-piston combustion engine system and a method of control of a free-piston combustion engine system are provided, as defined in the appended claims.

In some examples, a free-piston combustion engine system is provided, comprising: a cylinder comprising a combustion section; at least one free-piston assembly in contact with the combustion section; at least one driver section in contact with the at least one free-piston assembly; at least one linear electromagnetic machine for directly converting between kinetic energy of the at least one free-piston assembly and electrical energy; and processing circuitry that for the purpose of avoiding net electrical energy input over a subsequent stroke of the piston cycle, causes the at least one driver section to store at least a sufficient amount of energy from the at least one free-piston assembly during the expansion stroke to perform the subsequent stroke of the piston cycle.

In some examples, a free-piston combustion engine system is provided, comprising: a cylinder comprising a combustion section; at least one free-piston assembly in contact with the combustion section; at least one driver section in contact with the at least one free-piston assembly, wherein the at least one driver section is configured to store energy from the at least one free-piston assembly during an expansion stroke of a piston cycle; at least one linear electromagnetic machine for directly converting between kinetic energy of the at least one free-piston assembly and electrical energy; and processing circuitry that necessarily causes the at least one driver section to store at least a sufficient amount of energy from the at least one free-piston assembly during the expansion stroke to perform a subsequent stroke of the piston cycle without net electrical energy input over the subsequent stroke of the piston cycle.

In some examples, a system for controlling a free-piston combustion engine comprising at least one free-piston assembly in contact with a respective at least one driver section, and at least one linear electromagnetic machine for directly converting kinetic energy of the at least one free-piston assembly into electrical energy, the system comprising: at least one sensor coupled to the free-piston combustion engine for measuring a respective at least one operating characteristic of the engine and for outputting a respective at least one sensor signal; at least one control mechanism for adjusting a respective at least one operating characteristic of the free-piston combustion engine based on a respective at least one control signal; and processing circuitry that takes as input the at least one sensor signal and that outputs the at least one control signal, the processing circuitry configured to: process the at least one sensor signal to cause, using the control mechanism, the at least one driver section to store at least a sufficient amount of energy from the at least one free-piston assembly during the expansion stroke to perform a subsequent stroke of the piston cycle without net electrical energy input over the subsequent stroke of the piston cycle.

In some examples, a method of controlling a free-piston combustion engine comprising at least one free-piston assembly in contact with a respective at least one driver section, and at least one linear electromagnetic machine for directly converting kinetic energy of the at least one free-piston assembly into electrical energy, is provided, the method comprising: receiving at least one operating characteristic of the free-piston combustion engine; processing the at least one operating characteristic, using processing circuitry, to cause the driver section to store at least a sufficient amount of energy from the at least one free-piston assembly during an expansion stroke of a piston cycle to perform a subsequent stroke of the piston cycle; and causing, using the processing circuitry, the subsequent stroke of the piston cycle to be performed without net electrical energy input to the engine.

Other features and aspects will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features in accordance with various embodiments. The summary is not intended to limit the scope of the invention, which is defined solely by the claims attached hereto.

The present disclosure, in accordance with one or more various embodiments, is described in detail with reference to the following figures. The drawings are provided for purposes of illustration only and merely depict typical or example embodiments. These drawings are provided to facilitate an understanding of the concepts disclosed herein and shall not be considered limiting of the breadth, scope, or applicability of these concepts. It should be noted that for clarity and ease of illustration these drawings are not necessarily made to scale.

The figures are not intended to be exhaustive or to limit the disclosure to the precise form disclosed. It should be understood that the concepts and embodiments disclosed can be practiced with modification and alteration, and that the disclosure is limited only by the claims and the equivalents thereof.

Various embodiments of the present disclosure are directed towards a free-piston, linear combustion engine characterized by high thermal efficiencies. In at least one embodiment, the engine comprises: (i) a cylinder comprising a combustion section, (ii) at least one free-piston assembly in contact with the combustion section, (iii) at least one driver section in contact with the at least one free-piston assembly that stores energy during an expansion stroke of the engine (iv) and at least one linear electromagnetic machine (LEM) that directly converts between kinetic energy of the at least one free-piston assembly and electrical energy. It should be noted, however, that further embodiments may include various combinations of the above-identified features and physical characteristics.

Generally, free-piston combustion engine configurations can be broken down into three categories: <NUM>) two opposed pistons, single combustion chamber, <NUM>) single piston, dual combustion chambers, and <NUM>) single piston, single combustion chamber. A diagram of the three common free-piston combustion engine configurations is shown in <FIG>. Several illustrative embodiments of linear free-piston combustion engines are illustrated in <CIT>, and entitled "High-efficiency linear combustion engine". It will be understood that while the present disclosure is presented in the context of certain specific illustrative embodiments of linear free-piston combustion engines, the concepts discussed herein are applicable to any other suitable free-piston combustion engines, including, for example, non-linear free piston engines. Free-piston engines generally include one or more free-piston assemblies that are free from mechanical linkages that translate the linear motion of the piston assembly into rotary motion (e.g., a slider-crank mechanism) or free from mechanical linkages that directly control piston dynamics (e.g., a locking mechanism ). Free-piston engines have a number of benefits over such mechanically-linked piston engines, which lead to increased efficiency. For example, due to the inherent architectural limitations of mechanically-linked piston engines, free-piston engines can be configured with higher compression ratios and expansion ratios, which lead to higher engine efficiencies. Moreover, free-piston engines allow for increased variability in the compression and expansion ratios, including allowing for the compression ratio to be greater than the expansion ratio and allowing for the expansion ratio to be greater than the compression ratio, which may also increase the engine efficiency. The free-piston engine architecture also allows for increased control of the compression ratio on an engine cycle-to-cycle basis, which allows for adjustments due to variable fuel quality and fuel type. Additionally, due to the lack of mechanical linkages, free-piston engines result in substantially lower side loads on the piston assemblies, which allows for oil-less operation, and in turn, reduced friction and losses resulting therefrom.

<FIG> is a cross-sectional drawing illustrating one embodiment of a two-piston, single-combustion section, integrated gas springs, and separated LEM free-piston internal combustion engine <NUM>. This free-piston, internal combustion engine <NUM> directly converts the chemical energy in a fuel into electrical energy via an LEM <NUM>. As used herein, the term "fuel" refers to matter 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; (iii) hydrogen; and (iv) mixtures of any of the above. The engines described herein are suitable for both stationary power generation and mobile power generation (e.g., for use in vehicles).

Engine <NUM> includes a cylinder <NUM> with two opposed piston assemblies <NUM> dimensioned to move within the cylinder <NUM> and meet at a combustion section <NUM> in the center of the cylinder <NUM>. Each piston assembly <NUM> may include a piston <NUM> and a piston rod <NUM>. The piston assemblies <NUM> are free to move linearly within the cylinder <NUM>.

With further reference to <FIG>, the volume between the backside of the piston <NUM>, piston rod <NUM>, and the cylinder <NUM> is referred to herein as the driver section <NUM>. As used herein, a "driver section" refers to a section of an engine cylinder capable of storing energy and providing energy to displace the piston assembly without the use of combustion. The driver section <NUM>, in some embodiments, may contain a non-combustible fluid (i.e., gas, liquid, or both). In the illustrated embodiment, the fluid in the driver section <NUM> is a gas that acts as a gas spring. Driver section <NUM> stores energy from an expansion stroke of the piston cycle and provides energy for a subsequent stroke of the piston cycle, i.e. the stroke that occurs after an expansion stroke. For example, kinetic energy of the piston may be converted into potential energy of the gas in the driver section during an expansion stroke of the engine. As used herein, the term "piston cycle" refers to any series of piston movements that begin and end with the piston <NUM> in substantially the same configuration. One common example is a four-stroke piston cycle, which includes an intake stroke, a compression stroke, a power stroke, and an exhaust stroke. Additional alternate strokes may form part of a piston cycle as described throughout this disclosure. A two-stroke piston cycle is characterized as having a power stroke and a compression stroke. As used herein, an "expansion stroke" refers to a stroke of a piston cycle during which the piston assembly moves from a top-dead-center ("TDC") position to a bottom-dead-center ("BDC") position, where TDC refers to the position of the piston assembly, or assemblies, when the combustion section volume is at a minimum and BDC refers to the position of the piston assembly, or assemblies, when the combustion section volume is at a maximum. As noted above, since the compression ratio and expansion ratio of a free-piston engine can vary or be varied from cycle-to-cycle, the TDC and BDC positions can also vary or be varied from cycle-to-cycle, in some embodiments. Accordingly, as will be described below in further detail, an expansion stroke may refer to an intake stroke, a power stroke, or both. In some embodiments, the amount of energy to be stored by the driver section during an expansion stroke may be determined based on various criteria and controlled by a controller and associated processing circuitry as will be described below in further detail. For example, in some embodiments, the amount of energy to be stored by a driver section during an expansion stroke may be determined based on the energy required in a subsequent stroke, i.e., the stroke that occurs after the expansion stroke. In some embodiments, the controller and associated processing circuitry may, for the purpose of avoiding net electrical energy input over a subsequent stroke of the piston cycle, cause the driver section to store at least a sufficient amount of energy from the free-piston assembly during the expansion stroke to perform the subsequent stroke. In some embodiments, the controller and associated processing circuitry may necessarily cause the driver section to store at least a sufficient amount of energy from the free-piston assembly during an expansion stroke to perform the subsequent stroke without net electrical energy input over the subsequent stroke. In some embodiments, the amount of energy stored by the driver section during an expansion stroke may be greater than the amount required for a subsequent stroke. For example, in the case of a two-stroke piston cycle, the driver section may store, during a power stroke, an amount of energy greater than the amount of energy required for the subsequent compression stroke. In some embodiments, for example, in the case of a four-stroke piston cycle, the driver section may store an amount of energy, during a power stroke, greater than the amount of energy required for the subsequent exhaust stroke. In some embodiments, for example, in the case of a four-stroke piston cycle, the driver section may store an amount of energy, during an intake stroke, greater than the amount of energy required for the subsequent compression stroke. In some embodiments, the amount of energy stored in excess of that required for the subsequent stroke may be converted into electrical energy by LEMs <NUM> as will be described in more detail below. In some embodiments, the amount of energy stored by the driver section during an expansion stroke may be determined so as to enable the engine to operate continuously across consecutive piston cycles without electrical energy input from the LEMs <NUM>. For example, the amount of energy stored by the driver section during an expansion stroke may be determined so as to enable the engine to operate continuously across piston cycles without external electrical energy input other than that which may be required for initial start-up of the engine.

For purposes of brevity and clarity, the driver section will primarily be described herein in the context of a gas spring and may be referred to herein as the "gas section," "gas springs" or "gas springs section. " It will be appreciated that in some arrangements, the driver section <NUM> may include one or more other mechanisms in addition to or in place of a gas spring. For example, such mechanisms can include one or more mechanical springs, magnetic springs, or any suitable combination thereof. In some arrangements, a highly efficient linear alternator may be included that operates as a motor, which may be used in place of or in addition to a spring (pneumatic, hydrodynamic, or mechanical) for generating compression work. It will be understood by those skilled in the art that in some embodiments, the geometry of the driver section may be selected to minimize losses and maximize the efficiency of the driver section. For example, the diameter and/or dead volume of the driver section may be selected to minimize losses and maximize the efficiency of the driver section. As used herein, the term "dead volume" refers to the volume of the driver section when the piston assembly is at a BDC position. In some embodiments, for example, if the driver section is a gas or hydraulic spring, the diameter of the section may be different than the combustion section in order to provide for increased efficiency. Certain embodiments of gas springs will be described below in further detail with reference to <FIG>.

Combustion ignition can be achieved via, for example, compression ignition and/or spark ignition. Fuel can be directly injected into the combustion chamber <NUM> via fuel injectors ("direct injection") and/or mixed with air prior to and/or during air intake ("premixed injection"). The engine <NUM> can operate with lean, stoichiometric, or rich combustion using liquid fuels, gaseous fuels, or both, including hydrocarbons, hydrogen, alcohols, or any other suitable fuels as described above.

Cylinder <NUM> may include injector ports <NUM>, intake ports <NUM>, exhaust ports <NUM>, and driver gas exchange ports <NUM>, for exchanging matter (solid, liquid, gas, or plasma) with the surroundings. As used herein, the term "port" includes any opening or set of openings (e.g., a porous material) which allows matter exchange between the inside of the cylinder <NUM> and its surroundings. It will be understood that the ports shown in <FIG> are merely illustrative. In some arrangements, fewer or more ports may be used. The above-described ports may or may not be opened and closed via valves. The term "valve" may refer to any actuated flow controller or other actuated mechanism for selectively passing matter through an opening. Valves may be actuated by any means, including but not limited to: mechanical, electrical, magnetic, camshaft-driven, hydraulic, or pneumatic means. The number, location, and types of ports and valves may depend on the engine configuration, injection strategy, and piston cycle (e.g., two- or four-stroke piston cycles). In some embodiments, the matter exchange of the ports may be achieved by the movement of the piston assembly, which may cover and/or uncover the ports as necessary to allow exchange of matter.

In some embodiments, the operation of driver section <NUM> may be adjustable. In some embodiments, driver gas exchange ports <NUM> may be utilized to control characteristics of the driver section. For example, driver gas exchange ports <NUM> may be used to control the amount, temperature, pressure, any other suitable characteristics, and/or any combination thereof of the gas in the driver section. In some embodiments, adjusting any of the aforementioned characteristics and thus adjusting the amount of mass in the cylinder may vary the effective spring constant of the gas spring. In some embodiments, the geometry of driver section <NUM> may be adjusted to obtain desirable operation. For example, the volume of the driver section <NUM> may be increased or decreased by controlling the driver gas exchange ports <NUM> and the characteristics of the driver gas flowing therein. In some embodiments, the dead volume within the cylinder may be adjusted to vary the spring constant of the gas spring. It will be understood that any of the aforementioned control and adjustment of the driver section <NUM> and the gas therein may provide for control of the amount of energy stored by driver section <NUM> during an expansion stroke of engine <NUM>. It will also be understood that the aforementioned control of the characteristics of the gas in driver section <NUM> also provides for variability in the frequency of engine <NUM>.

Engine <NUM> may include a pair of LEMs <NUM> for directly converting the kinetic energy of the piston assemblies <NUM> into electrical energy (e.g., during a compression stroke, during an expansion stroke, during an exhaust stroke, and/or during an intake stroke). Each LEM <NUM> is also capable of directly converting electrical energy into kinetic energy of the piston assembly <NUM>. In some embodiments, the LEMs <NUM> may convert electrical energy into kinetic energy of the piston in order to start-up the engine, but need not convert electrical energy into kinetic energy during operation once the engine has started and sufficient fuel chemical energy is being converted into kinetic energy of the piston, at least part of which may be stored in the driver section <NUM> during expansion strokes. In some embodiments, start-up of the engine may be achieved by any other suitable technique, including, for example, the use of stored compressed gas. As illustrated, the LEM <NUM> includes a stator <NUM> and a translator <NUM>. Specifically, the translator <NUM> is coupled to the piston rod <NUM> and moves linearly within the stator <NUM>, which may remain stationary. In addition, the LEM <NUM> can be a permanent magnet machine, an induction machine, a switched reluctance machine, or any combination thereof. The stator <NUM> and translator <NUM> can each include magnets, coils, iron, or any suitable combination thereof. Because the LEM <NUM> directly transforms the kinetic energy of the pistons to and from electrical energy (i.e., there are no mechanical linkages), the mechanical and frictional losses are minimal compared to conventional engine-generator configurations. Furthermore, because the LEM <NUM> is configured to convert portions of the kinetic energy of the piston assemblies into electrical energy during any stroke of a piston cycle, and engine <NUM> includes an adjustable driver section <NUM> configured to store energy from an expansion stroke that can be converted to electrical energy during a subsequent stroke, the LEM <NUM> may be configured to have a lower electrical capacity than, for example, an LEM or other device that requires conversion of all energy within a single stroke of a piston cycle (e.g., only within the expansion stroke). Accordingly, in some embodiments, the associated linear alternator and power electronics of the LEM <NUM> may be reduced in size, weight, and/or electrical capacity. This may result in decreased size and cost of components, increased efficiency, increased reliability, and increased utilization as will be understood by one of ordinary skill in the art. Accordingly, the frequency and therefore power output of the engine may be increased in some embodiments.

It will be understood by one of ordinary skill in the art that each LEM <NUM> may be operated as both a generator and a motor. For example, when LEMs <NUM> convert kinetic energy of piston assemblies <NUM> into electrical energy they operate as generators. When acting as generators, the forces applied to translators <NUM> are in the opposite direction of the motion of piston assemblies <NUM>. Conversely, when LEMs <NUM> convert electric energy into kinetic energy of piston assemblies <NUM> they operate as motors. When acting as motors, the forces applied to translators <NUM> are in the same direction as the motion of piston assemblies <NUM>. For ease of reference, the center line in <FIG> (near injector ports <NUM>) and corresponding figures may be considered the origin, with the positive direction for each piston assembly being away from the center, in the outward direction.

The embodiment shown in <FIG> operates using a two-stroke piston cycle. A diagram illustrating the two-stroke piston cycle <NUM> of the two-piston integrated gas springs engine <NUM> of <FIG> is illustrated in <FIG>. As illustrated in <FIG>, engine <NUM> may operate using a two-stroke piston cycle including a compression stroke and a power stroke, with the pistons located at BDC prior to the compression stroke, and at top-dead-center TDC prior to the power stroke. As used herein with reference to the two-piston embodiment, BDC may refer to the point at which the pistons are furthest from each other. As used herein with reference to the two-piston embodiment, TDC may refer to the point at which the pistons are closest to each other. When at or near BDC, and if the driver section is to be used to provide compression work, the pressure of the gas within the driver section <NUM> is greater than the pressure of the combustion section <NUM>, which forces the pistons <NUM> away from BDC and inwards towards each other, i.e., in the negative direction. The gas in the driver section <NUM> can be used to provide some or all of the energy required to perform a compression stroke. As described above, in some embodiments, the piston <NUM> may be forced away from BDC by any other suitable mechanism, including a mechanical spring, a magnetic spring, or any other suitable mechanism that may be used to provide compression work. While the LEM <NUM> may also provide some of the energy required to perform a compression stroke, in a preferred embodiment, when sufficient energy is being produced during combustion, enough energy may be stored in the driver section <NUM> such that LEM <NUM> need not convert any electrical energy into kinetic energy of the piston <NUM> because the energy stored in driver section <NUM> may be transferred to the piston to provide the requisite compression work. The LEM <NUM> may also extract energy during the compression stroke. For example, if the gas in the driver section <NUM> (or other suitable means as described above) provides excess energy for performing the compression stroke, the LEM <NUM> may convert a portion of the kinetic energy of the piston assembly <NUM> into electrical energy.

The amount of energy required to perform a compression stroke may depend on the desired compression ratio, the pressure and temperature of the combustion section <NUM> at the beginning of the compression stroke, and the mass of the piston assembly <NUM>. As described above, driver section <NUM> may provide all of the energy needed for the compression stroke so that no other energy input (from LEM <NUM> or any other source) is necessary. In some embodiments, some energy may be input during the compression stroke, but the net energy during the compression stroke is still positive. A compression stroke continues until combustion occurs, which typically occurs at a time when the velocities of the pistons <NUM> are at or near zero. Combustion causes an increase in the temperature and pressure within the combustion section <NUM>, which forces the pistons <NUM> outward toward the LEMs <NUM>. During a power stroke, a portion of the kinetic energy of the piston assembly <NUM> may be converted into electrical energy by the LEM <NUM> and another portion of the kinetic energy does compression work on the gas (or other compression mechanism) in the driver section <NUM>. Alternatively, all of the kinetic energy of the piston assembly may be stored in driver section <NUM>. A power stroke continues until the velocities of the pistons <NUM> are zero. After the power stroke and before the subsequent compression stroke, with pistons <NUM> at or near BDC, the engine may exhaust combustion products and intake air, an air/fuel mixture, or an air/fuel/combustion products mixture. This process may be referred to herein as "breathing" or "breathing at or near BDC. " It will be appreciated by those of ordinary skill in the art that breathing may be achieved in any suitable manner, such as uni-flow or cross-flow scavenging, as described in previously referenced <CIT>. It will also be appreciated that although described as occurring after the power stroke, in some embodiments breathing may occur during the end of the power stroke and/or the beginning of the compression stroke. Similarly, in some embodiments, combustion may occur during the end of the compression stroke and/or the beginning of the power stroke.

<FIG> illustrates one exemplary port configuration <NUM> in which the intake ports <NUM> and exhaust ports <NUM> are in front of both pistons near BDC. The opening and closing of the exhaust ports <NUM> and intake ports <NUM> may be independently controlled. The location of the exhaust ports <NUM> and intake ports <NUM> can be chosen such that a range of compression ratios and/or expansion ratios is possible. The times in a cycle when the exhaust ports <NUM> and intake ports <NUM> are activated (opened and closed) can be adjusted during and/or between cycles to vary the compression ratio and/or expansion ratio and/or the amount of combustion product retained in the combustion section <NUM> at the beginning of a compression stroke. Retaining combustion gases in the combustion section <NUM> is called residual gas trapping (RGT) and can be utilized to effect combustion timing, peak combustion temperatures, and other combustion and engine performance characteristics.

Although operation of a two-stroke cycle is described above, the embodiment of <FIG> may also be operated using a four-stroke piston cycle, which includes an intake stroke, a compression stroke, a power (expansion) stroke, and an exhaust stroke. In some embodiments, any suitable modification may be made to operate using a four-stroke piston cycle. For example, as described in the previously referenced <CIT>, the location of the ports may be modified to operate the engine using a four-stroke piston cycle.

In some embodiments, in a four-stroke piston cycle, just as in the two-stroke cycle described above, driver section <NUM> may provide all of the work necessary for the compression stroke. In some embodiments, the driver section <NUM> may provide enough work to avoid net electrical energy input during the compression stroke. The compression stroke may continue until combustion occurs, e.g., when the velocities of pistons <NUM> are at or near zero. Combustion may be followed by a power stroke, during which kinetic energy of the piston assemblies <NUM> may be stored in driver section <NUM> and/or converted into electrical energy by LEMs <NUM> as described above with respect to the two-stroke cycle. At some point at or near the power-stroke BDC, exhaust ports may be opened, and an exhaust stroke may occur until the velocities of pistons <NUM> are at or near zero, which marks the exhaust stroke TDC for that cycle. As described above, the energy stored in driver section <NUM> during the power stroke may provide the work required to perform the exhaust stroke. At some point prior to reaching exhaust stroke TDC, the combustion section <NUM> closes the exhaust valves while there is still exhaust in the cylinder. In some embodiments, this trapped exhaust gas may store enough energy to perform the subsequent intake stroke. As with the power stroke, the kinetic energy of the piston assemblies <NUM> may be stored in driver section <NUM> and/or converted into electrical energy by LEMs <NUM> during the intake stroke, which occurs until the velocities of the pistons <NUM> are at zero. In some embodiments, driver section <NUM> may store enough energy during the intake stroke to perform the subsequent compression stroke. In some embodiments, any suitable amount of energy stored in the driver section in excess of the amount required for a subsequent compression stroke or a subsequent exhaust stroke may be converted into electrical energy by LEMs <NUM>.

<FIG> is a cross-sectional drawing illustrating an alternative two-piston, separated gas springs, and separated LEM engine, in accordance with the principles of the disclosure. It will be understood that the illustrated configuration is merely for purposes of example, and that any other suitable configuration of a two-piston, separated gas springs, and separated LEM engine may be used in accordance with the present disclosure. Engine <NUM> includes a main cylinder <NUM>, two opposed piston assemblies <NUM>, and a combustion section <NUM> located in the center of main cylinder <NUM>. The illustrated engine <NUM> has certain physical differences when compared with engine <NUM>. Specifically, engine <NUM> includes a pair of outer cylinders <NUM> that contain additional pistons <NUM>, and the LEMs <NUM> are disposed between the main cylinder <NUM> and the outer cylinders <NUM>. Each outer cylinder <NUM> includes a driver section <NUM> located between the piston <NUM> and the distal end of the outer cylinder <NUM> and a driver back section <NUM> located between the piston <NUM> and the proximal end of the outer cylinder <NUM>. Main cylinder <NUM> includes a pair of combustion back sections <NUM> disposed between the pistons <NUM> and the distal ends of the main cylinder <NUM>. In some embodiments, the driver back section <NUM> and the combustion back section <NUM> are maintained at or near atmospheric pressure. In some embodiments, the driver back section <NUM> and the combustion back section <NUM> are not maintained at or near atmospheric pressure. In the illustrated configuration, the main cylinder <NUM> has ports <NUM> for removal of blow-by gas, injector ports <NUM>, intake ports <NUM>, and exhaust ports <NUM>. Driver gas exchange ports <NUM> are located in the outer cylinders <NUM>. Each piston assembly <NUM> includes two pistons <NUM> and a piston rod <NUM>. The piston assemblies are free to move linearly between the main cylinder <NUM> and the outer cylinders <NUM> as depicted in <FIG>. It will be understood that the embodiment of <FIG> can operate using a two-stroke piston cycle using, for example, the methodology as set forth above with respect to <FIG>, and a four-stroke piston cycle as described above and in previously referenced <CIT>.

The configuration of <FIG>, as shown, includes a single unit referred to as the engine <NUM> and defined by the cylinder <NUM>, the piston assemblies <NUM> and the LEMs <NUM>. Similarly, the configuration of <FIG>, as shown, includes a single unit referred to as the engine <NUM> and defined by the main cylinder <NUM>, the piston assemblies <NUM>, the outer cylinders <NUM>, and the LEMs <NUM>. However, multiple units can be placed in parallel, which could collectively be referred to as "the engine. " This type of modular arrangement in which engine units operate in parallel may be used to enable the scale of the engine to be increased as needed by the end user. Additionally, not all units need be the same size, operate under the same conditions (e.g., frequency, stoichiometry, or breathing), or operate simultaneously (e.g., one or several units could be deactivated while one or several other units operate). When the units are operated in parallel, there exists the potential for integration between the engines, such as, but not limited to, gas exchange between the units and/or feedback between the units' respective LEMs <NUM>.

<FIG> illustrate further embodiments featuring integrated internal gas springs in which the gas spring is integrated inside of the piston assembly and the LEM is separated from the combustor cylinder. As illustrated in <FIG>, the integrated internal gas spring (IIGS) architecture may be similar in length to the integrated gas spring with separated LEM architecture illustrated in <FIG>. However, the IIGS architecture may eliminate issues with respect to the blow-by gases from the combustion section entering the gas spring, which also occurs in the fully integrated gas spring and LEM architecture.

<FIG> is a cross-sectional drawing illustrating a single-piston, integrated internal gas spring engine, in accordance with some embodiments of the present disclosure. Many components such as the combustion section <NUM> are similar to the components in previous embodiments (e.g., <FIG> and <FIG> ), and are labeled accordingly. The engine <NUM> comprises a cylinder <NUM> with piston assembly <NUM> dimensioned to move within the cylinder <NUM> in response to reactions within combustion section <NUM> near the bottom end of the cylinder <NUM>. Piston assembly <NUM> comprises a piston <NUM>, piston seals <NUM>, and a spring rod <NUM>. The piston assembly <NUM> is free to move linearly within the cylinder <NUM>. In the illustrated embodiment, the piston rod <NUM> moves along bearings <NUM> and is sealed by piston rod seals <NUM> that are fixed to the cylinder <NUM>. The cylinder <NUM> includes exhaust/injector ports <NUM>, <NUM> for intake of air, fuel, exhaust gases, air/fuel mixtures, and/or air/exhaust gases/fuel mixtures, exhaust of combustion products, and/or injectors. Some embodiments do not require all of the ports depicted in <FIG>. The number and types of ports depends on the engine configuration, injection strategy, and piston cycle (e.g., two- or four-stroke piston cycles).

In the illustrated embodiment, the engine <NUM> further comprises an LEM <NUM> (including stator <NUM> and magnets <NUM>) for directly converting the kinetic energy of the piston assembly <NUM> into electrical energy. It will be understood that LEM <NUM> may be configured to operate substantially the same as LEMs <NUM> described above with respect to <FIG>.

With further reference to <FIG>, piston <NUM> comprises a solid front section (combustor side) and a hollow back section (gas spring side). The area inside of the hollow section of the piston assembly <NUM>, between the front face of piston <NUM> and spring rod <NUM>, comprises a gas that serves as the gas spring <NUM>, which provides at least some of the work required to perform a compression stroke. Piston <NUM> moves linearly within the combustor section <NUM> and the stator <NUM> of the LEM <NUM>. The piston's motion is guided by bearings <NUM>, <NUM>, which may be solid bearings, hydraulic bearings, and/or air bearings. In the illustrated embodiment, the engine <NUM> includes both external bearings <NUM> and internal bearings <NUM>. In particular, the external bearings <NUM> are located between the combustion section <NUM> and the LEM <NUM>, and the internal bearings <NUM> are located on the inside of the hollow section of the piston <NUM>. The external bearings <NUM> are externally fixed and do not move with the piston <NUM>. The internal bearings <NUM> are fixed to the piston <NUM> and move with the piston <NUM> against the spring rod <NUM>.

With continued reference to <FIG>, the spring rod <NUM> serves as one face for the gas spring <NUM> and is externally fixed. The spring rod <NUM> has at least one seal <NUM> located at or near its end, which serves the purpose of keeping gas within the gas spring section <NUM>. Magnets <NUM> are attached to the back of the piston assembly <NUM> and move linearly with the piston assembly <NUM> within the stator <NUM> of the LEM <NUM>. The piston assembly <NUM> may have seals to keep gases in the respective sections. The illustrated embodiment includes (i) front seals <NUM> that are fixed to the piston <NUM> at or near its front end to keep to gases from being transferred from the combustion section <NUM>, and (ii) back seals <NUM> that are fixed to the cylinder <NUM> and keep intake gases and/or blow-by gases from being transferred to the surroundings.

<FIG> is a cross-sectional drawing illustrating an embodiment of a gas spring rod, in accordance with some embodiments of the present disclosure. Specifically, the spring rod <NUM> includes a central lumen <NUM> that allows mass to be transferred between the gas spring section <NUM> to a reservoir section <NUM> that is in communication with the surroundings. The communication with the surroundings is controlled through a valve <NUM>. The amount of mass in the gas spring <NUM> may be regulated to control the pressure within the gas spring <NUM> in accordance with some embodiments of the present disclosure.

<FIG> is a cross-sectional drawing illustrating a two-piston, integrated internal gas springs engine, in accordance with some embodiments of the present disclosure. Most of the elements of the two-piston embodiment are similar to those of the single-piston embodiment of <FIG>, and like elements are labeled accordingly. In addition, the operating characteristics of the single- and two-piston embodiments are similar as described in previous embodiments, including all the aspects of the linear alternator, breathing, combustion strategies, etc..

As described above, a driver section may be implemented as a gas spring, and may include one or more other mechanisms as one of ordinary skill in the art would understand. Various implementations of driver sections will be described with reference to <FIG> below. It will be understood to one of ordinary skill in the art that any of the driver sections and associated mechanisms illustrated in <FIG> may be suitably implemented in the free-piston engines described in <FIG> or any other suitable free-piston engines with driver sections.

<FIG> is a cross-sectional drawing illustrating a gas spring with an inlet port with a passive valve (referred to as a "passive inlet port"), in accordance with some embodiments of the present disclosure. As depicted in <FIG>, gas spring <NUM> is in contact with piston assembly <NUM>. It will be understood that in some embodiments, piston assembly <NUM> may be a free-piston assembly in contact with a combustion section as described above with respect to <FIG>. As described above with respect to the driver sections depicted in <FIG>, gas spring <NUM> may be capable of storing energy and providing energy to displace piston assembly <NUM> without the use of combustion. For example, energy may be stored in gas spring <NUM> as a result of compression of the gas therein by piston assembly <NUM> during an expansion stroke, and the stored energy may be used to displace piston assembly <NUM> to perform the subsequent stroke, such as a compression stroke or an exhaust stroke.

In some embodiments, it may be desirable to adjust the operation of gas spring <NUM>. For example, in some embodiments, it may be desirable to adjust the pressure of the gas spring by adding or removing gas from the gas spring. Accordingly, as depicted in <FIG>, an intake manifold <NUM> may be configured to provide make-up gas <NUM> to gas spring <NUM> via an inlet port <NUM>. It will be understood that intake manifold <NUM> may be coupled to any suitable source of pressurized gas such as an air compressor, and that the pressure of said gas may be controlled by any suitable technique and mechanism. In some embodiments, the opening and closing of inlet port <NUM> may be dictated by the operation of a passive valve <NUM>. As depicted, valve <NUM> may be coupled to a mechanical spring <NUM>. In some embodiments, valve <NUM> may be biased to a closed position by mechanical spring <NUM> and may move to an open position based on changes in the pressure of the gas in gas spring <NUM> or the pressure of the gas in intake manifold <NUM>. For example, valve <NUM> may move to an open position when the force applied to back surface <NUM> of valve <NUM> is greater than the force applied to front surface <NUM> of valve <NUM>. It will be understood that the force applied to back surface <NUM> may be dependent on the pressure of the gas in intake manifold <NUM>, the area of back surface <NUM>, the spring constant associated with mechanical spring <NUM>, and the distance required to move the valve from the closed position to the open position, and the force applied to front surface <NUM> may be dependent on the pressure of gas in gas spring <NUM> and the area of front surface <NUM>. Accordingly, in some embodiments, when the pressure of the gas in gas spring reduces past a certain threshold minimum, mechanical spring <NUM> may "crack" and cause valve <NUM> to move to an open position, allowing make-up gas <NUM> to flow through inlet port <NUM> until the pressure of the gas in gas spring <NUM> is sufficient to cause valve <NUM> to move back to the closed position. As one of ordinary skill in the art would understand, the areas of front surface <NUM> and back surface <NUM>, the spring constant of mechanical spring <NUM>, and the distance required to move the valve from the closed position to the open position may be selected and/or designed to determine the relevant "cracking pressure" that may cause valve <NUM> to open as described above. It will be understood that the simplified mechanical spring shown in <FIG> is illustrative and in some embodiments, any suitable spring or springs may be used in place of or in addition thereto, including but not limited to one or more compression springs, tension springs, torsion springs, and any combination thereof. For example, the mechanical spring may include one or more coil or helical compression springs, one or more coil or helical tension springs, one or more coil or helical torsion springs, one or more leaf springs, any other suitable spring, and any suitable combination thereof.

<FIG> is a cross-sectional drawing illustrating a gas spring with an inlet port with an active valve (referred to as an "active inlet port"), in accordance with some embodiments of the present disclosure. Similar to <FIG> described above, <FIG> depicts gas spring <NUM> in contact with piston assembly <NUM>. As described above with respect to piston assembly <NUM> of <FIG>, in some embodiments, piston assembly <NUM> may be a free-piston assembly in contact with a combustion section as described above with respect to <FIG> and gas spring <NUM> may be capable of storing energy and providing energy to displace piston assembly <NUM> without the use of combustion.

Gas spring <NUM> may operate similarly to gas spring <NUM> described above, with an intake manifold <NUM> configured to provide make-up gas <NUM> to gas spring <NUM> via an inlet port <NUM>. In some embodiments, the opening and closing of inlet port <NUM> may be dictated by the operation of an active valve <NUM>. Contrary to valve <NUM> depicted in <FIG>, valve <NUM> may be configured to be actively actuated by force applied by any suitable actuator, including an electric actuator, mechanical actuator, or both. For example, an electric actuator may be coupled to a controller which may generate a control signal to cause the actuator to apply a force on valve <NUM> to move it from the closed position to the open position or from the open position to the closed position. In some embodiments, an optional mechanical spring <NUM> may be coupled to valve <NUM>, and may bias the valve to be in the open or closed position by default.

<FIG> is a cross-sectional drawing illustrating a gas spring with intake ports, in accordance with some embodiments of the present disclosure. Similar to <FIG> and <FIG> described above, <FIG> depicts driver section or gas spring <NUM> in contact with piston assembly <NUM>. As described above with respect to piston assembly <NUM> of <FIG>, in some embodiments, piston assembly <NUM> may be a free-piston assembly in contact with a combustion section as described above with respect to <FIG> and gas spring <NUM> may be capable of storing energy and providing energy to displace piston assembly <NUM> without the use of combustion. <FIG> depicts inlet ports <NUM> which may be utilized to provide gas to gas spring <NUM>. It will be understood that inlet ports <NUM> may be coupled to any suitable sources of pressurized gas such as a compressor, and that the pressure of the pressurized gas may be controlled by any suitable technique and mechanism. As will be understood by one of ordinary skill in the art, the flow of gas into gas spring <NUM> may be controlled by controlling the pressure of the gas provided at inlet ports <NUM>. For example, in some embodiments, gas may flow into gas spring <NUM> via inlet ports <NUM> if the pressure of the gas provided at inlet ports <NUM> is greater than the pressure of the gas in gas spring <NUM>. Accordingly, in some embodiments, the pressure of the gas in gas spring <NUM> may be detected by any suitable pressure sensor, and may be adjusted by controlling the pressure of gas provided via inlet ports <NUM>. As described above, in some embodiments, the driver section and the combustion section, and in turn, inlet ports <NUM> may be maintained at or near atmospheric pressure. In such embodiments, it will be understood that seals <NUM> may be optional because gas will not likely tend to leak past any clearances between piston assembly <NUM> and the surrounding housing. In some embodiments, however, the driver section, the combustion section, and inlet ports <NUM> need not be maintained at or near atmospheric pressure. In some embodiments, for example, if the inlet ports are maintained substantially above atmospheric pressure, seals <NUM> may be used to keep gas from wasting away from driver section <NUM> through any clearances between piston assembly <NUM> and the surrounding housing.

<FIG> is a cross-sectional drawing illustrating a gas spring with an adjustable head, in accordance with some embodiments of the present disclosure. Similar to <FIG>, <FIG> depicts driver section or gas spring <NUM> in contact with piston assembly <NUM>, inlet ports <NUM> which may be utilized to provide gas to gas spring <NUM>, and optional sealing elements <NUM> to keep gas from escaping. <FIG> also depicts an adjustable head <NUM> and corresponding sealing elements <NUM>. In some embodiments, adjustable head <NUM> may be configured to change the geometry of gas spring <NUM>. For example, adjustable head <NUM> may translate or otherwise extend or retract in the directions indicated by the arrows in order to increase or decrease the dead volume of gas spring <NUM>. The translation, extension, retraction, or other suitable transformation of adjustable head <NUM> may be controlled by a controller coupled to adjustable head <NUM>. It will be understood that by controlling the dead volume, the pressure of the gas in gas spring <NUM> may also be controlled, provided that gas is kept in gas spring <NUM> by use of sealing elements <NUM> and/or optional sealing elements <NUM>. Accordingly, adjustable head <NUM> may allow for additional control and adjustment of gas spring <NUM>, in accordance with some embodiments. It will be understood that the control and adjustment of the gas spring as described above may allow for control of the effective spring constant of the gas spring.

<FIG> is a cross-sectional drawing illustrating a gas spring with adjustable components, in accordance with some embodiments of the present disclosure. Similar to <FIG> and <FIG>, <FIG> depicts gas spring <NUM> in contact with piston assembly <NUM>, inlet ports <NUM> which may be utilized to provide gas to gas spring <NUM>, and optional sealing elements <NUM> to keep gas from escaping. <FIG> also depicts adjustable components <NUM>. Although shown for illustrative purposes as three components, it will be understood that any suitable number and configuration of adjustable components <NUM> may be used in accordance with embodiments of the present disclosure. In some embodiments, adjustable components <NUM> may be configured to change the geometry of gas spring <NUM>. For example, adjustable components <NUM> may be screws, bolts, lugs, or other mechanical structures that are configured to translate or otherwise extend or retract in the directions indicated by the arrows in order to increase or decrease the dead volume of gas spring <NUM>. The translation, extension, retraction, or other suitable transformation of adjustable components <NUM> may be controlled by a controller coupled to adjustable components <NUM>. It will be understood that by controlling the dead volume, the pressure of the gas in gas spring <NUM> may also be controlled. Accordingly, adjustable components <NUM> may allow for additional control and adjustment of gas spring <NUM>, in accordance with some embodiments. As noted above, it will be understood that the control and adjustment of the gas spring as described above may allow for control of the effective spring constant of the gas spring.

<FIG> is a diagram illustrating the position, force, and power profiles of a free-piston engine, in accordance with some embodiments of the present disclosure. As shown, the diagram illustrates exemplary position <NUM>, force <NUM>, and power <NUM> profiles over time for a free-piston engine with a two-stroke piston cycle including a compression stroke and a power stroke. With reference to position profile <NUM>, as labeled in <FIG>, for reference purposes, the positive direction corresponds to the direction from TDC to BDC. For example, in the free-piston assemblies of <FIG>, the centerline would correspond to the origin, and the direction away from the centerline would be the positive direction for each free-piston assembly. As can be seen by position profile <NUM>, the piston assembly starts the compression cycle at BDC and progresses to TDC, at which point the power cycle begins. During the power cycle, the piston assembly progresses back to BDC.

With reference to force profile <NUM>, the force is positive when applied in a direction from TDC to BDC. For example, in the free-piston assemblies of <FIG>, force applied in the direction away from the centerline would be a positive force. As can be seen in force profile <NUM>, during the compression cycle, a relatively constant positive force may be applied to the piston assembly, and during the power cycle, the force may be negative (in the direction towards the centerline). It will be understood that the force applied need not be constant, and that in some embodiments, a variable force profile may be applied, for example, to produce a relatively constant power output. It will also be understood that in some embodiments, and as depicted herein, forces may not be applied when the piston assembly velocity is relatively low, due to the inefficiency of doing so.

The power output is the negative product of the force and velocity of the piston assembly. Referring specifically to power profile <NUM>, it can be seen that, in the ideal case illustrated, no power need be input to the system in order to perform the compression and power strokes of the piston cycle. Rather, as described above, in the ideal case, there is sufficient energy stored in the at least one driver section during the power stroke to perform the subsequent compression stroke without additional energy input into the system during the compression stroke.

Although in an ideal scenario, it may be desirable to avoid any power input during operation of the compression and power strokes as described with respect to <FIG>, in some embodiments it may be necessary or desirable to provide some power input. Accordingly, <FIG> is another diagram illustrating the position, force, and power profiles of a free-piston engine, in accordance with some embodiments of the present disclosure. Similar to <FIG>, <FIG> illustrates exemplary position <NUM>, force <NUM>, and power <NUM> profiles over time for a free-piston engine with a two-stroke piston cycle including a compression stroke and a power stroke. While the position profile <NUM> is generally similar to that of position profile <NUM> illustrated in <FIG>, it will be understood that the force profile <NUM> and the power profile <NUM> may differ from those illustrated in <FIG>. With reference to force profile <NUM> during the compression stroke, it can be seen at <NUM> that a force may be applied in the opposite direction as originally applied for a brief period. This is also reflected in power profile <NUM>, where a negative power showing power input for the same brief period may be seen at <NUM>. While this force application and power input may occur for a number of reasons, in some embodiments, this may be done in order to control the speed of the piston assembly or otherwise ensure that the piston assembly reaches the appropriate TDC before the power stroke. For example, a force may be applied to increase the speed of the piston assembly. Similarly, with further reference to force profile <NUM> during the power stroke, it can be seen at <NUM> that a force may be applied in the opposite direction as the rest of the power stroke for a brief period, which is also reflected in power profile <NUM>, where a negative power showing power input for the same brief period may be seen at <NUM>. As described above, this applied force and input power may occur for a number of reasons, but in some embodiments, force may be applied in this way and power input in order to control the speed of the piston assembly or otherwise ensure that the piston assembly reaches the appropriate BDC point before the subsequent compression stroke. For example, a force may be applied to increase the speed of the piston assembly as described above.

Although the provision of input power during compression and or power stroke described with respect to <FIG> is not necessarily ideal operation, it will be understood that the net electrical energy output over each stroke is still greater than zero (i.e., there is no net electrical energy input over each stroke). This is evident from power profile <NUM>, in which it can be seen that the integral over each stroke, represented by the area of the curve above zero subtracted by the area of the curve below zero, is substantially greater than zero. Accordingly, the amount of electrical energy output by the system over each stroke is greater than the electrical energy input to control the piston assembly position as described above. As used herein, the "net electrical energy" refers to the electrical energy transfer into or out of the LEM such as that described above with respect to <FIG>. In some embodiments, the LEM may include a stator coupled to power electronics (including, e.g., any DC bus, IGBTs, and/or any other suitable components) and/or a grid-tie inverter. Accordingly, in some embodiments, while some electrical energy may be input into the LEM via power electronics and/or a grid-tie inverter coupled to the LEM, the net electrical energy over a given stroke as described above would be output from the LEM to the power electronics and/or grid-tie inverter.

As stated, the embodiment described above with respect to <FIG> includes a two-piston, single-combustion section, two-stroke internal combustion engine <NUM>. Described below, and illustrated in the corresponding figures, is a control system applicable to a free-piston combustion engine generally. Accordingly, as described above, the control system is applicable to other free-piston combustion engine architectures, such as those described in the previously referenced <CIT>. As would be appreciated by those of ordinary skill in the art, various modifications and alternative configurations may be utilized, and other changes may be made, without departing from the scope of the disclosure. For example, in addition to the two-piston architectures described above with respect to <FIG><NUM>, the control system described herein is applicable to, for example, single-piston architectures. Similarly, in addition to the two-stroke engine described above with respect to <FIG>, the control system described herein is also applicable to, for example, four-stroke engines.

It will be understood from the above disclosure that the driver section may be configured (e.g., including by way of control circuitry) to avoid any need for electrical energy input or net electrical energy input during a stroke subsequent to an expansion stroke, from, for example, an LEM. As opposed to avoiding the use of an LEM for energy input coincidentally only under certain conditions, in some embodiments, the free-piston engine may be specifically configured for the purpose of avoiding net energy input during the stroke that occurs following an expansion stroke (e.g., the compression stroke following a power stroke). In some embodiments, the free-piston engine may be specifically configured to necessarily cause the stroke following an expansion stroke to be performed without net electrical energy input.

<FIG> is a block diagram of an illustrative piston engine system <NUM> having control system <NUM> for a piston engine <NUM>, in accordance with some embodiments of the present disclosure. Piston engine <NUM> may be, for example, any suitable free-piston engine as described above with respect to <FIG>. Control system <NUM> may communicate with one or more sensors <NUM> coupled to piston engine <NUM>. Control system <NUM> may be configured to communicate with auxiliary systems <NUM>, which may be used to adjust aspects or properties of piston engine <NUM>. In some embodiments, more than one piston engine may be controlled by control system <NUM>. For example, control system <NUM> may be configured to communicate with auxiliary systems and sensors corresponding to any number of piston engines. In some embodiments, control system <NUM> may be configured to interact with a user via user interface system <NUM>.

Control system <NUM> may include processing equipment <NUM>, communications interface <NUM>, sensor interface <NUM>, control interface <NUM>, any other suitable components or modules, or any combination thereof. Control system <NUM> may be implemented at least partially in one or more integrated circuits, ASIC, FPGA, microcontroller, DSP, computers, terminals, control stations, handheld devices, modules, any other suitable devices, or any combination thereof. In some embodiments, the components of control system <NUM> may be communicatively coupled via individual communications links or a communications bus <NUM>, as shown in <FIG>. Processing equipment <NUM> may include any suitable processing circuitry, such as one or more processors (e.g., a central processing unit), cache, random access memory (RAM), read only memory (ROM), any other suitable hardware components or any combination thereof that may be configured (e.g., using software, or hard-wired) to process information regarding piston engine <NUM>, as received by sensor interface <NUM> from sensor(s) <NUM>. Sensor interface <NUM> may include a power supply for supplying power to sensor(s) <NUM>, a signal conditioner, a signal pre-processor, any other suitable components, or any combination thereof. For example, sensor interface <NUM> may include a filter, an amplifier, a sampler, and an analog to digital converter for conditioning and pre-processing signals from sensor(s) <NUM>. Sensor interface <NUM> may communicate with sensor(s) <NUM> via communicative coupling <NUM>, which may be a wired connection (e.g., using IEEE <NUM> ethernet, or universal serial bus interface), wireless coupling (e.g., using IEEE <NUM> "Wi-Fi", or Bluetooth), optical coupling, inductive coupling, any other suitable coupling, or any combination thereof. Control system <NUM>, and more particularly processing equipment <NUM>, may be configured to provide control of piston engine <NUM> over relevant time scales. For example, a change in one or more temperatures may be controllable in response to one or more detected engine operating characteristics, and the control may be provided on a time scale relevant to operation of the piston engine (e.g., fast enough response to prevent overheating and/or component failure, to adequately provide apex control as described below, to allow for shutdown in the case of a diagnostic event, and/or for adequate load tracking).

Sensor(s) <NUM> may include any suitable type of sensor, which may be configured to sense any suitable property or aspect of piston engine <NUM>. In some embodiments, sensor(s) may include one or more sensors configured to sense an aspect and/or property of a system of auxiliary systems <NUM>. In some embodiments, sensor(s) <NUM> may include a temperature sensor (e.g., a thermocouple, resistance temperature detector, thermistor, or optical temperature sensor) configured to sense the temperature of a component of piston engine <NUM>, a fluid introduced to or recovered from piston engine <NUM>, or both. In some embodiments, sensor(s) <NUM> may include one or more pressure sensors (e.g., piezoelectric pressure transducers, strain-based pressure transducers, or gas ionization sensors) configured to sense a pressure within a section of piston engine <NUM> (e.g., a combustion section, or gas driver section), of a fluid introduced to or recovered from piston engine <NUM>, or both. In some embodiments, sensor(s) <NUM> may include one or more force sensors (e.g., piezoelectric force transducers or strain-based force transducers) configured to sense a force within piston engine <NUM> such as a tensile, compressive or shear force (e.g., which may indicate a friction force or other relevant force information, pressure information, or acceleration information). In some embodiments, sensor(s) <NUM> may include one or more current and/or voltage sensors (e.g., an ammeter and/or voltmeter coupled to a LEM of piston engine <NUM>) configured to sense a voltage, current, power output and/or input (e.g., current multiplied by voltage), any other suitable electrical property of piston engine <NUM> and/or auxiliary systems <NUM>, or any combination thereof. In some embodiments, sensor(s) <NUM> may include one or more sensors configured to sense the position of the piston assembly and/or any other components of the engine, the speed of the piston assembly and/or any other components of the engine, the acceleration of the piston assembly and/or any other components of the engine, the rate of flow, oxygen or nitrogen oxide emission levels, other emission levels, any other suitable property of piston engine <NUM> and/or auxiliary systems <NUM>, or any combination thereof.

Control interface <NUM> may include a wired connection, wireless coupling, optical coupling, inductive coupling, any other suitable coupling, or any combination thereof, for communicating with one or more of auxiliary systems <NUM>. In some embodiments, control interface <NUM> may include a digital to analog converter to provide an analog control signal to any or all of auxiliary systems <NUM>.

Auxiliary systems <NUM> may include a cooling system <NUM>, a pressure control system <NUM>, a gas driver control system <NUM>, and/or any other suitable control system <NUM>. Cooling/heating system <NUM> may include a pump, fluid reservoir, pressure regulator, bypass, radiator, fluid conduits, electric power circuitry (e.g., for electric heaters), any other suitable components, or any combination thereof to provide cooling, heating, or both to piston engine <NUM>. Pressure control system <NUM> may include a pump, compressor, fluid reservoir, pressure regulator, fluid conduits, any other suitable components, or any combination thereof to supply (and optionally receive) a pressure controlled fluid to piston engine <NUM>. Gas driver control system <NUM> may include a compressor, gas reservoir, pressure regulator, fluid conduits, any other suitable components, or any combination thereof to supply (and optionally receive) a driver gas to piston engine <NUM>. In some embodiments, gas driver control system may include any suitable components to control any of the gas spring components described above with respect to <FIG>. In some embodiments, other system <NUM> may include a valving system such as, for example, a cam-operated system or a solenoid system to supply oxidizer and/or fuel to piston engine <NUM>.

User interface <NUM> may include a wired connection, wireless coupling, optical coupling, inductive coupling, any other suitable coupling, or any combination thereof, for communicating with one or more of user interface systems <NUM>. User interface systems <NUM> may include display <NUM>, input device <NUM>, mouse <NUM>, audio device <NUM>, a remote interface accessed via website, mobile application, or other internet service, any other suitable user interface devices, or any combination thereof. In some embodiments, a remote interface may be remote from the engine but in proximity to the site of the engine. In other embodiments, a remote interface may be remote from both the engine and the site of the engine. Display <NUM> may include a display screen such as, for example, a cathode ray tube screen, a liquid crystal display screen, a light emitting diode display screen, a plasma display screen, any other suitable display screen that may provide graphics, text, images or other visuals to a user, or any combination of screens thereof. In some embodiments, display <NUM> may include a touchscreen, which may provide tactile interaction with a user by, for example, offering one or more soft commands on a display screen. Display <NUM> may display any suitable information regarding piston engine <NUM> (e.g., a time series of a property of piston engine <NUM>), control system <NUM>, auxiliary systems <NUM>, user interface system <NUM>, any other suitable information, or any combination thereof. Input device <NUM> may include a QWERTY keyboard, a numeric keypad, any other suitable collection of hard command buttons, or any combination thereof. Mouse <NUM> may include any suitable pointing device that may control a cursor or icon on a graphical user interface displayed on a display screen. Mouse <NUM> may include a handheld device (e.g., capable of moving in two or three dimensions), a touchpad, any other suitable pointing device, or any combination thereof. Audio device <NUM> may include a microphone, a speaker, headphones, any other suitable device for providing and/or receiving audio signals, or any combination thereof. For example, audio device <NUM> may include a microphone, and processing equipment <NUM> may process audio commands received via user interface <NUM> caused by a user speaking into the microphone.

In some embodiments, control system <NUM> may be configured to receive one or more user inputs to provide control. For example, in some embodiments, control system <NUM> may override control settings based on sensor feedback, and base a control signal to auxiliary system <NUM> on one or more user inputs to user interface system <NUM>. In a further example, a user may input a set-point value for one or more control variables (e.g., temperatures, pressures, flow rates, work inputs/outputs, or other variables) and control system <NUM> may execute a control algorithm based on the set-point value.

In some embodiments, operating characteristics (e.g., one or more desired property values of piston engine <NUM> or auxiliary systems <NUM>) may be pre-defined by a manufacturer, user, or both. For example, particular operating characteristics may be stored in memory of processing equipment <NUM>, and may be accessed to provide one or more control signals. In some embodiments, one or more of the operating characteristics may be changed by a user. Control system <NUM> may be used to maintain, adjust, or otherwise manage those operating characteristics.

As described above, in some implementations, the driver section may be configured to store a particular amount of energy during an expansion stroke of the engine. In some embodiments, as described above, the driver section may be configured to store enough energy during expansion to provide the energy required for a subsequent stroke, i.e. the stroke that occurs after the expansion stroke. For example, in an engine with a two-stroke cycle, the driver section may be configured to store enough energy during expansion to provide the energy required for a subsequent compression stroke. In an engine with a four-stroke cycle, for example, the driver section may be configured to store enough energy during the expansion stroke to provide the energy required for a subsequent exhaust stroke. In some embodiments, the driver section may be configured to store more than the amount required for a subsequent stroke. In some embodiments, the excess amount of energy, or a portion of the excess amount of energy, stored in the driver section may be converted by one or more LEMs into electrical energy during the subsequent stroke. For example, one or more LEMs may be configured to extract work during the power stroke of the free-piston combustion engine by converting a portion of the kinetic energy of the piston assembly into electrical energy. In some embodiments, the one or more LEMs may be further configured to extract at least some of the work provided by a driver section during the compression stroke of the free-piston combustion engine. That is, the potential energy stored in the driver section during the expansion stroke is converted into kinetic energy of the piston assembly during the subsequent stroke. At least some of this kinetic energy may be converted during the subsequent stroke into electrical energy by one or more LEMs. It will be understood, as described above, that when the LEMs are configured to extract electrical energy during expansion strokes and the subsequent strokes, they may be reduced in size and/or weight, thereby saving on material weight and costs.

In some implementations, the amount and manner of energy stored in the driver section and energy extracted by the LEMs may be controlled by, for example, control system <NUM>. For example, sensors <NUM> may be used to measure any one or more operating characteristics of the free-piston combustion engine, such as the position of the piston assembly, the speed of the piston assembly, the acceleration of the piston assembly, the pressure in the combustion section, the temperature of the combustion section, the potential energy of the combustion section, the chemical energy in the combustion section, the pressure in the driver section (e.g., the pressure of driver gas or the pressure of springs used as the driver section as described above), the potential energy of the driver section (e.g., the potential energy of the driver gas or the force of the springs used as the driver section as described above), the temperature of gas in the driver section, electric output, indicated work of the combustor or the driver section, the electrical efficiency, the indicated efficiency of the combustor or the driver section, the temperature of the LEM (e.g., stator or magnets), the combustor air flow rate, the combustor fuel flow rate, the driver section make-up air flow rate, the temperature of the piston assembly, the previous cycle performance, environmental temperature and pressure (e.g., the temperature and pressure of areas surrounding the engine), emissions characteristics, any other suitable characteristic, or any suitable combination thereof. Using sensor interface <NUM>, control system <NUM> may generate one or more signals indicative of the sensed one or more characteristics to be input into processing equipment <NUM>.

Processing equipment <NUM> may generate one or more control signals based at least in part on the signals received from sensors <NUM> and sensor interface <NUM>. In some embodiments, the processing equipment <NUM> may determine the amount of energy required for a given piston stroke based on signals received from sensors <NUM> and sensor interface <NUM>, and control signals may be used by processing equipment <NUM> to control the amount of kinetic energy of the piston assembly to be stored in the driver section as potential energy. Processing equipment <NUM> may also determine how much of the kinetic energy of the piston assembly to convert into electrical energy and cause that conversion to occur using any suitable control mechanism. As used herein, the term "control mechanism" may refer to any suitable software, hardware, and technique for controlling of any of the aforementioned operating characteristics and any suitable combination thereof to obtain the desired outcome. For example, the one or more control signals may control operating characteristics of the engine in order to store, in the driver section, the requisite energy for a subsequent stroke that was determined to be needed by the processing equipment <NUM>. For example, the one or more control signals may control the operating characteristics of the engine in order to cause the desired amount of kinetic energy of the piston assembly to be stored in the driver section during an expansion stroke of the engine, and subsequently to cause the desired amount of kinetic energy of the piston assembly to be converted into electrical energy by the LEM. As described above, the amount of energy required for a subsequent stroke (e.g., either a compression or exhaust stroke), may depend on the desired compression ratio, the pressure and temperature of the combustion section at the beginning of the subsequent stroke, the mass of the piston assembly, the desired combustion timing, barometric pressure, ambient temperature, and desired phasing characteristics with respect to other engines. The amount of kinetic energy to be converted into electrical energy may be determined based on a difference between the amount stored in the driver section during the expansion stroke and the amount needed for the subsequent stroke, which may depend at least in part on desired parameters associated with the engine. In some embodiments, the amount of kinetic energy to be converted into electrical energy may be determined based on the desired power output from the engine, the desired emissions output from the engine, the desired efficiency of the engine, the desired load tracking, any other desired parameter, or any suitable combination thereof. For example, if the driver section becomes less efficient, the amount of kinetic energy converted into electrical energy during the power stroke may be increased, and the amount of kinetic energy converted into electrical energy during the compression stroke may be decreased. Alternatively, for example, if the driver section becomes more efficient, the amount of kinetic energy converted into electrical energy during the power stroke may be decreased, and the amount of kinetic energy converted into electrical energy during the subsequent stroke may be increased.

In addition to controlling an amount of kinetic energy of the piston assembly to convert into electrical energy, the control signals may be used to control the manner in which the LEM converts kinetic energy into electrical energy. For example, the control signal may cause the conversion to take place in either direction at a constant rate, a non-constant rate, a variable rate or any combination thereof.

In some implementations, one or more parameters of the free-piston combustion engine may be used by processing equipment <NUM> to determine the amount of work to extract during the compression stroke of the engine. In some embodiments, the desired parameter may be input by a user via user interface system <NUM>. For example, a user may input a desired power output for the free-piston combustion engine via user interface system <NUM>. In other embodiments, a desired parameter may be received from an external device via communications interface <NUM>. For example, desired power output may be received from an external device indicating a desired power output based on historical power requirements, future forecasted power requirements, or any suitable combination thereof.

In some embodiments, processing equipment <NUM> may determine one or more operating characteristics of the engine that yield the desired parameter based on any suitable relationship between the parameter and the one or more operating characteristics. For example, processing equipment <NUM> may determine the velocity, acceleration, or other operating characteristic of the piston(s) based on the desired power output and the relationship of the operating characteristic to the desired power output. Processing equipment <NUM> may then determine the amount of compression work required to generate the operating characteristics determined by processing equipment <NUM>. Based on the required amount of compression work, processing equipment <NUM> may control the engine to extract a suitable amount of work during the compression stroke of the engine such that the remaining compression work acting on the piston will yield the desired operating characteristic or characteristics, which will in turn yield the desired power output. Although the embodiments are described above in terms of a desired power output, as described above, the processing equipment may optimize operating characteristics of the engine based on a desired efficiency, a desired emission output, desired load tracking, or any other suitable parameter of the engine.

In some embodiments, the aforementioned work extraction, engine parameters, and operating characteristics may be coordinated amongst several piston engines controlled by control system <NUM>. For example, kinetic energy of one piston engine may be converted into electrical energy and the resulting electrical energy may be converted into kinetic energy of another piston engine based on the desired engine parameters, the corresponding operating characteristics, and the amount of work required for compression and/or exhaust strokes.

Although embodiments are described above in terms of work extraction during a compression stroke or exhaust stroke of the free-piston combustion engine, it will be readily understood by those with skill in the art that in some embodiments the conversion of kinetic energy to electric energy and electric energy to kinetic energy may be more generally applied by control system <NUM>. In some embodiments, kinetic energy of the piston may be converted into electric energy continuously during operation of the engine, irrespective of the stroke or cycle of the engine. In some embodiments, kinetic energy of the piston assemblies may be converted into electrical energy continuously during operation of the engine, irrespective of the stroke or cycle of the engine. In other embodiments, the control system <NUM> may apply an arbitrary force on one or more piston assemblies of the engine based on any desired engine parameter or operating characteristic, and irrespective of any desired or required work extraction. For example, the control system <NUM> may control operation characteristics of the engine to apply forces on two pistons in order to synchronize the pistons such that they reach TDC and/or BDC at substantially the same time. As another example, the control system <NUM> may control operation characteristics of the engine to apply forces on pistons in order to phase separate engines such that they do not simultaneously operate at the same engine cycles in order to provide for a more continuous power flow. As another example, the control system <NUM> may control operation characteristics of the engine to obtain a desired apex point of the piston.

<FIG> shows a flow diagram <NUM> of illustrative steps for controlling a free-piston engine in accordance with some embodiments of the present disclosure. It will be understood that the foregoing steps may be implemented with any suitable free-piston engine and/or free-piston engine systems or components thereof as described above with respect to <FIG>, or any other suitable free-piston engine or free-piston engine systems.

Step <NUM> includes receiving engine operating characteristics from sensors. In some embodiments, engine operating characteristics may be received by processing equipment <NUM> or any processing circuitry thereof from sensors <NUM> via sensor interface <NUM> as described above with respect to <FIG>. In some embodiments, engine operating characteristics may include any of the operating characteristics described above or any suitable combination thereof. For example, processing equipment <NUM> may receive the compression ratio, the pressure and temperature of the combustion section, and the mass of the piston assembly. In some embodiments, processing equipment <NUM> may receive engine operating characteristics that provide information regarding the kinetic energy of the piston assembly from sensors <NUM> via sensor interface <NUM> as described above. In some embodiments, processing equipment <NUM> may receive engine operating characteristics that provide information regarding the amount of energy that can be stored in the driver section from sensors <NUM> via sensor interface <NUM> as described above.

Step <NUM> includes generating at least one control signal based on the operating characteristics received in step <NUM>. In some embodiments, processing equipment <NUM> or any processing circuitry thereof may generate one or more control signals based on the operating characteristics received in step <NUM>. For example, processing equipment <NUM> may generate control signals usable to adjust any of the aspects or properties of piston engine <NUM> discussed above with respect to <FIG> required in order to store the requisite amount of energy in the driver section to perform a subsequent stroke of a piston cycle In some embodiments, processing equipment <NUM> or any processing circuitry thereof may generate control signals to cause the driver section of piston engine <NUM> to store a sufficient amount of energy during an expansion stroke of the piston cycle for the purpose of avoiding net electrical energy input over the subsequent stroke of the piston cycle. In some embodiments, processing equipment <NUM> or any processing circuitry thereof may generate control signals that necessarily cause the driver section of piston engine <NUM> to store a sufficient amount of energy during an expansion stroke of the piston cycle to perform the subsequent stroke of the piston cycle without net electrical energy input over the subsequent stroke. In some embodiments, the subsequent stroke may comprise a compression stroke. In some embodiments, the subsequent stroke may comprise an exhaust stroke.

In some embodiments, processing equipment may receive any of the operating characteristics described above and generate control signals in steps <NUM> and <NUM> in a manner that takes into account changes in operating characteristics over time. For example, processing equipment may receive the position, speed, and/or acceleration of the piston assembly over time and generate control signals to adjust the operating characteristics accordingly. In some embodiments, processing equipment may receive engine operating characteristics that provide information regarding the kinetic energy of the piston assembly from sensors <NUM> via sensor interface <NUM> as described above on a periodic basis and generate updated control signals accordingly. In some embodiments, processing equipment may receive engine operating characteristics that provide information regarding the amount of energy that can be stored in the driver section from sensors <NUM> via sensor interface <NUM> as described above on a periodic basis and determine updated control signals accordingly. In some embodiments, relevant operating characteristics may be received and control signals may be generated at any suitable frequency such that the changes in operating characteristics over time can be taken into account before the subsequent stroke occurs. For example, the receipt and analysis of operating characteristics may occur at frequencies allowing for evaluation of the operating characteristics multiple times per stroke (e.g. <NUM> to <NUM>).

In some embodiments, processing equipment may take into account losses expected to occur in the energy storage and conversion process in generating any of the control signals in step <NUM>. For example, processing equipment may determine the amount of energy required for a subsequent stroke or the amount to be stored in the driver section based on known or predictable frictional losses, heat losses, or any other suitable losses associated with the energy storage and/or conversion. In some embodiments, processing equipment may allow for unexpected losses in generating any of the control signals in step <NUM>. For example, the processor may add a buffer amount of energy to account for unexpected losses during performance of the subsequent stroke when determining the amount of energy required for the subsequent stroke. As another example, the processor may add a buffer amount of energy to account for unexpected losses during storage of energy in the driver section during the expansion stroke when determining the amount of energy to be stored in the driver section during the expansion stroke.

Step <NUM> includes causing an amount of energy to be stored in the driver section during the expansion stroke based on one or more of the control signals generated in step <NUM>. In some embodiments, processing equipment <NUM> or any processing circuitry thereof may communicate control signals to any of the auxiliary systems <NUM> via control interface <NUM> in order to adjust the aspects or properties of piston engine <NUM> so that the requisite amount of energy is stored in the driver section during the expansion stroke. For example, control signals may act to adjust the pressure of the driver section by instructing gas driver control system <NUM> to add or remove gas to the driver section via an inlet gas port in order to store an amount of energy in the driver section during the expansion stroke. In some embodiments, control signals may act to adjust the dead volume of the cylinder by adjusting the settings of any of auxiliary systems <NUM>. In some embodiments, control signals may act to adjust any suitable properties of a gas spring using any of the mechanisms described above with respect to <FIG>. In some embodiments, as described above with respect to steps <NUM> and <NUM>, processing equipment may generate control signals and communicate with the piston engine and/or auxiliary systems thereof with any suitable frequency such that the changes in operating characteristics over time can be taken into account before the subsequent stroke occurs. For example, processing equipment may generate control signals and communicate with the piston multiple times per stroke to ensure response to changing operating characteristics.

Step <NUM> includes causing an amount of kinetic energy of the piston assembly to be converted into electrical energy based on one or more of the control signals generated in step <NUM>. In some embodiments, processing equipment <NUM> or any processing circuitry thereof may determine an amount of kinetic energy of the at least one free-piston assembly to convert into electrical energy and may cause at least one LEM to convert an amount of kinetic energy of the free-piston assembly into electrical energy based thereon. In some embodiments, processing equipment <NUM> may cause at least one LEM to directly convert an amount of kinetic energy of the at least one free-piston assembly into electrical energy during an expansion stroke of the piston cycle. In some embodiments, one or more processors of processing equipment <NUM> may cause at least one LEM to convert kinetic energy of the at least one free-piston assembly into electrical energy during the subsequent stroke of the piston cycle. For example, processing equipment <NUM> may cause at least one LEM to convert kinetic energy of the at least one free-piston assembly into electrical energy during any of the expansion stroke, the compression stroke, the exhaust stroke, the intake stroke, or any combination thereof. For example, one or more processors of processing equipment <NUM> may cause at least one LEM to convert the same amount of kinetic energy of the at least one free-piston assembly into electrical energy during both the expansion stroke and the subsequent stroke of the piston cycle. In some embodiments, the amount of kinetic energy converted into electrical energy by the at least one LEM may be determined such that it accounts for at least a predetermined minimum percentage of a total output power of the free-piston engine. In some embodiments, the amount of kinetic energy converted into electrical energy by the at least one LEM may be determined in order to maximize at least one of engine efficiency, engine power output, and engine emissions. In some embodiments, the amount of kinetic energy converted into electrical energy by the at least one LEM may be based on a difference between the first amount of energy stored in step <NUM> and the amount of energy required for the subsequent stroke. For example, if the amount of energy stored in step <NUM> exceeds the amount of energy required for the subsequent stroke, the amount of kinetic energy converted into electrical energy by the at least one LEM may be equal to or otherwise based on the excess stored amount.

Step <NUM> includes causing the subsequent stroke following the expansion stroke to be performed without net electrical energy input. In some embodiments, the energy stored in the driver section during the expansion stroke may provide at least some of the energy required for the subsequent stroke. In some embodiments, the energy stored in the driver section during the expansion stroke may provide all of the energy required for the subsequent stroke, such that no electrical energy input is needed for the subsequent stroke. In some embodiments, some electrical energy may be input during the subsequent stroke, but not so much as to amount to net electrical input over the subsequent stroke. For example, as described above with respect to <FIG>, energy may be input in order to increase the speed of the piston assembly or otherwise ensure that the piston assembly reaches a desired position. In some embodiments, the subsequent stroke may be a compression stroke. In some embodiments, the subsequent stroke may be an exhaust stroke.

As shown in <FIG>, steps <NUM> through <NUM> may be repeated for each piston cycle. In some embodiments, any or all of steps <NUM> through <NUM> may be repeated for each piston cycle over a consecutive number of piston cycles such that the engine operates continuously across the consecutive number of piston cycles without net electrical energy input. For example, after receiving electrical input from an LEM for start-up of the engine, steps <NUM> through <NUM> may be repeated for each piston cycle over a consecutive number of piston cycles to store enough energy in the driver section during each expansion stroke so as to avoid any further input from the LEM during the consecutive number of piston cycles. In some embodiments, steps <NUM> through <NUM> may be repeated such that operating conditions are continually being checked and the various amounts of energy to be stored and/or converted are continually updated to ensure that no external electrical energy input is needed.

It will be understood that while the processing equipment is able to determine values that correspond to amounts of energy to be stored in the driver section, the amounts that are actually stored in some cases may not be exactly as determined due to unforeseen engine losses, tolerances, environmental factors, or any other suitable condition. It is expected, however, that the actual stored amounts will be sufficiently close to the calculated values so that operation of the engine will only be minimally affected, if at all. As described above, in some embodiments, the processing equipment may account for these unknown losses or other suitable conditions by including buffers in the various amounts of energy to be stored.

For ease of reference, the figures may show multiple components labeled with identical reference numerals. It will be understood that this does not necessarily indicate that the multiple components identically labeled are identical to one another. For example, the pistons labeled <NUM> may have different sizes, geometries, materials, any other suitable characteristic, or any combination thereof.

Claim 1:
A free-piston combustion engine system, comprising:
a cylinder (<NUM>) comprising a combustion section (<NUM>);
at least one free-piston assembly in contact with the combustion section (<NUM>);
at least one gas spring (<NUM>) in contact with the at least one free-piston assembly, wherein the at least one gas spring (<NUM>) is configured to store energy from the at least one free-piston assembly during an expansion stroke of a piston cycle;
at least one linear electromagnetic machine for directly converting between kinetic energy of the at least one free-piston assembly and electrical energy; and
processing circuitry configured to:
determine a sufficient amount of energy for performing a subsequent stroke, and
to cause the at least one gas spring (<NUM>) to store at least the determined amount of energy from the at least one free-piston assembly during the expansion stroke to perform the subsequent stroke of the piston cycle by controlling one or more properties of the at least one gas spring (<NUM>) to avoid net electrical energy input, wherein the linear electromagnetic machine is configured to convert at least some of the energy stored in the gas spring (<NUM>) into electrical energy during the subsequent stroke of the piston cycle.