Control of the piston trajectory in a free-piston combustion engine

Abstract

To provide the suitable control of a piston trajectory in a free piston combustion engine.SOLUTION: In a variety of embodiments in this disclosure, a free piston combustion engine is set as an object. As explained in this specification, a free piston assembly is displaced by a method and a system, a position / force trajectory is repeatedly determined over a propagation path, and desired engine performance can be achieved by displacing the free piston assembly on the basis of the position / force trajectory at least partially. In the double-piston assembly free piston engine, two piston assemblies are synchronized with each other.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] This disclosure relates to a free-piston combustion engine, and more specifically, to the control of the piston trajectory within a free-piston combustion engine. [Background technology]

[0002] Some free-piston engines rely on piston position-time control, where the desired position-time trajectory of the piston is determined based on the piston's initial position. As the system moves the piston, the control strategy measures the degree to which the piston deviates from the desired position-time trajectory and attempts to compensate for any deviations to bring the piston closer to the desired trajectory. Some free-piston engines rely on a control strategy that measures the degree to which the piston deviates from other preferred trajectories (e.g., position-velocity) and attempts to compensate for any deviations to bring the piston closer to the desired trajectory.

[0003] These approaches typically rely on open-form solutions to control piston movement based on a previously determined trajectory, often failing to consider changing conditions within the engine that might affect piston movement. For example, after a desired trajectory has been determined, conditions within the engine may change to the point where the desired trajectory is no longer applicable. However, piston movement would still be based on the original desired trajectory and any deviations therefrom. [Overview of the project] [Means for solving the problem]

[0004] Various embodiments of this disclosure relate to the steps of controlling a free-piston linear combustion engine. In at least one embodiment, the engine comprises (i) a cylinder having a combustion chamber, (ii) at least one free-piston assembly in contact with the combustion chamber, (iii) at least one drive chamber in contact with the at least one free-piston assembly for storing energy during the expansion stroke of the engine, and (iv) at least one linear electromagnetic machine (LEM) for directly converting the kinetic energy of the at least one free-piston assembly into electrical energy. However, it should be noted that further embodiments may include various combinations of the features and physical properties identified above.

[0005] This disclosure relates to control techniques for determining and implementing trajectories for one or more piston assemblies in a free-piston engine. As used herein, the term “trajectory” refers to a set of data pairs representing the motion of a piston assembly in a free-piston engine, such as a position-force trajectory (a set of position-force pairs), a time-position trajectory (a set of time-position pairs), or a position-velocity trajectory (a set of position-velocity pairs). A position-force trajectory defines the forces acting on a piston assembly at a specified position in one or more of the piston assemblies; a time-position trajectory defines the position of a piston assembly at a specified time instant in one or more of the piston assemblies; and a position-velocity trajectory defines the velocity of a piston assembly at a specified position in one or more of the piston assemblies. At least one of the elements in the data pair of trajectories can be considered a cross coordinate in a functional relationship with the other data element, which is a vertical coordinate. In the case of multiple free-piston assemblies in one engine (for example, arranged as opposing pistons with a shared combustion section), the trajectory may include a data pair for each individual piston assembly. While orbits are generally represented as a series of data pairs, it will be understood that, under certain conditions, an orbit may contain only a single data pair (for example, a single position-force pair in the case of a position-force orbit).

[0006] According to this disclosure, a processing subsystem of a free-piston engine calculates a position-force trajectory for one or more piston assemblies in the free-piston engine based on the current position of at least one or more free-piston assemblies and the desired engine performance. As used herein with respect to the control of a free-piston engine, the term “desired engine performance” means operating the engine such that one or more piston assemblies reach a vertex at a desired distinct position, one or more piston assemblies reach a desired distinct target position at a distinctly specified velocity or acceleration, one or more piston assemblies reach a desired distinct target position with any other preferred parameters or conditions, or any combination thereof. The processing subsystem determines a specific force value based on the position-force trajectory generated on one or more piston assemblies as a function of their positions along their distinct propagation paths between distinct vertices. While this disclosure describes the situation of determining a force value generated on a piston assembly, it will be understood that any other preferred parameter values ​​may be calculated to generate movement of the piston assembly. For example, any suitable gas pressure value can be used to cause movement of the piston assembly, for example, by supplying gas pressure from an external compressed gas source, or by generating gas pressure by adjusting the sides of a gas spring. As used herein, the term “propagation path” refers to a position path along which the piston assembly traverses. For example, a processing subsystem may first calculate a position-force trajectory for one or more piston assemblies based on the current position of at least one or more piston assemblies and the desired engine performance, and then subsequently determine, based on the calculated position-force trajectory, force values ​​to be applied to one or more piston assemblies over a specified time or position interval to achieve the desired engine performance. The force values ​​may be applied to one or more piston assemblies, for example, by applying an electromagnetic force to one or more piston assemblies.In some embodiments, the processing subsystem calculates position-force trajectories based on the operating state of the free-piston engine. The operating state of the free-piston engine refers to calculations, measurements, or estimates or indicators of the engine's state (i.e., its dynamic system state), and any other suitable calculations, measurements, or estimates or indicators of the engine's operating characteristics, performance, parameters, and the environment. For example, one or more sensors may be used to measure pressure, temperature, force, velocity, acceleration, position, any other suitable parameters or conditions, or any combination thereof, in individual sections or components of the free-piston engine. This sensor information can be processed by the processing subsystem to calculate position-force trajectories and achieve desired engine performance.

[0007] In one preferred approach, the processing subsystem calculates the position-force trajectory for the piston assembly when a specific trigger is activated (e.g., in response to a specific event, at a specific threshold crossing, at any other preferred trigger, or any combination thereof). In another preferred approach, the processing subsystem calculates the position-force trajectory throughout the entire iterative engine stroke or cycle. For example, the calculation may be performed at specific time intervals (e.g., 1 kHz, 10 kHz, etc.) or at specific discrete position intervals (e.g., every 1 millimeter, every 1 micron, etc.). In yet another preferred approach, when the operating state of the free-piston engine changes, the processing subsystem may calculate a new position-force trajectory.

[0008] Each position-force trajectory calculation is performed regardless of deviation from previously calculated trajectories (position-force trajectory, time-position trajectory, or any other suitable trajectory). It will be understood that the position-force trajectory calculation is determined using, for example, one or more calculations, one or more specifications, or any combination thereof, including the use of lookup tables, curve fitting, or both. This aspect provides a control technique for free-piston engines that allows changes and modifications in the operating state of the free-piston engine (high or low speed, intentional or unintentional) to be considered in each new position-force trajectory calculation, thereby making it possible to exclude malfunctions in the operating state of the free-piston engine. Each position-force trajectory calculation may also be calculated regardless of the timing of the desired engine performance. That is, each position-force trajectory is defined without a time component and calculated without specifying the time at which the desired engine performance occurs (e.g., the time at which the piston assembly reaches its peak or otherwise reaches the target position). In some cases, using suitable assumptions about engine gas properties, conditions, and parameters, the position-force trajectory calculation may rely on a closed-form solution. In other cases, the calculation of position and force trajectories may rely on numerical iterative solutions (for example, by using a solution method to compute the solution). This specification also provides, for example, the following items: (Item 1) A method implemented by a computer system programmed to control the displacement of a free piston assembly in a free piston engine based on desired engine performance, wherein the method is: a) Determining the current position of the free piston assembly, b) Determining a position and force trajectory to displace the free piston assembly, regardless of a previously determined position and force trajectory, based on the current position of the free piston assembly and the desired engine performance. c) To cause displacement of the free piston assembly based on the position and force trajectory, d) Repeat steps a) through c) until it is determined that the programmed computer system has stopped. A method comprising (Item 2) Element b) further comprises calculating the speed of the free piston assembly, and determining the position-force trajectory further comprises determining the position-force trajectory based on the speed of the free piston assembly, the method according to item 1. (Item 3) Element b) further comprises performing one or more pressure measurements in one section of the free piston engine, and determining the position-force trajectory further comprises determining the position-force trajectory based on the one or more pressure measurements, the method according to item 1. (Item 4) Element b) further comprises determining one or more pressure estimates in one or more individual sections of the free piston engine, and determining the position-force trajectory further comprises determining the position-force trajectory based on the one or more pressure estimates, the method according to item 1. (Item 5) Determining the position-force trajectory comprises using a closed-form solution to determine the position-force trajectory, the method according to item 1. (Item 6) Determining the position-force trajectory comprises determining the position-force trajectory regardless of the timing of the desired engine performance, the method according to item 1. (Item 7) Determining a position-force trajectory for displacing the free piston assembly based on the desired engine performance comprises determining the position-force trajectory such that the free piston assembly reaches a desired target position at a specified speed, the method according to item 1. (Item 8) The specified speed is zero, the method according to item 7. (Item 9) The free piston assembly is a first free piston assembly, the position-force trajectory is a first position-force trajectory, the free piston engine comprises a second free piston assembly facing the first free piston assembly, element a) further comprises determining the current position of the second free piston assembly, and element b) further comprises determining a second position-force trajectory for displacing the second free piston assembly based on the current position of the second free piston assembly and the desired engine performance, regardless of a previously determined second position-force trajectory, and element c) further comprises causing displacement of the second free piston assembly based on the second position-force trajectory, the method according to item 1. (Item 10) Element b) further comprises calculating a synchronization force, the synchronization force being for the first free piston assembly and for the second free piston assembly respectively, and element c) further comprises causing displacement of the first free piston assembly and the second free piston assembly based on the individual synchronization forces, the method according to item 9. (Item 11) The programmed computer system determines to stop in step d) based on detecting that the conditions are sufficiently steady, and the method further comprises e) switching to an iterative adaptive control technique for controlling the displacement of the free piston assembly, the method according to item 1. (Item 12) A method implemented by a programmed computer system for controlling the displacement of a free piston assembly within a free piston engine based on desired engine performance, the method comprising: repeatedly determining one or more force values for displacing the free piston assembly based on the desired engine performance, regardless of deviation from a previously determined trajectory and regardless of the timing of the desired engine performance; Each iteration, based on one or more individual force values, the displacement of the free piston assembly is produced. Methods that include... (Item 13) The repeated determination of one or more of the aforementioned force values ​​is performed in each iteration. Measuring one or more measurements that indicate the state of the free piston engine during individual iterations, Determining one or more force values ​​based on individual measurements and the desired engine performance. The method described in item 12, including the method described in item 12. (Item 14) The method according to item 13, wherein one or more of the above measurements include at least one of the following: free piston assembly position, free piston assembly velocity, free piston assembly acceleration, combustion chamber gas pressure, gas spring gas pressure, drive chamber force, piston assembly compression force, piston assembly axial deflection, airflow, fuel flow, exhaust oxygen concentration, and any combination thereof. (Item 15) The repeated determination of one or more of the aforementioned force values ​​is performed in each iteration. To estimate one or more estimates of the state of the free piston engine during individual iterations, Determining one or more force values ​​based on individual estimates and the desired engine performance. The method described in item 12, including the method described in item 12. (Item 16) The method described in item 12, which involves repeatedly determining one or more of the aforementioned force values, including using a closed-form solution. (Item 17) The method according to item 12, wherein the free piston assembly is a first free piston assembly, and the free piston engine comprises a second free piston assembly opposite to the first free piston assembly, and the method further includes synchronizing the movement of the first free piston assembly with the movement of the second free piston assembly. (Item 18) To detect that the conditions are sufficiently steady, Based on detecting that the above conditions are sufficiently steady, it is determined to stop determining one or more force values, Switching to an iterative adaptive control technique for controlling the displacement of the free piston assembly. The method described in item 12, further including the method described in item 12. (Item 19) A method implemented by a computer system programmed to control the displacement of a free piston assembly in a free piston engine based on desired engine performance, wherein the method is: a) Determining the current position of the free piston assembly, b) Determining a position-force trajectory for displacing the free piston assembly, regardless of deviation from the previously determined trajectory, based on the current position of the free piston assembly, the desired engine performance, and the force values ​​from the previously determined position-force trajectory. c) To cause displacement of the free piston assembly based on the position and force trajectory, d) Repeat steps a) through c) until it is determined that the programmed computer system has stopped. Methods that include... (Item 20) The method of item 19, further comprising: element b) calculating the velocity of the free piston assembly, and determining the position and force trajectory, further comprising determining the position and force trajectory based on the velocity of the free piston assembly. (Item 21) The method of item 19, further comprising determining one or more pressure estimates in one or more individual compartments of the free piston engine, and determining the position-force trajectory, further comprising determining the position-force trajectory based on the one or more pressure estimates. (Item 22) The method of item 19, wherein determining the position and force trajectory includes determining the position and force trajectory using a closed-form solution. (Item 23) The method of item 19, wherein determining the position and force trajectory includes determining the position and force trajectory regardless of the timing of the desired engine performance. (Item 24) The method of item 19, wherein determining the position and force trajectory for displacing the free piston assembly based on the desired engine performance includes determining the position and force trajectory such that the free piston assembly reaches a desired target position at a specified speed. (Item 25) The method according to item 19, wherein determining the position-force trajectory for displacing the free piston assembly involves using a smoothing technique, wherein the force value comprises the force value from the immediately preceding determined position-force trajectory. (Item 26) The method according to item 19, wherein the free piston assembly is a first free piston assembly, the position-force trajectory is a first position-force trajectory, the free piston engine comprises a second free piston assembly opposite to the first free piston assembly, element a) further comprising determining the current position of the second free piston assembly, element b) further comprising determining a second position-force trajectory for displacing the second free piston assembly based on the current position of the second free piston assembly, the desired engine performance, and force values ​​from a previously determined second position-force trajectory, and element c) further comprising causing displacement of the second free piston assembly based on the second position-force trajectory. (Item 27) The method of item 26, further comprising element b) calculating synchronous forces, the synchronous forces being for the first free piston assembly and the second free piston assembly, respectively, and element c) further comprising causing displacements of the first free piston assembly and the second free piston assembly based on the individual synchronous forces. (Item 28) The method according to item 19, wherein the programmed computer system determines to stop in step d) based on detecting that the conditions are sufficiently steady, and the method further includes e) switching to an iterative adaptive control technique for controlling the displacement of the free piston assembly. (Item 29) A method implemented by a computer system programmed to control the displacement of a free piston assembly in a free piston engine based on desired engine performance, wherein the method is: Based on the desired engine performance, one or more force values ​​are repeatedly determined to displace the free piston assembly, regardless of any deviation from the previously determined trajectory. Each iteration, a displacement of the free piston assembly is produced based on one or more individual force values, While repeatedly determining one or more of the aforementioned force values, it is detected that the conditions are sufficiently steady. When the above conditions are sufficiently steady, switch to an iterative adaptive control technique for controlling the displacement of the free piston assembly. Methods that include... (Item 30) Using the aforementioned iterative adaptive control technique, while controlling the displacement of the free piston assembly, it is possible to detect when the conditions are not sufficiently steady, When the above conditions are not sufficiently steady, switch to a position-force trajectory control technique to control the displacement of the free piston assembly. The method described in item 29, further including the method described in item 29. [Brief explanation of the drawing]

[0009] The present invention will be described in detail with reference to the following drawings according to one or more embodiments. The drawings are provided solely for illustrative purposes and depict typical or exemplary embodiments. These drawings are provided to facilitate the understanding of the concepts disclosed herein and should not be considered as limiting the scope, scope, or applicability of these concepts. For clarity and ease of illustration, it should be noted that these drawings are not necessarily drawn to scale.

[0010] [Figure 1] Figure 1 shows three illustrative free-piston combustion engine configurations. [Figure 2] Figure 2 is a cross-sectional view illustrating a two-piston, single-combustion-chamber, integrated-gas-spring, and separate-type linear electromagnetism engine according to some embodiments of the present disclosure. [Figure 3] Figure 3 illustrates a two-stroke piston cycle of the two-piston integrated gas spring engine of Figure 2, according to some embodiments of the present disclosure. [Figure 4] Figure 4 is a cross-sectional view illustrating an alternative two-piston, separate gas spring, and separate linear electromagnetism engine according to some embodiments of the present disclosure. [Figure 5] Figure 5 is a cross-sectional view illustrating a single-piston integrated internal gas spring engine according to some embodiments of the present disclosure. [Figure 6] Figure 6 is a cross-sectional view illustrating an embodiment of a gas spring rod according to several embodiments of the present disclosure. [Figure 7] Figure 7 is a cross-sectional view illustrating a two-piston integrated internal gas spring engine according to several embodiments of the present disclosure. [Figure 8] Figure 8 illustrates exemplary position, force, and power diagrams of a free-piston engine over compression and expansion strokes according to several embodiments of the present disclosure. [Figure 9]Figure 9 illustrates other exemplary position, force, and power diagrams of a free-piston engine over its compression and expansion strokes, according to some embodiments of the present disclosure. [Figure 10] Figure 10 is a block diagram of an illustrative piston engine system according to several embodiments of the present disclosure. [Figure 11] Figure 11 illustrates exemplary position-velocity and position-force trajectories of a free-piston engine over compression and expansion strokes according to some embodiments of the present disclosure. [Figure 12] Figure 12 shows a flowchart illustrating illustrative steps for causing movement of a free piston assembly along a propagation path, according to some embodiments of the present disclosure. [Figure 13] Figure 13 illustrates other exemplary position-velocity and position-force trajectories of a free-piston engine over its compression and expansion strokes, according to some embodiments of the present disclosure. [Figure 14] Figure 14 illustrates other exemplary position-velocity and position-force trajectories of a free-piston engine over its compression and expansion strokes, according to some embodiments of the present disclosure. [Figure 15] Figure 15 shows illustrative state diagrams of hybrid control techniques according to several embodiments of the present disclosure. [Modes for carrying out the invention]

[0011] The figures are comprehensive and not intended to limit this disclosure to the precise form disclosed. The concepts and embodiments disclosed may be practiced with modifications and alterations, and this disclosure is limited only by the claims and their equivalents.

[0012] In some embodiments, the current operating parameters of a free-piston engine may be estimated based on preceding forces applied to one or more piston assemblies, calculated as part of a previous position-force trajectory. The estimated engine operating parameters may be used in conjunction with the current position of one or more piston assemblies to calculate a new position-force trajectory. For example, the most recent force value, whether determined or actually applied to the piston assembly, may be used to update the estimate of the current gas pressure in the combustion or drive section of the free-piston engine by applying a smoothing technique (e.g., an IRR or FIR filter) to the previously estimated or measured gas pressure to adjust for the change in gas pressure at least partially caused by the most recent applied force. This aspect avoids the need for expensive and unreliable sensors (e.g., pressure sensors) in the free-piston engine, thereby providing a low-cost and highly reliable control technique for the free-piston engine.

[0013] In some embodiments, with respect to a free-piston engine having multiple piston assemblies (for example, arranged as opposing pistons with a shared combustion section), in addition to a processing subsystem that calculates the position and force trajectory for each individual piston assembly, the processing subsystem may also calculate a synchronization force for the multiple piston assemblies and, based on the calculation, cause a certain force to be applied to the multiple piston assemblies to synchronize the movement of the multiple piston assemblies as desired.

[0014] In some embodiments, the processing subsystem may employ a hybrid control strategy that switches between multiple control techniques, at least one of which is based on the step of calculating a position-force trajectory as disclosed herein. The processing subsystem may, for example, utilize a position-force trajectory control technique during times when the engine operating state is non-steady (e.g., during engine startup) and utilize a different, less robust control technique during times when the engine operating state is sufficiently steady (e.g., delivering a constant steady power). The processing subsystem may switch from a less robust control technique to a more robust position-force trajectory control technique when, for example, an unintentional change in the engine operating state is detected (e.g., a misfire event, a higher-than-expected friction event, a change in fuel quality event, any other suitable change in the engine operating state, or any combination thereof). In some cases, the less robust control technique may rely on a time-position trajectory calculated based on a previously determined position-force trajectory (e.g., measured over an entire engine stroke or cycle) that was calculated while the processing subsystem had previously employed a position-force trajectory control technique. In some cases, less robust control techniques may rely on deviations from previously determined trajectories (position-force trajectories, time-position trajectories, or any other suitable trajectories).

[0015] Generally, free-piston combustion engine configurations can be classified into three categories: 1) two opposing pistons, single combustion chamber; 2) single piston, double combustion chamber; and 3) single piston, single combustion chamber. Diagrams of the three general free-piston combustion engine configurations are shown in Figure 1. Several illustrative embodiments of a linear free-piston combustion engine are illustrated in U.S. Patent No. 8,662,029, issued March 4, 2014, titled "High-efficiency linear combustion engine," by the same applicant, which is incorporated herein by reference in its entirety. While this disclosure is presented in connection with a specific illustrative embodiment of a linear free-piston combustion engine, it will be understood that the concepts discussed herein are applicable to any other suitable free-piston combustion engine, including, for example, a nonlinear free-piston engine. A free-piston engine generally includes one or more free-piston assemblies that lack mechanical linkages (e.g., slider-crank mechanisms) that convert the linear motion of the piston assembly into rotational motion, or mechanical linkages (e.g., locking mechanisms) that directly control the piston dynamics. Free-piston engines have several advantages over such mechanically coupled piston engines, which lead to increased efficiency. For example, due to the inherent structural limitations of mechanically coupled piston engines, free-piston engines can be configured with higher compression and expansion ratios, which lead to higher engine efficiency, as described in U.S. Patent No. 8,662,029, which is referenced and incorporated earlier. Furthermore, free-piston engines allow for increased variability in compression and expansion ratios, including allowing the compression ratio to exceed the expansion ratio and the expansion ratio to exceed the compression ratio, which can also increase engine efficiency. Free-piston engine architectures also allow for increased control of the compression ratio on a per-engine-cycle basis, which allows for adjustments due to variable fuel quality and fuel type.In addition, due to the absence of mechanical linkage, a free-piston engine results in substantially lower lateral loads on the piston assembly, which enables oil-less operation and, consequently, reduces friction and the resulting losses.

[0016] While this disclosure is presented in connection with a free-piston internal combustion engine, it will be understood that the teachings and concepts presented herein are also applicable to other types of free-piston devices, such as free-piston compressors without combustion or free-piston compressors with internal combustion. In such systems without combustion, electrical energy is converted into mechanical energy by the LEM to compress a fluid (liquid or gaseous) in a compression chamber or compression compartment. In such systems with combustion, fuel energy is converted into mechanical energy, possibly in conjunction with the conversion of electrical energy, to compress a fluid in a compression chamber or compression compartment. In addition, the teachings and concepts presented herein are also applicable to free-piston heat engines that convert an external thermal resource into electricity or to compress a fluid.

[0017] Figure 2 is a cross-sectional view illustrating one embodiment of a two-piston, single-combustion-chamber, integrated-gas-spring, and separate-type LEM free-piston internal combustion engine 100. This free-piston internal combustion engine 100 directly converts chemical energy in the fuel into electrical energy via the LEM 200. As used herein, the term “fuel” refers to a substance that reacts with an oxidizer. Such fuels include, but are not limited to, (i) hydrocarbon fuels such as natural gas, biogas, gasoline, diesel, and biodiesel; (ii) alcohol fuels such as ethanol, methanol, and butanol; (iii) hydrogen; and (iv) mixtures of any of the above. The engines described herein are suitable for both steady-state power generation and mobile power generation (for example, for use in vehicles).

[0018] The engine 100 includes a cylinder 105 with two opposing piston assemblies 120 that move within the cylinder 105 and are sized to abut in a combustion chamber 130 at the center of the cylinder 105. Each piston assembly 120 may include a piston 125 and a piston rod 145. The piston assemblies 120 move freely and linearly within the cylinder 105.

[0019] Referring further to Figure 2, the volume between the rear of the piston 125, the piston rod 145, and the cylinder 105 is referred to herein as the drive compartment 160. As used herein, “drive compartment” refers to a compartment of an engine cylinder capable of storing and supplying energy to displace the piston assembly without using combustion. In some embodiments, the drive compartment 160 may contain a non-combustible fluid (i.e., gas, liquid, or both). In the illustrated embodiments, the fluid in the drive compartment 160 is a gas acting as a gas spring. The drive compartment 160 stores energy from the expansion stroke of the piston cycle and provides energy for subsequent strokes of the piston cycle, i.e., strokes that occur after the expansion stroke. For example, the kinetic energy of the piston may be converted into potential energy of the gas in the drive compartment during the expansion stroke of the engine. In some embodiments, the potential energy stored in the drive compartment may be sufficient to carry out the compression stroke (or exhaust stroke, or any other preferred stroke that occurs following the expansion stroke) without any additional net electricity input, for example, by a motor force. As used herein, the term “piston cycle” refers to any series of piston movements that begin and end using piston 125 in substantially the same configuration. One common embodiment is a four-stroke piston cycle comprising an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke. Additional alternative strokes may form part of a piston cycle as described throughout this disclosure. A two-stroke piston cycle is characterized as having an expansion stroke and a compression stroke. As used herein, “expansion stroke” refers to the stroke of a piston cycle in which a piston assembly moves from the top dead center ("TDC") position to the bottom dead center ("BDC") position, where TDC is the position of one or more assemblies when the combustion compartment volume is minimum, and BDC is the position of one or more assemblies when the combustion compartment volume is maximum.As described above, since the compression ratio and expansion ratio of a free-piston engine may vary from cycle to cycle, in some embodiments the TDC and BDC positions may also vary from cycle to cycle. Therefore, as will be described in more detail below, the expansion stroke may refer to the intake stroke, the expansion stroke, or both. In some embodiments, the amount of energy stored by the drive compartment during the expansion stroke may be determined based on various criteria and controlled by a controller and associated processing circuits, as will be described in more detail below.

[0020] For the purposes of simplification and clarity, the drive compartment will be described herein primarily in relation to the gas spring and may be referred herein to as “gas compartment,” “gas spring,” or “gas spring compartment.” In some configurations, it will be understood that the drive compartment 160 may include one or more other mechanisms in addition to, or instead of, the gas spring. For example, such mechanisms may include one or more mechanical springs, magnetic springs, or any preferred combination thereof. In some configurations, a highly efficient linear AC machine acting as a motor may be included, which may be used in place of, or in addition to, the spring (pneumatic, dynamic, or mechanical) to generate compression work. In some embodiments, it will be understood by those skilled in the art that the geometry of the drive compartment may be selected to minimize losses and maximize the efficiency of the drive compartment. For example, the diameter and / or dead volume of the drive compartment may be selected to minimize losses and maximize the efficiency of the drive compartment. As used herein, the term “dead volume” refers to the volume of the drive compartment when the piston assembly is at its furthest possible BDC position (i.e., when the volume of the combustion compartment is at its maximum before the piston assembly contacts the physical stop). In some embodiments, for example, when the drive compartment is a gas or hydraulic spring, the diameter of the compartment may differ from that of the combustion compartment to provide increased efficiency. An embodiment with a gas spring will be described in further detail below with reference to Figure 8-12.

[0021] Combustion ignition can be achieved, for example, via compression ignition and / or spark ignition. Fuel can be injected directly into the combustion chamber 130 ("direct injection") or the intake port 180 ("port fuel injection") via a fuel injector, and / or mixed with air prior to and / or during intake ("pre-mixed injection"). Engine 100 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.

[0022] Cylinder 105 may include an injector port 170, an intake port 180, an exhaust port 185, and a drive gas exchange port 190 for exchanging material (solid, liquid, gas, or plasma) with its surroundings. As used herein, the term “port” includes any opening or set of openings (e.g., porous material) that allow for the exchange of material between the inside of cylinder 105 and its surroundings. It will be understood that the ports shown in Figure 2 are illustrative only. In some arrangements, fewer or more ports may be used. The ports described above 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 material 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 type of ports and valves may depend on the engine configuration, injection strategy, and piston cycle (e.g., a two- or four-stroke piston cycle). In some embodiments, the exchange of material in the port may be achieved by the movement of a piston assembly, which may cover and / or expose the port as necessary to allow for the exchange of material.

[0023] In some embodiments, the operation of the drive compartment 160 may be adjustable. In some embodiments, the drive gas exchange port 190 may be used to control the characteristics of the drive compartment. For example, the drive gas exchange port 190 may be used to control the amount, temperature, pressure, any other preferred characteristics, and / or any combination thereof of the gas in the drive compartment. In some embodiments, adjusting any of the aforementioned characteristics, and therefore adjusting the mass in the cylinder, may vary the effective spring constant of the gas spring. In some embodiments, the geometry of the drive compartment 160 may be adjusted to obtain the desired operation. In some embodiments, the dead volume in the cylinder may be adjusted to vary the spring constant of the gas spring. It will be understood that any of the aforementioned controls and adjustments of the drive compartment 160 and the gas in it may provide control over the amount of energy stored by the drive compartment 160 during the expansion stroke of the engine 100. It will also be understood that the aforementioned controls of the characteristics of the gas in the drive compartment 160 may also provide frequency variability of the engine 100.

[0024] The engine 100 includes a pair of LEMs 200 for directly converting the kinetic energy of the piston assembly 120 into electrical energy (for example, during the compression stroke, expansion stroke, exhaust stroke, and / or intake stroke). Each LEM 200 can also directly convert electrical energy into the kinetic energy of the piston assembly 120. In some embodiments, the LEMs 200 may convert electrical energy into piston kinetic energy to start the engine, but once the engine has started and sufficient fuel chemical energy has been converted into piston kinetic energy, at least a portion of which can be stored in the drive compartment 160 during the expansion stroke, there is no need to convert electrical energy into kinetic energy during operation. In some embodiments, starting the engine may be achieved by any other preferred technique, including, for example, the use of stored compressed gas. As shown, the LEMs 200 include a stator 210 and a transducer 220. Specifically, the transducer 220 moves linearly within the stator 210, coupled to the piston rod 145, and may remain steady. In addition, the LEM200 can be a permanent magnet machine, an induction machine, a switched reluctance machine, or any combination thereof. The stator 210 and the transducer 220 can each include a magnet, a coil, iron, or any preferred combination thereof. Because the LEM200 directly converts the kinetic energy of the piston to electrical energy and vice versa (i.e., there is no mechanical linkage), mechanical and friction losses are minimized compared to conventional engine-generator configurations. Furthermore, since the LEM200 is configured to convert a portion of the kinetic energy of the piston assembly into electrical energy during any stroke of the piston cycle, and the engine 100 includes an adjustable drive compartment 160 configured to store energy from the expansion stroke that can be converted into electrical energy during subsequent strokes, the LEM200 may be configured to have a lower electrical capacity than an LEM or other device that requires all energy conversion to occur within a single stroke of the piston cycle (e.g., only within the expansion stroke).Therefore, in some embodiments, the linear AC motor and power electronics associated with the LEM200 can be reduced in size, weight, and / or electrical capacity. This can result in reduced size and cost of components, increased efficiency, increased reliability, and increased utilization, as will be understood by those skilled in the art. Thus, the engine frequency, and therefore the power output, can be increased in some embodiments.

[0025] It will be understood by those skilled in the art that each LEM200 can operate as both a generator and a motor. For example, when the LEM200 converts the kinetic energy of the piston assembly 120 into electrical energy, they operate as generators. When acting as generators, the force applied to the transducer 220 is in the opposite direction to the motion of the piston assembly 120. Conversely, when the LEM200 converts electrical energy into kinetic energy of the piston assembly 120, they operate as motors. When acting as motors, the force applied to the transducer 220 is in the same direction as the motion of the piston assembly 120. For ease of reference, the centerlines in Figure 2 and the corresponding figures (near the injector port 170) can be considered as the origin, and the positive direction for each piston assembly is away from the center outward.

[0026] The embodiment shown in Figure 2 operates using a two-stroke piston cycle. A diagram illustrating the two-stroke piston cycle 300 of the two-piston integrated gas spring engine 100 of Figure 2 is shown in Figure 3. As shown in Figure 3, the engine 100 may also operate using a two-stroke piston cycle including a compression stroke and an expansion stroke, where the pistons are located at BDC prior to the compression stroke and at top dead center TDC prior to the expansion stroke. As used herein with reference to the two-piston embodiment, BDC may refer to the point where the pistons are furthest from each other. As used herein with reference to the two-piston embodiment, TDC may refer to the point where the pistons are closest to each other. When at or near BDC, and the drive compartment is used to provide compression work, the pressure of the gas in the drive compartment 160 exceeds the pressure in the combustion compartment 130, which pushes the piston 125 inward toward each other, i.e., in the negative direction, away from BDC. The gas in the drive compartment 160 may be used to provide some or all of the energy required to carry out the compression stroke. As described above, in some embodiments, the piston 125 may be pushed away from the BDC by any other preferred mechanism, including a mechanical spring, a magnetic spring, or any other preferred mechanism that can be used to provide compression work. The LEM 200 may also provide some of the energy required to perform the compression stroke, but in a preferred embodiment, when sufficient energy is produced during combustion, the energy stored in the drive compartment 160 may be transferred to the piston to provide the essential compression work, so that the LEM 200 does not need to convert any electrical energy into kinetic energy for the piston 125. The LEM 200 may also extract energy during the compression stroke. For example, if the gas in the drive compartment 160 (or other preferred means as described above) provides excess energy to perform the compression stroke, the LEM 200 may convert some of the kinetic energy of the piston assembly 120 into electrical energy.

[0027] The amount of energy required to perform the compression stroke may depend on the desired compression ratio, the pressure and temperature of the combustion chamber 130 at the start of the compression stroke, the mass of the piston assembly 120, system losses, and other properties and operating conditions of the engine. As described above, the drive chamber 160 may provide all the energy required for the compression stroke so that no other energy input (from the LEM 200 or any other source) is required. In some embodiments, some energy may be input from the LEM 200 during the compression stroke, but the net energy during the compression stroke is still positive (e.g., more energy is converted to electricity than input over the stroke). The compression stroke continues until combustion occurs, which typically occurs when the velocity of the piston 125 is zero or close to zero. Combustion causes an increase in temperature and pressure in the combustion chamber 130, which pushes the piston 125 outward toward the LEM 200. During the expansion stroke, a portion of the kinetic energy of the piston assembly 120 may be converted into electrical energy by the LEM 200, and another portion of the kinetic energy may perform compression work on the gas (or other compression mechanism) in the drive compartment 160. Alternatively, all of the kinetic energy of the piston assembly may be stored in the drive compartment 160. The expansion stroke continues until the velocity of the piston 125 becomes zero. After the expansion stroke and before the subsequent compression stroke, with the piston 125 at or near the BDC, the engine may exhaust combustion products and draw in air, an air / fuel mixture, or an air / fuel / combustion product mixture. This process may be referred to herein as “breathing” or “breathing at or near the BDC.” It will be understood by those skilled in the art that breathing may be achieved in any preferred manner, such as single-flow or cross-flow scavenging, as described in U.S. Patent No. 8,662,029, which is referenced and incorporated herein. Furthermore, although described as occurring after the expansion stroke, it will also be understood that in some embodiments, breathing may occur during the end of the expansion stroke and / or the beginning of the compression stroke.Similarly, in some embodiments, combustion may occur at the end of the compression stroke and / or at the beginning of the expansion stroke.

[0028] Figure 3 illustrates one exemplary port configuration 300 in which the intake port 180 and exhaust port 185 are located in front of both pistons near the BDC. The opening and closing of the exhaust port 185 and the intake port 180 may be controlled independently. The locations of the exhaust port 185 and the intake port 180 can be selected to allow a range of compression and / or expansion ratios. The time during a cycle when the exhaust port 185 and the intake port 180 are activated (opened and closed) can be adjusted during and / or between cycles to vary the compression and / or expansion ratio and / or the amount of combustion products retained in the combustion compartment 130 at the start of the compression stroke. Retaining combustion gases in the combustion compartment 130 is called residual gas trapping (RGT) and can be used to produce combustion timing, peak combustion temperature, and other combustion and engine performance characteristics. Alternatively, or in addition, exhaust gas recirculation (EGR) can be used to recirculate combustion gases to produce combustion timing, peak combustion temperature, and other combustion and engine performance characteristics.

[0029] While the operation of a two-stroke cycle has been described above, the embodiment in Figure 2 may also operate 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 preferred modifications may be made to operate using a four-stroke piston cycle. For example, as described in U.S. Patent No. 8,662,029, which has been referenced and incorporated earlier, the port locations may be modified to operate the engine using a four-stroke piston cycle.

[0030] In some embodiments, in a four-stroke piston cycle, the drive compartment 160 may provide all the work necessary for the compression stroke, just as in the two-stroke cycle described above. In some embodiments, the drive compartment 160 may provide enough work to avoid net electrical energy input during the compression stroke. In some embodiments, the drive compartment 160 may provide enough work to enable net electrical energy output during the compression stroke. The compression stroke may continue until combustion occurs, for example, until the velocity of the piston 125 becomes zero or approximates it. After combustion, a power stroke may follow, during which the kinetic energy of the piston assembly 120 may be stored in the drive compartment 160 and / or converted into electrical energy by the LEM 200, as described above with respect to the two-stroke cycle. At some point in or near the power stroke BDC, the exhaust port may be opened, and the exhaust stroke may occur until the velocity of the piston 125 becomes zero or approximates it, marking the exhaust stroke TDC for that cycle. As described above, the energy stored in the drive compartment 160 during the expansion stroke may provide the work required to perform the exhaust stroke. At some point prior to reaching the exhaust stroke TDC, the combustion compartment 130 closes the exhaust valve, while exhaust gases are still present in the cylinder. In some embodiments, this trapped exhaust gas may store enough energy to perform the subsequent intake stroke. Similar to the expansion stroke, the kinetic energy of the piston assembly 120 may be stored in the drive compartment 160 during the intake stroke and / or converted into electrical energy by the LEM 200, which occurs until the velocity of the piston 125 becomes zero. In some embodiments, the drive compartment 160 may store enough energy during the intake stroke to perform the subsequent compression stroke.In some embodiments, any preferred amount of energy stored in the drive compartment in excess of the amount required for a subsequent compression stroke or subsequent exhaust stroke may be converted into electrical energy by the LEM200.

[0031] Figure 4 is a cross-sectional view illustrating an alternative two-piston, separate gas spring, and separate LEM engine according to the principles of the present disclosure. The illustrated configuration is for the purposes of the embodiment only, and it will be understood that any other suitable configuration of a two-piston, separate gas spring, and separate LEM engine may be used in accordance with the present disclosure. Engine 400 includes a main cylinder 105, two opposing piston assemblies 120, and a combustion chamber 130 located in the center of the main cylinder 105. The illustrated engine 400 has some physical differences when compared to engine 100. Specifically, engine 400 includes a pair of outer cylinders 405 containing an additional piston 125, and the LEM 200 is positioned between the main cylinder 105 and the outer cylinders 405. Each outer cylinder 405 includes a drive chamber 410 located between the piston 125 and the distal end of the outer cylinder 405, and a drive rear chamber 420 located between the piston 125 and the proximal end of the outer cylinder 405. The main cylinder 105 includes a pair of combustion rear compartments 430 located between the piston 125 and the distal end of the main cylinder 105. In some embodiments, the drive rear compartment 420 and the combustion rear compartment 430 are maintained at or near atmospheric pressure. In some embodiments, the drive rear compartment 420 and the combustion rear compartment 430 are not maintained at or near atmospheric pressure. In the illustrated configuration, the main cylinder 105 has a port 440 for blow-by gas removal, an injector port 170, an intake port 180, and an exhaust port 185. A drive gas exchange port 190 is located in the outer cylinder 405. Each piston assembly 120 includes two pistons 125 and a piston rod 145. The piston assembly moves freely linearly between the main cylinder 105 and the outer cylinder 405, as depicted in Figure 4. It will be understood that the embodiment in Figure 4 can operate using, for example, a two-stroke piston cycle using the methodology described above with respect to Figure 3, and a four-stroke piston cycle as described in U.S. Patent No. 8,662,029, which is incorporated above and previously referenced.

[0032] The configurations in Figures 2 and 3, as shown, are referred to as engine 100 and include a single unit defined by cylinder 105, piston assembly 120, and LEM 200. Similarly, the configuration in Figure 4, as shown, is referred to as engine 400 and includes a single unit defined by main cylinder 105, piston assembly 120, outer cylinder 405, and LEM 200. However, multiple units can be installed in parallel, which may collectively be referred to as “engines.” This type of modular arrangement, in which engine units operate in parallel, may be used to allow the scale of the engine to be increased by the end user as needed. In addition, all units do not need to be the same size, operate under the same conditions (e.g., frequency, stoichiometry, or breathing), or operate simultaneously (e.g., one or more units may be deactivated while one or more other units are operating). When units operate in parallel, there is potential for integration between engines, such as gas exchange between units and / or feedback between individual LEM 200s of units, but not limited to these.

[0033] Figure 5-7 illustrates a further embodiment featuring an integrated internal gas spring, in which the gas spring is integrated inside the piston assembly and the LEM is separated from the combustion cylinder. As illustrated in Figure 5-7, the integrated internal gas spring (IIGS) architecture may be similar in length to the integrated gas spring with the separated LEM architecture illustrated in Figure 2-3. However, the IIGS architecture can eliminate the problem of blow-by gases from the combustion chamber entering the gas spring, which also occurs in fully integrated gas spring and LEM architectures.

[0034] Figure 5 is a cross-sectional view illustrating a single-piston, integrated internal gas spring engine according to several embodiments of the present disclosure. Many components, such as the combustion chamber 130, are similar to those in previous embodiments (e.g., Figures 1 and 2) and are labeled accordingly. The engine 500 comprises a cylinder 105 with a piston assembly 520 that is sized to move within the cylinder 105 in response to reactions in the combustion chamber 130 near the bottom end of the cylinder 105. The piston assembly 520 comprises a piston 530, a piston seal 535, and a spring rod 545. The piston assembly 520 moves freely linearly within the cylinder 105. In the illustrated embodiments, the piston rod 545 moves along a bearing 560 and is sealed by a piston rod seal 555 fixed to the cylinder 105. Cylinder 105 includes intake ports 570, 580 for air, fuel, exhaust gas, air / fuel mixture, and / or air / exhaust gas / fuel mixture, exhaust ports 570, 580 for exhaust of combustion products, and / or injectors. Some embodiments do not require all of the ports depicted in Figure 5. The number and type of ports depend on the engine configuration, injection strategy, and piston cycle (e.g., a two- or four-stroke piston cycle).

[0035] In the illustrated embodiment, the engine 500 further includes an LEM 550 (including a stator 210 and a magnet 525) for directly converting the kinetic energy of the piston assembly 520 into electrical energy. It is understood that the LEM 550 may be configured to operate substantially identically to the LEM 200 described above with respect to Figure 2-4.

[0036] Referring further to Figure 5, the piston 530 comprises a solid front compartment (combustor side) and a hollow rear compartment (gas spring side). The area inside the hollow compartment of the piston assembly 520 between the front of the piston 530 and the spring rod 545 contains gas that acts as a gas spring 160, providing at least some of the work required to carry out the compression stroke. The piston 530 moves linearly within the stator 210 of the combustor compartment 130 and the LEM 550. The motion of the piston is guided by bearings 560, 565, which may be integral bearings, hydraulic bearings, and / or pneumatic bearings. In the illustrated embodiment, the engine 500 includes both an external bearing 560 and an internal bearing 565. In particular, the external bearing 560 is located between the combustion compartment 130 and the LEM 550, and the internal bearing 565 is located on the inside of the hollow compartment of the piston 530. The external bearing 560 is fixed from the outside and does not move with the piston 530. The internal bearing 565 is fixed to the piston 530 and moves with the piston 530 relative to the spring rod 545.

[0037] Continuing to refer to Figure 5, the spring rod 545 serves as one face for the gas spring 160 and is fixed from the outside. The spring rod 545 has at least one seal 585 located at or near its end, which serves to retain gas within the gas spring compartment 160. The magnet 525 is mounted on the rear surface of the piston assembly 520 and moves linearly with the piston assembly 520 within the stator 210 of the LEM 550. The piston assembly 520 may have seals that retain gas within individual compartments. The illustrated embodiment includes (i) a front seal 535 fixed to the piston 530 at or near its front end to prevent gas from being transmitted from the combustion compartment 130, and (ii) a rear seal 555 fixed to the cylinder 105 to prevent intake gas and / or blow-by gas from being transmitted to the surroundings.

[0038] Figure 6 is a cross-sectional view illustrating an embodiment of a gas spring rod according to some embodiments of the present disclosure. Specifically, the spring rod 645 includes a central lumen 610 that allows mass to be transferred between a gas spring section 160 and a reservoir section 620 that communicates with the surroundings. Communication with the surroundings is controlled through a valve 630. The amount of mass in the gas spring 645 may be adjusted to control the pressure in the gas spring 645 according to some embodiments of the present disclosure.

[0039] Figure 7 is a cross-sectional view illustrating a two-piston, integrated internal gas spring engine according to several embodiments of the present disclosure. Most of the elements of the two-piston embodiment are similar to those of the single-piston embodiment in Figure 5, and similar elements are labeled accordingly. In addition, the operating characteristics of the single and two-piston embodiments are similar to those described in the earlier embodiments, including all aspects such as linear AC operation, breathing, and combustion strategy.

[0040] Figure 8 illustrates the position, force, and power of a free-piston engine according to several embodiments of the present disclosure. As shown, Figure 8 illustrates exemplary position 820, force 840, and power 860 figures over time for a free-piston engine with a two-stroke piston cycle including a compression stroke and an expansion stroke. Referring to position figure 820, for reference purposes, as indicated in Figure 8, the positive direction corresponds to the direction from TDC to BDC. For example, in the free-piston assembly of Figure 2-4, the centerline corresponds to the origin, and the direction away from the centerline would be the positive direction for each free-piston assembly. As can be seen from position figure 820, the piston assembly begins the compression stroke at BDC, proceeds to TDC, and at that point the expansion (or power) stroke begins. During the expansion stroke, the piston assembly proceeds to return to BDC.

[0041] Referring to force diagram 840, a force is positive when applied in the direction from TDC to BDC. For example, in the free piston assembly of Figure 2-4, a force applied away from the centerline would be a positive force. As can be seen in force diagram 840, a relatively constant positive force may be applied to the piston assembly during the compression stroke, and during the expansion stroke, the force may be negative (towards the centerline), allowing the LEM to extract energy during both strokes. The applied force does not need to be constant, and it will be understood 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, as described herein, a force may not be applied when the piston assembly speed is relatively low, due to the inefficiency of doing so.

[0042] The power output is the negative product of the force and velocity of the piston assembly. Referring specifically to power diagram 860, it can be seen that, in the illustrated ideal case, no power needs to be input to the system to carry out the compression and expansion strokes of the piston cycle. Rather, as described above, in the ideal case, sufficient energy is stored in at least one drive compartment during the expansion stroke to carry out the subsequent compression stroke without any additional energy input to the system during the compression stroke.

[0043] In an ideal scenario, it may be desirable to avoid any power input during the compression and expansion strokes as described with respect to Figure 8, but in some embodiments, it may be necessary or desirable to provide some power input. Thus, Figure 9 illustrates the position, force, and power of a free-piston engine according to some other embodiments of the present disclosure. Similar to Figure 8, Figure 9 illustrates exemplary position 920, force 940, and power 960 figures over time for a free-piston engine with a two-stroke piston cycle, including compression and expansion strokes. It will be understood that the position figure 920 is substantially similar to that of the position figure 820 illustrated in Figure 8, but the force figure 940 and power figure 960 may differ from those illustrated in Figure 8. Referring to the force figure 940 during the compression stroke, it can be seen that at 902, the force may be applied in the opposite direction to the direction in which it was initially applied over a short period. This is also reflected in the power figure 960, where a negative power may be seen at 904, indicating power input over the same short period. The application of force and input may occur for several reasons, but in some embodiments, it may be done to control the speed of the piston assembly or otherwise ensure that the piston assembly reaches a suitable or desired TDC position before the subsequent expansion stroke. For example, a force may be applied to increase the speed of the piston assembly. Similarly, referring further to force diagram 940 during the expansion stroke, it can be seen that in 906 a force may be applied in the opposite direction to the rest of the expansion stroke over a short period, which is also reflected in power diagram 960, where a negative force may be seen in 908 indicating a power input over the same short period. As described above, the applied force and input power may occur for several reasons, but in some embodiments, a force may be applied and power may be input to control the speed of the piston assembly or otherwise ensure that the piston assembly reaches a suitable or desired BDC position before the subsequent compression stroke. For example, a force may be applied to increase the speed of the piston assembly, as described above.

[0044] While the provision of input power during the compression and / or expansion strokes described with respect to Figure 9 is not necessarily ideal operation, it should be understood that the net electrical energy output over each stroke is still greater than zero (i.e., no net electrical energy input exists over each stroke). This is evident from power figure 960, where it can be seen that the integral over each stroke, represented by the area of ​​the curve above zero minus the area of ​​the curve below zero, is substantially greater than zero. Therefore, the amount of electrical energy output by the system over each stroke exceeds the electrical energy input to control the piston assembly position, as described above. As used herein, “net electrical energy” refers to the transfer of electrical energy in and out of the LEM, such as those described above with respect to Figure 2-4. In some embodiments, the LEM may include power electronics (e.g., including a DC bus, IGBTs, capacitors, and / or any other preferred components), a battery, and / or a stator coupled to a grid-tie inverter. Therefore, in some embodiments, some electrical energy may be input to the LEM via power electronics, batteries, and / or grid-tie inverters coupled to the LEM, while the net electrical energy over a given stroke as described above will be output from the LEM to power electronics, batteries, and / or grid-tie inverters.

[0045] Figures 8 and 14 illustrate the operation of a free-piston engine with no net electrical input over a given stroke, but it is understood that the principles of this disclosure can be applied to any suitable free-piston engine, including free-piston engines that operate with net electrical input during a stroke, such as during a compression stroke (e.g., during starting).

[0046] As described above, the embodiments with respect to Figure 2-4 include a two-piston, single-combustion-chamber, two-stroke internal combustion engine 100. Generally, control systems applicable to free-piston combustion engines are described below and illustrated in the corresponding figures. Thus, as described above, the control systems are also applicable to other free-piston combustion engine architectures, such as those described in U.S. Patent No. 8,662,029, which has been previously referenced and incorporated. As will be understood by those skilled in the art, various modifications and alternative configurations may be used and other changes may be made without departing from the scope of this disclosure. For example, in addition to the two-piston architecture described above with respect to Figure 2-4, the control systems described herein are also applicable to, for example, a single-piston architecture. Similarly, in addition to the two-stroke engine described above with respect to Figure 3, the control systems described herein are also applicable to, for example, a four-stroke engine.

[0047] Figure 10 is a block diagram of an illustrative piston engine system 1000 having a control system 1010 for a piston engine 1040, according to some embodiments of the present disclosure. The piston engine 1040 may be any preferred free piston engine, such as those described above with respect to Figure 2-7. The control system 1010 may communicate with one or more sensors 1030 coupled to the piston engine 1040. The control system 1010 may be configured to communicate with an auxiliary system 1020, which can be used to adjust the operating aspects or characteristics of the piston engine 1040. In some embodiments, one or more piston engines may be controlled by the control system 1010. For example, the control system 1010 may be configured to communicate with auxiliary systems and sensors corresponding to any number of piston engines. In some embodiments, the control system 1010 may be configured to interact with a user via a user interface system 1050.

[0048] The control system 1010 may include processing equipment 1012, a communication interface 1014, a sensor interface 1016, a control interface 1018, any other suitable components or modules, or any combination thereof. The control system 1010 may be implemented at least in part with one or more integrated circuits, ASICs, FPGAs, microcontrollers, DSPs, computers, terminals, control stations, handheld devices, modules, any other suitable devices, or any combination thereof. In some embodiments, the components of the control system 1010 may be communicatively coupled via individual communication links or communication buses 1011, as shown in Figure 10. The processing equipment 1012 may include any suitable processing circuitry, such as one or more processors (e.g., a central processing unit), a cache, random access memory (RAM), read-only memory (ROM), any other suitable hardware components, or any combination thereof, which can be configured (e.g., using software or via a wire) to process information about the piston engine 1040, such as that received from the sensor 1030 by the sensor interface 1016. The sensor interface 1016 may include a power source for supplying power to the sensor 1030, a signal modifier, a signal preprocessor, any other suitable components, or any combination thereof. For example, the sensor interface 1016 may include filters, amplifiers, samplers, and analog / digital converters for modulating and preprocessing the signal from the sensor 1030. The sensor interface 1016 may communicate with the sensor 1030 via a communication coupling 1019, which may be a wired connection (e.g., using IEEE 802.3 Ethernet® or a universal serial bus interface), a wireless coupling (e.g., using IEEE 802.11 "Wi-Fi" or Bluetooth®), an optical coupling, an inductive coupling, any other suitable coupling, or any combination thereof.The control system 1010, more specifically the processing unit 1012, may be configured to provide control of the piston engine 1040 over relevant time scales. For example, a temperature change of one or more 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 the operation of the piston engine (e.g., a response fast enough to prevent overheating and / or component failure, to properly provide peak control as described below, to enable shutdown in the event of a diagnostic event, and / or to provide proper load tracking).

[0049] Sensor 1030 may include any preferred type of sensor that can be configured to sense any preferred properties or aspects of the piston engine 1040. In some embodiments, the sensor may include one or more sensors configured to sense aspects and / or properties of the auxiliary system 1020. In some embodiments, sensor 1030 may include a temperature sensor (e.g., a thermocouple, resistance temperature detector, thermistor, or optical temperature sensor) configured to sense the temperature of the components of the piston engine 1040, the fluid introduced into or recovered from the piston engine 1040, or both. In some embodiments, sensor 1030 may include one or more pressure sensors (e.g., a piezoelectric pressure transducer, a strain-based pressure transducer, or a gas ionization sensor) configured to sense the pressure in a compartment of the piston engine 1040 (e.g., a combustion compartment or a gas-driven compartment), the pressure of the fluid introduced into or recovered from the piston engine 1040, or both. In some embodiments, sensor 1030 may include one or more force sensors (e.g., piezoelectric force transducers or strain-based force transducers) configured to sense forces within the piston engine 1040, such as tensile, compressive, or shear forces (which may indicate friction or other related force information, pressure information, or acceleration information). In some embodiments, sensor 1030 may include one or more current and / or voltage sensors (e.g., ammeters and / or voltmeters coupled to the LEM of the piston engine 1040) configured to sense voltage, current, power output and / or input (e.g., current multiplied by voltage), any other suitable electrical properties of the piston engine 1040 and / or auxiliary system 1020, or any combination thereof.In some embodiments, the sensor 1030 may include one or more sensors configured to sense the position of the piston assembly and / or any other component of the engine, the speed of the piston assembly and / or any other component of the engine, the acceleration of the piston assembly and / or any other component of the engine, the flow velocity, the oxygen or nitrogen oxide emission level, other emission levels, any other desirable properties of the piston engine 1040 and / or auxiliary system 1020, or any combination thereof.

[0050] The control interface 1018 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 the auxiliary systems 1020. In some embodiments, the control interface 1018 may include a digital-to-analog converter that provides analog control signals to one or all of the auxiliary systems 1020.

[0051] The auxiliary system 1020 may include a cooling system 1022, a pressure control system 1024, a gas-driven control system 1026, and / or any other suitable control system 1028. The cooling / heating system 1022 may include a pump, a fluid reservoir, a pressure regulator, a bypass, a radiator, fluid conduits, a power circuit (e.g., for an electric heater), any other suitable components, or any combination thereof, providing cooling, heating, or both to the piston engine 1040. The pressure control system 1024 may include a pump, a compressor, a fluid reservoir, a pressure regulator, fluid conduits, any other suitable components, or any combination thereof, supplying (and optionally receiving) pressure-controlled fluid to the piston engine 1040. The gas-driven control system 1026 may include a compressor, a gas reservoir, a pressure regulator, fluid conduits, any other suitable components, or any combination thereof, supplying (and optionally receiving) drive gas to the piston engine 1040. In some embodiments, the gas-driven control system may include any suitable component for controlling any of the gas spring components described above with respect to Figure 2-7. In some embodiments, other systems 1028 may include a valve system, such as a cam-operated system, a solenoid system, or any other electromechanical or electromachine device, for supplying oxidizer and / or fuel to the piston engine 1040. Valves may also be used to regulate exhaust outflow from the engine, for example, in a portless engine having a single-piston assembly array or a double-piston assembly array. The exhaust valve may be controlled using a voice coil (e.g., a linear motor) to enable single-flow scavenging.

[0052] The user interface 1015 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 the user interface systems 1050. The user interface system 1050 may include a display 1052, an input device 1054, a mouse 1056, an audio device 1058, a remote interface accessed via a website, a mobile application, or other internet service, any other suitable user interface device, or any combination thereof. In some embodiments, the remote interface may be remote from the organization but close to the organization's site. In other embodiments, the remote interface may be remote from both the organization and the organization's site. The display 1052 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 capable of providing graphics, text, images, or other images to the user, or any combination thereof. In some embodiments, the display 1052 may include a touchscreen that can provide tactile interaction with the user, for example, by providing one or more soft commands on the display screen. The display 1052 may display any suitable information relating to the piston engine 1040 (e.g., a time series of properties of the piston engine 1040), the control system 1010, the auxiliary system 1020, the user interface system 1050, any other suitable information, or any combination thereof. The input device 1054 may include a QWERTY keyboard, a numeric keypad, any other suitable set of hard command buttons, or any combination thereof. The mouse 1056 may include any suitable pointing device that can control a cursor or icon on a graphical user interface displayed on the display screen.The mouse 1056 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. The audio device 1058 may include a microphone, a speaker, headphones, any other suitable device for providing and / or receiving audio signals, or any combination thereof. For example, the audio device 1058 may include a microphone, and the processing unit 1012 may process audio commands received via the user interface 1015, triggered by the user speaking into the microphone.

[0053] In some embodiments, the control system 1010 may be configured to receive one or more user inputs and provide control. For example, in some embodiments, the control system 1010 may override control settings based on sensor feedback and set control signals to the auxiliary system 1020 in response to one or more user inputs to the user interface system 1050. In further embodiments, the user may input fixed values ​​for one or more control variables (e.g., temperature, pressure, flow rate, work input / output, or other variables), and the control system 1010 may execute a control algorithm based on these fixed values.

[0054] In some embodiments, operating characteristics (e.g., one or more desired properties of the piston engine 1040 or the auxiliary system 1020) may be predefined by the manufacturer, the user, or both. For example, certain operating characteristics may be stored in the memory of the processing unit 1012 and accessed to provide one or more control signals. In some embodiments, one or more of the operating characteristics may be modified by the user. The control system 1010 may be used to maintain, adjust, or otherwise manage those operating characteristics. For example, the control system 1010 may be used to modify operation based on environmental conditions such as temperature and pressure.

[0055] In some embodiments, the control system 1010 calculates a position-force trajectory for one or more piston assemblies in a free-piston engine, at least partially, based on desired engine performance (e.g., desired peak, position) and the current position of one or more piston assemblies. Based on the calculated position-force trajectory, the control system 1010 causes a displacement of one or more piston assemblies by applying a specific force to one or more piston assemblies over a specified time or position interval. Each position-force trajectory calculation by the control system 1010 is performed regardless of deviation from a previously determined trajectory (position-force, time-position, or any other preferred trajectory). The control system 1010 may calculate the position-force trajectory repeatedly over an engine stroke or cycle, after a change to the engine's operating state, or any combination thereof, when a specific trigger is activated (e.g., in response to a specific event). In some embodiments, the control system 1010 may also calculate the position-force trajectory regardless of the timing of the desired engine performance. In some cases, the control system 1010 may calculate the position-force trajectory based on the operating state of the engine. In some embodiments, the control system 1010 may estimate the current operating parameters of the engine based on preceding forces calculated as part of a previous position-force trajectory, or based on preceding forces applied to one or more piston assemblies. In some cases, the control system 1010 may calculate the position-force trajectory using a closed-form solution, a numerical iterative solution, or a combination of both. In embodiments using multiple piston assemblies, the control system 1010 may also calculate a synchronization force for the multiple piston assemblies in addition to calculating the position-force trajectory for each individual piston assembly, and based on the synchronization calculation, apply a force to the multiple piston assemblies to synchronize the movement of the multiple piston assemblies as desired.In some embodiments, the control system 1010 may employ a hybrid control strategy that switches between a position-force trajectory control technique and another control technique (for example, a control technique that relies on calculations of deviations from a previously determined trajectory) depending on the operating state of the engine.

[0056] The following is a discussion of several illustrative embodiments implemented in accordance with the concepts described above. These embodiments generally relate to single and double-piston free-piston internal combustion engines with drive compartments, such as those illustrated in Figure 2-7 and discussed above. In these embodiments, the control system 1010 is used to cause displacement of individual piston assemblies based on desired engine performance. It will be understood that the implementations and concepts discussed with reference to these specific embodiments are generally applicable to other embodiments as well. This discussion is provided for illustrative purposes and is not intended to limit the applicability of the disclosed implementations and concepts to these embodiments only.

[0057] Figure 11 shows exemplary position-velocity and position-force trajectories (1110 and 1120, respectively) of a piston assembly in a free-piston engine over its compression and expansion strokes. The force values ​​shown in 1120 correspond to force values ​​calculated by the control system 1010 and applied to the piston assembly by exerting an electromagnetic force on it via the LEM. The profiles illustrated in Figure 11 are idealized, simplified, or both for the purpose of clarity and ease of illustration. It will be understood that actual profiles may differ. The electromagnetic force is referred to herein as LEM force, LEM force value, motor force, motor force value, force, or force value. Referring to Figure 11 and the subsequent trajectory diagrams, the positive direction corresponds to the direction from TDC to BDC (for example, positive velocity corresponds to the piston assembly moving from TDC to BDC, and positive force corresponds to the force applied in the direction toward BDC). In addition, referring to Figure 11 and the subsequent orbital diagrams, the zero position point corresponds to the centerline for opposing-piston free-piston engines (e.g., Figures 2-4 and 7) or the end of the combustion compartment (i.e., the front of the combustion compartment) for single-piston free-piston engines (e.g., Figure 5). As shown in Figure 11, while the piston assembly circulates between the BDC and TDC (its apex), the LEM applies a force in the opposite direction to the motion of the piston assembly, thereby producing a net electrical energy output over both strokes. Producing a net electrical energy output over both strokes requires that the drive compartment be sized so that sufficient energy can be stored from the expansion stroke to provide more than enough energy required to carry out the subsequent compression stroke. This paradigm is generally assumed in the following discussion, but it will be understood that the control techniques disclosed herein apply to free-piston engines in which the drive compartment is sized so that the net electrical energy input is required during the compression stroke, and to free-piston engines in which there is no drive compartment and all the energy required to carry out the compression stroke is provided by the LEM.The single motor force values ​​per stroke shown in 1120 are an idealized representation of how a free-piston engine can operate. The following is a discussion of specific embodiments in which a control system 1010 may be used to control the displacement of the piston assembly in a free-piston engine and achieve desired engine performance.

[0058] Figure 12 shows a flowchart 1200 of illustrative steps for a control system 1010 that controls the displacement of one or more piston assemblies along a propagation path in a free-piston engine, according to some embodiments of the present disclosure. As illustrated, the control system 1010 first determines the current position of one or more piston assemblies in the free-piston engine in step 1202. Next, in step 1204, the control system 1010 calculates a position-force trajectory based on the desired engine performance and the current position of one or more piston assemblies. Finally, the control system 1010 causes a displacement of one or more piston assemblies by applying the one or more force values ​​calculated in step 1204 to one or more piston assemblies. The sequence of steps 1202, 1204, and 1206 is repeated until the control system 1010 sends a command to stop. A stop command may be transmitted for any preferred reason, including, for example, switching to a different control technique, turning off the engine, the control system 1010 determining that a mechanical or electronic safety switch has been activated, any other preferred reason, or any combination thereof. The sequence steps 1202, 1204, and 1206 can be repeated based on the activation of a specific trigger or iteration throughout the engine stroke or cycle. For example, the sequence steps 1202, 1204, and 1206 can be repeated in response to a specific event, at a specific threshold crossing, at any other preferred trigger, or any combination thereof. In another embodiment, the sequence steps 1202, 1204, and 1206 can be repeated at specific time intervals (e.g., 1 kHz, 10 kHz, etc.) or at specific discrete position intervals (e.g., every 1 millimeter, every 1 micron, etc.). The particular control technique illustrated by flowchart 1200 is referred to herein as the position-force trajectory control technique.

[0059] The control system 1010 determines the current position of one or more piston assemblies in step 1202 using any suitable sensor 1030. Suitable sensors 1030 for determining the position of one or more piston assemblies include magnetic encoders, optical encoders, optical diffraction grating encoders, laser-based encoders, any other suitable sensors for determining position, or any combination thereof. The current position can be any position between BDC and TDC, including the boundary. In the case of a linear free-piston engine, the current position of one or more piston assemblies can be represented as a single dimension along a single propagation axis for each piston assembly; however, it will be understood that the teachings of this disclosure may also be applicable to free-piston engines in which piston assemblies can move in more than one dimension and the current position can be represented multidimensionally.

[0060] In step 1206, the control system 1010 transmits one or more commands to the free piston engine and / or its auxiliary devices to cause displacement of one or more piston assemblies by applying one or more force values ​​calculated in step 1204 to one or more piston assemblies. The force may also be applied to one or more piston assemblies by, for example, applying an electromagnetic force to one or more piston assemblies via a LEM. The following discussion will focus on applying a force through a LEM, but it will be understood that the application of a force to one or more piston assemblies may also be applied through other techniques, such as by adjusting the properties of the drive compartment (e.g., by adjusting the spring stiffness or spring constant of the drive compartment). In some embodiments, the application of motor force may be implemented using techniques such as those described in U.S. Patent No. 8,624,542, issued January 7, 2014, by the same applicant, which is incorporated herein by reference in its entirety.

[0061] The force values ​​generated on one or more piston assemblies in step 1206 are based on the position-force trajectory previously calculated in step 1204. The reference to forces "generated" on piston assemblies will be understood as referring to the control system 1010 causing the mechanism that imparts force to the piston assemblies to impart a force (including positive, negative, or zero force) as indicated by the control system 1010. In step 1204, the control system calculates the position-force trajectory for one or more piston assemblies, at least in part, based on the desired engine performance (e.g., desired apex position) and the current piston of one or more piston assemblies determined in step 1202. The calculation of the position-force trajectory by the control system 1010 is performed regardless of any deviation from previously determined trajectories (position-force, time-position, or any other preferred trajectory). For example, instead of using the trajectory calculated at the start of the stroke (i.e., the previously calculated trajectory) and then compensating for deviations from this previously calculated trajectory during propagation, a completely new trajectory is calculated each time the sequence steps 1202, 1204, and 1206 are repeated. This type of solution allows changes and modifications in the operating state of a free-piston engine to be taken into account using each new position-force trajectory calculation. The control system 1010 may also calculate the position-force trajectory based on the current or past operating state of the engine. For example, the control system 1010 may calculate the position-force trajectory based on any preferred properties of one or more piston assemblies (e.g., speed, acceleration, dimensions, mechanical properties), any preferred properties of the combustion chamber gas (e.g., pressure, temperature, density, specific heat, dimensions), any preferred properties of the drive chamber (e.g., gas properties in the case of gas springs, mechanical properties in the case of mechanical springs, dimensions), any preferred properties of the LEM (e.g., motor force constant, motor force limit, motor current limit, motor resistance), any preferred properties of the engine performance (e.g., efficiency, power output, airflow, fuel flow, exhaust flow, fuel composition, exhaust composition, temperature, pressure), any other preferred calculations, measurements, or estimates or indicators of the engine's operating characteristics, performance, parameters, and environment, or any combination thereof.

[0062] Figure 13 illustrates one embodiment of the position-force trajectory control technique disclosed herein, showing the position-velocity trajectory and the position-force trajectory (1310 and 1320, respectively). In this embodiment, the desired engine condition (based on the calculation of the position-force trajectory) is the desired apex position of the piston assembly. [ka] Therefore, the control objective is to cause displacement of the piston assembly so that it has zero velocity at the desired TDC and BDC positions. The actual vertex position of the piston assembly (x TDC and x BDC) is shown in Figure 13 as being different from the desired position of the piston assembly for illustrative purposes. However, it will be understood that the difference between the desired vertex position and the actual vertex position of the piston assembly can be zero, positive, negative, or any combination thereof, and may vary depending on the specific implementation of the position-force trajectory control technique. In this embodiment, the new position-force trajectory is calculated at fixed time intervals, as illustrated by the force values ​​shown in the position-force trajectory plot 1320 (i.e., at higher velocities, the force values ​​are applied to the piston assembly over longer distances, and at lower velocities, the force values ​​are applied to the piston assembly over shorter distances). That is, the sequential steps 1202, 1204, and 1206 in the flowchart 1200 in Figure 12 are repeated at fixed time intervals (e.g., 1, 5, 100 kHz). All force values ​​in the position-force trajectory plot 1320 are shown in Figure 13 as being in the opposite direction to the motion of the piston assembly for illustrative purposes (i.e., the LEM is always converting the kinetic energy of the piston assembly into electrical energy). However, it will be understood that each force value can be any suitable force value, including positive force values ​​(i.e., promoting displacement of the piston assembly during the expansion stroke and suppressing displacement of the piston assembly during the compression stroke), negative force values ​​(i.e., promoting displacement of the piston assembly during the compression stroke and suppressing displacement of the piston assembly during the expansion stroke), or zero or neutral force values ​​(i.e., allowing the piston assembly displacement to continue using its current momentum without any force being applied).

[0063] In this embodiment, referring to Figure 13, the first position-force trajectory of the compression stroke is calculated in the BDC as illustrated by the force value F1 in the position-force trajectory plot 1320. The control system 1010 determines at least partially (determined in step 1202) the current position of the piston assembly and the desired vertex position of the piston assembly. [ka] Based on this, the first force value is calculated (in step 1204 of the position-force trajectory in flowchart 1200 of Figure 12), and then, in this embodiment, the force is applied to the piston assembly via the engine's LEM (in step 1206) until the new position of the piston assembly is determined based on a specified time interval and the new position-force trajectory is calculated. These sequential steps are performed until the piston assembly reaches TDC(x TDC The process is repeated until the vertex is reached at ), at which point the control system 1010 then controls the new desired vertex position at BDC. [ka] The sequence of steps is repeated based on this. The desired vertex position may remain constant across the cycle, remain constant within the stroke, change across the cycle, change within the stroke, or any combination thereof.

[0064] In some embodiments, the control system 1010 may rely on the first law of thermodynamics (i.e., conservation of energy) to calculate the position-force trajectory in each step 1204. For example, with respect to a single-piston free-piston engine, the position-force trajectory can be calculated by recognizing that over the idealized stroke of the engine (i.e., without losses from heat transfer, gas blow-by, or friction), the work done from / to the LEM, the work done from / to the combustion chamber gases, the kinetic energy of the piston assembly, and the work done from / to the drive chamber should sum up to zero. This is, for example, W LEM However, this is work from / to LEM, W c However, this is work done from / to the combustion chamber gas, KE p However, this is the kinetic energy of the piston assembly, W d However, this can be captured by equation 1, which is the work done from / to the drive section.

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[0065] As shown in equations 6 and 7, each position-force trajectory is calculated, at least in part, based on the current position of the piston assembly and the desired apex position of the piston assembly (i.e., the desired target position), regardless of deviation from a previously determined trajectory, regardless of the time at which the new position-force trajectory will be calculated, and regardless of the time at which the piston assembly reaches the desired apex position. Iteratively calculating the position-force trajectory using this model over the stroke of the engine cycle allows changes and alterations in the operating state of the free-piston engine (high or low speed, intentional or unintentional) to be taken into account in each new position-force trajectory calculation, thereby providing a control technique for a free-piston engine that makes it possible to exclude obstacles in the operating state of the free-piston engine. The control technique makes it possible to exclude obstacles resulting, for example, from combustion variability, misfires, changes in fuel energy content, changes in gas temperature or pressure, LEM phase losses, changes or alterations in the drive compartment spring constant, or any other suitable obstacle, or any combination thereof. Equations 6 and 7 were derived assuming, for example, that there are no energy losses in the engine from heat transfer, gas blow-by, or friction. However, it will be understood that energy losses may be included in the position-force-trajectory control techniques disclosed herein. For example, heat transfer losses in the gaseous compartment of an engine can be modeled as a function of gas temperature (which may be modeled as a function of position or volume), heat transfer losses in the LEM can be modeled as a function of current and resistance, gas blow-by losses in the gaseous compartment of an engine can be modeled as a function of gas pressure (which may be modeled as a function of position or volume), and friction losses can be modeled as a function of constant force, material properties, position, and / or velocity.

[0066] Solving equation 6 in this embodiment requires the integral of pressure over volume change for both the combustion chamber gas and the gas spring gas. These integrals can be calculated using numerical iterative solutions (e.g., solving ordinary differential equations) based on a thermodynamic property model, a heat transfer model, a gas blow-by model, a friction model, or any other suitable model, or any combination thereof. These integrals can also be calculated using closed-form solutions based on thermodynamic models, which can incorporate the effects of heat transfer, gas blow-by, friction, and other losses in the system. Calculating position and force trajectories using closed-form solutions saves computation time compared to numerical iterative solutions. This allows the control system 1010 to calculate new position and force trajectories at shorter time intervals (i.e., at faster frequencies), which can better account for malfunctions in the engine's operating state. For example, the compression and expansion of gas in the combustion chamber and gas spring can be modeled as reversible. The reversible work done for the compression and expansion of a gas can be calculated using equation 8, where p1 is the pressure of the gas in state 1, V1 is the volume of the gas in state 1, V2 is the volume of the gas in state 2, and k is the ratio of the specific heats.

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[0067] For the control system 1010 to solve equation 9 or 10, the gas pressure in the combustion chamber and gas spring must be measured or estimated, or both, at each current position of the piston assembly. The gas pressure at the current position of the piston assembly can be measured using any suitable sensor 1030, such as a piezoelectric pressure transducer, a strain-based pressure transducer, a gas ionization sensor, any other suitable pressure sensor, or any combination thereof. The gas pressure at the current position of the piston assembly can also be estimated. In general, relying on pressure estimates (as opposed to pressure measurement) can save costs and lead to more reliable engine operation by avoiding the need for expensive and often unreliable pressure sensors. For example, gas compression and expansion are [ka] However, this is the estimated gas pressure at the current position of the piston assembly, and p p However, this is the measured or estimated gas pressure at a previously determined position of the piston assembly, V p However, the measured or estimated volume of gas at the same previously determined position in the piston assembly can be modeled as isentropic or polytropic using equations 11 or 12, respectively. Equations 11 and 12 are applicable to estimating the current gas pressure in any compartment of the engine, including the combustion compartment and the drive compartment.

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[0068] In some embodiments, the control system 1010 may estimate the current gas pressure in the free-piston engine compartment by integrating the energy equilibrium over the stroke of the engine cycle from the fixed previous position of the free-piston assembly to the current position, where the fixed previous position may be, for example, the apex position, the port open or closed position, a combustion event, any other preferred position, or any combination thereof. For example, with respect to a single-piston free-piston engine with a gas spring driven compartment, the current gas pressure is: [ka] However, this is the work done from the fixed previous position to the current position, [ka] However, this is the work done from / to the combustion chamber gas from the fixed previous position to the current position. [ka] However, this can be estimated by using equation 15, which models the energy equilibrium of the free piston assembly from a fixed previous position to its current position, and is the work done from / to the gas spring compartment gas from a fixed previous position to its current position.

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[0069] Estimating current values ​​(e.g., current gas pressure) using previously calculated values ​​(e.g., force, acceleration, pressure, velocity, position) may require the use of smoothing filters such as infinite impulse response (IIR) filters or finite impulse response (FIR) filters with suitable coefficients for the value of interest, or dynamic estimators such as the Ruwenberger detector or Kalman filter. The gas pressure at the current or previous position of the piston assembly can be estimated using thermodynamic relation models (e.g., equations 11 or 12), force equilibrium models (e.g., equations 13 or 14), or first-law analysis (e.g., equations 6, 7, 9, 10, or 19), or a combination thereof. For example, the gas pressure at the current or previous position of the piston assembly can be estimated using two models, one of which is used as the primary estimate, and the other model is used to refine the secondary estimate using estimation techniques such as the Kalman filter, Ruwenberger detector, or model predictive estimation. In another embodiment, the gas pressure at the current or previous position of the piston assembly can be estimated based on minimizing the error between estimates from any two models. This minimization may include other costs such as, for example, acceleration estimates based on several position measurements, deviations from previous pressure measurements or estimates, deviations from pressure measurements or estimates from previous cycles or strokes, calculation time, information about noise or fault statistics, any other suitable costs, or any combination thereof. In some embodiments, the estimation of the gas pressure at the current or previous position of the piston assembly can be improved by pressure measurements from any otherwise unsuitable sensors that may provide insufficient, noisy, or slow measurements.

[0070] When the absolute velocity of the piston assembly is low and its absolute acceleration is high, the efficiency of the LEM may be low, and the LEM's ability to cause displacement of the piston assembly may be limited. To avoid the LEM applying force to the piston assembly when its efficiency is low and its control authority is limited, in some embodiments, the control system 1010 may reduce or eliminate the magnitude of the force applied to the piston assembly based on specified operating parameters of the free-piston engine. The specified operating parameters may include the position, velocity, or acceleration of the piston assembly, the temperature of the LEM's stator or transducer, the gas pressure in the engine compartment, any other suitable parameters, or a combination thereof. For example, the control system 1010 may shut off the LEM's ability to apply force to the piston assembly based on the position of the piston assembly, as shown in Figure 14, which shows the position-velocity trajectory 1410 and the position-force trajectory 1420. In this embodiment, the control system 1010 calculates the position-force trajectory in accordance with the disclosure, but when the position of the piston assembly is outside the cutoff position, the control system 1010 determines not to apply the force value calculated in the position-force trajectory calculation step 1204 to the piston assembly. In some embodiments, the control system 1010 may determine, based on specified operating conditions of the free-piston engine, to apply a force to the piston assembly that is different from the force value calculated in the position-force trajectory calculation step 1204. For example, the control system 1010 may apply a force reduction function to the force calculated in the position-force trajectory calculation step 1204 based on the position of the piston assembly (e.g., outside the cutoff position) to avoid abrupt changes in the operating state of the engine. In some embodiments, the control system 1010 may determine, based on specified operating conditions of the free-piston engine, to not calculate the position-force trajectory and not to apply a force to the piston assembly.

[0071] While various models for calculating position-force trajectories and estimating gas pressure (i.e., equations 1-19) are applicable to single-piston free-piston engines, it will be understood that the same models can be extended and applied to free-piston engines with multiple piston assemblies, such as opposed-piston free-piston engines with separate drive compartments, separate LEMs, and shared combustion compartments (as illustrated in Figures 2-4 and 7, for example). For instance, the same first-law analysis used to derive equation 1 can be applied to each piston assembly in an opposed-piston free-piston engine with separate drive compartments, separate LEMs, and shared combustion compartments. This is W LEM,1 and W LEM,2 , a job from / to two LEMs, W c However, this is work done from / to the combustion chamber gas, KE p,1 YobiKE p,2 However, this is the kinetic energy of the two piston assemblies, W d,1 and W d,2 However, this yields energy equilibrium equations 20a and 20b, which are the work done to and from the two drive sections. Equations 20a and 20b can be used by the control system 1010 to calculate the position-force trajectory for each individual piston assembly using the same or similar models as those used to derive equations 6, 7, 9, and 10.

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[0072] A consideration arising in the control of a free-piston engine with opposing piston assemblies is the synchronization of the piston assemblies. In some opposing-piston free-piston engines, it may be desirable that the vertices (at both TDC and BDC) of the two piston assemblies be at least substantially synchronized to maintain system stability. In other opposing-piston free-piston engines, a certain level of desynchronization may be desired for engine performance purposes, such as engine breathing, gas exchange, or any other preferred engine operating conditions. In some embodiments of the opposing-piston free-piston engine, the control system 1010 may adjust for differences between the positions of the individual piston assemblies. As used herein, the term “adjust” means to control to a reference such as zero. The control system 1010 may employ any preferred control technique for adjustment, such as proportional-integral-derivative (PID) control, optimal control, robust control, linear quadrant regulator control, model predictive control, adaptive control, any other preferred technique, or any combination thereof. In some embodiments, the control system 1010 may use PID control to adjust and synchronize the positions of the piston assemblies. For example, the control system 1010 may use PID control to determine the control input (e.g., force values ​​applied to the piston assemblies by individual LEMs) and adjust for the positional differences between the piston assemblies relative to their centers of motion. Counterforces may be added to each piston assembly to synchronize them substantially equally and minimize interference at the apex position. This may be done continuously to maintain a substantially net force balance and thus sufficient synchronization. In some embodiments, the control system 1010 may use a defined Poincaré diagram at the zero-velocity position of the piston assemblies (i.e., at the individual apex). For example, the control system 1010 may divide the stroke into two halves, applying an additional motor force in one direction during the first half of the stroke, and then applying an additional motor force in the opposite direction during the second half of the stroke.The control system 1010 may, prior to the expansion stroke, determine that the first piston assembly will lag behind the BDC (for example, using any preferred expected phase matching of the two piston assemblies, based on the timing of the previous stroke, based on any other preferred technique, or any combination thereof), and apply an additional motor force to the first piston assembly in the direction of motion during the first half of the expansion stroke (i.e., to encourage displacement), and then apply an additional motor force to the first piston assembly in the opposite direction of motion during the second half of the expansion stroke (i.e., to suppress displacement). Conversely, with respect to the second piston assembly, the control system 1010 may apply an additional motor force to the second piston assembly in the opposite direction of motion during the first half of the expansion stroke (i.e., to suppress displacement), and then apply an additional motor force to the second piston assembly in the direction of motion during the second half of the expansion stroke (i.e., to encourage displacement). In some embodiments, the control system 1010 may determine the synchronization force based on a desired timing for the desired engine performance. For example, the control system 1010 may determine the synchronization force to be applied to one or both piston assemblies so that the apex of each individual piston assembly occurs within a sufficiently small time difference.

[0073] In some embodiments, the control system 1010 may use an iterative adaptive control technique. Iterative adaptive control may be advantageous when the operating state, conditions, performance, and / or parameters of the free-piston engine are relatively steady and inter-cycle variations are limited. In some embodiments, the control system 1010 may use an iterative adaptive control technique in which it determines the position-force trajectory in each step 1204 for the current engine cycle based on the position-force trajectory from the previous engine cycle. In some embodiments, the control system 1010 may use an iterative adaptive control technique in which it drives force values ​​toward a known desired propagation path (e.g., to implement a smoother or more continuous force profile). For example, the control system 1010 may first estimate the position-force trajectory as a series of discrete force values ​​over the engine cycle based on information from the previous cycle (e.g., force, value, engine performance, etc.). The control system 1010 may then apply discrete force values ​​to the piston assembly over each stroke of the engine cycle, and at the end of each cycle, the control system 1010 may adjust the discrete force values ​​based on engine operating characteristics, measurements, performance, and / or conditions. The control system 1010 may modify all or some of the discrete force values ​​prior to a subsequent cycle, for example, if the piston assembly does not sufficiently achieve the desired target position for a given stroke. For example, if the piston reaches its peak before the desired target TDC in the previous cycle, the control system 1010 may reduce the magnitude of some or all of the discrete force values ​​in the subsequent cycle. In embodiments using a opposed-piston free-piston engine with a shared (or common) combustion compartment, the control system 1010 may, during a subsequent cycle, depend on or independently modify the discrete force values ​​in one or more portions of the stroke for one or both of the piston assemblies.For example, if, in the current engine cycle, the exhaust piston assembly reaches its peak at TDC after the intake piston assembly reaches its peak at TDC, the control system 1010 can adjust the discrete force values ​​applied to the exhaust piston assembly in subsequent cycles to achieve sufficient synchronization at TDC, but cannot adjust the discrete force values ​​applied to the intake piston assembly. This can be achieved, for example, by the control system 1010 reducing the magnitude of the discrete force values ​​applied to the exhaust piston assembly over the first half of the stroke, thereby allowing the midpoint velocity of the piston to increase, and then increasing the magnitude of the discrete force values ​​applied to the exhaust piston assembly over the second half of the stroke, thereby achieving sufficient synchronization at TDC. In some embodiments, the control system 1010 may use an iterative adaptive control technique based on calculating deviations from a previously determined trajectory (position-force, position-velocity, time-position, or any preferred trajectory).

[0074] In some embodiments, the control system 1010 may use a hybrid control technique that allows switching between multiple control techniques. The hybrid control technique may be advantageous for controlling a free-piston engine across a wide variety of operating conditions, controlling the free-piston engine when sufficiently fast and large failures of engine operation may occur (e.g., misfire, mechanical failure, gas quality change, or any other suitable change), and controlling the free-piston engine under steady or stable operating conditions (e.g., during steady and continuous power output). For example, the control system 1010 may employ a position-force trajectory control technique during engine startup, and then switch to an iterative adaptive control technique when engine operation becomes sufficiently stable or steady. The control system 1010 may then switch back to the position-force trajectory control technique if a sufficiently large failure is detected or if new engine operating conditions are desired (e.g., more or less power output, engine shutdown). Figure 15 illustrates one possible implementation of the hybrid control technique. The control system 1010 uses the position-force trajectory control technique in 1502. If the control system 1010 determines, based on any preferred criterion (e.g., absence of misfires, stable power output, stable efficiency, thermal equilibrium, or other preferred conditions), that the conditions have become sufficiently steady, the control system 1010 switches to the iterative adaptive control technique at 1504. If the control system 1010 determines, based on any preferred criterion, that the operating conditions have become sufficiently unsteady, or will become unsteady, the control system 1010 switches back to the position-force-trajectory control technique at 1502.

[0075] For ease of reference, the diagram may show multiple components labeled with the same reference number. It will be understood that this does not necessarily mean that multiple components labeled in the same way are identical to one another. For example, pistons labeled 125 may have different sizes, geometric shapes, materials, any other desirable properties, or any combination thereof.

[0076] The foregoing is merely an illustration of the principles of the present disclosure, and various modifications may be made by those skilled in the art without departing from the scope of the present disclosure. The embodiments described above are presented for illustrative purposes only and not limiting purposes. The present disclosure can also take many forms other than those expressly described herein. It is therefore emphasized that the present disclosure is not limited to the methods, systems, and apparatus expressly disclosed, but is intended to include variations and modifications thereof, which are within the spirit of the following claims.

Claims

1. A method carried out by a computer system programmed to control the displacement of opposing free piston assemblies, wherein both opposing free piston assemblies are configured to move in translational motion within the same cylinder, and a reaction compartment is located between the opposing free piston assemblies, and the method is Using the control system, a first position and force trajectory is determined for the first free piston assembly among the opposing free piston assemblies, Using the control system, a second position and force trajectory is determined for the second free piston assembly among the opposing free piston assemblies, To calculate the first synchronous force for the first free piston assembly, To calculate the second synchronization force for the second free piston assembly, Based on the first position-force trajectory, the second position-force trajectory, the first synchronization force, and the second synchronization force, displacement of the first free piston assembly and the second free piston assembly is generated. Methods that include...

2. The first free piston assembly and the second free piston assembly circulate between separate vertices defining two strokes, and the method is The method according to claim 1, further comprising using a linear electromagnetic machine to produce a net electrical energy output over both of the two strokes.

3. The method according to claim 1, wherein causing displacement of the first free piston assembly and the second free piston assembly includes adjusting the difference between the position of the first free piston assembly and the position of the second free piston assembly.

4. The method according to claim 1, wherein causing displacement of the first free piston assembly and the second free piston assembly includes synchronizing the vertex of the first free piston assembly with the vertex of the second free piston assembly.

5. The method according to claim 1, wherein the first synchronous force and the second synchronous force are opposing forces.

6. a) The first position-force trajectory includes the force applied to the first free piston assembly, regardless of deviation from a previously determined trajectory, based on the current position and target position of the first free piston assembly. b) Apply the force to the first free piston assembly over a first time interval, c) The method according to claim 1, wherein the first free piston assembly repeats a) and b) until it reaches at least one of the target position or vertex position.

7. The method according to claim 6, wherein the target position includes a desired vertex position.

8. a) The method of claim 6, wherein the force is determined at least in part on an estimated pressure in a compression section in contact with the first free piston assembly.

9. d) Determining a new target position for the first free piston assembly, e) Determining a new force to apply to the first free piston assembly based on the new current position of the first free piston assembly and the new target position, f) Applying the new force to the first free piston assembly over a second time interval The method according to claim 6, further comprising:

10. d) Determining a new force to apply to the first free piston assembly based on the new current position of the first free piston assembly and the target position, e) If the current position is outside the cutoff position threshold, determine that the new force will not be applied to the first free piston assembly over a second time interval. The method according to claim 6, further comprising:

11. A first free piston assembly configured to move in translational motion within a cylinder, A second free piston assembly configured to move in translation within the cylinder and facing the first free piston assembly, Control system and A system comprising, wherein the reaction compartment is located between the first free piston assembly and the second free piston assembly, and the control system is To determine the first position and force trajectory for the first free piston assembly, To determine the second position and force trajectory for the second free piston assembly, To calculate the first synchronous force for the first free piston assembly, To calculate the second synchronization force for the second free piston assembly, Based on the first position-force trajectory, the second position-force trajectory, the first synchronization force, and the second synchronization force, displacement of the first free piston assembly and the second free piston assembly is generated. A system configured to perform the following actions.

12. The first free piston assembly and the second free piston assembly circulate between separate vertices that define two strokes, and the control system, The system according to claim 11, wherein a linear electromagnetic machine is used to generate a net electrical energy output over both of the two strokes.

13. The system according to claim 11, wherein the control system is configured to cause displacement of the first free piston assembly and the second free piston assembly by adjusting the difference between the position of the first free piston assembly and the position of the second free piston assembly.

14. The system according to claim 11, wherein the control system is configured to cause displacement of the first free piston assembly and the second free piston assembly by synchronizing the vertex of the first free piston assembly with the vertex of the second free piston assembly.

15. The system according to claim 11, wherein the first synchronous force and the second synchronous force are opposing forces.

16. The control system is a) Determining the first position-force trajectory by determining the force to be applied to the first free piston assembly based on the current position and target position of the first free piston assembly, regardless of deviation from a previously determined trajectory, b) Applying the force to the first free piston assembly over a first time interval, c) Repeating a) and b) until the first free piston assembly reaches at least one of the target position or vertex position. The system according to claim 11, further configured to perform the following:

17. The system according to claim 16, wherein the target position includes a desired vertex position.

18. The control system is Based on the new current position of the first free piston assembly and the target position, a new force to be applied to the first free piston assembly is repeatedly determined, The new force is repeatedly applied to the first free piston assembly over individual new time intervals. The system according to claim 16, further configured to repeat a) and b).

19. The control system is d) Determining a new target position for the first free piston assembly, e) Determining a new force to apply to the first free piston assembly based on the new current position of the first free piston assembly and the new target position, f) Applying the new force to the first free piston assembly over a second time interval The system according to claim 16, further configured to perform the following:

20. The control system is d) Determining a new force to apply to the first free piston assembly based on the new current position of the first free piston assembly and the target position, e) If the current position is outside the cutoff position threshold, determine that the new force will not be applied to the first free piston assembly over a second time interval. The system according to claim 16, further configured to perform the following:

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