Heat engine

A heat engine comprising compressor and expander displacement elements (210, 211) reciprocating in respective compression and expansion chambers (111, 111′, 102, 102′) and arranged in a linear, free piston configuration, a combustor (116) separate from the compression and expansion chambers (111, 111′, 102, 102′), and a linear energy conversion device (212, 213) providing conversion of solid, liquid, or gaseous fuel into hydraulic, electric, or pneumatic energy by means of subjecting a working fluid to a thermodynamic cycle with substantially constant pressure combustion. The inlet and outlet valves of the compression chamber (102, 102′) and the rate of fuel injection to the combustor (116) are actively controlled by an electronic controller to avoid engine damage, and to maintain thermodynamic efficiency over a wide range of loads.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT International Application No. PCT/GB2010/050581 filed on Apr. 01, 2010, which claims priority to Great Britain Application No. 0905959.3 filed on Apr. 07, 2009, both of which are fully incorporated by reference herein.

The present invention relates to a heat engine.

Efficient conversion of heat into mechanical work has concerned researchers and engineers for more than a century, and recent years have seen an increasing focus on pollutant emissions from power generation. While internal combustion engines in many cases provide superior fuel conversion efficiencies, external combustion engines have unrivalled performance with respect to exhaust gas emissions levels, mainly due to significantly lower combustion temperatures. Exhaust gas components commonly accepted to pose human health risks, such as nitrogen oxides, carbon monoxide, and particulate matter, are increasingly being regulated by governments worldwide, particularly in densely populated areas. An external combustion engine with a fuel efficiency competitive to that of the internal combustion engine would have significant appeal due to the environmental benefits which could be realised.

Warren (U.S. Pat. No. 3,577,729) described a heat engine operating according to the Joule (also known as Brayton) thermodynamic cycle, that is, with essentially constant pressure combustion. The engine has similarities in operation to a conventional gas turbine, howeveruses reciprocating piston-cylinder arrangements for the compressor and expander units. The use of reciprocating machinery for these components improves compression and expansion efficiencies compared with the rotodynamic machinery used in gas turbine engines, however this also dramatically reduces system power to weight ratio. This “reciprocating Joule cycle engine concept” was discussed by Bell and Partridge (Bell M A; Partridge T. Thermodynamic design of a reciprocating Joule-cycle engine. Proc. Institution of Mechanical Engineers: Journal of Power Energy vol. 217, pages 239-246, 2003) and Moss et al. (Moss R W; Roskilly A P; Nanda S K. Reciprocating Joule-cycle engine for domestic CHP systems. Applied Energy vol. 80, pages 169-185, 2005), who demonstrated the engine's potential for high fuel efficiency. These reports also showed a high sensitivity to frictional losses and advised that great care must be taken in the design of the engine in order to minimise mechanical friction.

Benson (U.S. Pat. No. 4,044,558) described a closed cycle reciprocating Joule cycle engine using a linear, free-piston engine configuration and a linear load. This configuration is more compact than a crankshaft engine, and significantly reduces frictional losses in the system through utilising the linear power output directly. The use of a closed cycle gives flexibility in the choice of working fluid, benefiting system performance and increasing lifetime. However, a closed cycle engine requires a heat exchanger for transferring heat from an external source to the working fluid. Materials properties in the heat exchanger limit the permitted maximum cycle temperature in closed cycle engines, which limits the cycle efficiency that can be achieved. The use of an open cycle, as that proposed by Warren, in the system described by Benson appears desirable to improve fuel efficiency, but is associated with a number of challenges.

The free-piston engine principle is described extensively in the literature. The main challenge with free-piston machinery is well documented: due to the absence of a crankshaft mechanism, as that known from conventional engines, other means of controlling piston motion is required. Highly accurate control is required in order to avoid stroke lengths that can lead to mechanical contact between the piston and the cylinder head (“over-stroke”), which may cause catastrophic damage to the engine. At the same time, a low cylinder clearance volume is required to achieve efficient compression and expansion with high volumetric efficiencies, to maintain high engine efficiency. Moreover, the powering and control of engine accessories, such as valves, fuel injection, cooling pump, and lubrication pumps must be resolved by alternative means in a free-piston engine. In a conventional engine, rotating pumps can readily be driven by the crankshaft, and the timing of valves and fuel injection can be controlled by the crank position. The free-piston engine does not have a rotating power output or the positional reference that the crank angle offers, and, moreover, the piston stroke length is not fixed.

A further potential challenge in the reciprocating Joule cycle engine is the pulsating nature of the flow through the combustion chamber, which is a result of the reciprocating compression and expansion devices. In order to ensure efficient combustion, low emissions formation, and combustion stability, one may need to vary the rate of fuel injection according to the working fluid flow. In a crankshaft engine, it is relatively straight-forward mechanically to implement pulsating fuel injection to increase fuel flow subsequent to the compressor cylinder discharge, since both these components are controlled by crankshaft position and no timing difficulties will occur. In the free-piston engine, an alternative method must be developed.

The present invention relates to a highly efficient engine concept for the conversion of energy from solid, liquid, or gaseous fuels into electric, hydraulic, or pneumatic energy. It is intended for use in applications such as electric power generation, combined heat and power systems, propulsion systems, and other applications in which conventional combustion engines are presently used.

According to the present invention, there is provided a heat engine comprising: a compression chamber; a first positive displacement element reciprocable within said compression chamber; an expansion chamber; a second positive displacement element reciprocable within said expansion chamber; wherein said first and second positive displacement elements are mechanically coupled to reciprocate in unison in a free-piston configuration; conduit means for conducting said working fluid from said compression chamber to said expansion chamber; heating means for supplying heat to a working fluid in a heating section of said conduit means; first valve means for controlling the flow of said working fluid into said compression chamber; second valve means for controlling the flow of said working fluid from said compression chamber to said heating section; third valve means for controlling the flow of working fluid from said heating section to said expansion chamber; fourth valve means for controlling the flow of said working fluid out of said expansion chamber; a sensor adapted to output a signal corresponding to a position and/or velocity of the first/second positive displacement element; and a controller for continuously controlling the third and/or fourth valve means and/or the rate of supply of heat to the working fluid in accordance with the signal output by the sensor.

By providing a sensor adapted to output a signal corresponding to a position and/or velocity of the first/second positive displacement element, and a controller for variably controlling the third and/or fourth valve means and/or the rate of supply of heat to the working fluid in accordance with the signal output by the sensor, the engine is able to achieve higher fuel efficiency, enhanced control of the displacement elements, and greater operational flexibility, in particular greater adaptability to load variations. The sensor signal can be used to identify a danger of over-stroke or engine stalling, or fluctuations in operating conditions. Accordingly, the controller allows accurate control of valve timings and/or rate of heat supply, thereby maintaining high fuel efficiency for a wide range of loads, allowing its use in applications with rapidly changing load demands, and avoiding stalling or engine damage.

Preferably, the heat engine operates on an open cycle.

Using an open cycle enables a higher engine cycle efficiency to be achieved. When a closed cycle is used, a heat exchanger is required to transfer heat to the working fluid, and materials properties of the heat exchanger limit the maximum cycle temperature. Using an open cycle, higher temperatures can be used, increasing the fuel efficiency of the engine. In an open cycle system, fuel can be injected directly into the working fluid, offering much faster heat transfer and therefore better control and adaptability of the engine to changing conditions. The enhanced controllability resulting from the use of an open cycle constitutes a major advantage of this engine over the prior art.

Preferably, the heating means is a combustor.

Preferably, the controller is adapted to continuously control the supply of heat to the working fluid by outputting a signal for continuously controlling a rate of fuel injection to the combustor.

Advantageously, this allows the rate of supply of heat to the working fluid to be changed rapidly, enabling rapid response of the engine to load changes. Load changes are identified from unexpected changes in the velocity of the displacement elements monitored by the sensor. The controller adapts the rate of fuel injection to the combustor in response to such changes, thereby maintaining efficient engine operation. Furthermore, this feature advantageously provides a means for controlling the rate of supply of heat to the working fluid to compensate the pulsating nature of the flow of the working fluid through the combustion chamber.

In one embodiment, the controller controls the first, second, third and fourth valve means.

Although the first and second valve means may be controlled passively, engine control can be further enhanced by controlling all the valve means using the controller.

The second displacement member may divide the expansion chamber into two expansion subchambers, the third valve means being adapted to control the flow of working fluid alternately to each expansion subchamber.

Advantageously, configuring the second displacement element as a double-acting piston in this manner improves the efficiency of the engine.

The first displacement member may divide the compression chamber into two compression subchambers, the first valve means being adapted to control the flow of working fluid alternately to each compression subchamber.

The heat engine may further comprise an energy conversion device comprising at least one reciprocable element coupled for reciprocation with said first and second displacement members.

Advantageously, this enables the reciprocating motion of the displacement members to be converted to electrical, hydraulic or pneumatic energy for example.

The energy conversion device may be positioned between the compression chamber and the expansion chamber.

Advantageously, positioning the energy conversion device between the compression and expansion chambers means that the mechanical coupling between the first and second displacement members is only required to extend through one end of the compression and expansion chambers, minimising system friction and leakage.

The heat engine may further comprise a heat exchanger for transferring heat from working fluid conducted from the expansion chamber to working fluid conducted from the compression chamber.

Advantageously, the inclusion of a regenerative heat exchanger or recuperator causes the efficiency of the engine to peak at a significantly lower pressure ratio.

Preferably, the controller is adapted to adjust the timings of opening and/or closing the third and/or fourth means and/or to adjust the rate of input of heat to the working fluid to maintain stable engine operation when the signal output by the sensor indicates a change in kinetic energy of the first/second displacement member corresponding to a change in load force on the first/second displacement member.

In this way, the engine is advantageously adapted to a wider range of loads, and to changing loads.

In one embodiment, the controller is adapted to advance closure of the fourth valve means and to delay the opening of the third valve means, when the signal output by the sensor indicates an increase in kinetic energy of the first/second displacement element sufficient for the second displacement member to travel past a predefined end point.

Advantageously, this avoids engine damage due to over-stroke of the displacement elements.

In one embodiment, the controller is adapted to delay closure of the third valve means, when the signal output by the sensor indicates a decrease in kinetic energy of the first/second displacement element sufficient for the second displacement member to fail to reach a predefined end point.

Advantageously, this reduces the likelihood of engine stalling due to a sudden load change on the displacement elements.

FIG. 1shows a heat engine system according to a first embodiment of the invention. The system operates on an external combustion cycle with essentially constant pressure combustion, similar to that of conventional gas turbine engines. The compression and expansion devices consist of double-acting reciprocating cylinders arranged in a linear, free-piston configuration, and load is extracted using a linear-acting load device such as a linear electric generator or a hydraulic cylinder. An electronic controller is used to control the opening and closing of cylinder valves, as well as the rate of fuel injection.

The system consists of an expansion cylinder100with a reciprocable piston101therein. The piston101provides sealing against the walls of cylinder100through accurate machining or with the use of piston rings as is common in conventional engines, and divides the cylinder100into two working chambers102and102′. The piston101is fixed to a rod103, and the rod103extends through one or both ends of cylinder100, preferably supported by a bushing with appropriate sealing. Lubrication of the surfaces inside the cylinder100should be provided through the injection of lubricating oil, as known from conventional engines, or with the addition of a lubricating layer on the surface during manufacturing (also known as solid film lubrication). On each end of the cylinder100, a valve system104or104′ provides control of a flow connection between the respective working chambers102or102′ and an exhaust channel105. Similarly, on each end of the cylinder, a valve system106or106′ provides control of a flow connection between the respective working chamber102or102′ and a combustion products channel107. The valve systems104,104′,106and106′ are inFIG. 1illustrated as having conventional poppet-type valves, however they can be of any type suitable for operation at high temperature, such as rotating or sliding valves. The valve systems104,104′,106and106′ incorporate actuators which drive the opening and closing of the connection between working chambers102and102′ and combustion products channel107and exhaust channel105by means of electric, hydraulic, or pneumatic energy. Preferably, electro-magnetic valve actuators should be employed. The operation of valve systems104,104′,106and106′ is electronically controlled and the required position of each valve at any time (open or close) is transmitted by control signals108a-d.

The system further incorporates a compression cylinder109with a reciprocable piston110therein, dividing the cylinder109into two working chambers111and111′. The rod103extends through one or both ends of cylinder109, and is fixed to the piston110. Lubrication of the in-cylinder surface of cylinder109and sealing between the piston110and the cylinder109are provided similarly as described above. On each end of cylinder109, a valve system113or113′ connects the respective working chamber111or111′ to an intake air channel112, and a valve system114or114′ connects the respective working chamber111or111′ to a compressed air channel115. The operation of the valve systems113,113′,114and114′ is similar to that described above, but with the opening and closing of the valve systems being controlled by control signals108e-h.

Connecting the compressed air channel115and the combustion products channel107is a combustor116. The combustor116is assumed to have a design similar to those combustors used in conventional gas turbine engines. The combustor incorporates a combustion chamber117, a fuel injector118, and internal means for igniting a combustible mixture. Fuel is supplied through a fuel line119which has an electronically controllable valve120for control of the fuel flow rate to the injector118. The electronic control signal for the valve120is supplied by a control signal121.

A position sensor consists of a stationary part122and a non-stationary part122′. Fixed to the rod103is the non-stationary position sensor part122′. The stationary position sensor122records the position of the non-stationary part122′ and generates a position sensor signal124which identifies the position of the rod103at any time. The sensor may be a Hall effect sensor, although the skilled person will appreciate that other types of sensor may be used. The rod103further has a load connection123, to which a linear-acting load can be coupled. The load can be of any type, such as a linear electric machine, a hydraulic, pump, or a pneumatic compressor. An electronic controller125receives the position signal124and, based on the instantaneous and previous values of this signal, generates valve signals108a-fand fuel injection signal121, thereby controlling the opening and closing of the cylinder valves and the fuel flow rate.

Through the use of an open cycle with infinitely variable valve timings and accurate control of fuel injection rate, high fuel efficiency and operational flexibility can be realised. The linear engine configuration gives inherently low system frictional losses as well as a compact system with high power to weight ratio.

Accurate valve control combined with the direct control of the heat flow rate through fuel injection control also gives significantly enhanced mechanical control of the engine. The challenges associated with piston motion control are resolved by identifying a danger of over-stroke using a piston position sensor and an electronic controller to adjust valve timing accordingly, to eliminate any risk of engine damage. This also gives the system superior response to changes in operating conditions, allowing use in applications with rapidly changing load demands, in which prior art systems would be unsuitable. The enhanced controllability resulting from the use of an open cycle constitutes a major advantage of the proposed system over prior art.

FIG. 2shows an alternative embodiment of the system. In addition to those components described above, the embodiment shown inFIG. 2incorporates a regenerative heat exchanger204(also known as a recuperator), air intake filters209and209′, and a linear electric machine load device212and213. For clarity, the control system has been omitted and the valve systems have been simplified in the figure. The direction of fluid flow through the engine is indicated by the arrows. The valve systems104and106(seeFIG. 1) is replaced with a three-way valve system201and the valve systems104′ and106′ is replaced with a three-way valve system201′. Each valve system201or201′ is electronically controlled and includes an actuator, and can be commanded in one of three positions: closed, in which no flow through the valve is permitted; intake, in which fluid can only flow between combustion products channel107and the respective working chamber102or102′; and exhaust, in which fluid can only flow between the respective working chamber102or102′ and the flow channel202. The expansion cylinder piston101is fitted with piston rings211, of conventional design, in order to minimise leakage between chambers102and102′.

The intake air channel112(seeFIG. 1) is replaced with two separate intake ducts209and209′ which include intake air filters. This allows atmospheric air to be used directly in the engine without the risk of any impurities entering the system, similarly to conventional combustion engines. The valve systems113,113′,114, and114′ consist in this embodiment of passive, one-way valves, that is, their opening and closing are controlled by the instantaneous pressure difference across the individual valves. (Such valves are also known as check valves or non-return valves.) The settings of one-way valves113,113′,114, and114′ should be such that, as the compression cylinder piston110reciprocates, atmospheric air is pumped into the compressed air channel115.

The recuperator204works as a conventional heat exchanger, i.e. having two flow passages separated by a thin wall of large surface area, allowing heat to be transferred between fluids in the two passages. The recuperator204is positioned such that the fluid in flow channel202is led through the first passage through inlet205and exhausted to the exhaust channel105through outlet206. Similarly, fluid in compressed air channel115is permitted to enter the second recuperator passage through inlet207and is exhausted to flow channel203through outlet208. The flow channel203is connected to combustor116and the combustor outlet is connected to combustion products channel107, similarly as described above.

The embodiment illustrated inFIG. 2includes a linear electric generator acting as the load, comprising a stationary part212(the stator) and a moving part213(the translator). The electric machine is of conventional design, using coils positioned in the stator and permanent magnets positioned in the translator. In the embodiment shown, the translator213is embedded into the rod103to minimise system overall weight and size. For the same reason, in the embodiment illustrated inFIG. 2the load device is positioned between the compression and expansion cylinders. Using this configuration, the rod103is only required to extend through one end of compression cylinder109and expansion cylinder100, minimising system friction and leakage.

Basic System Operation

Referring toFIG. 1, the operation of the engine can be described as follows. The piston assembly consists of rod103, expansion cylinder piston101, compression cylinder piston110, and position sensor122′. The piston assembly attains a linear, reciprocating motion, driven by the net force which at any time is acting on it and constrained by the design of the expansion cylinder100, compression cylinder109, and load device coupled to load connection123. Assume that the piston assembly is moving towards the left hand side (LHS), as the arrow indicates. Atmospheric air is admitted to the intake air channel112and from that channel to compression cylinder chamber111′ through valve system113′ which is in the “open” position. Air in compression cylinder chamber111is being compressed and, at some point during the right-to-left stroke, valve system114is commanded open and the compressed air is discharged from chamber111into compressed air channel115.

During operation, air compressed in compression cylinder109flows from compressed air channel115to combustor116. In combustor116, fuel is injected by injector118and ignited, and high-temperature combustion products result. The combustion products flow through combustion products channel107to expansion cylinder100. As the piston assembly commences its motion towards the LHS, inlet valve system106′ is open and allows combustion products from combustion products channel107to enter expansion cylinder chamber102′. At some point during the stroke, inlet valve106′ closes, and the combustion products trapped in expansion cylinder chamber102′ expand down to a lower pressure level while performing work on piston101. During the complete leftwards motion of the piston assembly, valve system104is open and combustion products from the previous stroke are discharged from chamber102to exhaust channel105and disposed of through the exhaust outlet.

As the piston assembly reaches its LHS endpoint, the second part of the cycle commences. Expansion cylinder valve system104closes and combustion products are admitted to expansion cylinder chamber102through opening of valve system106. The pressure from the combustion products acting on piston101accelerates the piston assembly towards the RHS. At the same time, expanded combustion products from the previous stroke are discharged from expansion cylinder chamber102′ to the exhaust channel105through opening of valve system104′. In compression cylinder109the closing of valve system114and opening of valve system113allows atmospheric air to be admitted into chamber111, while closing of valve system113′ and subsequent opening of valve system114′ allows air admitted into chamber111′ in the previous stroke to be compressed and discharged into compressed air channel115.

The opening and closing of the valve systems104,104′,106,106′,113,113′,114, and114′ are controlled by electronic controller125, based on the piston assembly position signal124.

The increase in internal energy of the working fluid due to combustion in combustor116subjects the working fluid to a thermodynamic cycle. The amount of energy generated by the expansion of the working fluid in cylinder100is larger than that required for compression in cylinder109, which ensures continuous operation of the system and allows surplus energy to be extracted through a load device coupled to connection123and converted into high-level energy such as electric, hydraulic, or pneumatic energy.

The operation of the embodiment illustrated inFIG. 2follows that described above, with the following exceptions:

As compressor cylinder piston110reciprocates, the opening and closing of each compressor cylinder valve system113,113′,114, and114′ is controlled by the instantaneous pressure difference across each valve system. The valve systems113and113′ are configured such that if the pressure in the associated chamber111or111′ is lower than the pressure in the respective intake duct209or209′, the valve is open; otherwise the valve is closed. The valve systems114and114′ are configured such that if the pressure in the respective chamber111or111′ is higher than the pressure in compressed air channel115, the valve is open; otherwise the valve is closed.

As the piston assembly travels towards the LHS endpoint, three-way valve201is set such that the expanded combustion products can be discharged from chamber102to channel202. As the piston assembly reaches its LHS endpoint, three-way valve201switches to the “intake” setting so that fluid is allowed to flow from combustion products channel107into chamber102. At some point during the motion of the piston assembly towards the RHS endpoint, three-way valve201closes and the fluid in chamber102expands down to a lower pressure level. Three-way valve201′ operates similarly as valve201during the piston motion in the opposite direction.

As the expanded combustion products are discharged from expansion cylinder100, they are led through channel202to the first passage of the recuperator204before being discharged from the recuperator outlet206to exhaust channel105. As the compressed air is discharged from compression cylinder109to the compressed air channel115, it is led through the second passage of recuperator204before being supplied to the combustor116through channel203. In recuperator204, heat is transferred from the expanded combustion products to the compressed air.

FIG. 3illustrates the pressure in expansion cylinder chamber102and compression cylinder chamber111′ over one full engine cycle. The pressure in chambers102′ and111will be the mirror images of the plots shown inFIG. 3. The pressure p1denote the fluid pressure in the low-pressure side, which includes exhaust channel105and intake air channel112. The pressure p2denote the pressure in the high-pressure side, which includes compressed air channel115, combustor116, and combustion products channel107, as well as channels202and203for a configuration as shown inFIG. 2.

Assume that the piston assembly starts at the left-hand endpoint (LEP), at point1in the figure. At this point, valve106opens and the pressure in chamber102(shown inFIG. 3a) becomes equal to p2. As the piston assembly moves towards the right-hand endpoint (REP), combustion products from channel107is admitted into chamber102at pressure p2until valve106closes at point3. Thereafter, the pressure in chamber102drops as the fluid inside the chamber is expanded, and reaches a pressure equal to pl at REP (point5). Compression cylinder chamber111′ (FIG. 3b) is closed at LEP and, as the piston assembly moves towards REP, the fluid in chamber111′ is compressed and the pressure increases. As the pressure reaches p2, at point4, valve114′ opens and compressed fluid is discharged into compressed air channel115. At REP, valve114′ closes (point5) and valves113′ and104open (point6). During the return stroke from REP to LEP, expanded combustion products in chamber102are discharged into exhaust channel105through valve104, while air is admitted into chamber111′ from intake air channel112through valve113′. This completes one cycle of engine operation. The opposing chambers102′ and111mirror this operation.

Other Operational Issues

Starting. Several methods exist for the starting of the system. A connection on rod103can allow the driving of the piston assembly between the endpoints using external means, until self-sustained system operation is achieved. This is equivalent to those starting systems used in conventional engines. An alternative is to inject pressurised air into the compressed air channel115. This will start the motion of the piston assembly and, with controller125in operation, fuel can be injected and ignited to start the system. A third alternative is the use of the load device in motoring mode. Depending on the type of load device, stored hydraulic, pneumatic, or electric energy can be supplied to the system through appropriate load device control to drive the piston assembly until starting is achieved. In the second embodiment, shown inFIG. 2, this can be achieved using appropriate power electronics circuits to allow the electric machine212and213to operate in motoring mode. The most suitable starting method will depend on the specific design of the system and the plant in which it is employed.

Driving of accessories. Engine accessories, such as water pump, lubrication oil pump, and fuel pump, can be powered by external means, through a direct linkage from the piston assembly, or through using part of the produced energy, be it in electric, pneumatic, or hydraulic form. It is anticipated that the latter option will be preferred in most cases.

Operational optimisation. By allowing the controller to adjust the timing of the valve systems and the rate of fuel injection, the operation of the engine can be optimised for any operating condition. In particular, this relates to the “cut-off point” in the expansion cylinder, point3in FIG.3a. Varying the cut-off point according to the load level and other operating conditions to give an expansion of the combustion products down to the exhaust channel pressure exactly maximises the extraction of energy from the combustion products and thereby the fuel efficiency of the system. Similar control can be applied for the compression cylinder, however with the use of one-way valves, as illustrated inFIG. 2, such control follows automatically. By optimising the cut-off points, the system is capable of maintaining high fuel efficiency for a wide range of loads, which has been a limitation of prior art systems.

Piston motion control. The use of an open cycle with controllable valves and fuel injection gives significantly enhanced piston motion control possibilities and resolves the widely reported problems associated with the control of free-piston engines. Due to the low inertia of the system (compared to e.g. the crank system and flywheel in a conventional engine), a load change will have a much more direct influence in a free-piston engine. A closed cycle system, such as that described by Benson (U.S. Pat. No. 4,044,558), has a slow response to load changes as the heat addition is done through heat transfer in a heat exchanger, an inherently slow process. Hence, for a rapidly changing load, there is a risk of the engine stalling. An open cycle system in which fuel is injected directly into the working fluid will have superior control of the heat flow to the engine and therefore a much quicker response to load changes. The system presented here is therefore better suited for applications with varying load demands.

However, since there is no large energy storage, such as the flywheel in conventional engines, severe load changes may still compromise the operational stability of the engine. Both a rapid load increase and a rapid load decrease may lead to stability problems in free-piston engines, and these situations will be discussed separately here.

In the situation of a rapid load reduction, there will be an increase in the kinetic energy of the piston assembly and a risk for over-stroke. Consider the stroke between points6and1as illustrated inFIG. 3a. This stroke is driven by the high-pressure combustion products admitted into chamber102′, while the expanded combustion products in chamber102are discharged as illustrated in the figure. If, during this stroke from REP to LEP, the load is rapidly reduced, the kinetic energy of the piston assembly will be higher than normal when approaching LEP. This may lead to over-stroke and, in the worst case, the piston hitting the cylinder head. Even the scheduled opening of valve106at LEP to admit high-pressure fluid into chamber101may not provide a sufficiently large pressure force to retard the piston assembly and avoid a critical situation.

This situation is in the invention resolved with the use of the instantaneous piston position measurements and electronically controlled valve systems. If a reduction in the load occurs, this influences the acceleration of the piston assembly. Through the position measurements, a change in velocity is detected by the controller and any risk of over-stroke is identified. If there is such a risk, the controller advances the closing of valve104and delays the opening of valve106such that chamber102effectively forms a gas spring when the piston assembly approaches LEP. The degree to which the valve timings are adjusted will depend on the severity of the situation. This situation is illustrated inFIG. 4a. A load reduction which would cause the piston assembly to reach is mechanical limit is identified between points6and1′. At point l′, valve104is closed prematurely and the pressure in chamber102rises rapidly. The high pressure force contributes to retarding the piston assembly with no or only a minor over-stroke as a result. As the piston assembly velocity is reversed, intake valve106is opened and the next stroke continues unaffected.

Conversely, a rapid load increase may lead to the piston assembly not reaching the nominal endpoint and in the worst case the engine stalling. Such as situation is predicted similarly by the controller, based on the measured velocity of the piston assembly. Illustrated inFIG. 4b, a load increase is identified between points2and3. In this case, the closing of valve106is delayed until point3′ such that the pressure in chamber102remains high for a longer portion of the stroke, and thereby more work is done on piston101. (The additional work is shaded in the figure.) While this leads to a reduction in fuel efficiency since the fluid is not fully expanded at point5, it will only occur for a few cycles and therefore have little effect on the overall efficiency of the engine. In both the load reduction and the load increase cases, as soon as steady operation is achieved after the load change, the valve timing return to those values required for optimal fuel efficiency.

Hence, in addition to providing a fuel efficiency and power density advantage over prior art systems, the invention provides a solution for accurate control of piston motion, particularly in relation to emergency braking or response to rapid load changes. This reduces the risk of engine damage or unstable operation and allows use in a significantly wider range of applications, including those with highly varying load demands.

Design Considerations

The design requirements for the valve systems and flow channels are similar to those in conventional engines: low heat transfer losses, low flow pressure losses, and a compact design. The same will apply for the combustor and regenerative heat exchanger (if used), however some additional design requirements will apply for these components. Due to the reciprocating compressor and expander, the flow characteristics of the current system will, unlike conventional gas turbine engines, be pulsating. This does not rule out the use of conventional components; Moss et al. advised that these characteristics only requires a slightly larger heat exchanger. For the combustor, the implementation of pulsating fuel injection may need to be considered, depending on the volume of the flow channels between the combustor and cylinders; a large flow volume will reduce pressure oscillations and permit the use of a conventional combustor.

The main design considerations are the volume of the compressor and expander cylinders, and the maximum cycle temperature, that is in practice the fluid temperature at the combustor exit. These variables will determine the system pressure ratio, i.e. the ratio between the pressures on the high-pressure and low-pressure sides, and the cycle thermal efficiency.

FIG. 5ashows the influence of the pressure ratio on cycle efficiency for the first embodiment, as shown inFIG. 1, and the second embodiment, as shown inFIG. 2. The use of a recuperator gives a peak efficiency value at a significantly lower pressure ratio compared to the “simple cycle” without the regenerative heat exchanger. Bell and Partridge recommended a ratio of volumes between the expansion cylinder and compression cylinder of around3to achieve optimal efficiency in the recuperated system.

As is known from standard thermodynamic cycle analyses, a high maximum cycle temperature improves thermal efficiency. The permitted maximum cycle temperature in the system is limited by the materials properties in the combustion products channel, expansion cylinder valve systems, and expansion cylinder. It is recommended that materials suitable for high-temperature operation be used in these components.FIG. 5billustrates the theoretical cycle efficiency (i.e. not considering mechanical or gas flow losses) for maximum cycle temperatures of 750K, 1000K, and 1250K. Temperatures of above 1000K should in most cases be permitted with the use of standard metallic alloys; the use of e.g. ceramic materials may allow higher operating temperatures.

The power output of the engine depends heavily on the reciprocating speed. Unlike a conventional engine, the free-piston engine behaves similar to a mass-spring system, and the reciprocating speed is heavily influenced by the moving mass. Hence, the use of light-weight components in the piston assembly and load device is required for applications requiring a high engine power to weight ratio.

As with all heat engines, the minimising of heat transfer losses, leakage, and mechanical losses is of critical importance to obtain optimal fuel efficiency.

Finally, it is expected that the invention will be suitable for use in large plants in which several individual units provide the power outputs required in large-scale applications. Such a configuration allows significant operational benefits: individual units can be switched on or off according to the load demand of the plant; operation of several units with a common combustor is possible; operation of several units with a common recuperator is possible; and the positioning of the units and control of their operating speeds allow minimisation of system vibrations and noise.

With the use of an efficient thermodynamic cycle, a mechanically simple engine design, and electronic control of engine operation, a compact system with a fuel efficiency superior to that of prior art is presented. The system is suitable for energy conversion in a wide range of applications and sizes. The use of an open cycle with electronically controllable valves provides a solution to the piston motion control challenges in free-piston engine systems, which to date has hindered widespread commercial success of the free-piston engine concept. The invention is therefore suitable for applications which require a wide engine load range and have rapidly varying load demands.