A Four-Stroke Free Piston Internal Combustion Engine

Field of the Invention

The present invention relates to the field of free piston internal combustion engines, and particularly to a four-stroke free piston internal combustion engine.

Background of the Invention

Otto cycle four-stroke internal combustion engines have been in use for over a century, and are still the main mobile power source. This is mainly because of their relatively high efficiency and relatively high power-to-weight ratio. Over this period, many variations of the basic design have been developed, and some have seen widespread usage.

Existing internal combustion engines suffer from various disadvantages. For example, standard crank-operated, spark ignition (SI), four-stroke, internal combustion engines, such as those that currently drive the majority of cars, are limited to a compression ratio (CR) of roughly 10:1, because of "knocking" at higher CRs. This limitation on CR fundamentally limits the efficiency of an SI engine. A diesel engine, on the other hand, is not subject to knocking because the fuel is not injected until near maximum compression is achieved. As a result, diesel engines can achieve a higher CR than SI engines, and therefore higher efficiency. Unfortunately, the non-uniform mixing of fuel and air during fuel injection in a diesel engine typically creates particulate emissions ("soot") as well as polluting gases, such as nitrous oxides, and this pollution is generally unacceptable, despite the higher efficiency.

Both conventional SI and diesel engines transform the linear motion of the piston(s) into rotational motion of a shaft by operation of a crank. A free piston engine, on the other hand, is able to transform the piston's linear reciprocating motion directly into other forms of energy, such as pneumatic, electrical or hydraulic energy, without first converting it into rotational mechanical energy. A free piston engine has a number of advantages over crank engines. In particular, in free piston engines all the force of the expanding gases typically acts in the direction of motion, without significant side force acting to push the piston against the cylinder walls. The crank in a crank-operated engine on the other hand transmits a significant fraction of piston force to the walls and the crank bearing. At top dead centre (TDC), when the expansion force is at a maximum, practically all the expansion force is transmitted to the crank bearing instead of accelerating the piston. This leads to wear on the bearing and very high bearing loads. It also means that the piston spends a significant fraction of its cycle near TDC compared to

a free piston, and so a crank engine loses more heat to the walls, thus decreasing efficiency.

Yet another advantage of free piston engines over crank engines is that they can easily take advantage of an approach commonly referred to in the literature as homogeneous charge compression ignition (HCCI). In HCCI, the piston compresses a pre-mixed, lean fuel-air mixture adiabatically until the increasing temperature uniformly ignites the mixture. Unlike diesel combustion, HCCI avoids particulate emissions because the fuel and air are fully mixed before ignition. Like diesel ignition, HCCI relies on the high temperature created by compression to ignite the fuel-air mixture (charge). Because HCCI typically burns a lean charge, the combustion temperature is relatively low, and so nitrous oxides and other polluting gases are reduced compared to SI engines, and the compression ratio is much higher, giving efficiency comparable to diesel engines. In view of these advantages, one might expect wide spread usage of HCCI in internal combustion engines, but currently this is not so. The primary reason for this is the practical difficulty of timing HCCI in synchrony with the piston at TDC in a crank engine. The combination of a free piston engine with HCCI is particularly advantageous, because in a free piston engine, the TDC of the piston is not fixed by the geometry of the crank mechanism, but determined by the energy/momentum of the piston. That is, the energy of the moving free piston is absorbed by the compressed charge, so that if the piston energy is high enough, the resulting compressed charge will ignite, throwing the piston back. The exact point at which ignition occurs can vary from stroke to stroke without affecting the engine operation, so long as the piston has sufficient energy to cause ignition. Use of the HCCI combustion mode combined with free piston operation allows the use of any fuel, so long as it can be vaporized. In addition to the ability of free piston engines to use HCCI, they have other advantages, including: no side loading on the chamber wall; shorter dwell time at TDC; fewer moving parts; typically lower mass (due to the absence of a crank shaft and associated parts); and no load-bearing bearings.

Despite the above advantages of free piston internal combustion engines relative to crank-operated engines, they are still not widely accepted, as they are typically two- stroke engines. Two-stroke engines suffer from the problem that there is inevitably some mixing of the fresh charge with the exhaust gas stream. This not only leads to lower efficiency, but generally produces unacceptable levels of pollution. This is the primary reason that two-stroke SI engines are not widely used, except in small-scale applications where the level of pollution is not a serious concern. Another reason free piston engines

have not seen wide use is the difficulty of extracting the kinetic energy of the moving piston as other useful forms of energy.

US Patent No. 6,582,204 to Charles L. Gray Jr (hereinafter referred to as "Gray") discloses various embodiments of a four-stroke free piston internal combustion engine. These are depicted in Figures 8, 9, 10, 11, 12 and 13 of Gray. All these engines have a dual piston arrangement, with an hydraulic piston in between the pistons as the power output means. The embodiments shown in Figure 8 and 11 of Gray have four chambers, and operate on a four-stroke cycle. In the embodiment shown in Figure 8, there are essentially two free-piston assemblies coupled through a rack-and-pinion arrangement so that they reciprocate in opposite directions. The rack and pinion coupling allows one piston assembly to alternately drive the other. This method of coupling has many disadvantages. Firstly, the pinions exert a strong side load on both pistons, because they necessarily act on the piston's sides, rather than along the piston's axes. This side- loading removes a major advantage of free piston engines, which normally have no side loading. The main problems caused by side loading are increased wear and difficulties of lubricating under high load. Another disadvantage of the rack and pinion coupling is the wear and friction on both the rack and the pinion, as all the force on the piston must be transmitted through this coupling.

One way of eliminating the coupling problem inherent in Gray has been previously proposed by the present inventor in WIPO Patent Publication Number WO

2007/059565. Like the engines disclosed in Gray discussed above, this engine uses a four chamber, four-stroke cycle, but the pistons in the four chambers are rigidly coupled into a single piston assembly, with no side-loading. This engine has a disadvantage of being very long, and of the piston assembly being relatively massive, leading to slow operation. WIPO Patent Publication Number WO 2006/091682 discloses another four combustion chamber, four-stroke, free-piston engine, with a different coupling mechanism between the pistons in their separate chambers. The piston coupling mechanism of this engine, however will also create large side-loading on the chamber walls. German patent publication no. DE 3438687 Al discloses a further four combustion chamber, four-stroke, free piston engine. In this engine, combustion occurs on both sides of the piston head, so it is very difficult to lubricate, as the lubricating fluid must be exposed to the hot combustion gases.

Each of the four-stroke free piston engines referred to above has a heavy piston, because this piston must absorb and transmit the maximum force generated in all four combustion chambers. This heavy piston leads to lower piston speeds, and so to a lower power to weight ratio. Also, these engines require four sets of exhaust and inlet valves - one set for each of the four combustion chambers, resulting in excessive cost and complexity.

Object of the Invention

It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above described disadvantages, or at least to provide the useful alternative to prior art internal combustion engines.

Summary of the Invention

In one aspect, the present invention provides a four-stroke free piston internal combustion engine extending along a longitudinal axis and comprising: an engine block structure defining first and second axially opposing cavities; a piston structure mounted in said engine block structure for reciprocating motion relative to said engine block structure along said longitudinal axis in opposing first and second directions, said piston structure having a first piston head located in said first cavity, a second piston head located in said second cavity and a piston body connecting said first piston head and said second piston head; a first combustion chamber defined in said first cavity by a primary face of said first piston head and said engine block structure, said first combustion chamber being configured to expand on displacement of said piston structure in said first direction; a second combustion chamber defined in said second cavity by a primary face of said second piston head and said engine block structure, said second combustion chamber being configured to expand on displacement of said piston structure in said second direction; a first inlet valve communicating with said first combustion chamber; a first exhaust valve communicating with said first combustion chamber; a second inlet valve communicating with said second combustion chamber; a second exhaust valve communicating with said second combustion chamber; a first spring device adapted to absorb energy associated with displacement of said piston structure in said first direction and release energy to drive said piston structure in said second direction during a subsequent stroke;

a second spring device adapted to absorb energy associated with displacement of said piston structure in said second direction and release energy to drive said piston structure in said first direction during a subsequent stroke; an energy extraction and delivery system configured to: extract energy from said engine during a combustion/expansion stroke of said first combustion chamber and during a combustion/expansion stroke of said second combustion chamber, and deliver energy to said engine to drive said piston structure in said first direction during a compression stroke of said second combustion chamber. In one embodiment, said first spring device is in the form of a first gas spring and said second spring device is in the form of a second gas spring; said first gas spring comprising a first compressor chamber defined by a secondary face of said first piston head opposing said primary face of said first piston head and said engine block structure, said first compressor chamber being configured to expand on displacement of said piston structure in said second direction; said second gas spring comprising a second compressor chamber defined by a secondary face of said second piston head opposing said primary face of said second piston head and said engine block structure, said second compressor chamber being configured to expand on displacement of said piston structure in said first direction. In a first embodiment, said energy extraction and delivery system is further configured to deliver energy to said engine to drive said piston structure in said second direction during a compression stroke of said first combustion chamber.

In the first embodiment, said energy extraction and delivery system comprises: a pressurized primary pressure reservoir, a pressurized secondary pressure reservoir, said secondary pressure reservoir being maintained at lower pressure than said primary pressure reservoir, a first extraction valve operable to communicate said first compressor chamber with said primary pressure reservoir, a second extraction valve operable to communicate said second compressor chamber with said primary pressure reservoir, a first delivery valve operable to communicate said first compressor chamber with said secondary pressure reservoir, a second delivery valve operable to communicate said second compressor chamber with said secondary pressure reservoir.

Typically, said primary pressure reservoir is coupled to said secondary pressure reservoir via an expander engine.

Typically, said engine block structure is mounted to a mounting for reciprocating motion along said longitudinal axis. In an alternate embodiment, said first spring device is in the form of a first mechanical spring and said second spring device is in the form of a second mechanical spring.

In a second embodiment, said energy extraction and delivery system comprises a rack and gear arrangement configured to convert relative longitudinal displacement of said piston structure to rotational displacement of a shaft and vice versa.

Typically, said shaft is engageable with a flywheel configured to extract energy from, and deliver energy to, said shaft.

In a third embodiment, said energy extraction and delivery system comprises a linear alternator configured to convert relative longitudinal displacement of said piston structure to electric current and vice versa.

Typically, said alternator is coupled to a capacitor.

In another aspect, the present invention provides a method of operating the internal combustion engine defined above through a four-stroke cycle comprising a first piston stroke, a second piston stroke, a third piston stroke and a fourth piston stroke, said method comprising the steps of:

1 ) during said first piston stroke : a) combusting a compressed fuel-air mixture in said first combustion chamber, thereby expanding said first combustion chamber, driving said piston structure in said first direction; b) opening said second exhaust valve, exhausting combustion product from said second combustion chamber and subsequently closing said second exhaust valve; c) absorbing energy associated with displacement of said piston structure in said first direction with said first spring device; d) releasing energy from said second spring device, driving said piston structure in said first direction; e) extracting energy from said engine via said energy extraction and delivery system;

2) during said second piston stroke:

a) opening said first exhaust valve, exhausting combustion product from said first combustion chamber and subsequently closing said first exhaust valve; b) opening said second inlet valve, delivering a fuel-air mixture to said second combustion chamber via said second inlet valve and subsequently closing said

5 second inlet valve; c) releasing energy from said first spring device, driving said piston structure in said second direction d) absorbing energy associated with displacement of said piston structure in said second direction with said second spring device; o 3) during said third piston stroke: a) opening said first inlet valve, delivering a fuel-air mixture to said first combustion chamber via said first inlet valve and subsequently closing said first inlet valve; b) compressing said fuel-air mixture in said second combustion chamber;s c) absorbing energy associated with displacement of said piston structure in said first direction with said first spring device; d) releasing energy from said second spring device, driving said piston structure in said first direction; e) delivering energy to said engine via said energy extraction and delivery0 system driving said piston in said first direction

4) during said fourth piston stroke: a) compressing said fuel-air mixture in said first combustion chamber; b) combusting said compressed fuel-air mixture in said second combustion chamber, thereby expanding said second combustion chamber, driving said pistonS structure in said second direction; c) absorbing energy associated with displacement of said piston structure in said first direction with said first spring device; d) releasing energy from said first spring device, driving said piston structure in said second direction 0 e) absorbing energy associated with displacement of said piston structure in said second direction with said second spring device; and f) extracting energy from said engine via said energy extraction and delivery system.

In a first embodiment, said method further comprises, during said fourth piston stroke, delivering energy to said engine via said energy extraction and delivery system, driving said piston in said first direction

Brief Description of the Drawings s Preferred embodiments of the present invention will now be described, by way of an example only, with reference to the accompanying drawings wherein:

Figure 1 is a schematic cross-sectional view of a four-stroke free piston internal combustion engine according to a first embodiment:

Figure 2a is a schematic cross-sectional view of the internal combustion engineo of Figure 1 at the beginning of a first piston stroke;

Figure 2b is a schematic cross-sectional view of the internal combustion engine of Figure 1 at the beginning of a second piston stroke;

Figure 2c is a schematic cross-sectional view of the internal combustion engine of Figure 1 at the beginning of a third piston stroke; s Figure 2d is a schematic cross-sectional view of the internal combustion engine of Figure 1 at the beginning of a fourth piston stroke;

Figure 3 is a graph depicting pressure in each of the chambers of the internal combustion engine of Figure 1 through a full four-stroke cycle;

Figure 4 is a graph depicting relative velocity of the piston structure of the0 internal combustion engine of Figure 1 through a full four-stroke cycle;

Figure 5 is a pressure- volume graph of the left and right compressor chambers of the internal combustion engine of Figure 1 during operation;

Figure 6 is a schematic cross-sectional view of a four-stroke free piston internal combustion engine according to a second embodiment; and S Figure 7 is a schematic cross-sectional view of a four-stroke free piston internal combustion engine according to a third embodiment.

Detailed Description of the Preferred Embodiments Figure 1 schematically depicts a four-stroke free piston internal combustion engine 1 according to a first embodiment. The engine 1 extends along a longitudinal axis0 L and includes an engine block structure 2 that defines first and second axially opposing cavities 3a, 3b. In the orientation depicted throughout the drawings, the first cavity 3a is on the left and the second cavity 3b is on the right. Accordingly, hereinafter the first cavity 3 a will be referred to as the left cavity 3 a and the second cavity 3b will be referred to as the right cavity 3b. It will be understood, however, that the engine 1 may be ₅ oriented in any desired orientation. The engine 1 has various other axially opposing

duplicated features, with the engine 1 depicted generally being symmetrical about a transverse axis. Accordingly, for ease of understanding, the first and second of such features will generally be referred to as left and right features, although again other orientations are envisaged. A piston structure 4 is mounted in the engine block structure 2 for reciprocating motion relative to the engine block structure 2 along the longitudinal axis L in opposing first and second directions. The first direction is from left to right in Figure 1 , and the second direction is from right to left. The piston structure 4 has a left (first) piston head 5a located in the left cavity 3a, a right (second) piston head 5b located in the right cavity 3b and a piston body 6 connecting the left piston head 5a and right piston head 5b. The piston body 6 extends through a tunnel 7 defined in the engine block structure 2 extending between the left and right cavities 3a, 3b. In the embodiment depicted, the left and right cavities 3 a, 3b, left and right piston heads 5a, 5b, piston body 6 and tunnel 7 are all cylindrical, each having an axis of symmetry defined by the longitudinal axis L. A left (first) combustion chamber 8a is defined in the left cavity 3a by a primary face 9a of the left piston head 5a and the engine block structure 2. Specifically, the primary face 9a of the left piston head 5a is the circular end face of the piston head 5a facing the end wall 10a of the left cavity 3a. The left combustion chamber 8a expands on displacement of the piston structure 4 to the right (that is, in the first direction) and, conversely, contracts on displacement of the piston structure 4 to the left (that is, in the second direction).

Similarly, a right (second) combustion chamber 8b is defined in the right cavity 3b by a primary face 9b of the right piston head 5b and the engine block structure 2. Specifically, the primary face 9b of the right piston head 5b is the circular end face of the piston head 5b facing the end wall 10b of the right cavity 3b. The right combustion chamber 8b expands on displacement of the piston structure 4 to the left and, conversely, contracts on displacement of the piston structure 4 to the right.

A left (first) compressor chamber 1 Ia is defined by a secondary face 12a of the left piston head 5a and the engine block structure 2. The secondary face 12a of the left piston head 5a opposes the primary face 9a of the left piston head 5a, and is annular in form, extending about the end of the piston body 6. The left compressor chamber 11a expands on displacement of the piston structure 4 to the left, and conversely contracts on displacement of the piston structure 4 to the right. The left compressor chamber 11a forms a left gas spring, the operation of which will be described below. The left compressor chamber 1 Ia is sealed from the left compressor chamber 8a by way of a

sliding piston seal 13 extending about the left piston head 5a. The piston seal 13 may be in any of various forms, such as a known piston ring or lip seal. Lubrication for the piston seal 13 may be introduced on the left compressor chamber 11a side, given that this side of the left piston head 5a is not subjected to hot combustion gases as is the left combustion chamber 8a.

Similarly, a right (second) compressor chamber 1 Ib is defined by a secondary face 12b of the right piston head 5b and the engine block structure 2. The secondary face 12b of the right piston head 5b opposes the primary face 9b of the right piston head 5b, and is annular in form, extending about the end of the piston body 6. The right compressor chamber l ib expands on displacement of the piston structure 4 to the right, and conversely contracts on displacement of the piston structure 4 to the left. The right compressor chamber l ib forms a right gas spring, the operation of which will be described below. The right compressor chamber 1 Ib is sealed from the right compressor chamber 8b by way of another sliding piston seal 13 extending about the right piston head 5b. The right compressor chamber 1 Ib is sealed from the left compressor chamber 11 a by a further seal 14 located on the wall of the tunnel 7, sealing against the piston body 6.

A left (first) inlet valve 15a of standard configuration communicates with the left combustion chamber 8a. The left inlet valve 15a is mounted in the engine block structure 2 and is operable between open and closed configurations in the usual manner to communicate the left combustion chamber 8a with a fuel-air supply via an intake manifold (not depicted). A left (first) exhaust valve 16a, again of standard configuration, also communicates with the left combustion chamber 8a. The left exhaust valve 8a is mounted in the engine structure 2 and is operable between open and closed positions to communicate the left combustion chamber 8 a with an exhaust system via an exhaust manifold (not depicted).

Similarly, a right (second) inlet valve 15b of standard configuration communicates with the right combustion chamber 8b. The right inlet valve 15b is mounted in the engine block structure 2 and is operable between open and closed configurations in the usual manner to communicate the right combustion chamber 8b with a fuel-air supply via the intake manifold. A right (second) exhaust valve 16b, again of standard configuration, also communicates with the left combustion chamber 8b. The right exhaust valve 16b is mounted in the engine structure 2 and is operable between open and closed positions to communicate the right combustion chamber 8b with an exhaust system via the exhaust manifold.

The engine 1 further comprises an energy extraction and delivery system which, in the first embodiment, includes a pressurized primary pressure reservoir 17 and pressurized secondary pressure reservoir 18 that is maintained at lower pressure than the primary pressure reservoir 17. The energy extraction and delivery system also includes a left (first) extraction valve 19a that is operable between a closed configuration and an open configuration in which the left extraction valve 19a communicates the left compressor chamber 11a with the primary pressure reservoir 17. The energy extraction and delivery system also includes a left (first) delivery valve 20a that is operable between a closed configuration and an open configuration in which the left delivery valve 20a communicates the left compressor chamber l la with the secondary pressure reservoir 18.

The energy extraction and delivery system further includes a right (second) extraction valve 19b that is operable between a closed configuration and an open configuration in which the right extraction valve 19b communicates the right compressor chamber 1 Ib with the primary pressure reservoir 17. A right (second) delivery valve 20b is operable between a closed configuration and an open configuration in which the right delivery valve 20b communicates the right compressor chamber l ib with the secondary pressure reservoir 18.

The engine block structure 2 is mounted to a mounting 21 that allows the engine block structure 2 to freely reciprocate along the longitudinal axis L. The mounting 21 itself may be very compliant to allow motion on the engine block structure.

Alternatively, the engine block structure 2 may be mounted to the mounting 21 via low friction slider bearings or the like to allow reciprocating motion of the engine block structure. As the primary and secondary pressure reservoirs 17, 18 will typically be stationary in relation to the engine mounting 21, the connections 22, 23 between the engine block structure 2 and primary and secondary pressure reservoirs 17, 18 provide for the reciprocating motion of the engine block structure 2 relative to the engine mounting 21. Similarly, the intake manifold and exhaust manifolds will also typically be fixed in relation to the engine mounting 21 and thus the connections 24a, 24b, 25a, 25b between these manifolds and the engine block structure 2 at the left intake 15 a, right intake value 15b, left exhaust valve 16a and right exhaust value 16b also provide for the reciprocating motion of the engine block structure 2. In the embodiment depicted, each of these connections is provided by way of a sliding sleeve type connector. Alternatively, bellows type connectors or moving flexible hoses are also envisaged.

Operation of the engine 1 of the first embodiment will now be described with reference to Figures 2a to 2d and 3 to 5. Figures 2a to 2d depict the engine 1 at the

beginning of each of the four piston strokes of the four-stroke cycle, thus representing the extreme positions of the piston structure 4 that occur over a full cycle. Figure 3 depicts pressure in each of the four chambers of the engine 1 throughout a simulated cycle with a fuel-air ratio (relative to storiometric) of 0.4. Figure 4 depicts the relative velocity of the piston structure 4 relative to the engine block structure 2 throughout the simulated cycle. Figure 4 depicts a pressure- volume plot of the left and right compressor chambers 11a, l ib.

During the four-stroke cycle, the left combustion chamber 8a undergoes the following strokes in sequence: combustion/expansion, exhaust, intake and compression. The start of each stroke in this sequence is shown in Figures 2a, 2b, 2c and 2d respectively. The right combustion chamber 8b similarly undergoes the following strokes in sequence: exhaust, intake, compression and combustion/expansion, with the start of each stroke in this sequence also shown in Figures 2a, 2b, 2c and 2d. These sequences show that both the left and right combustion chambers 8 a, 8b undergo the same four- stroke sequence, but with the right chamber 8b leading the left chamber 8a by one stroke. Figure 2a depicts the engine 1 at the beginning of the first piston stroke, with the piston structure 4 located at its far left position relative to the engine block structure 2. At this position, at the beginning of the first piston stroke, combustion of the compressed fuel-air charge (compressed during the preceding stroke) has just occurred in the left combustion chamber 8a, which is thus filled with hot gas at high pressure. This high pressure is approximately 175 atmospheres (17.7 MPa) in the example represented in Figure 3. The left inlet valve 15a and left exhaust valve 16a are closed at the start of the first piston stroke, and remain closed throughout the first piston stroke. At the beginning of the first stroke, the right combustion chamber 8b contains expanded relatively low pressure exhaust gases resulting from combustion at the beginning of the previous piston stroke. The right exhaust valve 16b opens at the beginning of the first piston stroke to allow exhaust of these exhaust gases during the first piston stroke.

At the beginning of the first piston stroke, the left compressor chamber 11a contains a gas (typically air) at an initial pressure lower than that of the right compressor chamber l ib which also contains a gas (typically air), at high pressure. In the example represented in Figure 3, the left compressor chamber 1 Ia is initially at a pressure of about 15 atmospheres (1.5MPa), whilst the right compressor chamber 1 Ib is at a pressure of about 100 atmospheres (10. IMPa). At the beginning of the first piston stroke, the left and right extraction valves 19a, 19b and left and right delivery valves 20a, 20b are closed. The primary pressure reservoir 17 is at a pressure, in the example represented in

Figure 4, of about 80 atmospheres (8.1MPa), whilst the secondary pressure reservoir 18 is at a pressure of about 20 atmospheres (2.0MPa).

During the first piston stroke, the high pressure energy generated by combusting the compressed fuel-air mixture in the left combustion chamber 8 a acts against the primary face 9a of the left piston head 5a to expand the left combustion chamber 8a, driving the piston structure 4 to the right relative to the engine block structure 2. As the piston structure 4 moves to the right, the engine block structure 2 recoils to the left as a result of the engine block structure 2 being able to freely reciprocate relative to the mounting 21. The high pressure in the right compressor chamber l ib, acting against the secondary face 12b of the right piston head 5b, also tends to expand the right compressor chamber l ib releasing energy to further drive the piston structure 4 to the right. During the first piston stroke, as the piston structure 4 moves to the right, contracting the right combustion chamber 8b, exhaust gas in the right combustion chamber 3b is pushed out through the right exhaust valve 16b into the exhaust manifold. As the piston structure 4 moves to the right, the pressure in the left combustion chamber 8a rapidly drops, as depicted in Figure 3, as does the pressure in right compressor chamber 1 Ib. The pressure in the right combustion chamber 8b remains at close to atmospheric pressure, given that the right exhaust valve 16b remains open throughout the first piston stroke. The pressure in the left compressor chamber 11a gradually increases as it contracts, thereby absorbing energy associated with displacement of the piston structure 4. Part way through the first piston stroke, as can be best seen referring to Figure 3, the increase in pressure in the left compressor chamber 1 Ia and rapidly decreasing pressure in the left combustion chamber 8a and right compressor chamber 1 Ib result in the net force acting on the piston structure 4 being to the left, attempting to brake the piston structure 4 as it overshoots an equilibrium position at which the forces generated by pressures in the left and right compressor chamber l la, 1 Ib are equal. At this point, however, the piston structure 4 is travelling at maximum velocity, and thus has high momentum which results in the piston structure 4 continuing movement to the right, overshooting the equilibrium position. Accordingly, the pressure within the left compressor chamber l la continues to increase.

Once the pressure within the left compressor chamber l la reaches the pressure of the primary pressure reservoir 17, being a pressure of approximately 60 atmospheres (6.1MPa) in the example represented in Figure 3, the left extraction valve 19a is opened. As the piston structure 4 is driven further to the right, further contracting the left compressor chamber l la, gas within the left compressor chamber 11 a is driven through

the left extraction valve into the primary pressure reservoir 17 at effectively constant pressure, as reflected in Figure 3. Driving this high pressure gas into the primary pressure reservoir 17 accordingly results in extracting energy from the engine, increasing the available energy in the primary pressure reservoir 17. Later in the first piston stroke, the first extraction valve 19a closes, allowing pressure within the left compressor chamber 1 Ia to continue increasing, as depicted in Figure 3. This pressure in the left compressor chamber 11a, which by now far exceeds the now relatively low pressure in the right compressor chamber l ib, continues to act as a gas spring further decelerating the piston structure 4 as it nears the end of the first stroke. Accordingly, during this first piston stroke, part of the energy generated by combustion in the left combustion chamber 8a is extracted into the primary pressure reservoir 17 and part of the energy is utilized to energize the gas spring defined by the left compressor chamber 11a.

At the end of the first piston stroke/beginning of the second piston stroke, the right exhaust valve 16b closes and the second inlet valve 15b is opened. The left exhaust valve 16a is also opened. This configuration at the start of the second piston stroke is depicted in Figure 2b, as the piston structure 4 starts moving back to the left relative to the engine block structure 2.

At the beginning of the second piston stroke, and in fact throughout the second piston stroke, the left and right combustion chambers 8a, 8b remain at close to atmospheric pressure as the left exhaust valve 16a and right intake valve 15b remain open during the second piston stroke. At the beginning of the second piston stroke, the left compressor chamber 1 Ia is at a high pressure, being about 70 atmospheres (7.1 MPa) for the example represented in Figure 3. At this point, the expanded right compressor chamber 1 Ib is at a relatively low pressure. During the second piston stroke, the high pressure in the left compressor chamber

11a, acting against the secondary face 12a of the left piston head 5a, tends to expand the left compressor chamber 11a, releasing energy to drive the piston structure 2 to the left. During the second piston stroke, as the piston structure 4 moves to the left contracting the left combustion chamber 8a, exhaust gas in the left combustion chamber 8a is pushed out through the left exhaust valve 16a into the exhaust manifold. Fuel-air mixture is drawn from the intake manifold through the right intake valve 15b into the right combustion chamber 8b as the right combustion chamber 8b expands during the second piston stroke.

During the second piston stroke, the extraction and delivery valves 19a, 19b, 20a, 20b remain closed, such that the left and right compressor chambers 11a, l ib remain sealed. They thus act as gas springs throughout the second piston stroke. As the piston

structure 4 moves to the left, the pressure within the left compressor chamber 11a decreases as it expands, whilst the pressure within the right compressor chamber l ib increases as it contracts. Accordingly, the energy released by the left compressor chamber 11a driving the piston structure 4 is primarily absorbed by the right compressor chamber 1 Ib. Once the pressure within the left compressor chamber 11a decreases to below the pressure in the right compressor chamber l ib, past the pressure equilibrium position, the net force acting on the piston structure is to the right so it tends to brake the displacement of the piston structure 4. Again, however, the momentum of the piston structure 4 causes it to overshoot the equilibrium position, building up a relatively high pressure in the right compressors chamber l ib, being a pressure of approximately 55 atmospheres (5.6 MPa) by the end of the second stroke.

The second piston stroke is thus a non-power stroke, with the full stroke of the piston structure 4 being achieved by releasing the energy associated with the high pressure in the left compressor chamber 11a. This energy is primarily transferred to the right compressor chamber 1 Ib in preparation for the third stroke.

At the end of the second piston stroke/beginning of the third piston stroke, the left exhaust valve 16a and right intake valve 15b are closed, whilst the left intake valve 15a is opened. This configuration at the start of the third piston stroke is depicted in Figure 2c, as the piston structure 4 starts moving to the right again relative to the engine block structure 2.

At the beginning of the third piston stroke, and in fact throughout the third piston stroke, the left combustion chamber 8a remains at close to atmospheric pressure given that the left intake valve 15a remains open during the third piston stroke. The expanded right combustion chamber 8b and expanded left compressor chamber 1 Ia are each at a relatively low pressure at the beginning of the third piston stroke, whereas the right compressor chamber 1 Ib is at a relatively high pressure, of about 55 atmospheres (5.6 MPa) as noted above.

During the third piston stroke, the high pressure in the right compressor chamber l ib, acting against the secondary face 12b of the right piston head 5b, tends to expand the right compressor chamber l ib, releasing energy stored in the right compressor chamber 1 Ib to drive the piston structure 4 to the right. A fuel-air charge is drawn from the intake manifold through the left intake valve 15a into the left combustion chamber 8a as the left combustion chamber 8a expands. The fuel-air charge in the right combustion chamber 8b is compressed during the third piston stroke, increasing the pressure in the right-

combustion chamber 8b. The pressure in the left compressor chamber 11a also increases as it contracts during the third piston stroke.

As the piston structure 4 moves to the right during the third piston stroke, the pressure in the right compressor chamber l ib, which at this stage is the only driving force acting on the piston structure 4, decreases. Without the delivery of further energy to drive the piston structure 4 during the third piston stroke, the building pressure in the left compressor chamber 11a and right combustion chamber 8b would act against the decreasing pressure in the right compressor chamber 1 Ib to decelerate the piston structure 4 prematurely, preventing adequate compressor of the fuel-air charge within the right combustion chamber 8b. To provide additional driving force to the piston structure 4, the energy extraction and delivery system delivers energy to the engine to continue driving the piston structure 4 to the right. This is achieved by opening the right delivery valve 20b communicating the right compressor chamber l ib with the secondary pressure reservoir 18 once the pressure within the right compressor chamber 8b drops to the pressure level of the secondary pressure reservoir 18. In the example represented in Figure 3, this is a pressure of approximately 20 atmospheres (2.0 MPa). As the piston structure 4 continues moving to the right, gas flows into the right compressor chamber 1 Ib at a relatively constant pressure, thereby delivering energy to the engine 1 to continue driving the piston structure 4. Pressure within the right compressor chamber 1 Ib is thus maintained at a level above that in the remaining chambers for much of the third stroke, as depicted in Figure 3.

Later in the third stroke, as the pressure within both the left compressor chamber 11a and right combustion chamber 8b starts to rapidly increase, the left delivery valve 20a closes again and the rapidly increasing pressure within the compressed left compressor chamber 11 a and right combustion chamber 8b decelerate the piston structure 4. The energy released from the originally compressed right compressor chamber 1 Ib and additional energy delivered from the secondary pressure reservoir 18 is thus absorbed through compression of the left compressor chamber 11a and compression of the fuel-air charge within the right combustion chamber 8b. The third piston stroke is thus again a non-power stroke, with the energy required to displace the piston structure 4 and compress the fuel-air charge in the right combustion chamber 8b being delivered by the release of pressure within the right compressor chamber l ib and the delivery of additional energy delivered from the secondary pressure reservoir. The addition of compressed gas into the right compressor chamber l ib from the secondary pressure

reservoir 18 also serves to replace the gas that is expelled into the primary pressure reservoir 17 during the fourth stroke (discussed below).

At the end of the third piston/beginning of the fourth piston stroke, the compressed fuel-air charge in the right combustion chamber 8b is ignited and combusts. The left intake valve 15a also closes. This configuration at the start of the fourth piston stroke is depicted in Figure 2d as the piston structure 4 starts moving to the left again relative to the engine block structure 2.

At the beginning of the fourth stroke, with combustion of the air- fuel charge in the right combustion chamber 8b having just occurred, the pressure is elevated to approximately 175 atmospheres (17.7MPa) in the example represent in Figure 3. The left compressor chamber 8a is also at a relatively high pressure being approximately 100 atmospheres (10. IMPa) in the example. The left combustion chamber 8a is initially at close to atmospheric pressure, whilst the right compressor chamber 1 Ib is pressurized at a relatively low level as compared to the right combustion chamber 8b and left compressor chamber 11a.

The high pressures within the right combustion chamber 8b and left compressor chamber 11a provide a driving force to drive the piston structure 4 back to the left as they expand. Accordingly, pressure within both the right combustion chamber 8b and left compressor chamber 11a rapidly reduces during the fourth stroke. Conversely, the pressure with the contracting left combustion chamber 8a and right compressor chamber 1 Ib increase.

Just as the right delivery valve 20b opens during the third piston stroke, the left delivery valve 20a opens during the fourth piston stroke when the pressure within the left compressor chamber 11a drops to be equal that of the secondary pressure reservoir 18, being 20 atmospheres (2.0MPa) in the example represented in Figure 3. Gas at substantially constant pressure is thus delivered into the left compressor chamber 11 a, delivering further energy to the engine to continue driving the piston structure 4 to the left as pressure in the right combustion chamber 8b decreases, and replacing the gas previously expelled into the primary pressure reservoir 17 during the first stroke. The left delivery valve 21 a closes later in the fourth stroke as pressure in the left combustion chamber 8 a begins to rapidly increase toward the end of the stroke.

Just as the left extraction valve 19a opens during the first piston stroke, the right extraction valve 19b opens during the fourth piston stroke when the pressure within the right compressor chamber 1 Ib reaches the pressure of the primary pressure reservoir 17. As the right compressor chamber l ib further contracts, gas within the right compressor

chamber 1 Ib is driven into the primary pressure reservoir 17 at substantially constant pressure, as again reflected in Figure 3, thereby extracting energy from the engine. Later in the fourth piston stroke, the second extraction valve 19b closes, allowing pressure within the right compressor chamber 1 Ib to continue increasing. Again this increasing pressure acts as a gas spring, further decelerating the piston structure 4 as it nears the end of the fourth piston stroke.

Accordingly, during the fourth stroke, the energy to drive the piston structure 4 is provided by combustion of the compressed fuel-air charge in the right combustion chamber 8b, the expansion of the high pressure gas within the left compressor chamber 11a and delivery of pressurized gas to the left compressor chamber 1 Ia from the secondary pressure reservoir 18. This energy is used to compress the fresh fuel-air charge within the left combustion chamber 8a and compress gas within the right compressor chamber l ib with the remainder of the energy being extracted from the engine via the right extraction valve 19b. At the end of the fourth piston stroke, the compressed fuel-air charge in the left combustion chamber 8a is ignited and combusted and the four-stroke cycle restarts.

As is apparent from the above, the left and right combustion chambers 8a, 8b undergo a standard four-stroke cycle, but with the left combustion chamber 8a lagging the right combustion chamber 8b by one stroke. Only two of the four-strokes, being the first and fourth strokes, are power strokes. With only two power strokes and two combustion chambers, only two sets of inlet and exhaust valves are required, simplifying the engine. The two non-power strokes, being the second and third piston strokes, are driven by the left and right compressor chambers 11a, l ib acting as gas springs to reciprocate the piston structure 4 back and forth during each stroke, with the compression energy of those springs alternating between the left and right compressor chambers 11a, l ib. The left compressor chamber 11a also lags the right compressor chamber by one stroke in the engine operation described above. An alternative operating mode is for the left combustion chamber 8a and left compressor chamber 11 a to lead the right combustion chamber 8b and right compressor chamber 1 Ib by one stroke. That is, by exchanging the left and right designations, an equivalent operation to the one described above can be achieved. There is no particular reason for preferring left lagging or leading operation, and the engine controller can start the engine in either mode. Energy is delivered to the engine from the secondary pressure reservoir 18 by opening the left and right delivery valves 20a, 20b during the third and fourth piston strokes, respectively. This aids in driving the piston structure 4 to compress the respective adjacent combustion chamber

during its compressor stroke. Energy is extracted from the engine by opening the right and left extraction valves 19b, 19a during the fourth and first piston strokes, extracting part of the energy generated by expansion of the combusted fuel-air charge during the combustion strokes of the right and left combustion chambers 8b, 8a respectively. Operation of the left compressor chamber 11 a is represented in Figure 5 by way of a pressure- volume diagram. The right compressor chamber 1 Ib follows an identical sequence, only shifted in phase by one stroke.

During the first piston stroke, the pressure trajectory traces the following point sequence: a-b-c-(left extraction valve 19a opens)-(left extraction valve 19a closes)-d-e. During the sequence c-d, whilst the left extraction valve 19a is opened, the pressure is constant at the pressure of the primary pressure reservoir 17. After the left extraction valve 19a closes (point d), the remaining gas trapped in the left compressor chamber 11a is compressed along trajectory d-e. The corresponding pressure trajectory followed by the right compressor chamber l ib for this first piston stroke is: e-d-f-g. During the second piston stroke, the left compressor chamber 11a traces the following pressure trajectory: e-d-f-g, while the right compressor chamber 1 Ib traces the trajectory: g-f-d-e-h.

During the third piston stroke, the left compressor chamber 11a traces the following pressure trajectory: g-f-d-e-h, while the right compressor chamber 1 Ib traces the trajectory: h-e-d-f-(right delivery valve 20b opens)-(right delivery valve 20b closes)- b-a.

During the fourth piston stroke, the left compressor chamber 11a traces the following pressure trajectory: h-e-d-f-(left delivery valve 20a opens)-(left delivery valve 20a closes)-b-a, whilst the right compressor chamber 1 Ib traces the trajectory: a-b-c- (right extraction valve 19b opens)-(right extraction valve 19b closes)-d-e.

Over the entire cycle, the left pressure chamber 11 a traces the following point sequence: a-b-c-d-e=d=f=g-f-d-e-h=e=d=f=b=a, where "-" and "=" are used to distinguish the different strokes. The right compressor chamber l ib traces a similar closed trajectory, but phase shifted, so it begins at point e, not a. The energy extracted by each compressor chamber 11a, 1 Ib is indicated by the cross-hatched area in the pressure- volume diagram of Figure 5, and depends on the pressure in the primary pressure reservoir 17 and the pressure in the secondary pressure reservoir 18 and represents the available energy per cycle.

An external expander engine (not shown) may be used to harness the compressed gas energy stored in the primary and secondary pressure reservoirs 17, 18 by allowing gas

to flow from the higher pressure primary pressure reservoir 17 to the lower pressure secondary pressure reservoir 17 through the expander engine, and so provide mechanical or other forms of energy. Such compressed gas driven engines include gas turbines, rotary engines (such as Roots or Lysholm type rotary engines) or piston expansion engines (such as crank engines similar to steam engines). This expander engine could be used in reverse to charge up the primary and secondary pressure reservoirs 17, 18, should they fall outside of the engine operating range. The coupled engine 1 and expander engine moves the gas in the primary and secondary pressure reservoirs 17, 18 around in a closed cycle, extracting energy via the expander engine. Although the engine 1 described above utilizes gas as the working fluid for the left and right compressor chambers 11a, 1 Ib, an hydraulic fluid could be used as an alternative, with high and low pressure gas accumulators acting to store energy as is typical in hydraulic power systems.

Ignition of the compressed fuel-air charges in the left combustion chamber 8a at the beginning of the first piston stroke and in the right combustion chamber 8b at the beginning of the fourth stroke, may be achieved by any of various ignition methods. One envisaged method is spark ignition (SI), where an electric spark initiates a flame that spreads across the entire compressed charge. Another envisaged method is diesel ignition (often referred to as compressor ignition) where it is air that is compressed, and fuel is injected near the point of maximum compressor. The high temperature of the compressed air is enough to ignite the injected fuel. The preferred method of ignition is homogeneous charge compressor ignition (HCCI). In HCCI, a lean premixed fuel-air charge is compressed until it auto-ignites. HCCI has advantages of higher efficiency and lower pollution than both SI and diesel ignition. Other more complex methods of ignition, such as stratified charge ignition, are also envisaged. It would also be possible to switch ignition modes in the engine for different operating requirements, even while it is running.

The engine 1 described above is ideally suited to using HCCI. This is because the compression ratio in the engine 1 is not fixed, but depends only on pressure developed in the relevant combustion chamber 8 a, 8b as it nears the end of the compression stroke (the third and fourth piston strokes). This pressure is dependant on the momentum of the piston structure 4 during the stroke. If there is insufficient momentum/pressure, the compression of the fuel-air charge in the combustion chamber at the end of the compression stroke may not be enough to ignite the fuel-air charge, providing a misfire due to under-compression. Similarly, if there is excessive momentum / pressure, the fuel- air charge will not only ignite, but continue to be compressed after ignition, providing

over-compression. Preferably, the engine using HCCl is controlled to operate in tne slightly over-compressed mode to ensure no misfiring.

In many engine applications such as in automotive drive systems, the engine 1 is operated to control the power generated by the engine 1, up to the maximum power that the engine 1 is designed to deliver. Any of various power control methods may be utilised.

One suitable method of power control is varying the fuel-to-air ratio. This method changes the amount of energy released by each fuel-air charge. How much power control can be achieved by this method depends on the ignition mode used. An SI engine has a narrow range of fuel-to-air ratios in which it can operate, because too lean a mixture will not ignite. Diesel (CI) and HCCI modes of operation are able to use a wide range of fuel-to-air ratios, and so this control method is well suited to these modes of operation.

A further suitable method of power control is varying the mass of fuel-air charge trapped in the combustion chamber. The amount of energy released per combustion stroke is directly proportional to the fuel-air charge mass (for a fixed fuel-to-air ratio). Specific methods for varying the fuel-air charge mass include supercharging (that is, pressurizing the charge before it enters the combustion chamber); choking (that is, constricting the charge flow into the combustion chamber); early intake valve closing (that is, cutting off the charge flow into the combustion chamber before it is fully expanded) and late intake valve closing (that is, pushing some of the charge back out of the combustion chamber). These last two methods of charge mass control are known in the literature as the Miller cycle.

Another suitable method of power control is by utilizing dead strokes. This method of power control works by not admitting a fresh fuel-air charge into the left and right combustion chambers 8a, 8b by deliberately failing to open the left and right inlet valves 15 a, 15b when they would normally open during the third and second piston strokes respectively. Some exhaust gas is also trapped in the left and right combustion chambers 8a, 8b by early closing of the left and right exhaust valves 16a, 16b. Also, the extractor and delivery valves 19a, 19b, 20a, 20b remain closed throughout a dead stroke. The result of these modified valve operations for a dead stroke is to cause the piston structure 4, to oscillate due to compression and expansion in both the combustion chambers 8a, 8b and the compressor chambers 11a, 1 Ib. This oscillation can be repeated for as many dead strokes as desired, limited only by friction losses eventually stopping the reciprocating motion of the piston structure 4. When power cycles are interleaved with dead strokes, the compressor chambers 11a, 1 Ib are used in reverse so that they act

as expander chambers, taking gas from the higher pressure primary pressure reservoir and returning it to the lower pressure secondary pressure reservoir, as described below for start up operation. This results in a net delivery of energy from the energy extraction and delivery system to make up for friction losses during the dead strokes. If an odd number of dead strokes are used, then the relative phase of the left and right operation is reversed. That is, which side is leading or lagging by one stroke reverses.

A still further suitable method of power control is by stop / start operation. When power demand temporarily ceases, or becomes very low, the engine 1 may be stopped and then restarted when power demand returns. This stop / start operation is particularly easy for the engine 1, as the primary and secondary pressure reservoirs 17, 18 act as an energy buffer between the engine power supply and the application power demand, so the demand side does not experience significant power fluctuations due to engine stop / start operation.

The use of the primary and secondary pressure reservoirs 17, 18 coupled by an expander engine allows the maximum power delivered by the expander engine operating off the primary and secondary pressure reservoirs 17, 18 to exceed, for short durations, the maximum power of the engine 1. That is, the primary and secondary pressure reservoirs 17, 18 act as an energy storage device which can deliver very high power so long as some pressure difference between the pressure reservoirs 17, 18 remains. How long this high power output can be maintained depends on the storage capacity of the pressure reservoirs 17, 18, and by how much the power demand exceeds that delivered by the engine 1.

The engine 1 is operated by a control system to ensure optimal operation, as well as providing for stopping and starting of the engine 1. The control system senses the position and speed of the piston structure 4, and the pressure in the primary and secondary pressure reservoirs 17, 18 and utilizes these parameters to provide feedback control. In particular, the control system senses when a combustion chamber misfires. This may be sensed indirectly by sensing reduced velocity of the piston structure 4 or insufficient pressure in a combustion chamber 8 a, 8b at the end of its combustion stroke. The control system also senses when a combustion chamber 8a, 8b is significantly over-compressed. The control system is able to correct the mis-firing or over-compressed states by adjusting the timing of the extractor and delivery valves 19a, 19b, 20a, 20b. Since it is the operation of the extractor and delivery valves 19a, 19b, 20a, 20b that determines the amount of compressed gas that is transferred from the lower pressure secondary pressure reservoir 18 to the higher pressure primary pressure reservoir 17 (or the reverse during

startup), then the energy extracted (or added) per cycle can be controlled by timing of these valves alone.

As mentioned above, the left and right compressor chambers 11a, l ib may be used in reverse so as to provide a net energy input rather than output to the engine. This is applicable for startup operation, and to makeup energy lost to friction during dead strokes. This reverse operation involves briefly opening the left and right extractor valves 19a, 19b to respectively communicate the left and right compressor chambers 1 Ia, 1 Ib with the primary pressure reservoir 17 during an expansion stroke of the compressor chamber 11a, 1 Ib when the pressure in the compressor chamber l la, 1 Ib drops to the same as the pressure in the primary pressure reservoir 17, thereby adding gas / energy to the compressor chamber 1 Ia, 1 Ib. Similarly, the delivery valves 20a, 20b open to respectively communicate the left and right compressor chambers l la, 1 Ib with the secondary pressure reservoir 18 during a compression stroke of the compressor chamber l la, 1 Ib when the pressures are equal so as to push the added gas into the secondary pressure reservoir 18. The net effect is to move gas from the primary pressure reservoir 17 to the secondary pressure reservoir 18, while adding kinetic energy to the piston structure 4 in the process. A stopping process for this engine 1 is similar to running dead stroke s, controlling the left and right inlet valves 15a, 15b to ensure no fresh fuel-air charge is admitted and absorbing some of the piston structure kinetic energy through the left and right compressor chambers l la, l ib.

Any of various types of valve and valve actuator may be utilized for the inlet, exhaust, delivery and extraction valves of the engine 1, provided that it seals properly, its timing can be controlled, and it can act fast enough. Because the engine 1 is a free-piston engine, a standard crank shaft is not available to drive the valves. However, the motion of the engine block structure 2, relative to the mounting 21 could be used to actuate all of the valves, and especially the inlet and exhaust valves.

As discussed above, the engine block structure 2 is mounted on the mounting 21 in a manner enabling reciprocating motion of the engine block structure 2 such that, when the piston structure 4 is displaced in one direction, the engine block structure 2 recoils in the opposing direction. Alternatively, a reciprocating counterweight could be provided that cancels the engine block structure 2 recoil. Yet another method of cancelling the engine block structure 2 recoil is to provide a second, mirrror version of the engine 1 that is synchronized to oscillate in opposite phase to the first engine 1. This synchronization would be provided by the engine controller through valve timing on both engines.

The engine 1 of the first embodiment utilizes the primary and second pressure reservoirs 17, 18 as the energy extraction and delivery system by moving a gas (or hydraulic fluid) between the primary and secondary pressure reservoirs 17, 18 through the left and right compressor chambers l la, l ib. However, in many other applications, other forms of energy extraction, such as mechanical or electrical, are preferable and it is envisaged to use other such forms of energy extraction to extract energy from the engine and deliver the necessary energy to assist in driving the piston structure 4. In the above description, the engine chambers 8a and 8b are on the far left and right respectively, along with their respective inlet and exhaust valves, while the compressor chambers l la and 1 Ib are on the inner left and inner right respectively. The roles of compressor and combustion chambers can be reversed without changing the engine operation described above (not shown). In this reversed configuration, chambers 8a and 8b become the compressor chambers, and chambers l la and 1 Ib become the combustion chambers (along with their associated valves). The basic operation of this engine is not affected by this reversal.

Figure 6 schematically depicts a four-stroke free piston internal combustion engine 101 according to a second embodiment that extracts energy in a mechanical form. Features of the engine 101 of the second embodiments that are common with the engine 1 of the first embodiment are provided with the same reference numerals. The engine 101 utilizes substantially the same engine block structure 2, piston structure 4 and inlet and exhaust valve 15a, 15b, 16a, 16b arrangements as the engine 1 of the first embodiment. The engine 101, however, has the left and right compressor chambers l la, 1 Ib permanently sealed, by omission of the delivery and extractor valves. Accordingly, the left and right compressor chambers l la, 1 Ib act as gas springs only. The energy extraction and delivery system of the engine 101 is in a mechanical form, utilizing a rack and gear arrangement (also known as a rack and pinion) configured to convert longitudinal displacement of the piston structure 4 relative to the engine block structure 2 to rotational displacement of a shaft and vice versa.

In particular, two shafts 102 are arranged to extend in a lateral direction on opposing sides of the piston body 6 and are connected to the mounting 21 using suitable low friction bearings. A gear and rack arrangement is associated with each shaft 102, with the two arrangements being mirror images to prevent sideloading on the piston body structure 4. Accordingly, only one of the arrangements will be described. The shaft 102 is fixed to first and second teethed gears 103, 104 concentrically mounted on the shaft 102. A longitudinally extending first rack of teeth 105 is mounted on an inner face 106 of

the engine block structure 2 and defines a cavity 107 with the piston body 6. A longitudinally extending second rack of teeth 108 is mounted on the piston body 6 so as to oppose the first rack 105. The teeth of the first gear 103 engage the teeth of the first rack 105, whereas the teeth of the second gear 104 engage the teeth of the opposing second rack 108. As the piston structure 4 moves in one direction, the mounting 21 recoils in the opposing direction. The relative velocities of the piston structure 4 and engine block structure 2 are inversely proportional to the relative masses of the piston structure 4 and engine block structure 2. The ratio of the radii of the first and second gears 103, 104 is chosen to match the mass ratio such that relative motion of the piston structure 4 and engine block structure 2 is converted to pure torque acting on the gears 102, 103 and therefore the shaft 102 that is mounted to the stationary mounting 21.

The two opposing shafts 102, being located on opposing sides of the piston body 6, counter-rotate, so there is no net torque acting on the piston structure 4 or engine block structure 2. Rotation of the shafts 102 reverses direction when the piston structure 4 reverses direction between piston strokes. The shafts 102 are removably coupled to two counter-rotating flywheels (not shown) which act as energy stores / buffers. The flywheels are engaged and disengaged by ratchets, or similar mechanisms, which couple the rotating shafts 102 to the flywheel when their respective speeds match, and disengage when sufficient energy has been extracted or is delivered as required. The flywheels may be coupled to drive an output shaft by appropriate gearing, or directly drive a rotary electric generator.

Energy is extracted from the piston structure 4 during the first and fourth piston strokes of the cycle described above in relation to Figures 2a to 2d by engaging the flywheels when the rotational speed of the shafts 102 has increased to be equal to the speed of the flywheels and disengaged later in the stroke when sufficient energy has been extracted. Conversely, energy is delivered to the engine 1 to assist in driving the piston structure 4 during the third piston stroke by engaging the flywheels when the rotational speed of the shafts 102 reduces to that of the flywheels, preventing further reduction of the velocity of the piston structure 4 during this phase of the third stroke. The flywheel is then disengaged toward the end of the third stroke, enabling braking of the piston structure 4.

Figure 7 schematically depicts a 4-stroke free piston internal combustion engine 201 according to a third embodiment. This engine 201 extracts energy electrically. Again, features of the engine 201 of the third embodiment that are identical to the engine 1 of the first embodiment are provided with the same reference numerals. Again, the

engine 201 of the third embodiment has substantially the same piston structure 4, engine block structure 2 and inlet and exhaust valve 15a, 15b, 16a, 16b arrangements as the engine 1 of the first embodiment.

The energy extraction and delivery system of the engine 201 is in the form of a linear alternator 202. The linear alternator 202 comprises a series of concentric rings 203 of alternating permanent magnetic fields mounted on the piston body 6 and associated coils 204 mounted in the wall of the tunnel 7 formed in the engine block structure 2. When the piston structure 4 is displaced relative to the engine block structure 2, a current is electromagnetically generated within the coils 204. Similarly, application of an electric current to the coils 204 electromagnetically generates a force acting to displace the piston structure 4 relative to the engine block structure 2. The coils 204 are coupled to a large capacitor which acts an energy store/buffer. The alternator 202 is controlled by an electric control system that extracts electrical energy from the engine 1 during the first and fourth piston strokes and delivers energy during the third piston stroke (by delivering a current to the coils 204 from the capacitor).

As with the engine 101 of the second embodiment, the left and right compressor chambers 11a, 1 Ib are not provided with any valves, and thus remain sealed so as to act only as gas springs over the entire cycle of the engine 201, acting to reciprocate the piston structure 4 during each of the 4-strokes. Rather than having the permanent magnets 203 mounted on the piston body 6 and coils 204 mounted on the engine block structure 2, the reverse configuration could be employed, with the permanent magnets 203 mounted on the engine block structure 2 and the coils 204 mounted on the piston body 6. Whilst the engine 201 of the third embodiment is mechanically simpler than either the engine 1 of the first embodiment or the engine 101 of the second embodiment, the magnets 203 result in making the piston structure 4 significantly heavier. Further, electrical power extraction methods may not be as efficient as the power extraction methods described in relation to the first and second embodiments.

Whilst three separate forms of energy extraction and delivery system have been described in relation to each of the three embodiments, that is pneumatic, mechanical and electrical based, it is also envisaged to combine two or more of the modes of power extraction/delivery, but with added complexity.

It is also envisaged that, rather than utilizing gas springs in the form of the left and right compressor chambers 11a, 1 Ib to absorb energy associated with displacement of the piston structure 4 and subsequently release energy to drive the piston structure 4 in the opposing direction during a subsequent stroke, mechanical springs could be utilised

instead to provide the desired oscillatory motion. Such springs (not depicted) would typically be coil springs and may be mounted within the left and right cavities 3 a, 3b between the secondary faces 12a, 12b of the left and right piston heads 5a, 5b and the opposing end wall of the respective cavity 3a, 3b.

A person skilled in the art will appreciate various other possible modifications and variations of the engines described above.