BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas turbine, the combustion chamber of which is replaced by a hydraulic free-piston engine, thereby increasing the efficiency of the gas turbine. The invention is also concerned with an improved design of free-piston engine.

2. Description of the Related Art

It is known to use a gas turbine to generate power, and gas turbines are widely used in power stations, aircraft, nautical vessels, trains and tanks. Combustion in gas turbines takes place at constant pressure. However, energy released during constant-pressure combustion is less than the energy released during constant-volume combustion.

Therefore, it is desirable to enable a gas turbine to operate with constant-volume combustion, instead of constant-pressure combustion, as this will increase the efficiency of the gas turbine. The present invention aims to provide a gas turbine which operates at constant-volume combustion, and also a new type of free-piston engine. BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a gas turbine configured to operate at close to constant-volume combustion, comprising an upstream compressor coupled to a downstream turbine with at least one free- piston engine in- between, the free-piston engine replacing the combustion chamber of the gas turbine. The free-piston engine preferably comprises two opposed pistons arranged in an elongate cylinder having at least one inlet port and at least one outlet port with uniflow scavenging; a synchronising linkage between the two opposed pistons; a connection from each of the two pistons to an air-filled bounce chamber and to a hydraulic bounce accumulator; and hydraulic transmission from said free-piston engine to a load such as an alternator.

Free-piston engines were developed in the first half of the 20 ^th century for use in air compressors, amongst other applications. The present applicant in GB 2500440 A proposed a CHP application using a two -stage hydraulic pump arrangement with each piston, one stage providing hydraulic bounce and the other the hydraulic output; a pneumatic bounce is preferably also included. A further aspect of the present invention relates to an improvement in the design of free-piston engines, in which a hydraulic bounce system is incorporated in parallel to the hydraulic power output, so that the pressures in the two systems are independent of each other. This functions are preferably integrated in a symmetrical fashion in a single cylinder block, reducing imbalances and allowing design freedom in the sizes of, and pressures in, the two hydraulic functions.

The theoretical Otto cycle for the free-piston engine comprises adiabatic compression during the compression stroke, close to instantaneous combustion at virtually constant volume, adiabatic expansion during the expansion stroke and heat rejection. The hydraulic free-piston engine offers the potential to come close to achieving a theoretical Otto cycle with resulting high efficiency. An optimum Otto cycle also requires rapid combustion and therefore the present invention deploys homogeneous charge compression ignition (HCCI) in which the fuel-air mixture is compressed to the point of auto-ignition. During HCCI, ignition occurs at many points on a molecular scale simultaneously throughout the combustion chamber, giving rise to extremely rapid heat release. This extremely rapid combustion and the resulting rapid increase in pressure lead to high acceleration of the pistons, which causes a rapid increase in combustion chamber volume. This allows much less time for the formation of nitrogen oxides, thereby advantageously reducing unwanted emissions. The HCCI is achieved by reaching a very high compression ratio. Therefore, optimal conditions in the engine comprise close to constant-volume combustion, a lean fuel: air mixture and a high compression ratio. To achieve and maintain this optimum cycle, it may be necessary to make adjustments to the apparatus, such as to adjust the stroke, compression ratio, bounce characteristics, supercharge pressure and fuehair ratio. These optimal conditions of HCCI, giving close to constant-volume combustion, a lean mix and a high compression ratio enable the high efficiency of the free-piston engine and of the resulting gas turbine combination.

For a better understanding of the invention, its background and some embodiments will now be described with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a typical gas turbine;

Figure 2 illustrates the main components of a standard gas turbine configured with a separate work turbine driving an alternator, as used in a power station; Figure 3 shows a graph depicting different pressure/volume diagrams for various thermodynamic cycles;

Figure 4 schematically shows a gas turbine embodying the present invention;

Figure 5 illustrates the elements of the free piston engine incorporated in the gas turbine; and

Figure 6 is a detail showing the hydraulic pump used in the embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1

A typical gas turbine 101 is illustrated in Figure 1 . Gas turbines are widely used in aeroplane engines, as well as power stations, nautical vessels, trains and tanks.

Figure 2

Figure 2 illustrates the main components of a standard gas turbine. A gas turbine typically comprises an upstream compressor 201 coupled to a downstream turbine 202, with a combustion chamber 203 in-between. In a typical gas turbine, atmospheric air passes through the compressor 201 which compresses the air to a higher pressure. The compressed air is then ignited in a fuel-air mixture in the combustion chamber 203, thereby producing high temperature gas which enters the turbine 202 and on to a work turbine 204. The work from the turbine 202 is used to drive the air compressor, and the work from the work turbine 204 may be used to drive devices such as an electric generator. The present invention has particular application to a gas turbine in which the combustion chamber 203 has been replaced with at least one hydraulic free-piston engine.

Figure 3

Different pressure/volume diagrams are illustrated in Figure 3. The y-axis is

representative of increasing pressure and the x axis is representative of increasing volume. A compression ratio of approximately 10:1 , which occurs in a typical spark- ignition engine, is shown at 301. Further, a compression ratio of approximately 20:1 , as occurs in a typical diesel engine, is shown at 302. Line 303 is representative of the constant-pressure combustion taking place at a compression ratio of approximately 40:1 which occurs in a standard gas turbine.

The energy released during constant-pressure combustion is less than the energy released during constant-volume combustion because, during constant-pressure combustion, some of the energy is used to change the volume of the system without doing useful work. Therefore, it is desirable to enable a gas turbine to operate with constant-volume combustion, instead of constant-pressure combustion, as this would increase the efficiency of the gas turbine. Line 304 is representative of the constant-volume combustion taking place in embodiments of the present invention, which uses a hydraulic free-piston engine to replace the combustion chamber of the gas turbine. The hydraulic free-piston engine operates at very close to constant-volume combustion because the air/fuel mixture is contained within the space between the pistons and the volume changes only after the moment of detonation, which is extremely rapid (microseconds). A gas turbine with a free-piston engine in place of the combustion chamber, hence using virtually constant- volume combustion, has an overall compression ratio of approximately 45:1 to 65:1 , and optimally 55:1 (or a combustion chamber pressure of between 50 and 75 bar) whilst a modern gas turbine with constant pressure combustion as used in a power station has a compression ratio of approximately 40:1 (line 303). The replacement of the combustion chamber of the gas turbine with a hydraulic free-piston engine allows the gas turbine to operate at a higher efficiency than a standard gas turbine. This is because free-piston engines advantageously make more efficient use of energy, thereby using less fuel and producing lower emissions in comparison to conventional gas turbines with constant- pressure combustion chambers. These advantages are due to the constructional simplicity of free-piston engines, in which output power from the engines is extracted by load devices directly coupled to the moving pistons. There are also fewer energy losses through friction and use of more efficient thermodynamic cycles (constant volume combustion operation) which contribute to the higher efficiency. Figure 4

Such an apparatus 401 , embodying the present invention, is shown in Figure 4. In the present example, at least one hydraulic free-piston engine 402 is deployed in place of the usual combustion chamber in the gas turbine. The hydraulic free-piston engine 402 is capable of homogeneous charge compression ignition (HCCI), has a very high compression ratio and uses a lean fuekair ratio as the power source. A free-piston engine can be crudely described as an internal combustion engine that runs without a crankshaft. In an opposed free-piston engine, as in many embodiments of the present invention, there are two oppositely acting pistons in the same cylinder, and the power cycle is a two-stroke cycle wherein the two pistons compress the charge and ignite it, a power stroke follows and the exhaust gases flow out of the cylinder. The combustion chamber is formed in the space between the two pistons after the compression stroke.

The free-piston engine 402 has at least one air inlet 528 and at least one outlet or exhaust 403, connected to a cylinder in which two opposed pistons, described later, move in opposition. The intake air is supplied from a compressor 410, driven by a turbine 409 itself driven by the exhaust from the free-piston engine 402. Each piston of the engine is connected to a two-function hydraulic pump, giving two hydraulic outputs. One of these functions is connected to a hydraulic bounce accumulator 408, and the other, being the hydraulic work output of the motor, is connected via non-return valves 452 and a further hydraulic accumulator 404 to a hydraulic motor 405, itself further connected to an alternator 406 and a cooler/filter 407. The outputs of the two hydraulic pumps driven by the two engine pistons are here connected to the same motor, though they could be separate. The further hydraulic accumulator 404 serves to smooth the output of the engine. A hydraulic accumulator is usually constituted by a closed container in which is a diaphragm which separates the hydraulic fluid from the pressurising gas in the closed container, thus storing energy. Piston versions can also be used.

As shown, the exhaust gas path through the turbine 409 continues to a work turbine 412, which drives a rotary alternator 411. Alternators 406 and 41 1 are shown as separate machines, but it is possible that they could be combined as one. Figure 5

The elements of the free-piston engine 402 shown in Figure 4 are illustrated in more detail in Figure 5. The hydraulic free-piston engine 402 has a casing 510 enclosing two opposed pistons 502 arranged to move in opposition in an elongate cylinder 503, forming a common combustion chamber 504. A mechanical synchronising linkage (here a rack- and-pinion mechanism, though a hydraulic or other linkage is conceivable) 505 maintains the two opposed pistons in exact synchronisation. The free-piston engine 402 comprises at least one inlet port 530 and at least one outlet port 531 with uniflow scavenging 506. The outlet port 531 is connected to the exhaust 403. Air inlets 528 at each end of the casing allow air into an outer annular chamber 520, from where it is drawn via non-return valves 522 into an annular chamber 509 located inwardly of the first chamber 520; the former chamber 509 constitutes the air bounce chamber to be described. Finally, air is pressed by the returning piston 508 through further non-return valves 525 into the interior of the casing (scavenge chamber 526). Each of the two free pistons 502 has a rebound device or bounce function which acts to store the energy necessary to create the next compression stroke. The rebound has two components, hydraulic and pneumatic. The former is provided by one part of each of a pair of four-cylinder hydraulic pumps 507, and the latter by the pneumatic bounce chambers and pistons 509, 508 already described. The other pair of cylinders in the hydraulic pumps 507, forming the output of the engine, is connected to the hydraulic accumulator 404 to smooth out pulsations in the hydraulic fluid flow before the motor 405.

The free engine pistons 502 are connected to the air bounce pistons 508 in the air bounce chambers 509. They are also connected via ball joints 542 to yokes 540, which drive four pistons 602, 603 in the hydraulic pumps 507, as described below. The ball joints compensate for any small mis-alignment between the engine cylinder and the hydraulic cylinders. There are also ball joints, not shown, between the yokes and the piston rods of the respective two-function pumps 507. With a total piston assembly length of 70cm or more, in the embodiment, and various stresses and temperature gradients, this articulation is necessary to avoid friction, heat and wear.

The opposed-piston arrangement is in almost perfect internal balance which minimises vibration and allows uniflow scavenging - easily the best scavenging method in a two- stroke engine. This configuration was devised by Hugo Junkers over 80 years ago and, despite many efforts over the decades, no-one has been able to improve on it. Figure 6

One of the hydraulic pumps 507 is illustrated in Figure 6. The pump is based around a cylinder block having four parallel cylinders arranged in two diametrically opposed pairs 602, 603. One pair 602 has a smaller diameter, and these cylinders are connected to the hydraulic bounce accumulators 408. The cylinders of the other, larger, pair 603 provide the power output and, via the non-return valves 452, supply the hydraulic motor 405 via the hydraulic accumulator 404. The symmetrical arrangement of the pairs of cylinders, in a diamond- or rhombus-shaped configuration (in cross-section, with respect to the axis of the engine), largely eliminates the possibility of imbalance. (The figure is a regular rhombus but is shown slightly skewed in the diagram for clarity.) The tapering outwards of the fluid-flow passages (after the point of maximum travel of their pistons) reduces the velocity of fluid flow and hence friction and turbulence losses in the system. The length of the hydraulic pistons is about 10 cm and that of the piston rods is a little more than the stroke, which can vary a little but averages about 25 cm. The combination of the two hydraulic functions in parallel in one pump or block also allows a shorter overall construction. Thus, power transmission between the free-piston engine 402 and the alternator 406 is by hydraulic means. A two-function hydraulic pump 507 is coupled to each of the two opposed pistons 502 of the free-piston engine. Connected to each hydraulic pump are two independent hydraulic accumulators, namely a bounce accumulator 408 and an accumulator 404 for the high-pressure fluid carrying the output power of the engine. The two hydraulic circuits operate independently of each other. The hydraulic accumulators each contain both air and hydraulic fluid, separated by a diaphragm.

Operation

In operation, intake air coming from the compressor 410 (also known as scavenging air) is moderately further compressed by the reverse side of the air bounce piston 508 and passes into the inlet port, or set of inlet ports, 530 of the free-piston engine 402. There is uniflow scavenging between the inlet ports and the exhaust port 531 , and therefore fresh charge flows in one end of the cylinder and the exhaust gases flow out of the other end at the same time. Uniflow scavenging is the most sensible way to run a two-stroke engine with minimal risk of losing unburnt fuel out of the exhaust. The exhaust port 531 must open before the inlet port 530 opens, so that exhaust gases begin to flow out before fresh air begins to flow into the cylinder via the inlet port(s).

During the expansion (working) stroke of the engine, air is drawn in by the reverse of the air bounce pistons 508 via the air inlets 528, outer annular chambers 520 and non-return valves 522, into the inner, air bounce, cylinders or chambers 509. Meanwhile, the air of the front (outer side) of the air bounce piston 508 becomes highly compressed and eventually returns the piston for the compression stroke.

During the compression stroke of the engine, the pistons 508 drive the air drawn into the cylinders 509 via axially facing non-return valves 525 into the scavenge air chamber 526, ready for the following scavenge operation. At a suitable point fuel is injected into a chamber (not shown) surrounding the inlet port 530 and the fuel air mixture is

compressed by the pistons 502. Homogeneous ignition takes place, following which the expansion stroke begins.

At the end of the engine working stroke the exhaust ports 531 open, followed, a moment later, by the inlet ports 530. With all the cylinder ports open, uniflow scavenging 506 occurs through the cylinder 503: air flows from the scavenge air chamber 526, through inlet posts 530, into the cylinder 503, and at the same time exhaust flows out though exhaust ports 531 to the exhaust 403. The fuel used is typically natural gas (methane) from the mains supply, which is at low pressure (less than 0.3 bar). Its pressure is raised by a gas compressor (not shown) to approximately 8 bar (800 kPa), suitable for injection 413 into the inlet ports, where it then mixes with incoming air. Fuel injection can be altered to adjust the fuekair ratio in order to achieve a lean mixture. Although it is envisaged that the present invention will typically use natural gas, this does not preclude alternative fuels being utilised by embodiments of the present invention.

After passing via the hydraulic accumulator 404, the high-pressure fluid powers the hydraulic motor 405 which in turn is configured to operate the rotary alternator 406.

There is electronic speed control of the hydraulic motor. After exiting the hydraulic motor, the hydraulic fluid passes through the cooling and filtering device 407 and then into a reservoir before being re- used by the two-function hydraulic pumps 507. Electrical output is in the form of alternating current which, although suitable for operating plant machinery and lighting, may not be of suitable quality for feeding into the grid system as it may not display sufficiently accurate frequency control. Therefore, a rectifier may convert this alternating current into direct current which is either stored in a battery or converted back into higher quality alternating current in an inverter before being fed into the grid.

The hydraulic system provides an element of the bounce mechanism required by the free-piston engine. On the working (expansion) stroke of the piston, the very high combustion pressure gives the pistons a high acceleration for a brief time (for example, 1 millisecond) resulting in a velocity of, for example, 10 metres/second. Towards the end of the expansion stroke, exhaust ports 531 open and it is only the momentum of the piston assembly that works against the combined load of the high-pressure output pump, the hydraulic bounce pump 602 and air bounce chamber 509. The pistons soon stop after working against this combined load. The bounce energy stored in the air bounce chambers 509 plus the bounce hydraulic accumulators 408 acting on the pistons begins to move the piston assembly back into the compression stroke. The use of air-filled bounce chambers 509 for part of the bounce mechanism is preferred (the rest coming from the hydraulic bounce). The combination of air-filled bounce chambers 509 and hydraulic bounce accumulators 408 helps create the high compression ratio required for HCCI to occur.

With an air-filled bounce chamber, the pressure in the bounce chamber increases quickly towards the end of the engine working stroke (or expansion stroke) and decelerates the piston rapidly. The bounce pressure reaches a peak at the end of this working stroke, thereby reversing and accelerating the piston quickly into the compression stroke. When the exhaust ports close, the pistons are already moving relatively fast. The air bounce then decreases exponentially but the speed of the pistons is sustained by the steady hydraulic bounce force for the full compression stroke. At the end of the compression stroke, the momentum of the pistons, plus the hydraulic bounce force, creates a higher pressure in the combustion chamber and helps to ensure a high compression ratio and spontaneous ignition. Therefore, the use of hydraulic accumulators to provide a bounce effect together with an air-filled bounce chamber help to achieve the very high compression ratio required for homogeneous charge compression ignition. Each of the two engine pistons 502 moves freely in the cylinder and is connected only to a hydraulic load, plus the combined air and hydraulic bounce devices.

Finally, the exhaust gases from the free piston engine (at about 7.5 bar) drive the turbine 409 which drives the air compressor 410. The gases leaving the turbine 409 enter the work turbine 412 which drives a rotary alternator 41 1. This alternator may be a separate alternator to that driven by the hydraulic motor (namely alternator 406), or may be the same alternator.

Thus, summarising the action of the two-function hydraulic pumps, the two functions of the pumps are to provide the hydraulic component of the bounce force for the free-piston engine and to provide the means to carry the output power out of the engine. The two functions are separate even though they are performed by two parts of the same pump. Each function is performed by two hydraulic pistons in each pump. The two bounce pistons/cylinders 602 are connected to the hydraulic bounce accumulator without any intervening valves. The two high-pressure pistons 603 draw hydraulic fluid through nonreturn valves from a reservoir 450 at approximately atmospheric pressure. The pistons then pump the fluid, through the non-return valves 452, via the high-pressure accumulators 404 to the hydraulic motor 405 which drives the alternator 406.

It can thus be seen that the two-function hydraulic pumps comprise a symmetrical four- piston arrangement which overcomes an imbalance problem that exists on standard hydraulic pumps. In this symmetrical four-piston arrangement, there are two high- pressure pistons 603 compressing the fluid for the power output of the engine and two bounce pistons 602, each pair diametrically opposed. The main advantage is that the pistons are always symmetrical and consequently always in balance, even if the forces acting on the high-pressure pistons are very different from the forces acting on the bounce pistons. It will be clear to one skilled in the art that many improvements and modifications can be made to the foregoing exemplary embodiments without departing from the scope of the present invention.

The engine can be used as a generator, or a compressor, or for a vehicle, or for

Combined Heat and Power, or for any other application.

An aspect of the invention therefore includes a free-piston engine having a cylinder (503) and a pair of opposed pistons (502) reciprocating in the cylinder, each piston having a hydraulic bounce function using a hydraulic accumulator (408) and a hydraulic output arrangement (603), wherein both the hydraulic bounce accumulator and the hydraulic output are supplied by a single two-function hydraulic pump (507). The ree-piston engine according may include a pneumatic bounce arrangement (508, 509) associated with each piston. Further, each of said two-function hydraulic pump (507) may comprise two high- pressure hydraulic pistons (603) compressing fluid for power output of the engine and two bounce pistons (602), the piston pairs being configured in a diametrically opposed arrangement. In such an arrangement the output pistons (603) and bounce pistons (602) are arranged in a rhombic layout, as seen in section.

In a preferred embodiment, the high-pressure and bounce pistons (603, 602) are mounted on a common yoke (540) connected to the respective engine piston (502) by way of an articulation such as a ball joint (542). Optionally, the cylinder (503) has at least one inlet port (530) and at least one outlet port (531 ) with uniflow scavenging.

Embodiments described herein may include a free-piston engine having a mechanical synchronising linkage (505) between the two opposed engine pistons (502). The mechanical linkage may be a rack-and-pinion mechanism or hydraulic.

Embodiments described herein may include a free-piston engine wherein air is mixed with fuel as it flows through said at least one inlet port (530). The fuel may be mains natural gas.

Further preferred embodiments include a free-piston engine wherein intake air is compressed by the reverse side of pistons (508) in the air bounce chambers (509).

Preferably, the hydraulic output is connected to a hydraulic motor (405) via a hydraulic accumulator (404). Preferably, the two hydraulic bounce arrangements are connected to respective bounce accumulators (408) with a balancing connection between the two accumulators. Preferably, the load includes an alternator (406), preferably a rotary alternator, and a hydraulic motor (405) configured to operate the alternator. More preferably, the free-piston engine may further comprise an electronic speed control between the hydraulic motor (405) and the alternator (406).

In another aspect the present invention provides a gas turbine engine configured to operate at constant-volume combustion, comprising; an upstream compressor (410) coupled to a downstream turbine (409) with a free-piston engine in between and acting the combustion chamber of the gas turbine engine.

Preferably, the downstream turbine (409) is coupled to a further turbine (412) which further turbine is coupled to an alternator (41 1 ). More preferably, the turbine-driven alternator (41 1 ) is the same as the alternator (406) operated by the hydraulic motor.