WO2008001050A1

WO2008001050A1 - Closed cycle engine

Info

Publication number: WO2008001050A1
Application number: PCT/GB2007/002327
Authority: WO
Inventors: John Barrington Pearson
Original assignee: Bae Systems Plc
Priority date: 2006-06-26
Filing date: 2007-06-22
Publication date: 2008-01-03
Also published as: EP2032821B1; DE602007008416D1; WO2008001050B1; ES2348280T3; AU2007263635B2; KR101380796B1; AU2007263635A1; CA2655866C; ATE477409T1; KR20090028781A; CA2655866A1; JP2009541662A; EP2032821A1

Abstract

A closed cycle engine system comprises an engine unit operable to combust fuel with combustion supporting gas, and a gas circuit providing fluid communication between the intake and the exhaust of the engine unit. In operation, the engine unit produces exhaust gases. The gas circuit is provided with an absorber to at least partially absorb the exhaust gases and a flow resistance. The flow resistance is located between the absorber and the intake, and is arranged such that the pressure at the intake is less than the pressure at the exhaust. Such an arrangement improves the efficiency of the absorber, and thus the overall efficiency of the engine. A method of operating such an engine system, and vehicles comprising such an engine system, are also disclosed.

Description

CLOSED CYCLE ENGINE

This invention relates to an improved closed cycle engine system. Closed cycle engine systems are operable independently of atmospheric air, and so are particularly useful where atmospheric air is not freely available. Such engines are therefore often used in underwater applications.

Closed cycle engines are known, for example, from European Patent Publication No. 0118284. Such known engines comprise a circuit through which at least some of the exhaust gas from a combustion chamber is ducted so as to return thereto. A supply of oxygen mixed with an inert carrier gas is, provided to the combustion chamber, in which fuel is combusted with the oxygen to produce carbon dioxide, amongst other combustion products. The circuit comprises an absorber in which the exhaust gas is treated with water to remove carbon dioxide from the exhaust gas.

Unfortunately, the absorption of carbon dioxide gas from the exhaust requires a large amount of energy, and this creates a large parasitic power loss for the engine. Reducing this parasitic loss is essential if the overall efficiency of the engine is to be improved. It is recognised in the prior art that it is important for the partial pressure of carbon dioxide at the absorber to be high, since the efficiency of absorption increases as the partial pressure of carbon dioxide increases. Therefore, it has been suggested in EP0118284 to compress the exhaust gas before it reaches the absorber, and then to expand the output gas from the absorber. Such expansion is necessary to reduce the pressure at the intake manifold of the engine to a pressure that is within the operating constraints of the engine. However, compression of the exhaust gas, as disclosed in EP0118284, requires a further parasitic energy loss to power a compressor. Without this additional compression, the maximum absorption pressure is constrained by the maximum intake manifold pressure that the engine can accept. This constraint occurs because the engine peak cylinder pressure is directly influenced by the intake manifold pressure, which, in a system configured to minimise energy loss, is close to the absorption pressure. The working life of the engine is directly influenced by the engine peak cylinder pressure: a higher peak cylinder pressures unfortunately results in a shorter working life.

Accordingly, there exists a need for a more efficient closed cycle engine system with fewer parasitic losses. It is therefore an object of the present invention to provide a closed cycle engine system that at least partially addresses this need, and that at least partially mitigates the above-described problems with prior-known closed cycle engine systems.

In broad terms, the present invention resides in the concept of configuring a closed cycle engine system such that a pressure difference evolves, in operation of the engine, between the intake and exhaust manifolds, and making use of a greater pressure on the exhaust side of the engine to improve the efficiency of the absorption of exhaust gases. By incorporating a flow resistance into the gas circuit linking the exhaust to the intake, such a pressure difference can be achieved, thereby improving the efficiency of carbon dioxide absorption, and thus reducing parasitic losses. Thus, despite the fact that an additional resistance, that might be expected to increase energy losses within the engine system, has been introduced into the gas circuit flow, the overall efficiency of the system is improved. Moreover, these benefits are advantageously achieved without the use of a compressor. According to a first aspect of the present invention, there is provided a closed cycle engine system comprising: an engine unit operable to combust fuel with combustion supporting gas, thereby producing exhaust gases, the engine unit having an intake and exhaust; and a gas circuit providing fluid communication between the intake and the exhaust, the gas circuit having an absorber to at least partially absorb the exhaust gases and a flow resistance, the flow resistance being located between the absorber and the intake and arranged such that the pressure at the intake is greater than the pressure at the exhaust. Locating the flow resistance between the absorber and the intake also ensures that the absorber remains at the higher exhaust pressure, rather than at the lower intake pressure. Surprisingly, the incorporation of an additional flow resistance into the gas circuit improves the efficiency of the engine system because the pressure at the absorber is increased without the need for additional power-consuming components, such as a compressor. Increasing the pressure at the absorber increases the efficiency of CO₂ absorption, and thus parasitic losses associated with the absorber are reduced. Since parasitic losses are reduced, the export power of the system for a given engine shaft power is increased.

The inclusion of the resistance allows the closed cycle system to make use of the natural capacity of the engine unit to accept a pressure difference between the intake and exhaust manifolds. Indeed, certain types of engine unit, such as diesel engines provided with exhaust driven turbo chargers, are designed to operate with a pressure difference between the intake and exhaust. By removing the turbo charger, this pressure difference can be utilised to increase absorption efficiency. Moreover, the presence of a pressure difference between the intake and exhaust manifolds of the engine unit enables a wider variety of engine units to be selected for use in the closed cycle engine system. Previously, the constraints imposed by the maximum allowable intake manifold pressure have reduced the number of engine units that can be used in the closed cycle engine system. This problem is mitigated by the present invention, since the flow resistance can be selected in dependence on the engine unit it is desired to use in a given closed cycle engine system. The flow resistance may advantageously be adjustable in response to the pressure at the intake, thus ensuring that the intake pressure is kept within a range acceptable to the engine, whilst also enabling a high pressure to be maintained at the absorber. Moreover, the presence of an adjustable flow resistance allows the system to account for any transient increases in the pressure at the intake. Such transients may otherwise exceed the maximum pressure that the intake is able to accept.

The flow resistance comprises a flow restrictor, such as an orifice plate, or a section of reduced diameter pipe. The use of an orifice plate provides an advantageously simple way of achieving the flow resistance, that can be incorporated in to existing closed cycle engine systems, or into existing designs for closed cycle engine systems, rapidly and cost effectively. In one specific embodiment described hereinbelow, the flow resistance further comprises a pressure reducing valve responsive to the intake pressure. The pressure at the intake manifold of the engine unit can then be adjusted so that the most efficient pressure values can be selected.

A supply of combustion supporting gas may then be provided to the gas circuit between the orifice plate and the pressure reducing valve. It is convenient for the gas supply to be introduced once the bulk pressure reduction has been accomplished at the orifice plate. The combustion supporting gas is likely to be a mixture of oxygen and an inert carrier gas, such as argon, from separate gas supply bottles, and by introducing these gases before the flow passes through the pressure reducing valve, it can be ensured that the components of the combustion supporting gas are well mixed before entry into the intake manifold.

The flow resistance may advantageously comprise power extraction means to extract power from the flow in the gas circuit. The extraction of power from the flow in the gas circuit further enhances the efficiency of the closed cycle engine system. The power extraction means may comprise a turbine. Alternatively, the power extraction means may comprise a vane or other positive displacement motor.

The pressure at the intake may be controllable independently from the pressure at the exhaust. Such independent control means allows the pressures within the engine system to be adjusted so that enhanced efficiency can be achieved.

According to a second aspect of the present invention, there is provided a method of operating a closed cycle engine system, the system comprising an engine unit having an intake and a exhaust, and a gas circuit providing fluid communication between the exhaust and the intake, the method comprising the steps of: operating the engine, thereby producing exhaust gases, which exhaust gases are ejected into the gas circuit at the exhaust; at least partially absorbing the exhaust gases at an absorber; and providing a flow resistance in the gas circuit, the flow resistance being located between the absorber and the intake arranged such that the pressure at the intake is less than the pressure at the exhaust. The invention extends to a submersible vehicle comprising a closed cycle engine system as described above. Such a submersible vehicle may, for example, be a submarine, or any form of underwater. vehicle requiring motive means. In order that the invention may be better understood, a specific embodiment will now be described, by way of example only, with reference to the accompanying drawing. In the drawing:

Figure 1 is a schematic illustration of an embodiment of the invention.

An argon cycle closed cycle diesel engine system 100 in accordance with an embodiment of the invention is shown schematically in Figure 1.

System 100 comprises a diesel engine unit 110, which unit has an intake manifold 112 and an exhaust manifold 114. The exhaust manifold is linked via appropriate ducting or piping to an absorber 120, which in turn is linked, via separator 130, orifice plate 140, and pressure reducing valve 150, back to the intake manifold 112 of the engine unit 110. Thus a gas circuit, linking the exhaust manifold back to the intake manifold, is defined. Between orifice plate

140 and pressure reducing valve 150 there are provided inlets 144 and 146 for supplying argon and oxygen to the circuit. An inlet (not shown) for supplying fuel to the engine unit 110 is also provided. The supplies of oxygen and argon may be provided from gas bottles, or other appropriate gas storage devices.

The system 100 can be operated in closed cycle. Engine unit 110 is aspirated with a combustion supporting gas comprising a mixture of argon and oxygen supplied from inlets 144 and 146. Combination of fuel with the oxygen in the engine unit 110 produces exhaust gases including carbon dioxide (CO2). At least some of the CO₂ is absorbed in chamber 120. Absorber 120 may comprise, as in prior known closed cycle engines, a rotor provided with wire mesh, or other material having a high surface area to volume ratio, through which water is thrown radially outward by centrifugal force, whilst the exhaust gas is caused to flow therethrough in counterflow. Varying the amount of water passing through absorber 120 through use of a variable speed water pump 125 allows the amount of CO₂ absorbed, and thus the pressure at the absorber 120 and at the exhaust 114, to be controlled. The thus-treated gases are then passed through separator 130, which removes water from the gas flow, to orifice plate 140. The orifice plate 140, in combination with pressure-reducing valve 150, serves to control the pressure at the intake manifold 112 of the engine unit 110. Pressure-reducing valve 150 is

5 controlled in response to the pressure at the intake 112, so that it can be ensured that the pressure at the intake does not rise above the maximum intake pressure of which the engine unit 110 is capable. This is indicated schematically in Figure 1 by dashed line 155 linking intake 112 to pressure reducing valve 150. The pressure at the intake 112 is therefore controlled o independently of the pressure at the exhaust 114.

Engine unit 110 is a conventional diesel engine of a kind normally fitted with an exhaust driven turbo charger. When running in its normal aerobic configuration, such an engine will be configured to operate with a pressure difference between the engine 110 and the turbine sufficient to drive the turbine5 to compress the intake air up to the design pressure. The exhaust pressure will therefore be higher than the intake pressure created. In order to be used in the system 100, the engine unit is adapted by removal of the turbo charger and direct connection of the intake 112 and the exhaust 114 to the engine 110.

In closed cycle operation, therefore, with the turbo charger removed, the capability of the engine to operate with a pressure difference across it can be exploited to enhance the absorption efficiency by increasing the pressure at the absorber 120, whilst maintaining the pressure at the intake manifold 112 at a value within the range acceptable for the engine unit 110. The pressure difference builds after initial start-up of the engine unit 110: at first, the system will be at a uniform pressure. The partial pressure of CO2 in the system is very low at start-up, and so the absorption efficiency is also low. As the engine continues to run, the partial pressure of CO₂ therefore builds, and the absorber efficiency increases, until equilibrium is reached. A pressure difference builds between the intake manifold and the absorber through action of the additional flow resistance that comprises orifice plate 140 and pressure reducing valve

150. At equilibrium, this difference is maintained at a constant value consistent with the maximum pressure that the intake manifold can accept, whilst maintaining the pressure at the absorber 120 at the higher pressure present at the exhaust manifold 114. Thus the absorber efficiency is increased. Increasing the absorption efficiency reduces parasitic losses in the engine, and thus, by introducing the orifice plate and pressure reducing valve into the gas circuit, the overall efficiency of the engine is improved.

As will be immediately obvious to those skilled in the art, variations and modifications to the above-described embodiment are possible. For example, whilst, in the above, it has been described to use a combination of a orifice plate and a pressure reducing valve to restrict to the flow and achieve control of the intake pressure, several other configurations may be used. Such other configurations could make use of reduced diameter pipes, or various actively and passively controlled flow restrictors known to those skilled in the art. In a particularly simple configuration, the flow resistance may comprise only the orifice plate, rather than including the pressure-reducing valve as described above. Such a flow resistance would be appropriate where the exact pressure difference required in operation of the engine system was known at manufacture, and further control not needed. It is also envisaged to use devices that can make use of the energy from the pressure reduction to provide further power to the plant, such as gas turbines and expanders, or any type of positive displacement motor. Finally, it is noted that it is to be clearly understood that such variants and modifications, and others that will be immediately obvious to those skilled in the art, are possible without departing from the scope of the invention which is defined in the accompanying claims.

Claims

1. A closed cycle engine system comprising: an engine unit operable to combust fuel with combustion supporting gas, thereby producing exhaust gases, the engine unit having an intake and exhaust; and a gas circuit providing fluid communication between the intake and the exhaust, the gas circuit having an absorber to at least partially absorb the exhaust gases and a flow resistance, the flow resistance being located between the absorber and the intake and arranged such that the pressure at the intake is less than the pressure at the exhaust.

2. An engine system as claimed in claim 1 wherein the flow resistance is adjustable in response to the pressure at the intake.

3. An engine system as claimed in claim 1 or claim 2 wherein the flow resistance comprises a flow restrictor.

4. An engine system as claimed in any one of claims 1 to 3 wherein the flow resistance comprises an orifice plate.

5. An engine system as claimed in claim 4 wherein the flow resistance further comprises a pressure reducing valve responsive to the intake pressure.

6. An engine system as claimed in claim 5 wherein a supply of combustion supporting gas is provided to the gas circuit between the orifice plate and the pressure reducing valve.

7. An engine system as claimed in claim 1 or claim 2 wherein the flow resistance comprises power extraction means to extract power from the flow in the gas circuit.

8. An engine system as claimed in claim 7 wherein the power extraction means comprise a turbine.

9. An engine system as claimed in any preceding claim wherein the pressure at the intake is controllable independently from the pressure at the exhaust.

10. A closed cycle engine system substantially as described herein with reference to the accompanying drawing.

11. A submersible vehicle comprising a closed cycle engine system as claimed in any of claims 1 to 9.

12. A method of operating a closed cycle engine system, the system comprising an engine unit having an intake and a exhaust, and a gas circuit providing fluid communication between the exhaust and the intake, the method comprising the steps of: a) operating the engine, thereby producing exhaust gases, which exhaust gases are ejected into the gas circuit at the exhaust; b) at least partially absorbing the exhaust gases at an absorber; and c) providing a flow resistance in the gas circuit, the flow resistance being located between the absorber and the intake arranged such that the pressure at the intake is less than the pressure at the exhaust.

13. A method of operating a closed cycle engine system substantially as described herein with reference to the accompanying drawing.