Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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.
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2a
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, wherein the flow resistance is adjustable in response to the pressure
at the intake. 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
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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
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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. lt 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.
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4a
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; 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; and adjusting the flow resistance in response to the pressure at the
intake.
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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.
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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
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
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
turbine
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
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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.
