Heat engine and heat pump

An internal combustion engine has a compression cylinder with a liquid spray apparatus for spraying sufficient liquid into the cylinder such that the liquid absorbs the heat of the gas as it is compressed without vaporizing. A separator removes the liquid from the gas/liquid mixture as it leaves the cylinder. The gas is then directed to a expansion cylinder for combustion with fuel delivered by a fuel supply apparatus. The cylinders being coupled together by a crankshaft.

This invention relates to heat engines and heat pumps, and in particular to 
those for providing power and/or heat appropriate to domestic appliances, 
service industries, commerce and manufacturing industry. 
BACKGROUND OF THE INVENTION 
The attainment of high thermal efficiency is nearly always an important 
consideration in the field of power generation for the reason that the 
fuel cost is generally responsible for about two thirds of the cost of the 
power produced. In addition to the cost incentive, enviromental 
considerations require that greater effort be directed towards the 
achievement of higher efficiencies in order to minimise the production of 
carbon dioxide and other undesirable emissions. 
In general it is possible to achieve a higher thermal efficiency and fewer 
emissions in large generating units than in small ones. This is partly 
because of heat losses, friction and leakage flows which tend to be 
proportionally less significant in large units than in small ones. Also 
economies of scale make it possible to have more sophisticated equipment 
in large units. In small units, the cost of such equipment may be 
prohibitive. 
In spite of these factors, there are circumstances where small generating 
units are needed and it is important that they should be as efficient and 
enviromentally benign as possible. This situation arises in the many parts 
of the world where no electricity grid is available. It may be that 
construction of a power station to supply electricity is beyond the 
financial capacity of the local population or it may be that the 
electricity demand is too small to justify its construction. The former 
situation arises in many less developed countries. The latter situation 
applies in many remote or thinly populated regions and on offshore 
islands. 
Another application for small efficient engines arises in connection with 
combined heat and power (CHP). The use of heat and power together usually 
results in a higher overall energy efficiency than the use of mains power 
from the electricity grid. Since heat cannot be transported economically 
over any significant distance, CHP systems have to be sized for the local 
heat load. This usually implies generating units of modest size. 
The invention described here can be applied either as a heat engine or in 
modified form as a heat pump. Heat pumps transfer heat from a low 
temperature heat source to a high temperature heat sink. For example, in 
cold weather a heat pump can extract heat from the atmospheric air and 
pump it to a higher temperature in order to heat a building. 
Alternatively, in hot weather, the heat pump can operate as an air 
conditioning unit to extract heat from the internal air of the building 
and reject it to the outside atmosphere, even though the outside 
temperature is higher than the inside temperature. The heat pump may also 
be used to cool air in order to condense the water vapour in it. The heat 
rejected from the heat pump may then be used to restore heat to the air. 
In this case the heat pump is used to de-humidify the air. As with CHP, 
heat pumps have to be sized in accordance with the local heat load. 
Consequently, most heat pump capacity will be required in the form of 
small rather than large units. 
Most types of heat pump, air conditioning unit or refrigeration system 
require the use of an evaporating/condensing fluid which boils at an 
appropriate temperature such as one of the chloro-fluoro-carbons (CFC's). 
These substances are known to deplete the earth's ozone layer which 
protects human and animal life from harmful ultra-violet radiation. 
Although certain alternatives to CFC's are known, some of these also cause 
ozone depletion, but to a lesser degree. Other alternatives have 
disadvantages such as flammability, toxicity, high cost, poor 
thermodynamic properties or a tendency to increase global warming. 
Engines and heat pumps based on the Stirling Cycle are well known. One form 
of Stirling engine includes a compression chamber and an expansion chamber 
connected together via a regenerative heat exchanger forming a gas space 
which contains a working gas. According to the ideal Stirling Cycle 
working gas in the compression chamber is compressed by a piston and 
undergoes isothermal compression, the heat of compression being rejected 
to a low temperature heat sink. After this process is complete the cold 
working gas is pushed through the regenerator where it is preheated before 
entering the expansion chamber. In the expansion chamber, the hot 
compressed working gas is allowed to expand by forcing the piston out of 
the expansion chamber. During expansion, heat is added to the working gas 
so that the gas expands isothermally. The hot expanded gas is then pushed 
back through the regenerator to which it gives up its heat before being 
admitted to the compression chamber to begin the next cycle. 
U.S. Pat. No. 4,148,195 describes a heat actuated heat pump which requires 
a high temperature heat source such as the combustion of fuel and another 
heat source at low temperature such as atmospheric air. The heat output is 
at an intermediate temperature. The purpose of the heat pump is to convert 
a certain amount of heat energy at high temperature to a larger amount of 
heat energy at the intermediate temperature. This is done by extracting 
heat energy from the low temperature heat source. The heat actuated pump 
described in U.S. Pat. No. 4,148,195 is a closed-cycle system without 
valves which approximates to the Stirling cycle. Liquid pistons contained 
in a series of four interconnected U-tubes and which are connected in a 
closed circuit displace the working gas between adjacent expansion and 
compression chambers formed in the arms of the U-tubes. The liquid pistons 
transmit power around the closed circuit directly from the expanding gas 
in the expansion chamber to the compressing gas in the adjacent 
compression chamber, an expansion chamber and a compression chamber being 
formed in opposed arms of the same U-tube. The four U-tubes are connected 
via the gas space with regenerators. Two of the four regenerators and the 
associated gas volumes work in a temperature range between the high 
temperature and the intermediate temperature. The other two regenerators 
and associated gas volumes work in a temperature range between the low 
temperature and the intermediate temperature. The cycle is operated in 
such a way that power is transmitted via the medium of the liquid pistons 
from the gas volumes working over the high temperature range to the gas 
volumes working over the low temperature range. 
21st Inter-society Energy Conversion Engineering Conference Volume 1 (1986) 
pages 377 to 382 describes a Stirling heat actuated heat pump similar to 
that described in U.S. Pat. No. 4,148,195, in which the working gas is 
heated or cooled by taking liquid from a liquid piston, heating or cooling 
the liquid externally and reinjecting it into the expansion or compression 
cylinder as an aerosol. 
One drawback of these known heat pumps is that the maximum working 
temperature of the high temperature heat source is very low in comparison 
to what can be achieved in modern advanced power generating technologies, 
such as the combined cycle gas turbine. For example the temperature of 
heat addition to the heat pump is likely to be limited to 400.degree. C., 
whereas the turbine inlet temperature of a modern power generating gas 
turbine is anything up to 1300.degree. C. Consequently the efficiency of 
conversion of the high temperature heat to internal work within the heat 
actuated heat pump is also low, as would be expected from considerations 
of Carnot's theorem. As a result the overall coefficient of performance is 
very low. 
Another disadvantage of the heat actuated heat pump described in U.S. Pat. 
No. 4,148,195 lies in the fact that the liquid pistons have to be very 
long in order to achieve a low natural frequency of oscillation. The 
frequency of oscillation must be low because sufficient time must be 
allowed for heat transfer between the droplet spray and the gas. The 
required length of liquid piston is particularly difficult to achieve in a 
small device operating at high pressure. Also friction losses arising from 
long liquid pistons are likely to become unnacceptably high in a small 
device. Furthermore a high value for the ratio of length to stroke is 
required to avoid the so-called shuttle loss which arises from the 
transfer of heat from one end of each liquid piston to the other end. The 
shuttle loss occurs because the two ends of each liquid piston are at 
different temperatures and there is consequently some mixing of the liquid 
and transport of heat. 
U.S. Pat. No. 3,608,311 describes an engine whose operation is based on the 
Carnot Cycle, in which gas is successively compressed and expanded in a 
single cylinder by a liquid displacer. Hot and cold liquid from the liquid 
displacer is alternately injected into the cylinder to heat the gas during 
part of the expansion process, and to cool the gas during part of the 
compression process. 
One drawback of this known heat engine is that the power output per cycle 
is relatively low because it requires an extremely high compression ratio 
to raise the temperature of the working gas to a reasonable value during 
adiabatic compression, and such a compression ratio is not possible in 
practice. A further drawback of this engine is that the working gas is 
continually cycled between high and low temperatures while remaining in 
the same cylinder throughout the process. Therefore the walls of the 
cylinder also cycle from low to high temperatures and back again which 
implies large entropy changes and a reduction in thermodynamic efficiency. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention there is provided a heat 
engine comprising a compression chamber to contain gas to be compressed 
and a first piston to compress said gas by movement of the piston in said 
compression chamber and driving means arranged to drive said first piston 
into the compression chamber to compress said gas, an expansion chamber 
and a second piston to allow gas to expand therein by movement of the 
second piston out of the expansion chamber, means to feed compressed gas 
from said compression chamber to said expansion chamber, and means to heat 
said compressed gas from the compression chamber, transmission means 
operatively coupled to said second piston to permit power from the engine 
to be drawn, and means forming a spray of liquid in said compression 
chamber to cool the gas on compression therein. 
One advantage of this arrangement is that heat is rejected efficiently to 
the liquid in the liquid spray, at the lowest temperatures in the heat 
engine cycle. Furthermore, expansion is done in a separate chamber so that 
temperatures in each chamber and therefore the various parts of the 
chamber and of the pistons do not cycle between high and low temperatures, 
and thus reduceing the efficiency. 
In a preferred embodiment, the engine further comprises means to add heat 
to the gas in the expansion chamber during expansion thereof. Thus, the 
expansion process may be approximately isothermal. 
Preferably, the heating means also includes heat exchanger means arranged 
to pre-heat compressed gas from the compression chamber with heat from gas 
expanded in the expansion chamber. Thus, expanding the gas isothermally in 
the expansion chamber provides an opportunity of recovering some of this 
heat in a heat exchanger which is used to pre-heat the compressed gas from 
the compression chamber prior to expansion. The heat exchanger may for 
example be a regenerative heat exchanger if expanded gas from the 
expansion chamber flows along the same flow path as the incoming 
compressed gas from the compression chamber, or a recuperative heat 
exchanger if the gases flow along different flow paths. A recuperative 
heat exchanger is particularly advantageous where heat exchange is 
required between two gases where mixing of the gases is undesirable and/or 
the two gases are at substantially different pressures. 
One embodiment includes means for returning expanded gas leaving the 
expansion chamber to the compression chamber for recompression. The 
returning means may be separate from the means for feeding compressed gas 
to the expansion chamber, or the working gas may flow back and forth 
between the compression and expansion chambers along the same flow path. 
Embodiments in which the same body of working gas is continuously recycled 
between the compression and expansion chambers will be referred to as a 
closed-cycle engine. Because the working gas is sealed within the engine, 
the gas can be pre-pressurised so that the minimum pressure attained by 
the gas during the cycle is much greater than atmospheric. 
In one embodiment of the engine, the means to add heat to the gas in the 
expansion chamber comprises means forming a spray of hot liquid in the 
expansion chamber. The liquid used in the spray may be heated using an 
external heat exchanger and the source of heat may be waste heat e.g., 
industrial waste heat, solar energy or heat from a combustion chamber 
cooling system. Using a hot liquid spray to transfer heat into the 
expansion chamber is particularly advantageous when used in closed-cycle 
engines which have a heat source at relatively low temperature. Liquid 
sprays are not suitable for use at very high temperatures. 
An alternative embodiment includes first valve means operative to admit air 
or other oxidising gas into the compression chamber, second valve means 
operative to prevent gas in the expansion chamber returning to the 
compression chamber through said means for feeding compressed gas to the 
expansion chamber and wherein the means to add heat comprises means to 
provide a combustible fuel in the expansion chamber. In this embodiment, 
the mixture of fuel and hot compressed gas in the expansion chamber 
ignites and after expansion the combustion products are expelled from the 
engine via the heat exchanger means. A fresh supply of working gas is 
therefore required at the beginning of each cycle. Embodiments in which 
the working gas is renewed each cycle will be referred to as an open-cycle 
engine. One form of this embodiment may include means to control the rate 
of flow of combustible fuel into the expansion chamber to provide 
substantially isothermal expansion. 
It is generally preferable that the first and second pistons provide a good 
seal for the working gas and this is particularly important in the 
closed-cycle engine. Advantageously, the first and/or second pistons may 
comprise a liquid thus eliminating the sealing difficulties which may 
otherwise be present if the pistons are solid. A preferred embodiment 
comprises a pair of generally U-shaped conduits each containing a body of 
liquid as a piston, a compression chamber formed in each arm of one 
conduit and an expansion chamber formed in each arm of the other conduit, 
and means feeding compressed gas from one of said compression chambers to 
one of said expansion chambers and separate means feeding compressed gas 
from the other compression chamber to the other expansion chamber. In this 
embodiment, expansion and compression each occur twice per cycle and the 
timing of the liquid pistons is preferably arranged so that the expansion 
process in one of the expansion chambers drives the compression process in 
one of the compression chambers. This may be achieved by appropriate 
coupling between the drive means and the transmission means. A preferred 
embodiment comprises another pair of said generally U-shaped conduits 
whereby in use, the liquid piston in one U-shaped conduit containing 
expansion chambers is substantially 90.degree. out of phase with the 
liquid piston in the corresponding U-shaped conduit containing the other 
expansion chambers. It will thus be appreciated that this arrangement can 
provide a net positive power output at each stage during a complete cycle 
of the engine, thereby removing the need for a fly wheel or other means to 
sustain the operation of the engine between power strokes. 
When expanded gas is forced out of the expansion chamber by movement of the 
second piston into the expansion chamber, the gas pressure is increasing. 
A preferred embodiment of the engine includes means to provide liquids of 
at least two different temperatures for use in the liquid spray in the 
expansion chamber and includes means forming a spray of liquid during 
compression of gas in the expansion chamber to control the gas 
temperature. The temperature of the liquid spray is preferably such that 
the temperature of the gas remains constant during compression thereof. 
Advantageously, if said second piston comprises a liquid, said means to 
provide may be arranged to supply liquid from the liquid piston directly 
to the spray forming means. 
After compression of gas in the compression chamber, the gas pressure 
decreases and the gas expands as a result of both pistons moving out of 
their respective chambers. A preferred embodiment includes means to 
provide liquids of at least two different temperatures in the liquid spray 
in the compression chamber and includes means forming a spray of liquid 
during expansion of gas in the compression chamber to control gas 
temperature. Preferably, the temperature of the liquid spray is such that 
the gas temperature is maintained constant during expansion. 
Advantageously, if said first piston comprises a liquid, said means to 
provide may be arranged to supply liquid from said first piston directly 
to the spray forming means. 
Where any of the first pistons comprise a liquid, the drive means may 
comprise a member arranged to cooperate with the first piston such that 
motion of the member imparts motion in at least one direction to the 
piston. The member may comprise a solid piston and may be immersed in the 
liquid piston or floating on the surface thereof. The solid piston may be 
coupled to a shaft extending through the wall of the conduit containing 
the liquid piston. 
Likewise, where the or one of the second pistons comprises liquid, the 
transmission means may comprise a member arranged to cooperate with said 
second piston such that motion of the liquid piston in at least one 
direction is imparted thereto. The member may comprise a solid piston 
which is immersed in the liquid piston or arranged to float on the surface 
thereof. A shaft may be coupled to the solid piston and extend through the 
wall of the conduit containing the second piston. 
Alternatively, the first and second piston may comprise a solid material. 
One embodiment includes a pair of compression chambers and a pair of 
expansion chambers wherein in use the pistons in the compression chambers 
are arranged to move substantially in antiphase with each other and the 
pistons in the expansion chambers are arranged to move substantially in 
antiphase with each other. In a preferred embodiment, another said pair of 
compression chambers and another said pair of expansion chambers are 
provided wherein in use, the pistons in one pair of compression chambers 
are arranged to move substantially 90.degree. out of phase with the 
pistons in said other pair of compression chambers and the pistons in one 
pair of expansion chambers are arranged to move substantially 90.degree. 
out of phase with the pistons in the other said pair of expansion 
chambers. 
Preferably, in a closed-cycle engine the heat exchanger means comprises a 
regenerator. The purpose of the regenerator is to enable heat to be 
transferred to and from the working gas efficiently. 
In a preferred embodiment, separator means are provided to separate liquid 
from the gas leaving the or each compression chamber. In embodiments 
operating in a closed-cycle, a separator means may also be provided to 
separate liquid from the gas leaving the or each expansion chamber. 
Where the first and/or second pistons comprise a liquid, means are 
preferably provided to supply the or each means forming a spray with 
liquid from the liquid pistons. Advantageously, said means to supply may 
include a pump arranged to be driven by a respective piston. 
In one embodiment said driving means includes coupling means coupled to 
said transmission means so that in use, said first and second pistons move 
in predetermined phase relationship. It will be appreciated that coupling 
the first and second pistons together by for example a mechanical means 
such as a crankshaft is a convenient method to enable large compression 
ratios to be achieved, and at the same time maintain the phasing of the 
pistons. The phase angle between the first and second pistons may be such 
that the second piston leads the first piston by at least 90.degree. 
Alternatively the pistons could be driven independently and may each be 
adapted together with any means for coupling to an external drive, to 
withstand substantial forces against the pressures in their respective 
chambers. 
In one embodiment, the engine may further comprise a combustion chamber for 
the combustion of fuel, wherein the heating means comprises means to heat 
compressed gas from said compression chamber with heat conducted across at 
least one of the surfaces defining the combustion chamber of the engine. 
Thus, advantageously the present invention may readily be adapted to 
provide a cooling apparatus for a conventional combustion engine (e.g. 
petrol, diesel or gas) which recovers heat, normally wasted by 
conventional cooling apparatus and converts this heat into useful power. 
Cold compressed gas is produced in the compression chamber and heat lost 
to the combustion chamber walls is transferred to the compressed gas to 
provide cooling of the engine. The same method can be used to recover heat 
from the exhaust gases of a conventional combustion engine, for example by 
putting compressed air cooling channels through the exhaust manifold or by 
including a heat exchanger through which the exhaust gases would pass. The 
pre-heated compressed gas is then injected into the expansion chamber 
which expands forcing the piston out of the chamber and thereby generating 
useful mechanical work. In one embodiment, the expansion piston may be 
connected to an external output drive of the engine. This arrangement has 
the advantage of increasing the efficiency of conventional combustion 
engines. 
According to another aspect of the present invention there is provided a 
heat pump comprising an expansion chamber to contain gas to be expanded 
and a first piston to allow the gas to expand by movement of the piston 
out of the expansion chamber, a compression chamber to contain gas to be 
compressed and a second piston to compress said gas by movement of said 
second piston in the compression chamber, means to feed gas from one of 
said expansion chamber and said compression chamber to the other chamber, 
and means to form a spray of liquid in said compression chamber to absorb 
heat from said gas during compression, wherein said second piston is 
adapted to be driven by an external source of power into said compression 
chamber to compress the gas. 
This form of heat pump enables the pumped heat to be transferred to an 
external heat sink extremely efficiently via the medium of a liquid spray 
in the hot compression chamber and at the same time can be driven through, 
for example a mechanical coupling, by an external source of power and in 
particular an electric motor to provide a heat pump with a higher 
coefficient of performance than can be achieved by known heat pumps. 
Advantageously, this form of heat pump can perform heating or cooling in 
either a closed-cycle or an open-cycle. For example, one embodiment may be 
adapted for air conditioning in which air is drawn into the compression 
chamber from an external source, compressed substantially isothermally 
using the liquid spray and passed to the expansion chamber in which it 
expands, so that it does work, returning some of the energy used for 
compression. The expansion may be adiabatic so that the gas cools, and the 
cool gas may then be ejected from the heat pump to provide air 
conditioning. Alternatively, another embodiment of the heat pump may 
further include means to supply heat to the gas during expansion thereof 
in the expansion chamber so that the expansion is approximately 
isothermal. This may be done efficiently by employing a liquid spray in 
the expansion chamber. Heat is absorbed from the liquid droplets, which 
cool, and the cooled spray liquid may be used for cooling, e.g., air 
conditioning. The liquid spray injection into the expansion chamber also 
allows efficient heat transfer from a low temperature heat source so that 
the heat pump can pump this heat to higher temperature sink, for the 
purpose of heating. The heat pump can be modified for either open or 
closed-cycles. 
In another embodiment the heat pump may further comprise heat exchanger 
means arranged to pre-heat said expanded gas with heat from compressed gas 
leaving the compression chamber. This is particularly advantageous in the 
closed cycle in which the same gas is pumped back and forth between the 
expansion and compression chambers. 
A preferred embodiment includes coupling means for coupling the second 
piston to the external source of power, wherein the coupling means is 
adapted to withstand substantial force against the pressure of gas in the 
compression chamber. Coupling the heat pump to an external source of power 
in this way, enables much higher pressures and therefore a higher 
compression ratio to be achieved in the compression chamber so that a 
greater amount of heat can be pumped per cycle than achieved by prior art 
heat pumps. At the same time, the use of such a coupling enables the heat 
pump to be compact, since the attainment of high pressures (and therefore 
output) does not rely on the inertia of the pistons which would have to be 
relatively massive and therefore large in size. The coupling means may for 
example comprise a crank shaft. 
In a preferred embodiment, the first and second pistons are coupled 
together by a mechanical coupling means, e.g. a crank shaft so that the 
phasing of the pistons can be easily controlled. 
Another important advantage of the heat pump according to the present 
invention is that it does not require an evaporating or condensing fluid, 
and can be used with a gas which does not condense and a liquid which does 
not evaporate to any significant degree. There is no requirement for a 
specific boiling point. Indeed, it is possible to choose a gas such as 
helium and a liquid such as water, which will cause no harm to the 
environment should they be released. This is also an important advantage 
of the heat pump according to the present invention. An additional 
advantage of not requiring a specific boiling point is that the heat pump 
can work over a wider range of operating temperatures than conventional 
heat pumps. 
The heat pump may include any one or more of the preferred or alternative 
features mentioned above in association with the heat engine. 
Embodiments of the heat engine and heat pump may include any number of 
compression and expansion chambers and the number of compression and 
expansion chambers need not be equal.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 to 3, a pair of U-shaped conduits 1 and 3 each contain 
a body of liquid 5 and 7. A compression chamber 9, 11 is formed in each of 
the arms 13 and 15 of one of the U-shaped conduits 1 and an expansion 
chamber 17, 19 is formed in each arm 21 and 23 of the other U-shaped 
conduit 3. One of the compression chambers 9 is connected through a 
regenerator 25 to one of the expansion chambers 19 and the other 
compression chamber 11 is connected through another regenerator 27 to the 
other expansion chamber 17. In practice, the U-shaped conduits shown in 
FIGS. 1 and 2 would each be rotated 90.degree. to face each other, with 
the regenerators having the same length, as shown in FIG. 3. The two 
U-shaped conduits and regenerators are thus configured as a saddle and 
will be referred to as "saddle loop". An engine or a heat pump which 
consists of a single inter-connected mass of gas with a single 
regenerator, a single compression chamber and a single expansion chamber, 
each with a liquid or solid piston and each with means for addition or 
removal of heat is described as a "half saddle loop". 
Liquid sprays are provided in both compression chambers and both expansion 
chambers. Liquid used in the sprays 29 and 31 in the compression chambers 
is preferably drawn from the body of liquid in the conduit 1 and the 
liquid sprays 33 and 35 in the expansion chambers 17 and 19 is preferably 
drawn from the liquid in the corresponding conduit 3, as shown in FIG. 2. 
The liquid drawn from conduit 1 may be passed through a cooler 36 (FIG. 2) 
prior to injection in the compression chambers 9 and 11 and liquid drawn 
from conduit 3 may be passed through a heater prior to injection in the 
expansion chambers 17 and 19. A working gas fills the space formed by the 
compression chambers 9 and 11 and their corresponding expansion chambers 
19 and 17 with which they communicate via a respective regenerator 25 and 
27. Separators 37, 39, 41 and 43 are provided between the chambers and 
corresponding regenerators to remove any liquid in the working gas before 
the fluid passes through the regenerator concerned. 
Each U-shaped conduit 1 and 3 has a linear section 45 and 47 joining the 
adjacent arms. Mechanical means coupled to each liquid piston is provided 
to transmit power to and from the pistons. In the embodiment shown in 
FIGS. 1 to 3, a solid piston 49 and 51 is disposed in each of the linear 
sections of the conduit and is free to execute linear motion along the 
length thereof with the liquid pistons formed either side. A drive shaft 
53, 55 is connected to each solid piston 49 and 51 and extends through the 
wall of each conduit to provide means for driving or transmitting power 
from the liquid pistons. 
The two drive shafts 53 and 55 are coupled together by an external drive 
mechanism so that the displacement of each piston is approximately 
sinusoidal with time and so that a predetermined phase relationship is 
maintained between the pistons in different conduits. This can be achieved 
for example by coupling the drive shafts 53 and 55 to a crankshaft as for 
petrol or diesel engines, as shown in FIGS. 2 and 3. The crankshaft may be 
coupled to drive an electricity generator 52. 
The engine operates by passing the working gas through a thermodynamic 
cycle which involves repeated compressions and expansions. The compression 
is done when most of the working gas is in the compression chamber 9 and 
11 while the expansion is done when most of the working gas is in the 
expansion chamber 17 and 19. This may be achieved by arranging for the 
pistons in the expansion chambers to lead the pistons in the compression 
chambers by a phase angle of 90.degree. The phase angle between the 
pistons in the expansion chambers or compression chambers is 180.degree. 
With this arrangement, the expansion process in one of the expansion 
chambers will drive the compression process in the other compression 
chamber. For example expansion in chamber 19 will drive the compression in 
chamber 11 and the expansion in chamber 17 will drive the compression in 
chamber 9. 
One complete cycle of the engine will now be described in relation to one 
compression chamber and one expansion chamber only, beginning with 
compression in compression chamber 9. At the start of compression the 
liquid piston in the compression chamber 9 is at the bottom of its stroke 
and the piston in the expansion chamber 19 is at the mid-point of its 
stroke and moving upwards. Most of the working gas shared between the 
compression chamber 9 and the expansion chamber 19 is in the compression 
chamber 9. The compression piston moves into the compression chamber 9 and 
compresses the working gas against the gas pressure resulting from 
movement of the expansion piston into the expansion chamber 19. Cold 
liquid is sprayed into the compression chamber to cool the working gas 
during compression. This liquid may be obtained by drawing off liquid from 
the cold liquid piston (i.e. the compression piston) and then passing it 
through an external cooler 36 (shown in FIG. 2) before injecting it into 
the compression chamber. When the compression piston in compression 
chamber 9 is at the mid-point of its stroke the expansion piston in 
expansion chamber 19 will be at the top of its stroke and about to reverse 
direction. As the compression piston continues moving upwards in the 
compression chamber, compression of the working gas continues but at the 
same time the cool compressed gas begins to flow through the regenerator 
towards the expansion chamber 19 as the expansion piston begins to move 
downwards. The cool compressed gas leaving the compression chamber 9 is 
pre-heated with heat from the expanded gas which left the expansion 
chamber at the end of the previous cycle. 
When the compression piston in compression chamber 9 has reached the top of 
its stroke, the expansion piston in expansion chamber 19 is at the 
mid-point of its stroke and moving downward, out of the expansion chamber. 
Hot liquid is sprayed into the expansion chamber to maintain the 
temperature of the gas as it expands on continued downward movement of the 
expansion piston. This liquid may be obtained by drawing off liquid from 
the hot liquid piston (i.e. the expansion piston) and then passing it 
through an external heater 38 (FIG. 2) before injecting it into the 
expansion chamber. At the same time, the compression piston has reversed 
direction and is moving out of the compression chamber 9. To prevent the 
gas in the compression chamber from cooling during expansion it may be 
advantageous to spray liquid drawn directly from the liquid piston rather 
than liquid which has been pre-cooled in an external cooler. 
When the expansion piston has reached the bottom of its stroke in the 
expansion chamber 19 the compression piston will be at the mid-point of 
its stroke in the compression chamber 9 and moving downwards. The 
expansion piston reverses direction and the two pistons move in opposite 
directions forcing the working gas out of the expansion chamber, through 
the regenerator and into the compression chamber. The hot expanded gas 
leaving the expansion chamber is pre-cooled in the regenerator before 
returning to the compression chamber. As the expansion piston moves 
upwards into the expansion chamber, the gas remaining in that chamber 
undergoes some compression. To prevent heating of the gas, liquid may be 
sprayed into the expansion chamber. This liquid should preferably be taken 
directly from the hot liquid piston without passing through the external 
heater. When the compression piston in the compression chamber 9 reaches 
the bottom of its stroke, the expansion piston in the expansion chamber 19 
is at the mid-point of its stroke and travelling upwards into the 
expansion chamber, the compression piston reverses direction and the cycle 
is repeated. 
As mentioned above, the thermodynamic cycle in chambers 9 and 19 is 
180.degree. out of phase with the cycle in chambers 11 and 17. Thus, the 
expansion stroke in chamber 19 drives the compression stroke in chamber 11 
and the expansion stroke in chamber 17 drives the compression stroke in 
chamber 9. However, there are points in the cycle between the compression 
and expansion strokes where no net power output from the engine is 
occuring. Thus, to sustain the operation of the engine over the cycle a 
fly wheel may be used or it may be possible to rely on the inertia of the 
pistons themselves if they are massive enough. However, the need for a fly 
wheel can be avoided by providing a second saddle loop whose operating 
cycle is arranged to be 90.degree. out of phase with that of the first 
saddle loop. This may be achieved by incorporating an appropriate external 
drive mechanism. This embodiment of the heat engine is then capable of 
providing a net energy output at all stages of the cycle. An example of 
such an embodiment with an appropriate external drive mechanism 58 is 
shown in FIG. 4, in which similar parts to those shown in FIGS. 1 to 3 are 
designated by like numerals. 
One of the most important features of the engine described above is the use 
of hot and cold liquid sprays to maintain the temperature of the working 
gas within each chamber at the desired value. As stated above, the liquid 
sprays may be maintained throughout the cycle, although the liquid passes 
through the heat exchangers during only part of the injection cycle. The 
reason for this can be explained in connection with each chamber 
separately. 
During compression, the function of the spray is to keep the working gas 
temperature in the compression chamber as low as possible. Thus the liquid 
should be passed through the external cooler during this part of the 
cycle. When the gas is expanded, in a later part of the cycle, the 
function of the spray is to prevent the gas from cooling too much. During 
this part of the cycle, it is better to take the liquid directly from the 
liquid piston and not to cool it. 
The converse argument applies to the expansion chamber. During expansion 
the gas must be as hot as possible and therefore the liquid spray should 
be passed through the external heater. During compression, it is important 
to prevent the gas from becoming too hot. Therefore, the liquid should be 
taken directly from the liquid piston during this stage. 
In one embodiment, the pumping of the liquid used for the spray may be 
achieved by making direct use of the reciprocating motion of the piston 
and drive shaft. The pump which may be mounted within the conduit 
comprises a small piston driven by the liquid piston, the solid piston or 
the drive shaft, for example, as shown in FIG. 2, and which is arranged to 
slide in a cylinder incorporating non-return valves. A single pump 60,62 
in each conduit may be provided if the pump is double ended i.e. fills and 
pumps at both ends. This enables liquid to be supplied from each end 
alternately while the other end is filling. One double ended pump would 
serve two liquid spray injectors associated with that particular conduit. 
Each end of the pump may have two outlets, one which leads to the spray 
nozzle in one of the chambers associated with the particular conduit, 
while the other leads directly to the spray nozzle in the other chamber. 
Thus, although a liquid spray would be maintained almost continuously, the 
temperature of the injected liquid would vary during the cycle according 
to whether it had passed through the heat exchanger or not. 
The separators situated above the spray injector nozzles and which may 
comprise corrugated plates, also play an important part in the heat 
transfer process between the liquid spray and working gas, since the 
corrugated surfaces are expected to be cooled or heated by contact with 
the liquid from the spray, and will extend the contact area between the 
working gas and the liquid. When the gas flow in a particular chamber is 
upward, then most of the droplets injected at that time will be carried 
upwards into the separator. However there will still be many droplets in 
the lower gas space, resulting from the injection at earlier times. When 
the gas flow is downward, most of the liquid that has been separated onto 
the corrugated plates will be swept downwards into the chamber. In this 
way, it is expected that the separators will repeatedly collect then 
discard the liquid carried over into them. The separators may in addition, 
or alternatively be arranged to cause the working gas to swirl to 
facilitate the removal of liquid droplets, while at the same time 
minimising the pressure loss of the gas flow. 
The purpose of the regenerators is to change the temperature of the working 
gas from hot to cold or vice versa in a thermodynamically efficient way. 
The regenerator may comprise an array of narrow channels of various cross 
sectional geometries designed to provide a large heat transfer area 
between the gas and the material of the regenerator. The narrow channels 
may be formed using for example plates or tubes. The regenerator stores 
the heat from the working gas until the working gas reverses its direction 
of flow, after which the heat is restored to the working gas. The 
regenerator should also be designed to minimise the pressure drop over its 
length. 
The choice of working gas and heat transfer liquid in the liquid pistons 
depends on the application and the temperature range over which the engine 
needs to work. Because the engine operates in a closed-cycle and the 
liquid pistons form a perfect seal, the choice of working gas is not 
necessarily restricted by availability or cost and may be chosen for its 
thermodynamic properties. Thus, the working gas may be for example helium 
or hydrogen, which have excellent heat transfer characteristics. Helium 
may be preferred to hydrogen on safety grounds, although it would be more 
expensive. Another advantage of the closed-cycle engine is that the 
operating pressures of the working gas can be relatively high and would 
generally be in the range of 1-20 MPa (10-200 bar). 
At operating temperatures up to about 200.degree. C., water may be used as 
the heat transfer liquid. However, at higher temperatures water would 
probably not be suitable because of the high pressures needed to maintain 
it in the liquid state. For operating temperatures up to about 400.degree. 
C., commercial heat transfer fluids which are also liquid at low 
temperatures may be used. It is likely that helium would again be selected 
as the working gas for this higher temperature range. For operating 
temperatures above 400.degree. C. a liquid metal such as the 
sodium-potassium eutectic mixture (NaK) may be used with helium as the 
working gas. Eutectic NaK remains liquid down to -12.degree. C. and boils 
at 785.degree. C. (at atmospheric pressure). Molten salts are possible 
high temperature alternatives to liquid metals. However, because of the 
likely engineering difficulties in designing an engine suitable for use 
with high temperature liquids at temperatures above 400.degree. C., it may 
be better not to use a hot liquid at all. Instead, heat may be transferred 
into the engine through the walls of a heat exchanger enabling the engine 
to be driven from much higher temperature heat sources including the 
combustion of fuel. This fuel could be heavy oil, coal, biomass or 
domestic waste, since the products of combustion do not enter the engine. 
Thus, embodiments of the heat engine which employ hot liquid injection, 
are very suitable for power generation from relatively low temperature 
heat sources such as industrial waste heat or solar energy. 
The closed-cycle heat engine can be modified to operate as a heat pump in 
which mechanical energy is used to pump heat from a low temperature source 
to a high temperature sink. Thus, in contrast to the heat engine, 
compression is done on the working gas when the gas is hot and expansion 
is done when the working gas is cold. One embodiment of the heat pump may 
be described with reference to FIG. 1 or 2. In these embodiments, 
mechanical energy to drive the heat pump is imparted to the solid pistons 
49 and 51 via drive shafts 53 and 55. In contrast to the heat engine, the 
liquid piston in the compression chamber leads the piston in the 
associated expansion chamber by a predetermined phase angle, e.g. 
90.degree. instead of vice versa. Referring to FIGS. 1 and 2, liquid 
sprays 29 and 31 in chambers 9 and 11 are used to transfer heat to the 
heat pump from a low temperature heat source. Cool liquid is injected into 
chambers 9 and 11 during expansion of the working gas in the chambers 
which is driven by the liquid pistons. During expansion, heat from the 
spray is transferred to the working gas and the expansion process may be 
approximately isothermal. After heat has been extracted from the droplets 
in the liquid spray, the now cooler droplets recombine with the liquid in 
the liquid piston whose temperature will decrease as a result. Cool liquid 
from the liquid piston is passed to a suitable heat exchanger 38 (FIG. 2) 
in which heat is transferred to the liquid from a heat source. The heat 
source for the cold liquid could be atmospheric air, the ground, a river, 
stream or other body of water. Another possibility is to use extracted 
stale air from a ventilation system as the heat source. Alternatively warm 
waste water from baths etc. may be used. This is the converse of the 
operation of the heat exchanger in the heat engine in which the heat 
exchanger transfers heat from the liquid to a low temperature heat sink. 
Liquid sprays 33 and 35 in chambers 17 and 19 spray hot liquid into the 
chambers during compression of the working gas which is driven by the 
liquid piston. The hot liquid spray serves as a heat sink to the working 
gas, absorbing the heat produced by the work of compression. After 
compression, the now hotter liquid droplets in the spray recombine with 
the liquid piston whose temperature is thereby increased. Hot liquid from 
the liquid piston is passed to a suitable heat exchanger (not shown) in 
which heat from the liquid is transferred to the point of use. This is the 
converse of the operation of the heat exchanger in the heat engine in 
which the heat exchanger transfers heat from a hot source to the liquid. 
The heat may, for example be supplied to a hot water system similar to 
those used in many households. Alternatively the heat may be supplied to a 
ducted air system. 
A cycle of the heat pump in relation to one of the cold chambers 9 and the 
associated hot chamber 19 proceeds as follows, beginning with the liquid 
piston in the hot chamber 19 at the top of its stroke and reversing 
direction. 
As the liquid piston reaches the top of its stroke in the hot chamber 19, 
the liquid piston in the cold chamber 9 is reaching the mid-point of its 
stroke and moving out of the cold chamber 9. On continued movement of the 
liquid piston out of chamber 9, the cool gas expands and at the same time, 
cool liquid is injected into the cold chamber via the spray 29. The 
working gas in chamber 9 absorbs heat from the liquid spray and the gas 
expands approximately isothermally. When the liquid piston in cold chamber 
9 reaches the bottom of its stroke and reverses direction, the liquid 
piston in hot chamber 19 reaches the mid-point of its stroke and is moving 
out of the chamber. As the liquid piston in chamber 9 moves into the 
chamber, cool working gas is forced out of the chamber, passes through the 
regenerator in which it is preheated with heat from the working gas which 
left the hot chamber at the end of the previous cycle, and enters the hot 
chamber 19. When the liquid piston in chamber 19 reaches the bottom of its 
stroke and reverses direction, hot liquid is sprayed into chamber 19 via 
spray nozzle 35. At this point the liquid piston in chamber 9 reaches the 
mid-point of its stroke and most of the working gas is in the hot chamber 
19. The liquid piston in chamber 19 moves upwards into the chamber and 
compresses the working gas. The heat of compression is transferred to the 
liquid droplets in the hot spray and the compression process may be 
approximately isothermal. As the liquid piston in chamber 19 reaches the 
mid-point of its stroke, the liquid piston in the cold chamber 9 reaches 
the top of its stroke and reverses direction. On continued movement of the 
liquid piston into chamber 19 the working gas is forced out of the chamber 
and through the regenerator 25 to which it gives up its heat. The cool gas 
leaving the regenerator returns to the cold chamber where the cycle begins 
again. 
When the piston in the cold chamber 9 is moving into the chamber and 
forcing gas out, the gas pressure increases, tending to increase the gas 
temperature. Liquid may be sprayed into the cold chamber as the gas is 
being compressed to prevent the gas from heating too much and preferably 
to maintain the gas temperature constant. If a liquid piston is used, 
liquid for the spray may advantageously be drawn directly from the liquid 
piston. Similarly, when the piston in the hot chamber is moving out of the 
chamber and drawing gas in, the gas pressure drops, tending to lower the 
gas temperature. To prevent this, liquid may be sprayed into the hot 
chamber as the gas expands, so as to maintain the gas temperature 
constant. If a liquid piston is used, liquid for the spray may 
advantageously be drawn directly from the liquid piston. 
As for the heat engine, two saddle loops may be used and these will be 
90.degree. out of phase with each other, as shown in the embodiment of 
FIG. 4. Preferably, the working gas is a gas which does not pass through a 
phase transition (i.e. condense or evaporate) within the range of 
operating temperatures and pressures used in the heat pump. The working 
gas may, for example, be helium or hydrogen as for the heat engine. The 
heat transfer liquid may be water, and depending on the temperature of the 
cold source, anti-freeze may have to be added. If air is used as the heat 
source, then the heat source heat exchanger may have to be regularly 
de-frosted. 
The heat pump may be used for example for domestic or commercial 
applications for air-conditioning, refrigeration, space heating or for 
heating water. The heat pump may be driven by an electric motor 52 as 
shown in FIGS. 2 to 4. The efficiency of a heat pump is usually expressed 
as the co-efficient of performance, or COP, which is the conversion ratio 
of electricity to heat. The COP also depends on the temperatures of the 
heat source and the required heat supply. For heating of water for space 
heating and other domestic purposes, a conventional heat pump might be 
able to achieve a COP of about 3. The heat pump cycle described above, is 
expected to achieve COP's of about 3.5 in a domestic application when the 
heat source is just above freezing temperatures. The achievable COP should 
be about 4 with the heat source temperatures increased by the use of solar 
panels or by heat recovery from domestic waste water. Alternatively a heat 
pump as described above could extract heat from the atmosphere at near 
freezing point to provide ducted warm air for space heating at a COP of 
about 4. The COP could be improved above 4 if some heat was recovered from 
waste water, from stale ventilation air or from solar warming. 
Returning to the heat engine, another embodiment relies on the combustion 
of fuel to add heat to the working gas. A combustible fuel is injected 
into the expansion chamber, mixes with the hot compressed gas and ignites. 
The fuel is preferably a clean fuel such as gas or light distillate oil. 
An embodiment of this version of the heat engine is shown schematically in 
FIG. 2. Many of the features in the embodiment shown in FIG. 2 are similar 
to those of the embodiment shown in FIGS. 1 and 2 and like features are 
represented by like numerals. 
Referring to FIG. 5, the heat engine comprises a pair of U-shaped conduits 
1 and 3 each partially filled with liquid each of which serves as a liquid 
piston. Compression chambers 9 and 11 are formed in the arms 13 and 15 of 
one of the conduits 1 and combustion chambers 17 and 19 are formed in the 
arms 21 and 23 of the other conduit 3. One of the compression chambers 11 
is arranged to communicate with one of the combustion chambers 17 through 
a heat exchanger which is preferably a regenerator 27 and the other 
compression chamber 9 is arranged to communicate with the other combustion 
chamber 19 through another heat exchanger 25 which may also be a 
regenerator. The compression chambers 9 and 11 are provided with gas inlet 
valves, to admit air or other oxidising gas into the chambers and these 
may, for example be non-return valves. Each compression chamber 9 and 11 
has a liquid spray injector 29 and 31, the liquid used in the spray being 
drawn from the liquid piston, as before. Another valve 61, 63 is 
positioned between the compression chamber 9, 11 and the regenerator 25, 
27 to prevent exhaust gases from the combustion chamber 19, 17 via the 
regenerator 25, 27 returning to the compression chamber 9, 11. An exhaust 
port 65, 67 operated by an exhaust valve 69, 71, is provided between valve 
61, 63 and the regenerator 25, 27 to enable exhaust gases to be expelled 
after passing through and giving up their heat to the regenerator 25, 27. 
A fuel inlet port 73, 75 is provided in each combustion chamber 17, 19 to 
enable fuel to be introduced into the chamber. Each exhaust valve 69, 71 
is operated by a suitable timing mechanism (not shown). 
The engine cycle in relation to one of the compression chambers and the 
associated combustion chamber is as follows. When the level of liquid in 
the compression chamber 9 falls to the point at which the internal 
pressure becomes less than the pressure on the other side of the 
non-return valve 57, the inlet valve 57 opens and oxidizing gas is drawn 
in. If the gas source is atmospheric air, then the inlet valve will open 
when the pressure in the compression chamber is less than atmospheric. As 
the piston in the compression chamber reaches and falls beyond the 
mid-point of its stroke, the piston in the combustion chamber 19 reaches 
the bottom of its stroke and reverses direction. The exhaust valve 69 is 
opened and as the combustion piston moves into the combustion chamber, the 
exhaust gases are forced through the regenerator giving up their heat in 
the process. The non-return valve 61 prevents the exhaust gases from 
entering the compression chamber 9. 
When the combustion piston reaches and goes beyond the mid-point of its 
stroke in the combustion chamber, the compression piston reaches the 
bottom of its stroke and reverses direction. When the compression piston 
reaches its lower limit and starts to move upwards, the inlet valve closes 
so that the oxidising gas that was drawn in becomes compressed. The liquid 
spray maintains the gas close to ambient temperature, thus providing an 
approximately isothermal compression. During compression when the 
compression piston is between its lower limit and the mid-point of its 
stroke, the expansion piston continues to move into the expansion chamber 
19 forcing the hot combustion gases through the exhaust port 65 via the 
regenerator 25. When the pressure in the compression chamber exceeds that 
of the combustion chamber, the non-return valve 61 connecting the chambers 
opens and cool compressed gas passes through the regenerator, extracting 
heat so that it enters the combustion chamber at high temperature. The 
combustion piston reverses direction and moves out of the combustion 
chamber while the compression piston approaches the top of its stroke in 
the compression chamber. Shortly before the liquid piston reaches the top 
of its stroke in the compression chamber, and shortly before the 
combustion piston in the combustion chamber reaches the mid-point of its 
stroke, fuel is injected into the combustion chamber 19 and ignites either 
spontaneously or with the help of a pilot flame or spark (not shown). At 
some point during the continued downward movement of the combustion piston 
out of the combustion chamber, the fuel is turned off. The rate of 
injection of fuel may be regulated to provide approximately isothermal 
expansion. The compression piston will have reversed direction drawing a 
fresh supply of gas into the chamber and as the combustion piston 
approaches the bottom of its stroke the exhaust valve 65 opens and the 
cycle is repeated. 
To avoid the need for a fly wheel, two saddle loops may be provided which 
are arranged to operate 90.degree. out of phase from each other, as, for 
example, shown in FIG. 4. A mechanical drive system would be used as for 
the closed-cycle engine. The liquid forming the liquid piston in the 
conduits containing the combustion chambers and the compression chambers 
may be oil, water or possibly another fluid. The liquids in the two 
conduits are not necessarily the same. Floats 22, 24 comprising a solid 
material which float on the surface of the liquid piston in each 
combustion chamber may be provided to limit the contact of the combustion 
gases with the liquid. Some means of cooling the combustion chamber walls 
may also be provided. 
Both the closed-cycle engine and the open-cycle engine described above 
produce a work output involving large reciprocating forces at low 
frequency, for example about 1 Hz. If the engines are to be used in 
electrical power generation, a means would generally have to be provided 
to convert the slow speed form of mechanical energy into a suitable form 
to drive an electric generator. For modest unit sizes with a generating 
capacity up to about 1 MW, a slow speed crank shaft could be used, 
connected to a generator by appropriate gearing. Alternatively, a 
hypo-cyclic gear mechanism or worm drive gearing may be used. In the case 
of hypo-cyclic gears, the drive shaft of the engine is connected to a 
planet wheel having gear teeth around its external circumference. The 
planet wheel rolls around the inside of a fixed wheel having gear teeth on 
its internal circumference. The planet wheel is mounted on an arm which 
rotates as the planet wheel rolls around the inside of the fixed wheel. 
The rotating arm drives a generator via a speed-up gearing. This achieves 
the same kind of motion as crankshaft, but with the advantage that large 
side thrusts otherwise produced by a crankshaft, are avoided. It is also 
possible to make the hypo-cyclic gear more compact than a conventional 
crankshaft. Alternatively, the engine could be adapted to pump a hydraulic 
fluid through a turbine connected to a generator. This technique would be 
suitable for both large and small unit sizes. 
In another embodiment the liquid pistons may be replaced by solid pistons. 
Although it is possible to use solid pistons in the closed-cycle engine in 
which the working gas is passed back and forth between the expansion and 
compression chambers, it may be difficult to achieve adequate sealing of 
the enclosed high pressure gas, which is likely to be helium or hydrogen. 
Sealing is less critical for the open cycle engine in which a fresh supply 
of air or other oxidising gas is used at every cycle and consequently the 
use of solid pistons might be more appropriate for this case. FIG. 6 shows 
one embodiment of this form of heat engine. 
Referring to FIG. 6, an embodiment of the engine is generally indicated at 
100, and comprises four cylinders 113, 115, 121 and 123. A piston is 
provided for each cylinder and each piston is connected to a crankshaft 
169 by a connecting rod 171. In this embodiment, the engine is oriented 
such that the crankshaft is above the cylinders. Compression chambers 109 
and 111 are formed in two of the cylinders 113 and 115 and expansion 
chambers 117 and 119 are formed in the other cylinders 121 and 123. Each 
compression chamber has a gas inlet port 156, 158 controlled by gas inlet 
valves 157, 159 and a compressed gas outlet port 173, 175. A gas feed line 
177, 179 connects a compression chamber 109, 111 with a respective 
expansion chamber 119, 117 via a compressed gas inlet port 181, 183, each 
controlled by a gas inlet valve 185, 187 in the expansion chamber 119, 
117. Each expansion chamber 117, 119 has an exhaust gas outlet port 167, 
165 controlled by an exhaust valve 193, 191. All the gas inlet and outlet 
ports are situated near the bottom of the expansion and compression 
chambers. 
A spray nozzle 129, 131 is provided in each compression chamber 109, 111 
for injecting a liquid spray into each chamber 109, 111 during 
compression. A separator 137, 139 is mounted within each compression 
chamber 109, 111 to remove liquid from the compressed gas before the gas 
leaves the compression chamber. Thus the separator 137, 139 is situated 
above the compressed gas outlet port 173, 175. Various kinds of separator 
may be used, but it is important for the separator to be as compact as 
possible without causing too great a pressure drop in the gas entering the 
chamber or the compressed gas leaving the chamber. To avoid the separator 
causing a pressure drop in the flow of inlet gas, the gas inlet port may 
be situated on the piston side of the separator. To achieve small pressure 
loss, the separator may comprise a number of small swirl vanes mounted in 
short pipe sections with the pipe sections mounted in parallel. The 
induced swirl of gas causes entrained droplets to be thrown outwards and 
collected at the pipe walls. Swirl vane separators are often used for 
example in the steam generators and steam to steam reheaters of 
pressurised water reactors. 
Each separator 137, 139 is connected to an external cooler 197, 199 by a 
duct 201, 203. The flow of liquid from the separator to the cooler is 
controlled by valves 205 and 207, which may be non-return valves. Cooled 
liquid from the cooler is returned to a compression chamber via a duct 
209, 211 and a valve 129, 131 which may be of the non-return type. The 
flow of liquid around this circuit may be driven by the cyclic pressure 
variation in the compression chamber, which forces the liquid through the 
non-return valves in the required direction. It is necessary to maintain a 
gas space above the liquid level within the cooler to allow this process 
to occur. This could be done by the use of a level controller, such as a 
ball valve, mounted in the cooler. A separate supply of liquid may be 
connected to the cooler to replace any liquid which is lost in the gas 
flow to the combustion chamber. The replacement of liquid may also be 
controlled by the level controller, if this is used. 
The separator and cooling circuit described above provides for the 
separation, re-circulation and pumping of cooled liquid as a fine spray 
into the compression chamber without the use of external pumps. A similar 
arrangement may also be implemented in heat engines having liquid pistons. 
For some applications, it may be appropriate not to use a non-return valve 
upstream of the spray injector, but to control the injection using for 
example, a cam which would allow better control of the timing of the 
spray. Preferably, the timing is optimised to take account of the pressure 
difference between the cooler and the compression chamber and the finite 
transit time of the droplets within the chamber. Alternatively, internal 
or external pumps may be used to drive the flow of liquid through the 
spray injectors. In this case the pumps are preferably mechanically 
coupled to the piston shafts so that a separate power source is not 
needed. Spray pumps are more likely to be appropriate for use with engines 
or heat pumps in which there is a liquid piston, because of the slower 
operating speed. In these cases, the transit time of the droplets may be 
rather short in comparison with the time to complete one cycle of the 
engine. 
Each expansion chamber 119, 117 has a regenerative heat exchanger 125, 127 
mounted so that gas passes through the heat exchanger before entering or 
leaving the expansion chamber via the inlet and outlet ports respectively. 
Each expansion chamber has a fuel injection valve 174, 176 controlled by a 
suitable timing mechanism and a spark plug 178 to ignite the fuel/gas 
mixture which may be used for starting the engine or for both starting and 
continuously during running. 
The regenerative heat exchanger may consist of a large number of parallel 
channels of small diameter and short length cast for example in a 
honeycomb structure. The heat exchanger is mounted inside the combustion 
chamber in order to simplify the design and minimise the unswept gas 
volumes, but a separate regenerator light be preferred for some 
applications. 
The chambers are arranged in pairs, each pair comprising one compression 
chamber feeding cool compressed gas to one expansion chamber. The 
operating cycle of the pairs of chambers are separated by 180.degree. In 
this embodiment, this is accomplished by an appropriate design of 
crankshaft 169. In each pair the expansion process in the expansion 
chamber leads the compression process in the compression chamber by a 
predetermined phase angle which in this particular embodiment is 
90.degree. Again, the phase angle is fixed by appropriate design of the 
crankshaft 169. In this way, compression takes place when most of the gas 
is in the compression chamber, and expansion takes place when most of the 
gas is in the expansion chamber. Also, the expansion process occurring in 
the expansion chamber of one pair of chambers directly drives the 
compression process occurring in the compression chamber of the other 
pair. 
The operating cycle of one pair of chambers proceeds as follows, beginning 
with gas induction into the compression chamber. As the compression piston 
reaches the bottom of its stroke in the compression chamber, (i.e. 
farthest point from the crankshaft 169) the gas inlet port 157 opens and 
gas is drawn into the compression chamber as the piston moves out of the 
compression chamber 109. At the same time, the compressed gas inlet valve 
185 in the expansion chamber is closed and fuel is injected into the 
expansion chamber 119 as the expansion piston reaches mid-stroke moving 
out of the expansion chamber. The mixture of fuel and gas in the expansion 
chamber ignites and the combustion gases expand driving the expansion 
piston to the top of its stroke, (i.e. nearest point relative to 
crankshaft 169). 
The expansion piston reverses direction and the exhaust valve 191 opens and 
the exhaust gases pass through the regenerator 125 and are expelled 
through the exhaust port 189. Gas continues to be drawn into the 
compression chamber until the compression piston reaches the top of its 
stroke when the gas inlet valve 157 closes. The compression piston 
reverses direction and moves into the compression chamber at which point 
cool liquid is sprayed into the chamber cooling the gas during 
compression. 
As the compression piston reaches mid-stroke, the expansion piston reaches 
the bottom of its stroke in the expansion chamber and reverses direction. 
At this point the exhaust valve 191 closes and the compressed gas inlet 
valve 185 opens, allowing cool compressed gas from the compression chamber 
to flow into the expansion chamber. The compressed gas passes through the 
regenerator 125 in which it is pre-heated with heat from the exhaust 
gases. 
As the compression piston in the compression chamber reaches the bottom of 
its stroke, the compressed gas inlet valve 185 in the expansion chamber 
119 closes and fuel is injected into the expansion chamber, mixes with the 
pre-heated compressed gas and ignites. The combustion gas expands forcing 
the expansion piston to the top of its stroke and the cycle is repeated. 
Liquid removed from the compressed gas before leaving the compression 
chamber is forced out of the compression chamber through valve 205. The 
liquid is cooled in the cooler 197 before being returned and injected into 
the compression chamber. 
The other pair of chambers progress through a similar cycle but as 
mentioned above the operating cycles of each pair are separated by 
180.degree. Such an engine could run satisfactorily if the motion was 
sustained throughout the cycle with a large fly wheel. However, the engine 
may comprise two sets of four cylinders connected to a single crankshaft, 
with the operation of each set of four cylinders being out of phase by 
90.degree. This would allow positive drive at all stages of the cycle, 
with the result that a fly wheel would not be necessary to achieve 
continuous operation. 
In addition, it may also be possible to design an engine comprising one 
compression chamber and one expansion chamber as long as some means are 
provided to sustain the operation of the engine over the cycle period 
between the expansion or combustion strokes. 
The orientation of an engine with solid pistons may be as shown in FIG. 6, 
with the crankshaft above the cylinders. This has the advantage that the 
separation and removal of liquid droplets from the cylinder is assisted by 
gravity. On the other hand it may not be so easy to provide lubrication to 
the crankshaft and there may be other practical disadvantages to this 
arrangement. An alternative arrangement is to place the crankshaft below 
the cylinders and to arrange the piston to push the spent spray liquid out 
through the valve leading to the expansion cylinder. Means of separating 
the liquid could then be provided in the pipe leading to the expansion 
chamber. An alternative method of separation for the configuration with 
the crankshaft below the cylinders is for the piston to push the liquid 
over an internal weir at the top of the cylinder. The liquid would then be 
drained away by gravity. This would avoid the need for a large connecting 
pipe and external separator. 
The attraction of using solid pistons instead of liquid pistons is that it 
should be possible to run the engine at higher speeds. This implies a 
higher output for a given unit size, such that this engine could be 
suitable for mobile applications, for example in boats and road vehicles, 
in addition to static power generation. The sealing of the pistons will in 
general not be as good as that if liquid pistons were used, but the 
sealing in an open-cycle engine is not as important as it is in a 
closed-cycle engine. It is also possible to devise an engine comprising 
both liquid and solid pistons, for example with liquid pistons in the 
compression chambers and solid pistons in the combustion chambers. 
FIGS. 7 to 9 show other embodiments of a heat engine which are similar to 
that shown in FIG. 6 but which have been modified in a number of ways for 
improved performance including better efficiency and a much higher output 
in terms of work rate. 
Embodiment of the heat engine shown in FIGS. 7 to 9 comprise a pair of 
compression cylinders 113, 115 each having associated spray liquid cooling 
and re-circulating apparatus, and a pair of expansion or combustion 
cylinders 121, 123 and the description of these components described above 
in relation to the embodiment shown in FIG. 6 applies to corresponding 
components shown in FIGS. 7 to 9 and like components are designated by 
like reference numerals. The modifications to the heat engine which 
contribute to the improved performance of the embodiment shown in FIGS. 7 
to 9, will now be described. 
The moisture separators 137 and 139 have been removed from the interior of 
the compression chambers 109 and 111 and instead placed externally of the 
compression chambers and are connected in the compressed air feed lines 
177, 179 between the compressed gas outlet port 173, 175 of the 
compression chambers and the hot compressed gas inlet ports 165, 167 of 
the expansion chambers 119 and 117. Placing the moisture separators 
outside the compression chambers removes the dead volume within the 
chambers which would otherwise be present throughout compression and 
contribute to a lower compression ratio. Compressed gas outlet valves 204 
and 206 have been added to seal the compression chambers 109 and 111 from 
the volume enclosed by the external pipework leading from the compressed 
gas outlet ports 173, 175 of the compression chambers to the inlet ports 
of the expansion chambers, and to control the final pressure of the 
compressed gas in each compression chamber before the gas is passed to a 
respective expansion chamber and also to control the timing of the flow of 
compressed gas to the expansion chambers. Both the addition of the outlet 
valves 204 and 206 and the removal of the moisture separators from the 
inside of the compression chambers enables much higher compression ratios 
to be achieved. 
The regenerative heat exchangers 125 and 127 which are housed within the 
expansion chambers in the embodiment shown in FIG. 6, have been replaced 
by recuperative heat exchangers 244 and 246 mounted externally of the 
expansion chambers in the embodiment shown in FIGS. 7 to 9. Again, this 
greatly reduces the dead volume within the expansion chambers so that the 
energy of expansion of the hot compressed gas admitted into the expansion 
chambers is not wasted by firstly expanding into the dead volume of 
exhaust gas, from the previous cycle, trapped within the regenerative heat 
exchangers, and thereby reducing the temperature of the gas. Thus, much 
higher temperatures can be achieved in the expansion chamber. 
The recuperative heat exchangers 244 and 246 are each connected in a 
respective compressed gas feedline 177, 179 between a respective moisture 
separator 137, 139 and the hot compressed gas inlet port 181, 183 of a 
respective expansion chamber and are arranged to pre-heat the cool 
compressed gas from the compression chambers with exhaust gas leaving the 
expansion chambers through the exhaust ports 165, 167. The increased 
compression ratio obtainable from the engine shown in FIGS. 7 to 9 means 
that the ratio of the absolute temperatures before and after expansion is 
increased also. The temperature after the expansion is likely to be 
similar for both the engines shown in FIG. 3 and FIGS. 7 to 9 since this 
is determined by the materials of the heat exchanger. Hence the peak 
temperature of the engine shown in FIGS. 7 to 9 will be higher and the 
average temperature of heat addition during the expansion will be higher 
also. The above mentioned improvements enable both higher pressure 
differences and high temperatures to be achieved within the cycle, with 
heat being rejected at the lowest temperature within the cycle and heat 
being added at the highest temperature, which leads to an increase in 
power output. 
Further modifications have been made the embodiment shown in FIGS. 7 and 8 
to recover waste or excess heat in various parts of the cycle and to 
convert this heat into useful power, to increase the efficiency of the 
engine. In particular, each of the combustion cylinders 123, 121 is 
surrounded by a cooling jacket 212, 214 for recovering heat conducted 
through the combustion chamber walls. A bypass line 208, 210 is connected 
into the compressed gas feedline 177, 179 between the moisture separator 
137, 139 and the recuperative heat exchanger 244, 246 to supply cool 
compressed air from the compression chamber 109, 111 to the cooling jacket 
212, 214. The bypass line 208, 210 is connected near the bottom on the 
cooling jacket 212, 214 where the temperature of the combustion chamber 
walls is least. A pair of expansion cylinders 220, 222 are provided with 
associated pistons 224, 226 which are also connected to the crank shaft 
169 via connecting rods 171. Each expansion chamber has a gas inlet port 
216, 218 controlled by an inlet valve 232, 234 and a gas outlet port 236, 
238 controlled by an outlet valve 240, 242. The inlet port 216, 218 is 
connected to a point near the top of the cooling jacket 212, 214, the 
uppermost part of which surrounds the exhaust port and extends to the hot 
side of the recuperative heat exchanger 244, 246, where the temperatures 
are expected to be greatest. 
Thus, heat lost to the walls at the top of the combustion chamber is 
recovered and converted into useful work by directing part of the cool 
compressed gas from the compression chambers to the combustion chamber 
walls. Compressed air is much more effective as a cooling medium than air 
at atmospheric pressure. The cool compressed air enters the cooling jacket 
near the bottom in order to first cool the combustion chamber walls since 
the combustion chamber walls have to be kept below a temperature which is 
determined by the lubricating oil. The compressed gas is pushed upwards in 
the cooling jacket towards the top of the combustion chamber, absorbing 
heat and gradually rising in temperature. Having gained some heat in this 
cooling process, the compressed air is then used to cool the hotter parts 
of the system, such as the cylinder head and valves. Finally, the hot 
compressed air is intermittently extracted from the cooling system by 
opening the inlet valve into the expansion chamber in which it expands, 
driving the associated piston out of the chamber, thereby generating 
additional mechanical work. 
Because, in practice, the heat capacity of the exhaust gas leaving the 
combustion chambers will generally be larger than the compressed gas from 
the compression chambers, there will be more heat available in the exhaust 
gas than is required to pre-heat the cool compressed gas in the 
recuperative heat exchangers. This excess heat may also be recovered by 
compressing more gas than is required for combustion, directing this gas 
through the recuperative heat exchangers in which it is pre-heated with 
the excess heat available in the exhaust gas and then directing this 
pre-heated compressed gas to one or more of the expansion chambers. 
The advantage of this modification is a reduction in the final temperature 
of the exhuast gases, and an increase in the fuel efficiency of the 
engine. 
One or more expansion chambers to recover waste or excess heat from various 
parts of the engine may also be used in any of the other embodiments 
described herein. 
The embodiment of the heat engine shown in FIG. 7 is essentially 
symmetrical about the vertical centre line A with the right hand half of 
the engine being a mirror image of the left hand half. In this particular 
embodiment, the three pistons to the left of the centre line A are 
180.degree. out of phase from the three pistons to the right of the centre 
line, since this is expected to give the most uniform torque on the crank 
shaft 169. Also, the combustion chamber pistons in each half of the engine 
are arranged, via the crank shaft, to lead the corresponding compression 
chamber pistons by about 90.degree. This will provide a high torque to the 
crank shaft at the time when it is needed to achieve a high pressure in 
the compression chamber. This arrangement also has the possible advantage 
that compressed air is drawn into the combustion chamber from the feed 
line and heat exchanger before this gas is replenished by the opening of 
the outlet valve from the compression chamber. 
A complete operating cycle of the heat engine shown in FIG. 7 will now be 
described with reference to the three cylinders to the left of the centre 
line only, as the operation of the right hand side of the engine is 
essentially the same but is 180.degree. out of phase. In this example, air 
is used as the oxidising gas for combustion, although this need not 
necessarily be the case. 
When the piston 112 in the compression chamber 109 reaches the top of its 
stroke and begins to reverse direction, the compressed gas outlet valve 
204 closes and the inlet valve 157 opens and atmospheric air is drawn into 
the compression chamber through the air inlet port 156. At the same time 
as the compression piston 112 reaches the top of its stroke, the piston 
122 in the combustion chamber and the piston 224 in the expansion chamber 
are at the midpoint of their strokes and moving downward. The combustion 
chamber, at this point, contains pressurized hot combustion gases which 
are expanding and driving the piston out of the chamber. Likewise, the 
expansion chamber 228 contains hot pressurized air which is also expanding 
and driving the expansion piston 224 out of the chamber. The outlet valves 
in both the combustion chamber and expansion chamber are closed, and the 
inlet valves may also be closed. 
As the compression piston 112 reaches the mid point of its stroke, the 
combustion and expansion pistons reach the bottom of their strokes and 
reverse direction. At this point, the exhaust outlet valve 191 in the 
combustion chamber and the gas outlet valve 240 in the expansion chamber 
both open. As the pistons move into their respective chambers, exhaust gas 
is expelled from the combustion chamber through the outlet port 165 and 
passes through the heat exchanger 244 and out into the atmosphere. 
Likewise, the expanded gas is pushed out of the expansion chamber through 
the gas outlet port 236. 
Reduction of nitrogen oxides in the exhaust gases can be achieved, if 
desired, by injecting amonia upstream of or directly into the heat 
exchanger, and/or by incorporating a catalytic surface within the heat 
exchanger itself. 
When the combustion and expansion chamber pistons 122, 224 reach the 
midpoint of their upward stroke, the compression piston 112 reaches the 
bottom of its stroke and reverses direction. At this point, the air inlet 
valve 157 closes and a spray of cool liquid is injected into the 
compression chamber 109 through the spray injection valve 129 so that the 
air in the compression chamber is compressed approximately isothermally. 
When the combustion and expansion pistons reach the top of their stroke, 
their respective outlet valves 191, 240 both close and their respective 
air inlet valves 185, 232 open, admitting pre-heated compressed air into 
the chambers via a respective air inlet port 181, 216. At a predetermined 
point, the inlet valve supplying pre-heated compressed air into the 
combustion chamber is closed and fuel is injected into the chamber via the 
fuel injection valve 174. An ignition source 178, such as a spark plug, 
may be used to ignite the fuel, or the ignition may be spontaneous as the 
fuel mixes with the pre-heated compressed air. The piston 122 is driven 
out of the combustion chamber 119 by the pressure of the hot combustion 
gases, which cool to some degree as a result of doing work against the 
piston. 
The gas inlet valve 232 in the expansion chamber 228 is also closed at some 
predetermined point and the air expands adiabatically, driving the piston 
224 downward and out of the chamber. 
As the piston 112 in the compression chamber 109 approaches the top of its 
stroke, the compressed gas outlet valve 204 opens and the mixture of air 
and spray liquid is expelled from the chamber into the moisture separator 
137, in which the air and liquid are separated. The moisture separator 137 
is sized not only to achieve separation of the air/liquid mixture, but 
also to act as a reservoir for the liquid and a pressure accumulator for 
the compressed air. 
Liquid flows from the moisture separator 137 to the cooler 197 where the 
heat absorbed during the process of compression is liberated to the 
atmosphere or to some other heat sink. The liquid from the cooler 197 then 
flows back to the liquid spray injection valve 129 which controls the 
injection of liquid during compression. Since the injection of the spray 
will normally occur while the pressure in the compression chamber is below 
its maximum, it should be possible to achieve sufficient injection during 
this time. By the time the pressure has risen to the injection pressure 
and cut off the injection flow, sufficient liquid droplets will already be 
present in the compression chamber. Hence the compress ion chamber piston 
112 can effectively provide the means to pump the liquid around the 
cooling circuit and through the spray injection nozzles. 
Cool compressed air flows from the moisture separator 137 to the 
recuperative heat exchanger 224 in which it is pre-heated by the exhaust 
gases from the combustion chamber 119. 
When the piston 112 in the compression chamber 109 has reached the top of 
its stroke, the compressed gas outlet valve 204 closes, the air inlet 
valve 157 opens and the cycle is repeated. 
Phasing of the pistons in the various chambers is not too critical, 
particularly if the engine has a large fly-wheel to maintain its motion. 
However, it will generally be desirable to even out the torque on the 
crank shaft to minimise the operating stresses, to maintain an even motion 
and to minimise vibration. The phasing of the pistons will also affect the 
"breathing" i.e., the flow of air from the compression chamber to the 
combustion chamber and the pressure variations in the moisture separator 
and the heat exchanger. Although the phase angle between the combustion 
chamber pistons and the compression chamber pistons is about 90.degree. in 
the embodiment shown in FIG. 4, the phase angle may be different in other 
embodiments, but the choice of phase angle is a matter for careful 
optimization in the light of practical experience and measurements. 
Although the embodiments shown in FIGS. 7 and 8 has two moisture separators 
and two heat exchangers, the heat engine may be arranged with fewer 
separators and/or heat exchangers so that a single separator and/or heat 
exchanger is shared between two or more cylinders. This may have the 
advantage of reducing the size of these components, improve the uniformity 
of air flow and possibly reduce the costs. An embodiment of a heat engine 
having a heat exchanger 248 shared between two expansion cylinders is 
shown in FIG. 10. 
A further embodiment of any of the open cycle engines described above, 
incorporates a turbo-charger in the cycle, such as is often used for 
petrol and diesel engines and as shown, for example, in FIG. 9. The 
turbo-charger 249 may consist of a rotary compressor 251 and a rotary 
expander 253 on the same shaft 255. The compressor boosts the pressure of 
the atmospheric air before it is admitted into the isothermal compression 
chamber. The compressor is preferably driven by the expander, which is 
arranged between the exhaust outlet of the combustion chamber and the 
exhaust inlet to the heat exchanger. The overall effect of the 
turbo-charger is to raise the average pressure of the gases in both the 
compression and combustion chambers, so that an engine of given size has 
more power. The use of a turbo-charger will tend to reduce the efficiency 
of the engine slightly, because of the lower efficiencies of the rotary 
compressor and expander and because the turbo-compressor compresses 
adiabatically rather than isothermally. However, the incorporation of a 
turbo-charger may well be attractive because the reduced efficiency may be 
more than offset by a large increase in the power output from the same 
size of engine. 
Although the embodiment shown in FIGS. 7 to 10 shows the crank shaft 
driving a generator 247, the engine could alternatively be used to drive 
road or rail wheels or a ship's propellor. 
In an alternative embodiment, the pistons may be coupled together and 
driven by a rotating mechanical system other than a crank shaft, for 
example a hypo-cyclic gear. 
In a further embodiment, it may be advantageous to arrange the engine such 
that the compression process in the compression chambers take place at a 
slower speed than combustion in the combustion chambers. In other words, 
the engine may be arranged such that there are more combustion cycles as 
shown in the embodiments of FIGS. 7 to 10 per unit time than compression 
cycles. This may be achieved by providing appropriate gearing 170 between 
the crank shaft 168 of the compression chamber and that of the combustion 
chamber 172. If the engine also has an air expansion chamber to recover 
waste or excess heat in various parts of the cycle, it is also possible to 
arrange the engine such that the air expansion cycle is faster than the 
isothermal compression cycle, as shown in FIG. 8. The advantages of such 
an arrangement would be that the compression process may always be 
maintained at a moderate rate to allow sufficient time for the transfer of 
heat between the gas and the liquid droplets so that the compression 
process may always be substantially isothermal, that the heat loss per 
cycle from the combustion chamber is reduced to provide higher efficiency 
and that the power output from the engine can be higher. 
In an alternative embodiment, the present invention may be adapted to 
provide cooling for a conventional petrol, diesel or gas engine for the 
purpose of recovering heat and converting this heat into useful work. In 
its basic form, such an embodiment includes a compression chamber and an 
associated piston for compressing gas isothermally by liquid spray 
injection during compression, an expansion chamber and an associated 
piston connected either to an output drive of the engine or to some other 
drive which could benefit from additional power and a heat exchanger to 
pre-heat the cool compressed gas from the isothermal compression chamber 
with heat from the engine (which would otherwise be wasted) and means to 
feed the pre-heated compressed gas into the expansion chamber. The heat 
exchanger may simply consist of passages formed in the combustion chamber 
walls of the engine to allow the compressed air to circulate before being 
admitted into the expansion chamber. The isothermal compression and 
expansion chambers may be similar to those illustrated in FIG. 7, the main 
difference of this embodiment from that shown in FIG. 7 being that all the 
isothermally compressed air is used for heat recovery, not just a part of 
it. A specific embodiment of a heat engine which may be used for heat 
recovery in a conventional combustion engine is shown in FIG. 10. Cold 
compressed gas from the isothermal compression chambers 113, 115 is passed 
through heat exchanger 248 in which the compressed gas is heated by heat 
from the combustion engine, which may be heat from the exhaust gas and/or 
heat conducted through the surface of the combustion engine. The heated 
compressed gas is then expanded in the expansion chambers 228, 230 
providing work to the pistons which may be arranged to drive a generator 
247 or be coupled to the output drive of the combustion engine. 
Any of the engines described above can readily be adapted for use with 
combined heat and power systems, if required. The use of a non-condensing 
gas as the working gas allows much greater flexibility over the choice of 
operating temperatures than does a condensing vapour cycle. The system is 
simply adjusted to reject heat at a higher temperature than would be used 
for power generation only. 
Another option which could be used to produce the maximum amount of low 
temperature heat for drying, space heating or heating water, is to arrange 
a heat engine to drive a heat pump. The rejected heat from the engine may 
provide some of the low temperature heat. In addition, the mechanical 
output of the engine could drive a heat pump which would provide more 
heat. Calculations have indicated that it should be possible with an 
open-cycle combustion-driven engine to produce twice as much low 
temperature heat as is consumed in terms of the calorific value of the 
fuel. The additional heat may be pumped from the atmosphere, from the 
ground or from a large body of water. 
The heat pump with both hot and cold liquid spray injection would be very 
suitable for domestic or commercial space and water heating. However, 
there would also be scope for the design of a heat pump operating at a 
much higher temperature. An advantage of this particular type of heat pump 
is that it is not tied so tightly to a particular range of temperatures as 
is the case for heat pumps which rely on the evaporation of a liquid and 
the condensation of the vapour. 
Other embodiments of the heat pump may include valves so as to operate in 
open cycle similar to the systems shown in FIGS. 5, 6 or 7. However in 
this case, there would be no combustion in the expansion chamber and there 
may not be any form of recuperative or regenerative heat exchanger or 
droplet injection in the cool expansion chamber. For example, the air in 
the expansion chamber could be expanded adiabatically. The air in the 
compression chamber would be compressed isothermally by use of a piston 
and a droplet spray and the excess heat would be transferred to a 
convenient heat sink. This form of heat pump might be used as an air 
conditioning and ventilation unit with the expanded air leaving the system 
significantly cooler than the incoming air. The system would not be very 
suitable for the pumping of heat into a building from a cold atmosphere, 
because of the problem of icing inside the expansion chamber. 
Further embodiments of the heat pump would be similar to those described 
herein but without the liquid pistons. All the compression and expansion 
would be performed using solid pistons only. For example, it is possible 
to have liquid seals without necessarily having liquid pistons. 
It will be appreciated by those skilled in the art that there are many 
alternative mechanical arrangements for converting the linear motion of a 
piston to rotation of a drive shaft. Where a liquid piston is used and 
part of the mechanical drive comprises a drive or transmission shaft 
extending through a wall in the conduit, as shown in FIGS. 1 and 2, a seal 
must be provided between the wall and the reciprocating drive shaft. 
However, one possible disadvantage of this arrangement is that there may 
be considerable friction between the seal and drive shaft. An alternative 
arrangement which would perhaps reduce the friction involves a 
rack-and-pinion mounted within the horizontal section of the conduit. The 
pinion would be rotatably mounted with its axis transverse to direction 
motion of the piston, and the rack would be appropriately coupled or 
connected to the solid piston or pistons. The pinion may be arranged to 
drive a rotatable shaft which extends through the wall of the conduit via 
a seal, to transmit power from the piston externally. The solid piston 
which is coupled to the motion of the liquid piston would be arranged to 
move back and forth in one or other of the arms of the conduit and more 
than one such solid piston may be used within one conduit. 
Alternatively, linear motion of the piston may be converted to rotational 
motion of the drive shaft by mounting some form of fluid screw such as a 
propellor or turbine blade inside the conduit which is rotatably mounted 
on a drive shaft extending through the conduit. In this case the drive 
shaft is parallel to the direction of motion of the piston. Where 
reciprocating drive shafts are used in two saddle loops, it may be 
convenient to couple the drive shaft of one compression loop to the drive 
shaft of the other expansion loop. A hydraulic drive system may be used 
instead of a mechanical system. Thus, in the above case, each combined 
drive shaft of the saddle loop would drive an external reciprocating 
piston in an external hydraulic cylinder to pump hydraulic fluid. The 
predetermined phase angle (for example 90.degree.) between the two 
combined drive shafts could be achieved by timing the opening of valves in 
the hydraulic cylinders so as to prevent either shaft departing too far 
from its desired position at a particular stage of the cycle. 
In the engines or heat pumps in which liquid pistons are used, solid floats 
may be arranged to float on the surface of the liquid pistons. 
Modifications to the embodiments described will be apparent to those 
skilled in the art.