[0001] The present invention relates to a reciprocating external combustion engine, i.e.,
an engine of the type having a cylinder or cylinders whose reciprocating motion provides
a source of power and wherein the heat powering the cylinder is generated externally
of the cylinder. In particular, the invention provides a novel operating cycle.
[0002] Many attempts have been made to produce an engine which combines high thermal efficiency
in terms of converting applied heat energy into useful work, with acceptable power
to weight and power to volume ratios for the engine. The internal combustion engine
has a good power to weight ratio but a relatively low thermal efficiency. The diesel
engine has the best thermal efficiency (up to around 40 per cent). Thermodynamically
more efficient engines based on the Carnot, Stirling and Ericsson cycles have been
built but these have not in general been commercial successes, largely on account
of the problem of providing a small and efficient heat exchanger enabling the working
gas to become quickly and efficiently heated by the external heat source.
[0003] The steam engine is a well known form of external combustion engine but its power
to weight ratio is generally low, owing to its requiring a separate steam boiler and
condenser. The steam engine generally uses dried steam or other dry vapor as the working
fluid. However, the present invention is not concerned with such an engine but is
concerned with an external combustion engine which uses a gas such as air as the working
fluid.
[0004] Thus, the present invention provides a reciprocating external combustion engine wherein
energy is transferred to a working gas from a heated liquid heat-transfer medium,
which comprises
a cylinder having a piston reciprocatable therein and defining a working end space;
a heat exchanger for heating the heat transfer medium externally of the cylinder under
a pressure such as to maintain the medium in the liquid state;
induction means for inducting gas into the working end space;
an injector arranged to inject heated liquid medium into the gas before or after the
gas is inducted into the end space;
the cylinder having an outlet controlled to exhaust heat transfer medium from the
working end space near the end of an expansion stroke of the piston.
[0005] The external combustion engine of the present invention includes a cylinder which
may comprise a single double-acting cylinder having a piston therein defining on one
side of the piston (usually the rod- end side) a compressor end space and on the other
side of the piston the working end space, as in a two-stroke engine. However, this
would not preclude the use-of mechanical equivalents to this arrangement, .for example
the use of two cylinders coupled to a common shaft, one of the cylinders providing
by its piston the compressor end space and the other cylinder providing with its separate
piston the working end space. The engine may also be arranged to work according to
a four-stroke cycle wherein one stroke is an induction stroke. The compression ratio
employed may vary widely depending on the particular application of the engine. Thus,
in some applications a compression ratio as low as 1.5:1 or perhaps lower may be employed
and in other applications, the ratio may be as high as 20:1.
[0006] The engine might alternatively comprise a pair of opposed pistons reciprocatable
within a common cylinder, such that the two piston crowns and the cylinder wall define
the working end space.
[0007] Means are provided for inducting gas into each working space. In its simplest form
the inducting means may be a ram together with an inlet port into the working space
for scavenging the exhaust gas and replacing it with a. fresh gas charge, usually
around bottom dead center. Alternatively, a compressor may be provided to feed compressed
gas to the working space. In a two-stroke engine the compressor may be provided by
the compressor end space of the cylinder. However, a separate rotary or reciprocating
compressor might be provided, such as a vane or turbine compressor. In a four-stroke
engine the induction stroke serves to induct the gas.
[0008] Various inlet and outlet valves of conventional construction are provided as necessary,
and may be in the form of check valves or may be driven by means of a cam operated
by the engine. However, this would not preclude the absence of valves, for example,
the piston may be arranged to open and close inlet and outlet ports as in a two-stroke
engine.
[0009] An injector is also provided for injecting preheated liquid heat-transfer medium
into the gas. The purpose of the injected liquid medium is to enable heat transfer
from the heat exchanger to the gas to be achieved efficiently. Thus, very much smaller
heat exchanger surfaces are required in order to heat a given weight of liquid in
comparison to the surface area required to heat the same mass of gas. Consequently,
the present invention envisages heating the medium in the liquid state and allowing
the gas to become heated by contact with the hot liquid medium.
[0010] The heat-transfer medium may be one which vaporizes or does not vaporize under the
engine working conditions, after injection into the working gas. A non-vaporizing
liquid will generally be
[0011] introduced into the working space in the form of droplets having a high surface area.
A vaporizing liquid may evaporate at least partially to form a vapor thereby enabling
extremely good heat transfer to be achieved between the hot vapor derived from the
heat-transfer medium and the working gas. The liquid heat-transfer medium may be injected
into the gas before or after the gas is inducted into the working space. If the heat-transfer
medium is not vaporizable it is preferably sprayed into the gas in the form of droplets.
When a vaporizable medium is used, it may vaporize completely after injection or vaporize
incompletely. Although the liquid medium may be injected into unpressurized working
gas, it is well known that greater thermal efficiency is achieved by injecting the
liquid medium into the compressed gas.
[0012] To avoid confusion the following terms as used herein will be clarified. The working
gas into which liquid medium has been injected will be referred to generally as wet
gas. Gas into which liquid medium has not been injected will be referred to as dry
gas. The injected heat-transfer medium may be present in the gas in its liquid or
vapor state.
[0013] The heating of the liquid heat-transfer medium and its injection into the working
gas may be achieved in a variety of different ways. Generally, the heat exchanger
comprises a fuel-burner for heating the liquid medium.
[0014] Firstly, the liquid may be heated in a compact heat exchanger, for example a coil
of narrow bore tubing, to a high pressure and high temperature (i.e., to a high internal
energy). Since such narrow bore tubing can withstand great pressures, it is usually
possible to heat the liquid up to its critical point. For special applications where
the rate of heat transfer is to be high, it may be preferred to heat the medium to
a temperature and pressure above its critical point. The hot pressurized liquid medium
is then injected into the gas in a mixing chamber. A non-vaporizing liquid medium
is preferably injected by means of an atomizing injector. Internal energy of the medium
is rapidly transferred from the hot liquid droplets to the gas, thereby increasing
its pressure. The heated and pressurized wet gas is then fed into the working end
space of the cylinder where it expands (usually polytropically, i.e., non-adiabatically)
to drive the piston.
[0015] However, in a second most advantageous arrangement, the mixing chamber is dispensed
with and hot high pressure liquid medium which has been heated in the heat exchanger
is injected directly into the working end space of the cylinder. Thus, a charge of
scavenging gas is compressed to a pressure sufficient to enable it to be fed quickly
into the working end space when the piston is near bottom dead center. Then, the dry
scavenging gas is compressed adiabatically and so becomes heated as the piston travels
towards top dead center. At close to top dead center the heated pressurized liquid
medium is injected into the working end space, thereby causing the compressed gas
pressure to increase still further and its expansion to drive the piston downwards
again towards bottom dead center.
[0016] Heat transfer between the hot heat-transfer medium and the working gas is very rapid.
As the piston approaches bottom dead center, the gas expands (usually polytropically)
and becomes cooled causing the liquid or vapor to give up internal energy.
[0017] Preferably, the liquid is a vaporizable liquid, such as water, which at least partially
flashes to vapor immediately it is injected into the working space. Thus, heat transfer
between the hot water vapor and the working gas is very rapid.
[0018] Therefore, it may be seen that in this second arrangement the injected liquid medium
is merely acting as a heat transfer fluid which may enable the compressed gas to convert
internal energy to mechanical work. If a vaporizable medium is employed, the heat
transfer process is particularly effective provided that most of the vapor leaves
the cylinder in the liquid state, so that latent heat of vaporization is not lost.
[0019] The present invention is to be distinguished from a steam engine in that the medium
is maintained in its liquid form and not allowed to vaporize until it is-introduced
into the gas. This is in sharp contrast to a steam engine wherein, even if a flash
boiler is used, the water is always introduced into the cylinder in the form of steam.
In fact, since it is necessary to superheat the steam to remove water droplets in
a conventional steam engine, it is not possible to directly flash liquid water into
the cylinder of a steam engine since this would give rise to water droplets in the
cylinder. However, in the engine according to the present invention, the presence
of water droplets in the working space may be tolerated. Indeed, in some cases it
may be desirable to construct the piston and/or cylinder so as to retain liquid medium
in the working space after exhaust. Thus, the piston or cylinder may be provided with
suitable recesses.
[0020] It is necessary that the heated medium be maintained in the liquid state prior to
injection. Although this may be achieved by using appropriate sensors to ensure that
the temperature at a given pressure never exceeds the liquid boiling point, it has
been found that if an orifice of suitable size is connected to the heat exchanger
in which the liquid medium is heated and a flow of liquid medium is maintained through
the heat exchanger, then the application of heat to the medium does not cause the
liquid to boil. Thus, by correct choice of orifice size, complex temperature and pressure
sensing devices may be avoided. So long as.the orifice provides a pressure drop, the
pressure in the heat exchanger will at all times be such that, as the temperature
is increased, the pressure of the water in the heat exchanger will also increase and
thereby be always below the boiling point. The orifice normally forms part of the
injection means through which the liquid medium is injected into the gas.
[0021] The rate of working of the engine may be controlled by any of several means. For
example, it may be controlled by varying the amount of heat-transfer medium injected
into the cylinder. The rate of working of the engine may be controlled by controlling
the amount of heat supplied to the heating coil by the burner, for example, by controlling
the fuel supply to the burner (for a constant liquid volume injection rate). The rate
of working of the engine may also be controlled by controlling the rate of injection
of liquid medium, e.g. by using a variable displacement pump.
[0022] Usually, the heat-transfer medium is recovered from the exhaust gas after the gas
has been exhausted from the working space. The recovered medium which will still be
somewhat heated, may be recycled again to the heat exchanger so that its internal
energy is not lost. In this way, the medium acts merely has a heat transfer fluid
and is not substantially used up.
[0023] Water is a preferred heat transfer fluid, not only because it is vaporizable, but
also because it , has a thermal conductivity which is high compared to the other liquids,
for example heat transfer oils. Moreover, as will be explained later, means may be
provided for recovering water produced by combustion in the burner. Thus, it may be
possible to avoid any need for make-up water since this will be provided by water
from combustion in the burner. Of course, it is possible to use other liquids, such
as mercury, which has a thermal conductivity 10 times that of water, and sodium. However,
mercury has other obvious disadvantages, such as cost and toxicity. When water is
used an oil may be added to form a dispersion, emulsion or solution to assist lubrication
of the engine.
[0024] In a particularly preferred embodiment of the present invention, the working gas
is a gas which is capable of taking part in the combustion process which occurs in
the burner. In this way, the internal energy of the gas exhausted from the cylinder
is able to be recovered. The gas may be a gas capable of supporting combustion, such
as oxygen, air or other oxygen-containing gas, or nitrous oxide. Altneratively, the
gas may itself be a combustible gas chosen from all known combustible gases, such
as gaseous hydrocarbons, carbon monoxide, or hydrogen. Thus, some or all of the exhaust
gas may be fed to the burner.
[0025] The fuel burnt in the burner itself may be chosen from known combustible fuels such
as gasolines, fuel oils, liquefied or gaseous hydrocarbons, alcohols, wood, coal or
coke.
[0026] It is in general preferred to use various heat recovery means. Thus, the whole engine
may be enclosed in a heat-insulating enclosure and be provided with heat exchangers
to pick up stray heat and transfer it to, for example, the compressed gas or to preheat
the fuel for the burner. It is also preferred to recover the heat remaining in the
burner flue gases, and this may be achieved by passing the flue gases through a spray
chamber in which a stream of liquid (generally the same liquid as that injected into
the engine) is sprayed through the flue gas. When injection of a vaporizable liquid
medium is employed, it is preferred that the vaporizable liquid be sprayed through
the flue gases to heat the liquid medium close to its boiling point prior to being
passed to the heat exchanger. Moreover, when water is employed as the injected heat-transfer
medium, the use of a water spray chamber or a condenser is advantageous in that water
from the burner may be condensed out of the flue gases so that it is not necessary
to provide make-up water to the engine.
[0027] The construction of an engine according to the present invention is considerably
simplified in certain respects in comparison with known engines, such as internal
combustion engines. Thus, the temperatures encountered in the working space are generally
reduced, thereby simplifying sealing around the piston. It will be appreciated that
power may be provided in the engine of the present invention at lower temperatures
than, for example an internal combustion engine. Moreover, the internal combustion
engine is less thermally efficient in that means must be provided to cool the cylinders
and prevent seizing up.
[0028] Moreover, since the temperatures encountered in the engine are relatively low, for
example up to 350°C., it is not usually necessary to construct the cylinder of metal.
Plastics such as polytetrafluorethylene (PTFE), fiber-reinforced resins, and other
plastics used in engineering, are particularly advantageous due to their cheapness
and ease of use. Other heat insulating materials such as wood, concrete, glass or
ceramics may also be used.
[0029] In a preferred embodiment, the hot liquid medium is injected into one end of the
working end space and the inlet and outlet are at the other end of the piston stroke.
The use of low conductivity plastics materials, allows the one end of the cylinder
to be hot while the inlet and outlet region is relatively cool.
[0030] Power is taken from the engine by means of a piston rod attached to the reciprocating
piston means. The free end of the piston rod may be connected to an eccentric shaft
on a rotary fly wheel directly or by using a crankshaft so as to convert the reciprocating
motion into a rotary motion.
[0031] Although the invention has been described in relation to an engine having a single
cylinder, it will be appreciated that multicylinder engines of two or more cylinders
will generally be preferred in practice. In a two-stroke engine there will in general
be advantages in connecting the compressor end space of one cylinder so that it feeds
the working end space of a different cylinder, thus enabling the compressed gas to
be delivered at the most appropriate moment in the working cycle of a given cylinder.
[0032] The invention also relates to a method of operating a reciprocating external combustion
engine, and to a kit of parts for converting an engine (e.g. an internal combustion
engine such as a diesel engine) to an engine according to the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Embodiments of the invention will now be described with reference to the accompanying
drawings wherein:
Figure 1 is a schematic view of a first embodiment of external combustion engine according
to the present invention;
Figure 2 is a simplified view of the first embodiment illustrating its principle of
operation;
Figure 3 is a schematic cross-sectional view of the cylinder of the engine;
Figure 4 is a schematic cross-sectional view of a heat exchanger of the engine;
Figure 5 is a schematic cross-sectional view of, a spray device for cooling flue gas
from the burner;
Figure 6 shows a schematic diagram of a four cylinder arrangement according to the
present invention;
Figure 7 shows pressure (P) versus volume (V), and temperature (T) versus entropy
(S) diagrams for the first embodiment;
Figure 8 shows for comparison the PV and TS diagrams for the known two-stroke internal
combustion engine;
Figure 9 is a schematic view of a second embodiment of the present invention;
Figure 10 is a schematic view of a third practical embodiment of the present invention;
and
Figure 11 is a schematic cross-sectional view of the third embodiment.
. DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] In carrying out the invention, in one form thereof, as shown in Figure 1, the external
combustion engine comprises a cylinder 5 having piston 6 defining a compressor end
space C and a working end space P, a heating coil H of a heat exchanger for heating
liquid water under pressure by means of a burner B, an optional preheater PH for preheating
fuel F for the burner by means of burner flue heat, a spray device S for cooling flue
gas from the burner, pump X for feeding water under pressure to the heating coil,
a trap T for recovering liquid water from the wet exhaust gas from the working space,
and a gas dryer D for recovering liquid water from the combustion gas supplied to
the burner.
[0035] The external combustion engine works in the following manner. Air A at atmospheric
temperature and pressure is inducted into compressor end space
C of the cylinder 5 by moving piston 6 to the right (as viewed in:Figure 1) and thereby
opening inlet check valve 4. The outlet from the compressor end space C is closed
by means of check valve 2. When the piston 6 has reached the extreme right of this
travel (top dead center - TDC), inlet valve 4 closes. Continued movement of the reciprocating
piston back towards the left causes the air to become compressed.
[0036] Compression is continued to provide a sufficient pressure of air in space C in order
to efficiently scavenge exhaust gas from working space P when the compressed air is
admitted to the working space P somewhat before BDC via inlet valve 2. Thus, as the
piston approaches BDC outlet, valve 3 opens to release wet exhaust air, and shortly
afterwards check valve 2 is opened to admit compressed and slightly heated air to
scavenge and fill the working space P with dry air at substantially atmospheric pressure.
[0037] Shortly after BDC valves 2 and 3 are closed and as the piston moves toward TDC again,
the dry air is compressed adiabatically and isoentropically.
[0038] Around top dead center, hot pressurized water is injected through a valve and associated
injector 51 causing a rapid increase in pressure within the cylinder (along line bc
in Figure 7). The piston then moves back towards bottom dead center, the gas becoming
depressurized and cooled in the process. The expansion of the gas in the cylinder
is represented by the line cd in Figure 7. Around bottom dead center the gas is expelled
from the cylinder by incoming air and passes to the trap
T having a baffle 10. In the trap T, the liquid water is recovered and recycled to
the heating coil H ..wherein it is pressurized and heated. The exhaust air and water
vapor is led from the trap via dryer D to the burner where its internal energy is
recovered. Any condensate from the dryer is returned via line 7. Any water condensed
in the preheater is returned to the pump X via line 9.
[0039] Depending on the compression ratio and the rate of working at the time, the temperature
of the injected water may be above or equal to the temperature of compressed air in
the working space.
[0040] Figure 2 emphasizes the fact that the water itself acts principally as a heat transfer
fluid which is recycled after use. The only water lost from the system is that carried
out in the cooled flue gases from the spray chamber S. The cycle will now be described
in more detail.
[0041] In a specific embodiment of this invention, heated water at atmospheric pressure
and a temperature of below 100°C. is fed from the trap T (and possibly from the spray
chamber and preheater) to the pressure pump X whence it is delivered at a high pressure
to the heating coil H. The water in the heating coil H is heated to a temperature
of around 300°C. and a pressure of around 86 bar. The water is usually heated to temperature
below its critical temperature and pressure (220.9 bar and 374°C.), however, because
of the orifice provided, the pressure will always be such that at any temperature
it will maintain the water in its liquid state.
[0042] Ambient air is inducted into the compressor end space C via inlet valve 4 and delivered
into the working end space P during the period 45° before to 45° after BDC. This sweeps
the spent air from the working end space P and replaces it by cool air. As the piston
moves towards TDC again, the air is compressed to around 12 bar and (for a 6:1 compression
ratio) to a temperature of around 330°C. at top dead center. Typically, the compression
ratio of the cylinder is from 2:1 to 10:1, but, as pointed out earlier, it may be
somewhat lower or higher than this range.
[0043] At TDC, hot pressurized water at around 86 bar and 300°C. is injected into the working
end space P via injector 51 and some water immediately flashes to become vapor, thereby
atomizing the remaining injected liquid water and rapidly increasing the pressure
in the space P. Water injection is continued for around 5% to 25% of the whole stroke.
The pressure reached depends on the amount and temperature of the liquid water injected
and on how much of that vaporizes.
[0044] The rapid rise in pressure causes the piston 6 to move towards BDC again. Around
45° before BDC the exhaust valve 3 and inlet valve 2 are opened again to discharge
wet exhaust gas from the space P. The temperature of the wet exhaust gas is controlled
to be low so as to ensure that most of the water vapor in the space P is condensed
again to the liquid phase and its latent heat of vaporization recovered. The exhaust
air and water droplets are scavenged from the cylinder by the incoming flow of charge
air and passed to the trap T where the liquid water is separated from the spent air,
before the spent air is fed to the burner. The hot recovered liquid water is then
returned to the heat exchanger.
[0045] While the present invention has been described using a piston compressor in either
the same or a different cylinder from the working end space, it will be appreciated
that any other type of compressor may be used, such for example a rotary or separate
reciprocating compressor.
[0046] This embodiment allows a particularly simple cylinder construction, such as the one
shown in Figure 3. The relatively low temperatures encountered allow the use of engineering
plastics materials in the construction of the cylinder, and indeed such materials
have important low heat conductivity advantages.
[0047] The cylinder shown in Figure 3 comprises a uniflow cylinder body 52 having a row
of circumferentially arranged ports 53 which constitute the inlet and outlet to the
working end space P of the cylinder. A cylinder head 54 having the water injector
51 mounted therein is attached to one end of the body 52 and an'end plate 55 having
therein an inlet 56 and outlet 57 (and respective check valves) is provided at the
other end of the cylinder. A piston 58 and piston rod 59 are provided within the cylinder.
The ports 53 are arranged to be uncovered by the piston 58 as the piston approaches
the end'of its expansion stroke.
[0048] It will be noted that the piston has a contoured upper surface so that the charge
air to the working end space P is caused to follow the dotted path through the working
end space P and thereby efficiently scavenge the water laden spent air from the space
P.
[0049] It will be appreciated that the end of the cylinder adjacent the injector 51 is at
a relatively high temperature, whereas the end of the cylinder adjacent the inlet
and outlet ports 52 is at a relatively low temperature. The use of plastics materials
having a low thermal conductivity allows this advantageous temperature differential
to be maintained. Thus, were heat to be allowed to be conducted towards the outlet
ports 53, the temperature of the spent gas would be raised, thereby resulting in loss
of thermal efficiency.
[0050] The cylinder schematically represented in Figure 3 includes a circumferential recess
59a in the cylinder wall for retaining liquid medium in the working space after exhaust.
[0051] In addition, as shown in Figure 3, at least two seals 59b are mounted in circumferential
recesses in the cylinder wall..The piston of this invention need not fit closely against
the cylinder wall, since communication between the working end space and the compressor
end space can be blocked by the seals 59b, as illustrated by the dotted line view
of piston 58, in Figure 3 which shows the piston at the end of its compression stroke.
Having the piston slightly spaced from the cylinder wall provides an advantage in
that any scale deposited on the cylinder wall from the water will not interfere with
the operation of the engine until a substantial amount has accumulated, and maintenance
is thereby reduced.
[0052] The external combustion engine according to the invention features good power to
weight ratio comparable to internal combustion engines. Although the power to cylinder
volume ratio may not be as good, the overall engine power to volume ratio is comparable
to that of internal combustion engines. Since it is possible to arrange the combustion
conditions in the burner to an optimum, it is possible to achieve almost complete
combustion of the fuel to carbon dioxide and water and thus avoid carbon monoxide
or unburnt fuel impurities in the exhausted flue gases. In particular, since the combustion
occurs substantially at atmospheric pressure, there is practically no generation of
nitrogen oxides during the combustion process. Therefore, this engine represents an
improvement over internal combustion engines not only in terms of thermal efficiency
but also as regard pollutant emissions.
[0053] Moreover, the engine is capable of utilizing a wide variety of fuels, for example
gasoline, fuel oil, gaseous or liquefied hydrocarbons (including methane, butane and
propane), alcohols, and even solid fuels such as wood, coal or coke. The burner parameters
may be adjusted to ensure substantially complete and pollution-free combustion. Furthermore,
such an engine could be made to run more quietly than conventional internal combustion
engines.
[0054] Figure 4 shows the construction of the heat exchanger which combines the heating
coil H and the burner B. The heat exchanger comprises inner and outer coaxial sleeves
60 and 61, respectively, defining a double path for flue gas from the burner. Insulation
64 is provided around the outside of the heat exchanger. A fuel inlet jet is provided
for burning fuel F in air A admitted via an air inlet. Water W passes through a heating
coil H which comprises an inner coil 62 and outer coil 63 in the direction indicated
by the arrows such that water exits from inner coil 62 at a position close to the
highest temperature of the burner. The hot pressurized water is then fed along pipe
50 prior to injection into the working space P.
[0055] .When a multicylinder engine is used, individual cam-operated injector valves may
be provided on each cylinder. Alternatively, a distributor may be provided to periodically
distribute hot pressurized water to the appropriate cylinder. The injectors may deliver
a constant volume of water at a variable temperature. However, injectors delivering
a variable volume of water at constant temperature might also be used - particularly
when a more rapid change in working rate is required.
[0056] Figure 5 shows a spray device for cooling and washing the flue gases from the burner
B and thus recovering some of the heat and some water produced by combustion. It comprises
a spray chamber 17 having therein a funnel 18 onto which water is sprayed by spray
42 through the stream of hot flue gases. The flue gases are inducted via inlet 19
and arranged to flow tangentially around the chamber before exiting through the exit
20 as cooled flue gas. The flue gases thus pass through the spray and then through
a curtain of water falling from the inside aperture of the funnel 18. Preferably the
flue gases are cooled to below 100°C. so as to recover the latent heat of vaporization
of water from the burner. Water at substantially 100° exits through the outlet 21
before being fed by pump X into the heat exchanger. Cold feed water W is introduced
into the chamber via a ballcock 40 for maintaining a constant level of water in the
bottom of the spray chamber. A recycle pump R and associated ducting 22 is provided
for recycling the water through the spray to bring it up to its boiling point. However,
in practice if it is desired to cool the, flue gases below 100°C., it may be necessary
to withdraw water through the outlet 21 at a substantially lower temperature, e.g.,
50°C.
[0057] Figure 6 shows one arrangement of four cylinder external combustion engine according
to the present invention. The arrangement shown consists of a flat- four arrangement
of cylinders 40, 41, 42 and 43. Each cylinder comprises a piston and associated piston
rod attached to a comon crank shaft 44. It will be noted that each pair of adjacent
cylinders is arranged to be 180° out of.phase. The arrangement is generally similar
to that shown for a single cylinder in Figure 1, so that certain details are omitted.
Each cylinder has its own heat exchanger - burner assemble HX. However, each opposed
pair of cylinders 40, 41, and 42, 43 share a common exhaust air feed manifold to the
burners so as to damp out fluctuations in the air pressure in the burner.
[0058] Figure 7 shows the idealized thermodynamic operation of the engine of Figure 1. Figure
8 shows for comparison the operation of a conventional two-stroke engine.
[0059] Figure 7 (i) is the PV diagram for the case when hardly any of the injected water
flashes to vapor, the majority remaining in the liquid phase as droplets. This will
occur when the rate of vaporization is slow compared to the stroke time of the piston.
[0060] Figure 7 (ii) is the theoretical PV and TS diagrams for the case when all the injected
water vaporizes to the gaseous state. This might occur in a slow working engine.
[0061] In Figure 7 (i) air in the working space P is compressed during the compression stroke
adiabatically (i.e., the gas constant is approximately l.39) along line ab. The compression
is also isoentropic and heats the air. At constant volume liquid water is injected
and a small amount of water vapor produced at the same temperature as the compressed
air so that the pressure increases along bc. Considering only the air in the working
space, there is no change in T provided the injected water is at the same temperature.
As the piston descends the wet air expands along cd, however, due to the presense
of hot liquid water droplets the expansion is not adiabatic but polytropic (typically
the gas constant is between 1.33 and 1.35) so that the curve cd on the PV diagram
is flattened. The expansion also produces a fall in T and increase in S. The gas is
then exhausted from the working space so that the pressure of gas in the working space
falls along da.
[0062] This replacement of hot pressurized exhaust air by cooler charge air constitutes
a fall in both T and S.
[0063] Figure 7 (ii) shows the situation wherein all the water flashes to the vapor state.
In this case, the rise in pressure along bc is much greater, but the rate of pressure
drop along cd is also quicker since the absence of liquid water droplets ensures that
the air expands almost adiabatically. Thus the work done (i.e., the area of the figure
abed) in both cases (i) and (ii) is the same.
[0064] Without wishing to be limited by any theoretical discussion, the PV and TS diagrams
show the theoretical equilibrium situation when all the injected water is vaporized,
i.e., in a slow working engine when less than the amount of water required to saturate
the air is injected. For the sake of illustration, the injected water is at a slightly
lower temperature than the compressed air in the cylinder.
[0065] As before, air is compressed adiabatically (gas constant is about 1.39) along ab
at constant entropy. Typically, the pressure P
a at a is 1 bar and the temperature T
a is 300K (27°C.). At a compression ratio of 6:1 the air pressure P
b and temperature T
b at b rise to around 12 bar and 603K (330°C.).
[0066] Liquid water at 573K (300°C.) and 86 bar is then injected into the compressed air
and all becomes vapor. This causes an increase in pressure along be (typically P =
25 bar) and a decrease in temperature due to injection of the slightly colder water
(T
c = 586K (313°C.)). If the water is at the same temperature as the compressed air the
line bc on the TS diagram is horizontal. The reduction in entropy along bc of the
air in the cylinder arises from the added partial pressure of the water vapor.
[0067] As the piston moves back towards BDC, the wet gas expands (gas constant is about
1.34) along cd to a pressure P
d of about 2 bar and a theoretical temperature T
d of about 319K (46°C.). In practice, due to non-theoretical behaviour the temperature
will be higher, e.g., 80 - 100°C.
[0068] The gas is then scavenged from the working space along da as before causing a decrease
in temperature, pressure, and entropy of gas in the working space.
[0069] In the TX diagram P to P
d indicate the constant pressure curves. The net area of the two closed figures in
the TS diagram represents the heat added to the air. In the case shown this is negative
since injection of the water cools the air. When the water is at the same temperature
as the compressed air at b the areas of the two closed figures on the TS diagram cancel
out, i.e., no heat is added.
[0070] Figure 8 shows PV and TS diagrams for a known two-stroke cycle internal combustion
engine for comparison. It is analogous to the cycle of case (ii) above. The line ae
represents the opening of the exhaust valve before the end of the stroke in a conventional
two-stroke engine.
[0071] Figure 9 shows a second embodiment of the present invention which is similar to the
embodiment shown in Figure 1 except that the water passes to a mixing chamber M where
the water is injected into the compressed air so as to increase its pressure and temperature.
The hot compressed air and water vapor are then passed into the working end space
P of the cylinder, as before.
[0072] The trap T is provided in order to recover liquid water droplets from the exhaust
gas from working end space P. The trap T is of a construction known in steam engine
technology for removing liquid water from a gas. Alternatively, the trap may be a
cyclone dryer. Water from the trap is returned to the spray chamber S.
[0073] Thus, the operation of the engine is as follows. Preheated water from the spray chamber
S is fed by means of a high pressure pump X (for example a positive displacement piston
pump) to a heating coil H formed of narrow bore tubing. The water is then heated by
means of the burner B to a high temperature and pressure, for example 300°C.` and
86 bar. The hot pressurized water then passes through pipe 50 to an injection valve
51 in mixing chamber M. The mixing chamber M contains compressed and somewhat heated
air which has been delivered from the compressor end space C through the outlet valve
2. When the outlet valve 2 and the inlet valve 1 are closed, hot pressurized water
is injected via the injector 51 into the chamber M, thereby raising the temperature
and pressure of the air therein. When the piston 6 has reached top dead center, the
hot.pressurized water vapor - containing air from the mixing chamber M - is admitted
through inlet valve 1 into the piston end space P; the outlet valve 3 being closed.
The admitted hot pressurized air expands in the cylinder driving the piston 6 towards
bottom dead center, the air becoming cooled in the process. As the piston approaches
bottom dead center, valve 3 is opened to allow spent air which is still heated and
somewhat pressurized to be vented to the burner B.
[0074] Figures 10 and 11 illustrate a practical form of the invention, which is similar
in principle to the embodiment shown schematically in Figure 1 except that no spray
chamber is used.
[0075] The engine comprises four cylinders arranged in a 90° V-configuration. Water is pumped
from a storage tank 100 by a high pressure pump 101 along a pipe 102 to a two-stage
counter flow heat exchanger 103 of a construction as shown in Figure 4. A pressure
relief valve 104 is provided between pipe 102 and trap 100. Exhaust air is directed
to the heat exchanger 103 along duct 105 from the trap 100. The air flow is controlled
by valve 107. Fuel (e.g., propane gas) is introduced from a canister 106 via a preheater
126 into the air flow through fuel valve 108. Flue gases leave the heat exchanger
via flue 109.
[0076] Each piston 110 runs in a respective double-acting cylinder 111 and is connected
to a crosshead 112 by a piston rod 113. The crosshead is connected to crankshaft 114
by a further rod 115. Each cylinder has a cylinder head l16 provided with an injector
117 which includes a poppet valve operated by a cam on a camshaft 118 by means of
a rocker arm 119. The rod end -space of the cylinder acts as a compressor, air being
inducted via inlet valve 129, and is connected to the inlet 127 of the piston-end
space by a pipe 128. Each cylinder also has an exhaust port 120 into common exhaust
manifold 121 which returns air and liquid exhaust water to the trap 100. A flywheel
124 is mounted on the crankshaft. The exhaust port shown is controlled by the piston
110 shown in Figure 11, as in the form of invention shown in Figure 3, but in either
case a valve may be employed for controlling flow through the exhaust or outlet port.
[0077] An engine having a 6:1 compression ratio, a 9" diameter piston and a 4" stroke and
each cylinder delivers 10 horsepower at a water injection temperature of around 300°C.
and a pressure of 86 bar. The inclination of the cylinders assists exhaust of liquid
water by gravity. At 300°C. typically about 5 grams of water would be injected per
injection. The entire engine is contained within a heat-insulated enclosure.
[0078] Hot liquid water leaves the heat exchanger along pipe 122 and is fed to the injector
117. A pressure control valve 123 is provided between pipe 122 and the tank.
[0079] It has previously been pointed out that recesses may be provided in the cylinder
or piston to retain liquid medium in the working space after exhaust. In Figure 3
there has been shown a recess 59a in the cylinder for this purpose. The engine shown
in Figure 11 has recesses 130 provided in the piston head for this purpose.
[0080] The external combustion engine of this invention shown is capable of very high thermal
efficiency. Theoretically, cold air A and cold water W (if any) are inducted into
the engine, and cold flue gas is vented. Therefore, almost all the heat given out
by the'burner may become converted into work. In practice, thermal efficiencies of
the order of 50 to 60% appear to be attainable.
[0081] While it is contemplated that this invention will be carried out by manufacturing
new engines incorporating the features disclosed in this invention, it may also be
carried out by converting some existing internal combustion engines to operate in
accordance with the principles of this invention. For this purpose a kit may be supplied
incorporating the necessary components for making such a conversion. Such a kit would
include a heat exchanger, including a fuel-air burner, for heating water to the necessary
temperature and pressure; an insulated cylinder and piston, the cylinder having an
inlet for gas and an outlet for wet exhaust gas; a compressor for inducting gas into
the cylinder; a pump for transmitting water from the cylinder to the heat exchanger,
an injector for injecting liquid water directly or indirectly under pressure from
the heat exchanger into the cylinder, a metering device for controlling the amount
of water injected into the cylinder, and a chamber for separating condensed water
from dry saturated vapor. The kit could also include, optionally, a mixing chamber
for mixing compressed gas with the liquid heat-transfer medium.
1. A reciprocating external combustion engine wherein energy is transferred to a working
gas from a heated liquid heat-transfer medium, which comprises
a cylinder, a piston within the cylinder and reciprocatable therein, a working end
space being defined by the cylinder and piston;
a heat exchanger for heating the heat-transfer medium externally of the cylinder under
a pressure such as to maintain the medium in the liquid state, the heat exchanger
having an inlet for receiving heat-transfer medium and an outlet for delivering heated
liquid heat-. transfer medium;
induction means connected to-the cylinder for inducting gas into the working end space;
an injector connected to the outlet of the heat exchanger and arranged to inject heated
liquid medium into the gas before expansion of the gas in the working end space; and
an outlet from the cylinder for exhausting heat transfer medium and working gas from
the working end space near the end of an expansion stroke of the piston.
2. An engine according to claim 1 wherein the injector is mounted on the cylinder
so as to inject heated liquid medium directly into the working gas in the working
end space.
3. An engine according to claim 1 which further comprises a mixing chamber having
an inlet for working gas, the injector being mounted in the chamber for injecting
heated liquid medium into the gas, prior to the wet gas being inducted into the working
end space.
4.. An engine according to claim 1, wherein the gas is compressed before the heated
liquid medium is injected into the gas.
5. An engine according to claim 4, wherein the gas is compressed by means of a rotary
compressor.
6. An engine according to claim 4, wherein the cylinder is a double-acting cylinder
defining on one side of the piston the working end space and defining on the other
side of the piston a compressor end space, the compressor end space having an inlet
for working gas and an outlet.
7. An engine according to claim 4 arranged to operate according to a four-stroke cycle,
which includes an induction stroke and a compression stroke for the working gas.
8. An engine according to claim 1 wherein the injector is an atomizing injector, which
atomizes the injected liquid medium so as to facilitate heat transfer to the gas.
9. An engine according to claim 1, wherein the heat exchanger comprises at least one
tube for containing the heat-transfer medium and a fuel burner for heating the medium
in said at least one tube under a pressure such as to maintain the medium in the liquid
phase.
10. An engine according to claim 1, which further comprises a trap connected to the
outlet from the cylinder for recovering. liquid heat-transfer medium from the wet
exhaust gas.
11. An engine according to claim 1-wherein recycle means are provided for recycling
the exhausted heat-transfer medium to the heat exchanger.
12. An engine according to claim 11 wherein the recycle means comprises a spray chamber
having an inlet for heat transfer medium and an inlet for flue gases connected to
the heat exchanger, the chamber having a snray for spraying liquid heat-transfer medium
through the flue gas from the burner so as to preheat the liquid medium, the chamber
further having an outlet connected for feeding heat-transfer medium to the heat exchanger,
and an outlet for flue gas.
13. An engine according to claim 1 which further comprises speed control means for
controlling the rate of working of the engine by controlling the amount of liquid
heat-transfer medium injected.
14. An engine according to claim 13 wherein the speed control is a variable displacement
pump.
15, An engine according to claim 1 which further comprises speed control means for
controlling the rate of working of the engine by controlling the temperature of the
injected heat-transfer medium.
16, A reciprocating external combustion engine wherein heat energy is transferred
to air acting as a working gas by means of pressurized liquid water at a temperature
greater than the boiling point of water at atmospheric pressure, which comprises
a cylinder, a piston.within the cylinder and reciprocatable therein, a working end
space being defined by the cylinder and piston;
a heat exchanger for heating the liquid water externally of the working space to a
temperature above the boiling point of water at atmospheric pressure, the heat exchanger
having
1) an inlet for receiving liquid water and an outlet for delivering heated water,
2) at least one tube for containing said liquid water, and
3) a fuel-burner disposed for heating the liquid water in said at least one tube;
pressurizing means connected to said at least one tube of the heat exchanger for maintaining
said heated water in the liquid state;
induction means connected to the cylinder for inducting air into the working end space
near the beginning of a compression stroke of the piston;
an injector mounted on said cylinder and connected to the outlet of the heat exchanger
for receiving heated pressurized liquid water;
control means for controlling the injector to inject said heated pressurized liquid
water into the working end space near the end of the compression stroke of the piston;
and .
an outlet from the cylinder for exhausting cooled water and air from the working end
space near the end"of an expansion stroke of the piston, the majority of said cooled
water being exhausted in the liquid state.
17. A reciprocating external combustion engine wherein energy is transferred to a
working gas from a heated liquid heat-transfer medium, which comprises
a cylinder, a piston within the cylinder and reciprocatable therein, and a working
end space on at least one end of the piston;
means for heating the heat-transfer medium externally of the cylinder under a pressure
such as to maintain the medium in the liquid state;
induction means connected to the cylinder for inducting gas into the working end space;
an injector arranged to inject heated liquid medium into the gas before expansion
of the gas in the working end space; and
an outlet from the cylinder for exhausting heat transfer medium and working gas from
the working end space.
18. A method of operating a reciprocating external combustion engine having a cylinder,
a piston therein and a working end space on at least one end of the piston, wherein
energy is transferred to a working gas from a heated liquid heat transfer medium,
which comprises
1) inducting working gas into the end space;
2γ generating externally of the cylinder heated heat-transfer medium under a pressure
such as to maintain the medium in the liquid state;
3) before or after induction, injecting heated liquid medium into the working gas
so as to raise the internal energy of the gas;
4) in an expansion stroke of the piston, allowing the wet gas containing the heat-transfer
medium to expand thereby driving the piston, and
5) exhausting wet gas from the end space near the end of the expansion stroke.
19. A method according to claim 18 wherein the heat transfer medium is selected from
the group consisting of water, oil, sodium, mercury and mixtures thereof.
20. A method according to claim 18 or 19 carried out under such conditions of temperature
and pressure that at least part of the injected liquid medium vaporizes on injection.
21. A method according to any of claims 18 to 20 wherein the heated liquid medium
has a temperature and a pressure below its critical point but greater than its boiling
point at atmospheric pressure.
22, A kit of parts for converting an internal combustion engine to a reciprocating
external combustion engine according to claim 1, which comprises
means for heating liquid under pressure;
an insulated cylinder and piston, the cylinder having an inlet for gas and an outlet
for wet exhaust gas;
a compressor for inducting gas into the cylinder;
a pressure pump for feeding liquid to the heat exchanger;
an injector for injecting liquid directly or indirectly under pressure into the cylinder;
a metering device for controlling the amount of liquid injected; and
a separating chamber for receiving wet exhaust gas and separating dry saturated vapor
and gas from liquid.
23. A kit of parts for converting an internal combustion engine to a reciprocating
external combustion engine according to claim 1, which comprises
. a heat exchanger including a fuel-air burner for heating water under pressure;
an insulated cylinder and piston, the cylinder having an inlet for gas and an outlet
for wet exhaust gas;
a compressor for inducting gas into the cylinder;
a pressure pump for feeding water to the heat exchanger;
an injector for injecting liquid water directly or indirectly under pressure into
the cylinder;
a metering device for controlling the amount of water injected; and
a separating chamber for receiving wet exhaust gas and separating dry saturated vapor
and gas from liquid.