U.S. patent number 3,708,979 [Application Number 05/133,105] was granted by the patent office on 1973-01-09 for circuital flow hot gas engines.
This patent grant is currently assigned to Massachusetts Institute of Technology. Invention is credited to Vannevar Bush, Joseph L. Smith, Jr..
United States Patent |
3,708,979 |
Bush , et al. |
January 9, 1973 |
CIRCUITAL FLOW HOT GAS ENGINES
Abstract
A compressor, heater, expander and cooler are connected to form
a closed circuit. Valves control the pulsating flow of a working
gas about this circuit. An interchanger may be included in the
circuit to allow use of a low expansion ratio with a high ratio of
extreme temperatures. Relatively large volumes of heater and cooler
may be used. Power output is controlled by altering valve timing.
Alternately, power output may be controlled by the use of auxiliary
chambers and altering the mass of working gas in the circuit.
Inventors: |
Bush; Vannevar (Belmont,
MA), Smith, Jr.; Joseph L. (Concord, MA) |
Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
|
Family
ID: |
22457034 |
Appl.
No.: |
05/133,105 |
Filed: |
April 12, 1971 |
Current U.S.
Class: |
60/522; 60/682;
60/516 |
Current CPC
Class: |
F02G
1/043 (20130101); F02G 2270/50 (20130101); F02G
2258/10 (20130101); F02G 2244/50 (20130101); F02G
2244/00 (20130101) |
Current International
Class: |
F02G
1/00 (20060101); F02G 1/043 (20060101); F01b
001/00 () |
Field of
Search: |
;60/59R,59T,57R,39.6,36 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Geoghegan; Edgar W.
Claims
What is claimed is:
1. A hot gas engine comprising a compressor, a heater, an expander,
and a cooler in closed circuit connection, valves in the circuit to
control flow of working gas in the circuit, and variable control
means connected to at least one of the valves for varying operation
of the valves and thereby varying output of the engine.
2. The hot gas engine of claim 1 wherein the expander is a
cyclically variable volume chamber, and wherein the control means
is connected to the chamber for selectively controlling timing of
the valves in varied relation to cyclical variations of the chamber
volume.
3. The hot gas engine of claim 1 wherein the variable control means
is connected to an expander exhaust valve between the expander and
the cooler.
4. The hot gas engine of claim 3 wherein the variable control means
is connected to an expander intake valve between the heater and the
expander.
5. The hot gas engine of claim 1 wherein the variable control means
is connected to an expander intake valve between the heater and the
expander.
6. The hot gas engine of claim 1 wherein the variable control means
is connected to a compressor intake valve between the cooler and
the compressor.
7. The hot gas engine of claim 6 wherein the variable control means
is connected to an expander exhaust valve between the expander and
the cooler.
8. The hot gas engine of claim 6 wherein the variable control means
is connected to an expander intake valve between the heater and the
expander.
9. The hot gas engine of claim 8 wherein the variable control means
is connected to expander intake and exhaust valves.
10. The hot gas engine of claim 1 further comprising a shaft
connected to the expander and wherein at least one of the valves
comprises a pilot valve, and wherein the variable control means
comprises a variable pilot valve operator connected to the
shaft.
11. The hot gas engine of claim 1 further comprising an
interchanger in the circuit between the expander and cooler and the
compressor and heater for removing heat from gas flowing between
the expander and the cooler and transferring heat to gas flowing
between the compressor and the heater.
12. The hot gas engine of claim 11 further comprising a valve
controlled interchanger bypass in the circuit between the expander
and the cooler.
13. In hot gas energy conversion apparatus having sequentially
connected heater means, expander means, and compressor means for
flowing gas between the means and repeatedly heating, expanding and
compressing gas and thereby converting energy, the improvement
comprising variable valve means connected between at least two
sequentially connected means for varying gas flow between the means
and for varying energy conversion.
14. The apparatus of claim 13 wherein the variable valve means
comprises variable expander intake valve means connected between
the heater means and the expander means for selectively varying
amount of gas which is expanded in the expander means.
15. The apparatus of claim 14 wherein the variable valve means
further comprises variable expander exhaust valve means connected
between the expander means and the compressor means for varying the
period of gas flow to the cooler from the expander means.
16. The apparatus of claim 15 further comprising check valve means
connected between the compressor means and the heater means for
permitting free flow of gas toward the heater means.
17. The apparatus of claim 15 wherein the expander means comprises
cylinder and piston means, and further comprising shaft means
connected to the piston means for receiving energy therefrom, and
first and second variable valve operating means connected to the
shaft means and respectively connected to the expander intake and
exhaust valve means for varying timing of the valve means in
relation to movement of the piston means.
18. The apparatus of claim 13 further comprising cooler means
sequentially connected between the expander means and the
compressor means for cooling gas flowing between the expander means
and the compressor means.
19. An engine valve operator comprising a piston connected to an
engine valve, a cylinder forming a chamber with the piston, a gas
line connected to the chamber, a pilot valve connected to the gas
line, an operator connected to the pilot valve, and connected to an
engine shaft, a pressure reservoir connected to the pilot valve for
supplying pressurized gas through the pilot valve to the chamber
for driving the piston and changing state of an engine valve in a
timing sequence in relation to the engine shaft as controlled by
the operator.
20. The apparatus of claim 19 wherein the operator has a
selectively variable size for selectively varying dwell of the
engine valve.
21. The method of operating a hot gas engine comprising
compressing, heating, expanding and cooling working gas and
variably controlling flow of the working gas in a closed
circuit.
22. The method of claim 21 wherein the controlling comprises
varying flow of working gas to an expander and thereby varying the
expanding step.
23. The method of claim 22 wherein the controlling step comprises
varying flow of working gas from an expander and thereby varying
the expanding step.
24. The method of claim 21 wherein the controlling step comprises
varying flow of working gas from an expander and thereby varying
the expanding step.
25. The method of claim 21 wherein the controlling step comprises
varying inward flow to a compressor, thereby varying the
compressing step.
26. The method of claim 25 wherein the controlling step comprises
varying flow of working gas to an expander and from an expander,
thereby varying the expanding step.
27. The method of claim 21 wherein the controlling step comprises
opening an expander intake valve and admitting gas during a portion
of expansion of a cyclically variable expansion chamber, closing
the intake valve at varied points in expansion of the chamber and
expanding the gas during completion of the expansion of the
chamber, and operating the expander intake valve in timed
relationship with the cyclic variation of the chamber.
28. The method of claim 21 wherein the expanding step comprises
cyclically varying an expander volume, and wherein the controlling
step comprises opening an expander exhaust valve at a
volumetrically cyclic point between before reaching maximum
expander volume and before reaching minimum expander volume and
holding the expander exhaust valve open until about minimum
expander volume.
29. The method of claim 21 wherein the compressing step comprises
cyclically varying a compressor volume, and wherein the controlling
step comprises opening an intake valve in a compressor at about
minimum compressor volume, holding the compressor intake valve
open, and selectively closing the compressor intake valve between a
cyclical point before reaching maximum compressor volume and a
cyclical point after passing maximum compressor volume.
30. The method of claim 29 wherein the expanding step comprises
cyclically varying an expander volume, and wherein the controlling
step further comprises opening an expander intake valve at about
minimum expander volume, holding the expander intake valve open,
selectively closing the expander intake valve at a variable cyclic
point between minimum and maximum expander volume, selectively
opening an expander exhaust valve at a variable cyclic point
between the closing of the expander intake valve and before
reaching minimum expander volume holding the exhaust valve open and
closing the exhaust valve at about minimum expander volume.
31. The method of claim 21 wherein the controlling step comprises
changing open periods of the expander and compressor intake valves
thereby changing power.
32. The method of claim 31 wherein the controlling step comprises
maintaining a constant ratio of expander and compressor active
volumes.
33. The method of claim 21 wherein the controlling step comprises
changing open periods of the expander and compressor intake valves,
permitting free exhaust from the compressor and permitting free
exhaust from the expander during limited cyclic periods.
34. The method of hot gas energy conversion comprising flowing
working gas through a compressor, heater and expander and cooler,
removing working gas between the cooler and compressor to reduce
power, storing working gas at high pressure, and adding working gas
between the compressor and heater to increase power.
Description
BACKGROUND OF THE INVENTION
This invention presents a new form of hot gas engine. Hot gas
engines are a form of external combustion engines, that is engines
in which heat, to be partially converted into work, is applied
externally, not as in engines in which combustion occurs inside of
a cylinder.
The usual hot gas engine comprises a gas filled volume and a means
by which the bulk of the gas is shifted cyclically, alternately in
one direction and then the other direction from one region to
another, and an arrangement by which the consequent change of
temperature and pressure is caused to perform work, usually by
means of a piston operating a crankshaft. There is also a
regenerator to cause the changes of gas temperature to occur
gradually rather than abruptly.
In another known form of hot gas engine, gas is caused to move in
the enclosed volume about a circuit rather than to move cyclically
back and forth through the same channel. Compressed gas flows from
a compressor through a heater to an expander and then through a
cooler to the compressor. The regenerator commonly found on cyclic
flow hot gas engines is omitted as no longer necessary. For certain
modes of operation, the regenerator is replaced by a heat
interchanger.
Conventional hot gas engines may be employed in constant output
applications in which power requirements remain uniform. Constant
load pumps and generators are some examples of satisfactory
applications of constant output engines. However, where power
requirements rapidly vary, such as in automotive engines,
conventional hot gas engines have been unsatisfactory.
Methods of adjusting power in hot gas engines have concentrated on
changing heater temperature or changing the gas charge and hence
changing system pressure. Known power control methods have several
drawbacks, including inherent slowness of response.
SUMMARY OF THE INVENTION
The broad objectives of the invention are realized in hot gas
energy conversion apparatus which heats, expands and compresses gas
repetitively in a closed circuit and which controls the energy
output by variably controlling flow of working gas around the
circuit.
The primary elements in the preferred forms of the new engine are
compressors, heaters, expanders and coolers connected in closed
circuits, and variable valves between at least two adjacent
elements. In one embodiment, an interchanger is added, and the
interchanger may be used or bypassed as conditions require.
A gas compressor may be of any of the usual forms of such a device,
for example single or multiple reciprocating or rotating devices of
variable or uniform volumes. For convenience, a preferred form of
the invention is described as a compressor consisting of piston and
cylinder, with inlet and exhaust valves.
A heater is preferably in the form of a bundle of tubes in a
furnace, so arranged that the working gas enters at one end, is
gradually heated to a high temperature as it flows through the
heater tubes, and emerges at the other end. The heater internal
volume, containing the working gas, is preferably large compared to
the active volume, presently to be described. The furnace is
designed so that good furnace efficiency may be attained. There are
no strict limits on the area available for heat transfer between
furnace gases and working gas. That is in contrast with the
conventional hot gas engine in which all non-active volumes, such
as heater volume, must be held to a minimum in order to secure
improved output, thus placing severe restrictions on design.
A cooler is preferably of a type similar to the heater, and also
preferably is of relatively large volume. Rejected or unused heat
is transferred from the working gas to a cooling medium, usually
air or water. As in the heater there is a temperature gradient
along the path of working gas flow in the cooler.
One or more gas expanders may be of any form of variable internal
volume, but will be described as piston and cylinder units adapted
to operate with a crankshaft in the usual manner to produce
mechanical power.
A set of valves admits high pressure gas from the heater to an
expander for a chosen interval, and exhausts the gas to the cooler
after expansion. These valves may be operated by mechanical,
electrical or pneumatic methods, and may be controlled by any
convenient device, preferably by cams rotating at the speed of the
drive shaft. Provisions for altering the timing of the valves are
described herein.
The valves may be mechanically operated, as in a conventional
internal combustion engine. In this discussion however, it will be
assumed that they are pilot operated, that is that the power shaft
operates a set of small pilot valves which in turn operate the main
valves, or that the power shaft operates a set of contacts, and
corresponding solenoids operate the main valves.
A heat interchanger, during certain aspects of engine operation,
cools gas flowing from the expander to the cooler and
correspondingly heats gas flowing from the compressor to the
heater. The two gas flows in the interchanger are separated by
relatively thin walls so that the gas entering from the expander
can readily give up heat to the gas passing from the compressor to
the heater. The use of an interchanger is an important feature
since is allows the effective use of a relatively low pressure
ratio with a high temperature ratio between heater and cooler. When
a low pressure ratio is used, an interchanger is needed; it allows
good output and efficiency under those conditions. But if a high
pressure ratio is produced, as when the cutoff point is made
earlier, and compressor valves are altered in timing, the
interchanger is no longer needed, in fact becomes a distinct
disadvantage. Hence, it is well to bypass the interchanger when
valve control produces low output.
The flow of working gas about a circuit is thus as follows: The
compressor draws gas from the cold end of the cooler, compresses it
nearly adiabatically, with consequent increase of temperature, and
delivers it to the low temperature end of the heater, with the
modification that, during certain phases of engine operation, it is
further heated on its way to the heater by heat transfer in the
interchanger. The gas then flows through the heater, and its
temperature rises to a maximum. Working gas flows into an expander
during a controlled interval through which the expander volume has
increased from the point of minimum volume, near zero, to cut off
volume. During the balance of the increase of volume of the
expander, the gas is expanded nearly adiabatically and is
correspondingly cooled to a lower temperature and pressure. On the
return stroke during the decrease of volume of the expander, the
gas is forced into the warm end of the cooler, or, under certain
conditions, the gas loses heat in the interchanger before reaching
the cooler. The gas then flows through the cooler, and reaches its
minimum temperature, to complete the circuit.
The expander and compressor may operate on the same shaft; there
may be a number of expanders and compressors operating with a
common heater and cooler. The total active volume of compressors
will be less than that active volume of expanders. Both expander
and compressor cylinders are preferably double acting.
In a specific embodiment of a four cylinder engine, one cylinder is
used as a compressor, and the other three are used as expanders.
The diameter or stroke, or both, of a compressor may be different
from those of an expander. Pistons in the cylinders are connected
to a single crankshaft at convenient phase intervals. Each expander
has individual valves connected to intake and exhaust headers,
which are respectively connected to the heater and to the
interchanger.
Inlet valves of the expanders open at the dead center of the
pistons when cylinder volume is a minimum, and in the present
embodiment remain open for an adjustable fraction of the stroke. A
cam arrangement for this purpose is on a camshaft which is directly
driven by the power shaft to turn at the same speed, and in the
same or opposite direction, by gearing from the power shaft. In
this train of gears there is included a differential. When the
third shaft of this differential is held stationary, the drive
occurs as described. By adjusting the angle of the third shaft,
however, the angular relation between the two part cam and dwell
may be adjusted. When the elevated cam portion is adjusted, the
valve can be made to close at any desired angle of the power shaft.
In preferred embodiments, expander intake and exhaust and
compressor intake valve timing is varied. Adjusting cam dwell is
one example of a convenient method for changing valve timing.
For certain embodiments, both valves of the compressor may be
simple check valves. For the present embodiment, it is desirable
that the delivery of the compressor be adjustable. For this
purpose, the compressor outlet may be a check valve, and the inlet
valve may be operated by a cam mechanism such as is described for
expander valves. For full delivery, the inlet valve will be open
during the entire interval when cylinder volume is increasing, and
the check valve will be open during an interval when it is
decreasing. For partial delivery, however, the inlet valve will be
caused to close before full cylinder volume has been reached. In
this condition the gas in the cylinder will be expanded before it
is compressed to heater pressure to open the check valve. In an
alternate arrangement, the compressor inlet valve may be held open
after the cylinder has reached full volume. In this condition, a
part of the gas charge flows back to the compressor before
compression begins. The compressor cylinder may be of the same
diameter and stroke as the expander cylinders; they may be of
different diameters.
Adjustment of the expander valves alters the point of cutoff, and
the mass circulated per stroke, and hence alters the expansion
ratio and the power output. Adjustment of the compressor valves
serves to maintain mass balance, that is to ensure that the same
mass of gas per cycle is delivered to the heater as is extracted
therefrom, thus preserving pressure relations.
It is desirable that maximum temperature of the heater tubes be
maintained constant, independent of load, for this improves
efficiency. This is provided for by thermostatic control of the
furnace.
Preferably, the cylinders of the engine are double acting, that is
with operating gas in both ends of the cylinder, as in
reciprocating steam engines, rather than single acting as is
conventional in internal combustion engines. The maximum power
output of an engine, for a given weight and size, depends upon
speed, allowable pressures, and allowable forces on bearings. The
use of double acting cylinders not only introduces directly a
factor of two, but also greatly decreases the load on bearings, for
pressures in the two ends of the cylinder tend to partially offset
one another. In this connection it should be noted that, while in a
nonsupercharged internal combustion engine the maximum cylinder
pressure is determined largely by atmospheric pressure and
compression ratio, there is no such limit with the present engine.
When double acting cylinders are used there is employed a crosshead
and crankshaft such as has long been used with reciprocating steam
engines. The piston rod emerges from the cylinder through a sleeve
or packing. Since there is bound to be some leakage past this
sleeve, it is preferable to use a sealed crankbase full of the
working gas. A small engine driven pump may be used to transfer
working gas from the crankbase to the operating volume or to a
reservoir. The crankbase, with its bearings, is lubricated in the
usual manner, but there is preferably no oil in the working volume.
When helium is used as a working gas, and the crankbase is sealed,
there is no tendency for lubricating oil to deteriorate.
The engine herein described differs markedly from the internal
combustion engine. That engine is restricted by maximum allowable
compression ratios. The maximum pressures used in the latter
engines depend on mechanical strength of cylinders and allowable
compression ratios, unless they are supercharged. In the present
engine, maximum cylinder pressure can be chosen merely from
strength considerations. Thus, one can halve cylinder cross section
and double pressure to obtain the same output, without increasing
load on the bearings. In engine design, it is the maximum load on
bearings which is usually determining.
This leads to a second advantage of double acting cylinders. The
maximum pressure may be increased by the value equal to the minimum
pressure without increasing the maximum load on bearings. This
renders it favorable to operate with large temperature ratio and
with late cutoff.
In the embodiment discussed above, if it is made double acting,
each cylinder is concerned only with either compressor or expander
action. In an alternate embodiment a single cylinder or multiple
cylinder engine may be used. One end of each cylinder acts as an
expander; the other end acts as a compressor. The end containing
the piston rod is preferably the compressor. Even so, the
compressor volume, if fully used, is too large for appropriate mass
balance. Hence, the compressor side has idle volume. That is,
during piston movement the minimum volume is larger on the
compressor side than on the expander side. Preferably, a single
piston seal is placed near the compressor end of the piston, where
the temperature is low. With this configuration, the engine may
operate without a high temperature seal.
VALVE OPERATION
A preferred way of altering output is by altering valve timing.
When full output is to be maintained at all times, as might be the
case of an engine driving a water pump operating with a fixed head,
the valves on the compressor may be simple check valves, such as
are commonly used in compressors. When the ratio of compressor to
expander volume is correcttly chosen, in the light of cutoff point
and the temperatures of operation, the gas will be expanded in the
expander to approximately the pressure in the cooler at the end of
the expander stroke. When the expander exhaust valve is opened at
the dead center of the expander cylinder, there is very little
pressure across it. It can therefore readily be opened by a small
pneumatic actuator.
The engine expander and compressor valves are operated by use of
small cylinders and pistons, using the working gas which is
controlled by pilot valves. That has some advantages, for example
in some types of application the pressures developed in the engine
may be utilized to provide pressures in reservoirs, from which the
engine valves may be operated. Also, the use of helium in valves,
which may be at very high temperatures, avoids problems of
oxidation. Helium provides rapid engine valve operation in response
to pilot valve changes.
Preferably, there are two small auxiliary reservoirs, in which
pressure of working gas is maintained by a small pump. One
reservoir is at a pressure close to atmospheric, and the other is
at a high pressure. The auxiliary pump for this purpose may be
operated by a battery when the engine is idle, to have proper
conditions for starting the engine.
Small pilot valves are driven synchronously with the expanders and
compressor. The pilot valves selectively connect actuators for
expander and compressor valves with the high and low pressure
reservoirs to open and close the engine valves. Preferably gas
driven actuators operate the compressor intake and expander intake
and exhaust valves, for example:
The exhaust valve is constructed with a piston and cylinder with
connections to the pilot valve. A chamber below the actuator
cylinder has a passage connected to the interchanger. Any leak
along the actuator rod merely allows gas to flow to the chamber and
to the low pressure side of the system. The actuator cylinder may
be of very small diameter if a high pressure is maintained in the
small reservoir noted above. By placing the ports at proper points
in the actuator cylinder, there is a dashpot effect at the end of
the stroke in either direction. Thus, although the engine valve
operates very rapidly, undesirable impact is avoided at the end of
the stroke.
In a preferred form, the actuator cylinder is operated by a spool
type pilot valve which selectively connects ends of the actuator
cylinder to the high pressure reservoir and to the low pressure
reservoir. The spool is moved by a cam on a shaft which turns at
the same rate as the engine. This cam causes the pilot operated
expander exhaust valve to move up at one dead center and down at
the other. The stem of the spool valve is sealed by a metal
bellows, the interior of which is connected to the low pressure
reservoir to avoid loss of working gas. A spring causes the rod to
follow the cam. The use of a spring at the expander valve actuator
is avoided, as valves in the head of the expander are at high
temperature. Only a small amount of gas, per revolution flows
through the pilot valve; the dimensions hence can be very small.
Since the actuator cylinder is very hot, it is desirable to line it
with a ceramic coating, finished to a true cylindrical form at
operating temperature, as described in copending application Ser.
No. 15,462 titled "Piston Sealing", filed Oct. 7, 1970, by Vannevar
Bush.
In an alternative procedure and apparatus, two small reservoirs are
kept filled with air at appropriate high and low pressures by an
auxiliary pump, and the actuator cylinders are operated by air
pressure. The actuator cylinder is separated mechanically from the
expander valve, except for the valve rod. This allows the actuator
cylinder to be kept relatively cool. It can even be lubricated if
this is desirable. Bellows can be omitted from the spool type
valve.
While the expander exhaust valve is oriented in one direction,
namely so that expander pressure tends to hold it closed, the
intake valve is oriented in the other direction, so that heater
pressure tends to hold it closed.
The inlet valve to the expander opens at dead center and closes at
the cutoff point. The construction may be similar to that already
described for the exhaust valve. One further point is considered.
When the inlet valve opens at dead center, if there is a large
difference of pressure differential across it, the actuator
cylinder needs to be large to ensure the outward opening of the
inlet valve.
This difficulty is avoided in one of several ways. One is as
follows. When the closure of the expander exhaust valve is timed to
occur just before dead center, the gas remaining in the cylinder
will be compressed during the remainder of the stroke, thus
reducing the pressure difference tending to hold the inlet valve
closed. It may be arranged so that the cylinder pressure will
exceed that in the heater, and this excess pressure will itself
open the valve.
A second method of ensuring positive action of the intake valve is
as follows. A projection of the valve is actually hit by the
expander piston as it nears the end of its stroke, thus ensuring
very positive opening. The speed of the piston at that time, near
dead center, is relatively low, and the impact thus is
moderate.
In another valve opening apparatus, a cylindrical projection on the
valve enters a closely fitting recess in the piston of the expander
as this piston approaches the end of its stroke. The confined, and
hence compressed, gas provides a force to open the valve. To avoid
a force in the other direction as the piston recedes, a small check
valve is added in a passage connecting with the piston recess.
When the engine is started, in cold condition after a period of
inactivity, the pressures in heater and cooler are nearly equal,
but if the auxiliary small reservoir has been maintained at proper
pressures, there is no problem of proper valve action on starting.
As the engine is turned over, on starting, by a starting motor,
operating pressures are gradually established. The furnace is
simultaneously brought up to operating temperature.
When an engine runs constantly at full power, such as might be the
case in operating a pump or a generator having a constant load,
valve timing may remain uniform throughout the engine operation.
Power output of the engine is preferably varied by changing timing
of one or more valves. The coordinating of timing of a compressor
valve and all expander valves is preferred so that the active
volumes of the compressor and the expanders remain in constant
ratio. For example, for half load operation the compressor exhaust
valve remains a check valve, permitting flow toward the heater when
compressor pressure exceeds heater pressure, but the compressor
intake valve is held open for only one half of the filling stroke
of the compressor or is held open for the first half of the
compression stroke. Thus, only half a compressor volume is
compressed. At the same time, the expander intake valve is cut off
at one half of the full load cut off point so that only one half as
much gas is expanded, and the expander exhaust valve is open for
only the last half of the exhaust stroke. Thus, the gas is expanded
to a low cooler pressure and then compressed to cooler pressure
before the expander exhaust valve opens to release the gas to the
cooler or heat exchanger.
A second method of operation of the expander exhaust valve at part
load is to open the exhaust valve at the point in the expansion
where the pressure is equal to the cooler pressure. The valve
remains open for the remainder of the expansion stroke and for all
of the exhaust stroke. With this method of operation, the pressure
difference across the valve is always in the direction to hold the
valve closed. For this reason, holding the valve open for the
additional time is preferred in both the expander and the
compressor.
There are three principal ways in which power output may be varied
by changing timing of valves between elements. The effective
expander volume may be reduced, the effective compressor volume may
be reduced, or both effective volumes may be reduced.
Assume that, at full load, the temperature at the outlet of the
heater is T.sub.1 and the pressure P.sub.1. Similarly, the cooler
has temperature T.sub.2 and pressure P.sub.2. The volume of the
expander up to the point of cut off is V.sub.1, and the active
volume of the compressor is V.sub.2. In the steady state, the mass
of gas moved from the heater, per cycle, must be equal to the mass
of gas moved from the cooler to the heater.
This requires that
P.sub.1 V.sub.1 /T.sub.1 = P.sub.2 V.sub.2 /I.sub.2
Pressures alter until this condition is met.
For example, if a temperature ratio T.sub.1 /T.sub.2 = 3, in terms
of absolute temperature, is to be used, and if it is wished to
employ a pressure ratio P.sub.1 / P.sub.2 = 2 at full load, V.sub.2
/ V.sub.1 = 2/3; that is the active volume of the compressor should
be two-thirds the volume of the expander up to the point of cut off
at full load. The actual volume ratio is somewhat larger because of
flow losses in the valves, heat transfer to the cylinder walls,
clearance volume in the compressor and recompression in the
expander clearance volume.
At full load, the expander pressure should equal the cooler
pressure at the end of the power stroke. After cut off, the gas in
the expander expands by a volume ratio V.sub.1 / V, where V is
total expander cylinder volume. Since the expansion is nearly
adiabatic, this involves a pressure ratio (V.sub.1 /V).sup.k where
k, for helium, is 5/3. Since pressure decreases to one-half of
maximum pressure, this requires that V.sub.1 /V = 0.648.
To decrease power output, the point of cutoff is advanced, the
expander inlet valve is caused to close at an earlier point in the
cycle. The new value of expander volume at cutoff is designated
V.sub.1.sup.1. Pressures alter until
P.sub.1.sup.1 /P.sub.2.sup.1 = 2/3 (T.sub.1 /T.sub.2) (V.sub.1
/V.sub.1.sup.1)
or, on the assumed temperature ratio,
P.sub.1.sup.1 /P.sub.2.sup.1 = 2 V.sub.1 /V.sub.1.sup.1
Thus, as cutoff is advanced, the pressure ratio rises. When volume
at cut off is halved, the pressure ratio is doubled.
The mass of gas in the system is assumed constant. When the volumes
of heater and cooler are assumed equal, and each is assumed large
compared to cylinder volume,
(P.sub.1.sup.1 /T.sub.1)+(P.sub.2.sup.1 /T.sub.2) = constant
(P.sub.1.sup.1 /T.sub.1)+(P.sub.2.sup.1 /T.sub.2) = (P.sub.1
/T.sub.1)+(P.sub.2 /T.sub.2)
These expressions can be combined to show that, when cut off occurs
earlier, the heater pressure rises. In fact, in our example, if
expander volume at cut off is halved, the heater pressure will rise
to 10/7 of its full load value. This is undesirable, for one wishes
to use the maximum allowable heater pressure under conditions of
full load.
If the heater volume is made larger than the cooler volume, the
situation is somewhat improved, but the pressure will still rise as
cutoff is advanced.
As cut off is advanced, pressure ratio increases, so that
P.sub.2.sup.1 = (V.sub.1.sup.1 /2V.sub.1) P.sub.1.sup.1
but the expansion is then in the volumetric ratio V.sub.1.sup.1 /V,
and the pressure in the cylinder at the end of the power stroke is
(V.sub.1.sup.1).sup.k P.sub.1.
To ensure against back flow from the cooler to the expander, the
exhaust valve of the expander may be a check valve, which is held
closed for a proper interval by pressure in an actuator cylinder.
Pressure should be applied to hold the exhaust valve closed during
an interval from just before the inlet valve opens until the end of
the power stroke. A fixed cam operating a pilot valve may be used
for this purpose. The exhaust valve will then open only when
pressures become equalized.
Altering the operation of the valves of the compressor, the inlet
check valves of the compressor are replaced by pilot controlled
valves. The active volume of the compressor is reduced to decrease
output.
The active volume may be reduced in one of two ways. The inlet
valve may close late, that is, after dead center, in which case
part of the gas drawn from the cooler will be forced back into the
cooler before the remainder is compressed and is forced into the
interchanger. The inlet valve may close early, in which case, the
gas in the compressor is first expanded and then compressed before
being transferred to the interchanger.
From the expression above
(P.sub.1 /P.sub.2) = (T.sub.1 /T.sub.2) (V.sub.2 /V.sub.1)
it is evident that the pressure ratio will be decreased when the
effective volume of the compressor V.sub.2 is decreased. With fixed
expander cutoff, the pressure ratio in the expander will remain
constant. Hence, at the end of the expander stroke, the pressure in
the expander will be below that in the cooler. To avoid a rush of
gas when the expander exhaust valve is opened, it is necessary to
delay that opening beyond dead center until the gas in the expander
has been compressed to the pressure existing in the cooler. Thus,
the timing of two valves needs to be altered to use efficiently the
changes of the effective compressor volume for control.
Both of these methods (change of only V.sub.1 or change of only
V.sub.2) require changes in pressure ratio for changes in power.
Since a significant amount of gas must be compressed or expanded to
change the pressure ratio, there is a delay before the power
adjusts to a signal to change power. In the preferred method of
control, the timing of the expander inlet valve controls V.sub.1
(cut off volume), the timing of the expander exhaust valve controls
V (the maximum effective volume of the expander), and the timing of
the compressor inlet valve timing controls V.sub.2 (the effective
compressor volume).
When the timing of these three valves is adjusted so that V.sub.1,
V and V.sub.2 always remain in the same ratios, the power of the
engine is changed without any change in P.sub.1 or P.sub.2. This
eliminates all delays in changing the power of the engine. With
this method of control, the thermodynamic cycle of the gas remains
fixed, and power level is set by only changing the mass of gas
circulated per stroke. Thus, the thermodynamic efficiency of the
engine can be maintained over the entire power range.
Every unit mass of gas flowing about the circuit extracts from the
heater an amount of heat proportional to (T.sub.1 - T.sub.a), and
deposits in the cooler an amount proportional to (T.sub.b -
T.sub.2). With no interchanger, the temperature after compression
is T.sub.a, the temperature into the heater, and the temperature
after expansion is T.sub.b, the temperature into the cooler. With
an interchanger, the temperature after expansion is T.sub.a +
.DELTA.T and the temperature after compression is T.sub.b -
.DELTA.T, where .DELTA.T is the heat exchange temperature
difference in the interchanger.
The efficiency is thus:
.epsilon. = 1 - [(T.sub.b - T.sub.2)/(T.sub.1 - T.sub.a)]
This may be transformed to
.epsilon. = 1 - (T.sub.b /T.sub.1) = 1 - (T.sub.2 /T.sub.a)
Since the pressure ratio remains constant, the temperatures of gas
flowing into the cooler and into the heater, and without
interchanger, after expansion and compression, T.sub.b and T.sub.a,
also remain constant. The efficiency is hence constant during
change of load.
The power output, at a given speed, is proportional to
V.sub.1 (T.sub.1 - T.sub.a) [1 - (T.sub.2 /T.sub.a)]
and hence is directly proportional to V.sub.1.
All of the above applies when T.sub.1 and T.sub.2 are at full
operating values, and when the engine has operated long enough to
produce full operating pressures. There is then no appreciable
delay when V.sub.1 is altered to change output, for the mass of gas
in heater and cooler does not alter. We assume in above that heater
and cooler volumes are large compared to cylinder volumes.
The expressions above for output and efficiency hold whenever the
pressures remain constant. They hold for large heater and cooler
volumes, provided the ratios T.sub.1 /T.sub.2 and P.sub.1 /P.sub.2,
and V.sub.1 /V.sub.2 are appropriately chosen. There is an
approximation here, for actually the heater pressure changes during
expander intake, and the cooler pressure changes during compressor
intake. This effect apparently results merely in small change of
P.sub.1 /P.sub.2 under operating conditions.
But, when the above relations are widely departed from, under
starting conditions there is a net transfer of gas between heater
and cooler, each cycle, and P.sub.1 and P.sub.2 gradually alter. If
pressures at starting are equal, the heater is pumped up, and the
work needed for this needs to be considered in examining starting
torque.
Under starting conditions, three cases are considered.
The first case involves starting when both temperatures and
pressures are at nearly full value. The engine is now, of course,
ready to develop full power at once. For a traffic stop, one takes
his foot off the accelerator and the engine idles with early
cutoff. But one wishes to stop the engine, briefly or for a long
period. For a short stop, one would leave the furnace operating
under its thermostatic control. For a brief stop, one could stop
the engine by leaving the car in gear and applying the brake. This
would not be desirable, for the engine might start again on brake
release, if valves happen to be in correct position. It is better
to have a positive stop controlled by the car key. This would
operate a shutoff valve in the line from the heater, or it would
move cutoff back to nearly zero position. In either case, on
turning the key, the engine would automatically start and idle, or
a small action by the starting motor would cause it to do so.
In the second case, the heater temperature is fully established,
but cooler and heater pressures are equal. This condition arises
when the furnace is left ignited and the engine is stopped for a
considerable period, while gas leaks from heater to the cooler past
valves and pistons. The effect is reduced if a shut off valve above
is used, but valves can be expected to leak somewhat. Automobile
operators probably will not wish to shut off their furnace when
they make a relatively short stop.
In one example, the ratio of heater volume to total effective
expander volume is about 40 in a multicylinder engine. If the
accelerator is fully depressed and the cutoff hence is at full load
position, the initial negative starting torque is about half of
full load torque. That calls for a powerful starting motor, or one
well geared down. The pressures change rapidly to develop positive
power, and the net torque arrives at zero after only about 10
cycles.
In another example, the accelerator is at idling position, and
cutoff is at one fifth of its full load value. The initial negative
torque is one fifth of the previous value, but the engine has to
turn over for 50 cycles before positive net torque appears. That is
not a serious matter. The conditions of this case will not occur
often. And if the starting motor turns the engine over at 500 rpm,
one would need to wait only a fraction of a minute before the
engine would idle when the accelerator was depressed.
The third case is the usual one, starting with a cold furnace and
with pressures uniform throughout. The furnace and starting motor
are turned on at the same time. The time constant for the furnace
heating is about 30 seconds. The engine is turned over at 60 rpm;
there is hence one cycle per second.
At first, pumping is in the revere direction, that is to decrease
P.sub.1 rather than to increase it. Net torque becomes positive
after 27 seconds. The burden on the starting motor is not large. At
the end of one minute half of full load torque is available,
assuming the accelerator to be fully depressed throughout and
assuming the slow starting turnover to continue. Of course, as with
present engines, when positive torque appears, the engine speeds
up, and the accelerator is released. Thus, nearly full load torque
is available after about 40 seconds.
It is of advantage to delay operation of the starting motor until
the furnace has heated up a bit. When the motor is started when
T.sub.1 /T.sub.2 has arrived at 1:5, 1.5, is very little negative
net torque, and positive torque begins after 20 seconds. The
sequence readily is made automatic on turning the car key, or a
light indicates when the key should be turned from a furnace
ignition position to a second motor starting position.
ALTERNATE POWER ADJUSTMENT
There are a number of ways in which the power output of this engine
may be controlled with no adjustment of valve timing on either
expander or compressor. The engine operates at fixed cutoff, and
mass balance is secured by choosing a proper diameter of compressor
compared to expander cylinders. Power output is then adjusted by
altering the temperatures or the total mass of gas in the working
volume. These can be accomplished in several ways.
The fuel supply to the heater may be controlled to vary the maximum
temperature in the heater. This is an undesirable method, first
because there is a long time lag in response when a change of
output is called for, and second because efficiency is reduced when
heater temperature is lowered.
Another power-changing method is to alter the total mass of gas in
the system, thus altering the pressure during the cycle. This may
involve a time lag, but may be employed when such a lag is not of
great importance, as in a marine engine.
In one form of mass adjustment as described in U.S. Pat. No.
3,527,049 (FIG. 4), a reservoir is provided filled with working
gas. The pressure of this gas is intermediate between maximum and
minimum pressures in the working volume. This reservoir is
connected to the system, preferably to the cooler, by two pipes.
Each pipe includes a check valve and a second valve, preferably, a
solenoid operated shutoff. One check valve allows flow only into
the reservoir, and the second allows flow only out of the
reservoir. When the first solenoid valve is operated, the mass of
working gas is decreased, and when the second is operated, it is
increased. The power output is thus under control over a wide
range. The action of these valves may readily be compounded, by
well understood means, so that a given position of a lever or pedal
will correspond to a definite working gas mass and hence power
output.
In another method for varying system mass, there is supplied a high
pressure reservoir, and a low pressure reservoir, containing
working gas, which is preferably helium. Pipes from each of the
reservoirs, with a valve in each pipe, connect to the system,
preferably to the cooler. Opening one valve for an interval
increases system mass and output; opening the other decreases
system mass and output. An auxiliary pump may be supplied to pump
gas from the low to the high pressure reservoir, in order to
maintain a desired pressure difference.
Another method, as follows, may be used. A pipe is connected from
the heater to a high pressure reservoir, with a check valve in the
pipe to allow flow toward the reservoir. A second pipe, with check
valve, connects the cooler and a low pressure reservoir, allowing
flow from this reservoir to the cooler. These two pipes may be
passed through an interchanger to allow heat interchange. Under
steady conditions of operation, one reservoir will have a pressure
which is approximately equal to maximum system pressure, and the
other to minimum system pressure. Change of output can then be
obtained as before. There is, however, a limit to the amount of
output control which can be obtained in this manner. Consider an
example. Let us assume that the system is set to operate with a
pressure ratio of 2:1, at full power the HP reservoir will be at
maximum pressure, and the LP reservoir at half of that. Assume we
now wish to decrease output. Even if the reservoirs are very large,
we can decrease maximum operating pressure in the system only to
half its previous value. The consequent change in output may not be
sufficient, especially if one wishes to go from full power to
idling power. Of course, one may combine the two methods of
maintaining reservoir pressures.
A preferable method of altering system mass and output is as
follows. The control of engine power by changing the mass of gas
does not involve a time lag when the new gas is added to the system
from a high pressure reservoir into the high pressure part of the
system, preferably between the compressor outlet and the
interchanger. This method requires a high pressure reservoir which
is always at a pressure above the maximum pressure for the engine.
An auxiliary compressor with the required high pressure ratio adds
to engine weight and consumes power.
OBJECTS OF THE INVENTION
One object of the invention is the provision of hot gas energy
conversion methods and apparatus having a heater, expander and
compressor connected in a circuit with at least one variable valve
controlling flow of working gas around the circuit and thereby
varying energy conversion.
Another object of the invention is to provide hot gas engines
having gas flowing about a circuit from a compressor to a heater to
an expander to a cooler and back to the compressor, with valves
between the elements controlling the flow, and with at least one of
the valves being variable.
Another object of the invention is the provision of a hot gas
engine having a circuit flow with variable valves controlling the
flow of gas around the circuit.
This invention has an another object provision of a hot gas engine
having circuitous gas flow with valves operating in adjustable
timed relationship with the compressor and with the expander for
adjusting power output of the engine.
The invention has as other objects the provision of pilot valves
and valve actuators and check valve disabling actuators for
controlling hot gas engine valves.
Another object of the invention is the provision of working gas
mass varying systems which remove gas from a circuit between a
cooler and compressor and which add gas to a circuit between a
compressor and heater for reducing or increasing power.
These and other objects of the invention are apparent in the
disclosure which includes the foregoing and ongoing specification
with the appended claims and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a hot gas engine constructed
according to the present invention showing a general
innerrelationship of the parts.
FIG. 2 is a schematic detail of a hot gas engine showing valve
operations.
FIG. 3 is a schematic detail of a cam mechanism for varying valve
dwell and timing to control power output of the engine.
FIG. 4 is a schematic detail showing the engine circuit and valve
control for an embodiment of the engine of this invention.
FIG. 5 is a schematic detail of a method of controlling power by
varying the mass of gas moving through the circuit.
FIG. 6 is a detail of an alternative form of mass-varying power
control.
DETAILED DESCRIPTION OF THE DRAWINGS
A hot gas engine is generally referred to by the numeral 1. Four
cylinders are mounted on a crankbase 4. Pistons within the
cylinders are connected to crossheads within the base 4. Four sets
of crosshead guides are mounted internally within crankbase 4
beneath head 6 to constrain crosshead and piston movement to pure
reciprocation, eliminating side loading of pistons in the
cylinders. Connecting rods connect the crossheads to a crankshaft
in the crankbase.
The engine is made up of a compressor, heater, expander, cooler,
interchanger which may be removed from the circuit, and valves to
control flow around the circuit. Each expander or compressor
cylinder has two valves, four valves when double ended. Variable
control is provided for the expander and compressor intake valves
to operate the engine at a selected power output between full and
idling power. The compressor exhaust valves are check valves, and
the expander exhaust valves are check valves which are held closed
for part of each cycle.
Expander cylinders 12, 14 and 16 and compressor cylinder 18 are
thermally insulated from the environment with jackets which conduct
heat over external surfaces of the cylinders for providing uniform
temperatures throughout. The expander cylinders 12, 14 and 16 and
compressor cylinder 18 are spaced from the crankbase head 6 by
thermal isolating tubes 20. Preferably, all of the cylinders are
double-acting with duplicate sets of valves at opposite ends of the
cylinders. For clarity in the present description of the invention,
valve systems and working gas supplies are shown only for the
connections at upper ends of the cylinders.
Pistons reciprocate and the cylinders operate in a relationship
which is selected for kinematic engine balance and design
convenience. Timing relationship between the compressor and
expanders is not critical. It is not necessary that the compressor
be driven at the same speed as the expanders, and it is not
necessary that all of the expanders be driven at the same speed.
Varying speeds of expanders and compressors may be employed, and
the elements may have varied bores and strokes. An important
consideration is the volumetric compressor and expander
relationship which is described herein.
In a cycle of operation, compressed gas is exhausted from the
compressor through valve 22, which is a check valve in a preferred
form of the embodiment. Gas passes through conduit 24 into a thin
walled channel within heat interchanger 26, and the gas continues
through conduit 28 into heater 30.
Heated gas under the high pressure supplied by the compressor
leaves the heater through conduit 32 and flows into header 34, from
which the gas is valved into the expanders.
When the piston within expander cylinder 12 approaches top dead
center, valve 36 is opened, and gas passes into the cylinder 12 as
the piston begins its descent. At a selected cut-off point, valve
36 is closed, and the gas entrapped in the upper end of cylinder 12
is expanded, as the piston continues its descent.
Near the maximum volume of the upper end of cylinder 12, valve 38
is opened. As the piston within cylinder 1 ascends, the expanded
gas within the upper end of the cylinder flows through valve 38
into exhaust header 40 and exhaust conduit 42, into a second thin
walled channel within heat interchanger 26. There the expanded gas
gives up heat to the incoming compressed gas before the expanded
gas continues its flow through conduit 44 into cooler 46.
While the piston in the upper end of compressor 18 is descending,
compressor intake valve 48 is opened, filling the compressor and
completing the cycle. During the cycle, the expander intake valves
for expanders 14 and 16 are opened at appropriate times in
relationship to pistons moving within the cylinders, and the
expander exhaust valves are opened in similar timed relationship,
for filling the expanders with heated high pressure gas from header
34 and for exhausting expanded gas into header 40.
Expander intake and exhaust valves 36 and 38 and corresponding
intake and exhaust valves on expanders 14 and 16 and the intake
valve 48 for compressor 18 are operated in timed relation to the
cyclic volumetric changes of the cylinders. In one embodiment of
the invention, the crankshaft 50, which is driven by the expanders
and which drives the compressor, is provided with two gear toothed
pulleys 52. Timing belts 54 and 56 drive gear toothed pulleys 58
and 59, which are keyed to cam shafts 60 and 62. The cam shafts in
turn drive pilot valves of servo valve assemblies which operate the
expander and compressor valves.
Pilot valves 64, 66 and 68 are driven by separate cams on cam shaft
60. A high pressure reservoir 70 supplies a valve-operating gas,
preferably helium, the same as the engine working gas, through
lines 72 to the pilot valves. Lines 74 connect central portions of
the pilot valves to a low pressure reservoir which is located, for
example within the crankbase. An auxiliary compressor, driven by
one of the cam shafts or the crankshaft, pumps gas from the low
pressure reservoir to the high pressure reservoir 70. As the spool
piston within valve 64 is driven up and down, lines 76 and 78 are
alternately pressurized and exhausted by communication with lines
72 and 74. Thus, the alternating high and low pressures in lines 76
and 78 drive the valve operating piston within cylinder 79 up and
down to open and close valve 36 in timed relation with cam shaft 60
and crankshaft 50.
Interchanger bypass 80 may be used between the expander and the
cooler by opening valves 81.
Expander exhaust valve 38 is operated by a piston within cylinder
82. The valve operating cylinder 82 is supplied with pressure by
pilot valve 84, which has a spool piston driven by cam shaft
62.
All expander intake valves are operated by pilot pistons driven
from cam shaft 60. All expander exhaust valves and compressor
intake valve 48 are driven by cams on cam shaft 62. Two cam shafts
are preferred because power reduction requires different
adjustments for the compressor intake and expander exhaust than for
the expander intakes. For example, half power adjustment requires
approximately quarter revolution adjustment for the expander
exhaust valves and compressor intake valve and requires less than a
quarter revolution adjustment for the expander intake valve. As
shown in FIG. 1, the compressor intake valve 48 is operated by a
piston within cylinder 86, which is driven by alternations of
pressures in lines 88 as controlled by a pilot valve operated by
cam shaft 62. In FIG. 1, 89 generally indicates a high pressure
reservoir for supplying the pilot valves driven by cam shaft
62.
FIG. 2 is a schematic representation of the elements shown in FIG.
1. Gas flows from compressor 18 through check valve 22 and
interchanger 26 to the heater 30 and thence to header 34 which
communicates with valve 36. Expanded gas leaving expander 12 exits
through exhaust valve 38 and interchanger 26 on its way to cooler
46 and back to compressor 18 through valve 48, completing the
circuit. As shown in the drawings, valve 22 is a check valve
permitting flow outward from compressor 18 into conduit 24. The
remaining valves are piston operated valves. The operation of valve
36 is described in detail. The operation of valves 38 and 48 are
similar to that of 36.
Gears 52 and 58 and an intermediate timing belt drive cam shaft 60
at the same speed as crankshaft 50. An adjustable cam 90 on the cam
shaft drives a cam follower 92 on a piston rod 94. A spool piston
96 is mounted on the piston rod within valve 64. Spring 98 ensures
positive contact of follower 92 with cam 90. A cam 100 on cam shaft
60 drives a piston pump 102 which takes gas from low pressure
reservoir 104 and supplies gas to the high pressure reservoir 70
via check valves 106 and 108.
As shown in FIG. 2, cam 90 has raised follower 92 and spool piston
96 to the upper position. Upper high pressure line 72 is
communicated through line 76 with the upper end of valve operating
cylinder 80. Pressure is thus applied to the upper surface of
piston 110, holding poppet valve 112 seated so that valve 36 is
closed.
As cam 90 turns and spring 98 forces piston 96 to its lower
position, line 76 is communicated with line 74 for returning gas
from the upper end of cylinder 80 to the low pressure reservoir. At
the same time, the lower high pressure line 72' is communicated
with line 78 for lifting piston 110 and opening poppet valve
112.
Poppet valve 112 opens upward against full system pressure of the
compressed and heated working gas. At the time that the valve is
opened, piston 114 is approaching top dead center. To provide
additional force to open valve 112 against the system pressure, a
downward projection 116 is formed on the base of the valve.
Projection 116 fits with close tolerance in recess 118, building
pressure within the recess to force valve 112 upward. Check valve
120 and fine tube 122 prevent recess from drawing projection 116
downward as piston 114 descends with valve 112 open. A similar
problem does not exist in the opening of the expander exhaust valve
or the compressor intake valve.
The expander intake valve 36 remains open for the initial part of
the expander cycle when volume begins to increase. At a cut off
point, valve 36 is closed, and the gas within the expander is
expanded. As the piston reaches its lowest point, valve 38 is
opened and remains open until the piston 114 approaches the top
dead center. Compressor intake valve is open for the entire intake
stroke at full power. As shown in FIG. 2, the bores of compressors
and expanders may differ. Relative piston position in FIG. 2 is of
no significance.
Those valve openings apply for full power operation. When the
engine is operated at less than full power, active volumes of the
expanders and compressors are reduced, preferably maintaining the
volumetric ratio.
A means to control the time of the valve opening is shown in FIG.
3. Cam shaft 60 is driven by a belt 54 and gear 58. The gear has a
bevel gear arranged on its rear face for driving a differential
unit. Bevel gears 130 and 132 turn freely on slanted axles 134 and
136, which are mounted in slide 138. The large bevel gear on the
rear of gear 58 turns a small bevel gear 130, which in turn spins
small bevel gear 132, thereby turning bevel gear 140 in the same
direction as gear 58. Gear 140 drives hollow shaft 142, which has
fixed at intervals therealong cams 144. Each cam has a lifting
portion 146 and an adjacent arcuate slot 148. Cam 150, which is
attached to the cam shaft 60, extends through slot 148. As long as
slide 138 is held fixed, shaft 142 turns with the shaft 60, and cam
elements 146 and 150 remain relatively fixed. When slide 138 is
moved in the direction of arrow 152, gear 140 and shaft 142 and cam
146 are caused to rotate in the direction of arrow 154, increasing
the raised circumferential cam surface and reducing the time over
which the pilot valve is depressed for opening the expander intake
valve 36.
FIG. 4 is a simplified schematic representation of the engine of
the present invention in which compressor 18 has an intake valve
160, which is operated by a solenoid 162. Gas flows from compressor
18 through check valve 22, interchanger 26 and heater 30, and
through intake valve 164 which is opened by solenoid 166. After gas
is expanded in expander 12, solenoid 168 opens exhaust valve 169,
flowing gas through interchanger 26 and cooler 46, completing the
cycle. Solenoids 162, 166 and 168 are controlled electrically by
contacts or magnetic switches operated by cam shafts similar to the
gas pilot valve operation.
Rather than controlling power by valve timing, or in addition to
controlling power by controlling valve timing, power may be
controlled by increasing or decreasing the gas supply in the
circuit. As shown in FIG. 5, one preferred method of increasing
system power is by maintaining gas at a high pressure in reservoir
170. When increased power is desired, valve 172 is opened and gas
flows from high pressure reservoir 170 through check valve 174 into
conduit 24 between the compressor 18 and the interchanger 26.
System pressure may be reduced by opening valve 176, allowing gas
to flow from the circuit conduit between cooler 46 and compressor
18. Gas removed from the circuit flows into low pressure reservoir
180, where it may be pumped by auxiliary compressor 182 into the
high pressure reservoir 170.
An alternate form of changing circuit pressure and controlling
power is shown in FIG. 6. When a power reduction is required, valve
184 is opened, allowing gas to flow from heater 30 through check
valve 186 and interchanger 188 to reservoir 190. When increased
power is required, valve 192 is opened, allowing gas to flow from
reservoir 194 through interchanger 188 and check valve 196 to
cooler 46. An auxiliary compressor, not shown, transfers gas from
low pressure reservoir 190 to high pressure reservoir 194. Although
power output can be controlled by increasing or decreasing mass
within the system, the preferred method of power control is by
varying valve timing.
While the invention has been particularly shown and described with
reference to a preferred embodiment thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and
scope of the invention.
* * * * *