Shape memory element engine

An engine for conversion of low grade thermal energy into useful rotational energy incorporates a plurality of hollow deformable shape memory elements coupled in series with the crankpins of a crankshaft and with load limiting members. Hot and cold fluids are alternately directed through the crankshaft to the inner surface of the shape memory elements to provide rotation of the crankshaft.

This invention relates generally to the field of devices for conversion of 
low grade thermal energy to useful mechanical energy and, more 
specifically, to engines adapted for such conversion which incorporate 
shape memory elements. 
b. Description of the Prior Art 
This nation and, indeed, the majority of industrial nations, faces a major 
problem in conservation of energy, a problem commanding sharply increasing 
priorities as petroleum feedstocks become scarce and alternative energy 
sources such as nuclear power encounter political resistance or 
technological problems. 
While high grade energy sources are in short supply, low grade energy 
sources, such as spent industrial cooling water, geothermal hot water, 
power plant waste heat, solar energy and oceanic thermal gradients, are 
abundant. Since by definition such low grade energy sources involve 
relatively low temperature differentials, Carnot efficiencies are low. As 
a result, exixting technology for utilization of such low grade energy 
sources is limited. 
In the 1960's researchers began to recognize the potential of certain shape 
memory materials in the conversion of such low grade energy sources to 
useful mechanical power. Such shape memory materials exhibit a transition 
temperature or a narrow transition temperature range above and below which 
separate crystalline states apparently exist. Once annealed to a 
particular shape, these materials may be readily deformed while below the 
transition temperature. Upon subsequent reheating to above the transition 
temperature, the material returns to its original shape, exerting higher 
stress upon return that was required for low temperature deformation. The 
reversible cycle thus completed may potentially be used to do useful work. 
While several alloys and bimetallic combinations are known to exhibit such 
shape memory characteristics, the most promising results to date have been 
produced with alloys containing near stoichiometric ratios of titanium and 
nickel. The advantages of such NiTi alloys, known commonly as 55-Nitinol, 
for low grade energy conversion systems stem primarily from an inherently 
narrow transition temperature range and secondarily from an ability to 
shift the transition temperature range up or down by minor composition 
changes. Many systems for utilizing 55-Nitinol have been proposed since 
its unique properties were disclosed, including systems using stretchable 
55-Nitinol belts, asymmetric rotating wheels with 55-Nitinol spokes, and 
crankshafts driven by 55-Nitinol elements. These systems generally have 
however, failed to prove useful as a result of one or more inherent 
problems including low efficiencies, low power densities, low cycle 
speeds, complex fluid paths and sealing requirements and nonuniform 
heating and cooling of the shape memory elements. 
For all their unique advantages, 55-Nitinol and other shape memory 
materials are not without limiting parameters. If subjected to a 
deformation of more than approximately 8% strain, 55-Nitinol is limited to 
less than 100% recovery of its original shape. However, the maximum work 
output of 55-Nitinol elements occurs at a strain level approaching such 
maximum value. In addition, the transition temperature or temperature 
range expands as the recovery tensile stress is increased (i.e., as load 
is increased during recovery to original shape). Thus, while it is 
necessary to operate a 55-Nitinol engine at or near maximum stress and 
strain in order to achieve high power density, both of these limiting 
factors dictate that each 55-Nitinol element be consistently controlled as 
to both temperature and load in order to avoid localized temperature 
variations which might result in overstressing or overstraining portions 
of an element, causing irreversible limitations upon the ability of the 
element to recover its original shape. 
In addition, large surface areas of the shape memory elements must be 
exposed to the heating and cooling fluids in order to provide reasonable 
heat transfer rates and high cycle speeds while using low grade 
temperature sources. This requirement is further exacerbated by the need 
to provide sufficient length changes of the shape memory elements to 
achieve a reasonable power density. 
Providing maximum heat transfer area, adequate separation of hot and cold 
fluids, and sufficient length changes of the shape memory elements while 
operating at or near maximum loading and maximum strain with consistent 
temperature control is a major problem which has not been adequately 
solved by the prior art. 
SUMMARY OF THE INVENTION 
The applicant proposes a device for conversion of low grade thermal energy 
into mechanical energy consisting of a plurality of hollow shape memory 
materials formed as helical coils or other spring shaped elements. The 
shape memory elements are rotatably connected to the crank pins of a crank 
shaft, the crankshaft incorporating at least one and preferably two 
passages therethrough adapted for transporting and directing heating or 
cooling fluids or both. The passages of the crankshaft further cooperate 
with the shape memory elements such that the heating and cooling fluids 
may be directed within the bores of the shape memory elements at 
predetermined intervals in the rotational cycle of the crankshaft. A low 
temperature strain is induced in the shape memory elements by the rotation 
of the crankshaft followed by high temperature stress induced by contact 
with the hot fluid. Rotation of the crankshaft may thus be sustained and 
useful work produced. Each shape memory element further cooperates 
serially with load limiting means, preferably consisting of spring means, 
piston means, or a movable weight. 
By utilizing hollow tubes of shape memory material, coupled with fluid 
injection from the crankshaft end, the applicant succeeds in providing 
maximum heat surfaces for the shape memory elements while providing even 
temperature control across the length of the individual elements. Load 
control is effectively accomplished by providing load limiting means in 
series with each element.

DETAILED DESCRIPTION OF THE DRAWINGS 
As shown in FIG. 1, the shape memory element engine of this invention 
incorporates at least one and preferably a plurality of shape memory 
elements 30, preferably constructed of 55-Nitinol having a transition 
temperature range chosen to respond to the maximum and minimum 
temperatures of the low grade thermal energy source to be utilized. The 
shape memory elements 30 are further preferably annealed to a tubular 
helix configuration as shown in FIG. 1 and more clearly in FIG. 2. The 
helix configuration is chosen to provide nominal strain of the shape 
memory material while allowing reasonable total flexure of the element 30 
as a unit. The tubular configuration is chosen to provide maximum heat 
transfer area and to simplify fluid flow and hot and cold fluid separation 
problems. 
One end of each shape memory element 30 incorporates a bearing 35 rotatably 
surrounding and cooperating with a crankpin 22. The other end of each 
element 30 is connected in series to a load limiting member 40. As shown 
in FIG. 1, the load limiting member 40 may incorporate a piston 42 
sealably engaging a cylinder 41, the function and operation of which will 
be more fully considered in conjunction with the discussion of FIGS. 6 and 
7. 
As shown schematically in FIG. 1, the shape memory elements 30 cooperate 
with a crankshaft 20 such that expansion and contraction of the shape 
memory elements 30 may cause rotation of the crankshaft journals 21 within 
the main bearings 23 producing useful mechanical output. A flywheel 24 
provides inertial dampening utilizing well known principles. While a 
plurality of shape memory elements is preferred, the flywheel 24 allows 
for cooperation with only a single element. 
Now referring to FIG. 2, the web 25 and the crankpin 22 of the crankshaft 
20 may be seen to surround two fluid passages 26 and 27 adapted for 
transporting cold and hot fluids, respectively. Each fluid passage 26 and 
27 preferably runs continuously throughout or along the entire length of 
the crankshaft 20 and is preferably maintained under positive pressure. 
Each passage 26 and 27 incorporates a branch 28 and 29, respectively, to 
the surface of the crankpin 22 at opposed sides thereof such that hot 
fluid from one passage 27 may be injected into the bore 37 of a shape 
memory element 30 at approximately the top of the crankpin 22 rotation as 
shown, and cold fluid, from the other passage 26, may be injected into the 
bore 37 of the shape memory element at approximately the bottom of the 
crankpin's 22 travel. After injection, the fluids may, if desired, be 
collected and used to operate other shape memory elements having different 
transition temperatures. Alternately, the previously used fluids may be 
injected through additional passageways in the crankshaft 20 in order to 
pre heat or pre cool the shape memory elements, thereby more fully 
utilizing the thermal energy of the spent fluids. In order to reduce the 
thermal losses between the hot and cold fluids, the passageways 26 and 27 
would generally be insulated. 
The shape memory elements 30 shown in FIG. 1 are connected to the 
crankshaft 20 in an inline arrangement. However, to maximize the energy 
density of the engine, a plurality of shape memory elements 30 would 
generally be connected radially to each bearing 35. That is, the 
configuration at each crankpin 22 would resemble the spokes of a wagon 
wheel except that the spokes would be coiled shape memory element helical 
springs. As the entrance or bore 37 of each shape memory element 30 passes 
over each opening 28 and 29 in the crankpin 22, hot or cold fluids are 
injected into the interior of said elements 30. Effectively, the 
crankshaft 20 functions as a valve to meter the flow of the hot and cold 
fluids to the bore 37 of the elements 30 without the mechanical complexity 
of providing typical fluid valves, such as solenoid valves, at each 
entrance of each said shape memory elements. However, where only one 
coiled shape memory element is used to preform work, a mechanical fluid 
valve may be used effectively. 
As shown in FIGS. 3 and 4, slots 58 may be cut axially into the surface of 
the crankpin 22 between the fluid ports. The slots 58 allow the bore 37 of 
the shape memory element 30 to be exposed, following hot or cold fluid 
injection, to the surrounding atmosphere, thereby allowing ambient or 
pressurized air to replace the fluid which may thus be drained or forced 
from the element 30. This feature minimizes instantaneous temperature 
gradients across the length of the shape memory element 30 which might 
otherwise encourage overstress or overstrain. The function of the slots 58 
may be explained by the following example. If a thin elongated vertical 
tube is filled with water and a thumb is placed firmly over the top exit, 
the water will not flow out the lower exit. However, if the thumb is 
lifted, the water will then start flowing downward, but if the thumb is 
rapidly replaced, the water will stop flowing in the tube due to a vacuum 
pressure being induced behind the water. Similarily, when hot or cold 
fluids are injected through the shape memory element 30, said fluid motion 
will come to a halt when the fluid injection is stopped unless air is 
allowed to follow the injected fluid, usually hot or cold water. Because 
it is desirable to have the hot fluid to pass through the entire tube 30 
length before the cold fluid is injected (or vice verse) in order to 
maximize the energy extraction within said fluids, means of allowing air 
to follow said fluids is therefore preferred. 
In order for the injected liquids to flow through the tubing 30 without 
stopping, the slots 58 should be located on the crankpin 22 immediately 
behind (rotationally) the opening 28 and 29 in said crankpin. This allows 
the air to flow through the slots 58 and into the bore 37 immediately 
after the hot or cold fluid is injected. An alternate means of providing 
air or other gaseous fluid injection following the hot and cold liquid 
fluid injection would be to use additional passageways in the crankshaft 
20 to carry the air or pressurized air. Another alternate means of 
providing pressurized air to the bore 37 would be to enclose the instant 
invention, except for the tube 30 exit, within a container and then 
pressurize said container. Therefore pressurized air would be available to 
enter the slot 58 and will then push the hot and cold fluid through the 
shape memory element tubing 30. 
Now referring to FIG. 5, a refinement may be shown which discourages 
instantaneous temperature gradients. A cross-section of a shape memory 
element 30 may be seen to include a coating 38 of a high heat conductor 
material interposed between the bore 37 and the wall 36 of the element 30 
such that during fluid injection, fluid distribution problems will be 
discouraged from causing localized temperature variations within the shape 
memory element 30. A variety of additional or alternative methods may be 
employed to minimize fluid distribution problems and maximize heat 
transfer rates such as increasing the roughness of the inner tubular 
surface, and introducing the hot and cold fluids with a swirling motion 
such as by the use of vanes at the entrance to the shape memory elements. 
Now referring to FIGS. 1, 6, and 7, the operation of the load limiting 
member 40 may be shown. In FIGS. 1, 6, and 7 the load limiting member 40 
is thus coupled in series with the respective shape memory element 30. The 
load limiting member 40 shown in FIG. 1 incorporates means for providing a 
pressure differential across the piston 42. A maxinum stress may thus be 
provided for each shape memory element 30, additional stress above the 
maximum being avoided by displacement of the piston 42. The piston volume 
74 on the side nearest the shape memory element 30 is pressurized 
generally with a gaseous fluid through the opening 72. A gaseous fluid is 
preferred as it has low mass and may be readily moved in and out of the 
volume 74 due to translation of the piston 42. The volume 76 on the 
opposite side of the piston 42 is vented to the atmosphere through the 
opening 78. During the cold fluid injection phase or low stress capability 
phase of the shape memory element 30, the piston 42 will generally rest 
against the piston wall 79, and then lift off from the wall 79 during the 
high stress or high temperature phase. The pressure difference between the 
volumes 74 and 76 will then limit the maximum loading on the shape memory 
element 30. By allowing the gaseous fluid to move in and out of the volume 
74 through the opening 72, nearly constant loading will occur on the shape 
memory element 30 when said element is above its transition temperature. 
This approximate constant loading on the element 30 during the high 
element 30 temperature and high stress phase will then result in 
maximizing the energy output of said elements. Generally the opening 72 
from each of the piston load limiters 40 would be connected to a common 
pressure vessel, not shown. Alternate to having an opening 72, the volume 
74 may be made adequately large such that the volume change due to piston 
42 translation and therefore the pressure change within said volume 74 is 
sufficiently small such that the shape memory element 30 load limit is not 
exceeded. 
The usefulness for a load limiter occurs due to the desire to prevent the 
over stressing of the shape memory elements, while at the same time 
maximizing the energy output of said element. The possibility of over 
stressing the shape memory elements occurs due to the large difference in 
the stress-strain characteristics of said elements when in the hot and 
cold phases. When cold, the elements can be safely strained about 3 to 8 
percent of its length. However when hot, the element strain limit is less 
than one percent. 
Therefore, if the cold element 30 in FIG. 1 is stretched out at the top of 
the crankshaft cycle and then the element 30 is exposed to a hot fluid 
while at the same time said element is restrained at its outboard 
attachment, the hot element 30 will be strained beyond its capability and 
will either break or yield. 
Now to further describe the engine cycle shown in FIG. 1. Initially the 
piston 42 rests against the wall 79 with the element 30 in a cold state 
and the crankpin 22 is nearest the load limiter 40. As the crankshaft 20 
rotates, the cold element 30 is stretched out while the piston 42 remains 
against the wall 79. At the top of the cycle, the element 30 is injected 
with a hot fluid and the piston lifts off from the wall 79. The piston 42 
translation prevent the over stressing of the element 30. As the crankpin 
22 continues its rotation from the top of the cycle to near the bottom of 
the cycle, the piston 42 and element 30 moves towards the wall 79. Near 
the bottom of the cycle, the element is injected or exposed to the cold 
fluid. If the piston 42 is not already in recontact with the wall 79, then 
the pressure differential across the piston 42 will cause a small 
translation such that piston recontact with said wall will occur. To 
prevent the hard contact between the piston 42 and the wall 79, a shock 
absorber, not shown, may be used. 
Note that the wall 79 will prevent over straining the element 30 by 
limiting the outward translation of the piston 42. The crankpin 22 offset 
is selected such that the element strain is maintained within the 3 to 8 
element limit during the stretching when the element 30 is in its cold 
phase. Also note that to allow the hot and cold fluids to flow completely 
through the element 30, an appropriate hole may be required in the piston, 
or alternately, an opening may be provided near where the element attaches 
to the load limiter. A further note is that since the element may creep 
with time, the piston position should be adjustable. 
The spring 43 shown in FIG. 6 and the weight shown in FIG. 7 provide 
similar, alternative methods of constructing load limiting members 40, the 
spring 43 and the weight 44 each providing design simplicity but possibly 
sacrificing functional flexibility or efficiency. 
As previously noted, both the hot and cold fluids are preferably injected 
through the interior 37 of the tubular element 30. However, the element 30 
may also be heated or cooled externally. For example, if a hot fluid is 
injected internally, the tubular surface may then be cooled externally if 
the external air temperature is below the element transition temperature 
provided the engine rotation is sufficiently low so as to allow adequate 
time for said element to reject its thermal energy to the external 
atmosphere. Since the heat transfer coefficient of air is relatively 
small, an external fan may be used to increase the external heat transfer 
coefficient, said fan preferably rotating with the crankshaft. 
A cross-section of a tubular shape memory element 80 configuration with a 
plurality of longitudinal internal passageways is shown in FIG. 10. The 
tube 80 includes an outer shape memory element ring 82, inner shape memory 
element rings 83 and 84, shape memory element webbing 84 or fins, and 
fluid passageways 88. Hot and cold fluids are injected through the fluid 
passageways 88. This configuration with two or more shape memory element 
rings include several advantages such as (1) significant increase in the 
shape memory element material per unit tube cross-sectional area and 
therefore greater power output from the engine, (2) greater internal heat 
transfer coefficient, (3) significant increase in tube stiffness thereby 
reducing any tendancy for the tube to buckle, and (4) more even fluid 
distribution within the coiled tube. The fluid injection may be through 
all of the passageways 88 or only through the annulus between the rings. 
The tubing 80 may be constructed as a single piece of tubing, as shown, or 
using multiple rings with fins, said rings being slipped over each other. 
A similar product using copper or stainless steel is manufactured by 
Noranda Metal Industries, Inc. of Newtown, Conn. 
Referring now to FIGS. 8 and 9, an alternate embodiment may be seen wherein 
the helical coil shape memory elements described previously are replaced 
by a hollow torsional shape memory element 60. The hollow torsional 
element 60 is connected in series with a spring-type load limiting member 
43, in turn connected to a bearing 35 rotatably engaging a crankpin 22. 
The rotational center of a crankshaft 20 is shown by a journal 21 in FIG. 
9 and a center mark 21' in FIG. 8. 
The principle of operation of the hollow torsional shape memory element 60 
is similar to that previously discussed. Cold fluid is injected into an 
inner bore 61 through a fluid passage 62 prior to the torsional element 60 
being strained by rotation of the crankshaft 20. Upon reaching maximum 
rotation and, consequently, maximum strain, the hot fluid is injected into 
the bore 61 of the torsional element 60, urging it to return to its 
original shape while causing it to exert a force, translatable into useful 
work, on the rotating crankshaft 20, While only hollow helical coil and 
hollow torsional elements are illustrated, a variety of other spring 
configurations may be utilized such as hollow plate springs, hollow leaf 
springs and hollow spiral springs, with power strokes in either the 
compression or tension portion of the applicable cycle. 
Now referring to the coiled element 30 shown in FIG. 1. When the hot fluid 
is injected into the bore 37 of the element 30, the portion of the element 
closest to the crankshaft will be heated before the element material near 
the load limiter 40. This is due to the finite time for the fluid to 
traverse through the element 30. This will cause the element coils nearest 
the crankshaft to contract before the downstream coils and can result in 
these downstream coils being over-strained before they can be heated. 
One means of preventing this over-straining is to use a tubular, flexible 
shroud into which the coiled element 30 is imbeded. The shroud thereby 
allows the contraction of the element 30 coils while limiting the 
expansion of the coils. That is, the shroud interconnects the element 
coils thereby limiting the maximum strain on the said coils. 
While the above description contains many specificities, these should not 
be construed as limitation on the scope of the invention, but rather as an 
exemplification of one preferred embodiment thereof. Many other variations 
are possible, for example springs may be placed within the piston 42 
chamber or flow restrictors may be placed in the orficies 72 and 78 in 
order to modify the stress-strain characteristics of the piston load 
limiter. Another variation is to use the piston 42 to pump a fluid. 
Accordingly, the scope of the invention should be determined not by the 
embodiment illustrated, but by the appended claims and their legal 
equivalents.