Abstract:
A heat-driven engine includes a thermally conductive path into the engine, from a heat source and a working medium of a thermostrictive material, having a first temperature of transformation, positioned adjacent to the thermally conductive path. Also, a heat pump of phase change material is positioned adjacent to the working medium and an actuator is controlled to apply stimulus to the heat pump, causing a phase change and an associated release of thermal energy, to drive the working medium above its low-to-high temperature of transformation and controlled to alternatingly remove the stimulus from the heat pump, causing the phase change to reverse, and an associated intake of thermal energy, to drive the working medium below its high-to-low temperature of transformation. Also, heat flow through the thermally conductive path maintains the working medium at a temperature range permitting the heat pump to drive the working medium temperature, in the manner noted.

Description:
BACKGROUND 
       [0001]    Many ideas for improvements in the world&#39;s energy usage focus on increasing the efficiency of existing types of engines. Most heat engines are limited in their efficiency by the theoretical efficiency of the Carnot cycle, which requires an increase in operating temperature in order to increase operating efficiency. A typical application for a heat engine is to generate electricity by boiling water to create superheated steam and using the expansion of the steam to drive a turbine attached to a generator. This works very well if two temperature reservoirs can be created with a large temperature difference between them to facilitate a large expansion ratio of the superheated steam as it cools. Other gaseous working mediums having different specific heats and boiling points may be used, but in all cases the maximum efficiency of the heat engine is defined by the increase in temperature which can be achieved in the heat source over the temperature of the heat sink. 
         [0002]    If, however, one wishes to harvest a source of thermal energy with a low temperature relative to any available cooling reservoir, then low efficiencies and low power output must be accepted when using currently available heat engine technologies. Accordingly, additional methods of harvesting energy from relatively low temperature sources of thermal energy are desirable. 
         [0003]    Some heat engines using phase change materials, such as Nickel-Titanium alloys known as nitinol, have been designed in which the engine efficiency does not depend on the difference in temperature between the heat source and the heat sink. These engines are theoretically capable of utilizing relatively low-temperature sources of heat. These engines, however, tend to be rather inefficient and do not take advantage of the full phase change expansion that nitinol undergoes. Many of the existing designs do not fully insulate the heat source from the heat sink and therefore do not efficiently use the available heat. Accordingly, there is a need for a more efficient engine that utilizes a phase change material. 
       SUMMARY 
       [0004]    The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements. 
         [0005]    In a first separate aspect, the present invention may take the form of a heat-driven engine that includes a thermally conductive path into the engine, from a heat source and a working medium of a working medium phase change material, having a low-to-high temperature of transformation and a high-to-low temperature of transformation, positioned adjacent to the thermally conductive path. Also, a heat pump of phase change material is positioned adjacent to the working medium and an actuator is controlled to apply stimulus to the heat pump, causing a phase change and an associated release of thermal energy, to drive the working medium above its low-to-high temperature of transformation and controlled to alternatingly remove the stimulus from the heat pump, causing the phase change to reverse, and an associated intake of thermal energy, to drive the working medium below its high-to-low temperature of transformation. Also, heat flow through the thermally conductive path maintains the working medium at a temperature range that permits the heat pump to drive the working medium temperature, in the manner noted previously. 
         [0006]    In a second separate aspect, the present invention may take the form of a method of operating a heat-driven engine that utilizes a heat spreader, to permit a heat path into the engine, from a heat source, a working medium of phase change material, having a low-to-high temperature of transformation and high-to-low temperature of transformation, positioned adjacent to the thermally conductive path and a heat pump of phase change material positioned adjacent to the working medium. A stimulus is applied to the heat pump, causing a phase change and an associated release of thermal energy, to drive the working medium above its low-to-high temperature of transformation. Alternately the stimulus is removed from the heat pump, causing a reverse phase change and an associated intake of thermal energy to drive the working medium below its high-to-low temperature of transformation. Also, heat flow is permitted through the thermally conductive path to maintain the working medium at a temperature range that permits the heat pump to drive the working medium temperature above and below its temperature triggers. 
         [0007]    In a third separate aspect, the present invention may take the form of a heat-driven engine that includes a thermally conductive path into the engine, from a heat source; a working medium of phase change material, having a low-to-high temperature of transformation and a high-to-low temperature of transformation, positioned adjacent to the thermally conductive path; a heat pump of phase change material positioned adjacent to the working medium. A stimulus is applied to the heat pump, causing a phase change and the associated release of thermal energy, to drive the working medium above its low-to-high temperature of transformation and then the stimulus is removed from the heat pump causing the phase change to reverse, along with an associated intake of thermal energy, to drive the working medium below it high-to-low temperature of transformation. Further causing heat flow through the thermally conductive path and maintaining the working medium at a temperature range that permits the heat pump to so drive the working medium temperature. Also, the thermally conductive path includes a heat flow constricting element, to avoid heat flow that does not conform to desired characteristics. 
         [0008]    In a fourth separate aspect, the present invention may take the form of a cam assembly, for translating rotary movement of a first cycle type and producing from it linear movement having a second cycle type. The assembly includes a slider plate, supported by a pair of linear bushings and defining an aperture having a non-round shape; a first shaft being driven rotationally through movement of the first cycle type; and a cam-following projection joined to the first shaft by a crank that is fit into the aperture and follows the outline of the aperture as the first shaft moves through the first cycle type, causing the slider plate to move through its second cycle type. 
         [0009]    In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view of a proof-of-concept heat engine, according to the present invention. 
           [0011]      FIG. 2  is a perspective view of the interior elements of the heat engine of  FIG. 1   
           [0012]      FIG. 3  is a sectional view of the shaft of the interior elements of  FIG. 2 , taken at line  3 - 3  of  FIG. 2 . 
           [0013]      FIG. 4  is a schematic side view of a heat engine, according to the present invention. 
           [0014]      FIG. 4A  is a schematic view of a heat engine that is similar to that of  FIG. 4 , except for that shape memory alloy elements have been replaced by cylinders containing phase change materials. 
           [0015]      FIG. 5  shows a sectional view of the engine of  FIG. 4 , taken along line  5 - 5  of  FIG. 4 . 
           [0016]      FIG. 5A  is a sectional view of the engine of  FIG. 4 , taken from the same perspective as  FIG. 5 , showing a heat throttle mechanism that could be used with any of the embodiments. 
           [0017]      FIG. 6  is a schematic side view of an alternative embodiment of a heat engine according to the present invention. 
           [0018]      FIG. 7  is a sectional view of the engine of  FIG. 6 , taken along line  7 - 7 . 
           [0019]      FIG. 8  is a schematic side view of an additional alternative embodiment of a heat engine, according to the present invention. 
           [0020]      FIG. 8A  is a schematic side view of a heat engine, similar to that of  FIG. 8 , but wherein the shape memory alloy starter has been replaced by a starter of a differing form. 
           [0021]      FIG. 9  is a sectional view of the engine of  FIG. 8 , taken along line  9 - 9  of  FIG. 8 . 
           [0022]      FIG. 10  is a sectional view of a variant of the engine of  FIG. 8 , taken along line  9 - 9  of  FIG. 8 . 
           [0023]      FIG. 11  is a sectional view of the engine of  FIG. 4 , taken from the same perspective as  FIG. 5 , showing a heat reservoir that could be used with any of the embodiments. 
           [0024]      FIG. 12  is a schematic side view of the engine of  FIG. 4 , showing a cam mechanism that could be used with any of the embodiments to alter the timing of the motion created by the engine, thereby making this motion useful as an engine input. 
           [0025]      FIG. 13  is a top view of a pair of heat engines, joined together to achieve improved performance. 
           [0026]      FIG. 14  is a graph of temperature and heat flow between the major elements of the present invention. 
       
    
    
       [0027]    Exemplary embodiments are illustrated in referenced drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
       DETAILED DESCRIPTION 
       [0028]    Referring to  FIG. 1 , a proof-of-concept preferred embodiment of a heat engine  10  includes a working mechanism  12 , which is rigidly attached at either end to a set of four support columns  14 . Referring to  FIGS. 2 and 3 , a central shaft  16 , includes thermally insulating portions  18 , thermally conductive heat spreader  20  and thermally insulating core  22 . Between conductive heat spreader  20  and insulator  22  four pairs of a shape memory alloy (SMA) working medium  30  and an SMA heat pump  32  are arranged. Working medium  30  moves moveable clamp  28  and heat pump  32  is held by a stationary clamp  26 . By applying heat to conductive portion  20 , the working mediums  30  expand and contract, causing a magnetically permeable core (not shown) to move back and forth within a solenoid  24 , mounted on solenoid mounting plate  25 , to generate electricity. This movement is explained below, with reference to  FIGS. 4 ,  5  and  14 . 
         [0029]    A preferred embodiment of a heat engine  110  uses a working medium  30 , made of nitinol, to convert heat energy into kinetic energy. An adjacent heat pump  32 , also made of nitinol, is kept above its temperature of transformation (lower than that of the working medium  30 ). Stationary clamps  34  keep working medium  30  and heat pump  32  fixed in position at one end, whereas moveable clamps  36  permit motion on the other end. A heat spreader  44 , is driven by heat source  46  ( FIG. 5 ) and periodically warms working medium  30 , as described further below. Thermal insulation  47  prevents heat from escaping into the environment. Referring to  FIG. 14 , at time TO, both working medium temperature (curve  52 ) and heat pump temperature (curve  54 ) are below the working medium low-to-high trigger temperature. The heat pump  32  is then stressed by actuator  40 , forcing heat pump  32  into its pliable, Martensite, low-enthalpy state, and releasing heat energy (shown by a heat pump-to-working medium heat flow shaded area  56 ) that had been stored in the crystalline structure of the nitinol. Heat pump  32  heats up, and heat flow from heat pump  32  to working medium  30  begins. This causes the temperature of the working medium  30  to pass the low-to-high trigger  50 , causing the working medium  30  to change phases into its shorter, Austenite, high-enthalpy state. As the working medium  30  contracts, it exerts mechanical force that is harvested by kinetic energy harvesting mechanism  42 . 
         [0030]    While the transition of working medium  30  to its shortened, high-enthalpy state progresses, the temperature of the heat pump  32  and working medium  30  continue to rise, until the heat pump  32  transition to its low enthalpy state is largely complete. The transition of working medium  30  to its high-enthalpy state, continues (this transition began after the start of the transition of heat pump  32 ), causing medium  30  to absorb its latent heat of transformation, which is stored in its high-enthalpy crystalline structure. This phenomenon begins to cool down both working medium  30  and heat pump  32 . 
         [0031]    While heat pump  32  is in its low-enthalpy Martensite state, it has the physical characteristic of a spring in tension, exerting force on actuator  40 . At time T 3 , to push the temperature of working medium  30  below its high-to-low enthalpy trigger  62 , the actuator  40  of heat pump  32 , permits itself to be pulled by heat pump  32 , thereby permitting heat pump  32  to transition to its shorter, Austenite, high-enthalpy state, absorbing its latent heat of transformation, thereby and causing its temperature to plunge and drawing heat from working medium  30 . Energy may be harvested from the heat pump  32  at this time, compensating in part for the expenditure of kinetic energy at time T 0 . Soon, the temperature of working medium  30  falls below the working medium high-to-low trigger  62 , which causes working medium  30  to undergo the phase change to its pliable, Martensite, low-enthalpy state. Mechanism  42  exerts a relatively small force on working medium  30 , causing it to elongate and resetting it for the purpose of creating productive work against mechanism  42  during the next cycle. While the temperature of working medium  30  is below the temperature of the heat spreader  44 , medium  30  is warmed by the heat spreader  44  (shown in  FIG. 14  by a heat source-to-working medium heat flow shaded area  64 ), and in turn heats heat pump  32 . Moreover, as working medium  30  transitions to its pliable, Martensite, low-enthalpy phase, it releases its latent heat of transformation (less the thermal energy which has been converted to mechanical work), also contributing to the warming of medium  30  and heat pump  32 . This leads the heat and transformation cycle back to the starting point, at which point heat pump  32  is pulled, beginning the cycle over again. 
         [0032]    Referring to  FIG. 4A , In an alternative preferred, either  30  or the heat pump  32  can comprise an expandable cylinder that is filled with a phase-change material that undergoes a phase change, from liquid-to-gas, from liquid-to-solid or from solid-to-gas. In all of these cases (with the exception of water) the phase change material is compressed or cooled to cause phase-change to a lower volume phase, and is expanded or heated to cause phase change to a higher volume phase. 
         [0033]    More specifically, in the case of a working medium  30 , incorporating a liquid-to-vapor phase change material, the cycle begins with the working medium  30  in its liquid phase, at a relatively low pressure and a temperature at the operating temperature of the engine  110 . Heat energy from heat spreader  44  together with heat from actuating the heat pump  32 , causes a rapid vaporization (a flash boil), which causes the working medium cylinder to expand, thereby doing work against mechanism  42 . This expansion causes the temperature of working medium  30  to fall and heat absorption from a de-actuated heat pump causes the temperature of medium  30  to fall below the condensation point, causing contraction of cylinder  30 , and bringing the mechanism back to the beginning of the cycle. If heat pump  32  is also a liquid-to-vapor expandable cylinder, it is actuated by a sudden contraction, causing a rapid expression of heat, and heat pump  32  is de-actuated by a sudden expansion, causing its temperature to drop and causing it to absorb heat from working medium  30 . 
         [0034]    Skilled persons will recognize that this same principal of operation could be used with an expandable cylinder filled with a material that expands during a solid-to-liquid transformation or a solid-to-gas transformation for either working medium  30  or heat pump  32  or both. If water, or some other material that contracts when transforming from solid-to-liquid is used, the mechanism is constructed to accommodate this difference. 
         [0035]    Referring to  FIG. 5 , a heat source  46  sends heat into heat spreader  44 , but is partially isolated from working medium  30 , by area of lower total thermal conductivity  48 , to limit any back flow of heat from working medium  30  into heat source  46 , when working medium  30  is at its hottest, and to limit the heat flow from the heat source  46  to no more than that which can be converted to mechanical energy. Referring to  FIG. 5A , other devices, such as a mechanical shutter  150 , adapted to decrease heat flow in a partially closed or closed position, could also be used to limit the heat flow to and from the heat source  46 . A sensing and control assembly reads the temperature in the heat spreader  44  and places shutter  150  in a further closed position if the heat spreader is too hot. In an alternative preferred embodiment, element  150  schematically represents a variable-conductance heat pipe, controlled responsively to the temperature of heat spreader  44 , heat source  46 , and to the requirement for output of mechanical energy or the actual contemporaneous output of mechanical energy. Additionally, in one preferred embodiment, more than one heat source is connected, each by way of a separate variable-conductivity heat pipe, to heat spreader  44 . In an example of such an embodiment a first heat source is a solar collector and a second heat source is a backup fuel-burning heat source. When the fuel-burning heat source is in use, the variable-conductivity heat pipe to the solar collector can be set to lowest conductivity, to prevent the heat from the fuel-burning heat source from flowing to the solar collector. In yet another alternative, a thermal diode is used to prevent heat flow from heat spreader  44  back to a heat source  46 , or in the case of multiple heat sources, from one heat source to another. Referring to  FIG. 6 , in an alternative embodiment of a heat engine  210 , working medium  130  is warmed and cooled by a heat pump  132  that is magneto-caloric, and is caused to change phase by the application of a magnetic field, by electromagnet  140 , serving as the actuator for heat pump  132  and separated from pump  132  by insulator  136 . Alternatively, heat pump  132  is pyro-electric, with electric field generator  140  actuating it by creating an electric field, and insulator  136  again providing physical separation. Element  142  both harvests kinetic energy from working medium  130  and powers and controls field generator  140 .  FIG. 7  shows heat source  146 , area of lower total thermal conductivity  148  and heat spreader  144 , which perform in similar manner to the like elements of  FIG. 5 . 
         [0036]    Referring to  FIG. 8 , an embodiment of a heat engine  310  is shown which, unlike the embodiment of  FIG. 5 , includes a starter element  250 , intended for use in designs where the working medium  30  and heat pump  32  may increase in temperature above the desired operating temperature during time periods when the heat engine  310  is not operating, resulting in more difficult heat engine starting. The starter element thereby relieves the design requirement that the heat pump  32  be sized adequately to start the engine cycle by absorbing the extra heat required to lower the system temperature to less than the low-enthalpy to high-enthalpy transition temperature of the working medium  30 . Starter  250 , typically also made of nitinol shape memory alloy, is kept in tension in its low-enthalpy pliable state by the latch  252  during periods when the engine is not operating. When it is desired to start the engine cycle, element  250  is released by latch  252 , thereby transforming to its shortened high-enthalpy state, absorbing its latent heat of transformation and causing a drop in temperature in itself, working medium  30  and heat pump  32 . This causes working medium  30  to undergo a transformation from its high-enthalpy, shortened state, to its low-enthalpy, pliable state. In this embodiment, working medium  30  and heat pump  32  are linked by a power transmitting element, as will be explained further below in reference to  FIG. 12 . The forces applied to working medium  30  and heat pump  32  by actuators  40  and  42  are further controlled arbitrarily by a timing device, one example of which will be explained further below in reference to  FIG. 12 . Accordingly, when working medium  30  becomes pliable and is caused to elongate, this causes, after a delay, heat pump  32  to be pulled into its elongated, low-enthalpy state, causing a release of heat. The engine cycle is now in a state approximately equal to T 1  in  FIG. 14 , and the cycle is now self-perpetuating with heat flowing through starting element  250  to working medium  30 , as shown in  FIG. 9 .  FIG. 10  shows, however, that the starting element  250 , working medium  30  and heat pump  32  can be arranged in any one of a number of different configurations, some of which avoid the necessity of heat flowing through starting element  250 , to reach working medium  30 . 
         [0037]      FIG. 8A  depicts a heat engine  110 ′, similar to engine  110  of  FIG. 8 , but wherein starter element  250  can take any one of several different forms including one or more magneto-caloric elements, stimulated by a magnetic field producing actuator  252 ; one or more pyro-electric elements, stimulated by an electric-field producing actuator  252 ; an electro-mechanical heat pump, controlled by an actuator  252  in the form of an electrical switch; or a volume adjustable cylinder filled with a phase change material, such as water. Any cooling device which, when brought into thermal contact directly or indirectly, will absorb enough thermal energy to lower the temperature of the working medium down to a starting temperature can serve as a starting element  250 . In the volume-adjustable cylinder embodiment a starting element actuator  252  pulls on starter element cylinder  250 , causing a sudden drop in temperature of element  250 , which in turn causes a drop in temperature of working medium  30 , which starts heat engine  110 ′. Alternatively starting element  250  takes the form of a fluid passageway and actuator  252  is a cold-fluid blower, which creates a stream of cold fluid that cools working medium  30  and starts engine  110 ′. 
         [0038]    Referring to  FIG. 11 , in an instance in which a heat source, such as source  46  is inconstant, a thermal mass or other sort of heat stabilizer  370 , can be used to provide a constant-temperature heat source to engine  310  at a temperature close to the operating temperature of the engine  310 . In one embodiment stabilizer  370  includes a medium having a relatively large specific heat or a medium which undergoes a phase change (thus absorbing or releasing a relatively large latent heat) at a temperature near the operating temperature of the engine. Examples of these mediums could be cast iron or a large volume of water, having a large capacity to absorb heat. Further examples may include eutectic salts or organic chemicals having a phase change at approximately the operating temperature of engine  310 . Heat transfer into and out of the stabilizer  370  may be assisted by a heat exchange device, composed of, alternately, a solid finned structure, pipes through which liquid is pumped, or a series of evacuated heat pipes. 
         [0039]    Referring to  FIG. 12 , which shows an embodiment of a heat engine  410  in which working medium  30  is mechanically linked to heat pump  32 , in such a manner that after engine  410  is started heat pump  32  is mechanically driven by working medium  30  so that heat pump  32  is pulled, causing elongation and a release of heat, a fixed delay after working medium  30  has reached its maximum length and heat pump  32  is permitted to shorten, causing absorption of heat, a fixed delay after working medium  30  has reached its shortest length. 
         [0040]    Working medium  30  drives a shaft  440  hinged at the top and connected by hinge  444  to both a clockwise rotating flywheel and a counterweight  446  that supports a cam follower  448 . Cam follower  448  is constrained in its movement by cam aperture  450 , which is defined by a slider plate  452 , supported and permitted to slide by linear bushings  454 . A driving shaft  456 , which is driven by plate  452  alternatingly pulls and pushes heat pump  32 . 
         [0041]      FIG. 12  shows heat engine  410  at time T 3  in  FIG. 14 , with working medium  30  in its shortened high-enthalpy Austenite phase and heat engine  32  constrained to be in its lengthened, low enthalpy Martensite phase by the pressure of the cam follower  448  on the inside of cam aperture  450 . As the inertia of flywheel  440  causes it and the cam follower  448  to continue their clockwise rotation, cam follower  448  slips into a first notch  460  and the circular path of the cam follower  448  allows the slider plate  452  to move upwards under the influence of the force exerted on it through shaft  456  by heat pump  32 , which displays, at this phase in the cycle, characteristics similar to those of a spring held in tension. This causes the phase change and rapid cooling of heat pump  32 , which also cools the working medium  30  as shown on  FIG. 14  between T 3  and T 4 . Further constriction of heat pump  32 , allowed by working medium  30  crossing its high-to-low trigger temperature  62  and undergoing the phase change to its low-enthalpy, pliable state, causes cam follower  448  to follow the upper curve of cam aperture  450  until it enters second notch  462 . This is approximately time T 0  on  FIG. 14 . At this point the inertia of the flywheel  442  causes cam follower  448  to enter second notch  462  and exert force downward on slider plate  452 , causing the elongation of heat pump  32  and the release of its latent heat of transformation; thus driving the temperature of working medium  30  above its low-to-high trigger  50  and causing working medium  30  to undergo a phase change to its constricted, high-enthalpy state and the cycle to begin anew. In a preferred embodiment, over a complete cycle, the thermal energy converted to kinetic energy by the working medium  30 , is greater than the net kinetic energy input into the heat pump  32 , thereby creating a self-sustaining cycle. 
         [0042]      FIG. 13  shows two heat engines  410  mechanically linked together and with their cycles arranged out of phase so that each one complements the other. Further, there may be more heat engines  410  mechanically linked in like manner, arranged with their cycles at various phase relationships to each other. 
         [0043]    There are two criteria which are critical for choosing a nickel-titanium (nitinol) alloy for use as the shape memory allow material. The first is the relationship of the Austenite start temperature to the operating temperature of the engine. The Austenite start temperature (As) is the temperature at which the phase change in the nickel-titanium crystalline structure from Martensite (the low-enthalpy state) to Austenite (the high-enthalpy state) begins to take place. 
         [0044]    Nitinol is an alloy of nickel and titanium with approximately 50% Nickel and 50% Titanium by atomic count. The As of a nitinol can be reduced by increasing the ratio of nickel to titanium, and increased by reducing the ratio of nickel to titanium. As can be further affected by the heat treatment applied to the alloy during fabrication. Increasing the aging time and temperature of the heat treatment depletes nickel from the Ni—Ti lattice, thus increasing As. Using these methods, the Austenite start for the working medium  30  and heat pump  32  can be set to a temperature which allows for the operation of the heat engine. 
         [0045]    The average mechanical work output by the working medium  30  is determined by the average rate of heat flow from the heat spreader  44  into the working medium  30 . This rate of heat flow is determined by the difference between the temperature of the heat source and As of the Working Medium. If the operating temperature of the heat source is known, the As of the working medium  30  (and the proportion of Ni to Ti) can be specified so as to balance the heat input to the working medium  30  with the mechanical work output (plus inefficiencies). 
         [0046]    The As of the heat pump  32  is specified in a different fashion; the heat pump  32  alloy is superelastic, undergoing the stress-induced transformation rather than a temperature-induced transformation. In order to satisfy this condition, the Heat Pump alloy must have an As lower than any temperature it will be subjected to during the operation of the heat engine, such that it will always be in the high-enthalpy state unless subjected to enough stress to cause it to transition to the low-enthalpy state. 
         [0047]    The second criteria by which the nickel-titanium alloy should be chosen is the hysteresis temperature. The hysteresis temperature is defined as the difference between the Austenite start temperature and the Martensite start temperature (Ms). The Martensite start temperature is the temperature at which the temperature-induced transformation of the Nitinol crystalline structure from Austenite to Martensite begins to occur. 
         [0048]    The hysteresis temperature of the working medium  30  should be as small as is practical, because the thermal energy required to raise or lower the temperature of the Working Medium by the hysteresis temperature is wasted heat energy, which must be absorbed and released by the heat pump  32  in order to cause the working medium  30  to change phase. A larger hysteresis temperature requires a larger heat pump  32 , and increased inefficiencies in the total system. 
         [0049]    One method of reducing the hysteresis temperature of nitinol is to add a small amount of a third element to the alloy; often this third element is copper. 
         [0050]    The present device and method provide broader applicability for a process of converting thermal energy to mechanical energy by eliminating the requirement for a “cold reservoir”. The elimination of the step of cold reservoir cooling of a working medium also yields a significant increase in efficiency. Thermal energy from the heat source  46  is converted directly into mechanical energy by working medium  30  during its phase change from the low-enthalpy state to the high-enthalpy state. That portion of the latent heat of transformation of working medium  30  which is not so converted is released by working medium  30  during its phase change from the high-enthalpy state to the low-enthalpy state, and is absorbed by heat pump  32  during its phase change from the low-enthalpy state to the high-enthalpy state. The thermal energy thus absorbed by heat pump  32  is released again and flows back to working medium  30  during its phase change from the low-enthalpy state to the high-enthalpy state. Disregarding system inefficiencies such as heat loss to the environment through insulated or non-insulated portions of the device, all thermal energy that might otherwise be “waste heat” is thus recycled into working medium  30  in the course of one cycle, making a “cold reservoir” to receive waste heat unnecessary. As long as the thermal energy flowing from the heat source into the working medium  30  can be limited to the quantity that is converted to mechanical energy while the engine is operating plus losses due to system inefficiencies, the device will continue to operate as designed. The conversion of thermal energy to mechanical energy occurs during a phase change in which the crystalline structure of the working medium  30  changes from a low-entropy state to a high-entropy state, and the mechanical energy acts on the environment as work. The thermal energy so converted is no longer contained within the working medium  30 , and is therefore not absorbed by heat pump  32 . Thus over the course of a cycle of operation, entropy increases. No claim is made that would violate the Clausius Inequality by any of the embodiments of a heat engine according to the present invention. 
         [0051]    While a number of exemplary aspects and embodiments have been discussed above, those possessed of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.