DESCRIPTION 
     The present invention relates to a process for operating a steam thermal engine and a device for operating a steam thermal engine. In particular, the invention relates to a steam engine, which can be operated with waste heat, especially from a burner or a combustion engine. 
     The DE 196 10 382 A1 discloses a steam engine, whose working medium is converted into superheated steam in an exhaust gas heat exchanger. The exhaust gas heat exchanger obtains its energy from the lost energy or from the waste heat of a combustion engine, which is coupled with the steam engine. To this end, the coolant and exhaust gas of the combustion engine pass through the exhaust gas heat exchanger. 
     Consequently, in this system an attempt is made to utilize the lost heat of a combustion engine in order to increase the total efficiency of the system. Under optimal operating conditions, one-third of the energy supplied in the form of fuel is converted into mechanical energy by a combustion engine, one-third is dissipated by way of the cooling water, and the other one-third is dissipated by way of the hot exhaust gas. 
     The use of the waste heat of a combustion engine is complex because there are two media as the transfer agents of the lost energy, which are the cooling water and the exhaust gas. The energy, present in both media, can be fed only inadequately to a single system for energy recovery or energy conversion, because their respective thermodynamic parameters are different. The temperature level of the cooling water is approximately 100° C. In contrast, the temperature of the exhaust gas is approximately 300° C. at operating points with a low load up to 900° C. at operating points with a high load. 
     If, for example, water is used as the medium in the circulation of the thermal engine, the result is a boiling temperature of 100° C. at a pressure of 1 bar. To realize a desired pressure level of 10 bar, however, a temperature of 180° C. is necessary. This eliminates the cooling water of the combustion engine as the energy supplier, because the cooling water temperature would have to reach a minimum of 200° C. to guarantee the requisite energy transfer by a heat exchanger. 
     Another problem is that the design of the entire system must be simple in order to achieve a low weight so that use in the automotive field is possible. 
     The present invention provides a solution to the problem by providing a thermal engine that enables a fast and efficient conversion of thermal energy, which is stored in media and has varying thermodynamic parameters, into mechanical energy. 
     According to the present invention, the thermal energy from the media with different thermodynamic parameters can be converted efficiently into a hot steam, which in turn can be converted efficiently into mechanical energy in the expansion unit, by taking the following steps: heating the working medium of a thermal engine in a boiler to a low temperature of preferably boiling temperature at low pressure; feeding steam from the boiler into a pressure vessel, in which the steam is heated to a high temperature; injecting the liquid working medium (or condensate) from the boiler into the pressure vessel, whereby the working medium is instantaneously evaporated, whereby the pressure in the pressure vessel increases rapidly; and by feeding the hot steam from the pressure vessel to an expansion unit. 
     Injecting the boiling working medium into the pressure vessel, which is heated with, for example, hot exhaust gas, and in which there is already a certain amount of hot steam, enables a spontaneous and enormously large pressure increase in the pressure vessel. The increase in pressure can be passed onto the expansion unit. 
     In this manner the boiler can be heated especially advantageously with the cooling water of a combustion engine and the pressure vessel can be heated with the exhaust gas stream of the combustion engine, whereby a high pressure level can be generated in the circulation of the thermal engine. Alternatively, the boiler is heated with the steam, which has already been expanded by the expansion unit, and the pressure vessel is heated by a burner. 
     In an especially advantageous design the steam from the boiler is pre-compressed and then fed to the pressure vessel, whereby both the pressure and the temperature of the steam in the pressure vessel increase before the liquid working medium is injected into the pressure vessel. In this manner the pressure level is also increased in the pressure vessel with the temperature increase. 
     In an especially advantageous design, not only the boiler is heated by the hot coolant of a combustion engine and/or by the steam, which has already been expanded in the expansion unit, and the pressure vessel is heated by the exhaust gas of the combustion engine, but also the exhaust gases are even used to heat the boiler, working on a low temperature level, after the pressure vessel has been heated up. Thus, the waste heat from a combustion engine can be used more completely for the expansion unit. 
     By coupling the shaft of a combustion engine, whose waste heat is recovered for the expansion unit, with the shaft of the expansion unit with a coupling and/or with a reduction gear, the expansion unit and the combustion engine can be joined together in a simple manner when adjusting the different speed levels or decoupled from each other. 
     In an especially advantageous design, at least two cylinders of a piston machine are provided with one allocated pressure vessel each as the expansion unit, whereby a reciprocating motion of the piston machine can be used to pre-compress the steam coming from the boiler; and a subsequent reciprocating motion of the piston can be used to expand the high pressure steam from the pressure vessel. In so doing, it is possible to allow the steam generating process in a pressure vessel and its expansion in one of the expansion units to take place alternatingly, for example, between the two cylinder units in such a manner that enough time remains for the respective steam conditioning in the respective pressure vessel. 
     Three preferred embodiments are described in detail below in the following with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of an inventive thermal engine, according to a first embodiment. 
     FIG. 2 is a schematic drawing, which shows the working cycle of the thermal engine, according to the first embodiment of FIG.  1 . 
     FIG. 3 a  is a sectional view of the pressure vessel, according to the embodiment of FIG.  1 . 
     FIG. 3 b  is a sectional view of the pressure vessel along the cutline A—A, according to FIG. 3 a.    
     FIG. 4 is a schematic drawing of an inventive thermal engine, according to a second embodiment. 
     FIG. 5 is a schematic drawing of an inventive thermal engine, according to a third embodiment. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The construction and the operating mode of a first preferred embodiment of the thermal engine are explained with the aid of FIGS. 1 to  3 . 
     According to FIG. 1, the thermal engine has an expansion unit  1  with a first cylinder  2  and a second cylinder  3 , in which a piston  4  and  5  (shown as a schematic drawing in FIG. 2) is arranged so as to move reciprocally. Superheated steam, which is generated in a steam generator and belongs to a working medium that is preferably water here, is expanded in the cylinders  1  and  2 , in order to deliver mechanical energy to a shaft (not illustrated) by way of the pistons  4  and  5  that move back and forth. 
     The steam generator has a boiler  6 , which is designed as a heat exchanger and in which the working medium is heated preferably to boiling temperature, preferably to approximately 80 to 130° C. at 1 to 3 bar. The steam generator further includes first and second pressure vessels  7 ,  8 , which are designed as a heat exchanger and in which superheated steam, that is the vaporous phase of the working medium, is generated at a temperature of preferably 300° C. to 600° C., at a pressure ranging from 6 to 18 bar. 
     The first and the second pressure vessel  7 ,  8  are assigned first and second valve units  9 ,  10  respectively. Each valve unit  9 ,  10  has first  9   a ,  10   a ; second  9   b ,  10   b ; third  9   c ,  10   c ; and fourth valves  9   d ,  10   d.    
     The pressure vessels  7 ,  8  are heated with hot exhaust gas and a burner  11 . The burner  11  is supplied, on the one hand, by an air line  12  and a fan  13 , with ambient air and/or oxygen and, on the other hand, by a fuel line  14  and a pump  15  with fuel (gasoline, diesel gas, etc.). After the exhaust gas from the burner has dissipated the bulk of its quantity of heat for producing steam in the pressure vessel, the exhaust gas arrives, after passing through the pressure vessels  7 ,  8 , by way of a line  16  into the boiler  6 , in order to heat the water, which is present there in the liquid and partially vaporous state, whereby the residual heat from the exhaust gas is further utilized. The expanded steam is fed by way of a valve block  17  to a condenser  18 , where the steam is further liquefied. The condensate is fed by a pump  19  to the boiler  6 . The valve block  17  can be switched in such a manner that the expanded steam, coming from the expansion unit  1 , is passed directly by the throttle  20 , bypassing the condenser  18 , into the boiler  6 . The valve block  17  controls the further passage of the expanded steam either to the condenser  18  or to the boiler  6  as a function of the temperature and/or the liquid state in the boiler  6 . 
     Boiling working medium from the boiler  6  is injected by way of a line  21  and a high pressure pump  22  and an injection nozzle  23  into the pressure vessel  7 ,  8 . 
     The first  9  and the second valve unit  10  is designed preferably as an integral part in the cylinder head (not illustrated) of the respectively assigned first and second cylinder  2 ,  3 . The four valves  9   a  to  9   d ,  10   a  to  10   d  of a valve unit  9 ,  10  are preferably conventional mushroom-shaped valves, as also used in a four valve combustion engine. However, any type of channel opening and closing unit, such as a rotary disk valve unit or sliding valve unit, can be used as the valves  9   a  to  9   d ,  10   a  to  10   d . The valve is controlled mechanically, for example, by a camshaft or electrically by controlled electromagnets or pneumatically or hydraulically by corresponding actuators or by any other appropriate actuating unit. 
     The first valve  9   a ,  10   a  of each valve unit  9 ,  10  is connected to the boiler  6  by a line  24 . The second valve  9   b ,  10   b  of each valve unit  9 ,  10  is connected to the valve block  17  or the condenser  18  and/or the boiler  6  by a line  25 . The third valve  9   c  of the first valve unit  9  is connected to the second pressure vessel  8  by a line  26 . A fourth valve  9   d  of the first valve unit  9  is connected to the first pressure vessel  7  by a line  27 . A third valve  10   c  of the second valve unit  10  is connected to the first pressure vessel  7  by a line  28 . A fourth valve  10   d  of the second valve unit  10  is connected to the second pressure vessel  8  by a line  29 . 
     The operating mode of the thermal engine with its two cylinders  2 ,  3  and its assigned pressure vessels  7 ,  8  is explained in detail below with reference to FIG.  2 . 
     First, the operating sequence of the first cylinder  2  is described: 
     Cycle 1: Take in steam by way of the first valve  9   a  of the first valve block  9  from the boiler  6  with the piston  4 , which travels starting from the upper dead center in the downward direction and belongs to the first cylinder  2 . In the case of the water vapor used in the embodiment, the typical thermodynamic parameters are t=100° C., p=2 bar. The first valve  9   a  is opened during a crank angle of approximately 180°. 
     Cycle 2: Compress the water vapor by the piston  4 , which travels starting from the bottom dead center toward the top and belongs to the first cylinder  2 . And withdraw the compressed steam by way of the fourth valve  9   d  of the first valve block  9  and the line  27  into the first pressure vessel  7 . The fourth valve  9   d  of the first valve block  9  is opened during the compression phase lasting at a crank angle of approximately 180°. 
     After closing the line  27 , heated liquid working medium (at a preferred temperature of t=100° C.) which is supplied from the boiler  6  by way of the line  21 , the high pressure pump  22  and the injection valve  23 , is injected into the first pressure vessel  7 . Owing to the high temperature, prevailing in the pressure vessel  7 , this water can be used instantaneously for evaporation, whereby the pressure in the pressure vessel  7  is significantly increased, typically to 6-18 bar. 
     Cycle 3: The third valve  9   c  of the first valve block  9  is opened and passes the superheated steam, which is already conditioned in the second pressure vessel  8  and ranges from typically 6 to 18 bar, into the first cylinder  2 , whose piston  4  is still located in the upper dead center, in order to expand the hot steam, located in the second pressure vessel  8 . The resulting mechanical work is delivered to the crankshaft (not illustrated). 
     Cycle 4: When the piston  4  of the first cylinder  2  arrives at the bottom dead center, the fourth valve  9   b  of the first valve block  9  is opened; and the expanded steam is fed to the boiler  6  either by way of the condenser  18  and the pump  19  or directly by way of the throttle  20  over the piston  4 , which moves again toward the top, over the line  25 , the valve block  17 . 
     These working cycles run analogously in the second cylinder  3 , whereby the first pressure vessel  7 , pre-compressed by the first cylinder  2 , is expanded by the second cylinder  3 . The two cylinders  2  and  3  work offset in time in such a manner, that following injection of the boiling water or the working medium into the respective pressure vessel  7 ,  8 , there exists the maximum time for conditioning the pressure. 
     Respectively while the first cylinder  2  takes in steam from the boiler, and feeds to the second pressure vessel  8  in the pre-compressed form, the second cylinder  3  expands the superheated steam from the first pressure vessel  7  and produces mechanical work and vice versa, so that the result is a dwell time of the steam in the pressure vessel  7 ,  8  of 180° crankshaft angle. The dwell time can be used to evaporate the liquid working medium, injected into the pressure vessel  7 ,  8 , from the boiler  6 . 
     FIGS. 3 a  and  3   b  show in detail the pressure vessel  7 , which has tubes  30 , which are arranged in an inner ring  31 , and other tubes  32 , which are arranged in an outer ring  33  around the inner ring  31 . The face side  34  of the inner ring  31  of tubes  30  is connected to the outer ring  33  of tubes  32  by an overflow chamber  35 . On the one face side  34  of the pressure vessel  7  there is centered an injection unit  36 , whose purpose is to inject liquid working medium and which injects the liquid working medium into the space  37 , surrounded by the inner ring  31 . The other face side  38  exhibits an exhaust gas inlet  44 , which is open in the direction to the tubes  30  of the inner ring  31 . On the same face side  38  there is a collecting chamber  39  around the end side of the tubes  32  of the outer ring  33 , said chamber being provided with an exhaust gas outlet  40 . The entire construction is enveloped by a pressure-proof jacket  41 , which is provided with a steam inlet  42  and a steam outlet  43 . 
     Hot gases flow over the exhaust gas inlet  44  into the tubes  30  of the inner ring  31  and flow to the opposite face side  34  into the tubes  32  of the outer ring  33  over the overflow chamber  35 , through the tubes  32  of the outer ring  33  and exit again by way of the collecting chamber  39  and the exhaust gas outlet  40  from the pressure vessel  7 ,  8 . Preferably pre-compressed steam is supplied at the steam inlet  42 . Liquid, but boiling working medium, for example water, is injected by the injection nozzle  23 . The injected working medium evaporates instantaneously due to the high temperature in the pressure vessel  7 ,  8 , and mixes with the pre-compressed steam. After a predetermined dwell period, which guarantees that steam was formed at the desired temperature and the desired pressure in an adequate quantity, the steam is fed by the steam outlet  43  to the expansion unit  1 . 
     FIG. 4 shows a second embodiment, which differs from the first embodiment, according to FIGS. 1 to  3 , only in that, instead of the burner  11 , there is a combustion engine  45 , which is coupled directly to the expansion unit  1 , or by way of a coupling (not illustrated) and/or a gear (not illustrated). The cooling water of the combustion engine  45  is fed to the boiler  6 , in order to heat it, whereby the hot exhaust gas of the combustion engine  45  is fed to the pressure vessels  7 ,  8 , in order to heat them to the desired high temperature. 
     FIG. 5 shows a third embodiment, which is a combination of the first embodiment, according to FIGS. 1 to  3 , and the second embodiment, according to FIG.  4 . That is, that the thermal engine, according to the third embodiment, has both a burner  11  and a combustion engine  45 . The burner  11  is switched on, as necessary. This is especially the case during the warm up period of the combustion engine  45 , or when maximum power is required. 
     In the second (FIG. 4) and third embodiment (FIG. 5) the combustion engine  45  can be turned off in the event of a low load, or if the thermal engine is used in a vehicle, in overrun condition, whereby the thermal energy, stored in the pressure vessel  7 ,  8 , can still be used to operate the expansion unit  1 . 
     The above described embodiments can be provided with the following modifications. 
     Both the expansion unit  1  and the combustion engine  45  can also be designed, instead of as a piston machine, as a rotary piston machine, according to the Wankel principle, or as a turbine. 
     As the working medium, water or also any other appropriate working medium, such as hydrocarbons, can be used that have at normal pressure of 1 bar, an evaporation temperature ranging from 70° C. to 110° C. and a freezing point of below −40° C. 
     When coupling the steam thermal engine with a liquid cooled combustion engine it is advantageous for the dissipation of heat from the engine coolant into the boiler to design the combustion engine with cooling with performance data. 
     For the pressure vessel  7 ,  8  as the second heat exchanger at a relatively high temperature level, the external waste heat can also or additionally be supplied by a fuel cell, in particular of the SOFC type.
The invention relates to a method for the operation of a steam thermal engine, whereby the hot steam from a working medium is converted into kinetic energy, by means of a pressure-releasing device ( 1). The working medium is heated in a boiler ( 6) to a low temperature, preferably boiling point at a low pressure, steam is taken from the boiler ( 6) to a pressure chamber ( 7, 8), in which the steam is heated to a higher temperature. Liquid working medium (or condensate) is injected from the boiler ( 6) into the pressure chamber ( 7, 8) whereupon the working medium is instantaneously evaporated, such that the pressure in the pressure chamber ( 7, 8) rises markedly and the steam is fed from the pressure chamber ( 7, 8) to the pressure-releasing device ( 1).