The present invention relates to steam engines in general, and more particularly to improvements in engines of the type which operate with steam at a pressure above the critical point. Still more particularly, the invention relates to improvements in steam engines wherein the fluid medium whose heat energy is converted into mechanical energy is circulated along a closed path.
Presently known steam engines of the above outlined character operate with a working medium whose pressure is below the critical point. The upper limits of steam temperature and pressure in such engines are respectively about 500.degree. C. and 140 atmospheres superatmospheric pressure. This is satisfactory for certain applications; however, these engines also exhibit a number of serious drawbacks. Thus, the conversion of heat energy into mechanical energy takes place in the lower steam-liquid range even though it is well known that the efficiency of steam engines which operate in the higher range is much more satisfactory. When the operation takes place in the lower steam-liquid range, the medium must be supplied with additional evaporation heat subsequent to each passage through the engine cylinder or cylinders. In the so-called gas range (i.e., at a temperature above the critical temperature of 374.degree. C. and above the critical pressure of 225 kg/cm.sup.2), there is no need to supply evaporation heat for conversion of liquid into steam. Presently known steam engines cannot operate in the gas range for a number of reasons. Thus, in order to insure that the relatively small temperature gradient which is available in conventional machines will be converted into work to a maximum extent such engines are equipped or provided with means for condensing spent steam by cooling. According to the Sankey diagram, heat losses which develop in the condenser amount to 62 percent. Boiler losses equal or approximate 21 percent and other losses amount to 3 percent. Even though the heat energy of spent steam can be put to use, only 14 percent of the total heat energy is available for conversion into mechanical energy. It was also proposed to resort to condensation by compression (instead of cooling); however, the procedure is impractical because of the low pressure gradient which is available for conversion into mechanical energy.
Steam boilers which are used in combination with conventional engines operate at a relatively low pressure; this, combined with the low heat content of the working medium, necessitates the use of huge boilers if the engine requires a substantial energy input. A large boiler requires a long period of time for heating to operating temperature and the losses owing to radiation are very pronounced due to the large surface areas of such boilers. Moreover, explosion of a large steam boiler can cause substantial damage and/or injuries.
Boiler feed pumps which are used in connection with steam engines for vehicles are operated individually and directly by steam. Such pumps are not suited for operation in parallel which would be desirable in order to insure a more uniform admission of fluid. Also, the bodies of sliding and lifting valves which are used in conventional steam engines contain large dead spaces which cause substantial losses in flow. The lack of efficiency of such valves would be even more pronounced if the valves were used in or on the relatively small cylinders which are operated with steam whose pressure is in the supercritical range, i.e., the ratio of the aforementioned dead spaces to the volume of the cylinders would be even less satisfactory. Also, the valve regulating devices of presently known steam engines operate with pronounced inertia so that they cannot insure an accurate and reproducible rate of steam flow into and/or from the cylinders of the engine. As a rule, such regulating devices operate exclusively in dependency on the RPM of the crankshaft. If one were to use steam which is maintained at a pressure in the supercritical range, the RPM could be increased very substantially and the conventional regulating slide devices would be capable of controlling the movements of valves at the increased speed. However, once the speed of moving parts of valves exceeds a certain limit, they are subjected to excessive wear, mainly because of poor lubrication. Moreover, the valves are normally biased to closed position by springs, especially steel springs, which are subjected to pronounced thermal stresses. Furthermore, the characteristic resonant frequency of such springs is limited so that the closing of valves is not assured once the RPM of the crankshaft exceeds a certain limit. Certain other types of valves, e.g., Meier-Mattern valves and others which are operated with pressurized oil, exhibit many advantages over mechanically operated valves, particularly as concerns the speed of movement of valving elements between open and closed positions, lower inertia and simplicity of reversing the direction of movement. The pressure of oil is regulated substantially in dependency on the load upon the engine by resorting to an oil pressure regulator which insures that the duration of intervals during which the valves remain open varies with the speed of the engine. However, such regulating devices also exhibit a number of drawbacks. Thus, when the pressurized oil opens a valving element, oil flows into a container (which is mounted at a high level) and thence back to the oil pump. The circuit for the flow of oil is open and, therefore, one cannot compensate for pressure losses due to elasticity of oil and resistance which the oil encounters during flow in the piping, especially in the supercritical range when the rotational speed is extremely high and strokes are very short.