The present invention pertains to regulating the pressure in the work-space of a pressurized engine, such as a Stirling engine.
Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), and incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression.
A Stirling cycle engine operates under pressurized conditions. Stirling engines contain a high-pressure working fluid, preferably helium, nitrogen or a mixture of gases at 20 to 140 atmospheres pressure. A Stirling engine may contain two separate volumes of gases, a working gas volume containing the working fluid, called a work-space or working space, and a crankcase gas volume, the gas volumes separated by piston seal rings. The crankcase encloses and shields the moving portions of the engine as well as maintains the pressurized conditions under which the Stirling engine operates (and as such acts as a cold-end pressure vessel). A pressurized crankcase removes the need for high pressure sliding seals to contain the work-space working fluid and halves the load on the drive component for a given peak-to-peak work-space pressure, as the work-space pressure oscillates about the mean crankcase pressure. The power output of the engine is proportional to the peak-to-peak work-space pressure while the load on the drive elements is proportional to the difference between the work-space and the crankcase pressures.
The action of the piston rings can raise or lower the mean working pressure above or below the crankcase pressure, substantially mitigating the above-mentioned advantages of a pressurized crankcase. For example, manufacturing marks, deviations and molding details of the rings can produce preferential gas flow in one direction between the work-space and the crankcase. The resulting difference in pressure between the work-space and the crankcase can produce as much as double the load on engine, while peak-to-peak pressure and thus engine power increases only fractionally (see, e.g.,
In embodiments of the present invention, a device is provided that reduces the mean pressure difference between a work-space and a pressurized engine crankcase of an engine, such as a Stirling engine. The device includes a valve connecting the work-space and crankcase of the engine. The pressure difference between work-space and crankcase is monitored. When the mean pressure of the work-space differs from the crankcase pressure by a predetermined amount, the valve opens, allowing the pressure difference between the two spaces to equalize. When the pressure difference between the spaces is reduced sufficiently, the valve closes, isolating the work-space from the crankcase. This closure maximizing power production, while minimizing wear on drive components.
In a specific embodiment of the invention, pressure at which the valve opens is determined by a preloaded spring. In a further specific embodiment of the invention, the mean pressure is monitored by including a constriction in the passageway from the valve to the work-space so that a mean work-space pressure is presented to a pressure monitoring device. In a further specific embodiment of the invention, the device further includes a constriction in the passageway from the crankcase to the pressure monitoring device such that the monitoring device is presented with a mean crankcase pressure.
The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which:
In embodiments of the present invention, a device is provided that reduces the pressure difference between a work-space and a pressurized engine crankcase of an engine, such as a Stirling engine. Referring to
Stirling engine 50 contains two separate volumes of gases, a working gas volume 80 and a crankcase gas volume 78, that will be called hereinafter, a “work-space” and a “crankcase,” respectively. These volumes are separated by piston rings 68, among other components. In the work-space 80, a working gas is contained by a heater head 52, a regenerator 54, a cooler 56, a compression head 58, an expansion piston 60, an expansion cylinder 62, a compression piston 64 and a compression cylinder 66. The working gas is contained outboard of the piston seal rings 68. The crankcase 78 contains a separate volume of gas enclosed by the cold-end pressure vessel 70, the expansion piston 60, and the compression piston 64. The crankcase gas volume is contained inboard of the piston seal rings 68.
In the Stirling engine 50, the working gas is alternately compressed and allowed to expand by the compression piston 64 and the expansion piston 60. The pressure of the working gas oscillates significantly over the stroke of the pistons. During operation, fluid may leak across the piston seal rings 68 because the piston seal rings 68 do not make a perfect seal. This leakage results in some exchange of gas between the work-space and the crankcase. A work-space pressure regulator (“WSPR”) 84 serves to restore the pressure balance between the work-space and the crankcase. In embodiments of the invention, the WSPR is connected to the work-space by passageway 82, which may be a pipe or other equivalent connection, and to the crankcase by another passageway 86. When the work-space mean pressure 80 differs sufficiently from the mean crankcase pressure, the WSPR connects the two volumes via vent, 88 until the differential between the mean pressures diminishes.
For example, an exemplary work-space pressure regulator is shown in
When the mean work-space pressure is higher than the crankcase pressure, the higher pressure moves the shuttle 100 to the right, compressing spring 104. If the pressure difference is large enough to expose port 88 the work-space and the crankcase become connected. Some of the work-space gas flows into the crankcase until the two mean pressures are equalized, which allows the shuttle 100 to return to the original position, closing the port 88. Note that orifice from the work-space to the WSGR 92 may be sized to allow the pressure to equalize between work-space and crankcase quickly when port 88 is exposed, while still small enough to present a mean work-space pressure to the shuttle 100.
When the mean crankcase pressure is higher than the work-space pressure, the shuttle will move to the left, compressing spring 102. If the pressure difference is large enough, port 88 will be exposed to channel 112, connecting space 94 with the crankcase 78. Some of the crankcase gas flows into the work-space until the two mean pressures are equalized, which allows the shuttle 100 to return to its centered position, closing port 88.
The shuttle isolates the work-space 80 from the crankcase 78 in its centered position. The seal may be provided by two cup seals 122 located at the end of shuttle nearest the crankcase vent 86 or by equivalent seals as are known in the art. Two ring seals 120 center and guide the shuttle 88 in the WSPR body 114.
Another embodiment of the invention is shown in
When the two pressures are equal, the springs keep the piston centered such that the cup seal seals against the piston's sealing surface 255, preventing any flow between the two housings. When the pressure differential between the two housings becomes great enough, the force imbalance on the piston will cause the piston to move away from the region of high pressure, compressing the spring on the low-pressure side and relaxing the spring on the high-pressure side. Equilibrium is reached when the pressure force imbalance equals the spring force imbalance. If the pressure differential is great enough, the piston will be displaced enough that the cup seal 220 no longer contacts the sealing surface and instead loses sealing force against the decreasing diameter of the piston. Once the seal is broken, gas can flow from the high-pressure side, through the vent hole 240 or vent slot 245, past the cup seal 220, and into the adjacent housing. Gas will continue to flow until the pressure has equalized enough for the springs to return the piston to a position where the cup seal 220 seals against the sealing surface 255.
Another embodiment of the invention is shown in
The Preloaded WSPR relieves a mean overpressure in the work-space in the following manner. The oscillating work-space pressure, which is partially damped by the orifice 392, is applied to the face 380 of the inner piston 340 and to the face of the outer piston 342 that are proximate to the work-space. If the net mean pressure on the pistons is enough to overcome the preload on spring 302, then the inner and outer pistons move to the left and open the valve at 382. The released gas flows past the open seal at 382 around the outside of the outer piston 342, through spring 302 and into the crankcase via port 393. Once the difference between the work-space and the crankcase pressures drops below the preload on spring 302, the outer piston 342 moves back to the right and seals at 382. Seal 372 provides a compliant seat for piston 342.
The Preloaded WSPR relieves excess crankcase pressure by a similar method. When the net pressure times the inner piston's 340 area is greater than the preload on spring 304, the inner piston 340 moves to the right and opens the valve at 370, which provides a compliant seal for the inner piston 340. Gas from the crankcase flows between the outer and inner pistons and into the work-space via the orifice at 392 reducing the pressure differential. Once the difference between the work-space and the crankcase pressures drops below the preload on spring 304, the inner piston 340 moves back to the left and seals at 370.
In another preferred embodiment of the invention, the preloads in springs 302 and 304 may be preloaded to different force levels. The different forces applied by the springs would allow the workspace pressure to “pump-up” (i.e., increase) reaching a higher mean pressure, thereby allow the engine to produce higher mechanical power. This embodiment allows the design to add engine power without raising the crankcase mean pressure. Thus the power can be increased without redesigning or perhaps requalifying the crankcase pressure vessel.
The functioning of the Preloaded WSPR can be understood by considering the pressures difference between the two orifices 392 and 393 in
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