Abstract:
A device and method for equalizing the pressure between work-space and crankcase in a pressurized engine, such as a Stirling engine. The device consists of a two-way valve connected between the work-space and the crankcase. The valve is connected to the work-space with a passageway including a constriction to provide an mean pressure for monitoring purposes. The valve connects the work-space and crankcase allowing the pressure to equalize when the mean pressure of the work-space exceeds the crankcase pressure by a predetermined amount.

Description:
TECHNICAL FIELD 
   The present invention pertains to regulating the pressure in the work-space of a pressurized engine, such as a Stirling engine. 
   BACKGROUND OF THE INVENTION 
   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.  FIG. 1  shows typical pressures in the gas volumes for such an engine. 
   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.,  FIG. 2 ). In summary, pumping up the workspace mean pressure significantly increases engine wear with only a small attendant increase in power production. 
   SUMMARY OF THE INVENTION 
   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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
       FIG. 1  shows a graph of work-space and crank-case pressure for a Stirling engine with a pressurized crankcase; 
       FIG. 2  shows a graph of pressure between a work-space and a crankcase for a Stirling engine when the work-space is pumped-up; 
       FIG. 3  shows a side view in cross section of a sealed Stirling cycle engine; 
       FIG. 4  shows a pressure regulator for an engine according to an embodiment of the invention; 
       FIG. 5  shows a pressure regulator for an engine according to another embodiment of the invention; 
       FIG. 6  shows a pressure regulator for an engine according to a further embodiment of the invention; and 
       FIG. 7  shows the pressure difference that may develop across a valve according to the embodiment shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   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  FIG. 3 , a sealed Stirling cycle engine  50  is shown in cross section. While this embodiment of the present invention will be described with reference to the Stirling engine shown in  FIG. 3 , it should be understood that other engines, coolers, and similar machines may likewise benefit from embodiments of the present invention and such combinations are within the scope of the invention, as described in the appended claims. A sealed Stirling cycle engine operates under pressurized conditions. Stirling engine  50  contains a high-pressure working fluid, preferably helium, nitrogen or a mixture of gases at 20 to 140 atmospheres pressure. Typically, a crankcase  70  encloses and shields the moving portions of the engine as well as maintains the pressurized conditions under which the Stirling engine operates (and acts as a cold-end pressure vessel.) A heater head  52  serves as a hot-end pressure vessel. 
   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  FIG. 4 . Pipe or passageway  82  connects the pressure regulator  84  to the work-space  80 . A restrictive orifice  92  damps the oscillating work-space pressure applying the mean work-space pressure to one end of the shuttle,  100 . The orifice  92  is sized to be significantly larger than the piston seal ring leak. As used in this specification including any appended claims, the term “constriction” will be used to denote a narrowing in a pipe or passageway, including such a constriction at the end of a pipe or passageway or any place within the pipe or passageway. The other end of the shuttle  100  is exposed to the crankcase pressure via a pipe  86 , which pipe may include a restrictive orifice  93  or other constriction. Orifice  93  may be sized much smaller than orifice  92 , in which case the combination of the shuttle  100  and the orifice  93  act to damp movement of the shuttle from work-space pressure swings applied through orifice  92 . In a specific embodiment of the invention, orifice  92 , from WSGR to work-space is approximately 0.031 inches in diameter, while orifice  93 , from WSGR to the crankcase, is approximately 0.014 inches in diameter. In other embodiments of the invention, the constriction from shuttle to crankcase may be omitted. Note that the crankcase pressure is approximately constant over the piston&#39;s cycle, while the work-space pressure swings significantly during the cycle. Two springs  102 ,  104  keep the shuttle  100  centered, when the mean work-space and the crankcase pressures are equal. 
   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  FIG. 5  and labeled generally  200 . Work-space housing  205  and crankcase housing  210  are bolted together capturing piston  215 , work-space spring  225 , and crankcase spring  230  in their bores. The interface of the two housings creates cup seal gland  260  into which seats a bidirectional cup seal  220 , and an O-ring gland  265  into which seats an O-ring  270 . The O-ring seals the interior of the housings from the crankcase pressure. Two orifices  235  allow the pressures inside the two housings to remain equal to the mean crankcase pressure and the mean work-space pressure, respectively, without large pressure oscillations or large mass flows into/out of the housings. The piston is free to move axially within the housings by sliding on its bearing surfaces  250 . 
   When the two pressures are equal, the springs keep the piston centered such that the cup seal seals against the piston&#39;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  FIG. 6  and will be referred to as the Preloaded WSPR ( 300 ). This embodiment of the invention uses preloaded springs  302 ,  304  connected to an inner piston  340  and an outer piston  342  to control working gas flow into and out of the work-space  80 . The springs are open-coil springs and, thus, gas flows freely through these springs. WSPR  300  communicates with the work-space  80  via an orifice  392 . Likewise, the crankcase volume  78  is connected to WSPR  300  via port  393 . Work-space pressure oscillations are damped out by the constriction of the orifice  392  together with the force of the pre-loaded springs  302 ,  304  acting on the pistons  340 ,  342 . Seals  370 ,  372  provide a compliant seat for pistons  340 ,  342 . The orifice  392  is sized to be significantly larger than the piston seal ring leak. WSPR  300  may be mounted on the compression cylinder head of the engine  58  (see  FIG. 3 ). 
   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&#39;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  FIG. 6 . As an example, consider the pressure across valve  310 , as shown in  FIG. 7 . (It should be noted that  FIG. 7  is exemplary only and does not represent measured data on a WSPR.) The pressure difference between the two orifices can be better described as the pressure difference across regulator valve  310  where the regulator valve is composed of the two pistons  340 ,  342 , the two springs  302 ,  304  and the two valve seats  370 ,  372 .  FIG. 7  shows the pressure across valve  310  for two cases. In one case, the preload on each spring  302 ,  304  is the same, and the workspace does not “pump-up,” as shown by graph  402 . The workspace and crank case remain at approximately the same mean pressure. In the second case, the preload on spring  302  is greater than the preload on spring  304 . Graph  404  shows the pressure across the valves, when the workspace has a mean pressure that is 100 psi above the crankcase pressure. In the latter case, the pressure difference may become large enough to overcome the preload on valve  302 , opening valve  310  and allowing gas to flow out of the workspace into the crankcase, reducing the pressure in the workspace. The horizontal line in  FIG. 7  shows the pressure at which the preload on spring  304  is overcome. At that pressure, the WSPR opens allowing gas to pass between workspace and crankcase. The devices and methods described herein may be used in combination with components comprising other engines besides the Stirling engine in terms of which the invention has been described. The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims