Abstract:
A hydraulic actuator for a multicylinder Stirling engine provided to enable modulation of the displacement of the engine. The hydraulic actuator incorporates a rotary vane configuration which provides relative rotational adjustment between components of a swashplate assembly. The relative rotation provides adjustments to the angle formed by the swashplate relative to its angle of rotation, and thus varies the stroke of each piston connecting rod, which thereby modulates the swept volume of the respective piston within its cylinder bore.

Description:
BACKGROUND OF THE INVENTION 
     This invention is related to a heat engine and particularly to an improved Stirling cycle engine incorporating a mechanism for modulating the displacement of the engine. 
     In order that a Stirling engine meet the output requirements demanded for a particular operating condition, some means of power modulation is required. One approach is through adjusting the swept volume or displacement of the reciprocating pistons of the machine. The Assignees of the present invention have developed numerous approaches toward providing such modulation adjustment. In the Stirling engine of the type described in this specification, modulation adjustment is achieved by changing the angle which the swashplate forms from its axis of rotation. As the swashplate face surfaces approach a plane perpendicular to its axis of rotation, the swept volume of the pistons decrease. Conversely, when the swashplate face surfaces are inclined from a plane perpendicular to its rotational axis, the swept volume of the pistons increase. 
     The Assignees of the present application have incorporated various mechanical, electrical and hydraulic systems for causing the swashplate angle to be varied in a desired manner. One series of devices provides hydraulically actuated swashplate adjustment as described by U.S. Pat. No. 4,532,855. Various electrically driven actuators have also been described by the Assignee, including those described in U.S. Pat. Nos. 4,994,004; 5,611,201; and 5,836,846. Although the devices described by those previously referenced patents are viable designs, there is a continuing need to provide such adjustment systems which have the features of simplicity, rapid transient response, and reliability. This invention is aimed at achieving those desirable features. This invention further addresses the need to provide a measure of swashplate angle, needed as part of a variable swashplate control system. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a swashplate actuator system is described incorporating a hydraulic actuation system. The mechanism uses hydraulic pressure to move a rotary vane for providing swashplate angle adjustments. 
     The present invention further provides two approaches toward measuring swashplate angle, each using one or more proximity probes interacting with portions of the rotating driveshaft or the reciprocating motion of the cross heads of the engine. 
     Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a representative longitudinal cross sectional view of a Stirling engine of a prior art type suited for incorporation of the present invention; 
     FIG. 2 is a longitudinal cross sectional view through the hydraulic swashplate actuator in accordance with this invention; 
     FIG. 3 is a cross sectional view taken from FIG. 2 showing the internal pressure cavities of the rotary vane actuator; and 
     FIG. 4 is a diagrammatic view of a hydraulic actuator circuit for controlling the swashplate actuator of this invention in accordance with a first embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A Stirling engine of a type suited for use with the present invention is shown in assembled condition in FIG.  1  and is generally designated by reference number  10 . Stirling engine  10  incorporates a number of primary components, including drive case assembly  12 , cylinder block assembly  14 , and heater assembly (not shown). 
     Drive case assembly  12  incorporates housing  18  with drive shaft  20  journaled for rotation within the housing. Swashplate  22 , which will be described in greater detail below, provides a pair of opposed generally parallel face surfaces  24  and  26 . Each face surface  24  and  26  will preferably be provided with a slight taper in the radial direction in the order of 0.6°, to thereby facilitate establishing a hydrodynamic film between the surfaces  24 ,  26  and the respective adjacent cross head bearings  29 . Cross heads  28  engage the opposed face surfaces  24  and  26  and are connected with connecting rods  30  which are in turn coupled with pistons  32 . Cross heads  28  are maintained to reciprocate along an axis through the use of guide rods  34 . Through this mechanical linkage, reciprocating motion of pistons  32  are translated into rotation of drive shaft  20 . As is also evident, the angle which swashplate face surfaces  24  and  26  form with respect to the longitudinal axis of rotation  36  of drive shaft  20  (the plane of the swashplate) defines the stroke or displacement distance for the pistons  32 . 
     Cylinder block assembly  14  incorporates a number of cylinder bores  40  through which pistons  32  reciprocate. In the well known Stirling thermodynamic cycle, the pistons  32  shuttle a working gas such as helium or hydrogen between a cold space and a hot space. In this instance, the volume of gas above the dome of pistons  32  and the heater assembly (not shown) constitute the hot space of the engine. The cold space is defined, in part, by gas cooler  42 . Regenerator  44  is placed between gas cooler  42  and the heater assembly. The Stirling engine  10  illustrated in this description is a multi-cylinder, double acting type. In this instance, there is a gas volume connection between the hot space of one piston  32  and the cold space of the adjacent cylinder and piston. Engine  10  of FIG. 1 incorporated a swashplate actuator  46  of the electrically actuated type. FIGS. 2 and 3 illustrate swashplate actuator  48  of this invention which replaces actuator  46 . 
     Additional details regarding the construction of Stirling engine  10  may be provided with reference to U.S. Pat. No. 5,611,201 which is incorporated herein by reference. 
     Now with specific reference to FIG. 2, the components of swashplate actuator  48  are shown in more detail. Drive shaft  20  rotates within suitable journal bearing which include bearing shells  50  and  52 . These journal bearings are supplied with lubricating oil in a conventional manner. Drive shaft  20  incorporates swashplate journal  54  which is a cylindrical surface having its central longitudinal axis  56  inclined at angle a with respect to drive shaft axis  36 . Swashplate ring  58  is rotatably mounted on swashplate journal  54  via a pair of rolling element bearing assemblies  60  and  62 . The swashplate face surfaces  24  and  26  define parallel planes which are displaced from a plane perpendicular to journal axis  56  by angle β as shown in FIG.  2 . In this manner, relative rotation between swashplate journal  54  and swashplate ring  58  cause the angle of the plane formed by face surfaces  24  and  26  to vary with respect to the longitudinal axis  36  of the drive shaft, shown as angle φ. The relative rotated positions of swashplate ring  58  and swashplate journal  54  determine the extent to which angles α and β add to increase the swashplate angle φ, or subtract to reduce angle φ. As shown in FIG. 2, angle φ is at its maximum, in which angles α and β add at their full values to maximize angle φ. It is preferred that angles α and β are equal to one another. 
     As one means of measuring the angular position of swashplate face surfaces  24  and  26  and therefore the displacement of swashplate actuator  48 , a pair of electrical signal outputs are provided from proximity probes. As shown in FIG. 2, an extending shoulder of driveshaft  20  forms a projecting tab  49 . Tab  49  interacts with an electrical induction proximity probe  51 . Each time tab  49  rotates past proximity probe  51 , an electrical output signal is provided. In a similar manner, swashplate ring  58  forms protruding arcuate shaped tab  59 . Tab  59  interacts with electrical induction proximity probe  63  and provides an electrical output signal each time tab  59  passes across proximity probe  63 . Tab  59  has an arcuate shape since it needs to interact with probe  63  over a range of angular positions. Since the relative angular position between drive shaft  20  and swashplate ring  58  is directly related to the swashplate angle φ, the phase difference in the outputs between proximity probes  51  and  63  may be used to provide such an indication. Through the use of a suitable control system, the phase difference between the outputs from proximity probes  51  and  63  allow the swashplate angle to be continuously monitored. This output is used by a suitable control system to control the swashplate actuator  48  to provide a desired displacement for engine  10 . An alternative technique for instantaneously computing displacement is that of measuring linear displacement of any two cross heads  28  that are 90° from one another with an appropriately located proximity probe or sensor  51  for each cross head and equating displacement or swashplate angle, or both as desired. 
     As best shown in FIG. 3, drive shaft  20  and swashplate ring  58  cooperate to define a divided generally annular hydraulic cavity  64 . This cavity  64  is divided into four discrete isolated chambers  66 ,  68 ,  70  and  72 . In part, these chambers are isolated by a pair of diametrically arranged radially outwardly extending vanes  74  and  76  which extend from swashplate journal  54 . Another pair of radially oriented vanes  78  and  80  extend in a radially inward direction from swashplate ring  58 . Fluid sealing access across vanes  74 ,  76 ,  78 , and  80  is provided by tip seals  75 ,  77 ,  79 , and  81 , respectively. 
     Chambers  66 ,  68 ,  70  and  72  operate as opposed pairs. Hydraulic fluid is supplied to the coupled pair of chambers  66  and  68  via supply passage  82 , and chambers  70  and  72  via oil supply passage  84 . As best shown in FIGS. 2 and 3, a central oil passageway  86  is supplied by separate ports  88  and  90  which communicate with the outside diameter of driveshaft  20 . A central tube  92  divides oil passageway  86  into two discrete passages. Oil flowing into port  88  flows around the outside of tube  92  and through passage  82 . Conversely, oil supplied to port  90  travels through the interior of tube  92  and flows into passage  84 . Passageway  98  is provided to provide lubricating oil to bearings  60  and  62 . 
     The positions of passages  82  and  84  are best shown with reference to FIG.  3 . Passageway  82  extends diametrically across the drive shaft  20  and opens into cavities  66  and  68  at a position just adjacent to vanes  74  and  76 . Passageway  84  also extends diametrically across drive shaft  20  and communicates with chambers  70  and  72  at positions also just adjacent to vanes  74  and  76 , but on the opposite sides of the vanes as passageway  82 . 
     By controlling the pressure of applied hydraulic fluid in passages  82  and  84 , the angle of swashplate ring  58  with respect to drive shaft  20  and therefore the stroke of the engine can be modulated. FIG. 3 illustrates a condition in which the volume of fluid is supplied through passage  82  as compared with passage  84  is roughly equal, causing the volumes of chambers  66  and  68  to be nearly the same as that of chambers  70  and  72 . This condition corresponds with an engine displacement between the minimum and maximum volumes by controlling the stroke. When hydraulic fluid is supplied at greater pressure to passageway  82 , hydraulic fluid fills chambers  66  and  68  and they expand. This causes the swashplate ring  58  to rotate relative to the drive shaft  20  in a clockwise direction, until vane  78  reaches the phantom line position illustrated in FIG. 3 designated by reference number  78   a  (vane  80  undergoes the same angular change in position). At that position of vane  78   a , stop block  94  is contacted and continued relative rotation is not permitted. This position represents an extreme position of either maximum of minimum swashplate angle and corresponding piston  32  stroke. 
     When it is desired to rotate swashplate actuator  48  to the opposite extreme position, hydraulic fluid is sent through passageway  84 . In that condition, chambers  70  and  72  expand as fluid from chambers  66  and  68  is drained. This causes swashplate journal  54  to rotate in a counterclockwise direction relative to drive shaft  20 , eventually reaching the position shown in FIG. 3 where vane  78  reaches the position designated by reference number  78   b , at which point stop block  96  is contacted. While the intermediate and extreme positions were previously described, it is possible to place the components in any desired relative angular position between the extremes through appropriate control of applied pressures. 
     Now with reference to FIG. 4, a hydraulic actuator circuit is shown which supply hydraulic fluid to swashplate actuator  48  enabling it to undergo its change in position as described previously. FIG. 4 illustrates hydraulic actuator circuit  102 . As shown in FIG. 4, hydraulic fluid is stored in reservoir  104  and its pressure is increased through the use of pump  106 . Accumulator  103  provides a storage volume maintained at pressure. High pressure fluid is supplied on line  108  to a port of four-way directional control valve  110 . Solenoid  112  controls the position of a spool of directional control valve  110  to provide the fluid port connections  88  and  90  diagrammatically illustrated in FIG.  4 . In one position of the spool, line  108  becomes connected with line  114  which connects with port  88  and passageway  82 . Another line  116  is connected with passageway  84  via port  90 . Return line  118  allows hydraulic fluid to return back to reservoir  104 . Pressure control valves  120  and  122  are plumbed into lines  114  and  116 , respectively to control the outflow of hydraulic fluid into return line  118 . Pressure relief valve  123  drains fluid to reservoir  104  in the event of an overpressure condition. Filter  105  is provided to remove contaminants from the hydraulic fluid. 
     In operation of hydraulic actuator circuit  102 , when it is desired to change the swashplate angle, a control signal is directed to directional control valve solenoid  112 . By shifting the spool between the positions illustrated diagrammatically in the left and right hand sections of valve  110 , lines  114  and  116  are selectively connected with supply line  108  and return line  118  pressurized or provide a return fluid path as desired. Since there will generally be a slow leak of hydraulic fluid across actuator vanes  76  and  78 , there will be continuous need to actuate valve  110  as the actuator position deviates from a desired set position. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.