Drive mechanism for Stirling engine displacer and other reciprocating bodies

A mechanism is disclosed for driving the displacer of a crank-type Stirling engine at the same frequency but out of phase with the power piston of the engine. A biasing means is linked to the displacer for applying a biasing force which urges the displacer in one direction of its reciprocation. A flexible band is secured to the displacer for applying a force opposite to the biasing force. The other end of the band is secured to an anchor which is mounted on another part of the apparatus. The band extends slideably across at least one and preferably two bearing surfaces intermediate its secured ends.

TECHNICAL FIELD 
This invention relates generally to mechanisms for converting rotary motion 
to reciprocating motion and more particularly relates to a drive mechanism 
for driving the displacer of a crank-type Stirling engine. 
BACKGROUND ART 
There are two general types of Stirling engines. In the first, the piston 
and displacer are mechanically linked to a rotating shaft by means of a 
linkage means for converting between rotary motion and reciprocating 
motion. The most common linkage of this type is the conventional 
crankshaft and connecting rod linkage means. The second type of Stirling 
engine is the free piston Stirling engine. The present invention is not 
applicable to free piston Stirling engines but is applicable to the first 
type as well as to other devices in which there is a need for a linkage 
which converts rotary motion to reciprocating motion and in which it is 
desired to have a selected phase relationship between two reciprocating 
bodies linked to a rotating shaft. 
Although a variety of linkages are shown in the prior art, those which have 
been applied to Stirling engines include not only the conventional crank 
and connecting rod, but also the rhombic drive, bell cranks, cams and 
wobble plates. 
In the Stirling engine, the purpose of the linkage means which connects the 
piston to the rotating shaft is to transfer energy from the moving piston 
to the rotating shaft. The purpose of the linkage means which connects the 
displacer to the rotating shaft is to reciprocate the displacer in the 
working fluid at the proper phase and with the proper waveform or 
translation characteristic in order to obtain the desired operation of the 
engine. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention is a new linkage means for converting rotary motion 
to reciprocating motion. It is particularly useful for controlling the 
displacer of a Stirling engine or heat pump but is also applicable to 
other reciprocating bodies. 
The Stirling engine may be described as comprising a housing or support 
frame in which a power piston and displacer are mounted for reciprocation 
in one or more cylinders formed in the housing. Each of these two 
reciprocating bodies are linked to a rotating shaft by means of a linkage 
for converting between rotary motion and reciprocating motion. 
Embodiments of the present invention link a reciprocating body such as the 
displacer of a Stirling engine to the rotating shaft so that the displacer 
or other body will be reciprocated at the same frequency as the power 
piston but out of phase with it. 
The invention comprises a biasing means, such as a spring, which is linked 
to the displacer for applying a biasing force upon the displacer urging it 
in one direction of its reciprocation. A flexible band is secured at a 
place along its length to the displacer and extends away from the 
displacer in a direction for applying a force on the displacer opposite to 
the biasing force when the band is in tension. A band can not, of course, 
apply a significant compressive force. It can pull but not push. The other 
end of the band is secured to an anchor. The anchor is mounted either to 
the housing or support frame of the engine or the anchor is mounted to the 
linkage means which connects the power piston to the crank shaft. 
Intermediate the place on the band where it is attached to the displacer 
and the places where it is secured to an anchor, the band extends 
slideably across at least one bearing surface and preferably across two 
bearing surfaces. The bearing surface is formed on another part of the 
apparatus which is different from the parts to which the band is secured. 
Thus, if the band is secured to the displacer and the housing, at least 
one bearing surface will be formed on the power piston linkage means. In 
the alternative, if the band is secured between the displacer and the 
power piston linkage means, then at least one such bearing surface will be 
formed on or mounted to the housing. 
This is a major simplification of the Stirling engine because it can use a 
single throw crank, have less reciprocating mass while also reducing 
lubrication requirements. Additionally, embodiments of the present 
invention permit adjustment of the phase between the reciprocating piston 
and the displacer in an extremely easy manner which, with some 
embodiments, can even be done while the engine is operating. Further, the 
invention facilitates the design and adjustment of displacer stroke and 
average position.

DESCRIPTION OF THE PREFERRED AND ALTERNATIVE EMBODIMENTS 
FIG. 1 diagrammatically illustrates a Stirling engine having a housing 10, 
a crankshaft 12 which is rotatably journalled to the housing 10, a 
cylinder 16 formed in the housing 10 and a power piston 14 mounted for 
reciprocation in the cylinder 16. The power piston 14 is linked to the 
crankshaft 12 by means of a linkage means 18 for converting the 
reciprocating motion of the piston to the rotary motion of the crankshaft 
12. 
The particular linkage means for the power piston 14 is a conventional 
crank 20 formed on the crankshaft 12 and linked to the piston 14 by means 
of the connecting rod 22. The connecting rod is connected to the piston 14 
by means of a wrist pin 24 and to the crankshaft 12 by a crank pin 26. 
A displacer 28 is also reciprocally mounted in the same cylinder 16. The 
displacer 28 is slideably mounted on a coaxial shaft 32 extending upwardly 
from the power piston 13, all in the conventional manner. 
A biasing means in the form of a compression spring 30 is positioned 
between the displacer 28 and the power piston 14. The spring 30 is a 
biasing means which urges the displacer in an upward direction away from 
the piston 14. 
A flexible band 34 is connected at one end 36 to the displacer 28 and 
extends slideably across a bearing surface formed at the wrist pin 24 and 
a second bearing surface formed at the crank pin 26 to a fixed connection 
at an anchor 38 mounted to the housing. 
The band 34 pulls downwardly in tension on the displacer 28 to apply a 
force opposite to the force applied by the spring 30. The band 34 has a 
length which will maintain at least some minimum compression on the spring 
30 during the entirety of the cycle so that there is always a tension on 
the band 34 providing a static force equal and opposite to the static 
force of the spring 30. 
Although the term "band" has been selected to describe the structure, the 
band 34 merely need be a flexible, tension-transmitting member. The band 
may be embodied in a variety of shapes and materials and could be termed a 
belt, chain, rope, cable, cord, strip, tape, strand, wire, etc., or any 
one of a variety of equivalent structures. 
In FIG. 1 the band extends across bearing surfaces formed at wrist pin 24 
and crank pin 26. These bearing surfaces can be embodied in a variety of 
different structures. They can, in the simpliest form be formed of 
extensions of the actual wrist pin and crank pin. 
Alternatively, and preferably, they are separate structures mounted at the 
same region of the mechanism. Additionally, while they may be smooth 
surfaces machined on a pin or other part of the machine, they are 
preferably anti-friction devices such as rotatable pins or pulleys mounted 
to the linkage means or housing by appropriate anti-friction bearnings. 
Desirably, a second pin 24A is mounted adjacent the portion of the wrist 
pin which contacts the band to retain the sliding band against one of the 
pins during the entire cycle. 
As a further alternative, the band may have cogs, teeth or spaced openings 
instead of being smooth and the bearing surfaces may be mating gears or 
sprockets, etc., so that the band and the bearing surfaces are matingly 
meshed. 
Therefore, the term "extending slideably" over a bearing surface are merely 
words selected to include the variety of different forms that the 
invention may take and is not limited to an actual, direct, frictional 
sliding between the band and the bearing surfaces. The term includes 
equivalent structures. 
The motion of a displacer in a Stirling engine embodying the present 
invention is illustrated in FIGS. 2A and 2B. The motion may be described 
from geometrical considerations with reference to FIG. 2A by the equation: 
##EQU1## 
where y is the position of the displacer above the power piston, r and 
.theta. are shown in FIG. 2A and e is a dimensionless term for normalizing 
the results and equals b/r. 
I have found that, as the value of e becomes larger, the motion of the 
displacer more closely approximates sinusoidal motion. To demonstrate 
this, I have plotted in FIG. 2B the value of the expression 
e-.sqroot.1+e.sup.2 -2e cos .theta. for a value of e=2 and for a value of 
e=5. I also plotted the value of cos .theta. representing a pure 
sinusoidal motion. It is apparent from FIG. 2B that when e is 5 or greater 
the difference between perfect sinusoidal motion and the motion of a 
displacer in an embodiment of the invention is minimal. It is also 
apparent that the boundary conditions are identical. Furthermore, the 
motion of a piston with a crank and connecting rod linkage means is also 
not purely sinusoidal. 
Referring to FIG. 2A, the phase of the displacer 28 may be varied by moving 
the anchor pin 38 along a locus formed by a circle of radius b centered at 
the axis of rotation of the crankshaft 20. Movement of the anchor point 38 
along this locus does not change either variable b or r. It does, however, 
change .beta. in direct proportion to the angular adjustment of the anchor 
38. 
I have mathematically determined that as .beta. is varied from 
##EQU2## 
the phase angle between the displacer and the piston goes from .pi./2 to 
0. 
Additionally, outward radial movement of the anchor point 38 or drawing the 
band through the anchor point 38 will lower or change the average position 
of the displacer. Thus, the position of the anchor or the length of the 
band can be radially adjusted to adjust the average position of the 
displacer 28. 
FIG. 3 illustrates an embodiment of the invention which is quite similar to 
that of FIG. 1 with the exception that the direction of the forces applied 
by the band and the spring are interchanged and the spring is grounded to 
the housing. The spring 39 exerts a bias in the downward direction on the 
displacer 40 through its center displacer rod 42. The band 44 extends 
upwardly from its connection point 46 to the central displacer rod 42 and 
then extends slideably over bearing surfaces 48 and 50, which are the same 
bearing surfaces utilized in FIG. 1. Thus, in the embodiment of FIG. 3, 
the band 44 moves the displacer upwardly while the spring 39 urges it 
downwardly but otherwise the motion and operating characteristics are 
similar to those of the embodiment of FIG. 1. 
FIG. 4 illustrates a Stirling engine in which the piston 60 is linked 
through a conventional scotch yoke mechanism 62. It is at the slot pin 64 
that the bearing surface is formed across which the band 66 slideably 
extends to connection with an anchor 68 mounted to the housing. This 
embodiment utilizes a single bearing surface. 
In the embodiments of FIGS. 5, 6 and 7 the band extends from the displacer 
to attachment at the linking means which links the power piston to the 
rotating shaft. The band extends slideably across two bearing surfaces 
which are mounted to the housing. 
In FIG. 5 the bearing surfaces 80 and 82 are of the type described in 
connection with the embodiments of FIGS. 1-4. 
The embodiment of FIG. 6 is like that of FIG. 5 except that one of the 
bearing surfaces 100 is a roller having an eccentric shape. The eccentric 
shape can be used to modify the displacer motion, for example to cause it 
to more closely approximate perfect sinusoidal motion or to create a 
dwell. 
With the embodiment of FIG. 6 a band which matingly meshes with the 
eccentric roller 100 should be used in order to maintain the proper phase 
of the motion modifications. Thus, the mating is important where the 
motion modifications introduced by the eccentric are to be positioned at a 
particular angular relationship to the displacer cycle. With this 
embodiment as well as the other embodiments, the preferred band is a 
timing belt comprising a rubber based material with fabric or steel 
reinforcement and formed with cogs or teeth. 
In the embodiment of FIG. 7, the band 110 is connected to the displacer at 
its one end 112, slideably extends over the bearing surfaces 114 and 116 
which are mounted to the housing of the engine and then is anchored at the 
crank pin 118. A second band is also used extending between the same 
points of attachment but along a different path. It extends across an 
additional pair of bearing surfaces 120 and 122. The second band has an 
intermediate tension spring 124 and together with the spring serves as a 
dynamic biasing means for biasing the displacer with a downward force. 
In these embodiments of the invention it is not essential that a particular 
bearing surface or anchor for the band be located at the crank pin. They 
may be located at any position along the linkage means which joins the 
rotating crank to the piston. For example, referring to the embodiments of 
FIG. 1, positioning of the bearing surface closer to the wrist pin 24 
would reduce the stroke of the displacer 28. 
Similarly extensions can be formed on the linkage means to provide 
increased stroke. 
It should also be noted that any anchor which is mounted to the crankshaft 
must be rotatably mounted to avoid winding up the band as the crankshaft 
rotates. 
It can therefore be seen that a novel linkage means is disclosed which is 
capable of being formed in a variety of embodiments without departing from 
the spirit of the invention.