Note: Descriptions are shown in the official language in which they were submitted.
~ CA 02224345 1997-12-10
P 1803-1 PCT
Tri-Lobed Cam En~ine
FIELD OF INVENTION
This invention relates to reciprocating piston engines which include a
5 tri-lobed cam for converting the reciprocating piston movement to rotary movement
~ or vice versa depending upon whether the engine is used in a motor or compressor
mode.
BACKGROUND OF INVENTION
Reciprocating piston tri-lobed cam engines are disclosed in the
10 following patents:
U.S. ~atents 793,270 1,810,688 4,727,749
1,765,237 1,825,417 5,035,221
1,792,062 2,124,604 5,281,104
4,697,552
French Patent2,~37,132
In each of the foregoing patents a pair of diametrically opposed pistons are provided
which are coupled together in push-pull relationship by an inextensible link. Each
piston has a cam-follower, the link serving to maintain the cam-followers in contact
with the cam at all positions of rotation thereof. The coupling together of the pistons
20 in this manner necessitates the shaping of the tri-lobed cam such that the
dimension between diametrically opposed portions is substantially constant.
Generally speaking, such shaping includes a flattening of the lobes of the cam and
the formation of a concavity between adjacent pairs of lobes. ~hese engines havea relatively large angular interval over which they are no~ self-starting when
25 operated as external combustion engines.
In U.S. patent 1,203,855 there is disclosed a tri-lobed cam engine
wherein the pistons are not connected ~ogether, whereby they are free-floating.
The cam of this engine is asymmetrically shaped, whereby ~he engine would be
. . .
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suited for operation in one direction only. Moreover, the interaction between the
cam-followsrs and the cam would generate a severe reactive force which urges thepiston into contact with the wall of the cylinder in which it reciprocates, promoting
a rapid wear.
U.S. Patent 3,648,528 describes a rotary cam piston engine wherein a
pair of rollers are mounted to the piston rod above the axis of rotation of the cam
follower, which rollers bear on fixed guides. Although this arrangement will reduce
the lateral reactive forces transmitted to the piston, it will not be tota~ly eliminated.
Moreover, only one roller will bear on a guide at any given time in the course of
10 travel of the piston.
WO93/11342 describes a rotary cam piston engine wherein slide
bearings are mounted both above and below the axis of rotation of the cam follower
to contain the lateral reactive forces.
It is an object of this invention to provide improved tri-lobed cam
15 engines.
It is another object of this invention to provide tri-lobed cam engines
which utilize free-floating pistons wherein inter-reactive forces between the piston
and cylinder wall are reduced.
It is still another object of this invention to provide tri-lobed cam
20 engines wherein cams of different profile may be substituted, without necessitating
other change, to vary the engine characteristics.
It is yet another object of this invention to provide tri-lobed cam
engines that have a simple modular construction whereby the number and size of
the cylinders can be easily altered.
It is a further object of this invention to provide tri-lobed cam engines
that may be self-starting when operated as external combustion motors.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, an engine comprises
a housing with a shaft and tri-lobed cam and four free-floating piston means
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disposed on the housing in equi-spaced relationship. Each piston means includes
a cylinder and a free floating piston for reciprocal movement within the cylinder and
a cam-follower associated with each piston. The cam-follower has associated
therewith a guide bearing means and the housing has track means along which the
5 guide bearing means is movable.
The cam-follower is rotatable and the guide bearing means comprises
a pair of rotatable bearings which are respectively disposed on axially opposed
sides of the cam-follower in collinear axial relationship therewith. Accordingly the
lateral reactive forces generated at the cam follower may be transmitted to the track
10 means without transmitting any significant lateral force to the piston. Moreover,
each one of the pair of bearings will at all times transmit the lateral reactive force
to the track means, so reducing the stress load on the assembly.
With the reduction of piston~ylinder interaction and with the rolling
motion of the cam follower and associated guide bearing means, the engin~ is
15 particularly adapted for use as a high torque, essentially oil free air motor for use
in the food processing trades.
In accordance with the preferred embodiment, the four piston means
are arranged to form two diametrically opposed pairs, the cylindrical axes of the
pairs of cylinders intersecting at the axis of the engin~ shaft, so as to provide a
20 symmetry and reversibility of direction of the engine.
Suitably for general purpose use of the engine, the cam means is in
the form of an equilateral triangle, the sides of which are essentially rectilinear. The
lobes of the cam means are relatively sharply rounded with a radius suitably of
about 6 mm ~0.25 in.), which engine, when operated as an external combustion
25 motor, may be self-starting and reversing. However, the shape of the cam means
will influence the torque and other characteristics of the engine, and under
appropriate circumstances a triangular cam means with sinuously formed sides maybe preferred, particularly where the sides have a convex shape on approach to a
cam lobe.
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In further accordance with the preferred embodiment the housing is
provided with eight openings symmetrieally arranged therearound, and conveniently
each opening has an associated track means. Four of the openin~s may be elosed
with the above described piston means, which may be ref~rred to as the primary
5 piston means, with the remaining four openings capped off. Such engine is easily
modified to form an eight cylinder engine merely be removing the caps and
replacing them with secondary piston means which are essentially identical to the
primary piston means.
The foregoing objects and aspects of the invention, together with other
10 objects, aspects and advantages thereof will be more apparent from a consideration
of the following description of the preferred embodiment thereof taken in conjunction
with the drawings annexed hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
IN THE DRAWINGS:~5 FIG. 1- is a schematic transverse mid-seetional view of a four
cylinder engine in accordanc3 with the invention;
FIG. 2- is a sehematic axial mid-seetional view through one
eylinder of the engine of FIG. 1, with valve eomponents
shown in addition;
~0 FIG. 3 - is a schematic axial elevation showing further detail of a
valve arrangement with hidden detail shown in dashed
outline;
FIG. 4 - is a plan view of the cylinder of FIG. 3;
FIG. 5/6- are similar to FIGS. 3 and 4 respectively, but show a
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modified cylinder;
FIG. 7 - is similar to FIG. 1 but shows an eight cylind~r fonn of the
engine;
FIG. 8 - is a schematic illustration of an electrically operated valve
s arrangement associated with a cylindor, and
FIGS. 9A-9C- show variants of a cam means for use with
the engine of FIG. 1, with a basic triangular
shape being shown in dashed outline for the
purpose of comparison.
0 DESCRIPTION OF THE PREFERRED EMBODIMENT
Engine 10 comprises a housing 12 including a pair of opposed, spaced
apart side plates 14 with a shaft 16 mounted therefrom by bearings 18 for rotation.
A cam 20 is mounted on shaft 16 for rotation therewith. Housing 12 includes eight
facets forming mounts 22 disposed in equi-spaced relationship on a circle centred
15 on the axis of rotation of shaft 16, with stiffening spacers 26 being disposed between
the side plates 14 at each ~djacent pair of mounts. A piston means 28 which
includes a cylinder 30 is disposed on alternate ones of mounts 22 and secured
thereto by bolts 32 which conveniently screw into side plat~s 14, those of mounts
22 not having a cylinder disposed thereon being generally closed off by a cap 34.
20 Piston means 28 also includes a piston 40 from which is rigidly dependent a piston
rod 42. Each piston rod 42 has a clevis opening 44 within which is mounted a camfollower 46 on a bearing pin 48 adjacent the distal end of the piston rod. Bearing
pin 48 projects outwardly on opposed sides of clevis opening 44 to provide a mount
for a pair of guide bearings 50 disposed on axially opposed sides of cam follower
25 46. Each mount 22 has associated therewith a pair of tracks 54 which are
conveniently machined into side plates 14 and along which guide bearings 50 will
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roll as a piston 40 reciprocates in its cylinder 30. It will be understood that cylinders
30 and tracks 54 and bearing pins 48 are all centred on diameters passing through
the axis of rotation of shaft 16.
Engine 10, where in the form of an external combustion motor includes
a valve assembly 60 conveniently in the form of a rotating oscillating inlet valve 62
operated by a push rod assembly 64 disposed on the outside of housing 12 in
association with each cylinder 30 and which includes a push rod 66 driven by a
timing valve cam 68 disposed on engine shaft 16 and secured thereto by key 70.
An exhaust port 72is disposed in the wall of each cylinder 30 and an inlet port 74
10 in the head thereof.
Cam 20 is g~nerally in the form of an isoscsles triangle with rectilinear
sides 80 and lobes 82 which are sharply rounded with a radius 84 of approximately
6 mm, which dimension may be relatively independent of the size of cam 20, at least
over the range wherein sides 80 have a dimension in the range of about 5 cm to
lS about 50 cm (2 in. to 20 in.). Timing cam 68 has a shape that is generally
complementary to that of main cam 20, ie. is in the form of an isosceles triangle,
although with the lobes 86 thereof substantially flattened as will be subsequently
discussed.
Considering engine 10 operating as a motor from a source (not shown)
20 of expandable gas, and differentiating the cylinders 30 for the purposes of the
ensuing description with the letters A, B, C and D, and with the components in their
relative positions as shown in FIG. 1, at start-up, assuming the engine valve cam
68 to be adjusted to provide a cloc~wise movement of shaft 16, the piston 40 of
cylinder A will be marginally beyond its top dead centre position. The inl~t valve 62
25 to cylinder A will be marginally open, and those to cylinders B, C and D will be
closed. Accordingly, the piston 40 of cylinder A will be urged downwardly by theexpandable gas introduced into cylinder A, causing cam 20 and shaft 16 therewithto rotate in a clockwise direction. The valve 62 to cylinder A will suitably close when
piston 40 of cylinder A has descended approximately one third of its stroke with30 lobes 86 being shaped accordingly. Concomitantly with the downward movement
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of the piston of cylinder A, the piston of cylinder B will be urged upwardly, trappins
a volume of gas in the cyiinder. Ultimately, the piston 40 of cylinder B will assume
the position of the piston 40 of cylinder A illustrated in FIG. 1 and the expandable
gas wTII be introduced into the cylinder B. At this time engine 10 will be operating
S dynamically, and a work output will be generated by the piston 40 of both cylinder
A as this moves towards the bollo,l, of its stroke, and the piston of cylinder B. The
pistons of cylinders C and D will operate in analogous manner to pistons A and Band in general, when engine 10 is dynamically operational as a motor, two adjacent
pistons will provide power on an expansion stroke and two adjacent pistons will be
10 driven by cam 20.
In view of the relatively high torque output from engine 20, sha~t 16 may
often be coupled directly to a unit to be driven without any intermediate gear box.
Where it is desired that the engine of FIG. 1 be operated in an anti-clockwise
direction, it is merely required to flip timing cam 68 through 180~. It will be
15 understood that other, somewhat more complex variations may be used for shifting
timing cam 68 relative to shaft 16 for reversing the direction of rotatiori of the engine.
Considering now valve cam 68 to be adjusted to operate engine 10 as
a motor turning in an anti-clockwise direction and considering the parts to be in the
relative positions as seen in FIG. 1, at start-up the piston of cylinder A will be. in a
20 position marginally before top dead centre and the inlet valves 62 to cylinders A, C
and D will be closed. The inlet valve 62 to cyiinder B will be open, urging the piston
40 thereof downwardly, ther~by causing cam 20 to rotate in an anti~lockwise
direction and shaft 16 therewith. When cam 20 is rotated to a position to urge the
piston 40 of cylinder A to its top dead centre position, the inlet valve 62 to cylinder
25 A will open and the sequence of operations described above in relation to the engine when operated in a clockwise direction is repeated in reverse.
The rolling action of cam followers 46 and guide bearings 52 and the
reduction of side forces on pistons 40 permits engine 10 to be operated under
certain conditions without lubrication, or with lubrication provided only through the
30 use of sealed bearings, which is highly advantageous under adverse conditions.
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The maintenance of engine 10 is particularly facilitated due to the free
fioating action of the pistons 40, which permits the cylinders 30 and pistons 40 to be
removed simply by the remoYal of bolts 32. Cylinders 30 may accordingly be
replaced by cylinders 130 for example, as seen in FIGS. 5 ~nd 6, which have a
5 substantially grsater internal diameter than cylinders 30. In this instance,
securement bolts 132 will not intersect the side plates 14 of housing 12.
Accordin~ly, cylinders 130 are supported from mounts 22 by a pedestal 133 havingan internal diameter smaller than that of cylinder 130, whereby bolts 32 passingthrough an outwardly turned flange 135 serve to secure the cylinder to housing 120
10 in place of a cylinder 30.
The conversion of engine 10 to an eight cylinder engine is equally
simple, and involves the removal of caps 34 from the engine 10 of FIG. 1 and thesecurement of cylinders 30 and related c~mponents in their place, to form en~ine110 of FIG. 7.
A four cylinder motor 10 will have twelve power strokes per revolution
of shaft 16, and this will be doubled for the eight cylinder motor 110. Accordingly,
it will be appreciated that this results in motors having an exceptionally high torque
and smooth operation.
Although the materials of construction of engine 10 are not critical,
20 much of the structure thereof, including housing 10 is particularly amenable to
manufacture from plastic materials, and it is contemplated that the tracks 54 be lined
with replaceable liners 56 to facilitate maintenance.
As seen in FIG. 8, the engine of the invention may have an electrically
operated valve assembly 160 associated with each cylinder 130, which takes the
25 place of mechanically operated valve assembly 60 earlier described. Valve
assembly 160 includes an associated switch mechanism 162 including switch
contacts 164a, 164b which are actuated by rotcr 120, and reversing switch 166 which
permits the selection of either of switch contacts 164a, 164b.
Referring now to FIG. 9A, a second embodiment of main cam 20 is
30 identified thsrein by the numeral 120A, with the basic equilateral, rectilinearly sided
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shape of similar to that of cam 20 being superimposed in dashsd outline denoted
by the IQttsr O. Cam 120A has lobes 182, and sides 180 extending between
adjacent pairs of lobes. Sides 180 include a first portion 181A extending between
a lobe and a mid zon~ of the side denoted by the letter M, and a second portion
5 183A extending between the mid zone and the adjacent lobe. In this second
embodiment, cam side portions 181A and 183A are identically shaped whereby the
sides 180 are fully symmetrical, and cam 120A may rotate in either direction.
Assuming an anticlockwise direction, cam side portions 181A will control the
movement of a piston such as piston 40 on the power stroke of the engine, and cam
10 side portions 183A will control the movement on the exhaust stroke. Portion 181A
has a shallow S-shape, being initially convexly curved; this has the effect of
reducing the acceleration of piston 40 in the vicinity of lobe 182 on the power stroke
relative to the acceleration produced using cam 20; it also has the effect of flattening
the torque output curve whereby the maximum torque output occurs at a later
15 interval in the output stroke, while being sustained over an increased interval. Cam
side portion 181A changes to a concave shape on approach to mid-zone M, which
zone is disposed closer to the centre of rotation of cam 120A than in the
corresponding cam 20. This has the effect of increasing the length of the power
stroke and also the angular interval over which a relatively high torque output is
20 maintained in the output stroke. Cam side portion 183A also has a shallow S-shape.
Given that some gas will be trapped within a cylinder to serve as a cushion for a
piston within that cylinder on the exhaust stroke and that the gas will be
compressed by an effort applied through a piston follower such as 46, this shapeof cam side portion 183A serves to locate the angular interval over which the
25 maximum effort is applied in a generally diametric opposition to that over which a
maximum torque is output from another cylinder of the engine, to assist in the
smooth operation thereof. It will in addition serve to dirninish the deceleration of a
piston on approach to a lobe 182 on the exhaust stroke.
A cam having the shape shown in Fig. 9A may be preferred for
30 moderately high speed, reversible engines 10. However, it will be appreciated that
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other cam shapes may be preferred, for example that shown in Fig. 9B wherein theinitial portion 181B of cam 120B has a flat and neutral shape in comparison to the
basic triangular shape, a cam of this form being suited for medium speed operation.
In Fig. 9C a cam 120C is shown wherein the initial portion has a negative incline
5 which is best suited for low speed, high torque engines.
It will be apparent that many changes may be made to the illustrative
embodiment while falling within the scope of the invention, and it is intended that
all such changes be covered by the claims appended hereto.
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