Note: Descriptions are shown in the official language in which they were submitted.
20~8i9
This invention relates generally to internal combustion
engines, and more particularly to a piston assembly
comprising a piston head adapted for reciprocating movement
in a cylinder and a connectlng rod for connecting the piston
head to a crankshaft.
In a conventional internal combustion engine, the
piston head, which reciprocates in the cylinder, is
connected to the crankshaft by a connecting rod ~or
con~erting the reciprocating motion of the pi~ton into the
rotary motion of the cranksha~t. The connecting rod is
pivotally attached at one end, namely the big end, to the
crankshaft, and at the other end, namely the little end, to
the piston head by means of a pivot-shaft or wrist pin~ The
wrist pin is inserted into tha walls of the piston below the
piston ring. This arrangement limits the langth of the
connecting rod relative to the total dimension ~rom the
piston top to the bottom bearing centre of the connecting
rod. The connecting rod length determines the rod
angularity, or angular displacement, due to arankshaft
rotation. The rod angularity i~ related to the mechanical
efficiency of transforming linear motion during the power
stroke into rotary crank motion and its related torque. The
greater the angularity of the connecting rod, the lower the
e~ficiency o~ the conversion process.
An object of the present invention is to improve the
conversion efficiency relative to conventional designs.
Accordingly, the present invention provides a piston
assembly comprising a piston head adapted for reciprocating
movement in a cylinder, a connecting rod for connecting said
piston head to a crankshaft, said connecting rod having a
big end for pivotal attachment to the crankshaft and a
little end for attachment to the piston head, and coupling
means for pivotally attaching said little end to said piston
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2 ~ 9
head while en~uring lateral displacement of the pivo~ point
in the same direction as the lateral movement of the
connecting rod during rotation of the crankshaft so as to
reduce the angular displacement o~ the connecting rod.
By later~lly displacing the pivot point during the
crankshaft cycle, the connecting xod can be made to pivot
effectively about a virtual pivot point located above the
top o~ the piston with a consequential decrease in rod
angularity. As a result, the efficiency of the mechanical
conversion process is increased~
In a preferred embodiment, the connecting rod
terminates in a dual-ridge abutment engaging in a
complementary pocket formed on the underside o~ the piston
head. During the crankshaft cycle, the connecting rod rolls
between two pivot points defined by the abutment ridges.
Roller bearings carried by brackets attached to the piston
head engage bearing surfaces on the underside of the
abutment to retain the abutment in the receiving pocket at
at least three points.
Preferably one ridge i~ higher than the other. This
arrangement serves to provide a shorter travel distance for
the piston during the first 90of crankshaft rotation of the
power stroke (downward travel~. During this phase of the
cycle the lower pivot is in contact. On the upward stroke
the higher pivot is in contact.
Since the Pffective connecting rod length is
considerably greater than the actual rod length, trne
assembly offers a numker of advantages in addition to
reduced rod angularity for greatPr conversion ef~iciency.
There is a significant increase in piston dwell time at TDC
(top dead centre) as well as a significant decrea~e in dwell
time at BDC (bottom dead centre).
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In addition, the assembly offers a significant decrease
in the side loadings of the piston because the piston can be
maintained in a stable horizontal position by ensuring that
the abutment is always supported at at least three points
defined by the active abutment and the roller bearings.
Since no wri~t pin is required and the abutment i5
received directly in the pocket on the underside of the
piston, the piston can be made very ~hallow. This means
that piston can be approximately ~0 percent of the weight of
a conventional piston of the same bore diameter with a wrist
pin due to the shallow profile and reduction in material
volume. As a result, the reciprocating mass is reduced and
consequently the efficiency increased since the inertia of
the mass whose motion has to be reversed at each extremity
is less. The connecting rod in the novel piston assembly
has approximately the same overall weight as a conventional
connecting rod.
The absence of piston oscillation, due to the high-low
dual ridge arrangement, results in a substantial reduction
in friction compared to conventional designs.
Desirably, the pocket is made of a very hard material~
such as ceramics or ceramic-alloy compounds. These
materials provide much greater heat capacity and dimensional
stability than materials used in conventional designs. As a
result, the piston rings can be mounted ver~ close to the
edge of the piston crown, thus reducing crevice volume, iOe.
the volume of the space between the crown of the piston and
the uppermost piston ring. The abutment can be made of a
slightly softer material such as steel.
The piston assembly can be adapted to a variety of
geometries to suit internal combustion or engines of
different types, including four cycle, two cycle, diesel,
and multi-fuel engines, such as stratified charge engines
and the like.
The invention will now be described in more detail by
way of example only, with reference to the accompanying
drawings, in which:-
C-
Tigure 1 ~ is~diagrammatic illu~tration o~ a
conventional piston and connecting rod assembly;
Figure 2 is a similar illustration of a piston assembly
in accordance with one c~bodiment of the in~ention;
Figure 3 i5 a partial section of a practical embodiment
of a piston assembly in accordance with the invention;
Figure 4 is a section through the piston assembly taken
at right angles to the section in figure 3, showing the
connecting rod abutment in side view;
Figure 5 is an underside plan view o~ the piston head;
and
Figure 6 is a diagrammatic illustration helpful in
understanding the invention of the ronnecting rod in the 90
position;
Figure 7 is a diagrammatic illustration helpful in
understanding the invention of the connecting rod in the TDC
position.
Figure 8 is a detailed view of the dual ridge abutment;
and
Figure 9 shows a three-ridge abutment.
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2~6819
Referring now to Figure l, a conventional piston head
l, which can be of ceramic or ceramic composite, has a
connecting rod diagrammatically illustrated as 5 pivoted
about a rod pivot 100 to the piston head 1 and at its lower
5 end, or big end, to the crankshaft, diagrammatically
illustrated as 101. The piston head has conventional piston
rings 102 to provide a gas tight and oil tight seal for the
piston head.
As the crankshaft lO1 rotates, the ~onnecting rod 5 of
course undergoes angular displacement as the piston 1
reciprocates in the cylinder. In the illustrated prior art
design, with the connecting rod ratio of 02.56 the maximum
angular displacement of the connecting rod, when the
crankshaft is at 90 , is 11.3 . Additional impurtant
parameters ara the dwell time at top dead centre (TDC) and
bottom dead centre (BDC), which in the example shown are 12
and 20 respectively. Piston dwell occurs at top dead
centre and bottom dead centre due to the end of the
connecting rod describing an arc substantially coincident
with the path of the crankshaft and is a function o~ rod
length divided by piston stroke. A large rod provides
longer swell at TDC and shorter dwell at BDC.
In the embodiment according to the present invention,
as shown in Figure 2, the pivot point 100 is displaced
laterally as the piston reciprocates, which as illustrated
in Figures 6 and 7 has the effec~ of increasing the
effective length of the connecting rod such that it pivots
about a virtual pivot point 60 above the piston head l.
Figures 6 and 7, which are purely diagrammatic and
exaggerated for the purposes of illustration, show the
principle of operation of the invention by showing the
angular deflection of a connecting rod with a laterally
displaceable pivot as compared with a fixed pivot in the 90
position. As will be seen in the figures, the angularity o~
-- 5 --
20S6~19
the connecting rod 5 connected to a laterally displaceable
pivot is substantially less than the angularity of a
connecting rod 5' assumed to pivot about a fixed point on
the median plane 102' (Compare angle ~ with angle ~)~ The
limit planes 102 in the extreme positions of the connecting
rod 5, which pass through the displaced pivot point~ lOOa,
lOOb converge at a point 60, which constitutes a virtual
pivot point for the connecting rod. The conn~cting has an
effective length with regard to its angularity determined by
the location of the pivot point 60. By making le~t pivot
point slightly higher than the right, th~ travel distance o~
the piston 1 can be made slightly shorter for the first 90
of the crankshaft cycle.
As shown in Figure 2, the effect of the increase in
effective length of the connecting rod 5 is to increase top
dwell time to 23 and decrease bottom dwell time to 15 .
Figure 2 shows the arrangement for providing the
pivotal connection with lateral displacement of the pivot
point. The connecting rod 5 terminates in an elongate
29 abutment 15 having a pair of space ridges 51~ 52, the ridge
52 being higher than the ridge 51. The ridges 51, 52,
engage complementary depressions 61,62 in pocket 12 formed
on the underside of the piston head 1. The abutment 50 ha~
wing portions 56 extending beyo~d the main stem of the
connecting rod and definin~ bearing surfaces 53, 54 on
eith~r side of the stem that bear against roller bearing 41
carried in bracket 4 mounted on the underside 10 of the
piston head 1.
More details of the piston assembly can be seen in
Figures 3 to 5 where the actual profile connecting rod 5 is
-~shown. Figure 3 shows tha cylinder liner 6 and part of the
cylinder head 7. As can be seen in Figure 3, the piston
head 1 is of very shallow design, with a triangular
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20~68~9
compression ring 2 communicating with the cylinder space
through gas transfer holes 20 in the piston crown. The oil
ring 3 is located immediately below the compression ring.
The entire piston head assembly is preferably made of
ceramic or ceramic composite material, which allows the
compression ring 2 to be located immediately ad~acent the
piston crown.
The mode of operation can be understood with reference
to Figure 2. In the median position, when the piston is
either at TDC or BDC, the ridges 51, 52 fit snugly in the
corresponding matching depressions 61,62 in pocket 12. As
the piston head 1 moves from top dead centre to bottom dead
centre, the connecting rod 5 is angularly displaced to the
right, and as a result the abutment 5 undergoes an anti-
clockwise rocking motion causing the connecting rod to pivotabout ridge 52 during the downward stroke as seen in the
central position, where the connecting rod is at 90 crank
rotation. At BDC, both ridges 51, 52 engage their
complementary depressions, and then as the piston rises the
abutment 50 rolls in a clockwise direction so that the
connection rod pivots about ridge 51 duriny th0 upward
stroke. As explained with reference to Figure 6, this
results in the effective length of the connecting rod 5
being increased with a consequential reduction in the
angularity of the connecting rod during the crankshaft
cycle. The fact that the ridge 52 is lower than the ridge
51 has the effect of slightly shortening the travel distance
of the piston 1 during the powex stroke, which is desirable
for improved efficiency.
A further effect of the increase in effective length of
the connecting rod is to reduce bottom dwell time to 15 and
increase top dwell time to 23 . The described cooperating
abutment and pocket effectively cause the pivot point of the
connecting rod to be displaced durin~ the crankshaft cycle
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20~8~
and raise the effective pivot point to a point 60 above the
piston head 1. The effective pivot point is where the
connecting rod would have to be pivoted to undergo the same
lateral displacement for the same angular displacement i~
the lateral position of the pivot point did not move during
the crankshaft cycle.
The action of the high 51 and low 52 pivot-ridge within
the corresponding twin pivot-pocket 12 in the cylinder
undersi~e 10, and the roller tracks 53, 54 at the upper end
of the connecting rod, ensures that the abutment is retained
at at least three points and that the piston 1 remains in a
constant horizontal position regardless of connecting rod
angularity. This prevents piston oscillation within the
cylinder, thus reducing friction compared to con~entional
pistons, where oscillation in the cylindar tend to occur.
As previously indicated the piston 1 can be made very
shallow since there is no wr~stpin. Furthexmore, when very
hard materials, such a ceramics or ceramic-alloy compounds
are employed, these piston materials lnherently provide much
higher heat capacity and much greater dimensional stability.
As a result the piston rings 2 can be mounted very close to
the edge of the piston crown, thus reducing crevice volume.
The compression ring is triangular with a 45 inward
reaction sur~ace directed toward the gas pressure, which is
~5 transferred from the combustion chamber through piston-crown
transfer holes 2~. The 45 reaction surface on the interior
surface of the compression ring assures equal downward and
outward sealing pressure against piston-groove and cylinder
wall. The placement and shape of the compression ring
groove reduces the crevice volume by at least 50% compared
to conventional designs.
Figure 8 shows the effect of the height variation of
2 0 ~ 9
the ridges 51, 52. The lower ridge 52 always lies in the
direction of rotation of the crankshaftO As the abutment 15
pivots during crankshaft rotation, a gradual smooth rolling
contact is made between point A, B, C. I~ dual ridges o~
the same height are employed, the point of contact swings
between A, C without intermediate contact B and is therefore
somewhat jerky. This arrangement can be employed, however,
in low r.p.m. engines.
As shown in Figure 9, it is possible to have three or
more ridges, creating contact points A, B, C, D, E. The
ridges and corresponding groo~es then act somewhat in the
manner of a meshing gear train to smooth out the transition
between contact points during crankshaft rotation.
The piston assembly is approximately 60~ of the weight
of a conventional piston of the same bore diameter due to
its shallow profile and great reduction in material voluma.
This results in significant xeduction in overall
reciprocating mass. The connecting rod, with its special
twin-ridge pivot-head and conventional lower end, is roughly
equal in overall weight to a conventional rod.
The oil ring 3 is generally conventional in profile,
and mounted below the single compression ring described
above.
The piston assembly can be adaptsd in a variety of
geometries to suit internal combustion engines of different
types includîngo 4-cycles, 2-cycles, diesel of different
types and multi-fuel engines (stratifi2d charge etc.~O
In the described piston~assembly, the TDC dwell time is
almost double that of the best current engines and BDC dwell
time is much shorter. The combustion space remains smallex
for a longer period of the powerstroke which results in more
2 ~ 1 9
fuel-efficient power production.
The piston timing requires less ~alve overlap, which
implies a less aggressive camshaft. There is less gas ~low-
throu~h, which results in a truer compression ratio and a
lower intake airflow demand~ Less aggressive cam profiles
can be employed to provide higher and flatter torque curve.
Less radical valve-lift allows lower spring pressures,
~riction and weight.
The limit of breathing for a specific engine head
appears at higher RPM, the peak air intake flow occurs later
after ~DC, and the powerstroke piston acceleration is lower,
resulting in a reduced peak velocity during downward
movement.
BDC direction reversal is faster and upward piston
acceleration is higher, which proYides better response ~o
efficient exhaust tuning. No pis*on lubrication is required
at the rod junction and the pistons can be made ultra-
compact with no skirts or no wristpin. The piston shape ls
ideal for a cerami moulded construction. Ceramic pi~ton
are stable at high temperatures, which allows a closer fit
inside the cylinder. The piston remains in a constant
horizontal position and as a result there is no tendency for
piston to oscillate inside the cylinder. This results in
reduced stress on piston rings, the ability to operate with
two rings (rather than 3), reduced side thrust on piston, a
better piston seal, and reduced crevice volume. The lo.nyer
rod has reduced angularity, resulting in greater mechanical
efficiency (mcre torque~. The reciprocating mass is
reduced. The piston assembly has this potential ~or C05t
reduction in mass market.
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