U.S. patent number 3,641,988 [Application Number 05/007,927] was granted by the patent office on 1972-02-15 for valve-actuating mechanism for an internal combustion engine.
This patent grant is currently assigned to Fiat Societa per Azioni. Invention is credited to Dante Giacosa, Giovanni Torazza.
United States Patent |
3,641,988 |
Torazza , et al. |
February 15, 1972 |
VALVE-ACTUATING MECHANISM FOR AN INTERNAL COMBUSTION ENGINE
Abstract
A valve-actuating mechanism for an internal combustion engine
has a number of rocker arms for operating the respective valves,
each rocker arm having a profiled cam surface for engaging the
respective valve, and means, preferably hydraulically operated, for
varying the valve movement produced by the rocker arm in dependence
upon the engine speed and load to vary the valve timing for optimum
efficiency.
Inventors: |
Torazza; Giovanni (Turin,
IT), Giacosa; Dante (Turin, IT) |
Assignee: |
Fiat Societa per Azioni (Turin,
IT)
|
Family
ID: |
11273318 |
Appl.
No.: |
05/007,927 |
Filed: |
February 2, 1970 |
Foreign Application Priority Data
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Feb 3, 1969 [IT] |
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50581 A/69 |
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Current U.S.
Class: |
123/90.16;
123/90.48; 123/90.27; 123/90.31; 123/90.39; 123/90.6 |
Current CPC
Class: |
F01L
1/34 (20130101); F01L 13/0021 (20130101); F01L
1/12 (20130101); F02D 13/0211 (20130101); Y02T
10/12 (20130101); Y02T 10/18 (20130101); F01L
2820/035 (20130101) |
Current International
Class: |
F01L
13/00 (20060101); F01L 1/34 (20060101); F01L
1/12 (20060101); F02D 13/02 (20060101); F01l
001/34 (); F01l 001/04 () |
Field of
Search: |
;123/90.15,90.16,90.17,90.18,90.24,90.27,90.31,90.39,90.48,90.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1,284,700 |
|
Jan 1962 |
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FR |
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311,884 |
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Apr 1919 |
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DD |
|
Primary Examiner: Smith; Al Lawrence
Claims
What is claimed is:
1. In a valve-actuating mechanism for an internal combustion
engine, of the kind having a rotary drive shaft, a plurality of
rocker arms, means converting rotary movement of the shaft into
rocking movement of the rocker arms, and respective valves operable
by the rocker arms, the improvement comprising the rocker arm
having a cam surface the contour of which includes a profiled valve
operating portion, means varying the amplitude of the valve
movement caused by said profiled valve operating portion, means
varying the amplitude of the valve movement caused by said profiled
cam portion in dependence upon the speed of and load upon the
engine including an oscillating lever interposed between the cam
surface and the valve effective to amplify the valve displacement
caused by the valve operating portion of the said cam surface, and
means effecting a relative rotation between the rocker arm and the
oscillating lever, said rotation having a magnitude determined by
the speed of and load on the engine, wherein the means effecting
the relative rotation between the rocker arm and the oscillating
lever displace the fulcrum of the oscillating lever along an
arcuate path which is centered upon the axis of rocking movement of
the rocker arm.
2. Mechanism according to claim 1, wherein the fulcrum of the
oscillating lever is carried by a movable support which is in turn
rotatably mounted on the shaft bearing the rocker arms, and
including means rotating the movable support in dependence upon the
speed of and the load on the engine.
3. Mechanism according to claim 2, including an hydraulic actuator
device arranged to cause rotation of the support in dependence upon
the speed of and load on the engine.
4. Mechanism according to claim 3, wherein the hydraulic actuator
device comprises means defining a bore in the movable support, a
piston slidably mounted in the bore, a fixed reaction surface
engaging the piston, and means supplying said bore with oil the
pressure of which is dependent upon the speed of and load on the
engine.
5. Mechanism according to claim 4, wherein said means supplying
pressurized oil comprises a pump in the lubrication system of the
engine.
6. Mechanism according to claim 4, including a valve regulating the
pressure of the oil supplied to said bore, a centrifugal governor
controlling the operation of said valve in dependence on the engine
speed and a vacuum-operated capsule, connected to the induction
system of the engine, controlling the operation of said valve in
dependence on the engine load.
7. Mechanism according to claim 1, wherein the point of contact
between the cam surface and the oscillating lever shifts towards
the fulcrum of the oscillating lever upon increase of the amplitude
of valve displacement.
8. Mechanism according to claim 1, wherein the angular speeds of
the rocker arm and of the oscillating lever are in opposite
directions throughout the whole duration of the displacement of the
valve (6).
9. Mechanism according to claim 1, wherein the oscillating lever
has a rounded surface comprising an arc of a circle by which said
lever contact the respective valve.
10. Mechanism according to claim 9, wherein the center of said
rounded surface coincides with the axis of oscillation of the
rocker arm.
11. Mechanism according to claim 9, wherein the center of the said
rounded surface is displaced with respect to the axis of
oscillation of the rocker arm.
12. Mechanism according to claim 1, including respective shafts
mounted in the cylinder head of the engine, and respective crank
means, for example an eccentric and a connecting rod connecting the
induction and exhaust valves to the respective shafts, opening of
the respective valves being effected when the crank means are in
top dead center positions.
13. Mechanism according to claim 2, including a cam shaft on which
the rocker arms are rotatably mounted, the movable support
comprising an oscillating arm mounted for rotation about the cam
shaft, and further including a shaft carried by the oscillating arm
and acting as a fulcrum for the oscillating lever, means being
provided for rotating said arm in dependence upon the speed of and
load on the engine.
14. Mechanism according to claim 13, wherein one single actuator
device effects simultaneous rotation of the oscillating arms
associated with the induction and exhaust valves in dependence upon
the speed of and load on the engine.
15. Mechanism according to claim 14, wherein the actuator device is
hydraulic and includes a piston which contacts simultaneously the
respective oscillating arms constituting the movable supports
associated with the induction and the exhaust valves.
16. Mechanism according to claim 15, wherein the piston has an
outer surface with an axial section which is trapezium shaped, the
oscillating arms bearing upon the inclined sides of the said
trapezium-shaped surface.
Description
This invention concerns a device for driving the valve gear of
internal combustion engines, from a rotary drive shaft.
It is known that in internal combustion engines with a four-stroke
cycle the induction and exhaust strokes, which theoretically should
last only for the time taken by the piston to make the relative
strokes, are generally augmented, by causing each valve to open
before the bottom dead center position of the piston, and to close
after the top dead center position: that is to say, an advance and
a delay are given to the induction and exhaust strokes, so selected
as to allow for delays inherent in filling and exhausting the
cylinder, due to the resistance to the passage of gas through the
inlet and exhaust valves.
From this it follows that the total period during which each valve
is open has to be large (that is, with a large advance and a large
delay in valve opening and closing respectively) with large engine
speeds, while this period has to be much smaller at low speeds if
the engine power is to be maintained at low speed.
Since the engine, especially if it is used for motor vehicles, is
required to operate over a fairly wide range of speeds, the timing
used in practice is usually a compromise, giving moderate engine
powers at both low and high speeds; that is to say, the performance
of the engine will only be good within a fairly limited speed
range.
Similar considerations also apply in the case of two-stroke engines
which have operated valves.
There also arises the problem of restricting the outflow from the
engine cylinders of poisonous gases made up of unburnt hydrocarbon,
carbon monoxide and oxides of nitrogen; this outflow diminishes
with reduction of the changeover period--that is, the period during
which both the exhaust valve and the inlet valve of any one
cylinder are open.
From the foregoing considerations it will be apparent that, both
from the point of view of good performance (that is, high torque
and low fuel consumption at all speeds), and from the point of view
of low atmospheric contamination (that is, small amounts of
poisonous gases in the engine exhaust), the timing of the engine
valve gear ought to be variable in accordance with the speed of and
the load on the engine.
To this end various mechanisms have been patented, none of which,
however, either through constructional difficulties, or because of
their inherent disadvantages and limitations, proved satisfactory
in practice.
Among the best known of such mechanisms are the following:
2. Mechanisms which allow variation of the clearance between each
cam and the respective valve tappet, or which in order to reduce
timing increase the play: such arrangements have a serious
disadvantage in that they lead to unacceptable cam impact
velocities as soon as the tappet clearance becomes noticeable;
b. Mechanisms with frustoconical cams: each cam has a variable lift
law along its width so that by shifting the cam shaft axially,
different valve timings are achieved. From this it necessarily
arises that the tappets engaged by the cams must be spherical, and
in consequence there is punctiform contact between each cam and
tappet, with very high contact pressures, if the valve springs are
so proportioned as to allow high valve operating speeds;
c. Mechanisms which consist in controlling separately the opening
and the closing of each valve with two cams mounted on different
shafts: by varying the relative angular settings of the two shafts
the timing can be varied. With this system the laws of valve
opening and of closing remain constant and one has to accept a
valve lift which is constant in relation to the maximum valve lift
through a certain angle of rotation of the cam shafts, the constant
lift section being the more extensive the greater is the variation
of timing which is required. Mechanisms of this sort, whose
operation seems more reliable than mechanisms (a) and (b) are,
however, complicated in construction and expensive;
d. Mechanisms which produce a variation of the keying between the
engine shaft and each valve cam shaft: such mechanisms are
obviously applicable only to engines having twin cam shafts for the
induction and for the exhaust valves, driven by a chain or a
toothed belt. The aforesaid variation of the keying can be
obtained, for example, by rocking arms carrying sprocket wheels
which engage parts of the chain or the belt, to change the chain or
belt tension. Systems of this sort only make possible a scaling
down of the timing, whilst the duration of the strokes remains
unchanged, and
e. Mechanisms in which there is interposed, between each cam and
the respective valve tappet, a drum which shifts angularly on a
radius the center of which coincides with the axis of the cam: with
this system there is achieved only the same effect as in the
previous case, that is, a scaling down of the timing with strokes
of constant duration.
With the aim of obviating the disadvantages of the various
mechanisms listed above, the present invention provides a valve
actuating mechanism for an internal combustion reciprocating
engine, including a rotary drive shaft, and characterized in that
the mechanism includes means for converting the rotary movement of
the drive shaft into rocking movement of a cam having a cam surface
the contour of which includes a profiled valve operating portion,
and means for varying the amplitude of the valve movement caused by
said profiled cam portion in dependence upon the speed of and load
upon the engine.
Further characteristic features and advantages of this invention
will be apparent from the description which follows, with reference
to the accompanying drawings which illustrate by way of example
some practical embodiments, and in which:
FIG. 1 is a sectional elevation of a first embodiment of a valve
actuating mechanism according to the invention with induction and
exhaust valves closed;
FIG. 2 is a part-sectional elevation of part of the mechanism
illustrated in FIG. 1, on a larger scale, with the valve open;
FIG. 3 is a diagrammatic illustration of an hydraulic device which
is part of the mechanism illustrated in FIG. 1;
FIG. 4 consists of a number of diagrams illustrating graphically
the variations of the lift, speed and acceleration of the valves
plotted against the angular position of the rotating shaft of the
mechanism illustrated in FIG. 1, for two different positions of the
hydraulic device illustrated in FIG. 3;
FIG. 5 is a view in elevation of a variant of the embodiment
illustrated in FIG. 1;
FIG. 6 is a diagrammatic representation of an hydraulic device
forming part of the mechanism illustrated in FIG. 5;
FIG. 7 consists of a number of diagrams illustrating graphically
the variations of the lift, speed and acceleration of the valves
plotted against the angular position of the rotating shaft of the
mechanism illustrated in FIG. 5 for two different positions of the
device illustrated in FIG. 6;
FIG. 8 is a sectional elevation of a second embodiment of a valve
actuating mechanism according to the invention;
FIG. 9 is a sectional elevation of part of a third embodiment of
the invention, and
FIG. 10 comprises a number of diagrams illustrating graphically the
variations of the lift, speed and acceleration of the valves
plotted against the angular position of the rotating shaft of the
mechanism illustrated in FIG. 9 for two different positions of an
hydraulic device similar to that illustrated in FIG. 6.
Throughout the drawings, the same reference numerals indicate the
same or corresponding parts.
FIGS. 1 and 2 illustrate the valve gear of a four-stroke internal
combustion engine of the kind having overhead induction and exhaust
valves arranged in a V configuration and driven by two respective
overhead camshafts driven by the engine.
In FIG. 1, 1 indicates part of the internal combustion engine,
showing the upper part of a cylinder 2 housing a slidable piston 3
driven by a connecting rod 4.
The cylinder 2 has at its upper end two ports 5 for induction and
exhaust respectively, said ports 5 cooperating respectively with an
induction valve 6 and an exhaust valve 6a.
Each valve 6, 6a comprises a head 7 which is shaped in the
conventional manner to make sealing contact with seats ground on
the respective ports 5. Each vale 6, 6a further has a stem 8 formed
integrally with the head 7 and surmounted by a cotter 9.
On each valve 6, between the cotter 9 and the port 5, and
surrounding the stem 8, there are provided a number of springs 10
(two in the example illustrated) to urge the valves 6-6a towards
their closed positions (FIG. 1).
In order to effect opening of the valves 6, 6a against the biasing
action of the springs 10, there are housed in the cylinder head 11
of the engine 1, above the valves, two cam shafts 12, shown in
broken outline in FIG. 1, one for each valve 6-6a. The cam shafts
12 rotate at a speed which is directly related to the speed of the
engine. In this particular case, where the engine is of the
four-stroke type, the shafts 12 rotate at a speed equal to half
that of the engine shaft.
Each of the two cam shafts 12 supports, at axially spaced
positions, a number of circular eccentrics 13 the number of which
is equal to the number of valves on the shafts 12.
To each eccentric 13 there is keyed or secured a connecting rod 15
having a small end 14 to which is pivotally connected, by means of
a pin 16, a cam in the form of a rocker arm 17. The rocker arms 17
associated with each cam shaft 12 are pivotally supported on a
shaft 18.
On the side of the shaft 18 opposite to the pivot pin 16 each
rocker arm 17 has a shaped cam surface 19 having a first section 20
concentric with the axis of the shaft 18, and a second section,
hereinafter referred to as the gauge portion 21, the shape of which
determines the law of operation of the respective valves 6-6a.
Each rocker arm 17 acts with its cam surface 19 on the upper
surface 22 of an oscillating lever 23 pivotally mounted at one end
on a shaft 24. At its other end, on its lower surface, each
oscillating lever 23 has a rounded surface 25, the profile of which
is an arc of a circle, which rests upon the cotter 9 of the
respective valve stem 8.
The shaft 24, which acts as a fulcrum for the oscillating levers
23, is carried by a support 26 which is able to rotate around the
shaft 18 on which the rocker arms 17 are supported. Each support 26
(FIG. 3) has an internal cylindrical bore 27 in which a piston 28
is slidingly mounted. The piston 28 has at its lower end an
integral projection 29 which rests upon a bearing face 30 formed in
the cylinder head 11.
The upper part of the cylindrical bore 27 communicates, through a
port 31, with a duct 32a through which flows oil under pressure
from the lubrication system of the engine, supplied by a pump 33.
The flow or the pressure of the oil, or both, are regulated by a
valve 34 in dependence upon the speed of the engine and the load on
the latter, that is, the torque delivered thereby.
A branch duct 35 including a flow restrictor 36 branches from the
duct 32a, downstream of the pump 33. The branch duct 35 supplies
oil to a centrifugal governor 37 which in turn controls, in
accordance with the speed of the engine, the oil pressure upstream
of the restrictor 36, that is, the input oil pressure at the valve
34. The oil pressure, determined by the centrifugal governor 37,
controls the opening of the valve 34.
The opening of the valve 34 is at the same time controlled by a
vacuum-operated capsule 38, communicating with the induction
manifold of the engine, shown diagrammatically at 39. The capsule
38 controls the valve 34 in dependence on the load on the
engine.
In the mechanism thus far described, given the considerable
amplitude of angular oscillation of the rocker arms 17, which is
necessary to be able to accommodate a cam surface 19 which gives
the necessary movement, the oscillation of each rocker arm 17 is
effected by means of a desmodromic drive system comprising an
articulated quadrilateral linkage formed by the respective
eccentric 13 which acts as a crank, the connecting rod 15 and the
rocker arm 17.
By this means, the reactions of the valve springs 10 in the
negative acceleration parts of each piston stroke can be reduced
compared with valve-actuating mechanisms generally used. Moreover a
more rigorous law of movement for the valves is obtained insofar as
it is influenced by errors in construction of a single cam
only.
To justify the introduction of the oscillating lever 23 between the
rocker arm 17 and the cotter 9 of the respective valve 6, it should
be noted that drive of the valve 6 directly via the rocker arm 17
would require excessive dimensions for the rocker arm, if one
wished to obtain the law of acceleration usually adopted for the
valves of reciprocating engines.
In order to obtain the most favorable law of movement for each
valve 6, while keeping the dimensions of each rocker arm cam 17 and
of its cam surface 19 within acceptable limits, it has been found
preferable for the connection between the rocker arm 17 and the
valve 6 to be made through the intermediary of oscillating lever
23, placed in such a way that the area of contact of the cam
surface 19 with the upper surface 22 of the oscillating lever 23,
conjugate with it, is shifted towards the axis of rotation of the
oscillating lever 23 in order to increase of the lift. This leads
to angular velocities of the oscillating lever 23 contrary to those
of the rocker arm 17 during the whole valve-lifting movement, and
to an obvious reduction in the overall dimensions of the
device.
In the device described, since the shaft 24 is carried by the
movable supports 26, which in their turn are pivotable about the
shaft 18, the shaft 24 can shift through a circular arc having a
radius equal to the distance between the axes of the shafts 18 and
24, and it is precisely this shifting which achieves the required
variation of the valve timing.
In fact, upon displacement of the shaft 24 towards the plane 30,
reduction in the amplitude of the timing is obtained, since the
beginning of the gauge portion 21 of the cam surface 19 then comes
into contact with the oscillating lever 23 at a delayed angular
position of the drive shaft 12, and, consequently the last part of
the gauge portion 21 is not utilized.
In FIGS. 1 and 2 the oscillating lever 23 is shown as a continuous
line in the position which gives the maximum timing, and in broken
outline in the position which gives the minimum timing.
If the center of curvature of the rounded surface 25 of the
oscillating lever 23, when the associated valve 6 is closed,
coincides with the axis of the shaft 18, then the operating play or
tappet clearance is not influenced by the position of the axis of
the shaft 24; conversely, it is possible to obtain an operating
play or clearance which is linearly variable with the valve timing
by suitably displacing the center of curvature of the rounded
surface 25 from the axis of the shaft 18. This can be useful if,
for example, the engine is subject to thermal expansions of the
kinematic fall of the valve 6 bringing about variations in the play
which are proportional to the engine speed and load.
In the device illustrated in FIGS. 1 and 2 it should also be
remarked that the position of the cam shafts 12 is of considerable
importance. The placing which is shown permits advantageous
exploitation of the low ratio between the length of the connecting
rod 15 and the crank arm constituted by the radius of eccentricity
of the eccentric 13; in fact the lift of the valve is effected when
the crank mechanism comprising the eccentric 13 and the connecting
rod 15 is situated around the top dead center position which
corresponds to the greatest angular accelerations of the rocker arm
17.
Moreover, also as a result of the low value of the ratio between
the length of the connecting rod 15 and of the crank arm, with
clockwise rotation of the cam shaft 12, the angle through which the
shaft 12 has to rotate to effect the opening of the valve 6 is in
fact less than that necessary to bring about closing of the valve;
hence on closing of the valve there are lower speeds and
accelerations than on opening of the valve.
In this manner it is possible to reduce the tendency towards valve
bounce; moreover lower impact speeds are obtained between the head
7 of the valve 6 and the respective seat upon valve closure, such
as one tries to get in normal valve drives to reduce noise to a
minimum.
With regard to the hydraulic device for effecting rotation of the
support 26, the oil under pressure which enters the bore 27 is the
same as is used to lubricate the engine. In fact the oil supply
duct 32a connected to the bore 27 is branched from a duct 32 which
supplies the lubricating oil to the engine; the quantity and the
pressure of the oil, or both, are governed by the valve 34,
comprising a governor, which is attached to the device and which
senses the speed and the load of the engine, as hereinbefore
described.
With a drive of this kind one can regulate, in an entirely
independent manner, the relative timing of the induction and the
exhaust valves, since there are two separate drives for these
valves.
In the practical embodiments just described and in those described
hereafter, variation of the timing is achieved, as has been seen,
by means of an hydraulic drive actuator. It will be appreciated,
however, that the variation of the timing can be effected by means
of mechanical, pneumatic or electrical actuators responsive to the
speed and the load of the engine.
The diagrams illustrated in FIG. 4 show clearly the effect of the
relationship which exists between the length of the connecting rod
15 and the aforesaid crank arm in the device illustrated in FIGS. 1
and 2. As can be seen, the graphs showing the variations of the
lift, velocity and acceleration of the valve are asymmetrical.
Specifically, the absolute values of the velocity and the
acceleration are greater when the value is open, with the advantage
of obtaining, when the valve closes, low noise and efficient
working of the cooperating members.
In FIG. 4 the continuous curves refer to a wide timing position,
whereas the dashed curves refer to a position corresponding to a
narrower timing.
In the variant of the FIG. 1 embodiment illustrated in FIG. 5, a
single rotating cam shaft 12 drives both the valves 6; this cam
shaft 12 is housed in the engine cylinder block on one side of the
row of cylinders 2.
Mounted on the rotating shaft 12, and integral with it, are
arranged, for each cylinder 2, two circular eccentrics 13 each of
which is connected to or integral with a respective connecting rod
15 which, in this case, is of considerable length. In fact the
small end of the connecting rod 15 is linked, by means of a pin 16,
to a rocker arm cam 17 placed near the top of the cylinder head 11
of the engine 1.
Each rocker arm 17 is mounted on a shaft 18 which is arranged to
rotate within a number of supports 45 fixed to a bearing face 30 in
the head 11, by means of screws 46 (see FIG. 6).
The rocker arm 17, in contrast with that shown in FIGS. 1 and 2,
has its shaped cam surface on the same side of the shaft 18 as the
pin 16.
A support arm 47 is rotatably supported on the shaft 18 and in turn
supports a shaft 24 which acts as a fulcrum for an oscillating
lever 23.
The support arm 47 is formed on the opposite side of the shaft 24
from the shaft 18 with a nose 48 which bears upon a
trapezium-shaped upper surface 49 of a piston 50 of an hydraulic
actuator device 51 illustrated in FIG. 6. The piston 50 is mounted
slidingly in a cylindrical bore 52 in the device 51, the bore 52
being closed at its lower end by the bearing face 30.
Oil under pressure is supplied to the lower part of the bore 52
through an aperture 53. This oil is supplied, as in the embodiment
illustrated in FIG. 3, from the lubrication system via a duct 32a.
In this case also the pressurized oil is circulated by a pump 33,
and a valve 34 governs the delivery and the pressure of the oil in
accordance with the speed of the engine and the load to which the
engine is subjected, that is, the torque delivered by the
engine.
The upper surface 49 of the piston 50 has, on diametrically
opposite sides with respect to the axis of the piston 50, two
inclined plane surfaces, shown as 54 and 55, on which the noses 48
associated with the induction valve 6 and the exhaust valve 6a bear
respectively.
In the valve mechanism just described variation of the timing in
dependence upon the speed of and load on the engine is effected, as
in the device illustrated in FIGS. 1 and 2, by rotation of the
shaft 24, which acts as a fulcrum for the oscillating levers 23,
around the axis of the shaft 18.
In this case, in view of the arrangement of the members, it is
convenient to achieve the variation of timing with a single
actuator device 51, as illustrated in FIG. 6.
It will be observed that in this case too, where a single cam shaft
is provided, one can regulate independently the induction and
exhaust timing. This can be effected by varying the inclination of
the surfaces 54 and 55, or by making the nose 48 associated with
the induction valve 6 of a different length from the nose 48
associated with the exhaust valve 6a.
In the mechanism illustrated in FIG. 5 the cam shaft 12 is situated
in the cylinder block, and in view of the consequent considerable
length of the connecting rods 15, difference in the ratios between
the length of each connecting rod 15 and the effective crank arm
connected thereto is scarcely detectable. As a result the laws of
movement of the valve opening and closing operations will be almost
symmetrical as can be seen from the diagrams illustrated in FIG.
7.
In view of their length, it is more advantageous to make the
connecting rods 15 work as tie rods; this can be done as
illustrated in FIG. 5, by making each rocker arm 17 operative to
lift its respective valve when the crank gear consisting of the
eccentric 13 and of the connecting rod 15 is situated around the
bottom dead center.
The embodiment illustrated in FIG. 8 relates to a four-stroke
internal combustion engine which has induction valves 6 and exhaust
valves 6a arranged in a V formation. A single rotating cam shaft 12
is housed in the cylinder block to one side of the row of cylinders
2, and governs the opening and the closing of said valves 6 and
6a.
Analogously to the embodiment of FIG. 5, on the rotating cam shaft
12 and integral with it there are mounted or formed, for each
cylinder 2, two circular eccentrics 13, on each of which a
connecting rod 15 is mounted.
The small end of each connecting rod 15 remote from the eccentric
13 is joined by means of a pin 56 to one end of one arm 57 of a
small bellcrank lever 58 acting as a rocker. The levers 58,
associated with the two connecting rods 15, are mounted on a common
shaft 59 which acts as a fulcrum.
To the end of the other arm 60 of each lever 58 there is pivotally
joined, by means of a pin 61, one end of a link 62 which is
pivotally connected at its other end, by means of a pin 63, to the
rocker arm 17. The rocker arm 17 has a cam surface 19 which acts on
an oscillating lever 23 which in turn acts upon the cotter 9 of the
respective valve 6.
The shafts 18 and 24, acting as respective fulcra for the rocker
arm 17 and the oscillating lever 23, are carried by supports 45
attached to the cylinder head 11 of the engine 1 by means of
screws, not shown.
The shaft 59, acting as a fulcrum for the rocker levers 58 is
supported by a support 64 mounted rotatably upon one of the shafts
18, which relates to the cam 17 driving one of the two valves, for
example, that carrying the rocker arms 17 associated with the
exhaust valves 6a.
The shaft 59 is supported by the piston of an hydraulic actuator
device of the type illustrated in FIGS. 3 and 6; by means of this
device the shaft 59 can be displaced in dependence upon the speed
of and load on the engine 1.
The result of the displacement of the shaft 59 is a variation of
the effective length of the connecting rod 15 and the link 62 and,
therefore, a different profile of the cam surface 19 of the rocker
arm 17.
A disadvantage of this arrangement is that is comprises more moving
parts than the embodiment described previously. Moreover the
embodiment just described has a single actuator device, and from
this it follows that, while the drive is achieved with greater
ease, there is the disadvantage that the same variation of stroke
for both the induction and exhaust valves. It is clear that to
avoid this disadvantage it would be necessary to have the rocker
levers 58 rotatable about fulcra on two separate shafts and to
displace their axes of oscillation with independent actuators.
It should also be noted that in the embodiment described above, the
oscillating lever 23, the pivotal axis of the shaft of which
remains fixed, is not strictly indispensible and could be replaced
by a flat or cylindrical cotter, but its use makes it possible to
reduce the dimensions of the rocker arm 17.
Naturally the laws of opening and closing of the valves are almost
symmetrical, in view of the length of the connecting rod 15; as a
close approximation one can say that the diagram illustrated in
FIG. 7 is valid for this case as well.
The embodiment illustrated in FIG. 9 relates to a four-stroke
internal combustion engine 1 with induction and exhaust valves 6,
6a in line. In this case, also, the single rotating shaft 12
arranged in the cylinder block on one side of the cylinder 2,
drives the opening and closing of the valves referred to above.
The shaft 12 carries, for each valve, the integral circular
eccentric 13 on which the lower end of the connecting rod 15 is
rotatably mounted. The rocker arm 17 is pivotably connected to the
upper end of the connecting rod 15 by means of a pivot pin 65, the
cam 17 being mounted for rocking movement on a shaft 18.
The cam 17 has a cam surface 19 which acts on a contoured surface
66 made on a face of one arm 67 of a rocker 68 pivotally mounted on
a shaft 24. The other arm of the rocker 68 consists of the lever 23
which acts upon the cotter 9 of the respective valve 6.
The shaft 18 on which the rocker arms 17 are rotatably mounted is
supported by an oscillating arm 69 rotatably mounted upon a shaft
70. The shaft 70 and the shaft 24 are in turn rotatably supported
upon the support 45 which is fixed to the cylinder head 11 of the
engine 1 by means by screws (not shown).
The shaft 18 on which the rocker arms 17 are mounted bears upon an
hydraulic actuator device of the same type as those illustrated in
FIGS. 3 and 6 and is thus displaceable in accordance with the speed
of and load on the engine 1.
In FIG. 9 is illustrated the position of the shaft 18 corresponding
to the maximum cam lift; for a displacement of the shaft 18
downwardly a reduction in timing results, so that the said
displacement causes a rotation of the rocker arm 17 around the
pivot pin 65 and hence a given portion of the cam surface 19 comes
into operation with a delay angle equal to the angle through which
the rocker arm 17 has rotated around the pin 65.
The contoured surface 66 made upon the face of the arm 67 of the
rocker 68 must have its center of curvature coincident, when the
respective valve 6 is closed, with the axis of the shaft 70, if the
valve operating clearance or play is to be constant with variations
in the timing. Analogously with the mechanism illustrated in FIG.
1, a displacement of the center of curvature of the contoured
surface 66 of the axis of the shaft 70 causes the valve operating
play to vary linearly with valve timing.
It should be noted that the displacement of the axis of oscillation
of the rocker arm 17, apart from inducing rotation of the rocker
arm 17 itself, with consequent change in the angle of opening of
the valve 6, also causes a variation in the effective length of the
arm 67, as a result of which the loss of maximum valve lift,
corresponding to reduction of the valve opening angle, is less than
it is in the other devices previously described herein.
The diagrams illustrated in FIG. 10 show the laws of movement of
the valve in two different positions of the shaft 18. In view of
the length of the connecting rod 15 it is evident that the diagrams
are practically symmetrical.
It will be understood that details of practical embodiments of the
invention can be widely varied from those described and illustrated
herein by way of example, without departing from the scope of the
appended claims.
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