[0001] The invention relates to cam-driven valve-control mechanisms and to machines having
such mechanisms.
[0002] In machines in which cam-driven valves are used as flow-controlling devices, these
valves function by periodically influencing the flow of medium. The medium may be
any fluid, such as a liquid, a gas, or some other substance of a rheological nature,
such as, for example, a paste. Usually this periodic motion of the valve is derived
from a rotating cam in contact with the valve-mechanism, whereby the profile of the
cam controls the motion of the valve in an intended manner.The cam may drive the valve
directly by contact with the valve stem, or alternatively indirectly via a rocker,
a rocker- pushrod mechanism or any other suitable means. As a consequence of production
tolerances, temperature variations, and other factors, the mechanism usually shows
some geometrical play. Due to the motion there are, apart therefrom, the effects of
dynamics and deformation of the constituent parts of the valve train.For the period
that the valve should be at rest, i.e. outside its working- cycle, contact between
the valve and its cam has to be interrupted, resulting in additional play in the mechanism.
All types of followers, such as rollers, flat or knife-edges suffer from the disadvantage
that they do not provide the required compensation of the play, nor have any positive
effect on the other influences mentiond.
[0003] These cam-driven valve-control mechanisms are found for example, in pumps, compressors,
internal combustion engines with two-stroke or four-stroke working cycles, and hydraulic
and pneumatic motors.Though not limited thereto, there application is usually related
to machines with at least one reciprocating piston.
[0004] The play and all other negative factors in the above-mentioned mechanisms cause a
number of drawbacks when operating a machine comprising such cam-controlled valve.
Thus, on closing, as a consequence of the above factors in the mechanism, the valve
will hit its seat with considerable force. In combination with the springs, which
are present in valve-systems of this type, this may result in highly undesirable valve
bouncing or vibrations during the total valve motion cycle. This will result in undue
wear and may even result in mechanical failure due to breakage. Also the bouncing
of the valve causes inadvertent opening and closing of the valve, resulting in disrupted
control of the medium flow. The above discussed consequences during closing of the
valve equally occur during opening of the valve with similar drawbacks, resulting
from the play and the other negative factors.
[0005] The combined effects of loss of adequate control over opening and closing of the
valves are, together with increased wear, particularly prominent in those machines
in which these valves are used for controlling both the inlet as well as the outlet
flow. In most instances an overlap will occur, such as for example in reciprocating
piston compressors of some types. In those cases the motion of the valves in their
actual behaviour results in substantial variations in the pressure and the time at
which the compression is started, and similarly with respect to the time and pressure
at which the compression is released. This causes quite considerable variations in
conditions as is illustrated in Fig.l of the accompanying drawings. In Fig.l the pressure
is indicated along the vertical axis and the volume along the horizontal axis. The
solid line represents the desired situation whereas the dashed line represents what
is actually obtained. Since all instabilities in flow, variations in ultimate pressure
and wear, result in a considerable loss of energy, it is evident, that remedial measures
producing improvements of these damaging factors, will result in machines with a higher
reliability, with lower wear and with better efficiency. This last result applies
especially to internal combustion engines, as the result is a better control of the
cylinder contents, when the effect of valve play and valve-dynamics on valve-controlled
flow are eliminated or at least substantially reduced. These improvements would result
in a lower waste of energy and consequently in improved efficiency. This effect would
be especially noteworthy for diesel-engines.In diesel-engines the commencement of
combustion, which is entirely controlled by the pressure in the cylinder at the beginning
of the fuel- injection, as well as the duration thereof, which is also a function
of the pressure at the end of the compression stroke, markedly affects the efficiency.
In addition uncontrolled overlap may well result in loss of medium. These effects
are illustrated in Fig.2 of the accompanying drawings in which the same parameters
are plotted as in
Fig.l, and in which the solid line again represents the desired situation, whereas
the dotted line represents what is actually obtained.
[0006] According to the invention there is provided a valve-control mechanism comrpising
at least one cam for controlling the motion of a respective valve, wherein the or
each cam is provided with a post-cam at the descending part of its profile, causing
compensation of the play of the valve-control mechanism while the valve is still resting
on its seat, preceded by the main valve actuating portion, which is corrected for
the interaction of medium and valve dynamics, and/or with a pre-cam at the ascending
part of its profile causing compensation of the play of the valve-control mechanism
while the valve is still resting on its seat prior to opening of the valve.
[0007] The invention thus provides a way of compensating the play which is present in a
conventional cam-driven valve-mechanism, together with the control over valve-dynamics,
and accordingly the drawbacks resulting therefrom. This is achieved by altering the
ascending and/or descending slope of the driving cam in such a way that the influence
of existing plays and vibrations in the cam-driven mechanism are compensated for to
the extent that they no longer affect the critical actions of the valve. Thus full
control of the functioning of the valve can be established. The hitherto encountered
valve-bouncing and vibrations are prevented. The closing of the valve is dealt with
by the post-cam, and the opening of the valve is dealt with by the pre-cam.
[0008] As a consequence of the-post-cam, the valve will smoothly settle on its seat before
any play present in the valve-control mechanism is released. In a similar way the
opening of the valve can be controlled more accurately by the provision of the pre
-cam at the ascending side near the beginning of the profile. Thus it is possible to
cause the valve to open at the very moment this action is required, without this being
affected by any play present in the valve-control mechanism. Apart from the above
the main valve-actuating portion should of course be shaped in such a way that an
optimal positioning and motion of the valve with respect to flow-control is achieved.
[0009] In a preferred embodiment of the invention the or each cam is generally pear-shaped
with a dimple in at least the descending side of the cam, the narrower part serving
as the main valve-actuating portion, and most preferably the or each cam is symmetrical
along its longitudinal axis for the majority of applications.
[0010] It is of importance for good functioning of the valve-control mechanism that the
or each cam is designed and arranged so that it compensates the play and the dynamics
in the'valve-control mechanism up to the moment of closing of the valve or beyond
that moment.
[0011] In the valve-control mechanism the or each cam is preferably in contact with a roller,
a slider, a flat or knife-edge urging the valve. A preferred embodiment is a valve-control
mechanism in which the or each cam is in contact with one end of a rocker, the other
end of the rocker being in contact with a pushbar carrying a valve at its other end,
the pushbar being provided with a spring urging the valve into engagement with its
seat.
[0012] This preferred embodiment is especially suitable for an internal combustion engine
with a two-stroke or four-stroke working cycle.
[0013] From a practical viewpoint it is advantageous that each cam is substantially symmetrical
in shape.
[0014] The invention is further described below with reference to the accompanying drawings
in which
Fig.1, as mentioned above, is a graph showing pressure versus volume for a compressor,
Fig.2, as mentioned above, is a graph showing pressure versus volume for a diesel-engine,
Fig.3 presents a preferred embodiment of the cam according to the invention in sectional
view, and
Fig.4 presents a sectional view of a cam-driven valve-mechanism according to the invention.
[0015] Fig.3 shows a cam in which the solid line profile in the descending part (right hand
side) of the cam forms the after-bulge or post-cam and the dashed line represents
the conventional shape.In the ascending part (left hand side) the dashed line shows
a pre-bulge or pre- am and the solid line represents the conventional form. It will
be understood that the extent and the dimensions of every bulge are governed by the
prevailing conditions in the particular mechanism in which the cam is used, as will
be discussed more fully hereinbelow in connection with Fig.4. In most practical applications
the cam is substantially symmetrical in shape, being provided with a pre-cam and a
post-cam, the main profile of the cam being adapted to match the dynamics of medium
and valve.
[0016] The valve-control mechanism according to the invention can be used advantageously
in compressors, expanders, fluid pumps, hydraulic or pneumatic motors and in particular
diesel-engines and spark ignited engines.
[0017] Referring now to Fig.4, there is shown a valve 11 out of contact with its seat 12,
driven by a cam 4, which is shaped substantially as illustrated in Fig.3, via a rocker
7. As the cam rotates counterclockwise about its centre of rotation 5, the after-bulge
or post-cam 1 will initially compensate any play present. The after-bulge or post-cam
1 will come into contact with a pick-up point 6 of the rocker 7, which rotates about
its axis 8. This induced motion of the rocker 7 is passed on by point 9 to a pushrod
10 of the valve 11. A spring 13 has the customary function of inducing the valve 11,
once it has left its seat 12, to return to that seat 12. When the after-bulge or post-cam
1 passes the pick-up point 6, the valve 11 is returned to its seat 12, preferably
with zero acceleration, thus causing a smooth return of the valve 1
1 on its seat 12. Only after return of the valve 11 to its seat 12 is the drive-mechanism
further released. Any play that may then arise can no longer affect the valve 11 and
any occurrence of vibrations is thus avoided.
[0018] After having passed the section of the cam between post-cam 1 and pre-cam II in the
direction of the rotation, during which period the valve 11 will remain closed, pick-up
point 6 enters into contact with pre-cam II (indicated with the dotted line, the solid
line being the conventional shape). While passing this pre
-cam II any play present will initially be compensated. Only after any play has been
compensated will the valve 11 commence to open at exactly the required moment. The
ensuing motions of the valve 11 are from thereon controlled by the subsequent section
of the cam profile as indicated by 1, 2 and 3 respectively. This main part 1, 2, 3
of the cam profile is adapted in a way that valve dynamics and medium dynamics interact
in the favourable way anticipated. Thus the valve will make motions which are dictated
by this cam profile. Having gone through this part of the profile pick-up point 6
of rocker 7 will again come into contact with post-cam I, and the cycle as described
above will be repeated.
[0019] From the above it will be understood that the profiles of the main cam together with
the respective bulges, that is post-cam, and if desired also pre-cam, should be adapted
to the particular system and within a particular system to the respective dimensions
of the mechanism.As a consequence the valve will open smoothly at the desired moment
as dictated by the pre-cam, whereafter the valve port will show a variation with time
as dictated by the subsequent cam-profile 1, 2 and 3, and subsequently the valve is
returned to its seat as dictated by the post-cam, avoiding bouncing and hard hitting
of the seat of the valve. During this motion cycle valve dynamics and medium dynamics
interact as desired, detrimental wear and damage to valve and seat are prevented as
well. Once the valve has thus closed the valve port, during further following of the
post-cam the entire valve-control mechanism is smoothly released, thus avoiding any
detrimental vibration.
[0020] Depending on the machine on which the cam-driven valve-control mechanism is used,
the cam will be either symmetrical or asymmetrical. As an example of a machine in
which the cam will usually be substantially symmetrical mention may be made of a reversible
engine, such as for the propulsion of ships.
1. A valve-control mechanism comprising at least one cam for controlling the motion
of a respective valve, wherein the or each cam is provided with a post-cam-at the
descending part of its profile, causing compensation of the play of the valve-control
mechanism until the valve is resting on its seat, preceded by the main valve actuating
portion, and/or with a pre-cam at the ascending part of its profile causing compensation
of the play of the valve-control mechanism while the valve is still resting on its
seat prior to opening of the valve.
2. A mechanism according to claim 1, wherein the or each cam is generally pearshaped
with a dimple in at least the descending side of the cam, the narrower part serving
as the main valve-actuating cam portion.
3. A mechanism according to claim 1 or 2, wherein the or each cam is substantially
symmetrical along its longitudinal axis.
4. A mechanism according to any one of the preceding claims, wherein the or each cam
is arranged to compensate the play and dynamics of the valve-control system up to
the moment of closing of the valve or beyond that moment.
5. A mechanism according to any one of the preceding claims, wherein the or each cam
is in contaot-with a respective roller, a slider, a flat or a knife-edge urging the
valve.
6. A mechanism-according to any one of claims 1 to 4, wherein the or each cam is in
contact with one end of a rocker, the other end of the rocker being in contact with
a pushbar being provided with a spring urging the valve into engagement with its seat.
7. A compressor or expander which comprises a valve-control mechanism according to
any one of the- preceding claims.
8. A fluid pump or hydraulic or pneumatic motor, which comprises a valve-control mechanism
according to any one of claims 1 to 6.
9. An internal combustion engine, which comprises a valve-control mechanism according
to any one of claims 1 to 6.
10. An internal combustion engine according to claim 9, wherein the engine is a diesel-engine.
11. An internal combustion engine according to claim 9, wherein the engine is spark
ignited.