[0001] The present invention relates to a timing system for regulating, via respective camshaft-controlled
valves, fluid input or output to/from one or more cylinders of an engine and/or machine.
In particular, the present invention relates to an overhead camshaft timing system
for an internal combustion engine featuring a number of valves, e.g. five, per cylinder.
[0002] To improve the volumetric efficiency of internal combustion engines, particularly
high-performance engines of sports cars or so-called touring cars, the common design
practice is to provide timing systems which, instead of the usual two valves (supply
and exhaust) per cylinder, feature for example four (two supply and two exhaust) or
five (three supply and two exhaust) valves per cylinder. The latter type in particular
involves serious design problems in terms of valve control and/or arrangement of the
valves to prevent interference, particularly between the tappets of the three supply
valves. To overcome this problem, USA Patent n. 4.615.309 provides for a spread arrangement
of the supply valves, the middle one being arranged obliquely in relation to the two
lateral valves, thus enabling all three to be controlled by a single camshaft, and
for using tappets with smaller than normal diameter caps, in the example shown, smaller
than the caps on the exhaust valves.
[0003] Such a solution, however, still involves a number of drawbacks. Firstly, the supply
valve springs are difficult, when not impossible, to seat inside the respective tappet
caps, thus resulting in reduced compactness. And, secondly, the small diameter caps,
particularly in the presence of valves inclined in relation to the camshaft, and therefore
in the presence of possible lateral thrust on the tappets, may result in increased
contact pressure between the caps and the cap guides on the cylinder head, thus resulting
in impairment of the lubricating oil film and/or greater wear on moving parts.
[0004] The aim of the present invention is to provide a timing system, particularly for
high-performance internal combustion engines, designed to overcome the aforementioned
drawbacks, i.e. which is both compact and straightforward to produce, and enables
the obtainment of valve ports of as large a size as possible.
[0005] With this aim in view, according to the present invention, there is provided a timing
system, particularly for an internal combustion engine, whereby a camshaft controls,
against the action of elastic means, the translation of respective stems of a number
of adjacent service valves for regulating the input or output of a fluid from a respective
cylinder; each said valve being controlled by a respective cam of said camshaft via
the interposition of a respective tappet; characterised by the fact that the respective
axes of said valve stems are arranged eccentrically and on alternate opposite sides
in relation to the axis of the respective tappet, so as to define a first predetermined
eccentricity between the axis of each tappet and the axis of the corresponding valve
stem.
[0006] A non-limiting embodiment of the present invention will be described by way of example
with reference to the accompanying drawings, in which:
Fig.1 shows a lateral section of an internal combustion engine featuring a timing
system in accordance with the present invention;
Fig.2 shows a larger-scale view of a detail in Fig.2;
Fig.3 shows a schematic layout in perspective of a number of components on the timing
system according to the present invention.
[0007] Number 1 in Fig.s 1 to 3 indicates a known internal combustion engine of which is
illustrated, for the sake of simplicity, only the top portion comprising a cylinder
head 2 and, underneath, a single cylinder 3 defining, together with cylinder head
2, one of the combustion chambers of engine 1. The other combustion chambers defined
by any other cylinders 3 of engine 1 are not shown. Engine 1 presents a timing system
indicated as a whole by 4 and comprising, in the non-limiting example shown, five
service valves for each cylinder 3: three adjacent supply valves 5 and 6; and two
adjacent known exhaust valves (not shown). All the above valves are mushroom types
for opening/closing respective intake ports 7 and exhaust ports 8 of cylinder 3. Said
ports are formed in a ring on cylinder head 2, on top of the combustion chamber defined
by cylinder head 2 and respective cylinder 3, and are arranged substantially in the
form of a pentagon.
[0008] Timing system 4 also comprises a respective camshaft 10 for simultaneously controlling
all the valves 5 and 6 of all the cylinders 3 on engine 1; and a similar known camshaft
(not shown) for controlling in known manner the valves of exhaust ports 8 of all the
cylinders 3 on engine 1. For each valve 5 and 6, shaft 10 comprises a respective control
cam 11 of predetermined shape, for controlling translation of a respective valve stem
13 via the interposition of a respective known tappet 12 and against the action of
respective elastic means 15.
[0009] According to the non-limiting example shown, timing system 4 according to the present
invention presents an overhead camshaft arrangement, i.e. wherein cams 11 of camshaft
10 cooperate directly with tappets 12, camshaft 10 being arranged so as to turn about
an axis 14 (indicated in Fig.1 by a point defining its projection) corresponding directly
with ports 7. Tappets 12 may be mechanical types with no slack adjustment, as in the
non-limiting example shown, or any known hydraulic type with automatic slack takeup.
Whichever the case, each tappet 12 comprises a cylindrical cap 16 housed in axially
sliding manner inside a guide seat 18 formed in cylinder head 2; and a plate 19 on
top of cap 16 and between this and respective cam 11. Cylindrical cap 16 defines the
outer body of each tappet 12, the axis of which is therefore defined by the longitudinal
axis of symmetry 20 of cap 16 (Fig.2).
[0010] According to the present invention, each stem 13 of valves 5 and 6 presents its longitudinal
axis of symmetry 21 offset in relation to axis 20 of respective tappet 12. In particular,
axes 21 are arranged eccentrically on alternate opposite sides of axis 20 of respective
tappet 12, so as to define a first predetermined eccentricity e1 (Fig.2) between axis
20 of each tappet 12 and axis 21 of respective valve stem 13. Moreover, axes 20 of
tappets 12 are shifted laterally on alternate opposite sides in relation to the axis
of rotation 14 (the transverse position of which is shown by perpendicular line 14
a in Fig.2) of camshaft 10 on the side of respective valve stems 13, so as to define
a second predetermined eccentricity e₂ between axis 14 of camshaft 10 and axis 20
of each tappet 12.
[0011] According to a further characteristic of the present invention, valve 5, which is
located between said two valves 6, is also inclined in relation to valves 6, so that
axis 21 of stem 13 of valve 5 diverges towards respective cylinder 3 in relation to
those of valves 6 (Fig.1). By combining the above two characteristics, axes 21 of
stems 13 of all three valves 5, 6, when projected in a transverse plane in relation
to axis 14, converge at a point (not shown) some distance from and over axis 14 of
camshaft 10. Moreover, eccentricity e₂ and the size of respective plate 19 are so
selected that each cam 11 of camshaft 10 cooperates exclusively with, and exclusively
within the horizontal confines of, plate 19 of respective tappet 12. This provides
for limiting any localized wear exclusively to plate 19, which is replaceable, as
well as for minimising lateral thrust on guides 18, despite off-centering of the axis
along which the thrust of cams 11 is applied (axis 14
a) and axis 20 of respective tappet 12.
[0012] Said elastic contrasting means of valves 5 and 6 comprise respective helical springs
15 wound coaxially about stems 13 and housed inside the top of tappets 12, in the
example shown, inside caps 16, together with a respective shoulder plate 30 for and
integral with each valve stem 13. Via a conical surface 31 flaring towards respective
plate 19, each shoulder plate 30 cooperates with a pair of known cotters 32 secured
axially integral with stem 13 via engagement inside a groove 33 on the same. The end
of stem 13 cooperates with cap 16, while, opposite tappet 12, spring 15 rests on the
bottom wall of guide seat 18, thus providing for a continuous control mechanism for
controlling translation of valves 5 and 6 along the axis of stems 13.
[0013] The advantages of the present invention will be clear from the foregoing description.
The off-centered arrangement of valve stems 13 in relation to the axis of tappets
12 and of tappets 12 in relation to the axis of rotation of camshaft 10, and the diverging
arrangement of central supply valve 5 and the two lateral supply valves 6 provide
for obtaining a five-valve timing system featuring three large, widely spaced supply
ports (for adequate cooling and safeguarding the cylinder head) despite the three
valves being controlled by a single, and what is more, overhead camshaft. This therefore
results in a highly efficient, high-performance internal combustion engine timing
system, which is also highly compact. What is more, off-centering of the various valve
control components may be fairly small, thus minimising lateral thrust on the guide
surfaces of tappets 12, which, more importantly, is predictable and, if necessary,
may be counteracted at the design stage. On engines with known timing systems, on
the other hand, wherein spacing of the intake ports depends solely on the degree of
inclination of the valves, in-service lateral thrust may be unpredictable and, therefore,
far more difficult to counteract.
[0014] To those skilled in the art it will be clear that changes may be made to the system
as described and illustrated herein without, however, departing from the scope of
the present invention. For example, a similar timing system to the one described herein
may provide for indirect valve control via rocker arms, in which case, using rocker
arms of different lengths, the control thrust on the tappets may be applied axially.
1. A timing system, particularly for an internal combustion engine, whereby a camshaft
controls, against the action of elastic means, the translation of respective stems
of a number of adjacent service valves for regulating the input or output of a fluid
from a respective cylinder; each said valve being controlled by a respective cam of
said camshaft via the interposition of a respective tappet; characterised by the fact
that the respective axes of said valve stems are arranged eccentrically and on alternate
opposite sides in relation to the axis of the respective tappet, so as to define a
first predetermined eccentricity between the axis of each tappet and the axis of the
corresponding valve stem.
2. A timing system as claimed in Claim 1, characterised by the fact that the cams of
said camshaft cooperate directly with the respective said valve tappets according
to an overhead camshaft arrangement; the axes of said tappets being shifted laterally
on alternate opposite sides of the rotation axis of said camshaft on the valve stem
side, so as to define a second predetermined eccentricity between the axis of said
camshaft and the axis of each said tappet.
3. A timing system as claimed in Claim 2, characterised by the fact that it comprises
three supply valves for each said cylinder; said three valves being controlled simultaneously
by said camshaft, and one of said three valves being located between and inclined
in relation to the other two, so that the stem axis of said central valve diverges
towards the respective said cylinder and in relation to the stem axes of the other
two valves.
4. A timing system as claimed in Claim 3, characterised by the fact that the stem axes
of said three valves converge at a point some distance from and over the axis of said
camshaft.
5. A timing system as claimed in any one of the foregoing Claims, characterised by the
fact that said tappets are mechanical, each comprising a cylindrical cap cooperating
in sliding manner with a guide seat formed in the cylinder head of said respective
cylinder; and a plate on top of said cap; said second eccentricity and the size of
said plate being so selected that the respective cam of said camshaft cooperates exclusively
with and within the confines of said plate.
6. A timing system as claimed in any one of the foregoing Claims, characterised by the
fact that said elastic contrasting means comprise respective helical springs wound
about said valve stems and housed at least partially inside said tappets together
with a respective shoulder plate for and integral with each said valve stem.
7. A timing system as claimed in Claim 6, characterised by the fact that said shoulder
plate is secured integral with said respective valve stem via a pair of cotters cooperating
with a conical inner surface of said plate and a respective groove on said valve stem.