Valve system for engine

A valve system for an engine comprises a plurality of intake or exhaust valves, adapted to be driven by cams of which the profiles are different from each other to make maximum valve lifts of the valves different from each other, and compression springs biassing the valves toward the corresponding closing directions, the springs being identical to each other. An initial length of the spring is made shorter as the maximum valve lift of the valve becomes smaller, to make an initial load of the spring larger as the maximum valve lift of the valve becomes smaller.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a valve system for an engine. 
2. Description of the Related Art 
Japanese Unexamined Patent Publication No. 1-159417 discloses a valve 
system for an engine having a pair of exhaust valves, in which valve 
opening periods of the exhaust valves are different from each other to 
suppress the pulsation of the exhaust gas flow, to thereby reduce the 
pumping loss of the engine. Note that, in such a valve system, each 
exhaust valve is biassed by a compression spring toward the closing 
direction thereof, and opens when the opening force due to the associated 
cam becomes larger than the spring force of the compression spring. 
In the valve system, to make the valve opening periods of the exhaust 
valves different from each other, the profiles of the cams are made 
different from each other. In this case, the maximum valve lift of the 
exhaust valve becomes larger as the valve opening period thereof becomes 
longer, if the profiles of the cams are defined considering the durability 
and reliability of the exhaust valves as in the usual manner. Thus, the 
maximum valve lifts are different from each other, if the valve opening 
periods are made different from each other, as in the above-mentioned 
valve system. 
In the valve system in which the maximum valve lifts are made different 
from each other in this manner, the dynamic characteristic of each exhaust 
valve can be made optimum if the elements used with the exhaust valve such 
as the compression spring, are optimized for the respective exhaust valve. 
However, if the different elements are used for the different exhaust 
valves, the number of the elements and the costs therefor increase. 
Further, the elements are preferably identical for every exhaust valve, 
considering the assembly of the valve system. 
However, if the compression spring optimum for the exhaust valve having the 
shorter valve opening period is used with the exhaust valve having the 
longer valve opening period, a minimum excess load, which is a minimum 
value of the difference between the spring force of the compression spring 
and the inertia of the exhaust valve, becomes excessively larger. This 
results in increasing the friction on the exhaust valve having the longer 
valve opening period, and thus the fuel consumption rate may become low 
and the surface of the cam may wear out. On the other hand, if the 
compression spring optimum for the exhaust valve having the longer valve 
opening period is used with the exhaust valve having the shorter valve 
opening period, the minimum excess load becomes excessively smaller. As a 
results, the response of the exhaust valve having the shorter valve 
opening period with respect to the corresponding cam may deteriorate and 
thus the exhaust valve may jump or bounce. Further, in this case, the 
maximum engine speed must be limited because the surging of the spring for 
the shorter valve opening period may occur if the engine speed becomes 
higher. JPP'417 mentioned above does not teach a solution to any of the 
above problems. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a valve system for an 
engine capable of ensuring a good operation of the valves, while 
simplifying the structure and the assembly of the valve system. 
According to the present invention; there is provided a valve system for an 
engine comprising: a plurality of intake or exhaust valves adapted to be 
driven by cams, of which profiles are different from each other to make 
maximum valve lifts of the valves different from each other; and 
compression springs biassing the valves toward the corresponding closing 
directions, the springs being identical to each other; wherein an initial 
length of the spring is made shorter as the maximum valve lift of the 
valve becomes smaller, to make an initial load of the spring larger as the 
maximum valve lift of the valve becomes smaller.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 illustrate a case where the present invention is applied to a 
pair of exhaust valves of an engine. Alternatively, the present invention 
may be applied to more than two exhaust valves, a plurality of intake 
valves of an engine, or both the exhaust and intake valves. 
Referring to FIG. 1, first and second intake valves 2a and 2b are arranged 
on one side of an inner wall 1a of a cylinder head 1 of an engine, and 
first and second exhaust valves 3a and 3b are arranged on the other side 
of the inner wall 1a of the cylinder head 1. The intake valves 2a and 2b 
are formed by the identical members, and the exhaust valves 3a and 3b also 
formed by the identical members. Further, a spark plug 4 is arranged 
generally at the center of the inner wall 1a. 
Referring to FIG. 2, the reference numerals 5 and 6 respectively designate 
a cylinder block and a combustion chamber of the engine, 7a and 7b 
respectively designate first and second exhaust ports formed in the 
cylinder head 1, and 8a and 8b respectively designate first and second 
cams for respectively driving the first and second exhaust valves 3a and 
3b. The cams 8a and 8b are formed on a common cam shaft 9, which rotates 
about an axis K--K. 
Valve lifters 10a and 10b are arranged between the tops of the exhaust 
valves 3a and 3b and the associated cams 8a and 8b, and slidably 
reciprocate within guide holes 11a and 11b formed in the cylinder head 1, 
while being guided by the corresponding guide holes 11a and 11b. Further, 
valve spring retainers 12a and 12b are connected to the tops of the 
exhaust valves 3a and 3b, via locks (not shown). 
Valve spring seats 13a and 13b are formed in the cylinder head 1 around the 
stem portions of the exhaust valves 3a and 3b, each having a recessed 
configuration. Valve springs 14a and 14b are inserted between the valve 
spring retainers 12a and 12b and the corresponding valve spring seats 13a 
and 13b, in a compressed state. The valve springs 14a and 14b urge the 
associated exhaust valves 3a and 3b toward the respective closed position 
of the valves 3a and 3b. 
As shown in FIG. 2, the valve spring seat 13a for the first exhaust valve 
3a is formed to make the distance H from the cam axis K--K equal to H1, 
and the valve spring seat 13b for the second exhaust valve 3b is formed to 
make the distance H from the cam axis K--K equal to H2, which is longer 
than H1 by DH. When the exhaust valves 3a and 3b are closed, the distances 
between the cam axis K--K and the bottom surfaces of the valve spring 
retainers 12a and 12b are identical to each other, and are represented by 
"h" in FIG. 2. Therefore, if the lengths of the valve springs 14a, 14b 
when the exhaust valves 3a and 3b are kept closed is referred to as an 
initial length, the initial length of the valve spring 14a is represented 
by H1-h, and that of the valve spring 14b is represented by H2-h, which is 
longer than that of the valve spring 14a by DH. 
The valve lifters 10a and 10b, the valve spring retainer 12a and 12b, and 
the valve springs 14a and 14b are respectively formed by the identical 
members. In other words, the elements for the first exhaust valve 3a and 
those for the second exhaust valve 3b are identical. This avoids assembly 
errors. 
The opening forces of the cams 8a and 8b act on the associated exhaust 
valves 3a and 3b via the corresponding valve lifter 10a and 10b, 
respectively, and the exhaust valve 3a, 3b will open when the opening 
force acting thereon becomes larger than the closing force of the 
corresponding valve spring 14a, 14b. 
FIG. 3 illustrates valve lift curves of the exhaust valves 3a and 3b, the 
valve lift curves illustrating the relationships between the valve lift of 
a valve and the rotational angle of a cam. In FIG. 3, the curve L1 shows 
the valve lift curve of the first exhaust valve 3a and the curve L2 shows 
the valve lift curve of the second exhaust valve 3b. As shown in FIG. 3, 
the first exhaust valve 3a opens for a period corresponding to the cam 
rotational angle CA1, and the second exhaust valve 3b opens for a period 
corresponding to the cam rotational angle CA2. Namely, the valve opening 
period of the second exhaust valve 3b is longer than that of the first 
exhaust valve 3a. Making the valve opening periods of the first and second 
exhaust valves 3a and 3b different from each other in this manner, reduces 
the pulsation of the exhaust gas flow and thereby the pumping loss of the 
engine, as mentioned at the beginning of the specification. Note that, as 
shown in FIG. 3, the closing timings of the first and second exhaust 
valves 3a and 3b substantially conform to each other. This ensures the 
stability of the engine during the engine idling operation. 
To make the valve opening periods of the exhaust valves 3a and 3b different 
from each other, the profiles of the cams 8a and 8b are made different 
from each other, as shown in FIG. 4. This simplifies the structure of the 
valve system. Note that the radius R of the basic circles of the cams 8a 
and 8b are identical to each other. 
Generally, a profile of a cam is defined to make the maximum valve lift of 
the associated valve as large as possible, while preventing the response 
of the valve from deteriorating and preventing the striking sound of the 
cam from being loud. Thus, if the profiles of the cams are defined to make 
the valve opening periods different from each other under these 
limitations, the maximum valve lifts are made different from each other. 
Namely, as shown in FIG. 3, the maximum valve lift M1 of the first exhaust 
valve 3a is smaller than the maximum valve lift M2 of the second exhaust 
valve 3b, by DL. Further, when the profiles of the cams 8a and 8b are 
defined under the above-mentioned limitations, the relationships between 
the accelerations of the exhaust valves 3a and 3b and ratios of the valve 
lifts of the exhaust valves 3a and 3b to the corresponding maximum valve 
lifts are made substantially identical to each other. 
Next, the method for setting the distance H between the cam axis K--K and 
the valve spring seat 13a, 13b will be explained, with reference to FIG. 
5, as well as FIG. 2. In FIG. 5, curves I1 and I2 partly illustrate the 
inertia of the first and second exhaust valve 3a and 3b, respectively. 
Conventionally, the valve spring seats are formed to make the distances H 
identical to each other, and thus the initial lengths of the valve springs 
14a and 14b are identical to each other. In this case, if the identical 
valve springs are used, initial loads acting on the valve springs 14a and 
14b are identical to each other, and thereby spring force curves of the 
valve springs 14a and 14b are made identical to each other. Note that an 
initial load is a load acting on the valve spring 14a, 14b when the 
corresponding exhaust valve 3a, 3b is kept closed, and a spring force 
curve illustrates the relationships between the spring force of the valve 
spring 14a, 14b and the cam rotational angle. 
Namely, if the valve spring seats 13a and 13b are formed to make both the 
initial lengths of the valve springs 14a and 14b equal to H1-h, both the 
initial loads of the valve springs 14a and 14b are made equal to IL1, and 
thus both the spring force curves of the valve springs 14a and 14b in this 
case conform to the curve S1 shown in FIG. 5. Contrarily, if the valve 
spring seats 13a and 13b are formed to make both the initial lengths of 
the valve springs 14a and 14b equal to H2-h, both the initial loads are 
made equal to IL2, and thus both the spring force curves conform to the 
curve S2 shown in FIG. 5. 
However, when the maximum valve lifts of the exhaust valves 3a and 3b are 
different from each other as in the present embodiment, the inertia curves 
I1 and I2 of the exhaust valves 3a and 3b are different from each other, 
as shown in FIG. 5. In this condition, if both the spring force curves of 
the valve springs 14 and 14b are made identical to the curve S1 in FIG. 5, 
the minimum excess load of the second exhaust valve 3b is equal to the 
excessively larger value MALx, while the minimum excess load of the first 
exhaust valve 3a is equal to the optimum value MALS. If the minimum excess 
load becomes excessively large as in this case, the friction between the 
cams 8a and 8b and valve lifts 10a and 10b increases, and thus the fuel 
consumption rate becomes low and the surfaces of the cams or the valve 
lifters wear out. 
Contrarily, if both the spring force curves of the valve springs 14 and 14b 
are made identical to the curve S2 in FIG. 5, the minimum excess load of 
the first exhaust valve 3a is made equal to the excessively smaller value 
MALy, while the minimum excess load of the second exhaust valve 3b is made 
equal to the optimum value MALS. If the minimum excess load becomes 
excessively smaller as in this case, the responses of the exhaust valves 
3a and 3b with respect to the cams 8a and 8b deteriorate, and thus jumping 
and bouncing of the exhaust valves occurs. Further, in this case, the 
maximum engine speed should be limited because the surging of the spring 
may occur if the engine speed becomes higher. 
Accordingly, when the maximum valve lifts of the exhaust valves 3a and 3b 
are different from each other and thereby the inertia curves of the 
exhaust valves 3a and 3b are different from each other, it is necessary to 
make the spring force curves of the valve springs 14a and 14b different 
from each other, to thereby optimize the minimum excess load of the 
respective exhaust valve 3a, 3b. Namely, when the valve springs are 
identical to each other and thereby the spring constants thereof are 
identical, it is necessary to make the initial length shorter as the 
maximum valve lift becomes smaller, to make the initial load larger as the 
maximum valve lift becomes smaller. 
Therefore, in the present embodiment, the valve spring seat 13b for the 
exhaust valve 3b having the larger maximum valve lift is formed to make 
the distance H equal to H2 to make the initial lengths of the valve spring 
14b equal to H2-h, and the valve spring seat 13a for the exhaust valve 3a 
having the smaller maximum valve lift is formed to make the distance H 
equal to H1, which is shorter than H2 by DH, to make the initial lengths 
of the valve spring 14a equal to H1-h, which is shorter than H2 by DH. As 
a result, the initial load of the valve spring 14b is made equal to IL2, 
and the spring force curve of the valve spring 14b conforms to the curve 
S2, and the initial load of the valve spring 14a is made equal to IL1, 
which is IL2, than IL2, and the spring force curve of the valve spring 14a 
conforms to S1. Accordingly, both the minimum excess loads of the exhaust 
valves 3a and 3b are made equal to the optimum value MALS. 
According to the present embodiment, the initial lengths and initial loads 
of the valve springs 14a and 14b are adjusted by adjusting the distance H 
of the valve spring seats 13a and 13b, and the distance H is simply 
adjusted by the design of the mold for molding the cylinder head 1. 
Namely, the special machining and the additional elements are unnecessary, 
and thus the valve system is constructed at low cost and easily. 
FIG. 6 illustrates another embodiment of the present invention. 
Referring to FIG. 6, the distance H of the valve spring seats 13a and 13b 
are made equal to H2. However, a spacer 20 having a thickness of DH is 
inserted between the bottom end of the valve spring 14b and the associated 
valve spring seat 13b. As a result, the initial lengths of the valve 
spring 14b is made equal to H2-h, and that of the valve spring 14a is made 
equal to H1-h, which is shorter than H2 by DH, as in the previous 
embodiment. Such a spacer makes the initial lengths and initial loads of 
the valve springs 14a and 14b adjustable in the conventional cylinder 
head. 
Alternatively, the initial length of the valve spring may be adjusted by 
adjusting the radius of the basic circle of the associated cam. 
According to the present invention, it is possible to provide a valve 
system for an engine capable of ensuring a good operation of the valves, 
while simplifying the structure and the assembly of the valve system. 
While the invention has been described by reference to specific embodiments 
chosen for purposes of illustration, it should be apparent that numerous 
modifications could be made thereto by those skilled in the art without 
departing from the basic concept and scope of the invention.