VALVES
It has been stated in the preceding chapter that the valves of the
gasoline motor are the sentinels placed on guard at the entrance to and
exit from each cylinder to make certain that the mixture follows its
proper course at the proper time.
Therefore, if we accept the definition that a valve is a mechanical
appliance for controlling the flow of a liquid or a gas, strictly
speaking no such thing as a "valveless" motor exists. Two-cycle
motors are sometimes said to be valveless because of the fact that
the movement of the piston automatically regulates the flow of the
exhaust and intake gases, but in this case the piston is in reality
the valve. On the four-cycle motor, however, like events take place
only on alternate strokes in the same direction, and consequently some
controlling mechanism that operates but once for every four strokes of
the piston is needed to time the flow of the gases.
As has been stated in the previous chapter, the most common form of
valve is known as the poppet type from the fact that its action is
a lifting one. Such a valve may be located in a projection cast on
either side of the top of each cylinder, or it may be inverted from
this position and placed in the cylinder head. When in the former
location, the valve is opened by an upward push on the rod to which it
is attached at its center, while a valve placed in the cylinder head is
forced down to allow the escape or entrance of the exhaust or intake
gases. The ordinary type of poppet valve is somewhat similar in shape
to a mushroom, having a very thin and flat head and a slender stem. The
disc portion of the valve is known as its head, while the rod forged
with the valve and by which the head is raised and lowered is called
the stem.
The projections cast in the cylinders of a "T"-head or "L"-head motor,
and in which the valves are placed, are known as the valve pockets.
Valves so located are lifted by a direct upward push caused by the
rotation of a cam and are returned to their closed position by means of
the extension of a stiff spiral spring surrounding each valve stem. It
is only the outer edge of the lower side of the valve head that comes
in contact with the surrounding surfaces of the opening which is closed
when the valve is returned to its ordinary position by the spring.
This surface of contact surrounding the opening is known as the valve
seat, and it is this, together with the edge of the valve which rests
against it, that must be ground smooth in order to insure a tight joint
when the valve is closed. On the majority of poppet valves the edge of
the head and the seat against which it rests are beveled to an angle
of approximately forty degrees in order to conform to the natural
direction taken by the gases when they are admitted or expelled. In a
few cases, however, the seat angle is ninety degrees, which means that
the edge of the head is ground flat, or straight, at right angles to
the stem.
One of the chief advantages found in the use of a poppet valve is the
fact that a large opening can be obtained after the valve head has
been raised but a comparatively short distance. This means that the
valve stem need travel only a fraction of an inch between the full
open and the full closed position of the valve and that the operating
mechanism for obtaining this lift is simple. Practically every poppet
valve, therefore, is lifted by means of a cam, which is a thick,
irregularly-shaped piece of steel mounted on a shaft known as the cam
shaft. If the end of the valve stem, or a rod connected to it, is
held against the periphery of the cam while the latter is revolved by
its shaft, the valve will be forced up, or away, rather, an amount
corresponding to the increase in distance between the periphery of the
cam at this point of contact and its axis.
In other words, if the cam were a true circle with its axis passing
through its center, there would be no motion of the valve, for all
points of the periphery of a circle are at the same distance from the
center. Consequently a portion of the periphery of the cam is extended
in the shape of a "nose," the projection of this beyond the smallest
diameter of the cam being the distance that the valve will be lifted
when this point of the cam surface comes in contact with the stem or
push rod. The broader, or more blunt, the nose of the cam, the longer
will the valve remain open as the cam shaft is revolved, while the
"slope" of the sides of the nose determines the rapidity with which the
valve will be pushed out and back. Inasmuch as the valve should remain
closed throughout two-thirds or three-quarters of every two revolutions
of the flywheel, the greater part of the periphery of the cam is
circular, or at the same distance from the axis at all points.
As has been mentioned before, the cam serves only to lift the valve,
the return of the latter to its seat being obtained by the force from a
spring that is coiled around the stem. Thus the spring holds the end of
the push rod at all times against the periphery of the cam. This push
rod, in some instances, is a small bar of special steel that slides in
guides of long-wearing bearing alloy. The upper end of the push rod
is in contact with the lower end of the valve stem, while its other
extremity is oftentimes designed in the form of a small steel roller
that thus serves to create a rolling contact with the periphery of the
cam.
In other designs, the lower extremity of the push rod may be in the
form of a specially-hardened steel pin with a rounded end, while still
a third type consists of a flat disc slightly "offset" on the end of
the push rod so that various points of its surface will come in contact
with the periphery of the cam and the wear will be evenly distributed.
Whatever the particular design, however, the cam is well lubricated
and both it and the push rod are intended to last as long as any part
of the motor.
Many motors are designed with one valve at the side and the other,
usually the intake, in the head. There are also many motors
manufactured that have both the intake and the exhaust valves located
in the head, in which case the valve pockets, or projections, are
eliminated. Such valves may be operated by the same type of cams and
cam shaft as those used to open the valves at the side. As the opening
of a valve located in the head is downward, however, the motion
produced by the cam on the push rod must be reversed in direction.
This reversal of motion is obtained by means of a lever mounted at
its center and placed in contact with the upper extremity of the push
rod at its outer end. The other end of this lever operates in contact
with the end of the valve stem, and thus an upward push on the rod is
converted into a downward thrust on the stem. This lever that reverses
the direction of the push rod motion is known as a rocker arm and is
mounted in a yoke cast with the cylinder head.
Inasmuch as a spring is used to keep the valve tightly closed when
the cam is not lifting the latter, it is the contact of the valve head
with its seat that must form the stop to the motion of the spring. It
will be seen that the force of the spring is communicated through the
valve stem to the push rod, and thence to the periphery of the cam when
the latter is in a position to lift the valve. The push rod should not
be forced tightly against the periphery of the cam when the valve is
closed, however, for this would prevent perfect contact between the
valve and its seat. Consequently there should be a certain amount of
"play" between the end of the push rod and valve stem so that it will
be certain that the head is forced against the seat with the full power
of the spring and without the cam serving as a stop.
On the other hand, this play should not be too great, for the cam and
push rod will then move an appreciable distance before the valve is
raised. This will cause the opening of the valve to occur late and
will reduce the distance that the stem is raised, thus restricting the
size of the opening. Furthermore, an undue amount of play between the
ends of the push rod and stem will result in a pound or "hammer blow"
between the two that is liable to wear the surfaces rapidly.
The "happy medium" that will give the best results may be obtained by
properly setting the small valve "tappets" that are secured to the
end of the stems or push rods. By turning the nut of the tappet in
one direction, the length of the push rod will be reduced, while the
reverse operation will increase the length of the rod or stem. This is
primarily intended for taking up any wear that may occur at the ends of
the push rod or valve stem. In the case of engines having the valves in
the head, the long push rod of each valve should be so loose as to move
perceptibly when shoved up and down by the thumb and finger.
When the rocker arm is pressed down against the valve stem, the space
between the other end of the rocker arm and the push rod should be
sufficiently wide to admit a piece of tissue paper. The same test may
be made in connection with valves located at the side, after first
ascertaining that the end of the short push rod is resting firmly
against the periphery of the cam. The play will be apparent, of course,
only when the valve is tightly closed, and in order to make certain
that their cams are in the "inactive" position, the piston should be
set at the beginning of the explosion stroke when testing the intake
or exhaust valve. This is at the point of ignition and is the time at
which both valves should be tightly closed.
The cam shaft to which the cams that operate the valves are attached
is generally placed inside the crank case. If the motor is of the
"T"-head type, having valves on opposite sides of the cylinders, the
cam shaft operating the exhaust valves will be found on one side of the
crank case, while that for opening the inlet valves will be located on
the other. If the motor is of the "L"-head type, all the cams will be
placed on the one shaft. The cams are sometimes forged with their shaft
in a solid piece, while in other designs they are keyed in place, but
whatever type is used, the cams and their shaft may be considered as
integral with each other.
The cam shafts are generally driven by a gear meshing with a smaller
one attached to the front end of the crank shaft of the motor, which
forms one of the forward train of gears that are enclosed in an
aluminum case. If the cam shaft is driven at the same speed as is
the crank shaft of the motor, it will be seen that the valves will
open once at every revolution of the flywheel. In a four-cycle motor,
however, the explosion and other events occur but once in each cylinder
for every two revolutions of the flywheel, and consequently the cam
shaft must be driven at one-half the speed of the crank shaft.
To obtain the proper speed ratio, each cam shaft is driven by a
"two-to-one" gear, which means that the gear on the end of the crank
shaft has but one-half as many teeth as have those attached to the cam
shafts. There is thus one revolution of each cam shaft gear for every
two of the crank shaft gear, and consequently each cam shaft is driven
at the required half speed.
The cam shafts may be driven by a chain, the links of which fit over
teeth cut on sprocket wheels, but there must always be a constant
relation between the position of the cam shaft and that of the crank
shaft. This constant relation is necessary in order that the valves
will open and close at the proper points during the travel of the
piston. For example, the exhaust valve should open toward the end of
the explosion stroke in order to allow the burned gases to be forced
out, and the cam for operating this valve should always be in the
lifting position at exactly the proper moment.
If the cam shaft is not positively driven, this position may change and
the exhaust valve might be opened at the beginning of the ignition
of the charge, in which case the force of the explosion would be
wasted almost entirely. On the other hand, the inlet valve should
open at about the beginning of the suction stroke in order that the
fresh charge may be drawn in by the downward travel of the piston;
it is evident that this cannot be opened at any other time without a
resulting loss in the power developed by the motor.
The proper timing of the action of the valves is consequently one of
the most important adjustments of a motor. When the motor is assembled
and tested at the factory, the valves are properly timed and there
is no possibility that they will require further adjustment in this
respect until after the engine is "taken down" for the purpose of
cleaning or the renewal of a broken part. If it should ever become
necessary to remove one of the cam shafts or any of the gears
constituting the forward train, the greatest care should be taken
to make certain that all are returned to _exactly_ their original
position. A difference of one tooth in the relative meshing of the
gears may result in a loss of fifty per cent. of the power developed by
the motor.
Absolute rules for the proper timing of the valves cannot be given
here, for various motors are designed with slightly different
positions at which the exhaust and inlet valves should be opened and
closed. A cam shaft should never be removed, however, without first
marking the intermeshing teeth of its driving gear and those of its
companions. This may best be done by means of a small prick punch
which, when tapped lightly with a hammer, will make a permanent mark
at the desired point on the surface of the gear. If the motor is of
the "T"-head type, having its valves operated by two cam shafts, care
should be taken to designate the right and left-hand gears so that
their positions will not be reversed if both have been removed at the
same time.
A safe method to pursue is to indicate the right-hand gear with one
punch mark, while two should be used for the gear at the left. Three
teeth should be marked on each pair of intermeshing gears. That is, a
tooth on one gear should be marked, and then each of the teeth between
which it meshes on the other gear. The second cam shaft gear should be
marked before the motor is turned.
As has been stated, the cams on many motors are forged integral with
their shafts, and there is consequently no possibility of the removal
of one from the other. Those cams which are keyed to their shafts are
accurately and rigidly set and the keyways so cut that there is slight
chance of a mistake in returning a cam that has been removed. It should
seldom be necessary to remove a cam from its shaft, however.
Many motors are provided with timing marks on the flywheel to indicate
the positions of the latter at which the valves of the various
cylinders should open and close. In connection with these marks a
pointer attached to the crank case and indicating the top of the
flywheel is used. When the marked, for example, 4
Ex 0, is under the pointer, it indicates that the exhaust valve on the
fourth cylinder should be about to open. If the motor is turned but
very little beyond this point, a lifting should be felt at the proper
push rod or valve stem.
It is well to test the setting of the valves occasionally by means of
these marks, for wear at the rocker arms, the push rods, the valve
stem, or the cam travelers will result in unevenly-timed valves. It
should be remembered that it is the valve itself that should open
after the proper mark on the flywheel has been passed, and that the
movement of a long push rod is not sufficient evidence that the valve
is beginning to leave its seat. There may be so great an amount of lost
motion between the push rod, cam, rocker arm, and valve stem that the
flywheel may be turned several degrees beyond the proper point before
this "play" will be taken up and the valve itself will begin to move.
Although the timing of a motor may be given in inches of piston travel
beyond a certain dead center, at which point an exhaust or inlet valve
should open or close, it is generally expressed in degrees of flywheel
revolution. Suppose, for example, it is said that the inlet valve
should open ten degrees after the beginning of the suction stroke. This
would indicate that the flywheel should be turned through an arc of ten
degrees from the point at which the piston is at its upper dead center
before the inlet valve for that particular cylinder should begin to
open. Expressed in terms of flywheel revolution, the total travel of
the piston during each stroke is 180 degrees, and as in the proximity
of its dead centers the piston moves but a short distance in comparison
with the size of the arc through which the flywheel swings, valves may
be set very accurately by this method.
Not all cam shafts for operating the valves are located in the crank
case. On several designs of motors the cam shaft extends along the top
of the cylinders and is driven by a vertical shaft and two sets of
bevel gears. On such motors both inlet and exhaust valves are located
in the cylinder heads, and owing to the proximity of the cam shaft, but
short push rods and valve stems are needed. The valves are sometimes
operated by means of a bell crank or rocker arm that acts directly
against the cam surface and end of the valve stem.
On some designs a double cam is used which serves to operate both the
inlet and exhaust valves of the cylinder. The bearings and cams of such
a shaft are generally enclosed in oil and dustproof casing screwed to
the top of the cylinders. Such a cam shaft should never be dismounted
without first marking intermeshing teeth of all spur and bevel gears
that are concerned in its operation.
All poppet valves must be accessible and readily removable for the
purpose of cleaning and grinding the contact surfaces of the head and
seat. The pockets in which the valves placed at the side of a cylinder
are located are generally provided with large screw plugs at the top.
Such a plug may be removed with a heavy wrench, and as the opening
which it fills is larger than the head of the valve, the latter may be
removed after first loosening the spiral spring surrounding its stem.
It is not necessary to remove the valve entirely from its pocket in
order to grind its surfaces, but the pin holding the spring stop in
place must be withdrawn so that the tension of the spring on the valve
will not be so great as to prevent the latter from being lifted to
permit the introduction of the abrasive and turning the head with the
grinding tool.
Valves located in the head of the cylinder must be removed entirely
before their surfaces can be ground. This, however, is not a difficult
operation, as the valve and its seat are generally placed in a
removable "cage" that either screws in place or is held firmly in
position by means of a clamp or like device. Inasmuch as the seat is
contained in this removable cage in which the valve operates, the
grinding may be done at a work bench or on the bed of any convenient
tool, independently of the location of the motor.
If a valve seems sluggish in its action at high speeds of the motor, it
is possible that its spring has become somewhat weakened. These springs
are designed to be exceedingly stiff and heavy, some of them requiring
a pressure of two hundred and fifty pounds to compress the coils one
inch. With such a spring, a special tool is required to compress it
sufficiently to enable the valve to be removed. A spiral spring that
has become weakened may sometimes be strengthened by "stretching," but
it is not to be supposed that this would be of great avail in the case
of a spring as heavy as those used on some valves. If, however, a flat
tool is introduced between the various coils and each is separated
slightly so that the ultimate length of the entire spring is greater
than it was formerly, it will exert a more powerful force on the valve
when it is returned to its place surrounding the stem.
Stiffening the spring, however, will be of but little help if the stem
or push rod is tight in the guides through which it slides. These
guides are often made of a special bearing bronze and are designed to
withstand a large amount of wear, but the friction surfaces must be
lubricated if satisfactory service is to be obtained. The lower guide
is generally lubricated by the oil from the cams, while the guide
near the valve may receive its oil from the engine cylinder. It is
not necessary that these guides shall be packed or that they shall be
particularly tight, as they are not called upon to retain any gas or
air pressure, but they must hold the stem and rod sufficiently rigid to
prevent any perceptible side motion and thus cause imperfect seating
of the valve. In replacing valve stems and push rods, it should be
made certain that each works freely in its guide before the spring is
installed. If there is a slight tendency for the guide to grip the rod
or stem, the latter should be smoothed with emery paper at the point
at which it comes in contact with the guide and plenty of oil applied
until the surfaces are well "worked down." As the distance that the
rods and stems travel through the guides is comparatively short, the
wear is slight and only a small amount of lubricant is needed, provided
the rubbing surfaces are smooth and well-fitted to each other.
The mechanism of a sleeve valve motor is slightly different from that
of the poppet valve type. Each sleeve is operated by a connecting
rod and eccentric mounted on a shaft driven by a chain or gears from
the crank shaft of the motor. The eccentric replaces the cams of the
poppet valve motor, and as it must maintain a certain relation with the
position of the piston in order that the operation of the valves shall
be timed correctly, the same care must be observed in replacing the
eccentric shaft with the proper teeth of the sprocket or gear in mesh
as has already been described in connection with the cam shaft of the
poppet valve motor.