ENGINE PARTS
The sudden and powerful outward movements of the piston under the
pressure from the combustion are transmitted to a =crank shaft=,
which must be of great strength in order to resist the heavy strains
under which it operates. It is made of the best steel available for
the purpose, and has as many =cranks= as the engine has cylinders.
The cranks are generally made in one piece with the shaft for the
sake of strength, and for stiffness there are as many =bearings= as
possible. The number of bearings for the crank shaft of an engine
with four or more cylinders depends on the arrangement of the
cylinders. If the cylinders are evenly spaced, there will be room for
a bearing between each pair of cranks, so that a four-cylinder engine
will have five bearings, one at each end, the other three being
between the cranks. If the cylinders are in pairs, there will not be
room between the cranks of a pair for a bearing, the only space for
it being between the pairs; a four-cylinder engine built in this way
will thus have but three bearings, one at each end and one in the
center. Crank shafts are described by their bearings as three, five,
etc., =point= crank shafts.
[Illustration: FIG. 3.—GASOLINE ENGINE IN SECTION.]
[Illustration: FIG. 4.—_A_, Two-throw crank shaft; _B_, four-throw
crank shaft, 180°.]
The relative positions of the cranks of a crank shaft are expressed
in degrees of a circle; if, for instance, the cranks project from
opposite sides of the shaft so that they are a half revolution apart,
it is called a 180-degree crank shaft.
The outer ends of the crank arms, which correspond to the cranks of a
bicycle, support the =crank pin=, which may be likened to the pedal,
and to this the large end of the =connecting rod= is attached, the
small end being connected to the piston. The connecting rod must be
of great strength, tough but not brittle, and is made of steel or
bronze.
[Illustration: FIG. 5.—ONE-THROW CRANK SHAFT.]
[Illustration: FIG. 6.—CONNECTING ROD.]
The piston is a trifle smaller than the bore of the cylinder, and
its length is usually greater than its diameter. It is hollow, with
one end closed, the closed end being that against which the pressure
is exerted. A =wrist pin= passes through it, and through the small
end of the connecting rod, to enable the latter to swing from side
to side in following the turning of the crank shaft. A tight joint
is maintained between it and the cylinder walls by cast-iron =piston
rings=, which are of square or rectangular cross-section, split so
that they may spring open, and fitted into grooves cut around the
piston. They are of such shape that their tendency to expand keeps
them pressed against the cylinder walls, but being split, their
elasticity prevents their binding; they fit the grooves snugly, and
while they may move freely in them, they hold the pressure from
escaping. The number of rings varies with the design of the engine,
but the most usual arrangement is three to a piston, placed around
the upper end.
[Illustration: FIG. 7.—_A_, Piston; _B_, piston in section.]
[Illustration: FIG. 8.—PISTON RINGS.]
The cylinder should be of the highest grade of cast iron, with the
smoothest possible surface for the piston to slide against.
[Illustration: FIG. 9.—CONICAL VALVE SEAT.]
[Illustration: FIG. 10.—AUTOMATIC INLET VALVE IN CAGE.]
The =valve= openings, or =seats=, are circular, and are usually made
slightly funnel shaped, the =disks= that cover them being slightly
conical to fit. The large end of the funnel is toward the combustion
space, so that when the disk is lifted from its seat it moves inward.
Valves are held against their seats by coil springs that surround
the valve =stems=, which are rods extending from the center of the
disks, and there are two methods by which they are opened. In an
=automatic valve=, the spring that holds the disk against its seat
is weak, and the higher pressure outside of the cylinder during
the suction stroke forces the disk away from its seat against the
pressure of the spring. The valve remains open until the pressure
in the combustion space is about equal to that outside, when the
spring draws the disk back to its seat, to which it is held as long
as the pressure inside is higher than that of the atmosphere. This
arrangement is only possible for inlet valves, and is largely used,
but exhaust valves, and often inlet valves as well, are =mechanically
operated=; that is, they are opened and held open by a mechanism
driven by the crank shaft, in the form of a =cam=. A cam can best be
described as a “wheel with a hump on it,” or, in other words, it is
a piece of metal mounted on a shaft, cylindrical in form except for
one portion, which projects farther from the shaft than the rest. The
cam revolves with the shaft, and the projection, called the =nose=,
will displace anything resting against it. The illustration shows a
cam in three positions of its revolution, with the end of a valve
stem resting against it—the roller being attached to the stem to
reduce the friction. The valve stem is held in =guides=, so that the
only movement it may have is up and down; when the cam revolves, the
nose lifts the stem and opens the valve, holds it open as long as the
flat end of the cam is under the stem, and when the nose passes from
under, the valve is drawn to its seat by the spring.
[Illustration: FIG. 11.—CAM ACTION.]
The moment at which an automatic valve opens is governed partly by
the tension of its spring; if it is too strong, greater pressure will
be required to open it, and it will close sooner than if the tension
is light. Accurate adjustment of this spring is necessary in order
that the charge may enter the combustion space without delay, and
continue to enter as long as possible. The opening and closing of
mechanically operated valves depend on the shape of the cam, and not
being affected by the more or less uncertain action of a spring, they
are more positive in action.
The =cam shaft= on which the cam is mounted is driven by the crank
shaft, but as the valve opens but once during two revolutions, the
cam shaft revolves at half speed, making one revolution while the
crank shaft makes two. This is done by means of =gears=.
If two gears running together, or in mesh, have the same number of
teeth, they will make the same number of revolutions, but if one has
twice as many teeth as the other, the smaller will revolve twice
while the larger revolves once. As the cam shaft must revolve but
once while the crank shaft revolves twice, its gear must have twice
the number of teeth as the gear on the crank shaft. The cam shaft is
also called the =secondary=, or =half-time shaft=, and the gears that
drive it the =two-to-one gears=.
In some designs of engines, the nose of the cam bears directly
against the valve stem, but it is more usual to place a =valve-lifter
rod=, or =push rod=, between them, the cam acting on the rod and
lifting it, and that in turn lifting the valve stem. When the nose of
the cam is not acting on the stem or rod, there must be a small space
between them, for if the stem or rod rests firmly against the cam at
all times, the valve disk might be prevented from seating firmly. The
space is left between the stem and lifter rod, the spring acting only
on the stem.
[Illustration: FIG. 12.—FOUR ARRANGEMENTS OF VALVES.]
The valves may open into the cylinder in a variety of ways; both may
be in one =pocket=, or one may be in a pocket and the other in the
head, or each in a separate pocket, or both in the head. The first
two illustrations show automatic inlet valves, and the third and
fourth mechanically operated valves; when two mechanically operated
valves are in the head, it is necessary to open them by means of
=rocker arms=, for because of their position it would be impossible
for the valve-lifter rod to act directly on their stems.
If after the explosion the burned gases were permitted to escape
directly into the open air from the cylinder, the effect would be the
same as the firing of a gun, and for the same reasons. The pressure
in the cylinder being higher than that of the atmosphere, the sudden
expansion of the gases would produce a report, and as this would be
most undesirable for an automobile, provision is made by which the
gases are cooled and permitted to expand gradually, so that when they
reach the open air they are at its pressure, or nearly so. This is
done in the =muffler=, or =silencer=, to which the =exhaust pipe=
conducts the products of combustion. The muffler consists of a series
of chambers of different sizes, one inside of the other; the gases
pass from the smaller to the larger, expanding as they go, until from
the largest they should escape without noise, having lost their heat
and pressure.
While the pressure exerted during the power stroke depends on the
heat of the gases, and it is necessary to have the engine hot in
order that there may be as little loss of heat as possible, the
temperature must not be permitted to rise to the point at which the
lubricating oil would burn. Lubricating oil for gasoline engines is
made to stand high heat, but if heated beyond its limit it will
burn, and then, besides the loss of its property of lubrication, a
deposit of carbon, hard or gummy, will form, fouling the combustion
space or piston rings, and interfering with the operation of the
engine. Overheating is prevented either by circulating water through
channels surrounding the combustion space, or by directing a blast of
air against it.
[Illustration: FIG. 13.—_A_, Force circulation water-cooling system;
_B_, thermosiphon circulation water-cooling system. (Flow of water
indicated by arrows.)]
The channels, called =water jackets=, provided for the circulation
of the water, are usually cast with the cylinder, or formed of sheet
metal. Cool water enters at the bottom and escapes at the top,
absorbing heat during its passage. Of the two systems of keeping the
water in circulation, the most usual consists of a rotary pump,
which forces the water through the jackets and then to a =cooler=,
or =radiator=, which is so placed that it is exposed to the air
currents set up as the car moves. In order to cool the water, the
radiator must have a large surface exposed to the air, and the water
must pass through it in small streams. The early types consisted of
coils of small copper tubing, on which were strung disks of copper,
the water flowing through the tubing, and the disks absorbing its
heat and giving it up to the air, but these are being abandoned in
favor of =cellular= or =honeycomb= radiators. These types, which are
usually placed at the extreme front of the car, are made up of a
great number of short lengths of small tubing, in any one of several
shapes, placed side by side, and held together either by plates or by
soldering their ends.
[Illustration: FIG. 14.—_A_, Centrifugal pump; _B_, vane pump; _C_,
gear pump and cover.]
[Illustration: FIG. 15.—RADIATOR CONSTRUCTIONS. _A_, Spiral flange
(water passes through the tube); _B_, cellular, and _C_, honeycomb
(air passes through the tubes; water passes between the tubes).]
[Illustration: FIG. 16.—RADIATOR AND FAN.]
The heated water enters at the top of the case in which the tubes
are contained, and flows to the bottom, finding passages between the
tubes, while the air passes through, being assisted by a fan driven
by the engine.
The second system of keeping the water in circulation follows the
principle that heated water tends to rise, its place being taken by
the cooler water that tends to sink. This, called the =thermosiphon=
or =gravity= system, requires all of the parts and connections to
be large and completely filled with water. The water in the jacket
rises as it absorbs heat from the cylinder walls, and flows out to
the radiator, which it enters at the top. Its place in the jacket
is taken by the cooled water from the bottom of the radiator, and
this circulation continues, being more rapid as the difference in
temperature between the heated and cooled water increases. It is
naturally not so rapid as circulation that is forced by a pump, and
more liable to become inoperative by the clogging of the pipes,
jacket, or radiator.
Of the methods of increasing the surface of a cylinder in order to
cool it by a blast of air, the most usual is to cast it with flanges
that project from all parts of the combustion space. These become
heated as the temperature of the cylinder walls increases, and the
air that is blown against them carries off the heat. Other methods
consist of setting pins or copper strips into the cylinder walls, of
such form that the air current strikes a surface composed of points,
by which the heat easily passes to the air. Another system consists
of surrounding the combustion chamber with a jacket open at the
bottom, air being blown into it from the top by a powerful blower.
Other things that are necessary for successful air cooling are large
valves by which the hot gases may be quickly discharged when their
period of usefulness is ended, and small cylinders rather than large,
as the heat from small quantities of gases may be carried off more
quickly than from large.
The lubrication of a gasoline engine must be carefully looked after,
as on its thoroughness depends the continued delivery of power. The
most usual method of lubricating the piston and cylinder walls is to
keep the crank case filled with oil to such a point that the end of
the connecting rod dips into it in turning. This spatters the oil to
all parts of the crank case, and a portion is caught in a groove
cut around the lower end of the piston. The inward movement of the
piston spreads the oil on the cylinder walls, and it is distributed
around the piston rings, so that they move easily in their grooves.
As the oil is used up, it is replaced from a =lubricator= so that a
constant level is maintained, and this operates either by gravity, or
by a small force pump driven by the engine, or by the maintaining of
pressure in the oil tank.
Mechanically operated or pressure lubricators supply oil to all
parts of the engine, and as the quantity passed to each bearing is
adjustable, a feed may be maintained that is exactly suited to the
requirements.
The bearings of an automobile operate under such widely different
conditions that one kind of lubricant will not be suitable for
all. The maker of the car has tested the different oils, and it is
advisable to follow his instructions on the brand and grade most
suitable for each bearing, rather than to try experiments. The
Lubrication Table on pages 244 and 245 gives the kind and quantity of
lubricant most suitable to the work performed by each bearing.