ENGINE DETAILS
=The Piston= is perhaps the most important detail to consider, for
it is on the piston that the force of the explosion acts when the
heat energy is converted into mechanical energy. It must be made
sufficiently strong to withstand the bursting effect of successive
explosions, and yet if we make the metal too thick it will retain too
much of the _waste heat_ and the piston may _seize_ in the cylinder
due to expansion. To understand why the piston is likely to seize in
the cylinder we have only to remember that when a metal body is heated
it gets larger in every direction, but if cooled it returns to its
original size. Now if we make the metal of the piston too thick so
that the waste heat cannot pass _quickly_ through it and dissipate
itself at cooler parts of the engine, then the bulk of this heat will
be concentrated in the piston head, causing it to expand and become a
tight fit in the cylinder, as the cylinder walls are fairly thin and in
contact with the jacket water which keeps them fairly cool and prevents
them expanding much above their normal size. The actual amount of
expansion is very small of course, but there is very little _clearance_
between the piston and the cylinder walls, even when the engine is all
cold—perhaps five-thousandths of an inch. The piston therefore must be
strong, yet as light as we can make it, having regard to the necessity
for its being amply stiff and rigid and able to _stand up to_ its work.
[Illustration: FIG. 15.—TWO VIEWS OF A CAST IRON PISTON WITH GUDGEON
PIN AND PACKING RINGS.]
Generally it will be an iron casting in the form of a small cylinder
(see Fig. 15), having provision in it for the packing rings P, and the
gudgeon pin G, with its fastening screws S_{1}, S_{2}. The piston
itself, as we have observed, must be a nice sliding fit in the bore of
the cylinder without any shake or side play when there are no packing
rings in the grooves. The packing rings are turned to size so as to fit
the cylinder exactly and prevent any gas leaking past the piston into
the crankchamber. These rings are very light, are made from cast iron,
and arranged to break joint, as indicated, by cutting the middle ring
in the opposite direction to the two outer ones. Bosses are cast on
the inside of the piston and afterwards bored out to receive the steel
_gudgeon pin_ or wrist pin G. This pin is best made of plain parallel
cylindrical form _ground_ true, and the bosses in the piston should
be reamered out to the exact size of the pin. When the pin has been
inserted the tapered screws are screwed hard up by means of a special
spanner and bear against the pin, preventing it from coming loose or
from shaking or knocking. There are many other methods of fixing the
gudgeon pin which are not shown here; each has some special point in
its favour, but the one illustrated is undoubtedly the best and affords
a positive adjustment for wear.
[Illustration: FIG. 16.—METHOD OF FIXING THE GUDGEON PIN WHICH ALLOWS
FOR ADJUSTMENT AFTER WEAR.]
[Illustration: FIG. 17.—THREE FORMS OF PISTON HEAD.]
An enlarged view of one of the bosses, showing the taper pin in detail
and how the split pin Q prevents it from slacking back by contact with
the wall of the piston, is shown in Fig. 16. Sometimes the lower part
of the piston is made lighter by drilling holes through the walls.
It is very important to reduce the weight of the piston as much as
possible, otherwise the engine cannot attain a high speed, so that it
becomes essential to bear this in mind when constructing engines for
racing purposes. Frequently we find _steel_ pistons used, as they may
be made lighter for the same strength, and then steel piston rings may
be used; they are not much in favour for ordinary motor-car engines
because the steel pistons expand at a greater rate than the cast iron
of the cylinder, so that there is more liability to seizure. The
crown of the piston is sometimes curved upwards and at other times
curved downwards, but more often it is flat as shown in Fig. 17. The
gudgeon pin is sometimes made of mild steel, and the surface is then
case-hardened in the centre where the connecting rod end bears. At the
present time it is quite as common to find gudgeon pins made of special
nickel steel or other steel alloys that do not require case-hardening.
On the whole these special steels make the best gudgeon pins and stand
the hardest wear.
[Illustration: FIG. 18.—CONNECTING ROD IN THE FORM OF A PHOSPHOR BRONZE
STAMPING.]
=The Connecting Rod= is another very important detail of the engine
mechanism, its function being to transmit the force of the explosion
from the piston to the crankshaft.
[Illustration: FIG. 19.—STEEL CONNECTING ROD TURNED OUT OF THE SOLID
BAR.]
One end of the connecting rod moves up and down with the piston and
oscillates (or swings to and fro) on the gudgeon pin, while the other
end of the connecting rod travels in a circle, being pivoted at the
crankpin and rotating in a circle which has for its centre the centre
line of the engine crankshaft. This is clearly indicated in Fig. 18.
On the _suction stroke_ of the engine the piston has to be _pulled_
down, as we have already seen; on the _explosion stroke_ the greatest
pressure acts on the piston and _pushes_ the connecting rod down. Thus
sometimes the connecting rod is being pulled and at other times it is
being pushed; in each case it has to overcome the resistance of the
engine and drive the car. It is evident, therefore, that the character
of the load carried by a connecting rod is just about as complex and
dangerous as it is possible for a system of loading to be, and great
care has to be taken in the design of such rods to ensure adequate
strength without undue weight, as this would tend to keep down the
maximum speed of the engine. Another important consideration is the
_cost of production_, and for this reason one often finds it in the
form of a phosphor bronze _stamping_ of I section, although the ideal
form is a round section of steel with a straight taper from gudgeon
pin to crankpin end, and having a hole bored right up the centre to
reduce the weight without sacrificing much strength. When the rod is
made in the form of a stamping between _dies_ it is possible to turn
out great quantities at very low cost and at a very rapid rate, whereas
the round steel rods would require to be machined from the solid bar
to compete in price with the others. When phosphor bronze is used it
is only necessary to bore out carefully and face the bearings at the
two ends for the gudgeon pin and crankpin; the bearing at the crankpin
end is always formed with a removable cap to facilitate fitting it
nicely to the crankpin, journal and also to allow for adjustment as the
bearing wears. With steel rods it is necessary to cast a white-metal
lining in the crankpin end and then bore it out to form the bearing,
but the crosshead bearing is usually formed by a phosphor bronze bush.
It is evident, therefore, that the steel rods are more expensive, but
they make a splendid mechanical job. A steel connecting rod is shown
complete in Fig. 19. Stamped steel rods of I section are also commonly
used and are much better and stronger than those made entirely of
phosphor bronze.
[Illustration: FIG. 20.—CRANKPIN AND CRANKWEBS.]
[Illustration: FIG. 21.—FOUR-THROW CRANKSHAFT]
=The Crankshaft=, as its name implies, is a shaft with one or more
_cranks_ or right-angled bends in it. Its function is to convert the
sliding motion of the piston into the rotary motion of the flywheel and
revolving shaft. A crankshaft with a single throw (or single crank)
is shown in Fig. 20; a four-throw crankshaft is shown in Fig. 21; and
Fig. 22 shows how an equivalent motion can be obtained by a single pin
fixed into the face of a flywheel. This device (Fig. 22) is frequently
used for motor-cycle engines. Crankshafts are always made of steel;
sometimes _mild_ steel is used, but more usually special alloys of
steel containing chrome, nickel, vanadium, etc., are used. The general
practice at the present time is to machine the crankshaft direct out of
a solid bar of steel; this requires special _jigs_ for holding the work
and special tools for cutting the metal, but is the quickest, cheapest,
and most satisfactory method to adopt. A few firms specialize in this
class of work with high-grade steel and can supply crankshafts from
stock.
[Illustration: FIG. 22.—MOTOR-CYCLE CRANKPIN FIXED INTO THE FLYWHEEL.]
It is easily seen by examining Fig. 18 that the crankshaft is being
twisted in overcoming the engine resistance, while Fig. 20 shows that
the crankshaft is being _bent_ under the push from the connecting rod,
so that we say the material of a crankshaft is subjected to _combined
bending and twisting_, and as such a combination is not easy to resist
we see now why special steel alloys are required for safety, combined
with economy in material and reduction of weight. In Fig. 20 the
_crankpin_ is shown at A, the crank cheeks or _webs_ at B_{1}, B_{2}
and the crankshaft proper at C. The portions of the crankshaft C which
work in the bearings D_{1}, D_{2} are termed _journals_. A crankshaft
must be very stiff and not bend or twist sensibly, otherwise the shaft
will vibrate when the engine runs up to speed—which would be very
undesirable. It must be perfectly true with all the bearings absolutely
in line and the journals well bedded down in their respective brasses
(or bearings), otherwise mechanical troubles will arise. Each crank
with its crankpin and webs forms a lop-sided or unbalanced mass, so
that either (1) each crank must have its own _balance weight_ as in
Fig. 23, or (2) special balancing masses must be fitted at each end of
the crankshaft. A convenient method of balancing the crankshaft is to
have a fan pulley at one end and the flywheel at the opposite end, so
that by drilling holes in the faces of these discs an amount of metal
may be removed from them sufficient to balance the excess weight of the
respective crankpins and webs. In Fig. 24 the shaded area indicates
that portion of the crank which constitutes an unbalanced mass.
Crankshafts for high-speed engines have always to be very carefully
balanced, otherwise the engines will never run satisfactorily, the want
of balance being greatly aggravated as the speed of rotation increases.
Fig. 25 shows how the crankshaft of a two-cylinder engine may be
balanced by drilling holes in the flywheel and fan pulley respectively,
but the same effect may be produced by attaching balancing masses—this
latter method would, however, be more inconvenient and expensive. The
crankpins and journals are _ground_ truly circular after being turned
in the lathe as true as possible.
[Illustration: FIG. 23.—SKETCH OF A BALANCED CRANK.]
[Illustration: FIG. 24.—SKETCH SHOWING THE UNBALANCED PORTION OF THE
CRANK BY SHADED LINES.]
[Illustration: FIG. 25.—TWO-THROW CRANKSHAFT BALANCED BY DRILLING HOLES
IN FLYWHEEL AND FAN PULLEY.]
[Illustration: FIG. 26.—SKETCH TO ILLUSTRATE THE FORCES ACTING ON A
FLYWHEEL RIM.]
=The Flywheel.=—We have already described how the force driving the
piston of a motor-car engine varies during the four strokes of the
cycle, but we must note that it also varies considerably during each
individual stroke. Thus, on what is known as the _explosion stroke_ (or
power stroke) of the cycle, the pressure at the commencement of this
stroke may be exceedingly great and yet towards the end of the stroke
the gases have expanded and the exhaust valve has been opened, so that
the pressure acting on the piston is then very small. Again, on the
compression, suction, and exhaust strokes, the piston has to be pushed
or pulled by some means, as _no power is being generated_. Therefore,
if the engine is to be self-acting and run continuously, some means
must be provided for storing up the great force of the explosions and
giving it out again on the idle strokes. The function of the flywheel
is to _store_ any energy given to it over and above that required to
drive the car and to give it out again when required for performing the
functions of compressing, exhausting, and sucking in gas, as well as
to keep the car running steadily. It is simply a heavy wheel mounted
on the end of the crankshaft which, when once started revolving at a
high speed, is not easily stopped, and which will give up part of
its energy each time its speed is reduced owing to the demands of the
engine; but when the engine is generating power the wheel will speed
up and store the excess—the mere fact that the wheel is heavy causes
these changes in speed to occur slowly, and therefore on the whole the
_fluctuation_ of speed is not great when a suitable flywheel is fitted.
The flywheel does _not_ limit the _maximum_ speed of the engine, as it
could go on slowly increasing in speed if no resistance was encountered
until the wheel finally _burst_ or flew to pieces. Thus the flywheel
does not regulate the _speed_ of the engine; it merely smooths out the
inequalities in the several _strokes_ of the “cycle.” Flywheels of
motor-car engines are now always made of steel, so that they can be
safely run at speeds up to 3,000 revolutions per minute without fear
of the rim bursting. All parts of the rim tend to fly off radially in
the direction of the arrows as shown in Fig. 26 under the action of
_centrifugal force_. A built-up flywheel is shown in Fig. 27, and one
made from a single stamping of steel is shown in Fig. 28. Generally
speaking, when a coned clutch is fitted one portion of it is formed
on the inside of the flywheel rim as indicated in these two figures.
When the construction shown in Fig. 28 is adopted the lining would be
inserted _after_ the clutch cone had been put into place; very often
the lining is made up of sections which can be readily inserted or
withdrawn after the cone is in position.
[Illustration: FIG. 27.—A FLYWHEEL BUILT UP FROM STEEL FORGINGS.]
[Illustration: FIG. 28.—A FLYWHEEL TURNED FROM A STEEL STAMPING.]