ENGINE BALANCE
A drawback to the use of reciprocating engines is that the weight
of the piston and connecting rod in sliding first one way and then
the other produces great vibration, and that the crank shaft in
bringing these parts to a stop at each end of the stroke is subjected
to violent shocks that in time wear it loose in its bearings. With
internal-combustion engines this vibration, and the shock on the
crank shaft, are greatly increased by the intensity with which the
pressure is exerted.
Engines with one cylinder may be balanced to some extent by the use
of counterweights attached to the crank shaft, and by the use of so
heavy a fly wheel that its momentum produces a comparatively steady
movement; but a perfect absorption of the vibration would require
the engine to be run at a constant speed, which is not possible with
those used on automobiles.
In two-cylinder engines the vibration may be reduced by so arranging
the parts that the pistons slide in opposite directions, the weight
of one being balanced by that of the other. This plan is used in
engines of the =horizontal double-opposed= type, which is considered
to be the most satisfactory for low powers. The cylinders are
horizontal, with their open ends toward each other, the crank shaft
lying between them. The crank shaft is =two-throw=, 180°; that is,
there are two pairs of crank arms, projecting from opposite sides of
the shaft, so that they are a half revolution apart.
Two cylinder engines are also built with vertical cylinders, and are
of two types, according to the construction of the crank shaft. In
one, the crank shaft is 180°, and in the other both pairs of crank
arms project from the same side of the shaft so that the crank pins
are in line, this being called a 360° crank shaft.
In the 180° type one piston moves up as the other moves down, so that
they balance, but it results in the power strokes occurring in both
cylinders during one revolution of the crank shaft, with no power
during the revolution that follows.
To understand the reason for this, the order in which the events of
the cycle occur must be recalled, and it must be remembered that of
the four strokes of the piston during which they are performed the
two outward strokes are inlet and power, and the two inward strokes
compression and exhaust. If the piston of a two-cylinder vertical
engine (Fig. 17) is moving downward on the power stroke, piston No. 2
will be ascending, and the only events that can then be performed in
its cylinder are compression or exhaust. If performing compression,
it will move under power during the next stroke (the other half of
the revolution), No. 1 then exhausting (Table No. 1). This brings the
two power strokes in one revolution, and during the next revolution
there will be no power stroke, for No. 1 will be performing suction
and compression, and No. 2 exhaust and suction.
[Illustration: FIG. 17.—ENGINE ARRANGEMENTS SHOWING ORDER OF FIRING.]
If, as shown in the second table, No. 2 is moving upward on exhaust
while No. 1 moves down under power, its previous stroke, the first
half of the revolution, will have been the power stroke, and the
same condition will exist of two power strokes occurring in the same
revolution.
In either case the balance of the moving parts is offset by the
irregular production of power, which produces bad results in the
setting up of strains in the engine, and the uneven running of the
car.
In two-cylinder vertical engines with 360° crank shaft (Fig. 17) the
pistons move up and down together, which of course results in bad
balance. In this arrangement, however, the applications of power may
be evenly spaced, for as the piston in cylinder No. 1 moves downward
on the power stroke, piston No. 2 moves in the same direction, and
may make either the inlet or power stroke. If it makes the inlet
stroke, it must move inward before it can again move outward on the
power stroke, and there will be an interval of one stroke between the
power strokes of the two cylinders. To have the power strokes in the
two cylinders occur together, as suggested as the alternative, would
obviously give bad results in the great strain imposed on the crank
shaft, the weight of the fly wheel that would be necessary to carry
two pistons through three dead strokes, and the jerky running of the
car.
The defects of two-cylinder vertical engines with either design of
crank shaft outweigh any possible advantage of that construction,
and the horizontal double-opposed type, in evenly occurring power
strokes, mechanical balance, and simplicity, is in almost universal
use for cars of low power.
A two-cylinder engine does not require so heavy a fly wheel as a
one-cylinder engine in proportion to the power delivered, because
there is a power stroke every revolution instead of in alternate
revolutions, and the parts must be carried over only one dead stroke
instead of three. Similarly, the fly wheel of a four-cylinder engine
may be still lighter, for in that type there are two power strokes
in every revolution, with no dead strokes. At the same time, it is
not possible to dispense with it entirely, for the pressure acting
on the piston at the beginning of the power stroke falls rapidly as
the piston moves before it, and the momentum by which the fly wheel
tends to revolve at a constant speed steadies and smooths what would
otherwise be a jerky motion.
The crank shaft of a four-cylinder engine is 180°, four-throw, with
the two end cranks projecting in the opposite direction to that
of the two inside cranks. This arrangement is used in preference
to having the cranks project alternately; that is, the first and
third to one side and the second and fourth to the other, because
of economy in manufacturing, and because practice has shown that it
results in less vibration in the running of the engine.
This construction of the crank shaft does not permit the power
strokes to occur in rotation, cylinder No. 2 following No 1, and then
Nos. 3 and 4, for it has been explained that when cranks are 180°
apart the power strokes occur during one revolution, and that when
360° apart there is a dead stroke between the two power strokes.
Cranks 1 and 2 are 180° apart, as are 3 and 4, and also 2 and 4,
but 2 and 3 are 360°; that is, their crank pins are in line, which
is also the case with 1 and 4. The power stroke in cylinder No. 2
may follow that in cylinder No. 1, but must be followed by that in
cylinder No. 4, as that is 180° away from No. 2, and No. 3 comes
last, being 180° from No. 4. The succession in which the power
strokes occur, called the =firing order=, is thus 1, 2, 4, 3, and
this is arranged for by the setting of the valves and the timing of
the ignition.
This firing order was used for the first four-cylinder automobile
engines, but it has lately been suggested that the firing order 1, 3,
4, 2 shows advantages in the reduction of strains and vibration, and
is being adopted.
Three-cylinder engines are built with 120° crank shafts; that is,
the cranks are one third of a revolution apart, instead of one
half of a revolution, as is the case with 180° crank shafts. This
gives three power strokes during two revolutions, and results in
excellent balance. Six-cylinder engines have crank shafts of similar
construction, and produce six power strokes during two revolutions,
with the best balance that it is possible to obtain without an
excessive number of cylinders. A power stroke occurs at every 120°
that the crank shaft revolves, and as each power stroke endures for
a half revolution, or 180°, it follows that one commences when the
previous one is only two thirds complete. This gives a very steady
application of power, for the crank shaft is at all times operated by
the driving effect of the combustion strokes, and it is possible to
run the engine smoothly at greatly varying speeds by control of the
position at which the spark occurs in the combustion space, and the
admission of a greater or less volume of the mixture during the inlet
stroke.
The type of engine that will give satisfaction depends on the work
that is demanded of it, the care that it will receive, and the
knowledge of the operator. A one-cylinder engine, having a minimum of
parts, may be successfully managed by a novice, but its vibration,
and the fact that it delivers power for only one fourth of the time
that it runs, make it unsatisfactory for any but light work. Cars
of medium power are almost universally equipped with horizontal
double-opposed engines, with excellent results, and they are not
difficult to maintain. More powerful cars have four-cylinder
vertical engines, which give a sufficiently constant pull for all
practical purposes, with a correspondingly greater demand for care
and attention. Six-cylinder engines are of doubtful advantage for
general use, without the services of a chauffeur, for while they
develop more constant power than engines with fewer cylinders, the
complication of parts and the care necessary are increased.