THE INTERNAL-COMBUSTION ENGINE.
The meaning of the term--Action of the internal-combustion
engine--The motor car--The starting-handle--The engine--The
carburetter--Ignition of the charge--Advancing the spark--Governing
the engine--The clutch--The gear-box--The compensating gear--The
silencer--The brakes--Speed of cars.
THE MEANING OF THE TERM "INTERNAL-COMBUSTION ENGINE."
In the case of a steam-boiler the energy of combustion is transmitted to
water inside an air-tight vessel. The fuel does not actually touch the
"working fluid." In the gas or oil engine the fuel is brought into
contact and mixed with the working fluid, which is air. It combines
suddenly with it in the cylinder, and heat energy is developed so
rapidly that the act is called an explosion. Coal gas, mineral oils,
alcohol, petrol, etc., all contain hydrogen and carbon. If air, which
contributes oxygen, be added to any of these in due proportion, the
mixture becomes highly explosive. On a light being applied, oxygen and
carbon unite, also hydrogen and oxygen, and violent heat is generated,
causing a violent molecular bombardment of the sides of the vessel
containing the mixture. Now, if the mixture be _compressed_ it becomes
hotter and hotter, until a point is reached at which it ignites
spontaneously. Early gas-engines did not compress the charge before
ignition. Alphonse Beau de Rochas, a Frenchman, first thought of making
the piston of the engine squeeze the mixture before ignition; and from
the year 1862, when he proposed this innovation, the success of the
internal-combustion engine may be said to date.
[Illustration]
[Illustration: FIG. 39.--Showing the four strokes that the piston of a
gas-engine makes during one "cycle."]
ACTION OF THE ENGINE.
The gas-engine, the oil-engine, and the motor-car engine are similar in
general principles. The cylinder has, instead of a slide-valve, two, or
sometimes three, "mushroom" valves, which may be described as small and
thick round plates, with bevelled edges, mounted on the ends of short
rods, called stems. These valves open into the cylinder, upwards,
downwards, or horizontally, as the case may be; being pushed in by cams
projecting from a shaft rotated by the engine. For the present we will
confine our attention to the series of operations which causes the
engine to work. This series is called the Beau de Rochas, or Otto,
cycle, and includes four movements of the piston. Reference to Fig. 39
will show exactly what happens in a gas-engine--(1) The piston moves
from left to right, and just as the movement commences valves G (gas)
and A (air) open to admit the explosive mixture. By the time that P has
reached the end of its travel these valves have closed again. (2) The
piston returns to the left, compressing the mixture, which has no way of
escape open to it. At the end of the stroke the charge is ignited by an
incandescent tube I (in motor car and some stationary engines by an
electric spark), and (3) the piston flies out again on the "explosion"
stroke. Before it reaches the limit position, valve E (exhaust) opens,
and (4) the piston flies back under the momentum of the fly-wheel,
driving out the burnt gases through the still open E. The "cycle" is now
complete. There has been suction, compression (including ignition),
combustion, and exhaustion. It is evident that a heavy fly-wheel must be
attached to the crank shaft, because the energy of one stroke (the
explosion) has to serve for the whole cycle; in other words, for two
complete revolutions of the crank. A single-cylinder steam-engine
develops an impulse every half-turn--that is, four times as often. In
order to get a more constant turning effect, motor cars have two, three,
four, six, and even eight cylinders. Four-cylinder engines are at
present the most popular type for powerful cars.
THE MOTOR CAR.
[Illustration: FIG. 40.--Plan of the chassis of a motor car.]
We will now proceed to an examination of the motor car, which, in
addition to mechanical apparatus for the transmission of motion to the
driving-wheels, includes all the fundamental adjuncts of the
internal-combustion engine.[8] Fig. 40 is a bird's-eye view of the
_chassis_ (or "works" and wheels) of a car, from which the body has been
removed. Starting at the left, we have the handle for setting the
engine in motion; the engine (a two-cylinder in this case); the
fly-wheel, inside which is the clutch; the gear-box, containing the cogs
for altering the speed of revolution of the driving-wheels relatively to
that of the engine; the propeller shaft; the silencer, for deadening the
noise of the exhaust; and the bevel-gear, for turning the
driving-wheels. In the particular type of car here considered you will
notice that a "direct," or shaft, drive is used. The shaft has at each
end a flexible, or "universal," joint, which allows the shaft to turn
freely, even though it may not be in a line with the shaft projecting
from the gear-box. It must be remembered that the engine and gear-box
are mounted on the frame, between which and the axles are springs, so
that when the car bumps up and down, the shaft describes part of a
circle, of which the gear-box end is the centre.
An alternative method of driving is by means of chains, which run round
sprocket (cog) wheels on the ends of a shaft crossing the frame just
behind the gear-box, and round larger sprockets attached to the hubs of
the driving-wheels. In such a case the axles of the driving-wheel are
fixed to the springs, and the wheels revolve round them. Where a Cardan
(shaft) drive is used the axles are attached rigidly to the wheels at
one end, and extend, through tubes fixed to the springs, to bevel-wheels
in a central compensating-gear box (of which more presently).
Several parts--the carburetter, tanks, governor, and pump--are not shown
in the general plan. These will be referred to in the more detailed
account that follows.
THE STARTING-HANDLE.
[Illustration: FIG. 41.--The starting-handle.]
Fig. 41 gives the starting-handle in part section. The handle H is
attached to a tube which terminates in a clutch, C. A powerful spring
keeps C normally apart from a second clutch, C^1, keyed to the engine
shaft. When the driver wishes to start the engine he presses the handle
towards the right, brings the clutches together, and turns the handle in
a clockwise direction. As soon as the engine begins to fire, the faces
of the clutches slip over one another.
THE ENGINE.
[Illustration: FIG. 42.--End and cross sections of a two-cylinder
motor.]
We next examine the two-cylinder engine (Fig. 42). Each cylinder is
surrounded by a water-jacket, through which water is circulated by a
pump[9] (Fig. 43). The heat generated by combustion is so great that the
walls of the cylinder would soon become red-hot unless some of the heat
were quickly carried away. The pistons are of "trunk" form--that is,
long enough to act as guides and absorb the oblique thrust of the piston
rods. Three or more piston rings lying in slots (not shown) prevent the
escape of gas past the piston. It is interesting to notice that the
efficiency of an internal-combustion engine depends so largely on the
good fit of these moving parts, that cylinders, pistons, and rings must
be exceedingly true. A good firm will turn out standard parts which are
well within 1/5000 of an inch of perfect truth. It is also a wonderful
testimony to the quality of the materials used that, if properly looked
after, an engine which has made many millions of revolutions, at the
rate of 1,000 to 2,000 per minute, often shows no appreciable signs of
wear. In one particular test an engine was run _continuously for several
months_, and at the end of the trial was in absolutely perfect
condition.
The cranks revolve in an oil-tight case (generally made of aluminium),
and dip in oil, which they splash up into the cylinder to keep the
piston well lubricated. The plate, P P, through a slot in which the
piston rod works, prevents an excess of oil being flung up. Channels are
provided for leading oil into the bearings. The cranks are 180° apart.
While one piston is being driven out by an explosion, the other is
compressing its charge prior to ignition, so that the one action deadens
the other. Therefore two explosions occur in one revolution of the
cranks, and none during the next revolution. If both cranks were in
line, the pistons would move together, giving one explosion each
revolution.
[Illustration: FIG. 43.--Showing how the water which cools the cylinders
is circulated.]
The valve seats, and the inlet and exhaust pipes, are seen in section.
The inlet valve here works automatically, being pulled in by suction;
but on many engines--on all powerful engines--the inlet, like the
exhaust valve, is lifted by a cam, lest it should stick or work
irregularly. Three dotted circles show A, a cog on the crank shaft; B, a
"lay" cog, which transmits motion to C, on a short shaft rotating the
cam that lifts the exhaust valve. C, having twice as many teeth as A,
revolves at half its rate. This ensures that the valve shall be lifted
only once in two revolutions of the crank shaft to which it is geared.
The cogs are timed, or arranged, so that the cam begins to lift the
valve when the piston has made about seven-eighths of its explosion
stroke, and closes the valve at the end of the exhaust stroke.
THE CARBURETTER.
A motor car generally uses petrol as its fuel. Petrol is one of the more
volatile products of petroleum, and has a specific gravity of about
680--that is, volume for volume, its weight is to that of water in the
proportion of 680 to 1,000. It is extremely dangerous, as it gives off
an inflammable gas at ordinary temperatures. Benzine, which we use to
clean clothes, is practically the same as petrol, and should be treated
with equal care. The function of a _carburetter_ is to reduce petrol to
a very fine spray and mix it with a due quantity of air. The device
consists of two main parts (Fig. 44)--the _float chamber_ and the _jet
chamber_. In the former is a contrivance for regulating the petrol
supply. A float--a cork, or air-tight metal box--is arranged to move
freely up and down the stem of a needle-valve, which closes the inlet
from the tank. At the bottom of the chamber are two pivoted levers, W W,
which, when the float rests on them, tip up and lift the valve. Petrol
flows in and raises the float. This allows the valve to sink and cut off
the supply. If the valve is a good fit and the float is of the correct
weight, the petrol will never rise higher than the tip of the jet G.
[Illustration: FIG. 44.--Section of a carburetter.]
The suction of the engine makes petrol spirt through the jet (which has
a very small hole in its end) and atomize itself against a
spraying-cone, A. It then passes to the engine inlet pipe through a
number of openings, after mixing with air entering from below. An extra
air inlet, controllable by the driver, is generally added, unless the
carburetter be of a type which automatically maintains constant
proportions of air and vapour. The jet chamber is often surrounded by a
jacket, through which part of the hot exhaust gases circulate. In cold
weather especially this is a valuable aid to vaporization.
[Illustration: FIG. 45.--Sketch of the electrical ignition arrangements
on a motor car.]
IGNITION OF THE CHARGE.
All petrol-cars now use electrical ignition. There are two main
systems--(1) by an accumulator and induction coil; (2) _magneto
ignition_, by means of a small dynamo driven by the engine. A general
arrangement of the first is shown in Fig. 45. A disc, D, of some
insulating material--fibre or vulcanite--is mounted on the cam, or
half-speed, shaft. Into the circumference is let a piece of brass,
called the contact-piece, through which a screw passes to the cam shaft.
A movable plate, M P, which can be rotated concentrically with D through
part of a circle, carries a "wipe" block at the end of a spring, which
presses it against D. The spring itself is attached to an insulated
plate. When the revolution of D brings the wipe and contact together,
current flows from the accumulator through switch S to the wipe; through
the contact-piece to C; from C to M P and the induction coil; and back
to the accumulator. This is the _primary, or low-tension, circuit_. A
_high-tension_ current is induced by the coil in the _secondary_
circuit, indicated by dotted lines.[10] In this circuit is the
sparking-plug (see Fig. 46), having a central insulated rod in
connection with one terminal of the secondary coil. Between it and a
bent wire projecting from the iron casing of the plug (in contact with
the other terminal of the secondary coil through the metal of the
engine, to which one wire of the circuit is attached) is a small gap,
across which the secondary current leaps when the primary current is
broken by the wipe and contact parting company. The spark is intensely
hot, and suffices to ignite the compressed charge in the cylinder.
[Illustration: FIG. 46.--Section of a sparking-plug.]
ADVANCING THE SPARK.
We will assume that the position of W (in Fig. 45) is such that the
contact touches W at the moment when the piston has just completed the
compression stroke. Now, the actual combustion of the charge occupies
an appreciable time, and with the engine running at high speed the
piston would have travelled some way down the cylinder before the full
force of the explosion was developed. But by raising lever L, the
position of W may be so altered that contact is made slightly _before_
the compression stroke is complete, so that the charge is fairly alight
by the time the piston has altered its direction. This is called
_advancing_ the spark.
GOVERNING THE ENGINE.
There are several methods of controlling the speed of
internal-combustion engines. The operating mechanism in most cases is a
centrifugal ball-governor. When the speed has reached the fixed limit it
either (1) raises the exhaust valve, so that no fresh charges are drawn
in; (2) prevents the opening of the inlet valve; or (3) throttles the
gas supply. The last is now most commonly used on motor cars, in
conjunction with some device for putting it out of action when the
driver wishes to exceed the highest speed that it normally permits.
[Illustration: FIG. 47.--One form of governor used on motor cars.]
A sketch of a neat governor, with regulating attachment, is given in
Fig. 47. The governor shaft is driven from the engine. As the balls, B
B, increase their velocity, they fly away from the shaft and move the
arms, A A, and a sliding tube, C, towards the right. This rocks the
lever R, and allows the valves in the inlet pipe to close and reduce the
supply of air and gas. A wedge, W, which can be raised or lowered by
lever L, intervenes between the end of R and the valve stem. If this
lever be lifted to its highest position, the governing commences at a
lower speed, as the valve then has but a short distance to travel before
closing completely. For high speeds the driver depresses L, forces the
wedge down, and so minimizes the effect of the governor.
THE CLUTCH.
The engine shaft has on its rear end the fly-wheel, which has a broad
and heavy rim, turned to a conical shape inside. Close to this,
revolving loosely on the shaft, is the clutch plate, a heavy disc with a
broad edge so shaped as to fit the inside of a fly-wheel. It is
generally faced with leather. A very strong spring presses the plate
into the fly-wheel, and the resulting friction is sufficient to prevent
any slip. Projections on the rear of the clutch engage with the gear-box
shaft. The driver throws out the clutch by depressing a lever with his
foot. Some clutches dispense with the leather lining. These are termed
_metal to metal_ clutches.
THE GEAR-BOX.
We now come to a very interesting detail of the motor car, the gear-box.
The steam-engine has its speed increased by admitting more steam to the
cylinders. But an explosion engine must be run at a high speed to
develop its full power, and when heavier work has to be done on a hill
it becomes necessary to alter the speed ratio of engine to
driving-wheels. Our illustration (Fig. 48) gives a section of a
gear-box, which will serve as a typical example. It provides three
forward speeds and one reverse. To understand how it works, we must
study the illustration carefully. Pinion 1 is mounted on a hollow shaft
turned by the clutch. Into the hollow shaft projects the end of another
shaft carrying pinions 6 and 4. Pinion 6 slides up and down this shaft,
which is square at this point, but round inside the _loose_ pinion 4.
Pinions 2 and 3 are keyed to a square secondary shaft, and are
respectively always in gear with 1 and 4; but 5 can be slid backwards
and forwards so as to engage or disengage with 6. In the illustration no
gear is "in." If the engine is working, 1 revolves 2, 2 turns 3, and 3
revolves 4 idly on its shaft.
[Illustration: FIG. 48.--The gear-box of a motor car.]
To get the lowest, or "first," speed the driver moves his lever and
slides 5 into gear with 6. The transmission then is: 1 turns 2, 2 turns
5, 5 turns 6, 6 turns the propeller shaft through the universal joint.
For the second speed, 5 and 6 are disengaged, and 6 is moved up the
page, as it were, till projections on it interlock with slots in 4; thus
driving 1, 2, 3, 4, shaft. For the third, or "solid," speed, 6 is pulled
down into connection with 1, and couples the engine shaft direct to the
propeller shaft.
The "reverse" is accomplished by raising a long pinion, 7, which lies in
the gear-box under 5 and 6. The drive then is 1, 2, 5, 7, 6. There being
an odd number of pinions now engaged, the propeller shaft turns in the
reverse direction to that of the engine shaft.
[Illustration: FIG. 49.]
THE COMPENSATING GEAR.
Every axle of a railway train carries a wheel at each end, rigidly
attached to it. When rounding a corner the outside wheel has further to
travel than the other, and consequently one or both wheels must slip.
The curves are made so gentle, however, that the amount of slip is very
small. But with a traction-engine, motor car, or tricycle the case is
different, for all have to describe circles of very small diameter in
proportion to the length of the vehicle. Therefore in every case a
_compensating gear_ is fitted, to allow the wheels to turn at different
speeds, while permitting them both to drive. Fig. 49 is an exaggerated
sketch of the gear. The axles of the moving wheels turn inside tubes
attached to the springs and a central casing (not shown), and terminate
in large bevel-wheels, C and D. Between these are small bevels mounted
on a shaft supported by the driving drum. If the latter be rotated, the
bevels would turn C and D at equal speeds, assuming that both axles
revolve without friction in their bearings. We will suppose that the
drum is turned 50 times a minute. Now, if one wheel be held, the other
will revolve 100 times a minute; or, if one be slowed, the other will
increase its speed by a corresponding amount. The _average_ speed
remains 50. It should be mentioned that drum A has incorporated with it
on the outside a bevel-wheel (not shown) rotated by a smaller bevel on
the end of the propeller shaft.
THE SILENCER.
The petrol-engine, as now used, emits the products of combustion at a
high pressure. If unchecked, they expand violently, and cause a partial
vacuum in the exhaust pipe, into which the air rushes back with such
violence as to cause a loud noise. Devices called _silencers_ are
therefore fitted, to render the escape more gradual, and split it up
among a number of small apertures. The simplest form of silencer is a
cylindrical box, with a number of finely perforated tubes passing from
end to end of it. The exhaust gases pouring into the box maintain a
constant pressure somewhat higher than that of the atmosphere, but as
the gases are escaping from it in a fairly steady stream the noise
becomes a gentle hiss rather than a "pop." There are numerous types of
silencers, but all employ this principle in one form or another.
THE BRAKES.
Every car carries at least two brakes of band pattern--one, usually
worked by a side hand-lever, acting on the axle or hubs of the
driving-wheel; the other, operated by the foot, acting on the
transmission gear (see Fig. 48). The latter brake is generally arranged
to withdraw the clutch simultaneously. Tests have proved that even heavy
cars can be pulled up in astonishingly short distances, considering
their rate of travel. Trials made in the United States with a touring
car and a four-in-hand coach gave 25-1/3 and 70 feet respectively for
the distance in which the speed could be reduced from sixteen miles per
hour to zero.
SPEED OF CARS.
As regards speed, motor cars can rival the fastest express trains, even
on long journeys. In fact, feats performed during the Gordon-Bennett and
other races have equalled railway performances over equal distances.
When we come to record speeds, we find a car, specially built for the
purpose, covering a mile in less than half a minute. A speed of over 120
miles an hour has actually been reached. Engines of 150 h.p. can now be
packed into a vehicle scaling less than 1-1/2 tons. Even on touring cars
are often found engines developing 40 to 60 h.p., which force the car up
steep hills at a pace nothing less than astonishing. In the future the
motor car will revolutionize our modes of life to an extent comparable
to the changes effected by the advent of the steam-engine. Even since
1896, when the "man-with-the-flag" law was abolished in the British
Isles, the motor has reduced distances, opened up country districts, and
generally quickened the pulses of the community in a manner which makes
it hazardous to prophesy how the next generation will live.
_Note._--The author is much indebted to Mr. Wilfrid J. Lineham, M. Inst.
C.E., for several of the illustrations which appear in the above
chapter.
[8] Steam-driven cars are not considered in this chapter, as their
principle is much the same as that of the ordinary locomotive.
[9] On some cars natural circulation is used, the hot water flowing from
the top of the cylinder to the tank, from which it returns, after being
cooled, to the bottom of the cylinder.
[10] For explanation of the induction coil, see p. 122