THE INTERNAL COMBUSTION ENGINE
It will be observed that in a steam engine the heat is developed outside
of the cylinders and the latter used solely for the purpose of taking
the steam and utilizing it, by causing its expansion to push a piston to
and fro.
We shall now consider that type of motor which creates the heat within
the cylinder itself and causes an expansion which is at once used and
discharged at the reciprocating motion of the piston.
The original method of utilizing what is called _Internal combustion_
Motors, was to employ a fixed gas. A _fixed_ gas is one which will
remain permanently in that condition, unlike a vapor made from gasoline.
The difference may be explained as follows:
Fixed Gases.--If the vapor of gasoline, or petroleum, is subjected to a
high heat, upwards of 1500 degrees, it is so changed chemically, that it
will not again return to a liquid state. This is called _fixing_ it. Gas
is made in that way from the vapor of coal, and fixed, producing what
is called illuminating gas.
Although the temperature of fixing it is fully three times greater than
is required to explode it, the fact that it is heated in closed retorts,
and oxygen is prevented from mixing with it, prevents it from burning,
or exploding.
Gas Engines.--Such a gas has been used for many years in engines which
were usually of the horizontal type, and were made exceedingly heavy and
cumbrous, and provided with enormous fly wheels. Gases thus made are not
as rich as those generated direct from the hydro-carbon fuels, because,
being usually made from coal they did not have a large percentage of
hydrogen.
Energy of Carbon and Hydrogen.--When a pound of carbon is burned, it
develops 14,500 heat units, and a pound of hydrogen over 52,000 heat
units. Assuming that 85 per cent. of a pound of petroleum is carbon, and
15 per cent. is hydrogen, the heat units of the carbon would be 12,225,
and the heat units of the 15 per cent. of hydrogen would be 12,800. The
combined value is, therefore, 25,025, which is almost double that of
coal gas.
This fact makes the gasoline engine so much more efficient, and for the
same horse power the cylinders can be made smaller, and the whole
structure much lighter in every way.
Gasoline motors are of two types, one in which an explosion takes place
at every revolution of the crank, called the _two-cycle_, and the other
the _four-cycle_, in which the explosion occurs at every other turn of
the crank.
The terms _two-cycle_ is derived from the movement of the piston, as
that moves downwardly during the period when the crank is making a half
turn, and returns in its upward stroke when the crank completes the
turn, or that two half turns of the crankshaft complete the cycle.
Four-cycle engines have two such complete movements at each impulse, or
require four half turns of the crankshaft to complete the cycle.
The Two-Cycle Type.--In order to clearly distinguish between this and
the four-cycle, it would be well to examine the diagram, Fig. 23. For a
clearer understanding the drawing is explained in detail.
The cylinder A, within which the piston works, has a removable cap B,
and at its lower end a removable crank case C. The case is designed to
entirely close the lower end of the cylinder so that it is air tight,
for reasons which will be explained.
The outer jacket, or casing D, at the upper end of the cylinder, is
designed to provide a space E, for the circulation of water, to cool
the cylinder during its working period. The crankshaft F passes through
the crank case, the latter having suitable bearings G for taking care of
the wear.
[Illustration: _Fig. 23. Two-cycle. First Position._]
The piston H is connected up with the rod I, the latter being hinged at
a point within the piston, as shown. The crank case has an inlet port,
provided with a valve which opens inwardly, so that when the piston
moves upwardly the valve will open and air will be drawn into the crank
case and space below the piston.
At one side is a vertical duct K, which extends from a point directly
above the crank case, to such a position that when the piston is at its
lowest point gas can be discharged into the space above the piston.
On the opposite side of the cylinder, and a little above the inlet port
of the duct K, is a discharge port M. The inlet port and the discharge
port, thus described, are both above the lower end of the piston when it
is at its highest point.
The spark plug is shown at N. On the upper end of the piston, and close
to the side wall through which the inlet port K is formed, is an
upwardly-projecting deflecting plate O, the uses of which will be
explained in the description of its operation.
Fig. 23 shows the piston at its highest point, and we will now assume
that ignition takes place, thus driving the piston downwardly until the
upper end of the piston has fully uncovered the discharge port M, as
shown in Fig. 24. This permits the exhaust to commence, and as the
piston proceeds down still further, so as to uncover the inlet port K,
the gas, which at the down stroke has been compressed in the space below
the piston, rushes in, and as it strikes the deflecting plate O, is
caused to flow upwardly, and thus helps to drive out the burnt gases
remaining at the upper end of the cylinder.
[Illustration: Two-cycle Engine.
Fig. 24. Second position. Fig. 25. Third position.]
This action is called scavenging the cylinder, and the efficiency of
this type of engine is largely due to the manner in which this is done.
It is obvious that more or less of the unburnt gases will remain, or
that some of the unburnt carbureted air will pass out at each discharge,
and thus, in either case, detract from the power of the subsequent
explosion.
As the piston now moves upwardly to complete the cycle, the piston
closes both of the ports, thus confining the gas which was previously
partly compressed, and as the piston proceeds the gas is still further
compressed until the piston again reaches the upward limit of its
motion.
Advantages of the Two-Cycle Engine.--This kind of engine has several
distinct advantages. It has less weight than the four-cycle; it gives
double the number of impulses for a given number of revolutions of the
crankshaft; and it dispenses with valves, springs, cam-shafts, stems and
push rods.
More or less danger, however, attends the operation of a two-cycle
engine, principally from the fact that an explosive mixture in a
partially compressed condition is forced into the space which the
instant before was occupied by a flame, and it is only because the
expansion of the burst gases at the previous charge has its temperature
decreased so far below the explosion point, that the fresh gas is not
ignited, although there have been occasions when explosions have taken
place during the upstroke.
The Four-Cycle Engine.--The most approved type is that which is known as
the _four-cycle_. This will also be fully diagrammed so as to enable us
to point out the distinctive difference.
[Illustration: Four-cycle Engine.
Fig. 26. First position. Fig. 27. Second position.]
Figs. 26 and 27 show sections of a typical four-cycle engine, in which
the inlet and the exhaust valves are mechanically operated. The cylinder
A is either cast with or separate from the crank case B, and has a
removable head C. The upper end of the cylinder has a water space formed
by the jacket D.
The inlet port E and the discharge port F are both at the upper end of
the cylinder. The crank shaft G passes horizontally through the crank
case, and it is not necessary, as in the case of the two-cycle-engine,
to have the case closed tight.
The piston H is attached to the connecting rod I, which is coupled to
the crank, as shown. The crank shaft has a small gear J, which meshes
with two gears of double size on opposite sides of the crank shaft, one
of the gears K, being designed to carry the cam L for actuating the stem
L´, which opens the valve M in the port that admits the carbureted air.
[Illustration: Four-cycle Engine.
Fig. 28. Third position. Fig. 29. Fourth position.]
The other large gear N is mounted on a shaft which carries a cam O that
engages the lower end of a push rod P, to open the valve Q in the
discharge port F. It should be observed that the stems L´, P, are made
in two parts, with interposing springs R, so the valves may be firmly
seated when the stems drop from the cams.
The spark plug S is located in the head, close to the inlet port. The
character of the igniting system is immaterial, as the object of the
present diagrams is to show the cycle and method of operating the engine
at each explosion, and to fully illustrate the manner in which it is
distinguished from the two-cycle type.
A fly wheel is necessary in this as in the other type, and in practice
the two gear wheels, K, N, are placed outside of the case B, and only
the small gear, and the cam shafts, on which the cams are mounted, are
within the case.
The operation is as follows: In Fig. 26 the piston is shown in a
position about to commence its downward movement, and we will assume
that the ignition has just taken place. Both valves M, Q, are closed, as
it will be noticed that the cams L, O, are not in contact with the lower
ends of the push rods.
The explosion drives the piston down to the position shown in Fig. 27,
when the cam O begins to raise the stem P, and thus opens the discharge
valve Q, permitting the burnt gases to escape as the piston travels
upwardly to the position shown in Fig. 28.
At this position the valve Q closes, and the cam L opens the inlet
valve M, so that as the piston descends the second revolution, the
carbureted air is drawn in until the crank has just turned at its lowest
limit of movement, as shown in Fig. 29.
The upward stroke of the piston now performs the work of compressing the
carbureted air in the cylinder, and it is ready for the ignition the
moment it again reaches the position shown in Fig. 26.
The Four Cycles.--The four distinct operations thus performed are as
follows: First, the explosion, and downward movement of the piston.
Second, the upward movement of the piston, and the discharge of the
burnt gases. Third, the down stroke of the piston, and the indrawing of
a fresh charge of carbureted air. Fourth, the upward movement of the
piston, and the compression of the charge of carbureted air.
The order of the engine performance may be designated as follows: 1.
Impulse. 2. Exhaust. 3. Admission. 4. Compression.
Ignition Point.--While the point of ignition, shown in the foregoing
diagrams, represents them as taking place after the crank has passed the
dead center, the firing, in practice, is so adjusted that the spark
flashes before the crank turns past the dead center.
The reason for this will be apparent on a little reflection. As the
crank turns very rapidly the spark should be _advanced_, as it is
called, because it takes an interval of time for the spark to take
effect and start the explosion. If the sparking did not take place until
the crank had actually passed the dead center, the full effect of the
compression and subsequent explosion pressure would not be had.
Advantage of the Four-Cycle Type.--The most marked advantage in the
four-cycle type is its efficiency. As it has one full stroke within
which to exhaust the burnt gases, the cylinder is in a proper condition
to receive a full value of the incoming charge, and there is no
liability of any of the unburnt gases escaping during the exhaust from
the previous explosion.
The next important advantage of this type is in the fact that it can be
operated at a higher speed than the two-cycle type, and this is a great
advantage, notwithstanding the less number of impulses in the four-cycle
type.
The Loss in Power.--The great disadvantage in all engines of this class
is the great loss resulting from their action. The explosion which takes
place raises the temperature to fully 2000 degrees of heat, and unless
some provision is made to keep the cylinder down to a much lower
temperature the engine would soon be useless.
High temperatures of this character absolutely prevent lubrication, a
thing which is necessary to insure proper working. For this reason a
water jacket is provided, although there are engines which are cooled by
the action of air.
In any event, the heat imparted to the cylinder is carried away and
cannot be used effectively, so that fully one-half of the power is
dissipated in this direction alone.
The next most serious loss is in the escape of heat through the burnt
gases, which amounts to seventeen per cent. If the expansive force of
the burnt gases at the time of ignition is 250 pounds per square inch,
and at the time of the discharge it is fifty pounds, only four-fifths of
its power is effectively used.
As, however, the discharge is against the air pressure of nearly fifteen
pounds per square inch, it is obvious that thirty-five pounds per inch
is driven away and lost.
The third loss is by conduction and radiation, which amounts to fifteen
per cent. or more, so that the total loss from all sources is about
eighty-four per cent., leaving not more than sixteen per cent. of the
value of the fuel which is converted into power.
Engine Construction.--In the construction of engines the utmost care
should be exercised in making the various parts. The particular
features which require special care are the valves, which should be
ground to fit tightly, the proper fitting of the piston rings, crank
shaft and connecting rod bearings as well as the accurate relining of
these bearings.
[Illustration: Fig. 30. Valve Grinding.]
Valve Grinding.--Fig. 30 shows a valve and valve seat. The valve has
usually a cross groove so that a screw driver in a drill stock may be
used to turn it and to exert the proper pressure. The finest emery
powder and a first class quality of oil should be used. The valve is
seated and after the oil and emery powder are applied the drill stock
is used to turn the valve.
After twenty or thirty turns, wipe off the parts and examine the contact
edges, to see whether the entire surfaces are bright, which will
indicate that the valve fits true on its seat. Never overgrind. This is
entirely unnecessary. It is better also to rock the crank of the drill
stock back and forth, instead of turning it in one direction only.
The Crank Shaft.--The crank shaft is the most difficult part of the
engine to build. It is usually made of a single forging of special steel
and the cranks and bearings are turned out of this, requiring the utmost
care. Formerly these were subject to breakage, but improved methods have
eliminated all danger in this direction.
The Cams.--Notwithstanding the ends of the push rods are provided with
rollers to make the contact with the cams, the latter will wear, and in
doing so they will open the valves too late. The slightest wear will
make considerable difference in the inlet valve, and it requires care
and attention for this reason, in properly designing the cams, so that
wear will be brought to a minimum.