ENGINE PRINCIPLES
The working part of a tractor is the engine; it is this that furnishes
the power that makes the machine go.
The engine gets its power from the burning of a mixture of fuel vapor
and air. When this mixture burns, it becomes heated, and, as is usual
with hot things, it tries to expand, or to occupy more room.
The mixture is placed in a cylinder, between the closed end and the
piston; it is then heated by being burned, and, in struggling to
expand, it forces the piston to slide down the cylinder. This movement
of the piston makes the crank shaft revolve, which in turn drives the
tractor.
The first step in making the engine run is to put a charge of mixture
into the cylinder, and it is clear that if the burning of the charge is
to move the piston, the piston must be in such a position that it is
able to move. When the mixture is burned, the piston must therefore be
at the closed end of the cylinder.
After the charge of mixture has been burned, the cylinder must be
cleared of the dead and useless gases that remain, in order to make
room for a fresh charge.
The charge of mixture is drawn into the cylinder just as a pump sucks
in water. At a time when the piston is at the closed end of the
cylinder, a valve is opened connecting the space above the piston with
the device that forms the mixture; then by moving the piston outward,
mixture is sucked into the space above it. When the piston reaches the
end of its stroke the cylinder has been filled with mixture, and the
valve then closes.
It would be useless to set fire to the mixture at that time, for the
piston is as far down the cylinder as it can be, and pressure could
not move it any farther. To get the piston into such a position that
the expanding mixture can move it, it is forced back to the closed end
of the cylinder. This squeezes or _compresses_, the cylinderful of
mixture into the small space, called the _combustion chamber_, between
the piston and the cylinder head.
If the mixture is now burned, the piston can move the length of the
cylinder, and in so doing it develops power.
The cylinder is cleared of the burned and useless gases by opening a
valve and pushing them out by moving the piston back to the inner end
of the cylinder. When this has been done, the valve is closed, and, by
opening the inlet valve and moving the piston outward, a fresh charge
is sucked in, and the several steps of the _gas engine cycle_ are
repeated.
The name _cycle_ is given to any series of steps or events that must be
gone through in order that a thing may happen. Thus the empty shell
must be taken out of a gun and a fresh cartridge put in before the gun
can be fired again, and that series of steps might be called the gun
cycle.
The gas engine cycle requires the piston to make four strokes. An
outward stroke sucks in a charge of mixture, and an inward stroke
returns the piston to the firing position and compresses the charge.
Then comes the outward stroke when the piston moves under power,
followed by the inward stroke that clears the cylinder of the burned
gases.
For every stroke of the piston the crank shaft makes a half-revolution;
the crank shaft therefore makes two revolutions to four strokes of the
piston and to each repetition of the gas engine cycle.
Of these four strokes of the piston only one produces power. The other
three strokes, called the _dead strokes_, are required to prepare for
another power stroke.
A gas engine cylinder thus produces power for only one quarter of the
time that it runs. This is one of the striking differences between the
gas engine and the steam engine, for the piston of a steam engine moves
under power all of the time that the engine runs.
A one-cylinder gas engine must have something to make the piston go
through the dead strokes, for otherwise the piston would stop at
the end of the power stroke; the piston is kept in motion by heavy
flywheels attached to the crank shaft. These, like any object, try to
continue in motion when once they are started; a power stroke starts
the crank shaft revolving and its flywheels keep it going.
Thus, the piston drives the crank shaft during the power stroke, and
the crank shaft drives the piston during the dead strokes.
To start an engine, the crank shaft is revolved to make the piston suck
in a charge of mixture and compress it; then the charge is burned, the
power stroke takes place, and the engine runs.
A clear idea of what goes on inside of the cylinder is quite necessary
in order to take proper care of an engine and to get the best work
out of it. The following description applies to any cylinder, for the
action in all cylinders of an engine is the same.
=Inlet Stroke.=—During the inlet stroke (No. 1, Fig. 1), the piston
moves outward; the inlet valve is open, and the exhaust valve is
closed. This movement of the piston creates suction, and if there
are leaks in the cylinder, air will be sucked in and will spoil the
proportions of the charge. This will prevent the proper burning of the
mixture, and the engine will lose power.
The piston moves at such high speed that the mixture cannot enter fast
enough to keep up with it; mixture is still flowing in when the piston
reaches the end of its stroke, and even when it begins to move inward
on the next stroke. The more mixture there is in the cylinder, the more
powerfully the engine will run; the inlet valve is therefore held open
for as long a time as the mixture continues to enter.
[Illustration]
[Illustration: FIG. 1.—THE GAS ENGINE CYCLE]
In slow-speed 1-cylinder and 2-cylinder engines the valve closes when
the piston reaches the end of its stroke; on high-speed engines the
valve does not close until the piston has moved ¼ inch or ½ inch on the
compression stroke.
=Compression Stroke.=—During the compression stroke (No. 2, Fig. 1)
the piston moves inward, and both valves are closed. This movement
places the piston in position to move outward on the power stroke.
As the outlets to the cylinder are closed, the charge of mixture
cannot escape, and is therefore compressed into the space between the
piston and the cylinder head when the piston is at the inner end of
its stroke. This space is usually about one quarter the volume of the
cylinder; the charge is therefore compressed to about one quarter of
its original volume.
This compression of the charge is very important in the operation of
the gas engine, and any interference with it will make the engine run
poorly.
In the first place, it improves the quality of the charge, and makes it
burn very much better. When the charge enters the cylinder, the fuel
vapor and air are not thoroughly mixed; much of the fuel is not turned
into vapor. By compressing the charge it becomes heated; this vaporizes
the fuel, and vapor and air become thoroughly mixed.
Compression also increases the power. Suppose that the cylinder
contains a quart of mixture which, when heated, will expand to a
gallon. If this quart of mixture is compressed to a half pint, it
will not lose its ability to expand to a gallon, and will exert more
pressure in expanding from a half pint to a gallon than from a quart to
a gallon.
A leaky cylinder will cause a further loss of power because some of the
charge will escape during the compression stroke, which will leave less
to be burned and to develop power.
=Ignition.=—Setting fire to the charge of mixture is called the
_ignition_ of the charge, and it takes place close to the end of the
compression stroke. To get the greatest power, all of the mixture
should be on fire and heated most intensely as the piston begins the
power stroke.
When the mixture is set on fire, it does not explode like gunpowder,
but burns comparatively slowly; the charge is ignited by an electric
spark, and the flame spreads from that point until it is all on
fire. In order to give the flame time to spread, the spark passes
sufficiently before the end of the compression stroke to have the
entire charge on fire as the power stroke begins. This is called the
_advance_ of the ignition.
The flame takes the same time to spread through the charge when the
engine is running fast as when it is running slow. Therefore if the
engine is speeded up, the spark must be advanced, for otherwise the
piston would be on the power stroke before the flame would have time to
spread all through the mixture.
When the engine is slowed down, the spark must have less advance, or
must be _retarded_, for, if it were not, the charge would all be in
flame, and exerting its full pressure, before the piston reached the
end of its compression stroke.
The subject of ignition, which is of great importance, is covered more
fully in Chapter VI.
=Power Stroke.=—During the power stroke (No. 3, Fig. 1) the piston
moves outward, and both valves are closed. As it begins, the mixture is
all on fire, and great pressure is exerted against the piston.
As the piston moves outward the combustion space becomes larger, and
the gases obtain the room for expansion that they seek. As they expand,
the pressure that they exert becomes less. By the time the piston
is three quarters the way down the power stroke, the pressure is so
reduced that it has little or no effect; the gases are still trying to
expand, however, so the exhaust valve is opened at that point, and they
begin to escape.
=Exhaust Stroke.=—During the exhaust stroke (No. 4, Fig. 1) the piston
moves inward and the exhaust valve is open. This movement of the piston
pushes the burned gases out of the cylinder, and it is clear that the
more thoroughly the cylinder is emptied of them, the more room there
will be for a fresh charge.
In high-speed engines the gases cannot escape as fast as the piston
moves; they are still flowing out when the end of the stroke is
reached. Therefore the valve is closed, not at the end of the stroke,
but when the piston has moved about ⅛ inch outward on the inlet stroke.
The inlet valve opens as the exhaust valve closes.
It can be seen that through the inlet and compression strokes a leak
will reduce the charge and so interfere with the production of full
power. The piston must make a tight fit in the cylinder, the valves
must seat tightly, and gaskets and other parts must be in proper
condition.
[Illustration: FIG. 2.—1-CYLINDER POWER DIAGRAM]
Figure 2 shows a power diagram for a 1-cylinder engine, in which the
crank shaft moves under power during one stroke out of every four. An
engine with two cylinders can be built so that first one cylinder
applies power and then the other, in which case the crank shaft moves
under power during two strokes out of every four.
[Illustration: FIG. 3.—2-CYLINDER POWER DIAGRAM]
Figure 3 is a power diagram of an engine of this sort. If piston 1 is
moving down under power, piston 2 is also moving down, but on the inlet
stroke. The following stroke is exhaust in cylinder 1 and compression
in cylinder 2, and cylinder 2 will then deliver a power stroke while
cylinder 1 is on inlet. Thus the crank shaft will receive a power
stroke, followed by a dead stroke; then another power stroke and
another dead stroke, and so on.
There will be the disadvantage, however, that the pistons, moving up
and down together, will cause vibration, which in the course of time
will be likely to give trouble. To overcome this, a 2-cylinder engine
can be built as indicated in Figure 4.
In this engine the cranks project on opposite sides of the crank shaft
instead of on the same side, as in Figure 3; the pistons thus move in
opposite directions, and produce no vibration. Power will be unevenly
applied, however, for both power strokes occur in one revolution, with
two dead strokes in the succeeding revolution.
[Illustration]
[Illustration: FIG. 4.—2-CYLINDER POWER DIAGRAM, 180 SHAFT]
With piston 1 moving down on power, piston 2, moving upward, can only
be performing compression or exhaust. If it is on compression, its
power stroke will follow the power stroke of piston 1, while if it is
on exhaust its power stroke will have occurred immediately before the
power stroke of piston 1. In either case one power stroke follows the
other, taking place in one revolution of the crank shaft, while in the
following revolution both pistons will be performing dead strokes.
While there is no vibration from the movement of the pistons in this
engine, the uneven production of power will produce vibration of
another kind.
These two types may be built with the cylinders standing up or lying
down; that is, they may be either _vertical engines_ or _horizontal
engines_. The _double opposed_ engine, which is built only in horizontal
form, is free from either kind of vibration, but has the disadvantage
of occupying more room than the others. The cylinders, instead of being
side by side, and on the same side of the crank shaft, are placed end
to end, with the crank shaft between them, as shown in Figure 5.
The pistons make their inward and outward strokes together, but in so
doing they move in opposite directions. Thus every power stroke is
followed by a dead stroke, as in the engine shown in Figure 3, while
the movement of one piston balances that of the other, as is the case
with the engine shown in Figure 4.
[Illustration: FIG. 5.—H. D. O. POWER DIAGRAM]
[Illustration]
[Illustration: FIG. 6.—4-CYLINDER POWER DIAGRAM]
In a 4-cylinder engine one power stroke follows another without any
dead stroke intervals, which, of course, makes the crank shaft revolve
more smoothly and with a steadier application of power. The power
diagram is shown in Figure 6; in studying this it should be remembered
that if two pistons move in opposite directions, as in Figure 4,
one power stroke follows another, while if they move in the same
direction, as in Figure 3, there is an interval of one stroke between
their power strokes.
The crank shaft of a 4-cylinder engine is so made that the middle
pistons move in the same direction, and opposite to the end pistons.
This construction has been found to make a smoother running engine than
if pistons 1 and 3 moved one way while pistons 2 and 4 moved the other.
If piston 1 is on the power stroke, either piston 2 or piston 3 can
follow, for they are moving in the opposite direction. If we say that
piston 2 is the next, then piston 4 must be the third to give a power
stroke, for it is the only one left that is moving in the opposite
direction to piston 2. Piston 3 is thus the fourth to move under power,
and it is followed by another power stroke by piston 1; the _firing
order_ is then said to be 1, 2, 4, 3.
If it is piston 3 that follows piston 1, piston 4 will again be the
third to produce power, and piston 2 will be the fourth. The firing
order will then be 1, 3, 4, 2. There is no other order in which a
4-cylinder engine can produce power, and there is no choice between
them.
The firing order of an engine is established by the manufacturer, and
depends on the order in which the valves are operated.