TWO-CYCLE MOTORS
There has always been a strong prejudice in favor of the four-cycle
motor for the power plant of the gasoline automobile. This may be due
to the fact that designers have spent most of their time and energy on
the development of this engine, and that therefore the two-cycle type
has not yet been sufficiently "tried out" in the motor car to enable us
to judge fairly as to its real merits. Certain it is that in the few
instances in which the two-cycle motor has been used as an automobile
power plant, the results have been highly satisfactory, and the present
vogue of the four-cycle motor--with well over 98 per cent. of the
automobiles now made adhering to this type--is largely due to popular
prejudice in its favor.
As has been described in the first chapter of the present volume,
the four-cycle motor devotes a separate stroke to each of the events
of expansion, scavenging or expulsion of the burned gases, suction,
and compression. The two-cycle motor, on the other hand, devotes but
two strokes to these four events, and there is therefore an explosion
twice as often in the two-cycle engine cylinder as is the case with the
four-cycle type. But in lieu of the suction stroke of the four-cycle
motor, there must be some method of forcing the charge into the
cylinder of the two-cycle engine. The base, or compartment below the
piston, in which the crank revolves, is used for this purpose. As the
piston travels upward on its compression stroke, a partial vacuum is
formed in the base, and if a passage is opened between this compartment
and the carburetor, the charge will be sucked in.
All outside connections with the base are tightly closed on the
down-stroke of the piston, and consequently the recently-inhaled charge
will be compressed, ready for its entrance into the cylinder above the
piston as soon as the connecting passage is opened. This passage is
opened, as has already been described, at the bottom of the stroke and
the compressed charge rushes in and fills the space in the cylinder
that at that time is being vacated by the exhaust gases.
The majority of two-cycle motors are made without any valve mechanism,
the opening and closing of the passages being entirely automatic.
These passages are cast with the engine and lead into the cylinder
through openings in the walls called "ports." The opening leading from
the cylinder to the exhaust pipe, or exhaust port, is placed near
the bottom of the stroke so that it is covered by the piston, except
at the lower extremity of the travel of the latter. Just below the
exhaust port, and on the opposite side of the interior of the cylinder,
is placed the intake port, or opening of the passage connecting the
cylinder with the base.
Now, as the piston is forced downward, it uncovers the exhaust port and
an easy means of escape is furnished for the burned gases. Immediately
after this, the intake port on the opposite side is uncovered by the
still-descending piston, and the previously compressed charge, which
is only awaiting the opportunity in the base, "blows" in. The exhaust
gases are still escaping when this happens, and therefore it is
necessary to prevent the incoming charge from passing directly across
the top of the piston and out through the exhaust port before use has
been made of its explosive qualities.
Consequently, to keep it in its proper path, a baffle plate is attached
to the top of the piston which serves to deflect the incoming charge
toward the top of the cylinder, and this not only prevents the loss of
the mixture, but also furnishes a blast of air that helps to blow out
the burned gases. On the return of the piston to the top of its stroke,
it first passes over the intake port and then covers the exhaust port,
effectually closing both and preventing the escape of the charge during
compression. While this is going on, it must be remembered, the piston
is forming the partial vacuum in the base, which serves to draw in the
charge for the succeeding explosion.
If the charge is drawn directly into the base from the carburetor, a
check valve must be used in the pipe connecting the two; otherwise
the mixture would be forced back into the carburetor the instant the
piston began its descent. A two-cycle motor drawing its charge in this
manner is known as the two-port type, for there are only the exhaust
and the inlet ports in the interior of the cylinder walls. The passage
connecting the carburetor with the base may enter at the bottom of the
cylinder, for this space and the base are the same when the piston is
at the top of its stroke. Thus if this port is placed so that it is
uncovered when the piston is at the top of its stroke, it will admit
the charge to the base at a time when a partial vacuum has been created
in this compartment by the upward movement of the piston.
This port is again covered as soon as the piston starts on its
downward journey, and thus the charge is prevented from escaping until
the intake port connecting the base with the top of the cylinder is
opened. Such a two-cycle motor is known as the three-port type, and
it will be seen that not even an automatic check valve is used in its
passages--and it is consequently a "valveless" motor in the liberal
interpretation of the term.
The high velocity of the charge recompenses for the short time that the
port is uncovered, and consequently the base is filled with nearly as
large an amount of charge as is the case with the two-port motor--which
allows the incoming gases to enter the crank case during the entire
upward stroke of the piston.
It will thus be seen that the piston of the two-cycle motor acts as
a pump in two ways. First, the vacuum is formed that serves to draw
the charge into the crank case, or base, of the motor; and second,
the return stroke of the piston compresses this recently-inhaled
charge and makes it ready to be "shot" up into the cylinder as soon
as the piston has uncovered the port that forms the upper terminal of
the communicating passage. There can, of course, no greater amount of
fresh charge enter the cylinder than is drawn into the crank case.
Consequently, the amount to which the cylinder will be filled depends
upon the vacuum formed and the pressure exerted upon the charge by
the succeeding down-stroke of the piston. It is to be supposed that
the piston rings will be tight and that none of the charge can escape
by them, and therefore the vacuum formed and pressure exerted in the
crank case will depend entirely upon the displacement of the piston
in its travel compared with the total capacity of the crank case. In
other words, if the crank case is large and the piston is small and
travels but a short distance, its pump action on the entire volume will
be small. But if the crank case is small and the travel of the piston
alternately doubles and halves the volume, the motion of the piston
will cause the pressure in the crank case to vary greatly.
In a preceding paragraph it has been described in what manner the
incoming charge in the two-cycle motor was used to "scavenge" the
cylinder, or rid it of burned gases, by deflecting the mixture and
allowing this to force out the remaining exhaust before the exhaust
port was closed by the upward motion of the piston. It is evident
that the greater the force, within certain limits, with which the
charge enters the cylinder, the more perfect will be the scavenging
action. But there is a limit to the pressure that can be attained by
the mixture when it is compressed in the crank case previous to its
discharge into the cylinder. This limit is determined by the size
of the space required for the revolution of the crank and "big end"
of the connecting rod, and by the volume displaced by the motion of
the piston. The crank must have room in which to revolve, and the
displacement of the piston can only be the area of its top multiplied
by its length of stroke. Thus eight pounds per square inch is about
the usual limit of crank case compression with this type of two-cycle
motor. This may be varied slightly one way or the other by the
arrangement of the ports, but it makes slight difference whether the
motor is of the two- or three-port type so far as this consideration is
concerned.
Two-cycle motors have been designed which combine the principles of
action of both the two- and three-port types. The most important
departure from the generally-accepted type of two-cycle motor, however,
is the design in which the charge is fed into the cylinder from a
chamber that is absolutely independent of the crank case proper. This
may be accomplished in several ways. There may be what is termed a
"differential piston" in which a separate plunger operates in the
interior of the hollow "trunk" piston, and by means of the proper
connection with the crank shaft compresses the charge in the chamber
thus formed at the time it is to be forced into the cylinder.
Another design for obtaining intake compression independent of the
crank case consists of a collar, or circular enlargement at the base of
the piston. This collar reciprocates within the lower portion of the
piston in a chamber which has been bored to the exact size. The collar
consequently forms a variable base for this compartment, and as the
piston descends, the collar travels with it, thus drawing in a charge
of the fresh mixture. On the upward stroke, this mixture is compressed
by the collar as it reduces the size of the compartment. It will be
seen that such a motor can be designed to compress the charge to almost
any amount.
Inasmuch as the mixture, as mentioned above, is compressed on the
up-stroke of the piston, it is evident that it cannot be discharged
into that particular cylinder at that time--for the mixture should be
delivered to its cylinder only when the piston is at the bottom of
its stroke. In the case of a four-cylinder engine, however, one of
the pistons would be in the proper position for the entrance of the
charge, and it is into this cylinder, that the compressed mixture is
forced. The compression space in each cylinder, therefore, works for
its neighbor, rather than for itself.
This interchange of courtesies is obtained through the good offices of
a distributor in the form of a rotating, hollow cylinder having ports
cut throughout its length that register with corresponding passages
leading to the various cylinders. This distributor is timed with the
crank shaft of the motor, and may be driven either by a gear or by a
silent chain. As the mixture is compressed in the separate chamber of
one cylinder, the passage leading to the distributor is opened by the
revolution of the latter, and the charge is led through this passage,
the distributor, and thence through another passage--also opened by the
distributor--to the proper cylinder. The cylinders thus operate in
pairs, one receiving its charge while the other is about to begin its
explosion stroke--and vice versa.
The force of the explosion in a gasoline engine cylinder is not only
dependent upon the amount and nature of the inflammable mixture
admitted, but upon the force with which it is compressed, as well.
The average compression pressure of a two- or four-cycle engine of
the ordinary type, is from 60 to 70 pounds per square inch. Inasmuch
as this pressure, assuming that the rings and valves are tight, is
proportional to the displacement of the piston stroke compared with the
volume of the clearance space, the amount of compression is constant at
all speeds and loads of the motor. Should it be possible to increase
this compression at will, it would be found that, with a warm motor, a
pressure in the neighborhood of 100 pounds per square inch would serve
to generate sufficient heat to ignite the mixture before the formation
of the spark--for it is one of the elementary laws of physics that a
gas will become heated when compressed. It is for this reason that the
compression pressure of the ordinary automobile motor is kept in the
neighborhood of 70 pounds per square inch.
A method of varying compression pressure to meet individual load
requirements has been devised for some motors, however, and while such
types are not as yet in general use in automobiles, it is probable that
the near future will find much advancement along these lines. One such
two-cycle motor that has been designed especially for automobile use
employs a separate air compressor driven by the engine itself and used
as the clutch and variable speed transmission of the car. The amount of
pressure generated in the compressor is dependent upon the resistance
offered to its operation--or, in other words, it increases with
additional load carried by the motor. The compression, or compressed
air, rather, is carried directly from the compressor to the cylinders
of the motor, being admitted at the proper time by a rotary valve
driven by the crank shaft. Thus the compression in each cylinder is
automatically regulated by the load, and a motor of this type possesses
a high "overload" capacity.
The motor mentioned above operates on somewhat the same principles as
those found in the Diesel engine, which will be, as many predict, the
ultimate type of internal combustion motor. The Diesel motor is not
necessarily a particular make of engine, but bears the name of the
originator of the principles involved. These are distinct from those
of the Otto cycle, which is the principle upon which practically all
automobile motors operate. The Otto cycle consists of the well-known
series of events in the cylinder, as follows: Ignition, followed by the
explosion, or expansion of the burned charge; discharge of the exhaust
gases, or scavenging; admission of the fresh charge, suction; and
compression of the newly-received mixture previous to ignition and the
repetition of the cycle. In speaking of the Otto and Diesel engines, it
must be borne in mind that they are referred to as a class, rather than
as a particular make--as one would mention poppet valve or sleeve valve
engines--for there may be many manufacturers of each type.
Although the Diesel principle may be applied to either the two or
four-cycle type of motor, it is to the former design that it lends
itself unusually well. This motor operates a two-stage air compressor
in conjunction with a storage tank. At the beginning of the compression
stroke, pure air under high pressure is admitted to the cylinder.
In its upward travel, the piston compresses this air to a pressure
approximating 500 pounds per square inch. While it has been shown that
such a pressure is about five times more than enough to generate
sufficient heat to cause premature ignition, it must be remembered
that, unlike the ordinary type of motor, this is only pure air that is
injected into the cylinder and contains none of the explosive gasoline
vapor. At the top of the stroke, however, when the compression is at
its maximum, the fuel is injected directly into the cylinder without
having been previously vaporized.
This is another feature in which the Diesel motor is entirely different
from the Otto type, for the latter must employ a carburetor to vaporize
the fuel before it can be admitted to the cylinder. But inasmuch as
there is already a pressure approximating 500 pounds per square inch
in the cylinder of the Diesel motor at the time the fuel is injected,
there must be a force behind the latter of 750 or 1,000 pounds per
square inch in order to enable it to overcome the resistance of the
highly-compressed air in the cylinder. In short, the liquid fuel is
sprayed directly into the cylinder at a pressure of 750 or 1,000 pounds
per square inch. This tremendous pressure is sufficient, not only to
vaporize the particles of fuel as soon as they enter the cylinder
from the nozzle, or "atomizer," but to cause them to burst into
flame, as well. In other words, the compression of the air previously
has generated sufficient heat in the cylinder to ignite the fuel
immediately on its admission.
The fuel continues to be injected into the cylinder during the greater
part of the down-stroke of the piston. In this respect, also, is the
Diesel motor radically different from the Otto type, for the latter
receives its full charge at one time and fires the entire amount in
a single "explosion." In the Diesel motor, on the other hand, the
ignition continues as long as fuel is admitted, and thus this engine
is of the internal _combustion_ type in the strictest sense of the
word. It is, after all, the expansion of the gases due to the heat of
combustion that produces the power in a gasoline engine, and if the
fuel can be so admitted that it can burn during the greater part of the
stroke, a high efficiency will be obtained.
The exhaust gases of the ordinary two-cycle motor pass out of the
exhaust port as it is uncovered by the descent of the piston. Those
that remain are forced out by the sudden admission of the fresh charge,
which is deflected upward and is intended to scavenge the top of the
cylinder. But it is claimed that thus employing the fresh mixture as
a scavenging agent is wasteful of the fuel-permeated charge and does
not conduce to efficient running. The system is simple in the extreme,
however, and does its work well in small installations in which fuel
economy is not of vital importance. But in the two-cycle Diesel type
of engine, the high pressure of the pure air is used for scavenging,
and as this is admitted with so large an initial force, the exhaust
port may remain uncovered for a longer period than would be the case
were the air to rely entirely on the up-stroke of the piston for its
compression. Then too, whatever air may escape contains no fuel, and
consequently efficient scavenging may be obtained without waste.
At the high pressure at which the fuel is injected into the cylinder
of the Diesel engine, practically any grade of gasoline, naphtha,
kerosene, crude oil, or other form of petroleum can be vaporized.
The compressed air employed in the compression and injection of the
fuel is also used for starting the motor, for this is not a type that
is amenable to hand cranking. Thus the Diesel type of engine can be
run in any weather on any grade of oil fuel, and as the carburetor
and electrical ignition system are absolutely eliminated, two of the
great sources of trouble of the automobile motor are absent--and this
feature, alone, even more than the superior economy of operation, will
appeal to the average motorist.
Just when this type of motor will be taken up by automobile designers
is difficult to state. The Diesel type of engine has proved so
wonderfully successful for large stationary power plants and for marine
purposes, and its reliability is so absolute on all grades of fuel,
that this motor may solve the failing-gasoline-supply problem. As yet,
about 100 horsepower is the smallest unit that has been made in any
quantities, but it was recently announced that this type would, in
the very near future, be built for motor trucks and other commercial
vehicles. Consequently, it is well for all those interested in the
application of the two-cycle motor to the automobile to understand the
elementary principles on which this radically-different type operates.
* * * * * *
Transcriber's note:
Inconsistent hyphenation has been retained unless one form predominated.
The following corrections have been made:
p. 19 This give a much -> give changed to gives
p. 34 purpose of cleanning -> cleanning changed to cleaning
Everything else has been retained as printed.