TWO-CYCLE ENGINE
The two-cycle type of gasoline engine differs from the four-cycle
type described in the foregoing chapters in that the five events
composing the cycle are performed during one revolution of the crank
shaft, or two strokes of the piston, power being developed during
every outward stroke of the piston instead of alternate outward
strokes.
[Illustration: FIG. 18.—TWO-CYCLE ENGINE.]
In order that this result may be attained, the construction of the
engine is changed, and, as will be seen in Fig. 18, the crank case
is utilized as a receiver for the mixture before it passes to the
combustion space. The valves are replaced by =ports=, which are
openings into the combustion space that are covered and uncovered
by the piston as it slides in the cylinder. The =inlet port= is
uncovered when the piston is at the inmost point of its stroke
(Fig. 18, A), and then admits the mixture to the crank case; the
=by-pass port= and the =exhaust port= are uncovered when the piston
is at the outmost point of its stroke (Fig. 18, B), the former then
permitting the mixture to pass from the crank case to the combustion
space, and the latter is that through which the burned gases escape
after combustion has taken place.
During an inward stroke, the pressure in the crank case is reduced
as the piston slides away from it, and fresh mixture is forced into
it by the higher atmospheric pressure as soon as the inlet port is
uncovered. This port is covered when the piston makes an outward
stroke, and the mixture, not being able to escape, is compressed. Its
tendency to expand causes it to flow to the combustion space when
the by-pass port is uncovered, and in entering it strikes a ledge
on the piston so that it is deflected to the top of the combustion
space instead of being able to shoot across the cylinder and out the
open exhaust port. The inward stroke of the piston covers these two
ports and compresses the mixture, ignition occurring in the regular
manner. The pressure developed by the combustion drives the piston
outward, and as soon as the exhaust port is uncovered (which is
slightly before the uncovering of the by-pass port), the gases, which
are still expanding, begin to escape, and are further expelled by the
fresh charge that enters and drives them before it. Thus the five
events of the cycle are performed during an inward and an outward
stroke of the piston, the crank case end of the piston drawing a
charge of fresh mixture into the crank case and forcing it into the
combustion space, and the combustion chamber end compressing it and
being acted on by the pressure from the combustion.
At slow speeds, two-cycle engines have advantages over the four-cycle
type in the production of a power stroke at every revolution of the
crank shaft, and the absence of valves and valve mechanism with their
weight and possibility of giving trouble. This simplicity makes the
two-cycle engine popular for motor boats, where they are run at
slow and constant speed, but for higher and changing speeds these
advantages are outweighed by disadvantages that show little sign of
being overcome.
With the engine running at a thousand revolutions a minute, it can be
understood that the ports will be open for only a brief period during
each stroke, and that the faster the engine runs the shorter will be
the period during which the gases may enter or leave the combustion
space. The inefficiency of two-cycle engines as compared with engines
of the four-cycle type is due entirely to the fact that the burned
gases have not sufficient time in which to escape from the combustion
space, nor the fresh charge time to enter. The fresh charge that does
enter being incomplete, and being contaminated by the portion of the
burned gases that has not been able to escape, result in the “choking
up” of the engine, and in the production of lower power than the
dimensions and weight of the engine should warrant.