Figure 12-5.-Events in a two-cycle, internal combustion engine.
A two-cycle engine is shown in figure 12-5. Every
other stroke in this engine is a power stroke. Each time
the piston moves down, it is on the power stroke. Intake,
compression, power, and exhaust still take place; but
they are completed in just two strokes. Figure 12-5
shows that the intake and exhaust ports are cut into the
cylinder wall instead of at the top of the combustion
chamber as in the four-cycle engine. As the piston moves
down on its power stroke, it first uncovers the exhaust
port to let burned gases escape and then uncovers the
intake port to allow a new fuel-air mixture to enter the
combustion chamber. Then on the upward stroke, the
piston covers both ports and, at the same time,
compresses the new mixture in preparation for ignition
and another power stroke.
In the engine shown in figure 12-5, the piston is
shaped so that the incoming fuel-air mixture is directed
upward, thereby sweeping out ahead of it the burned
exhaust gases. Also, there is an inlet into the crankcase
through which the fuel-air mixture passes before it
enters the cylinder. This inlet is opened as the piston
moves upward, but it is sealed as the piston moves
downward on the power stroke. The downward moving
piston slightly compresses the mixture in the crankcase.
That gives the mixture enough pressure to pass rapidly
through the intake port as the piston clears this port. This
action improves the sweeping-out, or scavenging, effect
of the mixture as it enters and clears the burned gases
from the cylinder through the exhaust port.
FOUR-CYCLE VERSUS TWO-CYCLE
You have probably noted that the two-cycle engine
produces a power stroke every crankshaft revolution;
the four-cycle engine requires two crankshaft
revolutions for each power stroke. It might appear that
the two-cycle engine could produce twice as much
power as the four-cycle engine of the same size,
operating at the same speed. However, that is not true.
With the two-cycle engine, some of the power is used to
drive the blower that forces the air-fuel charge into the
cylinder under pressure. Also, the burned gases are not
cleared from the cylinder. Additionally, because of the
much shorter period the intake port is open (compared
to the period the intake valve in a four-stroke-cycle is
open), a smaller amount of fuel-air mixture is admitted.
Hence, with less fuel-air mixture, less power per power
stroke is produced compared to the power produced in
a four-stroke cycle engine of like size operating at the
same speed and under the same conditions. To increase
the amount of fuel-air mixture, we use auxiliary devices
with the two-stroke engine to ensure delivery of greater
amounts of fuel-air mixture into the cylinder.