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Two-Cycle Engine
Theory of Operation
Introduction
All internal
combustion engines
are
characterized
by
a
four part
cycle of
operation
.
The four parts
are:
1. Intake,
2
.
Compression,
3. Combustion,
and 4. Exhaust.
A fuel
/
air mixture must be drawn into the
engine,
compressed,
burned,
and exhausted
.
A two -stroke
engine uses only
two strokes to
ac
-
complish all
four
parts.
Each of
the
four
parts can be
easily
understood
by
following the path
of
the fuel
/
air mixture through
the engine
.
(See Figure 2-1 .)
1.
Intake:
On the upstroke of the piston
a charge
of fuel mixed with air is drawn into the engine
through an open
port in the cylinder.
2.
Compression: Near the bottom of the down-
stroke,
the intake port is closed, and the piston
starts back
up.
Th is upstroke compresses the
fuel
/
air mixture to a fraction of its former
volume
.
3
.
Combustion: When the mixture has been com-
pressed, it
is
ignited
and
forces the piston
down.
This is called
the power stroke.
4
.
Exhaust:
After the power
stroke,
the burned
gases exit through
the
now open
eX.ha
ust port.
and the cycle is ready to start again
.
The piston
is carried through each step
from
one
power stroke
to the
next
by the
rotating inertia of
the
crankshaft.
The
intake
of a two -stroke
engine
is perhaps the
most
complex
part of
the
cycle. After the fuel and
air have been m ixed
in the
carburetor,
it is
drawn
into the
crankcase.
As the
piston rises, a hole in
the
cylinder
wall
(the intake port)
is uncovered
by
the lower
edge of the piston.
The rising
piston
effectively
increases
the
volume
of
the
crankcase,
drawing
in the mixture
.
On
its
return
tr ip,
the
piston blocks the intake
port and lightly com-
presses the
mixture
in
the
crankcase
.
At about
55
0
-60
0
BBDC
(before
bottom dead
center),
the
upper edge of
the piston uncovers ports
in
the cyl-
inder
wall that
are
connected
to the crankcase
.
The
mixture
in
the
crankcase,
under
pressure, rushes
through these transfer ports into the
cylinder.
o
Figure 2
-
1
THEORY OF OPERATION
2 -5
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