Section 2
Troubles
hooti
ng
The
2-Stroke
Cycle
Engine
Theory
01
Operation
Figures
2.
1 and 2 2
A two-stroke cycle or
"two-cycle" engine
produces one
power
stroke for every
revolution
of
its
crankshaft. Intake
and
compression
occur during the
piston's "up " stroke;
power
and
exhaust
occur during the
"down" stroke.
Six distinct pro-
cesses occur during
every revolution
of
the crankshaft, and a
1055
or
weakness
of
even one
of
these
processes wi/l affect
the other five.
Piston Ported Induction
1.
Intake
A.S Ihe piston moves up towar the c
yl
lnder
head a low pressu
re
(vacuum)
is
created
In
the crankcase .
As
the movlng pistan uncovers
he
i
ltake
por
t,
atmosphenc pr
es
su
re
pushes
a fresh charge
01
fuel-air nixture
in
ta
the
crankcase.
The
mi
xture accomp1
is
hes two functions
befare il is moved
to
he
combu
sti
on
chamber:
•
Oi
1
su
spended
in
lhe mi
xt
ure coats all
Internal engine parts
in
cludlng cylmder
walls crankshaft. and bearings.
• T
he
atomlzed
fue
l
mi
xture absorbs hea! as
il enters !he crankcase. lowerlng lhe
engine's opera
ti
ng temperalu
re
.
2.
Compression
As t
he
piston continues lo move upward and
cover lhe cylind
er
ports, the combust
lo
n
c
ha
mb
er
is
sealed and compression begins
The
ajr-fu
el
mi
xture introduced duri
ng
Ihe
previous
rev
olution's lfans/er cycle
is
now
t
ra
pped and becomes compressed between
Ihe piston and cy
li
nder head
3.lgnition
As the piston a proaches the lap t Ihe
cylind
er
, lhe spark
pl
ug
fi
res and gnl es lhe
fu
el
-air
mi
xture .
To
compensale lar increases
in eng
in
e rpm.
spar
k liming
is
advanced
electr
on
ical1y.
Spark plug
Intake
port
open
Vacuum
F/9ure Z 1 Crankcase vac
m;8
creat
ed
dunng
Ihe
compression stroke When Ihe spark
plllg
(¡res ¡he
compressed
mixture al>nve l/
le
Dlston ¡gil/les
and
begms
ro
e
ano
Shlndaiwa Inc 1995
6
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