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THE 2-CYCLE
PRINCIPLE
The two steps
(strokes) in a complete power cycle of a
2-stroke-cycle engine.
CLICK HERE FOR A
COMPARISON OF 2 STROKE AND 4 STROKE ENGINES
[ A
] The crankshaft has rotated away from bottom
dead center (BDC), moving the piston toward the
cylinder head and closing the transfer and exhaust
ports. Trapped above the piston is the air fuel
charge inducted from the previous cycle. Below the
piston, the rotary disk has started to uncover the
induction port that leads to the carburetor and
the outside atmosphere. Because the volume of the
crankcase beneath the piston is increasing, its
pressure relative to the outside air is
decreasing. This pressure differential causes
outside air to rush into the carburetor where it
mixes with available fuel before continuing
through the induction port and rotary disk, into
the crankcase.
[ B
] As the piston nears top dead center (TDC)
on the secondary compression operation, the air
fuel mixture reaches the ignition point. Ignition
is achieved by the heating effect of compression
and the heat retained by the glow plug element
from the previous cycle, among other things.
Combustion occurs between the top of the piston
and the cylinder head. This volume is known as the
combustion chamber. As combustion progresses and
the gas increases, pressure rises proportionally.
The momentum of the rotating engine components
carries the piston beyond TDC in the direction of
crankshaft rotation. The rotary disk is almost
closed as pressure caused by combustion (several
100 pounds per square inch), drives the piston
away from the cylinder head on the power event.
[ C
] Diagram C shows the piston moving to a
point at which the power (or expansion) event has
just ended with the opening of the exhaust port.
The period of crankshaft rotation necessary to
move the piston from exhaust opening to transfer
opening is called exhaust lead. Exhaust lead is
necessary in most engines to provide time for the
relatively high exhaust-gas pressure to blow down
to atmospheric pressure. If exhaust lead were
eliminated, exhaust gas would probably back-flow
through the transfer port, contaminating the cool,
fresh air-fuel mixture in the crankcase. Notice
that the rotary disk valve is still closed,
allowing the descending piston to continue
compressing the fresh air-fuel mixture in the
crankcase (primary compression), as it awaits the
opening of the transfer port.
[ D
] Diagram D depicts the exhaust and transfer
ports open, with cylinder and crankcase
scavenging taking place. The previously compressed
fresh air-fuel mixture is flowing through the
bypass channel and transfer port-chasing the
tail-end exhaust gases out of the exhaust port. |
