106
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
the crankcase, but not enough volume enters to
raise the crankcase pressure. Pressure in the
crankcase is still lower than atmospheric pres-
sure. At the point when the piston skirt is about
to uncover the intake port, the crankcase has
reached its lowest pressure.
As the piston continues to travel upward, the pis-
ton skirt uncovers the intake port. SeeFigure 5-39.
Because atmospheric pressure outside the engine
is greater than the pressure in the crankcase,
air outside the engine rushes into the crankcase
in the form of the air-fuel charge. This flow of
charge continues as the piston reaches top dead
center (TDC) and changes direction to travel
down in the cylinder. Top dead center is the
location of the piston at its highest point in the
cylinder. The flow of the charge into the crank-
case continues until the piston, moving down in
the cylinder, closes off the intake port. The tops
of the transfer ports are momentarily covered
by the piston. The air-fuel charge is trapped in
the crankcase. As the piston continues to move
down, the air-fuel charge trapped under the pis-
ton is compressed.
Intake
port
Impulse
port
Top of
transfer
port
Exhaust
port
A B
Volume
under piston
increases
Goodheart-Willcox Publisher
Figure 5-38. The intake event. A—The intake event begins with the piston at bottom dead center (BDC).
B—As the intake event continues, the piston moves up from the crankcase. The volume below the pis-
ton expands. The pressure below the piston decreases.
Intake
port is
uncovered
Low
pressure
Goodheart-Willcox Publisher
Figure 5-39. The intake port is uncovered. The
air-fuel charge rushes into the crankcase.