In the classic symmetrical port timing, opening
and closing of the ports by the piston is symmetrical.
The mechanism is very simple, cross-flow and
loop-flow are symmetrical porting. The drawbacks
are short circuiting, translating into charge loss, poor
intake and exhaust eciency, and the impossibility
of super charging. The process of symmetrical port
timing is as follows: exhaust port open; intake port
open; intake port close; exhaust port close. The replacement of the products of combustion in the cylinder
from the previous power stroke with fresh air
charge to be burned in the next cycle must be completed
in the very short time duration available between
the end of the expansion stroke and the start
of the charging process. If the scavenging is insu-
cient, it results in high fuel consumption, lubricating
oil contamination, and large internal exhaust gas recirculation.
In non-symmetrical porting timing, the
opening and closing of the intake and exhaust is nonsymmetrical,
but the design is much more complex.
The process of non-symmetrical port timing is as follows:
exhaust port open; intake port open; exhaust
port close; intake port close as in a four-stroke engine.
The advantages for non-symmetrical port timing
are: super charging can be possible, and scavenging
is more ecient.