108
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
piston and ring(s) seal the bottom of the combus-
tion chamber. The spark plug, which should be
torqued into the threaded hole according to the
manufacturer’s specifications, seals off the top
of the combustion chamber. As the piston moves
up, the sides of the piston close off the tops of
the transfer ports, stopping the flow of air-fuel
charge into the combustion area. When the top-
side of the piston covers the exhaust port, sec-
ondary compression begins. See Figure 5-38B.
The volume of air-fuel charge above the pis-
ton is quickly compressed into a very small
area. Before it enters the crankcase, the air-fuel
charge is the same temperature as the ambient
temperature of the air entering through the car-
buretor. Ambient temperature is the tempera-
ture of the air outside the piece of equipment as
it is being used. Once the air-fuel charge enters
the crankcase, it absorbs heat from the crank-
case, transfer ports, combustion chamber, and
piston crown. Then, the process of compress-
ing the air-fuel charge causes its temperature to
increase further.
As the charge is compressed during second-
ary compression, the temperature continues to
increase in proportion to the compression ratio.
The compression ratio is a comparison of the
volume of air-fuel charge brought into the cyl-
inder to its volume after final compression. For
example, in an engine with 8:1 compression
ratio, as the piston moves up in the cylinder
a volume of 8 parts of air-fuel charge is com-
pressed into a volume of 1 part. In other words,
in an engine with an 8:1 compression ratio, the
air-fuel charge will be compressed to 1/8 of its
original volume. This also means that the heat
of the charge will also be increased proportion-
ally to the compression ratio. An 8-to-1 com-
pression ratio could elevate the temperature of
the air-fuel charge to approximately eight times
ambient temperature.
The spark plug fires before the piston reaches
top dead center even though secondary com-
pression is not yet completed. The compression
of the air-fuel charge continues until the piston
reaches top dead center.
Power
Before secondary compression is completed,
the spark from the spark plug begins the power
event by igniting the air-fuel charge. This cre-
ates a flame front. The flame front is a controlled
burning of the air-fuel charge starting at the
point of ignition, the spark plug. The heat from
the flame front continues igniting the adja-
cent unburned charge. The flame front contin-
ues growing until the entire charge is burned.
During this burn, the exhaust port is closed and
the tops of the transfer ports are closed.
As the flame front starts, the burned fuel cre-
ates heated exhaust gases. The heated exhaust
gases expand, creating pressure. As the flame
front grows, the pressure increases. Eventu-
ally there is enough pressure from the burning
gases to push the piston down. By this time, the
piston has passed top dead center and started to
move down in the cylinder. As the remainder of
the air-fuel charge burns, combustion pressure
is forcing the piston down.
The power portion of the downstroke of the
piston begins at top dead center and continues to
the point where the crown of the piston is at the
top edge of the exhaust port. See Figure 5-42.
The burning of the air-fuel charge must end
before the exhaust port is uncovered; otherwise,
flames will exit through the exhaust port. When
the side of the piston at the crown uncovers the
exhaust port, the combustion pressure that had
Exhaust
port
closed
GTS Productions/Shutterstock.com, Goodheart-Willcox Publisher
Figure 5-42. Pressure from heated exhaust
gases pushes the piston down during the power
event. All the fuel must be burned before the pis-
ton uncovers the exhaust port.