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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
Fuel Lines
Fuel lines, also referred to as fuel pipes, serve as
a conduit to bring fuel from the fuel tank filter
to the carburetor and, in some cases, to return
fuel from the carburetor back to the tank. The
material used for fuel lines must be rated for
fuel. To be rated for fuel means the material
will not break down, dissolve, or deteriorate
when in contact with gasoline. The inside diam-
eter (ID) of the fuel line determines how much
fuel can flow through the line at any given time.
Engines with larger displacement require more
fuel when operating and need fuel lines with
a larger ID. Smaller engines can use fuel lines
with a smaller ID.
Fuel lines meeting the EPA evaporative emis-
sions standards will be identified by printing on
the fuel line or by the fact that the fuel line is
installed on an EPA-compliant engine. Print-
ing on the fuel line should read: “EPA-(fuel line
manufacturer’s name)-(emission family).” Fuel
line on an EPA-compliant engine may not be
labeled, but the engine itself will have a “Emis-
sion Control (Exhaust and Evaporative)” label.
Molded fuel lines are manufactured to be
used on a specific engine. See Figure 5-37. The
curves and bends in the lines, the inside diam-
eter of the ends, the length of each line, the
mounting grommet, and the part that secures
the fuel lines to the fuel tank are designed to fit
one engine series.
Two-Stroke Engine Operation
A two-stroke engine performs the four events of
the operating cycle in two strokes of the piston.
Keep in mind that when any one event is taking
place, other events are happening at the same
time. A power event refers to the process of pro-
ducing one downward push of the piston using
combustion gases. The operating cycle, also
called a “power cycle,” begins with the intake
of the air-fuel charge, and continues by com-
pressing it, igniting it, and exhausting the spent
gases. One additional process, specific to two-
stroke engines, is the process of transfer of fuel
from the crankcase to the combustion chamber.
In a two-stroke engine, the four events overlap
and yet are separate from each other. We will
look at the operating cycle by examining each
of the four events for a piston-ported engine:
intake, compression, power, and exhaust.
Intake
The purpose of the intake event is to bring air
plus fuel into the engine for combustion. The air
must travel through the carburetor to pick up a
precise amount of fuel. The fuel has been pre-
mixed with two-stroke engine oil to be used to
lubricate the internal parts of the engine. This
air, fuel, and oil mixture is called the air-fuel
charge, or the charge. The intake event begins
by lowering the pressure in the crankcase below
atmospheric pressure. This low pressure area
is created when the piston moves up in the
cylinder.
The piston starts at BDC (bottom dead cen-
ter). See Figure 5-38. Bottom dead center is
the location of the piston at its lowest point of
travel in the cylinder. The piston skirt covers the
intake port. The lower ends of the transfer ports
are open to the crankcase. The crankcase and
transfer ports are filled with the remnants of
the air-fuel charge left over from the previous
power cycle. Low pressure is created as the pis-
ton moves up in the cylinder, increasing the vol-
ume of space open to the crankcase. This allows
the existing air-fuel charge to expand.
As the piston begins moving up in the cyl-
inder, no air-fuel charge enters the crankcase
because the intake port is still sealed off by the
piston. The piston crown covers the top of the
transfer ports. The transfer ports are filled with
air only, if the engine is just being started, or
an air-fuel charge left over from the previous
cycle of power. As the piston moves up, what-
ever is in the transfer ports is pulled down into
Fuel filter Return fuel line Inlet fuel line
Goodheart-Willcox Publisher
Figure 5-37. Molded fuel lines are designed to
fit one series of engines. Their premolded shape
makes installation simple.