Service Manual
SECTION
6
Engine Description
Two-cycle engines have special advantages which
make their use more practical in certain applications.
Two cycle engines are lightweight with an excellent
power-to-weight ratio and can be operated in any
position. They are also notably easy
to maintain and
service because of their uncomplicated design. The
Lawn-Boy Two-cycle
V
Engine used on Lawn-Boy
rotary motors is a reed valve design. This design
name describes the path of the fuel/air mixture into
the cylinder/crankcase and combustion chamber, and
the exhausting of spent gases.
Engine Theory
of
Operation
Two two-cycle (or two-stroke) engine is one of the
simplest and most efficient power systems ever
developed.
Figures 62 and 63
below show what happens inside
the engine during one full crankshaft revolution. Fuel
intake, fuel ignition and the exhaust of burned gases
all take place during a single 360-degree rotation of
the crankshaft.
Reed
first stroke
I
I
Figure 62
Beginning at a point where the top of the piston is just
below the exhaust port, the piston moves forward
toward the crankcase.
As
the transfer port is
uncovered, the air/fuel mixture stored in the
pressurized crankcase is forced into the combustion
chamber.
The crankshaft continues its rotation and the piston
begins its travel toward the spark plug.
As
it moves,
the piston seals off the transfer port first, then the
exhaust port. When both ports are sealed, the
remaining travel compresses the air/fuel mixture to
prepare it for ignition. The travel of piston towards the
spark plug is called the compression stroke
(Figure 62).
Engine
46
ENGINE
The movement of the piston towards the spark plug
also has an important effect in the crankcase. Once
the transfer port is sealed, the crankcase is under
vacuum. This action pulls the reed valve open and
draws in a fresh charge of air/fuel mixture.
Just before the piston reaches top dead center (TDC),
the spark plug fires. When the engine is functioning
properly, the air/fuel mixture ignites, causing a burn
that occurs evenly through the power stroke.
(If
the
compression ratio is too high, the air/fuel mixture
actually explodes, which can be heard and is
identified as detonation). The burn continues as the
piston changes direction and begins its travel towards
the crankcase.
When the piston opens the exhaust port, high-
pressure exhaust gases exit the exhaust port. Further
movement of the piston towards the crankcase
uncovers the transfer port which allows a fresh charge
of oil/fuel mixture to enter the combustion chamber.
As
the exhaust gases continue to exit the exhaust port
the engine is readied for another cycle.
The 2-cycle engine
is
always well lubricated as long a
the correct oil/fuel mixture is maintained in the fuel
tank. Oil suspended in the fuel vapor adheres to all
moving parts, keeping them continually coated,
regardless of operating angle.
Figure 63
illustrates the
second (or power) stroke
of
a 2-cycle engine.
Second
Stroke
Figure 63
Service Section