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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
manufacturers modify the crankshaft so the
engine produces more torque at lower engine
speeds.
Two-Stroke Engine
Components
The basic parts of a two-stroke engine are:
• Cylinder.
• Cylinder bore.
• Crankcase.
• Piston.
• Connecting rod and crankshaft.
• Bearings.
• Muffler.
• Ignition coil.
• Flywheel.
• Starter.
• PTO.
• Carburetor.
• Air filter.
• Fuel tank.
Typical two-stroke engine cylinders have basic
parts that are common to all engines. Depend-
ing on the engine design, some cylinders may
have additional components built into them or
added to them.
Cylinder
The following are the basic parts of a cylinder
and are identified in Figure 5-6:
• Cooling fins.
• Cylinder bore.
• Exhaust port.
• Spark plug hole.
• Transfer ports.
Cooling Fins
The cooling fins are thin parallel “plates” cast
on the outside of the cylinder. Their sole pur-
pose is to increase the surface area of the cyl-
inder so more heat can be carried away from
the engine. The walls of the cylinder bore are
relatively thin and heat can be quickly trans-
ferred from the walls to the cooling fins. There
are curved fins called “vanes” on the flywheel.
These fins move air when the flywheel is rotat-
ing. Air from the flywheel vanes blows across
the cylinder cooling fins to remove engine heat.
The cylinder is almost completely encased by
the engine housing. The engine housing is a
thin metal shroud for the engine and serves as
an air duct, directing flywheel air across the
cooling fins. The engine housing is designed to
maximize air movement across the fins to carry
off heat, but to exhaust the hot air away from
the operator. Because there is very little space
between the engine housing and cooling fins,
excessive debris can be pulled in by the flywheel,
collect on the fins, obstruct air movement across
the fins, and cause the engine to overheat.
Cylinder Bore
The cylinder bore provides a machined surface
within which the piston reciprocates. Depend-
ing on the manufacturer, the surface of the cyl-
inder may be plated. Chrome plating provides a
good antifriction surface when coated with the
two-stroke oil premixed with the gasoline. The
plating is hard and helps minimize wear during
the life of the engine.
Exhaust Port
The exhaust port allows exhaust gases to
leave the cylinder after combustion. The
cross-sectional area of the port opening, the
shape of the exhaust port walls, and the texture
of the port walls are engineered to allow the
most efficient removal of the exhaust gases. The
exhaust port design is specific to each engine
series to match the engine’s displacement, oper-
ating speed range, and power requirements.
Crankcase/insulator
block gasket Reed valve Backing plate
Insulator
block
Insulator block/
carburetor gasket
Goodheart-Willcox Publisher
Figure 5-5. The amount the reed can open is
limited by the backing plate.