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THE GUIDE TO SMALL ENGINES 3
The four-stroke small engine that powers
your lawn mower, tiller, generator and many
other types of equipment is a gasoline engine
that generates the precise amount of power
necessary to get the job done. Here’s how the
components in your engine interact.
Engine components &
their function
The rewind cord is pulled to start the
combustion process. On some models, a
starter motor replaces the rewind, drawing
on battery power to start the engine.
Revolving magnets work in conjunction with the
ignition armature and spark plug to produce a
spark in the combustion chamber.
The carburetor draws in fuel from the fuel tank
and outside air to form a combustible vapor that
is fed into the combustion chamber.
Intake and exhaust valves open and close at
precisely timed intervals to let air and fuel enter
the engine and to let spent gases exit.
The piston is pushed through the cylinder by the
force of expanding gases. The piston’s motion
causes the crankshaft to turn. Momentum then
carries the piston back toward the top of
the cylinder.
Oil stored in the crankcase circulates through
the engine to lubricate key components like the
piston and crankshaft and to provide generalized
cooling by drawing away heat from internal
engine surfaces.
A flywheel
brake and stop switch are included
on engines for equipment such as mowers that
require constant supervision. The two compon-
ents are designed to stop the engine if you
release the controls.
An air vane or flyweights monitor engine
RPMs so the governor can maintain the selected
engine speed.
Cooling fins help reduce engine tempera-
tures when air circulates across the hottest
engine surfaces.
INTRODUCTION TO ENGINES
Parts of the small engine – front view Parts of the small engine – side view
Fuel tank and shroud
Fuel cap
Air cleaner cover
Cylinder head
Piston
Engine block
Crankshaft
Muffler
Intake valve
Exhaust valve
Rewind rope
Rewind
Flywheel brake
Blower housing
Flywheel
Carburetor
Air cleaner element
Fuel tank and shroud
Spark plug
Cylinder head
Intake valve spring
Intake valve
Exhaust valve spring
Exhaust valve Crankshaft
Dipstick
Piston
Connecting rod
Crankcase
Oil fill cap
Blower housing
Rewind
Flywheel
Ignition armature
Spark plug lead
Primer bulb