Section 2 Basics of Engine Operation
92
The oil in a four-stroke engine must be drained
periodically and replaced with clean oil. Also worth
noting, four-stroke engines must be operated in an
upright position or the oil will fl ow away from the
pump or splash fi nger, preventing lubrication.
Two-Stroke Engine
The two-stroke engine (commonly called
two-cycle engine) performs the same cycle of events
as the four-stroke engine. The main difference is
that the intake, compression, power, and exhaust
functions take place during only two strokes of
the piston. The two strokes occur during each
revolution of the crankshaft. Therefore, it takes
only one revolution of the crankshaft to complete a
two-stroke cycle.
A two-stroke engine has several advantages
over a four-cycle unit. It is much simpler in design
than the four-stroke engine because the conven-
tional camshaft, valves, and tappets are unneces-
sary. See Figure 5-13.
Additionally, a two-stroke engine is smaller
and lighter than a four-stroke engine of equivalent
horsepower. Unlike the four-stroke engine, the
two-stroke engine will get adequate lubrication
even when operated at extreme angles. It receives
its lubrication as fuel mixed with oil is passed
through the engine.
Installing the correct mixture of fuel and oil is
a critical factor in maintaining a two-stroke engine
in good working condition. The prescribed type
and grade of engine oil must be mixed with the
fuel in proper proportion before being placed in
the fuel tank.
Lubrication
Lubrication of the four-stroke engine is pro-
vided by placing the correct quantity and grade of
engine oil in the crankcase. Several methods are
used to feed the oil to the correct locations. The
two most common methods are the splash system
and the pump system. Some engines employ one
or the other; others use a combination of both.
Goodheart-Willcox Publisher
Figure 5-12.
The four-stroke cycle diagram shows the exact number
of degrees each valve is open or closed and the time
spark ignition occurs. Note that both valves are open
(overlap) through an arc of 40°, permitting exhausting
gases to create a partial vacuum in the cylinder and
help draw a mixture of fuel into the cylinder.
Valve
overlap
Exhaust
valve
closes
Intake
valve
opens
Compression
stroke
Intake
stroke
10 °
TDC
Spark
Power
stroke
Exhaust
stroke
Exhaust
valve
open
B
30°
30°
Intake
valve
closes
60°
45°
BDC
A
Lawn-Boy Power Equipment, Gale Products
Figure 5-13.
The number of moving parts in a four-stroke engine is greater than in a two-stroke engine. Other differences are
listed in the chart at the end of this chapter.
Moving Parts—Two-Cycle Engine Moving Parts—Four-Cycle Engine
Copyright Goodheart-Willcox Co., Inc.