Chapter 5 Principles of Engine Operation, Two- and Four-Stroke Engines
89
Two- and Four-Stroke
Engines
A basic design feature that aids in small engine
identifi cation is the number of piston strokes
required to complete one operating cycle. A four-
stroke engine, for example, requires four strokes
per cycle; a two-stroke engine requires two.
A stroke of the piston is its movement in the
cylinder from one end of its travel to the other.
When the piston is at the bottom of its travel, it is
said to be at bottom dead center (BDC). When it is
at the top of its stroke, it is at top dead center (TDC).
Each stroke of the piston, then, is either toward the
rotating crankshaft or away from it. Each stroke is
identifi ed by the job it performs (intake, exhaust, etc.).
Four-Stroke Engine
In a four-stroke engine (called a four-cycle
engine), four strokes are needed to complete the
operating cycle. The four strokes are as follows:
intake stroke
compression stroke
power stroke
exhaust stroke
Two strokes occur during each revolution of the
crankshaft. Therefore, a four-stroke cycle requires
two revolutions of the crankshaft to complete one
operating cycle. Figure 5-10 illustrates each of the
four strokes taking place in proper sequence.
Intake Stroke
Figure 5-10A shows the piston traveling down-
ward in the cylinder on the intake stroke. As the
piston moves down, the volume of space above it
is increased. This creates a partial vacuum that
draws the air-fuel mixture through the intake
valve port and into the cylinder.
With the intake valve open during the intake
stroke, atmospheric pressure outside the engine
forces air through the carburetor. This gives a
Note
Four- F o r -s troke r ok e engin ne s use intake n and exhaust valves F F i i k d h l
to to co o on tro o ol th he flow w of gases s into and out of the
combustion co o b s n cham c mb er.
the cylinder, yet it is free to slide on the lubricated
walls of the cylinder. One end of the connecting
rod is attached to the piston; the other end is
fastened to an offset journal, or crankpin, on the
crankshaft. As the piston moves up and down, the
connecting rod forces the journal to follow a circular
path, rotating the crankshaft.
When the engine is cranked, gasoline is atom-
ized and mixed with air. This mixture is forced
through an intake port and into the cylinder, where
it is compressed by the piston on the upstroke and
ignited by an electrical spark.
Burning rapidly, the heated gases trapped
within the cylinder (combustion chamber) expand
and apply pressure to the walls of the cylinder and
to the top of the piston. This pressure drives the
piston downward, causing the crankshaft to turn.
This downward movement of the piston is called
the power stroke.
As the piston and connecting rod push the
crankshaft journal downward, the pressure of the
burned gases is released through an exhaust port.
Meanwhile, a fresh air-fuel charge enters the cylin-
der and momentum pushes the crankshaft journal
past the bottom of its travel, carrying it into the
upstroke on another operating cycle.
Goodheart-Willcox Publisher
Figure 5-9.
Combustion forces the piston down to rotate the
crankshaft.
Spark plug
Cylinder
Combustion
Piston
Connecting rod
Journal
Crankshaft
Exhaust
ports
Copyright Goodheart-Willcox Co., Inc.
Previous Page Next Page