Chapter 5 Principles of Engine Operation, Two- and Four-Stroke Engines
93
Ignition-Power
As the piston travels upward, it also com-
presses the air-fuel charge brought into the cylin-
der during the previous cycle to about one-tenth of
its original volume. See Figure 5-15A. The spark
is timed to ignite the air-fuel mixture when the
piston approaches TDC. See Figure 5-15B.
On some small engines, spark occurs almost
at TDC during starting, and then automatically
advances so that it occurs earlier. This is done to
get better effi ciency from the force of combustion
at higher speeds.
Peak combustion pressure is applied against the
piston top immediately after TDC. Driving down-
ward with maximum force, the piston transmits
straight line motion through the connecting rod
to create rotary motion of the crankshaft. See
Figure 5-15C.
Exhaust
The exhaust phase of the two-stroke cycle is
shown in Figure  5-15C. As the piston moves to
expose the exhaust port, most of the burned gases
are expelled. Complete exhausting of gases from
the cylinder and combustion chamber takes place
when the transfer ports are opened and the new
air-fuel charge rushes in.
Fuel Transfer
Figure 5-15C and Figure 5-15D show the piston
moving downward, compressing the air-fuel charge
in the crankcase. When the piston travels far
enough on the downstroke, the transfer port is
In this way, there is clean oil continuously sup-
plied to all moving parts while the engine is running.
The oil eventually burns in the combustion chamber
and is exhausted with other gases.
Two-stroke engines are popular in string
trimmers, leaf blowers, and other high-rpm
applications.
Principles of Two-Stroke
Operation
The location of the ports in a two-stroke engine
is essential to correct timing of the intake, transfer,
and exhaust functions, Figure 5-14. The cutaway
cylinder in Figure 5-14A shows the exhaust port
at the highest point, the transfer port next, and the
intake port at the lowest point. Some engines have
more than one transfer port. See Figure  5-14B.
Figure 5-15 illustrates the principles of two-stroke
engine operation.
Intake into the Crankcase
As the piston moves upward in the cylinder of
a two-stroke engine, crankcase pressure drops and
the intake port is exposed. Because atmospheric
pressure is greater than the crankcase pressure, air
rushes through the intake port and into the crank-
case to equalize the pressures. See Figure 5-15A.
While passing through the carburetor, the
intake air pulls a charge of fuel and oil along with
it. This charge remains in the crankcase to lubricate
ball and needle bearings until the piston opens the
transfer port on the downstroke.
Kohler Co.
Figure 5-14.
A cutaway cylinder block shows the location of the intake, exhaust, and transfer ports of a two-cycle engine.
A—Due to the cutaway, only one of two transfer ports is shown. B—The section is revolved 90° to show both ports.
Intake
port
Exhaust
port
A B
Transfer
port (1 of 2)
Transfer
ports
Intake
port
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