General Information; Section 1 General Information; Two-Cycle Engine Theory - Briggs & Stratton 276535 Repair Manual

2-Cycle Snow Engine
Section 1 - General Information
SECTION 1

GENERAL INFORMATION

TWO-CYCLE ENGINE THEORY

In a piston-ported engine, the fuel mixture enters the
crankcase through a port that comes directly from the
carburetor (commonly referred to as a "third port"), See
item figure 1.
Intake Ports Closed
Ignition
Fig. 1
At about 28° before top-dead-center (BTDC), the spark
plug ignites and forces the piston away from the spark
plug. As the piston moves away from the spark plug, it
exposes the exhaust port (Item
expanding exhaust gases escape. As the piston
continues to move away from the spark plug, it
Third Port Open
Partial Vacuum
Figure 2) and the
compresses the fuel mixture (Item
the bottom side of the piston in the crankcase.
Intake Ports Closed
Exhaust Port Opening
Fig. 2
Near the bottom of its travel, the piston uncovers the
pressure transfer port and allows this compressed fuel/
air mixture to expand into the combustion chamber
(Item Figure 3).
Intake Ports Open
Exhaust Port Open
Fig. 3
Now the piston begins its travel in the opposite direction
and first closes off the transfer port and then the exhaust
Figure 2) that is on
Third Port Closed
Pressure Building Up
Fresh Fuel Change
Third Port Closed
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