Section 2 Basics of Engine Operation
94
Stratified Scavenge Engine
To reduce two-cycle engine emissions and
increase fuel economy, manufacturers have
designed a two-cycle engine that effectively elimi-
nates scavenge loss of the air-fuel charge. This
engine, called a stratifi ed scavenge engine, uses
an extra intake port and a specially shaped piston
to introduce a small amount of fresh air into the
transfer port ahead of the air-fuel charge. When the
transfer port is uncovered during the fuel transfer
phase of two-cycle operation, the fresh air fl ows
into the cylinder before the air-fuel charge, push-
ing the spent exhaust gases from the cylinder.
This prevents the unburned fuel in the air-fuel
charge from escaping through the exhaust port.
opened and the compressed air-fuel charge rushes
through the port and into the cylinder. The new
charge cools the combustion area and pushes
(scavenges) the remaining exhaust gases out of the
cylinder. This completes one cycle of operation.
Scavenge Loss
In a conventional two-stroke engine, a signifi cant
portion of the air-fuel charge (in some cases, over
25%) fl ows out through the open exhaust port as it
pushes the exhaust gases from the cylinder. This
occurrence is called scavenge loss. Scavenge loss
results in increased hydrocarbon emissions and
reduced fuel economy.
Rupp Industries, Inc.
Figure 5-15.
These illustrations show the sequence of events that take place in a two-stroke engine. Compression and intake
occur simultaneously, and then ignition occurs. Exhaust precedes the transfer of fuel during the lower portion of
the power stroke. The piston functions as the only valve in the engine.
A
Fuel Intake into Crankcase
B
Ignition-Power Segment
C
Exhaust Port Uncovered
D
Fuel Transfer into Cylinder
Carburetor
Vacuum
Exhaust
Transfer
port
Positive
pressure
Compressed
fuel mixture
Copyright Goodheart-Willcox Co., Inc.