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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
Impulse Port
The impulse port is a passage that gives the car-
buretor access to the crankcase pressure and
vacuum pulses. The carburetor requires pres-
sure and vacuum pulses to pump fuel while the
engine is running. Figure 5-6 shows the loca-
tion of the impulse port below the intake port
on a piston ported engine. On this engine, when
the carburetor is mounted onto the cylinder, the
impulse port is connected by a passage to the
carburetor. On other engines, the port could be
a drilled passage with a brass fitting pressed
into it. A vacuum line would be used to connect
the impulse port to the carburetor. If the engine
does not have an impulse port on the cylinder,
then the engine would have an impulse port on
the crankcase.
Decompression Valve
On some handheld two-stroke engines, a decom-
pression valve is used to release compression
pressure from the engine when the operator
is trying to start the engine, Figure 5-7. These
devices are used on engines with very high com-
pression ratios. The engine starter rope is hard
to pull. Sometimes the engine feels like it is
“locked up” because the rope is so hard to pull.
The owner’s manual tells the operator how to
use the decompression valve when starting the
engine. The decompression valve is also referred
to as a compression release device.
Stratified Scavenging Port
The stratified scavenging port allows a small vol-
ume of fresh air (not mixed with fuel) to enter
the tops of the transfer ports to reduce exhaust
emissions from the engine. When the air-fuel
charge transfers from the crankcase, the fresh
air is pushed into the cylinder above the piston
first and is allowed to exit through the exhaust
port just before the end of the exhaust event.
This reduces the amount of unburned hydrocar-
bons emitted in the exhaust. In Figure 5-8, the
fresh air port is located above the intake port.
Crankcase
The function of the crankcase is to support the
crankshaft and seal the engine. The piston,
connecting rod, and crankshaft are inside the
crankcase, but the ends of the crankshaft pro-
trude outside the crankcase. A ball bearing
inside the crankcase supports each end of the
crankshaft. See Figure 5-9. The bearings in a
two-stroke engine crankcase are lubricated by
the two-cycle oil premixed with the fuel.
The ball bearings in a two-stroke engine are
used to reduce friction between a rotating shaft
(the crankshaft) and a stationary support (the
crankcase). Each ball bearing is composed of
Decompression valve Spark plug
Cylinder block
Goodheart-Willcox Publisher
Figure 5-7. The decompression valve is mounted
on the cylinder to release pressure from the cylin-
der when starting the engine.
Cylinder
Stratified
scavenging
port
Intake port
Goodheart-Willcox Publisher
Figure 5-8. The port above the intake port allows
fresh air to enter the engine to help reduce
emissions.