15-11-2012, 06:26 PM
Two-Cycle and Four-Cycle Engines
Two-Cycle.pdf (Size: 1.02 MB / Downloads: 288)
Exhaust stroke
After the piston has completed the power
stroke, the burned gases must be removed from the
cylinder before introducing a fresh charge. This
takes place during the exhaust stroke. The exhaust
valve opens and the rising piston pushes the
exhaust gases from the cylinder. See Figure 5-1D.
The exhaust valve has to function much like the
intake valve. When closed, the valve must seal.
When open, it must allow a streamlined flow of
exhaust gases out through the port. See Figure 5-2.
The removal of gases from the cylinder is called
scavenging.
The passageway that carries away exhaust
gases is referred to as the exhaust manifold or
exhaust port. Like the intake manifold, the exhaust
manifold must be designed for smooth flow of
gases.
The heat absorbed by the exhaust valve must
be controlled or the valve will deteriorate rapidly.
See Figure 5-3. Some valve heat is carried away
by conduction through the valve stem to the guide.
However, the hottest part of the valve, the valve
head, transfers heat through the va
Principles of operation
The location of the ports in a two-cycle
engine is essential to correct timing of the intake,
transfer, and exhaust functions. The cutaway
cylinder in Figure 5-9A shows the exhaust port at
the highest point, the transfer port next, and the
intake port at the lowest point. Some engines, particularly
loop-scavenged engines, have more than
one transfer port. See Figure 5-9B.
Intake into crankcase
As the piston moves upward in the cylinder of
a two-cycle engine, crankcase pressure drops and
the intake port is exposed. Because atmospheric
pressure is greater than the crankcase pressure, air
rushes through the carburetor and into the crankcase
to equalize the pressures. See Figure 5-10A.
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.
Scavenging and tuning
When properly designed, the exhaust system
scavenges all exhaust gases from the combustion
chamber. The system allows the new fuel charge
to move in more rapidly for cleaner and more
complete combustion.
For best efficiency, the fuel charge should be
held in the cylinder momentarily while the exhaust
port is open. This helps prevent fuel from being
drawn out of the cylinder with exhaust gases.
Suggested Activities
Look up additional information about internal
combustion engine development. Names to
look up: Christian Huygens, Philip Lebon,
Samuel Brown, William Barnett, Pierre
Lenoir, Beau DeRochas, Dr. N. A. Otto,
Atkinson, Gottlieb Daimler, Priestman and
Hall, Herbert Akroyd Stuart, Rudolph Diesel.
2. Begin a collection of engine repair and service
manuals.
3. Using a worn out engine, cut away portions
that will make the working parts visible while
still enabling them to move. Report on the
operation and timing of each part. After
further study, replace the spark plug of the
cutaway engine with a small lightbulb
switched on and off by the ignition switching
system to simulate ignition.
4. Make a bulletin board display that illustrates
the principles of two- and four-cycle engines