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[Figures 4-1 through 4-3] Two common means of classifying
reciprocating engines are the:
1. Number of piston strokes needed to complete a
cycle—two or four.
2. Method of cooling—liquid or air.
Refer to the Pilot’s Handbook of Aeronautical Knowledge
for a comprehensive review of how reciprocating four-stroke
engines operate.
Two-Stroke Engines
Two-stroke engines are commonly used in WSC aircraft. Twostroke
aviation engines evolved from two-stroke snowmobile
and watercraft engines, the difference being that an aircraft
engine is optimized for reliability with dual ignition often
installed for each cylinder. Two-stroke engines are popular
because they have fewer components than four-stroke engines
which makes them less expensive to manufacture and lighter,
thus increasing the power-to-weight ratio.
Two-stroke engines require that oil be mixed into the fuel
to lubricate the engine, instead of being held in a sump and
requiring a separate pressurized recirculating system like that
of a four-stroke engine. Details on two-stroke oil mixing are
covered in the lubrication section. One stroke as the piston
moves up is intake and compression, while the second stroke
as the piston moves down is power and exhaust. The two4-
3
Figure 4-2. Two-stroke water-cooled engine.
Figure 4-3. Four-stroke water- and oil-cooled engine.
4-4
Crank shaft operates/rotates rotary valve
Open port shown for air/fuel intake
Figure 4-5. Intake rotary valve for a two-stroke engine.
High crankcase pressure closes reed valve
Exhaust port
Reed valve closed
Transfer port
Low crankcase pressure opens reed valve
Reed valve open
Transfer port
Fuel/Air/Oil Mixture
Figure 4-4. Reed valve is open with low pressure and closes when
the pressure increases in a two-stroke engine.
stroke engine performs the same functions as a four-stroke
engine in half the number of strokes.
A wide range of valve systems are found on two-stroke
engines for the purpose of opening and closing ports in
the cylinder to let fuel in and exhaust out at the proper
time. This is similar to the intake and exhaust valves on a
four-stroke engine. One-way pressure valves, called spring,
reed, or poppet valves, open when the pressure drops within
the crankcase, pulling the fuel from the carburetor into the
crankcase. [Figure 4-4]
Mechanical rotary valves are driven off the engine, rotate
to provide an opening at the precise time, and can be on the
intake and exhaust ports. [Figure 4-5]
Piston porting does not use any valves. The fuel inlet port is
opened and closed by the piston position as it moves up and
down in the cylinder. This is called a “piston ported inlet” and
is used in the two-stroke process description that follows.
Two-Stroke Process
The two-stroke process begins with the fuel entering the
engine and concludes as it exits as exhaust.
Crankcase Vacuum Intake Stroke—Piston Moving
Up
The upward stroke of the piston [Figure 4-6A] creates a
vacuum in the crankcase and pulls the fuel/air/oil mixture
into the crankcase through the intake valve system from the
carburetor. [Figure 4-6B] This can be a pressure-actuated
reed valve, a rotary valve, or a ported inlet system where
the lower piston skirt provides an opening for the fuel/air/oil
mixture to fl ow in when the piston is reaching its highest
point of top dead center (TDC). At this point, the greatest
portion of the fuel/oil/air mixture has fi lled the crankcase.
[Figure 4-6B]
Crankcase Compression Stroke—Piston Moving
Down
During the downward stroke, the pressure valve is forcibly
closed by the increased crankcase pressure, the mechanical
rotary valve closes, or the piston closes off the fuel/air oil
mixture intake port as shown. The fuel/oil/air mixture is then
compressed in the crankcase during the downward stroke of
the piston. [Figures 4-6B to 4-6D]
4-5
Figure 4-6. Piston ported inlet cycles for a two-stroke engine.
Piston moves up Piston is at top Piston moves down Piston is at bottom
Lower crankcase chamber Upper combustion chamber
Vacuum created in
crankcase
A Fuel/Oil/Air mixture
enters from carburetor
B Fuel/Oil/Air mixture
pressurized
C Pressurized Fuel/Oil/
Air mixture transferred
D
Fuel/Oil/Air mixture
pressure
E F Fuel/Oil/Air ignited G Piston forced down Fuel/Air enters and
exhaust exits
H
Piston Ported Two-Stroke Cycles
1
2
Connecting Rod
Piston
Exhaust port
Crank
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Weight-Shift Control Aircraft Flying Handbook(39)
