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reciprocating engines are:
1. By the number of piston strokes needed to
complete a cycle: two-stroke or four-stroke; and
2. By the method of cooling: liquid or air-cooled.
Refer to Chapter 5 of 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 powered
parachutes. Two-stroke aviation engines evolved
from two-stroke snowmobile and watercraft engines,
4-2
Crankcase Vacuum Intake Stroke—Piston
Moving up: Figure 4-3 a to b
The upward stroke of the piston [Figure 4-3a] 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 4-3b] This
can be a pressure-actuated reed valve, a rotary valve,
or a third ported inlet system where the lower piston
skirt provides an opening for the fuel/air/oil mixture
to flow in when the piston is reaching its highest
point Top Dead Center (TDC). At this point, the
greatest portion of the fuel/air/oil mixture has filled
the crankcase.
Crankcase Compression Stroke—Piston
Moving down: Figure 4-3 b to c
During the downward stroke, the pressure valve is
forced closed by the increased crankcase pressure,
the mechanical rotary valve closes, or the piston
closes off the fuel/air oil mixture intake port. The
fuel mixture is then compressed in the crankcase
during the downward stroke of the piston.
Crankcase Transfer/Exhaust—Piston at lowest:
Figure 4-3 d
When the piston is near the bottom of its stroke, the
transfer port opening from the crankcase to the combustion
chamber is exposed, and the high pressure
fuel/air mixture in the crankcase transfers around the
piston into the main cylinder.
This fresh fuel/air/oil mixture pushes out the exhaust
(called scavenging) as the piston is at its lowest point
and the exhaust port is open. Some of the fresh fuel/
air/oil mixture can escape out the exhaust port resulting
in the higher fuel use of the two stroke engine.
Cylinder start of Compression Stroke—Piston
initially Moving up: Figure 4-3 e
As the piston starts to move up, covering the transfer
port, the tuned exhaust bounces a pressure wave at
the precise time across the exhaust port (more on
this in the exhaust system discussion) to minimize
Figure 4-1. Reed valve is open with low pressure and closes when the pressure increases.
Figure 4-2. Intake rotary valve for a two cycle engine.
4-3
the fuel/air/oil mixture from escaping out the exhaust
port.
Cylinder Compression Stroke—Piston Moving
Up: Figure 4-3 e to f
The piston then rises, and compresses the fuel mixture
in the combustion chamber. During this piston
compression process, the crankcase vacuum intake
process is happening simultaneously, as described
earlier. This is why four processes can happen in two
strokes.
Cylinder Power Stroke—Piston Moving Down:
Figure 4-3 f to g
At the top of the stroke, the spark plug ignites the
fuel mixture and drives the piston down as the power
stroke of the engine.
Cylinder Power Stroke—Piston Moving Down:
Figure 4-3 g to h
As the piston passes the exhaust port, the exhaust
starts to exit the combustion chamber. As the piston
continues down, the transfer port opens and the
swirling motion of the air/fuel/oil mixture pushes the
exhaust out the exhaust port.
Piston Reverses Direction From Down Stroke to
Up Stroke: Figure 4-3 h to a
As the piston reverses direction from the down
stroke to the up stroke the process is complete. Figure 4-4. The cycles in a four-stroke engine.
Figure 4-3. Piston ported inlet for a two cycle engine.
4-4
Four-Stroke Engines
Four-stroke engines are very common in most aircraft
categories, and are becoming more common in
powered parachutes. [Figure 4-4] Four-stroke engines
have a number of advantages, including reliability,
fuel economy, longer engine life, and higher horsepower
ranges.
These advantages are countered by a higher acquisition
cost, lower power-to-weight ratios, and a higher
overall weight. The increased weight and cost are
the result of additional components, e.g., camshaft,
valves, complex head to house the valve train, etc.,
incorporated in a four-stoke engine.
Exhaust Systems
Engine exhaust systems vent the burned combustion
gases overboard, reduce engine noise, and (in the
case of two-stroke engines) help keep the fresh fuelair
mixture in the cylinders. An exhaust system has
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