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Crankcase
Tuned exhaust pressure wave
Fuel/Oil/Air intake port
Transfer port
Crankcase Transfer/Exhaust—Piston at Lowest
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/oil/air 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/oil/air mixture can escape
through the exhaust port, resulting in the higher fuel use of
the two-stroke engine. [Figure 4-6D]
Cylinder Start of Compression Stroke—Piston
Initially Moving Up
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 to minimize the fuel/air/oil mixture
escaping through the exhaust port. [Figure 4-6E]
Cylinder Compression Stroke—Piston Moving Up
The piston then rises and compresses the fuel mixture in
the combustion chamber. [Figure 4-6E to 4-6F] 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.
[Figures 4-6B and 4-6C]
Cylinder Power Stroke—Initial Piston Moving
Down
At the top of the stroke, the spark plug ignites the fuel/oil/air
mixture and drives the piston down as the power stroke of
the engine. [Figures 4-6F and 4-6G]
Cylinder Power Stroke—Final Piston Moving Down
As the piston passes the exhaust port, the exhaust exits the
combustion chamber. As the piston continues down, the
transfer port opens and the swirling motion of the fuel/
oil/air mixture pushes the exhaust out of the exhaust port.
[Figures 4-6H]
Piston Reverses Direction From Down Stroke to Up
Stroke
As the piston reverses direction from the down stroke to the up
stroke, the process is complete. [Figures 4-6H and 4-6A]
4-6
EGT Probes
Tuned Exhaust System
Exhaust Silencer
Figure 4-8. Two-stroke tuned exhaust system with EGT probes
installed where the exhaust enters the exhaust system.
Figure 4-7. The cycles in a four-stroke engine.
1. Intake 2. Compression
3. Power 4. Exhaust
Intake valve Exhaust valve
Piston
Spark plug
Crankshaft Connecting rod
Four-Stroke Engines
Four-stroke engines are very common in most aircraft
categories and are becoming more common in WSC
aircraft. [Figure 4-7] 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) 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 twostroke
engines) help keep the fresh fuel/oil/air mixture in the
cylinders. An exhaust system has exhaust piping attached
to the cylinders, as well as a muffl er. The exhaust gases
are pushed out of the cylinder and through the exhaust pipe
system to the atmosphere.
Some exhaust systems have an exhaust gas temperature
probe. This probe transmits an electric signal to an instrument
in front of the pilot. This instrument reads the signal and
provides the exhaust gas temperature (EGT) of the gases at
the exhaust manifold. This temperature varies with power and
with the mixture (ratio of fuel to air entering the cylinders),
and is used to make sure the fuel/air mixture is within
specifi cations. When there is a problem with carburetion,
the EGT gauge will normally be the fi rst notifi cation for a
pilot. [Figure 4-8]
Two-Stroke Tuned Exhaust Systems
In two-stroke engines, the exhaust system increases the fuel
economy and power of the engine. The two-stroke exhaust
system is an integral part of any two-stroke engine design,
often controlling peak power output, the torque curve, and
even the revolutions per minute (RPM) limit of the engine.
The exhaust system must be tuned to produce a back pressure
wave at the exhaust port to act as an exhaust valve as shown
in Figure 4-6E. When hot spent gases are vented out of the
exhaust port, they are moving fast enough to set up a high
pressure wave. The momentum of that wave down the exhaust
　
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