Introduction
This chapter covers the engines found on most weight-shift
control (WSC) aircraft and includes the exhaust, ignition,
lubrication, cooling, propeller, gearbox, induction, charging,
and fuel systems. Reciprocating engine operating theory is
covered for both two-stroke and four-stroke engines. The
WSC engine and propeller, often referred to as a powerplant,
work in combination to produce thrust. The powerplant
propels the aircraft and charges the electrical system that
supports WSC operation.
Powerplants
Chapter 4
Figure 4-1. Two-stroke air-cooled engine.
The powerplant system is composed of the engine, gearbox,
and propeller. It is a key component of a WSC aircraft and
should be maintained according to both the engine and
airframe manufacturer recommendations.
Prefl ight information, along with maintenance schedules
and procedures, can be found in the pilot’s operating
handbook (POH) for Special Light-Sport Aircraft (S-LSA),
and/or maintenance references from the manufacturers.
Engine inspections and maintenance must be performed and
documented in a logbook. A pilot should review this logbook
before fl ying an unfamiliar aircraft.
Reciprocating Engines
WSC aircraft are designed with reciprocating engines.
[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. Two-
stroke 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 two-
Figure 4-2. Two-stroke water-cooled engine.
Figure 4-3. Four-stroke water- and oil-cooled engine.
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]
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
crankcaseA Fuel/Oil/Air mixture
enters from carburetorB Fuel/Oil/Air mixture
pressurizedC Pressurized Fuel/Oil/
Air mixture transferredD
Fuel/Oil/Air mixture
pressureE Fuel/Oil/Air ignitedF Piston forced downG Fuel/Air enters and
exhaust exitsH
Piston Ported Two-Stroke Cycles
Connecting Rod
Piston
Exhaust port
Crank
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]
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 two-
stroke 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
pipe diffuser lowers the pressure behind it. That low pressure
is used to help suck out all of the residual, hot, burnt gas
from the power stroke and at the same time help pull a fresh
fuel/air charge into the cylinder. This is called scavenging
and is an important function of a tuned two-stroke exhaust
system. [Figure 4-6H]