The design of the exhaust converging section causes a
returning pressure wave to push the fresh fuel/air charge
back into the exhaust port before the cylinder closes off that
port. Called pulse charging, it is another important function
of the exhaust system. [Figure 4-6E]
Tuned exhaust systems are typically tuned to a particular
rpm range. The more a certain rpm range is emphasized, the
less effective the engine will operate at other rpm. Vehicles
like motorcycles take advantage of this with the use of
transmissions. Motorcycle exhaust pipe builders can optimize
a certain rpm range and then the driver shifts gears to stay in
that range. Aircraft, with no transmission, do not have this
ability. On an aircraft, an exhaust pipe has to be designed to
operate over a broad range of rpm from idle to full speed, a
reason that simply putting a snowmobile engine on a WSC
does not work well.
Overall, the two-stroke exhaust system for a WSC aircraft is a
specifi c design and must be matched to the engine to operate
properly and obtain the rated power. It also reduces noise
and directs the exhaust to an appropriate location. Exhaust
silencers can be added to reduce noise, but additional weight,
cost, and slight power reduction are the byproducts.
Four-Stroke Engine Exhaust Systems
Four-stroke engines are not as sensitive as two-stroke engines
because they have exhaust valves and, therefore, do not
need the precision pulse tuned exhaust system. However,
directing the exhaust out appropriately and reducing
the noise are important considerations. Again, using the
manufacturer’s recommended confi gurations is required
for S-LSA and recommended for Experimental Light-Sport
Aircraft (E-LSA).
Engine Warming
Two-Stroke Engine Warming
Two-stroke engines must be warmed because different metals
expand at different rates as they are heated. When heating
steel and aluminum, the aluminum parts expand faster than
the steel parts. This becomes a problem in two different
areas of many two-stroke engines. The fi rst place is in the
cylinders of the engine.
The cylinders have steel walls that expand slowly, compared
to aluminum pistons that expand quickly. If an engine is
revved too quickly during takeoff before warming up, a lot of
heat is generated on top of the piston. This quickly expands
the piston, which can then seize in the cylinder. A piston
seizure will stop the engine abruptly.
The second area of concern is lower in the engine around
the crankshaft. This is an area where parts may get too loose
with heat, rather than seizing up. Additionally, the crankcase
has steel bearings set into the aluminum which need to
expand together or the bearings could slip. Many two-stroke
engines have steel bearings that normally hug the walls of
the aluminum engine case. The crank spins within the donuts
of those steel bearings.
If the engine heats too quickly, the aluminum case out-
expands those steel bearings and the crank causes the bearings
to start spinning along with it. If those steel bearings start
spinning, it can ruin the soft aluminum walls of the case,
which is very expensive. If heat is slowly added to an engine,
all parts will expand more evenly. This is done through a
proper warm-up procedure. Many two-stroke engines are
best warmed up by running the engine at a set rpm for a set
amount of time. Follow the instructions in the POH; however,
a good rule of thumb is to start the engine initially at idle
rpm, get it operating smoothly at 2,500 rpm for 2 minutes
for initial warm up, and then warm the engine at 3,000 rpm
for 5 minutes. The cylinder head temperature or coolant
temperature must be up to the manufacturer’s recommended
temperatures before takeoff. This may require running the
engine at higher rpm to reach required temperatures on some
engines.
Once the engine is warmed up and the aircraft is fl ying, it is
still possible to cool down the engine too much. This happens
when the engine is idled back for an extended period of time.
Even though the engine is running, it is not generating as
much heat as the cooling system is effi ciently dumping engine
heat into the atmosphere. An immediate power application
with a cooled engine can seize the engine just as if the engine
had not been warmed in the fi rst place.
In water-cooled engines, on a long descent at idle, the
coolant cools until the thermostat closes and the engine is not
circulating the radiator fl uid through the engine. The engine
temperature remains at this thermostat closed temperature
while the radiator coolant continues to cool further. If full
throttle is applied, the thermostat can open, allowing a blast
of coolant into the warm engine. The piston is expanding due
to the added heat, and the cylinder is cooling with the cold
radiator water, resulting in piston seizure. To prevent this,
slowly add power well before getting close to the ground
where power is needed. This gives the system a chance to open
the thermostat gradually and warm up the radiator water.
Just as it takes time for the engine crankcase and bearings
to warm up, it also takes those steel parts a long time to cool
down. If a pilot lands, refuels, and wants to take off again
quickly, there is no need to warm up again for 5 minutes.
The lower end of the engine stays warmed up after being
shut down for short periods.
rpm
3 4 5
ENGINE HOURS
Shown when engine is shut off
ENGINE RPM
6500MAX CON 6800MAX
Figure 4-9. Engine rpm is indicated on the analog gauge (top) and
the digital gauge (bottom).
Any engine restart is an example in which it would be
appropriate to warm the engine up until the gauges reach
operating temperatures. The lower end of the engine is warm
and now a pilot needs to be concerned only with preventing
the pistons from seizing.
Four-Stroke Engine Warming
A four-stroke engine must also be warmed up. The four-
stroke engine has a pressurized oil system that provides
more uniform engine temperatures to all of its components.
Takeoff power can be applied as soon as the water, cylinder
head temperature (CHT), oil temperatures, and oil pressure
are within the manufacturer’s recommended tolerances for
takeoff power applications.
Gearboxes
Gearboxes are used on most WSC reciprocating engines to
take the rotational output of an internal combustion engine
which is turning at a high rpm and convert it to a slower (and
more useful) rpm to turn the propeller. Gearboxes come in
different gear ratios depending on the output speed of the
engine and the needed propeller turning speeds.
Some examples are a two-stroke rpm reduction from 6,500
engine rpm with a 3.47 to 1 reduction, resulting in 1,873
propeller rpm. A four-stroke rpm reduction could be from
5,500 engine rpm with a 2.43 to 1 reduction, resulting in
2,263 propeller rpm. A gearbox is a simple device that bolts
directly to the engine and, in turn, has the propeller bolted
directly to it.
A two-cycle engine gearbox is kept lubricated with its own
built-in reservoir of heavy gearbox oil. The reservoir is
actually part of the gearbox case itself. The gearbox oil has
to be changed periodically since the meshing of the gears
will cause them to wear and will deposit steel fi lings into
the oil. If the oil is not changed, the abrasive fi lings cause
even more wear.
Some gearboxes have a built-in electric starter motor. When
activated, the motor turns the gearing which cranks the
engine.
Four-stroke propeller reduction gearboxes use oil from the
engine oil system for lubrication.
Some gearboxes come with a built-in centrifugal clutch
and others have allowances for installation. A centrifugal
clutch is very useful in a two-stroke engine because it allows
the engine to idle at a lower speed without the load of the
propeller. Otherwise, two-stroke engines can generate a great
deal of vibration at low rpm when loaded. As the engine
speeds up, the centrifugal clutch engages and smoothly starts
the propeller spinning. When the engine is brought back
to idle, the clutch disengages and allows the engine to idle
smoothly again; the propeller stops when on the ground and
windmills when fl ying.
Propeller
The propeller provides the necessary thrust to push the WSC
aircraft through the air. The engine power is used to rotate the
propeller, which generates thrust very similar to the manner
in which a wing produces lift. The amount of thrust produced
depends on the airfoil shape, the propeller blade angle of
attack (AOA), and the engine rpm. [Figure 4-9] Light-sport
aircraft (LSA) are equipped with either a fi xed-pitch or a
ground adjustable-pitch propeller.
Fixed-Pitch Propeller
The pitch of the fixed-pitch propeller is set by the
manufacturer and cannot be changed. Refer to the Pilot’s
Handbook of Aeronautical Knowledge for basic propeller
principles.
Full
Idle
100%
Power
Needle Jet
ain Jet
Idle Jet
50%
Figure 4-10. Throttle position and jetting system used.
Pilot (or idle) Jet
Idle Air
Pilot Air Screw Adjustment
Air Bleed Holes
Pilot Hole
Throttle Valve
Bypass Hole
Figure 4-11. Pilot or idle jet system.
Ground Adjustable-Pitch Propeller
Adjustable-pitch propellers for WSC aircraft can be adjusted
only on the ground with hand tools. If an engine is over-
revving, more pitch can be added to the propeller. If the
engine is not developing the full recommended rpm during
fl ight, then some pitch can be taken out of the blades. This
should be done according to the WSC aircraft’s POH and by
a qualifi ed technician.
Induction Systems
The induction system brings air in from the atmosphere,
mixes it with fuel, and delivers the fuel/air mixture (fuel/oil/
air mixture for two stroke engines) to the engine intake and
to the cylinders where combustion occurs. Outside air enters
the induction system through an air fi lter on the engine. The
air fi lter inhibits the entry of dust and other foreign objects.
Two types of induction systems are used in WSC engines:
1. The carburetor system is most common. It mixes the
fuel and air in the carburetor before this mixture enters
the engine intake.
2. The fuel injection system injects the fuel into the air
just before entry into each cylinder.
Carburetor Systems
WSC aircraft use fl oat-type carburetors. The “fl oat-type
carburetor” acquires its name from a fl oat that rests on fuel
within the carburetor fl oat chamber, commonly known as
the fuel bowls. The fl oat maintains the fuel level in the
fuel bowls. As fuel is used by the engine, the fuel and fl oat
levels drop, opening the valve letting more fuel into the
fuel bowls until the proper level of fuel in the fuel bowls
is achieved and the valve is closed. Reference the Pilot’s
Handbook of Aeronautical Knowledge for basic information
on fl oat carburetor operation. Modern two- and four-stroke
carburetors operate with three separate jetting systems
depending on engine power. [Figure 4-10]
When the throttle is closed for engine idling, the throttle
valve is closed and the fuel/air mixture is supplied through
the idle (pilot) jet and idle (pilot) air passage. The fuel/air
mixture is supplied to the cylinders through the bypass hole.
[Figure 4-11]
As the throttle is advanced and the throttle valve is raised,
the fuel is sucked up through the main jet but is controlled
by the opening and taper of the jet needle and needle jet.
This is effective throughout most of the midrange operation.
About half throttle, the main jet size starts to infl uence the
amount of fuel mixed with the air and this effect continues
until it is the main infl uence at the highest throttle settings.
[Figures 4-10 and 4-12]
Figure 4-12. Jet needle/needle jet and main jet system.
Main Jet
Air Inlet
Needle Jet
Jet Needle
Throttle Slide
Cutaway
Figure 4-13. Typical two-stroke carburetor.
This condition may occur at high elevation airports and
during climbs or cruise fl ight at high altitudes. To maintain
the correct fuel/oil/air mixture, the main jets are usually
changed for smaller jets based on the density altitude of
the base airport. Operating from low altitude airports and
climbing to altitude where the mixture becomes rich for short
periods is acceptable.
Operating an aircraft at a lower altitude airport with the jets
set for higher altitudes will create too lean of a mixture, heat
up the engine, and cause the engine to seize. The pilot must
be aware of the jetting for the machine to adjust the mixture.
Consult your POH for specifi c procedures for setting jets at
different density altitudes.
Four-Stroke Mixture Settings
Four-stroke engines typically have automatic mixture control
for higher altitudes or a mixture control that can be operated
by the pilot.
Carburetor Icing
One disadvantage of the carburetor system versus the fuel
injected system is its icing tendency. Carburetor ice occurs
due to the effect of fuel vaporization and the decrease in air
pressure in the venturi, which causes a sharp temperature
drop in the carburetor. If water vapor in the air condenses
when the carburetor temperature is at or below freezing, ice
may form on internal surfaces of the carburetor, including
the throttle valve.
Ice generally forms in the vicinity of the venturi throat. This
restricts the fl ow of the fuel/air mixture (fuel/oil/air mixture
for two stroke) and reduces power. If enough ice builds up,
the engine may cease to operate. Carburetor ice is most likely
to occur when temperatures are below 70 °F (21 °C) and
the relative humidity is above 80 percent. However, due to
the sudden cooling that takes place in the carburetor, icing
can occur even with temperatures as high as 100 °F (38 °C)
and humidity as low as 50 percent. This temperature drop
can be as much as 60 to 70 °F. Therefore, at an outside air
temperature of 100 °F, a temperature drop of 70 °F results in
an air temperature in the carburetor of 30 °F. [Figure 4-14]
The fi rst indication of carburetor icing is a decrease in engine
rpm, which may be followed by engine roughness. Although
carburetor ice can occur during any phase of fl ight, it is
particularly dangerous when using reduced power during a
descent. Under certain conditions, carburetor ice could build
unnoticed until trying to add power. To combat the effects of
carburetor ice, some engines have a carburetor heat option.
Some of the newer four-stroke engines have carburetor heat
turned on all the time to combat icing. Two-stroke engines
are typically less susceptible to icing but specifi c installations
Two-Stroke Carburetor Jetting for Proper Mixture
Carburetors are normally set at sea level pressure with
the jets and settings determined by the manufacturer.
[Figure 4-13] However, as altitude increases, the density
of air entering the carburetor decreases, while the density of
the fuel remains the same. This creates a progressively richer
mixture, same fuel but less air, which can result in engine
roughness and an appreciable loss of power. The roughness
is usually due to spark plug fouling from excessive carbon
buildup on the plugs. Carbon buildup occurs because the
excessively rich mixture lowers the temperature inside the
cylinder, inhibiting complete combustion of the fuel.
Relative humidity
Outside air temperature
100%
50%
80%
60%
70%
90%
20 °F/–7 °C 32 °F/0 °C 70 °F/21 °C 100 °F/38 °C
High carburetorHigh carburetor
icing potentialicing potential
High carburetor
icing potential
Carburetor icing possible
Figure 4-14. Although carburetor ice is most likely to form when
temperature and humidity are in ranges indicated by this chart,
carburetor ice is also possible under conditions not depicted.
Fuel tank
Fuel lines to each cylinder
Fuel manifold valve
Electric fuel pump
Fuel filter
Throttle control
Fuel/air control unit
Fuel
pressure
regulator
Figure 4-15. Fuel injection system.
dictate how susceptible the carburetor is to icing. Consult
the aircraft POH for the probability of carburetor ice for the
specifi c installation and for carburetor ice procedures.
Fuel Injection Induction Systems
In a fuel injection system, the fuel is injected either directly
into the cylinders or just ahead of the intake valve. A fuel
injection system usually incorporates these basic components:
engine-driven fuel pump, fuel/air control unit, fuel manifold
(fuel distributor), discharge nozzles, auxiliary fuel pump, and
fuel pressure/fl ow indicators. [Figure 4-15]
The engine-driven fuel pump provides fuel under pressure
from the fuel tank to the fuel/air control unit. This control
unit, which essentially replaces the carburetor, meters the fuel
and sends it to the fuel manifold valve at a rate controlled
by the throttle. After reaching the fuel manifold valve, the
fuel is distributed to the individual fuel discharge nozzles.
The discharge nozzles, which are located in each cylinder
head, inject the fuel/air mixture at the precise time for each
cylinder directly into each cylinder intake port.
Some of the advantages of fuel injection are:
• No carburetor icing.
• Better fuel fl ow.
• Faster throttle response.
• Precise control of mixture.
• Better fuel distribution.
• Easier cold weather starts.
Disadvantages include:
• Diffi culty in starting a hot engine.
• Vapor locks during ground operations on hot days.
• Problems associated with restarting an engine that
quits because of fuel starvation.
Figure 4-16. Keyed ignition system with integral starter.
Ignition System
The typical ignition system on WSC aircraft provides the
spark that ignites the fuel/air mixture in the cylinders and
is made up of magneto/generators, control boxes, spark
plugs, high-voltage leads, and the ignition switch. For most
LSA engines designed specifi cally for aircraft, a magneto/
generator uses a permanent magnet to generate an electric
current independent of the aircraft’s electrical system, which
might include a battery. The aircraft electrical system can
fail—the battery can go dead. However, this has no effect
on the ignition system.
The electricity from the ignition magneto/generator goes into
the ignition control box where the correct voltage is produced
and timed to fi re the spark plugs at the proper time. Modern
WSC aircraft use an electronic capacitance discharge system
that operates without any moving parts to increase reliability
and effi ciency. Capacitance Digital Systems (CDI) operate
similarly but they have the ability to change the timing of the
spark for different rpm. Consult the POH for the particular
system for each engine.
The system begins to fi re when the starter is engaged and the
crankshaft begins to turn. It continues to operate whenever
the crankshaft is rotating. Most WSC aircraft incorporate
a dual ignition system with two individual magneto/
generators, separate sets of wires, separate sets of control
boxes, and separate sets of spark plugs to increase reliability
of the ignition system. Each magneto/generator operates
independently to fi re one of the two spark plugs in each
cylinder. If one of the systems fails, the other is unaffected.
The engine will continue to operate normally, although a slight
decrease in engine power can be expected.
The operation of the magneto/generator output to the ignition
system is controlled in the fl ight deck by the ignition switch.
Since there are two individual ignition systems, there are
normally two separate ignition toggle switches or separate
positions on the ignition control, as shown in Figure 4-16.
Identifi cation of a malfunctioning ignition system during
the pretakeoff check is observed by the decrease in rpm that
occurs when fi rst turning off one ignition switch, turning it
back on, and then turning off the other. A noticeable decrease
in engine rpm is normal during this check. If the engine
stops running when switching to one ignition system or if
the rpm drop exceeds the allowable limit, do not fl y until
the problem is corrected. The cause could be fouled plugs,
broken or shorted wires between the magneto/generator and
spark plugs, or improperly timed fi ring of the plugs because
of a defective control box. It should be noted that “no drop”
in rpm is not normal, and in that instance, the aircraft should
not be fl own. Following engine shutdown, keep the ignition
switches in the OFF position. Even with the battery and
master switches OFF, the engine can fi re and turn over if
an ignition switch is left ON and the propeller is moved
because the magneto/generator requires no outside source
of electrical power. The potential for serious injury in this
situation is obvious.
Standard category aircraft engine systems are described in
the Pilots Handbook of Aeronautical Knowledge; however,
these engines are not typically used on WSC. Automobile
engines or other non aircraft engines may be used on WSC
where the ignition system runs off the battery rather than a
magneto/generator system. In this case if the battery system
fails, the engine ignition system will fail and the engine will
stop.
Combustion
During normal combustion, the fuel/air mixture burns in
a very controlled and predictable manner. Although the
process occurs in a fraction of a second, the mixture actually
begins to burn at the point where it is ignited by the spark
plugs, then burns away from the plugs until it is consumed
completely. This type of combustion causes a smooth buildup
of temperature and pressure and ensures that the expanding
gases deliver the maximum force to the piston at exactly the
right time in the power stroke.
Detonation is an uncontrolled, explosive ignition of the
fuel/air mixture within the cylinder’s combustion chamber.
It causes excessive temperatures and pressures which, if not