Chapter 4 — Powerplants
Mixing Two-Stroke Oil and Fuel
Two-stroke engines require special two-stroke oil to
be mixed into the fuel before reaching the cylinder of
the engine. In some engines, an oil injection pump is
used to deliver the exact amount of oil into the intake
of the engine depending on the throttle setting. An advantage
of an oil injection system is pilots don’t have
to premix any oil into the fuel. However, an important
preflight check is to make sure the two-stroke oil reservoir
is properly filled.
If a two-stroke engine doesn’t have an oil injection
system, it is critical to mix oil into fuel before it is
put into the tank. Just pouring oil into the fuel tank
doesn’t give it the proper chance to mix with the gas
and makes it difficult to measure the proper amount of
oil for mixing. To mix two-stroke oil you should:
• Find a clean, approved container. Pour a little
gas into it to help pre-dilute the two-stroke oil.
• Pour in a known amount of two-stroke oil
into the container. Oil should be approved for
air-cooled engines at 50:1 mixing ratio (check
the engine manufacturer for proper fuel to oil
ratio for your PPC). Use a measuring cup if
necessary. Shake the oil-gas mixture around a
little to dilute the oil with gasoline.
• Add gasoline until the 50:1 ratio is reached. If
you choose to use a water separating funnel,
make sure the funnel is grounded or at least in
contact with the fuel container.
• Put the cap on the fuel can and shake the
gasoline and oil mixture thoroughly.
Most small aircraft use a direct-cranking electric
starter system. This system consists of a source of electricity, wiring, switches, and solenoids to operate
the starter and a starter motor. The starter engages the
aircraft flywheel or the gearbox, rotating the engine
at a speed that allows the engine to start and maintain
Electrical power for starting is usually supplied
by an on-board battery. When the battery switch is
turned on, electricity is supplied to the main power
bus through the battery solenoid. Both the starter and
the starter switch draw current from the main bus, but
the starter will not operate until the starting solenoid
is energized by the starter switch being turned to the
“start” position. When the starter switch is released
from the “start” position, the solenoid removes power
from the starter motor. The starter motor is protected
from being driven by the engine through a clutch in
the starter drive that allows the engine to run faster
than the starter motor.
In a four-stroke engine, the engine oil system performs
several important functions, including:
• Lubrication of the engine’s moving parts.
• Cooling of the engine by reducing friction.
• Removing heat from the cylinders.
• Providing a seal between the cylinder walls and
• Carrying away contaminants.
Four-stroke engines use either a wet-sump or drysump
oil system. Refer to Chapter 5 of the Pilot’s
Handbook of Aeronautical Knowledge for more information
on four-stroke oil systems.
Engine Cooling Systems
The burning fuel within the cylinders produces intense
heat, most of which is expelled through the exhaust
system. Much of the remaining heat, however,
must be removed, or at least dissipated, to prevent the
engine from overheating.
While the oil system in a four-stroke engine and the
fuel-oil mix in a two-stroke engine is vital to the internal
cooling of the engine, an additional method of
cooling is necessary for the engine’s external surface.
Powered parachute engines operate with either aircooled
or liquid-cooled systems.
Many powered parachutes are equipped with a cylinder
head temperature (CHT) gauge. This instrument
indicates a direct and immediate cylinder temperature change. This instrument is calibrated in degrees Celsius
or Fahrenheit. Proper CHT ranges can be found
in the pilot’s operating handbook for that machine.
Air cooling is accomplished by air being pulled into
the engine shroud by a cooling fan. Baffles route this
air over fins attached to the engine cylinders where
the air absorbs the engine heat. Expulsion of the hot
air takes place through one or more openings in the
shroud. If cylinder head temperatures rise too much in
an air cooled engine, it is because of lubrication problems:
cooling fan drive belt damage or wear, or air
blockage in the cooling fins by a bird or insect nest.
Liquid cooling systems pump coolant through jackets
in the cylinders and head. The heated liquid is then
routed to a radiator where the heat is radiated to the
atmosphere. The cooled liquid is then returned to the
engine. If the radiator is mounted low and close to the
propeller, the propeller can constantly move air across
the radiator and keep the engine cool even when the
powered parachute is not moving. Radiators mounted
high and away from the propeller raise the center of
gravity and make it more difficult for the radiator to cool the engine unless the powered parachute is moving.
Breaking in an engine through ground runs on a
hot day is when radiator placement is most critical.
Liquid-cooled engines can overheat for a number of
reasons, such as coolant not at proper levels, a leak, a
failed water pump, or a blockage of the radiator. Operating
an engine above its maximum design temperature
can cause a loss of power and detonation. It will
also lead to serious permanent damage, such as scoring
the cylinder walls and damaging the pistons and
rings. Monitor the engine temperature instruments to
avoid high operating temperature.
Operating the engine lower than its designed temperature
range can cause piston seizure and scarring on
the cylinder walls. This happens most often in liquidcooled
powered parachutes in cold weather where
large radiators designed for summer flying may need
to be partially blocked off.