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Chapter 4 — Powerplants
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.
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.
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
exhaust piping attached to the cylinders, as well as
a muffler. 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 specifications.
When there is a problem with carburetion, the
EGT gauge will normally be the first notification for
a pilot.
Two-Stroke Tuned Exhaust Systems
In two-stroke engines, the exhaust system increases
the fuel economy and power of the engine. The twostroke
exhaust system is an integral part of any twostroke
engine design; often controlling peak power
output, the torque curve, and even the RPM limit of
the engine.
The exhaust system must be tuned to produce a back
pressure wave to act as an exhaust valve. 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.
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. That is called pulse-charging and
is another important function of the exhaust system.
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 RPMs. 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 RPMs from idle to full
speed. This is part of the reason that simply putting a
snowmobile engine on a powered parachute doesn’t
work well.
Overall, the two-stroke exhaust system for a PPC is
a specific 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 configurations
is required for Special Light Sport Aircraft
(S-LSA) and recommended for Experimental Light
Sport Aircraft (E-LSA).
Two-Stroke Engine Warming
Two-stroke engines must be warmed up because metals
expand at different rates as they heat up. If you
heat up steel and aluminum, you will find that the aluminum
parts expand faster than the steel parts. This
becomes a problem in two different areas of many
two-stroke engines. The first place is in the cylinders
of the engine.
The cylinders have steel cylinder 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. That 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 engine crankshaft. This is an area where
things 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 you heat up the engine two quickly, the aluminum
case will out-expand those steel bearings and the
crank will cause the bearings to start spinning along
with it. If those steel bearings start spinning, they can
ruin the soft aluminum walls of the case, which is
very expensive.
If heat is slowly added to an engine, all the 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
your POH; however, a good rule of thumb is to initially
start the engine at idle RPM, get it operating
smoothly, and then warm the engine at 3,000 RPM
for 5 minutes.
Once the engine is warmed up and the powered parachute
is flying, it is still possible to cool down the
engine too much. This will happen 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 efficiently dumping 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 first 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 fluid 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 because of
the added heat and the cylinder is cooling with the
cold radiator water resulting in a piston seizure. To prevent this, slowly add power well before you get
close to the ground where you will need power. This
will give the system a chance to gradually open the
thermostat and warm up the radiator water.
Just as it takes a while for the engine crankcase and
bearings to warm up, it also takes those steel parts a
long time to cool down. If you land, refuel and want
to take off again quickly, there is no need to warm up
again for 5 minutes. The lower end of the engine will
stay warmed up after being shut down for short periods.
An engine restart is an example where 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 you only need to be concerned
with preventing the pistons from seizing.
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