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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]
4-7
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 outexpands
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
　
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