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crankshaft moved, but earlier
engines made the crankshaft stay still
while the cylinders moved. As you
42 Canadian Private Pilot Studies
can imagine, maintenance was
difficult, to say the least.
Most modern light aircraft, including
helicopters, have their cylinders
opposite each other (horizontally or
vertically opposed), to cancel some
forces out, but many have them in
line and even in a V formation.
Some engines are even upside down,
as found with the Gypsy Major in
the Chipmunk (since the pistons in
this case are at the bottom, there is a
danger of oil leaking into the
combustion chambers, which is why
the engines are turned through one
complete cycle before being started,
to make sure the insides are free to
move. You will break the starter if
the cylinders are blocked with oil).
One big difference the average car
driver will find is that an aero engine
has two spark plugs per cylinder,
which are powered by independent
magnetos (see below). When doing
power checks before takeoff, they
are checked against each other for
power and whether they actually are
independent. Another is that car
engines are a lot smoother (there are
some diesel engines that won't upset
a coin standing on its edge). As
cylinders in an engine are all the
same, we will look at just one in a
moment to see how they work (see
The 4 Stroke Cycle, below).
The cylinder is just that, but it is
closed at one end to provide an
airtight seal (there are valves in the
cylinder head to allow the mix of air
and fuel in, and exhaust gases out,
but only at specified times). Inside
the cylinder is a piston, which slides
up and down to provide an action
like a pump, since it pulls air and fuel
in, and pushes the exhaust out.
As the piston is meant to be gastight,
and no fit is perfect, the piston will
have two or three rings round it (the
scraper is for cleaning) to mate against
the cylinder wall and stop movement
of anything from one side of it to
the other because, on the one hand,
the engine will not produce full
power if the burnt gases leak out
and, on the other, oil will get
through to the head from the
lubrication system, mix with the fuel
and air and cause a lot of bluish grey
smoke (if you are getting mysterious
oil leaks from your car, and
everything appears to be done up
underneath, check your piston rings,
as they may be allowing pressurised
gases through to the sump to force
the oil out).
The piston is attached to the
crankshaft with the connecting rod (or
conrod, for short), with a big and a
small end, for the crankshaft and
piston, respectively (if either end
goes, the engine will suddenly start
clattering loudly):
The crankshaft in an in-line engine
(that is, one that is not a radial,
including V8s and horizontally
opposed) is not straight– it is offset
for each piston connected to it, one
Airframes, Engines & Systems 43
after the other, so the up and down
(reciprocating) movement of the
piston is translated into rotary
motion, to drive a propeller or main
rotor gearbox:
The length of the piston's travel
inside the cylinder is known as the
stroke, and the peaks of its travel up
and down are known as top- and
bottom dead centre. The former is an
important factor in the timing of the
spark, discussed below.
When the piston is at the top of its
stroke, there is a small space
between it and the cylinder head,
into which the fuel/air mixture is
pulled and compressed (it's called
the combustion chamber). In being
compressed, the mixture gets
warmer, making it more disposed to
ignite when the spark plug fires:
Unfortunately, some mixtures get so
warm they can ignite without the
spark (diesels work this way), which
will not only increase the operating
temperature unnecessarily, but cause
real harm to the engine from shock
waves, caused when the piston gets
the effects of the power stroke when
it doesn't expect it (pound for
pound, fuel is more explosive than
dynamite). This pre-ignition also
comes from hotspots in the cylinder,
such as the end of the spark plug, or
lead oxybromide deposits that result
from leaning, which are likely to be
glowing bright red. This may happen
if cooling is inadequate, as when
climbing too steeply, and is why an
engine sometimes doesn't stop.
Once the mixture has ignited this
　
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